U.S. patent application number 12/700713 was filed with the patent office on 2010-07-15 for quantum dot white and colored light-emitting devices.
This patent application is currently assigned to Massachusetts Institute of Technology. Invention is credited to Moungi G. Bawendi, Jason Heine, Klavs F. Jensen, Jeffrey N. Miller, Ronald L. Moon.
Application Number | 20100176715 12/700713 |
Document ID | / |
Family ID | 26785282 |
Filed Date | 2010-07-15 |
United States Patent
Application |
20100176715 |
Kind Code |
A1 |
Bawendi; Moungi G. ; et
al. |
July 15, 2010 |
QUANTUM DOT WHITE AND COLORED LIGHT-EMITTING DEVICES
Abstract
A light-emitting device comprising a population of quantum dots
(QDs) embedded in a host matrix and a primary light source which
causes the QDs to emit secondary light and a method of making such
a device. The size distribution of the QDs is chosen to allow light
of a particular color to be emitted therefrom. The light emitted
from the device may be of either a pure (monochromatic) color, or a
mixed (polychromatic) color, and may consist solely of light
emitted from the QDs themselves, or of a mixture of light emitted
from the QDs and light emitted from the primary source. The QDs
desirably are composed of an undoped semiconductor such as CdSe,
and may optionally be overcoated to increase photoluminescence.
Inventors: |
Bawendi; Moungi G.; (Boston,
MA) ; Heine; Jason; (Cambridge, MA) ; Jensen;
Klavs F.; (Lexington, MA) ; Miller; Jeffrey N.;
(Los Altos Hills, CA) ; Moon; Ronald L.;
(Atherton, CA) |
Correspondence
Address: |
STEPTOE & JOHNSON LLP
1330 CONNECTICUT AVENUE, N.W.
WASHINGTON
DC
20036
US
|
Assignee: |
Massachusetts Institute of
Technology
Cambridge
MA
Lumileds Lighting US, LLC
San Jose
CA
|
Family ID: |
26785282 |
Appl. No.: |
12/700713 |
Filed: |
February 4, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11787152 |
Apr 13, 2007 |
7692373 |
|
|
12700713 |
|
|
|
|
10877698 |
Jun 25, 2004 |
7264527 |
|
|
11787152 |
|
|
|
|
09350956 |
Jul 9, 1999 |
6803719 |
|
|
10877698 |
|
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|
09167795 |
Oct 7, 1998 |
6501091 |
|
|
09350956 |
|
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|
60092120 |
Apr 1, 1998 |
|
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|
Current U.S.
Class: |
313/503 |
Current CPC
Class: |
H01L 33/06 20130101;
H01S 3/169 20130101; H01L 33/504 20130101; C09K 11/02 20130101;
C09K 11/565 20130101; Y02B 20/181 20130101; H01L 33/502 20130101;
B82Y 10/00 20130101; Y02B 20/00 20130101; Y10S 977/95 20130101;
C09K 11/883 20130101 |
Class at
Publication: |
313/503 |
International
Class: |
H01J 99/00 20060101
H01J099/00 |
Goverment Interests
[0002] This invention was made with government support under
Contract Number 9400034 awarded by the National Science Foundation.
The government has certain rights in the invention.
Claims
1. A white light emitting device comprising: a laser or UV primary
light source; and at least a first layer in optical communication
with the primary light source and comprising a matrix including at
least a first population of quantum dots of a material and size
adapted to emit red secondary light, a second population of quantum
dots of a material and size adapted to emit green secondary light,
and a third population of quantum dots of a material and size
adapted to emit blue secondary light.
2. The white light emitting device of claim 1, wherein the quantum
dots in the first, second and third populations each comprise a
core and one or more shell layers disposed about the core.
3. The white light emitting device of claim 2, wherein the core
comprises a material independently selected from the group
comprising CdS, CdSe, CdTe, InP, alloys thereof, and mixtures
thereof.
4. The white light emitting device of claim 3, wherein the one or
more shell layers comprises a material independently selected from
the group comprising ZnS, ZnSe, ZnTe, alloys thereof, and mixtures
thereof.
5. The white light emitting device of claim 1, wherein the matrix
comprises a polymer.
6. The white light emitting device of claim 5, wherein the polymer
includes a polystyrene.
7. The white light emitting device of claim 6, wherein the
polystyrene is functionalized with amine groups that bind to the
quantum dots.
8. The white light emitting device of claim 2, wherein the quantum
dots in the first population have a size distribution having less
than 10% rms deviation in diameter of the core.
9. The white light emitting device of claim 2, wherein the quantum
dots in the second population have a size distribution having less
than 10% rms deviation in diameter of the core.
10. The white light emitting device of claim 2, wherein the quantum
dots in the third population have a size distribution having less
than 10% rms deviation in diameter of the core.
11. The white light emitting device of claim 1, wherein the matrix
is physically separated from the primary light source.
12. The white light emitting device of claim 11, comprising a
medium between the primary light source and the matrix.
13. The white light emitting device of claim 12, wherein the medium
comprises air.
14. The white light emitting device of claim 12, wherein the medium
comprises a liquid.
15. The white light emitting device of claim 12, wherein the medium
comprises a polymer.
16. The white light emitting device of claim 12, wherein the medium
comprises a glass.
17. The white light emitting device of claim 1, wherein the matrix
is formed in a concentric conformation.
18. The white light emitting device of claim 17, wherein the matrix
is formed in a cylindrical conformation.
19. The white light emitting device of claim 18, wherein light from
the primary light source is configured to hit the side or base of
the cylinder.
20. The white light emitting device of claim 1, wherein the primary
light source is in physical contact with the matrix.
21. The white light emitting device of claim 1, wherein the matrix
is selected from the group comprising a liquid, a polymer, an
epoxy, a silica, a glass, a silica gel, and combinations
thereof.
