U.S. patent application number 10/212001 was filed with the patent office on 2003-04-10 for quantum dots of group iv semiconductor materials.
Invention is credited to Lee, Howard Wing Hoon.
Application Number | 20030066998 10/212001 |
Document ID | / |
Family ID | 29220075 |
Filed Date | 2003-04-10 |
United States Patent
Application |
20030066998 |
Kind Code |
A1 |
Lee, Howard Wing Hoon |
April 10, 2003 |
Quantum dots of Group IV semiconductor materials
Abstract
The invention relates to a quantum dot. The quantum dot
comprises a core including a semiconductor material Y selected from
the group consisting of Si and Ge. The quantum dot also comprises a
shell surrounding the core. The quantum dot is substantially defect
free such that the quantum dot exhibits photoluminescence with a
quantum efficiency that is greater than 10 percent.
Inventors: |
Lee, Howard Wing Hoon;
(Fremont, CA) |
Correspondence
Address: |
COOLEY GODWARD, LLP
3000 EL CAMINO REAL
5 PALO ALTO SQUARE
PALO ALTO
CA
94306
US
|
Family ID: |
29220075 |
Appl. No.: |
10/212001 |
Filed: |
August 2, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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60309898 |
Aug 2, 2001 |
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60309905 |
Aug 2, 2001 |
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60309979 |
Aug 2, 2001 |
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60310090 |
Aug 2, 2001 |
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60310095 |
Aug 2, 2001 |
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Current U.S.
Class: |
257/19 ;
257/E21.09; 257/E29.071; 257/E29.082 |
Current CPC
Class: |
G02B 2006/12195
20130101; G02F 2202/32 20130101; G02B 2006/12145 20130101; G02B
2006/12159 20130101; G02F 1/3517 20130101; G02F 1/365 20130101;
G02F 1/3521 20130101; B82Y 10/00 20130101; H01L 29/1602 20130101;
G02F 1/01791 20210101; H01L 33/20 20130101; G02B 6/1225 20130101;
G02F 1/3556 20130101; B82Y 20/00 20130101; G02F 2/006 20210101;
G02F 2202/36 20130101; H01L 29/127 20130101; Y10S 977/814 20130101;
G02B 6/122 20130101; G02B 2006/12097 20130101; G02F 1/217 20210101;
H01L 29/16 20130101; H01L 33/34 20130101; G02B 2006/12147 20130101;
G02F 1/3515 20130101 |
Class at
Publication: |
257/19 |
International
Class: |
H01L 029/06 |
Claims
What is claimed is:
1. A quantum dot comprising: a) a core including a semiconductor
material Y selected from the group consisting of Si and Ge; and b)
a shell surrounding said core, said quantum dot being substantially
defect free such that said quantum dot exhibits photoluminescence
with a quantum efficiency that is greater than 10 percent.
2. The quantum dot of claim 1, wherein Y is Si, and said core has a
diameter between approximately 1 nm and 20 nm.
3. The quantum dot of claim 1, wherein Y is Ge, and said core has a
diameter between approximately 1 nm and 50 nm.
4. The quantum dot of claim 1, wherein said core has, a diameter
between approximately 1 nm and 10 nm.
5. The quantum dot of claim 1, wherein said core is substantially
spherical with an aspect ratio between approximately 0.8 and
1.2.
6. The quantum dot of claim 1, wherein said shell has a thickness
between approximately 0.1 nm and 5 nm.
7. The quantum dot of claim 1, wherein said shell includes an
oxide.
8. The quantum dot of claim 1, wherein said shell includes an oxide
YO.sub.n with n being between approximately 0 and 2.
9. The quantum dot of claim 8, wherein n is between approximately
1.8 and 2.
10. The quantum dot of claim 1, wherein said shell completely
surrounds said core.
11. The quantum dot of claim 1, wherein said quantum dot exhibits
photoluminescence with a quantum efficiency that is at least 20
percent.
12. The quantum dot of claim 1, wherein said quantum dot exhibits
photoluminescence with a quantum efficiency that is at least 50
percent.
13. The quantum dot of claim 1, further comprising at least one
surface ligand coupled to said shell.
14. The quantum dot of claim 1, further comprising a plurality of
surface ligands coupled to said shell to provide a surface coverage
between approximately 50 percent and 100 percent.
15. A quantum dot comprising: a) a core including a semiconductor
material Y selected from the group consisting of Si and Ge; and b)
a ligand layer surrounding said core, said ligand layer including a
plurality of surface ligands, said quantum dot exhibiting
photoluminescence with a quantum efficiency that is greater than 10
percent.
16. The quantum dot of claim 15, wherein said plurality of surface
ligands are coupled to said core to provide a surface coverage
between approximately 50 percent and 100 percent.
17. The quantum dot of claim 15, wherein said plurality of surface
ligands are coupled to said core to provide a surface coverage
between approximately 80 percent and 100 percent.
18. The quantum dot of claim 15, wherein a surface ligand of said
plurality of surface ligands contains an atom selected from the
group consisting of C, O, S, Si, N, P, and Se, and said core forms
a covalent bond with said atom.
19. The quantum dot of claim 15, wherein a surface ligand of said
plurality of surface ligands is selected from the group consisting
of alkyls, alkenyls, alkynyls, aromatics, aromatic heterocycles,
conjugated aromatics, polyenes, cyanides, hydroxys, alkoxys,
carboxylates, phenoxys, siloxys, cyanates, thioalkyls, thioaryls,
thiocyanates, silylthios, substituted silyl groups, amino groups,
mono-substituted amines, di-substituted amines, imino groups, and
silylaminos.
20. The quantum dot of claim 15, wherein said, quantum dot exhibits
photoluminescence with a quantum efficiency that is at least 20
percent.
21. The quantum dot of claim 15, further comprising a shell
positioned between said core and said ligand layer and surrounding
said core, said shell including an oxide YO.sub.n with n being
between approximately 0 and 2.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application Serial No. 60/309,898, filed on Aug. 2, 2001, U.S.
Provisional Application Serial No. 60/309,905, filed on Aug. 2,
2001, U.S. Provisional Application Serial No. 60/309,979, filed on
Aug. 2, 2001, U.S. Provisional Application Serial No. 60/310,090,
filed on Aug. 2, 2001, and U.S. Provisional Application Serial No.
60/310,095, filed on Aug. 2, 2001, the disclosures of which are
incorporated herein by reference in their entirety.
FIELD OF THE INVENTION
[0002] This invention relates generally to quantum dots. More
particularly, this invention relates to quantum dots of Group IV
semiconductor materials.
BACKGROUND OF THE INVENTION
[0003] Over the past several years, there has been an increasing
interest in exploiting the extraordinary properties associated with
quantum dots. As a result of quantum confinement effects,
properties of quantum dots can differ from corresponding bulk
values. These quantum confinement effects arise from confinement of
electrons and holes along three dimensions. For instance, quantum
confinement effects can lead to an increase in energy gap as the
size of the quantum dots is decreased. Consequently, as the size of
the quantum dots is decreased, light emitted by the quantum dots is
shifted towards higher energies or shorter wavelengths. By
controlling the size of the quantum dots as well as the material
forming the quantum dots, properties of the quantum dots can be
tuned for a specific application.
[0004] Previous attempts at forming quantum dots have largely
focused on quantum dots of direct band gap semiconductor materials,
such as Group II-VI semiconductor materials. In contrast to such
direct band gap semiconductor materials, Group IV semiconductor
materials such as Si and Ge have energy gaps, chemical properties,
and other properties that render them more desirable for a variety
of applications. However, previous attempts at forming quantum dots
of Si or Ge have generally suffered from a number of shortcomings.
In particular, formation of quantum dots of Si or Ge sometimes
involved extreme conditions of temperature and pressure while
suffering from low yields and lack of reproducibility. And, quantum
dots that were produced were generally incapable of exhibiting
adequate levels of photoluminescence that can be tuned over a broad
spectral range. Also, previous attempts have generally been
unsuccessful in producing quantum dots of Si or Ge that are
sufficiently stable under ambient conditions or that can be made
sufficiently soluble in a variety of matrix materials.
[0005] It is against this background that a need arose to develop
the quantum dots and methods for forming quantum dots described
herein.
SUMMARY OF THE INVENTION
[0006] In one innovative aspect, the present invention relates to a
quantum dot. In one embodiment, the quantum dot comprises a core
including a semiconductor material Y selected from the group
consisting of Si and Ge. The quantum dot also comprises a shell
surrounding the core. The quantum dot is substantially defect free
such that the quantum dot exhibits photoluminescence with a quantum
efficiency that is greater than 10 percent.
[0007] In another embodiment, the quantum dot comprises a core
including a semiconductor material Y selected from the group
consisting of Si and Ge. The quantum dot also comprises a ligand
layer surrounding the core. The ligand layer includes a plurality
of surface ligands. The quantum dot exhibits photoluminescence with
a quantum efficiency that is greater than 10 percent.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] For a better understanding of the nature and objects of the
invention, reference should be made to the following detailed
description taken in conjunction with the accompanying drawings, in
which:
[0009] FIGS. 1(a), 1(b), 1(c), and 1(d) illustrate quantum dots
according to some embodiments of the invention.
[0010] FIG. 2 illustrates the energy gap of quantum dots fabricated
from silicon plotted as a function of the size of the quantum dots,
according to an embodiment of the invention.
[0011] FIG. 3 illustrates photoluminescence (PL) spectra from six
samples with different sizes of silicon quantum dots, according to
an embodiment of the invention.
[0012] FIG. 4(a) illustrates the energy gap of quantum dots
fabricated from germanium plotted as a function of the size of the
quantum dots, according to an embodiment of the invention.
[0013] FIG. 4(b) illustrates size-selective photoluminescence (PL)
spectra for different sizes of germanium quantum dots, according to
an embodiment of the invention.
[0014] FIG. 5(a) illustrates concentration dependence of the linear
index of refraction of engineered nonlinear nanocomposite materials
doped with silicon and germanium quantum dots, according to an
embodiment of the invention.
[0015] FIG. 5(b) illustrates concentration dependence of the
optical nonlinearity of engineered nonlinear nanocomposite
materials doped with silicon and germanium quantum dots, according
to an embodiment of the invention.
[0016] FIGS. 6(a), 6(b), 6(c), 6(d), 6(e), and 6(f) illustrate
nonlinear directional couplers comprising engineered nonlinear
nanocomposite materials, according to some embodiments of the
invention.
[0017] FIGS. 7(a), 7(b), 7(c), 7(d), 7(e), and 7(f) illustrate an
embodiment of a nonlinear Mach-Zehnder (MZ) interferometer
comprising an engineered nonlinear nanocomposite material.
[0018] FIGS. 8(a), 8(b), 8(c), and 8(d) illustrate an alternative
embodiment of a nonlinear MZ interferometer comprising an
engineered nonlinear nanocomposite material.
[0019] FIG. 9 illustrates a figure-of-merit (FOM) for all-optical
switching with an engineered nonlinear nanocomposite material as a
function of quantum dot size, according to an embodiment of the
invention.
[0020] FIGS. 10(a) and 10(b) illustrate photoluminescence spectra
of silicon quantum dots made in accordance with an embodiment of
the invention.
[0021] FIGS. 11(a) and 11(b) illustrate photoluminescence spectra
of germanium quantum dots made in accordance with an embodiment of
the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0022] Definitions
[0023] The following definitions may apply to some of the elements
described with regard to some embodiments of the invention. These
definitions may likewise be expanded upon herein.
[0024] As used in this specification and the appended claims, the
singular forms "a", "an", and "the" include plural references
unless the content clearly dictates otherwise. Thus, for example,
reference to "a quantum dot" includes a mixture of two or more such
quantum dots and may include a population of such quantum dots.
[0025] "Optional" or "optionally" means that the subsequently
described event or circumstance may or may not occur and that the
description includes instances where the event or circumstance
occurs and instances in which it does not. For example, the phrase
"optionally surrounded with a shell" means that the shell may or
may not be present and that the description includes both the
presence and absence of such a shell.
[0026] Embodiments of the invention relate to a class of novel
materials comprising quantum dots. As used herein, the terms
"quantum dot", "dot", and "nanocrystal" are synonymous and refer to
any particle with size dependent properties (e.g., chemical,
optical, and electrical properties) along three orthogonal
dimensions. A quantum dot can be differentiated from a quantum wire
and a quantum well, which have size-dependent properties along at
most one dimension and two dimensions, respectively.
[0027] It will be appreciated by one of ordinary skill in the art
that quantum dots can exist in a variety of shapes, including but
not limited to spheroids, rods, disks, pyramids, cubes, and a
plurality of other geometric and non-geometric shapes. While these
shapes can affect the physical, optical, and electronic
characteristics of quantum dots, the specific shape does not bear
on the qualification of a particle as a quantum dot.
[0028] For convenience, the size of quantum dots can be described
in terms of a "diameter". In the case of spherically shaped quantum
dots, diameter is used as is commonly understood. For non-spherical
quantum dots, the term diameter, unless otherwise defined, refers
to a radius of revolution (e.g., a smallest radius of revolution)
in which the entire non-spherical quantum dot would fit.
[0029] A quantum dot will typically comprise a "core" of one or
more first materials and can optionally be surrounded by a "shell"
of a second material. A quantum dot core surrounded by a shell is
referred to as a "core-shell" quantum dot.
[0030] The term "core" refers to the inner portion of the quantum
dot. A core can substantially include a single homogeneous
monoatomic or polyatomic material. A core can be crystalline,
polycrystalline, or amorphous. A core may be "defect" free or
contain a range of defect densities. In this case, "defect" can
refer to any crystal stacking error, vacancy, insertion, or
impurity entity (e.g., a dopant) placed within the material forming
the core. Impurities can be atomic or molecular.
[0031] While a core may herein be sometimes referred to as
"crystalline", it will be understood by one of ordinary skill in
the art that the surface of the core may be polycrystalline or
amorphous and that this non-crystalline surface may extend a
measurable depth within the core. The potentially non-crystalline
nature of the "core-surface region" does not change what is
described herein as a substantially crystalline core. The
core-surface region optionally contains defects. The core-surface
region will preferably range in depth between one and five
atomic-layers and may be substantially homogeneous, substantially
inhomogeneous, or continuously varying as a function of position
within the core-surface region.
[0032] Quantum dots may optionally comprise a "shell" of a second
material that surrounds the core. A shell can include a layer of
material, either organic or inorganic, that covers the surface of
the core of a quantum dot. A shell may be crystalline,
polycrystalline, or amorphous and optionally comprises dopants or
defects. The shell material is preferably an inorganic
semiconductor with a bandgap that is larger than that of the core
material. In addition, preferred shell materials have good
conduction and valence band offsets with respect to the core such
that the conduction band is desirably higher and the valence band
is desirably lower than those of the core. Alternatively, the shell
material may have a bandgap that is smaller than that of the core
material, and/or the band offsets of the valence or conduction
bands may be lower or higher, respectively, than those of the core.
The shell material may be optionally selected to have an atomic
spacing close to that of the core material.
[0033] Shells may be "complete", indicating that the shell
substantially completely surrounds the outer surface of the core
(e.g., substantially all surface atoms of the core are covered with
shell material). Alternatively, the shell may be "incomplete" such
that the shell partially surrounds the outer surface of the core
(e.g., partial coverage of the surface core atoms is achieved). In
addition, it is possible to create shells of a variety of
thicknesses, which can be defined in terms of the number of
"monolayers" of shell material that are bound to each core. A
"monolayer" is a term known in the art referring to a single
complete coating of a shell material (with no additional material
added beyond complete coverage). For certain applications, shells
will preferably be of a thickness between approximately 0 and 10
monolayers, where it is understood that this range includes
non-integer numbers of monolayers. Non-integer numbers of
monolayers can correspond to the state in which incomplete
monolayers exist. Incomplete monolayers may be either homogeneous
or inhomogeneous, forming islands or clumps of shell material on
the surface of the quantum dot. Shells may be either uniform or
nonuniform in thickness. In the case of a shell having nonuniform
thickness, it is possible to have an "incomplete shell" that
contains more than one monolayer of shell material. For certain
applications, shell thickness will preferably range between
approximately 1 .ANG.and 100 .ANG.A.
[0034] It will be understood by one of ordinary skill in the art
that there is typically a region between the core and shell
referred to herein as an "interface region". The interface region
may comprise an atomically discrete transition between the material
of the core and the material of the shell or may comprise an alloy
of the materials of the core and shell. The interface region may be
lattice-matched or unmatched and may be crystalline or
noncrystalline. The interface region may contain one or more
defects or be defect-free. The interface region may be homogeneous
or inhomogeneous and may comprise chemical characteristics that are
graded between the core and shell materials such that a gradual or
continuous, transition is made between the core and the shell.
Alternatively, the transition can be discontinuous. The width of
the interface region can range from an atomically discrete
transition to a continuous graded alloy of core and shell materials
that are purely core material in the center of the quantum dot and
purely shell material at the outer surface. Preferably, the
interface region will be between one and five atomic layers
thick.
[0035] A shell may optionally comprise multiple layers of a
plurality of materials in an onion-like structure, such that each
material acts as a shell for the next-most inner layer. Between
each layer there is optionally an interface region. The term
"shell" is used herein to describe shells formed from substantially
one material as well as a plurality of materials that can, for
example, be arranged as multi-layer shells.
[0036] A quantum dot may optionally comprise a "ligand layer"
comprising one or more surface ligands (e.g., organic molecules)
surrounding a core of the quantum dot. A quantum dot comprising a
ligand layer may or may not also comprise a shell. As such, the
surface ligands of the ligand layer may bind, either covalently or
non-covalently, to either the core or the shell material or both
(in the case of an incomplete shell). The ligand layer may comprise
a single type of surface ligand (e.g., a single molecular species)
or a mixture of two or more types of surface ligands (e.g., two or
more different molecular species). A surface ligand can have an
affinity for, or bind selectively to, the quantum dot core, shell,
or both at least at one point on the surface ligand. The surface
ligand may optionally bind at multiple points along the surface
ligand. The surface ligand may optionally contain one or more
additional active groups that do not interact specifically with the
surface of the quantum dot. The surface ligand may be substantially
hydrophilic, substantially hydrophobic, or substantially
amphiphilic. Examples of the surface ligand include but are not
limited to an isolated organic molecule, a polymer (or a monomer
for a polymerization reaction), an inorganic complex, and an
extended crystalline structure.
[0037] It will be understood by one of ordinary skill in the art
that when referring to a population of quantum dots as being of a
particular "size", what is meant is that the population is made up
of a distribution of sizes around the stated "size". Unless
otherwise stated, the "size" used to describe a particular
population of quantum dots will be the mode of the size
distribution (i.e., the peak size).
[0038] As used herein, the "size" of a quantum dot will refer to
the diameter of a core of the quantum dot. If appropriate, a
separate value will be used to describe the thickness of a shell
surrounding the core. For instance, a 3 nm silicon quantum dot with
a 1.5 nm SiO.sub.2 shell is a quantum dot comprising a 3 nm
diameter core of silicon surrounded by a 1.5 nm thick layer of
SiO.sub.2, for a total diameter of 6 nm.
[0039] For certain applications, the thickness of the ligand layer
is a single monolayer or less and can sometimes be substantially
less than a single monolayer.
[0040] As used herein, the term "photoluminescence" refers to the
emission of light of a first wavelength (or range of wavelengths)
by a substance (e.g., a quantum dot) that has been irradiated with
light of a second wavelength (or range of wavelengths). The first
wavelength (or range of wavelengths) and the second wavelength (or
range of wavelengths) can be the same or different.
[0041] As used herein, the term "quantum efficiency" refers to the
ratio of the number of photons emitted by a substance (e.g., a
quantum dot) to the number of photons absorbed by the
substance.
[0042] As used herein, the term "monodisperse" refers to a
population of quantum dots wherein at least about 60% of the
population, preferably 75% to 90% of the population, or any integer
or noninteger therebetween, falls within a specified particle size
range. A population of monodispersed particles deviates less than
20% root-mean-square (rms) in diameter, more preferably less than
10% rms, and most preferably less than 5% rms.
[0043] "Optically pure" refers to a condition in which light
passing through or past a material is substantially unchanged in
mode quality as a result of inhomogeneities in the material or
modulations at the interface between materials. This does not
include mode disruption resulting from changes in index of
refraction of waveguides. For instance, a material with large
aggregates of quantum dots capable of scattering light would not be
optically pure. The same material with aggregates of a size that do
not significantly scatter light, however, would be optically pure.
It will be apparent to one of ordinary skill in the art that what
is meant above by "substantially unchanged" will depend on the
optical requirements of a particular application. To this end,
"optically pure" refers to the level of optical purity required for
the application in which the material is to be used.
[0044] "Optically homogeneous" is defined as being homogeneous
across a length scale that is significant for optical waves,
preferably greater than 250 nm, more preferably greater than 4
.mu.m, and most preferably greater than .about.1000 .mu.m.
[0045] A "waveguide structure" is a term of art and refers to an
optical device capable of transmitting light from one location to
another. A waveguide structure can transmit light through the use
of guiding by localized effective index differences. One example of
this involves total internal reflection within a "waveguide core",
with an index of refraction n.sub.1, surrounded by a "cladding",
with an index of refraction n.sub.2, wherein n.sub.1>n.sub.2.
Another example of a waveguide structure involves appropriately
micro or nanostructured materials such as photonic bandgap
materials where the guiding results from the periodic micro- or
nano-structure of the materials.
[0046] "Cladding" is any material that surrounds the waveguide core
in a waveguide structure such that n.sub.1>n.sub.2. In a typical
waveguide structure, light propagates as a traveling wave within
and along the length of the "waveguide core" and evanescently
decays within the cladding with a decay constant related to the
ratio of n.sub.1 to n.sub.2. Light trapped within, and traveling
along, the length of a waveguide core is referred to as being
"guided".
[0047] The shape of a waveguide core or a cladding can typically be
described in terms of its "cross-section". The cross-section is the
shape created by cutting the waveguide core or the cladding along
the axes perpendicular to the longitudinal axis of the waveguide
structure. The longitudinal axis is the axis in which guided light
travels.
