U.S. patent application number 14/734081 was filed with the patent office on 2015-09-24 for semiconductor nanoparticle-based materials for use in light emitting diodes, optoelectronic displays and the like.
The applicant listed for this patent is Nanoco Technologies Ltd.. Invention is credited to Siobhan S. Daniels, Emma Hogarth, Mark McCairn, Imad Naasani, Hao Pang.
Application Number | 20150270455 14/734081 |
Document ID | / |
Family ID | 44310593 |
Filed Date | 2015-09-24 |
United States Patent
Application |
20150270455 |
Kind Code |
A1 |
Naasani; Imad ; et
al. |
September 24, 2015 |
Semiconductor Nanoparticle-Based Materials For Use in Light
Emitting Diodes, Optoelectronic Displays and the Like
Abstract
A formulation incorporates nanoparticles, particularly quantum
dot (QD) nanoparticles, into an optically clear medium (resin) to
be used as a phosphor material in lighting and display
applications, and as a down converting phosphor material in LEDs
(light emitting diodes). The resin is compatible with QDs to allow
high performance and stability of QD-based LEDs, lighting and
display applications.
Inventors: |
Naasani; Imad; (Manxhester,
GB) ; Pang; Hao; (Sale, GB) ; Daniels; Siobhan
S.; (Hadfield, GB) ; Hogarth; Emma;
(Clayton-Le-Woods, GB) ; McCairn; Mark;
(Littleborough Shore, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nanoco Technologies Ltd. |
Manchester |
|
GB |
|
|
Family ID: |
44310593 |
Appl. No.: |
14/734081 |
Filed: |
June 9, 2015 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
13485292 |
May 31, 2012 |
9082941 |
|
|
14734081 |
|
|
|
|
61493719 |
Jun 6, 2011 |
|
|
|
Current U.S.
Class: |
257/98 ;
252/519.34; 438/27 |
Current CPC
Class: |
H01L 33/507 20130101;
C09K 11/883 20130101; H01L 2933/005 20130101; H01L 2224/48091
20130101; C09K 11/70 20130101; H01L 2933/0041 20130101; C09K 11/703
20130101; H01L 33/502 20130101; H01L 2224/48247 20130101; H01L
2924/16195 20130101; C08L 33/06 20130101; H01L 33/501 20130101;
C08F 222/1006 20130101; C09K 11/02 20130101; H01L 2224/48091
20130101; C09K 11/025 20130101; C08F 220/1812 20200201; H01L 33/56
20130101; C09K 11/565 20130101; H01L 2924/00014 20130101 |
International
Class: |
H01L 33/50 20060101
H01L033/50; C09K 11/02 20060101 C09K011/02; C09K 11/88 20060101
C09K011/88; H01L 33/56 20060101 H01L033/56; C09K 11/70 20060101
C09K011/70 |
Foreign Application Data
Date |
Code |
Application Number |
May 31, 2011 |
GB |
1109054.5 |
Claims
1. A method of preparing a formulation for use in the fabrication
of a light emitting device, said method comprising or consisting
essentially of: incorporating a population of semiconductor
nanoparticles comprising or consisting essentially of ions from
groups 13 and 15 of the Periodic Table into an optically
transparent poly(meth)acrylate encapsulation medium.
2. A method according to claim 1, wherein said poly(meth)acrylate
encapsulation medium is derived from a (meth)acrylate monomer and a
trivalent crosslinking compound.
3. A method of preparing a formulation for use in the fabrication
of a light emitting device, said method comprising or consisting
essentially of: incorporating a population of semiconductor
nanoparticles into an optically transparent poly(meth)acrylate
encapsulation medium derived from a (meth)acrylate monomer and a
trivalent crosslinking compound.
4. A method according to claim 2 or 3, wherein said monomer and
crosslinking compound are reacted in the presence of a
photoinitiator.
5. A method of preparing a formulation for use in the fabrication
of a light emitting device, said method comprising or consisting
essentially of: incorporating a population of semiconductor
nanoparticles into an optically transparent polymeric encapsulation
medium derived from a laurylmethacrylate monomer and a trivalent
crosslinking compound reacted in the presence of a
photoinitiator.
6. A method according to claim 5, wherein said monomer,
crosslinking compound and photoinitiator are combined to provide an
encapsulant precursor mixture to which is then added the
semiconductor nanoparticles prior to polymerisation of said monomer
to provide said optically transparent polymeric encapsulation
medium.
7. A method according to claim 5, wherein said semiconductor
nanoparticles are produced by converting a nanoparticle precursor
composition to the material of the nanoparticles in the presence of
a molecular cluster compound under conditions permitting seeding
and growth of the nanoparticles on the cluster compound.
8. A method according to claim 7, wherein the nanoparticles
incorporate first and second ions and the nanoparticle precursor
composition comprises or consists essentially of separate first and
second nanoparticle precursor species containing said first and
second ions respectively for incorporation into the growing
nanoparticles.
9. A method according to claim 7, wherein the nanoparticles
incorporate first and second ions and the nanoparticle precursor
composition comprises or consists essentially of a single molecular
species containing said first and second ions for incorporation
into the growing nanoparticles.
10. A light emitting device including or consisting essentially of
a primary light source in optical communication with a formulation
comprising or consisting essentially of a population of
semiconductor nanoparticles comprising or consisting essentially of
ions from groups 13 and 15 of the Periodic Table, said
nanoparticles being incorporated into an optically transparent
(meth)acrylate encapsulation medium.
11. A light emitting device including or consisting essentially of
a primary light source in optical communication with a formulation
comprising or consisting essentially of a population of
semiconductor nanoparticles incorporated into an optically
transparent poly(meth)acrylate encapsulation medium derived from a
(meth)acrylate monomer and a trivalent crosslinking compound.
12. A light emitting device including or consisting essentially of
a primary light source in optical communication with a formulation
comprising or consisting essentially of a population of
semiconductor nanoparticles incorporated into an optically
transparent polymeric encapsulation medium derived from a
laurylmethacrylate monomer and a trivalent crosslinking compound
reacted in the presence of a photoinitiator.
13. A device according to claim 10, wherein said primary light
source is selected from the group consisting of a light emitting
diode, a laser, an arc lamp, and a black-body light source.
14. A device according to claim 10, wherein said formulation is in
accordance with claim 1.