22. The white light emitting device of claim 1, wherein the quantum
dots in the first, second, and third populations are coated with a
material having an affinity for the matrix.
23. The white light emitting device of claim 1, wherein the primary
light source comprises a UV light source.
Description
[0001] This application is a continuation of U.S. application Ser.
No. 11/787,152 filed on Apr. 13, 2007, which is a divisional of
U.S. application Ser. No. 09/350,956, filed Jul. 9, 1999, which
claims benefit of U.S. application Ser. No. 09/167,795, filed Oct.
7, 1998, which claims benefit of U.S. Provisional Application
60/092,120, filed Apr. 1, 1998, the disclosures of which are
incorporated herein by reference in their entirety.
[0003] A portion of the disclosure of this patent document contains
material that is subject to copyright protection. The copyright
owner has no objection to the facsimile reproduction by anyone of
the patent document or the patent disclosure, as it appears in the
Patent and Trademark Office patent file or records, but otherwise
reserves all copyrights whatsoever.
FIELD OF THE INVENTION
[0004] The present invention relates to the use of quantum dots in
light-emitting devices. The invention further relates to
light-emitting devices that emit light of a tailored spectrum of
frequencies. In particular, the invention relates to a
light-emitting device, wherein the device is a light-emitting
diode.
BACKGROUND OF THE INVENTION
[0005] Light-emitting devices, in particular, light-emitting diodes
(LEDs), are ubiquitous to modern display technology. More than 30
billion chips are produced each year and new applications, such as
automobile lights and traffic signals, continue to grow.
Conventional devices are made from inorganic compound
semiconductors, typically AlGaAs (red), AJGaInP
(orange-yellow-green), and AlGaTnN (green-blue). These devices emit
monochromatic light of a frequency corresponding to the band gap of
the compound semiconductor used in the device. Thus, conventional
LEDs cannot emit white light, or indeed, light of any "mixed"
color, which is composed of a mixture of frequencies. Further,
producing an LED even of a particular desired "pure"
single-frequency color can be difficult, since excellent control of
semiconductor chemistry is required.
[0006] Light-emitting devices of mixed colors, and particularly
white LEDs, have many potential applications. Consumers would
prefer white light in many displays currently having red or green
light-emitting devices. White light-emitting devices could be used
as light sources with existing color filter technology to produce
full color displays. Moreover, the use of white LEDs could lead to
lower cost and simpler fabrication than red-green-blue LED
technology.
[0007] White LEDs are currently made by combining a blue LED with a
yellow phosphor to produce white light. However, color control is
poor with this technology, since the colors of the LED and the
phosphor cannot be varied. This technology also cannot be used to
produce light of other mixed colors.
[0008] It has been proposed to manufacture white or colored
light-emitting devices by combining various derivatives of
photoluminescent polymers such as poly(phenylene vinylene) (PPVs).
One device that has been proposed involves a PPV coating over a
blue GaN LED, where the light from the light-emitting device
stimulates emission in the characteristic color of the PPV, so that
the observed light is composed of a mixture of the characteristic
colors of the device and the PPV. However, the maximum theoretical
quantum yield for PPV-based devices is 25%, and the color control
is often poor, since organic materials tend to fluoresce in rather
wide spectra. Furthermore, PPVs are rather difficult to manufacture
reliably, since they are degraded by light, oxygen, and water.
Related approaches use blue GaN-based LEDs coated with a thin film
of organic dyes, but efficiencies are low (see, for example, Guha
et al. (1997) J. Appl. Phys. 82(8):41264128; Ill-Vs Review 10(1):4,
1997).
[0009] It has also been proposed to produce light-emitting devices
of varying colors by the use of quantum dots (QDs). Mattoussi et
al. (1998)1 Appl. Phys. 83:7965-7974; Nakamura et al. (1998)
Electronics Lett. 34:2435-2436; Schlamp et al. (1997) J. Appl.
Phys. 82:5837-5842; Colvin et al. (1994) Nature 370:354-357.
Semiconductor nanocrystallites (i.e., QDs) whose radii are smaller
than the bulk exciton Bohr radius constitute a class of materials
intermediate between molecular and bulk forms of matter. Quantum
confinement of both the electron and hole in all three dimensions
leads to an increase in the effective band gap of the material with
decreasing crystallite size. Consequently, both the optical
absorption and emission of QDs shift to the blue (higher energies)
as the size of the QDs gets smaller. It has been found that a CdSe
QD, for example, can emit light in any monochromatic color, in
which the particular color characteristic of the light emitted is
dependent only on the QD's size.
[0010] Currently available light-emitting diodes and related
devices that incorporate quantum dots use QDs that have been grown
epitaxially on a semiconductor layer. This fabrication technique is
most suitable for the production of infrared light-emitting
devices, but devices in higher-energy colors have not been achieved
by this method. Further, the processing costs of epitaxial growth
by currently available methods (molecular beam epitaxy and chemical
vapor deposition) are quite high. Colloidal production of QDs is a
much more inexpensive process, but QDs produced by this method have
generally been found to exhibit low quantum efficiencies, and thus
have not previously been considered suitable for incorporation into
light-emitting devices.
[0011] A few proposals have been made for embedding colloidally
produced QDs in an electrically conductive layer in order to take
advantage of the electroluminescence of these QDs for a
light-emitting device. Mattoussi et al. (1998), supra; Nakamura et
al. (1998), supra; Schlamp et al. (1997), supra; Colvin et al.
(1994), supra. However, such devices require a transparent,
electrically conductive host matrix, which severely limits the
available materials for producing devices by this method. Available
host matrix materials are often themselves light-emitting, which
may limit the achievable colors using this method.
SUMMARY OF THE INVENTION
[0012] In one aspect, this invention comprises a device, comprising
a light source and a population of QDs disposed in a host matrix.