[0048] "Optical fibers" and "planar waveguides" are two common
forms of waveguide structures known in the art. "Optical fiber", as
the term is commonly used, typically refers to a structure
comprising a substantially cylindrical waveguide core surrounded by
a substantially cylindrical cladding and optionally comprising a
flexible, protective outer-coating. Alternatively, or in
conjunction, an optical fiber can comprise a non-cylindrical
waveguide core with a cross-section shaped as a trapezoid, a
circle, an oval, a triangle, or another geometric and nongeometric
shape.
[0049] "Planar waveguides" are waveguide structures fabricated on a
substrate by a variety of methods. "Planar waveguides" typically
comprise a substantially rectangular waveguide core. Alternatively,
or in conjunction, planar waveguides can comprise non-rectangular
waveguide cores with cross-sections of trapezoids, circles, ovals,
triangles, or a plurality of other geometric and nongeometric
shapes. While the term "planar" suggests a flat structure, the term
"planar waveguide", as used herein, also refers to structures
comprising multiple flat layers. Optionally, one or more layers in
a planar waveguide are not flat. One of skill in the art will
appreciate that the key aspect of a "planar waveguide" is that it
is a waveguide structure fabricated on a "substrate". Unless
otherwise stated, the term "waveguide structure" will be used
herein to describe a planar waveguide.
[0050] "Waveguide substrate" or "substrate" is used herein to
describe the material on which a planar waveguide is located. It is
common that a planar waveguide is fabricated directly on the
surface of the substrate. The substrate typically comprises a solid
support such as, for example, a silicon wafer and optionally
comprises an additional "buffer layer" that separates the,
waveguide structure from the solid support. The buffer layer
optionally comprises a plurality of layers comprising one or more
materials or combination of materials. The buffer layer may
optionally act, in part, as a cladding. Alternatively, the
waveguide substrate may be a flexible substrate serving the same
purpose.
[0051] "Single mode" waveguide structures are those waveguide
structures (either planar or fiber optic) that typically support a
single optical mode (e.g., TEM00). Such waveguide structures are
preferred according to some embodiments of the invention.
"Multi-mode" waveguide structures are those waveguide structures
that typically support multiple optical modes simultaneously.
[0052] "Waveguide diameter" is herein used to describe the diameter
of a substantially cylindrical waveguide core of an optical fiber.
Waveguide diameter is also used to describe the diameter of a
substantially cylindrical core on a planar waveguide.
[0053] "Waveguide width" or "width" is used herein to describe the
cross-sectional dimension of a substantially rectangular waveguide
core that is oriented parallel to the substrate surface. This is
also referred to as the "horizontal dimension" of the waveguide
core. "Waveguide height" or "height" is used herein to describe the
cross-sectional dimension of a substantially rectangular waveguide
core that is oriented perpendicular to the substrate surface. This
is also referred to as the "vertical dimension" of the waveguide
core. Based on the definitions of "width" and "height" described
here, one of ordinary skill in the art will understand the
translation of these terms to other geometrically or
nongeometrically shaped waveguide cores. Unless otherwise stated,
the standard definitions of width and height used in geometry will
be used to describe geometric cross-sectional shapes.
[0054] "Core taper" refers to a region of the waveguide core in
which the geometry of the waveguide core is changed. This may
comprise changing the size and/or shape of the waveguide core in
one or two dimensions. A core taper, for example, may comprise a
transition of a waveguide core with a square cross-section of 15
.mu.m.times.15 .mu.m to a waveguide core with a square
cross-section of 7 .mu.m.times.7 .mu.m. A core taper may also, for
example, comprise a transition from a waveguide core with a square
cross-section of 15 .mu.m.times.15 .mu.m to a waveguide core with a
circular cross-section of 10 .mu.m in diameter. Many other forms of
core-tapers are possible and will be understood from the above
definition.
[0055] A "core taper" is typically engineered to gradually change
the characteristics of the waveguide structure over a defined
distance, referred to as the "taper length". Ideally, the taper
length will be long enough so that the transition preserves the
mode structure of an optical signal through the taper. In
particular, it is preferred, but not required, that a single
optical mode entering a taper remains a single mode after exiting
the taper. This retention of the mode-structure is referred to as
an "adiabatic transition". While the term "adiabatic transition" is
commonly used, those of ordinary skill in the art will recognize
that it is typically not possible to have a perfectly adiabatic
transition, and that this term can be used to describe a transition
in which the mode structure is substantially undisrupted.
[0056] A "cladding taper" is a novel embodiment disclosed herein
that is similar to a core taper; however, it refers to a change in
width of the cladding around the waveguide core. Similar to a core
taper, a cladding taper can be used to change the size and/or shape
of the cladding and can be defined to have a taper length. The
taper length can be such as to produce an adiabatic or nonadiabatic
transition.
[0057] Both core and cladding tapers may optionally refer to the
case in which the index of refraction of the materials in the core
or cladding are gradually changed, or "graded" over the taper
length. As used herein, the term "gradually" refers to changes that
occur continuously or in small steps over a given nonzero distance.
Core and cladding tapers may optionally comprise changes to the
index, size, and/or shape of the core or cladding,
respectively.
[0058] A "bend" is used herein to describe a portion of a planar
waveguide in which the planar waveguide displays a degree of
curvature in at least one dimension. Typically, the cross-section
of the waveguide is substantially unchanged within the bend.
Typically, bends will be smooth and continuous and can be described
in terms of a radius of curvature at any given point within the
bend. While bends can curve the planar waveguide both parallel and
perpendicular to the substrate (e.g., horizontal or vertical bends,
respectively), unless otherwise stated, the term "bend" will herein
refer to horizontal bends. Optionally, bends can also comprise
tapers.
[0059] A "multimode interference device" or multimode
interferometer (MMI) refers to an optical device in which the
cross-section of the waveguide core is substantially changed
(typically increased) within a short propagation length, leading to
a region of waveguide core in which more than one mode (but
typically fewer than 10 modes) may propagate. The interaction of
these propagating multiple modes defines the function performed by
the MMI. MMI devices include fixed ratio splitters/combiners and
wavelength multiplexers/demultiplexers.
[0060] As used herein, a "waveguide coupler", "optical coupler",
and "directional coupler" are synonymous and refer to a waveguide
structure in which light is evanescently coupled between two or
more waveguide cores within a coupling region such that the
intensity of the light within each of the individual cores
oscillates periodically as a function of the length of the coupling
region. A more detailed description of a waveguide coupler is
disclosed below.
[0061] A "nonlinear waveguide coupler" is a waveguide coupler in
which the region between and/or around two or more coupled
waveguide cores is filled with a material (e.g., an "active
material") with an index of refraction that can be changed. By
changing the index of refraction of the active material, the
coupling characteristics of the nonlinear waveguide coupler can be
modified. Alternatively, the active material may be contained
within one or more of the coupled waveguide cores (e.g., as a
section of one of the waveguide cores).
[0062] A "Mach-Zehnder interferometer" or "MZ interferometer" (MZI)
is a waveguide structure in which light from a waveguide core
(e.g., an "input waveguide core") is split into two or more
separate waveguide cores (e.g., "waveguide arms" or "arms"). Light
travels a defined distance within the arms and is then recombined
into a waveguide core (e.g., an "output waveguide core"). In a MZ
interferometer, the history of the optical signals in each arm
affects the resulting signal in the output waveguide core. A more
detailed description of a MZ interferometer is disclosed below.
[0063] A "nonlinear MZ interferometer" is a MZ interferometer in
which one or more of the waveguide arms comprise an active
material. The active material may be in the core and/or cladding of
the waveguide arm. Modifying the index of refraction of the active
material modulates the signal in the output waveguide core by
changing the degree of constructive and/or destructive interference
from the waveguide arms.
[0064] "Active material" refers to any material with nonlinear
optical properties that can be used to manipulate light in
accordance with some embodiments of the invention. While the term
active material will typically be used to refer to an engineered
nonlinear nanocomposite material as described herein, the term may
also be used to describe other nonlinear materials known in the
art.
[0065] "Active region" refers to the region of an optical device in
which the index of refraction of the active material is modulated
in order to manipulate light. In the case of an electro-optic
modulator, the active region is that area of the device where a
voltage is applied. In a .sub..chi..sup.(3) based device, the
active region is that area to which a trigger-signal is applied.
Note that while the active region can be the only region of the
device in which an intentional change in optical properties occurs,
it does not restrict, the location of the active material, which
may extend beyond the active region. Regions containing active
materials outside the active region are typically not modulated
during normal operation of the device. "Active length" describes
the length of the active region along the longitudinal axis of the
device.
[0066] In the case of optical devices employing evanescent coupling
of light between two waveguide cores (e.g., a waveguide coupler),
the "interaction region" or "coupling region" is the region of the
optical device in which the coupling occurs. As is typically
understood in the art, all waveguides can couple at some
theoretically non-zero level. The interaction region, however, is
typically considered to be that region of the optical device in
which evanescent fields of the waveguides overlap to a significant
extent. Here again, the interaction region does not restrict the
extent of either the active region or the active material, which
may be greater or lesser in extent than the interaction region.
[0067] "Interaction length" describes the length of the interaction
region. "Interaction width" is the spacing between two coupled
waveguides within the interaction region. Unless otherwise stated,
the interaction width is assumed to be substantially constant
across at least a portion of the interaction length.
[0068] "Trigger pulse", "trigger signal", "control pulse", "control
signal", "control beam", and "activation light" are synonymous and
refer to light that is used to create a transient change in the
index of refraction in the materials of some embodiments of the
present invention. A trigger pulse can either be pulsed or CW.
[0069] "Data pulse", data signal", and "data beam" are synonymous
and refer to light used to transmit information through an optical
device. A data pulse can optionally be a trigger pulse. A Data
pulse can either be pulsed or CW.
[0070] "CW light" and "CW signal" are synonymous and refer to light
that is not pulsed.
[0071] "Wavelength range-of-interest" refers to any range of
wavelengths that will be used with a particular optical device.
Typically, this will include both the trigger and data signals,
where the ranges for the trigger and data signals can be the same
or different. For instance, if a device is fabricated for use in
the 1550 nm telecom range, the data wavelength range-of-interest
may be defined as 1.5 .mu.m to 1.6 .mu.m, and the trigger
wavelength range-of-interest may be defined as 1.5 .mu.m to 1.6
.mu.m (or a different range). For devices in the 1300 nm range, the
data wavelength range-of-interest may be defined as 1.25 .mu.m-1.35
.mu.m. While these are preferred wavelength range-of-interests, it
will be understood that the specific wavelength range-of-interest
can be different depending on the specific application. The ability
to tune the materials of embodiments of the current invention
implies that any wavelength range-of-interest may be used. In
general, 300 nm to 4000 nm is a preferred wavelength
range-of-interest, more preferably 300 nm to 2000 nm, more
preferably 750 nm to 2000 nm, more preferably 1260 nm to 1625 nm,
most preferably 1310.+-.50 nm and 1580.+-.50 nm.
[0072] Quantum Dots
[0073] Embodiments of the current invention, in part, exploit the
extraordinary properties of quantum dots. Quantum dots have optical
and electronic properties that can be dependent (sometimes strongly
dependent) on both the size and the material forming the quantum
dots.
[0074] In nature, it is the size range on the order of a few
nanometers in which the quantum mechanical characteristics of atoms
and molecules often begin to impact and even dominate the classical
mechanics of everyday life. In this size range, a material's
electronic and optical properties can change and become dependent
on size. In addition, as the size of a material gets smaller, and
therefore more atomic-like, many characteristics change or are
enhanced due to a redistribution of oscillator strength and density
of states. These effects are referred to as "quantum confinement"
effects. For example, quantum confinement effects can cause the
energy gap of the quantum dot or the energy of the light emitted
from the quantum dot to increase as the size of the quantum dot
decreases. These quantum confinement effects result in the ability
to finely tune many properties of quantum dots (e.g., optical and
electronic properties) by carefully controlling their size. This
control provides one critical aspect of some embodiments of the
current invention.
[0075] A quantum dot will typically be in a size range between
about 1 nm and about 1000 nm in diameter or any integer or fraction
of an integer therebetween. Preferably, the size will be between
about 1 nm and about 100 nm, more preferably between about 1 nm and
about 50 nm or between about 1 nm to about 20 nm (such as about 2,
3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20
nm or any fraction of an integer therebetween), and more preferably
between about 1 nm and 10 nm.
[0076] FIGS. 1(a), 1(b), 1(c), and 1(d) illustrates quantum dots
according to some embodiments of the invention. In particular, FIG.
1(a) illustrates a quantum dot 100 comprising a core 102, according
to an embodiment of the invention. A core (e.g., the core 102) of a
quantum dot may comprise inorganic crystals of Group IV
semiconductor materials including but not limited to Si, Ge, and C;
Group II-VI semiconductor materials including but not limited to
ZnS, ZnSe, ZnTe, ZnO, CdS, CdSe, CdTe, CdO, HgS, HgSe, HgTe, HgO,
MgS, MgSe, MgTe, MgO, CaS, CaSe, CaTe, CaO, SrS, SrSe, SrTe, SrO,
BaS, BaSe, BaTe, and BaO; Group III-V semiconductor materials
including but not limited to AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs,
GaSb, InN, InP, InAs, and InSb; Group IV-VI semiconductor materials
including but not limited to PbS, PbSe, PbTe, and PbO; mixtures
thereof; and tertiary or alloyed compounds of any combination
between or within these groups. Alternatively, or in conjunction, a
core can comprise a crystalline organic material (e.g., a
crystalline organic semiconductor material) or an inorganic and/or
organic material in either polycrystalline or amorphous form.
[0077] A core may optionally be surrounded by a shell of a second
organic or inorganic material. FIG. 1(b) illustrates a quantum dot
104 according to another embodiment of the invention. Here, the
quantum dot 104 comprises a core 106 that is surrounded by a shell
108. A shell (e.g., the shell 108) may comprise inorganic crystals
of Group IV semiconductor materials including but not limited to
Si, Ge, and C; Group II-VI semiconductor materials including but
not limited to ZnS, ZnSe, ZnTe, ZnO, CdS, CdSe, CdTe, CdO, HgS,
HgSe, HgTe, HgO, MgS, MgSe, MgTe, MgO, CaS, CaSe, CaTe, CaO, SrS,
SrSe, SrTe, SrO, BaS, BaSe, BaTe, and BaO; Group III-V
semiconductor materials including but not limited to AlN, AlP,
AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, and InSb;
mixtures thereof; and tertiary or alloyed compounds of any
combination between or within these groups. Alternatively, or in
conjunction, a shell can comprise a crystalline organic material
(e.g., a crystalline organic semiconductor material) or: an
inorganic and/or organic material in either polycrystalline or
amorphous form. A shell may be doped or undoped, and in the case of
doped shells, the dopants may be either atomic or molecular. A
shell may optionally comprise multiple materials, in which
different materials are stacked on top of each other to form a
multi-layered shell structure.
[0078] As illustrated in FIGS. 1(c) and 1(d), a quantum dot may
optionally comprise a ligand layer comprising one or more surface
ligands (e.g., organic molecules) surrounding a core, according to
some embodiments of the invention. In FIG. 1(c), a quantum dot 110
comprises a core 112 and a ligand layer 114 surrounding the core
112. In FIG. 1(d), a quantum dot 116 comprises a core 118 and a
ligand layer 122 surrounding the core 118. Here, the quantum dot
116 also comprises a shell 120 surrounding the core 118, where the
shell 120 is positioned between the core 118 and the ligand layer
122.
[0079] Optical Properties
[0080] Linear Optical Properties:
[0081] One of the most dramatic examples of "quantum confinement"
effects is that, for a semiconductor material, the energy gap
shifts as a function of size. This can be seen in FIG. 2, where the
energy gap of quantum dots fabricated from silicon, referred to
herein as "silicon quantum dots", is plotted as a function of the
size (e.g., diameter) of the quantum dots, according to an
embodiment of the invention. The silicon quantum dots were made as
described herein. The vertical axis represents the energy gap of
the silicon quantum dots, and the horizontal axis represents the
size of the silicon quantum dots. The observed values for the
energy gap (dots with error bars) are compared against
pseudopotential and tight-binding models (solid line) and against
the simple effective mass theory (dashed line).
[0082] The same effect can be seen for the emission wavelength as a
function of the size of quantum dots. FIG. 3 illustrates
photoluminescence (PL) spectra from six samples with different
sizes of silicon quantum dots, according to an embodiment of the
invention. The silicon quantum dots were made as described herein
and include shells formed of an oxide. The vertical axis represents
a normalized PL signal, and the horizontal axis represents the
emission wavelength. The PL spectra illustrated in FIG. 3 is
obtained by optically exciting the silicon quantum dots with
ultraviolet light. The wavelength of the optical excitation is
shorter than the wavelength at the absorption edge of the silicon
quantum dots. FIG. 3 demonstrates the range of sizes that can be
made with the methods described herein. The quantum dots shown at
the top of FIG. 3 are not drawn to scale and are meant to
illustrate the relative size of the quantum dots responsible for
the PL spectra. FIGS. 2 and 3 demonstrate the unprecedented control
that can be obtained over absorption and emission characteristics
of the silicon quantum dots.
[0083] Through a series of relations called the Kramers-Kroenig
equations, the properties of refractive index and dielectric
constant can be related to absorption. As such, size-dependent
control of absorption allows control of refractive index.
[0084] In addition to the size of a quantum dot, the optical and
electronic properties are also strongly influenced by the material
from which it is fabricated. Quantum confinement effects represent
a modulation of the bulk properties of the material. As such, any
changes resulting from a reduction in size are made relative to the
bulk properties of the material. By selecting (e.g., independently
selecting) the appropriate combination of quantum dot size and
material, an even greater control of the optical and electronic
properties of a quantum dot is provided. As an example, FIGS. 4(a)
and (b) show the size dependent absorption and emission of
germanium quantum dots, which differ from those of silicon quantum
dots, according to an embodiment of the invention. The germanium
quantum dots were made as described herein. In FIG. 4(a), the
vertical axis represents the energy gap of the germanium quantum
dots, and the horizontal axis represents the size of the germanium
quantum dots. The observed values for the energy gap (open dots
with error bars) are compared against theoretical predictions
(solid dots and solid line). In FIG. 4(b), a size-selective PL
spectrum is shown, where the vertical axis represents a normalized
PL signal, and the horizontal axis represents the emission
wavelength. The far right curve is offset vertically for clarity.
The PL spectra shown in FIG. 4(b) are collected using different
excitation wavelengths, such that only quantum dots with energy
gaps less than or equal to the photon energy of the excitation
light (i.e., greater than a certain quantum dot size) are
excited.
[0085] Relation of Size and Material to Dielectric Constant and
Index of Refraction
[0086] For most materials, the index of refraction far from
resonance decreases as the energy gap of the material increases (a
consequence of the Kramers-Kroegnig equations). This explains, for
example, why the index of refraction of transparent materials
(e.g., silica, metal halides, and organics) is less than that for
inorganic semiconductors with smaller relative absorption energies.
This effect also typically applies to quantum dots. In this case,
as the size of the quantum dot decreases, the energy gap increases,
decreasing the index of refraction. Thus, for quantum dots, the
off-resonant index of refraction (at a fixed wavelength) typically
correlates with size, affording another method to control the
optical properties of the quantum dots.
[0087] Relation of Concentration of Quantum Dots to Dielectric
Constant and Index of Refraction
[0088] Embodiments of the invention involve altering the index of
refraction of a material by varying the concentration of quantum
dots in the material. An example of this is shown in FIG. 5(a),
which illustrates concentration dependence of the linear index of
refraction of engineered nanocomposite materials doped with silicon
and germanium quantum dots, according to an embodiment of the
invention. The silicon and germanium quantum dots were made in
accordance with the methods described herein. The index of
refraction is plotted as a function of the quantum dot
concentration expressed in weight percent. In this figure, the
index of refraction is measured in the visible range (sodium D
line).
[0089] This concentration dependence provides yet another method of
controlling the overall refractive index of a material by utilizing
the properties of quantum dots. The ability to embed quantum dots
into a variety of host materials will be discussed in a later
section.
[0090] Nonlinear Optical Properties
[0091] In general, a wide variety of nonlinear optical phenomena
can arise when materials are exposed to high-intensity light. Some
of these nonlinear phenomena are used in certain aspects of
telecommunications (e.g., Raman amplifiers) and many are being
considered for future use (e.g., four-wave mixing, cross-phase
modulation, and solitons). Although nonlinear phenomena are
typically associated with high-intensities, these phenomena are
also observed at lower intensities due to phase matching, resonant
enhancement, and/or long interaction lengths.
[0092] Light incident on a material can induce a polarization (P),
which can be expressed as (in SI units)
P=.epsilon..sub.0.chi.E=.epsilon..sub.0.left
brkt-bot..sub..chi..sup.(1)E+-
.sub..chi..sup.(2)E.times.E+.sub..chi..sup.(3)E.times.E.times.E+. .
. .right brkt-bot.,
[0093] where E is the electric field strength, .epsilon..sub.0 is
the electric permittivity, .sub..chi. is the overall optical
susceptibility, and .sub..chi..sup.(n) is the nth order optical
susceptibility. Since .sub..chi..sup.(2) phenomena are typically
only present in materials that lack inversion symmetry (e.g.,
non-centrosymmetry), certain embodiments of the invention primarily
exploit .sub..chi..sup.(3) phenomena, which can be exhibited by all
materials. It should be recognized that tensor elements of
.sub..chi..sup.(3) are in general complex quantities. The induced
refractive index change .DELTA.n and the nonlinear index of
refraction .gamma. are related to the real part of appropriate
tensor elements of .sub..chi..sup.(3) e.g.,
Re[.sub..chi..sup.(3).sub.1111], while the two-photon absorption
coefficient .beta. is related to the imaginary part of appropriate
tensor elements of .sub..chi..sup.(3) , e.g.,
Im[.sub..chi..sup.(3).sub.1111]). In particular, certain
embodiments of the invention exploit phenomena that change the
index of refraction of a material by creating an effective optical
susceptibility
.sub..chi..sub..sub.eff=.sub..chi..sup.(1)+.sub..chi..sup.(3)E.times.E=.su-
b..chi..sup.(1)+.sub..chi..sup.(3)I,
[0094] where I is the intensity of the particular light beam
creating the effective optical susceptibility (and where the higher
order terms are assumed to be small and are therefore neglected
here, although they can be utilized as well), which can affect the
same light beam or another light beam at the same or different
frequency. This leads to an effective or overall index of
refraction given by
n(.omega.')=n.sub.0+.gamma.(.omega.',.omega.)I(.omega.),
[0095] and an operational definition for a nonlinear index of
refraction .gamma. given by
.gamma.(.omega.',.omega.)=n(.omega.')-n.sub.0/I(.omega.)
