U.S. patent application number 13/663300 was filed with the patent office on 2014-05-01 for semiconductor structure having nanocrystalline core and nanocrystalline shell pairing with compositional transition layer.
The applicant listed for this patent is Juanita N. Kurtin. Invention is credited to Juanita N. Kurtin.
Application Number | 20140117311 13/663300 |
Document ID | / |
Family ID | 50546171 |
Filed Date | 2014-05-01 |
United States Patent
Application |
20140117311 |
Kind Code |
A1 |
Kurtin; Juanita N. |
May 1, 2014 |
SEMICONDUCTOR STRUCTURE HAVING NANOCRYSTALLINE CORE AND
NANOCRYSTALLINE SHELL PAIRING WITH COMPOSITIONAL TRANSITION
LAYER
Abstract
Semiconductor structures having a nanocrystalline core and
nanocrystalline shell pairing compositional transition layers are
described. In an example, a semiconductor structure includes a
nanocrystalline core composed of a first semiconductor material. A
nanocrystalline shell composed of a second semiconductor material
surrounds the nanocrystalline core. A compositional transition
layer is disposed between, and in contact with, the nanocrystalline
core and nanocrystalline shell and has a composition intermediate
to the first and second semiconductor materials. In another
example, a semiconductor structure includes a nanocrystalline core
composed of a first semiconductor material. A nanocrystalline shell
composed of a second semiconductor material surrounds the
nanocrystalline core. A nanocrystalline outer shell surrounds the
nanocrystalline shell and is composed of a third semiconductor
material. A compositional transition layer is disposed between, and
in contact with, the nanocrystalline shell and the nanocrystalline
outer shell and has a composition intermediate to the second and
third semiconductor materials.
Inventors: |
Kurtin; Juanita N.;
(Hillsboro, OR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kurtin; Juanita N. |
Hillsboro |
OR |
US |
|
|
Family ID: |
50546171 |
Appl. No.: |
13/663300 |
Filed: |
October 29, 2012 |
Current U.S.
Class: |
257/22 ; 257/14;
257/E29.071; 257/E29.075 |
Current CPC
Class: |
H01L 33/502 20130101;
C09K 11/025 20130101; H01L 29/127 20130101; H01L 33/06 20130101;
H01L 29/221 20130101; B82Y 10/00 20130101; C09K 11/883 20130101;
H01L 29/0665 20130101; B82Y 20/00 20130101 |
Class at
Publication: |
257/22 ; 257/14;
257/E29.071; 257/E29.075 |
International
Class: |
H01L 29/15 20060101
H01L029/15; H01L 29/12 20060101 H01L029/12 |
Claims
1. A semiconductor structure, comprising: a nanocrystalline core
comprising a first semiconductor material; a nanocrystalline shell
comprising a second, different, semiconductor material at least
partially surrounding the nanocrystalline core; and a compositional
transition layer disposed between, and in contact with, the
nanocrystalline core and nanocrystalline shell, the compositional
transition layer having a composition intermediate to the first and
second semiconductor materials.
2. The semiconductor structure of claim 1, wherein the
compositional transition layer is an alloyed layer comprising a
mixture of the first and second semiconductor materials.
3. The semiconductor structure of claim 1, wherein the
compositional transition layer is a graded layer comprising a
compositional gradient of the first semiconductor material
proximate to the nanocrystalline core through to the second
semiconductor material proximate to the nanocrystalline shell.
4. The semiconductor structure of claim 1, wherein the
compositional transition layer has a thickness approximately in the
range of 1.5-2 monolayers.
5. The semiconductor structure of claim 1, wherein the first
semiconductor material is cadmium selenide (CdSe), the second
semiconductor material is cadmium sulfide (CdS), and the
compositional transition layer comprises CdSe.sub.xS.sub.y, where
0<x<1 and 0<y<1.
6. The semiconductor structure of claim 1, wherein the first
semiconductor material is cadmium selenide (CdSe), the second
semiconductor material is zinc selenide (ZnSe), and the
compositional transition layer comprises Cd.sub.xZn.sub.ySe, where
0<x<1 and 0<y<1.
7. The semiconductor structure of claim 1, wherein the
compositional transition layer passivates or reduces trap states
where the nanocrystalline shell surrounds the nanocrystalline
core.
8. The semiconductor structure of claim 1, wherein the
nanocrystalline core is an anisotropic nanocrystalline core having
an aspect ratio between, but not including, 1.0 and 2.0.
9. The semiconductor structure of claim 8, wherein the
nanocrystalline shell is an anisotropic nanocrystalline shell
having an aspect ratio approximately in the range of 4-6.
10. The semiconductor structure of claim 8, wherein the aspect
ratio of the anisotropic nanocrystalline core is approximately in
the range of 1.01-1.2.
11. The semiconductor structure of claim 1, further comprising: an
insulator coating surrounding and encapsulating the nanocrystalline
core/nanocrystalline shell pairing.
12. The semiconductor structure of claim 11, wherein the insulator
coating comprises an amorphous material selected from the group
consisting of silicon dioxide (SiO.sub.2), silicon oxide
(SiO.sub.x), aluminum oxide (Al.sub.2O.sub.3), zirconia
(ZrO.sub.x), titania (TiO.sub.x), and hafnia (HfO.sub.x).
13. The semiconductor structure of claim 1, wherein the
nanocrystalline shell completely surrounds the nanocrystalline
core.
14. The semiconductor structure of claim 1, wherein the
nanocrystalline shell only partially surrounds the nanocrystalline
core, exposing a portion of the nanocrystalline core.
15. The semiconductor structure of claim 1, wherein the
nanocrystalline core is disposed in an asymmetric orientation with
respect to the nanocrystalline shell.
16. The semiconductor structure of claim 1, wherein the
nanocrystalline core, nanocrystalline shell and compositional
transition layer form a quantum dot.
17. The semiconductor structure of claim 16, wherein the quantum
dot is a down-converting quantum dot.
18. The semiconductor structure of claim 16, wherein the quantum
dot is an up-shifting quantum dot.
19. The semiconductor structure of claim 1, further comprising: a
nanocrystalline outer shell at least partially surrounding the
nanocrystalline shell, the nanocrystalline outer shell comprising a
third semiconductor material different from the first and second
semiconductor materials.
20. The semiconductor structure of claim 19, further comprising: a
second compositional transition layer disposed between, and in
contact with, the nanocrystalline shell and the nanocrystalline
outer shell, the second compositional transition layer having a
composition intermediate to the second and third semiconductor
materials.
21. The semiconductor structure of claim 20, wherein the second
compositional transition layer is an alloyed layer comprising a
mixture of the second and third semiconductor materials.
22. The semiconductor structure of claim 20, wherein the second
compositional transition layer is a graded layer comprising a
compositional gradient of the second semiconductor material
proximate to the nanocrystalline shell through to the third
semiconductor material proximate to the nanocrystalline outer
shell.
23. The semiconductor structure of claim 20, wherein the second
compositional transition layer has a thickness approximately in the
range of 1.5-2 monolayers.
24. The semiconductor structure of claim 20, wherein the first
semiconductor material is cadmium selenide (CdSe), the second
semiconductor material is cadmium sulfide (CdS), the third
semiconductor material is zinc sulfide (ZnS), and the second
compositional transition layer comprises Cd.sub.xZn.sub.yS, where
0<x<1 and 0<y<1.
25. The semiconductor structure of claim 20, wherein the first
semiconductor material is cadmium selenide (CdSe), the second
semiconductor material is zinc selenide (ZnSe), the third
semiconductor material is zinc sulfide (ZnS), and the second
compositional transition layer comprises ZnSe.sub.xS.sub.y, where
0<x<1 and 0<y<1.
26. The semiconductor structure of claim 20, wherein the second
compositional transition layer passivates or reduces trap states
where the nanocrystalline outer shell surrounds the nanocrystalline
shell.
27. A semiconductor structure, comprising: a nanocrystalline core
comprising a first semiconductor material; a nanocrystalline shell
comprising a second, different, semiconductor material at least
partially surrounding the nanocrystalline core; a nanocrystalline
outer shell at least partially surrounding the nanocrystalline
shell, the nanocrystalline outer shell comprising a third
semiconductor material different from the first and second
semiconductor materials; and a compositional transition layer
disposed between, and in contact with, the nanocrystalline shell
and the nanocrystalline outer shell, the compositional transition
layer having a composition intermediate to the second and third
semiconductor materials.
28. The semiconductor structure of claim 27, wherein the
compositional transition layer is an alloyed layer comprising a
mixture of the second and third semiconductor materials.
29. The semiconductor structure of claim 27, wherein the
compositional transition layer is a graded layer comprising a
compositional gradient of the second semiconductor material
proximate to the nanocrystalline shell through to the third
semiconductor material proximate to the nanocrystalline outer
shell.
30. The semiconductor structure of claim 27, wherein the
compositional transition layer has a thickness approximately in the
range of 1.5-2 monolayers.
31. The semiconductor structure of claim 27, wherein the first
semiconductor material is cadmium selenide (CdSe), the second
semiconductor material is cadmium sulfide (CdS), the third
semiconductor material is zinc sulfide (ZnS), and the compositional
transition layer comprises Cd.sub.xZn.sub.yS, where 0<x<1 and
0<y<1.
32. The semiconductor structure of claim 27, wherein the first
semiconductor material is cadmium selenide (CdSe), the second
semiconductor material is zinc selenide (ZnSe), the third
semiconductor material is zinc sulfide (ZnS), and the compositional
transition layer comprises ZnSe.sub.xS.sub.y, where 0<x<1 and
0<y<1.
33. The semiconductor structure of claim 27, wherein the
compositional transition layer passivates or reduces trap states
where the nanocrystalline outer shell surrounds the nanocrystalline
shell.
34. The semiconductor structure of claim 27, wherein the
nanocrystalline shell is an anisotropic nanocrystalline shell
having an aspect ratio approximately in the range of 4-6.
35. The semiconductor structure of claim 27, further comprising: an
insulator coating surrounding and encapsulating the nanocrystalline
core/nanocrystalline shell/nanocrystalline outer shell
combination.
36. The semiconductor structure of claim 35, wherein the insulator
coating comprises an amorphous material selected from the group
consisting of silicon dioxide (SiO.sub.2), silicon oxide
(SiO.sub.x), aluminum oxide (Al.sub.2O.sub.3), zirconia
(ZrO.sub.x), titania (TiO.sub.x), and hafnia (HfO.sub.x).
37. The semiconductor structure of claim 27, wherein the
nanocrystalline outer shell completely surrounds the
nanocrystalline shell.
38. The semiconductor structure of claim 27, wherein the
nanocrystalline outer shell only partially surrounds the
nanocrystalline shell, exposing a portion of the nanocrystalline
shell.
39. The semiconductor structure of claim 27, wherein the
nanocrystalline core, nanocrystalline shell, nanocrystalline outer
shell, and compositional transition layer form a quantum dot.
40. The semiconductor structure of claim 39, wherein the quantum
dot is a down-converting quantum dot.
41. The semiconductor structure of claim 39, wherein the quantum
dot is an up-shifting quantum dot.
42. A composite, comprising: a matrix material; and a plurality of
semiconductor structures embedded in the matrix material, each
semiconductor structure comprising: a nanocrystalline core
comprising a first semiconductor material; a nanocrystalline shell
comprising a second, different, semiconductor material at least
partially surrounding the nanocrystalline core; a compositional
transition layer disposed between, and in contact with, the
nanocrystalline core and nanocrystalline shell, the compositional
transition layer having a composition intermediate to the first and
second semiconductor materials; and an amorphous insulator coating
surrounding and encapsulating the nanocrystalline
core/nanocrystalline shell pairing.
43. The composite of claim 42, wherein each of the plurality of
semiconductor structures is cross-linked with, polarity bound by,
or tethered to the matrix material.
44. The composite of claim 42, wherein each of the plurality of
semiconductor structures is bound to the matrix material by a
covalent, dative, or ionic bond.
45. The composite of claim 42, wherein one or more of the
semiconductor structures further comprises a coupling agent
covalently bonded to an outer surface of the amorphous insulator
coating.
46. The composite of claim 42, wherein the compositional transition
layer is an alloyed layer comprising a mixture of the first and
second semiconductor materials.
47. The composite of claim 42, wherein the compositional transition
layer is a graded layer comprising a compositional gradient of the
first semiconductor material proximate to the nanocrystalline core
through to the second semiconductor material proximate to the
nanocrystalline shell.
48. The composite of claim 42, wherein the compositional transition
layer has a thickness approximately in the range of 1.5-2
monolayers.
49. The composite of claim 42, wherein the first semiconductor
material is cadmium selenide (CdSe), the second semiconductor
material is cadmium sulfide (CdS), and the compositional transition
layer comprises CdSe.sub.xS.sub.y, where 0<x<1 and
0<y<1.
50. The composite of claim 42, wherein the first semiconductor
material is cadmium selenide (CdSe), the second semiconductor
material is zinc selenide (ZnSe), and the compositional transition
layer comprises Cd.sub.xZn.sub.ySe, where 0<x<1 and
0<y<1.
51. The composite of claim 42, wherein the compositional transition
layer passivates or reduces trap states where the nanocrystalline
shell surrounds the nanocrystalline core.
52. The composite of claim 42, wherein the amorphous insulator
coating comprises a material selected from the group consisting of
silicon dioxide (SiO.sub.2), silicon oxide (SiO.sub.x), aluminum
oxide (Al.sub.2O.sub.3), zirconia (ZrO.sub.x), titania (TiO.sub.x),
and hafnia (HfO.sub.x).
53. The composite of claim 42, wherein each of the semiconductor
structures is a down-converting quantum dot.
54. The composite of claim 42, wherein each of the semiconductor
structures is an up-shifting quantum dot.
55. The composite of claim 42, wherein each semiconductor structure
further comprises: a nanocrystalline outer shell at least partially
surrounding the nanocrystalline shell, the nanocrystalline outer
shell comprising a third semiconductor material different from the
first and second semiconductor materials.
56. The composite of claim 55, each semiconductor structure further
comprising: a second compositional transition layer disposed
between, and in contact with, the nanocrystalline shell and the
nanocrystalline outer shell, the second compositional transition
layer having a composition intermediate to the second and third
semiconductor materials.
