U.S. patent application number 15/922104 was filed with the patent office on 2018-07-19 for anistropic semiconductor nanoparticles.
This patent application is currently assigned to Yissum Research Development Company of the Hebrew University of Jerusalem Ltd.. The applicant listed for this patent is Yissum Research Development Company of the Hebrew University of Jerusalem Ltd.. Invention is credited to Uri BANIN, Itai LIEBERMAN, Amit SITT, Adiel ZIMRAN.
Application Number | 20180201834 15/922104 |
Document ID | / |
Family ID | 44903316 |
Filed Date | 2018-07-19 |
United States Patent
Application |
20180201834 |
Kind Code |
A1 |
BANIN; Uri ; et al. |
July 19, 2018 |
ANISTROPIC SEMICONDUCTOR NANOPARTICLES
Abstract
The present invention provides seeded rod (SR) nanostructure
systems including an elongated structure embedded with a seed
structure being a core/shell structure or a single-material rod
element. The SR systems disclosed herein are suitable for use in a
variety of electronic and optical devices.
Inventors: |
BANIN; Uri; (Mevasseret
Zion, IL) ; ZIMRAN; Adiel; (Efrat, IL) ;
LIEBERMAN; Itai; (Tel Aviv, IL) ; SITT; Amit;
(Tel Aviv, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Yissum Research Development Company of the Hebrew University of
Jerusalem Ltd. |
Jerusalem |
|
IL |
|
|
Assignee: |
Yissum Research Development Company
of the Hebrew University of Jerusalem Ltd.
Jerusalem
IL
|
Family ID: |
44903316 |
Appl. No.: |
15/922104 |
Filed: |
March 15, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13809185 |
Jan 9, 2013 |
9957442 |
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PCT/IL2011/000734 |
Sep 15, 2011 |
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15922104 |
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61383413 |
Sep 16, 2010 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C09K 11/025 20130101;
C09K 11/7492 20130101; C09K 11/883 20130101; Y10S 977/892 20130101;
C09K 11/70 20130101; B82Y 40/00 20130101; Y10S 977/773 20130101;
Y10T 428/2933 20150115; C09K 11/565 20130101 |
International
Class: |
C09K 11/88 20060101
C09K011/88; C09K 11/74 20060101 C09K011/74; C09K 11/70 20060101
C09K011/70; C09K 11/56 20060101 C09K011/56 |
Claims
1. A seeded rod (SR) nanostructure comprising an elongated
structure embedding a single spherical core/shell structure,
wherein: at least one material of said core, shell, and elongated
structure is independently selected from the group consisting of a
semiconductor material, an insulator material, and a metal oxide
material.
2. The nanostructure according to claim 2, wherein the peak
structure in the XRD spectrum of the spherical core/shell structure
is different from the peak structure in the XRD structure of the
seeded nanostructure embedding the spherical core/shell
structure.
3. The nanostructure according to claim 1, wherein the material of
said elongated structure and the material of said spherical
core/shell structure is selected, independently, amongst
semiconductor materials.
4. The nanostructure according to claim 1, wherein the spherical
core/shell structure is positioned concentrically or
non-concentrically within the elongated structure.
5. The nanostructure according to claim 1, wherein: the elongated
structure comprises a first material, the core of the spherical
core/shell structure comprises a second material, at least one
shell of the spherical core/shell structure independently comprises
a further material, and each of said first, second, and further
materials is selected such that adjacent materials are different
from each other.
6. The nanostructure according to claim 5, wherein the at least one
shell material is selected to have a polymorphic crystal form to
enable anisotropic growth thereonto.
7. The nanostructure according to claim 6, wherein: the material
enabling anisotropic growth has a cubic or a non-cubic crystal
structure, and the non-cubic structure is selected from the group
consisting of hexagonal, monoclinic, orthorhombic, rhombohedral,
and tetragonal crystal structure.
8. The nanostructure according to claim 5, wherein each of the
first, the second, and the further materials is independently
selected from the group consisting of metal oxides, insulators, and
semiconducting materials.
9. The nanostructure according to claim 5, wherein each of the
first, the second, and the further materials comprises an element
of Group IIIB, IVB, VB, VIB, VIIB, VIIIB, IB, IIB, IIIA, IVA or VA
of block d of the Periodic Table of the Elements.
10. The nanostructure according to claim 9, wherein each of the
first, the second, and the further materials comprises a Group
III-V semiconductor material selected from the group consisting of
InAs, InP, InN, GaN, InSb, InAsP, InGaAs, GaAs, GaP, GaSb, AlP,
AIN, AlAs, AlSb, CdSeTe, ZnCdSe, CdSe, CdS and any combination
thereof.
11. The nanostructure according to claim 1, being selected from the
group consisting of InAs/CdSe/CdS, InP/ZnTe/ZnS, InP/ZnSe/ZnTe,
InP/ZnSe/CdS, InP/ZnSe/ZnS, ZnTe/ZnSe/ZnS, ZnSe/ZnTe/ZnS,
ZnSeTe/ZnTe/ZnS, CdSe/CdS Se/CdS, CdSe/CdS/CdZnS,
CdSe/CdZnSe/CdZnS, and CdSe/CdZnS/ZnS.
12. The nanostructure according to claim 1, wherein the
semiconductor material is a Group III-V material selected from the
group consisting of InAs, InP, InN, GaN, InSb, InAsP, InGaAs, GaAs,
GaP, GaSb, AlP, AIN, AlAs, AlSb, CdSeTe, ZnCdSe, and any
combination thereof.
13. A process for manufacturing the seeded rod nanostructure
according to claim 1, the process comprising contacting the
spherical core/shell structure in solution with at least one
precursor of the material of the elongated structure under
conditions permitting elongated growth of said elongated structure
material onto a surface of the spherical core/shell structure to
thereby obtain the seeded rod nanostructure.
14. A device comprising the nanostructure according to claim 1.
15. A seeded rod (SR) nanostructure comprising an elongated
structure embedding a single spherical core/shell structure,
wherein the nanostructure is selected from the group consisting of
InAs/CdSe/CdS, InP/ZnTe/ZnS, InP/ZnSe/ZnTe, InP/ZnSe/CdS,
InP/ZnSe/ZnS, ZnTe/ZnSe/ZnS, ZnSe/ZnTe/ZnS, ZnSeTe/ZnTe/ZnS,
CdSe/CdS Se/CdS, CdSe/CdS/CdZnS, CdSe/CdZnSe/CdZnS, and
CdSe/CdZnS/ZnS.
16. The nanostructure according to claim 1, wherein the spherical
core/shell structure is selected from the group consisting of
InAs/CdSe, InP/ZnTe, InP/ZnSe, ZnTe/ZnSe, ZnSe/ZnTe, ZnSeTe/ZnTe,
CdSe/CdSSe, CdSe/CdS, CdSe/CdZnSe, and CdSe/CdZnS.
17. The nanostructure according to claim 1, wherein the elongated
structure is of a material selected from the group consisting of
CdS, ZnS, ZnTe, CdS, and CdZnS.
18. The nanostructure according to claim 1, wherein the spherical
core/shell structure comprises a single shell or multiple shells,
wherein each of the multiple shells is different from any shell
adjacent thereto.
Description
FIELD OF THE INVENTION
[0001] This invention generally relates to nanomaterials,
particularly anisotropic semiconductor nanoparticles.
BACKGROUND OF THE INVENTION
[0002] Colloidal semiconductor nanocrystals (NCs) have attracted
great interest due to the ability to tailor their absorption and
photoluminescence (PL) over a wide spectral range, by changing
their size, shape and composition. In particular, II-VI and III-V
semiconductors NCs are of importance due to their fluorescence,
covering the visible to the near infrared (NIR) spectrum, which is
appealing for a variety technological applications [1,2].
[0003] The optical behavior of the particles can be further
modified by controlling their shape. For example, unlike spherical
NCs, nanorods have been found to have linearly polarized emission.
In addition, the rod shape of the particles enables electric field
induced switching of the fluorescence [3,4]. These properties make
semiconductor nanorods (NRs) highly desirable [3,5,6].
[0004] Furthermore, NRs display unique characteristics including
low lasing thresholds associated with increased Auger lifetimes
[7,8], large absorbance cross-sections, and linearly polarized
absorption and emission [9,10]. These properties show promise for
using NRs in applications such as lasing [7,11,12], bio-labeling
[13,14], and polarized single-photon sources [15].
[0005] While II-VI semiconductor nanorods are grown via a
surfactant control growth approach, this approach is difficult to
realize for cubic structured semiconductor NCs such as the III-V
semiconductors. In these NCs, chemically dissimilar surfaces are
not obviously present due to the high symmetry of lattice, and as a
result preferential binding of ligands, which is essential for the
surfactant controlled growth mechanism, cannot be obtained.
Previous works showed that III-V semiconductor rods can be grown
via a solution-liquid-solid (SLS) mechanism, by using small metal
NCs as catalyst for rod growth [16,17]. However, the presence of
the metal particles strongly quenches the photoluminescence.
[0006] Recently, a new type of core-shell nanoparticles, known as
seeded rods (SRs) was introduced [18-20], where spherical
nanoparticle of one material is embedded within a rod of another
material. Several SR systems were reported including CdSe cores
embedded in CdS rods, forming type-I and quasi-type-II systems
[20-24], and ZnSe cores embedded in CdS rods forming a type-II
system [18]. These particles exhibit several advantageous
properties typical for 1d systems, including linearly polarized
emission [23,25], suppression of Auger nonradiative recombination
[26], and large absorbance cross-sections [18,20,23].
[0007] The synthesis of such structures is performed by two
consecutive steps, where NCs seeds are first synthesized, and then
the seeds are rapidly injected into a hot solution of precursors
and ligands for the formation of a rod shell around them. Such
dot-rod heterostructures are highly crystalline and uniform and
exhibit strong and stable PL emission. However, in order to achieve
good optical properties, the core and shell materials should have a
low lattice-mismatch and generally also similar crystal type, which
limits the variety of structures that can be constructed in this
manner. Core/multishell NPs were introduced for spherical shaped
particles, but the utilization of this concept in rod shaped
systems and in particular in seeded rod was never previously
performed.
[0008] Core\shell semiconductors nanoparticles are more stable for
photoluminescence and have higher quantum efficiency due to the
shell passivation of the dangling bonds. Bawendi et al. [27]
presented synthesis of CdSe quantum dots coated with a ZnS shell
and having photoluminescence quantum yield of up to 50% and a
narrow size distribution. These CdSe dots were synthesized through
a typical TOP\TOPO synthesis. A size selective precipitation of
these dots was performed by means of centrifugation. A ZnS shell
was deposited by a drop-wise addition of the Zn and S precursor
mixture. It was further contemplated that the process may be
applicable also for CdTe and CdS cores and for ZnSe shell. Banin et
al. [16] discloses different III-V II-VI core shell
combinations.
[0009] Treadway et al. [28] expanded the method for various kinds
of semiconductors materials including II-VI III-V families. In this
method all the layer precursors are added simultaneously as
described therein. In 2003, Peng et al. [29] demonstrated a method
for preparing core/shell structures. In the method the layers are
added successively and each layer is added in two steps, one for
the anion and second for the cation.
[0010] In 2005, Banin et al. [30], using a layer by layer method,
showed a new kind of material, composited of a core coated with a
multilayered shell. This work presented III-V\II-VI\II-VI spherical
core\shell\shell2 materials, having high quantum efficiency and
high photostabilty improved over the previous core/shell. In
addition, this method enables a variety of combinations between
different materials and crystal structures. Peng et al. [29]
described syntheses of II-VI\II-VI, III-V\II-VI, II-VI\III-V and
III-V\III-V spherical core\shell nanocrystals, creating quantum
dots and quantum shells (reverse type-I). The same method is used
for synthesizing of core\multishell structures, creating quantum
wells and dual emitting quantum dots. Furthermore, core\shell and
core\multishell nanocrystals doped with transition metals (Mn, Fe,
Cu, etc.) are synthesized using the same method.
[0011] Rod-shaped semiconductors nanoparticles have unique
properties due to their 1D confinement. These materials hold high
polarization which can be used in different applications.
[0012] Alivisatos et al. [31] describes a process for the formation
of II-VI semiconductor nanorods, in which shape control is achieved
by adding different surfactants. The balance between the surfactant
induces different shapes. Group II metal (Zn, Cd, Hg) and group VI
element (S, Se, Te) are added, together or separately, to the
heated mixture of surfactants, followed by decreasing the
temperature to allow crystal growth.
[0013] Alivisatos et al. [32] suggested the same process for the
formation of III-V semiconductor nanorods.
[0014] Rod/shell materials have better optical properties. However,
this presents a significant synthetic challenge, as the rod shape
must be preserved even though it is not thermodynamically stable.