15. A method of fabricating a light emitting device comprising or
consisting essentially of: providing a population of semiconductor
nanoparticles comprising or consisting essentially of ions from
groups 13 and 15 of the Periodic Table in an optically transparent
poly(meth)acrylate encapsulation medium to produce a
nanoparticle-containing formulation, and depositing said
formulation onto a primary light source such that said primary
light source is in optical communication with said population of
semiconductor nanoparticles.
16. A method of fabricating a light emitting device comprising or
consisting essentially of: providing a population of semiconductor
nanoparticles in an optically transparent (meth)acrylate
encapsulation derived from a (meth)acrylate monomer and a trivalent
crosslinking compound medium to produce a nanoparticle-containing
formulation, and depositing said formulation onto a primary light
source such that said primary light source is in optical
communication with said population of semiconductor
nanoparticles.
17. A method of fabricating a light emitting device comprising or
consisting essentially of: providing a population of semiconductor
nanoparticles in an optically transparent polymeric encapsulation
medium derived from a laurylmethacrylate monomer and a trivalent
crosslinking compound reacted in the presence of a photoinitiator
to produce a nanoparticle-containing formulation, and depositing
said formulation onto a primary light source such that said primary
light source is in optical communication with said population of
semiconductor nanoparticles.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a division of U.S. patent application
Ser. No. 13/485,292 filed May 31, 2012, which claims priority to
U.S. provisional application 61/493,719, filed Jun. 6, 2011, and
also claims priority to Great Britain application 1109054.5, filed
May 31, 2011, the contents of each of these applications are hereby
incorporated by reference in their entireties.
FIELD OF THE INVENTION
[0002] Embodiments of the present invention relate to semiconductor
nanoparticle--based materials for use in light emitting devices,
such as, but not limited to, light emitting diodes (LEDs) and
optoelectronic displays. Particularly, but not exclusively,
embodiments of the present invention relate to resin formulations
for use in the fabrication of quantum dot (QD) based light emitting
devices, methods for producing said formulations, processes for
fabricating such devices employing said formulations and the
devices thus formed.
BACKGROUND
[0003] Light-emitting diodes (LEDs) are becoming more important to
modern day life and it is envisaged that they will become one of
the major applications in many forms of lighting such as automobile
lights, traffic signals, general lighting, liquid crystal display
(LCD) backlighting and display screens. Currently, LED devices are
made from inorganic solid-state compound semiconductors, such as
AlGaAs (red), AlGaInP (orange-yellow-green), and AlGaInN
(greenblue), however, using a mixture of the available solid-state
compound semiconductors, solid-state LEDs which emit white light
cannot be produced. Moreover, it is difficult to produce "pure"
colors by mixing solid-state LEDs of different frequencies.
Therefore, currently the main method of color mixing to produce a
required color, including white, is to use a combination of
phosphorescent materials which are placed on top of the solid-state
LED whereby the light from the LED (the "primary light") is
absorbed by the phosphorescent material and then re-emitted at a
different frequency (the "secondary light"), i.e. the
phosphorescent materials down convert the primary light to the
secondary light. Moreover, the use of white LEDs produced by
phosphor down-conversion leads to lower cost and simpler device
fabrication than a combination of solid-state red-green-blue
LEDs.
[0004] Current phosphorescent materials used in down converting
applications absorb UV or mainly blue light and converts it to
longer wavelengths, with most phosphors currently using trivalent
rare-earth doped oxides or halophosphates. White emission can be
obtained by blending phosphors which emit in the blue, green and
red regions with that of a blue or UV emitting solid-state device.
i.e. a blue light emitting LED plus a green phosphor such as,
SrGa.sub.2S.sub.4:Eu.sup.2+, and a red phosphor such as,
SrSiEu.sup.2+ or a UV light emitting LED plus a yellow phosphor
such as, Sr.sub.2P.sub.2O.sub.7:Eu.sup.2+;Mu.sup.2+, and a
blue-green phosphor. White LEDs can also be made by combining a
blue LED with a yellow phosphor, however, color control and color
rendering is poor when using this methodology due to lack of
tunability of the LEDs and the phosphor. Moreover, conventional LED
phosphor technology uses down converting materials that have poor
color rendering (i.e. color rendering index (CRI)<75).
[0005] There has been substantial interest in exploiting the
properties of compound semiconductors consisting of particles with
dimensions in the order of 2-50 nm, often referred to as quantum
dots (QDs) or nanocrystals. These materials are of commercial
interest due to their size-tuneable electronic properties which can
be exploited in many commercial applications such as optical and
electronic devices and other applications that ranging from
biological labeling, photovoltaics, catalysis, biological imaging,
LEDs, general space lighting and electroluminescent displays
amongst many new and emerging applications.
[0006] The most studied of semiconductor materials have been the
chalcogenides II-VI materials namely ZnS, ZnSe, CdS, CdSe, CdTe;
most noticeably CdSe due to its tuneability over the visible region
of the spectrum. Reproducible methods for the large scale
production of these materials have been developed from "bottom up"
techniques, whereby particles are prepared atom-by-atom, i.e. from
molecules to, clusters to particles, using "wet" chemical
procedures.
[0007] Two fundamental factors, both related to the size of the
individual semiconductor nanoparticle, are at least in part
responsible for their unique properties. The first is 30 the large
surface to volume ratio; as a particle becomes smaller, the ratio
of the number of surface atoms to those in the interior increases.
This leads to the surface properties playing an important role in
the overall properties of the material. The second factor being,
with many materials including semiconductor nanoparticles, that
there is a change in the electronic properties of the material with
size, moreover, because of quantum confinement effects the band gap
gradually becomes larger as the size of the particle decreases.
This effect is a consequence of the confinement of an `electron in
a box` giving rise to discrete energy levels similar to those
observed in atoms and molecules, rather than a continuous band as
observed in the corresponding bulk semiconductor material. Thus,
for a semiconductor nanoparticle, because of the physical
parameters, the "electron and hole", produced by the absorption of
electromagnetic radiation, a photon, with energy greater than the
first excitonic transition, are closer together than they would be
in the corresponding macrocrystalline material, moreover the
Coulombic interaction cannot be neglected. This leads to a narrow
bandwidth emission, which is dependent upon the particle size and
composition of the nanoparticle material. Thus, QDs have higher
kinetic energy than the corresponding macrocrystalline material and
consequently the first excitonic transition (band gap) increases in
energy with decreasing particle diameter.