57. The composite of claim 56, wherein the second compositional
transition layer is an alloyed layer comprising a mixture of the
second and third semiconductor materials.
58. The composite of claim 56, wherein the second compositional
transition layer is a graded layer comprising a compositional
gradient of the second semiconductor material proximate to the
nanocrystalline shell through to the third semiconductor material
proximate to the nanocrystalline outer shell.
59. The composite of claim 56, wherein the second compositional
transition layer has a thickness approximately in the range of
1.5-2 monolayers.
60. The composite of claim 56, wherein the first semiconductor
material is cadmium selenide (CdSe), the second semiconductor
material is cadmium sulfide (CdS), the third semiconductor material
is zinc sulfide (ZnS), and the second compositional transition
layer comprises Cd.sub.xZn.sub.yS, where 0<x<1 and
0<y<1.
61. The composite of claim 56, wherein the first semiconductor
material is cadmium selenide (CdSe), the second semiconductor
material is zinc selenide (ZnSe), the third semiconductor material
is zinc sulfide (ZnS), and the second compositional transition
layer comprises ZnSe.sub.xS.sub.y, where 0<x<1 and
0<y<1.
62. The composite of claim 56, wherein the second compositional
transition layer passivates or reduces trap states where the
nanocrystalline outer shell surrounds the nanocrystalline
shell.
63. A composite, comprising: a matrix material; and a plurality of
semiconductor structures embedded in the matrix material, each
semiconductor structure comprising: a nanocrystalline core
comprising a first semiconductor material; a nanocrystalline shell
comprising a second, different, semiconductor material at least
partially surrounding the nanocrystalline core; a nanocrystalline
outer shell at least partially surrounding the nanocrystalline
shell, the nanocrystalline outer shell comprising a third
semiconductor material different from the first and second
semiconductor materials; a compositional transition layer disposed
between, and in contact with, the nanocrystalline shell and the
nanocrystalline outer shell, the compositional transition layer
having a composition intermediate to the second and third
semiconductor materials; and an amorphous insulator coating
surrounding and encapsulating the nanocrystalline
core/nanocrystalline shell/nanocrystalline outer shell
combination.
64. The composite of claim 63, wherein each of the plurality of
semiconductor structures is cross-linked with, polarity bound by,
or tethered to the matrix material.
65. The composite of claim 63, wherein each of the plurality of
semiconductor structures is bound to the matrix material by a
covalent, dative, or ionic bond.
66. The composite of claim 63, wherein one or more of the
semiconductor structures further comprises a coupling agent
covalently bonded to an outer surface of the amorphous insulator
coating.
67. The composite of claim 63, wherein the compositional transition
layer is an alloyed layer comprising a mixture of the second and
third semiconductor materials.
68. The composite of claim 63, wherein the compositional transition
layer is a graded layer comprising a compositional gradient of the
second semiconductor material proximate to the nanocrystalline
shell through to the third semiconductor material proximate to the
nanocrystalline outer shell.
69. The composite of claim 63, wherein the compositional transition
layer has a thickness approximately in the range of 1.5-2
monolayers.
70. The composite of claim 63, wherein the first semiconductor
material is cadmium selenide (CdSe), the second semiconductor
material is cadmium sulfide (CdS), the third semiconductor material
is zinc sulfide (ZnS), and the compositional transition layer
comprises Cd.sub.xZn.sub.yS, where 0<x<1 and 0<y<1.
71. The composite of claim 63, wherein the first semiconductor
material is cadmium selenide (CdSe), the second semiconductor
material is zinc selenide (ZnSe), the third semiconductor material
is zinc sulfide (ZnS), and the compositional transition layer
comprises ZnSe.sub.xS.sub.y, where 0<x<1 and 0<y<1.
72. The composite of claim 63, wherein the compositional transition
layer passivates or reduces trap states where the nanocrystalline
outer shell surrounds the nanocrystalline shell.
73. The composite of claim 63, wherein the amorphous insulator
coating comprises an amorphous material selected from the group
consisting of silicon dioxide (SiO.sub.2), silicon oxide
(SiO.sub.x), aluminum oxide (Al.sub.2O.sub.3), zirconia
(ZrO.sub.x), titania (TiO.sub.x), and hafnia (HfO.sub.x).
74. The composite of claim 63, wherein each of the semiconductor
structures is a down-converting quantum dot.
75. The composite of claim 63, wherein each of the semiconductor
structures is an up-shifting quantum dot.
Description
TECHNICAL FIELD
[0001] Embodiments of the present invention are in the field of
quantum dots and, in particular, semiconductor structures having a
nanocrystalline core and nanocrystalline shell pairing with one or
more compositional transition layers.
BACKGROUND
[0002] Quantum dots having a high photoluminescence quantum yield
(PLQY) may be applicable as down-converting materials in
down-converting nanocomposites used in solid state lighting
applications. Down-converting materials are used to improve the
performance, efficiency and color choice in lighting applications,
particularly light emitting diodes (LEDs). In such applications,
quantum dots absorb light of a particular first (available or
selected) wavelength, usually blue, and then emit light at a second
wavelength, usually red or green.
SUMMARY
[0003] Embodiments of the present invention include semiconductor
structures having a nanocrystalline core and nanocrystalline shell
pairing with one or more compositional transition layers.
[0004] In an embodiment, a semiconductor structure includes a
nanocrystalline core composed of a first semiconductor material. A
nanocrystalline shell composed of a second, different,
semiconductor material at least partially surrounds the
nanocrystalline core. A compositional transition layer is disposed
between, and in contact with, the nanocrystalline core and
nanocrystalline shell. The compositional transition layer has a
composition intermediate to the first and second semiconductor
materials.
[0005] In another embodiment, a semiconductor structure includes a
nanocrystalline core composed of a first semiconductor material. A
nanocrystalline shell composed of a second, different,
semiconductor material at least partially surrounds the
nanocrystalline core. A nanocrystalline outer shell at least
partially surrounds the nanocrystalline shell and is composed of a
third semiconductor material different from the first and second
semiconductor materials. A compositional transition layer is
disposed between, and in contact with, the nanocrystalline shell
and the nanocrystalline outer shell. The compositional transition
layer has a composition intermediate to the second and third
semiconductor materials.
[0006] In another embodiment, a composite includes a matrix
material and a plurality of semiconductor structures embedded in
the matrix material. Each semiconductor structure includes a
nanocrystalline core composed of a first semiconductor material. A
nanocrystalline shell composed of a second, different,
semiconductor material at least partially surrounds the
nanocrystalline core. A compositional transition layer is disposed
between, and in contact with, the nanocrystalline core and
nanocrystalline shell. The compositional transition layer has a
composition intermediate to the first and second semiconductor
materials. An amorphous insulator coating surrounds and
encapsulates the nanocrystalline core/nanocrystalline shell
pairing.
[0007] In another embodiment, a composite includes a matrix
material and a plurality of semiconductor structures embedded in
the matrix material. Each semiconductor structure includes a
nanocrystalline core composed of a first semiconductor material. A
nanocrystalline shell composed of a second, different,
semiconductor material at least partially surrounds the
nanocrystalline core. A nanocrystalline outer shell at least
partially surrounds the nanocrystalline shell and is composed of a
third semiconductor material different from the first and second
semiconductor materials. A compositional transition layer is
disposed between, and in contact with, the nanocrystalline shell
and the nanocrystalline outer shell. The compositional transition
layer has a composition intermediate to the second and third
semiconductor materials. An amorphous insulator coating surrounds
and encapsulates the nanocrystalline core/nanocrystalline
shell/nanocrystalline outer shell combination.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 depicts a plot of prior art core/shell absorption
(left y-axis) and emission spectra intensity (right y-axis) as a
function of wavelength for conventional quantum dots.
[0009] FIG. 2 illustrates a schematic of a cross-sectional view of
a quantum dot, in accordance with an embodiment of the present
invention.
[0010] FIG. 3 illustrates a schematic of an integrating sphere for
measuring absolute photoluminescence quantum yield, in accordance
with an embodiment of the present invention.
[0011] FIG. 4 is a plot of photon counts as a function of
wavelength in nanometers for sample and reference emission spectra
used in the measurement of photoluminescence quantum yield, in
accordance with an embodiment of the present invention.
[0012] FIG. 5 is a plot including a UV-Vis absorbance spectrum and
photoluminescent emission spectrum for red CdSe/CdS core/shell
quantum dots, in accordance with an embodiment of the present
invention.
[0013] FIG. 6 is a plot including a UV-Vis absorbance spectrum and
photoluminescent emission spectrum for a green CdSe/CdS core/shell
quantum dot, in accordance with an embodiment of the present
invention.
[0014] FIG. 7 illustrates operations in a reverse micelle approach
to coating a semiconductor structure, in accordance with an
embodiment of the present invention.
[0015] FIG. 8 is a transmission electron microscope (TEM) image of
silica coated CdSe/CdS core/shell quantum dots having complete
silica encapsulation, in accordance with an embodiment of the
present invention.
[0016] FIGS. 9A-9C illustrate schematic representations of possible
composite compositions for quantum dot integration, in accordance
with an embodiment of the present invention.
[0017] FIG. 10 is a transmission electron microscope (TEM) image of
a sample of core/shell CdSe/CdS quantum dots, in accordance with an
embodiment of the present invention.
[0018] FIG. 11 is a plot including a UV-Vis absorbance spectrum and
photoluminescent emission spectrum for a CdSe/CdS core/shell
quantum dot having a PLQY of 96%, in accordance with an embodiment
of the present invention.
[0019] FIG. 12 is a transmission electron microscope (TEM) image of
a sample of CdSe/CdS quantum dots having a PLQY of 96%, in
accordance with an embodiment of the present invention.
[0020] FIG. 13 illustrates a cross-sectional view of a
semiconductor structure having a nanocrystalline core and
nanocrystalline shell pairing with one compositional transition
layer, in accordance with an embodiment of the present
invention.
[0021] FIG. 14 illustrates a cross-sectional view of a
semiconductor structure having a nanocrystalline
core/nanocrystalline shell/nanocrystalline outer shell combination
with two compositional transition layers, in accordance with an
embodiment of the present invention.
[0022] FIG. 15 illustrates a cross-sectional view of a
semiconductor structure having a nanocrystalline
core/nanocrystalline shell/nanocrystalline outer shell combination
with one compositional transition layer, in accordance with an
embodiment of the present invention.
[0023] FIG. 16 is a plot of increasing photoluminescence (PL)
intensity as a function of time (seconds) for non-alloyed rod
materials, in accordance with an embodiment of the present
invention.
DETAILED DESCRIPTION
[0024] Semiconductor structures having a nanocrystalline core and
nanocrystalline shell pairing with one or more compositional
transition layers are described herein. In the following
description, numerous specific details are set forth, such as
specific quantum dot geometries and efficiencies, in order to
provide a thorough understanding of embodiments of the present
invention. It will be apparent to one skilled in the art that
embodiments of the present invention may be practiced without these
specific details. In other instances, well-known related
apparatuses, such as the host of varieties of applicable light
emitting diodes (LEDs), are not described in detail in order to not
unnecessarily obscure embodiments of the present invention.
Furthermore, it is to be understood that the various embodiments
shown in the figures are illustrative representations and are not
necessarily drawn to scale.
[0025] Also disclosed herein are quantum dots having high
photoluminescence quantum yields (PLQY's) and methods of making and
encapsulating such quantum dots. A high PLQY is achieved by using a
synthetic process that significantly reduces the defects and self
absorption found in prior art quantum dots. The resulting
geometries of the quantum dots may include non-spherical quantum
dot cores shelled with a rod-shaped shell. The aspect or volume
ratio of the core/shell pairing may be controlled by monitoring the
reaction process used to fabricate the pairing. Uses of quantum dot
compositions having high PLQYs are also disclosed, including solid
state lighting. Other applications include biological imaging and
fabrication of photovoltaic devices.
[0026] As a reference point, quantum dots based on a spherical
cadmium selenide (CdSe) core embedded in a cadmium sulfide (CdS)
nanorod shell have been reported. Such quantum dots do not have a
high PLQY. Typically, prior art core/shell quantum dots suffer from
several structural deficiencies which may contribute to a reduced
PLQY. For example, prior art core/shell quantum dots used for
down-shifting applications typically have overlapping absorption
and emission profiles. Profile overlap may be attributed to core
material selection such that both the absorption and emission of
the quantum dot is controlled by the size, shape, and composition
of the core quantum dot, and the shell, if any, is used only as a
passivating layer for the surface. However, the prior art
arrangement leads to a significant amount of self-absorption
(re-absorption of the down-shifted light), which decreases the
measured PLQY. Accordingly, a typical prior art core/shell quantum
dot PLQY is below 80% which is often not high enough for device
applications. Also, prior art core/shell quantum dots suffer from
self absorption due in part to inappropriate volume of core/shell
material.
[0027] As an example, FIG. 1 depicts a plot 100 of prior art
core/shell absorption and emission spectra intensity as a function
of wavelength for conventional quantum dots. The absorption spectra
(102a, 102b, 102c) are of CdSe core nanorods for a same core size
with different thickness shells (a, b, c). FIG. 1 also depicts the
emission spectra (104a, 104b, 104c) of the three core/shell quantum
dots after exposure to blue or UV light. The absorption spectrum
and the emission spectrum overlap for each thickness of shell.
[0028] The low PLQY of prior art quantum dots is also attributed to
poor nanocrystal surface and crystalline quality. The poor quality
may result from a previous lack of capability in synthetic
techniques for treating or tailoring the nanocrystal surface in
order to achieve PLQYs above 90 percent. For example, the surface
may have a large number of dangling bonds which act as trap states
to reduce emission and, hence, PLQY. Previous approaches to address
such issues have included use of a very thin shell, e.g.,
approximately 1/2 monolayer to 5 monolayers, or up to about 1.5 nm
of thickness, to preserve the epitaxial nature of the shell.
However, a PLQY of only 50-80% has been achieved. In such systems,
considerable self-absorption may remain, decreasing the PLQY in
many device applications. Other approaches have included attempts
to grow a very large volume of up to 19 monolayers, or about 6 nm
of shell material on a nanometer-sized quantum dot. However, the
results have been less than satisfactory due to mismatched lattice
constants between the core and shell material.