Several examples for core/shell NRs were synthesized, showing
improved quantum efficiencies of around 30% [33,34]. These
materials were reported by Alivisatos et al. [35] describing
methods for synthesizing CdSe rod shaped core coated with a graded
CdS\ZnS shell, and by Banin et al [34] growing ZnS shells. The
shell precursors are added drop-wise. In this case the resulting
structure has a thin shell layer and coats the rod essentially with
even thickness on all sides. The invention contemplates the same
structure for rod shaped cores and conformal shells of different
semiconductor groups (II-VI, III-V, IV).
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SUMMARY OF THE INVENTION
[0067] Rod-shaped nanocrystals are of great interest because the
rod shape produces polarized light emission. A drawback of
rod-shaped structures such as nanorods (NRs), in comparison to
spherical nanocrystals, is their lower fluorescence quantum
efficiencies. This originates, on the one hand, from the increased
delocalization of the carriers in the NRs which reduces the
electron-hole overlap, decreasing the radiative decay rate, and, on
the other hand, from the large surface area of the NRs that
increases the probability for surface trapping, leading to higher
non-radiative decay rate.
[0068] The present invention provides a novel family of Seeded Rod
(SR) nanostructures (referred also herein as "SR systems"), in
which such deficiencies, known for the spherical nanocrystals as
well as for nanorod systems, are minimized or completely
diminished. The SR systems of the invention, as will be further
demonstrated hereinbelow, exhibit the following
characteristics:
[0069] 1. Seeded rod with seeds that have a core/multi-shell
structure have higher quantum efficiency because of improved
compatibility of interfaces and better surface electronic
passivation.
[0070] 2. Buffer layers between the seed inner core and the outer
rod structure enable deposition of materials of different lattice
constants by relaxing stress and strain between the different
layers and, therefore, enable a better shell growth with improved
optical characteristics.
[0071] 3. The formation of the rod-shell imparts a rod-like
behaviour on electronic states originating from the spherical seed,
including polarization of the absorption and the emission, and
increased Auger lifetime.
[0072] 4. Buffer layers between the inner core and outer rod enable
deposition of materials of different crystal structures with
different reactive facets, through which selective growth in
particular directions can be obtained. For example, the growth of
layer with hexagonal crystal structure on top of a core with cubic
crystal structure enables the growth of a third rod shaped shell,
otherwise difficult to synthesize in cubic systems.
[0073] 5. Spatial electron and hole distributions along the
particle can be controlled through applied electronic and
electromagnetic fields, leading to controlled changes in the
emission behaviour.
[0074] Thus, the invention provides in one of its aspects a seeded
rod (SR) nanostructure comprising an elongated structure (rod-like
structure) embedded (seeded) with a single seed structure, said
seed structure being a core/shell structure or a single-material
rod element (not a core/shell structure, the material of the rod
element being different from the material of the elongated
structure);
[0075] where said seed structure is a rod element, the thickness of
the elongated structure material embedding said rod element, along
one axis of the nanostructure (as further explained hereinbelow) is
greater on one end as compared to the other end along the same
axis, or the thickness along one axis is greater as compared to the
thickness along another axis; and
[0076] where the seed is a core/shell structure, the material of at
least one of said core and shell is a semiconductor material.
[0077] The SR system of the invention is generally depicted in FIG.
21. In FIG. 21, the SR system is designated (100), the elongated
structure is designated (10) and the single seed structure embedded
within said elongated structure is generally designated (20).
[0078] In some embodiments, the seed structure (20) is a core/shell
structure such as (21), (22), (23), (24), and (26). In other
embodiments, the seed structure (20) is a single material rod
element, depicted by element (25).
[0079] As used herein, the term "seeded rod", SR, or any lingual
variation thereof, refers to a nano-size elongated heterostructure
(100), being typically between about 5 nm and 500 nm in length and
between about 2 and 50 nm in width (thickness). The SR
nanostructure may have an aspect ratio (length/thickness) between
1.8 and 20. In some embodiments, the aspect ratio is larger than
1.8. In further embodiments, the aspect ratio is larger than 2. In
still further embodiments, the aspect ratio is larger than 3 and in
further embodiments, the aspect ratio is larger than 10.
[0080] In the SR the chemical composition may change with
position.
[0081] The SR (100) of the invention is made of a "seed structure"
(20), which is embedded (seeded or contained or enclosed) within an
"elongated structure" (10). The seed structure (20) and the
elongated structure (10) which contains the seed are of different
materials/compositions and may be of different structures and
forms. The outermost (as defined herein) material of the seed
structure (20) is in direct contact with the material of the
elongated structure (10).
[0082] The elongated structure is a non-spherical shaped structure
which may be of any elongated shape, such as a rod, rice,
cylindrical, arrow, an elongated rectangle, or a branched system
with two arms (bipod), three arms (tripod) four arms (tetrapod) and
so on. The seed structure embedded within the elongated structure
may be a core/shell system, referred to herein as a "core/shell SR
system" or may be a non-core/shell rod element, referred to herein
as a "rod-in-rod SR system".
[0083] In some embodiments, the SR system of the invention is a
core/shell SR system, in which case the core/shell seed may be
spherical or non-spherical in shape and may be positioned
concentrically or non-concentrically within the elongated structure
in which it is embedded. Where the core/shell or any of the shells
of the core/shell structure are non-spherical, they may be of a
form selected from rod-like shape, rod, rice, cylindrical, arrow,
an elongated rectangle, or a branched system with two arms (bipod),
three arms (tripod) four arms (tetrapod) and so on.
[0084] As used herein, the term "spherical", or any lingual
variation thereof, refers generally to a substantially (nearly)
round-ball geometry. The term generally reflects on the spherical
non-elongated shape of a nanoparticle (core seed, shell, etc),
which need not be perfectly round in shape. The size of the
spherical nanoparticle is typically the average diameter thereof. A
"non-spherical" nanoparticle is such which is "elongated" in shape
and has a defined long and short axis. The size of the elongated
nanoparticle (such as the SR systems of the invention, a
rod-element in rod-in-rod SR systems according to the invention, or
elongated shell structures), is given as the length of the long
axis of the particle, the width (short axis) and/or the particle
aspect ratio.
[0085] As stated herein, the core/shell structure or the
rod-element structure may be positioned concentrically or
non-concentrically within the elongated structure of the SR system.
An element or structure is said of being "concentrically"
positioned with respect of a further element or structure when the
geometrical center of each of the elements of structures are
substantially coaxially aligned. When the centers are not so
aligned, the elements or structures are regarded as being
non-concentric with respect of each other, namely their geometrical
centers do not coaxially align.
[0086] In some embodiments, the overall shape of the core/shell
structure is spherical and constitutes a core and one or more
shells, each shell may have a thickness of at least about 0.2 nm.
In other embodiments, the thickness of each of said shells is at
least about 1 nm. In further embodiments, the thickness is at most
20 nm.
[0087] In some embodiments, the thickness ranges from about 0.2 nm
to about 20 nanometers. In some embodiments, the thickness may
range from about 0.2 nm to about 4 nm. In other embodiments, the
thickness may range from about 0.2 nm to about 8 nm. In further
embodiments, the thickness may range from about 0.2 nm to about 12
nm. In other embodiments, the thickness may range from about 0.2 nm
to about 16 nm. In some embodiments, the thickness may range from
about 1 nm to about 4 nm. In other embodiments, the thickness may
range from about 1 nm to about 8 nm. In further embodiments, the
thickness may range from about 1 nm to about 12 nm. In some
embodiments, the thickness may range from about 1 nm to about 16
nm. In additional embodiments, the thickness may range from about 1
nm to about 20 nm. In some embodiments, the thickness may range
from about 2 nm to about 4 nm. In other embodiments, the thickness
may range from about 2 nm to about 8 nm. In further embodiments,
the thickness may range from about 2 nm to about 16 nm. In some
additional embodiments, the thickness may range from about 2 nm to
about 20 nm.
[0088] In other embodiments, the overall shape of the core/shell
structure is non-spherical but rather elongated. In such cases, the
length of the longest axis of the structure is between 6 to 100 nm,
and the thickness (width) may range from 1.5 to 10 nm.
[0089] In other embodiments, the elongated shaped core/shell
structure may have an aspect ratio (length/diameter) between 1.8
and 50. In some embodiments, the aspect ratio is larger than 1.8.
In further embodiments, the aspect ratio is larger than 2. In still
further embodiments, the aspect ratio is larger than 3 and in
further embodiments, the aspect ratio is larger than 10.
[0090] The SR system of the invention is a rod-like nanostructure
(100), the nanostructure comprising an elongated structure (10) of
a first material (which in some embodiments, is a semiconductor
material), embedding a seed structure (20) which may be core/shell
structure (such as, (21), (22), (23), (24), and (26)) comprising a
core of a second (e.g., semiconductor) material and at least one
shell of a further (e.g., semiconductor) material, each of said
first, second and further (e.g., semiconductor) materials being
selected such that adjacent materials are different from each
other.
[0091] Thus, the core/shell structure has a core of one material
and at least one shell of a different material, one or more of said
materials is a semiconductor material. Where the core/shell
structure comprises of more than one shell, the structure may then
be referred to as a "core/multishell" structure, and may then be
designated as core/shell(1)/shell(2) . . . /shell(n), wherein (n)
is the number of consecutive shells from 1 to 30. Thus, the number
of shells (n) is selected from 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,
12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28,
29 and 30. The number of shells, depending inter alia on the SR
system of the invention and the intended application, is 1, or 2,
or 3, or 4, or 5, or 6, or 7, or 8, or 9, or 10, or 11, or 12, or
13, or 14, or 15, or 16, or 17, or 18, or 19, or 20, or 21, or 22,
or 23, or 24, or 25, or 26, or 27, or 28, or 29 or 30.
[0092] In some embodiments, the number of shells ranges from 1 and
5. In further embodiments, the number of shells ranges from 2 and
5. In some embodiments, the number of shells ranges from 2 and 4.
In other embodiments, the number of shells is 2 or 3. In further
embodiments, the number of shells is between 5 and 30.
[0093] Adjacent shells are of different materials/compositions. In
a core/multishell structure, at least one of the core and shell(1),
shell(2) . . . shell(n) materials is a semiconductor material. In
some embodiments, in a core/multishell structure, the material of
each of core and shell(1), shell(2) . . . shell(n) is a
semiconductor material.
[0094] Each shell may be spherical or non-spherical in shape,
depending, inter alia, on the shape of the core or any of the inner
shells. Where one or more of the shells is non-spherical, the
subsequent shell(s) (namely the adjacent shell(s) which is/are
further removed from the core) to the non-spherical shell are also
non-spherical. Where one or more of the shells is spherical, the
subsequent shell(s) (namely the adjacent shell(s) which is/are
further removed from the core) may be either spherical or
non-spherical in shape.
[0095] As the at least one shell may be spherical or elongated in
shape, the core/shell structure may thus be selected from spherical
core/spherical shell, spherical core/elongated shell, and elongated
core/elongated shell.
[0096] In some embodiments, the core is spherical and each of the
at least one shells is spherical. In other embodiments, the core is
spherical and each of the at least one shells is elongated. In
still other embodiments, the core is elongated in shape and each of
the at least one shells is elongated.
[0097] The invention thus provides SR systems wherein the seed is a
core/shell structure that is selected from: [0098] spherical
core/spherical shell (e.g., 21, FIG. 21) [0099] spherical
core/spherical shell(1)/spherical shell(2) (e.g., 22, FIG. 21)
[0100] spherical core/elongated shell (e.g., 23, FIG. 21) [0101]
spherical core/spherical shell(1)/elongated shell(2) (e.g., 24,
FIG. 21) [0102] spherical core/elongated shell(1)/elongated
shell(2) [0103] elongated core/elongated shell (e.g., 26, FIG. 21)
[0104] elongated core/elongated shell(1)/elongated shell(2).
[0105] In some embodiments, the elongated structure (10) and the
core/shell structure (20) embedded therein are concentric.
Non-limiting examples of such concentric systems are presented in
FIGS. 1B, 1D, 1H, 1L, and 1O.
[0106] In some embodiments, the elongated structure (10) and the
core/shell structure (one or both of core and shell) embedded
therein are non-concentric. Non-limiting examples of such
non-concentric systems are presented in FIGS. 1A, 1C, 1E, 1F, 1G,
1I, 1J, 1K, 1P, 1Q and 1R.
[0107] The "core" is the innermost material contained in a
core/shell structure. The core may be of various shapes, i.e.,
spherical (nearly spherical), rod, pseudo-pyramid, cube, octahedron
and others. The core may be positioned concentrically or
non-concentrically with respect of each or some of the shell(s),
and/or with respect of the elongated structure in which the
core/shell seed is embedded. In some embodiments, the core is
spherical and may have a diameter ranging from about 1 to about 20
nm.