[0008] Core semiconductor nanoparticles, which consist essentially
of a single semiconductor material along with an outer organic
passivating layer, tend to have relatively low quantum efficiencies
due to electron-hole recombination occurring at defects and
dangling bonds situated on the nanoparticle surface which can lead
to non-radiative electron-hole recombinations. One method to
eliminate defects and dangling bonds on the inorganic surface of
the QD is to grow a second inorganic material, having a wider
band-gap and small lattice mismatch to that of the core material
epitaxially on the surface of the core particle, to produce a
"core-shell" particle. Coreshell particles separate any carriers
confined in the core from surface states that would otherwise act
as non-radiative recombination centers. One example is a ZnS shell
grown on the surface of a CdSe core. Another approach is to prepare
a core-multi shell structure where the "electron-hole" pair is
completely confined to a single shell layer consisting of a few
monolayers of a specific material such as a quantum dot-quantum
well structure. Here, the core is of a wide band gap material,
followed by a thin shell of narrower band gap material, and capped
with a further wide band gap layer, such as CdS/HgS/CdS grown using
substitution of Hg for Cd on the surface of the core nanocrystal to
deposit just a few monolayers of HgS which is then over grown by a
monolayer of CdS. The resulting structures exhibit clear
confinement of photo-excited carriers in the HgS layer. To add
further stability to QDs and help to confine the electron-hole pair
one of the most common approaches is by epitaxially growing a
compositionally graded alloy layer on the core to alleviate strain
that could otherwise led to defects. Moreover for a CdSe core in
order to improve structural stability and quantum yield, rather
growing a shell of ZnS directly on the core a graded alloy layer of
Cdi.sub.1-xZn.sub.xSe.sub.1-yS.sub.y can be used. This has been
found to greatly enhance the photoluminescence emission of the
QDs.
[0009] Doping QDs with atomic impurities is an efficient way also
of manipulating the emission and absorption properties of the
nanoparticle. Procedures for doping of wide band gap materials,
such as zinc selenide and zinc sulfide, with manganese and copper
(ZnSe:Mn or ZnS:Cu), have been developed. Doping with different
luminescence activators in a semiconducting nanocrystal can tune
the photoluminescence and electroluminescence at energies even
lower than the band gap of the bulk material whereas the quantum
size effect can tune the excitation energy with the size of the QDs
without having a significant change in the energy of the activator
related emission.
SUMMARY AND DETAILED DESCRIPTION
[0010] Embodiments of the present invention obviate or mitigate one
or more of the problems with current methods for fabricating
semiconductor nanoparticle--based light emitting devices. In a
first aspect, embodiments of the present invention provide a
formulation for use in the fabrication of a light emitting device,
said formulation comprising or consisting essentially of a
population of semiconductor nanoparticles comprising or consisting
essentially of ions from groups 13 and 15 of the Periodic Table,
said nanoparticles being incorporated into an optically transparent
poly(meth)acrylate encapsulation medium.
[0011] Embodiments of the present invention relate to the
incorporation of fluorescent semiconductor nanoparticles (e.g.
quantum dots (QDs) into an optically clear and chemically stable
medium, which may be referred to herein as a "resin"--this term may
encompass any suitable host material in which the semiconductor
nanoparticles are incorporated. Embodiments of the present
invention provide formulations or resins incorporating the
nanoparticles alone (the nanoparticles being embedded directly in
the encapsulation medium or resin), nanoparticles contained in or
associated with beads or bead-like architectures, or combinations
thereof.
[0012] The formulation according to various embodiments of the
present invention allows QDs particularly, in particular III-V QDs,
to be used as a phosphor material with high performance, minimal
aggregation, and preserved quantum yield in the final
light-emitting device.
[0013] In a second aspect, embodiments of the present invention
provide a formulation for use in the fabrication of a light
emitting device, said formulation comprising or consisting
essentially of a population of semiconductor nanoparticles
incorporated into an optically transparent poly(meth)acrylate
encapsulation medium derived from a (meth)acrylate monomer and a
trivalent crosslinking compound.
[0014] In a third aspect, embodiments of the present invention
provide a formulation for use in the fabrication of a light
emitting device said formulation comprising or consisting
essentially of a population of semiconductor nanoparticles
incorporated into an optically transparent polymeric encapsulation
medium derived from a laurylmethacrylate monomer and a multivalent
crosslinking compound reacted in the presence of a
photoinitiator.
[0015] The poly(meth)acrylate may be any suitable
(meth)acrylate-based polymer. It preferably incorporates a
medium-to-long carbon chain, such as a C.sub.8-C.sub.20 carbon
chain, more preferably a C.sub.12-C.sub.18 carbon chain. It is
preferred that the poly(meth)acrylate is selected from the group
consisting of polylauryl (meth)acrylate, polystearyl
(meth)acrylate, polytrimethylsilyl (meth)acrylate,
polytrimethylsilyloxyalkyl (meth)acrylate (e.g.
2-(Trimethylsilyloxy)ethyl methacrylate),
polyglycidyl(meth)acrylate, methyl (meth)acrylate, and combinations
thereof. Each of the aforementioned acylates may be substituted or
unsubstituted with one or more chemical groups, such as an alkyl
group, for example a methyl, ethyl or propyl group. Structures of
the monomers from which these polymers may be obtained are set out
below. It is particularly preferred that said poly(meth)acrylate is
polylaurylmethacrylate.
##STR00001##
[0016] In various embodiments, it is preferred that said
poly(meth)acrylate encapsulation medium is derived from a
(meth)acrylate monomer, preferably laurylmethacrylate, and a
trivalent crosslinking compound.
[0017] In embodiments of the present invention, any suitable
multivalent crosslinking agent may be used provided it is
compatible with the (meth)acrylate monomer(s) being used and the
photoinitiator. A trivalent crosslinking compound is preferred,
such as trimethylolpropanetrimethacrylate. The structures of
trimethylolpropanetrimethacrylate and another preferred trivalent
crosslinking compound are set forth below.
##STR00002##
[0018] In various embodiments, the monomer and crosslinking
compound are preferably reacted in the presence of a
photoinitiator. Any suitable photoinitiator may be used provided it
is compatible with the (meth)acrylate monomer(s) and crosslinking
compound being used. A preferred type of photoinitiator is a
bis-acylphosphine oxide (BAPO) photoinitiator, such as
bis(2,4,6-trimethylbenzoyl)phenylphosphineoxide, which is marketed
as Irgacure 819 .RTM.