The QDs are characterized by a band gap energy smaller than the
energy of at least a portion of the light from the light source.
The matrix is disposed in a configuration that allows light from
the source to pass therethrough. When the QD disposed in the host
matrix is irradiated by light from the source, that light causes
the QDs to photoluminesce secondary light. The color of the
secondary light is a function of the size, size distribution and
composition of the QDs.
[0013] In one embodiment of this aspect, the QDs comprise a core of
CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, GaAs, GaP, GaAs, GaSb, HgS, HgSe,
HgTe, InAs, InP, InSb,_AlAs, AIP, AlSb, an alloy thereof, or a
mixture thereof, and are, optionally, overcoated with a shell
material comprising ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe,
MgS, MgSe, GaAs, GaN, GaP, GaAs, GaSb, HgO, HgS, HgSe, HgTe, InAs,
InN, InP, InSb, AlAs, AIN, AlP, AlSb, an alloy thereof, or a
mixture thereof. Preferably, the band gap energy of the overcoating
is greater than that of the core. The core or core-shell QD may be
further coated with a material having an affinity for the host
matrix. The host matrix may be any polymer, such as polyacrylate,
polystyrene, polyimide, polyacrylamide, polyethylene, polyvinyl,
poly-diacetylene, polyphenylene-vinylene, polypeptide,
polysaccharide, polysulfone, polypyrrole, polyimidazole,
polythiophene, polyether, epoxies, silica glass, silica gel,
siloxane, polyphosphate, hydrogel, agarose, cellulose, and the
like. The primary light source may be a light-emitting diode, a
laser, an arc lamp or a black-body light source. The color of the
device is determined by the size, size distribution and composition
of the QDs. The size distribution may be a random, gradient,
monomodal or multimodal and may exhibit one or more narrow peaks.
The QDs, for example, may be selected to have no more than a 10%
rms deviation in the diameter of the QDs. The light may be of a
pure color, or a mixed color, including white.
[0014] In a related aspect, the invention comprises a method of
producing a device as described above. In this method, a population
of QDs is provided, and these QDs are dispersed in a host matrix. A
light source is then provided to illuminate the QDs, thereby
causing them to photoluminesce light of a color characteristic of
their size, size distribution and composition. The QDs may be
colloidally produced (i.e., by precipitation and/or growth from
solution), and may comprise a core of CdS, CdSe, CdTe, ZnS, ZnSe,
ZnTe, GaAs, GaP, GaAs, GaSb, HgS, HgSe, HgTe, InAs, InP, lnSb,
AlAs, AlP, AISb, an alloy thereof, or a mixture thereof. The QDs
are, optionally, overcoated with a shell material comprising ZnS,
ZnSe, ZnTe, CdS, CdSe, CdTe, MgS, MgSe, GaAs, GaP, GaAs, GaSb, HgS,
HgSe, HgTe, InAs, InP, InSb, AlAs, AlP, AlSb, an alloy thereof, or
a mixture thereof. The host matrix may be any material in which QDs
may be dispersed in a configuration in which they may be
illuminated by the primary light source. Some examples of host
matrix materials include polyacrylate, polystyrene, polyimide,
polyacrylamide, polyethylene, polyvinyl, poly-diacetylene,
polyphenylene-vinylene, polypeptide, polysaccharide, polysulfone,
polypyrrole, polyimidazole, polythiophene, polyether, epoxies,
silica glass, silica gel, siloxane, polyphosphate, hydrogel,
agarose, cellulose, and the like. Any light source capable of
causing the QDs to photoluminesce may be used; some examples are
light-emitting diodes, lasers, arc lamps and black-body light
sources.
[0015] It may be desirable to tailor the size distribution of the
QDs of a particular core composition to tailor the color of light
which is produced by the device. In one embodiment, referred to
herein as a "monodisperse size distribution," the QDs exhibit no
more than a 10% rms deviation in diameter. The light may be of a
pure color using a monodisperse size distribution of QDs or of a
mixed color using a polydisperse size distribution of QDs,
including white.
[0016] In a further aspect, the invention comprises a QD
composition, in which QDs are disposed in a host matrix. The QDs
are, optionally, coated with a material having an affinity for the
host matrix. When illuminated by a source of light of a higher
energy than the band gap energy of the QDs, the QDs photoluminesce
in a color characteristic of their size, size distribution and
composition.
[0017] In one embodiment, the QDs comprise a core of CdS, CdSe,
CdTe, ZnS, ZnSe, ZnTe, GaAs, GaP, GaAs, GaSb, HgS, HgSe, HgTe,
InAs, InP, InSb, AlAs, AlP, AlSb, an alloy thereof, or a mixture
thereof, and are, optionally overcoated with a shell material
comprising ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgS, MgSe,
GaAs, GaN, GaP, GaAs, GaSb, HgO, HgS, HgSe, HgTe, InAs, InN, InP,
InSb, AIAs, MN, ALP, AlSb, an alloy thereof, or a mixture thereof.
The host matrix may be a polymer such as polyacrylate, polystyrene,
polyimide, polyacrylamide, polyethylene, polyvinyl,
polydiacetylene, polyphenylene-vinylene, polypeptide,
polysaccharide, polysulfone, polypyrrole, polyimidazole,
polythiophene, polyether, epoxies, silica glass, silica gel,
siloxane, polyphosphate, hydrogel, agarose, cellulose, and the
like. In one embodiment, the QDs are coated with a monomer related
to a polymer component of the host matrix. The QDs may be selected
to have a size distribution exhibiting an rms deviation in diameter
of less than 10%; this embodiment will cause the QDs to
photoluminesce in a pure color.