[0108] In further embodiments, the core is a rod-like particle,
namely a nanocrystal with extended growth along one of its axis,
while maintaining very small dimensions of the other two axes. The
length of the longest axis of the rod-like core typically ranges
from about 5 nm to about 400 nm. In some embodiments, the length of
the longest axis is between 10 to 100 nm. In other embodiments, the
thickness (width) of the rod-like core may range from about 2 to
about 50 nm. In additional embodiments, the thickness is between
about 2 and about 10 nm.
[0109] In other embodiments, the rod-like core may have an aspect
ratio (length/diameter) between 1.8 and 20. In some embodiments,
the aspect ratio is larger than 1.8. In further embodiments, the
aspect ratio is larger than 2. In still further embodiments, the
aspect ratio is larger than 3 and in further embodiments, the
aspect ratio is larger than 10.
[0110] The material of the outermost shell of the core/shell
structure, namely the shell which is in direct contact with the
material of the elongated structure (in which it is embedded) is
selected, in some of the embodiments disclosed herein, to have a
lattice constant substantially similar to the lattice constant of
any of the materials in contact therewith. Optionally further, the
outermost shell is selected to have a polymorphic crystal form to
enable e.g., facile anisotropic growth thereonto.
[0111] In fact, any of the shells' materials are selected to have a
lattice constant substantially similar to the two material zones
(shells or core and shells) which are adjacent form either of its
sides (inside or outside) thereto so as to decrease the lattice
strain between the material of, e.g., one shell and the material of
an adjacent shell. Such a selection of core, shell and elongated
structure materials significantly improves fluorescence quantum
yield in comparison to standard SR structures lacking core/shell
structures. This permits facile growth of various semiconductor
combinations with anisotropic seeded-rod architecture and new
combinations between different crystal structures, providing a
better control over the anisotropic growth of the nanorod as well
as the electronic potential profile and yielding improved optical
characteristics.
[0112] As such, depending on the intended use, each shell material,
being intermediate to two other materials, is in fact a buffer
material which is selected to decrease the lattice strain between
the materials. For example, in a SR system having a spherical
core/spherical shell structure, the lattice constant of the
material of the spherical shell (being the outermost shell of the
core/shell structure) is intermediate to the lattice constant of
the spherical core material and the material of the elongated
structure in which the core/shell is embedded. Where the SR system
comprises a spherical core/spherical shell(1)/spherical shell(2)
structure, the lattice constant of the shell(1) material is
intermediate to the lattice constant of the spherical core material
and the material of shell(2). Also, the lattice constant of the
shell(2) material is intermediate to the lattice constant of the
shell(1) material and the material of elongated structure in which
the core/shell is embedded.
[0113] Similarly, where the SR system comprises a spherical
core/elongated shell structure, the lattice constant of the
elongated shell material is intermediate to the lattice constant of
the spherical core material and the material of the elongated
structure.
[0114] For example, in cases where the core material is InAs, and
shell(2) is CdS, shell(1) may be of CdSe. Where the core material
is InP and shell(2) is ZnS, the shell(1) material may be ZnSe.
[0115] As stated above, the material of a core and/or a shell of a
core/shell structure may be selected to have a polymorphic
morphology so as to enable, e.g., facile elongated growth onto the
seed (being the core or core/shell structure onto which elongated
growth is directed). In some embodiments, the material enabling
elongated growth, i.e., the material onto which direct elongated
growth is to proceed, is selected amongst materials having both a
cubic (zinc blend) and a non-cubic morphologies, wherein the
non-cubic morphology is selected from hexagonal (wurtzite),
monoclinic, orthorhombic, rhombohedral, and tetragonal crystal
structure. For example, where the SR system comprises a spherical
core/spherical shell(1)/elongated shell(2) structure, the material
of spherical shell(1) may be selected to adopt an elongated
morphology. Where the SR system comprises a spherical
core/elongated shell(1)/elongated shell(2) structure, the core
material may be selected to adopt an elongated morphology.
[0116] In other embodiments, the material of a core and/or a shell
of a core/shell structure may be selected to have a crystal
morphology compatible with facile elongated growth onto the seed
(being the core or core/shell structure onto which elongated growth
is directed), the crystal form may be selected from hexagonal
(wurtzite), monoclinic, orthorhombic, rhombohedral, and
tetragonal.
[0117] In another aspect of the invention, the SR system is a
rod-in-rod system, wherein the elongated structure (10, FIG. 21) is
seeded with a non-core/shell structure rod element (25) of a
different material. The rod element endows the rod-in-rod SR system
with significantly improved properties in terms of increased
polarized emission and Auger recombination processes. The inner
elongated element offers polarized emission such that the degree of
emission polarization of the rod-in-rod structure is significantly
higher as compared to that of a seeded rod with a spherical seed.
The outer elongated shell also increases the absorption
polarization at wavelengths related to its absorption. The outer
elongated shell effectively passivates the surface of the embedded
rod element, eliminating trap states at the interface, and thus
leads to high emission quantum efficiencies and to improved
photostability. Unlike rod/shell structures, wherein the shell acts
as a thin coating over the rod element, the rod-in-rod SR system of
the invention is of excellent optical properties, demonstrating
strong and stable emission. As will be further demonstrated
hereinbelow, the rod-in-rod SR systems of the invention are also
prepared via a facile and fast approach that enables
up-scaling.
[0118] Thus, the invention provides a seeded rod (rod-in-rod SR)
system comprising an elongated structure (rod-like structure, 10)
seeded with a rod element (25) of a single material (the material
of the rod element being different from the material of the
elongated structure). As used herein, the term "single material"
refers to a rod element having a single chemical composition. As
may be understood, the rod element is neither a core/shell
structure nor a segmented (barcode) structure of two or more
materials or compositions.
[0119] In some embodiments, the elongated structure (10) of the
rod-in-rod SR system consists of a rod element of a single material
(25), wherein said rod element may be positioned concentrically or
non-concentrically with respect to the elongated structure (10). In
some embodiments, the elongated structure of the rod-in-rod SR
system consists of a rod element, which is positioned
non-concentrically with respect to the elongated structure. In
other words, the thickness of the elongated structure material
along one of the axis, e.g., the long axis, of the SR system is
greater on one end as compared to the other end along the same
axis. This is clearly demonstrated in FIG. 22. In such embodiments,
where the rod element is positioned non-concentrically, the
thickness of the elongated structure material along one of the
axis, e.g., the long axis (LX), as annotated tk.sub.1 or tk.sub.2,
is greater than the thickness tk.sub.2 or tk.sub.1, respectively,
along the same axis (the thickness being one or more of
tk.sub.1>tk.sub.2, tk.sub.1<tk.sub.2, tk.sub.3<tk.sub.4,
tk.sub.3>tk.sub.4).
[0120] In other embodiments, the elongated structure (10) of the
rod-in-rod SR system consists of a rod element (25), wherein said
rod element is positioned concentrically with respect to the
elongated structure (10), with the thickness of the elongated
structure material along one of the axis, e.g., the long axis (LX
in FIG. 22), in comparison to the other axis, e.g., the short axis
(SX in FIG. 22), is substantially different. In other words, the
thickness tk.sub.1 is substantially the same as tk.sub.2, and the
material thickness tk.sub.3 is substantially the same as tk.sub.4,
wherein tk.sub.1 (or tk.sub.2) is different from tk.sub.3 (or
tk.sub.4).
[0121] In some embodiments, the material thickness, tk.sub.1 or
tk.sub.2, along the long axis is greater than the material
thickness, tk.sub.3 or tk.sub.4, along the short axis.
[0122] In some embodiments, the thickness along the long axis
(i.e., tk.sub.1 and/or tk.sub.2) of the rod-in-rod SR system is at
least twice as large as the thickness along the short axis (i.e.,
tk.sub.3 and/or tk.sub.4). In other embodiments, the thickness
along the long axis is at least three times as large as the
thickness of the short axis.
[0123] As stated above, the invention provides a SR system
comprising an elongated structure (10) embedded with a seed
structure (20) being an elongated core/shell seed (such as element
26 in FIG. 21).
[0124] In some embodiments, the elongated core/shell structure
comprises an elongated core and a number of elongated shells
ranging from 1 to 30. The thickness of each of said shells, i.e.,
in an elongated core/multishell system, is as defined above for a
spherical core/multishell system.
[0125] In some embodiments, the number of elongated shells is
selected from 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,
16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 and 30. The
number of shells, depending inter alia on the SR system of the
invention and its intended application, is 1, or 2, or 3, or 4, or
5, or 6, or 7, or 8, or 9, or 10, or 11, or 12, or 13, or 14, or
15, or 16, or 17, or 18, or 19, or 20, or 21, or 22, or 23, or 24,
or 25, or 26, or 27, or 28, or 29 or 30.
[0126] In some embodiments, the number of shells ranges from 1 and
5. In further embodiments, the number of shells ranges from 2 and
5. In further embodiments, the number of shells ranges from 2 and
4. In other embodiments, the number of shells is 2 or 3. In further
embodiments, the number of shells is between 5 and 30.
[0127] In some embodiments, the number of the elongated shells is
one. In other embodiments, the number of said elongated shells is 2
or 3.
[0128] The invention thus provides SR systems wherein the
core/shell structure is selected from: [0129] elongated
core/elongated shell (e.g., 26, FIG. 21) [0130] elongated
core/elongated shell(1)/elongated shell(2).
[0131] In some embodiments, the elongated structure (10) and the
elongated core/shell structure (20) embedded therein are
concentric. Non-limiting examples of such concentric systems are
presented in FIG. 10.
[0132] In some embodiments, the elongated structure (10) and the
elongated core/shell structure (20) embedded therein are
non-concentric. Non-limiting examples of such non-concentric
systems are presented in FIGS. 1P, 1R and 1Q.
[0133] As used herein with reference to any of the SR systems of
the invention, or any component (shell, core, element) thereof, the
term "material" refers to a single material or a combination of at
least one material(s). The material may be a metal, a metal oxide,
a metal alloy, an insulator, a semiconductor material or any
combination thereof. In some embodiments, the term material refers
to a semiconductor material; in such cases, the nanostructure of
the invention is wholly of a semiconductor material, namely each of
said core, shells and elongated structure materials, independently,
is selected amongst semiconductor materials, as disclosed
herein.
[0134] The material of one component of a SR system according to
the invention may be different from the material of another
component, or may be of the same material but of a different
chemical composition. For example, the material of one shell may be
a metal while the material of another shell may be a different
metal or a semiconductor material. Similarly, the material of the
core of a core/shell SR system may be a metal and the material of
shell(1) may be a metal alloy or metal oxide of the metal from
which the core is composed of.
[0135] The materials may be selected on the basis of their
structural and electronic properties. In some cases, a
material-gradient may be constructed whereby the material
composition of, e.g., an inner shell as compared to, e.g., an outer
shell, changes such that the concentration of one material within
said chemical composition is continuously reduced while the
concentration of another material increases. For example, a core
material may be CdSe, a shell(n) material may be CdS, and the
materials of any of the shells positioned therebetween is a graded
alloy of the formula CdSe.sub.xS.sub.1-x, wherein x varies from
slightly below 1 to slightly above zero (e.g.,
0.9999>.times.>0.0001).
[0136] As stated above, in a SR system of the invention, where the
seed structure is a core/shell or core/multishell structure, at
least one of the second (core) material and a further (shell)
material is a semiconductor material. In fact, each of the SR
materials, e.g., the first material (of the elongated structure),
the second (core) material, and the further material(s) (of each of
the at least one shells), of any of the SR systems of the invention
(core/shell SR system or rod-in-rod SR system), independently of
the other, may be selected amongst metals, metal alloys, metal
oxides, insulators, and semiconducting materials. In some
embodiments, the material is or comprises an element of Groups
IIIB, IVB, VB, VIB, VIIB, VIIIB, IB, IIB, IIIA, IVA and VA of block
d of the Periodic Table of the Elements.
[0137] In some embodiments, the material is or comprises a
transition metal selected from Groups IIIB, IVB, VB, VIB, VIIB,
VIIIB, IB and IIB of block d the Periodic Table. In some
embodiments, the transition metal is a metal selected from Sc, Ti,
V, Cr, Mn, Fe, Ni, Cu, Zn, Y, Zr, Nb, Tc, Ru, Mo, Rh, W, Au, Pt,
Pd, Ag, Mn, Co, Cd, Hf, Ta, Re, Os, Ir and Hg.
[0138] In some embodiments, the material is a semiconductor
material selected from elements of Group I-VII, Group II-VI, Group
III-V, Group IV-VI, Group and Group IV semiconductors and
combinations thereof.
[0139] In other embodiments, the semiconductor material is a Group
I-VII semiconductors are CuCl, CuBr, Cul, AgCl, AgBr, AgI and the
like.
[0140] In other embodiments, the semiconductor material is a Group
II-VI material being selected from CdSe, CdS, CdTe, ZnSe, ZnS,
ZnTe, HgS, HgSe, HgTe, CdZnSe, ZnO and any combination thereof.