[0019] The photoinitiator (e.g., Irgacure 819.RTM.) is preferably
dissolved in the crosslinker (e.g.,
trimethylolpropanetrimethacrylate). This may then be added to the
(meth) acrylate monomer(s) to provide an encapsulant precursor
mixture. An aliquot of the mixture is then added to a sample of the
desired semiconductor nanoparticles (e.g. cadmium-free QDs, such as
III-V (e.g. InP) QDs). The nanoparticle-containing mixture may then
be used to fabricate the desired light emitting device. By way of
example, an appropriate volume of the nanoparticle-containing
mixture may be deposited into a cup of an LED or used to make a
phosphor sheet using any appropriate technique for generating a
nanoparticle film (e.g. inkjet printing, casting, doctor blade,
roller coating, screen printing etc). The filled LED or printed
display device may then be irradiated to provide a cured, optically
transparent matrix that incorporates the desired type(s) of
nanoparticles.
[0020] The formulation according to any aspect of the present
invention may include one or more additives to aid the preparation
of the formulation, processibility of the formulation and/or to
enhance the performance of the final device. Additives may include
one or more from the following group: polymerization enhancers
(e.g. benzophenones, BF3); wave-guiding materials (e.g. fumed
silica and its derivatives, polymethylmethacrylate (PMMA)); agents
for increasing viscosity (e.g. fumed silica, hydrophobic polymers,
polylaurylmethacrylate (PLMA), dextrin palmitate); agents for
enhancing light transmittance; and agents for improving the
solubility of the nanoparticles in the encapsulant precursor
mixture (which, in a preferred embodiment, is basic). Additionally
or alternatively the formulation may include additives to enhance
the mechanical and/or tensile properties or the finally cured
material, and/or the weathering of the final device, e.g. an LED
(e.g. TiO.sub.2 nanopowders, silicone hydride containing siloxanes
and siloxane polymers, fumed silica).
[0021] The semiconductor nanoparticles in the first aspect which
contain ions from groups 13 and 15 of the Periodic Table preferably
contain one or more semiconductor material selected from the group
consisting of InP, InAs, InSb, AlP, AlS, AlAs, AlSb, GaN, GaP,
GaAs, GaSb and combinations thereof. In the second and third
aspects of the present invention the semiconductor nanoparticles
may contain ions selected from group 11, 12, 13, 14, 15 and/or 16
of the periodic table, or said semiconductor nanoparticles may
contain one or more types of transition metal ion or d-block metal
ion. By way of example, said semiconductor nanoparticles may
contain one or more semiconductor material selected from the group
consisting of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, InP, InAs, InSb,
AlP, AlS, AlAs, AlSb, GaN, GaP, GaAs, GaSb, PbS, PbSe, Si, Ge, MgS,
MgSe, MgTe, and combinations thereof.
[0022] The semiconductor nanoparticles may be dispersed directly
into the encapsulation medium, incorporated into a plurality of
discrete microbeads which are then dispersed or embedded within
said encapsulation medium, or a combination may be used.
[0023] The term "beads" is used for convenience and is not intended
to impose any particular size or shape limitation. Thus, for
example, the beads may be spherical but other configurations are
possible, such as disc- or rod-like. Where reference is made herein
to "microbeads" this is intended to refer to "beads" as defined
above having a dimension on the micron scale. The
nanoparticle-containing optically transparent medium is generally
provided in the form of a plurality of discrete, i.e. separate or
distinct, microbeads. For the avoidance of doubt, reference to
microbeads as being "discrete" is not intended to exclude composite
materials formed by aggregations of microbeads since even in such
materials each microbead retains its original bead-like structure
despite being in contact with one or more other microbeads. By
pre-loading small microbeads, which may range in size from 50 nm to
500 .mu.m or more preferably 25 nm to 0.1 mm or more preferably
still 20 nm to 0.5 mm in diameter, with QDs, then incorporating one
or more of these QD-containing beads into an LED encapsulation
material on a UV or blue LED, changing, in a controllable and
reproducible manner, the color of the light emitted by the LED
device is facilitated. Moreover, it has been shown that this
approach may be much simpler than attempting to directly
incorporate the QDs directly into an LED encapsulate in terms of
ease of color rendering, processing, and reproducibility and offers
greater QD stability to photooxidation. This approach may lead to
better processing; the QD-containing beads may be made to the same
size as the currently employed YAG phosphor material which range
from 10 to 100 .mu.m and may thus be supplied to commercial
manufacturers in a similar form to that of the current commercially
used phosphor material. Moreover, the QD-containing beads are in a
form that is compatible with the existing LED fabrication
infrastructure.
[0024] The material from which the beads or microbeads are made is
preferably optically transparent medium and may be made in the form
of a resin, polymer, monolith, glass, sol gel, epoxy, silicone,
(meth)acrylate or the like using any appropriate method. It is
preferred that the resulting nanoparticle-containing bead is
suitably compatible with the optically transparent encapsulating
medium to enable the nanoparticle-containing beads to be embedded
within the encapsulant such that the chemical and physical
structure of the resulting composite material (i.e. the encapsulant
with nanoparticle-containing beads embedded therein) remains
substantially unchanged during further processing to incorporate
the composite into a light emitting device and during operation of
the resulting device over a reasonable lifetime for the device.
Suitable bead materials include: poly(methyl (meth)acrylate)
(PMMA); poly(ethylene glycol dimethacrylate) (PEGMA); poly(vinyl
acetate) (PVA); poly(divinyl benzene) (PDVB); poly(thioether);
silane monomers; epoxy polymers; and combinations thereof. A
particularly preferred bead material which has been shown to
exhibit excellent processibility and light emitting device
performance comprises or consists essentially of a copolymer of
PMMA, PEGMA and PVA. Other preferred bead materials employ
polystyrene microspheres with divinyl benzene and a thiol
co-monomer; silane monomers (e.g. 3-(trimethoxysilyl)
propylmethacrylate (TMOPMA) and tetramethoxy silane (TEOS)); and an
epoxy polymer (e.g. Optocast.TM. 3553 from Electronic Materials,
Inc., USA).
