U.S. patent application number 12/750039 was filed with the patent office on 2011-03-03 for surface structures for enhancement of quantum yield in broad spectrum emission nanocrystals.
Invention is credited to James R. McBride, Sandra J. Rosenthal, Michael A. Schreuder.
Application Number | 20110049442 12/750039 |
Document ID | / |
Family ID | 44720400 |
Filed Date | 2011-03-03 |
United States Patent
Application |
20110049442 |
Kind Code |
A1 |
Schreuder; Michael A. ; et
al. |
March 3, 2011 |
SURFACE STRUCTURES FOR ENHANCEMENT OF QUANTUM YIELD IN BROAD
SPECTRUM EMISSION NANOCRYSTALS
Abstract
Disclosed are inorganic nanoparticles comprising a body
comprising cadmium and/or zinc crystallized with selenium, sulfur,
and/or tellurium; a multiplicity of phosphonic acid ligands
comprising at least about 20% of the total surface ligand coverage;
wherein the nanocrystal is capable of absorbing energy from a first
electromagnetic region and capable of emitting light in a second
electromagnetic region, wherein the maximum absorbance wavelength
of the first electromagnetic region is different from the maximum
emission wavelength of the second electromagnetic region, thereby
providing a Stokes shift of at least about 20 nm, wherein the
second electromagnetic region comprises an at least about 100 nm
wide band of wavelengths, and wherein the nanoparticle exhibits has
a quantum yield of at least about 10%. This abstract is intended as
a scanning tool for purposes of searching in the particular art and
is not intended to be limiting of the present invention.
Inventors: |
Schreuder; Michael A.;
(Burnsville, MN) ; McBride; James R.; (Nashville,
TN) ; Rosenthal; Sandra J.; (Nashville, TN) |
Family ID: |
44720400 |
Appl. No.: |
12/750039 |
Filed: |
March 30, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61165283 |
Mar 31, 2009 |
|
|
|
Current U.S.
Class: |
252/519.2 ;
428/402; 556/24 |
Current CPC
Class: |
B82Y 30/00 20130101;
C09K 11/02 20130101; C09C 1/10 20130101; C09C 1/12 20130101; C09K
11/883 20130101; C09K 11/565 20130101; C01P 2004/64 20130101; H05B
33/14 20130101; C09C 1/04 20130101; C01P 2002/82 20130101; C09K
11/025 20130101; Y10T 428/2982 20150115 |
Class at
Publication: |
252/519.2 ;
428/402; 556/24 |
International
Class: |
H01B 1/12 20060101
H01B001/12; B32B 5/16 20060101 B32B005/16; C07F 9/09 20060101
C07F009/09 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] This invention was made with government support under Grant
DEFG0202ER45957 awarded by the United States Department of Energy
and Grant IIP0646322 awarded as a SPIR Phase 2 grant by the
National Science Foundation. The United States government has
certain rights in the invention.
Claims
1. An inorganic nanoparticle comprising: a. a body comprising Cd
and/or Zn crystallized with selenium, sulfur, and/or tellurium; b.
a surface covered with a multiplicity of surface ligands, wherein
phosphonic acid ligands comprise at least about 20% of the total
surface ligand coverage; and c. a diameter of less than about 3.0
nm, wherein the nanocrystal is capable of absorbing energy from a
first electromagnetic region and capable of emitting light in a
second electromagnetic region, wherein the maximum absorbance
wavelength of the first electromagnetic region is different from
the maximum emission wavelength of the second electromagnetic
region, thereby providing a Stokes shift of at least about 20 nm,
wherein the second electromagnetic region comprises an at least
about 100 nm wide (FWHH) band of wavelengths, and wherein the
nanoparticle exhibits has a quantum yield of at least about
10%.
2. The nanoparticle of claim 1, wherein the second emission
electromagnetic region comprises wavelengths of from about 420 nm
to about 710 nm.
3. The nanoparticle of claim 1, wherein the first electromagnetic
region comprises light with a wavelength of less than about 450
nm.
4. The nanoparticle of claim 1, wherein the first electromagnetic
region comprises light with a wavelength of about 414 nm.
5. The nanoparticle of claim 1, wherein the nanoparticle comprises
at least one of cadmium selenide, cadmium sulfide, or cadmium
telluride, or a mixture thereof.
6. The nanoparticle of claim 1, wherein the nanoparticle comprises
at least one of zinc sulfide, zinc selenide, zinc telluride,
magnesium sulfide, magnesium selenide, magnesium telluride, or a
mixture thereof.
7. The nanoparticle of claim 1, further comprising an optically
transparent inorganic shell at the surface of the nanoparticle.
8. The nanoparticle of claim 7, wherein the optically transparent
inorganic shell comprises zinc sulfide or magnesium sulfide.
9. The nanoparticle of claim 1, further comprising an organic
ligand coating at the surface of the nanoparticle.
10. The nanoparticle of claim 9, wherein the organic ligand coating
comprises at least one of a hexadecylamine residue, a
dodecylphosphonic acid residue, a tri-n-butylphosphine residue, a
tri-n-octylphosphine oxide residue, or a mixture thereof.
11. The nanoparticle of claim 1, wherein the nanoparticle was not
produced by an etching process.
12. A method of preparing an inorganic nanoparticle comprising the
steps of: a. heating to a temperature of greater than about
300.degree. C. a reaction mixture comprising a C.sub.4 to C.sub.22
alkyl- or aryl-phosphonic acid and a source of cadmium or zinc in a
molar ratio of from about 1:5 to about 1:1 Cd/Zn:phosphonic acid;
b. adding to the reaction mixture an injection mixture comprising a
C.sub.2 to C.sub.16 trialkyl- or triarylphosphine and a source of
selenium, sulfur, or tellurium; and c. decreasing the temperature
of the reaction mixture to less than about 300.degree. C.
13. The method of claim 12, wherein the injection mixture further
comprises a C.sub.6 to C.sub.24 hydrocarbon, provided in a ratio in
the injection solution of less than about 90:10 to the C.sub.2 to
C.sub.16 trialkyl- or triarylphosphine.
14. The method of claim 13, wherein the C.sub.6 to C.sub.24
hydrocarbon is octadecene.
15. The method of claim 13, wherein the C.sub.2 to C.sub.16
trialkyl- or triarylphosphine is tri-n-butylphosphine.
16. The method of claim 12, wherein the inorganic nanoparticle
prepared has a quantum yield of at least about 10%.
17. The method of claim 12, further comprising the step of adding a
solvent to the reaction mixture so as to decrease the temperature
of the reaction mixture to less than about 250.degree. C.
18. The method of claim 12, wherein the source of cadmium or zinc
comprises cadmium oxide.
19. The method of claim 12, wherein the source of cadmium or zinc
comprises zinc oxide.
20. The method of claim 12, wherein the source of selenium, sulfur,
or tellurium comprises selenium powder.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Application No.
61/165,283, filed Mar. 31, 2009; which is hereby incorporated
herein by reference in entirety.
BACKGROUND
[0003] In response to ever increasing energy demands and subsequent
costs, a tremendous emphasis is being placed on energy-saving
solid-state lighting devices in the form of light emitting diodes
(LEDs). Specifically, a need exists for pure white-light LEDs as a
more efficient replacement for conventional lighting sources.
Switching to solid state lighting would reduce global electricity
use by 50% and reduce power consumption by 760 GW in the United
States alone over a 20 year period. Solid-State Lighting
http://lighting.sandia.gov/; The Promise of Solid State Lighting
for General Illumination, Optoelectronics Industry Development
Association, Washington, D.C. (2001).
[0004] The complications associated with design and fabrication of
such devices have generated great interest in developing
white-light phosphors that do not depend on complex doping schemes
or combinations of materials. One proposed solution is to use a
mixture of semiconductor nanocrystals as the intrinsic emitting
layer for an LED device. Achermann, M.; Petruska, M. A.; Kos, S.;
Smith, D. L.; Koleske, D. D.; Klimov, V. I. Nature 2004, 429,
642-646; Mueller, A. H.; Petruska, M. A.; Achermann, M.; Werder, D.
J.; Akhadov, E. A.; Koleske, D. D.; Hoffbauer, M. A.; Klimov, V. I.
Nano Lett. 2005, 5, 1039-1044. Semiconductor nanocrystals can
exhibit high fluorescence quantum efficiencies and large molar
absorptivities. Alivisatos, A. P. Science 1996, 271, 933-937; Yu,
W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15, 2854-2860.
However, mixtures of such nanocrystals merely approximate white
light by mixing the traditional red, green, and blue colors and
result in a loss in total device efficiency due to self absorption
for a device of more than a few monolayers. Mueller, A. H.;
Petruska, M. A.; Achermann, M.; Werder, D. J.; Akhadov, E. A.;
Koleske, D. D.; Hoffbauer, M. A.; Klimov, V. I. Nano Lett. 2005, 5,
1039-1044.
[0005] Therefore, there remains a need for methods and compositions
that overcome these deficiencies and that effectively provide
broad-emission nanocrystals having improved quantum yield.
SUMMARY
[0006] In accordance with the purpose(s) of the invention, as
embodied and broadly described herein, the invention, in one
aspect, relates to ultra-small nanocrystals exhibiting a quantum
yield of at least about 10%.
[0007] Disclosed are inorganic nanoparticles comprising a body
comprising Cd and/or Zn crystallized with selenium, sulfur, and/or
tellurium; a surface covered with a multiplicity of surface
ligands, wherein phosphonic acid ligands comprise at least about
20% of the total surface ligand coverage; and a diameter of less
than about 3.0 nm, wherein the nanocrystal is capable of absorbing
energy from a first electromagnetic region and capable of emitting
light in a second electromagnetic region, wherein the maximum
absorbance wavelength of the first electromagnetic region is
different from the maximum emission wavelength of the second
electromagnetic region, thereby providing a Stokes shift of at
least about 20 nm, wherein the second electromagnetic region
comprises an at least about 100 nm wide (FWHH) band of wavelengths,
and wherein the nanoparticle exhibits has a quantum yield of at
least about 10%.
[0008] Also disclosed are methods of preparing an inorganic
nanoparticle comprising the steps of heating to a temperature of
greater than about 300.degree. C. a reaction mixture comprising a
C.sub.4 to C.sub.22 alkyl- or aryl-phosphonic acid and a source of
cadmium or zinc in a molar ratio of from about 1:5 to about 1:1
Cd/Zn:phosphonic acid; adding to the reaction mixture an injection
mixture comprising a C.sub.2 to C.sub.16 trialkyl- or
triarylphosphine and a source of selenium, sulfur, or tellurium;
and decreasing the temperature of the reaction mixture to less than
about 300.degree. C.
[0009] Also disclosed are the products produced by the methods of
the invention.
[0010] Also disclosed are frequency converters comprising at least
one quantum dot of the invention, nanoparticle of the invention,
plurality of nanoparticles of the invention, or product of the
invention, or mixture thereof dispersed within a matrix, for
example, a polymeric matrix or a glass matrix.
[0011] Also disclosed are light emitting diode devices comprising a
light emitting diode capable of emitting energy of a first
wavelength, and the frequency converter of the invention positioned
within an emission path of the light emitting diode, wherein the
frequency converter is capable of absorbing energy of the first
wavelength.
[0012] Also disclosed are methods for producing a light emitting
diode device comprising the step of positioning the frequency
converter of the invention within an emission path of a light
emitting diode capable of emitting energy of a first wavelength,
wherein the frequency converter is capable of absorbing energy of
the first wavelength.
[0013] Also disclosed are modified fluorescent light sources
comprising a substantially optically transparent and substantially
hermetically sealed tube having a first end, a second end, an
interior surface, an exterior surface, and a lumen extending
therethrough; a first electrode positioned at the first end; a
second electrode positioned at the second end; inert gas vapor and
mercury vapor within the lumen of the tube; a phosphor substitute
comprising at least one frequency converter of the invention,
quantum dot of the invention, nanoparticle of the invention,
plurality of nanoparticles of any of the invention, or product of
the invention, or mixture thereof substantially coating the
interior surface or the exterior surface of the tube.
[0014] Also disclosed are electroluminescent devices comprising an
n-type semiconductor, a p-type semiconductor, and a quantum dot
layer in electrical or photonic communication with the n-type
semiconductor and the p-type semiconductor, wherein the quantum dot
layer comprises at least one quantum dot of the invention,
nanoparticle of the invention, plurality of nanoparticles of any of
the invention, product of the invention, or frequency converter of
the invention, or mixture thereof.
[0015] Also disclosed are energy cascade systems comprising at
least two quantum dots in photonic or energetic communication,
wherein the first quantum dot is capable of absorbing energy from a
first absorption electromagnetic region and capable of emitting
energy in a first emission electromagnetic region, wherein the
second quantum dot is capable of absorbing energy from a second
absorption electromagnetic region and capable of emitting energy in
a second emission electromagnetic region, and wherein the first
emission electromagnetic region overlaps with the second absorption
electromagnetic region.
[0016] While aspects of the present invention can be described and
claimed in a particular statutory class, such as the system
statutory class, this is for convenience only and one of skill in
the art will understand that each aspect of the present invention
can be described and claimed in any statutory class. Unless
otherwise expressly stated, it is in no way intended that any
method or aspect set forth herein be construed as requiring that
its steps be performed in a specific order. Accordingly, where a
method claim does not specifically state in the claims or
descriptions that the steps are to be limited to a specific order,
it is no way intended that an order be inferred, in any respect.
This holds for any possible non-express basis for interpretation,
including matters of logic with respect to arrangement of steps or
operational flow, plain meaning derived from grammatical
organization or punctuation, or the number or type of aspects
described in the specification.
[0017] Additional advantages of the invention will be set forth in
part in the description which follows, and in part will be obvious
from the description, or may be learned by practice of the
invention. The advantages of the invention will be realized and
attained by means of the elements and combinations particularly
pointed out in the appended claims. It is to be understood that
both the foregoing general description and the following detailed
description are exemplary and explanatory only and are not
restrictive of the invention, as claimed.
BRIEF DESCRIPTION OF THE FIGURES
[0018] The accompanying figures, which are incorporated in and
constitute a part of this specification, illustrate several
embodiments and together with the description serve to explain the
principles of the invention.
[0019] FIG. 1 shows absorption (dashed line) and emission (solid
line, .lamda..sub.ex=367 nm) spectra of ultra-small CdSe. Static
absorption and emission spectra were obtained using a Cary Bio 50
UV-Visible spectrometer and an ISS photon counting fluorescence
spectrometer, respectively.
[0020] FIG. 2 shows a C. I. E. 1931 chromaticity diagram. The
chromaticity diagram illustrates all of the colors of the visible
spectrum corrected for the response of the human eye. The dot
indicates the chromaticity coordinates of the nanoparticles of the
invention. This figure illustrates that the nanoparticles produce a
slightly warm white light that is comparable to that of a tungsten
light bulb.
[0021] FIG. 3 shows white-light emission from CdSe nanoparticles of
the invention. (Left panel) Thin film of ultra-small CdSe in
polyurethane excited by a frequency doubled titanium:sapphire laser
(400 nm) with white-light clearly seen reflecting off the table
surface. (Right panel) 5 mm commercial UV LED (400 nm) illuminating
a thin coating of ultra-small CdSe in polyurethane.
[0022] FIG. 4 shows a comparison of conventional tungsten and
fluorescent lighting emission spectra with the AM 1.5 solar
spectrum, a commercial white LED and ultra-small CdSe nanocrystals.
The CdSe nanoparticles of the invention exhibit a broad emission
covering most of the visible spectrum. Unlike conventional tungsten
sources, no energy is wasted producing wavelengths beyond the
visible spectrum.
[0023] FIG. 5 shows a schematic of an electroluminescent device
with nanoparticles of the invention as the emitting layer.
[0024] FIG. 6 shows a schematic of the geometry for
electroluminescent and frequency converting LED device
characterization.
[0025] FIG. 7 shows an absorption/emission spectrum of CdSe
nanocrystals prepared by an etching process.
[0026] FIG. 8 shows an absorption/emission spectrum of ultra-small
cadmium selenide nanocrystals having improved quantum yield.
[0027] FIG. 9 shows images of (a) three 385 nm LEDs powered at 20
mA each, and white-light CdSe nanocrystals in PFCB polymer at
concentrations of (b) 1% nanocrystals in PFCB and (c) 2%
nanocrystals in PFCB, with three 385 nm LEDs illuminating the
samples from underneath.
[0028] FIG. 10 shows absorption spectra of PFCB, and 1%, 2%, 5%,
and 10% by weight ultra-small CdSe nanocrystals dispersed in
PFCB.
[0029] FIG. 11 shows emission spectra of PFCB, and 1%, 2%, 5%, and
10% by weight ultra-small CdSe nanocrystals dispersed in PFCB. The
PFCB only curve was multiplied by 1/2 for scaling purposes.
[0030] FIG. 12 shows the first emission peak of ultra-small CdSe
nanocrystals plotted vs. the band-edge absorption peak. Note that
below 1.7 nm the emission ceases to blue-shift, though the
absorption feature does blue-shift.
[0031] FIG. 13 shows the Michaelis-Arbuzov reaction used to
synthesize various phosphonic acids and the structures of said
phosphonic acids. Phenyl and hexyl phosphonic acid are not
included, as they were purchased.
[0032] FIG. 14 shows the first emission feature .lamda. vs.
band-edge absorption feature .lamda.. A) Traditional and pinned
nanocrystal sizes; the boxed area is the pinned region. B) Enlarged
area from A showing the pinned emission feature.
[0033] FIG. 15 shows the wavelength of pinned first emission
feature vs. number of carbons in the alkyl chain of the phosphonic
acid used during synthesis.
[0034] FIG. 16 shows the chemical structures and the corresponding
calculated grouped electronegativities for each structure.
[0035] FIG. 17 shows the normalized energy of pinned emission
feature and electronegativity of each phosphonic acid vs. number of
carbons in the alkyl chain. The experimental and theoretical values
are normalized to the respective butyl phosphonic acid energies of
each set.
[0036] FIG. 18 shows the quantum yield of dirty and clean
nanocrystals: (top) ultra-small size and (bottom) traditional size
regime. Clean and dirty refer to nanocrystal before and after the
cleaning process.
DETAILED DESCRIPTION
[0037] The present invention may be understood more readily by
reference to the following detailed description of aspects of the
invention and the Examples included therein and to the Figures and
their previous and following description.
[0038] Before the present compounds, compositions, articles,
devices, and/or methods are disclosed and described, it is to be
understood that they are not limited to specific synthetic methods
unless otherwise specified, or to particular reagents unless
otherwise specified, as such may, of course, vary. It is also to be
understood that the terminology used herein is for the purpose of
describing particular embodiments only and is not intended to be
limiting. Although any methods and materials similar or equivalent
to those described herein can be used in the practice or testing of
the present invention, example methods and materials are now
described.
[0039] All publications mentioned herein are incorporated herein by
reference to disclose and describe the methods and/or materials in
connection with which the publications are cited. The publications
discussed herein are provided solely for their disclosure prior to
the filing date of the present application. Nothing herein is to be
construed as an admission that the present invention is not
entitled to antedate such publication by virtue of prior invention.
Further, the dates of publication provided herein may be different
from the actual publication dates, which may need to be
independently confirmed.
A. DEFINITIONS
[0040] As used in the specification and the appended claims, the
singular forms "a," "an" and "the" include plural referents unless
the context clearly dictates otherwise. Thus, for example,
reference to "a component," "a polymer," or "a particle" includes
mixtures of two or more such components, polymers, or particles,
and the like.
[0041] Ranges can be expressed herein as from "about" one
particular value, and/or to "about" another particular value. When
such a range is expressed, another embodiment includes from the one
particular value and/or to the other particular value. Similarly,
when values are expressed as approximations, by use of the
antecedent "about," it will be understood that the particular value
forms another embodiment. It will be further understood that the
endpoints of each of the ranges are significant both in relation to
the other endpoint, and independently of the other endpoint. It is
also understood that there are a number of values disclosed herein,
and that each value is also herein disclosed as "about" that
particular value in addition to the value itself. For example, if
the value "10" is disclosed, then "about 10" is also disclosed. It
is also understood that when a value is disclosed that "less than
or equal to" the value, "greater than or equal to the value" and
possible ranges between values are also disclosed, as appropriately
understood by the skilled artisan. For example, if the value "10"
is disclosed the "less than or equal to 10" as well as "greater
than or equal to 10" is also disclosed. It is also understood that
throughout the application, data is provided in a number of
different formats and that this data represents endpoints and
starting points, and ranges for any combination of the data points.
For example, if a particular data point "10" and a particular data
point 15 are disclosed, it is understood that greater than, greater
than or equal to, less than, less than or equal to, and equal to 10
and 15 are considered disclosed as well as between 10 and 15. It is
also understood that each unit between two particular units are
also disclosed. For example, if 10 and 15 are disclosed, then 11,
12, 13, and 14 are also disclosed.
[0042] A residue of a chemical species, as used in the
specification and concluding claims, refers to the moiety that is
the resulting product of the chemical species in a particular
reaction scheme or subsequent formulation or chemical product,
regardless of whether the moiety is actually obtained from the
chemical species. Thus, an ethylene glycol residue in a polyester
refers to one or more --OCH.sub.2CH.sub.2O-- units in the
polyester, regardless of whether ethylene glycol was used to
prepare the polyester. Similarly, a sebacic acid residue in a
polyester refers to one or more --CO(CH.sub.2).sub.8CO-- moieties
in the polyester, regardless of whether the residue is obtained by
reacting sebacic acid or an ester thereof to obtain the
polyester.
[0043] As used herein, the terms "optional" or "optionally" means
that the subsequently described event or circumstance may or may
not occur, and that the description includes instances where said
event or circumstance occurs and instances where it does not.
[0044] As used herein, the term "copolymer" refers to a polymer
formed from two or more different repeating units (monomer
residues). By way of example and without limitation, a copolymer
can be an alternating copolymer, a random copolymer, a block
copolymer, or a graft copolymer.
[0045] As used herein, the term "visible spectrum" or "visible
light" or "visible light spectrum" refers to the portion of the
electromagnetic spectrum to which the human eye is sensitive, i.e.,
light with wavelengths from about 400 nm to about 700 nm.
[0046] As used herein, the term "white light" refers to light
energy integrated over the visible portion of the spectrum (i.e.,
from about 400 nm to about 700 nm) so that all colors are blended
to appear white to the eye. Such light contains approximately equal
amounts of the primary additive colors of light; the human eye
perceives this light as colorless.
[0047] As used herein, the term "quantum dot" refers to a
semiconductor crystal with a nanoscale diameter, also called a
nanocrystal, that because of its small size behaves like a
potential well that confines electrons in three dimensions to a
region on the order of the electrons' de Broglie wavelength in
size, a few nanometers in a semiconductor. Typically, a quantum dot
can absorb energy within a first electromagnetic region and emit
light in a second electromagnetic region; the particular absorbance
or emission regions can depend upon the material and diameter of
the quantum dot. In one aspect, the nanoparticle of the invention
can be considered a quantum dot or nanocrystal; however, the
nanoparticle of the invention can differ from conventional quantum
dots or nanocrystals in that the nanoparticle of the invention can
emit broad band visible light, for example, white light.
[0048] Disclosed are the components to be used to prepare the
compositions of the invention as well as the compositions
themselves to be used within the methods disclosed herein. These
and other materials are disclosed herein, and it is understood that
when combinations, subsets, interactions, groups, etc. of these
materials are disclosed that while specific reference of each
various individual and collective combinations and permutation of
these compounds may not be explicitly disclosed, each is
specifically contemplated and described herein. For example, if a
particular compound is disclosed and discussed and a number of
modifications that can be made to a number of molecules including
the compounds are discussed, specifically contemplated is each and
every combination and permutation of the compound and the
modifications that are possible unless specifically indicated to
the contrary. Thus, if a class of molecules A, B, and C are
disclosed as well as a class of molecules D, E, and F and an
example of a combination molecule, A-D is disclosed, then even if
each is not individually recited each is individually and
collectively contemplated meaning combinations, A-E, A-F, B-D, B-E,
B-F, C-D, C-E, and C-F are considered disclosed. Likewise, any
subset or combination of these is also disclosed. Thus, for
example, the sub-group of A-E, B-F, and C-E would be considered
disclosed. This concept applies to all aspects of this application
including, but not limited to, steps in methods of making and using
the compositions of the invention. Thus, if there are a variety of
additional steps that can be performed it is understood that each
of these additional steps can be performed with any specific
embodiment or combination of embodiments of the methods of the
invention.
[0049] It is understood that the compositions disclosed herein have
certain functions. Disclosed herein are certain structural
requirements for performing the disclosed functions, and it is
understood that there are a variety of structures that can perform
the same function that are related to the disclosed structures, and
that these structures will typically achieve the same result.
B. BROAD-EMISSION NANOCRYSTALS
[0050] In one aspect, the invention relates to ultra-small cadmium
selenide (CdSe) nanocrystals capable of emitting white light. The
inorganic particles or nanocrystals can also be referred to as
quantum dots. More specifically, in one aspect, the invention
relates to a quantum dot capable of emitting white light.
Typically, these ultra-small nanocrystals exhibit broadband
emission (e.g., from about 420 nm to about 710 nm) throughout the
visible light spectrum while not suffering from self absorption due
to an unusually narrow particle size distribution and an unusually
large Stokes shift (see, e.g., FIG. 1), making these nanocrystals
ideal materials for devices currently under development and also an
ideal platform to study the molecule-to-nanocrystal transition.
Achermann, M.; Petruska, M. A.; Kos, S.; Smith, D. L.; Koleske, D.
D.; Klimov, V. I. Nature 2004, 429, 642-646.
[0051] Without wishing to be bound by theory, it is believed that
this broadband emission is the result of the size of the
nanoparticle. In one aspect, the nanoparticles of the invention are
so small that the surface is intimately involved in the emission;
that is, the nanoparticle is essentially all surface, which is not
the case with larger nanocrystals. Further, without wishing to be
bound by theory, it is believed that the broadband emission is the
result of the surface structure of the nanoparticle. That is,
nanoparticles prepared by conventional techniques but of comparable
size to the nanoparticles of the invention typically do not exhibit
the broadband emission observed with the nanoparticles of the
invention.
[0052] In a further aspect, the invention relates to an inorganic
nanoparticle having a surface and a diameter and capable of
absorbing energy from a first electromagnetic region and capable of
emitting light in a second electromagnetic region, wherein the
diameter is less than about 5.0 nm, and wherein the second
electromagnetic region comprises an at least about 50 nm wide
(FWHH) band of wavelengths.