[0141] In further embodiments, Group III-V material are selected
from InAs, InP, InN, GaN, InSb, InAsP, InGaAs, GaAs, GaP, GaSb,
AlP, AlN, AlAs, AlSb, CdSeTe, ZnCdSe and any combination
thereof.
[0142] In additional embodiments, the semiconductor material is
selected from Group IV-VI, the material being selected from PbSe,
PbTe, PbS, PbSnTe, Tl.sub.2SnTe.sub.5 and any combination
thereof.
[0143] In other embodiments, the material is or comprises an
element of Group IV. In some embodiments, the material is selected
from C, Si, Ge, Sn, Pb.
[0144] In some embodiments, the material is metal, metal alloys, or
metal oxide. Non-limiting examples include ZnO, CdO,
Fe.sub.2O.sub.3, Fe.sub.3O.sub.4, and In.sub.2O.sub.3.
[0145] In other embodiments, the material is selected amongst metal
alloys and intermetallics of the above metal and/or transition
metals.
[0146] In further embodiments, the material is selected from copper
sulfides, selected in a non-limiting manner from Cu.sub.2S,
Cu.sub.2Se, CuInS.sub.2, CuInSe.sub.2, Cu.sub.2(ZnSn)S.sub.4,
Cu.sub.2(InGa)S.sub.4, CuInS.sub.2, CuGaS.sub.2, CuAlS.sub.2 and
mixed copper-iron sulfides such as Cu.sub.5FeS.sub.4 (Bornite) and
CuFeS.sub.2 (chalcopyrite).
[0147] In further embodiments, the material is or comprises a
semiconductor material.
[0148] In some embodiments, in the core/shell SR systems of the
invention, the core material is selected from InAs, InP, CdSe,
ZnTe, ZnSe, and ZnSeTe. In other embodiments, each of the shell
materials is selected from CdSe, CdS, CdSSe, CdZnSe, CdZnS, ZnSe,
and ZnTe.
[0149] In some embodiments, the material of the elongated structure
is selected from CdS, CdZnS, ZnS, ZnTe, and ZnTe/ZnS.
[0150] In some embodiments, the core/shell SR system of the
invention is a core/shell(1)/SR system selected from InAs/CdSe/CdS,
InP/ZnTe/ZnS, InP/ZnSe/ZnTe, InP/ZnSe/CdS, InP/ZnSe/ZnS,
ZnTe/ZnSe/ZnS, ZnSe/ZnTe/ZnS, ZnSeTe/ZnTe/ZnS, CdSe/CdS Se/CdS,
CdSe/CdS/CdZnS, CdSe/CdZnSe/CdZnS, and CdSe/CdZnS/ZnS.
[0151] In some embodiments, the core/shell SR system of the
invention is a core/shell(1)/shell(2) SR system selected from
InAs/CdSe/ZnSe/CdS, and InP/ZnSe/ZnTe/ZnS.
[0152] As stated above, in the SR systems of the invention each of
said first, second and further materials are selected such that
each two adjacent materials are different from each other. In some
embodiments, the first material may be a semiconductor and the
further material may be a metal. In other embodiments, the first
material may be one semiconductor material and the further material
may be another semiconductor material. In further embodiments, each
of the core and shell materials is a semiconductor material,
provided that two adjacent materials are not the same.
[0153] In some embodiments, at least one of the first, second and
further materials is a semiconductor. In further embodiments, at
least one of the first, second and further materials is a
metal.
[0154] In another aspect of the invention, there is provided a
process for the manufacture of the SR systems of the invention. In
FIG. 23, a process for the manufacture of a spherical
core/spherical shell SR system is depicted. As a person of skill in
the art would understand, this depicted process may be similarly
adapted to the manufacture of other SR systems of the
invention.
[0155] Generally, in the first step of the exemplified process of
FIG. 23, the core is obtained by direct synthesis or from a
commercial source. As noted above, the core may be a spherical core
(as exemplified in FIG. 23) or an elongated core. The shells are
grown on the core prior to the formation of the SR system. In some
instances, a core/shell structure is pre-made, following known
processes for the manufacture of core/shell nanostructures, and
thereafter used in accordance with a process of the invention.
[0156] Thus, the invention provides a process for the production of
a SR system comprising an elongated structure (rod-like structure,
10) embedded with a seed structure (20), wherein a core/shell
structure is employed for obtaining a core/shell SR system, and a
rod element is employed for obtaining a rod-in-rod SR system, the
process comprising contacting said seed structure, in solution,
with precursors of the elongated structure material.
[0157] In some embodiments, the process comprising: [0158]
obtaining a pre-made solution of seed structures (20), each of said
seed structures (20) being selected from a rod element seed, a
core/shell (spherical or non-spherical) seed and a core/multishell
(spherical or non-spherical) seed; [0159] contacting said seed
structures (20), in solution, with a precursor solution of an
elongated structure (10) material under conditions permitting
elongated growth of said material onto the surface of said seed
structure;
[0160] to thereby afford the SR systems.
[0161] In some embodiments, the process further comprises isolating
the SR systems.
[0162] In some embodiments, the seed structure is a spherical (or
elongated) core/shell(n) nanostructure and the process comprises:
[0163] providing a solution of core/shell(n) structures; [0164]
contacting said core/shell(n) structures with a precursor solution
of the elongated structure material under conditions permitting
elongated growth of said material onto the surface of shell(n); to
thereby obtain a core/shell(n) SR system in accordance with the
present invention.
[0165] As used herein, the core/shell(n) designates a core/shell
seed having n shells, wherein n is selected from 1 to 30.
[0166] In other embodiments, the seed structure is a rod element
and the process comprises: [0167] providing a solution of rod
elements; [0168] contacting said rod elements with a precursor
solution of the elongated structure material under conditions
permitting elongated growth of said material onto the surface of
said rod elements; to thereby obtain a rod-in-rod SR system in
accordance with the present invention.
[0169] The contacting step of the seed structure (core/shell or rod
element) with precursors of the elongated structure material is
carried out, in solution, by adding, e.g., injection, the seed
structure together with precursors of the elongated structure
material into a growth solution, at an appropriate temperature. In
some embodiments, the temperature is higher than 80.degree. C. In
other embodiments, the temperature is higher than 200.degree.
C.
[0170] In some embodiments, the temperature range is between
80.degree. C. to 400.degree. C. In other embodiments the
temperature range is between 200.degree. C. to 400.degree. C. In
further embodiments the temperature range is between 120.degree. C.
to 380.degree. C. In still other embodiments the temperature range
is between 200.degree. C. to 300.degree. C.
[0171] In some embodiments, the solution is swiftly injected into a
growth solution.
[0172] The growth solution, in which the SR systems of the
invention are grown, may comprise of additional precursors for
building the elongated structure, organic ligands which support
elongated growth, and organic solvents and ligands used for
dispersing the seeds structures and dissolving the other ligands.
The growth solution provides the seeds, which act as nucleation
center for the formation of the enveloping shell(s), and can also
provide ligands for directing the growth direction of the rod
shaped shell, and ligands for improving the solubility of ligands
or dispersing of seeds in the main solution flask.
[0173] Generally and without wishing to be bound by theory, the
elongated geometry of the nanoparticle SR system of the invention
may be controlled by several parameters, including: the crystalline
phase of the outer shell material, as discussed hereinabove, and
the conditions under which the elongated growth is carried out.
[0174] In some embodiments, the material of the outer shell of the
core/shell nanostructure is characterized by an elongated
crystalline phase, and is thus capable of inducing elongated
structure growth. In some other embodiments, the elongated growth
is achieved by employing a growth temperature to enable conversion
to the anisotropic crystalline phase.
[0175] In further embodiments, the anisotropic growth is achieved
by using ligands which attach preferentially to certain facets of
said seed nanostructure (to thereby render them less-reactive),
thus promoting elongated growth along reactive facets. In many
cases, combination of strong attaching ligands and weak attaching
ligands lead to anisotropic growth. In general, long tail
phosphonic acid ligands are strong attaching molecules used for
directional growth. Weak ligands include oleic acid, oleyl amine
and phosphine oxides. A broad spectrum selection of ligands, which
may be used in accordance with the present invention, may be
derived theoretically using the Bravais-Friedel Donnay-Harker
theory and the growth morphology method (see for example: (1) A.
Bravais, Etudes Crystall-ographiques, Academie des Sciences, Paris
(1913); (2) J. D. H. Donnay and D. Harker, Amer. Mineralogist, 22,
463 (1935); (3) Z. Berkovitch-Yellin, J. Am. Chem. Soc., 107, 8239
(1985) and R. Docherty, G. Clydesdale, K. J. Roberts, P. Bennema.,
J. Phys. D: Appl. Phys., 24, 89 (1991).
[0176] In some embodiments, the ligands used with anionic
precursors are selected amongst trioctylphosphine (TOP) and
tributylphosphine (TBP).
[0177] In some embodiments, the weak ligands are selected
from_trioctylphosphine oxide (TOPO), dodecyl amine (DDA),
tetradecyl amine (TDA), hexadecyl amine (HDA), octadecyl amine
(ODA) and oleic acid (OA).
[0178] In further embodiments, the strong ligands are selected from
dodecylphosphonic acid (DDPA), tridecylphosphonic acid (TDPA),
octadecylphosphonic acid (ODPA), hexylphosphonic acid (HPA) and
thiols.