[0029] Conventionally, a spherical shell is grown on a spherical
core in order to fabricate a core/shell quantum dot system.
However, if too much volume of shell material is added to the core,
the shell often will to crack due to strain. The strain introduces
defects and decreases the PLQY. Band-edge emission from the quantum
dots is then left to compete with both radiative and non-radiative
decay channels, originating from defect electronic states. Attempts
have been made to use an organic molecule as a passivating agent in
order to improve the size-dependent band-edge luminescence
efficiency, while preserving the solubility and processability of
the particles. Unfortunately, however, passivation by way of
organic molecule passivation is often incomplete or reversible,
exposing some regions of the surface of a quantum dot to
degradation effects such as photo-oxidation. In some cases,
chemical degradation of the ligand molecule itself or its exchange
with other ligands results in fabrication of poor quality quantum
dots.
[0030] One or more embodiments of the present invention address at
least one or more of the above issues regarding quantum dot quality
and behavior and the impact on PLQY of the fabricated quantum dots.
In one approach, the quality of quantum dot particle interfaces is
improved over conventional systems. For example, in one embodiment,
high PLQY and temperature stability of a fabricated (e.g., grown)
quantum dot is centered on the passivation or elimination of
internal (at the seed/rod interface) and external (at the rod
surface) interface defects that provide non-radiative recombination
pathways for electron-hole pairs that otherwise compete with a
desirable radiative recombination. This approach may be generally
coincident with maximizing the room-temperature PLQY of the quantum
dot particles. Thus, thermal escape paths from the quantum dot,
assisted by quantum dot photons, are mitigated as a primary escape
mechanism for thermally excited carriers. Although the chemical or
physical nature of such trap states has not been phenomenologically
explored, suitably tuning electron density at the surface may
deactivate trap states. Such passivation is especially important at
increased temperatures, where carriers have sufficient thermal
energy to access a larger manifold of these states.
[0031] In an embodiment, approaches described herein exploit the
concept of trap state deactivation. Furthermore, maintenance of
such a deactivation effect over time is achieved by insulating a
quantum dot interface and/or outer most surface from an external
environment. The deactivation of surface states is also important
for the fabrication of polymer composites including quantum dots,
particularly in the case where the polymer composite is exposed to
a high flux light-source (as is the case for solid-state lighting,
SSL) where it is possible for some of the particles to have more
than one exciton. The multi-excitons may recombine radiatively or
non-radiatively via Auger recombination to a single exciton state.
For non-passivated quantum dot systems, the Auger rate increases
with particle volume and with exciton population. However, in an
embodiment, a thick, high quality, asymmetric shell of (e.g., of
CdS) is grown on well-formed seeds (e.g., CdSe) to mitigate Auger
rate increase.
[0032] One or more embodiments described herein involve an
optimized synthesis of core/shell quantum dots. In a specific
example, high PLQY and temperature stable quantum dots are
fabricated from CdSe/CdS core-shell nanorods. In order to optimize
the quantum dots in place of light emitting diode (LED) phosphors,
the temperature stability of the quantum dots is enhanced, and the
overall PLQY increased. Such improved performance is achieved while
maintaining high absorption and narrow emission profiles for the
quantum dots. In one such embodiment, materials systems described
herein are tailored for separate optimization of absorption and
emission by employing a core/shell structure. The core material
predominantly controls the emission and the shell material
predominantly controls the absorption. The described systems enable
separate optimization of absorption and emission and provides very
little overlap of the absorption and emission to minimize
re-absorption of any emitted light by the quantum dot material
(i.e., self-absorption).
[0033] Several factors may be intertwined for establishing an
optimized geometry for a quantum dot having a nanocrystalline core
and naocrystalline shell pairing. As a reference, FIG. 2
illustrates a schematic of a cross-sectional view of a quantum dot,
in accordance with an embodiment of the present invention.
Referring to FIG. 2, a semiconductor structure (e.g., a quantum dot
structure) 200 includes a nanocrystalline core 202 surrounded by a
nanocrystalline shell 204. The nanocrystalline core 202 has a
length axis (a.sub.CORE), a width axis (b.sub.CORE) and a depth
axis (c.sub.CORE), the depth axis provided into and out of the
plane shown in FIG. 2. Likewise, the nanocrystalline shell 204 has
a length axis (a.sub.SHELL), a width axis (b.sub.SHELL) and a depth
axis (c.sub.SHELL), the depth axis provided into and out of the
plane shown in FIG. 2. The nanocrystalline core 202 has a center
203 and the nanocrystalline shell 204 has a center 205. The
nanocrystalline shell 204 surrounds the nanocrystalline core 202 in
the b-axis direction by an amount 206, as is also depicted in FIG.
2.
[0034] The following are attributes of a quantum dot that may be
tuned for optimization, with reference to the parameters provided
in FIG. 2, in accordance with embodiments of the present invention.
Nanocrystalline core 202 diameter (a, b or c) and aspect ratio
(e.g., a/b) can be controlled for rough tuning for emission
wavelength (a higher value for either providing increasingly red
emission). A smaller overall nanocrystalline core provides a
greater surface to volume ratio. The width of the nanocrystalline
shell along 206 may be tuned for yield optimization and quantum
confinement providing approaches to control red-shifting and
mitigation of surface effects. However, strain considerations must
be accounted for when optimizing the value of thickness 206. The
length (a.sub.SHELL) of the shell is tunable to provide longer
radiative decay times as well as increased light absorption. The
overall aspect ratio of the structure 200 (e.g., the greater of
a.sub.SHELL/b.sub.SHELL and a.sub.SHELL/c.sub.SHELL) may be tuned
to directly impact PLQY. Meanwhile, overall surface/volume ratio
for 200 may be kept relatively smaller to provide lower surface
defects, provide higher photoluminescence, and limit
self-absorption. Referring again to FIG. 2, the shell/core
interface 208 may be tailored to avoid dislocations and strain
sites. In one such embodiment, a high quality interface is obtained
by tailoring one or more of injection temperature and mixing
parameters, the use of surfactants, and control of the reactivity
of precursors, as is described in greater detail below.
[0035] In accordance with an embodiment of the present invention, a
high PLQY quantum dot is based on a core/shell pairing using an
anisotropic core. With reference to FIG. 2, an anisotropic core is
a core having one of the axes a.sub.CORE, b.sub.CORE or c.sub.CORE
different from one or both of the remaining axes. An aspect ratio
of such an anisotropic core is determined by the longest of the
axes a.sub.CORE, b.sub.CORE or c.sub.CORE divided by the shortest
of the axes a.sub.CORE, b.sub.CORE or c.sub.CORE to provide a
number greater than 1 (an isotropic core has an aspect ratio of 1).
It is to be understood that the outer surface of an anisotropic
core may have rounded or curved edges (e.g., as in an ellipsoid) or
may be faceted (e.g., as in a stretched or elongated tetragonal or
hexagonal prism) to provide an aspect ratio of greater than 1 (note
that a sphere, a tetragonal prism, and a hexagonal prism are all
considered to have an aspect ratio of 1 in keeping with embodiments
of the present invention).
[0036] A workable range of aspect ratio for an anisotropic
nanocrystalline core for a quantum dot may be selected for
maximization of PLQY. For example, a core essentially isotropic may
not provide advantages for increasing PLQY, while a core with too
great an aspect ratio (e.g., 2 or greater) may present challenges
synthetically and geometrically when forming a surrounding shell.
Furthermore, embedding the core in a shell composed of a material
different than the core may also be used enhance PLQY of a
resulting quantum dot.
[0037] Accordingly, in an embodiment, a semiconductor structure
includes an anisotropic nanocrystalline core composed of a first
semiconductor material and having an aspect ratio between, but not
including, 1.0 and 2.0. The semiconductor structure also includes a
nanocrystalline shell composed of a second, different,
semiconductor material at least partially surrounding the
anisotropic nanocrystalline core. In one such embodiment, the
aspect ratio of the anisotropic nanocrystalline core is
approximately in the range of 1.01-1.2 and, in a particular
embodiment, is approximately in the range of 1.1-1.2. In the case
of rounded edges, then, the nanocrystalline core may be
substantially, but not perfectly, spherical. However, the
nanocrystalline core may instead be faceted. In an embodiment, the
anisotropic nanocrystalline core is disposed in an asymmetric
orientation with respect to the nanocrystalline shell, as described
in greater detail in the example below.
[0038] Another consideration for maximization of PLQY in a quantum
dot structure is to provide an asymmetric orientation of the core
within a surrounding shell. For example, referring again to FIG. 2,
the center 203 of the core 202 may be misaligned with (e.g., have a
different spatial point than) the center 205 of the shell 204. In
an embodiment, a semiconductor structure includes an anisotropic
nanocrystalline core composed of a first semiconductor material.
The semiconductor structure also includes a nanocrystalline shell
composed of a second, different, semiconductor material at least
partially surrounding the anisotropic nanocrystalline core. The
anisotropic nanocrystalline core is disposed in an asymmetric
orientation with respect to the nanocrystalline shell. In one such
embodiment, the nanocrystalline shell has a long axis (e.g.,
a.sub.SHELL), and the anisotropic nanocrystalline core is disposed
off-center along the long axis. In another such embodiment, the
nanocrystalline shell has a short axis (e.g., b.sub.SHELL), and the
anisotropic nanocrystalline core is disposed off-center along the
short axis. In yet another embodiment, however, the nanocrystalline
shell has a long axis (e.g., a.sub.SHELL) and a short axis (e.g.,
b.sub.SHELL), and the anisotropic nanocrystalline core is disposed
off-center along both the long and short axes.
[0039] With reference to the above described nanocrystalline core
and nanocrystalline shell pairings, in an embodiment, the
nanocrystalline shell completely surrounds the anisotropic
nanocrystalline core. In an alternative embodiment, however, the
nanocrystalline shell only partially surrounds the anisotropic
nanocrystalline core, exposing a portion of the anisotropic
nanocrystalline core, e.g., as in a tetrapod geometry or
arrangement. In an embodiment, the nanocrystalline shell is an
anisotropic nanocrystalline shell, such as a nano-rod, that
surrounds the anisotropic nanocrystalline core at an interface
between the anisotropic nanocrystalline shell and the anisotropic
nanocrystalline core. The anisotropic nanocrystalline shell
passivates or reduces trap states at the interface. The anisotropic
nanocrystalline shell may also, or instead, deactivate trap states
at the interface.
[0040] With reference again to the above described nanocrystalline
core and nanocrystalline shell pairings, in an embodiment, the
first and second semiconductor materials (core and shell,
respectively) are each materials such as, but not limited to, Group
II-VI materials, Group III-V materials, Group IV-VI materials,
Group I-III-VI materials, or Group II-IV-VI materials and, in one
embodiment, are monocrystalline. In one such embodiment, the first
and second semiconductor materials are both Group II-VI materials,
the first semiconductor material is cadmium selenide (CdSe), and
the second semiconductor material is one such as, but not limited
to, cadmium sulfide (CdS), zinc sulfide (ZnS), or zinc selenide
(ZnSe). In an embodiment, the semiconductor structure further
includes a nanocrystalline outer shell at least partially
surrounding the nanocrystalline shell and, in one embodiment, the
nanocrystalline outer shell completely surrounds the
nanocrystalline shell. The nanocrystalline outer shell is composed
of a third semiconductor material different from the first and
second semiconductor materials. In a particular such embodiment,
the first semiconductor material is cadmium selenide (CdSe), the
second semiconductor material is cadmium sulfide (CdS), and the
third semiconductor material is zinc sulfide (ZnS).
[0041] With reference again to the above described nanocrystalline
core and nanocrystalline shell pairings, in an embodiment, the
semiconductor structure (i.e., the core/shell pairing in total) has
an aspect ratio approximately in the range of 1.5-10 and, 3-6 in a
particular embodiment. In an embodiment, the nanocrystalline shell
has a long axis and a short axis. The long axis has a length
approximately in the range of 5-40 nanometers. The short axis has a
length approximately in the range of 1-5 nanometers greater than a
diameter of the anisotropic nanocrystalline core parallel with the
short axis of the nanocrystalline shell. In a specific such
embodiment, the anisotropic nanocrystalline core has a diameter
approximately in the range of 2-5 nanometers. The thickness of the
nanocrystalline shell on the anisotropic nanocrystalline core along
a short axis of the nanocrystalline shell is approximately in the
range of 1-5 nanometers of the second semiconductor material.
[0042] With reference again to the above described nanocrystalline
core and nanocrystalline shell pairings, in an embodiment, the
anisotropic nanocrystalline core and the nanocrystalline shell form
a quantum dot. In one such embodiment, the quantum dot has a
photoluminescence quantum yield (PLQY) of at least 90%. Emission
from the quantum dot may be mostly, or entirely, from the
nanocrystalline core. For example, in an embodiment, emission from
the anisotropic nanocrystalline core is at least approximately 75%
of the total emission from the quantum dot. An absorption spectrum
and an emission spectrum of the quantum dot may be essentially
non-overlapping. For example, in an embodiment, an absorbance ratio
of the quantum dot based on absorbance at 400 nanometers versus
absorbance at an exciton peak for the quantum dot is approximately
in the range of 5-35.
[0043] In an embodiment, a quantum dot based on the above described
nanocrystalline core and nanocrystalline shell pairings is a
down-converting quantum dot. However, in an alternative embodiment,
the quantum dot is an up-shifting quantum dot. In either case, a
lighting apparatus may include a light emitting diode and a
plurality of quantum dots such as those described above. The
quantum dots may be applied proximal to the LED and provide
down-conversion or up-shifting of light emitted from the LED. Thus,
semiconductor structures according to the present invention may be
advantageously used in solid state lighting. The visible spectrum
includes light of different colors having wavelengths between about
380 nm and about 780 nm that are visible to the human eye. An LED
will emit a UV or blue light which is down-converted (or
up-shifted) by semiconductor structures described herein. Any
suitable ratio of emission color from the semiconductor structures
may be used in devices of the present invention. LED devices
according to embodiments of the present invention may have
incorporated therein sufficient quantity of semiconductor
structures (e.g., quantum dots) described herein capable of
down-converting any available blue light to red, green, yellow,
orange, blue, indigo, violet or other color. These structures may
also be used to downconvert or upconvert lower energy light (green,
yellow, etc) from LED devices, as long as the excitation light
produces emission from the structures.