[0025] By incorporating at least some of the QDs into an optically
transparent, preferably clear, stable bead material, the otherwise
reactive QDs may be protected from the potentially damaging
surrounding chemical environment. Moreover, by placing a number of
QDs into a single bead, for example in the size range from 20 nm to
500 .mu.m in diameter, the subsequent QD-bead may be more stable,
than free or "naked" QDs, to the types of chemical, mechanical,
thermal and photoprocessing steps which are required to incorporate
QDs in most commercial applications, such as when employing quantum
dots as down converters in a "QD-solid-state-LED" light emitting
device.
[0026] The formulation according to embodiments of the present
invention may contain a population of semiconductor nanoparticles
distributed across a plurality of beads embedded within the
optically transparent encapsulating medium. Any desirable number of
beads may be embedded, for example, the encapsulating medium may
contain 1 to 10,000 beads, more preferably 1 to 5000 beads, and
most preferably 5 to 1000 beads.
[0027] Some or all of the nanoparticle-containing microbeads may
include a core comprising or consisting essentially of a first
optically transparent material and one or more outer layers or
shells of the same or one or more different optically transparent
materials deposited on said core. Nanoparticles may be confined to
the core region of the microbeads or may be dispersed throughout
the core and/or one or more of the shell layers of the
microbeads.
[0028] Advantages of QD-containing beads over free QDs may include
greater stability to air and moisture, greater stability to
photo-oxidation and greater stability to mechanical processing.
Moreover, by pre-loading small microbeads, which may range in size
from a few 50 nm to 500 .mu.m, with QDs then incorporating one or
more of these QD-containing beads into the encapsulating medium on
a UV or blue LED, it may be a relatively simple process to change,
in a controllable and reproducible manner, the color of the light
emitted by the LED-based light emitting device.
[0029] While incorporating semiconductor nanoparticles into beads
may afford some or all of the aforementioned advantages, it is
preferred that the formulation according to the first aspect of the
present invention comprises or consists essentially of at least
some semiconductor nanoparticles comprising or consisting
essentially of ions from groups 13 and 15 of the Periodic Table
which are dispersed directly into the encapsulation medium. That
is, the formulation according to the first aspect preferably
contains semiconductor nanoparticles containing group 13 and 15
ions which are not incorporated into beads or microbeads, and
optionally contains further semiconductor nanoparticles, which may
or may not contain group 13 and/or 15 ions, 10 that are
incorporated into beads or microbeads. In this way, the optical
properties of the final device may be optimized and/or maximized
for a particular application. For example, by using a mixture of
bead-encapsulated QDs with non-bead encapsulated QDs (i.e. "bare"
or "naked" QDs), the final device may benefit from both the
advantages of the bead-encapsulated QDs in terms of robustness,
processibility etc. while also benefiting from advantages
associated with using QDs dispersed directly within the
encapsulating medium, such as increased brightness, optical clarity
and/or quantum yield due to the omission of the additional layer of
encapsulating bead material and the associated additional
processing steps required to combine the QDs with the beads. It is
further preferred that the formulations according to the second
and/or third aspects of the present invention comprise or consist
essentially of at least some semiconductor nanoparticles which are
dispersed directly into the encapsulation medium. In a particularly
preferred embodiment of the first, second and/or third aspect of
the present invention all of the semiconductor nanoparticles are
dispersed directly into the encapsulation medium, i.e. none of the
semiconductor nanoparticles are incorporated into beads or
microbeads dispersed within the encapsulation medium. In this way,
the advantages of providing the QDs directly into the encapsulation
medium, rather than first incorporating them into beads, may be
obtained.
[0030] In a fourth aspect, embodiments of the present invention
provide a method of preparing a formulation for use in the
fabrication of a light emitting device, said method comprising or
consisting essentially of incorporating a population of
semiconductor nanoparticles comprising or consisting essentially of
ions from groups 13 and 15 of the Periodic Table into an optically
transparent poly(meth)acrylate encapsulation medium.
[0031] The poly(meth)acrylate encapsulation medium is preferably
derived from a (meth)acrylate monomer and a trivalent crosslinking
compound. It is preferred that the fourth aspect of the present
invention is applied to produce a formulation according to the
first aspect of the present invention defined above or a preferred
embodiment thereof.
[0032] In a fifth aspect, embodiments of the present invention
provide a method of preparing a formulation for use in the
fabrication of a light emitting device, said method comprising or
consisting essentially of incorporating a population of
semiconductor nanoparticles into an optically transparent
poly(meth)acrylate encapsulation medium derived from a
(meth)acrylate monomer and a trivalent crosslinking compound.
[0033] The monomer and crosslinking compound are preferably reacted
in the presence of a photoinitiator. It is preferred that the fifth
aspect of the present invention is applied to produce a formulation
according to the second aspect of the present invention defined
above or a preferred embodiment thereof.
[0034] In a sixth aspect, embodiments of the present invention
provide a method of preparing a formulation for use in the
fabrication of a light emitting device, said method comprising or
consisting essentially of incorporating a population of
semiconductor nanoparticles into an optically transparent polymeric
encapsulation medium derived from a laurylmethacrylate monomer and
a trivalent crosslinking compound reacted in the presence of a
photoinitiator.
[0035] It is preferred that the sixth aspect of the present
invention is applied to produce a formulation according to the
third aspect of the present invention defined above or a preferred
embodiment thereof.
[0036] In the fourth, fifth and sixth aspects of the present
invention it is preferred that said monomer, crosslinking compound
and photoinitiator are combined to provide an encapsulant precursor
mixture to which is then added the semiconductor nanoparticles
prior to polymerization of said monomer to provide said polymeric
optically transparent encapsulation medium. In this way the need to
include a nanoparticle capping agent, such as TOP or TOPO, in the
mixture containing the nanoparticles and the polymerizable monomer
may be avoided.
[0037] In the fourth, fifth and sixth aspects of the present
invention it is preferred that said semiconductor nanoparticles are
produced by converting a nanoparticle precursor composition to the
material of the nanoparticles in the presence of a molecular
cluster compound under conditions permitting seeding and growth of
the nanoparticles on the cluster compound. The nanoparticles may
incorporate first and second ions, in which case the nanoparticle
precursor composition may comprise or consist essentially of
separate first and second nanoparticle precursor species containing
said first and second ions respectively for incorporation into the
growing nanoparticles or the first and second ions may be combined
into a single molecular species containing both types of ions for
incorporation into the growing nanoparticles.