[0179] In further embodiments, the anisotropic growth is achieved
by using high concentration of precursors in solution. The
precursors may be selected from the following: [0180] Metal
precursors as cations, where "M" represents the metal atom,
include: [0181] oxides selected from M.sub.2O, MO, M.sub.2O.sub.3,
MO.sub.2, M.sub.2O.sub.2, MO.sub.3, M.sub.3O.sub.4, MO.sub.5, and
M.sub.2O.sub.7; [0182] acetates (the group CH.sub.3COO.sup.-,
abbreviated AcO.sup.-) selected from AcOM, AcO.sub.2M, AcO.sub.3M,
and AcO.sub.4M; [0183] acetates hydrates (the group
CH.sub.3COO.sup.-, abbreviated AcO.sup.-) selected from
AcOM.xH.sub.2O, AcO.sub.2M.xH.sub.2O, AcO.sub.3M.xH.sub.2O, and
AcO.sub.4M.xH.sub.2O, wherein x varies based on the nature of M;
[0184] acetylacetonates (the group C.sub.2H.sub.7CO.sub.2.sup.-,
abbreviated AcAc.sup.-) selected from AcAcM, AcAc.sub.2M,
AcAc.sub.3M, and AcAc.sub.4M; [0185] acetylacetonate hydrates (the
group C.sub.2H.sub.7CO.sub.2.sup.-, abbreviated AcAc.sup.-)
selected from AcAcM.xH.sub.2O, AcAc.sub.2M.xH.sub.2O,
AcAc.sub.3M.xH.sub.2O, and AcAc.sub.4M.xH.sub.2O, wherein x varies
based on the nature of M; [0186] chlorides selected from MCl,
MCl.sub.2, MCl.sub.3, MCl.sub.4, MCl.sub.5, and MCl.sub.6; [0187]
chlorides hydrates selected from MCl.xH.sub.2O,
MCl.sub.2.xH.sub.2O, MCl.sub.3.xH.sub.2O, MCl.sub.4.xH.sub.2O,
MCl.sub.5.xH.sub.2O, and MCl.sub.6.xH.sub.2O, wherein x varies
based on the nature of M; [0188] bromides selected from MBr,
MBr.sub.2, MBr.sub.3, MBr.sub.4, MBr.sub.5, and MBr.sub.6; [0189]
bromides hydrates selected from MBr.xH.sub.2O, MBr.sub.2.xH.sub.2O,
MBr.sub.3.xH.sub.2O, MBr.sub.4.xH.sub.2O, MBr.sub.5.xH.sub.2O, and
MBr.sub.6.xH.sub.2O, wherein x varies based on the nature of M;
[0190] iodides selected from MI, MI.sub.2, MI.sub.3, MI.sub.4,
MI.sub.5, and MI.sub.6; [0191] iodides hydrates selected from
MI.xH.sub.2O, MI.sub.2.xH.sub.2O, MI.sub.3.xH.sub.2O, MI.sub.4
.xH.sub.2O, MI.sub.5.xH.sub.2O, and MI.sub.6 .xH.sub.2O, wherein x
varies based on the nature of M; [0192] carboxylates (abbreviated
RCO.sub.2.sup.-, and including acetates) selected from MRCO.sub.2,
M(RCO.sub.2).sub.2, M(RCO.sub.2).sub.3, M(RC.sup.O.sub.2).sub.4,
M(RCO.sub.2).sub.5, .sup.and M(RCO.sub.2).sub.6; [0193]
carboxylates hydrates (abbreviated RCO.sub.2.sup.-) selected from
MRCO.sub.2.xH.sub.2O, M(RCO.sub.2).sub.2.xH.sub.2O,
M(RCO.sub.2).sub.3.xH.sub.2O, M(RCO.sub.2).sub.4.xH.sub.2O,
M(RCO.sub.2).sub.5.xH.sub.2O, and M(RCO.sub.2).sub.6.xH.sub.2O,
wherein x varies based on the nature of M; [0194] nitrates selected
from MNO.sub.3, M(NO.sub.3).sub.2, M(NO.sub.3).sub.3,
M(NO.sub.3).sub.4, M(NO.sub.3).sub.5, and M(NO.sub.3).sub.6; [0195]
nitrates hydrates selected from MNO.sub.3.xH.sub.2O,
M(NO.sub.3).sub.2.xH.sub.2O, M(NO.sub.3).sub.3.xH.sub.2O,
M(NO.sub.3).sub.4.xH.sub.2O, M(NO.sub.3).sub.5.xH.sub.2O, and
M(NO.sub.3).sub.6.xH.sub.2O, wherein x varies based on the nature
of M; [0196] nitrites selected from MNO.sub.2, M(NO.sub.2).sub.2,
M(NO.sub.2).sub.3, M(NO.sub.2).sub.4, M(NO.sub.2).sub.5, and
M(NO.sub.2).sub.6; [0197] nitrites hydrates selected from
MNO.sub.2.xH.sub.2O, M(NO.sub.2).sub.2.xH.sub.2O,
M(NO.sub.2).sub.3.xH.sub.2O, M(NO.sub.2).sub.4.xH.sub.2O,
M(NO.sub.2).sub.5.xH.sub.2O, and M(NO.sub.2).sub.6.xH.sub.2O,
wherein x varies based on the nature of M; [0198] cyanates selected
from MCN, M(CN).sub.2, M(CN).sub.3, M(CN).sub.4, M(CN).sub.5,
M(CN).sub.6; [0199] cyanates hydrates selected from MCN.xH.sub.2O,
M(CN).sub.2.xH.sub.2O, M(CN).sub.3.xH.sub.2O,
M(CN).sub.4.xH.sub.2O, M(CN).sub.5.xH.sub.2O, and
M(CN).sub.6.xH.sub.2O, wherein x varies based on the nature of M;
[0200] sulfides selected from M.sub.2S, MS, M.sub.2S.sub.3,
MS.sub.2, M.sub.2S.sub.2, MS.sub.3, M.sub.3S.sub.4, MS.sub.5, and
M.sub.2S.sub.7; [0201] sulfides hydrates selected from
M.sub.2S.xH.sub.2O, MS.xH.sub.2O, M.sub.2S.sub.3.xH.sub.2O,
MS.sub.2.xH.sub.2O, M.sub.2S.sub.2.xH.sub.2O, MS.sub.3.xH.sub.2O,
M.sub.3S.sub.4.xH.sub.2O, MS.sub.5.xH.sub.2O, and
M.sub.2S.sub.7.xH.sub.2O, wherein x varies based on the nature of
M; [0202] sulfites selected from M.sub.2SO.sub.3, MSO.sub.3,
M.sub.2(SO.sub.3).sub.3, M(SO.sub.3).sub.2,
M.sub.2(SO.sub.3).sub.2, M(SO.sub.3).sub.3,
M.sub.3(SO.sub.3).sub.4, M(SO.sub.3).sub.5, and
M.sub.2(SO.sub.3).sub.7; [0203] sulfites hydrates selected from
M.sub.2SO.sub.3.xH.sub.2O, MSO.sub.3.xH.sub.2O,
M.sub.2(SO.sub.3).sub.3.xH.sub.2O, M(SO.sub.3).sub.2.xH.sub.2O,
M.sub.2(SO.sub.3).sub.2.xH.sub.2O, M(SO.sub.3).sub.3.xH.sub.2O,
M.sub.3(SO.sub.3).sub.4.xH.sub.2O, M(SO.sub.3).sub.5.xH.sub.2O, and
M.sub.2(SO.sub.3).sub.7.xH.sub.2O, wherein x varies based on the
nature of M; [0204] hyposulfite selected from M.sub.2SO.sub.2,
MSO.sub.2, M.sub.2(SO.sub.2).sub.3, M(SO.sub.2).sub.2,
M.sub.2(SO.sub.2).sub.2, M(SO.sub.2).sub.3,
M.sub.3(SO.sub.2).sub.4, M(SO.sub.2).sub.5, and
M.sub.2(SO.sub.2).sub.7; [0205] hyposulfite hydrates selected from
M.sub.2SO.sub.2.xH.sub.2O, MSO.sub.2.xH.sub.2O,
M.sub.2(SO.sub.2).sub.3.xH.sub.2O, M(SO.sub.2).sub.2.xH.sub.2O,
M.sub.2(SO.sub.2).sub.2.xH.sub.2O, M(SO.sub.2).sub.3.xH.sub.2O,
M.sub.3(SO.sub.2).sub.4.xH.sub.2O, M(SO.sub.2).sub.5.xH.sub.2O, and
M.sub.2(SO.sub.2).sub.7.xH.sub.2O, wherein x varies based on the
nature of M; [0206] sulfate selected from M.sub.2SO.sub.3,
MSO.sub.3, M.sub.2(SO.sub.3).sub.3, M(SO.sub.3).sub.2,
M.sub.2(SO.sub.3).sub.2, M(SO.sub.3).sub.3,
M.sub.3(SO.sub.3).sub.4, M(SO.sub.3).sub.5, and
M.sub.2(SO.sub.3).sub.7; [0207] sulfate hydrates selected from
M.sub.2SO.sub.3.xH.sub.2O, MSO.sub.3.xH.sub.2O,
M.sub.2(SO.sub.3).sub.3.xH.sub.2O, M(SO.sub.3).sub.2.xH.sub.2O,
M.sub.2(SO.sub.3).sub.2.xH.sub.2O, M(SO.sub.3).sub.3.xH.sub.2O,
M.sub.3(SO.sub.3).sub.4.xH.sub.2O, M(SO.sub.3).sub.5.xH.sub.2O, and
M.sub.2(SO.sub.3).sub.7.xH.sub.2O, wherein x varies based on the
nature of M; [0208] thiosulfate selected from
M.sub.2S.sub.2O.sub.3, MS.sub.2O.sub.3,
M.sub.2(S.sub.2O.sub.3).sub.3, M(S.sub.2O.sub.3).sub.2,
M.sub.2(S.sub.2O.sub.3).sub.2, M(S.sub.2O.sub.3).sub.3,
M.sub.3(S.sub.2O.sub.3).sub.4, M(S.sub.2O.sub.3).sub.5, and
M.sub.2(S.sub.2O.sub.3).sub.7; [0209] thioulfate hydrates selected
from M.sub.2S.sub.2O.sub.3.xH.sub.2O, MS.sub.2O.sub.3.xH.sub.2O,
M.sub.2(S.sub.2O.sub.3).sub.3.xH.sub.2O,
M(S.sub.2O.sub.3).sub.2.xH.sub.2,
M.sub.2S.sub.2O.sub.3).sub.2.xH.sub.2O,
M(S.sub.2O.sub.3).sub.3.xH.sub.2O,
M.sub.3(S.sub.2O.sub.3).sub.4.xH.sub.2O,
M(S.sub.2O.sub.3).sub.5.xH.sub.2O, and
M.sub.2(S.sub.2O.sub.3).sub.7.xH.sub.2O, wherein x varies based on
the nature of M; [0210] dithionites selected from
M.sub.2S.sub.2O.sub.4, MS.sub.2O.sub.4,
M.sub.2(S.sub.2O.sub.4).sub.3, M(S.sub.2O.sub.4).sub.2,
M.sub.2(S.sub.2O.sub.4).sub.2, M(S.sub.2O.sub.4).sub.3,
M.sub.3(S.sub.2O.sub.4).sub.4, M(S.sub.2O.sub.4).sub.5, .sup.and
M.sub.2(S.sub.2O.sub.4).sub.7; [0211] dithionites hydrates selected
from M.sub.2S.sub.2O.sub.4.xH.sub.2O, MS.sub.2O.sub.4.xH.sub.2O,
M.sub.2(S.sub.2O.sub.4).sub.3.xH.sub.2O,
M(S.sub.2O.sub.4).sub.2.xH.sub.2O,
M.sub.2(S.sub.2O.sub.4).sub.2.xH.sub.2O,
M(S.sub.2O.sub.4).sub.3.xH.sub.2O,
M.sub.3(S.sub.2O.sub.4).sub.4.xH.sub.2O,
M(S.sub.2O.sub.4).sub.5.xH.sub.2O, and
M.sub.2(S.sub.2O.sub.4).sub.7.xH.sub.2O, wherein x varies based on
the nature of M; [0212] phosphates selected from M.sub.3PO.sub.4,
M.sub.3(PO.sub.4).sub.2, MPO.sub.4, and M.sub.4(PO.sub.4).sub.3;
[0213] phosphates hydrates selected from M.sub.3PO.sub.4.xH.sub.2O,
M.sub.3(PO.sub.4).sub.2.xH.sub.2O, MPO.sub.4.xH.sub.2O, and
M.sub.4(PO.sub.4).sub.3.xH.sub.2O, wherein x varies based on the
nature of M; [0214] carbonates selected from M.sub.2CO.sub.3,
MCO.sub.3, M.sub.2(CO.sub.3).sub.3, M(CO.sub.3).sub.2,
M.sub.2(CO.sub.3).sub.2, M(CO.sub.3).sub.3,
M.sub.3(CO.sub.3).sub.4, M(CO.sub.3).sub.5,
M.sub.2(CO.sub.3).sub.7; [0215] carbonate hydrates selected from
M.sub.2CO.sub.3.xH.sub.2O, MCO.sub.3.xH.sub.2O,
M.sub.2(CO.sub.3).sub.3.xH.sub.2O, M(CO.sub.3).sub.2.xH.sub.2O,
M.sub.2(CO.sub.3).sub.2.xH.sub.2O, M(CO.sub.3).sub.3.xH.sub.2O,
M.sub.3(CO.sub.3).sub.4.xH.sub.2O, M(CO.sub.3).sub.5.xH.sub.2O, and
M.sub.2(CO.sub.3).sub.7.xH.sub.2O, wherein x varies based on the
nature of M; [0216] hypochlorites/chlorites/chlorates/cerchlorates
(abbreviated ClO.sub.n.sup.-, n=1, 2, 3, 4) selected from
MClO.sub.n, M(ClO.sub.n).sub.2, M(ClO.sub.n).sub.3,
M(ClO.sub.n).sub.4, M(ClO.sub.n).sub.5, and M(ClO.sub.n).sub.6;
[0217] hypochlorites/chlorites/chlorates/perchlorates hydrates
selected from MClO.sub.n.xH.sub.2O, M(ClO.sub.n).sub.2.xH.sub.2O,
M(ClO.sub.n).sub.3.xH.sub.2O, M(ClO.sub.n).sub.4.xH.sub.2O,
M(ClO.sub.n).sub.5.xH.sub.2O, and M(ClO.sub.n).sub.6.xH.sub.2O,
wherein x varies based on the nature of M, and n=1, 2, 3, 4; [0218]
hypobromites/bromites/bromates/berbromates (abbreviated
BrO.sub.n.sup.-, n=1, 2, 3, 4) selected from MBrO.sub.n,
M(BrO.sub.n).sub.2, M(BrO.sub.n).sub.3, M(BrO.sub.n).sub.4,
M(BrO.sub.n).sub.5, and M(BrO.sub.n).sub.6; [0219]
hypobromites/bromites/bromates/perbromates hydrates selected from
MBrO.sub.n.xH.sub.2O, M(BrO.sub.n).sub.2.xH.sub.2O,
M(BrO.sub.n).sub.3.xH.sub.2O, M(BrO.sub.n).sub.4.xH.sub.2O,
M(BrO.sub.n).sub.5.xH.sub.2O, and M(BrO.sub.n).sub.6.xH.sub.2O,
wherein x varies based on the nature of M, and n=1, 2, 3, 4; [0220]
hypoiodites/iodites/iodates/periodates (abbreviated IO.sub.n.sup.-,
n=1, 2, 3, 4) selected from MIO.sub.n, M(IO.sub.n).sub.2,
M(IO.sub.n).sub.3, M(IO.sub.n).sub.4, M(IO.sub.n).sub.5, and
M(IO.sub.n).sub.6; [0221]
hypochlorites/chlorites/chlorates/perchlorates hydrates selected
from MIO.sub.n.xH.sub.2O, M(IO.sub.n).sub.2.xH.sub.2O,
M(IO.sub.n).sub.3.xH.sub.2O, M(IO.sub.n).sub.4.xH.sub.2O,
M(IO.sub.n).sub.5.xH.sub.2O, and M(IO.sub.n).sub.6.xH.sub.2O,
wherein x varies based on the nature of M, and n=1, 2, 3, 4; [0222]
Metal alkyls; [0223] Metal alkoxides; [0224] Metal amines; [0225]
Metal phosphines; [0226] Metal thiolates; [0227] Combined
cation-anion single source precursors, i.e., molecules that include
both cation and anion atoms, for example of the formula
M(E.sub.2CNR.sub.2).sub.2 (M.dbd.Zn, Cd, Pb, Ga, In, Hg, E.dbd.S,
P, Se, Te, O, As, and R=alkyl, amine alkyl, silyl alkyl, phosphoryl
alkyl, phosphyl alkyl).
