U.S. patent application number 14/722845 was filed with the patent office on 2015-12-03 for nanostructure material methods and devices.
The applicant listed for this patent is The Board of Trustees of the University of Illinois, Dow Global Technologies LLC, Rohm and Haas Electronic Materials, LLC. Invention is credited to Brian T. Cunningham, Kishori Deshpande, Jaebum Joo, Jong Keun Park, Gloria G. See, Peter Trefonas, Jieqian Zhang.
Application Number | 20150349194 14/722845 |
Document ID | / |
Family ID | 54702772 |
Filed Date | 2015-12-03 |
United States Patent
Application |
20150349194 |
Kind Code |
A1 |
Cunningham; Brian T. ; et
al. |
December 3, 2015 |
NANOSTRUCTURE MATERIAL METHODS AND DEVICES
Abstract
In one aspect, structures are provided comprising: a substrate
having a first surface and a second surface; and a polymeric layer
disposed on the first surface of the substrate, the polymeric layer
comprising a polymer and a plurality of light-emitting
nanocrystals; the polymeric layer having a patterned surface, the
patterned surface having a patterned first region having a first
plurality of recesses and a patterned second region having a second
plurality of recesses, wherein the plurality of recesses in each
region has a first periodicity in a first direction, and a second
periodicity in a second direction which intersects the first
direction, wherein the first periodicity of the first region is
different from the first periodicity of the second region.
Inventors: |
Cunningham; Brian T.;
(Champaign, IL) ; See; Gloria G.; (Champaign,
IL) ; Trefonas; Peter; (Medway, MA) ; Park;
Jong Keun; (Westborough, MA) ; Deshpande;
Kishori; (Lake Jackson, TX) ; Zhang; Jieqian;
(Southborough, MA) ; Joo; Jaebum; (Somerville,
MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Board of Trustees of the University of Illinois
Dow Global Technologies LLC
Rohm and Haas Electronic Materials, LLC |
Urbana
Midland
Marlborough |
IL
MI
MA |
US
US
US |
|
|
Family ID: |
54702772 |
Appl. No.: |
14/722845 |
Filed: |
May 27, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62003258 |
May 27, 2014 |
|
|
|
Current U.S.
Class: |
257/96 ; 257/79;
438/29 |
Current CPC
Class: |
H01L 2251/5369 20130101;
H01L 31/02327 20130101; H01L 33/50 20130101; G02B 6/00 20130101;
H01L 31/0203 20130101; H01L 33/02 20130101; Y02E 10/52 20130101;
H01L 31/054 20141201; H01L 33/005 20130101; H01L 27/322
20130101 |
International
Class: |
H01L 33/00 20060101
H01L033/00; H01L 33/50 20060101 H01L033/50 |
Claims
1. A structure comprising: a substrate having a first surface and a
second surface; and a polymeric layer disposed on the film surface
of the substrate, the polymeric layer comprising a polymer and a
plurality of light-emitting nanocrystals; the polymeric layer
having a patterned surface, the patterned surface having a
patterned first region having a first plurality of recesses and a
patterned second region having a second plurality of recesses,
wherein the plurality of recesses in each region has a first
periodicity in a first direction, and a second periodicity in a
second direction which intersects the first direction, wherein the
first periodicity of the first region is different from the first
periodicity of the second region.
2. The structure of claim 1 wherein the second periodicity of the
first region is the same as the second periodicity of the second
region.
3. The structure of claim 1 wherein the polymeric layer comprises a
plurality of first regions and a plurality of second regions.
4. The structure of claim 3 wherein the first and second regions
are in an alternating relationship to each other.
5. The structure of claim 1 wherein the light-emitting nanocrystals
comprise one or more heterojunction.
6. The structure of claim 1 wherein the light-emitting nanocrystals
comprise quantum dots.
7. The structure of claim 1 wherein the substrate is optically
transparent.
8. The structure of claim 1 wherein a layer of a material having a
higher index of refraction than the polymeric layer is disposed on
the patterned surface of the polymeric layer.
9. The structure of claim 1 wherein the second direction intersects
the first direction at an angle of from 65 to 115.degree..
10. The structure of claim 1 wherein the second direction is
substantially orthogonal to the first direction.
11. A method of forming a light-emitting system comprising forming
the structure of claim 1.
12. The method of claim 11 wherein the polymeric layer comprises a
plurality of first regions and a plurality of second regions.
13. A light-emitting apparatus comprising: a structure comprising;
a substrate having a first surface and a second surface; a
polymeric layer disposed on the first surface of the substrate, the
polymeric layer comprising a polymer and a plurality of
light-emitting nanocrystals; the polymeric layer having a patterned
surface, the patterned surface having a patterned first region
having a first plurality of recesses and a patterned second region
having a second plurality of recesses; wherein the plurality of
recesses in each region has a first periodicity in a first
direction, and a second periodicity in a second direction which
intersects the first direction, wherein the first periodicity of
the first region is different from the first periodicity of the
second region; layer of a material having a higher index of
refraction than the polymeric layer is disposed on the patterned
surface of the polymeric layer; and a light source arranged to
provide light to the second surface of the substrate.
14. The light emitting apparatus of claim 13 wherein the first
periodicity is operable to outcouple light emission in a defined
direction.
15. The light emitting apparatus of claim 13 wherein the first
periodicity is operable to outcouple light emission in a direction
that is normal to the first surface of the substrate.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to and the benefit of U.S.
provisional patent Application No. 62/003,258 filed in the United
States Patent and Trademark Office on May 27, 2014, the entire
contents of which are incorporated herein by reference.