[0053] In a yet further aspect, the invention relates to an
inorganic nanoparticle having a surface and a diameter and capable
of absorbing energy from a first electromagnetic region and capable
of emitting light in a second electromagnetic region, wherein the
diameter is less than about 2.0 nm, and wherein the second
electromagnetic region comprises an at least about 100 nm wide
(FWHH) band of wavelengths.
[0054] In one aspect, the nanoparticle of the invention is not
produced by an etching process. In a further aspect, the
nanoparticle of the invention is not produced by an inverse
micellular process.
[0055] 1. Particle Size
[0056] In one aspect, the nanoparticle of the invention has a
diameter, which can also be referred to as a particle size. When,
the nanoparticle of the invention is provided as a collection of
nanoparticles, the diameter refers to the average diameter of the
nanoparticles in the collection. While referring to the particle
size as a diameter, it is understood that the particles can be
spherical, approximately spherical, or nonspherical. In
nonspherical cases, the diameter typically refers to the diameter
of a sphere having the same hydrodynamic volume of the
particle.
[0057] In one aspect, the diameter of nanoparticle of the invention
can be controlled by the method of preparation. For example, by
controlling the temperature of the preparation mixture, growth
kinetics can be favored relative to nucleation (also referred to as
nanoparticle formation or initiation) kinetics. Consequently, a
collection of nanoparticles can be provided with an unusually
narrow particle size distribution. In a further aspect, by further
controlling the temperature of the preparation mixture, growth
kinetics can be disfavored. Consequently, the size of the
nanoparticle or collection of nanoparticles can be limited to a
particular size or diameter.
[0058] In one aspect, the diameter of the nanoparticle of the
invention is less than about 3.0 nm, less than about 2.5 nm, less
than about 2.0 nm, less than about 1.5 nm, or less than about 1.0
nm.
[0059] In a further aspect, the diameter of the nanoparticle of the
invention can be from about 5.0 nm to about 0.5 nm, from about 5.0
nm to about 1.0 nm, from about 5.0 nm to about 2.0 nm, from about
5.0 nm to about 3.0 nm, from about 4.0 nm to about 0.5 nm, from
about 4.0 nm to about 1.0 nm, from about 4.0 nm to about 2.0 nm,
from about 4.0 nm to about 3.0 nm, from about 3.0 nm to about 0.5
nm, from about 3.0 nm to about 1.0 nm, from about 3.0 nm to about
2.0 nm, from about 2.0 nm to about 0.5 nm, or from about 2.0 nm to
about 1.0 nm. In a further aspect, the diameter of the nanoparticle
of the invention can be about 11 .ANG., about 12 .ANG., about 13
.ANG., about 14 .ANG., about 15 .ANG., about 16 .ANG., about 17
.ANG., about 18 .ANG., or about 19 .ANG..
[0060] 2. Absorption Region
[0061] In one aspect, the nanoparticle of the invention is capable
of absorbing energy in a first electromagnetic region. That is, the
nanoparticle can absorb light having one or more wavelengths. In a
further aspect, the first electromagnetic region comprises light
with a wavelength of less than about 450 nm, for example less than
about 425 nm, less than about 400 nm, less than about 375 nm, less
than about 350 nm, less than about 325 nm, less than about 300 nm,
less than about 275 nm, less than about 250 nm, less than about 225
nm, or less than about 200 nm. In a further aspect, the first
electromagnetic region comprises light of a wavelength of from
about 100 nm to about 450 nm, for example, from about 100 nm to
about 170 nm, from about 170 nm to about 290 nm, from about 100 nm
to about 290 nm, from about 290 nm to about 400 nm, from about 100
nm to about 320 nm, from about 290 nm to about 320 nm, from about
320 nm to about 400 nm, or from about 320 nm to about 450 nm. In a
yet further aspect, the first electromagnetic region comprises
light with a wavelength of from about 400 nm to about 430 nm, from
about 410 nm to about 420 nm, or about 414 nm.
[0062] The absorption of a photon of light by the semiconducting
material of the nanoparticle of the invention and subsequent
emission of a lower energy photon by the nanoparticle of the
invention results in fluorescence.
[0063] 3. Emission Region
[0064] In one aspect, the nanoparticle of the invention is capable
of emitting light in a second electromagnetic region. That is, the
nanoparticle can emit light having one or more wavelengths. When
the emission comprises more than one wavelength, such emission
comprises an emission band and can be characterized by a maximum
emission and an emission band width. Typically, absorbance band
width or emission band width can be reported as a band measurement
of full-width at half-height (FWHH). For example, an emission band
can comprise light of wavelengths from about 420 nm to about 710
nm, having a maximum emission at about 560 nm and an emission band
width of about 210 nm (FWHH) (see, e.g., FIG. 1).
[0065] In one aspect, the emission band comprises one or more
wavelengths within the second electromagnetic region. In a further
aspect, the emitted wavelengths are continuous. In a yet further
aspect, the emitted wavelengths are discontinuous. In a further
aspect, the emission band comprises all wavelengths within the
second electromagnetic region. In a yet further aspect, the
emission band comprises less than all wavelengths within the second
electromagnetic region.
[0066] In one aspect, the second electromagnetic region comprises
an at least about 50 nm wide (FWHH) band of wavelengths, for
example, at least about 60 nm wide (FWHH), at least about 70 nm
wide (FWHH), at least about 80 nm wide (FWHH), at least about 90 nm
wide (FWHH), at least about 100 nm wide (FWHH), at least about 125
nm wide (FWHH), at least about 150 nm wide (FWHH), at least about
175 nm wide (FWHH), at least about 200 nm wide (FWHH), at least
about 250 nm wide (FWHH), at least about 300 nm wide (FWHH), at
least about 350 nm wide (FWHH), or at least about 400 nm wide
(FWHH). In a further aspect, the second electromagnetic region
comprises an emission band at least about 100 nm wide, at least
about 125 nm wide, at least about 150 nm wide, at least about 175
nm wide, at least about 200 nm wide, at least about 250 nm wide, at
least about 300 nm wide, at least about 350 nm wide, or at least
about 400 nm wide.
[0067] In a further aspect, the second electromagnetic region can
comprise a band of wavelengths at least about 50 nm wide (FWHH), at
least about 60 nm wide (FWHH), at least about 70 nm wide (FWHH), at
least about 80 nm wide (FWHH), at least about 90 nm wide (FWHH), at
least about 100 nm wide (FWHH), at least about 125 nm wide (FWHH),
at least about 150 nm wide (FWHH), at least about 175 nm wide
(FWHH), at least about 200 nm wide (FWHH), at least about 250 nm
wide (FWHH), at least about 300 nm wide (FWHH), at least about 350
nm wide (FWHH), or at least about 400 nm wide (FWHH) in the region
between about 400 nm and about 700 nm, about 420 nm and about 710
nm, about 350 nm and about 750 nm, about 400 nm and about 600 nm,
about 500 nm and about 700 nm, about 300 nm and about 800 nm, about
500 nm and about 750 nm, or about 350 nm and about 600 nm.
[0068] In further aspects, the second electromagnetic region can
comprise light of all wavelengths from about 400 nm to about 700 nm
or the visible light spectrum.
[0069] 4. Nanocrystal Materials
[0070] In one aspect, the nanoparticle of the invention can be
provided from materials known to those of skill in the art of the
preparation of nanocrystals or quantum dots. For example, the
nanoparticle of the invention can comprise at least one of cadmium
selenide, cadmium sulfide, or cadmium telluride, or a mixture
thereof. In another example, the nanoparticle of the invention can
comprise at least one of zinc sulfide, zinc selenide, zinc
telluride, magnesium sulfide, magnesium selenide, magnesium
telluride, or a mixture thereof.
[0071] In a further aspect, the nanoparticle of the invention can
be provided as part of a collection of nanoparticles. In such an
aspect, the collection of nanoparticles can comprise nanoparticles
comprising at least one of cadmium selenide, cadmium sulfide,
cadmium telluride, zinc sulfide, zinc selenide, zinc telluride,
magnesium sulfide, magnesium selenide, magnesium telluride, or a
mixture thereof. The individual nanoparticles can be the same or
different in composition.
[0072] 5. Stokes Shift
[0073] In one aspect, absorbance region or first electromagnetic
region can be different from the emission region or second
electromagnetic region, thereby providing a Stokes shift.
Typically, the Stokes Shift is reported as the difference between a
wavelength of maximum absorbance and a wavelength of maximum
emission and can be expressed in nanometers (nm). For example, a
nanoparticle can have an absorbance band with a maximum absorbance
of about 414 nm and an emission band with a maximum emission at
about 441 nm, thereby providing a Stokes Shift of about 27 nm (see,
e.g., FIG. 1).
[0074] In a further aspect, the Stokes shift can be at least about
10 nm, at least about 15 nm, at least about 20 nm, at least about
25 nm, at least about 30 nm, at least about 35 nm, at least about
40 nm, at least about 45 nm, or at least about 50 nm. In a further
aspect, the Stokes shift can be from about 15 nm to about 50 nm,
from about 20 nm to about 50 nm, from about 30 nm to about 50 nm,
from about 40 nm to about 50 nm, from about 20 nm to about 30 nm,
from about 20 nm to about 40 nm, from about 20 nm to about 50 nm,
from about 30 nm to about 40 nm, or from about 30 nm to about 50
nm.
[0075] 6. Quantum Yield
[0076] In one aspect, the quantum dots of the invention exhibit a
quantum yield. As used herein, the "quantum yield" of a
radiation-induced process refers to the number of times that a
defined event occurs per photon absorbed by the system. Thus, the
quantum yield is a measure of the efficiency with which absorbed
light produces some effect.
[0077] With respect to the quantum dots of the invention, quantum
yield generally refers to a "light in" to "light out" ratio. Such a
ratio can be measured by any of the methods for measuring
electromagnetic absorbance and emission known to those of skill in
the art. For example, the "light in" can be measured by a
spectrophotometer set at the emission wavelength(s) of an
electromagnetic energy source (e.g., an ultraviolet light emitter).
As another example, the "light out" can be measured by a
spectrophotometer set at the wavelengths of white light (i.e., one
or more wavelengths of visible light) emitted by the quantum dots
of the invention.
[0078] The term "quantum yield" can be applied to a single quantum
dot or to an average of a plurality of quantum dots. In theory, a
quantum yield must be greater than 0% and is typically less than
100%. However, for photo-induced or radiation-induced chain
reactions, in which a single photon may trigger a long chain of
transformations, quantum yields greater than 100% are possible.
[0079] In one aspect, the quantum yield of the quantum dots of the
invention are greater than about 2%, for example, greater than
about 3%, greater than about 4%, greater than about 5%, greater
than about 6%, greater than about 7%, greater than about 8%,
greater than about 9%, greater than about 10%, greater than about
12%, greater than about 15%, greater than about 20%, greater than
about 25%, greater than about 30%, greater than about 35%, greater
than about 40%, greater than about 45%, greater than about 50%,
greater than about 55%, greater than about 60%, greater than about
65%, greater than about 70%, greater than about 75%, greater than
about 80%, greater than about 85%, greater than about 90%, or
greater than about 95%.
[0080] 7. Nanoparticle Surface
[0081] In one aspect, due to a very small particle size, the
nanoparticles of the invention can have an unusually high surface
area to volume ratio. Consequently, the surface composition can
have an influence on the observed properties of the nanoparticles
of the invention. For example, the surface composition can be
functionalized to enhance nanoparticle stability or to facilitate
formation of dispersions or suspensions of collections of
nanoparticles of the invention. Further, the surface of the
nanocrystal can play an intimate role in the optical properties of
the nanocrystal, such as the emission quantum yield (i.e., light in
to light out ratio) and the width of the spectrum (i.e., emission
band width).
[0082] The nanoparticles of the invention can be further
functionalized at the surface. Such functionalization can occur
during preparation or subsequent to preparation. In one aspect, the
nanoparticle of the invention can further comprise an inorganic
shell at the surface of the nanoparticle. The inorganic shell can
comprise, for example, cadmium selenide, cadmium sulfide, cadmium
telluride, zinc sulfide, zinc selenide, zinc telluride, magnesium
sulfide, magnesium selenide, magnesium telluride, zinc sulfide,
magnesium sulfide, or a mixture thereof. In a further aspect,
inorganic shell can comprise, for example, cadmium oxide, magnesium
oxide, zinc oxide, aluminum oxide, titanium dioxide, cadmium
sulfoselenide, or cadmium selenium oxide. In one aspect, the
inorganic shell can be optically transparent. In a further aspect,
the inorganic shell can be absent from the nanoparticle of the
invention.
[0083] In one aspect, the nanoparticle of the invention can further
comprise an organic ligand coating at the surface of the
nanoparticle. The organic ligand coating can comprise, for example,
at least one of an alkylamine or arylamine residue, an alkyl- or
arylphosphonic acid residue, a trialkyl- or triarylphosphine oxide
residue, a trialkyl- or triarylphosphine residue, or a mixture
thereof. In a further aspect, the organic ligand coating can
comprise at least one of a hexadecylamine residue, a
dodecylphosphonic acid residue, a tri-n-octylphosphine oxide
residue, or a mixture thereof. In a yet further aspect, the organic
ligand coating can comprise at least one alkyl or aryl phosphine;
for example, the organic ligand coating can comprise
tributylphosphine or triphenylphosphine. In an even further aspect,
the organic ligand coating can comprise at least one alkylthiol or
carboxylic acid or mixture thereof; for example, the organic ligand
coating can comprise 1-octanethiol or decanoic acid. In a further
aspect, organic ligand coating can be absent from the nanoparticle
of the invention.
[0084] In a yet further aspect, the nanoparticle of the invention
can further comprise an inorganic shell and an organic ligand
coating at the surface of the nanoparticle.
[0085] 8. Collections of Nanocrystals
[0086] In one aspect, the nanoparticle of the invention can be
provided as part of a collection of nanoparticles comprising a
plurality of nanoparticles of the invention. In such an aspect, the
plurality of nanoparticles can exhibit the absorption properties
and the emission properties disclosed herein for individual
nanoparticles of the invention.
[0087] For example, the plurality of nanoparticles is capable of
emitting light in a second electromagnetic region. In one aspect,
the second electromagnetic region comprises an at least about 50 nm
wide (FWHH) band of wavelengths, for example, at least about 60 nm
wide (FWHH), at least about 70 nm wide (FWHH), at least about 80 nm
wide (FWHH), at least about 90 nm wide (FWHH), at least about 100
nm wide (FWHH), at least about 125 nm wide (FWHH), at least about
150 nm wide (FWHH), at least about 175 nm wide (FWHH), at least
about 200 nm wide (FWHH), at least about 250 nm wide (FWHH), at
least about 300 nm wide (FWHH), at least about 350 nm wide (FWHH),
or at least about 400 nm wide (FWHH). In a further aspect, the
second electromagnetic region comprises an emission band at least
about 100 nm wide, at least about 125 nm wide, at least about 150
nm wide, at least about 175 nm wide, at least about 200 nm wide, at
least about 250 nm wide, at least about 300 nm wide, at least about
350 nm wide, or at least about 400 nm wide.
[0088] In a further aspect, the second electromagnetic region can
comprise a band of wavelengths at least about 50 nm wide (FWHH), at
least about 60 nm wide (FWHH), at least about 70 nm wide (FWHH), at
least about 80 nm wide (FWHH), at least about 90 nm wide (FWHH), at
least about 100 nm wide (FWHH), at least about 125 nm wide (FWHH),
at least about 150 nm wide (FWHH), at least about 175 nm wide
(FWHH), at least about 200 nm wide (FWHH), at least about 250 nm
wide (FWHH), at least about 300 nm wide (FWHH), at least about 350
nm wide (FWHH), or at least about 400 nm wide (FWHH) in the region
between about 400 nm and about 700 nm, about 420 nm and about 710
nm, about 350 nm and about 750 nm, about 400 nm and about 600 nm,
about 500 nm and about 700 nm, about 300 nm and about 800 nm, about
500 nm and about 750 nm, or about 350 nm and about 600 nm. In
further aspects, the second electromagnetic region can comprise
light of all wavelengths from about 400 nm to about 700 nm or the
visible light spectrum.
[0089] In one aspect, the plurality of nanoparticles of the
invention can have an unusually narrow size distribution.
Typically, the size distribution for a collection of nanoparticles
of the invention can be such that at least about 80% of the
nanoparticles have diameters within about 10% of the average
diameter of the collection of nanoparticles. For example, the size
distribution for a collection of nanoparticles of the invention can
be such that at least about 50%--for example, at least about 60%,
at least about 70%, at least about 80%, at least about 90%, or at
least about 95%--of the nanoparticles have diameters within about
50%--for example, within about 40%, within about 30%, within about
20%, within about 10%, or within about 5%--of the average diameter
of the collection of nanoparticles.
C. METHODS OF PREPARING BROAD-EMISSION NANOCRYSTALS
[0090] Several reported synthetic schemes have produced ultra-small
nanocrystals by growing larger nanocrystals, then etching them with
various chemical etchants. Landes, C.; Braun, M.; Burda, C.;
El-Sayed, M. A. Nano Lett. 2001, 1, 667-670; Landes, C.; El-Sayed,
M. A. J. Phys. Chem. A 2002, 106, 7621-7627. Reports of ultra-small
nanocrystals grown into the ultra-small, in contrast to ultra-small
nanocrystals etched from larger nanocrystals, are relatively rare.
Qu, L.; Yu, W. W.; Peng, X. Nano Lett. 2004, 4, 465-469; Chen, X.;
Samia, A. C. S.; Lou, Y.; Burda, C. J. Am. Chem. Soc. 2005, 127,
4372-4375. Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123,
183-184. Inverse micellular methods and low temperature
organometallic preparations can also yield ultra-small
nanocrystals; however, none of these methods produce nanocrystals
with comparable optical properties to high-temperature
pyrolytically-synthesized nanocrystals. Kasuya, A.; Sivamohan, R.;
Barnakov, Y. A.; Dmitruk, I. M.; Nirasawa, T.; Romanyuk, V. R.;
Kumar, V.; Mamykin, S. V.; Tohji, K.; Jeyadevan, B.; Shinoda, K.;
Kudo, T.; Terasaki, O.; Liu, Z.; Beloludov, R. V.; Sundararajan,
V.; Kawazoe, Y. Nature Materials 2004, 3, 99-102. Rogach, A., L.;
Kornowski, A.; Gao, M.; Eychmuller, A.; Weller, H. J. Phys. Chem. B
1999, 103, 3065-3069.
[0091] In one aspect, the invention relates to a method of
preparing an inorganic nanoparticle comprising the steps of heating
a reaction mixture comprising a C.sub.4 to C.sub.22 alkylphosphonic
acid and a source of cadmium or zinc to a temperature of greater
than about 300.degree. C.; adding to the reaction mixture an
injection mixture comprising a C.sub.2 to C.sub.16
trialkylphosphine and a source of selenium, sulfur, or tellurium;
and decreasing the temperature of the reaction mixture to less than
about 300.degree. C. In a further aspect, the reaction mixture can
further comprise at least one of a C.sub.4 to C.sub.20
trialkylphosphine oxide, or a C.sub.8 to C.sub.20 alkylamine or
arylamine, or a mixture thereof. In a yet further aspect, the
injection mixture can further comprise a C.sub.6 to C.sub.24
hydrocarbon.
[0092] In a further aspect, the invention relates to a method of
preparing an inorganic nanoparticle comprising the steps of
providing an approximately 1.0 M injection mixture comprising
selenium and tri-n-butylphosphine; diluting the injection mixture
with octadecene to approximately 0.10 M concentration; providing a
reaction mixture comprising tri-n-octylphosphine oxide,
hexadecylamine, cadmium oxide, and dodecylphosphonic acid; heating
the reaction mixture to a temperature of about 330.degree. C.;
adding the injection mixture to the reaction mixture so as to
decrease the temperature of the reaction mixture to from about
260.degree. C. to about 270.degree. C.; and adding toluene to the
reaction mixture so as to decrease the temperature of the reaction
mixture to less than about 150.degree. C. within about ten seconds
after adding the injection mixture to the reaction mixture.
[0093] In a further aspect, the nanocrystals of the invention are
not prepared by an etching process. In a yet further aspect, the
nanocrystals of the invention are not prepared by an inverse
micellular process.
[0094] 1. Reaction Mixture
[0095] In one aspect, the reaction mixture comprises a C.sub.4 to
C.sub.22 alkylphosphonic acid and a source of cadmium or zinc. In a
further aspect, the reaction mixture comprises a C.sub.4 to
C.sub.20 trialkylphosphine oxide, a C.sub.8 to C.sub.20 alkylamine
or arylamine, a C.sub.4 to C.sub.22 alkylphosphonic acid, and a
source of cadmium or zinc. In one aspect, the reaction mixture
comprises a solution. In a further aspect, the reaction mixture
comprises a suspension. It is understood that these components can
be added to the reaction mixture in any order. In a further aspect,
one or more of these components can be absent from the reaction
mixture.
[0096] In general, the reaction mixture can be provided at any
concentration. However, in one aspect, the alkylphosphonic acid is
provided in molar excess relative to the source of cadmium or zinc.
For example, the alkylphosphonic acid to source of cadmium or zinc
ratio can be about 1:1, about 1.05:1, greater than about 1:1, about
1.5:1, about 2:1, about 3:1, about 4:1, about 5:1, or greater than
about 5:1.
[0097] In a further aspect, the trialkylphosphine oxide is provided
relative to the amount of the alkylamine. For example, the
trialkylphosphine oxide can present in the reaction mixture in an
amount of about 0%, about 5%, about 10%, about 20%, about 30%,
about 40%, about 50%, about 60%, about 70%, about 80%, about 90%,
about 5%, or about 100% of the total weight of the
trialkylphosphine oxide and the alkylamine. Conversely, in one
aspect, the alkylamine can present in the reaction mixture in an
amount of about 0%, about 5%, about 10%, about 20%, about 30%,
about 40%, about 50%, about 60%, about 70%, about 80%, about 90%,
about 5%, or about 100% of the total weight of the
trialkylphosphine oxide and the alkylamine.
[0098] In one aspect, a typical reaction mixture can comprise the
components in the following exemplary amounts: about 1 mmol of the
source of cadmium or zinc, about 2 mmol of the alkylphosphonic
acid, about 18.6 mmol of the trialkylphosphine oxide, and about
12.25 mmol of alkylamine. It is understood, however, that the
relative amounts of the various components can, of course, be
varied. For example, relative to about 1 mmol of the source of
cadmium or zinc, from about 2 mmol to about 5 mmol--for example,
about 2.5 mmol, about 3 mmol, about 3.5 mmol, about 4 mmol, or
about 4.5 mmol--of the alkylphosphonic acid can be present; from
about 10 mmol to about 40 mmol--for example, from about 15 mmol to
about 20 mmol or from about 15 mmol to about 25 mmol--of the
trialkylphosphine oxide can be present; and from about 5 mmol to
about 30 mmol--for example, from about 10 mmol to about 15 mmol,
from about 10 mmol to about 20 mmol, or from about 15 mmol to about
20 mmol--of alkylamine can be present.
[0099] a. Alkylamine or Arylamine
[0100] In one aspect, the reaction mixture comprises a C.sub.8 to
C.sub.20 alkylamine or arylamine. The C.sub.8 to C.sub.20
alkylamine or arylamine can be any C.sub.8 to C.sub.20 alkylamine
or arylamine known to those of skill in the art. The C.sub.8 to
C.sub.20 alkylamine or arylamine can be, for example, a C.sub.8,
C.sub.9, C.sub.10, C.sub.11, C.sub.12, C.sub.13, C.sub.14,
C.sub.15, C.sub.16, C.sub.17, C.sub.18, C.sub.19, or C.sub.20
alkylamine or arylamine. In a further aspect, the alkylamine or
arylamine can be a C.sub.12 to C.sub.20 alkylamine or arylamine, a
C.sub.12 to C.sub.18 alkylamine or arylamine, or a C.sub.14 to
C.sub.18 alkylamine or arylamine. In a further aspect, the
alkylamine or arylamine comprises hexadecylamine. In one aspect,
the alkylamine or arylamine can comprise mixtures of C.sub.8 to
C.sub.20 alkylamines or arylamines. In a further aspect, the alkyl
group of an alkylamine can be straight-chain or branched and can be
substituted or unsubstituted. In a further aspect, the alkyl group
of an alkylamine can be cyclic and/or the nitrogen of the
alkylamine can comprise a heteroatom of a cyclic compound, for
example, pyrimidine.
[0101] In one aspect, the C.sub.8 to C.sub.20 alkylamine or
arylamine can comprise a primary amine, a secondary amine, a
tertiary amine, or a mixture thereof. In one aspect, wherein the
C.sub.8 to C.sub.20 alkylamine or arylamine is provided as a
secondary amine, the alkylamine can have two C.sub.4 to C.sub.10
alkyl or aryl groups, which can be the same or different. In a
further aspect, wherein the alkylamine or arylamine is provided as
a secondary amine, the alkylamine or arylamine can have two C.sub.8
to C.sub.20 alkyl or aryl groups, which can be the same or
different. In one aspect, wherein the C.sub.8 to C.sub.20
alkylamine or arylamine is provided as a tertiary amine, the
alkylamine or arylamine can have three C.sub.2 to C.sub.7 alkyl or
aryl groups, which can be the same or different and total to 8 to
20 carbon units. In a further aspect, wherein the alkylamine or
arylamine is provided as a tertiary amine, the alkylamine can have
three C.sub.8 to C.sub.20 alkyl or aryl groups, which can be the
same or different.
[0102] In one aspect, the C.sub.8 to C.sub.20 alkylamine or
arylamine can comprise an aromatic amine. That is, the nitrogen of
the amine can be bound to one or more aromatic moieties, for
example, to one or more benzene, naphthalene, pyridine, imidazole,
chlorobenzene, toluene, or aniline residues. In a further aspect,
the aryl group(s) of an arylamine can be the same or different and
can be substituted or unsubstituted. In a further aspect, the
nitrogen of the arylamine can comprise a heteroatom of a cyclic
aromatic compound, for example, pyridine.
[0103] It is also understood that the C.sub.8 to C.sub.20
alkylamine or arylamine can be absent from the reaction
mixture.