[0018] A related aspect of the invention comprises a prepolymer
composition comprising a liquid or semisolid precursor material,
with a population of QDs disposed therein. The composition is
capable of being reacted, for example by polymerization, to form a
solid, transparent or translucent host matrix, i.e., a host matrix
that allows light to pass therethrough. Optionally, the QDs are
coated with a material having an affinity for the precursor
material or with a prepolymeric material. For example, if the
prepolymer composition forms a polyacrylate upon polymerization,
the QD can be coated with an acrylate monomer which, optionally,
allows the QD to become incorporated into the backbone structure of
the polymer. The precursor material may be a monomer, which can be
reacted to form a polymer. The QDs may comprise a core of CdS,
CdSe, CdTe, ZnS, ZnSe, ZnTe, GaAs, GaP, GaAs, GaSb, HgS, HgSe,
HgTe, InAs, InP, InSb, AlAs, AlP, AlSb, an alloy thereof, or a
mixture thereof, and are, optionally, overcoated with a shell
material comprising ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe,
MgS, MgSe, GaAs, GaN, GaP, GaAs, GaSb, HgO, HgS, HgSe, HgTe, InAs,
InN, InP, InSb, AlAs, MN, ATP, AlSb, an alloy thereof, or a mixture
thereof. The QDs may be selected to have a size distribution having
an rms deviation in diameter of less than 10%.
[0019] In yet another aspect, the invention comprises a method of
producing light of a selected color. The method comprises the steps
of providing a population of QDs disposed in a host matrix, and
irradiating the QDs in the host matrix with a source of light
having an energy higher than the band gap energy of a QD in the
host matrix such that the QDs are caused to photoluminesce. The QDs
may comprise a core of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, GaAs, GaP,
GaAs, GaSb, HgS, HgSe, HgTe, InAs, InP, InSb, AlAs, AlP, AlSb, an
alloy thereof, or a mixture thereof, and are, optionally overcoated
with shell material comprising ZnO, ZnS, ZnSe, ZnTe, CdO, CdS,
CdSe, CdTe, MgS, MgSe, GaAs, GaN, GaP, GaAs, GaSb, HgO, HgS, HgSe,
HgTe, InAs, InN, InP, InSb, AlAs, AIN, AlP, AISb, an alloy thereof,
or a mixture thereof. The host matrix may comprise polymers such as
polyacrylate, polystyrene, polyimide, polyacrylamide, polyethylene,
polyvinyl, poly-diacetylene, polyphenylene-vinylene, polypeptide,
polysaccharide, polysulfone, polypyrrole, polyimidazole,
polythiophene, polyether, epoxies, silica glass, silica gel,
siloxane, polyphosphate, hydrogel, agarose, cellulose, and the
like.
[0020] The host matrix containing the QDs may be formed by reacting
a precursor material having QDs disposed therein (for example by
polymerization or physically entrapping). Alternatively, two or
more precursor materials may be provided, each having QDs of a
different sizes, size distributions and/or compositions disposed
therein. These precursors may be mixed and reacted to form a host
matrix, or alternatively, they may be layered to form a host matrix
having different sizes, size distributions and/or compositions of
QDs in different layers.
BRIEF DESCRIPTION OF THE DRAWING
[0021] The file of this patent contains at least one drawing
executed in color. Copies of this patent with color drawings(s)
will be provided by the Patent and Trademark Office upon request
and payment of the necessary fee.
[0022] The invention is described with reference to the several
figures of the drawing, which are presented for the purpose of
illustration only, and in which,
[0023] FIG. 1 represents one embodiment of a light-emitting device
according to the invention;
[0024] FIG. 2 represents another embodiment of a light-emitting
device according to the invention;
[0025] FIG. 3 represents yet another embodiment of a light-emitting
device according to the invention; and
[0026] FIG. 4 is a color photograph of several suspensions of QDs
in hexane, illustrating the wide range of colors that can be
achieved by the methods and devices of the invention.
DETAILED DESCRIPTION
[0027] The Practice of the present invention will employ, unless
otherwise indicated, conventional methods of chemistry within the
skill of the art. Such techniques are explained fully in the
literature.
[0028] As used in this specification and the appended claims, the
singular forms "a," "an" and "the" include plural references unless
the content clearly dictates otherwise. Thus, for example,
reference to "a quantum dot" includes a mixture of two or more such
quantum dots, a "layer" includes more than one such layer, and the
like.
[0029] In describing the present invention, the following terms
will be employed, and are intended to be defined as indicated
below.
[0030] The term "quantum dot" or "QD" as used herein is intended to
encompass a core nanocrystal, an overcoated core ("core-shell")
nanocrystal, a coated core-shell nanocrystal or a coated core,
unless the context clearly indicates otherwise.
[0031] The phrase "colloidally grown" quantum dots is used herein
to refer to QDs which have been produced by precipitation and/or
growth from a solution. A distinction between these QDs and quantum
dots epitaxially grown on a substrate is that colloidally grown QDs
have a substantially uniform surface energy, while epitaxially
grown QDs usually have different surface energies on the face in
contact with the substrate and on the remainder of the QD
surface.
[0032] As used herein, the terms "pure" or "monochromatic" color
refers to a color which is composed of light of a narrow
distribution of wavelengths having a spectral width between about
10-100 nm, preferably between about 10-50 nm, and more preferably
about 10-30 nm. A "mixed" or "polychromatic" color refers to a
color which is composed of light of a mixture of different
monochromatic colors.
[0033] The term "monomer" is intended to refer to a substance that
can be polymerized according to techniques known in the art of
materials science, and may include oligomers. A "related monomer"
of a polymer is a component monomer of the polymer, or a compound
capable of being incorporated into the backbone of the polymer
chain.
[0034] The term "affinity" is meant to describe the adherence
between a QD with a coat material and a host matrix. The adherence
may comprise any sort of bond including, but not limited to,
covalent, ionic, or hydrogen bonding, Van der Waals' forces, or
mechanical bonding, or the like.