FIELD
[0002] In one aspect, structures are provided, comprising: a
substrate having a first surface and a second surface; and a
polymeric layer disposed on the first surface of the substrate, the
polymeric layer comprising a polymer and a plurality of
light-emitting nanocrystals; the polymeric layer having a patterned
surface, the patterned surface having a patterned first region
having a first plurality of recesses and a patterned second region
having a second plurality of recesses, wherein the plurality of
recesses in each region has a first periodicity in a first
direction, and a second periodicity in a second direction which
intersects the first direction, wherein the first periodicity of
the first region is different from the first periodicity of the
second region.
BACKGROUND
[0003] There are a broad range of application-specific needs for
lighting and display technologies used in homes, workplaces and
consumer products. Precise control of the output spectrum of
lighting products is desirable to match the requirements for color
temperature and output directionality, while at the same time
optimizing power efficiency and manufacturing cost, See U.S. Pat.
No. 8,692,446 and U.S. 2013/0051032. For video display
applications, controlling the blend of primary colors in each pixel
is necessary, while the control of pixel output directionality must
be tailored for a range of viewing methods that may be either
tightly confined (for privacy) or widely dispersed (for wide
viewing angle).
[0004] By varying the duty cycle, period, and refractive index, the
resonant characteristics of photonic crystal (PC) structures can be
tuned to interact with wavelengths extending from the ultraviolet
to the infrared. Certain photonic crystal structures have been used
for a variety of applications including polarizers, filters,
biosensors, optical communication components, displays, and
lighting, PCs have been incorporated into light emitting diodes
(LEDs) in order to increase extraction efficiency, and to control
the directionality of light output, either normal to the device or
into angular sidelobes.
[0005] While certain photonic crystal structures have been
reported, improved light-emitting structures are needed for many
applications.
SUMMARY
[0006] We now provide new light-emitting structures and devices,
and methods of making such structures and devices.
[0007] The power efficiency, spectral characteristics, and output
directionality of light emitting diodes (LEDs) used for lighting
and video display may be tailored by integrating nanostructures
that interact with photon emitters. It has now been found that
visible-wavelength-emitting nanostructure materials can be
integrated within a polymer-based photonic crystal (PC) and excited
by an ultraviolet-emitting LED. As discussed herein, the term
nanostructure material includes quantum dot materials as well as
nanocrystalline nanoparticle nanocrystals that comprise one or more
heterojunctions such as heterojunction nanorods.
[0008] The PC design incorporates distinct periods in orthogonal
directions, enabling simultaneous resonant coupling of ultraviolet
excitation photons to the one or more nanostructure materials and
visible nanostructure material(s) emission to efficiently extract
photons normal to the PC surface. The combined excitation and
extraction enhancements result in an increase in the nanostructure
material(s) output intensity. Multiple nanostructure material-doped
PCs can be combined on a single surface to optimally couple with
distinct populations of nanostructure materials, offering an
ability for blending color output and directionality of multiple
wavelengths. Devices can be fabricated upon flexible plastic
surfaces by a manufacturable replica molding approach. More
particularly, in a first aspect, a light-emitting structure is
provided that comprises: a substrate having a first surface and a
second surface; and a polymeric layer disposed on the first surface
of the substrate, the polymeric layer comprising a polymer and one
or more light-emitting nanostructure materials such as a plurality
of light-emitting crystals; the polymeric layer having a patterned
surface, the patterned surface having a patterned first region
having a first plurality of recesses and a patterned second region
having a second plurality of recesses, wherein the plurality of
recesses in each region has a first periodicity in a first
direction, and a second periodicity in a second direction which
intersects the first direction, wherein the first periodicity of
the first region is different from the first periodicity of the
second region. In typical aspects, the first periodicity of a first
region will be different from the first periodicity of a second
region whereby distinct light-emitting nanocrystals in each region
are selectively excited so as to emit light of distinct wavelengths
in each region, for instance where light of a first wavelength
(e.g. red) is emitted in the first region and light of a second
wavelength (e.g. blue) is emitted in the second region. In
preferred aspects, the first periodicity of a first region will be
different from the first periodicity of a second region where the
two respective first periodicities differ by more than 5% in the
same measured value (e.g. distance between mid-points of nearest
neighbor recesses in each of the first regions), more typically
where the two respective first periodicities differ by at least
10%, 15%, 20%, 25%,30%, 35%, 40%, 45%, 50% or 70% in the same
measured value (e.g. distance between mid-points of nearest
neighbor recesses in each of the first regions).
[0009] In preferred embodiments, the second periodicity of the
first region is at least substantially the same as the second
periodicity of the second region. In typical aspects, the second
periodicity of a first region will be at least substantially the
same as the second periodicity of a second region so as to
outcouple the same source of incoming light, including UV
radiation. For instance, in certain preferred aspects, the second
periodicity of a first region will be at least substantially the
same as the second periodicity of a second region where the two
respective second periodicities differ no more than 5% in the same
measured value (e.g. distance between mid-points of nearest
neighbor recesses in each of the second regions), more typically
where the two respective second periodicities differ no more than
4%, 3%, 2%, 1% or 0.5% in the same measured value (e.g. distance
between mid-points of nearest neighbor recesses in each of the
second regions). Limitations on fabrication techniques may result
in the two respective second periodicities being not precisely the
same.