[0104] b. Alkyl- or Arylphosphonic Acid
[0105] In one aspect, the reaction mixture comprises a C.sub.4 to
C.sub.22 alkyl- or arylphosphonic acid. The C.sub.4 to C.sub.22
alkyl- or arylphosphonic acid can be any C.sub.4 to C.sub.22 alkyl-
or arylphosphonic acid known to those of skill in the art. The
C.sub.4 to C.sub.22 alkyl- or arylphosphonic acid can be, for
example, a C.sub.4, C.sub.5, C.sub.6, C.sub.7, C.sub.8, C.sub.9,
C.sub.10, C.sub.11, C.sub.12, C.sub.13, C.sub.14, C.sub.15,
C.sub.16, C.sub.17, C.sub.18, C.sub.19, C.sub.20, C.sub.21, or
C.sub.22 alkyl- or arylphosphonic acid. In a further aspect, the
alkyl- or arylphosphonic acid can be a C.sub.4 to C.sub.16 alkyl-
or arylphosphonic acid, or a C.sub.10 to C.sub.14 alkyl- or
arylphosphonic acid. In a further aspect, the alkyl- or
arylphosphonic acid comprises dodecylphosphonic acid. In one
aspect, the alkyl- or arylphosphonic acid can comprise mixtures of
C.sub.4 to C.sub.22 alkyl- or arylphosphonic acids. In a further
aspect, the alkyl group of the alkyl- or arylphosphonic acid can be
straight-chain or branched and can be substituted or
unsubstituted.
[0106] In a further aspect, an alkyl group of an alkyl- or
arylphosphonic acid can be straight-chain or branched and can be
substituted or unsubstituted. In a further aspect, an alkyl group
of an alkyl- or arylphosphonic acid can be cyclic and/or the
phosphorous of an alkyl- or arylphosphonic acid can comprise a
heteroatom of a cyclic compound.
[0107] In one aspect, an alkyl- or arylphosphonic acid can comprise
an aromatic alkyl- or arylphosphonic acid. That is, the phosphorous
of the phosphonic acid can be bound to an aromatic moiety, for
example, to a benzene, a naphthalene, a pyridine, an imidazole, a
toluene, or an aniline residue. In a further aspect, the aryl
group(s) of an alkyl- or arylphosphonic acid can be substituted or
unsubstituted. In a further aspect, the phosphorous of the alkyl-
or arylphosphonic acid can comprise a heteroatom of a cyclic
aromatic compound.
[0108] In one aspect, alkyl- or aryl-phosphonic acid moieties
comprise at least about 20% of the total surface ligand coverage at
the nanoparticle surface. For example, alkyl- or aryl-phosphonic
acid moieties can comprise at least about 25%, at least about 30%,
at least about 35%, at least about 40%, at least about 45%, at
least about 50%, at least about 55%, at least about 60%, at least
about 65%, at least about 70%, at least about 75%, at least about
80%, at least about 85%, at least about 90%, at least about 95%, or
substantially all of the total surface ligand coverage.
[0109] In a further aspect, the C.sub.4 to C.sub.22 alkyl- or
aryl-phosphonic acid and the source of cadmium or zinc can be
provided in the reaction mixture in a molar ratio of from about 1:5
to about 1:1 Cd/Zn:phosphonic acid. For example, the C.sub.4 to
C.sub.22 alkyl- or aryl-phosphonic acid and the source of cadmium
or zinc can be provided in a molar ratio of from about 1:5 to about
1:1, of from about 1:4.5 to about 1:1, of from about 1:4 to about
1:1, of from about 1:3.5 to about 1:1, of from about 1:3 to about
1:1, of from about 1:2.5 to about 1:1, of from about 1:2 to about
1:1, of from about 1: to about 1:1, of from about 1:5 to about
1:1.5, of from about 1:5 to about 1:2, of from about 1:4.5 to about
1:1.5, of from about 1:4 to about 1:2, of from about 1:3.5 to about
1:2, of from about 1:3 to about 1:2, of from about 1:2.5 to about
1:2, of from about 1:2.5 to about 1:1.5, of about 1:5, of about
1:4.5, of about 1:4, of about 1:3.5, of about 1:3, of about 1:2.5,
of about 1:2, of about 1:1.5, of about 1:1 Cd/Zn:phosphonic
acid.
[0110] c. Trialkyl- or Triarylphosphine Oxide
[0111] In one aspect, the reaction mixture comprises a C.sub.4 to
C.sub.20 trialkylphosphine oxide. The C.sub.4 to C.sub.20
trialkylphosphine oxide can be any C.sub.4 to C.sub.20
trialkylphosphine oxide known to those of skill in the art. That
is, each alkyl group of the trialkylphosphine oxide can comprise a
C.sub.4 to C.sub.20 alkyl group. The C.sub.4 to C.sub.20
trialkylphosphine oxide can be, for example, a C.sub.4, C.sub.5,
C.sub.6, C.sub.7, C.sub.8, C.sub.9, C.sub.10, C.sub.11, C.sub.12,
C.sub.13, C.sub.14, C.sub.15, C.sub.16, C.sub.17, C.sub.18,
C.sub.19, or C.sub.20 trialkylphosphine oxide. In a further aspect,
the trialkylphosphine oxide can be a C.sub.4 to C.sub.12
trialkylphosphine oxide or a C.sub.6 to C.sub.10 trialkylphosphine
oxide. In a further aspect, the C.sub.4 to C.sub.20
trialkylphosphine oxide comprises tri-n-octylphosphine oxide. In
one aspect, the trialkylphosphine oxide can comprise mixtures of
C.sub.8 to C.sub.20 trialkylphosphine oxides. In a further aspect,
the alkyl groups of the trialkylphosphine oxide can be
straight-chain or branched, can be substituted or unsubstituted,
and can be the same or different.
[0112] In a further aspect, one or more alkyl group(s) of an
trialkyl- or triarylphosphine oxide can be straight-chain or
branched and can be substituted or unsubstituted. In a further
aspect, one or more alkyl group(s) of an trialkyl- or
triarylphosphine oxide can be cyclic and/or the phosphorous of an
trialkyl- or triarylphosphine oxide can comprise a heteroatom of a
cyclic compound.
[0113] In one aspect, an trialkyl- or triarylphosphine oxide can
comprise an aromatic trialkyl- or triarylphosphine oxide. That is,
the phosphorous of the trialkyl- or triarylphosphine oxide can be
bound to one or more aromatic moieties, for example, to benzene,
naphthalene, pyridine, imidazole, chlorobenzene, toluene, or
aniline residues. In a further aspect, the aryl group(s) of an
alkyl- or arylphosphonic acid can be the same or different and can
be substituted or unsubstituted.
[0114] It is also understood that the C.sub.4 to C.sub.20
trialkylphosphine oxide can be absent from the reaction
mixture.
[0115] d. Source of Cadmium or Zinc
[0116] In one aspect, the reaction mixture comprises a source of
cadmium or zinc. That is, in one aspect, the reaction mixture
comprises at least one compound capable of providing cadmium, zinc,
or a mixture thereof. In a further aspect, the reaction mixture
comprises more than one compound capable of providing cadmium,
zinc, or a mixture thereof.
[0117] The source of cadmium or zinc can be any source of cadmium
or zinc known to those of skill in the art. For example, the source
of cadmium or zinc can comprise at least one of an oxide of cadmium
or zinc, a dialkyl cadmium or dialkyl zinc, a carboxylate salt of
cadmium or zinc or a phosphonate salt of cadmium or zinc or a
mixture thereof.
[0118] In one aspect, the source of cadmium or zinc can comprise at
least one of an oxide of cadmium or zinc. That is, the source of
cadmium or zinc can be, for example, at least one of zinc oxide or
cadmium oxide or a mixture thereof.
[0119] In one aspect, the source of cadmium or zinc can comprise at
least one of a dialkyl cadmium or dialkyl zinc or a mixture
thereof. In one aspect, the alkyl groups can be the same or
different, can be substituted or unsubstituted, and can be
straight-chain or branched. In a further aspect, the alkyl can be
one or more C.sub.1 to C.sub.6 alkyl groups, for example, at least
one of methyl, ethyl, propyl, butyl, pentyl, or hexyl groups or a
mixture thereof. In further aspects, the alkyl can be one or more
C.sub.1 to C.sub.12 alkyl groups or C.sub.6 to C.sub.12 alkyl
groups.
[0120] In one aspect, the source of cadmium or zinc can comprise at
least one of a carboxylate salt of cadmium or zinc or a phosphonate
salt of cadmium or zinc or a mixture thereof. In one aspect, the
carboxylate or a phosphonate group can be alkyl or aryl, can be
substituted or unsubstituted, and can be straight-chain or
branched. In a further aspect, the carboxylate or phosphonate group
can be one or more C.sub.1 to C.sub.6 carboxylate or phosphonate
groups, for example, at least one of methyl, ethyl, propyl, butyl,
pentyl, or hexyl carboxylate or phosphonate group or a mixture
thereof. In further aspects, the carboxylate or phosphonate group
can be one or more C.sub.1 to C.sub.12 carboxylate or phosphonate
groups or C.sub.6 to C.sub.12 carboxylate or phosphonate
groups.
[0121] In a yet further aspect, the source of cadmium or zinc
comprises at least one of zinc oxide, dimethyl zinc, cadmium oxide,
cadmium acetate, cadmium stearate, or dimethyl cadmium or a mixture
thereof. In a further aspect, the source of cadmium or zinc
comprises cadmium oxide.
[0122] In one aspect, the source of cadmium or zinc and the
alkylphosphonic acid can be provided in a molar ratio of from about
5:1 Cd/Zn:phosphonic acid to about 1:1 Cd/Zn:phosphonic acid. For
example, the source of cadmium or zinc and the alkylphosphonic acid
can be provided in a molar ratio of from about 4.5:1
Cd/Zn:phosphonic acid, about 4:1 Cd/Zn:phosphonic acid, about 3.5:1
Cd/Zn:phosphonic acid, about 3:1 Cd/Zn:phosphonic acid, about 2.5:1
Cd/Zn:phosphonic acid, about 2:1 Cd/Zn:phosphonic acid, or about
1.5:1 Cd/Zn:phosphonic acid. In a further aspect, the source of
cadmium or zinc and the alkylphosphonic acid can be provided in a
molar ratio of greater than about 5:1 Cd/Zn:phosphonic acid.
[0123] In another aspect, the source of cadmium or zinc and the
alkylphosphonic acid can be provided in a molar ratio of from about
1:5 Cd/Zn:phosphonic acid to about 1:1 Cd/Zn:phosphonic acid. For
example, the source of cadmium or zinc and the alkylphosphonic acid
can be provided in a molar ratio of from about 1:4.5
Cd/Zn:phosphonic acid, about 1:4 Cd/Zn:phosphonic acid, about 1:3.5
Cd/Zn:phosphonic acid, about 1:3 Cd/Zn:phosphonic acid, about 1:2.5
Cd/Zn:phosphonic acid, about 1:2 Cd/Zn:phosphonic acid, or about
1:1.5 Cd/Zn:phosphonic acid. In a further aspect, the source of
cadmium or zinc and the alkylphosphonic acid can be provided in a
molar ratio of greater than about 1:5 Cd/Zn:phosphonic acid. In
still other aspects, the molar ratio of the source of cadmium or
zinc and the phosphonic acid can be less than or greater than any
values specifically recited herein, and the present invention is
not intended to be limited to any particular ratio.
[0124] e. Reaction Temperature
[0125] In one aspect, the reaction mixture is heated to a
temperature before the injection mixture is added. In a further
aspect, the reaction mixture is heated to a temperature sufficient
to facilitate both initiation of nanocrystal formation and growth
of nanocrystals before the injection mixture is added.
[0126] For example, the temperature can be greater than about
200.degree. C., greater than about 225.degree. C., greater than
about 250.degree. C., greater than about 275.degree. C., greater
than about 300.degree. C., greater than about 310.degree. C.,
greater than about 320.degree. C., greater than about 330.degree.
C., greater than about 340.degree. C., or greater than about
350.degree. C. In a further aspect, the temperature can be less
than about 400.degree. C., less than about 450.degree. C., less
than about 400.degree. C., less than about 375.degree. C., less
than about 350.degree. C., less than about 325.degree. C., less
than about 300.degree. C., less than about 275.degree. C., less
than about 250.degree. C., or less than about 225.degree. C.
[0127] In a further aspect, the reaction mixture can be heated to a
temperature of from about 200.degree. C. to about 400.degree. C.
before the injection mixture is added. For example, the temperature
can be from about 250.degree. C. to about 400.degree. C., from
about 300.degree. C. to about 400.degree. C., from about
250.degree. C. to about 350.degree. C., from about 300.degree. C.
to about 350.degree. C., about 310.degree. C., about 320.degree.
C., about 330.degree. C., or about 340.degree. C.
[0128] 2. Injection Mixture
[0129] In one aspect, the injection mixture comprises a C.sub.2 to
C.sub.16 trialkyl- or triarylphosphine and a source of selenium,
sulfur, or tellurium. In a further aspect, the injection mixture
comprises a C.sub.2 to C.sub.16 trialkyl- or triarylphosphine, a
C.sub.6 to C.sub.24 hydrocarbon, and a source of selenium, sulfur,
or tellurium. In one aspect, the injection mixture comprises a
solution. In a further aspect, the injection mixture comprises a
suspension. It is understood that these components can be added to
the injection mixture in any order. In a further aspect, one or
more of these components can be absent from the injection
mixture.
[0130] In general, the injection mixture can be provided at any
concentration. However, in one aspect, the injection mixture can be
provided as a solution of the source of selenium, sulfur, or
tellurium in the C.sub.2 to C.sub.16 trialkyl- or triarylphosphine,
which can then be diluted with a C.sub.6 to C.sub.24 hydrocarbon.
In a further aspect, the injection mixture can be provided as a
suspension of the source of selenium, sulfur, or tellurium in the
C.sub.2 to C.sub.16 trialkyl- or triarylphosphine, which can then
be diluted with a C.sub.6 to C.sub.24 hydrocarbon. The mixture can
be provided at any desired concentration and is typically provided
at a concentration of from about 0.1 M to about 7 M. For example,
the source of selenium, sulfur, or tellurium can be provided as an
approximately 1.0 M (e.g., approximately 0.25 M, approximately 0.5
M, approximately 0.75 M, approximately 1.25 M, approximately 1.5 M,
approximately 1.75 M, approximately 2 M, approximately 2.5 M, or
approximately 3 M) mixture in the C.sub.2 to C.sub.16 trialkyl- or
triarylphosphine and can then be optionally diluted with a C.sub.6
to C.sub.24 hydrocarbon to, for example, an approximately 0.1 M
(e.g., approximately 0.025 M, approximately 0.05 M, approximately
0.075 M, approximately 0.125 M, approximately 0.15 M, approximately
0.175 M, approximately 0.2 M, approximately 0.25 M, or
approximately 0.3 M) concentration.
[0131] In one aspect, source of selenium, sulfur, or tellurium can
be provided in an amount and concentration so as to achieve a
Cd/Zn:Se/S/Te ratio of greater than about 1.01:1 after the
injection mixture is added to the reaction mixture. That is, in one
aspect, the overall reaction can be maintained cation-rich (i.e.,
Cd- or Zn-rich). For example, the Cd/Zn:Se/S/Te ratio can be
greater than about 1.01:1, greater than about 1.05:1, greater than
about 1.1:1, greater than about 1.2:1, greater than about 1.3:1,
greater than about 1.4:1, greater than about 1.5:1, greater than
about 2:1, greater than about 3:1, greater than about 4:1, or
greater than about 5:1.
[0132] In a further aspect, the Cd/Zn:Se/S/Te ratio can be from
about 1.01:1 to about 5:1, for example, about 1.05:1 to about 4:1,
about 1.1:1 to about 3:1, about 1.5:1 to about 2:1, about 1.5:1 to
about 2.5:1, about 2:1 to about 2.5:1, about 1.01:1, about 1.05:1,
about 1.1:1, about 1.2:1, about 1.3:1, about 1.4:1, about 1.5:1,
about 2:1, about 3:1, greater than about 4:1, or about 5:1.
[0133] In a yet further aspect, the Cd/Zn:Se/S/Te ratio can be from
about 1:1.01 to about 1:5, for example, about 1:1.05 to about 1:4,
about 1:1.1 to about 1:3, about 1:1.5 to about 1:2, about 1:1.5 to
about 1:2.5, about 1:2 to about 1:2.5, about 1:1.01, about 1:1.05,
about 1:1.1, about 1:1.2, about 1:1.3, about 1:1.4, about 1:1.5,
about 1:2, about 1:3, greater than about 1:4, or about 1:5. In a
still further aspect, the Cd/Zn:Se/S/Te ratio can be about 1:1.3,
for example, from about 1:1.0 to about 1:6, from about 1:1.1 to
about 1:5, from about 1:1.2 to about 1:4, or from about 1:1.25 to
about 1:35.
[0134] In a further aspect, the C.sub.2 to C.sub.16 trialkyl- or
triarylphosphine is provided relative to the amount of the C.sub.6
to C.sub.24 hydrocarbon. For example, the trialkyl- or
triarylphosphine can present in the reaction mixture in an amount
of about 1%, about 2.5%, about 5%, about 10%, about 15%, about 20%,
about 30%, about 40%, about 50%, about 60%, about 70%, about 80%,
about 90%, about 95%, or about 100%, of the total weight of the
trialkyl- or triarylphosphine and the hydrocarbon. Conversely, in
one aspect, the hydrocarbon can present in the reaction mixture in
an amount of about 99%, about 97.5%, about 95%, about 90%, about
85%, about 80%, about 70%, about 60%, about 50%, about 40%, about
30%, about 20%, about 10%, about 5%, or about 0% of the total
weight of trialkyl- or triarylphosphine and the hydrocarbon. It is
understood that the relative amounts can also be expressed as a
ratio.
[0135] In certain aspects, the C.sub.6 to C.sub.24 hydrocarbon and
the C.sub.2 to C.sub.16 trialkyl- or triarylphosphine are provided
in a ratio in the injection solution of less than about 70:30, of
about 70:30, of less than about 75:25, or of about 75:25. In
further aspects, the C.sub.6 to C.sub.24 hydrocarbon can be
octadecene, and the C.sub.2 to C.sub.16 trialkyl- or
triarylphosphine can be tri-n-butylphosphine. For example, the
C.sub.6 to C.sub.24 hydrocarbon can be octadecene, the C.sub.2 to
C.sub.16 trialkyl- or triarylphosphine can be tri-n-butylphosphine
are, and the octadecene and the tri-n-butylphosphine can be
provided in a ratio in the injection solution of from about 80:20
to about 60:40. In a further example, the C.sub.6 to C.sub.24
hydrocarbon can be octadecene, the C.sub.2 to C.sub.16 trialkyl- or
triarylphosphine can be tri-n-butylphosphine are, and the
octadecene and the tri-n-butylphosphine can be provided in a ratio
in the injection solution of from about 70:30 to about 75:25. In
certain aspects, these methods can prepare inorganic nanoparticles
having a quantum yield of at least about 5%, at least about 8%, or
at least about 10%.
[0136] Without wishing to be bound by theory, it is believed that
the ratio of the C.sub.2 to C.sub.16 trialkyl- or triarylphosphine
to the C.sub.6 to C.sub.24 hydrocarbon can affect the quantum yield
of the resultant nanocrystals. That is, certain ratios of the
C.sub.2 to C.sub.16 trialkyl- or triarylphosphine to the C.sub.6 to
C.sub.24 hydrocarbon can increase the rate of reduction of cadmium
oxide or zinc oxide, thereby facilitating availability of the
metal. Further, without wishing to be bound by theory, it is
believed that the disclosed ratios of the C.sub.2 to C.sub.16
trialkyl- or triarylphosphine to the C.sub.6 to C.sub.24
hydrocarbon can facilitate formation of the core crystal structure
of the nanocrystals, thereby providing superior structures.
[0137] a. Trialkyl- or Triarylphosphine
[0138] In one aspect, the reaction mixture comprises a C.sub.2 to
C.sub.16 trialkyl- or triarylphosphine. The C.sub.2 to C.sub.16
trialkyl- or triarylphosphine can be any C.sub.2 to C.sub.16
trialkyl- or triarylphosphine known to those of skill in the art.
That is, each alkyl group of a C.sub.2 to C.sub.16
trialkylphosphine can comprise a C.sub.2 to C.sub.16 alkyl group. A
C.sub.2 to C.sub.16 trialkylphosphine can be, for example, a
C.sub.2, C.sub.3, C.sub.4, C.sub.5, C.sub.6, C.sub.7, C.sub.8,
C.sub.9, C.sub.10, C.sub.11, C.sub.12, C.sub.13, C.sub.14,
C.sub.15, or C.sub.16 trialkylphosphine. In a further aspect, a
trialkylphosphine can be a C.sub.2 to C.sub.12 trialkylphosphine, a
C.sub.2 to C.sub.10 trialkylphosphine, a C.sub.2 to C.sub.8
trialkylphosphine, or a C.sub.2 to C.sub.6 trialkylphosphine. In a
further aspect, a C.sub.2 to C.sub.16 trialkylphosphine comprises
tri-n-butylphosphine. In one aspect, a C.sub.2 to C.sub.16
trialkylphosphine can comprise mixtures of C.sub.2 to C.sub.16
trialkylphosphines. In a further aspect, the alkyl groups of a
C.sub.2 to C.sub.16 trialkylphosphine can be straight-chain or
branched, can be substituted or unsubstituted, and can be the same
or different. In a further aspect, the alkyl group of a
trialkylphosphine can be cyclic and/or the phosphorous of a
trialkylphosphine can comprise a heteroatom of a heterocyclic
compound.
[0139] In one aspect, the C.sub.2 to C.sub.16 trialkyl- or
triarylphosphine can comprise an aromatic phosphine. That is, the
phosphorous of the phosphine can be bound to one or more aromatic
moieties, for example, to one or more benzene, naphthalene,
pyridine, imidazole, chlorobenzene, toluene, or aniline residues.
In a further aspect, the aryl group(s) of an arylphosphine can be
the same or different and can be substituted or unsubstituted. In a
further aspect, the phosphorous of the arylphosphine can comprise a
heteroatom of a cyclic aromatic compound.
[0140] It is also understood that the C.sub.2 to C.sub.16 trialkyl-
or triarylphosphine can be absent from the reaction mixture.
[0141] b. Hydrocarbon
[0142] In one aspect, the reaction mixture comprises a C.sub.6 to
C.sub.24 hydrocarbon. The hydrocarbon can be any C.sub.6 to
C.sub.24 hydrocarbon known to those of skill in the art. The
C.sub.6 to C.sub.24 hydrocarbon can be, for example, a C.sub.6,
C.sub.7, C.sub.8, C.sub.9, C.sub.10 C.sub.11, C.sub.12, C.sub.13,
C.sub.14, C.sub.15, C.sub.16 C.sub.17, C.sub.18, C.sub.19,
C.sub.20, C.sub.21, C.sub.22, C.sub.23, or C.sub.24 hydrocarbon. In
a further aspect, the C.sub.6 to C.sub.24 hydrocarbon can be a
C.sub.6 to C.sub.12 hydrocarbon, a C.sub.14 to C.sub.22
hydrocarbon, or a C.sub.16 to C.sub.20 hydrocarbon. In a further
aspect, the C.sub.6 to C.sub.24 hydrocarbon comprises octadecene.
In one aspect, the C.sub.6 to C.sub.24 hydrocarbon can comprise
mixtures of C.sub.6 to C.sub.24 hydrocarbons. In a further aspect,
the C.sub.6 to C.sub.24 hydrocarbon can be straight-chain or
branched and can be substituted or unsubstituted. For example,
halogenated hydrocarbons can be used. In a further aspect, the
C.sub.6 to C.sub.24 hydrocarbon can comprise a cyclic
hydrocarbon.
[0143] In a further aspect, the C.sub.6 to C.sub.24 hydrocarbon can
comprise an aromatic hydrocarbon. That is, the hydrocarbon can
comprise, for example, benzene, naphthalene, pyridine, imidazole,
chlorobenzene, toluene, or aniline. Further, in one aspect, the
hydrocarbon can comprise one or more benzene, naphthalene,
pyridine, imidazole, chlorobenzene, toluene, or aniline residues.
In a further aspect, the aryl group(s) of the C.sub.6 to C.sub.24
hydrocarbon can be the same or different and can be substituted or
unsubstituted.
[0144] It is also understood that the C.sub.6 to C.sub.24
hydrocarbon can be absent from the reaction mixture.
[0145] c. Source of Selenium, Sulfur, or Tellurium
[0146] In one aspect, the injection mixture comprises a source of
selenium, sulfur, or tellurium. That is, in one aspect, the
injection mixture comprises at least one compound capable of
providing selenium, sulfur, or tellurium or a mixture thereof. In a
further aspect, the reaction mixture comprises more than one
compound capable of providing selenium, sulfur, or tellurium or a
mixture thereof.
[0147] The source of selenium, sulfur, or tellurium can be any
source of selenium, sulfur, or tellurium known to those of skill in
the art. In one aspect, the source of selenium, sulfur, or
tellurium comprises at least one of selenium powder, sulfur powder,
or tellurium powder or a mixture thereof. In a further aspect, the
source of selenium, sulfur, or tellurium comprises at least one of
hexamethyl silylsulfide, hexamethyl silylselenide, or hexamethyl
silyltelluride or a mixture thereof.
[0148] d. Injection Temperature
[0149] In one aspect, the injection mixture can be provided at any
convenient temperature. In a further aspect, the injection mixture
can be provided at a temperature lower than that of the reaction
mixture temperature. For example, the injection mixture can be
provided at room temperature. It is also understood that the
injection mixture can be provided at a temperature higher than room
temperature. Typically, the injection mixture is provided at a
temperature of equal to or less than that necessary to maintain
nanocrystal growth. In one aspect, the injection mixture is
provided at a temperature of less than that necessary to maintain
nanocrystal formation initiation. By "the injection mixture can be
provided," it is meant that, in one aspect, the injection mixture
can be added to the reaction mixture at the provided
temperature.
[0150] In one aspect, the temperature of the injection mixture can
be maintained at a temperature of less than that necessary to
initiate nanocrystal formation. That is, an injection mixture
containing both a source of source of cadmium or zinc and a source
of selenium, sulfur, or tellurium can be provided at a temperature
of less than that necessary to initiate nanocrystal formation until
a heating step is performed, thereby initiating nanocrystal
formation. In a further aspect, the temperature of the injection
mixture can be maintained at a temperature of less than that
necessary to maintain nanocrystal growth. That is, an injection
mixture containing a source of selenium, sulfur, or tellurium can
be provided at a temperature of less than that necessary to
initiate nanocrystal formation until added to the reaction mixture,
thereby decreasing the temperature of the reaction mixture to less
than that necessary to initiate nanocrystal formation.
[0151] In a further aspect, the injection mixture can be provided
at less than about 300.degree. C., less than about 275.degree. C.,
less than about 250.degree. C., less than about 225.degree. C.,
less than about 200.degree. C., less than about 175.degree. C.,
less than about 150.degree. C., less than about 125.degree. C.,
less than about 100.degree. C., less than about 75.degree. C., or
less than about 50.degree. C. In a further aspect, the injection
mixture can be provided at about 300.degree. C., about 275.degree.