[0035] "Optional" or "optionally" means that the subsequently
described event or circumstance may or may not occur, and that the
description includes instances where the event or circumstance
occurs and instances in which it does not. For example, the phrase
"optionally overcoated with a shell material" means that the
overcoating referred to may or may not be present in order to fall
within the scope of the invention, and that the description
includes both presence and absence of such overcoating.
[0036] Light-emitting devices of almost any color visible to the
human eye can be produced by the techniques of the current
invention using a single undoped semiconductor material for the
QDs. Embodiments of the invention are illustrated in FIGS. 1 and 2,
and indicated generally at 10 and 100, respectively. In general
terms, the invention comprises a primary light source 13, for
example a light-emitting diode, a laser, an arc lamp or a
black-body light source. The primary light source 13 is desirably
chosen so that its energy spectrum includes light of higher
energies than the desired device color energy emitted by the QDs
The primary light source is disposed so as to irradiate a host
matrix 12 containing a population of QDs 14. The primary light
source is in optical communication with the host matrix. In one
embodiment, primary light source 13 is in physical contact with the
host matrix. Optionally, a medium 11 is interposed between host
matrix 12 and primary light source 13. Medium 11 may be a medium
transparent or translucent to or conductive of at least a portion
of the light emitted from primary light source 13, e.g., air, a
vacuum, a polymer, a glass, a liquid or the like. The interposition
of medium 11 between primary light source 13 and host matrix 12 can
result in the light source being physically separate from the host
matrix.
[0037] Host matrix 12 may be any material in which QDs can be
disposed and that is at least partially transparent or translucent
to, i.e., allows light to pass therethrough, or conductive of light
from primary light source 13; examples of suitable host matrices
are discussed further below. The host matrix 12 desirably contains
a dispersion of QDs 14, wherein the size, size distribution and/or
composition of the QDs has been selected to produce light of a
given color. Other configurations of QDs disposed in a host matrix,
such as, for example, a two-dimensional layer on a substrate with a
polymer overcoating, are also contemplated within the scope of the
invention. Techniques for producing QDs that fluoresce in a narrow
spectral distribution of a selected color are discussed further
below and in Dabbousi et al. (1997) J. Phys. Chem. B 101:9463-9475
and in copending U.S. patent application Ser. No. 08/969,302,
"Highly Luminescent Color Selective Materials," Bawendi et al,
filed Nov. 13, 1997; such techniques allow particularly fine color
control of the final light-emitting device. However, other
techniques for producing QDs and disposing them in a host matrix
are also encompassed within the scope of the invention.
[0038] The primary light source 13 and the size, size distribution
and composition of the QDs 12 are chosen in such a way that the
radiation emitted from the device is of the desired color. The
invention may be constructed with a density of QDs such that
substantially all light from the primary source is absorbed by the
QDs and the radiation emitted from the device is produced
principally by photoluminescence of the QDs. Alternatively, the
invention may be constructed with a lower density of QDs such that
the light emitted from the device is a mixture of unabsorbed
primary light and of secondary light produced by photoluminescence
of the QDs. A very wide range of both pure and mixed colors can be
produced by a device constructed according to the principles of the
invention.
[0039] For example, CdSe QDs can be produced that emit colors
visible to the human eye, so that in combination with a source of
higher energy than the highest energy of the desired color, these
QDs can be tailored to produce visible light of any spectral
distribution. FIG. 4 shows several suspensions of CdSe QDs made
according to the method of Dabbousi et al., supra, and U.S.
application Ser. No. 08/969,302, supra, and illustrates the very
wide range of colors which can be achieved using the
photoluminescence of these materials. The maxima of the
photoluminescent peaks in these solutions are (from left to right)
(a) 470 nm, (b) 480 nm, (c) 520 nm, (d) 560 nm, (e) 594 run, and
(f) 620 nm. The solutions are being irradiated by an ultraviolet
lamp emitting 356 nm ultraviolet light.
[0040] QDs can also be produced that emit in the ultraviolet and
infra red spectral ranges. Examples of ultraviolet- and
infrared-emitting QDs are, e.g., CdS, ZnS and ZnSe, and InAs, CdTe
and MgTe, respectively. Such UV and IR emitters can also be
incorporated into the device disclosed and claimed herein.
[0041] It is usually desirable that the QDs be isolated from each
other within the host matrix, particularly when the device is
intended to emit light of a mixed color. For example, when two QDs
of different sizes are in close contact, the larger QD, which has a
lower characteristic emission energy, will tend to absorb a large
fraction of the emissions of the smaller QD, and the overall energy
efficiency of the device will be reduced, while the color will
shift towards the red.
[0042] In one particular embodiment of the invention, a white
light-emitting device is provided. Such a device may be produced by
combining a combination of sizes of photoliiminescent QDs with a
standard blue primary light source. Referring to FIG. 1, the
device, generally indicated at 10, comprises a blue light source
13, for example an LED of the AIGaInN type, to provide primary
light. This light passes through a layer or layers comprising QDs
that luminesce in a lower-energy range than the blue LED embedded
in a polymeric matrix. In the embodiment shown in FIG. 1, the
primary light first passes through a layer 16 of QDs 18 of a
material and size adapted to emit red secondary light. The primary
light which has not been absorbed by the first layer and the
secondary light then pass through a second layer 20 of QDs 22 of a
material and size adapted to emit green secondary light. Once the
light has passed through this second layer, it will be composed of
a mix of unabsorbed blue primary light, green secondary light, and
red secondary light, and hence will appear white to the observer.
The relative amplitudes of the red, green, and blue components of
the light can be controlled by varying the thickness and QD
densities of the red and green layers to produce a light-emitting
device of a desired color.
[0043] In another preferred embodiment, the red-emitting QDs 22 and
green-emitting QDs 18 can be mixed in a common matrix 12, as shown
in FIG. 2. The color can be controlled by varying the relative
densities of the different sizes and compositions of QDs and the
thickness of the layer.