[0010] The polymeric layer can comprise a plurality of first
regions and a plurality of second regions. In certain embodiments,
the first and second regions are in an alternating relationship to
each other. In certain embodiments, the one or more light-emitting
nanostructure materials comprise one or more heterojunctions. In
certain embodiments, the one or more light-emitting nanostructure
materials comprise quantum dots. In certain embodiments, the
substrate is optically transparent. In certain embodiments, a layer
of a material having a higher index of refraction than the
polymeric layer is disposed on the patterned surface of the
polymeric layer. In certain embodiments, the second direction
intersects the first direction at an angle of from 65 to
115.degree.. In certain embodiments, the second direction is
substantially orthogonal to the first direction.
[0011] In another aspect, the invention provides a light-emitting
apparatus, comprising: a structure comprising a substrate having a
first surface and a second surface a polymeric layer disposed on
the first surface of the substrate, the polymeric layer comprising
a polymer and one or more light-emitting nanostructure materials
such as a plurality of light-emitting crystals; the polymeric layer
having a patterned surface, the patterned surface having a
patterned first region having a first plurality of recesses and a
patterned second region having a second plurality of recesses,
wherein the plurality of recesses in each region has a first
periodicity in a first direction, and a second periodicity in a
second direction which intersects the first direction, wherein the
first periodicity of the first region is different from the first
periodicity of the second region; a layer of a material having a
higher index of refraction than the polymeric layer is disposed on
the patterned surface of the polymeric layer; and a light source
arranged to provide light to the second surface of the
substrate.
[0012] As discussed above, in typical aspects, the first
periodicity of a first region will be different from the first
periodicity of a second region whereby distinct light-emitting
nanocrystals in each region are selectively excited so as to emit
light of distinct wavelengths in each region, for instance where
light of a first wavelength (e.g. red) is emitted in the first
region and light of a second wavelength (e.g. blue) is emitted in
the second region. In preferred aspects, the first periodicity of a
first region will be different from the first periodicity of a
second region where the two respective first periodicities differ
by more than 5% in the same measured value (e.g. distance between
mid-points of nearest neighbor recesses in each of the first
regions), more typically where the two respective first
periodicities differ by at least 10%, 15%, 20%, 25%, 30%, 35%, 40%.
45%, 50% or 70% in the same measured value (e.g. distance between
mid-points of nearest neighbor recesses in each of the first
regions).
[0013] In preferred embodiments, the first periodicity is operable
to outcouple light emission in a defined direction. In certain
embodiments, the first periodicity is operable to outcouple light
emission in a direction that is normal to the first surface of the
substrate.
[0014] In another aspect, a structure is provided comprising a
polymeric layer comprising a polymer and one or more light-emitting
nanostructure materials such as a plurality of light- emitting
nanocrystals; wherein the polymeric layer comprises first and
second regions, each region having a first periodicity in a first
direction, and a second periodicity in a second direction which
intersects the first direction.
[0015] In a further aspect, methods are provided for forming a
light-emitting system comprising forming a structure of the
invention. In certain embodiments, the polymeric layer of the
system comprises a plurality of first regions and a plurality of
second regions.
[0016] The invention also provides devices obtained or obtainable
by the methods disclosed herein, including a variety of
light-emitting devices, photodetectors, chemical sensors,
photovoltaic device (e.g. a solar cell), transistors and diodes, as
well as biologically active surfaces that comprise the systems
disclosed herein.
[0017] Other aspects of the invention are disclosed infra.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 shows an exemplary device according to the present
invention.
[0019] FIG. 2 (which includes FIGS. 2a through 2d) shows exemplary
embodiments of devices according to the present invention.
[0020] FIG. 3 shows simulated transmission spectra of target
resonances.
[0021] FIG. 4 shows the impact of quantum dot extraction with and
without a photonic crystal (PC) structure present.
[0022] FIG. 5 (which includes FIGS. 5A, 5B, 5C and 5D) shows a
comparison of the angle dependence of excitation at different
stages of TiO.sub.2 deposition.
[0023] FIG. 6 (which includes FIGS. 6A and 6B) shows angle
dependence of transmission measured with angle variations along 0
and .PHI..
[0024] FIG. 7 (which includes FIGS. 7A and 7B) shows
photomicrographs quantum dot enhancement within PC regions.
DETAILED DESCRIPTION
[0025] We have now found that the resonant modes of a photonic
crystal can be engineered to occur at specific combinations of
angle and wavelength, allowing light of a selected wavelength and
incident direction to couple to the photonic crystal and excite a
highly localized electromagnetic standing wave with an amplitude
that is substantially greater than the original illumination
source. This can be accomplished by e.g. an appropriate choice of
dielectric materials and material dimensions. Nanostructure
materials including quantum dots that down-convert light from a
broad band of excitation wavelengths to a very specific emission
wavelength, have been successfully incorporated into photonic
crystals with specific resonances designed to couple to the
relevant excitation and/or emission wavelengths of the
nanostructure material. We have found that by introducing asymmetry
into the photonic crystal structure, through the use of different
periods in orthogonal directions, a photonic crystal may
incorporate multiple resonances at widely varied wavelengths so as
to interact simultaneously with the excitation and emission spectra
of the integrated nanostructure material emitters to enhance the
number of photons generated by the nanostructure material (e.g.
quantum dots), while increasing the efficiency of emitted photons
that reach a viewer.
[0026] We have now demonstrated that structures comprising a
polymeric layer comprising a polymer and one or more light-emitting
nanostructure materials such as a plurality of light-emitting
nanocrystals, the polymeric layer having a patterned surface
including first and second regions, and disposed on a surface of a
substrate, can be tailored to provide a desired output spectrum of
light.
[0027] We have found the present structures can provide a number of
performance benefits. In particular, the present structures can
combine enhancement of the excitation and extraction of the light
output of light-emitting nanocrystals.