C., about 250.degree. C., about 225.degree. C., about 200.degree.
C., about 175.degree. C., about 150.degree. C., about 125.degree.
C., about 100.degree. C., about 75.degree. C., about 50.degree. C.,
or about 25.degree. C.
[0152] In a further aspect, the injection mixture can be provided
at a temperature of from about 25.degree. C. to about 300.degree.
C., from about 25.degree. C. to about 275.degree. C., from about
25.degree. C. to about 250.degree. C., from about 25.degree. C. to
about 225.degree. C., from about 25.degree. C. to about 250.degree.
C., from about 25.degree. C. to about 225.degree. C., from about
25.degree. C. to about 200.degree. C., from about 25.degree. C. to
about 175.degree. C., from about 25.degree. C. to about 150.degree.
C., from about 25.degree. C. to about 125.degree. C., from about
25.degree. C. to about 100.degree. C., from about 200.degree. C. to
about 300.degree. C., from about 100.degree. C. to about
200.degree. C., from about 150.degree. C. to about 250.degree. C.,
or from about 50.degree. C. to about 150.degree. C.
[0153] In a yet further aspect, the injection mixture can be
provided at temperature less than room temperature. For example,
the injection mixture can be provided at a temperature of from
about 0.degree. C. to about 25.degree. C., from about 5.degree. C.
to about 25.degree. C., from about 10.degree. C. to about
25.degree. C., from about 15.degree. C. to about 25.degree. C.,
from about 0.degree. C. to about 10.degree. C., or of less than
about 0.degree. C.
[0154] 3. Temperature Decrease
[0155] In one aspect, the injection mixture can be added to the
reaction mixture so as to decrease the temperature of the reaction
mixture and, thus, the temperature of the resulting combined
mixture. In one aspect, the addition of the injection mixture
lowers the temperature of the combined mixture to a temperature at
which nanocrystal initiation is disfavored, yet nanocrystal growth
is still favored. In a further aspect, the addition of the
injection mixture lowers the temperature of the combined mixture to
a temperature at which both nanocrystal initiation and nanocrystal
growth are disfavored.
[0156] In one aspect, addition of the injection mixture decreases
the temperature of the reaction mixture to less than about
300.degree. C., for example, to less than about 270.degree. C., to
less than about 250.degree. C., to less than about 225.degree. C.,
to less than about 200.degree. C., to less than about 175.degree.
C., to less than about 150.degree. C., or to less than about
125.degree. C. In a further aspect, addition of the injection
mixture decreases the temperature of the reaction mixture to from
about 150.degree. C. to about 300.degree. C., from about
150.degree. C. to about 250.degree. C., from about 150.degree. C.
to about 200.degree. C., from about 200.degree. C. to about
250.degree. C., or from about 250.degree. C. to about 300.degree.
C.
[0157] It is understood that both injection mixture temperature and
injection mixture addition rate can influence the temperature of
the combined mixture and, thus the kinetics of nanocrystal
initiation and nanocrystal growth. Typically, nucleation can occur
at relatively higher temperatures, and growth can occur at
relatively lower temperatures. If, for example, injection is
distributed over a longer time period, the temperature remains at
the nucleation temperature longer and, thus, more nanocrystals can
nucleate. If, as a further example, injection is distributed over a
shorter time period, the temperature remains at the nucleation
temperature for less time and, thus, fewer nanocrystals can
nucleate.
[0158] In one aspect, the temperature of the reaction mixture is
decreased to less than a temperature within a time after adding the
injection mixture to the reaction mixture. For example, the
temperature can be about 330.degree. C., about 320.degree. C.,
about 310.degree. C., about 300.degree. C., about 290.degree. C.,
about 280.degree. C., about 270.degree. C., about 260.degree. C.,
about 250.degree. C., about 240.degree. C., about 230.degree. C.,
about 220.degree. C., about 210.degree. C., about 200.degree. C.,
about 190.degree. C., about 180.degree. C., about 170.degree. C.,
about 160.degree. C., or about 150.degree. C. For example, the time
can be about 90 seconds, about 60 seconds, about 30 seconds, about
20 seconds, about 10 seconds, or about 5 seconds.
[0159] It is also understood that external cooling can be used to
decrease the temperature of the combined mixture, both in place of
and in concert with the cooling effect of the addition of injection
mixture.
[0160] 4. Temperature Control Solvents
[0161] In one aspect, a solvent can be added to the combined
mixture after addition of the injection mixture, or simultaneous
with the addition of the injection mixture, so as to decrease the
temperature of the combined mixture. That is, the addition of a
solvent can lower the temperature of the combined mixture. In one
aspect, the addition of a solvent mixture lowers the temperature of
the combined mixture to a temperature at which both nanocrystal
initiation and nanocrystal growth are disfavored.
[0162] In one aspect, a solvent can be provided at any convenient
temperature. In a further aspect, a solvent can be provided at a
temperature lower than that of the combined mixture temperature.
For example, a solvent can be provided at room temperature.
[0163] Typically, a solvent is provided at a temperature of less
than that necessary to maintain nanocrystal growth. By "a solvent
can be provided," it is meant that, in one aspect, a solvent can be
added to the combined mixture at the provided temperature.
[0164] In a further aspect, a solvent can be provided at less than
about 300.degree. C., less than about 275.degree. C., less than
about 250.degree. C., less than about 225.degree. C., less than
about 200.degree. C., less than about 175.degree. C., less than
about 150.degree. C., less than about 125.degree. C., less than
about 100.degree. C., less than about 75.degree. C., or less than
about 50.degree. C. In a further aspect, a solvent can be provided
at about 300.degree. C., about 275.degree. C., about 250.degree.
C., about 225.degree. C., about 200.degree. C., about 175.degree.
C., about 150.degree. C., about 125.degree. C., about 100.degree.
C., about 75.degree. C., about 50.degree. C., or about 25.degree.
C.
[0165] In a further aspect, a solvent can be provided at a
temperature of from about 25.degree. C. to about 300.degree. C.,
from about 25.degree. C. to about 275.degree. C., from about
25.degree. C. to about 250.degree. C., from about 25.degree. C. to
about 225.degree. C., from about 25.degree. C. to about 250.degree.
C., from about 25.degree. C. to about 225.degree. C., from about
25.degree. C. to about 200.degree. C., from about 25.degree. C. to
about 175.degree. C., from about 25.degree. C. to about 150.degree.
C., from about 25.degree. C. to about 125.degree. C., from about
25.degree. C. to about 100.degree. C., from about 200.degree. C. to
about 300.degree. C., from about 100.degree. C. to about
200.degree. C., from about 150.degree. C. to about 250.degree. C.,
or from about 50.degree. C. to about 150.degree. C.
[0166] In a yet further aspect, a solvent can be provided at
temperature less than room temperature. For example, a solvent can
be provided at a temperature of from about 0.degree. C. to about
25.degree. C., from about 5.degree. C. to about 25.degree. C., from
about 10.degree. C. to about 25.degree. C., from about 15.degree.
C. to about 25.degree. C., from about 0.degree. C. to about
10.degree. C., or of less than about 0.degree. C.
[0167] In one aspect, addition of the injection mixture decreases
the temperature of the reaction mixture to less than about
300.degree. C., for example, to less than about 270.degree. C., to
less than about 250.degree. C., to less than about 225.degree. C.,
to less than about 200.degree. C., to less than about 175.degree.
C., to less than about 150.degree. C., or to less than about
125.degree. C. In a further aspect, addition of the injection
mixture decreases the temperature of the reaction mixture to from
about 150.degree. C. to about 300.degree. C., from about
150.degree. C. to about 250.degree. C., from about 150.degree. C.
to about 200.degree. C., from about 200.degree. C. to about
250.degree. C., or from about 250.degree. C. to about 300.degree.
C.
[0168] It is understood that both solvent temperature and solvent
addition rate can influence the temperature of the combined mixture
and, thus the kinetics of nanocrystal initiation and nanocrystal
growth. In one aspect, the temperature of the combined mixture is
decreased to less than a temperature within a time after adding the
solvent to the combined mixture. For example, the temperature can
be about 330.degree. C., about 320.degree. C., about 310.degree.
C., about 300.degree. C., about 290.degree. C., about 280.degree.
C., about 270.degree. C., about 260.degree. C., about 250.degree.
C., about 240.degree. C., about 230.degree. C., about 220.degree.
C., about 210.degree. C., about 200.degree. C., about 190.degree.
C., about 180.degree. C., about 170.degree. C., about 160.degree.
C., or about 150.degree. C. For example, the time can be about 90
seconds, about 60 seconds, about 30 seconds, about 20 seconds,
about 10 seconds, or about 5 seconds.
[0169] It is also understood that external cooling can be used to
decrease the temperature of the combined mixture, both in place of
and in concert with the cooling effect of the addition of a
solvent.
[0170] In one aspect, the solvent comprises at least one C.sub.1 to
C.sub.12 alcohol. For example, the alcohol can be methanol,
ethanol, propanol, or butanol. The alcohol can be straight-chain or
branched and can be substituted or unsubstituted. In a further
aspect, mixtures of alcohols can be used. As a further example, the
C.sub.1 to C.sub.12 alcohol can be a diol or a triol. In a further
aspect, the solvent comprises at least one C.sub.5 to C.sub.12
hydrocarbon or C.sub.5 to C.sub.12 cyclic hydrocarbon or a mixture
thereof. The hydrocarbon can be straight-chain or branched and can
be substituted or unsubstituted. In a further aspect, mixtures of
hydrocarbons can be used. In a further example, the solvent can be
an aromatic solvent. For example, the solvent can be at least one
of benzene, toluene, or xylene or a mixture thereof. As a further
example, halogenated solvents can be used. In a further aspect,
toluene can be added to the reaction mixture so as to decrease the
temperature of the reaction mixture to less than about 150.degree.
C.
[0171] It is also understood that the use of temperature control
solvents can be absent from the invention.
[0172] 5. Combined Injection Method
[0173] In one aspect, the invention relates to a method of
preparing an inorganic nanoparticle comprising the sequential steps
of heating a composition comprising a source of cadmium or zinc and
a source of selenium, sulfur, or tellurium to a temperature
sufficient to initiate nanoparticle formation and sufficient to
support nanoparticle growth; cooling the composition to a
temperature insufficient to initiate nanoparticle formation but
sufficient to support nanoparticle growth; and cooling the
composition to a temperature insufficient to initiate nanoparticle
formation and insufficient to support nanoparticle growth. In a
further aspect, the composition can further comprise at least one
of a C.sub.4 to C.sub.20 trialkyl- or triarylphosphine oxide, a
C.sub.8 to C.sub.20 alkylamine or arylamine, a C.sub.4 to C.sub.22
alkyl- or arylphosphonic acid, a C.sub.2 to C.sub.16 trialkyl- or
triarylphosphine, or a C.sub.6 to C.sub.24 hydrocarbon or a mixture
thereof. It is understood that the various components, reactants,
solvents, temperatures, and conditions of the invention can also be
used in connection with the combined injection method to produce
the nanoparticles of the invention.
[0174] In a further aspect, a temperature sufficient to initiate
nanoparticle formation and sufficient to support nanoparticle
growth can be from about 250.degree. C. to about 400.degree. C.,
for example, from about 275.degree. C. to about 350.degree. C.,
from about 300.degree. C. to about 350.degree. C., from about
325.degree. C. to about 350.degree. C., from about 300.degree. C.
to about 375.degree. C., or from about 300.degree. C. to about
400.degree. C. In a further aspect, a temperature insufficient to
initiate nanoparticle formation but sufficient to support
nanoparticle growth can be from about 150.degree. C. to about
350.degree. C., for example, from about 200.degree. C. to about
300.degree. C., from about 225.degree. C. to about 300.degree. C.,
from about 250.degree. C. to about 300.degree. C., from about
275.degree. C. to about 300.degree. C., or from about 275.degree.
C. to about 325.degree. C. In a further aspect, a temperature
insufficient to initiate nanoparticle formation and insufficient to
support nanoparticle growth can be from about 100.degree. C. to
about 300.degree. C., for example, from about 150.degree. C. to
about 250.degree. C., from about 175.degree. C. to about
250.degree. C., from about 200.degree. C. to about 250.degree. C.,
from about 225.degree. C. to about 250.degree. C., from about
200.degree. C. to about 225.degree. C., from about 175.degree. C.
to about 200.degree. C., from about 150.degree. C. to about
200.degree. C., less than about 200.degree. C., less than about
175.degree. C., or less than about 150.degree. C.
[0175] In one aspect, the heating step can comprise the steps of
heating a reaction mixture to a temperature sufficient to initiate
nanoparticle formation and sufficient to support nanoparticle
growth, and adding to the reaction mixture an injection mixture
comprising a source of cadmium or zinc and a source of selenium,
sulfur, or tellurium. In a further aspect, the heating step can
comprise the steps of heating a reaction mixture comprising a
source of cadmium or zinc to a temperature sufficient to initiate
nanoparticle formation and sufficient to support nanoparticle
growth, and adding to the reaction mixture an injection mixture
comprising a source of selenium, sulfur, or tellurium.
D. THEORETICAL
[0176] Typically, the ultra-small nanocrystals of the invention do
not exhibit the strong band edge emission feature that is observed
in conventional CdSe nanocrystals, but do exhibit strong band edge
absorption features indicative of high quality CdSe nanocrystals.
Without wishing to be bound by theory, it is believed that the
broad emission can be attributed to charge recombination from
surface midgap states that arise from the presence of
non-coordinated surface selenium sites. See, e.g., Hill, N. A.;
Whaley, K. B. J. Chem. Phys. 1994, 100, 2831-2837. While band edge
emission can occur by direct recombination of the electron and hole
within a nanocrystal, deep trap emission can occur when a
photogenerated hole, trapped in a midgap state, encounters an
electron before it can relax non-radiatively to the ground state.
See, e.g., Underwood, D. F.; Kippeny, T. C.; Rosenthal, S. J.,
Journal of Physical Chemistry B 2001, 105, 436-443. That is,
without wishing to be bound by theory, it is believed that the
observed properties are the direct result of the extreme
surface-to-volume ratio forcing the electron and hole to
predominately interact at the nanocrystal surface.
[0177] This phenomenon of hole trapping to the selenium surface
sites has been studied by ultrafast fluorescence upconversion
spectroscopy. Underwood, D. F.; Kippeny, T. C.; Rosenthal, S. J.,
Journal of Physical Chemistry B 2001, 105, 436-443. Typically, as
nanocrystal size decreases, the amount of hole trapping increases.
Without wishing to be bound by theory, it is believed that this is
due not only to the reduced physical distance the hole must travel
to reach the surface, but also to an increased surface-to-volume
ratio resulting in more available surface sites. Accordingly, as
nanocrystal size continues to decrease, an even larger population
of photoexcited excitons can be funneled toward the hole-trapping
decay pathway, ultimately making it the dominant mode of radiative
relaxation. Ultra-small nanocrystals are so small that the electron
wave function can have significant overlap with the selenium
surface sites. Brus, L. E. J. Chem. Phys 1984, 80; Brus, L. J.
Phys. Chem. 1986, 90, 2555-2560. Therefore, any hole trapped on the
surface would likely encounter the electron before non-radiatively
relaxing to the ground state. Underwood, D. F.; Kippeny, T. C.;
Rosenthal, S. J. Journal of Physical Chemistry B 2001, 105,
436-443. Compounding this situation, in the present invention,
nanocrystal growth time is so short (typically, from about 10
seconds to about 20 seconds) that surface reconstruction and high
temperature annealing have little time to occur. Without wishing to
be bound by theory, it is believed that this results in a surface
that is likely defect-ridden. Furthermore, unlike larger
nanocrystals, the diameter of the ultra-small nanocrystal and the
length of the ligand are quite comparable. Again, without wishing
to be bound by theory, it is believed that coupling of vibrational
modes of the ligand to surface atoms, as well as collisional
relaxation, can provide further avenues for energy dissipation,
thereby providing more effective trap sites and potentially
contributing to the broad lognormal emission line shape.
[0178] While deep trap emission can be quite common in small
(<30 .ANG.) nanocrystals, it is typically accompanied by a large
band edge emission feature. Landes, C. F.; Braun, M.; El-Sayed, M.
A. J. Phys. Chem. B 2001, 105, 10554-10558; Murray, C. B.; Norris,
D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706-8715. In
contrast to that observed in the present invention, the presence of
a strong band edge feature can bias the white-light emission toward
a particular color, thereby reducing the quality of the white light
produced. The band edge emission feature of the nanoparticles of
the invention is greatly diminished, providing a more balanced
white-light emission with chromaticity coordinates of 0.322, 0.365,
which fall well within the white region of the 1931 CIE diagram
(FIG. 2). See also
http://hyperphysics.phy-astr.gsu.edu/hbase/vision/cie.html#c2.
Examples of the white-light emission are shown in FIG. 3. FIG. 4
compares the emission spectrum of ultra-small CdSe nanocrystals to
conventional tungsten (General Electric 100 W) and fluorescent
lighting (Sylvania Octron.RTM. #F032) emission spectra with the AM
1.5 solar spectrum, and a commercial white LED (Photon
Micro-Light.RTM., Photon Light.com). The visible spectrum is
denoted by the shaded area.
E. SURFACE-MODIFIED ULTRA-SMALL NANOCRYSTALS WITH ENHANCED QUANTUM
YIELD
[0179] The synthesis, and subsequent functionality, of
semiconductor nanocrystals for a variety of applications has become
a topic of broad scientific interest. [Kuno, M.; Lee, J. K.;
Dabbousi, B. O.; Mikulec, F. V.; Bawendi, M. G., The band edge
luminescence of surface modified CdSe nanocrystallites: Probing the
luminescing state. Journal of Chemical Physics 1997, 106, (23),
9869-9882.; Murray, C. B.; Norris, D. J.; Bawendi, M. G., Synthesis
and Characterization of Nearly Monodisperse Cde (E=S, Se, Te)
Semiconductor Nanocrystallites. Journal of the American Chemical
Society 1993, 115, (19), 8706-8715.; Peng, Z. A.; Peng, X. G.,
Formation of high-quality CdTe, CdSe, and CdS nanocrystals using
CdO as precursor. Journal of the American Chemical Society 2001,
123, (1), 183-184.; Yin, Y.; Alivisatos, A. P., Colloidal
nanocrystal synthesis and the organic-inorganic interface. Nature
2005, 437, (7059), 664-670.; Peng, X. G.; Manna, L.; Yang, W. D.;
Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P., Shape
control of CdSe nanocrystals. Nature 2000, 404, (6773), 59-61.;
Rosenthal, S. J.; McBride, J.; Pennycook, S. J.; Feldman, L. C.,
Synthesis, surface studies, composition and structural
characterization of CdSe, core/shell and biologically active
nanocrystals. Surface Science Reports 2007, 62, 111-157.] In
particular, colloidal nanocrystals have been investigated for use
as light harvesters in photovoltaic devices, [Greenham, N. C.;
Shinar, J.; Partee, J.; Lane, P. A.; Amir, O.; Lu, F.; Friend, R.
H., Optically detected magnetic resonance study of efficient
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characterization of a nanocrystal based photovoltaic device.
European Physical Journal D 2001, 16, (1-3), 275-277.; Smith, N.
J.; Emmett, K. J.; Rosenthal, S. J., Photovoltaic cells fabricated
by electrophoretic deposition of CdSe nanocrystals. Applied Physics
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in semiconductor quantum dots. Chemical Physics Letters 2008, 457,
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PbS quantum dots. Nano Letters 2005, 5, (5), 865-8711 as biological
labels, [Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos,
A. P., Semiconductor nanocrystals as fluorescent biological labels.
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Targeting cell surface receptors with ligand-conjugated
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in light-emitting diodes (LED). [Colvin, V. L.; Schlamp, M. C.;
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semiconductor nanocrystals using an epitaxial quantum well. Nature
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Allen, J. W., Optical properties of CdSe nanocrystals in a polymer
matrix. Applied Physics Letters 1999, 75, (20), 3120-3122.;
Schreuder, M. A.; Gosnell, J. D.; Smith, N. J.; Warnement, M. R.;
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[0180] Initial investigations of nanocrystals focused on the
crystal structure and size as the sole source of the unique
optoelectronic properties observed. However, it has been determined
that the surface ligands on the nanocrystal are not only useful for
their tethering and solubility properties, but are also a factor in
the photoluminescent (PL) properties. [Kuno, M.; Lee, J. K.;
Dabbousi, B. O.; Mikulec, F. V.; Bawendi, M. G., The band edge
luminescence of surface modified CdSe nanocrystallites: Probing the
luminescing state. Journal of Chemical Physics 1997, 106, (23),
9869-9882.; Murray, C. B.; Norris, D. J.; Bawendi, M. G., Synthesis
and Characterization of Nearly Monodisperse Cde (E=S, Se, Te)
Semiconductor Nanocrystallites. Journal of the American Chemical
Society 1993, 115, (19), 8706-8715.; Yin, Y.; Alivisatos, A. P.,
Colloidal nanocrystal synthesis and the organic-inorganic
interface. Nature 2005, 437, (7059), 664-670.; Pradhan, N.;
Reifsnyder, D.; Xie, R. G.; Aldana, J.; Peng, X. G., Surface ligand
dynamics in growth of nanocrystals. Journal of the American
Chemical Society 2007, 129, (30), 9500-9509.; Bullen, C.; Mulvaney,
P., The effects of chemisorption on the luminescence of CdSe
quantum dots. Langmuir 2006, 22, (7), 3007-3013.; Landes, C.;
Braun, M.; Burda, C.; El-Sayed, M. A., Observation of large changes
in the band gap absorption energy of small CdSe nanoparticles
induced by the adsorption of a strong hole acceptor. Nano Letters
2001, 1, (11), 667-670.; Munro, A. M.; Ginger, D. S.,
Photoluminescence quenching of single CdSe nanocrystals by ligand
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A. C. S.; Lou, Y. B.; Burda, C., Investigation of the
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control of growth, morphology, and capping structure of colloidal
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[0181] Initially, conventional methodology used dimethylcadmium as
the cadmium precursor in CdSe nanocrystal synthesis. [Murray, C.
B.; Norris, D. J.; Bawendi, M. G., Synthesis and Characterization
of Nearly Monodisperse Cde (E=S, Se, Te) Semiconductor
Nanocrystallites. Journal of the American Chemical Society 1993,
115, (19), 8706-87151 However, due to the cost and hazards
associated with the use of dimethylcadmium, other precursors have
been developed. [Peng, Z. A.; Peng, X. G., Formation of
high-quality CdTe, CdSe, and CdS nanocrystals using CdO as
precursor. Journal of the American Chemical Society 2001, 123, (1),
183-184.; Rosenthal, S. J.; McBride, J.; Pennycook, S. J.; Feldman,
L. C., Synthesis, surface studies, composition and structural
characterization of CdSe, core/shell and biologically active
nanocrystals. Surface Science Reports 2007, 62, 111-157.; Hambrock,
J.; Birkner, A.; Fischer, R. A., Synthesis of CdSe nanoparticles
using various organometallic cadmium precursors. Journal of
Materials Chemistry 2001, 11, (12), 3197-3201.; Qu, L. H.; Peng, Z.
A.; Peng, X. G., Alternative routes toward high quality CdSe
nanocrystals. Nano Letters 2001, 1, (6), 333-337.] Specifically,
CdO combined with a phosphonic acid to form a Cd-phosphonate
precursor complex was suggested as a more cost effective and green
alternative. [Peng, Z. A.; Peng, X. G., Formation of high-quality
CdTe, CdSe, and CdS nanocrystals using CdO as precursor. Journal of
the American Chemical Society 2001, 123, (1), 183-184.] As a result
of this synthetic innovation, the effects of the phosphonic acid as
a component of the Cd precursor and as a surface ligand have been
explored. [Nair, P. S.; Fritz, K. P.; Scholes, G. D., A multiple
injection method for exerting kinetic control in the synthesis of
CdSe nanorods. Chemical Communications 2004, 2084-2085.; Liu, H.
T.; Owen, J. S.; Alivisatos, A. P., Mechanistic study of precursor
evolution in colloidal group II-VI semiconductor nanocrystal
synthesis. Journal of the American Chemical Society 2007, 129, (2),
305-312.; Kanaras, A. G.; Sonnichsen, C.; Liu, H. T.; Alivisatos,
A. P., Controlled synthesis of hyperbranched inorganic nanocrystals
with rich three-dimensional structures. Nano Letters 2005, 5, (11),
2164-2167.] Phosphonic acid ligands were shown to affect the growth
kinetics and shape of the nanocrystals. [Wang, W.; Banerjee, S.;
Jia, S. G.; Steigerwald, M. L.; Herman, I. P., Ligand control of
growth, morphology, and capping structure of colloidal CdSe
nanorods. Chemistry of Materials 2007, 19, (10), 2573-2580.; Yong,
K. T.; Sahoo, Y.; Swihart, M. T.; Prasad, P. N., Growth of CdSe
quantum rods and multipods seeded by noble-metal nanoparticles.
Advanced Materials 2006, 18, (15), 1978-+.; Nair, P. S.; Fritz, K.
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[0182] Ab-initio and first principles calculations have since
determined that phosphonic acid is the most strongly bound ligand
on nanocrystals, leading to the surface coating being composed
almost exclusively of them. [Manna, L. et al., First-principles
modeling of unpassivated and surfactant-passivated bulk facets of
wurtzite CdSe: A model system for studying the anisotropic growth
of CdSe nanocrystals. Journal of Physical Chemistry B 2005, 109,
(13), 6183-6192.; Peng, Z. A.; Peng, X. G., Nearly monodisperse and
shape-controlled CdSe nanocrystals via alternative routes:
Nucleation and growth. Journal of the American Chemical Society
2002, 124, (13), 3343-3353.; Puzder, A.; Williamson, A. J.;
Zaitseva, N.; Galli, G.; Manna, L.; Alivisatos, A. P., The effect
of organic ligand binding on the growth of CdSe nanoparticles
probed by Ab initio calculations. Nano Letters 2004, 4, (12),
2361-2365.; Kopping, J. T.; Patten, T. E., Journal of the American
Chemical Society 2008, 130, (17), 5689-5698.]
[0183] Without wishing to be bound by theory, for CdSe
nanocrystals, the conventional paradigm has been that as the
nanocrystal diameter decreased, the band-edge absorption and
band-edge emission shifted to higher energies, due to quantum
confinement. [Rossetti, R.; Ellison, J. L.; Gibson, J. M.; Brus, L.
E., Size Effects in the Excited Electronic States of Small
Colloidal Cds Crystallites. Journal of Chemical Physics 1984, 80,
(9), 4464-4469.] In contrast, for white-light emitting CdSe
nanocrystals, as shown in FIG. 12 (ultra-small sizes, <1.7 nm),
the emission is no longer dominated by the band-edge emission.