[0044] Semiconductor structures according to embodiments of the
present invention may be advantageously used in biological imaging
in, e.g., one or more of the following environments: fluorescence
resonance energy transfer (FRET) analysis, gene technology,
fluorescent labeling of cellular proteins, cell tracking, pathogen
and toxin detection, in vivo animal imaging or tumor biology
investigation. Accordingly, embodiments of the present invention
contemplate probes having quantum dots described herein.
[0045] Semiconductor structures according to embodiments of the
present invention may be advantageously used in photovoltaic cells
in layers where high PLQY is important. Accordingly, embodiments of
the present invention contemplate photovoltaic devices using
quantum dots described herein.
[0046] Semiconductor structures according to embodiments of the
present invention may be advantageously used in display
applications as phosphor replacements where high PLQY is important.
Accordingly, embodiments of the present invention contemplate
display devices using quantum dots described herein.
[0047] There are various synthetic approaches for fabricating CdSe
quantum dots. For example, in an embodiment, under an inert
atmosphere (e.g., ultra high purity (UHP) argon), cadmium oxide
(CdO) is dissociated in the presence of surfactant (e.g.,
octadecylphosphonic acid (ODPA)) and solvent (e.g.,
trioctylphopshine oxide (TOPO); trioctylphosphine (TOP)) at high
temperatures (e.g., 350-380 degrees Celsius). Resulting Cd.sup.2+
cations are exposed by rapid injection to solvated selenium anions
(Se.sup.2-, resulting in a nucleation event forming small CdSe
seeds. The seeds continue to grow, feeding off of the remaining
Cd.sup.2+ and Se.sup.2- available in solution, with the resulting
quantum dots being stabilized by surface interactions with the
surfactant in solution (ODPA). The aspect ratio of the CdSe seeds
is typically between 1 and 2, as dictated by the ratio of the ODPA
to the Cd concentration in solution. The quality and final size of
these cores is affected by several variables such as, but not
limited to, reaction time, temperature, reagent concentration,
surfactant concentration, moisture content in the reaction, or
mixing rate. The reaction is targeted for a narrow size
distribution of CdSe seeds (assessed by transmission electron
microscopy (TEM)), typically a slightly cylindrical seed shape
(also assessed by TEM) and CdSe seeds exhibiting solution stability
over time (assessed by PLQY and scattering in solution).
[0048] For the cadmium sulfide (CdS) shell growth on the CdSe
seeds, or nanocrystalline cores, under an inert atmosphere (e.g.
UHP argon), cadmium oxide (CdO) is dissociated in the presence of
surfactants (e.g., ODPA and hexylphosphonic acid (HPA)) and solvent
(e.g. TOPO and/or TOP) at high temperatures (e.g., 350-380 degrees
Celsius). The resulting Cd.sup.2+ cations in solution are exposed
by rapid injection to solvated sulfur anions (S.sup.2-) and CdSe
cores. Immediate growth of the CdS shell around the CdSe core
occurs. The use of both a short chain and long chain phosphonic
acid promotes enhanced growth rate at along the c-axis of the
structure, and slower growth along the a-axis, resulting in a
rod-shaped core/shell nanomaterial.
[0049] CdSe/CdS core-shell quantum dots have been shown in the
literature to exhibit respectable quantum yields (e.g., 70-75%).
However, the persistence of surface trap states (which decrease
overall photoluminescent quantum yield) in these systems arises
from a variety of factors such as, but not limited to, strain at
the core-shell interface, high aspect ratios (ratio of rod length
to rod width of the core/shell pairing) which lead to larger
quantum dot surface area requiring passivation, or poor surface
stabilization of the shell.
[0050] In order to address the above synthetic limitations on the
quality of quantum dots formed under conventional synthetic
procedures, in an embodiment, a multi-faceted approach is used to
mitigate or eliminate sources of surface trap states in quantum dot
materials. For example, lower reaction temperatures during the
core/shell pairing growth yields slower growth at the CdSe--CdS
interface, giving each material sufficient time to orient into the
lowest-strain positions. Aspect ratios are controlled by changing
the relative ratios of surfactants in solution as well as by
controlling temperature. Increasing an ODPA/HPA ratio in reaction
slows the rapid growth at the ends of the core/shell pairings by
replacing the facile HPA surfactant with the more obstructive ODPA
surfactant. In addition, lowered reaction temperatures are also
used to contribute to slowed growth at the ends of the core/shell
pairings. By controlling these variables, the aspect ratio of the
core/shell pairing is optimized for quantum yield. In one such
embodiment, following determination of optimal surfactant ratios,
overall surfactant concentrations are adjusted to locate a PLQY
maximum while maintaining long-term stability of the fabricated
quantum dots in solution. Furthermore, in an embodiment, aspect
ratios of the seed or core (e.g., as opposed to the seed/shell
pairing) are limited to a range between, but not including 1.0 and
2.0 in order to provide an appropriate geometry for high quality
shell growth thereon.
[0051] In another aspect, an additional or alternative strategy for
improving the interface between CdSe and CdS includes, in an
embodiment, chemically treating the surface of the CdSe cores prior
to reaction. CdSe cores are stabilized by long chain surfactants
(ODPA) prior to introduction into the CdS growth conditions.
Reactive ligand exchange can be used to replace the ODPA
surfactants with ligands which are easier to remove (e.g., primary
or secondary amines), facilitating improved reaction between the
CdSe core and the CdS growth reagents.
[0052] In addition to the above factors affecting PLQY in solution,
self-absorption may negatively affect PLQY when these materials are
cast into films. This phenomenon may occur when CdSe cores
re-absorb light emitted by other quantum dots. In one embodiment,
the thickness of the CdS shells around the same CdSe cores is
increased in order to increase the amount of light absorbed per
core/shell pairing, while keeping the particle concentration the
same or lower in films including the quantum dot structures. The
addition of more Cd and S to the shell formation reaction leads to
more shell growth, while an optimal surfactant ratio allows
targeting of a desired aspect ratio and solubility of the
core/shell pairing.
[0053] Accordingly, in an embodiment, an overall method of
fabricating a semiconductor structure, such as the above described
quantum dot structures, includes forming an anisotropic
nanocrystalline core from a first semiconductor material. A
nanocrystalline shell is formed from a second, different,
semiconductor material to at least partially surround the
anisotropic nanocrystalline core. In one such embodiment, the
anisotropic nanocrystalline core has an aspect ratio between, but
not including, 1.0 and 2.0, as described above.
[0054] With reference to the above described general method for
fabricating a nanocrystalline core and nanocrystalline shell
pairing, in an embodiment, prior to forming the nanocrystalline
shell, the anisotropic nanocrystalline core is stabilized in
solution with a surfactant. In one such embodiment, the surfactant
is octadecylphosphonic acid (ODPA). In another such embodiment, the
surfactant acts as a ligand for the anisotropic nanocrystalline
core. In that embodiment, the method further includes, prior to
forming the nanocrystalline shell, replacing the surfactant ligand
with a second ligand, the second ligand more labile than the
surfactant ligand. In a specific such embodiment, the second ligand
is one such as, but not limited to, a primary amine or a secondary
amine.
[0055] With reference again to the above described general method
for fabricating a nanocrystalline core and nanocrystalline shell
pairing, in an embodiment, forming the nanocrystalline shell
includes forming the second semiconductor material in the presence
of a mixture of surfactants. In one such embodiment, the mixture of
surfactants includes a mixture of octadecylphosphonic acid (ODPA)
and hexylphosphonic acid (HPA). In a specific such embodiment,
forming the nanocrystalline shell includes tuning the aspect ratio
of the nanocrystalline shell by tuning the ratio of ODPA versus
HPA. Forming the second semiconductor material in the presence of
the mixture of surfactants may also, or instead, include using a
solvent such as, but not limited to, trioctylphosphine oxide (TOPO)
and trioctylphosphine (TOP).
[0056] With reference again to the above described general method
for fabricating a nanocrystalline core and nanocrystalline shell
pairing, in an embodiment, forming the anisotropic nanocrystalline
core includes forming at a temperature approximately in the range
of 350-380 degrees Celsius. In an embodiment, forming the
anisotropic nanocrystalline core includes forming a cadmium
selenide (CdSe) nanocrystal from cadmium oxide (CdO) and selenium
(Se) in the presence of a surfactant at a temperature approximately
in the range of 300-400 degrees Celsius. The reaction is arrested
prior to completion. In one such embodiment, forming the
nanocrystalline shell includes forming a cadmium sulfide (CdS)
nanocrystalline layer on the CdSe nanocrystal from cadmium oxide
(CdO) and sulfur (S) at a temperature approximately in the range of
120-380 degrees Celsius. That reaction is also arrested prior to
completion.
[0057] The aspect ratio of the fabricated semiconductor structures
may be controlled by one of several methods. For example, ligand
exchange may be used to change the surfactants and/or ligands and
alter the growth kinetics of the shell and thus the aspect ratio.
Changing the core concentration during core/shell growth may also
be exploited. An increase in core concentration and/or decrease
concentration of surfactants results in lower aspect ratio
core/shell pairings. Increasing the concentration of a shell
material such as S for CdS will increase the rate of growth on the
ends of the core/shell pairings, leading to longer, higher aspect
ratio core/shell pairings.
[0058] As mentioned above, in one embodiment of the present
invention, nanocrystalline cores undergo a reactive ligand exchange
which replaces core surfactants with ligands that are easier to
remove (e.g., primary or secondary amines), facilitating better
reaction between the CdSe core and the CdS growth reagents. In one
embodiment, cores used herein have ligands bound or associated
therewith. Attachment may be by dative bonding, Van der Waals
forces, covalent bonding, ionic bonding or other force or bond, and
combinations thereof. Ligands used with the cores may include one
or more functional groups to bind to the surface of the
nanocrystals. In a specific such embodiment, the ligands have a
functional group with an affinity for a hydrophobic solvent.
[0059] In an embodiment, lower reaction temperatures during shell
growth yields slower growth at the core/shell interface. While not
wishing to be bound by any particular theory or principle it is
believed that this method allows both core and shell seed crystals
time to orient into their lowest-strain positions during growth.
Growth at the ends of the core/shell pairing structure is facile
and is primarily governed by the concentration of available
precursors (e.g., for a shell of CdS this is Cd, S:TOP). Growth at
the sides of the core/shell pairings is more strongly affected by
the stabilizing ligands on the surface of the core/shell pairing.
Ligands may exist in equilibrium between the reaction solution and
the surface of the core/shell pairing structure. Lower reaction
temperatures may tilt this equilibrium towards more ligands being
on the surface, rendering it more difficult for growth precursors
to access this surface. Hence, growth in the width direction is
hindered by lower temperature, leading to higher aspect ratio
core/shell pairings.
[0060] In general consideration of the above described
semiconductor or quantum dot structures and methods of fabricating
such semiconductor or quantum dot structures, in an embodiment,
quantum dots are fabricated to have an absorbance in the blue or
ultra-violet (V) regime, with an emission in the visible (e.g.,
red, orange, yellow, green, blue, indigo and violet, but
particularly red and green) regime. The above described quantum
dots may advantageously have a high PLQY with limited
self-absorption, possess a narrow size distribution for cores,
provide core stability over time (e.g., as assessed by PLQY and
scattering in solution), and exhibit no major product loss during
purification steps. Quantum dots fabricated according one or more
of the above embodiments may have a decoupled absorption and
emission regime, where the absorption is controlled by the shell
and the emission is controlled by the core. In one such embodiment,
a dimension of the core correlates with emission color, e.g., a
core diameter progressing from 3-5.5 nanometers correlates
approximately to a green.fwdarw.yellow.fwdarw.red emission
progression.
[0061] With reference to the above described embodiments concerning
semiconductor structures, such as quantum dots, and methods of
fabricating such structures, the concept of a crystal defect, or
mitigation thereof, may be implicated. For example, a crystal
defect may form in, or be precluded from forming in, a
nanocrystalline core or in a nanocrystalline shell, at an interface
of the core/shell pairing, or at the surface of the core or shell.
In an embodiment, a crystal defect is a departure from crystal
symmetry caused by on or more of free surfaces, disorder,
impurities, vacancies and interstitials, dislocations, lattice
vibrations, or grain boundaries. Such a departure may be referred
to as a structural defect or lattice defect. Reference to an
exciton is to a mobile concentration of energy in a crystal formed
by an excited electron and an associated hole. An exciton peak is
defined as the peak in an absorption spectrum correlating to the
minimum energy for a ground state electron to cross the band gap.
The core/shell quantum dot absorption spectrum appears as a series
of overlapping peaks that get larger at shorter wavelengths.
Because of their discrete electron energy levels, each peak
corresponds to an energy transition between discrete electron-hole
(exciton) energy levels. The quantum dots do not absorb light that
has a wavelength longer than that of the first exciton peak, also
referred to as the absorption onset. The wavelength of the first
exciton peak, and all subsequent peaks, is a function of the
composition and size of the quantum dot. An absorbance ratio is
absorbance of the core/shell nanocrystal at 400 nm divided by the
absorbance of the core/shell nanocrystal at the first exciton peak.
Photoluminescence quantum yield (PLQY) is defined as the ratio of
the number of photons emitted to the number of photons absorbed.
Core/shell pairing described herein may have a Type 1 band
alignment, e.g., the core band gap is nested within the band gap of
the shell. Emission wavelength may be determined by controlling the
size and shape of the core nanocrystal, which controls the band gap
of the core. Emission wavelength may also be engineered by
controlling the size and shape of the shell. In an embodiment, the
amount/volume of shell material is much greater than that of the
core material. Consequently, the absorption onset wavelength is
mainly controlled by the shell band gap. Core/shell quantum dots in
accordance with an embodiment of the present invention have an
electron-hole pair generated in the shell which is then funneled
into the core, resulting in recombination and emission from the
core quantum dot. Preferably emission is substantially from the
core of the quantum dot.