[0028] Thus, in one aspect, the invention provides a structure
comprising: a substrate having a first surface and a second
surface; and a polymeric layer disposed on the first surface of the
substrate, the polymeric layer comprising a polymer and one or more
light-emitting nanostructure materials such as a plurality of
light-emitting nanocrystals; the polymeric layer having a patterned
surface, the patterned surface having a patterned first region
having a first plurality of recesses and a patterned second region
having a second plurality of recesses, wherein the plurality of
recesses in each region has a first periodicity in a first
direction, and a second periodicity in a second direction which
intersects the first direction, wherein the first periodicity of
the first region is different from the first periodicity of the
second region.
[0029] FIG. 1 is an illustration of an exemplary device according
to the invention. As seen in FIG. 1, a polymer layer is applied in
first region 12 and second region 14 on substrate 10. The polymer
layer incorporates a plurality of light-emitting nanocrystals. A
layer 16 of a material having a higher index of refraction than the
polymeric layer may be disposed on the patterned surface of the
polymeric layer. The substrate can be made of any rigid or flexible
material, suitably a material that is optically transparent in a
desired wavelength range. For example, the substrate can Ce made of
glass, cellulose acetate, or polymeric materials such as
polyethylene terephthalate, polyimides, polycarbonate,
polyurethane, and the like. The substrate can have any suitable
thickness, for example, from 1 micron to 1 mm in thickness. The
polymer applied to the substrate 10 can be any suitable polymeric
material, including polyethylene terephthalate, polyimides,
polycarbonate, polyurethane, and the like. Preferred polymeric
materials include lauryl methacrylate (LMA), ethyl glycol
dimethacrylate (EGDMA) and mixtures thereof. The polymer layer can
optionally be adhered to the substrate with an optically
transparent adhesive such as NOA 61 (Norland Products, Inc.).
[0030] The patterned first region 12 and the patterned second
region 14 of the patterned surface each have a plurality of
recesses. In each patterned region, the plurality of recesses has
periodicity; e.g., the plurality of recesses are spaced equally or
other regular or repeating arrangement along a specified dimension
on the surface. The plurality of recesses can be formed integrally
with the polymeric layer, e.g., by coating a polymer solution onto
a patterned master template. Alternatively, the plurality of
recesses can be formed by first forming a substantially flat or
planar polymer layer on the substrate, and then patterning the
polymeric layer, e.g., by stamping with a patterned die. In a
further alternative, microstructures such as ridges, lenslets,
pyramids, trapezoids, round or square shaped posts, or curved sided
cone structures (see, e.g., U.S. Patent Application Publication No.
2010/0128351) are formed or applied on the polymeric layer by
deposition of a material on the surface of the polymeric layer,
thereby defining the plurality of recesses on the polymeric
layer.
[0031] The plurality of recesses within each of the first region
and the second region are suitably periodic in two dimensions, that
is, nearest neighbor recesses are spaced equally or in other
regular or repeating pattern in two different directions (i.e., a
first direction and a second direction) along the surface. Thus,
the patterned first region 12 of the polymer layer has a first
periodicity in a first direction and a second periodicity in a
second direction, and the patterned second region 14 of the polymer
layer has a first periodicity in a first direction and a second
periodicity in a second direction. The first periodicity in the
first direction of the first patterned region and the first
periodicity in the first direction of the second patterned region
may be the same, or may advantageously be different. Similarly, the
second periodicity in the second direction of the first patterned
region and the second periodicity in the second direction of the
second patterned region may be the same, or may be different. In
certain embodiments, the second periodicity of the first region is
the same as the second periodicity of the second region.
[0032] The spacing of recesses can be selected to produce one or
more resonances at one or more selected wavelengths so as to
interact simultaneously with the excitation and emission spectra of
the integrated light-emitting nanocrystals in the polymer layer, as
discussed below, as a means of enhancing the number of photons
generated by each nanostructure material (e.g., quantum dot).
Rigorous coupled wave analysis can be used to predict the resonant
wavelengths and electromagnetic field distributions at the resonant
wavelengths for a given spacing or recesses. Thus, for example,
recesses having a spacing of 250 nm can provide a resonance at 490
nm, while recesses having a spacing of 340 nm can provide a
resonance at 590 nm. In certain embodiments, the spacings in both
first and second directions are less than 1 micron.
[0033] In certain embodiments, the second direction of periodic
recesses within the first and/or second region intersects the first
direction of periodic recesses at an angle of from 65 to
115.degree.. In certain embodiments, the second direction is
substantially orthogonal to the first direction.
[0034] In certain embodiments, the polymeric layer comprises a
plurality of first regions and a plurality of second regions. The
plurality of first and second regions can be arranged on the
substrate in any desired pattern, such as a checkerboard pattern.
In certain embodiments, the first and second regions are in an
alternating relationship to each other.
[0035] When the layer 16 is present, the layer can be of any
optically transparent material having a higher index of refraction
than material of the polymeric layer. Suitable materials for the
layer 16 include titanium dioxide (TiO.sub.2) or other suitable
high refractive index inorganic oxide. The layer 16 can be
deposited by coating (e.g., spin coating, spray coating, dip
coating), sputtering, or other methods for depositing a layer of
material on the polymeric layer without disturbing the patterning
of the polymeric layer. The thickness of the layer 16 can be used
to tune the resonant wavelength of the periodic recesses. When the
layer 16 is TiO.sub.2, a suitable thickness is from about 50 nm to
about 500 nm, e.g., about 85 nm.