[0044] In yet another preferred embodiment, layers of host matrix
containing QDs can be formed in a concentric conformation, e.g., a
spherical or cylindrical conformation, as illustrated in FIG. 3.
Indicated generally at 200, the device comprises layers of host
matrix 202, in which are dispersed QDs 204, and primary light
source 220. Inner layer 210 is prepared, for example, by providing
a precursor material having disposed therein a QD 216 having a
size, size distribution, composition, or combination thereof,
selected to emit in a predetermined spectral range. The precursor
material is reacted, e.g., polymerized, to form host matrix 210
having QDs 216 dispersed therein. These steps are repeated as often
as desired with the same or different precursor material having
disposed therein QDs of the same or different size, size
distribution, composition or combination thereof to form layers of
host matrix 208 and 206 having disposed therein QDs 214 and 212,
respectively, surrounding host matrix 210. If desired, a the QDs
may be omitted from any layer. Primary light source 220 is disposed
to be in optical communication with the layers of host matrix 202
so as to irradiate the QDs 204 disposed therein. In one embodiment,
primary light source 220 is in physical contact with the host
matrix. Optionally, medium 218, as described above, is interposed
between the layers of host matrix 202 and primary light source 220.
When the host matrix is conformed as a cylinder, the primary light
source can be disposed to irradiate the QDs in the host matrix from
the base or the side of the cylinder.
[0045] In still another embodiment, the primary light source may be
a light source such as a laser or a UV light source. In this
embodiment, the QD layer(s) may comprise QDs emitting in a spectral
range ranging from infrared to violet. By controlling the size,
size distribution and composition of the QDs, the spectral
distribution of the resulting light may be controlled.
[0046] When it is desired to produce a light-emitting device that
emits a particular color, rather than a white light-emitting
device, this also may be accomplished by the practice of the
invention. Although the invention is expected to be particularly
useful for the manufacture of a light-emitting device that produces
polychromatic light (mixed colors), which are difficult to produce
by traditional methods, light-emitting devices that produce
monochromatic light (pure colors) may also be prepared by the
practice of the invention. This may be desirable for purposes of
ease of manufacturing, since substantially the same set of
equipment is required to produce light-emitting devices of almost
any visible color, whether pure or mixed.
[0047] The perception of color by the human eye is well understood,
and formulae for mixing pure colors to produce any desired mixed
color can be found in a number of handbooks. The color of light
produced by a particular size and composition of QD may also be
readily calculated or measured by methods which will be apparent to
those skilled in the art. As an example of these measurement
techniques, the band gaps for QDs of CdSe of sizes ranging from 12
.ANG. to 115 .ANG. are given in Murray et al. (1993) J. Am. Chem.
Soc. 115:8706. These techniques allow ready calculation of an
appropriate size, size distribution and composition of QDs and
choice of primary light source to produce a light-emitting device
of any desired color.
[0048] When a white light-emitting device, e.g., a white LED, is
desired, an appropriate mix of QD sizes may be used. A white light
which appears "clean" to the observer may be achieved, for example,
by tailoring the spectral distribution to match a black body
distribution, e.g., as would be produced by a resistive lamp.
[0049] When a colored device, such as a blue AIGaTnN LED, is used
as the primary light source, the color of the light generated by
that device may or may not be included in the final spectrum
produced by the device according to the invention, depending on the
density of the QDs and the path length of the light. If a
sufficiently high density of QDs is provided, the QDs will absorb
substantially all of the primary light, and only secondary light in
the characteristic colors of the QDs will be observed. If a lower
density of QDs is provided, a significant quantity of primary light
may be mixed with the secondary light emitted by the QDs.
[0050] The host matrix will typically be a solid or liquid material
which is at least sufficiently transparent or translucent so that
light emitted by the QDs can be detected and in which QDs can be
dispersed. For example, the host matrix can be a polymer, an epoxy,
a silica glass, a silica gel, or a solvent, but any suitable
material may serve as the host matrix. The host matrix can be any
material that is at least partially transparent or translucent to
or conductive of light from the primary light source. An advantage
of the present invention compared to light-emitting devices based
on electroluminescence of QDs, rather than photoluminescence, is
that in the present invention the host matrix need not be
electrically conductive. Electroluminescent QD LEDs require a
transparent, electrically conductive material to serve as the host
matrix. Such materials are rare, compared to the very large number
of transparent or translucent materials available for use with the
present invention that are not necessarily conductive. Suitable
host matrix materials for the devices described herein include many
inexpensive and commonly available materials, such as polyacrylate,
polystyrene, polyimide, polyacrylaraide, polyethylene, polyvinyl,
poly-diacetylene, polyphenylene-vinylene, polypeptide,
polysaccharide, polysulfone, polypyrrole, polyimidazole,
polythiophene, polyether, epoxies, silica glass, silica gel,
siloxane, polyphosphate, hydrogel, agarose, cellulose, and the
like.
[0051] A further advantage of the present invention is the
manufacturing flexibility afforded by the use of multiple
populations of QDs to achieve both pure and mixed colors of light.
"Stock" solutions of different sizes, size distributions and
compositions of QDs suspended in a monomer or other precursor
material can be maintained, and mixed in varying amounts to produce
almost any desired color. For example, three suspensions of CdSe
QDs in a liquid monomer such as styrene could be produced: a first
suspension of QDs of approximately 5.5 nm diameter (which will
luminesce in the red), a second suspension of QDs of approximately
4.0 nm diameter (which will luminesce in the green), and a third
suspension of QDs of approximately 2.3 nm diameter (which will
luminesce in the blue). These suspensions function as a kind of
"light paint"; by varying the amounts of these three suspensions,
and polymerizing the resulting mixture, light-emitting devices of a
very wide range of colors can be produced using the same
manufacturing techniques, varying only the starting materials.