[0096] where n(.omega.') is the effective index of refraction at
.omega.', no is the low-intensity refractive index (e.g., the
linear index of refraction), and I(.omega.) is the intensity of
light with optical frequency .omega. that creates the effective
optical susceptibility or index change. The nonlinear index of
refraction .gamma.(.omega.',.omega.) is related to
.sub..chi..sup.(3).sub.ijkl(-.omega.', .omega.', .omega.,-.omega.),
e.g., .sub..chi..sup.(3).sub.1111(-.omega.', .omega.',
.omega.,-.omega.. If only one light beam is involved, then .omega.'
can be set equal to .omega.. If two light beams are involved, then
.omega.' and .omega. can be the same or different. Situations where
.omega.' and .omega. are the same can correspond to degenerate
conditions (which is further discussed herein), in which case the
nonlinear index of refraction .gamma. can be referred to as a
degenerate nonlinear index of refraction (or .gamma..sub.deg).
Situations where .omega.' and .omega. are different can correspond
to non-degenerate conditions (which is further discussed herein),
in which case the nonlinear index of refraction .gamma. can be
referred to as a non-degenerate nonlinear index of refraction (or
.gamma..sub.nondeg). As one of ordinary skill in the art will
understand, an optical frequency of a light beam (e.g., .omega. or
.omega.') is inversely related to a wavelength of the light beam
(e.g., .lambda. or .lambda.').
[0097] This intensity dependent refractive index n(.omega.') can be
exploited for all-optical switching and optical signal processing.
For certain applications, nonlinear absorption processes are of
particular importance, in which case, optimization of
Im[.sub..chi..sup.(3).sub.ijkl- ] is preferred.
[0098] Nonlinear Optical Properties of Quantum Dots
[0099] In general, three mechanisms are principally responsible for
.sub..chi..sup.(3) nonlinearities in quantum dots. These effects
fall into the broad categories of resonant, nonresonant, and
near-resonant effects. These categories can be further subdivided
into degenerate (e.g., all light beams have the same wavelength)
and non-degenerate (e.g., one or more light beams have different
wavelengths) cases.
[0100] 1) Resonant Effects:
[0101] Resonant processes typically result from a change in
electronic properties upon resonant excitation (e.g., the linear
absorption of light). This leads to a corresponding change in
refractive index, following the Kramers-Kroenig relations. The
magnitude of an absorption change, and hence the optical
nonlinearity, is directly related to the ground state absorption
cross-section modified by any excited state absorption. In the case
of a material with discrete states, such as molecules or quantum
dots, the optical nonlinearity results from state-filling and is
related to (.sigma..sub.g-.sigma..sub.e), where .sigma..sub.g and
.sigma..sub.e are the absorption cross sections of the material in
the ground and excited states respectively, with a reduction in
refractive index occurring for a reduction in absorption. For
quantum dots, further enhancement of .sub..chi..sup.(3) results
from unique physical phenomena such as quantum confinement, local
electric field effects, and quantum interference effects.
[0102] As indicated above, the optical nonlinearity is related to
(.sigma..sub.g-.sigma..sub.e), so that increasing the oscillator
strength of optical transitions from the ground state generally
increases the optical nonlinearity. In the case of quantum dots, a
decrease in size increases the spatial overlap of the electron and
hole wave functions, which in turn increases the oscillator
strength. Resonant nonlinearity therefore tends to increase with
decreasing size. This enhancement, however, can be limited by any
size dispersion.
[0103] Another important effect arises from the presence of one or
more defects in a quantum dot. Defects can be present as trap
states within the quantum dot. Due to the enormous surface to
volume ratio in the size range of quantum dots, most relevant traps
exist on the surface. If not passivated correctly, resonant
excitation of a quantum dot creates electron-hole pairs that
quickly relax into these surface-states. Holes, with their
relatively large effective mass, tend to trap more easily, while
the electrons, with their smaller effective mass, remain largely
delocalized. The result is a spatial separation of the electron and
hole wavefunctions and a decrease in oscillator strength, reducing
the magnitude of the resulting nonlinearity. Furthermore, by
tailoring the rate of relaxation between the delocalized quantum
dot states and the localized surface states, it is possible to
control the response time of the resonant optical nonlinearity.
[0104] Resonant nonlinearities can be utilized in both the
degenerate and non-degenerate cases with respect to the wavelength
of control and data beams. In the degenerate case, the wavelength
range-of-interest lies near the absorption edge. For a single beam,
the absorption can be saturated, leading to an intensity dependent
absorption, commonly known as saturable absorption. For degenerate
control and data beams, the control beam can modulate the
transmission of the data beam, leading to an optical modulator. The
refractive index change caused by the absorption change can also be
utilized. Due to the broad electronic absorption in semiconductors
in general and quantum dots in particular, resonant nonlinearities
can be observed for the case where the control beam and the data
beam are non-degenerate. In this case, the control beam can be of
higher photon energy, such that carriers are generated which relax
(primarily via phonon emission) towards the band edge, where the
absorption bleaching and/or excited state absorption can affect the
data beam of lower photon energy (but still resonant).
[0105] 2) Nonresonant Effects:
[0106] In contrast to resonant nonlinearities, where linear
absorption of light is typically required, non-resonant
nonlinearities typically do not require single-photon absorption of
light. As a result, nonresonant nonlinearities are intrinsically
fast since excited state relaxation is not required. However,
nonresonant nonlinearities are generally smaller than resonant
nonlinearities, due to the lack of strong single-photon resonance
enhancement (although multi-photon resonance can be utilized to
enhance the nonresonant nonlinearity).
[0107] There are three primary enhancement factors that can be
utilized for nonresonant nonlinearities in quantum dots: quantum
confinement, multi-photon resonance enhancement, and local-field
effects. Quantum confinement provides an increase in oscillator
strength due to enhanced wavefunction overlap (as described above),
which enhances .sub..chi..sup.(3). Multi-photon resonances can be
utilized in the absence of single-photon resonances to enhance the
nonresonant nonlinearity. However, multi-photon resonances can
introduce unwanted nonlinear absorptive losses. For certain
applications, the ideal situation is one where the relevant light
beams are just below the threshold of a multi-photon resonance,
thereby allowing some resonant enhancement without significant
nonlinear absorption loss. Finally, local field effects can be
utilized to enhance the nonresonant .sub..chi..sup.(3). In
particular, for a nanocomposite material in which quantum dots with
dielectric constant .epsilon..sub.1 are imbedded in a matrix
material with dielectric constant .epsilon..sub.2, an externally
applied electric field (such as that originating from an
electromagnetic light source) can be locally enhanced at the
quantum dots if .epsilon..sub.1>.epsilon..sub.2, with the
magnitude of the enhancement related to
.DELTA..epsilon.=.epsilon..sub.1-.epsilon..sub.2. Such a situation
can arise by embedding the quantum dots in a lower index matrix
material. When illuminated by light, the electric field at the
quantum dots is enhanced compared to the incident external field,
in turn leading to an increase in the overall nonlinear response.
This enhancement increases with size of the quantum dots as the
quantum dot bandgap energy decreases, resulting in an increase in
dielectric constant (.epsilon..sub.1).
[0108] Nonresonant nonlinearities can be utilized in the
non-degenerate case as well. In this case, the control beam can
have either higher or lower photon energy than the data beam. One
advantage of the non-degenerate case is that enhancement of
cross-phase modulation (the control beam inducing an index change
seen by the data beam) can occur without enhancement of self-phase
modulation (the data beam affecting itself by the self-induced
index change), which can cause some deleterious effects for
telecommunications data streams.
[0109] 3) Near-Resonant Effects:
[0110] Near-resonant nonlinearities can be classified into two
categories: degenerate (typically close to resonance) or
non-degenerate (typically with one beam resonant and the other beam
nonresonant). In the former case, the beams are typically very
close to the resonance edge, i.e., just above, just below, or
exactly at the edge of resonance, so that either no direct
excitation of the material occurs through linear absorption or very
little direct absorption occurs. The non-degenerate case is perhaps
the more useful situation, as the refractive index change induced
by resonant excitation via a control beam causes a phase change for
the data beam that is below resonance (so as to minimize losses due
to single- or multi-photon absorption). For example, the refractive
index change due to the absorption saturation that extends to
photon energies well below the absorption edge can be utilized,
where carriers can be directly generated using the control beam
instead of generating carriers via two-photon absorption using a
high-intensity data beam. In addition, the excitation of free
carriers in quantum dots due to absorption of control beam photons
can lead to a refractive index change caused by other free carrier
effects. For example, due to their small size, quantum dots
typically intrinsically have high free carrier densities for even
single photon absorption (e.g., .about.10.sup.18 carriers/cm.sup.3
for one photon absorption in a single quantum dot). This leads to
effects such as quantized Auger recombination and enhanced
reflectivity (due to a large plasma frequency) at high enough
carrier densities (e.g., .about.10.sup.20 carriers/cm.sup.3).
[0111] Size Dependence
[0112] From the discussion above, the size dependence (for a given
quantum dot material) of both resonant and nonresonant nonlinear
processes can be derived. Typically, for resonant optical
nonlinearity, the magnitude of the nonlinearity increases as the
quantum dot size decreases, decreases as the number of quantum dots
with traps that localize electrons or holes increases, and
decreases as the size dispersion increases.
[0113] Typically, for nonresonant processes, the optical
nonlinearity increases with increasing quantum dot size, increases
with increasing index of refraction of the quantum dot, increases
with decreasing index of refraction of the surrounding matrix
material. There is the caveat that these trends may not continue
indefinitely to all sizes of quantum dots but can be useful as aids
in practical design considerations. By carefully tailoring the
specific size of the quantum dot, resonant effects, nonresonant
effects, or both, can be used to optimize the resulting nonlinear
response.
[0114] Quantum Dot Material Dependence
[0115] One important consideration for a material forming a quantum
dot is that, for bound electrons, the optical nonresonant
nonlinearity typically depends on the energy gap of the material as
1/E.sub.g.sup.n, where n typically ranges from about 4 to about 6.
The nonresonant nonlinearity therefore can increase significantly
as the energy gap decreases. This trend favors a combination of
large quantum dot sizes and materials with intrinsically small
bandgap energies. At the same time, however, the photon energy in
the wavelength range-of-interest can affect the choice of material
and quantum dot size in order to avoid significant linear and
nonlinear absorption. Specifically, the material in the bulk form
desirably should have an energy gap roughly equal to or greater
than the photon energy in the wavelength range-of-interest for a
data beam in order to exploit quantum confinement effects that
shift the energy gap to higher energies. At the same time, to avoid
significant multi-photon absorption effects, the energy gap of the
material desirably should be sufficiently large that the energy gap
of the resulting quantum dot is greater than two times the photon
energy of the data beam photons.
[0116] For the case of nonresonant optical nonlinearities, these
two concerns specify opposing trends that bracket the energy gap of
the material of choice for quantum dots according to some
embodiments of the present invention. The material in the bulk form
desirably should have an energy gap less than this bracketed energy
in order to exploit quantum confinement effects that shift the
energy gap to higher energies. As an example, to avoid two-photon
losses in degenerate all-optical switching components operating
near 1550 nm (corresponding to a photon energy of 0.8 eV) and to
also take advantage of the 1/E.sub.g.sup.n behavior of the
nonlinear response, the quantum dot energy gap should be less than
but close to 775 nm (or greater than but close, to 1.6 eV).
[0117] Enhanced Optical Properties
[0118] In addition to size-dependent spectral characteristics,
quantum confinement can also result in an enhancement in the
magnitude of various optical and electronic properties due to a
redistribution of the density of states. Properties such as
absorption cross-section and excited-state polarizability have been
found to be enhanced by several orders of magnitude over bulk
materials. .sub..chi..sup.(3) can also be enhanced by quantum
confinement, as described previously.
[0119] Additional Effects
[0120] There are many practical definitions of a figure-of-merit
(FOM) that take into account the many parameters that can be
important and relevant for all-optical switching. One example of
such a FOM is defined as 1 n ,
[0121] where .DELTA.n is the induced refractive index change, a is
the linear and nonlinear absorption coefficient, and .tau. is the
response time of the material. For this FOM, which is particularly
relevant for resonant optical nonlinearities where light absorption
is used, the larger the FOM, the better will be the performance of
the all-optical switching. A definition of a FOM useful for
nonresonant optical nonlinearities, where ideally no or little
light absorption occurs, is 2.gamma./.beta..lambda., where .gamma.
is the nonlinear index of refraction, .beta. is the two-photon
absorption coefficient, and .lambda. is the wavelength of
operation. In this case, useful all-optical switching typically
occurs when FOM>1. According to some embodiments of the
invention, the following effects can be important for the formation
of nanocomposite materials with a FOM in a usable range for
practical optical switching.
[0122] The Effect of Defects on FOM:
[0123] Defects within quantum dot materials can have a substantial
negative impact on their performance as nonlinear optical
materials. Defects in the core and/or surface of the quantum dot
can yield direct absorption of below-bandgap photons, increasing
optical losses, and decreasing the overall FOM. As a result, while
.sub..chi..sup.(3) may be high, the material can still be
inappropriate for optical switching. The effect of defects on
optical switching using quantum dots has not been previously
considered as discussed herein.
[0124] One important aspect of some embodiments of the invention is
that, for quantum dots to be used as a nonlinear optical material,
they desirably should comprise a substantially defect-free core. In
this case, the term "defect" typically refers to defects with
energy below the energy gap of the quantum dot core or within the
energy range of the wavelength range-of-interest. Additionally, the
surface of quantum dots should be well passivated, such that there
are substantially no defect states. Passivation can be
accomplished, for example, through the inclusion of appropriate
surface ligands in the ligand layer to bind to defect sites and
remove them from the energy gap. Alternatively, or in conjunction,
passivation can be achieved by applying a shell to the quantum dot
core to fill or eliminate the defect sites. In this case, the shell
material is preferably a material with an energy gap that is higher
than that corresponding to the wavelength range-of-interest, and
more preferably higher than the energy gap of the quantum dot core.
Additionally, the shell desirably should be substantially
defect-free or should have defects that can be eliminated through
the inclusion of appropriate surface ligands.
[0125] Concentration Effects:
[0126] One important aspect of some embodiments of the invention is
that the nonlinear properties of a material including quantum dots
can be substantially affected by correlated interactions between
two or more quantum dots. In particular, while .sub..chi..sup.(3)
can be proportional to concentration of quantum dots at low
concentrations, as the concentration increases, the individual
quantum dots can get close enough to interact with each other,
producing collective phenomena that can further enhance
nonlinearity. This effect is seen in FIG. 5(b), which illustrates
concentration dependence of the optical nonlinearity of engineered
nonlinear nanocomposite materials doped with silicon and germanium
quantum dots, according to an embodiment of the invention. The
silicon and germanium quantum dots were made in accordance with
the, methods described herein. The vertical axis represents the
nonlinear index of refraction .gamma., and the horizontal axis
represents the relative concentration of quantum dots in a matrix
material. As shown in FIG. 5(b), .gamma. can increase superlinearly
with concentration at sufficiently high concentrations. The effect
of concentration (and particularly the superlinear concentration
dependence) on optical switching using quantum dots has not been
previously considered as discussed herein.
[0127] For FIG. 5(b), .gamma. arises as a result of nonresonant
degenerate nonlinearities. The values attained for .gamma. are
particularly large. As shown in FIG. 5(b), the nanocomposite
material doped with silicon quantum dots has .gamma. as high as
about 8.times.10.sup.-5 cm.sup.2/W, which is 9 orders of magnitude
larger than the bulk material from which the silicon quantum dots
are fabricated (bulk silicon has a nonresonant degenerate .gamma.
of about 8.times.10.sup.-14 cm.sup.2/W). Additional nonlinear
enhancement can be induced through the appropriate selection of
molecular species in the ligand layer (see discussion below on
Molecular Tethers).
[0128] Summary of Nonlinear Optical Properties of Quantum Dots
[0129] Enhancement and tunability of the optical nonlinearity in
individual quantum dots and multi-quantum dot nanocomposites,
combined with substantially defect free and/or well passivated
quantum dot cores, provide the engineered nonlinear nanocomposite
materials according to some embodiments of the current invention.
Such nanocomposite materials can satisfy various characteristics
for an ideal .sub..chi..sup.(3) based optical material that include
(but are not limited to): large Re[.sub..chi..sup.(3).sub.ijkl] in
the wavelengths range-of-interest; a multi-photon transition that
can be tuned to maximize near-resonance enhancement while
minimizing optical loss due to absorption; the use of
non-degenerate control and data beams where the control beam is
resonant and induces a large index change at the data beam
wavelength while introducing low optical loss at that wavelength;
the use of degenerate control and data beams to allow cascading of
devices; and low optical loss due to absorption by defects.
[0130] Colloidal Quantum Dots
[0131] Structures comprising quantum dots can be fabricated using
vapor deposition, ion-implantation, photolithography, spatially
modulated electric fields, semiconductor doped glasses,
strain-induced potential variations in quantum wells, atomic width
fluctuations in quantum wells, and a variety of other techniques.
Preferably, quantum dots are formed or used in a form that can be
easily incorporated into flexible or engineered optical materials
or devices. In addition, it is desirable to separate the optical
properties of the quantum dots from those of a matrix material to
achieve a sufficiently large FOM with reduced absorption and/or
scattering by the matrix material.
[0132] In a preferred embodiment, the current invention comprises
colloidal quantum dots. Colloidal quantum dots are freestanding
nanostructures that can be dispersed in a solvent and/or a matrix
material. Such colloidal quantum dots are a particularly preferred
material for some embodiments of the current invention because they
can be more easily purified, manipulated, and incorporated into a
matrix material.
[0133] It will be apparent to one of ordinary skill in the art that
the defining characteristic for a "colloidal" quantum dot is that
it is a freestanding nanostructure. The method of fabrication,
size, and shape of the particular colloidal quantum dot do not bear
on its classification.
[0134] Chemical Properties
[0135] Chemically Controllable Surface
[0136] According to some embodiments of the invention, a unique
physical characteristic of quantum dots is that, while the core can
comprise a crystalline semiconductor material, the surface can be
coated with a variety of different organic and/or inorganic
materials. These surface coatings (e.g., shells or ligand layers)
can impart stability and chemical activity, as well as passivation
of electrically and optically active defect sites on the quantum
dot surface. These surface coatings are optionally substantially
different in chemical nature than the inorganic core. As a result,
while quantum dots can comprise primarily a highly nonlinear
semiconductor material, they substantially appear to the
surrounding material as surface ligands. As such, the
processability and chemical, stability of this highly nonlinear and
tunable optical material can primarily be a function of the surface
layer and not a function of the material that provides the majority
of the optical characteristics.
[0137] Surface ligands are preferably bi-functional. By
bi-functional, it is meant that there are at least two portions of
the surface ligand such that one portion interacts primarily with
the quantum dot surface, while the second portion interacts
primarily with the surrounding environment (e.g., solvent and/or
matrix material). These at least two portions of the surface ligand
may be the same or different, contiguous or noncontiguous, and are
optionally contained within two or more different molecular species
that interact with each other to form the ligand layer. The at
least two portions can be selected from a group consisting of
hydrophilic groups, hydrophobic groups, or amphiphilic groups. The
interaction of each of the at least two portions and the quantum
dot or surrounding environment can be covalent or noncovalent,
strongly interacting or weakly interacting, and can be labile or
non-labile. The at least two portions can be selected independently
or together.
[0138] In some embodiments of the current invention, the surface
ligands are selected such that the portion that interacts with the
quantum dot passivates defects on the surface such that the surface
is made substantially defect-free. At the same time, the portion
that interacts with the environment is selected specifically to
impart stability and compatibility (e.g., chemical compatibility or
affinity) of the quantum dot within a matrix material that is
selected for a specific application. Simultaneously satisfying both
of these requirements is an important aspect of certain embodiments
of the current invention relating to the development of an
engineered nonlinear nanocomposite material. Alternative methods of
achieving these requirements include (but are not limited to): 1)
Passivating the surface of the quantum dot independent of the
ligand layer (e.g., using a shell or creating an intrinsically
defect free surface), while the environmental compatibility is
imparted by the surface ligands, or 2) imparting both passivation
and environmental compatibility independent of the ligand layer.
Achieving passivation of the surface of quantum dots is one
advantage of using colloidal quantum dots over alternate
approaches.
[0139] Through the appropriate selection of surface ligands,
quantum dots can be incorporated into a variety of matrix materials
such as, for example, liquids, glasses, polymers, crystalline
solids, and even close-packed ordered or disordered quantum dot
arrays. The resulting nanocomposite materials can be formed into
homogeneous, high-quality optical films of quantum dots.
Alternatively, the chemistry can be selected to allow dispersion of
the quantum dots into a matrix material with a controllable degree
of aggregation, forming micron or sub-micron sized clusters. The
result is an increased local fill-factor and an enhanced local
field effect that may further increase the nonlinear response of
the nanocomposite materials of embodiments of the present
invention.