[0062] Measurement of Photoluminescence Quantum Yield (PLQY) may be
performed according to the method disclosed in Laurent Porres et
al. "Absolute Measurements of Photoluminescence Quantum Yields of
Solutions Using an Integrating Sphere", Journal of Fluorescence
(2006) DOI: 10.1007/s10895-005-0054-8, Springer Science+Business
Media, Inc. As an example, FIG. 3 illustrates a schematic of an
integrating sphere 300 for measuring absolute photoluminescence
quantum yield, in accordance with an embodiment of the present
invention. The integrating sphere 300 includes a sample holder 302,
a spectrometer 304, a calibrated light source 306 and an
ultra-violet (UV) LED 308. FIG. 4 is a plot 400 of photon counts as
a function of wavelength in nanometers for sample and reference
emission spectra used in the measurement of photoluminescence
quantum yield, in accordance with an embodiment of the present
invention. Referring to plot 400, both excitation and emission
peaks for a sample are calibrated against corresponding excitation
and emission peaks for a reference.
[0063] In an embodiment, PLQY is measured with a Labsphere.TM. 6''
integrating sphere, a Labsphere.TM. LPS-100-0105 calibrated white
light source, a 3.8W, 405 nm Thorlabs.TM. M405L2 UV LED and an
Ocean Optics.TM. USB4000-VIS-NIR spectrometer. The spectrometer and
UV LED are coupled into the sphere using Ocean Optics.TM. UV-Vis
optical fibers. The spectrometer fiber is attached to a lens in a
port at the side of the sphere at 90 degrees relative to the
excitation source. The lens is behind a flat baffle to ensure only
diffuse light reaches the lens. The calibrated white light source
is affixed to a port in the side of the sphere, at 90.degree. to
both the excitation source and the spectrometer port. Custom made
sample holders are used to hold solid and solution (cuvette)
samples and to rotate samples between direct and indirect
measurement positions. Sample holders are coated with a barium
sulfate diffuse reflective material. Before measurements are
recorded, the calibrated white light source is used to calibrate
the spectrometer as a function of wavelength (translating counts
per second into relative intensity vs. wavelength). To measure
PLQY, a reference sample is inserted into the sphere, and the
excitation source LED signal is recorded. This reference sample is
generally a blank, such as a cuvette containing a solvent or a
sample without quantum dots, so as to only measure the properties
of the quantum dots. If it is desirable to measure the properties
of the matrix, the blank may be only the substrate. The sample is
then inserted into the sphere, in direct beam line for direct
measurements, and out of the beam for indirect measurements. The
spectrum is recorded and split into excitation and emission bands,
each is integrated, and the number of photons emitted per photons
absorbed is the photoluminescence quantum yield (PLQY), which is
equal to the difference between sample emission and reference
emission divided by the difference of reference excitation and
sample excitation.
[0064] Quantum dots according to embodiments of the present
invention have a PLQY between 90-100%, or at least 90%, more
preferably at least 91%, more preferably at least 92%, more
preferably at least 93%, more preferably at least 94%, more
preferably at least 95%, more preferably at least 96%, more
preferably at least 97%, more preferably at least 98%, more
preferably at least 99% and most preferably 100%. FIG. 5 is a plot
500 including a UV-Vis absorbance spectrum 502 and photoluminescent
emission spectrum 504 for red CdSe/CdS core/shell quantum dots, in
accordance with an embodiment of the present invention. The quantum
dots have essentially no overlapping absorption and emission bands
and having an absorbance ratio of about 24. The PLQY was determined
to be 94% at 617 nm. The average length (from transmission electron
microscopy (TEM) data) is 27 nm.+-.3.3 nm. The average width (from
TEM data) is 7.9 nm.+-.1.1 nm. The average aspect ratio (from TEM
data) is 3.5.+-.0.6. FIG. 6 is a plot 600 including a UV-Vis
absorbance spectrum 602 and photoluminescent emission spectrum 604
for a green CdSe/CdS core/shell quantum dot, in accordance with an
embodiment of the present invention. The quantum dot has a small
extent of overlapping absorption and emission bands and has an
absorbance ratio of 16 (plus or minus one).
[0065] In another aspect, semiconductor structures having a
nanocrystalline core and corresponding nanocrystalline shell and
insulator coating are described. Particularly, coated quantum dots
structures and methods of making such structures are described
below. In an embodiment, core/shell quantum dots are coated with
silica by a method resulting in compositions having
photoluminescence quantum yields between 90 and 100%. In one such
embodiment, semiconductor structures are coated with silica using a
reverse micelle method. A quantum dot may be engineered so that
emission is substantially from the core.
[0066] Prior art quantum dots may have poor nanocrystal surface and
crystalline quality as a result of prior art synthetic techniques
not being capable of treating the nanocrystal surface in ways
capable of achieving PLQYs above 90 percent. For example, the
surface of a nanocrystalline core/shell pairing may have a large
number of dangling bonds which act as trap states reducing emission
and, therefore, PLQY. Prior art techniques to modify the quantum
dot surface include coating quantum dots with silica. However,
prior art silica coated quantum dots do not achieve the PLQY
necessary for continued use in solid state lighting devices.
[0067] In conventional approaches, silica coatings can encapsulate
more than one particle (e.g., quantum dot structure) at a time, or
the approaches have resulted in incomplete encapsulation. One such
conventional approach included coating a quantum dot with silica
using self-assembled micelles. The approach requires the presence
of a majority of a polar solvent to form a micelle. The requirement
is for polar solvent environments to generate the encapsulating
micelle, and thus limits the technique to aqueous based
applications, such as biological tagging and imaging. Quantum dots
with a hydrophobic surfactant or ligand attached are aqueous
solution insoluble and thus silica cannot be precipitated with the
nanocrystals within the aqueous domains of the micro emulsion.
Ligand exchange reactions may be required which then leads to
surface quality degradation. However, conventional quantum dot
systems often rely on the weak dative Van der Waals bonding of
ligands such as phosphonic acids, amines, and carboxylic acids to
maintain the structures in solution and protect and passivate the
surface of the quantum dot.
[0068] The integration of a quantum dot into a product may require
protection for chemical compatibility with the solution environment
during processing, and ultimately the plastic or gel used for
encapsulation. Without such compatibility, particles are likely to
aggregate and/or redistribute themselves within the matrix, an
unacceptable occurrence in, for example, a solid state lighting
product. Protection of the surface and maintenance of an
electronically uniform environment also ensures that the density of
non-radiative pathways (traps) is minimized, and that the emission
energy (color) is as uniform as possible. Furthermore, the surface
is protected from further chemical reaction with environmental
degradants such as oxygen. This is particularly important for LED
applications, where the quantum dot must tolerate temperatures as
high as 200 degrees Celsius and constant high-intensity
illumination with high-energy light. However, the weak surface
bonding of prior art quantum dot ligands are non-ideal for the
processing and long-term performance required of an LED product, as
they allow degradants access to the quantum dot surface.
[0069] In accordance with an embodiment of the present invention,
core/shell quantum dots coated with silica and other ligands to
provide a structure having a high PLQY. One embodiment exploits a
sol-gel process which encapsulates each quantum dot individually in
a silica shell, resulting in a very stable high PLQY quantum dot
particle. The coated quantum dots disclosed herein may
advantageously possess a narrow size distribution for CdSe core
stability over time (assessed by PLQY and scattering in
solution).
[0070] In a general embodiment, a semiconductor structure includes
a nanocrystalline core composed of a first semiconductor material.
The semiconductor structure also includes a nanocrystalline shell
composed of a second, different, semiconductor material at least
partially surrounding the nanocrystalline core. An insulator layer
encapsulates, e.g., coats, the nanocrystalline shell and
nanocrystalline core. Thus, coated semiconductor structures include
coated structures such as the quantum dots described above. For
example, in an embodiment, the nanocrystalline core is anisotropic,
e.g., having an aspect ratio between, but not including, 1.0 and
2.0. In another example, in an embodiment, the nanocrystalline core
is anisotropic and is asymmetrically oriented within the
nanocrystalline shell. In an embodiment, the nanocrystalline core
and the nanocrystalline shell form a quantum dot.
[0071] With reference to the above described coated nanocrystalline
core and nanocrystalline shell pairings, in an embodiment, the
insulator layer is bonded directly to the nanocrystalline shell. In
one such embodiment, the insulator layer passivates an outermost
surface of the nanocrystalline shell. In another embodiment, the
insulator layer provides a barrier for the nanocrystalline shell
and nanocrystalline core impermeable to an environment outside of
the insulator layer. In any case, the insulator layer may
encapsulate only a single nanocrystalline shell/nanocrystalline
core pairing. In an embodiment, the semiconductor structure further
includes a nanocrystalline outer shell at least partially
surrounding the nanocrystalline shell, between the nanocrystalline
shell and the insulator layer. The nanocrystalline outer shell is
composed of a third semiconductor material different from the
semiconductor material of the shell and, possibly, different from
the semiconductor material of the core.
[0072] With reference again to the above described coated
nanocrystalline core and nanocrystalline shell pairings, in an
embodiment, the insulator layer is composed of a layer of material
such as, but not limited to, silica (SiO.sub.x), titanium oxide
(TiO.sub.x), zirconium oxide (ZrO.sub.x), alumina (AlO.sub.x), or
hafnia (HfO.sub.x). In one such embodiment, the layer is a layer of
silica having a thickness approximately in the range of 3-30
nanometers. In an embodiment, the insulator layer is an amorphous
layer.
[0073] With reference again to the above described coated
nanocrystalline core and nanocrystalline shell pairings, in an
embodiment, an outer surface of the insulator layer is ligand-free.
However, in an alternative embodiment, an outer surface of the
insulator layer is ligand-functionalized. In one such embodiment,
the outer surface of the insulator layer is ligand-functionalized
with a ligand such as, but not limited to, a silane having one or
more hydrolyzable groups or a functional or non-functional bipodal
silane. In another such embodiment, the outer surface of the
insulator layer is ligand-functionalized with a ligand such as, but
not limited to, mono-, di-, or tri-alkoxysilanes with three, two or
one inert or organofunctional substituents of the general formula
(R.sup.1O).sub.3SiR.sup.2; (R.sup.1O).sub.2SiR.sup.2R.sup.3;
(R.sup.1O)SiR.sup.2R.sup.3R.sup.4, where R.sup.1 is methyl, ethyl,
propyl, isopropyl, or butyl, R.sup.2, R.sup.3 and R.sup.4 are
identical or different and are H substituents, alkyls, alkenes,
alkynes, aryls, halogeno-derivates, alcohols, (mono, di, tri, poly)
ethyleneglycols, (secondary, tertiary, quaternary) amines,
diamines, polyamines, azides, isocyanates, acrylates, metacrylates,
epoxies, ethers, aldehydes, carboxylates, esters, anhydrides,
phosphates, phosphines, mercaptos, thiols, sulfonates, and are
linear or cyclic, a silane with the general structure
(R.sup.1O).sub.3Si--(CH.sub.2).sub.n--R--(CH.sub.2).sub.n--Si(R-
O).sub.3 where R and R.sup.1 is H or an organic substituent
selected from the group consisting of alkyls, alkenes, alkynes,
aryls, halogeno-derivates, alcohols, (mono, di, tri, poly)
ethyleneglycols, (secondary, tertiary, quaternary) amines,
diamines, polyamines, azides, isocyanates, acrylates, metacrylates,
epoxies, ethers, aldehydes, carboxylates, esters, anhydrides,
phosphates, phosphines, mercaptos, thiols, sulfonates, and are
linear or cyclic, a chlorosilane, or an azasilane. In another such
embodiment, the outer surface of the insulator layer is
ligand-functionalized with a ligand such as, but not limited to,
organic or inorganic compounds with functionality for bonding to a
silica surface by chemical or non-chemical interactions such as but
not limited to covalent, ionic, H-bonding, or Van der Waals forces.
In yet another such embodiment, the outer surface of the insulator
layer is ligand-functionalized with a ligand such as, but not
limited to, the methoxy and ethoxy silanes (MeO).sub.3SiAllyl,
(MeO).sub.3SiVinyl, (MeO).sub.2SiMeVinyl, (EtO).sub.3SiVinyl,
EtOSi(Vinyl).sub.3, mono-methoxy silanes, chloro-silanes, or
1,2-bis-(triethoxysilyl)ethane. In any case, in an embodiment, the
outer surface of the insulator layer is ligand-functionalized to
impart solubility, dispersability, heat stability, photo-stability,
or a combination thereof, to the semiconductor structure. For
example, in one embodiment, the outer surface of the insulator
layer includes OH groups suitable for reaction with an intermediate
linker to link small molecules, oligomers, polymers or
macromolecules to the outer surface of the insulator layer, the
intermediate linker one such as, but not limited to, an epoxide, a
carbonyldiimidazole, a cyanuric chloride, or an isocyanate.
[0074] With reference again to the above described coated
nanocrystalline core and nanocrystalline shell pairings, in an
embodiment, the nanocrystalline core has a diameter approximately
in the range of 2-5 nanometers. The nanocrystalline shell has a
long axis and a short axis, the long axis having a length
approximately in the range of 5-40 nanometers, and the short axis
having a length approximately in the range of 1-5 nanometers
greater than the diameter of the nanocrystalline core. The
insulator layer has a thickness approximately in the range of 1-20
nanometers along an axis co-axial with the long axis and has a
thickness approximately in the range of 3-30 nanometers along an
axis co-axial with the short axis.
[0075] A lighting apparatus may include a light emitting diode and
a plurality of semiconductor structures which, e.g., act to down
convert light absorbed from the light emitting diode. For example,
in one embodiment, each semiconductor structure includes a quantum
dot having a nanocrystalline core composed of a first semiconductor
material and a nanocrystalline shell composed of a second,
different, semiconductor material at least partially surrounding
the nanocrystalline core. Each quantum dot has a photoluminescence
quantum yield (PLQY) of at least 90%. An insulator layer
encapsulates each quantum dot.
[0076] As described briefly above, an insulator layer may be formed
to encapsulate a nanocrystalline shell and anisotropic
nanocrystalline core. For example, in an embodiment, a layer of
silica is formed using a reverse micelle sol-gel reaction. In one
such embodiment, using the reverse micelle sol-gel reaction
includes dissolving the nanocrystalline shell/nanocrystalline core
pairing in a first non-polar solvent to form a first solution.