[0036] In another aspect, the invention provides a light-emitting
apparatus comprising a structure comprising (i) a substrate having
a first surface and a second surface; (ii) a polymeric layer
disposed on the first surface of the substrate, the polymeric layer
comprising a polymer and one or more light-emitting nanostructure
materials such as a plurality of light-emitting nanocrystals; the
polymeric layer having a patterned surface, the patterned surface
having a patterned first region having a first plurality of
recesses and a patterned second region having a second plurality of
recesses, wherein the plurality of recesses in each region has a
first periodicity in a first direction, and a second periodicity in
a second direction which intersects the first direction, wherein
the first periodicity of the first region is different from the
first periodicity of the second region; and (iii) a layer of a
material having a higher index of refraction than the polymeric
layer is disposed on the patterned surface of the polymeric layer;
and a light source arranged to provide light to the second surface
of the substrate.
[0037] The light source can be any suitable source of ultraviolet
(UV) or visible light, e.g., light in the range of 200
nm<.lamda.<700 nm including an LED.
[0038] In preferred embodiments, the first periodicity is operable
to outcouple light emission in a defined direction. As used herein,
the term "outcouple" or "outcoupling" refers to conversion of
substrate and I/0 modes of light emission to external modes of
light emission, thereby enhancing light output from the device. In
certain embodiments, the first periodicity is operable to outcouple
light emission in a direction that is normal to the first surface
of the substrate. In certain embodiments, the second periodicity of
the first region is the same as the second periodicity of the
second region.
[0039] As discussed above, the term "nanostructure material", as
used herein, includes quantum dot materials as well as
nanocrystalline nanoparticles (nanoparticles or nanocrystals) that
comprise one or more heterojunctions such as heterojunction
nanorods. Nanostructure materials, including nanocrystals and
quantum dots, are semiconductor materials having a nanocrystal
structure and sufficiently small to display quantum mechanical
properties. See U.S. Published Application 2013/0056705 and U.S.
Pat. No. 8,039,847. See also U.S. 2012/0234460 and U.S.
20130051032.
[0040] Thus, as discussed above, the term nanostructure material as
used herein includes both quantum dot materials as well as
nanocrystalline nanoparticles (nanoparticles) that comprise one or
more heterojunctions such as heterojunction nanorods.
[0041] A quantum dot suitably may be Group II-VI material, a Group
III-V material, a Group V material, or a combination thereof. The
quantum dot suitably may include e.g. at least one selected from
CdS. CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, CiaN, Gal),
GaAs. InP and InAs. Under different conditions, the quantum dot may
include a compound including two or more of the above materials.
For instance, the compound may include two or more quantum dots
existing in a simply mixed state, a mixed crystal in which two or
more compound crystals are partially divided in the same crystal
e.g., a crystal having a core-shell structure or a gradient
structure, or a compound including two or more nanocrystals. For
example, the quantum dot may have a core structure with through
holes or an encased structure with a core and a shell encasing the
core. In such embodiments, the core may include e.g. one or more
materials of CdSe. CdS, ZnS, ZnSe, CdTe, CdSeTe, CdZnS, PhSe,
AgInZnS, and ZnO. The shell may include e.g. one or-more materials
selected from CdSe. ZnSe, ZnS, ZnTe, CdTe, PbS, TiO, SrSe, and
HgSe.
[0042] Passivated nanocrystalline nanoparticles (nanoparticles)
that comprise a plurality of heterojunctions suitably facilitate
charge carrier injection processes that enhance light emission when
used as a device. Such nanoparticles also may be referred to as
semiconducting nanoparticles and may comprise a one-dimensional
nanoparticle that has disposed at each end a single endcap or a
plurality of endcaps that contact the one-dimensional nanoparticle.
The endcaps also may contact each other and serve to passivate the
one-dimensional nanoparticles. The nanoparticles can be symmetrical
or asymmetrical about at least one axis. The nanoparticles can be
asymmetrical in composition, in geometric structure and electronic
structure, or in both composition and structure. The term
heterojunction implies structures that have one semiconductor
material grown on the crystal lattice of another semiconductor
material, The term one-dimensional nanoparticle includes objects
where the mass of the nanoparticle varies with a characteristic
dimension (e.g., length) of the nanoparticle to the first power.
This is shown in the following formula (1): M.alpha. I d where M is
the mass of the particle, I: is the length of the particle and d is
an exponent that determines the dimensionality of the particle.
Thus, for instance, when d=1, the mass of the particle is directly
proportional to the length of the particle and the particle is
termed a one-dimensional nanoparticle. When d=2, the particle is a
two-dimensional object such as a plate while d=3 defines a
three-dimensional object such as a cylinder or sphere. The
one-dimensional nanoparticles (particles where d=1) includes
nanorods, nanotubes, nanowires, nanowhiskers, nanoribbons and the
like. In one embodiment, the one-dimensional nanoparticle may be
cured or wavy (as in serpentine), i,e, have values of d that lie
between 1 and 1.5.
[0043] Exemplary preferred materials are disclosed in U.S. patent
application Ser. Nos. 13/834,325 and 13/834,363, both incorporated
herein by reference.
[0044] The one-dimensional nanoparticles suitably have
cross-sectional area or a characteristics thickness dimension
(e.g., the diameter for a circular cross-sectional area or a
diagonal for a square of square or rectangular cross-sectional
area) of about 1 nm to 10000 nanometers (nm), preferably 2 nm to 50
nm, and more preferably 5 nm to 20 nm (such as about 6, 7, 8, 9,
10, 11, 12, 13, 14, 15, 16, 17. 18, 19 or 20 nm) in diameter.