[0140] An important aspect of some embodiments of this invention
relates to effectively separating the optical properties of the
quantum dots from the optical, chemical, mechanical, and other
properties of the matrix material. In this aspect, it is possible
to combine the large nonlinearities of quantum dots with the ease
of handling and processability of a matrix material such as a
standard polymer. Thus, this aspect provides two additional
features of an ideal .sub..chi..sup.(3) based optical material:
physical and chemical compatibility with specific device
architectures and the ability to be easily processed for
incorporation.
[0141] Molecular Tethers:
[0142] In addition to conveying stability and chemical
compatibility with the surrounding environment, the ligand layer
can optionally be used to tailor the physical, optical, chemical,
and other properties of the quantum dots themselves. In this case,
it is not just the chemical nature of the surface ligand but also
the interaction of the surface ligand with the quantum dot that
imparts an additional level of control over the physical, optical,
chemical, and other properties of the resulting nanocomposite
material. We refer herein to any molecule, molecular group, or
functional group coupled (e.g., chemically attached) to the surface
of,a quantum dot that imparts additional functionality to the
quantum dot as a "molecular tether". In some cases, the molecular
tether can be electrically active, optically active, physically
active, chemically active, or a combination thereof. The inclusion
of molecular tethers into a quantum dot structure is an important
aspect of some embodiments of the present invention.
[0143] Active species are used to precisely control the electrical,
optical, transport, chemical, and physical interactions between
quantum dots and the surrounding matrix material and/or the
properties of individual quantum dots. For instance, a conjugated
bond covalently bound to the surface of one or more quantum dots
may facilitate charge transfer out of one quantum dot and into
another. Similarly, a physically rigid active group bound in a
geometry substantially normal to the surface of a quantum dot can
act as a physical spacer, precisely controlling minimum
interparticle spacing within an engineered nonlinear nanocomposite
material.
[0144] As described above, collective phenomena (e.g., at high
concentrations) are an important aspect of some embodiments of the
current invention. This aspect can be further enhanced by allowing
individual quantum dots to interact with one another using
molecular tethers that foster interactions between quantum dots. At
sufficiently high number densities, the molecular tethers begin to
make contact with molecular tethers from -other quantum, dots or
with other quantum dots directly. This can serve to augment
nonlinearity by controlling the interaction between quantum dots
and thus increasing the degree of collective phenomena compared to
single particle phenomena. Molecular tethers may include, but are
not limited, to conducting polymers, charge transfer species,
conjugated polymers, aromatic compounds, or molecules with
donor-acceptor pairs. These molecular tethers can foster electron
delocalization or transport and thus can increase the interaction
between quantum dots. Additionally, the molecular tethers can be
selected to facilitate high quantum dot number densities without
the detrimental aggregation that often plagues high concentration
systems.
[0145] Molecular tethers can also be selected to impart stability
of quantum dots under a variety of environmental conditions
including ambient conditions. Molecular tethers can optionally
contain chemically active groups to allow quantum dots to be
attached to polymer backbones, along with other active molecules.
This provides a method for controlling the density of quantum dots
within close proximity of molecules that influence a variety of
functions such as carrier transport or delocalization.
[0146] An additional aspect of the present invention is the use of
molecular tethers to physically connect two or more quantum dots in
a 1 dimensional, 2 dimensional, or 3 dimensional structure or
array. Such quantum dot superstructures can be created to initiate
multiple dot quantum interference interactions or collective
phenomena yielding new and useful properties such as enhanced,
nonsaturating optical nonlinearities. The length and properties of
these molecular tethers can be tailored to enhance or generate
specific quantum phenomena. These nanostructures can have the
properties of single quantum dots or an ensemble of quantum dots
depending on the nature of the molecular tethers. For certain
applications, more than one type of molecular tether can be used to
connect quantum dots.
[0147] The quantum dots according to some embodiments of the
invention exemplify microscopic conditions that enhance the
nonresonant optical nonlinearity arising from local electric field
effects described above. Whether the quantum dot surface is
terminated with oxide or ligand layer (e.g., molecular tethers),
the result is a particle (e.g., a core of the quantum dot) with
dielectric constant .epsilon..sub.1 surrounded by an environment
(e.g., the surface oxide layer or molecular tethers) with
dielectric constant .epsilon..sub.2 where
.epsilon..sub.1>.epsilon..su- b.2. Therefore, the enhancement of
the nonresonant optical nonlinearity can be engineered by the
judicious choice of oxide or molecular tether without resorting to
a surrounding bulk matrix material. In other words, a single
quantum dot as described in this patent should exhibit an enhanced
nonresonant optical nonlinearity since the surface layer functions
as a surrounding matrix material with a lower dielectric constant.
Optionally, molecular tethers can be used to connect quantum dots
together without a separate matrix material. In this case, an
extrinsic matrix material is not required since the individual
interconnected quantum dots exhibit an enhanced local electric
field effect.
[0148] A preferred approach of attaching appropriate molecular
tethers to a quantum dot surface can be thought of as essentially
treating a quantum dot as a very large molecule (e.g., a
macro-molecule) and the molecular tethers as functionalizations of
this large molecule. This creates a large three-dimensional
structure with enhanced nonlinear optical properties resulting from
the combination of quantum effects from the quantum dot and carrier
polarization and delocalization effects from the molecular tethers
and from the interaction of these two effects. These properties can
be tailored by the choice of molecular tethers. In addition, a
quantum dot can also represent a large and stable reservoir of
polarizable charge that also contributes to a large nonlinear
optical response.
[0149] Macroscopic Quantum Dot Solids
[0150] Macroscopic solids can be fabricated in which quantum dots
form a substantially close-packed array (e.g., a cubic
closed-packed array) in the absence of an extrinsic matrix
material. These "quantum dot solids" can either be crystalline,
polycrystalline, or amorphous. While containing a relatively high
density of quantum dots, quantum dot solids can still be easily
processed since, during formation, the quantum dots can be
dispersed in a solvent that is subsequently removed. Uniform solid
quantum dot films, for instance, can be formed using standard
spin-coating techniques as, for example, described in C. R. Kagan
et al., "Long-range resonance transfer of electronic excitations in
close-packed CdSe quantum-dot solids," Phys. Rev. B 54, 8633
(1996), the disclosure of which is incorporated herein by reference
in its entirety. In addition, surface ligands can still be selected
to impart solvent compatibility and appropriate chemical stability
to the final quantum dot solid. In contrast to the interconnected
material described above, these macroscopic quantum dot solids are
typically not held together by molecular bonds but rather by Van
der Waals forces.
[0151] High-quality optical materials can be fabricated from
quantum dot solids with substantially homogeneous optical
properties throughout the material. The density of quantum dots can
be tuned by modifying the length and/or structure of the surface
ligands. Careful selection of surface ligands can produce
continuously tunable densities up to a maximum fill-factor of about
75% by volume of the quantum dot solid, preferably between about
0.005% and 75% by volume (e.g., between about 10% and 75% by
volume, between about 30% and 75% by volume, between about 50% and
75% by volume, or between about 60% and 75% by volume). The surface
ligands are optionally removed partially or completely by heating
or chemical treatment after the quantum dot solid is formed. More
specifically, the length of the surface ligands can be used to
define the spacing between quantum dots. By combining the ability
to create density-controlled quantum dot solids with variable
density quantum dots in a matrix material, the concentration of
quantum dots, and therefore the nonlinear index of refraction of
the materials described herein, can be tuned over many orders of
magnitude.
[0152] In the case of quantum dot solids, the surface ligands can
take the place of an extrinsic matrix material according to some
embodiments of the current invention. In the case of close-packed
quantum dots in which the surface ligands have typically been
removed, the quantum dots themselves are considered to form their
own "intrinsic" matrix material. Quantum dot solids according to
some embodiments of the invention can be fabricated in a variety of
ways, such as, for example, described in C. B. Murray et al.,
"Self-Organization of CdSe Nanocrystallites into Three-Dimensional
Quantum Dot Superlattices," Science 270, 1335 (1995), C. R. Kagan
et al., "Long-range resonance transfer of electronic excitations in
close-packed CdSe quantum-dot solids," Phys. Rev. B 54, 8633
(1996), and U.S. Pat. No. 6,139,626 to Norris et al., entitled
"Three-dimensionally patterned materials and methods for
manufacturing same using nanocrystals" and issued on Oct. 31, 2000,
the disclosures of which are incorporated herein by reference in
their entirety. Quantum dot solids according to some embodiments of
the invention can be fabricated with a variety of different quantum
dot materials, sizes, and size distributions. It is also possible
to form mixed quantum dot solids comprising a plurality of quantum
dot materials, sizes, and size distributions.
[0153] Engineerable Nonlinear Nanocomposite Materials
[0154] One embodiment of the present invention comprises an
engineered nonlinear nanocomposite material that combines the large
nonlinear and size dependent optical properties of quantum dots
with the processability and chemical stability of a matrix material
and/or a chemically controlled quantum dot surface. By separately
selecting the size and material of the quantum dot, the surface
ligands, the matrix material, and the density of quantum dots
within the matrix material, one can independently tune various
significant characteristics in designing an ideal nonlinear optical
material.
[0155] In particular, this embodiment of the present invention
comprises the following characteristics that, taken together, in
part or in whole, provides a substantially improved nonlinear
optical material over what is known in the art.
[0156] The effects of quantum confinement and the specific
selection of quantum dot material is used to create extremely large
optical nonlinearities, specifically
Re[.sub..chi..sup.(3).sub.ijkl], in the data beam wavelength
range-of-interest, while the energies of single- and multi-photon
absorption features are selected to minimize absorptive loss of the
data beam and heating and to optimize resonant enhancement effects.
This optimization can include the use of appropriately chosen
non-degenerate control and data beams. Alternatively, the nonlinear
absorption mechanisms can be enhanced, e.g.,
Im[.sub..chi..sup.(3).sub.ij- kl] can be optimized, depending upon
the application.
[0157] The matrix material is selected, independent of the quantum
dot material and size, with the desired chemical and mechanical
properties to impart physical and chemical compatibility with the
specific device architecture and materials as well as the process
of incorporation into devices.
[0158] The surface ligands of the quantum dots are selected to
facilitate homogeneous incorporation of the quantum dots into the
selected matrix material and are optionally selected to facilitate
controlled aggregation of quantum dots within the selected matrix
material.
[0159] The density of quantum dots in the matrix material is
selected to precisely tune the linear index of refraction to match
the boundary conditions for a given device architecture (in the
case of high-index materials, a quantum dot solid can be used).
EXAMPLE 1
[0160] This example describes a preferred embodiment in which an
engineered nonlinear nanocomposite material is incorporated into a
nonlinear directional coupler that utilizes nonresonant or
near-resonant nonlinearities. In the current example, the waveguide
core is fabricated from doped silica with an index of refraction of
1.52 at 1.55 .mu.m. It will be recognized by one of ordinary skill
in the art that doped silica can have an index of refraction over a
wide range of values. The current example is not meant to limit the
scope of the invention, and it will be understood that variations
on this example can extend to waveguide cores with an arbitrary
index of refraction.
[0161] In the case of a nonlinear directional coupler, light is
evanescently coupled between two waveguide cores such that a signal
entering one waveguide core oscillates between the two as a
function of the interaction length. By choosing an appropriate
length, the light can be coupled completely into one or the other
of the two waveguide cores (i.e., the "off" state can be
transmission through one or the other waveguide core by appropriate
device design). By changing the index of refraction between the
waveguide cores, it is possible to switch the output waveguide core
from the "off" state to the other waveguide core (i.e., the "on"
state) for a fixed length device. An index change from a
.sub..chi..sup.(3) based nonlinear material can yield extremely
fast optical switching. However, so far no single material has been
appropriate for a commercial optical switch based on a nonlinear
direction coupler.
[0162] The active material in this optical device desirably should
have a large nonlinear response in the data beam wavelength
range-of-interest. It is also desirable (primarily for nonresonant
nonlinearities) to maximize resonant enhancement, while
simultaneously avoiding significant single-or multi-photon
absorption. At the same time, the linear index of refraction of the
active material desirably should be less than that of the core
material and be close to that of the rest of the cladding to avoid
disruption of the optical mode as light is guided into the active
region.
[0163] In this example, depicted in FIGS. 6(a) through 6(e), the
device comprises doped silica waveguide cores 602 and 604 (n=1.552)
fabricated on a doped silica substrate 606 (n=1.515). As shown in
FIG. 6(b), the other three sides of the waveguide cores 602 and 604
are initially surrounded by air (n=1), as is the space between the
waveguide cores 602 and 604 in the interaction region. The space
around the waveguide cores 602 and 604 is then filled with an
engineered nonlinear nanocomposite material 608 (n=1.515) to match
the waveguide boundary conditions of the substrate 606, as shown in
FIG. 6(c). By illuminating the interaction region with
trigger-pulses as shown in FIGS. 6(d) and 6(e), the index of
refraction between the waveguide cores 602 and 604 is changed,
activating the switch.
[0164] Operation of this switch is slightly different than what is
commonly described in the art. It is best understood by
presupposing that the directional coupler length is chosen such
that in the inactivated state, the two waveguide cores 602 and 604
exchange energy such that each output will receive substantially
half of the power from each input (acting as a 3 dB coupler). If
the illumination is such that the index of refraction increases in
the nonlinear nanocomposite material 608, the interaction between
the two propagating waveguide cores 602 and 604 will decrease,
leading to a reduction in the data energy transferred between the
cores 602 and 604, forcing the switch closer to a bar state. If the
illumination is such that the index of refraction decreases in the
nonlinear nanocomposite material 608, the interaction between the
two cores 602 and 604 increases, increasing the energy transferred
between the cores 602 and 604, forcing the switch closer to a cross
state. One skilled in the art will recognize that this transfer
function is cyclic and that further reduction of the index of
refraction of the nonlinear nanocomposite material 608 will result
in oscillations between the cross and bar states. If desired, the
length of nonlinear directional coupler may be chosen to include
several oscillations in the inactive state, leading to an effective
bias in the total oscillations.
[0165] The engineered nonlinear nanocomposite material 608 for this
example comprises silicon oxide coated silicon quantum dots or
organic-terminated silicon or germanium quantum dots dispersed in a
poly(methyl methacrylate) polymer matrix material (PMMA; n=1.49).
PMMA is chosen here due to its desirable optical properties for use
in the 1.55 .mu.m range and its ease of processing in waveguide
structures. Examples of these desirable optical properties include
high optical transmissivity in the visible wavelength, relatively
low absorption near 1550 nm, and low birefringence (as low as
0.0002 at 1550 nm has been observed).
[0166] In order to optimize degenerate nonresonant switching at
1.55 .mu.m, silicon quantum dots with a diameter of around 4 nm are
used, placing the 2-photon absorption peak at higher energy than
the spectral energy range-of-interest. This is sufficient to
minimize 2-photon absorption that may result in signal loss and
heating, while maintaining a significant resonance enhancement at
the wavelength of the trigger pulse. This particular combination of
quantum dot material and size also yields a maximum in
.sub..chi..sup.(3) at 1.55 .mu.m. To maximize near-resonant
switching, the appropriate choice of control wavelength and quantum
dot resonance at the control wavelength desirably should be chosen
that minimizes or reduces absorption loss at the data
wavelength.
[0167] To facilitate incorporation of the quantum dots into PMMA,
the silicon or germanium quantum dots can be coated with a ligand
layer comprising a long-chained hydrocarbon with a methacrylate
functional group at the end. Alternatively, any functional group
compatible with PMMA can be used. Quantum dots and PMMA are
dissolved in an organic solvent, such as toluene, and applied to
the device as shown in FIG. 6(c). The concentration of PMMA is
determined based on the desired thickness of the final
nanocomposite material and the method of application. In the case
of spin-coating, a 5% PMMA solution is appropriate. The
concentration of quantum dots is selected such that the final
nanocomposite material, after deposition, has a linear index of
refraction of 1.515. This is determined by calibrating the initial
concentration of quantum dots (as measured by the absorption
characteristics) to the final index of refraction of a PMMA-quantum
dot film deposited in the method to be used. The linear index of
the film can be measured using ellipsometry or the like.
[0168] After spin-coating the polymer-quantum dot solution over the
device, the solvent is allowed to evaporate, leaving an engineered
nonlinear nanocomposite coated device as shown in FIG. 6(c). The
index of refraction around all sides of the waveguide cores 602 and
604 is matched and optimized for the specific device. At the same
time, .sub..chi..sup.(3) and the resonance conditions for 1.55
.mu.m are independently tuned for optimum switching performance. As
a final aspect of the current example, based on the known intensity
of the trigger-pulse and the resulting nonlinear response of the
engineered nonlinear nanocomposite material 608, the active length
of the device is selected to provide optimal switching performance.
This can be done by limiting the illumination area of the
trigger-pulse to define the active area as in FIG. 6(d) or by
designing the specific waveguide structure with the appropriate
interaction length as in FIG. 6(e). The actual active length can be
determined empirically or through simulation.
[0169] By increasing the index of refraction of waveguide cores,
substantially larger concentrations of quantum dots can be
incorporated into the active material while retaining functionality
of the switch. This can yield substantially higher switching
efficiency. For example, as shown in FIG. 6(f), with silicon
waveguide cores 610 and 612 having an index of refraction of
.about.3.4, an active material 614 desirably should have an index
of refraction equal to or less than 3.39 to achieve efficient
waveguiding through the active region. This allows densities of
quantum dots as high as those of close-packed quantum dot solids
(either crystalline or amorphous).
EXAMPLE 2
[0170] To highlight the flexibility of embodiments of the current
invention, this example describes a second preferred embodiment in
which an engineered nonlinear nanocomposite material may be used in
a waveguide nonlinear Mach-Zehnder (MZ) interferometer. In this
case, as shown in FIGS. 7(a) through 7(f), a waveguide core is
fabricated from partially oxidized silicon with an index of
refraction of 2.4 at 1.55 .mu.m. Once again, it will be apparent to
one of ordinary skill in the art that partially oxidized silicon
can have a range of indices of refraction, and that 2.4 is not
meant to limit the scope of the invention. Variations on this
example comprising other possible indices can be used depending on
the specific application.
[0171] In the nonlinear MZI of the present example, a data signal
traveling along a waveguide core is split into two separate and
uncoupled waveguide arms with a defined phase relation between
them. The signals travel along the arms for a predetermined length
and are then recombined. Phase differences resulting from the
propagation of the light in each arm result in constructive or
destructive interference of the signals in the output waveguide
core. By modulating the index of refraction of one or both of the
arms, the output signal can be switched on or off by creating a
relative 0- to .pi.-phase shift between the signals. One of
ordinary skill in the art will realize that further changes in
index of refraction will result in cyclic exchange between the on
and off states. An index change from a .sub..chi..sup.(3) based
nonlinear material would yield extremely fast optical switching;
however, so far no single material has been appropriate for a
commercial switch based on this device.
[0172] As with the example above, the active material in this
device desirably should have a high nonlinear response in the
wavelength range-of-interest and no significant absorption. In this
case, however, the nonlinear material is incorporated directly into
the waveguide core. As such, the index of refraction of the
engineered nonlinear nanocomposite desirably should be greater than
that of the cladding material and be close to that of the core to
avoid disruption of the optical mode as light moves into the active
region.
[0173] In this example, as shown in FIG. 7(a), the device comprises
a partially oxidized waveguide core (n=2.4) fabricated on a silica
substrate (n=1.45) and surrounded by a silica cladding on three
sides. The top of the waveguide core is bounded by air (n=1). A
section of one of the waveguide arms is etched away as shown in
FIG. 7(b), filled with an engineered nonlinear nanocomposite
material (n=2.4) as shown in FIG. 7(c) to match the boundary
conditions of the waveguide core, and then polished as shown in
FIG. 7(d). By illuminating the active region with trigger-pulses as
shown in FIGS. 7(e) and 7(f), the index of refraction in one arm is
changed, thus activating the switch. A preferred engineered
nonlinear nanocomposite material for this example comprises silicon
oxide coated silicon quantum dots formed into a close-packed
quantum dot solid with index of refraction tuned to 2.4.
[0174] In order to optimize switching at 1.55 .mu.m, silicon
quantum dots with a diameter of 4 nm are used, placing the 2-photon
absorption peak at higher energy than the spectral energy
range-of-interest. This is sufficient to eliminate or reduce
2-photon absorption that may result in signal loss and potential
heating of the device by the trigger-pulse. This particular
combination of material and size also yields a maximum in
.sub..chi..sup.(3) at 1.55 .mu.m. To maximize near-resonant
switching, the appropriate choice of control wavelength and quantum
dot resonance at the control wavelength desirably should be chosen
that minimizes or reduces any absorption loss at the data
wavelength.
[0175] In order to achieve precise index of refraction control
within the waveguide arm, surface ligands desirably should be
selected to yield a specific particle-to-particle spacing within
the final quantum dot solid. This can be achieved by measuring the
index of refraction of many thin-films, formed by the method to be
used, with quantum dots comprising different types of surface
ligands. By using ellipsometry or the like, the index of refraction
resulting from each type of surface ligand and deposition method
can be determined and calibrated for determining the optimum
conditions for the final device deposition. In the case of the
present example, an index of 2.4 corresponds roughly to a packing
density of 70% by volume. A short-chained hydrocarbon is preferable
in this case, such as a butyl- or other alkyl group.
[0176] The quantum dots, in a solvent of hexane or toluene, are
spin-coated over the surface of the device, filling the open region
of the waveguide arm as shown in FIG. 7(c). A slow spin speed is
preferable, since the thickness of the material in the waveguide
arm can be controlled by polishing the overflow off the surface
(1000 rpm). The concentration of quantum dots in the solution
should be high, preferably in the range of 1 nM to 1M, more
preferably 10 .mu.M to 1 mM.,
[0177] After spin-coating, the solvent is allowed to evaporate,
creating a close-packed quantum dot solid filling the open region
of the waveguide arm as shown in FIG. 7(c). The surface is then
polished to provide an optical-quality interface on the topside of
the device in the active region as shown in FIG. 7(d). The index of
refraction of the engineered nonlinear nanocomposite is matched to
that of the waveguide core of the arm and optimized for the
specific device. At the same time, .sub..chi..sup.(3) and the
resonance conditions for 1.55 .mu.m are independently tuned for
optimum switching performance. As a final aspect of the current
embodiment, based on the known intensity of the trigger-pulse and
the resulting nonlinear response of the engineered nonlinear
nanocomposite material, the active length is selected to provide
optimal switching. This can be done by designing the etched length
of the waveguide arm to the desired active length as in FIG. 7(e)
or by limiting the illumination area of the trigger-pulse as in
FIG. 7(f). The specific active length can be determined empirically
or through simulation.