[0038] In a seventh aspect, embodiments of the present invention
provide a light emitting device including or consisting essentially
of a primary light source in optical communication with a
formulation comprising or consisting essentially of a population of
semiconductor nanoparticles comprising or consisting essentially of
ions from groups 13 and 15 of the Periodic Table, said
nanoparticles being incorporated into an optically transparent
(meth)acrylate encapsulation medium.
[0039] In a further aspect, embodiments of the present invention
provide a method of fabricating a light emitting device comprising
or consisting essentially of providing a population of
semiconductor nanoparticles comprising or consisting essentially of
ions from groups 13 and 15 of the Periodic Table in an optically
transparent poly(meth)acrylate encapsulation medium to produce a
nanoparticle-containing formulation, and depositing said
formulation onto a primary light source such that said primary
light source is in optical communication with said population of 30
semiconductor nanoparticles.
[0040] In an eighth aspect, embodiments of the present invention
provide a light emitting device including a primary light source in
optical communication with a formulation comprising or consisting
essentially of a population of semiconductor nanoparticles
incorporated into an optically transparent poly(meth)acrylate
encapsulation medium derived from a (meth)acrylate monomer and a
trivalent crosslinking compound.
[0041] In another aspect, embodiments of the present invention
provide a method of fabricating a light emitting device comprising
or consisting essentially of providing a population of
semiconductor nanoparticles in an optically transparent
(meth)acrylate encapsulation derived from a (meth)acrylate monomer
and a trivalent crosslinking compound medium to produce a
nanoparticle-containing formulation, and depositing said
formulation onto a primary light source such that said primary
light source is in optical communication with said population of
semiconductor nanoparticles.
[0042] In a ninth aspect, embodiments of the present invention
provide a light emitting device including a primary light source in
optical communication with a formulation comprising or consisting
essentially of a population of semiconductor nanoparticles
incorporated into an optically transparent polymeric encapsulation
medium derived from a laurylmethacrylate monomer and a trivalent
crosslinking compound reacted in the presence of a
photoinitiator.
[0043] In a further aspect, embodiments of the present invention
provide a method of fabricating a light emitting device comprising
or consisting essentially of providing a population of
semiconductor nanoparticles in an optically transparent polymeric
encapsulation medium derived from a laurylmethacrylate monomer and
a trivalent crosslinking compound reacted in the presence of a
photoinitiator to produce a nanoparticle-containing formulation,
and depositing said formulation onto a primary light source such
that said primary light source is in optical communication with
said population of semiconductor nanoparticles.
[0044] The primary light source in any of the above-defined devices
or methods may be selected from the group consisting of a light
emitting diode, a laser, an arc lamp, and a black-body light
source.
[0045] In each of the devices according to the seventh, eighth and
ninth aspects of the present invention it is preferred that said
formulation is in accordance with the first, second or third
aspects of the present invention respectively or preferred
embodiments thereof.
[0046] Embodiments of the current invention provide a semiconductor
nanoparticle formulation for use in the fabrication of light
emitting devices, preferably with the devices incorporating an LED
as a primary light source and the semiconductor nanoparticles as a
secondary light source. In a preferred embodiment the formulation
contains one or more types of semiconductor nanoparticles, such as
QDs, incorporated into a plurality of polymeric beads which are
embedded or entrapped within the optically transparent
poly(meth)acrylate encapsulation medium.
[0047] In the Comparative Example below an LED-based light emitting
device incorporating a formulation according to embodiments of the
present invention is tested against a light emitting device
incorporating similar QDs embedded directly in a commercially
available silicone LED encapsulant analogous to prior art devices.
The device incorporating the formulation according to embodiments
of the present invention was observed to perform significantly
better than the prior art device in that it exhibited a
significantly enhanced LED lifetime as compared to device
incorporating QDs dispersed in the silicone LED encapsulant.
[0048] The optically transparent medium may contain any desirable
number and/or type of semiconductor nanoparticles. The medium may
contain a single type of semiconductor nanoparticle, e.g. InP or
CdSe, of a specific size range, such that the composite material
incorporating the nanoparticles incorporated within the medium
emits monochromatic light of a pre-defined wavelength, i.e. color.
The color of the emitted light may be adjusted by varying the type
of semiconductor nanoparticle material used, e.g. changing the size
of the nanoparticle, the nanoparticle core semiconductor material,
and/or adding one or more outer shells of different semiconductor
materials. Moreover, color control may also be achieved by
incorporating different types of semiconductor nanoparticles, for
example nanoparticles of different size and/or chemical composition
within the optically transparent medium. Furthermore, the color and
colour intensity may be controlled by selecting an appropriate
number of semiconductor nanoparticles within the optically
transparent medium. Preferably the medium contains at least around
1000 semiconductor nanoparticles of one or more different types,
more preferably at least around 10,000, more preferably at least
around 50,000, and most preferably at least around 100,000
semiconductor nanoparticles of one or more different types.
Color Indexing
[0049] The color of the light output from the QD-bead-LED (the
"secondary light") may be measured using a spectrometer. The
spectral output (mW/nm) may then be processed mathematically so
that the particular color of the light emitting device may be
expressed as color coordinates on a chromaticity diagram, for
example the 2.degree. CIE 1931 chromaticity diagram.
[0050] The 2.degree. CIE 1931 chromaticity coordinates for a
particular spectrum may be calculated from the spectral power
distribution and the CIE 1931 color matching functions x, y, z. The
corresponding tristimulus values may be calculated thus
TABLE-US-00001 X = .intg. px d .lamda. Y = .intg. py d .lamda. Z =
.intg. pz d .lamda.
[0051] Where p is the spectral power, and x, y and z are the color
matching functions. From X, Y, and Z the chromaticity coordinates
x, y may be calculated according to
x = X X + Y + Z and y = Y X + Y + Z ##EQU00001##
[0052] Using x, y as the coordinates, a two-dimensional
chromaticity diagram (the CIE 1931 color space diagram) may be
plotted.
Color Rendering
[0053] Color rendering describes the ability of a light source to
illuminate objects such that they appear the correct color when
compared to how they appear when illuminated by a reference light
source. Usually the reference light source is a tungsten filament
bulb which is assigned a color rendering index (CRI) of 100. To be
acceptable for general lighting, a white light emitting device
source is typically required to have a CRI>80. An example of
poor color rendering is the sodium street lamp which has very poor
color rendering capability, i.e. it is difficult to distinguish a
red car from a yellow car illuminated by a sodium lamp, in the dark
under a sodium lamp they both appear grey.