Nanorods are suitably rigid rods that have circular cross-sectional
areas whose characteristic dimensions lie within the aforementioned
ranges. Nanowires or nanowhiskers are curvaceous and have different
or vermicular shapes. Nanoribbons have cross-sectional area that is
bounded by four or five linear sides. Examples of such
cross-sectional areas are square, rectangular, parallelopipeds,
rhombohedrals, and the like. Nanotubes have a substantially
concentric hole that traverses the entire length of the nanotube,
thereby causing it to be tube-like. The aspect ratios of these
one-dimensional nanoparticles are greater than or equal to 2,
preferably greater than or equal to 5, and more preferably greater
than or equal to 10.
[0045] The one-dimensional nanoparticles comprise semiconductors
that suitably include those of the Group II-VI(ZnS, ZnSe, ZnTe,
CdS, CdTe, I IgS, I IgSe, IlgTe, and the like) and III-V (GaN,
CiaP, GaAs, CiaSh, InN, InP, InAs, InSb, AlAs, AlP, AISb, and the
like) and IV (Ge, Si, Pb and the like) materials, an alloy thereof,
or a mixture thereof.
[0046] Nanostructure materials including quantum dot materials are
commercially available and also may be prepared for example by a
standard chemical wet method using a metallic precursor as well as
by injecting a metallic precursor into an organic solution and
growing the metallic precursor. The size of the nanostructure
material including quantum dot may be adjusted to absorb or emit
light of red (R), green (G), and blue (B) wavelengths, Thus, a
light-emitting nanocrystal may be selected to absorb or emit light
of a selected wavelength or wavelength range.
[0047] The light-emitting nanostructure materials such as
nanocrystals or quantum dots can be incorporated into the polymer
layer by addition of a suspension or solution of the nanostructure
materials (e.g. nanocrystals or quantum dots) to a monomer
solution, followed by coating of the polymer solution onto the
substrate and curing of the polymer solution to provide the polymer
with embedded nanocrystals or quantum dots.
[0048] The present structures can combine enhancement of the
excitation and extraction of the light output of light-emitting
nanocrystals. In certain embodiments, the enhancement of the
excitation and extraction of the light output of light-emitting
nanocrystals can be 2.times., 3.times., 4.times., 5.times., or
10.times. of the light output of the light-emitting
nanocrystals.
[0049] The following examples are illustrative of the
invention.
EXAMPLE 1
[0050] In this example, as also shown in Figure (a), quantum dots
(QDs) were incorporated into a replica-molded flexible
polymer-based PC structure that was excited by a UV backlight LED.
The UV excitation source couples to a resonant mode of the PC,
which creates an enhanced excitation at the coupling wavelength by
increasing the magnitude of the electric field experienced by the
QDs in the PC, thus producing greater photon output than would
occur without a PC structure. The asymmetric PC is designed to
produce a resonance at the wavelength of QD emission, resulting in
photon emission that is efficiently channeled normal to the PC
surface. As further shown in FIG. 2, an interleaved surface was
designed and fabricated in a checkerboard pattern, containing two
PC designs. While both regions were designed to produce resonances
for the same UV excitation wavelength, each region was optimized
for a different QD emission wavelength. Thus, a single surface,
populated with a mixture of QDs, can be tailored by selection of
the relative surface area represented by each PC design to produce
a specific overall output spectrum.
Materials and Methods
[0051] A silicon wafer was fabricated to serve as a "master"
template for the replica molding process, and thus contains a
negative surface image of the desired PC grating structure. The
master's grating structure was fabricated via electron beam
lithography on a layer of thermally-grown SiO.sub.2 on a Si wafer,
upon which reactive ion etching was used to produce 80 nm tall
pillars, as shown in FIG. 2(c). The patterned device area was
3.times.3 mm.sup.2. To facilitate the clean removal of the replica
from the master, the wafer was cleaned with a piranha solution (3:1
(v/v) mixture of sulfuric acid and hydrogen peroxide) for 20 min,
rinsed with DI water (MilliQ), and dried with N.sub.2 Next, a
vapor-phase deposition of (tridecafluoro-1,1,2,2-tetrahydrooctyl)
trichlorosilane (No-Stick, Alfa Aesar) was performed by placing the
wafer into an enclosed container with two drops of the No-Stick
solution for 1 h.
[0052] CdSeS/ZnS alloyed QDs were purchased from Sigma-Aldrich (6
nm. 1 mg/ml in toluene, oleic acid as ligand), or synthesized for
this application by coating with oleic acid ligand, then purified
twice using precipitation and centrifugation with ethanol and
methanol. Lauryl methacrylate (LMA) and ethylene glycol
dimethacrylate (EGDMA) (Sigma-Aldrich) were purified to remove the
inhibitor with an inhibitor-removal column (Sigma-Aldrich) before
their use.
[0053] The UV curable polymer, consisting of 182 ..mu.L, of LMA and
18 .mu.L, of EGDMA, was mixed in a flask, and 4 mL, of the QD
hexane solution and 8 uL. oleic acid was added and mixed well, then
20 .mu.L of PLMA monopolymer solution (Scientific Polymer Products,
Inc.) was added to increase the viscosity. The remaining solvent
was removed using a rotavap at room temperature and 2 .mu.L of
initiator (Darocur 1173, Sigma-Aldrich) was added immediately
before spin coating. The solution was spin coated onto the master
wafer at 600 rpm for 30 s, then immediately polymerized by exposure
to a high intensity UV lamp for 30 min in a nitrogen atmosphere
glovebox.