[0178] Alternatively, a nonlinear MZ interferometer can be
fabricated without etching a portion of a waveguide core as shown
in FIGS. 8(a) through 8(d). In this case, a engineered
nanocomposite material can be simply cast on top of the entire
device as shown in FIG. 8(b) with any excess removed as shown in
FIG. 8(c), such that the active material is in evanescent contact
with the signal passing through each of the arms (as well as
elsewhere). By illuminating a portion of one or both arms, the
active region can be defined as shown in FIG. 8(d). In this
preferred embodiment, the engineered nonlinear nanocomposite
desirably should be designed to have an index of refraction that is
compatible with waveguiding in the partially oxidized silicon core
(e.g., n<2.4). Again, this nanocomposite material is preferably
a close-packed quantum dot solid.
[0179] Had further chemical processing steps been required in
either of the above examples, it would also be possible to select
the matrix material and/or surface ligands to impart stability of
the engineered nonlinear nanocomposite under the required
conditions.
[0180] The current embodiments not only provide a nonlinear
material with a dramatically increased nonlinear response for use
in these optical devices, they simultaneously provide materials
that have been engineered to have optimum linear index of
refraction, 2-photon absorption, near-resonance enhancement, and
processability for each application. This level of independent
control of optical, chemical, and mechanical properties does not
exist in other materials.
[0181] Preferred Quantum Dot Materials
[0182] Preferred quantum dots according to some, embodiments of the
present invention comprise substantially defect free quantum dots
with a well-passivated surface. Preferred quantum dots also
comprise a bandgap energy that is preferably greater than the
photon energy range-of-interest (e.g., for the data beam), and more
preferably greater than twice the photon energy range-of-interest
(primarily for nonresonant nonlinearities) for its intended
applications. While maintaining these requirements, the material
and size of the quantum dots can be interchangeable. The specific
material and size can be selected as necessary to engineer the
optical characteristics for a particular application. The following
provides certain preferred characteristics according to some
embodiments of the invention:
[0183] Core-Shell Quantum Dots:
[0184] Core-shell quantum dots are particularly preferred because
defects can result in traps for electrons or holes at the surface
of a quantum dot core. These traps can degrade the electrical and
optical properties of the quantum dot, yielding low-energy states
within the bandgap of the material. An insulating layer at the
surface of the quantum dot core provides a rise in the chemical
potential at the interface, which can eliminate energy states that
serve as traps. Surprisingly, these trap states can actually
interfere with efficient switching or decrease the FOM of a
material by contributing to single or multi-photon absorption.
Additionally, shells act to physically protect the core material
from chemical interactions such as oxidation, reduction, or
dissolution. For instance, one embodiment of the present invention
relates to the use of a shell to stabilize intrinsically unstable
silicon or germanium quantum dots. Optionally, the shell can
provide an appropriate chemical surface for covalent or
non-covalent binding of molecules to the quantum dot, wherein the
core material may or may not provide an appropriate surface for
such binding.
[0185] Preferably, a quantum dot will be substantially defect free.
By substantially defect free, it is typically meant that within the
quantum dot there is fewer than 1 defect per quantum dot,
preferably substantially fewer than 1 defect per quantum dot, more
preferably less than 1 defect per 1000 quantum dots, more
preferably less than 1 defect per 10.sup.6 quantum dots, more
preferably less than 1 defect per 10.sup.9 quantum dots. Typically,
a smaller number of defects within a quantum dot translates into an
increased photoluminescence quantum efficiency. For certain
embodiments of the invention, a quantum dot that is substantially
defect free will typically exhibit photoluminescence with a quantum
efficiency that is greater than 6 percent, preferably greater than
10 percent, more preferably at least 20 percent, more preferably at
least 30 percent, more preferably at least 40 percent, and more
preferably at least 50 percent.
[0186] Preferably, the core will be substantially crystalline and
be substantially defect-free. By substantially defect free, it is
typically meant that within the core there is fewer than 1 defect
per quantum dot, preferably substantially fewer than 1 defect per
quantum dot, more preferably less than 1 defect per 1000 quantum
dots, more preferably less than 1 defect per 10.sup.6 quantum dots,
more preferably less than 1 defect per 10.sup.9 quantum dots.
[0187] In a similar manner, the shell and/or the interface region
preferably will be substantially defect free, where it is typically
meant that within the shell and/or the interface region there is
fewer than 1 defect per quantum dot, preferably substantially fewer
than 1 defect per quantum dot, more preferably less than 1 defect
per 1000 quantum dots, more preferably less than 1 defect per
10.sup.6 quantum dots, more preferably less than 1 defect per
10.sup.9 quantum dots.
[0188] Size and Size-Distribution:
[0189] Another preferred characteristic of the quantum dots of some
embodiments of the present invention is such that a figure-of-merit
(FOM) for all-optical switching or processing can be largely
insensitive to size dispersion, contrary to results and predictions
in the literature. FIG. 9 illustrates a figure-of-merit (FOM) for
all-optical switching with an engineered nonlinear nanocomposite
material as a function of quantum dot size, according to an
embodiment of the invention. Here, the nanocomposite material
includes germanium quantum dots made with methods described herein.
The FOM in this case is defined as 2.gamma./.beta..lambda., which
is applicable for nonresonant nonlinearities. The criteria for
effective all-optical switching is FOM>1. FIG. 9 shows how the
FOM for all-optical switching depends on the size of the quantum
dots. It can be seen that the FOM exceeds 1 for a large size
dispersion, e.g., for diameters ranging from 3 nm to 6 nm. Similar
results can be obtained with the other quantum dots described
herein, e.g., silicon quantum dots. Therefore, some embodiments of
the present invention avoid the need for a substantially
monodispersed size distribution of quantum dots while substantially
improving switching characteristics and efficiency over previous
uses of quantum dots as nonlinear materials. The effects of size
distribution and specifically how the FOM of switching depends on
the quantum dot size has not been previously considered in
detail.
[0190] Shape and Shape Distribution
[0191] Quantum dots can be fabricated in a variety of shapes,
including (but not limited to) spheroids, rods, pyramids, cubes,
and other geometric and non-geometric shapes. For shapes that are
not spherically symmetric, a distribution of orientations can
result in an effective broadening of the size distribution as seen
by incident light. To avoid the need for orientation of quantum
dots within a matrix material, the preferred quantum dot shape is
spherical, according to some embodiments of the invention.
Spherical quantum dots are also preferred for nanocomposites
comprising oriented quantum dots. Alternatively, another preferred
embodiment comprises spheroid or substantially spherical quantum
dots, with an aspect ratio restricted to between 1.+-.(% size
distribution) or with an aspect ratio between approximately 0.8 and
1.2. In this case, orientation plays an insignificant role in the
inhomogeneous broadening of the spectral features. For similar
reasons, the preferred quantum dot will also be substantially
monodisperse in shape. These considerations regarding the
importance of shape and/or shape-distribution constitute an
improvement in the use of quantum dots as a nonlinear material.
[0192] It should be recognized that an arbitrary shape may still be
preferred as long, as the relative orientation dependence of the
broadening of the linear and nonlinear optical properties is less
than the broadening resulting from the size distribution of the
quantum dot sample.
[0193] Crystal Structure of the Core
[0194] For reasons similar to those described above for shape,
preferred quantum dots according to some embodiments of the
invention will have a core with a crystal structure that is
spherically symmetric, more preferably a cubic or diamond crystal
structure. Alternatively, the crystal structure may be
non-spherically symmetric, preferably cylindrically symmetric, more
preferably a wurtzite crystal structure.
[0195] It should be recognized that an arbitrary crystal structure
may still be preferred as long as the relative orientation
dependence of the broadening of the linear and nonlinear optical
properties is less than the broadening resulting from the size
distribution of the quantum dot sample. Once again, the
considerations described here regarding the importance of crystal
structure constitute an improvement in the use of quantum dots as a
nonlinear material.
[0196] Semiconductor Materials
[0197] There are a variety of preferred quantum dot materials for
some embodiments of the current invention. For any given
application, the preferred materials can be determined based on the
specific optical requirements for that application. Examples of
such preferred materials include but are not limited to inorganic
crystals of Group IV semiconductor materials including but not
limited to Si, Ge, and C; Group II-VI semiconductor materials
including but not limited to ZnS, ZnSe, ZnTe, ZnO, CdS, CdSe, CdTe,
CdO, HgS, HgSe, HgTe, HgO, MgS, MgSe, MgTe, MgO, CaS, CaSe, CaTe,
CaO, SrS, SrSe, SrTe, SrO, BaS, BaSe, BaTe and BaO; Group III-V
semiconductor materials including but not limited to AlN, AlP,
AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs and InSb; Group
IV-VI semiconductor materials including but not limited to PbS,
PbSe, PbTe, and PbO; mixtures thereof; and tertiary or alloyed
compounds of any combination between or within these groups,
including but not limited to GeSe, SnS, SnSe, PbS, PbSe, PbTe
ZnGeAs.sub.2, ZnSnP.sub.2, ZnSnAs.sub.2, CdSiAs.sub.2, CdGeP.sub.2,
CdGaAs.sub.2, CdSnP.sub.2, and CdSnAs.sub.2.
[0198] Quantum dots of many semiconductor materials can be
fabricated, at least in part, using a variety of methods. Some
preferred synthetic methods include those described for Group Ill-V
and Group II-VI semiconductors as described in U.S. Pat. No.
5,990,479 to Weiss et al., entitled "Organo Luminescent
semiconductor nanocrystal probes for biological applications and
process for making and using such probes" and issued on Nov. 23,
1999; U.S. Pat. No. 5,262,357 to Alivisatos et al., entitled "Low
temperature thin films formed from nanocrystal precursors" and
issued on Nov. 16, 1993; U.S. Pat. No. 5,505,928 to Alivisatos et
al., entitled "Preparation of III-V semiconductor nanocrystals" and
issued on Apr. 9, 1996; C. B. Murray et al., "Synthesis and
characterization of nearly monodisperse CdE (E=sulfur, selenium,
tellurium) semiconductor nanocrystallites, " J Am. Chem. Soc. 115,
8706 (1993); and in the thesis of C. Murray, "Synthesis and
Characterization of II-VI Quantum Dots and Their Assembly into 3-D
Quantum Dot Superlattices" (Massachusetts Institute of Technology,
Cambridge, Mass., 1995), the disclosures of which are hereby
incorporated in their entireties by reference.
[0199] The fabrication of some types of shells on quantum dots can
be performed using a variety of methods. Preferred methods include
those described in X. Peng et al., "Epitaxial Growth of Highly
Luminescent CdSe/CdS Core/Shell Nanocrystals with Photostability
and Electronic Accessibility," J. Am. Chem. Soc. 119, 7019 (1997)
and B. O. Dabbousi et al., "(CdSe)ZnS Core-Shell Quantum Dots:
Synthesis and Characterization of a Size Series of Highly
Luminescent Nanocrystallites," J. Phys. Chem. B 101, 9463 (1997),
the disclosures of which are hereby incorporated by reference in
their entirety.
[0200] Two preferred materials for use in quantum dots are silicon
and germanium, according to some embodiments of the invention. Both
Si and Ge have bulk energy gaps that are less than 1.6 eV, making
them ideal materials from which to fabricate quantum dots that
exploit quantum confinement to enhance optical nonlinearities at
telecommunications wavelengths (where photon energies are typically
.about.0.8 eV). The ideal chemistry of Group IV materials (as
discussed below) further solidifies these choices.
[0201] In addition, the electron affinity or ionization potential
of Group IV materials (e.g., Si and Ge) makes them amenable to
forming strong and stable covalent bonds with organic and inorganic
surface ligands, making them ideal for this purpose and for
enabling quantum dots that are stable in ambient as well as
reasonably extreme environmental conditions. The significance of
this capability can be better appreciated by recognizing that the
surfaces of quantum dots comprised of more ionic materials often
require surfactants or ionic species to cap, which involves less
preferable and weaker van der Waals bonds, hydrogen bonds, or ionic
bonds. Examples of these more ionic quantum dot materials include
Group II-VI materials such as CdSe. These more ionic quantum dots
often require complex processing to modify the ionic quantum dots
so as to enable the more desirable covalent bonding between the
quantum dot surface and surface ligands, e.g., a surface layer or
layers comprised of a material different than the core quantum dot
material typically needs to be added to the ionic quantum dot
surface, wherein the attached surface layer or layers are amenable
to covalent bonding to surface ligands. An example of such a
surface layer is one comprised of CdS.
[0202] In addition, the chemical properties of Group IV materials
(e.g., Si and Ge) are such that a stable oxide can be formed that
serves to confine carriers and to passivate the surface to mitigate
surface traps.
[0203] In addition, the Bohr exciton is relatively large in Ge
(.about.12 nm), thus providing a large size range over which the
beneficial effect of quantum confinement, as discussed in various
sections herein, are relevant.
[0204] A Novel Quantum Dot Material
[0205] In one embodiment, the quantum dots are silicon quantum dots
or germanium quantum dots that are surface passivated (or
terminated) with an inorganic layer (such as oxides of silicon and
germanium) and/or organic and/or inorganic surface ligands, herein
sometimes referred to as SiQDs and GeQDs, respectively. SiQDs and
GeQDs as described herein are novel types of quantum dots that show
definitive quantum confinement effects as manifested by size
dependent properties such as size-dependent energy gaps that can be
tuned over a very broad range and in particular from the near
infrared to the near ultraviolet. In addition, SiQDs and GeQDs are
stable under a variety of environmental conditions including
ambient (e.g., pressure: .about.1 atmosphere; Gases: .about.70%
nitrogen, .about.30% oxygen; Temperature: .about.20-25.degree. C.)
for desired periods of time depending on the specific application.
A SiQD and a GeQD can be comprised of a substantially Si core for a
SiQD and a substantially Ge core for a GeQD. In addition, the
"surface" of the SiQD can be comprised of Si and inorganic elements
such as oxygen and/or organic ligands (R). In addition, the
"surface" of the GeQD can be comprised of Ge and inorganic elements
such as oxygen and/or organic ligands (R).
[0206] In one embodiment of the invention, a SiQD comprises a
substantially defect free silicon crystal core of diameter between
approximately 1 nm and 100 nm, preferably between approximately 1
nm and 20 nm, more preferably between approximately 1 nm and 10 nm,
while a GeQD comprises a substantially defect free germanium
crystal core of diameter between approximately 1 nm and 100 nm,
preferably between approximately 1 and 50 nm, more preferably
between approximately 1 and 20 nm. In the case of an inorganic
shell surrounding the silicon or germanium core, this shell
typically has a thickness of between approximately 0.1 and 5 nm.
One preferred inorganic shell is SiO.sub.n for SiQD and GeO.sub.n
for GeQD with n ranging between approximately 0 and 2, preferably
ranging between approximately 1.5 and 2, most preferably ranging
between approximately 1.8 to 2. The chemical composition of the
shell (e.g., relative amounts of Si (or Ge) and O) is potentially
varying continuously through a portion of the shell and optionally
varying discontinuously through a portion of the shell, in which
case n can represent an averaged value within the shell. In the
case of organic surface ligands terminating the surface, the SiQD
and GeQD can comprise ligand layers comprising organic molecules
with a structure R. R can be any one of a variety of hydrophobic,
hydrophilic, or amphiphilic molecules (a list of preferred surface
ligands is included below). The surface ligands can provide a
surface coverage of available silicon (or germanium) and oxygen
binding sites at the surface to provide between approximately 0%
and 100% surface coverage, preferably between approximately 20% and
100% surface coverage, more preferably between approximately 50%
and 100% surface coverage, more preferably between approximately
80% and 100% surface coverage, with a maximum of one or more
complete layers of surface ligands. R can optionally comprise a
plurality of different organic molecules at a plurality of absolute
and relative densities. Finally, a SiQD or GeQD may optionally
comprise additional R-groups that do not interact directly with the
quantum dot surface, but rather indirectly through other R-groups
interacting directly with the surface. In this case, surface
coverage greater than 100% is possible.
[0207] It has long been considered that the production of an
ambient-stable silicon quantum dot or germanium quantum dot with a
defined oxide shell could not be achieved due to difficulties in
growing a stable and trap-free surface oxide shell Thus, the SiQD
and GeQD described herein represent a substantial advance.
[0208] Methods for fabricating SiQDs and GeQDs in accordance with
some embodiments of the invention are discussed below. It should be
noted, however, that the current invention refers to SiQDs or GeQDs
synthesized by a variety of other methods in addition to those
described herein. Some embodiments of the invention encompass
various possible variations of composition of SiQD and GeQDs that
could be made while retaining the general characteristics of a
substantially crystalline Si or Ge core and a substantially
noncrystalline inorganic (e.g., oxide) shell or organic ligand
layer.
[0209] Method One--"Top Down" Approach
[0210] A general method for the formation of quantum dots of some
embodiments of this invention involves a "top down" approach in
which "bulk" material is converted to nanostructured material in
the form of quantum dots. In this approach, a form of energy is
applied to a form of a material from which the quantum dot is to be
made. The material can be in bulk form, hence the term "top down."
The material in the bulk form is preferably converted to a fine
powder, preferably as fine as possible, more preferably particles
in the nanometer size regime (e.g., between approximately 1 nm and
100 nm) comprised of the material from which the quantum dot is to
be made. One advantage of using a fine powder is that it leads to
shorter processing times to achieve the desired quantum dot. The
applied energy can be in the form of, for example, acoustic or
vibrational energy (e.g., sound energy), optical energy (e.g.,
light energy), electrical energy, magnetic energy, thermal energy,
chemical energy, or any combination thereof. More precise size
control of the final quantum dots can be achieved by applying more
than one source of energy to the starting material. Multiple
sources of energy can be applied simultaneously or they can be
applied in various sequential combinations. This also leads to
shorter processing times to achieve the desired quantum dot. It is
believed that the applied energy fractures or breaks the starting
material into smaller particles and/or "grows" the starting
material into larger particles by, for example, fracturing or
breaking (e.g., "consuming") the smaller particles. In essence,
this method evolves the starting material into the desired
nanostructured form. This results in the unique quantum dots
described herein in which a stable, well-formed, substantially
defect-free inorganic shell is formed on the surface of the QD, as
well as a defect-free interfacial region between the core and the
shell. This shell imparts a high stability to the QDs under a
variety of environmental conditions including ambient. In the
specific case of an oxide shell, the oxide is stable and
substantially defect-free.
[0211] Specific examples of "top-down" methods of formation of
quantum dots of some embodiments of this invention follow:
Example 1
Oxide-Terminated SiQDs
[0212] A powdered form of Si, from which Si quantum dots can made
for some embodiments of this invention, is derived from porous
silicon (PSi). A nanostructured PSi layer is removed and made into
a fine powder. Energy is then applied in the form of sound energy
through sonication and light energy through irradiation with a
light source. The size of the SiQDs is determined by the duration
and power of the sonication (with longer and higher power
sonication giving rise to smaller quantum dots) and by the
characteristics of the light source (with shorter wavelengths and
longer irradiation times giving rise to smaller quantum dots).
[0213] The method in this example uses sonication periods
sufficient to form stable and well-formed quantum dots with stable
oxide surface termination that were not previously available. In
addition, the method allows the size of the quantum dots to be
controlled by the sonication period. In addition, the method also
uses light irradiation as a means to control the size and the size
distribution of the quantum dots, where shorter wavelengths of
irradiation and longer irradiation times give smaller quantum dots
and narrower size distributions. This light irradiation allows
better control over the size and range of sizes of the quantum dots
than previously available, with sizes ranging from .about.1 nm to
.about.6 nm in diameter in one embodiment of the invention.
[0214] The result from the method of this invention is
oxide-terminated SiQDs that are stable under a variety of
environmental conditions including ambient. This stability results
largely from the stable and substantially defect-free oxide shell
and interfacial region between the core and shell.
[0215] PSi is formed using a variety of methods that include, but
are not limited to, anodic electrochemical etching of p-doped or
n-doped silicon as, for example, described in A. G. Cullis et al.,
"The structural and luminescence properties of porous silicon," J.
Appl. Phys. 82, 909 (1997), the disclosure of which is incorporated
herein by reference in its entirety. One preferred method includes
starting with p-type (e.g., Boron-doped) silicon (Si) wafers
comprising a plurality of orientations, with the (100) orientation
being preferred. The wafer resistivity preferably ranges from 0.02
.OMEGA.-cm to 30 .OMEGA.-cm. The wafer is preferably between
approximately 500-600 microns thick. Electrical contact to the
wafer is made through a thin layer of metal (e.g., aluminum or
platinum; preferably between approximately 100-500 microns thick)
deposited on the backside of the wafer. Anodic electrochemical
etching is performed on the wafer, which is placed in a solution
comprising aqueous hydrofluoric acid (HF, preferably 48 wt %) and
ethanol. The weight percentage of ethanol to aqueous HF ranges
between approximately 0% and 60%, preferably between approximately
45% and 55%. Various conducting materials can be used as the
counter electrode in which metals are an example. Examples of such
metals include, but are not limited to, aluminum, copper, brass,
and platinum.
[0216] The metal layer making electrical contact with the silicon
wafer may optionally be protected from erosion in the acidic
solution by isolating the metal layer from the solution. This can
be achieved by sealing the silicon surface with a gasket such that
the etching solution is substantially only in contact with the
silicon side of the substrate. Alternatively, the electrode metal
can be selected to be relatively inert under the selected etching
conditions or can be selected with a thickness great enough to
withstand the etching procedures described below.