Subsequently, the first solution is added along with a species such
as, but not limited to, 3-aminopropyltrimethoxysilane (APTMS),
3-mercapto-trimethoxysilane, or a silane comprising a phosphonic
acid or carboxylic acid functional group, to a second solution
having a surfactant dissolved in a second non-polar solvent.
Subsequently, ammonium hydroxide and tetraorthosilicate (TEOS) are
added to the second solution.
[0077] Thus, semiconductor nanocrystals coated with silica
according to the present invention may be made by a sol-gel
reaction such as a reverse micelle method. As an example, FIG. 7
illustrates operations in a reverse micelle approach to coating a
semiconductor structure, in accordance with an embodiment of the
present invention. Referring to part A of FIG. 7, a quantum dot
heterostructure (QDH) 702 (e.g., a nanocrystalline core/shell
pairing) has attached thereto a plurality of TOPO ligands 704 and
TOP ligands 706. Referring to part B, the plurality of TOPO ligands
704 and TOP ligands 706 are exchanged with a plurality of
Si(OCH.sub.3).sub.3(CH.sub.2).sub.3NH.sub.2 ligands 708. The
structure of part B is then reacted with TEOS (Si(OEt).sub.4) and
ammonium hydroxide (NH.sub.4OH) to form a silica coating 710
surrounding the QDH 702, as depicted in part C of FIG. 7. FIG. 8 is
a transmission electron microscope (TEM) image 800 of silica coated
802 CdSe/CdS core/shell quantum dots 804 having complete silica
encapsulation, in accordance with an embodiment of the present
invention. Thus, a reverse micelle is formed after adding ammonium
hydroxide and tetraethylorthosilicate (TEOS), the source for the
silica coating. TEOS diffuses through the micelle and is hydrolyzed
by ammonia to form a uniform SiO.sub.2 shell on the surface of the
quantum dot. This approach may offer great flexibility to
incorporate quantum dots of different sizes. In one such
embodiment, the thickness of the insulator layer formed depends on
the amount of TEOS added to the second solution.
[0078] With reference again to the above described method of
forming coated nanocrystalline core and nanocrystalline shell
pairings, in an embodiment, the first and second non-polar solvents
are cyclohexane. In an embodiment, forming the coating layer
includes forming a layer of silica and further includes using a
combination of dioctyl sodium sulfosuccinate (AOT) and
tetraorthosilicate (TEOS). In another embodiment, however, forming
the layer includes forming a layer of silica and further includes
using a combination of polyoxyethylene (5) nonylphenylether and
tetraorthosilicate (TEOS). In another embodiment, however, forming
the layer includes forming a layer of silica and further includes
using cationic surfactants such as CTAB (cetyltrimethylammonium
bromide), anionic surfactants, non-ionic surfactants, or pluronic
surfactants such as Pluronic F 127 (an ethylene oxide/propylene
oxide block co-polymer) as well as mixtures of surfactants.
[0079] Upon initiation of growth of a silica shell, the final size
of that shell may be directly related to the amount of TEOS in the
reaction solution. Silica coatings according to embodiments of the
present invention may be conformal to the core/shell QDH or
non-conformal. A silica coating may be between about 3 nm and 30 nm
thick. The silica coating thickness along the c-axis may be as
small as about 1 nm or as large as about 20 nm. The silica coating
thickness along the a-axis may be between about 3 nm and 30 nm.
Once silica shelling is complete, the product is washed with
solvent to remove any remaining ligands. The silica coated quantum
dots can then be incorporated into a polymer matrix or undergo
further surface functionalization. However, silica shells according
to embodiments of the present invention may also be functionalized
with ligands to impart solubility, dispersability, heat stability
and photo-stability in the matrix.
[0080] In another aspect, quantum dot composite compositions are
described. For example, the quantum dots (including coated quantum
dots) described above may be embedded in a matrix material to make
a composite using a plastic or other material as the matrix. In an
embodiment, composite compositions including matrix materials and
silica coated core/shell quantum dots having photoluminescence
quantum yields between 90 and 100% are formed. Such quantum dots
may be incorporated into a matrix material suitable for down
converting in LED applications.
[0081] Composites formed by conventional approaches typically
suffer from non-uniform dispersion of quantum dots throughout the
matrix material which can result in particle agglomeration.
Agglomeration may be so severe as to result in emission quenching
reducing light output. Another problem is lack of compatibility
between the quantum dots and the matrix reduces composite
performance. Lack of materials compatibility may introduce a
discontinuity at the polymer/quantum dot interface where composite
failure may initiate when it is deployed in ordinary use.
[0082] Accordingly, there remains a need for a composite material
having a quantum dot composition in a matrix that is strong,
resistant to thermal degradation, resistant to chemical
degradation, provides good adhesion between the coated quantum dot
and coupling agent and provides good adhesion between the coupling
agent and the polymer matrix. Embodiments described below include
quantum dots incorporated into composite matrixes to produce high
refractive index films having a high PLQY suitable for solid state
device lighting including light emitting diodes.
[0083] In an embodiment, an approach for incorporating quantum dots
into matrix materials includes coating the quantum dot with a
silica shell and reacting the silica shell with a silane coupling
agent having two reactive functionalities under the proper
conditions. Such an arrangement drives a condensation reaction,
binding one end of the silane to the silica surface and leaving the
other end of the molecule exposed for integration into a matrix.
Other approaches include using a curable material such as metal
oxide nanocrystals in a matrix material. In the curable material,
metal oxide nanocrystals are linked to a polymer matrix via
titanate or a zirconate coupling agents as well as a silane
coupling agent, where the metal atoms of the coupling agent link to
the oxygen atoms of the metal oxide nanocrystals. Since metal
oxides generally do not have a higher refractive index, the curable
material incorporating the metal oxide nanocrystals typically can
not achieve a refractive index sufficient to improve the light
extraction efficiency of photons emitted by an LED in a solid-state
device. A high refractive index material including zinc sulfide
(ZnS) in a matrix material is another approach attempted. In making
the high refractive index material, ZnS colloids are synthesized
with ligands having hydroxyl functional groups that are linked to
isocyanate function groups present on an oligomer backbone in the
matrix material.
[0084] In a general embodiment, a composite includes a matrix
material. A plurality of semiconductor structures (e.g., quantum
dot structures having a coated or non-coated core/shell pairing,
such as the structures described above) is embedded in the matrix
material. In an embodiment, a lighting apparatus includes a light
emitting diode and a composite coating the light emitting diode.
The composite may be formed by embedding quantum dots in a matrix
material described below.
[0085] With reference to the above described composite, in an
embodiment, each of the plurality of semiconductor structures is
cross-linked with, polarity bound by, or tethered to the matrix
material. In an embodiment, each of the plurality of semiconductor
structures is bound to the matrix material by a covalent, dative,
or ionic bond. By way of example, FIGS. 9A-9C illustrate schematic
representations of possible composite compositions for quantum dot
integration, in accordance with an embodiment of the present
invention. Referring to FIG. 9A, a nanocrystalline core 902A and
shell 904A pairing is incorporated into a polymer matrix 906A by
active cross-linking through multiple and interchain binding to
form a cross-linked composition 908A. Referring to FIG. 9B, a
nanocrystalline core 902B and shell 904B pairing is incorporated
into a polymer matrix 906B by polarity-based chemical similarity
and dissolution to form a polarity based composition 908B.
Referring to FIG. 9C, a nanocrystalline core 902C and shell 904C
pairing is incorporated into a polymer matrix 906C by reactive
tethering by sparse binding and chemical similarity to form a
reactive tethering based composition 908C.
[0086] With reference again to the above described composite, in an
embodiment, one or more of the semiconductor structures further
includes a coupling agent covalently bonded to an outer surface of
the insulator layer. For example, in one such embodiment, the
insulator layer includes or is a layer of silica (SiO.sub.x), and
the coupling agent is a silane coupling agent, e.g., having the
formula X.sub.nSiY.sub.4-n, where X is a functional group capable
of bonding with the matrix material and is one such as, but not
limited to, hydroxyl, alkoxy, isocyanate, carboxyl, epoxy, amine,
urea, vinyl, amide, aminoplast and silane, Y is a functional group
such as, but not limited to, hydroxyl, phenoxy, alkoxy, hydroxyl
ether, silane or aminoplast, and n is 1, 2 or 3. In another
embodiment, however, the coupling agent is one such as, but not
limited to, a titanate coupling agent or a zirconate coupling
agent. It is to be understood that the terms capping agent, capping
ligand, ligand and coupling agent may be used interchangeably as
described above and, generally, may include an atom, molecule or
other chemical entity or moiety attached to or capable of being
attached to a nanoparticle. Attachment may be by dative bonding,
covalent bonding, ionic bonding, Van der Waals forces or other
force or bond.
[0087] In the case that a silica surface of a silica coated quantum
dot is modified using silane coupling agents having multiple
functional moieties, coupling to the surface of the silica shell
and coupling to a matrix material and/or other matrix additives may
be enabled. Such an approach provides uniform dispersion throughout
the composite matrix using as little effort (e.g., reaction energy)
as possible. Stronger physical and/or chemical bonding between the
silica coated quantum dots and the matrix resin occurs. Also, the
silane coupling composition must be compatible with both the silica
coated quantum dot, which is inorganic, and the polymer matrix,
which may be organic. Without being bound by any particular theory
or principle, it is believed that the silane coupling agent forms a
bridge between the silica and the matrix resin when reactive
functional groups on the silane coupling agent interact with
functional groups on the surface of the silica and/or the matrix
resin. Because the functional groups involved are typically polar
in nature, the coupling agent tends to be hydrophilic and readily
dispersed in an aqueous size composition.
[0088] Matrix materials suitable for embodiments of the present
invention may satisfy the following criteria: they may be optically
clear having transmission in the 400-700 nm range of greater than
90%, as measured in a UV-Vis spectrometer. They may have a high
refractive index between about 1.0 and 2.0, preferably above 1.4 in
the 400-700 nm range. They may have good adhesion to an LED surface
if required and/or are sufficiently rigid for self-supporting
applications. They may able to maintain their properties over a
large temperature range, for example -40.degree. C. to 150.degree.
C. and over a long period of time (over 50,000 hours at a light
intensity typically 1-10 w/cm2 of 450 nm blue light).
[0089] Thus, with reference again to the above described composite,
in an embodiment, the insulator layer is composed of a layer of
silica (SiO.sub.x), and the matrix material is composed of a
siloxane copolymer. In another embodiment, the matrix material has
a UV-Vis spectroscopy transmission of greater than 90% for light in
the range of 400-700 nanometers. In an embodiment, the matrix
material has a refractive index approximately in the range of 1-2
for light in the range of 400-700 nanometers. In an embodiment, the
matrix material is thermally stable in a temperature range of
-40-250 degrees Celsius. In an embodiment, the matrix material is
composed of a polymer such as, but not limited to, polypropylene,
polyethylene, polyesters, polyacetals, polyamides, polyacrylamides,
polyimides, polyethers, polyvinylethers, polystyrenes, polyoxides,
polycarbonates, polysiloxanes, polysulfones, polyanhydrides,
polyamines, epoxies, polyacrylics, polyvinylesters, polyurethane,
maleic resins, urea resins, melamine resins, phenol resins, furan
resins, polymer blends, polymer alloys, or mixtures thereof. In one
such embodiment, the matrix material is composed of a polysiloxane
such as, but not limited to, polydimethylsiloxane (PDMS),
polymethylphenylsiloxane, polydiphenylsiloxane and
polydiethylsiloxane. In an embodiment, the matrix material is
composed of a siloxane such as, but not limited to,
dimethylsiloxane or methylhydrogen siloxane.
[0090] Additionally, with reference again to the above described
composite, in an embodiment, the plurality of semiconductor
structures is embedded homogeneously in the matrix material. In an
embodiment, the composite further includes a compounding agent
embedded in the matrix material. The compounding agent is one such
as, but not limited to, an antioxidant, a pigment, a dye, an
antistatic agent, a filler, a flame retardant, an ultra-violet (UV)
stabilizer, or an impact modifier. In another embodiment, the
composite further includes a catalyst embedded in the matrix
material, the catalyst one such as, but not limited to, a thiol
catalyst or a platinum (Pt) catalyst.
[0091] Accordingly, in an embodiment, a method of fabrication
includes forming a plurality of semiconductor structures embedded
in a matrix material (or embedding preformed semiconductor
structures in a matrix material). In one such embodiment, embedding
the plurality of semiconductor structures in the matrix material
includes cross-linking, reactive tethering, or ionic bonding the
plurality of semiconductor structures with the matrix material. In
an embodiment, the method further includes surface-functionalizing
an insulator layer for the semiconductor structures prior to
embedding the plurality of semiconductor structures in the matrix
material. In one such embodiment, the surface-functionalizing
includes treating the insulator layer with a silane coupling agent.
However, in an alternative embodiment, coated semiconductor
structures are embedded in a matrix by using a ligand-free
insulator layer.
[0092] In another embodiment, simple substitution at the surface of
the silica coated quantum dots is effective for stable integration
without undesired additional viscosity and is suitable to produce a
low-viscosity product such as a silicone gel. In one embodiment of
the present invention a composite incorporates quantum dots which
crosslink with the matrix through silane groups and which possess
an adequate number of silane groups in order to form an elastic
network. In addition, adequate adhesion to various substrates is
enabled. Furthermore, silicone-based matrixes may be used. A
structure of such polymers may be obtained which form
microstructures in the crosslinked composition, thereby yielding
cross-linked polymer compounds with an excellent mechanical
strength. Furthermore, because of the distribution of the reactive
silane groups, a high elasticity may be obtained after
cross-linking.