[0228] In some embodiments, each of the SR systems of the invention
is coated, partially or wholly with a plurality of passivating
ligands. Exemplary passivating lignads are trioctylphosphine (TOP),
tributylphosphine (TBP), trioctylphosphine oxide (TOPO), dodecyl
amine (DDA), tetradecyl amine (TDA), hexadecyl amine (HDA),
octadecyl amine (ODA), oleic acid (OA), dodecylphosphonic acid
(DDPA), tridecylphosphonic acid (TDPA), octadecylphosphonic acid
(ODPA), hexylphosphonic acid (HPA) and thiols.
[0229] The tailoring of the SR systems of the invention provides
the opportunity to control the optical and electronic properties of
the SR systems (nanoparticles), as demonstrated by the
nanoparticles' enhanced fluorescence efficiency and polarized
emission. Furthermore, the SR systems enable the tailoring of the
energetic electronic levels, i.e., aligning the band offset of the
different elements in the system.
[0230] Thus, the SR systems of the invention may be characterized
by any energy band configuration (type I, type II and type III).
Generally, for application purposes, the SR systems of the
invention are tailored to adopt either a type I or a type II band
configuration. "Type I" refers to the band configuration of a
nanostructure, wherein the band offset of two adjacent materials in
the SR system (e.g., core and shell) is such that the energetic
positions of the conduction and valance band edges of one material
are within the conduction and valance band-edges of the other
adjacent material. "Type II" refers to the staggered band
configuration, wherein the energetic position of the conduction
band edge of one material lies between the conduction and valance
band edges of the other material, and the valance band edge of the
first material lies below that of the second material. Quasi
type-II band configuration are also possible, where either
conduction or valance band edge are similar in energy.
[0231] The SR systems of the invention may be utilized for a
variety of electronic and optical applications, such in the
communication industries as well as in other optical applications
such as fluorescence, lighting, displays, marking, biomedicine,
sensors, absorbing or lasing materials, etc.
[0232] Thus, the invention also provides a device incorporating at
least one SR system according to the present invention. In some
embodiments, the device is selected from a light conversion layer,
a transmitter, a laser, a Q-switch, a switch, an optical switch, an
optical fiber, a gain device, an amplifier, a display, a detector,
a communication system, a light emitting diode, a solar cell, and a
sensor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0233] In order to understand the invention and to see how it may
be carried out in practice, embodiments will now be described, by
way of non-limiting example only, with reference to the
accompanying drawings, in which:
[0234] FIGS. 1A-R depict 18 different SR systems in accordance with
the present invention: [0235] FIG. 1A depicts a spherical
core/shell structure embedded within an elongated structure,
wherein the core is of a first material, a spherical shell of a
second material, the core/shell structure positioned
non-concentrically within the elongated structure composed of a
third material. [0236] FIG. 1B depicts a spherical core/shell
structure embedded within an elongated structure, wherein the core
is of a first material, a spherical shell of a second material, the
core/shell structure positioned concentrically within the elongated
structure composed of a third material. [0237] FIG. 1C depicts a
spherical core/shell structure embedded within an elongated
structure, wherein the core is of a first material, a spherical
shell(1) of a second material, a spherical shell(2) of a third
material, the core/shell structure positioned non-concentrically
within the elongated structure composed of a material which is
different from the shell(2) material but may be the same as any of
the core and shell(1) materials. [0238] FIG. 1D depicts a spherical
core/shell structure embedded within an elongated structure,
wherein the core is of a first material, a spherical shell(1) of a
second material, a spherical shell(2) of a third material, the
core/shell structure positioned concentrically within the elongated
structure composed of a material which is different from the
shell(2) material but may be the same as any of the core and
shell(1) materials. [0239] FIG. 1E depicts an elongated core/shell
structure embedded within an elongated structure, wherein the
spherical core is of a first material, an elongated shell of a
second material, the core/shell structure positioned within the
elongated structure composed of a material which is of a third
material. As may be noted, the core is positioned
non-concentrically with respect of the shell and the shell is
positioned non-concentrically with respect of the elongated
structure. [0240] FIG. 1F depicts an elongated core/shell structure
embedded within an elongated structure, wherein the spherical core
is of a first material, an elongated shell of a second material,
the core/shell structure positioned within the elongated structure
composed of a material which is of a third material. As may be
noted, the core is positioned concentrically with respect of the
shell and the shell is positioned non-concentrically with respect
of the elongated structure. [0241] FIG. 1G depicts an elongated
core/shell structure embedded within an elongated structure,
wherein the spherical core is of a first material, an elongated
shell of a second material, the core/shell structure positioned
within the elongated structure composed of a material which is of a
third material. As may be noted, the core is positioned
non-concentrically with respect of the shell and the shell is
positioned concentrically with respect of the elongated structure.
[0242] FIG. 1H depicts an elongated core/shell structure embedded
within an elongated structure, wherein the spherical core is of a
first material, an elongated shell of a second material, the
core/shell structure positioned within the elongated structure
composed of a material which is of a third material. As may be
noted, the elongated core/shell structure is positioned
concentrically with respect of the elongated structure. [0243] FIG.
1I depicts an elongated core/shell structure embedded within an
elongated structure, wherein the spherical core is of a first
material, a spherical shell(1) is a second material, an elongated
shell of a further material, the core/shell structure positioned
within the elongated structure composed of a material which is of a
third material. As may be noted, the spherical core/shell(1) is
positioned non-concentrically with respect of the elongated shell
and the elongated shell is positioned non-concentrically with
respect of the elongated structure. [0244] FIG. 1J depicts an
elongated core/shell structure embedded within an elongated
structure, wherein the spherical core is of a first material, a
spherical shell(1) is a second material, an elongated shell of a
further material, the core/shell structure positioned within the
elongated structure composed of a material which is of a third
material. As may be noted, the spherical core/shell(1) is
positioned concentrically with respect of the elongated shell which
is positioned non-concentrically with respect of the elongated
structure. [0245] FIG. 1K depicts an elongated core/shell structure
embedded within an elongated structure, wherein the spherical core
is of a first material, a spherical shell(1) is a second material,
an elongated shell of a further material, the core/shell structure
positioned within the elongated structure composed of a material
which is of a third material. As may be noted, the spherical
core/shell(1) is positioned non-concentrically with respect of the
elongated shell and the elongated shell is positioned
concentrically with respect of the elongated structure. [0246] FIG.
1L depicts an elongated core/shell structure embedded within an
elongated structure, wherein the spherical core is of a first
material, a spherical shell(1) is a second material, an elongated
shell of a further material, the core/shell structure positioned
within the elongated structure composed of a material which is of a
third material. As may be noted, the spherical
core/shell(1)/elongated shell is positioned concentrically with
respect of the elongated structure. [0247] FIG. 1M depicts a
rod-in-rod seeded rod system according to the invention, wherein
the rod-element is positioned concentrically with respect of the
elongated structure. [0248] FIG. 1N depicts a rod-in-rod seeded rod
system according to the invention, wherein the rod-element is
positioned non-concentrically with respect of the elongated
structure. [0249] FIG. 1O depicts an elongated core/shell structure
embedded within an elongated structure, wherein the elongated core
is of a first material, the elongated shell of a second material,
the core/shell structure positioned within the elongated structure
composed of a material which is of a third material. As may be
noted, the elongated core is positioned concentrically with respect
to the elongated shell and the elongated core/shell structure is
positioned concentrically with respect of the elongated structure.
[0250] FIG. 1P depicts an elongated core/shell structure embedded
within an elongated structure, wherein the elongated core is of a
first material, the elongated shell of a second material, the
core/shell structure positioned within the elongated structure
composed of a material which is of a third material. As may be
noted, the elongated core is positioned non-concentrically with
respect of the elongated shell and the elongated core/shell
structure is positioned non-concentrically with respect of the
elongated structure. [0251] FIG. 1Q depicts an elongated core/shell
structure embedded within an elongated structure, wherein the
elongated core is of a first material, the elongated shell of a
second material, the core/shell structure positioned within the
elongated structure composed of a material which is of a third
material. As may be noted, the elongated core is positioned
non-concentrically with respect of the elongated shell and the
elongated shell structure is positioned concentrically with respect
of the elongated structure. [0252] FIG. 1R depicts an elongated
core/shell structure embedded within an elongated structure,
wherein the elongated core is of a first material, the elongated
shell of a second material, the core/shell structure positioned
within the elongated structure composed of a material which is of a
third material. As may be noted, the elongated core is positioned
concentrically with respect of the elongated structure but
non-concentrically with respect of the elongated shell.
[0253] FIG. 2 presents the emission spectra of InAs NCs, InAs/CdS
SRs and InAs/CdSe/CdS core/shell/rods. All spectra are normalized
with respect to the peak intensity of the InAs/CdSe/CdS SRs.
[0254] FIG. 3 depicts the valence and conduction band edges
relative positions in InAs/CdSe/CdS core/shell/rod SR system in
accordance with the invention.
[0255] FIGS. 4A-B are TEM images depicting the growth of
InAs/CdSe/CdS SRs during synthesis: [0256] FIG. 4A--After 2 min,
59.6 nm.times.5.5 nm rods are obtained, and [0257] FIG. 4B--HR-TEM
and FFT indicating the direction of the growth. Inset:
magnification of the FFT area.
[0258] FIG. 5 depicts the Powder XRD measurements of: (a) InAs/CdSe
core/shell, and (b) InAs/CdSe/CdS core/shell/rod particles. The
expected positions and intensities of the most intense XRD
reflections for bulk fcc zincblende (ZB) InAs, bulk fcc (ZB) CdSe,
bulk hexagonal (wurtzite) CdSe, and bulk hexagonal (wurtzite) CdS
are depicted in upper and lower panels. The XRD pattern of the
InAs/CdSe core/shell (a) matches well the highly overlapping
reflections of ZB InAs and ZB CdSe. No indication for a wurtzite
CdSe structure can be found in the reflections, indicating that the
CdSe shell obtains a fcc structure. Upon formation of the CdS rod,
peaks which match hexagonal bulk CdS become dominant (b).
[0259] FIGS. 6A-6B are: [0260] FIG. 6A depicts normalized emission
of InAs NCs (dashed line), InAs/CdSe core/shell (thin solid line),
InAs/CdSe/CdS SRs of same core/shell seeds and different rod shells
(large dash line, thick solid line). [0261] FIG. 6B depicts
stability measurements of NCs suspended in toluene. The particles
were excited by 532 nm 66 mW laser under ambient atmosphere. InAs
cores (large dash line) lose up to 70% of their fluorescence after
3 hours, while InAs/CdSe core/shell (thick solid line) lose up to
15% of their fluorescence after 20 hours and InAs/CdSe/CdS SRs were
stable for 20 hours and did not lose their fluorescence (small dash
line).
[0262] FIGS. 7A-D present parallel (solid line) and perpendicular
(dashed line) polarized emission spectra measurements of: [0263]
FIG. 7A--spherical InAs/CdSe core/shell NCs embedded in a polymer
film, stretched.times.3.5 along the z-axis. [0264] FIG. 7B--seeded
rods embedded in a non-stretched stretched polymer film. In both
[0265] FIG. 7A and FIG. 7B no polarization was detected. [0266]
FIG. 7C--seeded rods embedded in a polymer film stretched.times.3.5
along the z-axis showing polarization of 30%. [0267] FIG.
7D--seeded rods embedded in a polymer film stretched.times.5 along
the z axis, showing polarization of 47%. [0268] Insets illustrate
the arrangement of the particles in the film. Polarization was
calculated by
P=(I.sub..parallel.-I.sub..perp.)/(I.sub..parallel.+I.sub..perp.).
[0269] FIG. 8 presents the emission spectra of InAs/CdSe/ZnSe
(dashed line) and InAs/CdSe/ZnSe/CdS (solid line).
[0270] FIG. 9 is an illustration of band structure of
InP\ZnSe\ZnTe.sub.xS.sub.1-x.
[0271] FIG. 10 is an illustration of band structure of
ZnSe\ZnTe\ZnTe.sub.xS.sub.1-x.
[0272] FIG. 11 is an illustration of band structure of
InP\ZnTe\ZnS.
[0273] FIG. 12A-B are: [0274] FIG. 12A a TEM image of core CdSe
rods with length of 8 nm and width of 2.4 nm, and [0275] FIG. 12B a
TEM image of the obtained CdSe/CdS rod-in-rod SRs of length 45 nm
and width of 5 nm. Scale bar 20 nm.