[0052] Preferably, colloidally produced QDs are coated such that
they can be dispersed in the host matrix without flocculation. In
the case of dispersal in a polymeric host matrix, use of a related
monomer with a pendent moiety possessing affinity for the QD's
surface has been found to allow good mixing of QDs into a polymer
matrix. Particular cases of this type of coating may be found in
the Examples. In the case of dispersal in a prepolymer host matrix,
use of a related monomer with a pendent moiety possessing affinity
for the QD's surface has been found to allow good mixing into a
monomer solution for subsequent polymerization to form the host
matrix. Particular cases of this type of coating may be found in
the Examples. In the case of dispersal into a silica glass or gel,
any coating that will bind at one end to the QD, and the other end
of which has an affinity for the matrix, may be used. The coating
may be applied directly to the surface of the QD or as a coating to
an overcoated QD.
[0053] A number of methods of producing QDs are known in the art.
Any method of producing QDs that will fluoresce with a desired
spectrum may be used in the practice of the invention. Preferably,
the methods described in Dabbousi et al., supra, and U.S.
application Ser. No. 08/969,302, supra, can be used to produce QDs
useful in devices as disclosed and claimed herein. Dabbousi et al.,
supra, discloses a method that can be used for overcoating QDs
composed of CdS, CdSe, or CdTe with ZnS, ZnSe, or mixtures thereof.
Before overcoating, the QDs are prepared by a method described in
Murray et al., supra, that yields a substantially monodisperse size
distribution. An overcoat of a controlled thickness can then be
applied by controlling the duration and temperature of growth of
the coating layer. The monodispersity of the core QDs results in
monochromatic emission. The overcoated QDs, optionally, have
improved quantum efficiency and emit more light than unovercoated
QDs.
[0054] The above method can be used to prepare separate populations
of QDs, wherein each population exhibits a different characteristic
photoluminescence spectrum. By mixing populations so prepared, a
device that fluoresces in any desired mixed color, including white,
may be produced. The overcoating on the QDs allows the device to
produce more light than would be possible using unovercoated
QDs.
[0055] Below are examples of specific embodiments of the present
invention. The examples are offered for illustrative purposes only,
and are not intended to limit the scope of the present invention in
any way.
[0056] Efforts have been made to ensure accuracy with respect to
numbers used (e.g., amounts, temperatures, etc.), but some
experimental error and deviation should, of course, be allowed
for.
Example 1
ODs in Polystyrene
[0057] A green light-emitting device has been constructed according
to the principles of the invention described above. The QDs used to
construct this device were composed of a CdSe core and a ZnS
overcoating. The absorption and luminescence properties of the QDs
were primarily determined by the size of the CdSe core. The ZnS
shell acted to confine electrons and holes in the core and to
electronically and chemically passivate the QD surface Both the
core and shell were synthesized using wet chemistry techniques
involving formation of CdSe or ZnS from precursors added to a hot
organic liquid as described below.
CdSe Core Synthesis
[0058] 16 ml of trioctylphosphine (TOP), 4 ml of 1 M
trioctylphosphine selenide (TOPSe) in TOP, and 0.2 ml
dimethylcadmium were mixed in an inert atmosphere (nitrogen-filled
glovebox). 30 g of trioctylphosphine oxide (TOPO) was dried under
vacuum at 180.degree. C. for 1 hour, and then heated to 350.degree.
C. under nitrogen. The precursor solution was then injected into
the TOPO. The temperature immediately fell to about 260.degree. C.
and CdSe nanocrystals immediately formed. The absorption peak of
the nanocrystals immediately after injection was found to be around
470 nm. The temperature was held at 250-260.degree. C. for about
10-15 minutes, allowing the nanocrystals to grow. During this time,
the absorption peak shifted from 470 nm to 490 nm. The temperature
was then dropped to 80.degree. C. and held with the solution under
nitrogen. The heat was removed and about 15 ml butanol was added to
prevent solidification of the TOPO as it cooled to room
temperature. This process produced 12.times.10 moles (12 .mu.moles)
of CdSe QDs.
[0059] The UV-Vis absorption spectrum of the CdSe nanocrystals
showed a first transition peak at 486 nm with a half-width half-max
(HWHM) measured on the red side of the peak, of 14 nm. This
absorption peak corresponded to a nanocrystalradius of 13 .ANG..
The actual size distribution can be determined experimentally via
small, angle x-ray scattering or TEM. The absorption spectrum gave
a rough estimate of the size distribution. The 14 nm HWHM suggested
a size distribution with a HWHM of about 1 .ANG..
ZnS Shell Synthesis
[0060] The CdSe core solution (15 ml; 2.22 .mu.moles) was used to
produce the overcoated QDs. The nanocrystals were precipitated out
of the solution by slowly adding 40-50 ml of methanol. The
precipitate was then redispersed in hexane and filtered with 0.2
micron filter paper. 40 g of TOPO was dried as described above and
then cooled to 80.degree. C. The nanocrystals in hexane were
injected into the TOPO, and the hexane was evaporated under vacuum
for 2 hours. A ZnS precursor solution was then prepared in an inert
atmosphere by mixing 4 ml of TOP, 0.28 ml of diethylzinc, and 0.56
ml of bistrimethylsilyl sulfide (TMSi).sub.2S. The amounts of
precursor were chosen to produce a ZnS shell thickness of about 9
angstroms, which corresponds to 4 monolayers at 2.3
angstroms/monolayer. The nanocrystal/TOPO solution was then heated
to 140.degree. C., and the precursor solution was added over 4
minutes. The temperature was then reduced to 100.degree. C. and
held at that temperature for at least two hours. Heat was removed
and butanol added to prevent solidification of the TOPO.
[0061] The UV-Vis absorption spectrum of the overcoated QDs showed
the first transition peak at 504 nm with a HWHM measured on the red
side of the peak of 20 nm. The photoluminescence peak was at 520
nm.