[Bowers, M. J.; McBride, J. R.; Rosenthal, S. J., White-light
emission from ultra-small cadmium selenide nanocrystals. Journal of
the American Chemical Society 2005, 127, (44), 15378-15379.] It has
been shown that although the absorption continues to blue-shift,
indicating smaller diameters, the bluest emission feature appears
to be pinned at a specific energy/wavelength (FIG. 12). [Dukes, A.
D.; Schreuder, M. A.; Sammons, J. A.; McBride, J. R.; Smith, N. J.;
Rosenthal, S. J., Pinned emission from ultra-small cadmium selenide
nanocrystals. Journal of Chemical Physics 2008, 129, (12).] This
pinned emission is believed to be due to trap states on the surface
of the nanocrystal, influenced by the surface ligands. [Bowers, M.
J.; McBride, J. R.; Rosenthal, S. J., White-light emission from
ultra-small cadmium selenide nanocrystals. Journal of the American
Chemical Society 2005, 127, (44), 15378-15379.; Dukes, A. D.;
Schreuder, M. A.; Sammons, J. A.; McBride, J. R.; Smith, N. J.;
Rosenthal, S. J., Pinned emission from ultra-small cadmium selenide
nanocrystals. Journal of Chemical Physics 2008, 129, (12).]
[0184] Previous reports have differed over whether the emission of
these ultra-small nanocrystals can be altered through ligand
modification. [Nag, A.; Sarma, D. D., White light from
Mn.sup.2+-doped Cds nanocrystals: A new approach. Journal of
Physical Chemistry C 2007, 111, 13641-13644.; Singh, S. et al.,
Structure, microstructure and physical properties of ZnO based
materials in various forms: bulk, thin film and nano. Journal of
Physics D-Applied Physics 2007, 40, 6312-6327.] These discussions
have generally centered on a difference of opinion as to the source
of this broad emission. Defect emission coming from either
interstitial atoms or vacancies in the core of the nanocrystals
would not be altered by varying the surface ligands. CdSe
nanocrystals with these types of defects would typically be more
amorphous than crystalline due to the small number of inner atoms.
Chen et al. have previously studied the absorption and emission of
nanocrystals of amorphous character. [Chen, X. B.; Samia, A. C. S.;
Lou, Y. B.; Burda, C., Investigation of the crystallization process
in 2 nm CdSe quantum dots. Journal of the American Chemical Society
2005, 127, 4372-4375.] This work showed that amorphous nanocrystal
defect emission should be accompanied by absorption of mid-gap
states, as well as a blue-shift in the absorption peak maxima due
to crystallization over time. [Chen, X. B.; Samia, A. C. S.; Lou,
Y. B.; Burda, C., Investigation of the crystallization process in 2
nm CdSe quantum dots. Journal of the American Chemical Society
2005, 127, 4372-4375.] However, it has been shown that with the
disclosed methods, these ultra-small nanocrystal's absorption
maxima red-shifts due to growth of the crystal and not preliminary
crystallization. [Dukes, A. D.; Schreuder, M. A.; Sammons, J. A.;
McBride, J. R.; Smith, N. J.; Rosenthal, S. J., Pinned emission
from ultra-small cadmium selenide nanocrystals. Journal of Chemical
Physics 2008, 129, (12).] This work, along with the large Stokes
shift seen in this ultra-small size regime, has led to the
discovery that the broad-emission comes from surface states,
indicating that this emission could be tuned by changing the ligand
environment around those states.
[0185] In the disclosed methods, the effects that eleven different
phosphonic acid ligands (FIG. 13) have on the photoluminescence
properties of white-light emitting nanocrystals are evaluated.
Changing the phosphonic acid ligand on the surface of these
ultra-small nanocrystals allows tuning of the highest energy peak
in the emission and affects the quantum yield. In addition to the
fundamental interest in controlling trap-state emission, tuning
this emission will allow for color control over white solid-state
light sources that employ nanocrystals as the emissive source.
F. FREQUENCY CONVERTERS
[0186] In one aspect, the invention relates to a frequency
converter. That is, at least one quantum dot of the invention,
nanoparticle of any of the invention, plurality of nanoparticles of
the invention, or product of the methods of the invention, or
mixture thereof, can be dispersed within a matrix, for example a
polymeric or glass matrix, and placed within the emission path of
an energy source providing light above the band gap energy of the
nanoparticle of the invention. Thus, the nanoparticles of the
invention can act as frequency downconverters when excited by light
above the band gap energy (see FIG. 1). Accordingly, the
nanoparticles can emit white-light when encapsulated in a polymer
matrix and excited by, for example, an ultraviolet (UV) emitting
light-emitting diode (LED) (see FIG. 3). Different polymers (e.g.,
polyurathane, polycarbonate, polymethylmethacralate, polyesters) as
well as varied nanocrystal loading, can be employed to optimize the
performance of a frequency converter device and, therefore, obtain
peak efficiencies. In one aspect, a commercially available
polyurethane, such as that found in MINWAX.RTM., can be used to
provide the polymeric matrix.
[0187] In one aspect, a frequency converter can be constructed by
encapsulating at least one nanoparticle of the invention within a
polymer matrix (e.g., polyurethane) and positioning the resulting
composition within the emission path of an energy source providing
light above the band gap energy of the nanoparticle of the
invention.
[0188] In one aspect, the polymer matrix can comprise any polymer
known to those of skill in the art. For example, the polymer matrix
can comprise at least one of a polyurethane, a polyacrylate, a
polymethacrylate, a polycarbonate, a polyester, a polyamide, a
polyimide, a polyether, a polyolefin, a polystyrene, a
polythiophene, a polysiloxane, or a polyfluoroethylene, or a
mixture thereof, or a copolymer thereof. In one aspect, the polymer
is substantially transparent to visible light. In a further aspect,
the polymer is substantially transparent to ultraviolet light. In
one aspect, the polymer matrix comprises a photocurable polymer. In
a further aspect, a condensation polymer is substantially absent
from the polymer matrix. It is also understood that the polymer can
include additives to modify the physical or chemical properties of
the polymer. In one aspect, the polymer matrix can be formed by any
method known to those of skill in the art, for example, by solution
casting or by injection molding. Further, the polymer matrix can be
provided in any desired shape, for example, the polymer matrix can
be provided as a film or a lens. Alternatively, the polymer matrix
can be formed by performing a polymerization reaction in the
presence of at least one nanoparticle of the invention.
[0189] In one aspect, the matrix can comprise any glass known to
those of skill in the art. For example, the glass matrix can
comprise borosilicate glass. In a further aspect, the glass matrix
can further comprise at least one additive known to those of skill
in the art to modify the physical or optical properties of the
glass matrix. In a further aspect, an additive can be selected and
included in the glass of the glass matrix such that the additive
improves the strength of the glass matrix or such that the additive
absorbs particular wavelengths of light emitted by the energy
source or the nanoparticles of the invention. For example, the
matrix--polymeric or glass--can be supplemented with an additive
that is opaque with respect to ultraviolet energy, yet transparent
to visible light.
[0190] In a further aspect, the frequency converter of the
invention can further comprise at least one phosphor, additional
quantum dot or mixture thereof. In such aspects, the at least one
phosphor, additional quantum dot, or mixture thereof can be
selected such that the at least one phosphor, additional quantum
dot, or mixture thereof can absorb energy from the energy source
providing light above the band gap energy of the nanoparticle of
the invention or from the emission of the nanoparticle of the
invention and can emit energy of the same or of a different
frequency as the nanoparticle of the invention, thereby modifying
the emission of the frequency converter. For example, the phosphor
can comprise at least one white phosphor or yellow phosphor or a
mixture thereof. In such an aspect, the phosphor can absorb energy
from the energy source (e.g., UV light) and emit visible light of a
frequency intrinsic to the at least one white phosphor or yellow
phosphor or a mixture thereof.
[0191] In a further aspect, the additional quantum dot can be a
quantum dot of other than the nanoparticle of the invention. In
such an aspect, the additional quantum dot can absorb energy from
the energy source providing light above the band gap energy of the
nanoparticle of the invention or from the emission of the
nanoparticle of the invention and can emit energy of the same or of
a different frequency as the nanoparticle of the invention, thereby
modifying the emission of the frequency converter.
[0192] In a further aspect, the additional quantum dot can function
as an energy cascade system of the invention, as disclosed herein,
and is capable of absorbing energy of a wavelength outside the
first electromagnetic region and capable of emitting energy in the
first electromagnetic region. That is, in such an aspect, the
additional quantum dot can absorb energy emitted by the energy
source providing light above the band gap energy of the
nanoparticle of the invention and can emit energy within the
absorption region of the nanoparticle of the invention, thereby
increasing the amount of energy for absorption by the nanoparticle
of the invention and, therefore, increasing the energy emitted by
the frequency converter. For example, the additional quantum dot
can be capable of absorbing energy of a wavelength of from about
100 nm to about 290 nm and capable of emitting energy of a
wavelength of from about 290 nm to about 400 nm. As another
example, the additional quantum dot can be capable of absorbing
energy of a wavelength of from about 100 nm to about 320 nm and
capable of emitting energy of a wavelength of from about 320 nm to
about 400 nm. In one aspect, the additional quantum dot comprises
at least one of cadmium sulfide, cadmium selenide, cadmium
telluride, zinc selenide, zinc sulfide, or zinc telluride or a
mixture thereof. For example, the quantum dot can comprise a single
material (e.g., cadmium sulfide), a core/shell or two or more
materials (e.g., cadmium sulfide/zinc sulfide), or an alloy of two
or more materials.
G. LIGHT EMITTING DIODE DEVICES
[0193] In one aspect, a light emitting diode device can be provided
by using the frequency converter of the invention with an energy
source, for example a light emitting diode, capable of providing
energy above the band gap energy of the nanoparticle of the
invention. That is, the frequency converter can be positioned
within the emission path of such, thereby providing a light
emitting diode device.
[0194] Therefore, in one aspect, the invention relates to a light
emitting diode device comprising a light emitting diode (LED)
capable of emitting energy of a first wavelength, and the frequency
converter of the invention positioned within an emission path of
the light emitting diode, wherein the frequency converter is
capable of absorbing energy of the first wavelength. In a further
aspect, the first wavelength comprises energy within the first
electromagnetic region of the nanoparticle of the invention
employed in the frequency converter of the invention. For example,
the first wavelength can comprise light having a wavelength of from
about 200 nm to about 500 nm, of from about 300 nm to about 450 nm,
of from about 100 nm to about 290 nm, of from about 290 nm to about
400 nm, of from about 100 nm to about 320 nm, or of from about 320
nm to about 400 nm.
[0195] In a further aspect, the light emitting diode device of the
invention can further comprise at least one phosphor, additional
quantum dot or mixture thereof. In such aspects, the at least one
phosphor, additional quantum dot, or mixture thereof can be
selected such that the at least one phosphor, additional quantum
dot, or mixture thereof can absorb energy from the energy source
providing light above the band gap energy of the nanoparticle of
the invention or from the emission of the nanoparticle of the
invention and can emit energy of the same or of a different
frequency as the nanoparticle of the invention, thereby modifying
the emission of the light emitting diode device. For example, the
phosphor can comprise at least one white phosphor or yellow
phosphor or a mixture thereof. In such an aspect, the phosphor can
absorb energy from the energy source (e.g., UV light) and emit
visible light of a frequency intrinsic to the at least one white
phosphor or yellow phosphor or a mixture thereof.
[0196] In a further aspect, the additional quantum dot can be a
quantum dot of other than the nanoparticle of the invention. In
such an aspect, the additional quantum dot can absorb energy from
the energy source providing light above the band gap energy of the
nanoparticle of the invention or from the emission of the
nanoparticle of the invention and can emit energy of the same or of
a different frequency as the nanoparticle of the invention, thereby
modifying the emission of the light emitting diode device.
[0197] In a further aspect, the additional quantum dot can function
as an energy cascade system of the invention, as disclosed herein,
and is capable of absorbing energy of a wavelength outside the
first electromagnetic region and capable of emitting energy in the
first electromagnetic region. That is, in such an aspect, the
additional quantum dot can absorb energy emitted by the energy
source providing light above the band gap energy of the
nanoparticle of the invention and can emit energy within the
absorption region of the nanoparticle of the invention, thereby
increasing the amount of energy for absorption by the nanoparticle
of the invention and, therefore, increasing the energy emitted by
the light emitting diode device. For example, the additional
quantum dot can be capable of absorbing energy of a wavelength of
from about 100 nm to about 290 nm and capable of emitting energy of
a wavelength of from about 290 nm to about 400 nm. As another
example, the additional quantum dot can be capable of absorbing
energy of a wavelength of from about 100 nm to about 320 nm and
capable of emitting energy of a wavelength of from about 320 nm to
about 400 nm. In one aspect, the additional quantum dot comprises
at least one of cadmium sulfide, cadmium selenide, cadmium
telluride, zinc selenide, zinc sulfide, or zinc telluride or a
mixture thereof. For example, the quantum dot can comprise a single
material (e.g., cadmium sulfide), a core/shell or two or more
materials (e.g., cadmium sulfide/zinc sulfide), or an alloy of two
or more materials.
[0198] In one aspect, the light emitting diode device can be
provided by a method comprising the step of positioning the
frequency converter of the invention within an emission path of a
light emitting diode (LED) capable of emitting energy of a first
wavelength, wherein the frequency converter is capable of absorbing
energy of the first wavelength. In a further aspect, the
positioning step can comprises the steps of dissolving at least one
polymer and dispersing at least one quantum dot of the invention,
nanoparticle of any the invention, plurality of nanoparticles of
the invention, or product of the invention, or mixture thereof into
a solvent, thereby producing a solution-dispersion; applying the
solution-dispersion within the emission path of the light emitting
diode; and removing the solvent. That is, in one aspect, a light
emitting diode device of the invention can be provided by
solution-casting a solution-dispersion of the nanoparticle of the
invention and a polymer onto at least a portion of the surface of
and within the emission path of a light emitting diode (LED).
[0199] In a further aspect, the positioning step can comprise the
steps of dissolving at least one polymer and dispersing at least
one quantum dot of the invention, nanoparticle of the invention,
plurality of nanoparticles of any of the invention, or product of
the methods of the invention, or mixture thereof into a solvent,
wherein the polymer is a photocurable polymer at the first
wavelength, thereby producing a photocurable solution-dispersion;
contacting the emission path of the light emitting diode with the
photocurable solution-dispersion; and energizing the light emitting
diode, thereby photocuring at least a portion of the photocurable
solution-dispersion. That is, in one aspect, a light emitting diode
device of the invention can be provided by photocuring (also
referred to as photocrosslinking) a solution-dispersion of the
nanoparticle of the invention and a polymer onto at least a portion
of the surface of and within the emission path of a light emitting
diode (LED). Photocurable polymers are well-known to those of skill
in the art, and it is understood that any photocurable polymer can
be used in connection with the light emitting diode device of the
invention. For example, the photocurable polymer can comprise
polybutadiene or a copolymer of polybutadiene. In one aspect, the
resulting photocured polymer is substantially transparent to
visible light. In a further aspect, the resulting photocured
polymer is substantially transparent to ultraviolet light.
[0200] In a further aspect, the positioning step can comprise the
steps of dissolving at least one monomer and at least one
photoactive initiator and dispersing at least one quantum dot of
the invention, nanoparticle of the invention, plurality of
nanoparticles of the invention, or product of the methods of the
invention, or mixture thereof into a solvent, wherein the
photoactive initiator is active at the first wavelength and the
photoactive initiator can initiate polymerization of the monomer,
thereby producing a photopolymerizable solution-dispersion;
contacting the emission path of the light emitting diode with the
photopolymerizable solution-dispersion; and energizing the light
emitting diode, thereby photopolymerizing at least a portion of the
monomer dissolved in the photopolymerizable solution-dispersion.
That is, in one aspect, a light emitting diode device of the
invention can be provided by photopolymerizing a
solution-dispersion of the nanoparticle of the invention and a
polymer onto at least a portion of the surface of and within the
emission path of a light emitting diode (LED). Photoactive
initiators and photopolymerizable polymers are well-known to those
of skill in the art, and it is understood that any photoactive
initiator and/or photopolymerizable polymer can be used in
connection with the light emitting diode device of the invention.
For example, the photopolymerizable polymer can comprise polymethyl
methacrylate initiated by 2,2-dimethoxy-2-phenylacetophenone and
1-hydroxycyclohexylphenyl ketone. In one aspect, the resulting
photopolymerizable polymer is substantially transparent to visible
light. In a further aspect, the resulting photopolymerizable
polymer is substantially transparent to ultraviolet light.
H. MODIFIED FLUORESCENT LIGHT SOURCES
[0201] In one aspect, the invention relates to a modified
fluorescent light source. That is, a modified fluorescent light
source can be provided by substituting the nanoparticle of the
invention for the phosphor employed within a conventional
fluorescent light source. In such an aspect, a modified fluorescent
light source can comprise a substantially optically transparent and
substantially hermetically sealed tube having a first end, a second
end, an interior surface, an exterior surface, and a lumen
extending therethrough; a first electrode positioned at the first
end; a second electrode positioned at the second end; inert gas
vapor and mercury vapor within the lumen of the tube; a phosphor
substitute comprising at least one frequency converter of the
invention, quantum dot of the invention, nanoparticle of the
invention, plurality of nanoparticles of any of the invention, or
product of the methods of the invention, or mixture thereof
substantially coating the interior surface or the exterior surface
of the tube.
[0202] In one aspect, the modified fluorescent light source
operates in a manner similar to the operation of a conventional
fluorescent light source. For example, by establishing a current
between the first electrode positioned at the first end and the
second electrode positioned at the second end, a plasma is
established from the inert gas vapor and mercury vapor within the
lumen of the substantially optically transparent and substantially
hermetically sealed tube. In one aspect, the inert gas can comprise
any inert gas capable of establishing a plasma under electrical
current sustainable under conventional electrical loads; for
example, the inert gas can comprise argon gas. As well-known to
those of skill in the art, a plasma comprising mercury emits
ultraviolet light at a wavelength of 254 nm. In a conventional
fluorescent light source, the emitted ultraviolet light excites the
phosphor coating at the interior surface of the tube, and the
phosphor absorbs the ultraviolet light and then emits visible light
at discrete wavelengths (see FIG. 4). It is also understood that
other ultraviolet sources--for example, Xenon or one or more
halogens--can be used in connection with the invention.
[0203] In contrast, in one aspect, in the modified fluorescent
light source of the invention, the phosphor is replaced or
supplemented by the nanoparticle of the invention. That is, the
phosphor substitute can be present at the surface of the tube in
combination with the phosphor or the phosphor substitute can
replace the phosphor. In one aspect, the phosphor is absent from
the modified fluorescent light source. In one aspect, the emitted
ultraviolet light excites the nanoparticle of the invention coating
at the surface(s) of the tube, and the nanoparticle absorbs the
ultraviolet light and then emits broad emission visible light (see
FIG. 4). In a further aspect, in the modified fluorescent light
source of the invention, the optically transparent and
substantially hermetically sealed tube can comprise an ultraviolet
light opaque material and the phosphor substitute can substantially
coat the interior surface of the tube. In a further aspect, the
optically transparent and substantially hermetically sealed tube
can comprise an ultraviolet light transparent material and the
phosphor substitute can substantially coat the exterior surface of
the tube.
[0204] In a further aspect, the modified fluorescent light source
of the invention can further comprise at least one phosphor,
additional quantum dot or mixture thereof. In such aspects, the at
least one phosphor, additional quantum dot, or mixture thereof can
be selected such that the at least one phosphor, additional quantum
dot, or mixture thereof can absorb energy from the mercury emission
or from the emission of the nanoparticle of the invention and can
emit energy of the same or of a different frequency as the
nanoparticle of the invention, thereby modifying the emission of
the modified fluorescent light source. For example, the phosphor
can comprise at least one white phosphor or yellow phosphor or a
mixture thereof. In such an aspect, the phosphor can absorb energy
from the mercury emission (e.g., UV light) and emit visible light
of a frequency intrinsic to the at least one white phosphor or
yellow phosphor or a mixture thereof.
[0205] In a further aspect, the additional quantum dot can be a
quantum dot of other than the nanoparticle of the invention. In
such an aspect, the additional quantum dot can absorb energy from
the mercury emission or from the emission of the nanoparticle of
the invention and can emit energy of the same or of a different
frequency as the nanoparticle of the invention, thereby modifying
the emission of the modified fluorescent light source.
[0206] In a further aspect, the additional quantum dot can function
as an energy cascade system of the invention, as disclosed herein,
and is capable of absorbing energy of a wavelength outside the
first electromagnetic region and capable of emitting energy in the
first electromagnetic region. That is, in such an aspect, the
additional quantum dot can absorb energy emitted by the mercury
emission and can emit energy within the absorption region of the
nanoparticle of the invention, thereby increasing the amount of
energy for absorption by the nanoparticle of the invention and,
therefore, increasing the energy emitted by the modified
fluorescent light source. For example, the additional quantum dot
can be capable of absorbing energy of a wavelength of from about
100 nm to about 290 nm and capable of emitting energy of a
wavelength of from about 290 nm to about 400 nm. As another
example, the additional quantum dot can be capable of absorbing
energy of a wavelength of from about 100 nm to about 320 nm and
capable of emitting energy of a wavelength of from about 320 nm to
about 400 nm. In one aspect, the additional quantum dot comprises
at least one of cadmium sulfide, cadmium selenide, cadmium
telluride, zinc selenide, zinc sulfide, or zinc telluride or a
mixture thereof. For example, the quantum dot can comprise a single
material (e.g., cadmium sulfide), a core/shell or two or more
materials (e.g., cadmium sulfide/zinc sulfide), or an alloy of two
or more materials.
I. ELECTROLUMINESCENT DEVICES BASED ON BROAD-EMISSION
NANOCRYSTALS
[0207] The unique optical properties of the nanoparticles of the
invention make them ideal materials for incorporation into solid
state lighting applications in the form of light emitting diodes
(LEDs). LEDs can be made by incorporating an intrinsic emitting
layer into a p-n junction device. When under forward bias,
electrons and holes are injected into the intrinsic layer, where
they recombine to emit light. Lumileds.TM.: Light From Silicon
Valley, Craford, M. G. (2004). The wavelength of the emitted photon
is determined by the intrinsic layer or phosphor. Typically,
phosphors are limited to monochromatic emission, requiring a
mixture of phosphors or a complex doping scheme to achieve
white-light. Yang, W.-J.; Luo, L.; Chen, T.-M.; Wang, N.-S. Chem.
Mater. 2005, 17, 3883-3888.
[0208] An ideal material for a white-light emitting intrinsic layer
would be robust, emit over the entire visible spectrum, have high
quantum efficiency, not suffer from self absorption, be easy to
produce on a large scale, and not waste energy producing
wavelengths beyond the visible spectrum (e.g., tungsten). Taking
these criteria into consideration, semiconductor nanocrystals, in
particular the nanoparticles of the invention, are attractive
candidate materials for applications in solid state lighting.
Unlike commercial phosphors, emission color can be tuned by simply
controlling the nanocrystal size. Further, white-light emission can
be provided by employing the nanoparticles of the invention.
[0209] Nanocrystal-based LEDs have been demonstrated showing
colored-light emission. One example developed by Klimov et al.
utilizes core/shell nanocrystals deposited onto an InGaN quantum
well. The quantum well is pumped by laser emission, generating
electrons and holes which, through energy transfer, cause the
nanocrystals to emit light at their fundamental band edge energy.
Achermann, M.; Petruska, M. A.; Kos, S.; Smith, D. L.; Koleske, D.
D.; Klimov, V. I. Nature 2004, 429, 642-646. While intuitive,
simply mixing several colors of nanocrystals together to achieve
white light results in an overall reduction of device efficiency
through self-absorption between the various sizes of nanocrystals.
This can be especially detrimental for device designs requiring a
thickness of more than several monolayers. Mueller, A. H.;
Petruska, M. A.; Achermann, M.; Werder, D. J.; Akhadov, E. A.;
Koleske, D. D.; Hoffbauer, M. A.; Klimov, V. I. Nano Lett. 2005, 5,
1039-1044. The large Stokes shift and narrow size distribution of
these ultra-small nanocrystals, along with the broad emission
spectrum, would alleviate any efficiency loss attributed to
self-absorption from mixing nanocrystal sizes.
[0210] In contrast to conventional mixed-nanocrystal-based
electroluminescent devices, the nanoparticle of the invention, in
one aspect, when used as an intrinsic layer, can provide a
broadband emission, thereby providing white light while avoiding an
overall reduction of device efficiency through self-absorption
between the various sizes of nanocrystals
[0211] 1. Structure
[0212] In one aspect, the electroluminescent device comprises an
n-type semiconductor, a p-type semiconductor, and a quantum dot
layer in electrical or photonic communication with the n-type
semiconductor and the p-type semiconductor, wherein the quantum dot
layer comprises at least one quantum dot of the invention,
nanoparticle of the invention, plurality of nanoparticles of any of
the invention, product of the methods of the invention, or
frequency converter of the invention, or mixture thereof (see,
e.g., an exemplary electroluminescent device structure in FIG. 5).
It is understood that n-type semiconductor and p-type semiconductor
materials--both inorganic and organic (e.g.,
N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine
(TPD), aluminum tris(8-hydroxyquinoline) (Alq.sub.3), or other
known polymeric semiconductors)--known to those of skill in the art
can be used in connection with the nanoparticles of the invention.
In a further aspect, the quantum dot layer can be in contact with
and in electrical communication with the n-type semiconductor and
the p-type semiconductor. That is, in one aspect, the nanoparticle
of the invention can be excited through contact-based energy
transfer interactions. In a further aspect, the quantum dot layer
can be in contact with the n-type semiconductor or the p-type
semiconductor and in electrical or photonic communication with the
n-type semiconductor and the p-type semiconductor. That is, in one
aspect, the nanoparticle of the invention can be excited through
non-contact-based energy transfer interactions.
[0213] In a further aspect, the electroluminescent device of the
invention can further comprise at least one phosphor, additional
quantum dot or mixture thereof. In such aspects, the at least one
phosphor, additional quantum dot, or mixture thereof can be
selected such that the at least one phosphor, additional quantum
dot, or mixture thereof can absorb energy from the light emitted by
recombination of electrons and holes injected into the intrinsic
layer under forward bias or from the emission of the nanoparticle
of the invention and can emit energy of the same or of a different
frequency as the nanoparticle of the invention, thereby modifying
the emission of the electroluminescent device. For example, the
phosphor can comprise at least one white phosphor or yellow
phosphor or a mixture thereof. In such an aspect, the phosphor can
absorb energy from the light emitted by recombination of electrons
and holes injected into the intrinsic layer under forward bias
(e.g., UV light) and emit visible light of a frequency intrinsic to
the at least one white phosphor or yellow phosphor or a mixture
thereof.