EXEMPLARY SYNTHETIC PROCEDURES
Example 1
Synthesis of CdSe Core Nanocrystals
[0093] 0.560 g (560 mg) of ODPA solid was added to a 3-neck 25 ml
round-bottom flask and 6 g TOPO solid was added to the flask. 0.120
g (120 mg) of CdO solid was added to the flask. With the flask
sealed and the reagents inside (CdO, ODPA, TOPO), heat the reaction
to 120.degree. C. under flowing UHP Argon gas. When the reaction
mixture becomes liquid, begin stirring at 800 RPM to completely
distribute the CdO and ODPA. When the temperature equilibrates at
around 120.degree. C., begin degassing the reaction mixture:
Standard degas is for 30 minutes at as low a vacuum as the system
can maintain, preferably between 10-30 torr. After the first degas,
switch the reaction back to flowing UHP Argon gas. The temperature
of the reaction was raised to 280.degree. C. to dissociate the CdO.
Dissociation is accompanied by a loss of the typical red color for
CdO. After dissociation of the CdO, cool the reaction to
120.degree. C. for the 2nd degassing step. Preferably this step is
done slowly. In one embodiment this is done in increments of 40
degrees and allowed to equilibrate at each step. When the reaction
mixture has cooled to about 120.degree. C., begin the second
degassing step. The second degassing is typically 1 hour at the
lowest vacuum level possible. After the second degassing, switch
the reaction back to flowing UHP Argon. Heat the reaction mixture.
Inject 3.0 g TOP into the reaction solution as temperature
increases above 280.degree. C. Equilibrate the reaction solution at
370.degree. C. When the reaction is equilibrated at 370.degree. C.,
inject 0.836 g of 14% Se:TOP stock solution into the solution. The
reaction is run until the desired visible emission from the core is
achieved. For CdSe cores the time is usually between 0.5 and 10
minutes. To stop the reaction: while continuing to stir and flow
UHP Argon through the reaction, rapidly cool the solution by
blowing nitrogen on the outside of the flask. When the reaction
temperature is around 80.degree. C., expose the reaction solution
to air and inject approximately 6 mL of toluene. Precipitate the
CdSe nanocrystals through the addition of 2-propanol (IPA) to the
reaction solutions. Preferably the mixture should be approximately
50/50 (by volume) reaction solution/IPA to achieve the desired
precipitation. Centrifuge for 5 minutes at 6000 RPM. Redissolve the
CdSe in as little toluene as possible solid (<2 mL). Precipitate
the CdSe again using IPA. Centrifuge. Decant the supernatant
liquid. Dissolve the CdSe solid in anhydrous toluene.
Example 2
[0094] Synthesis of CdSe/CdS core-shell nanocrystal
heterostructures having PLQY>90%. Transfer 0.290 g (290 mg) of
ODPA into a round bottom flask. Transfer 0.080 g (80 mg) of
hexylphosphonic acid (HPA) into the flask. Transfer 3 g TOPO into
the flask. Transfer 0.090 g (90 mg) of CdO solid into the reaction
flask. With the flask sealed and the reagents inside (CdO, ODPA,
TOPO, HPA), heat the reaction to 120.degree. C. under flowing UHP
Argon gas. When the reaction mixture becomes liquid, at about
60.degree. C., begin stiffing at 800 RPM to completely distribute
the CdO, ODPA, and HPA. When the temperature settles at 120.degree.
C., begin degassing the reaction mixture. After the degas step,
switch the reaction back to flowing UHP Argon gas. Raise the
temperature of the reaction to 280.degree. C. to dissociate the
CdO. Increase the temperature set-point of the reaction to
320.degree. C. Inject 1.5 g TOP into the reaction solution as
temperature increases above 280.degree. C. When the reaction is
equilibrated at 320.degree. C., inject a mixture of 1.447 g of 7.4%
S:TOP stock solution and 0.235 g concentration-adjusted CdSe seed
stock into the reaction solution. Immediately reduce the set point
of the temperature controller to 300.degree. C. Allow the reaction
to proceed for the requisite time to necessary to produce the
desired length and width of shell, yielding a rod having an aspect
ratio as between 1.5 and 10, more preferably between 3 and 6.
Reaction temperature for shell growth is between 120.degree. C. and
380.degree. C., preferably between 260.degree. C. and 320.degree.
C., more preferably between 290.degree. C. and 300.degree. C.
[0095] The reaction is monitored by testing a sample to determine
the absorbance at 400 nm and the at the CdSe exciton peak. Most
preferably the reaction is stopped when the absorbance at 400 nm
divided by the absorbance at the CdSe exciton peak is between about
25-30, but the invention contemplates that the absorbance ratio may
be between about 6 and about 100, preferably between about 15-35.
By "stopping the growth" it is meant that any method steps may be
employed known in the art if desired and available to cease the
growth of the shell. Some methods will lead to quicker cessation of
shell growth than others.
[0096] Absorbance measuring may be performed by UV-VIS
spectroscopic analytical method, such as a method including flow
injection analysis for continuous monitoring of the reaction. In an
embodiment, the reaction is stopped or arrested by removing a
heating mantle and allowing the reaction vessel to cool. When the
reaction temperature is around approximately 80 degrees Celsius,
the reaction solution is exposed to air and approximately 4-6 mL of
toluene is injected. The quantum dots are purified by transferring
the reaction solution into four small centrifuge tubes, so that an
equal volume is in each tube. The QDH product is precipitated
through the addition of 2-propanol (IPA) to the reaction solutions.
Following centrifuging, the supernatant liquid is decanted. The QDH
is redissolved in as little toluene as possible (e.g., less than
approximately 2 mL) and re-concentrated into one centrifuge tube.
The precipitation and centrifugation steps are repeated. The final
solid product is then dissolved in approximately 2 g of
toluene.
Example 3
[0097] Synthesis of CdSe/CdS quantum dot having an absorbance ratio
between 6-100. A quantum dot was fabricated according to Example 2
and having an absorbance ratio between 6-100. FIG. 10 is a
transmission electron microscope (TEM) image 1000 of a sample of
core/shell (1002/1004) CdSe/CdS quantum dots, in accordance with an
embodiment of the present invention. The TEM image 1000 indicates
that there are substantially no structural defects as can be
deduced from the low density of stacking faults and lack of other
visible defects along the semiconductor structure 1002/1004.
Example 4
[0098] Synthesis of CdSe/CdS red quantum dot with a PLQY=96%.
Quantum dots were fabricated according to Example 2 and having an
absorbance ratio between 6-100, and having a PLQY of 96% at 606 nm.
The average length (from TEM data) is 22.3 nm.+-.3.1 nm. The
average width (from TEM data) is 6.0 nm.+-.0.6 nm. The average
aspect ratio (from TEM data) is 3.8.+-.0.6. FIG. 11 is a plot 1100
including a UV-Vis absorbance spectrum 1102 and photoluminescent
emission spectrum 1104 for a CdSe/CdS core/shell quantum dot having
a PLQY of 96%, in accordance with an embodiment of the present
invention. The quantum dot has essentially no overlapping
absorption and emission bands. FIG. 12 is a transmission electron
microscope (TEM) image 1200 of a sample of CdSe/CdS quantum dots
1202 fabricated according to example 4, in accordance with an
embodiment of the present invention.
Example 5
Reactive Ligand Exchange for Quantum Dot Structures
[0099] 0.235 g of concentration-adjusted CdSe stock from Example 2
are exposed to a reactive exchange chemical,
trimethylsilylpyrollidine (TMS-Pyr), for 20 minutes in an air-free
environment and are mixed completely. After 20 minutes, an alcohol,
usually 2-propanol or methanol is added to the mixture to quench
the reactivity of the TMS-Pyr reagent, and to precipitate the
reactively exchanged CdSe particles. The precipitated particles are
centrifuged at 6000 RPM for 5 minutes. The resulting supernatant
liquid is decanted and the precipitate are re-dissolved in 0.235 g
of anhydrous toluene for use in the procedure described in Example
2. Reactive ligand exchange is used to introduce any number of
desired surface functionalities to the surface of quantum dot cores
prior to rod growth or the surface of the core/shell particles
after synthesis.
Example 6
[0100] Coating semiconductor nanocrystalline core/shell pairing
with silica using dioctyl sodium sulfosuccinate (AOT).
Approximately 4.5 g of AOT is dissolved in 50 mL of cyclohexane.
0.5 g of QDH is precipitated w/methanol, and then re-dissolved in
hexane. 20 .mu.L of 3-aminopropyltrimethoxysilane (APTMS) is added
and stirred for 30 minutes. 900 .mu.L of NH4OH (29 wt %) is added
into the solution immediately followed by 600 .mu.L of TEOS. The
solution is stirred for about 16 hrs which allows the mixture to
react until a silica shell coats the nanocrystal. The silica coated
particles are precipitated by MeOH and the precipitated particles
are separated from the supernatant using a centrifuge. The
SiO.sub.2 coated particles can be re-dispersed in toluene or left
in cyclohexane.
Example 7
[0101] Coating a semiconductor nanocrystal with silica using IGEPAL
CO-520. Approximately 4.46 g of Igepal CO-520 (Polyoxyethylene (5)
nonylphenylether) is dissolved in 50 mL of cyclohexane and allowed
to mix. "n" may be 3, 4, 5, 6, 7, 8, 9 or 10, preferably about 5.
0.5 grams of quantum dots dissolved in toluene are added. 20 .mu.L
of 3-APTMS is added and stirred for about 30 minutes. 900 .mu.L of
NH.sub.4OH (29 wt %) is added into the solution immediately
followed by 600 .mu.L of TEOS. The solution is stirred for about 16
hrs at 1600 rpm which allows the mixture to react until a silica
shell coats the nanocrystal. The micelles are broken up by IPA and
collected using a centrifuge. The SiO.sub.2 coated particles may be
re-dispersed in toluene or left in cyclohexane for polymer
integration.
Example 8
[0102] Methoxy silane coupling agent. Silica-shelled core-shell
quantum dots are dispersed in 20 parts toluene to 1 part
(MeO).sub.3SiR(R=allyl or vinyl), and constantly stirred to allow
the coupling reaction to take place. The functionalized particles
are separated and cleaned by precipitation with IPA and
centrifugation at 6000 rpm for 10 min. The process is repeated two
or more times. Cleaned particles are dispersed in a known amount of
toluene or polymer solution.
Example 9
Quantum Dot/Polymer Preparation
[0103] To prepare the films, a known mass of quantum dots in
toluene or cyclohexane is added to premade polymer solution,
depending on solvent compatibility of the polymer matrix used.
Other solvents may also be used for dissolution, if so desired for
polarity match with the matrix or to increase or decrease the
viscosity or rate of solvent evolution from the cast film.
Example 10
Film Casting
[0104] The composite compositions are prepared by drop casting
approximately 360 .mu.L of QDH polymer solution onto a 12 mm glass
round. The amount of quantum dots added to the polymer solution can
be adjusted for different optical densities in the final QDH film.
After casting films, the slow evaporation of solvent is important
to give a film free of large surface imperfections. QDH-polymer
solutions in toluene are allowed to evaporate in a vented fume
hood. The films are cast on a level stainless plate. Once films are
dried they are analyzed for PLQY and UV-Vis properties.
Example 11
[0105] The surface of silica-shelled quantum dot was functionalized
using a variety of methoxy and ethoxy silanes: (MeO).sub.3SiAllyl,
(MeO).sub.3SiVinyl, (MeO).sub.2SiMeVinyl, (EtO).sub.3SiVinyl,
EtOSi(Vinyl).sub.3. The functionalized silica-shelled quantum dot
was then used in the standard polymer formulation with additives
for crosslinking, as well as without any further crosslinking
co-agents such as TAIC in the case of EVA or divinylsilanes for
siloxanes.
Example 12
[0106] In one embodiment, it is preferred that the olefin group is
able to participate in a crosslinking process through radical
mechanism in the case of EVA or through hydrosilylation process in
the case of siloxanes. Allyl and vinyl are preferred, but other
olefins can be included.
Example 13
[0107] In one embodiment, the degree of crosslinking may be
increased using quantum dots with a higher density of the olefin
groups on silica surface of quantum dots.
Example 14
Using Polarity
[0108] The surface of a silica-shelled particle is modified with
organo-substituted silanes in order to maximize the compatibility
with a polymer matrix such as the polysiloxanes for LEDs. The
silica surface is modified with organo-substituted silanes, and its
properties are therefore modified by the grafted functional
groups.
Example 15
Pt Catalyst
[0109] A platinum-based catalyst may be introduced in Examples
9-14. In addition to the functionalized silica particles, two
competing or complementary catalysts are available for
cross-linking.
Example 16
Thiol Catalyst
[0110] The Pt catalyst of example 15 is replaced with a thiol
catalyst with a thiol-ene reaction. Di-thiols or multifunctional
thiols are used. The approach enables UV curing in place of heat
curing.
[0111] The above described semiconductor structures, e.g., quantum
dots, may be fabricated to further include one or more
compositional transition layers between portions of the structures,
e.g., between core and shell portions. Inclusion of such a
transition layer may reduce or eliminate any performance
inefficiency associated with otherwise abrupt junctions between the
different portions of the structures. For example, the inclusion of
a compositional transition layer may be used to suppress Auger
recombination within a quantum dot structure. Auger recombination
events translate to energy from one exciton being non-radiatively
transferred to another charge carrier. Such recombination in
quantum dots typically occurs on sub-nanosecond time scales such
that a very short multi-exciton lifetime indicates non-radiative
recombination, while higher nanosecond bi-exciton lifetimes
indicate radiative recombination. A radiative bi-exciton has a
lifetime approximately 2-4 times shorter than radiative single
exciton.
[0112] Temperature performance can be highly dependent on
semiconductor surface preparation, indicating that accessibility of
surface states may be responsible for high temperature behavior.
Light-induced intermittency is a feature of quantum dots which is
believed to lead to poor performance at high flux intensities. The
phenomenon is relatively temperature independent, which leads to a
surface state insensitive model for the behavior. Instead, it
appears to be controlled primarily by the composition and nature of
the internal particle interfaces.
[0113] Accordingly, in an embodiment, a quantum dot architecture is
fabricated to include one or more of a graded composition, multiple
layers, and asymmetry to control exciton dynamics. Such tailoring
of the quantum dot architecture may be performed to provide optimum
photoluminescent (PL) quantum yield performance at high
temperatures (e.g., up to approximately 200 degrees Celsius) and
high incident flux intensities (e.g., up to approximately 150
W/cm.sup.2). As a benchmark, a variety of architectures for quantum
dots have been shown to provide very high quantum yields (e.g., 80%
or higher) at room temperatures, but often exhibit temperature
droop and photobleaching at high temperatures and high fluxes. One
or more embodiments of the present invention are directed to the
fabrication of quantum dots having compositions and architectures
suitable to address the above described structural issues which
have been known to cause temperature droop and photobleaching.