Dispersal of QDs in Polymer
[0062] ZnS-overcoated QDs were dispersed in poly(styrene) as
follows. ZnS-overcoated QDs (0.44 .mu.moles CdSe QDs) in
TOPO/butanol were precipitated and then dispersed in hexane as
described above. Hexane was evaporated under vacuum from an aliquot
containing 0.09 .mu.moles QDs. The QDs were redispersed in 0.1 ml
of toluene. n-Functionalized, amine-terminated polystyrene
(molecular weight=2600; 0.05 g) was dissolved in 0.2 ml toluene.
0.05 ml of toluene solution containing QDs (0.04 .mu.moles CdSe
QDs) and 0.05 ml functionalized polystyrene in toluene (about 0.01
g) were mixed together and sonicated for about 10 minutes. A
solution of 1 g polystyrene (molecular weight=45,000) in 1 ml of
toluene was prepared. 0.1 ml of this concentrated polystyrene
solution (about 0.05 g polystyrene) was added to the
QD/functionalized-polystyrene solution. The resulting solution was
sonicated for 2 minutes to thoroughly mix the QDs and
polystyrene.
Production of Diode
[0063] The blue diode used as a primary light source was GaN based
and had a luminescence peak at 450 nm. The glass cap was a
shortened, thin-walled glass tube (OD=5 mm, ID=4.3 mm, length=
3/16''). The glass cap was filled with the QD/polymer solution and
allowed to dry under flowing nitrogen for over two hours. More
QD/polymer solution could be added and dried as needed, but only
one filling and drying step was needed for this diode. When dried,
the polymer left a void at the base of the cap. The emitting
portion of the blue diode was then placed in this void at the base
of the cap. The polymer itself did not contact the diode. Green
light was produced as the blue light from the GaN caused the QDs to
luminesce at 520 nm. The 520 nm light gave the device a green
appearance.
Example 2
QDs in an Epoxy Polymer Matrix
[0064] CdSe/ZnS QDs having a 14 .ANG. core radius were prepared as
described in Example 1. 0.01 .mu.moles of QDs in TOPO solution were
taken, and the QDs were precipitated and washed 2 times with
methanol. The QDs were then redispersed in 0.27 ml (2 mmoles) of a
capping monomer, 6-mercaptohexanol. In order to effectively
disperse the QDs in the capping monomer, the solutions were first
sonicated for about 10 minutes and then stirred for 2 hours at
50-60.degree. C.
[0065] The QD solution was then further reacted with epoxide
monomers. 0.56 ml (2 mmoles) of poly[(phenyl
glycidylether)-co-formaldehyde] (number average molecular
weight=345) and 0.08 ml (0 8 mmoles) of diethyltriamine were added
to the 6-mercaptohexanol solution. The resulting mixture was
thoroughly mixed and placed in a glass tube having an outside
diameter of 6 mm and a length of 50 mm. Air bubbles formed during
mixing were removed by sonicating for 10 minutes. The glass tube
containing the monomer mixture was then heated to 70.degree. C. in
an oil bath for 2 hours, forming a high molecular weight epoxy with
the QDs distributed therein. This formed composite could then be
used as described in Example 1 with a primary light source to make
a green LED.
Example 3
QDs in a Methacrylate Polymer Matrix
[0066] CdSe/ZnS QDs having core radii of 13, 15, 18, 21, 23, 29,
and 34 .ANG. were prepared as described in Example 1. Solutions of
between 0.01-0.05 .mu.moles of each diameter of QD in TOPO were
precipitated and washed with methanol 2 times. 50-100 .mu.l
(100-200 .mu.moles) of trioctylphosphine, freshly removed from a
nitrogen-atmosphere glove box, were then added to each QD
precipitate. 650 .mu.l of lauryl methacrylate (Sigma-Aldrich, 96%,
2.2 mmoles) was added to each QD-trioctylphosphine solution and
stirred for 2 minutes. Approximately 350 .mu.l of 1,6-hexanediol
dimethacrylate (Polysciences, 98%, 1.2 mmoles) was added to each
lauryl methacrylate solution and stirred for another 2 minutes to
form a monomer solution of each different diameter QD. 10-20 mg of
azobisisobutylonitrile (AIBN, 1% w/w) was then added to each
monomer solution. The resulting mixtures were individually mixed
thoroughly and placed in glass tubes having an outer diameter of 6
mm, an inner diameter of approximately 4.5 mm, and a length of 50
mm. In a separate experiment, blue gallium nitride LED primary
light sources (Nichia, NSPB300A, epoxy-polymer encapsulated) were
dipped into each of the monomer solutions until the monomer
solution completely covered the diode head.
[0067] Each of these two types of devices was then placed in an
oven, preheated at 70.degree. C., for approximately 2 hours. Care
was taken to avoid disturbing the monomer mixture during
polymerization. After 2 hours, the monomer was completely
polymerized, i.e., it was firm on contact and it was resistant to
deformation under applied pressure. For the polymerized specimens
without the LEDs, the glass tubes were scored with a file and
broken to yield polymerized QD-composite plastic sticks that
emitted blue, blue-green, green, yellow, orange, red, or deep red
light under UV excitation. The LED-containing specimens emitted in
the same colors, with the exception of blue, under excitation by
the blue LED.
[0068] Mixed colored and white emitters can be constructed by
mixing different monomer solutions having different core radii CdSe
QDs in them. Surprisingly, polymerization does not reduce the
quantum yields of the QDs, so the final color emitted by these
mixed QD-polymer composites is of the same energy and intensity as
the initial mixture of monomer solutions.
[0069] Other embodiments of the invention will be apparent to those
skilled in the art from a consideration of the specification or
practice of the invention disclosed herein. It is intended that the
specification and example be considered as exemplary only, with the
true scope and spirit of the invention being indicated by the
following claims.
* * * * *