[0214] In a further aspect, the additional quantum dot can be a
quantum dot of other than the nanoparticle of the invention. In
such an aspect, the additional quantum dot can absorb energy from
the light emitted by recombination of electrons and holes injected
into the intrinsic layer under forward bias or from the emission of
the nanoparticle of the invention and can emit energy of the same
or of a different frequency as the nanoparticle of the invention,
thereby modifying the emission of the electroluminescent
device.
[0215] In a further aspect, the additional quantum dot can function
as an energy cascade system of the invention, as disclosed herein,
and is capable of absorbing energy of a wavelength outside the
first electromagnetic region and capable of emitting energy in the
first electromagnetic region. That is, in such an aspect, the
additional quantum dot can absorb energy emitted by the light
emitted by recombination of electrons and holes injected into the
intrinsic layer under forward bias and can emit energy within the
absorption region of the nanoparticle of the invention, thereby
increasing the amount of energy for absorption by the nanoparticle
of the invention and, therefore, increasing the energy emitted by
the electroluminescent device. For example, the additional quantum
dot can be capable of absorbing energy of a wavelength of from
about 100 nm to about 290 nm and capable of emitting energy of a
wavelength of from about 290 nm to about 400 nm. As another
example, the additional quantum dot can be capable of absorbing
energy of a wavelength of from about 100 nm to about 320 nm and
capable of emitting energy of a wavelength of from about 320 nm to
about 400 nm. In one aspect, the additional quantum dot comprises
at least one of cadmium sulfide, cadmium selenide, cadmium
telluride, zinc selenide, zinc sulfide, or zinc telluride or a
mixture thereof. For example, the quantum dot can comprise a single
material (e.g., cadmium sulfide), a core/shell or two or more
materials (e.g., cadmium sulfide/zinc sulfide), or an alloy of two
or more materials.
[0216] 2. Device Fabrication
[0217] Electroluminescent devices of the invention can be prepared
by methods analogous to those reported in the work of Coe et al.,
who employ a relatively inorganic approach to nanocrystal-based
electroluminescent devices (FIG. 5). Coe, S.; Woo, W.; Bawendi, M.;
Bulovic, V.; "Electroluminescence from single monolayers of
nanocrystals in molecular organic devices," Nature 420 800-803
(2002). Briefly, a chloroform solution of ultra-small CdSe and
N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'diamine
(TPD), can be spin-cast onto a clean indium tin oxide (ITO) coated
glass substrate. The nanocrystal and TPD concentrations can be
adjusted to yield a monolayer of nanocrystals, on top of an about
35 nm TDP layer, when spin-cast. An approximately 40 nm thick film
of aluminum tris(8-hydroxyquinoline) (Alq.sub.3) can then be
thermally evaporated; followed by an approximately 1 mm diameter,
approximately 75 nm thick Mg:Ag (10:1 by mass) cathode with a 50 nm
Ag cap. An additional layer of
3-(4-biphenyl)-4-phenyl-5-t-butylphenyl-1,2,4-triazole (TAZ) and/or
bathocuprione (BCP) with a nominal thickness of 10 nm can be added
between the nanocrystals and the Alq.sub.3 layer or between the
Alq.sub.3 layer and the metal cathode layers. All of these
processes are typically carried out in a dry box under air-free
nitrogen atmosphere. More advanced designs can incorporate Bragg
mirrors or other photonic crystal structures, which can enhance the
brightness, efficiency, and directionality of light emission. In
these devices, a bottom mirror can essentially reflect back any
light emitted toward the substrate and therefore increase the light
extraction through top surface.
[0218] 3. Device Characterization
[0219] The properties that can be evaluated for the broad-emission
nanocrystal-based LEDs in solid state lighting applications are
luminous intensity, luminous efficacy, color temperature, and
lifetime. In order to measure these properties, photometric
experiments that measure optical radiation as perceived by the
human eye can be performed. A photometric quantity can be obtained
as follows: P=.intg.K(.lamda.)R(.lamda.)d.lamda., where P
represents the photometric quantity (e.g., luminous flux in
lumens), R represents the corresponding radiometric quantity that
can be measured directly (e.g., radiant flux in Watts), and
K(.lamda.) represents the visual response of the eye as a function
of wavelength. Therefore, photometric experiments can use either a
broadband detector with a special photopic filter that is
calibrated to mimic the spectral response of the human eye (i.e.,
photometer) or a photodiode array with associated computer software
that takes into account the spectral luminous efficacy of the human
eye (i.e., spectroradiometer). Spectroradiometers are more
accurate, especially at the edges of the visual response curve.
[0220] Traditional light source testing methods are typically
insufficient for characterizing LEDs due to the complicated spatial
light distribution that is typical for LEDs. Special consideration
is taken to avoid large discrepancies in measurements from
different metrology laboratories. Therefore, in addition to
maintaining a constant temperature environment and drive current,
specific geometrical measurement conditions are imposed to ensure
reproducibility. The International Commission on Illumination (CIE)
sets the standards for characterizing an LED's performance. FIG. 6
illustrates one such configuration. The LED can be placed in a
fixed position with its mechanical center aligned to the middle of
the detector, which is located at a distance, D, from the LED. The
optical center, meaning the location of the center of the emission
cone of light, is not necessarily coincident with the mechanical
center. Consequently, in order to enable reproducible measurements,
all LEDs are tested at their mechanical center. The CIE has
suggested the A and B criterion for setting the distance, D.
Measurements are taken at D equal to 316 mm (A) and 100 mm (B).
While the two conditions should give the same results for a
Lambertian source that emits light uniformly in all directions, the
emission of LEDs tends to be non-uniform and directional so the two
measurements need to be averaged to determine the standardized LED
performance. The configuration shown in FIG. 6 can be used to
measure average luminous intensity (lm/sr), which loosely
correlates to brightness as perceived by the human eye.
[0221] A second configuration is necessary to measure the total
luminous flux (lm), lifetime, and color. By placing the LED in the
center of an integrating sphere, the flux emitted in all directions
can be detected. Given the electrical power input to the LED and
the measured luminous flux output, the efficiency of the device can
be determined. The industry target for luminous efficacy, as
specified in the OIDA Solid-State LED Roadmap, is 75 lm/W by 2007
and 200 lm. Light Emitting Diodes (LEDs) for General Illumination:
An OIDA Technology Roadmap Update 2002
(http://lighting.sandia.gov/lightingdocs/OIDA_SSL_LED_Roadmap_Full.pdf).
For comparison, an incandescent lamp has an efficiency of
approximately 16 lm/W, a commercial white light LED operates at
approximately 30 lm/W, and a research-grade white light LED has
been demonstrated at approximately 80 lm/W for very low drive
currents. See http://www.lumileds.com; N. Narendran, Y. Gu, J. P.
Freyssinier-Nova, and Y. Zhu, "Extracting phosphor-scattered
photons to improve white LED efficiency," Phys. Stat. Sol. A 202,
R60-R62 (2005). Internal quantum efficiency, light extraction
efficiency, and phosphor down-conversion efficiency are all
contributing factors to luminous efficacy. The lifetime of a
solid-state LED, considered to be the 50% lumen depreciation level,
can also be determined using the integrating sphere. The target
lifetime for solid-state LEDs is greater than 100,000 hours by
2020. Current white light LEDs have lifetimes greater than 50,000
hours. http://www.lumileds.com. If the integrating sphere is
coupled with a spectroradiometer, a calculation of the chromaticity
coordinates and color temperature can also be accomplished, which
specifies the apparent color of the LED when viewed directly. The
spectroradiometer performs a complete spectral power distribution
of the source being measured from which the colorimetric parameters
can be mathematically calculated.
J. ENERGY CASCADE SYSTEMS
[0222] The nanoparticle of the invention typically absorbs energy
within a first electromagnetic region and emits energy within a
second electromagnetic region. When used in connection with an
energy source, the first electromagnetic region of the nanoparticle
of the invention can, in one aspect, can comprise wavelengths of
energy other than the entire emission region of the energy source.
That is, the energy source may emit light at one or more
wavelengths outside of the absorbance band of the nanoparticle of
the invention. In such aspects, an energy cascade system can be
employed to accumulate energy for absorption by the nanoparticle.
The resulting system can be termed a cascade of energy collected
for use by the longer-wavelength-absorbing nanoparticle or quantum
dot. Specifically, in one aspect, a nanocrystal or core/shell
nanocrystal, including for example a nanoparticle of the invention,
can be used to collect light at very low wavelengths, e.g., 250 nm.
The core/shell can then emit light at a higher wavelength (e.g.,
within a first electromagnetic region), wherein a nanoparticle in
the invention can absorb efficiently.
[0223] For example, in one aspect, when an energy source emits
energy within a first absorption electromagnetic region and the
nanoparticle or quantum dot absorbs energy within a second
absorption electromagnetic region, wherein the first absorption
electromagnetic region and the second absorption electromagnetic
region do not wholly overlap, the nanoparticle or quantum dot
cannot absorb all available energy emitted by the energy source. In
such an aspect, at least two quantum dots in photonic or energetic
communication, wherein the first quantum dot is capable of
absorbing energy from a first absorption electromagnetic region and
capable of emitting energy in a first emission electromagnetic
region, wherein the second quantum dot is capable of absorbing
energy from a second absorption electromagnetic region and capable
of emitting energy in a second emission electromagnetic region, and
wherein the first emission electromagnetic region overlaps with the
second absorption electromagnetic region can be employed to make a
greater amount of energy emitted by the energy source available for
absorption by the nanoparticle or quantum dot.
[0224] The energy cascade systems of the invention can be used in
connection with the nanoparticle of the invention as well as with
nanoparticles of other than the invention. That is, in one aspect,
the energy cascade system comprises a second quantum dot comprising
the quantum dot of the invention, the nanoparticle of the
invention, or the product of the methods of the invention. In a
further aspect, both the first quantum dot and the second quantum
dot are narrow band emission nanocrystals of other than the
invention.
[0225] In one aspect, the invention relates to an energy cascade
system comprising at least two quantum dots in photonic
communication, wherein the first quantum dot is capable of
absorbing energy from a first absorption electromagnetic region and
capable of emitting energy in a first emission electromagnetic
region, wherein the second quantum dot is capable of absorbing
energy from a second absorption electromagnetic region and capable
of emitting energy in a second emission electromagnetic region, and
wherein the first emission electromagnetic region overlaps with the
second absorption electromagnetic region.
[0226] In a further aspect, the first absorption electromagnetic
region comprises wavelengths of from about 100 nm to about 290 nm
and the first emission electromagnetic region comprises wavelengths
of from about 290 nm to about 400 nm. In a yet further aspect, the
first absorption electromagnetic region comprises wavelengths of
from about 100 nm to about 320 nm and the first emission
electromagnetic region comprises wavelengths of from about 320 nm
to about 400 nm. In a still further aspect, the second absorption
electromagnetic region comprises wavelengths of from about 290 nm
to about 400 nm or from about 320 nm to about 400 nm. In one
aspect, the second emission electromagnetic region comprises
wavelengths of from about 400 nm to about 700 nm or from about 420
nm to about 710 nm.
[0227] In one aspect, the first quantum dot comprises at least one
of cadmium sulfide, cadmium selenide, cadmium telluride, zinc
selenide, zinc sulfide, or zinc telluride or a mixture thereof. For
example, the quantum dot can comprise a single material (e.g.,
cadmium sulfide), a core/shell or two or more materials (e.g.,
cadmium sulfide/zinc sulfide), or an alloy of two or more
materials.
[0228] In one aspect, the second quantum dot comprises the
nanoparticle of the invention or at least one of cadmium sulfide,
cadmium selenide, cadmium telluride, zinc selenide, zinc sulfide,
or zinc telluride or a mixture thereof. For example, the quantum
dot can comprise a single material (e.g., cadmium sulfide), a
core/shell or two or more materials (e.g., cadmium sulfide/zinc
sulfide), or an alloy of two or more materials.
K. SOLID STATE LIGHTING
[0229] Solid state lighting, in the form of white light emitting
diodes (LEDs), is an attractive replacement for current lighting
technologies based on their potential for longer service lifetimes
and lower power consumption. [The Promise of Solid State Lighting
for General Illumination, Optoelectronics Industry Development
Association, Washington, D.C., 2001.] This requires an LED that
emits white light which is pleasing to the eye, similar to the
output from incandescent light bulbs or sunlight. Most current
white-light LEDs emit a harsh white color similar to fluorescent
lights. The quality of the emitted white light can be evaluated by
quantities such as CIE chromaticity coordinates, color rendering
index (CRI), and correlated color temperature (CCT). Each of these
measures is important for characterizing the unique properties of
white-light LEDs. The CIE chromaticity coordinates (x, y) allow
specification of the exact color of the emission as perceived by
the human eye, where (0.33, 0.33) is the ideal value for white
light. Color rendering index is a measure of the ability of a
source to reproduce the colors of a variety of objects that it
illuminates, presented on a scale of 0 to 100, representing the
worst (0) and best (100) color rendering. Correlated color
temperature is a measure of the temperature in degrees Kelvin where
a blackbody radiator best matches the emission spectrum of the
tested light source. Ideal CCT values for white-light LEDs range
from warmer white (2500 K to 4500 K) to cooler white (4500 K to
6500 K). [D'Andrade, B. W.; Forrest, S. R. "White Organic
Light-Emitting Devices for Solid-State Lighting." Adv. Mater. 16,
1585-1595, 2004.] The values for each of these quantities are given
for common white-light sources in Table 1.
[0230] A 2001 Department of Energy study found that transitioning
to solid state lighting, such as using white-light LEDs, would
reduce carbon dioxide emission by 258 million metric tons and
eliminate the need for over 100 new power stations over a 20 year
period. [The Promise of Solid State Lighting for General
Illumination, Optoelectronics Industry Development Association,
Washington, D.C., 2001.] Generally, the goal is to decrease the
global electricity usage for lighting by 50% while at the same time
provide higher quality lighting. Over the same 20 year period, this
could save 760 GW of energy in the U.S. alone, which would result
in a financial savings of over $115 billion dollars.
TABLE-US-00001 TABLE 1 CIE coordinates, CRI, and CCT for common
white-light sources. White-Light Source CIE x CIE y CRI CCT (K)
Incandescent Bulb 0.448 0.408 100 2854 Fluorescent, cool white
0.375 0.367 89 4080 Fluorescent, warm white 0.440 0.403 72 2940
Daylight (CIE Standard Illuminant D.sub.65) 0.313 0.329 90 6500
Inova commercial white-light LED 0.308 0.300 76 7137 [D'Andrade, B.
W.; Forrest, S. R. "White Organic Light-Emitting Devices for
Solid-State Lighting." Adv. Mater. 16, 1585-1595, 2004.] The Inova
commercial white-light LED was measured in a Labsphere
SLMS-LED-1050 integrating sphere system.
[0231] 1. White-Light LEDs
[0232] A current problem with transitioning to solid state lighting
is the lack of an efficient LED that is easy to manufacture and
possesses sufficient color properties for general lighting. Several
approaches have been reported toward the development of a viable
solid state white-light LED. One technique utilizes a blue
light-emitting InGaN LED coated with cerium-doped yttrium aluminum
garnet (Ce.sup.3+:YAG) crystals, which function as a yellow
phosphor. [Tamura, T.; Setomoto, T.; Taguchi, T. "Illumination
characteristics of lighting array using 10 candela-class white LEDs
under AC 100 V operation." Journal of Luminescence 87-89,
1180-1182, 2000.; Schlotter, P., et al., "Fabrication and
characterization of GaN:InGaN:AlGaN double heterostructure LEDs and
their application in luminescence conversion LEDs." Mat. Sci. and
Eng. B 59, 390-394, 1999.] The crystals convert some of the blue
LED light to yellow light, creating a mixture of yellow and blue
light that gives the appearance of white light. This light is
considered to be cool white because it has a color temperature of
about 5000 K. The lack of emission in the red region of the
spectrum results in an inferior white light compared to
incandescent bulbs, which is a significant obstacle to overcome
before achieving the transition to solid state lighting. In order
to enable a more tunable form of white light, a combination of red,
green, and blue phosphors can be pumped with a UV LED.
[Steigerwald, D. A., et al., "Illumination With Solid State
Lighting Technology." IEEE J. on Sel. Top. in Quant. Elec. 8,
310-320, 2002.; Sheu, J. K., et al., "White-Light Emission From
Near UV InGaN--GaN LED Chip Precoated With Blue/Green/Red
Phosphors." IEEE Photonics Technology Letters 15, 18-20, 2003.]
However, by using three phosphors instead of one, the complexity of
fabricating these devices is greatly increased.
[0233] White-light LEDs have also been demonstrated without the use
of phosphors by growing homoepitaxial zinc selenide (ZnSe) on a
ZnSe substrate. Some of the greenish-blue light (485 nm) from the
active region of the p-n junction is absorbed by the substrate and
re-emitted as yellow light (centered on 585 nm) via self-activated
emission, resulting from distant donor-acceptor pair transitions.
[Katayama, K., et al., "ZnSe-based white LEDs." Journal of Crystal
Growth 214/215, 1064-1070, 2000.; Shirakawa, T. "Effect of defects
on the degradation of ZnSe-based white LEDs." Mater. Sci. and Eng.
B 91-92, 470-475, 2002.; Thomas, A. E.; Russell, G. J.; Woods, J.
"Self-activated emission in ZnS and ZnSe." J. Phys. C: Solid State
Phys. 17, 6219-6228, 1984.] However, the color rendering of these
devices is poor due to a CRI of 68, which is not high enough for
general room illumination. White-light LEDs have also been
fabricated with higher color rendering by using separate red,
green, and blue LEDs to produce white light with a CRI greater than
80. [Muthu, S.; Schuurmans, F. J.; Pashley, M. D. "Red, Green, and
Blue LEDs for White Light Illumination." IEEE J. on Sel. Top. In
Quant. Elec. 8, 333-338, 2002.; Chhajed, S.; Xi, Y.; Li, Y.-L.;
Gessmann, Th.; Schubert, E. F. "Influence of junction temperature
on chromaticity and color-rendering properties of trichromatic
white-light sources based on light-emitting diodes." J. of Appl.
Phy. 97, 054506, 2005.] However, these devices are expensive and
complex to produce because of the complicated feedback system that
is necessary to maintain the proper color balance as the red,
green, and blue LEDs degrade at different rates over time.
[0234] 2. CdSe nanocrystal-based LEDs
[0235] For more than a decade, monochromatic light emitting diodes
have been fabricated using CdSe nanocrystals as the emitting layer
of the device. Electrons are injected into the nanocrystal layer
while holes are injected into a layer of a semiconducting polymer
such as p-paraphenylene vinylene (PPV) or polyvinylcarbazole (PVK),
resulting in light emission from both the polymer and the CdSe
layer. [Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P.
"Light-emitting diodes made from cadmium selenide nanocrystals and
a semiconducting polymer." Nature 370, 354-357, 1994.; Dabbousi, B.
O.; Bawendi, M. G.; Onitsuka, O.; Rubner, M. F.
"Electroluminescence from CdSe quantum-dot/polymer composites."
Appl. Phys. Lett. 66, 1316-1318, 1995.; Schlamp, M. C.; Peng, X.;
Alivisatos, A. P. "Improved efficiencies in light emitting diodes
made with CdSe.CdS.core/shell type nanocrystals and a
semiconducting polymer." J. of Appl. Phys. 82, 5837-5842, 1997.;
Mattoussi, H., et al., "Electroluminescence from heterostructures
of poly(phenylene vinylene) and inorganic CdSe nanocrystals." J. of
Appl. Phys. 83, 7965-7974, 1998.; Mattoussi, H., et al., "Composite
thin films of CdSe nanocrystals and a surface passivating/electron
transporting block copolymer: Correlations between film
microstructure by transmission electron microscopy and
electroluminescence." J. of Appl. Phys. 86, 4390-4399, 1999.]
Because of the poor charge conduction in CdSe nanocrystal layers,
utilizing a single monolayer of CdSe nanocrystals has been shown to
increase the external electroluminescence quantum efficiency of
these devices from <0.01% to 1.1%. [Coe, S.; Woo, W.-K.;
Bawendi, M.; Bulovi , V. L. "Electroluminescence from single
monolayers of nanocrystals in molecular organic devices." Nature
420, 800-803, 2002.; Coe-Sullivan, S.; Woo, W.-K.; Steckel, J. S.;
Bawendi, M. G.; Bulovi , V. L. "Tuning the performance of hybrid
organic/inorganic quantum dot light-emitting devices." Org.
Electron. 4, 123-130, 2003.] Recently, a new charge conduction
mechanism has been demonstrated where CdSe nanocrystals are
deposited onto an InGaN quantum well and electron-hole pairs are
indirectly injected into the nanocrystals, resulting in an energy
transfer and band edge emission by the nanocrystals. [Achermann,
M.; Petruska, M. A.; Kos, S.; Smith, D. L.; Koleske, D. D.; Klimov,
V. I. "Energy-transfer pumping of semiconductor nanocrystals using
an epitaxial quantum well." Nature 429, 642-646, 2004.] In
addition, CdSe nanocrystals have been used as the intrinsic layer
of an all-inorganic LED when surrounded by n- and p-doped GaN to
inject electrons and holes directly, avoiding the low carrier
mobilities of organic materials. [Mueller, A. H. et al. "Multicolor
Light-Emitting Diodes Based on Semiconductor Nanocrystals
Encapsulated in GaN Charge Injection Layers." Nano Lett. 5,
1039-1044, 2005.]
[0236] 3. White-Light Emitting CdSe Nanocrystals
[0237] Current white-light, nanocrystal-based devices rely on
either a combination of nanocrystal and polymer emission or the
mixing of several sizes of nanocrystals to achieve white light. It
has been widely published that the color of the light emitted
depends on the size of the nanocrystal. For example, CdSe
nanocrystals with a diameter of 7 nm emit red light while CdSe
nanocrystals with a diameter of 2 nm emit blue light. [Murray, C.
B., Norris, D. J., Bawendi, M. G. "Synthesis and Characterization
of Nearly Monodisperse CdE (E=S, Se, Te) Semiconductor
Nanocrystallites." J. Am. Chem. Soc. 115, 8706-8715, 1993.;
Alivisatos, A. P. "Semiconductor Clusters, Nanocrystals, and
Quantum Dots." Science 271, 933-937, 1996.] The efficiency of
white-light LEDs based on several sizes of nanocrystals is
compromised due to the overlap of the emission and absorption
spectra of the different size nanocrystals. The present approach to
achieving high quality white-light LEDs that can be fabricated at
low cost involves the use of white-light emitting CdSe nanocrystals
that are a single ultra-small size and exhibit broadband emission
from 420-710 nm when illuminated with UV light, as seen in FIG. 1.
These nanocrystals are monodisperse with a diameter of
approximately 1.5 nm. Without wishing to be bound by theory,
because the ultra-small nanocrystals are believed to be
approximately 1.5 nm in diameter, the emission in the blue part of
the spectrum is believed due to the inherent size of the
nanocrystals and is likely a result of internal exciton
recombination processes. The broad emission in the yellow and red
portions of the ultra-small CdSe nanocrystal emission spectrum has
been attributed to midgap states resulting from uncoordinated
selenium atoms on the surface of the nanocrystals. [Bowers, M. J.,
II; McBride, J. R.; Rosenthal, S. J. "White-Light Emission from
Ultra-small Cadmium Selenide Nanocrystals." J. Am. Chem. Soc. 127,
15378-15379, 2005.]
[0238] In one aspect, the invention relates to a broad emission LED
comprising at least one nanocrystal of the invention encapsulated
within a polymer, for example, a polymer as disclosed herein,
including but not limited to a perfluorocyclobutyl (PFCB) polymer.
In various aspects, LEDs comprising the nanocrystals of the
invention can emit produce white light from 420-710 nm.
[0239] 4. Advantages of White-Light Nanocrystal-Based Devices
[0240] Single, ultra-small CdSe nanocrystals, in one aspect, have a
large Stokes shift separating the absorption and emission peaks.
Consequently, devices based on these nanocrystals will not suffer
from self-absorption, potentially resulting in higher device
efficiency. Another advantage is that ultra-small CdSe nanocrystal
white-light LEDs would not require a mixture of phosphors or a
complex feedback system to generate white light, resulting in much
simpler devices. Furthermore, the addition of emission in the red
portion of the spectrum gives higher color rendering than in
devices emitting mostly blue and yellow light, such as the blue LED
with yellow phosphor and the ZnSe-based devices. Encapsulating
ultra-small CdSe nanocrystals coated onto UV LEDs act as frequency
downconverters to produce white light from 420-710 nm.
[0241] In the preparation of LEDs, the choice of encapsulant can
play a role in the white-light emission spectrum. In the examples,
the highest quality white-light spectra were observed when the
ultra-small CdSe nanocrystals are dispersed into PFCB polymer.
[0242] Increasing the concentration of nanocrystals in PFCB
typically results in the emission color changing from a cooler
white to a warmer white. Furthermore, varying the wavelength of the
LED used to excite the samples can have a small effect on the
emission spectrum of the nanocrystals, with longer wavelengths LEDs
causing a shift towards a cooler white color. Using a 385 nm LED to
excite 2% CdSe nanocrystals in PFCB, chromaticity coordinates of
(0.33, 0.30), CRI of 79, a color temperature of 5406 K, and
luminous efficacy slightly below 0.5 lm/W have been measured.
Incorporation of ultra-small nanocrystals into an
electroluminescent device can lead to higher quality and more
economical solid state lighting devices.
L. EXPERIMENTAL
[0243] The following examples are put forth so as to provide those
of ordinary skill in the art with a complete disclosure and
description of how the compounds, compositions, articles, devices
and/or methods claimed herein are made and evaluated, and are
intended to be purely exemplary of the invention and are not
intended to limit the scope of what the inventors regard as their
invention. Efforts have been made to ensure accuracy with respect
to numbers (e.g., amounts, temperature, etc.), but some errors and
deviations should be accounted for. Unless indicated otherwise,
parts are parts by weight, temperature is in .degree. C. or is at
ambient temperature, and pressure is at or near atmospheric.