[0217] Electrochemical etching of the Si wafer is carried out for
various time durations (which can range t from between
approximately 2 and 200 minutes, depending on the starting
parameters) using a constant current density ranging from between
approximately 5 and 1000 mA/cm.sup.2 with approximately 60
mA/cm.sup.2 as a preferred current density and approximately 30
minutes as a preferred etching time. After etching, the surface of
the Si wafer is left with a thin layer (between approximately 10
microns and 1 mm in thickness) of nanostructured material, which
comprises PSi. The peak of the luminescence of the PSi ranges
typically from 600 nm to 800 nm (or to greater than 800 nm).
[0218] The PSi is optionally rinsed with deionized water, dried
under a stream of nitrogen gas, and placed in a vacuum chamber. The
chamber is evacuated to a moderate pressure for several hours,
preferably less than 1 Torr, more preferably less than 500 mTorr,
and most preferably less than 100 mTorr. The samples are then
transferred to a solvent-free environment (e.g., a drybox). The
nanostructured PSi layer is then mechanically removed or scraped
(which can be accomplished, for example, with a knife edge or
scalpel) from the Si substrate, and the removed material is
collected. The nanostructured PSi layer can also be separated from
the Si wafer through a second electrochemical etching process in
which low concentration HF/H.sub.2O (preferably between 0.5 and 2%)
and a high current density (preferably greater than 160
mA/cm.sup.2) is used for a few minutes to separate the anodized and
nanostructured PSi layer from the Si substrate.
[0219] After the PSi layer has been separated from the silicon
substrate, the PSi is ground into a fine powder (using, for
example, a mortar and pestle and/or a mechanical agitator) yielding
about 25 to 40 mg of powdered PSi from a wafer surface area
measuring approximately 1 inch in diameter. The peak of the PL
spectra of the powder is in the red spectral region and ranges any
where from 600 nm to greater than 800 nm, depending on the
conditions of the electrochemistry. A solvent is then added to the
powdered PSi. Preferred solvents include, but are not limited to,
acetonitrile, toluene, hexane, methanol, ethanol, ethylene glycol,
and water. In the case of organic solvents, the solvent may be
dried over a dehydrant (e.g., calcium hydride or magnesium
sulfate), distilled, and degassed prior to being added to the
PSi.
[0220] The resulting mixture of PSi powder and solvent is placed in
a bath and sonicated with acoustic waves or sound energy for a
period of time. Although acoustic energy is being disclosed, it is
to be understood that other types of energy may be used, as
discussed above. The sonication can be accomplished with a variety
of equipment that emits acoustic waves or vigorously agitates or
shakes the powder, with an ultrasonic bath being a particularly
convenient method.
[0221] The size and size distribution of the quantum dots in the
mixture can be controlled by varying the duration of sonication.
The precise period of time required for sonication depends on a
number of factors that include the acoustic power of the sonicator,
the solvent used, the initial size and size distribution of the
nanostructures in the PSi powder, etc., and the characteristics of
the sonication should be calibrated for the specific processing
conditions used. A factor determining the optimum time duration of
the sonication is the time required to achieve the desired size of
the resultant quantum dots, i.e., the sonication is continued until
the desired quantum dot size is reached (e.g., until cores are
formed with diameters within a predetermined or desired range).
Generally, the size of the quantum dots decreases as the sonication
time increases, and the size of the quantum dots can be determined
by the energy gap or the peak wavelength of the photoluminescence
of the colloidal suspension of quantum dots. The relationship
between quantum dot size and energy gap and the relationship
between the PL peak wavelength and quantum dot size for silicon
quantum dots were previously described with respect to FIGS. 2 and
3. Therefore, the photoluminescence spectrum of the colloidal
suspension can be periodically taken during the sonication process
to monitor the progress of the sonication. Typically, the PL peak
wavelength shifts towards shorter wavelengths (corresponding to a
shift towards smaller peak sizes) during the sonication process.
Once the sonication time is calibrated for the processing
conditions used to give the desired quantum dot size, this method
can give very consistent results.
[0222] Another factor that can be used to determine the optimum
time duration of the sonication is the time required to achieve the
desired shells for the resultant quantum dots, e.g., the sonication
is continued until oxide shells are formed having desired
properties as discussed herein. If desired, the sonication time can
be calibrated for the processing conditions used to give the
desired photoluminescence quantum efficiencies.
[0223] As mentioned above, the precise relationship between
sonication time and the quantum dot size that results depends on
several parameters that may need to be calibrated with each
specific fabrication setup and conditions. The following is an
example that serves as a point of reference. With a sonication
power of 80 W and with methanol as the solvent, a sonication period
of 10 days resulted in oxide-terminated Si quantum dots with an
average size of .about.1.5-1.7 nm in diameter and emitting in the
near ultraviolet-blue; a sonication period of 3 days resulted in
quantum dots with an average size of .about.2.5 nm in diameter and
emitting in the green; a sonication period of 1 day resulted in
quantum dots with an average size of .about.3.6 nm in diameter and
emitting in the red.
[0224] Upon removal from the ultrasonic bath, the mixture is
allowed to settle and is centrifuged, and the supernatant is
filtered to remove any large particles. Preferred pore sizes of the
filter range between approximately 20 nm and 450 nm. Filters can
also be used to separate the different sizes of quantum dots.
Additionally, other separation techniques such as chromatography,
more specifically gel permeation chromatography or size exclusion
chromatography, can be used to separate the different sizes of
quantum dots. The result is a colloidal suspension of
oxide-terminated Si quantum dots (SiQDs) of various sizes that are
stable in a variety of environmental conditions that include
ambient (room temperature, pressure, and atmosphere).
[0225] As mentioned above, more precise size and size distribution
control of the final quantum dots can be achieved by applying more
than one source of energy to the starting material. In one
preferred method, two sources of energy are applied to the starting
material. One additional preferred source of energy is light
energy. In this example of a preferred method, the sample is
irradiated with light during or sequentially with sonication. The
light source can be a lamp (e.g., Tungsten, Xenon, or Mercury), a
light emitting diode (LED), a laser, or any other light source
capable of emitting light at the appropriate wavelengths, where
"appropriate wavelengths" is described below. Alternatively,
irradiation can be implemented during the electrochemical etching
process (in which the etched surface of the Si wafer is
irradiated). The size of the quantum dots that result is determined
by a number of parameters including wavelength, intensity, spectral
bandwidth, and duration of irradiation. Preferably, the wavelength
of irradiation should be within the spectral region where the light
is absorbed by at least a subset of the quantum dot sizes to be
controlled. Specifically, within a size distribution of quantum
dots, the longer the wavelength of the irradiation, the larger the
size of the resulting quantum dots. More specifically, to achieve a
specific size SiQD, the sample should be irradiated with photons of
energy approximately equal to the energy gap of the desired SiQD.
This effect can be accentuated by increasing the duration and/or
intensity of irradiation. In particular, the size and size
distribution of the quantum dots in the mixture can be controlled
by varying the duration of irradiation. The optimum time duration
of the irradiation is the time required to achieve the desired size
and/or size distribution of the resultant quantum dots, i.e., the
irradiation is continued until the desired quantum dot size is
reached (e.g., until cores are formed with diameters within a
predetermined or desired range) and/or until the desired size
distribution is reached (e.g., until substantially monodisperse
quantum dots are formed).
[0226] For any specific set of synthesis parameters, the precise
relationship between irradiation wavelength, irradiation intensity,
irradiation duration, and quantum dot size should be calibrated as
is done in the case of the sonication method alone. This can be
achieved by monitoring the energy gap or peak wavelength or spread
of the photoluminescence at various times during the irradiation as
an indicator of the progress toward the desired quantum dot.
Typically, the photoluminescence has a peak wavelength that shifts
towards shorter wavelengths (corresponding to a shift towards
smaller peak sizes) and a wavelength spread that narrows
(corresponding to a shift towards narrower spread in sizes) during
the irradiation process.
[0227] The following serves as examples or points of reference.
Simultaneously irradiating and sonicating the sample as described
above for 5 days with 50 mW of laser light at 400 nm results in
oxide-terminated SiQDs that luminesce in the near ultraviolet-blue
spectral region; simultaneously irradiating and sonicating the
sample as described above for 2 days with 100 mW of laser light at
532 nm results in oxide-terminated SiQDs that luminesce in the
green spectral region; simultaneously irradiating and sonicating
the sample as described above for 0.5 days with 150 mW of laser
light at 620 nm results in oxide-terminated SiQDs that luminesce in
the red spectral region.
[0228] The result of this "top down" approach is oxide-terminated
Si quantum dots that are stable in a variety of environmental
conditions, including ambient. This capability was previously
thought to not be possible. This is achieved in the "top down"
approach through the establishment of a stable and substantially
defect-free silicon oxide shell surrounding the Si quantum dot
core.
[0229] The defect-free nature of the resulting SiQDs is manifested
in the quantum efficiency of the photoluminescence from these
SiQDs. The presence of defects in quantum dots can trap excited
carriers (electrons and holes). These trapped carriers can either
nonradiatively relax, or they can radiatively recombine in a
defect. Both processes lead to a low quantum efficiency for the
photoluminescence from the quantum dots. Previous quantum dots
formed of Si or Ge typically exhibited photoluminescence quantum
efficiencies of .about.1-5%. In contrast, the photoluminescence
quantum efficiency of the SiQDs made with the methods of some
embodiments of this invention is greater than 6%, preferably at
least or greater than 10%, more preferably at least 20%, more
preferably at least 30%, more preferably at least 40%, and more
preferably at least 50% (e.g., as high as between approximately 50%
and 60%). This represents the largest photoluminescence quantum
efficiency observed for such quantum dots.
[0230] FIG. 3 shows global PL spectra of six samples of different
sized oxide-terminated Si quantum dots made with the method
described herein. FIG. 3 shows that the light emission can be
readily tuned from the red to the ultraviolet, and as a result, the
size can be readily tuned as well. And, this light emission is
stable in a variety of environmental conditions including
ambient.
[0231] The electronic and optical properties of these SiQDs that
are made in this fashion are unique in that they show size
dependent properties that are uniquely consistent with quantum
confinement. The optical and electronic properties of these Si
quantum dots are uniquely consistent with theoretical calculations
more sophisticated than Effective Mass approaches, such as the
Empirical Pseudopotential Method and the Tight Binding Method. A
comparison of the size dependent energy gap calculated by these
methods and with measurements taken on the SiQDs synthesized by the
method disclosed herein is shown in FIG. 2. The agreement is
extremely good and is the best observed for any quantum dots formed
of Si.
[0232] The use of light to control the physical size or size
distribution of the quantum dots in this synthetic process is a
particularly novel aspect of certain embodiments of the present
invention. Previous methods have typically used optical excitation
not for control of the physical parameters of the quantum dots but
to initiate the chemical reaction needed for quantum dot formation,
i.e., to photolyze the chemical precursors. As described above,
embodiments of the invention utilize optical control over the
physical parameters of quantum dots in a synthetic method. This
aspect is also applicable to other quantum dot synthetic procedures
and is not limited to those described herein with respect to SiQDs
and GeQDs.
[0233] Overall, the size and size distribution of the resulting
SiQDs can be precisely controlled by varying the duration of
sonication processing, the strength or intensity of the acoustic
energy of the sonication device, the photon wavelength (photon
energy) of irradiation, the intensity of irradiation, the spectral
width of irradiation, the duration of irradiation, the size and
size distribution of the starting material, and the solvent into
which the starting material is incorporated. For certain
embodiments of the invention, the average SiQD size can be varied
from .about.1 nm to greater than 6 nm with this technique. These
average sizes give rise to light emission from the infrared to the
ultraviolet.
[0234] Alternatively, PSi can be fabricated using n-doped Si
wafers. In this case, a process similar to that described above can
be followed. However, the electrochemical etching process may be
performed in the dark, and the wafer desirably should be
illuminated with a light source (UV light being preferred) during
etching in order to generate "holes" (as opposed to electrons)
needed in the etching process.
Example 2
Oxide-Terminated GeQDs
[0235] In the synthesis above, the reactants and starting materials
can be replaced with their germanium counterparts for the formation
of GeQDs. As shown in FIGS. 4(a) and 4(b), GeQDs of sizes ranging
from 1 nm to 16 nm have been synthesized using the method according
to an embodiment of the invention.
[0236] The electronic and optical properties of these GeQDs that
are made in this fashion are unique in that they show size
dependent properties that are uniquely consistent with quantum
confinement. The optical and electronic properties of these GeQDs
are uniquely consistent with theoretical calculations more
sophisticated than Effective Mass approaches, such as the Empirical
Pseudopotential Method and the Tight Binding Method. A comparison
of the size dependent energy gap calculated by these methods with
measurements taken on the GeQDs synthesized by the method disclosed
herein is shown in FIG. 4(a). The agreement is extremely good and
is the best observed for any quantum dots formed of Ge.
[0237] Method Two--"Bottoms Up" Approach
[0238] In another embodiment, quantum dots can be fabricated from
chemical precursors. This is essentially a "bottoms up" approach in
which the quantum dots can be assembled "atom-by-atom" through
chemical synthesis.
[0239] The present invention provides general high yield methods of
synthesizing surface-functionalized quantum dots and, in
particular, methods of synthesizing soluble quantum dots of a Group
IV, semiconductor material in a solution at relatively low
temperatures.
[0240] The methods can be described by reference to the
following:
[0241] Method 2-a:
YX.sub.a+Reducing Agent.fwdarw.(Y)X (1)
(Y)X+Capping Agent(R).fwdarw.(Y)R (2)
[0242] Method 2-b:
YX.sub.a+R.sub.bYX.sub.c+Reducing Agent.fwdarw.R(Y)X (3)
R(Y)X+Capping Agent(R').fwdarw.R(Y)R' (4)
[0243] wherein YX.sub.a is a source of Y, with Y being Si or Ge,
and X is selected from the group consisting of --F, --Cl, --Br,
--I, --O--CO--R.sup.(1), --NR.sup.(2)R.sup.(3), --O--R.sup.(4),
--S--R.sup.(5), and so forth, with R.sup.(1), R.sup.(2), R.sup.(3),
R.sup.(4), and R.sup.(5) independently selected from the group
consisting of alkyls, alkenyls, alkynyls, aryls, and so forth. The
reducing agent is selected from either activated metals (e.g.,
Group IA, Group IIA, transition metals, and lanthanides) or
hydrides (Group IIIB hydrides, Group IVB hydrides, and transition
metal hydrides). Electrochemical reduction can also be used for
reduction. The capping agent(R) and the capping agent(R') are
sources of surface ligands R and R', respectively, and can be
selected from organometallic reagents, e.g., RM (or R'M), with R
(or R') being a surface ligand (e.g., a linear or branched alkyls,
alkenyls, alkynyls, ether, ester, acid, amide or nitrile moiety
having between 1 and about 20 carbon atoms). It should be
recognized that the surface ligands R and R' can be the same or
different. The capping agents can also be an alcohol, amine, thiol,
and so forth. M is preferably from Group IA, Group IIA, or Group
IIB. In the above, "a" represents an oxidation state or
coordination number of Y in the source of Y, which is typically 2,
4, or 6, and "b" and "c" are integers that can each range from 1 to
6. "a" is typically equal to the sum of "b" and "c". In method 2-a,
(Y)X represent intermediate particles comprising cores including Y
and with surfaces terminated with X, and, (Y)R represent quantum
dots that are formed with surfaces terminated with R. In method
2-b, R(Y)X represent intermediate particles comprising cores
including Y and with surfaces terminated with R and X, and R(Y)R'
represent quantum dots that are formed with surfaces terminated
with R and R'.
[0244] The basic strategy involves solution phase reduction of
Si.sup.a+ or Ge.sup.a+, where a represents the oxidation state of
Si or Ge, and subsequent termination with organic or organometallic
reagents. The methods according to some embodiments of the
invention allow mild synthesis, precise manipulation,
functionalization, and interconnection of the Group IV quantum dots
to an extent not previously achieved. The key differentiations
between previously used methods and the methods according to some
embodiments of the invention include one or more of the
following:
[0245] Some embodiments can avoid arduous procedures typically
associated with the use of highly pyrophoric and air-sensitive
starting materials, such as Group IV Zintl compounds or sodium
metal. The Group IV Zintl salts are typically prepared by combining
starting materials (e.g., K and Si) at elevated temperature
(500-900.degree. C.) in a sealed tube for a few days. As an
example, a method of some embodiments of this invention uses milder
and air-stable reducing agents such as magnesium (Mg), other Group
IIA metals, transition metals, or lanthanides. This makes the
method more amenable to scale up and large scale manufacture.
[0246] Some embodiments provide a method in which the reaction
conditions are less extreme than required by previous methods. In
particular, a method of some embodiments of the invention avoids
the high pressure and high temperature conditions as sometimes
previously used that can produce large amounts of undesirable
insoluble materials.
[0247] Some embodiments need not utiltize high energy sonochemical
techniques for reduction of Si.sup.4+, which has typically produced
either-small amorphous particles with ill-defined surface
composition or larger insoluble aggregates with an irregular
network.
[0248] Some embodiments need not utilize highly toxic gaseous Group
IV hydrides and pyrophoric metal hydrides.
[0249] The yields from the method of some embodiments in this
invention are significantly higher than in previously reported
methods. In some embodiments, the yields that can be obtained are
between approximately 35% and 95%.
[0250] The size control that can be achieved is greater than
previous methods.
[0251] The range of sizes possible that can be produced is greater
than achievable with previously reported methods. Some embodiments
allow production of different sizes of quantum dots that can give
rise to infrared to ultraviolet light emission (e.g., not limited
to production of smaller quantum dots that emit primarily in the
blue and blue-green region).
[0252] The resultant quantum dots are not limited to certain size
distributions (e.g., the size distribution control that can be
achieved is greater).
[0253] Some embodiments afford quantum dots with defined surface
composition and high surface coverage with surface ligands.
[0254] The resultant quantum dots are more stable than those
produced from other methods and have the unique properties as
described herein.
[0255] The resultant quantum dots are more crystalline than those
produced from other methods.
[0256] Quantum dots can be produced with higher amounts than
achievable with other methods. (e.g., in quantities of at least ten
grams).
[0257] The functionalization of quantum dots using methods 2-a or
2-b allows functional group inter-conversion at the surface of
intermediate particles produced in equations 1 or 3 with
appropriate organic reagents in equations 2 or 4 to form ligands
layers. The intermediate particles typically will comprise cores
that include Y. The organic functionalization of these quantum dots
imparts favorable solubility in common organic solvents and
compatibility in various matrix materials such as organic polymers,
inorganic polymers, gels, glasses, and the like.
[0258] Method 2-a is based on controlled chemical reduction of
readily available molecular Si.sup.a+ and Ge.sup.a+ reagents, where
a ranges from 2 to 6 and typically from 2 to 4, and the quenching
of the corresponding intermediate particles (Y)X with different
reagents in a reaction medium. Two suitable families of reducing
agents are activated metals and hydrides. A nonaqueous reaction
medium desirably should be used for the reduction of silicon and
germanium reagents because of the large negative reduction
potential and high oxophilicity of Group IV compounds. The
controlled addition of a capping agent(R) such as RM (R=alkyl,
aryl, etc. and M=Li, Na, MgA, ZnA, with A being a halogen, and so
forth) to the corresponding intermediate particles at relatively
low temperatures produces the functionalized quantum dots in high
yield.
[0259] In a further effort to control quantum dot particles and
size distributions, terminating agents, R.sub.bYY.sub.c, can be
employed using method 2-b (equation 3). R group, in this method,
serves as the terminating agent for the quantum dots. The ratio of
R.sub.bYX.sub.c to the YX.sub.a reagent in equation 3 can be used
as a basic measure of the surface-to-volume ratio of the quantum
dots. A mixture of YX.sub.a and R.sub.bYX.sub.c, in the presence of
a reducing agent, yields the corresponding intermediate particles,
R(Y)X, which can be treated with a capping agent(R') such as R'M to
displace the remaining leaving groups, X, on the surface of the
intermediate particles (equation 4). This methodology can, in
principle, produce highly functionalized quantum dots.
[0260] A preferred method of this chemical synthetic method is
described as follows.
[0261] A silicon source, e.g., SiCl.sub.4, is reacted with a
reducing agent, e.g., Mg powder, under an inert atmosphere, e.g.,
argon. These materials are heated together in a liquid-phase
reaction medium. The reaction medium should desirably be aprotic.
It can be a hydrocarbon, or it could be aromatic. It could be a
cyclic or acyclic ether, an aromatic ether, or a polyether. It
could contain oxygen, nitrogen, sulfur, and/or phosphorous (so long
as it is compatible with the other reagents). It can include an
organic solvent with various combinations of more than one
hetero-atom or any combination of the solvents discussed
previously. Representative solvents include alkanes such as
heptane, decane, and octadecane; aromatics including benzene,
tetralin, and naphthalene; and alkylaromatics such as toluene,
xylene, and mesitylene; ethers such as dialkylethers, diarylethers,
alkylarylethers, and cyclic ethers; and polyethers like glymes.
[0262] In this process, a Group IV source, such as providing
Si.sup.4+, Ge.sup.4+, or Ge.sup.2+, especially in the form of
halides or with corresponding 1-20 carbon organic substituent (RA,
R=organic substituent, A=O, S, N, Si, etc.), is reacted with a
reducing agent, such as a Group IA compound, Group IIA compound,
transition metal, lanthanide, or hydride, in liquid phase reaction
medium at an elevated temperature. Representative Group IV sources
include SiF.sub.4, SiCl.sub.4, SiBr.sub.4, SiI.sub.4, GeF.sub.4,
GeCl.sub.4, GeBr.sub.4, GeI.sub.4, GeCl.sub.2, GeBr.sub.2,
GeI.sub.2, SiR.sub.4, Si(OR).sub.4, Si(SR).sub.4,
Si(NR.sup.(1)R.sup.(2)).sub.4, Si(O.sub.2R).sub.4, Si(SiR).sub.4,
GeR.sub.4, Ge(OR).sub.4, Ge(SR).sub.4,
Ge(NR.sup.(1)R.sup.(2)).sub.4, Ge(O.sub.2R).sub.4, Ge(SiR).sub.4,
Ge(NR.sup.(1)R.sup.(2)).sub.2, as well as the dimmers and the
higher oligomers of the above reagents (R, R.sup.(1),
R.sup.(2)=organic substituent). Representative reducing agents
include Li, Na, K, Na/K alloy, Rb, Cs, Be, Mg, Ca, Sr, Ba, Sc, Ti,
Zr, Mn, Fe, Co, Ni, Pd, Cu, Zn, Ce, Sm, Gd, Eu, LiAlH.sub.4,
NaBH.sub.4, Super-hydride, L-Selectride, RSiH.sub.3,
R.sub.2SiH.sub.2, R.sub.3SiH (R=organic substituent), and the like.