[0054] After the film was fully cured, a layer of NOA 61 (Norland
Products Inc,) was drop coated over the composite film. An acetate
sheet (Optigrafix Acetate) substrate, selected for low
birefringence, was then placed over the master wafer and brought
into contact with the uncured NOA drops to form a thin continuous
layer between the acetate sheet and the composite thin film. Next,
the NOA was cured for 10 min using a UV lamp under ambient
conditions. The acetate substrate, along with the NOA layer and
composite thin film, was then released from the master wafer with
the thin film of QD-PLMA containing the replicated 2D cavity
structure. After replica molding, TiO.sub.2 was deposited by
sputtering (K. J. Lesker Dual-Gun Sputter System) to the depth
required for resonance at the desired wavelength. Deposition times
were restricted to keep the substrate temperature from exceeding
40.degree. C., to avoid thermally induced damage to the polymer
materials, which sometimes required multiple layers of TiO.sub.2,
deposition to reach the correct thickness.
Device Structure
[0055] The device structure interleaves the regions of two distinct
2D PCs in a checkerboard pattern. Each region consists of
rectangular cavities, as shown in FIG. 2(b), with resonances
created by the periodic variation in the orthogonal directions on
the surface. Each region varies in one direction with dimensions
selected to provide enhancement from the same UV excitation source
(200 nm period with 40% and 70% duty cycles in Regions 1 and 2,
respectively), while the orthogonal directions have larger feature
sizes for producing resonances at visible wavelengths. The larger
features in Region 1 have a lateral width of 250 nm to produce
resonances at .lamda.=490 nm, while the features in Region 2 have a
lateral width of 340 nm, designed to produce resonances at
.lamda.590 nm, i.e. a first periodicity of the first region is
different from the first periodicity of the second region. In this
device, a second periodicity of that first region is at least
substantially the same as the second periodicity of that second
region. For both regions, the structure is formed from a QD-doped
polymer with a grating depth of 80 nm that is coated with an 85 nm
thin film of TiO.sub.2 While the period of the structure is the
main determinant of the resonant wavelength, the resonances can
also be tuned via control of the TiO.sub.2 thickness.
[0056] The PC structures were designed using rigorous coupled wave
analysis (Rsoft, DiffractMod) to predict the resonant wavelengths
and electromagnetic field distributions at the resonant
wavelengths, by evaluating a unit cell of the PC with periodic
boundary conditions in both the x- and y-directions, as indicated
in FIG. 2. Note that, due to the large difference between the
refractive index of TiO.sub.2 in the UV (n=2.87) and the visible
(n=2.61 at .lamda.=590 nm), separate simulations were carried out
for unpolarized incident light in two wavelength bands
(350<.lamda.<450 nm, and 450<.lamda.<800 nm) and
plotted together for each PC region. The simulation results (FIG.
3) show large dips in the transmission efficiency at the
wavelengths for which guided mode resonance occurs. Both regions
have resonances in the UV near .lamda.=370 nm, and Region 1 has a
resonance near .lamda.=490 nm, while Region 2 has a resonance near
.lamda.=600 nm. These visible wavelength resonances are designed to
overlap with the emission spectra of QDs incorporated into the PC.
The 2D PC structure produces several additional resonance modes
caused by the variation in feature sizes experienced by the
unpolarized incident light. Also, although in a one dimensional PC,
the TM mode (with both x and z-directional components) and the TE
mode (with components in the y-direction) can be isolated, they are
both present as TE- and TM-like modes in a 2D PC, which leads to
additional resonances at wavelengths other than those required by
the design.
Results
[0057] The emission properties of the devices were measured using a
UV LED (Thor Labs, Ultra Bright Deep Violet LED) centered at
.lamda.=375 with a 20 nm full-width half-maximum as the excitation
source. A 350<.lamda.<390 nm bandpass filter was used to
eliminate any non-UV emission from the LED. The LED output was
collimated before illuminating the PC. The device was mounted over
a cover with a 3 mm diameter aperture, assuring that only the
patterned PC region was excited and measured.
[0058] The device under test was mounted to a motorized rotary
stage, allowing the incident excitation angle to be varied. The
output passed through a UV filter to eliminate any light from the
excitation source, then was collected by a collimating lens
attached to an optical fiber. The fiber was connected to a
spectrometer (USB2000+, Ocean Optics) from which the emission can
be measured and observed through the LabView OmniDriver software
which also controlled the rotation position of the stage in 0.1
degree steps.
[0059] To measure the impact of the extraction angle, the same
equipment was used, but instead of mounting the PC sample to a
rotation stage and varying the excitation angle, the PC sample
position was fixed. The collimator coupled to the optical fiber was
instead mounted on the stage and rotated around the PC, allowing
extracted light to be collected over a range of angles with respect
to the PC surface.
[0060] The photonic band diagram of a device was determined using
the same experimental setup as that to measure the excitation
output, but the UV LED and associated bandpass filter were replaced
with a tungsten-halogen lamp coupled to an optical fiber that
outputs unpolarized light through a collimator, then the broadband
transmission was measured across a range of angles.
[0061] In the sample with QDs emitting at a peak wavelength of
.lamda.=505 nm, the extraction was measured before and after a
deposition of 20 nm of TiO.sub.2 to compare the output intensity
with and without a photonic crystal structure, as shown in FIG. 4.
There is an asymmetry in the UV-LED output beam, which creates as
asymmetry in the QD emission both with and without the PC. However,
with the PC present, an increase by a factor of 2 is present in the
measured output intensity of the extraction angle-dependent OD
emission. The narrow, angle dependent enhancement is due to the
variation in extraction angle, while the broader enhancement across
all measured angles is due to the enhanced excitation over the
entire PC area.