[0244] Acetonitrile, ethyl acetate, hexanes, toluene, methanol,
hexanol, and butanol, all reagent grade, were purchased from
Sigma-Aldrich. For the nanocrystal synthesis, phenyl phosphonic
acid (PPA, 98%, Aldrich), hexyl phosphonic acid (97%, Strem),
hexadecyl amine (HDA, technical grade 90%, Aldrich), CdO (99.99%,
Strem), Se powder (200 mesh, 99.99%, Strem), and tributyl phosphine
(TBP, 97%, Aldrich) were used as received, while other phosphonic
acids were synthesized as described below. The various phosphonic
acid syntheses required the use HCl (12.1 M, aqueous, EMD
Chemicals), triethyl phosphite (TEP, 98%, Aldrich), and either
Bromodocosane (96%, Aldrich), Bromohexadecane (97%, Aldrich),
Bromododecane (97%, Aldrich), Bromodecane (98%, Aldrich),
Bromooctane (99%, Acros-organics), Bromobutane (98%, Aldrich),
2-ethyl,hexyl bromide (95%, Aldrich), 3-methyl,butyl bromide (96%,
Aldrich), or 3-phenyl,propyl bromide (98%, Acros-organics).
[0245] All .sup.1H and .sup.13C spectra were recorded on a Bruker
300 MHz spectrometer equipped with a 7 Tesla Oxford magnet
controlled by a Bruker DPX-300 console at ambient temperature in
deuterated DMSO, methanol, or CDCl.sub.3. The spectra were analyzed
using TopSpin 2.0 software. The IR spectra for all the phosphonic
acids were recorded on a ThermoNicolet-IR300 spectrometer using
EZOMNIC version 6.1 software. ESI-MS measurements were done on a
FinniganMat-Electrospray Ionization LCQ Mass spectrometer system
with TunePlus version 1.3 software. All absorption measurements
were conducted on a Varian-Cary 50 Bio UV-Visible Spectrophotometer
with accompanying software. The emission spectra were recorded
using an ISS PC1 photon counting spectrofluorimeter, using Vinci
version 1.6.5P5 software.
[0246] 1. Synthesis of Ultra-Small Cadmium Selenide
Nanocrystals
[0247] a. Reagents
[0248] Tri-n-octylphosphine oxide (TOPO, 90% tech. grade),
hexadecylamine (HDA, 90% technical grade) and octadecene (ODE, 90%
technical grade) were purchased from Aldrich and used as received.
CdO (99.999% Puratrem), tri-n-butylphosphine (TBP, 97%), and
selenium powder (200 mesh) were purchased from Strem and used as
received. Dodecylphosphonic acid (DPA) was synthesized via the
Arbuzov reaction from triethylphoshite and 1-dodecylbromide
followed by acid work up with concentrated HCl and
recrystallization from cold ethyl acetate. [Maege, I.; Jaehne, E.;
Henke, A.; Adler, H.-J. P.; Bram, C.; Jung, C.; Stratmann, M.
"Self-assembling adhesion promoters for corrosion resistant metal
polymer interfaces." Progress in Organic Coatings 34, 1-12, 19981
All other solvents used were HPLC grade and purchased from Fisher
Scientific, unless otherwise noted.
[0249] b. Procedure 1
[0250] A 1.0 M solution of selenium in TBP was produced by
dissolving 7.896 g selenium powder in 100 mL of TBP. The 0.10 M
injection mixture was made by dilution of the 1.0 M Se:TBP solution
with ODE. This solution was stable for several days stored in a
capped bottle, in ambient conditions at room temperature. The
reaction solvent was mixed from a unit quantity of TOPO and HDA
(7.2 g and 2.97 g respectively) along with 0.128 g of CdO and 0.496
g of DPA (scaled as necessary). These contents were heated in a 100
mL three neck flask under argon with vigorous stirring to
310.degree. C. A needle was placed in the septum to allow for an
argon purge until the reaction mixture reached 150.degree. C., at
which point the reaction vessel was considered water- and
oxygen-free. Upon reaching reaction temperature (330.degree. C.), 5
mL of the Se:TBP:ODE solution were swiftly injected and the
temperature reduced to 270.degree. C. as quickly as was possible
without allowing the temperature to drop below 260.degree. C. To
achieve ultra-small nanocrystals (e.g., <20 .ANG.), a second
syringe of toluene (typically 5 mL) was injected to reduce the
reaction temperature to <150.degree. C. within about 2 to about
10 seconds after the initial injection.
[0251] c. Procedure 2
[0252] As before, a 0.10 M injection mixture of selenium in TBP was
produced by dissolving 7.896 g selenium powder in 100 mL of TBP and
dilution of the 1.0 M Se:TBP solution with ODE. The reaction
mixture was prepared from 2.97 g of HDA and 0.128 g of CdO and
0.496 g of DPA (scaled as necessary). These contents were heated in
a 100 mL three neck flask under argon with vigorous stirring to
330.degree. C., until the solution became colorless. The mixture
was then purged with argon and cooled to approximately room
temperature (i.e., about 25.degree. C.). Then, 7.2 g of TOPO was
added to the reaction mixture, which was then heated to reaction
temperature (e.g., about 330.degree. C.).
[0253] At reaction temperature, 5 mL of the Se:TBP:ODE injection
mixture were injected and the temperature reduced to 270.degree. C.
as quickly as was possible without allowing the temperature to drop
below 260.degree. C. To achieve ultra-small nanocrystals (e.g.,
<20 .ANG.), a second syringe of toluene (typically 5 mL) was
injected to reduce the reaction temperature to <150.degree. C.
within about 2 to about 10 seconds after the initial injection.
[0254] d. Characterization
[0255] FIG. 1 shows the absorption and emission properties of a
sample of ultra-small CdSe prepared by the methods of the
invention. The first absorption feature is at 414 nm which has been
assigned as a thermodynamically determined "magic size," on the
order of 15 .ANG., for CdSe. Qu, L.; Yu, W. W.; Peng, X. Nano Lett.
2004, 4, 465-469; Landes, C.; Braun, M.; Burda, C.; El-Sayed, M. A.
Nano Lett. 2001, 1, 667-670. The emission spectrum shows a band
edge emission followed by two features attributed to emission from
energetically different midgap states. The fluorescence quantum
yield for this example is on the order of 2% to 3%, with a
calculated extinction coefficient of 22311 per mole of
nanocrystals. Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater.
2003, 15, 2854-2860. Additionally, the material demonstrated good
photostability. Thin films of ultra-small CdSe (neat on a glass
slide) were able to maintain their optical properties after ten
days of exposure to intense UV light from a 1000 .mu.W, 370 nm LED
(LEDtronics) under ambient conditions (air at room
temperature).
[0256] 2. Preparation of Nanocrystals by Etching (Comparative)
[0257] A solution of 456 nm absorbing CdSe nanocrystals in toluene
was mixed with butylamine. The final butylamine concentration was
approximately 0.9M. The etching process was carried out at room
temperature over four days. The absorption and emission spectra
were collected once the primary absorption peak had reached 414 nm.
The spectrum of the resulting nanocrystals is shown in FIG. 7.
[0258] 3. Ultra-Small Cadmium Selenide Nanocrystals Having Improved
Quantum Yield
[0259] a. Reagents
[0260] Tri-n-octylphosphine oxide (TOPO, 90% tech. grade),
hexadecylamine (HDA 90% tech. grade) and octadecene (ODE, 90% tech.
grade) were purchased from Aldrich and used as received. CdO
(99.999% Puratrem), tri-n-butylphosphine (TBP, 97%), and selenium
powder (200 mesh) were purchased from Strem and used as received.
Dodecylphosphonic acid (DPA) was synthesized via the Abruzov
reaction from triethylphoshite and 1-dodecylbromide followed by
acid work up with concentrated HCl and recrystallization from cold
ethyl acetate. All other solvents were HPLC grade and purchased
from Fisher Scientific unless otherwise noted.
[0261] b. Synthesis
[0262] A 1.0 M solution of selenium in TBP was produced by
dissolving 7.896 g selenium powder in 100 mL of TBP. The 0.20 M
injection solution was made by dilution of the 1.0 M Se:TBP
solution with TBP. This solution was stable for several days stored
in a sealed bottle, in ambient conditions at room temperature. The
reaction solvent was mixed from a unit quantity of TOPO and HDA
(7.0 g and 3.0 g respectively) along with 0.128 g of CdO and 0.496
g of DPA. These contents were heated in a 100 mL three neck flask
under argon with vigorous stirring to 330.degree. C. A needle was
placed in the septum to allow for an argon purge until the reaction
solution reached 150.degree. C. at which point the reaction vessel
was considered water and oxygen free. Upon reaching reaction
temperature (330.degree. C.), the reaction continued stirring until
the red color from the CdO disappeared. The temperature was then
reduced to 315.degree. C. and the heating mantle was removed. 4.5
mL of the 0.2 M Se:TBP along with 2 mL ODE were swiftly injected. A
second syringe of butanol (typically 10 mL) was injected to reduce
the reaction temperature to <150.degree. C. within 2-10 seconds
after the initial injection. Time is determined by the observed
color of the reaction vessel. That is, for example, the alcohol
solution can be added at the first appearance of a yellow color in
the solution. This addition, in one aspect, can also have the
effect of precipitating the nanocrystals. This reaction may be
scaled to produce larger or smaller quantities.
[0263] c. Isolation
[0264] Nanocrystals are divided into equally into two 11 dram
vials. The vials are filled to the top with methanol and
centrifuged. The liquid is discarded and a small amount of hexyl
alcohol is added the vial and then centrifuged again. The
nanocrystals are removed by the hexyl alcohol leaving behind the
excess HDA. The liquid is poured into two clean 11 dram vials and
the nanocrystals are precipitated by addition of methanol and
collected by centrifugation. The solvent is discarded and the
nanocrystals are re-dissolved in the non-polar solvent of choice
(toluene, hexanes, chloroform, etc.).
[0265] d. Characterization
[0266] The nanocrystals of this example exhibit a broad emission
spectrum. An exemplary spectrum is shown as FIG. 8.
[0267] e. Quantum Yield
[0268] Nanocrystals produced in this example exhibit average
quantum yields (i.e., light in to light out ratios) of at least
10%. That is, in this example, nanocrystals produced by the methods
of the invention can have a quantum yield of greater than about
10%.
[0269] 4. Encapsulation
[0270] a. Nanocrystal Synthesis
[0271] A 1.0 M stock solution of selenium in TBP was produced by
dissolving 11.84 g selenium powder in 150 mL of TBP. The injection
solution was produced by further diluting a 0.2 M Se:TBP solution
with TBP. This solution was stable for several days stored in a
capped bottle, under nitrogen or argon at room temperature. The
reaction solvent was mixed from a unit quantity of TOPO and HDA
(7.0 g and 3.0 g respectively) along with 0.128 g of CdO and 0.496
g of DPA (scaled as necessary). These chemicals were placed in a
100 mL three neck flask, with a temperature probe, a bump trap, and
a septum in the three necks. A needle was placed in the septum to
allow for an argon purge until the reaction solution reached
150.degree. C., at which point the reaction vessel was considered
water and oxygen free.
[0272] The reaction continued to be heated under argon with
vigorous stirring up to 320.degree. C. Upon reaching reaction
temperature, 4.25 mL of the 0.2 M Se:TBP solution along with 2.00
mL of ODE solution were mixed in a syringe and swiftly injected.
Following this injection the temperature of the reaction flask was
maintained between 270.degree. C. and 240.degree. C. To achieve
ultra-small nanocrystals (e.g., <20 .ANG.), a second syringe of
butanol (typically 10 mL) was injected to reduce the reaction
temperature to <130.degree. C. within 2-10 seconds after the
initial injection (depending on the desired size). Further cooling
using compressed air, applied to the outside of the reaction flask,
cooled the solution to <100.degree. C. The nanocrystals were
then removed from the coordinating solvents using methanol and
hexanol washes, before being suspended in toluene.
[0273] b. Sample Preparation
[0274] A range of different encapsulant materials were chosen based
on their ability to add phosphor materials to them before curing
and on their color after curing, preferably clear. The encapsulants
were prepared according to the instructions provided using the
following materials: Easy Cast clear casting epoxy (Environmental
Technology Inc.), RTVS 61 clear silicone (Insulcast), Epo-Tek epoxy
resin (Epoxy Technology), perfluorocyclobutyl (PFCB) polymer
(Tetramer Technologies), EP 691 clear epoxy resin (Resinlab), and
EP 965 clear epoxy resin (Resinlab). Other materials were tested
but are not listed due to incomplete mixing with the nanocrystal
solution or insufficient hardening at room temperature after
addition of the nanocrystals. The CdSe nanocrystals were added
while in solution (toluene) and mixed thoroughly before the
encapsulant was cured. Aggregation was monitored via fluorescence
microscopy, where the emitted white light blinking on and off from
a thin film of nanocrystals in the encapsulant was observed. This
on/off or intermittency behavior is based on an Auger ionization
process and is only seen in single nanocrystal emission, indicating
little or no aggregation. [Nirmal, M.; Dabbousi, B. O.; Bawendi, M.
G.; Macklin, J. J.; Trautman, J. K.; Harris, T. D.; Brus, L. E.
"Fluorescence intermittency in single cadmium selenide
nanocrystals." Nature 383, 802-804, 1996.; Blanton, S. A.; Hines,
M. A.; Guyot-Sionnest, P. "Photoluminescence wandering in single
CdSe nanocrystals." Appl. Phys. Lett. 69, 3905-3907, 1996.]
[0275] For curing, vials of samples were placed inside a lab oven
at 65-100.degree. C. for 3-24 hours depending on the curing
schedule of the encapsulant. After hardening, the samples were
removed from the oven and left to cool to room temperature before
testing. If the encapsulant material did not adhere to the glass
vial, the sample was removed from the vial before testing. However,
if the encapsulant did adhere to the glass, the sample was tested
while still inside the vial. The Easy Cast, RTVS 61, Epo-Tek
samples were tested after being removed from the vial, while the
PFCB, EP 691, and EP 965 samples were tested in the vial.
[0276] c. Sample Testing
[0277] Absorption and photoluminescence were recorded for all
samples. The UV-V is absorbance measurements were performed in air
using a Varian Cary Bio 100 UV-Vis spectrophotometer.
Photoluminescence measurements were carried out using either a
spectrofluoremeter or an integrating sphere with a fiber-coupled
spectrometer. Multiple excitation sources were used for the
photoluminescence measurements, including a 300 W high-pressure
xenon arc lamp with monochromator, and UV or blue LEDs in
wavelengths of 365, 375, 385, 395, 405, 410, and 423 nm (Roithner
LaserTechnik). The LEDs were powered at 20 mA and 4 V using a
Keithley 2400 sourcemeter.
[0278] The first photoluminescence apparatus utilized an ISS PC1
photon counting spectrofluoremeter in air. The nanocrystals in
solution were excited by the xenon white-light source using a
monochromator. The samples of nanocrystals in encapsulant were
excited using commercial UV and blue LEDs. Images of the LEDs and
then the samples on top of the LEDs can be seen in FIG. 9, which
shows the white light being emitted by the samples. The absorption
and emission spectra of these samples can be seen in FIG. 10 and
FIG. 11, respectively. The second photoluminescence apparatus
consisted of a Labsphere SLMS-LED-1050 integrating sphere system.
The samples were placed on top of the LEDs in the center of the
integrating sphere. The integrating sphere was fiber-optically
coupled to a CDS 500 CCD-based spectrometer that measured emission
from 350-850 nm. The provided software calculated radiant flux,
luminous flux, CIE chromaticity coordinates, correlated color
temperature (CCT), and color rendering index (CRI).
[0279] Though many encapsulant materials were tested, the polymer
PFCB typically performed the best due to the high UV absorption by
the samples with CdSe nanocrystals at low thicknesses (several
millimeters). To achieve this same level of absorption in other
materials, most needed to be several centimeters thick, which is
much larger than is typically acceptable for a commercial product.
Ideally, either a very thin coating of the encapsulant with
nanocrystals would be applied to the outside of a commercial UV
LED, or the nanocrystals in the encapsulant would replace the
commercial clear epoxy bulb and encase a UV LED die. In either
case, it is typically desirable to have as little as possible UV
light leak out of the device. In addition to the high absorbance,
the nanocrystals mixed thoroughly and easily in PFCB while curing
in a matter of hours. Based on its better performance than the
other encapsulants, the data will be presented only for PFCB
samples.
[0280] d. Absorption
[0281] FIG. 10 shows the absorption spectra for PFCB alone, and
PFCB with 1%, 2%, 5%, and 10% nanocrystals by weight. The PFCB
curve shows a gradual decrease in absorption from 350 nm to 700 nm
with no visible peaks. The nanocrystals in solution show a strong
band edge absorption feature at 414 nm, which is typical for
ultra-small CdSe nanocrystals with a diameter of 1.5 nm, as seen in
FIG. 1. The nanocrystals with PFCB curves show the same general
trend of the PFCB only curve with the addition of the band edge
absorption feature at 414 nm, which shows that the nanocrystals
have not grown in size and are dispersed into the polymer matrix.
At only 1% nanocrystals by weight, the absorption spectrum is
almost identical to the polymer only absorption spectrum. At 2%
nanocrystals in PFCB, the band edge absorption peak of the CdSe
nanocrystals can be observed. As the ratio of the nanocrystals to
the polymer increases, the absorption spectrum more strongly
resembles that of pure, ultra-small CdSe nanocrystals in
solution.
[0282] e. Photoluminescence
[0283] FIG. 11 shows the emission spectra for PFCB alone and for
PFCB with 1%, 2%, 5%, and 10% nanocrystals by weight. A 365 nm LED
was used as the excitation source and the LED emission was detected
with the photon counting spectrofluoremeter. The PFCB curve shows a
strong emission in the blue portion of the spectrum up to about 500
nm, with very little emission at higher wavelengths. At only 1%
nanocrystals in PFCB, the emission spectrum shows a similar trend
to the polymer only curve but with a slightly lower amount of blue
emission. The samples with concentrations of 2%, 5%, and 10%
nanocrystals display a decreasing amount of blue emission at higher
concentrations of nanocrystals in PFCB. Like the absorbance, an
increase in the amount of nanocrystals in the sample causes the
emission to appear more like the nanocrystal only emission spectrum
and less like the PFCB only emission spectrum.
[0284] The color characteristics of CIE coordinates, CRI, and CCT
as well as the luminous efficacy for the PFCB with nanocrystal
samples are reported in Table 2. The luminous efficacy is
calculated as total luminous flux divided by the electrical power
used to light the UV LEDs, in lm/W. The 1% and 2% concentration of
nanocrystals in PFCB had CIE coordinates of (0.289, 0.260) and
(0.328, 0.296) respectively when illuminated with 385 nm LEDs, as
seen in Table 2, which are within the white-light range and are
similar to the other samples since the nanocrystals alone in
solution have coordinates of (0.322, 0.365). [Bowers, M. J., II;
McBride, J. R.; Rosenthal, S. J. "White-Light Emission from
Magic-Sized Cadmium Selenide Nanocrystals." J. Am. Chem. Soc. 127,
15378-15379, 2005.] The CRI and CCT were typically about 80 and
3500-5000 K, respectively, and are also in the ideal range for
white-light LEDs. [Steigerwald, D. A., et al., "Illumination With
Solid State Lighting Technology." IEEE J. on Sel. Top. in Quant.
Elec. 8, 310-320, 2002.] The luminous efficacy of the exemplary
devices is under 1 lm/W, which is partly due to the low quantum
yield (10%) of the white-light nanocrystals. This yield is
typically lower than the single-color CdSe nanocrystal emission
quantum yield.
TABLE-US-00002 TABLE 2 CIE coordinates, CRI, and CCT for 1%, 2%,
5%, and 10% by weight ultra-small CdSe nanocrystals in PFCB when
excited with three 365 nm, 375 nm, or 385 nm LEDs. Luminous CCT
Efficacy LED Concentration CIE x CIE y CRI (K) (lm/W) 365 nm 1%
0.3566 0.4234 71.4 4779 0.05 2% 0.3759 0.4143 78.1 4299 0.05 5%
0.4167 0.4896 66.7 3881 0.04 10% 0.4412 0.4993 60.4 3549 0.03 375
nm 1% 0.3390 0.3817 81.2 5065 0.29 2% 0.3855 0.4156 81.0 4029 0.29
5% 0.4217 0.4499 78.2 3559 0.27 10% 0.4377 0.4565 78.5 3346 0.25
385 nm 1% 0.2886 0.2603 76.1 9497 0.49 2% 0.3277 0.2955 79.0 5406
0.46 5% 0.3634 0.3407 80.5 4139 0.43 10% 0.3920 0.3675 81.1 3554
0.41
[0285] Results indicate that the excitation wavelength had a small
effect on the emission properties of the devices. As the peak
wavelength of the LED increased, more blue light was emitted by the
LED, resulting in a cooler white color. This trend can be seen in
Table 2, with higher color temperatures resulting from excitation
with longer wavelength LEDs. In contrast, an increase in the
concentration of nanocrystals in PFCB caused a small decrease in
the amount of blue emission from the device, partly due to self
absorption of the nanocrystals. This resulted in a warmer white
color or lower color temperature at higher concentrations of
nanocrystals in PFCB, also seen in Table 2. The luminous efficacy
of the devices was similarly affected by these same factors. The
level of UV light absorbance by the samples varies with wavelength
as shown in FIG. 10, which causes a change in the amount of
emission power and device efficiency depending on the peak
wavelength of the LED. In the same way, the concentration of
nanocrystals alters the efficiency of the samples. By adding more
nanocrystals to the same amount of polymer, more UV light can be
absorbed as well as emitted by the devices when their concentration
is below 10%. In addition to the LED wavelength and concentration,
the emission of the devices can be affected by the encapsulant
material used. Some of the materials used had a large amount of
emission in the visible spectrum and can cause the color emitted to
become much more blue or yellow, depending on the encapsulant
material. Though this effect can be undesirable since the
nanocrystals emit white light on their own, this additional
emission allows for tuning of the light to become cooler (more
blue) or warmer (more yellow) in future applications. However, UV
light absorbed by the encapsulant can affect the longevity of the
encapsulant.
[0286] 5. Surface Modification of Nanocrystals
[0287] a. Phosphonic Acid Preparation
[0288] A Michaelis-Arbuzov reaction followed by acidic hydrolysis
was used to synthesize the various phosphonic acids employed and is
described briefly as follows (FIG. 13). [Engel, R., Synthesis of
Carbon-Phosphorus Bonds. CRC Press, Inc.: Boca Raton, Fla., 1988; p
229.; Kosolapoff, G. M., Organophosphorus compounds. Wiley: New
York, N.Y., 1950; p 376.] Molar equivalents of the bromoalkane of
interest and TEP were refluxed at .about.150.degree. C. for at
least 24 hours in order to ensure near complete S.sub.N2
substitution. The reaction was considered complete when the
evolution of bromoethane was no longer evident. In general, a
slight yellow color in the reaction was also noted. The reaction
mixture was cooled to 80.degree. C., followed by the addition of
2.1 molar equivalents HCl. In a few cases a small amount of
deionized water was added to the reaction mixture in order to
increase the overall reaction volume. Post-hydrolysis, this excess
water was removed via vacuum distillation. The reaction was then
refluxed for at least 48 hours at 103.degree. C.; in the cases of
the longer chain phosphonic acids, solid product was noted falling
out of solution upon completion of the hydrolysis. The reaction was
then cooled, and the product phosphonic acid was recrystallized
using acetonitrile, ethyl acetate, or hexanes. The crystals were
collected via vacuum filtration, washed with cold solvent, and
allowed to dry. The collected crystals were then placed under
vacuum to remove any remaining solvent.
[0289] The purity of the phosphonic acid product was verified using
nuclear magnetic resonance spectroscopy (NMR), infrared
spectroscopy (IR), negative mode electrospray ionization mass
spectrometry (ESI-MS), and melting point measurements (Supplemental
information). Using negative mode ESI-MS for the phosphonic acids
commonly gave dimer, trimer, and sodiated peaks in addition to the
phosphonic acid minus one peak. The IR spectra of each phosphonic
acid showed alkyl stretches corresponding to the carbon chain,
P--O--H stretches in the 1040-910 cm.sup.-1 region, P.dbd.O
stretches in the 1200-1100 cm.sup.-1 region, in addition to the
2350-2080, 1740-1600, and 1040-917 cm.sup.-1 peak regions, common
to all phosphonic acids. [Nakanishi, K.; Solomon, P. H., Infrared
Absorption Spectroscopy. 2nd ed.; Holden-Day, Inc.: San Francisco,
Calif., 1977; p 287.; Nakamoto, K., Infrared and Raman Spectra of
Inorganic and Coordination Compounds. John Wiley and Sons: New
York, N.Y., 1978; p 448.; Gunzler, H.; Gremlich, H. U., IR
Spectroscopy. Wiley, VCH: Weinheim, Germany, 2002; p 361.] Proton
and carbon NMR spectra were consistent with the expected number,
location, and integration of peaks.
[0290] b. Nanocrystal Preparation
[0291] Nanocrystals were synthesized with slight modifications to
previously published syntheses. [Peng, Z. A.; Peng, X. G.,
Formation of high-quality CdTe, CdSe, and CdS nanocrystals using
CdO as precursor. Journal of the American Chemical Society 2001,
123, (1), 183-184.; Bowers, M. J.; McBride, J. R.; Rosenthal, S.
J., White-light emission from magic-sized cadmium selenide
nanocrystals. Journal of the American Chemical Society 2005, 127,
(44), 15378-15379.; Gosnell, J. D. et al., Eds. SPIE: 2006; p
63370A.] Since phosphonic acids have been found to be an impurity
in tri-octyl phosphine oxide (TOPO), [Peng, X. G.; Manna, L.; Yang,
W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P.,
Shape control of CdSe nanocrystals. Nature 2000, 404, (6773),
59-611 the synthesis was modified to use only HDA as the
surfactant. In a three-neck, round bottom flask 10 g HDA, 1 mmol
CdO, and 2 mmol phosphonic acid were heated to 145.degree. C. under
Ar purge. The flask contents were then heated to 325.degree. C.
under Ar with vigorous stirring. As the CdO and phosphonic acid
converted to a Cd-phosphonate complex, the flask's contents became
clear and colorless. After cooling to 310.degree. C., 4 mL of a 0.2
M Se:TBP solution were quickly injected into the flask. To produce
nanocrystals small enough to have band-edge absorption features
less than 420 nm, this initial injection was rapidly followed by a
second injection of 20 mL of butanol (kill-shot) to cool the
reaction to .about.130.degree. C., followed by further cooling to
below 100.degree. C. using compressed air applied to the outside of
the flask. [Bowers, M. J.; McBride, J. R.; Rosenthal, S. J.,
White-light emission from magic-sized cadmium selenide
nanocrystals. Journal of the American Chemical Society 2005, 127,
(44), 15378-153791 For the larger nanocrystal sizes, the reaction
was allowed to proceed until the desired size was achieved and
subsequently cooled in the same way. Small aliquots (<500 .mu.L)
were taken from each reaction and diluted in toluene. These "dirty"
nanocrystals were reserved for quantum yield (QY), UV-VIS
absorption, and PL testing.