[0054] Embodiments of the present invention provide a
light-emitting device comprising or consisting essentially of a
population of QDs incorporated into an optically transparent
medium. The QDs within the optically transparent medium are in
optical communication with a primary solid-state photon/light
source (e.g. an LED, laser, arc lamp or black-body light source)
such that, upon excitation by primary light from the primary light
source the QDs within the optically transparent medium emit
secondary light of a desired color. The required intensities and
emission wavelengths of the light emitted from the device itself
may be selected according to appropriate mixing of the color of the
primary light with that of the secondary light(s) produced from the
down conversion of the primary light by the QDs. Moreover, the size
(and thus emission) and number of each type of QD within the
optically transparent medium may be controlled, as may the size,
morphology and constituency of the optically transparent medium,
such that subsequent mixing of the QD-containing media enables
light of any particular color and intensity to be produced.
[0055] It will be appreciated that the overall light emitted from
the device may consist essentially of only the light emitted from
the QDs, i.e. just the secondary light, or a mixture of light
emitted from the QDs and light emitted from the solid-state/primary
light source, i.e. a mixture of the primary and secondary light.
Color mixing of the QDs may be achieved either within the
QDcontaining media or a mixture of differently colored optically
transparent media with all the QDs within a specific medium being
the same size/color (e.g. some containing all green QDs and others
containing all red QDs).
[0056] Embodiments of the present invention are illustrated with
reference to the following nonlimiting examples and figures in
which:
[0057] FIG. 1 is a schematic representation of a QD-based light
emitting device according to various embodiments of the present
invention;
[0058] FIG. 2 is an ambient light photo of an LED filled with a
cadmium-free QD acrylate resin according to various embodiments of
the present invention; and
[0059] FIG. 3 is a plot of QD-photoluminescence intensity expressed
as a percentage of the initial value versus time for the a device
according to various embodiments of the present invention and a
device prepared using a common silicone QD encapsulant resin.
EXAMPLES
[0060] The Example below describes the preparation of QD-containing
formulations for use in the fabrication of new, improved QD-based
light emitting devices in accordance with embodiments of the
present invention. In the Comparative Example a device in
accordance with embodiments of the present invention is tested
against a device based on prior art principles using the same type
of QDs to compare the performance of the two devices. Two methods
for producing QDs suitable for incorporation into said formulations
are first set out in the Synthetic Methods section below.
Synthetic Methods
Method 1
Preparation of CdSelHDA Capped Nanoparticles
[0061] HDA (500 g) was placed in a three-neck round bottomed flask
and dried and degassed by heating to 120.degree. C. under a dynamic
vacuum for >1 hour. The solution was then cooled to 60.degree.
C. To this was added 0.718 g of
[HNEt.sub.3].sub.4[Cd.sub.10Se.sub.4(SPh).sub.16] (0.20 mmols). In
total 42 mmols, 22.0 ml of TOPSe and 42 mmols, (19.5 ml, 2.15M) of
Me.sub.2Cd.TOP was used. Initially 4 mmol of TOPSe and 4 mmols of
Me.sub.2Cd.TOP were added to the reaction at room temperature and
the temperature increased to 110.degree. C. and allowed to stir for
2 hours. The reaction mixture was a deep yellow color. The
temperature was progressively increased at a rate of
.about.1.degree. C./5 min with equimolar amounts of TOPSe and
Me.sub.2Cd.TOP being added dropwise. The reaction was stopped when
the photoluminescence (PL) emission maximum had reached around 600
nm, by cooling to 60.degree. C. followed by addition of 300 ml of
dry ethanol or acetone. This produced a precipitation of deep red
particles, which were further isolated by filtration. The resulting
CdSe particles were recrystallized by re-dissolving in toluene
followed by filtering through Celite followed by reprecipitation
from warm ethanol to remove any excess HDA, selenium or cadmium
present. This produced 10.10 g of HDA capped CdSe nanoparticles.
Elemental analysis C=20.88%, H=3.58%, N=1.29%, Cd=46.43%. Max
PL=585 nm, FWHM=35 nm. 38.98 mmols, 93% of Me.sub.2Cd consumed in
forming the QDs.
Preparation of CdSe/ZnS-HDA Capped Nanoparticles
[0062] HDA (800 g) was placed in a three neck round-bottom flask,
dried and degassed by heating to 120.degree. C. under a dynamic
vacuum for >1 hour. The solution was then cooled to 60.degree.
C. To this was added 9.23 g of CdSe nanoparticles that have a PL
maximum emission of 585 nm. The HDA was then heated to 220.degree.
C. To this was added by alternate dropwise addition a total of 20
ml of 0.5M Me.sub.2Zn.TOP and 0.5M, 20 ml of sulfur dissolved in
octylamine. Three alternate additions of 3.5, 5.5 and 11.0 ml of
each were made, whereby initially 3.5 ml of sulfur was added
dropwise until the intensity of the PL maximum was near zero. Then
3.5 ml of Me.sub.2Zn.TOP was added dropwise until the intensity of
the PL maximum had reached a maximum. This cycle was repeated with
the PL maximum reaching a higher intensity with each cycle. On the
last cycle, additional precursor was added once the PL maximum
intensity been reached until it was between 5-10% below the maximum
intensity, and the reaction was allowed to anneal at 150.degree. C.
for 1 hour. The reaction mixture was then allowed to cool to
60.degree. C. whereupon 300 ml of dry "warm" ethanol was added
which resulted in the precipitation of particles. The resulting
CdSeZnS particles were dried before re-dissolving in toluene and
filtering through Celite followed by re-precipitation from warm
ethanol to remove any excess HDA. This produced 12.08 g of HDA
capped CdSe--ZnS core-shell nanoparticles. Elemental analysis
C=20.27%, H=3.37%, N=1.25%, Cd=40.11%, Zn=4.43%; Max PL 590 nm,
FWHM 36 nm.