[0276] FIG. 13 provides the Energy Dispersive X-ray Spectroscopy
(EDS line scan) of CdSe/CdS rod in a rod SR (CdSe core rod of 8
nm.times.2.4 nm and CdS outer rod of 45 nm.times.5 nm). X axis is
length in nm. The position of the CdSe inner core rod is identified
by the high concentration of Se which is apparent only between 15
nm and 25 nm (app. 5 nm from the position of the outer rod
boundary, at 10 nm). The outer CdS rod is depicted by the Cd and S
plateaus between 10 nm and 55 nm.
[0277] FIG. 14 depicts CdSe/CdS rod in a rod SRs (CdSe core rod of
8 nm.times.2.4 nm and CdS outer rod of 45 nm.times.5 nm) emission
(thin solid line), absorption (dashed line) and photoluminescence
emission (thick solid line).
[0278] FIG. 15 is an illustration of band diagram of CdSe/CdS
rod-in-rod particles.
[0279] FIGS. 16A-H are TEM images of CdSe/CdS rod-in-rod system
prepared from different CdSe seeds: [0280] FIG. 16A--6.5
nm.times.2.4 nm CdSe rods, [0281] FIG. 16B--rods (FIG. 16A)
embedded in 15 nm.times.4.5 nm CdS rod shells, [0282] FIG. 16C--9
nm.times.2.2 nm CdSe rods, [0283] FIG. 16D--rods (FIG. 16C)
embedded in 40 nm.times.3.8 nm CdS rod shell, [0284] FIG. 16E--20
nm.times.2.5 nm CdSe rods, [0285] FIG. 11F--rods (FIG. 11E)
embedded in 45 nm.times.5.2 nm CdS rod shells, [0286] FIG. 16G--40
nm.times.2.5 nm CdSe rods, and [0287] FIG. 16H--rods (FIG. 16G)
embedded in 60 nm.times.5.5 nm CdS rod shells. Scale bars are 20
nm. Insets show schematic diagrams of the particles.
[0288] FIGS. 17A-B are: [0289] FIG. 17A presents the elemental
analysis (EDS line scan, 0.5 nm step size, smoothed) along the
length of a single rod-in-rod (20 nm.times.2.5 nm CdSe rods in 45
nm.times.5.2 nm CdS rod). The particle's schematic diagram is
depicted at the bottom. [0290] FIG. 17B presents TEM images of Au
growth on the same rod-in-rod sample, showing the growth of gold
ellipsoids over the rods (scale bar is of 20 nm), as is depicted
schematically in FIG. 12A.
[0291] FIGS. 18A-C are: [0292] FIG. 18A--absorption and
photoluminescence spectra of 9 nm.times.2.2 nm CdSe rod embedded in
a 40 nm.times.3.8 nm CdS. [0293] FIG. 18B--absorption and
photoluminescence spectra of 20 nm.times.2.5 nm CdSe rod embedded
in a 45 nm.times.5.2 nm CdS. Insets shows core CdSe rod absorption
and photoluminescence. [0294] FIG. 18C illustrates the band
potential profile of CdSe@CdS rod-in-rod system (as generally
depicted in FIG. 15)
[0295] FIGS. 19A-D are: [0296] FIG. 19A--emission of CdSe/CdS
core/shell (core diameter 3.5 nm, shell thickness of 0.8 nm)
solution in toluene excited at their band edge using horizontally
polarized light, both parallel and perpendicular emission
components overlap, showing negligible emission polarization;
[0297] FIG. 19B--emission of CdSe/CdS rod-in-rod solution in
toluene excited at their band edge using horizontally polarized
light. The vertical emission component is shown in thin line and
horizontal component is shown thick line. The parallel and
perpendicular emission components do not overlap, resulting in a
significant emission polarization. [0298] FIG. 19C presents a
diagram indicating the lab and the rod coordinates system. The lab
coordinates: a--the excitation ray path, b--the collection path,
and c--the vertical polarization direction. The rod coordinates
system: z--the rod main axis and xy--the plane perpendicular to the
main axis. [0299] FIG. 19D--anisotropy of different core/shell
systems measured using the photoselection method excited at the
band-edge, at 530 nm, at 470 nm and 350 nm. Particles diagrams are
shown in relative aspect ratios.
[0300] FIGS. 20A-C are: [0301] FIG. 20A--photoselection anisotropy
map as a function of the single particle absorption and emission
polarizations. Four thick lines may be traced in the figure from
the top right-hand side corner. The 1.sup.st and 3.sup.rd thick
lines are the experimental anisotropy values obtained for
rod-in-rod 3 excited at the band edge (right) and at 355 nm (left),
and 2n.sup.d and 4.sup.th thick lines are the experimental
anisotropy values obtained for rod-in-rod 2 excited at the band
edge (right) and at 355 nm (left). Horizontal doted lines indicate
the polarization of the band edge emission under the assumption
that the absorption is completely polarized (p=1). [0302] FIG.
20B--polarization of the band-edge emission obtained from FIG. 20A
as a function of the core's aspect ratio. The X marks indicate
spherical core shell, circles indicate core/shell in rod samples,
and squares indicate rod-in-rod samples. Dashed line is a guide to
the eye, demonstrating the abrupt increase in polarization for an
AR of .about.3. [0303] FIG. 20C--polarization of the absorption
obtained from FIG. 20A as a function of the core AR for excitation
at 530 nm, 470 nm and 350 nm. Dashed lines show the average values
of the obtained polarization for each excitation wavelength.
[0304] FIG. 21 presents a depiction of a seeded rod system
according to an embodiment of the invention.
[0305] FIG. 22 presents the relative position of a seed structure
within an elongated structure system of the invention.
[0306] FIG. 23 depicts schematically a process for constructing
seeded rod systems of the invention.
DETAILED DESCRIPTION OF EMBODIMENTS
Core/Shell Seed Embedded in an Elongated Structure
Example A1: InAs/CdSe/CdS
[0307] The synthesis of InAs/CdS SR systems according to the
present invention was performed according to the seeded-growth
approach, by rapidly injecting a mixture of InAs NCs, grown
according to literature [36] and sulfur precursor into a hot
solution of cadmium precursor and two phosphonic acids in
tri-n-octylphosphine oxide.
[0308] As demonstrated in FIG. 2, in which the emission of InAs NCs
and of InAs/CdS SR is shown, the growth of the CdS shell directly
on top of the InAs seeds induced a large red shift (from 970 nm to
1500 nm) accompanied by a broadening of the emission peak and by a
significant reduction of its intensity. This reduction can be
attributed to formation of traps and intermediate states in the
InAs/CdS interface as a results of large lattice mismatch between
the two materials (6.058 .ANG. for InAs and 5.832 .ANG. for CdS fcc
structures). To overcome the lattice mismatch problem, an
intermediate layer was constructed on top of the core, forming a
core-shell structure, which acted as the seed for the rod growth
(FIG. 3). This additional intermediate layer provided a bridge for
lattice matching between the core and rod materials and also acted
as a passivation layer for dangling bonds of the InAs cores.
[0309] A buffer shell of CdSe was chosen for this task since the
lattice constant is close to that of InAs (CdSe=6.05 .ANG.). In
addition, the band gap of CdSe forms a potential barrier for both
electron and hole and it is expected to confine the exciton to the
InAs core forming a type I system (FIG. 3). The formation of the
buffer layer greatly increased the emission, as can be seen in FIG.
2.
[0310] The composition of the SR system was verified using energy
dispersive spectroscopy (EDS) analysis. Elemental ratios of
As:Se:S:Cd=1:1.6:36.5:46.2 were obtained, in good agreement with
the expected values for SRs with InAs seed's radius of 2.3 nm, a
two-monolayer thick CdSe shell and 5 nm.times.50 nm CdS rod.
[0311] FIGS. 4A and 4B present TEM images of the SRs with
core/shell particles used as seeds. High-Resolution TEM analysis
was performed on several SRs, as shown in FIG. 5 (line b). Fast
Fourier Transform (FFT) analysis of the images indicated that the
outer shell of the CdS had a hexagonal wurtzite structure. The
growth direction of the rods was along the c-axis of the hexagonal
structure, in agreement with XRD measurements, as can be seen in
FIG. 5, and with previous results obtained for other CdS SRs
systems [18-20]. The formation of a hexagonal rod was not trivial
when considering the fact that the InAs core has a cubic structure
[36,37], as epitaxial growth of hexagonal structures on top of
cubic structures tend to yield branched architectures such as
tetrapods [38,39,40].
[0312] Formation of CdSe shell on top of the InAs core causes
smearing of the absorption peaks accompanied by a red shift of the
excitonic peak. The growth of CdS rod on top of the core/shell
system significantly increases the absorption for wavelengths lower
than 500 nm, because of the onset of absorption into the CdS rod
transitions. In addition, upon the rod growth, the emission is red
shifted significantly. The effect of the rod thickness and shape
can be seen in FIG. 6A, where the emission of SRs with a diameter
of 5.2 nm and more matches-like is 1445 nm, while rods of 5.5 nm
diameter emit at 1600 nm. This effect can be attributed to changes
in the particles' dimensions and to other effects, such as lattice
strain.
[0313] Photo-stability measurements were performed by irradiation
of the NCs suspended in toluene using 66 mW 532 nm laser under
ambient atmosphere (FIG. 6B). The SRs exhibited a stable emission
for 20 hours, while InAs/CdSe core/shells show a 15% reduction in
the emission over the same period of time. The high stability of
both systems in comparison to InAs NCs can be ascribed to the good
passivation of the InAs core by the CdSe shell. However, the
existence of another shell in the SRs decreases the diffusion of
oxygen to the core, and thus reduces the arsenic oxidation in the
core and increases the stability.
[0314] Polarization measurements were performed on SRs embedded in
a stretched polymer film (FIGS. 7A-D). Previous works have shown
that type I SRs emit polarized light, in contrast to spherical
core/shell systems [9]. While InAs/CdSe core/shell dots (FIG. 7A)
and non-stretched disordered SRs film (FIG. 7B) did not show any
polarization, in stretched film the polarization goes up to 47%
(FIGS. 7C-D). Polarization measurements indicate that the
InAs/CdSe/CdS SRs emit polarized light, and that the polarization
is dictated by the spatial alignment of the rods within the polymer
matrix. The degree of the polarization (47%) is similar to that of
InAs nano rods of aspect ratio of approximately 1.5 as was shown in
recent theoretical works [41].
Example A2: InAs/CdSe/ZnSe/CdS
[0315] In this example, an additional shell of ZnSe was grown on
the InAs/CdSe core/shell, using a layer by layer (LBL) method.
Formation of the second shell was aimed at increasing the potential
barrier, thus leading to a decrease in the red shift of the
emission (from 1600 nm to 1400 nm) and enable further control of
the band gap energy. Controlling the shift is achieved by changing
the thickness of the different shells (FIG. 8).
Example A3: InP/ZnSe/CdS
[0316] Another kind of combination between II-VI and III-V
materials was achieved by using InP as core. The band gap of bulk
InP (1.34 eV) is higher than the InAs (0.35 eV), resulting a
shorter wavelength emission, and enabled to reach the visible-NIR
spectrum range (500-900). This range is of high interest for
lighting, displays and biological applications. InP/ZnSe quantum
dots were synthesized by a drop-wise addition of Zn and Se
precursors on the pre-synthesized InP cores. This was done either
by alternating injections of the Zn and the Se precursors (using
the layer-by-layer method) or by a continuous injection of a
mixture of both precursors. Another possible method for achieving
InP/ZnSe core/shell was by seeded growth of the ZnSe shell, in
which the InP cores were mixed with the Se precursor and injected
rapidly to a heated mixture of the Zn precursor and ligands (long
chained amines, phosphineoxides, etc.)
[0317] The core/shell seeds are injected to the growth solution of
the CdS rod as described above to form InP/ZnSe seeded CdS
nanorods.
Example A4: InP/ZnSe/ZnTe.sub.xS.sub.1-x
[0318] This example describes the synthesis of InP/ZnSe III-V/II-VI
core/shell spherical seed embedded in a ZnTe.sub.xS.sub.1-x rod
shaped shell nanoparticles. InP/ZnSe quantum dots were synthesized
by a drop-wise addition of Zn and Se precursors to the
pre-synthesized InP cores. This was done either by alternating
injections of the Zn and the Se precursors (using the
layer-by-layer method) or by a continuous injection of a mixture of
both precursors. Another possible method for achieving InP/ZnSe
core/shell was by seeded growth of the ZnSe shell, in which the InP
cores were mixed with the Se precursor and injected rapidly to a
heated mixture of the Zn precursor and ligands (long chained
amines, phosphoric acids, phosphineoxides, etc.) The resulting
InP/ZnSe nanodots were used as seeds for the growth of
ZnTe.sub.xS.sub.1-x rod shaped shell. This phase too was realized
by the same methods as described above. The anisotropic rod shaped
shell was achieved by choosing specific ligands, which support
anisotropic crystal growth. The Te and S ratios was such that the
resulting nanostructure is of type I (FIG. 9).