[0292] Unreacted precursor and solvents were removed from the
nanocrystals via a three wash process. Initially, 15 mL of each
reaction mixture was placed into a centrifuge tube and precipitated
with methanol. The supernatant was discarded and the pellet was
re-suspended in 8 mL of hexanol. This suspension was centrifuged in
order to remove HDA and Cd-phosphonate. The supernatant (containing
the nanocrystals) was then decanted into clean tubes, and the
initial methanol/centrifugation step was repeated. The supernatant
was again discarded and the nanocrystals suspended in toluene for
further measurements, these nanocrystals hereafter will be referred
to as "clean" nanocrystals.
[0293] c. Quantum Yield Determination
[0294] In order to determine the quantum yield of the nanocrystals
synthesized, the emission of nanocrystals in toluene, dirty and
clean, was compared to a standard dye. At nanocrystal sizes with
band-edge absorption of 400-420 nm (ultra-small nanocrystals), the
nanocrystal solutions were diluted to have an absorption <0.1 at
375 nm. A solution of Coumarin 152A in hexanes (QY=1.00) [Bowers,
M. J.; McBride, J. R.; Rosenthal, S. J., White-light emission from
magic-sized cadmium selenide nanocrystals. Journal of the American
Chemical Society 2005, 127, (44), 15378-153791 [Nad, S.; Kumbhakar,
M.; Pal, H., Photophysical properties of coumarin-152 and
coumarin-481 dyes: Unusual behavior in nonpolar and in higher
polarity solvents. Journal of Physical Chemistry A 2003, 107, (24),
4808-48161 was diluted to the same optical density. Emission
spectra were then collected of each solution using 375 nm as the
excitation wavelength. For the ultra-small nanocrystals, the
emission intensity was integrated and compared to that of the
Coumarin 152A solution. Larger nanocrystal sizes with band-edge
absorption features from 540-570 nm were diluted to have an optical
density <0.1 at 395 nm, the excitation wavelength used for the
PL. These "traditional" nanocrystals were compared to a solution of
Coumarin 503 in ethanol (QY=0.56). [Reynolds, G. A.; Drexhage, K.
H., New Coumarin Dyes with Rigidized Structure for Flashlamp-Pumped
Dye Lasers. Optics Communications 1975, 13, (3), 222-225.] The
quantum yield of the nanocrystal solutions was then found using
Equation 1:
QY NC = ( E NC / A NC E STD / A STD ) .times. ( ( .eta. NC ) 2 (
.eta. STD ) 2 ) .times. QY STD ( 1 ) ##EQU00001##
[0295] In this equation E.sub.NC is the nanocrystals integrated
emission intensity, A.sub.NC is the nanocrystals optical density at
the excitation wavelength, E.sub.STD is the standard's integrated
emission intensity, A.sub.STD is the standard's optical density at
the excitation wavelength, .eta..sub.NC is toluene's refractive
index, .eta..sub.STD the standard's solvent's refractive index, and
QY.sub.STD is the standard's literature quantum yield. All QY
measurements were performed on six different batches of
nanocrystals for each phosphonic acid to ensure accuracy.
[0296] In order to monitor emission pinning and avoid slight
batch-to-batch variations, an average of 21 batches per phosphonic
acid were synthesized that had band-edge absorption features below
420 nm. Absorption and PL spectra were acquired for each batch and
the data fit with a Gaussian curve to determine the center
wavelength for the band-edge absorption and the highest energy
emission feature.
[0297] d. Results and Discussion
[0298] Initially seven phosphonic acids with unbranched alkyl
chains (FIG. 13) were used to synthesize broad band-emitting
nanocrystals. These early experiments were intended to show pinning
of the emission of nanocrystals with diameters less than .about.1.7
nm, using different phosphonic acids compared to the pinning
previously seen with dodecyl phosphonic acid. [Dukes, A. D.;
Schreuder, M. A.; Sammons, J. A.; McBride, J. R.; Smith, N. J.;
Rosenthal, S. J., Pinned emission from ultra-small cadmium selenide
nanocrystals. Journal of Chemical Physics 2008, 129, (12).]
Plotting the wavelength of the first emission feature vs. the
wavelength of the band-edge absorption clearly showed a region of
pinned emission (FIG. 14). It is important to note the slope change
from the traditional size regime of nanocrystals (>1.7 nm) to
the ultra-small nanocrystals. In the traditional nanocrystal sizes
the emission red-shifts with red-shifting band-edge absorption and
increasing diameter. For the ultra-small nanocrystals, the
different sizes do not show a change in the wavelength of the
emission from the highest energy trap state, despite changes in the
wavelength of the band-edge absorption feature. [Dukes, A. D.;
Schreuder, M. A.; Sammons, J. A.; McBride, J. R.; Smith, N. J.;
Rosenthal, S. J., Pinned emission from ultra-small cadmium selenide
nanocrystals. Journal of Chemical Physics 2008, 129, (12).] The
average and standard deviation for the wavelength at which the
first emission feature was located were obtained for each set of
nanocrystal batches made with different phosphonic acids.
[0299] It was noted during the synthesis that it was possible to
make the smallest nanocrystals, as determined using Peng et al.'s
nanocrystal sizing equations, [Yu, W. W.; Qu, L. H.; Guo, W. Z.;
Peng, X. G., Experimental determination of the extinction
coefficient of CdTe, CdSe, and CdS nanocrystals. Chemistry of
Materials 2003, 15, (14), 2854-2860.] with the shortest chain
phosphonic acid. Table 3 lists the shortest wavelength absorption
feature achieved with each phosphonic acid. These smallest sizes
for each phosphonic acid were achieved by reducing the time between
the Se:TBP injection and the "kill-shot" described previously. The
time between the injections was approximately the same (.about.1
second) for all the smallest sizes, yet different sizes of
nanocrystals were achieved. This trend indicated that the shorter
the alkyl chain on the phosphonic acid, the slower the reaction
would proceed. The slower reaction kinetics allowed for smaller
sizes to be obtained using the short chain phosphonic acids than
were achievable with the longer alkyl chains.
TABLE-US-00003 TABLE 3 Shortest wavelength band-edge absorption
obtained for each phosphonic acid Bluest .lamda. band-edge
Phosphonic acid name absorption obtained (nm) Butyl PA 359 Hexyl PA
371 Octyl PA 373 Decyl PA 376 Dodecyl PA 384 Hexadecyl PA 395
Docosyl PA 392 2-ethyl,hexyl PA (2-EHPA) 378 3-methyl,butyl PA (3-
371 MBPA) Phenyl PA 374 3-phenyl,propyl PA (3- 376 PPPA)
[0300] The growth kinetics of the CdSe nanocrystals depend on the
reactivity of the Cd and Se precursors. [Peng, Z. A.; Peng, X. G.,
Nearly monodisperse and shape-controlled CdSe nanocrystals via
alternative routes: Nucleation and growth. Journal of the American
Chemical Society 2002, 124, (13), 3343-3353.; Qu, L. H.; Yu, W. W.;
Peng, X. P., In situ observation of the nucleation and growth of
CdSe nanocrystals. Nano Letters 2004, 4, (3), 465-469.]
Intuitively, the rate at which Cd atoms are added to the initial
nanocrystal seeds is dependent on the rate the Cd-phosphonate can
be separated, generating a reactive Cd species; this rate should
depend on the strength of the Cd--O--P bond. Accordingly, it was
hypothesized that the shorter ligands would bind the strongest to
Cd in the Cd-phosphonate precursor and to the nanocrystal
surface.
[0301] In addition to influencing the nanocrystal growth kinetics,
one would expect the Cd--O--P bond would affect the energy of a Cd
surface trap state. In a previous study, Whaley et al. calculated
that the density of states for the Cd rich surface of a nanocrystal
would be affected by the bonding of the Cd atoms on that surface,
including any dangling orbitals (Cd surface trap sites). [Hill, N.
A.; Whaley, K. B., A Theoretical-Study of the Influence of the
Surface on the Electronic-Structure of Cdse Nanoclusters. Journal
of Chemical Physics 1994, 100, (4), 2831-28371 The trend that was
observed in the current experiments was that the shorter alkyl
phosphonic acids blue-shift one of the emission features. This
trend was seen after locating the shortest wavelength (highest
energy) emission feature of the white-light emitting nanocrystals
for each of the phosphonic acids (FIG. 15). Each of these ligands
gives a statistically (99% confidence level) different emission
wavelength, except dodecyl, hexadecyl, and docosyl phosphonic acid.
These three phosphonic acid ligands give the nanocrystals' a
statistically similar emission feature, and by association the same
emission energy level. This is not surprising since an energetic
difference seen between chain lengths of 12, 16, or 22 carbons
would indicate the trap-states interact with atoms through more
than 14 bonds. Although, any trend past 3 bonds is surprising in
and of itself.
[0302] Four branched chain phosphonic acids were also evaluated.
The pinning results from these phosphonic acids indicated that it
is not only the number of carbons in the alkyl chain that is
important; the arrangement of these carbons also has an influence
on the wavelength of the light emitted from the nanocrystals. Table
4 illustrates the pinned wavelengths for all the phosphonic acids
tested. Of particular note, although 3-methyl, butyl PA, octyl PA,
and 3-phenyl, propyl PA all have different numbers of carbons in
their alkyl chain, the wavelength of the pinned emission they cause
are indistinguishable.
TABLE-US-00004 TABLE 4 Pinned emission .lamda., of nanocrystals
made with each phosphonic acid Phosphonic acid Emission .lamda.
(nm) Butyl PA 425.8 .+-. 2.1 Hexyl PA 430.2 .+-. 2.8 Octyl PA 436.3
.+-. 3.5 Decyl PA 440.0 .+-. 2.3 Dodecyl PA 445.1 .+-. 2.4
Hexadecyl PA 445.1 .+-. 3.8 Docosyl PA 445.2 .+-. 2.9 2-EHPA 441.3
.+-. 4.0 3-MBPA 434.8 .+-. 2.9 Phenyl PA 429.1 .+-. 1.7 3-PPPA
435.1 .+-. 2.6
[0303] Without wishing to be bound by theory, the effect seen in
the optical properties of ultra-small nanocrystals is believed to
be a consequence of electronegativity. The electronegativity of
individual atoms affects the strength of bonds between atoms. In
addition, the electronegativity of individual atoms is influenced
by the atoms that surround the atom of interest. This theory of
electronegativity modification is known as inductive
electronegativity and is a large factor in many analytical
measurements, such shielding and de-shielding in NMR spectroscopy.
[Jacobsen, N. E., NMR Spectroscopy explained: simplified theory,
applications, and examples for organic chemistry and structural
biology. Wiley-Interscience: Hoboken, N.J., 2007; p 668.; Kemp, W.,
Organic Spectroscopy. 2nd ed.; Macmillan: 1987; p 299.] Using
Bratsch's "Group Electronegativity Method," [Bratsch, S. G., A
Group Electronegativity Method with Pauling Units. Journal of
Chemical Education 1985, 62, (2), 101-103.] the electronegativity
of each phosphonic acid with an attached Cd was determined. First,
the ligands were broken into groups of ethyl carbons, methyl
carbons, branched carbons, hydroxyl groups, phosphorous atoms, and
single oxygens (FIG. 16). The electronegativity (X.sub.G) was
calculated for each of these groups using Equation 2, where N.sub.G
is the number of atoms in the group, .upsilon. is the number of a
specific type of atom, and .chi. is the electronegativity of each
atom in Pauling units. The individual group electronegativities
were then combined using Equation 3 to determine the ligand's
overall equalized electronegativity (X.sub.eq), where N is the
total number of atoms and N.sub.G is the number of atoms in each
group.
X G = N G v .chi. ( 2 ) X eq = N N G .chi. G ( 3 ) ##EQU00002##
[0304] Inductive electronegativity generally is not thought to have
an effect on atoms more than a few bonds away. However, for
electron traps on surface Cd atoms affected by the phosphonic acid
ligand, this does seem to be the case. When the experimental data
set and theoretical data set were normalized to the each set's
respective butyl PA ligand energy and setting this value to one,
this theoretical data matched up well with the experimental
emission data for the straight chain ligands and two branched chain
ligands (FIG. 17). For the two ligands tested that included phenyl
groups, this theoretical treatment did not yield consistent
results, in all likelihood due to the conjugation in the ligand's
alkyl chain.
[0305] The pinned emission peak in these broad band-emitting
nanocrystals is believed to be due to trap state emission. [Bowers,
M. J.; McBride, J. R.; Rosenthal, S. J., White-light emission from
magic-sized cadmium selenide nanocrystals. Journal of the American
Chemical Society 2005, 127, (44), 15378-15379.; Dukes, A. D.;
Schreuder, M. A.; Sammons, J. A.; McBride, J. R.; Smith, N. J.;
Rosenthal, S. J., Pinned emission from ultra-small cadmium selenide
nanocrystals. Journal of Chemical Physics 2008, 129, (12).]
Previously, surface cadmiums have been suggested as electron traps
in nanocrystals. [Lifshitz, E.; Dag, I.; Litvitn, I. D.; Hodes, G.,
Optically detected magnetic resonance study of electron/hole traps
on CdSe quantum dot surfaces. Journal of Physical Chemistry B 1998,
102, (46), 9245-9250.; Chestnoy, N.; Harris, T. D.; Hull, R.; Brus,
L. E., Luminescence and Photophysics of Cds Semiconductor
Clusters--the Nature of the Emitting Electronic State. Journal of
Physical Chemistry 1986, 90, (15), 3393-3399.] Additionally, it has
been shown that the trapping of charge carriers can cause emission
from discrete energy states. [Lifshitz, E.; Dag, I.; Litvitn, I.
D.; Hodes, G., Optically detected magnetic resonance study of
electron/hole traps on CdSe quantum dot surfaces. Journal of
Physical Chemistry B 1998, 102, (46), 9245-9250.; Chestnoy, N.;
Harris, T. D.; Hull, R.; Brus, L. E., Luminescence and Photophysics
of Cds Semiconductor Clusters--the Nature of the Emitting
Electronic State. Journal of Physical Chemistry 1986, 90, (15),
3393-3399.; Lifshitz, E.; Dag, I.; Litvin, I.; Hodes, G.; Gorer,
S.; Reisfeld, R.; Zelner, M.; Minti, H., Properties of CdSe
nanoparticle films prepared by chemical deposition and sol-gel
methods. Chemical Physics Letters 1998, 288, (2-4), 188-196.;
Underwood, D. F.; Kippeny, T.; Rosenthal, S. J., Ultrafast carrier
dynamics in CdSe nanocrystals determined by femtosecond
fluorescence upconversion spectroscopy. Journal of Physical
Chemistry B 2001, 105, (2), 436-443.] The trapping energy of an
electron on a surface Cd will be affected by the available orbitals
and the electron density around the nucleus. [Hill, N. A.; Whaley,
K. B., A Theoretical-Study of the Influence of the Surface on the
Electronic-Structure of Cdse Nanoclusters. Journal of Chemical
Physics 1994, 100, (4), 2831-2837.; Hill, N. A.; Whaley, B.,
Electronic-Structure of Semiconductor Nanoclusters--a
Time-Dependent Theoretical Approach. Journal of Chemical Physics
1993, 99, (5), 3707-3715.] The electron density around a surface Cd
will vary depending on the electron-donating or
electron-withdrawing nature of any molecules bound to it.
Accordingly, the difference in electronegativity of the phosphonic
acids would explain the change in emission wavelength seen in these
ultra-small nanocrystals (FIG. 17). Experimentally, it appears that
shorter alkane chain lengths cause higher energy emissions, due to
the increased electron withdrawing nature of these ligands exerted
on the Cd's adjacent dangling bond, previously suggested as being
an electron trap. [Hill, N. A.; Whaley, K. B., A Theoretical-Study
of the Influence of the Surface on the Electronic-Structure of Cdse
Nanoclusters. Journal of Chemical Physics 1994, 100, (4),
2831-2837.; Lifshitz, E.; Dag, I.; Litvitn, I. D.; Hodes, G.,
Optically detected magnetic resonance study of electron/hole traps
on CdSe quantum dot surfaces. Journal of Physical Chemistry B 1998,
102, (46), 9245-9250.; Klimov, V. I.; McBranch, D. W.; Leatherdale,
C. A.; Bawendi, M. G., Electron and hole relaxation pathways in
semiconductor quantum dots. Physical Review B 1999, 60, (19),
13740-13749.; Gomez, D. E.; van Embden, J.; Jasieniak, J.; Smith,
T. A.; Mulvaney, P., Blinking and surface chemistry of single CdSe
nanocrystals. Small 2006, 2, 204-208.]
[0306] In addition to altering the emission wavelength of the
ultra-small nanocrystals, the phosphonic acid ligand on the surface
affected the QY (FIG. 18, Table 5). Initially, the QY changes with
phosphonic acid were believed to be due only to the cleaning
process, used to separate the nanocrystals from the coordinating
solvent and precursors. However, it was noted that a similar trend
was seen in "dirty" and "clean" nanocrystals when using the
straight alkyl chain ligands. The branched and phenyl ring
containing ligands did not fit into this trend once cleaned (Table
5). This could be explained by the conjugation in the chain, the
cleaning process removing these ligands, or the steric size of
these phosphonic acids reducing the surface coverage as compared
with the straight chain ligands. It should be noted that the
ligand's steric size is a possible cause for the QY effects, but is
not likely to be the cause of the energetic differences, as
discussed below.
TABLE-US-00005 TABLE 5 QY of nanocrystal's synthesized with each
phosphonic acid QY of small, QY of small, QY of large, QY of large,
dirty clean dirty clean Phosphonic acid nanocrystals (%)
nanocrystals (%) nanocrystals (%) nanocrystals (%) Butyl PA 2.54
.+-. 0.17 1.03 .+-. 0.21 1.37 .+-. 0.18 0.37 .+-. 0.15 Hexyl PA
2.80 .+-. 0.37 2.44 .+-. 0.31 2.68 .+-. 0.19 1.40 .+-. 0.32 Octyl
PA 2.59 .+-. 0.47 4.84 .+-. 0.56 6.35 .+-. 0.94 2.70 .+-. 0.50
Decyl PA 2.96 .+-. 0.45 6.10 .+-. 0.40 9.20 .+-. 0.45 4.88 .+-.
0.14 Dodecyl PA 5.24 .+-. 1.24 7.37 .+-. 0.76 9.08 .+-. 0.88 4.93
.+-. 0.29 Hexadecyl PA 7.37 .+-. 1.19 9.12 .+-. 0.50 9.26 .+-. 0.35
5.79 .+-. 0.66 Docosyl PA 6.91 .+-. 0.71 8.64 .+-. 0.51 8.25 .+-.
0.18 7.18 .+-. 0.18 2-EHPA 3.18 .+-. 0.83 3.98 .+-. 0.99 10.17 .+-.
1.09 9.14 .+-. 1.74 3-MBPA 5.00 .+-. 1.18 0.75 .+-. 0.59 3.91 .+-.
0.69 2.55 .+-. 0.39 Phenyl PA 2.95 .+-. 0.34 0.50 .+-. 0.29 2.21
.+-. 0.44 1.32 .+-. 0.35 3-PPPA 2.45 .+-. 0.77 0.22 .+-. 0.05 5.47
.+-. 0.94 0.38 .+-. 0.17
[0307] Nanocrystals in the traditional size regime (>1.7 nm)
made with each phosphonic acid demonstrated the known trend of
longer emission wavelength with increased diameter. However, for
these nanocrystals, a similar trend in the QY was seen with the
straight chain phosphonic acids as in the small nanocrystals (FIG.
18, Table 5). Once again, the branched and phenyl containing
ligands did not fit with the rest of the data.
[0308] The quantum yield is most likely impacted by the physical
nature of the ligands rather than the electronic properties. For
the traditional size regime, the nanocrystals have a lower QY when
cleaned because some of the ligands are removed, decreasing the
surface passivation and increasing the dangling bonds that trap
charges. Since the emission in this size regime is mainly from
band-edge recombination, this leads to a decrease in the QY. The
increased QY caused by longer chain phosphonic acids could be due
to these longer chains passivating of any surface states with
portions of the alkyl ligand chain. [Berrettini, M. G.; Braun, G.;
Hu, J. G.; Strouse, G. F., NMR analysis of surfaces and interfaces
in 2-nm CdSe. Journal of the American Chemical Society 2004, 126,
(22), 7063-7070.; Garrett, M. D. et al., Band edge recombination in
CdSe, CdS and CdSxSe1-x alloy nanocrystals observed by ultrafast
fluorescence upconversion: The effect of surface trap states.
Journal of Physical Chemistry C 2008, 112, (33), 12736-12746.] For
the ultra-small sizes, the cleaned nanocrystals exhibit a higher
quantum yield due to the removal of ligands during the cleaning
process, creating more emission trap states. The low QY caused by
the shorter chain phosphonic acids may be due to the stronger bond
that shorter chains have with Cd, consequently they are less likely
to be removed, preventing trap states from being created.
[0309] Solvation dynamics and ligand steric were discarded as
sources of the emission changes. If the emission changes were due
to the solvation dynamics of the various ligands, then changing the
nanocrystals to various solvents would change the wavelength at
which the ultra-small nanocrystals were pinned. When the
nanocrystal emission was analyzed in hexanes, toluene, mesitylene,
and chloroform the emission peak wavelengths were unchanged.
Considering the data presented in Table 4 and FIG. 15, allowed for
the elimination of steric considerations. Assuming that the steric
size of the ligand is the most important factor in determining the
location of the first peak, the most bulky ligand is phenyl
phosphonic acid and butyl phosphonic acid is the smallest ligand.
This should put the phenyl phosphonic acid nanocrystals on one edge
of the pinning data, with lessening steric size down to butyl
phosphonic acid nanocrystals at the other extreme. However, the
phenyl nanocrystals had a pinned wavelength at .about.430 nm,
almost the exact middle of the data.
[0310] These ultra-small nanocrystals likely consist of less than
25 Cd--Se pairs. At this small size, it is possible that the
phosphonic acids not only affect the kinetics of the growth
reaction, but also influences the crystal structure. Without
wishing to be bound by theory, it is believed that a change in the
crystal structure is a potential cause for the modulated emission
shown here; although is less likely due to the ligand exchange data
previously presented. [Dukes, A. D.; Schreuder, M. A.; Sammons, J.
A.; McBride, J. R.; Smith, N. J.; Rosenthal, S. J., Pinned emission
from ultra-small cadmium selenide nanocrystals. Journal of Chemical
Physics 2008, 129, (12).]
[0311] e. Conclusions
[0312] The emission of ultra-small CdSe nanocrystals was shown to
be pinned at different wavelengths depending on the phosphonic acid
used during synthesis. For straight chain phosphonic acids, longer
alkyl chains pinned the bluest emission feature at longer
wavelengths than shorter chains. Surprisingly, the energy changes
are shown to correlate with the electronegativity of the phosphonic
acid ligands. In addition to shifting the wavelength of the pinned
emission, these ligands are shown to influence the quantum yield of
nanocrystals in both the ultra-small and traditional size regime.
The quantum yield for such white-light emitting nanocrystals can be
increased to about 10%, or even higher.
[0313] This new size regime of ultra-small nanocrystals has
stimulated the idea that the nanocrystal and ligands may no longer
be considered separately. At the sizes studied here, the ligand has
become essential to the overall crystal's optical properties. This
recognition requires a shift in thinking and experimentation. At
this ultra-small size, each atom becomes increasingly important
regardless of whether it is part of the inner crystal, the
nanocrystal surface, or the ligands. Further knowledge, along with
controlling the placement of atoms at specific locations in these
small crystals will allow for precise alteration of many of the
physical, electronic, and optical properties of these nanocrystals.
An advantage of utilizing the electronegativity effect is that it
allows for the fine-tuning of the white-light emission from these
nanocrystals.
[0314] 6. Example Preparation of Nanocrystals with Phosphonic Acid
Ligands
[0315] a. Synthesis
[0316] In a three-neck, round bottom flask 10 g hexadecylamine, 1
mmol CdO, and 2 mmol of hexadecyl phosphonic acid were heated to
145.degree. C. under Ar purge. The flask contents were then heated
to 325.degree. C. under Ar with vigorous stirring. As the CdO and
phosphonic acid converted to a Cd-phosphonate complex, the flask's
contents became clear and colorless. After cooling to 310.degree.
C., 4 mL of a 0.2 M Se:tri-butyl phosphine solution were quickly
injected into the flask. To produce nanocrystals small enough to
have band-edge absorption features less than 420 nm, this initial
injection was rapidly followed by a second injection of 20 mL of
butanol ("kill-shot") to cool the reaction to .about.130.degree.
C., followed by further cooling to below 100.degree. C. using
compressed air applied to the outside of the flask.
[0317] Unreacted precursor and solvents were removed from the
nanocrystals via a three wash process. Initially, 15 mL of each
reaction mixture was placed into a centrifuge tube and precipitated
with methanol. The supernatant was discarded and the pellet was
resuspended in 8 mL of hexanol. This suspension was centrifuged in
order to remove HDA and Cd-phosphonate. The supernatant (containing
the nanocrystals) was then decanted into clean tubes, and the
initial methanol/centrifugation step was repeated. The supernatant
was again discarded and the nanocrystals suspended in toluene for
further treatments.
[0318] b. Post-Cleaning Treatment
[0319] About 125 mL of a 10 .mu.M ultra-small nanocrystal solution
and a molar excess (2.5 mmol) of Zn acetate were added to a round
bottom flask. While stirring this solution was heated at 60.degree.
C. for 15 minutes. The solution was cooled and a majority of the
toluene was removed in vacuo (rotary evaporator). The remaining
solution was divided into centrifuge tubes and these were filled
with methanol. After centrifugation, the nanocrystals were
re-suspended in toluene for analysis.
[0320] c. Results
[0321] The nanocrystals post-synthesis had an average quantum yield
of 7.4%, which increased up to 9.2% after the initial cleaning
process. The post-cleaning treatment described above has been shown
to increase the quantum yield by as much as 80%, or yielding
nanocrystals with a quantum yield of nearly 17%. Some other metal
acetate treatments (Cd and Mn acetate) increased the quantum yield,
while others (Pb and Hg acetate) decreased the quantum yield of
these ultra-small nanocrystals; however, Zn acetate showed the
largest increase.
[0322] It will be apparent to those skilled in the art that various
modifications and variations can be made in the present invention
without departing from the scope or spirit of the invention. Other
embodiments of the invention will be apparent to those skilled in
the art from consideration of the specification and practice of the
invention disclosed herein. It is intended that the specification
and examples be considered as exemplary only, with a true scope and
spirit of the invention being indicated by the following
claims.
* * * * *
References