Reducing agents can be provided in a variety of forms (e.g., as a
powder, a liquid, a solid, and so forth). For certain reducing
agents (e.g., a Group IIA compound such as, for example, Mg), it is
desirable to provide such reducing agents in a powdered form to
facilitate reaction with the Group IV source. Alternatively, or in
conjunction, it is desirable to provide such reducing agents in
other forms such as in the form of chips, a mesh, dendritic pieces,
ribbons, rods, turning or activated (e.g., "Rieke magnesium",
etc.).
[0263] One or two of each of these two groups of materials are
mixed together in the reaction medium (e.g., an anhydrous aprotic
solvent) for at least few minutes. For some embodiments, the
reaction between a source of Si or Ge and a reducing agent is
performed by maintaining the reaction medium at a temperature
between approximately -78.degree. C. and 300.degree. C., preferably
between approximately 60.degree. C. and 280.degree. C., and at
around ambient pressure (e.g., about 1 atm) for a period of time
between approximately 2 and 48 hrs. For some embodiments, the
reaction between Na and silicon reagents can require an elevated
temperature and a prolonged period to complete. The reflux
temperature of the reaction medium can be used. Elevated pressures
of up to about 100 atmospheres can be used to obtain higher
temperatures. Suitable temperatures range between approximately 25
and 300 .degree. C.
[0264] In an additional step in the same pot, the intermediate
product, which is chemically labile, can be functionalized with
organic substituents when treated with an appropriate reactive
material (e.g., capping agent, surface ligands, molecular tethers,
terminating agents, passivator, etc.). These reagents can be
organometallic reagents, RM (R=surface ligand such as alkyl, aryl,
heteroaryl, and so forth and M=Li, Na, MgA, ZnA, with A being a
halogen, and so forth), alcohols, amines, amides, thiols,
phosphines, oxyphosphines, acids, silanes, germanes, oxides,
silanols, and germanols or their corresponding anion salts.
Representative ligand sources include organolithium reagents (e.g.,
n-butyllithium, sec-butyllithium, tert-butyllithium,
n-hexyllithium, and phenyl lithium); Grignard reagents (e.g.,
octylmagnesium halide, phenylmagnesium halide, and allylmagnesium
halide); alcohols (e.g., ethanol, isopropyl alcohol, and phenol);
amines and thiols (e.g., diethylamine, octylamine, and hexylthiol);
and the like.
[0265] For some embodiments, the reaction between the intermediate
particles or nanocrystallites and the source of surface ligands can
require a prolonged period to complete and can require an elevated
temperature. Suitable temperatures range from room temperature to
about 100.degree. C. The reaction can be completed in between
approximately 2 and 100 hours at ambient temperature. Subsequent
work-up affords the organically functionalized quantum dots as a
powder. The subsequent work-up preferably involves the addition
under an inert atmosphere (e.g. argon) of acidic water to destroy
the unreacted reducing agent or the organometallic reagent. The
product can be extracted with organic solvents. The solvent can be
a hydrocarbon, an aromatic, or a mixed hydrocarbon fraction. It
could be an ether or a polyether. It could be ester. It could
contain nitrogen, sulfur, and/or halides (so long as it is not very
soluble in water). Representative solvents include hexanes, decane,
toluene, xylene, diethyl ethers, glyme, dichloromethane,
chloroform, ethyl acetate, carbon disulfide, and the like. The
extraction process desirably should be repeated several times to
improve the yield.
[0266] The product is a quantum dot powder that can be isolated by
removing the solvent. This can be carried out by evaporation,
filtration, and the like.
[0267] The synthetic method described above is associated with
yields in the range of 35 to 95%, which are significantly higher
than previously obtainable. A broad range of particle sizes can be
achieved, e.g., between approxinately 1-100 nm.
[0268] Various factors can affect particle size, including the
nature of the reaction medium, the nature of the reducing agent,
the nature of the starting material, the ratio of reagents
employed, concentration, temperature, and pressure employed. The
reaction medium employed can play an important role in the physical
properties of the quantum dot product. More particularly,
coordinating solvents or agents such as oxygen or nitrogen or
sulfur or phosphorous containing organic compounds tend to yield
quantum dots with larger particle size. In particular, the size and
the size distribution of the quantum dots can be controlled by
varying the coordination ability of the solvent or the co-solvent.
As to the effect of temperature, higher reaction temperatures
improve the crystallinity of the quantum dots and aid in the
production of bigger quantum dots. Concentration also affects the
particle size, with lower concentrations tending to produce smaller
quantum dots (the larger amount of solvent effectively causes
better heat dispersion from the reacting species).
[0269] This invention will be further described by the following
examples. These examples are not to be construed as limiting the
scope of this invention, which is defined by the appended
claims.
Example 1
[0270] A 500-ml three-neck round bottom flask equipped with a
stirring bar, a reflux condenser, and a thermometer was purged with
argon and charged with 200 ml of the selected solvent (e.g., glymes
(n=1 to 5)) and the reducing agent (e.g., magnesium powder, 0.05 to
0.20 mol). Freshly distilled YX.sub.4 (0.05 to 0.20 mol) was added
dropwise, and the resulting brown-reddish solution was heated to
higher temperatures (e.g., between approximately 60 and 280.degree.
C.) for a period of time (e.g., between approximately 2 and 100
hrs, typically between approximately 2 and 48 hrs). The resulting
mixture was cooled to about -20.degree. C. and treated with an
excess amount of the capping agent (e.g., 1.8 M solution of
phenyllithium), which was added dropwise to keep the temperature
below room temperature. After the reaction mixture was stirred at
ambient temperature for a period of time (e.g., between
approximately 2 and 48 hrs), it was quenched with dilute protic
acid (pH.about.2) and extracted with an organic solvent (e.g.,
toluene). The combined organic extracts were washed with water and
dried over a drying agent (e.g., sodium sulfate). The solvents were
removed under reduced pressure, and traces of the solvents were
removed by precipitation with a nonsolvent (e.g., pentane). After
centrifugation or filtration, the product was collected and dried
in a vacuum oven. The product can be purified by column
chromatography (e.g., silica, CH.sub.2Cl.sub.2/methanol, 95/5).
Example 2
[0271] The preparation of Example 1 is repeated using sodium as the
reducing agent.
Example 3
[0272] The preparation of Example 1 is repeated using barium as the
reducing agent.
Example 4
[0273] The preparation of Example 2 is repeated using a mixture of
35%/65% (by volume) diglyme/xylenes as the reaction medium.
Example 5
[0274] The preparation of Example 1 is repeated using diphenyl
ether as the reaction medium.
Example 6
[0275] The preparation of Example 1 is repeated using tetraglyme as
the reaction medium.
Example 7
[0276] The preparation of Example 1 is repeated using
n-butyllithium as the capping agent.
Example 8
[0277] The preparation of Example 1 is repeated using
3-butenylmagnesium bromide as the capping agent.
Example 9
[0278] The preparation of Example 1 is repeated using
allylmagnesium bromide as the capping agent.
Example 10
[0279] The preparation of Example 1 is repeated using
4-methoxyphenylithium as the capping agent.
Example 11
[0280] The preparation of Example 1 is repeated using
pentafluorophenyllithium as the capping agent.
Example 12
[0281] The preparation of Example 1 is repeated using
perfluorohexyllithium as the capping agent.
Example 13
[0282] The preparation of Example 1 is repeated using sodium
ethoxide as the capping agent.
Example 14
[0283] The preparation of Example 1 is repeated using silicon
tetrabromide as the source of silicon.
Example 15
[0284] The preparation of Example 1 is repeated in a sealed
pressure reactor at 260.degree. C.
Example 16
[0285] The preparation of Example 1 is repeated using a mixture of
70%/10%/10%/10% (by molar ratio)
germaniumtetrachloride/phenyltrichloroge-
rmane/diphenyldichlorogermane/triphenylgermanium as germanium
source and terminating agents, R.sub.bYX.sub.c.
[0286] Preferred Surface Ligands and Molecular Tethers
[0287] As described in the previous sections, the ligand layer can
serve to passivate the surface of a quantum dot and eliminate
surface defects. It also facilitates compatibility with matrix
materials. This is further explained as follows. Fluoropolymers are
a group of desirable materials for optical applications because of
their unique properties. Fluoropolymers, in general, have low
indices of refraction (e.g., in comparison with regular hydrocarbon
polymers) and thus low intrinsic scattering loss. They also, in
general, exhibit low absorption loss as they are typically
comprised of little or no carbon-hydrogen bonds. They are
hydrophobic and thus low in moisture absorption. They, in general,
are chemically and thermally inert and thus compatible in demanding
environments and extreme process conditions in device fabrication.
Because of their inertness, fluoropolymers are nearly non-mixable
with many materials, such as conventional quantum dots. Embodiments
of the current invention provide a novel approach to circumvent the
compatibility issue by introducing fluorinated surface ligands to
the surface of quantum dots (e.g., as in Example 11 and 12 of the
preceding section). The quantum dots, which are terminated with a
ligand layer of fluorinated surface ligands, can now be
incorporated into, for example, Cytop.RTM. brand polymer (a
perfluorinated polymer from Asahi), facilitated, for example, by
using a solvent vehicle, CT-SOLV 180 from Asahi.
[0288] The following are preferred surface ligands of the ligand
layer, according to some embodiments of the invention. This list,
which is not intended to be exhaustive, describes a number of
surface ligands having desirable physical characteristics that can
be used to form ligand layers for SiQDs or GeQDs. In the following,
Y is Si or Ge, and Y--C, Y--O, Y--S, Y--Si, and Y--N denote
covalent bonds between Si or Ge and a C atom, an O atom, a S atom,
a Si atom, and a N atom, respectively. Other preferred surface
ligands, not listed below, can contain a P or a Se atom that is
covalently bonded to Si or Ge.
[0289] Y--C
[0290] A) Alkyls
[0291] a. Simple aliphatic alkyl groups (e.g., methyl, ethyl,
propyl, etc.)
[0292] b. Branched and cyclic alkyl groups (e.g., iso-propyl,
tert-butyl, cyclohexyl, etc.)
[0293] c. Substituted alkyl groups (e.g., 4-cyanobutyl,
3-ethoxy-3-oxopropyl, etc.)
[0294] d. Perfluorinated alkyl groups (e.g., linear, branched, or
cyclic)
[0295] B) Alkenyls
[0296] a. Simple isolated double bonds (e.g., 1-hexenyl,
1-dodecenyl, etc.)
[0297] b. Substituted alkenes (e.g., 6-heptenenitrile, etc.)
[0298] c. Conjugated polyenes (e.g., pentadienyl etc.)
[0299] d. Polymerizable alkenes (e.g., allyl, 3-butenyl, 2-butenyl
etc.)
[0300] C) Alkynyls
[0301] a. Simple isolated alkynes (e.g., hexynyl, octynyl,
etc.)
[0302] b. Substituted alkynes (e.g., phenylethynyl, etc.)
[0303] c. Polymerizable alkynyls
[0304] d. Perfluoro alkynyls
[0305] D) Aromatics and Aromatic Heterocycles
[0306] a. Phenyls, Pyridyls, Thienyl, etc.
[0307] b. Substituted Aromatics and Aromatic Heterocycles
[0308] i. With electron withdrawing groups (nitro, nitrile, fluoro,
perfluoro, carboxylate, e.g., 4-cyanophenyl, etc.)
[0309] ii. With electron donating groups (amino, alkoxy, e.g.,
4-methoxyphenyl, etc.)
[0310] E) Conjugated Aromatics, Aromatic Heterocycles, and Polyenes
(poly is referred to well defined oligomers)
[0311] a. Polyenes
[0312] b. Poly(p-phenylene),
[0313] c. Poly (diacetylene)
[0314] d. Poly(triacetylene)
[0315] e. Poly(p-phenylene vinylene)
[0316] f. Poly(p-phenylene ethynylene)
[0317] g. Polythiophene
[0318] h. Polypyrrol
[0319] i. Polyaniline
[0320] j. Poly(phenylene sulfide)
[0321] F) Cyanide
[0322] Y--O
[0323] A) Hydroxy, Alkoxy, etc.
[0324] a. Diol, triol, polyol, etc.
[0325] b. Cholesteryl group
[0326] c. Trisubstituted siloxy
[0327] B) Carboxylate
[0328] C) Phenoxy
[0329] D) Siloxy
[0330] E) Cyanate
[0331] F) Inorganic Oxides
[0332] Y--S
[0333] A) Thioalkyl
[0334] B) Thioaryl
[0335] C) Thiocyanate
[0336] D) Silylthio
[0337] Y--Si
[0338] A) Substituted silyl group
[0339] B) Tri-substituted silyl group with one or more functional
groups
[0340] Y--N
[0341] A) Amino group (e.g., linear, branched, aromatic, or
cyclic)
[0342] B) Mono and di-substituted amines
[0343] C) Imino group (e.g., linear, branched, aromatic, or
cyclic)
[0344] D) Silylamino
[0345] FIGS. 10(a) and 10(b) show PL spectra of organic-terminated
Si quantum dots, and FIGS. 11(a) and 11(b) show PL spectra of
organic-terminated Ge quantum dots. The Si and Ge quantum dots were
made with the methods described herein. The PL spectra show that
the light emission can be readily tuned from the red to the
ultraviolet by exciting quantum dots of different sizes. The PL
spectra are obtained by optically exciting the quantum dots with
wavelengths shorter than the wavelength at the absorption edge of
the quantum dots. This light emission is stable in ambient
conditions. This stability is due in large part to the relative
completeness and stability of the surface termination, e.g., the
surface termination and the interface between the core and the
surface termination is substantially defect free. In FIGS. 10(a)
and 10(b), the surfaces of the Si quantum dots are terminated with
4-methoxyphenyl groups. In FIG. 10(b), the vertical axis represents
a normalized photoluminescence signal from FIG. 10(a). In FIGS.
11(a) and 11(b), the surfaces of the Ge quantum dots are terminated
with butyl groups. In FIG. 11(b), the vertical axis represents a
normalized photoluminescence signal from FIG. 11(a). Similar
results can be seen for Si quantum dots having surfaces terminated
with ethoxy groups and Ge quantum dots having surfaces terminated
with methyl groups.
[0346] The electronic and optical properties of these
organic-terminated SiQDs and GeQDs that are made in this fashion
are unique in that they show size dependent properties that are
uniquely consistent with quantum confinement. The optical and
electronic properties of these SiQDs and GeQDs are uniquely
consistent with theoretical calculations more sophisticated than
Effective Mass approaches, such as the Empirical Pseudopotential
Method and the Tight Binding Method. A comparison of the size
dependent energy gap calculated by these methods with measurements
taken on the SiQDs and GeQDs synthesized by the method disclosed
herein show that the agreement is extremely good and is the best
observed for any quantum dot formed of Si or Ge.
[0347] According to some embodiments of the invention,
nanocomposite materials comprising quantum dots that are
surface-terminated with various organic groups and dispersed in
processible matrix materials such as organic polymers or sol-gels
can exhibit new quantum phenomena. This new quantum phenomena in
turn allow a large variety of new applications (such as all-optical
switching) and the fabrication of device structures using low cost
processing techniques (e.g., spin coating or dipping). Described
herein are several novel synthetic schemes to fabricate these
quantum dots and to functionalize their surfaces with molecular
species that are chemically bonded to the surface for stability and
robustness. Use of such functionalized quantum dots avoids the need
for expensive and specialized fabrication equipment and facilities.
The synthesis of these nanostructures can be readily implemented in
many laboratories.
[0348] As discussed above, the value of this quantum dot
nanostructure can derive from molecular tethers serving multiple
functions. The molecular tethers may be active in a variety of
ways, e.g., electrically, chemically, mechanically, or optically
active. This enables precise control of the electrical, optical,
transport, chemical, and physical interactions between quantum dots
and the surrounding matrix material or the properties of individual
quantum dots. These molecular tethers can be a key innovation
needed to develop new devices and applications. Examples of
particularly preferred embodiments of optically active molecular
tethers are molecules with polarized or polarizable sections or
with large polarizabilities, donor-acceptor molecules,
hetero-molecules, and charge transfer molecules.
[0349] Another major innovation comes from collective phenomena
resulting from nanocomposite materials that include coupled quantum
systems such as coupled quantum dots. The ability to attach active
molecular tethers to the quantum dot surface allows coupling
quantum dots together in various one, two, and three-dimensional
configurations or arrays to initiate multiple quantum interference
interactions between quantum dots that may be applied towards novel
devices. The length and properties of these molecular tethers can
be tailored to enhance or generate specific quantum phenomena such
as enhanced nonlinear optical properties. For instance, molecular
tethers can provide charge transport between two or more
interconnected quantum dots. For certain embodiments of the present
invention, the quantum dots can be massively interconnected to an
extent that is unlike previous efforts. The massively
interconnected quantum dot system can be comprised of more than 2
interconnected quantum dots, preferably more than 10 interconnected
quantum dots, preferably more than 1000 interconnected quantum
dots, and most preferably more than 10.sup.9 interconnected quantum
dots. For certain embodiments of the invention, the massively
interconnected quantum dot system can be comprised of two or more
massively interconnected quantum dot subsystems, which subsystems
may or may not be connected. The quantum dots can be interconnected
via the formation of chemical bonds between appropriate molecular
tethers on different quantum dot surfaces. This, in turn, can be
performed using the functionalization of the quantum dot surfaces
as described earlier herein. After the quantum dot surface is
functionalized, the interconnection can proceed via chemical
reaction between surface functional groups, e.g., conjugated
species, aromatics, etc. As a result of such interconnection, a
large variety of nanostructures is possible:
[0350] (1) n quantum dots coupled in a linear structure or
array.
[0351] (2) n quantum dots coupled in an arbitrary 2-dimensional
structure or array.
[0352] (3) n quantum dots coupled in an arbitrary 3-dimensional
structure or array (e.g., to produce new lattice structure and new
materials with tailorable properties.).
[0353] (4) n quantum dots attached to a polymer backbone to give
controllable densities of quantum dots. These quantum dots can be
coupled with other species (e.g., electron donating or accepting
molecules) onto the polymer backbone to generate other new
phenomena and applications.
[0354] These nanostructures can have the properties of single
quantum dots or an ensemble of quantum dots, which will be
determined by the nature of the molecular tethers. This approach
can be important for exploiting collective excitations in quantum
dot systems towards innovative devices. These new nanostructures
represent an important innovation in nanotechnology. Examples of
particularly preferred embodiments of molecular tethers that can be
used to interconnect quantum dots in this fashion and to generate
controllable collective phenomena include conjugated species such
as conjugated polymers (e.g., alkenes, alkynes, and aromatics).
[0355] The uniqueness of the synthetic process described above is
manifested in, but is not restricted to, the following properties
of the resultant quantum dots and nanocomposite materials formed of
such quantum dots: (1) extremely large optical nonlinearities are
manifested, e.g., in large values of
Re[.sub..chi..sup.(3).sub.ijkl], with values as high as 10.sup.-5
cm.sup.2/W to 10.sup.-4 cm.sup.2/W. Previous materials with optical
nonlinearities in the infrared and more specifically in the
important telecommunications region of 1500 nm to 1600 nm typically
have values of nonresonant degenerate .gamma.of .about.10.sup.-12
cm.sup.2/W to 10.sup.-11 cm.sup.2/W or less. As a result, the
optical nonlinearity, e.g. Re[.sub..chi..sup.(3).sub.ijkl], of the
quantum dots of this invention is .about.10.sup.6 to 10.sup.8 times
larger than such previous materials; (2) stability of the quantum
dots in a variety of environmental conditions including ambient;
(3) stability of the infrared to ultraviolet emission in a variety
of environmental conditions including ambient; (4) control over the
size of the quantum dots such that the light emission can be
size-tuned from the infrared to the ultraviolet; (5) control over
the size of the quantum dots such that the nonlinear optical
properties give large figures-of-merits that surpass those required
for effective all-optical switching; (6) the nonlinear optical
properties are such that all-optical switching occurs in a very
short time (depending on the nature of the nonlinear optical
mechanism (e.g., resonant or non-resonant, the switching time can
range from picoseconds to less than 60 femtoseconds); (7) low
switching energy (<<1.times.10.sup.-12 Joules); (8)
non-degenerate (e.g., control and data beams with different
wavelengths) all-optical switching where the wavelengths of the
relevant beams can be detuned from each other over a very broad
spectral range (>>100 nm) and still maintain effective
all-optical switching; (9) all-optical switching can occur
throughout a broad wavelength range (e.g., from 400 nm to 1600
nm).
[0356] Preferred Matrix Materials
[0357] According to some embodiments of the current invention, the
matrix material that is used to host quantum dots can be selected
from a broad range of materials due in large part to the versatile
surface termination of the quantum dots as discussed above. These
matrix materials can include, for example, organic and inorganic
polymers or glasses with different properties including mechanical
strength, optical transparency, lightwave transmissivity, thermal
stability, dimensional stability, low temperature flexibility,
moisture absorption, and chemical inertness.
[0358] The matrix materials in some embodiments of the current
invention are preferred to be highly transparent and low absorption
in the wavelength range from 600 nm to 2 .mu.m. Also, they are
preferred to be highly compatible with quantum dots so that a
desired amount of quantum dots can be readily incorporated into the
resulting nanocomposite material without degrading optical and
mechanical properties. Polymers with special functional groups may
be selected to facilitate solubility interactions and enhance
compatibility with quantum dots. This is further explained as
follows. Polymers with Lewis acid (base) functional groups, for
example, can be selected to host quantum dots which are
surface-functionalized with Lewis base (acid) surface ligands.