[0062] In order to demonstrate the ability of the PC to selectively
enhance a sub-population of embedded QDs, a sample containing a
homogeneous mixture QDs, with emissions centered at .lamda.=490 nm
and .lamda.=585 nm was fabricated. The emission was measured on a
QD doped grating structure without PC resonances by measuring the
emission of a structure without TiO.sub.2 (FIG. 5(a)) and after the
PC is formed by deposition of a 43 nm TiO.sub.2 thin film (FIG.
5(b)). The maximum QD emission increased by 4 times for the 490 nm
QDs and 5 times for the 585 nm QDs, shown in FIG. 5(b), but only
within the regions in which their emission matched their
corresponding PC resonance. To adjust the resonance conditions of
the PC for enhancing the emission wavelengths of both types of QDs,
an additional 42 nm of TiO.sub.2 was deposited, and that resulted
in a total increase of 4.2 times for the 490 nm QDs and 5.8 times
for the 585 nm QDs (FIG. 5(c), as the resonance conditions of the
PC were red-shifted by the thicker TiO.sub.2 layer, FIG. 5(d) shows
the photonic bandgap of the structure with the total of 85 nm of
TiO.sub.2, where the darker bands indicate the wavelength and angle
coupling leading to resonance within the PC. These bands correspond
to the bands of enhancement seen in FIG. 5(c) within the QD
emission.
[0063] Because the device structure has a different period in each
orthogonal direction, the transmission efficiency can be measured
over the range of angles across 0 that vary with the shorter, UV
resonant features or the .PHI. angle with the larger features that
couple to visible wavelengths. The difference in the two photonic
bands is shown in FIG. 6. In FIG. 6(a), the angle 0 is varied,
there is an angle-dependent resonance in the UV, while the
resonance in the visible is constant for all wavelengths,
regardless of angle. This occurs because there is no angle
variation experienced by the features responsible for coupling to
those wavelengths. A similar situation occurs in FIG. 6(b) with
constant wavelength resonance occurring in the UV wavelengths.
While varying the angle .PHI. experienced by the PC only changes
its coupling to the larger PC features and shows angle dependent
variation at wavelengths greater than .lamda.k=450 nm.
[0064] The enhancement of QDs in a region with PC coupling is
substantial enough to be easily visible to the naked eye. FIG. 7
shows photographs of two dual-region QD-doped PCs with emissions at
from .lamda.=490 nm and .lamda.=585 nm. The brighter regions are
providing both enhanced excitation and extraction for the embedded
QDs. The alternate regions have a resonance condition of the PC
that is coupling only to the excitation wavelength, and appears
darker due to the lack of an extraction enhancement.
Discussion
[0065] Devices using the PC structure demonstrated in this example
combine enhancement of the excitation and extraction of up to
5.8.times. the QD output produced with no PC structure present.
There is an expected difference between the improvements in
excitation and extraction, given that QDs are dispersed through
both regions of the PC structure. Therefore, the QDs in every
region experience enhancement of the UV excitation wavelength, but
the output wavelengths are enhanced only in one region, or half the
total device area.
[0066] The enhancements offered by this approach may be further
improved in a number of ways. For instance, by optimizing the
feature sizes for specific colors, the PCs may be designed to
better couple to the emission and excitation wavelengths of the
desired QDs, increasing the local electric field within the PC, and
thus the enhancement experienced in the QD output. Specifically
placing the QDs only in the PC pixel region where they would
experience both excitation and extraction would decrease the
quantities of QDs required and also extract light more
effectively.
[0067] A device also may be designed to utilize a non-UV excitation
source simply by adjusting the design parameters to couple to a
different wavelength. Pixel patterning also can create regions with
no PC structure at all, allowing only the excitation source light
to pass through, thus increasing the flexibility of color mixing
options for lighting.
[0068] The use of nonoreplica molding for fabrication makes it
possible to scale up to large area fabrication of flexible
substrates. With appropriate materials, large area, flexible
displays and light sources can be constructed to use pixelated PC
enhancement. The use of PCs in lighting and displays gives the
advantage of angle steering possible with PC enhancement to broaden
or narrow the output angles and control the directivity of light
output in both lighting and displays. Polarization control is also
possible with a PC, and could eliminate for example up to at least
50% loss of backlight power by providing an initially polarized
output in display technology.
[0069] The technological opportunities afforded by PCs combined
with the levels of enhancement possible using QD-embedded PC
devices may be a key enabler for the affordable incorporation of
QDs into novel lighting and display applications. The enhancements
require lower concentrations of QDs and could advance the color
purity and performance of QD-based light sources towards consumer
applications.
[0070] The devices in this example demonstrate the incorporation of
QDs into a replica molded 2-dimensional PC. The PC has distinct
periods in orthogonal axes, allowing one direction of the structure
to resonantly couple the UV LED excitation source to the embedded
QDs, The orthogonal direction resonantly couples to the OD emission
in the visible spectrum, enhancing the extraction of photons normal
to the device surface. These structures have demonstrated combined
excitation and extraction enhancements up to 5.8.times. output
intensity, using an approach that interleaves PC regions and
enables design-selectable resonant properties, allowing different
types of QDs to be embedded into the device and experiencing
simultaneous enhancement from the same excitation source, but
different extracted wavelengths. The resulting pixelated surface on
a flexible substrate enables blending of the color and directional
output of multiple QD emission wavelengths for potential lighting
or display applications.
* * * * *