[0114] More specifically, as is described in greater detail below
in association with FIGS. 13-15, an optimal particle (e.g., quantum
dot structure) may have one or more of a high aspect ratio, a large
volume relative to other quantum dot heterostructures, and graded
or alloyed transitions between different semiconductor materials.
The graded or alloyed transitions can be used to render a
compositional and structural transition from one component (such as
a quantum dot core) to another component (such as a quantum dot
shell) a smooth function rather than a step function. In one
embodiment, the terms "graded," "gradient," or "grading" are used
to convey gradual transitioning from one semiconductor to another.
In one embodiment, the terms "alloy," "alloyed," or "alloying" are
used to convey an entire volume having a fixed intermediate
composition. In more specific embodiments, core or seed volume is
maximized relative to shell volume for a given emission color. A
graded or alloyed core/shell transition layer may be included
between the two volumes.
[0115] Furthermore, temperature droop may be attributed to surface
states which are made more accessible to an excited state at high
temperatures. In an embodiment, temperature droop is mitigated or
eliminated by using large, crystalline volume in addition to a
large bandgap crystalline coating over the core and an amorphous
outer coating applied to the resulting to the particle. In an
embodiment, photo-bleaching is mitigated or eliminated by alloying
or grading a well-passivated particle at transitions between
semiconductor layers. Alloying or grading the transition between
different semiconductors may provide certainty for momentum of the
carriers, reducing non-radiative excited state relaxation
processes.
[0116] Past approaches to addressing quantum dot inefficiencies
have included the use of spherical alloyed particles which
demonstrate good photobleaching characteristics, but with starting
PLQYs in the 40% range. Such systems provide reasonable behavior at
high intensity but with emission efficiencies that may be too low
to be useful in a device such as a lighting device. Furthermore,
past approaches have not effectively addressed both photo-bleaching
and temperature droop in a same particle.
[0117] In an exemplary embodiment, a quantum dot is fabricated to
include core semiconductor nanoparticle is fabricated to have an
aspect ratio between 1 and 2, a graded transition of 1/2 to 2
monolayers between core and shell, a rod-shaped shell with a high
aspect ratio (e.g., between 4 and 6), a graded transition of 1/2 to
2 monolayers between first and second semiconductor shell
materials, a final intrinsic layer of the second semiconductor
shell material, and an outer coating of amorphous insulator, such
as SiO.sub.2. Such quantum dots exhibit very high PLQYs preserved
both at room temperature and under conditions of high temperature
and high flux for long periods of time. More generally, such
quantum dots may provide optimal temperature stability and high
temperature performance by stabilizing a seed/rod structure with a
coating of amorphous wide bandgap insulator. The addition of the
insulating layer may serve to passivate the surface of the quantum
dots, which may be a key to improving reliability. Additional
insulator coating may be included to provide extremely high room
temperature quantum yields, as well as isolation of an
electronically active core from its surroundings, excellent
temperature and light stability, and a pathway to a robust and
stable composite with relevant plastics for LED applications.
Essentially, in an embodiment, the insulator provides an extra
barrier between the active quantum dot and the matrix, preventing
oxidation of the surface, uncontrolled alloying, ligand loss and
other undesirable high temperature processes. Since surface defects
can be detrimental to room-temperature and high temperature
performance of quantum dots, the addition of an amorphous layer can
be key to their durability in the LED product. The above and other
examples of architectures for quantum dots having one or more
alloyed or graded compositional transition layer are illustrated in
and described below in association with FIGS. 13-15.
[0118] In a first example, FIG. 13 illustrates a cross-sectional
view of a semiconductor structure having a nanocrystalline core and
nanocrystalline shell pairing with one compositional transition
layer, in accordance with an embodiment of the present
invention.
[0119] Referring to FIG. 13, a semiconductor structure 1300
includes a nanocrystalline core 1302 composed of a first
semiconductor material. A nanocrystalline shell 1304 composed of a
second, different, semiconductor material at least partially
surrounds the nanocrystalline core 1302. A compositional transition
layer 1310 is disposed between, and in contact with, the
nanocrystalline core 1302 and nanocrystalline shell 1304. The
compositional transition layer 1310 has a composition intermediate
to the first and second semiconductor materials.
[0120] In an embodiment, the compositional transition layer 1310 is
an alloyed layer composed of a mixture of the first and second
semiconductor materials. In another embodiment, the compositional
transition layer 1310 is a graded layer composed of a compositional
gradient of the first semiconductor material proximate to the
nanocrystalline core 1302 through to the second semiconductor
material proximate to the nanocrystalline shell 1304. In either
case, in a specific embodiment, the compositional transition layer
1310 has a thickness approximately in the range of 1.5-2
monolayers. Exemplary embodiments include a structure 1300 where
the first semiconductor material is cadmium selenide (CdSe), the
second semiconductor material is cadmium sulfide (CdS), and the
compositional transition layer 1310 is composed of
CdSe.sub.xS.sub.y, where 0<x<1 and 0<y<1, or where the
first semiconductor material is cadmium selenide (CdSe), the second
semiconductor material is zinc selenide (ZnSe), and the
compositional transition layer 1310 is composed of
Cd.sub.xZn.sub.ySe, where 0<x<1 and 0<y<1.
[0121] In accordance with an embodiment of the present invention,
the compositional transition layer 1310 passivates or reduces trap
states where the nanocrystalline shell 1304 surrounds the
nanocrystalline core 1302. Exemplary embodiments of core and/or
shell parameters include a structure 1300 where the nanocrystalline
core 1302 is an anisotropic nanocrystalline core having an aspect
ratio between, but not including, 1.0 and 2.0 (in a specific
embodiment, approximately in the range of 1.01-1.2), and the
nanocrystalline shell is an anisotropic nanocrystalline shell
having an aspect ratio approximately in the range of 4-6.
[0122] In an embodiment, the nanocrystalline shell 1304 completely
surrounds the nanocrystalline core 1302, as depicted in FIG. 13. In
an alternative embodiment, however, the nanocrystalline shell 1304
only partially surrounds the nanocrystalline core 1302, exposing a
portion of the nanocrystalline core 1302. Furthermore, in either
case, the nanocrystalline core 1302 may be disposed in an
asymmetric orientation with respect to the nanocrystalline shell
1304. In one or more embodiments, semiconductor structures such as
1300 are fabricated to further include a nanocrystalline outer
shell 1306 at least partially surrounding the nanocrystalline shell
1304. The nanocrystalline outer shell 1306 may be composed of a
third semiconductor material different from the first and second
semiconductor materials, i.e., different from the materials of the
core 1302 and shell 1304. The nanocrystalline outer shell 1306 may
completely surround the nanocrystalline shell 1304 or may only
partially surround the nanocrystalline shell 1304, exposing a
portion of the nanocrystalline shell 1304.
[0123] For embodiments including a nanocrystalline outer shell, an
additional compositional transition layer may be included. Thus, in
a second example, FIG. 14 illustrates a cross-sectional view of a
semiconductor structure having a nanocrystalline
core/nanocrystalline shell/nanocrystalline outer shell combination
with two compositional transition layers, in accordance with an
embodiment of the present invention.
[0124] Referring to FIG. 14, a semiconductor structure 1400
includes the nanocrystalline core 1302, nanocrystalline shell 1304,
nanocrystalline outer shell 1306 and compositional transition layer
1310 of structure 1300. However, in addition, semiconductor
structure 1400 includes a second compositional transition layer
1412 disposed between, and in contact with, the nanocrystalline
shell 1304 and the nanocrystalline outer shell 1306. The second
compositional transition layer 1412 has a composition intermediate
to the second and third semiconductor materials, i.e., intermediate
to the semiconductor materials of the shell 1304 and outer shell
1306.
[0125] In an embodiment, the second compositional transition layer
1412 is an alloyed layer composed of a mixture of the second and
third semiconductor materials. In another embodiment, the second
compositional transition layer 1412 is a graded layer composed of a
compositional gradient of the second semiconductor material
proximate to the nanocrystalline shell 1304 through to the third
semiconductor material proximate to the nanocrystalline outer shell
1306. In either case, in a specific embodiment, the second
compositional transition layer 1412 has a thickness approximately
in the range of 1.5-2 monolayers. Exemplary embodiments include a
structure 1400 where the first semiconductor material is cadmium
selenide (CdSe), the second semiconductor material is cadmium
sulfide (CdS), the third semiconductor material is zinc sulfide
(ZnS), and the second compositional transition layer 1412 is
composed of Cd.sub.xZn.sub.yS, where 0<x<1 and 0<y<1,
or the first semiconductor material is cadmium selenide (CdSe), the
second semiconductor material is zinc selenide (ZnSe), the third
semiconductor material is zinc sulfide (ZnS), and the second
compositional transition layer 1412 is composed of
ZnSe.sub.xS.sub.y, where 0<x<1 and 0<y<1. In accordance
with an embodiment of the present invention, the second
compositional transition layer 1412 passivates or reduces trap
states where the nanocrystalline outer shell 1306 surrounds the
nanocrystalline shell 1304.
[0126] For other embodiments including a nanocrystalline outer
shell, an outer compositional transition layer may be included
without including an inner compositional transition layer. Thus, in
a third example, FIG. 15 illustrates a cross-sectional view of a
semiconductor structure having a nanocrystalline
core/nanocrystalline shell/nanocrystalline outer shell combination
with one compositional transition layer, in accordance with an
embodiment of the present invention.
[0127] Referring to FIG. 15, a semiconductor structure 1500
includes the nanocrystalline core 1302, nanocrystalline shell 1304,
and nanocrystalline outer shell 1306 of structure 1300. In
addition, the semiconductor structure 1500 includes the
compositional transition layer 1412 of structure 1400 disposed
between, and in contact with, the nanocrystalline shell 1304 and
the nanocrystalline outer shell 1306. However, structure 1500 does
not include the compositional transition layer 1310 of structure
1300, i.e., there is no compositional transition layer between the
core 1302 and shell 1304.
[0128] Referring to FIGS. 13-15, as depicted, the structures 1300,
1400 and 15500 may further include an insulator coating 1308
surrounding and encapsulating the nanocrystalline
core/nanocrystalline shell pairing or nanocrystalline
core/nanocrystalline shell/nanocrystalline outer shell combination.
In one such embodiment, the insulator coating is composed of an
amorphous material such as, but not limited to, silicon dioxide
(SiO.sub.2), silicon oxide (SiO.sub.x), aluminum oxide
(Al.sub.2O.sub.3), zirconia (ZrO.sub.x), titania (TiO.sub.x), or
hafnia (HfO.sub.x). In an embodiment, the structures 1300, 1400 and
1500 are quantum dot structures. For example, structures 1300, 1400
and 1500 may be used as a down-converting quantum dot or an
up-shifting quantum dot.
[0129] A plurality of quantum dot structures such as structures
1300, 1400 or 1500 may be included in a composite having a matrix
material and a plurality of the semiconductor structures embedded
in the matrix material. In one such embodiment, each of the
plurality of semiconductor structures is cross-linked with,
polarity bound by, or tethered to the matrix material. In another
such embodiment, each of the plurality of semiconductor structures
is bound to the matrix material by a covalent, dative, or ionic
bond. In either case, in a specific embodiment, one or more of the
semiconductor structures further includes a coupling agent
covalently bonded to an outer surface of the amorphous insulator
coating 1308.
[0130] In another aspect, generally, individual quantum dots
exhibit characteristic "on/off" events, termed blinking, which
represent emission from neutral and charged species, respectively.
It is this behavior that is believed to be responsible for the
photobleaching behavior often observed in quantum dots. The
formation of charged species is believed to arise from charge
transfer from within the quantum dot to trap states at the
core/shell interface, its surface, or the matrix, and increases as
a function of incident intensity. Core volume and surface have been
shown to be a significant parameter in blinking behavior. Of even
greater importance may be the formation of smooth transitions
between different epitaxial semiconductor layers.
[0131] Furthermore, changes to the core or seed size and overall
dimensions of the nanocrystalline semiconductor particle may affect
intrinsic particle performance and stability, and it is important
to understand the changes in exciton dynamics in order to prevent
or correct detrimental changes. Time resolved optical interrogation
techniques may thus be used as part of the characterization process
when changes to a quantum dot structure are made. Time resolved
photoluminescence (TRPL) provides the excited state lifetime, as
both a function of incident intensity and temperature. Transient
absorption spectroscopy measures the change in the absorption
coefficient of a sample after it is excited by a short (<1 ps)
pulse at 400-450 nm. By monitoring the time-dependence of this
change at different wavelengths (400 nm to 1200 nm) the radiative
and non-radiative dynamics of the decay pathways can be
investigated.
[0132] Another characterization technique for analyzing high
intensity behavior is PLQY at high flux and high temperature. By
analyzing photoluminescence quantum yield at high flux and high
temperature, the multi-exciton radiative efficiency and activation
energy of trap-states can be ascertained. An example of time
resolved photoluminescence of non-alloyed rod materials is given in
FIG. 16. FIG. 16 is a plot 1600 of increasing photoluminescence
(PL) intensity as a function of time (seconds), based on equation
1602 and data plot 1604, for non-alloyed rod materials. The time
resolved photoluminescence is obtained at high fluence, e.g., 0.016
and 0.05 mW, with a radiative lifetime of approximately 13-14
nanoseconds. As the incident flux is increased by 2 orders of
magnitude, a fast lifetime competes with regular lifetime, and
regular lifetime decreases to 5 nanoseconds. A shorter radiative
lifetime is better at high fluxes to reduce bi-exciton population.
One or more embodiments described herein, such as structures 1300,
1400 or 1500, described above, provide quantum dot architectures
suitable for multi-exciton radiative efficiency, e.g., for
up-shifting or down-converting layers in lighting devices.
[0133] Thus, semiconductor structures having a nanocrystalline core
and nanocrystalline shell pairing with one or more compositional
transition layers have been disclosed.
* * * * *