Method 2
Preparation of InP Core Quantum Dots (500-700 nm Emission)
[0063] Di-butyl ester (100 ml) and myristic acid (10.0627 g) were
placed in a three-neck flask and degassed at 70.degree. C. under
vacuum for one hour. After this period, nitrogen was introduced and
the temperature increased to 90.degree. C. ZnS molecular cluster
[Et.sub.3NH.sub.4][Zn.sub.10S.sub.4(SPh).sub.16] (4.7076 g) was
added and the mixture allowed to stir for 45 minutes. The
temperature was then increased to 100.degree. C. followed by the
dropwise addition of In(MA).sub.3 (1M, 15 ml) followed by
(TMS).sub.3P (1M, 15 ml). The reaction mixture was allowed to stir
while increasing the temperature to 140.degree. C. At 140.degree.
C., 35 further dropwise additions of In(MA).sub.3 (1M, 35 ml) (left
to stir for 5 minutes) and (TMS).sub.3P (1M, 35 ml) were made. The
temperature was then slowly increased to 180.degree. C. and further
dropwise additions of In(MA)3 (1M, 55 ml) followed by (TMS).sub.3P
(1M, 40 ml) were made. By addition of the precursor in the manner
above nanoparticles of InP could be grown with the emission maximum
gradually increasing from 520 nm up to 700 nm, whereby the reaction
may be stopped when the desired emission maximum has been obtained
and left to stir at this temperature for half an hour. After this
period, the temperature was decreased to 160.degree. C. and the
reaction mixture was left to anneal for up to 4 days (at a
temperature between 20-40.degree. C. below that of the reaction). A
UV lamp was also used at this stage to aid in annealing.
[0064] The nanoparticles were isolated by the addition of dried
degassed methanol (approx. 200 ml) via cannula techniques. The
precipitate was allowed to settle and then methanol was removed via
cannula with the aid of a filter stick. Dried degassed chloroform
(approx. 10 ml) was added to wash the solid. The solid was left to
dry under vacuum for 1 day. This produced 5.60 g of InP core
nanoparticles. Max PL=630 nm, FWHM=70 nm.
Post-Operative Treatments
[0065] The quantum yields of the InP QDs prepared above were
increased by washing with dilute HF acid. The dots were dissolved
in anhydrous degassed chloroform (.about.270 ml). A 50 ml portion
was removed and placed in a plastic flask, flushed with nitrogen.
Using a plastic syringe, the HF solution was made up by adding 3 ml
of 60% w/w HF in water and adding to degassed THF (17 ml). The HF
was added dropwise over 5 hours to the InP dots. After addition was
complete the solution was left to stir overnight. Excess HF was
removed by extracting through calcium chloride solution in water
and drying the etched InP dots. The dried dots were redispersed in
50 ml chloroform for future use. PL max 567 nm, FWHM 60 nm. The
quantum efficiencies of the core materials at this stage range from
25-90%.
Growth of a ZnS Shell to Provide InPlZnS Core/Shell Quantum
Dots
[0066] A 20 ml portion of the HF-etched InP core particles was
dried down in a 3-neck flask. 1.3 g myristic acid and 20 ml
di-n-butyl sebacate ester was added and degassed for 30 minutes.
The solution was heated to 200.degree. C. then 1.2 g anhydrous zinc
acetate was added and 2 ml 1M (TMS).sub.2S was added dropwise (at a
rate of 7.93 ml/hr) after addition was complete the solution was
left to stir. The solution was kept at 200.degree. C. for 1 hour
then cooled to room temperature. The particles were isolated by
adding 40 ml of anhydrous degassed methanol and centrifuged. The
supernatant liquid was disposed of and to the remaining solid 30 ml
of anhydrous degassed hexane was added. The solution was allowed to
settle for 5 hours and then re-centrifuged. The supernatant liquid
was collected and the remaining solid was discarded. PL emission
peak Max.=535 nm, FWHM=65 nm. The quantum efficiencies of the
nanoparticle core/shell materials at this stage ranged from
35-90%.
EXAMPLE
QD-Containing LED Preparation
[0067] A solution of cadmium-free quantum dots in toluene (e.g. 20
mg of InP/ZnS core/shell QDs produced as described above) is dried
under vacuum to leave a QD residue. To the residue an acrylate
monomeric mixture was added and the dots incubated until a clear
solution was formed. The acrylate mixture was composed of
laurylmethacrylate (1.85 ml, 6.6 mmol), a photoinitiator (Irgacure
819.RTM. (bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide), 9 mg)
dissolved in a crosslinker (trimethylolpropanetrimethacrylate (1.06
ml, 3.3 mmol)).
[0068] An aliquot of the monomer mixture (e.g. 1.5 to 3111)
containing the QDs was used to fill a cup of an LED. The filled LED
was then irradiated (Hamamatsu uv-LED lamp LC-L2, 365 nm, 7500
mW/cm.sup.2, 10 min, 1 LED above & 1 LED below 8cm distance) to
provide a QD-polymethacrylate filled LED having the structure shown
in FIGS. 1 and 2. With reference to FIG. 1 there is shown a light
emitting device 1 comprising a conventional LED package 2 with a
standard LED chip 3. Directly on top of the LED chip 3 within the
LED well 4 is provided a sufficient volume of a commercially
available silicone resin 5 so as to cover and submerge the LED chip
3. A sufficient volume of the QD-monomer mixture 6 is provided on
top of the silicone layer 5 so as to substantially fill the LED
well 4. Since the silicone resin 5 submerges the LED chip 3 there
is a space between the chip 3 and the QD-monomer mixture 6 that is
filled with the silicone resin 5. In this way, the QD-containing
mixture 6 is insulated from the potentially harmful high
temperatures generated by the chip 3 during operation. A UV curing
epoxy resin 7 is provided around the periphery of the opening to
the LED well 4, upon which is provided a thin layer of an
encapsulating material 8, such as glass. The epoxy resin 7 and
QD-monomer mixture 6 are then exposed to UV radiation as described
above to cure the resin 7 and seal the device 1, and to polymerize
and crosslink the QD-monomer mixture 6.
COMPARATIVE EXAMPLE
[0069] LEDs were fabricated using either a standard commercially
available silicon resin (SCR1011.RTM., ShinEtsu) or an acrylate
resin according to on the current invention. The LEDs were operated
at 20 mAmp and generated a 450 nm blue light with 22 mW intensity.
FIG. 3 demonstrates the difference in the stability and performance
of the LEDs. The acrylate based resin according to embodiments of
the present invention provided a remarkable increase in
stabilization of the encapsulated QDs on the LED as compared to the
LED incorporating QDs dispersed in the commercially available
silicon resin.
* * * * *