Another example is to take advantage of hydrogen-bonding
interactions. Polymers of hydrogen-bonding donors (acceptors) are
the preferred matrix materials for quantum dots which are surface
functionalized with hydrogen-bonding acceptors (donors).
Additionally, polymers with strong dipolar groups are the preferred
matrix materials for quantum dots which are engineered with strong
dipolar surface ligands. The strong intermolecular interactions
described above greatly enhance compatibility between the quantum
dots and the matrix materials. Therefore, high contents of quantum
dots can be readily incorporated into the matrix materials while
maintaining desired uniformity and homogeneity (e.g., allowing the
quantum dots to be substantially uniformly dispersed throughout the
matrix materials). Additionally, block copolymers can be used to
further enhance compatibility by skillful selection of monomer
units and block length. As a result, a nanocomposite material
comprising quantum dots and a matrix material can be engineered to
be of high optical quality and low scattering loss. More
importantly, the linear and nonlinear index of refraction can be
tuned for a variety of applications primarily by adjusting the
content of the quantum dots and by selecting the matrix material,
according to some embodiments of the invention.
[0359] In addition to the optical properties and compatibility with
the quantum dots, the preferred matrix material desirably should
meet other requirements for a specific application. Thus, other
properties can be considered in the selection of matrix
materials.
[0360] One preferred matrix material is selected from a group of
polymers with high glass transition temperature, T.sub.g, such as
polyimides, fluoropolymers (e.g., Teflon AF.RTM. brand
fluoropolymers available from DuPont), polymers derived from
B-staged bisbenzocyclobutene monomers (e.g., Cyclotene.RTM. brand
resins and Cyclotene.RTM. brand fluorinated resins available from
The Dow Chemical Company), phenolic resin, and fluorinated
poly(aryl ether sulfide), for applications where thermal stability
is important.
[0361] Another preferred matrix material is selected from a group
of polymers with low T.sub.g such as poly(isobutylene),
poly(diphenoxyphosphazene), and fluorinated acrylate (ZPU series
from Zen Photonics Co., LTD) for applications where low temperature
flexibility and low birefringence are desired.
[0362] Another preferred matrix material can be selected from
photosensitive polymers, such as fluoropolymers (e.g., Cytop.RTM.
brand fluoropolymers available from Asahi), poly(methyl
methacrylate), and photoresists to facilitate lithographical
fabrication of devices.
[0363] Another preferred matrix material is selected from a group
of cross-linkable polymers for applications where isotropic
homogeneity or dimensional stability is required.
[0364] Another preferred matrix material is a blend of two or more
polymers which are engineered to tailor the optical and mechanical
properties and thermal and chemical stability.
[0365] Another preferred matrix material is a copolymer including
random and block copolymer.
[0366] Another preferred matrix material is a homopolymer
including, but not limited to, the following:
[0367] Poly(vinyl alcohol)
[0368] Poly(vinyl butyral)--other
[0369] Poly(vinylcarbazol)
[0370] Poly(vinyl fluoride)
[0371] Poly methyl vinyl ether
[0372] Polyethylene
[0373] Polypropylene
[0374] Polystyrene
[0375] Poly(vinyl pyridine)
[0376] Polyimides
[0377] Poly(ethylene oxide)
[0378] Photoresist (positive or negative)
[0379] Cyclotene.RTM.
[0380] Fluorinated Cyclotene.RTM.
[0381] Cytop.RTM.
[0382] PMMA
[0383] Fluorinated acrylates
[0384] Poly(siloxanes)
[0385] Poly(silanes)
[0386] Poly(diphenoxyphosphazenes)
[0387] Poly(vinyl ferrocene)
[0388] Polycarbonate
[0389] Polystyrene
[0390] Poly(cyclic olefen) such as Zenor.RTM. and Zenex.RTM.
[0391] Teflon.RTM. AF.RTM.
[0392] Another preferred matrix material is a glass including, but
not limited to, the following:
[0393] Sol-gel derived glasses
[0394] Organically modified glasses
[0395] Spin-on glasses
[0396] Flow-glass
[0397] Dielectrics such as Low K FlowFill.TM. brand dielectrics of
Trikon Industries
[0398] Dielectrics such as Black Diamond.TM. brand dielectrics of
Applied Materials, Inc.
[0399] Preferred Methods of Use
[0400] The engineered nonlinear nanocomposite materials of some
embodiments of the present invention can be incorporated into an
optical device by a variety of methods, including a variety of
standard methods known in the art. The flexibility to process the
nanocomposite material of embodiments of the current invention,
desirably independent of the nonlinear optical properties, is a key
benefit of embodiments of the current invention. By selecting an
appropriate matrix material and solvent, engineered nonlinear
nanocomposites can be deposited using spin-coating, spin casting,
dip coating, spraying, blade application, screen printing, and
other methods commonly used in the process of standard
semiconductor micro-fabrication.
[0401] While processes like spin-coating have been used in other
contexts, the combination of tuning the optical and mechanical
properties of an engineered nonlinear nanocomposite material
followed by spin-coating, or the like, is unlike previous
capabilities. Traditional nonlinear materials known in the art have
chemical and mechanical properties that are directly linked to
their optical properties. The processing techniques that can be
used to incorporate these materials are therefore often limited to
those that are compatible with the materials themselves. For
instance, LiNbO.sub.3 is a crystal and can therefore not be
incorporated by spin coating.
[0402] The steps of incorporating a nonlinear nanocomposite
material into a device by selecting desired optical properties,
substantially independently selecting desired chemical and/or
mechanical properties to facilitate incorporation by a particular
technique (e.g., spin-coating), and then incorporating the
nanocomposite material using that technique represents a
substantial improvement over previous incorporation methods. While
spin-coating has been discussed herein as a specific example of a
standard method of materials incorporation, this is done strictly
for exemplary purposes and should not be considered to limit the
scope of the invention.
[0403] For example, desired optical properties such as linear index
of refraction and .gamma. can be established by selecting or tuning
at least one of a chemical composition of quantum dot cores, a
chemical composition of quantum dot shells, a peak size of quantum
dots, a thickness of the shells, a chemical composition of ligand
layers, a chemical composition of a matrix material, a
concentration of the quantum dots in the matrix material, and a
degree of interconnection of the quantum dots (e.g., using
molecular tethers). Other desired optical properties such as
single-photon and multi-photon absorption characteristics can be
established by selecting or tuning at least one of a chemical
composition of quantum dot cores, a chemical composition of quantum
dot shells, a peak size of quantum dots, and a thickness of the
shells. Desired chemical and mechanical properties can be
established by selecting or tuning at least one of a chemical
composition of ligand layers and a chemical composition of a matrix
material. As discussed previously, at least two of these desired
properties can be substantially independently established,
according to some, embodiments of the invention.
[0404] In addition to standard incorporation techniques, other
methods of deposition such as layer-by-layer growth using polymers
with alternating and complementary functionalities, as pioneered by
Gero Decher and described in T. Sasaki et al., "Layer-by-Layer
Assembly of Titania Nanosheet/Polycation Composite Films," Chem.
Mater. 13, 4661 (2001), the disclosure of which is incorporated
herein by reference in its entirety, can be used to create films
and coatings of laminated layer structures in the required
thickness with desired density of quantum dots.
[0405] All of the same processing techniques are also possible for
quantum dot solids, including the ability to perform layer-by-layer
growth. Here again, the process of selecting the chemical
properties of the surface ligands and solvent to facilitate
incorporation by a particular technique, desirably independent of
the optical properties, represents a significant improvement over
previous incorporation methods.
[0406] The following provides some additional preferred methods of
incorporating an engineered nonlinear nanocomposite material into a
variety of devices:
[0407] The engineered nonlinear nanocomposite material can be
dispersed in a polymer and subsequently dissolved in an appropriate
solvent to create a fluid of sufficient viscosity to generate the
desired thickness of a film. The film thickness can be easily
tailored by varying the solvent content and therefore the
viscosity. The specific quantum dot surface chemistry is selected
for compatibility with the selected polymer and solvent to be used.
Some preferred materials include: Dow Chemical's Cyclotene.RTM.,
which is B-staged divinylsiloxane-bis-benzocyclobutene with
Mesitylene and minor portions of other organic compounds; poly
(methyl methacrylate) (PMMA); photoresists (both positive and
negative) used in semiconductor manufacturing; and so forth.
[0408] The engineered nonlinear nanocomposite material is dispersed
in a suitable carrier fluid or solvent and applied evenly over the
desired surface. Heat, vacuum, IR radiation, and/or an inert
carrier gas are then used to remove the carrier fluid, giving rise
to a film of the engineered nonlinear nanocomposite material on the
device.
[0409] The engineered nonlinear nanocomposite material is dispersed
in a carrier gas, which is either reactive or inert. Appropriate
carrier gasses include, but are not limited to, SiH.sub.4, N.sub.2,
H.sub.2, O.sub.2, and N.sub.2O. The gases are allowed to react
under appropriate conditions of heat and/or plasma to cause a CVD
film to be deposited on a substrate of choice. In this embodiment,
a preferred substrate is a silicon wafer, optionally comprising
lithographic structures or patterns on the surface.
[0410] The engineered nonlinear nanocomposite material is
incorporated into a sputter target, optionally using procedure (i)
above. Alternatively, a pure target of a desired matrix material
could be used (e.g., organic or inorganic targets, preferably
SiO.sub.2), and the engineered nonlinear nanocomposite material is
introduced in a gas in a sputter chamber. The engineered nonlinear
nanocomposite material is then incorporated directly into a growing
sputtered film.
[0411] The engineered nonlinear nanocomposite material is heated
and caused to vaporize. The material vapors are then transported to
a desired surface and condensed by keeping the surface at a
suitable temperature. The result is a solid film deposited on a
device.
[0412] The same concepts can be, used in systems that deposit Low K
material such as Low K FlowFill.TM. brand dielectrics from Trikon
Industries or Black Diamond.TM. brand dielectrics from Applied
Materials, Inc., thus incorporating quantum dots into low k films
for even better control of the index of refraction and
processability.
[0413] Preferred Nanocomposite Materials
[0414] Embodiments of the current invention comprise a
nanocomposite material with a controllable set of optical,
mechanical, chemical, and electronic properties. The nanocomposite
material can comprise quantum dots dispersed in an organic and/or
inorganic matrix material. The matrix material may be either doped
or undoped with molecular species, with a density of quantum dots
therein such that the index of refraction (e.g., the linear index
of refraction or the overall index of refraction) of the
nanocomposite material falls between approximately 1.3 and 5.0.
Some embodiments of the nanocomposite material comprise at least or
more than 10% by weight of the quantum dots (e.g., at least 20% by
weight of the quantum dots, at least 30% by weight of the quantum
dots, at least 40% by weight of the quantum dots, or at least 50%
by weight of the quantum dots, such as between approximately 50%
and 60% by weight of the quantum dots). More particularly, some
embodiments of the nanocomposite material can comprise the above
discussed weight percentages of the quantum dots with little or no
agglomeration or aggregation of the quantum dots and with the
quantum dots substantially uniformly dispersed throughout the
matrix material. Also disclosed are nanocomposite materials (e.g.,
quantum dot solids) such that the density of quantum dots within
the nanocomposite material is between approximately 0.005% and 75%
by volume. Optionally, the index of refraction of the nanocomposite
material can be additionally tuned by selecting a matrix material
with a specific index of refraction and/or further doping the
matrix material to modify that index. This provides additional
control over the optical characteristics. Optionally, the matrix
material can be a polymerizable material with a desired index of
refraction. The index of the refraction can be further fine-tuned
by cross-linking via various activation mechanisms including
thermal, photo illumination, plasma, and high energy radiations.
The matrix material in which the quantum dots are dispersed may
optionally have an intrinsically high .sub..chi..sup.(3). The
matrix material may optionally be an intrinsic matrix of a quantum
dot solid.
[0415] The nanocomposite materials according to some embodiments
are preferably optically pure, with a homogeneous distribution of
quantum dots dispersed therewithin. These quantum dots may be
substantially uniformly dispersed as individual dots or as
aggregates of controlled sizes (e.g., smaller aggregates up to
massively interconnected quantum dot subsystems). The engineered
nanocomposite materials are preferably optically homogeneous and
uniform, so that little or no scattering and/or mode disruption
result from light passing through or past the material, as the
specific application demands. For certain applications,
close-packed micron- or sub-micron-sized clusters of quantum dots
dispersed in a polymer or other matrix material with a filling
fraction optimized to enhance local field effects may be
preferable.
[0416] In addition, nanocomposite materials of some embodiments
preferably have an optical nonlinearity, such as
Re[.sub..chi..sup.(3).sub.ijkl] contributing to .gamma.., e.g.,
under degenerate conditions, such as nonresonant degenerate
conditions in a wavelength range-of-interest of between
approximately 10.sup.-12 and 10.sup.-5 cm.sup.2/W, more preferably
between 10.sup.-10 and 10.sup.-5 cm.sup.2/W and most preferably
between 10.sup.-8 and 10.sup.-5 cm.sup.2/W. In particular, certain
embodiments of the nanocomposite material have .gamma. being at
least 10.sup.-9 cm.sup.2/W (e.g., at least 10.sup.-8 cm.sup.2/W or
at least 10.sup.-7 cm.sup.2/W) when irradiated with light having a
wavelength between approximately 3.times.10.sup.-5 cm and
2.times.10.sup.-4 cm. More particularly, certain embodiments of the
nanocomposite material have .gamma. being at least 10.sup.-9
cm.sup.2/W (e.g., at least 10.sup.-8 cm.sup.2/W or at least
10.sup.-7 cm.sup.2/W) when irradiated with light having a
wavelength between approximately 1.25.times.10.sup.-4 cm and
1.35.times.10.sup.-4 cm or between approximately
1.5.times.10.sup.-4 cm and 1.6.times.10.sup.-4. For .gamma. under
non-degenerate conditions, both relevant wavelengths (e.g.,
wavelengths corresponding to trigger and data signals) can lie
within the wavelength ranges stated above. Optical characteristics
of the disclosed nanocomposite materials can be evaluated in a
variety of configurations and are not restricted by the specific
examples described herein. One of skill in the art will appreciate
that the linear and nonlinear optical properties of a material can
be evaluated using methods such as Z-scan, FWM, cross-phase
modulation, nonlinear phase shift in an interferometer, nonlinear
etalons, and so forth.
[0417] The mechanical properties of nanocomposite materials are
preferably selected to be compatible with incorporation into
devices selected from the list of: planar waveguides, nonplanar
waveguides, optical fibers, waveguide cores, waveguide claddings,
free-space optics, and hybrid optical devices. Such nanocomposite
materials can be used in a variety of optical devices for
switching, modulating, and manipulating light in ways such as for
an optical switch, an optical cross-connect, a wavelength
converter, and the like, as well as combinations thereof.
[0418] The nanocomposite materials described herein can have a
number of key attributes lacking in other materials. For instance,
the nanocomposite materials can have an extremely large optical
nonlinearity. This optical nonlinearity can be represented by the
real part of various tensor elements of .sub..chi..sup.(3), which
include .sub..chi..sup.(3).sub.1111, .sub..chi..sup.(3).sub.1212,
.sub..chi..sup.(3).sub.1221, .sub..chi..sup.(3).sub.1122, and
various permutations of the energies of the optical fields
involved, e.g., .sub..chi..sup.(3).sub.ijkl (-.omega..sub.4,;
.omega..sub.1, .omega..sub.2, .omega..sub.3). According to some
embodiments of the invention, the value of the real part of these
tensor elements falls in the range of 10.sup.-9 cm.sup.2/W to
10.sup.-4 cm.sup.2/W. As a result, the nanocomposite materials
allows all-optical devices to be made that can be effectively
switched or controlled with very low intensity light such as light
from continuous wave laser diodes and also LEDs in some cases. This
capability is highly sought after but has not been previously
achieved in a satisfactory manner.
[0419] Also, the nanocomposite materials described herein can
exceed various FOM criteria for effective all-optical switching. In
particular, certain embodiments of the nanocomposite material has a
FOM that is at least 1 (e.g., at least 1.5 or 1.8), where this
particular figure-of-merit can be defined as
2.gamma./.beta..lambda., where .beta. is a two-photon absorption
coefficient of the nanocomposite material expressed in cm/W, and
.lambda. is a wavelength between approximately 3.times.10.sup.-5 cm
and 2.times.10.sup.-4 cm, preferably between approximately
1.25.times.10.sup.-4 cm and 1.35.times.10.sup.-4 cm or between
approximately 1.5.times.10.sup.-4 cm and 1.6.times.10.sup.-4. It
should be recognized that other definitions for the FOM may be used
instead. This is particularly differentiating since, though other
materials may possibly have large nonlinear refractive indices, the
linear or nonlinear losses such as that originating from two photon
absorption are often sufficiently large so that the FOM is
inadequate, the thermal properties are sufficiently poor such that
the FOM is inferior, and the temporal response is considerably
slower than with the nanocomposite materials described herein.
Another important consequence is that significantly shorter lengths
of the nanocomposite materials described herein are required for
effective all-optical switching devices. Thus, significantly
smaller and faster devices can be made. As an example, rather than
requiring centimeters or more of a conventional material to
effectively switch light, devices can be made with the
nanocomposite materials with, lengths of approximately ten microns
to a few millimeters.
[0420] In addition, the nanocomposite materials described herein
are relatively simple and inexpensive to make, are more easily
processed, are compatible with a large number of other material
systems, and can be incorporated more readily into various device
structure and in nearly any device size. Epitaxial growth of the
nanocomposite materials described herein is typically not required,
which can be an advantage since epitaxial growth is typically an
expensive process that is often not amenable to simple processing
or large area devices and is often incompatible with many other
material systems (since it requires epitaxial growth on material
systems that are lattice-matched to itself). The nanocomposite
materials can be deposited on various substrates, substantially
independently of their size, surface area, and surface nature, in
the form of films or coatings of varying thicknesses and can be
formed into structures of various shapes and sizes. Importantly,
these films, coatings, and structures made from -the nanocomposite
materials can be manufactured with a number of simple and
inexpensive fabrication techniques such as spin coating, spray
coating, doctor blading, and dip coating at ambient temperature and
pressure, or using conventional molding processes for engineering
plastics and elastomers.
[0421] And, the nanocomposite materials described herein can have
optical, chemical, thermal, and mechanical properties engineered to
suit device or application requirements. For certain embodiments,
these various desirable attributes result in large part from the
inherent flexibility in engineering the surface properties of
quantum dots, substantially independently of their optical and
electronic properties. In addition, these attributes can also
result from use of organic or inorganic polymers with tailored
optical, thermal, chemical, and mechanical properties suitable for
different devices and applications.
[0422] Alternative Applications for Engineered Nanocomposite
Materials
[0423] While the application of engineered nanocomposite materials
as a nonlinear material is described herein, it should be
recognized that such materials will also find applications in a
variety of areas such as, though not limited to, engineered
resonant nonlinear nanocomposite materials, engineered linear
nanocomposite materials, engineered absorptive nanocomposite
materials, engineered electro-optic nanocomposite materials,
engineered thermo-optic nanocomposite materials, engineered thermal
nanocomposite materials, engineered gain nanocomposite materials,
engineered magneto-optic nanocomposite materials, engineered
magnetic nanocomposite materials, engineered electronic
nanocomposite materials, engineered biological nanocomposite
materials, engineered optoelectronic nanocomposite materials, and
engineered mechanical nanocomposite materials. It should be
recognized that the tunable physical, chemical, electronic, and
optical properties of the materials described herein, as well as
the methods of incorporation thereof, can be used to create
materials with specific characteristics tailored to many other
applications.
[0424] At this point, one of ordinary skill in the art will
recognize various advantages associated with some embodiments of
the invention. Embodiments of this invention provide a method of
synthesizing nanocrystalline materials. Embodiments of the
invention also provide a method of synthesis that produces quantum
dots with organically functionalized surfaces and a method of
synthesis that produces quantum dots with surfaces passivated with
oxide. Embodiments of the invention provide a method of synthesis
that is safe, energy efficient, scalable, and cost-effective.
Embodiments of this invention also provide a method of synthesis
that employs environmentally benign starting materials or
commercially available or readily prepared materials.
Advantageously, embodiments of this invention provide a method of
synthesis that results in high yield and a method of synthesis that
results in very soluble and processable products. Also, embodiments
of the invention provide a method of synthesis that results in
highly crystalline material and that yields quantum dots with a
narrow size distribution, such as with a dispersion of the size
distribution is less than 15% rms. Embodiments of this invention
provide a method of synthesis that yields quantum dots with narrow
shape distribution and a method of synthesis that produces quantum
dots that are uniform in composition. In addition, embodiments of
the invention provide a method of synthesis that produces quantum
dots that are uniform in surface chemistry.
[0425] Each of the patent applications, patents, publications, and
other published documents mentioned or referred to in this
specification is herein incorporated by reference in its entirety,
to the same extent as if each individual patent application,
patent, publication, and other published document was specifically
and individually indicated to be incorporated by reference.
[0426] While the present invention has been described with
reference to the specific embodiments thereof, it should be
understood by those skilled in the art that various changes may be
made and equivalents may be substituted without departing from the
true spirit and scope of the invention as defined by the appended
claims. In addition, many modifications may be made to adapt a
particular situation, material, composition of matter, method,
process step or steps, to the objective, spirit and scope of the
present invention. All such modifications are intended to be within
the scope of the claims appended hereto. In particular, while the
methods disclosed herein have been described with reference to
particular steps performed in a particular order, it will be,
understood that these steps may be combined, sub-divided, or
re-ordered to form an equivalent method without departing from the
teachings of the present invention. Accordingly, unless
specifically indicated herein, the order and grouping of the steps
is not a limitation of the present invention.
* * * * *