[0319] Similarly, alloyed shell layers may be used for further
control of the structural and optical properties.
Example A5: ZnSe/ZnTe/ZnTe.sub.xS.sub.1-x
[0320] This example describes the synthesis of ZnSe/ZnTe core/shell
spherical seed embedded in a ZnTe.sub.xS.sub.1-x rod shaped shell
nanoparticles. ZnSe/ZnTe quantum dots were synthesized by a
drop-wise addition of Zn and Te precursors to the pre-synthesized
ZnSe cores. This was done either by alternating injections of the
Zn and the Te precursors (using the LBL method) or by a continuous
injection of a mixture of both precursors. Another possible method
for achieving ZnSe/ZnTe core/shell was by seeded growth of the ZnTe
shell, in which the ZnSe cores were mixed with the Te precursor and
injected rapidly to a heated mixture of the Zn precursor and
ligands (long chained amines, phosphoric acids, phosphineoxides,
etc.). ZnSe/ZnTe is a type II system, which causes a significant
redshift of the emission peak. By doing so, the emission peak was
shifted into the visible range (band gap of .about.2.0-2.4 eV) from
the UV-blue emission of the bare ZnSe dots.
[0321] The resulting ZnSe/ZnTe nanodots were used as seeds for the
growth of ZnTe.sub.xS.sub.1-x rod shaped shell. This phase too was
realized by the same methods as described above. The anisotropic
rod shaped shell was achieved by choosing specific ligands, which
support anisotropic crystal growth. The Te and S ratios are such
that the charge carriers are confined to the ZnSe/ZnTe core (FIG.
10).
[0322] Similarly, alloyed shell layers may be used for further
control of the structural and optical properties.
[0323] A summary of various combinations of structures in
accordance with the invention is presented in Table 1.
TABLE-US-00001 TABLE 1 A summary of various combinations of
structures in accordance with the invention. Advantages Emission
(polarized Buffer Elongated Wavelength emission in all material
Core structure structure (nm) cases) InAs/CdSe/CdS InAs CdSe CdS
800-3300 III-V II-VI combination + NIR InAs/CdSe/ZnSe/CdS InAs/CdSe
ZnSe CdS 800-3300 III-V II-VI combination + NIR InP/ZnTe/ZnS InP
ZnTe ZnS 400-1080 III-V II-VI combination + NIR + Cd Free
InP/ZnSe/ZnTe InP ZnSe ZnTe 400-950 III-V II-VI combination + NIR +
Cd Free InP/ZnSe/CdS InP ZnSe CdS 400-950 III-V II-VI combination +
NIR InP/ZnSe/ZnS InP ZnSe ZnS 400-950 III-V II-VI combination + Vis
+ Cd Free InP/ZnSe/ZnTe/ZnS InP ZnSe ZnTe/ZnS 400-950 III-V II-VI
combination + Red emission + Cd Free ZnTe/ZnSe/ZnS ZnTe ZnSe ZnS
350-630 Red light emitting Cd free nanorods ZnSe/ZnTe/ZnS ZnSe ZnTe
ZnS 350-630 Blue-green light emitting Cd free nanorods
ZnSeTe/ZnTe/ZnS ZnSeTe ZnTe ZnS 350-630 Blue-green light emitting
Cd free nanorods CdSe/CdSSe/CdS CdSe CdSSe CdS 400-750 Visible
emission, high QY, stability CdSe/CdS/CdZnS CdSe CdS CdZnS 400-750
Visible emission, high QY, stability CdSe/CdZnSe/CdZnS CdSe CdZnSe
CdZnS 400-750 Visible emission, high QY, stability CdSe/CdZnS/ZnS
CdSe CdZnS ZnS 400-750 Visible emission, high QY, stability
Rod-In-Rod Seeded Rod Systems
Example B1: CdSe Rod in a CdS Rod
[0324] To benefit from the facile seeded growth approach, while
further increasing the 1D characteristic of the combined
heterostructures, the seeded rod-in-rod systems have been
developed. One such example is the CdSe/CdS heterostructure. The
innovative approach yields rods with high degree of linear
polarization and with high photoluminescence (PL) quantum
efficiencies. As demonstrated, there is now the ability to control
the optical properties, and in particular the polarization of these
structures, by tailoring the core rod length and diameter.
Moreover, a study of the excitation wavelength dependence was
performed, providing insight to the interplay between electric and
dielectric contributions to the polarization properties of NRs.
[0325] CdSe/CdS rod-in-rod SRs were synthesized by injecting, e.g.,
swiftly injecting, a mixture of CdSe nanorods, and sulfur precursor
into hot solution of cadmium precursor and two phosphonic acids in
tri-n-octyl phosphine. The resulting rod-in-rod particles exhibit
high emission quantum yields of up to 80% and improved polarization
with respect to equivalent "sphere in a rod" systems, which is of
the overall scale of nanorods.
[0326] Synthesis of CdSe/CdS rod-in-rod was done according to the
seeded growth method reported by Carbone et al. [23] In the first
step, CdSe rods of several different lengths and diameters were
synthesized to serve as seeds [34]. The CdSe rods were cleaned by
repetitive precipitation in a toluene/methanol mixture, mixed with
elemental sulfur and dissolved in 1.5 g of tri-octyl phosphine
(TOP). In the second seeded growth step, the seeds mixture was
swiftly injected into a flask containing tri-octyl phosphine oxide
(TOPO), hexyl phosphonic acid (HPA), octadecyl phosphonic acid
(ODPA) and CdO, heated under argon atmosphere to 360.degree. C. The
reaction was kept at this temperature for a few minutes, after
which the reaction flask was allowed to cool to room temperature,
followed by separation of the rod-in-rod products from the growth
solution.
[0327] FIG. 12A shows TEM images of typical CdSe seeds (length of 8
nm and diameter of 2.4 nm) which were used as seeds for the
synthesis of CdSe/CdS rods in rods shown in FIG. 12B (length of 45
nm and diameter of 5 nm). In this example, while the diameter grew
only by 2.6 nm after the outer rod growth, the length grew much
more significantly, by 37 nm. The position of the CdSe rod seed
with in the CdS rod in this sample was obtained from energy
dispersive X-ray line scan spectroscopy (FIG. 13), which indicates
the material composition of a single particle along its main axis.
The graph in FIG. 13 shows the relative amounts of cadmium, sulfur
and selenium along the particle (x axis shows length in nm). In
this sample, it can be seen that the inner CdSe rod, which can be
identified by the selenium peak located between 15 and 25 nm,
resides asymmetrically (non-concentrically) within the outer rod,
which can be identified by the cadmium and sulfur plateaus from 10
to 55 nm.
[0328] Upon growth of CdS rod shell on top of the CdSe rod, a red
shift accompanied by a large increase in the emission intensity of
the particles was observed. The red shift is attributed to the
decrease in the barrier for the wave functions in the core rod,
leading to leakage of the electron and hole wavefunction to the
outer shell which results in a decrease of the band gap. The
increase in quantum yield is attributed to passivation of surface
traps in the core by the CdS shell. FIG. 14 shows the absorption
(dashed line), emission (thin solid line) and photoluminescence
emission (thick solid line) of the CdSe/CdS rod-in-rod systems
described above. Both photoluminescence emission and absorption
show a distinctive peak at .about.600 nm, followed by weaker peaks
at .about.570 nm and at .about.520 nm which are all attributed to
electronic transitions in the CdSe inner rod (FIG. 15). These
features, which are rarely seen in sphere in a rod seeded rods due
to the low amounts of CdSe in the particles, are clearly seen in
CdSe/CdS rod-in-rod systems of the invention and are more
pronounced as the CdSe rods lengths and diameters increase. Both
absorption and PLE are significantly increased for wavelengths
lower than 480 nm, because of the onset of absorption into the CdS
rod shell transitions.
[0329] Quantum efficiency measurements preformed on the rod-in-rod
seeded rods have shown a high increase of the QY. In the sample
described above (CdSe 8.times.2.4 nm in CdS 45.times.5), QY
increased from .about.4% in bare CdSe rods seeds to 78% in CdSe/CdS
rod-in-rod SRs. Similar effects were seen also for longer inner
seed rods. For example, in CdSe 23.times.3 nm embedded in CdS
56.times.5 QY increased from .about.3% in bare CdSe rods seeds to
38% in CdSe/CdS rods in rods SRs). At least a one order of
magnitude increase in the QY was obtained for all synthesized
samples between the bare CdSe rod and the CdSe/CdS rod in rod, both
when exciting to the CdSe states (excitation at 519 nm) and when
exciting into the CdS states (at 470 nm).
[0330] FIGS. 16A-H show several examples of CdSe NRs which were
used as seeds for formation of CdSe/CdS rod-in-rod systems.
Comparing the images show thickening and elongation of the
core/shell rod-in-rod systems with respect to the initial CdSe
rods. For example, FIG. 16A shows TEM images of 6.5 nm.times.2.4 nm
CdSe rods before, and after (FIG. 16B) the growth of CdS rod-shell.
Upon shell growth the length of the rod increased to 15 nm and its
thickness increased to 4.5 nm. The core and shell dimensions of
several examples are summarized in Table 2.
TABLE-US-00002 TABLE 2 Various rod-in-rod systems (R@Rs) and
core/shell systems (S@Rs) dimensions and spectroscopic
charachteristics Modeled CdSe core CdS shell Anisotropy single
particle length width length width in PS PL Sample (nm) (nm) (nm)
(nm) .lamda.-PL (nm) QY measurement polarization R@R1 6.5 2.4 15
4.5 606 62% 0.2293 0.71 R@R2 9 2.2 40 3.8 597 76% 0.2865 0.82 R@R3
20 2.5 45 5.5 626 38% 0.2974 0.82 R@R4 40 2.5 60 5.2 632 36% 0.2867
0.82 S@R1 3.7 (diameter) 20 4.8 624 65% 0.2015 0.65 S@R2 3.9
(diameter) 65 5 627 42% 0.2291 0.71
[0331] In S@Rs the core is usually positioned asymmetrically inside
the rod, at around a quarter to a third of its length, because of
the difference in growth rates of the different facets [23]. To
determine the position of the CdSe rod within the shell,
compositional mapping by scanning TEM electron dispersive X-ray
spectroscopy (STEM-EDS) was performed on the R@Rs (FIG. 17A). The
Cd composition was relatively uniform along the entire length of
the nanorods, while the Se and S are concentrated at the middle and
at the edges of the rod, respectively. In addition, the Se
concentration was positioned asymmetrically along the rod, closer
to one edge, resembling the behavior of the S@R system. These
results, indicating the higher reactivity of the rods ends, are in
also in agreement with previously reported syntheses of Cd
chalcogenides columnar heterostructures, in which the rods ends
acted as nucleation centers that promote the continuous growth of
rod of other composition. However, in the seeded growth approach
there is also growth of the shell over the entire seed, as can be
deduced from the increase in thickness and from the significant
improvement in fluorescence quantum yield.
[0332] Further indication for the CdSe rod position was obtained by
applying selective gold growth over the R@Rs. Briefly, in this
procedure gold growth is performed via a low-temperature reduction
of AuCl.sub.3 dissolved with seeded nanorods, dodecyl-amine (DDA)
and dodecyl-dimethyl-ammonium bromide (DDAB) in toluene. As was
shown in previous works [42,19], in CdSe/CdS S@Rs with a thin
shell, a spherical gold dot is grown over the rod in proximity to
the CdSe seed, marking its position along the rod.
[0333] This behavior was attributed to the tendency of the CdSe
seed to act as a sink for electrons, thereby promoting Au growth in
that region [19]. In contrast to the spherical Au dots which are
formed in S@R systems, carrying out the procedure on R@Rs of 20 nm
resulted in the growth of gold prolate ellipsoids over the CdS
rods, elongated in the direction of the rod's main axis (FIG. 17B).
The position of the gold ellipsoids along the roads closely matches
the position of Se concentration along the rod obtained from the
EDS measurements.
[0334] The buildup of the CdS shell was also apparent when
comparing the absorbance and PL of the CdSe/CdS R@Rs to that of the
bare CdSe rods seeds. FIG. 18A shows the absorption and emission of
9 nm.times.2.2 nm bare CdSe rods (inset) and of the same rods
embedded in a 40 nm.times.3.8 nm CdS shell. Upon the growth of the
shell, the emission peak red shifts from 551 nm to 597 nm. The
shift was accompanied by large increase in quantum efficiency from
4.3% for the bare cores to a high value of 76% for the core/shell
R@Rs (both excited at 510 nm). A red shift of the excitonic peak
was also apparent in the absorption spectrum (from 535 nm to 592
nm). As in the case of S@R systems, the growth of the CdS shell was
accompanied by an increase of the absorption at wavelengths below
500 nm, because of the onset of transitions in the CdS rod (FIG.
18C). However, in R@R systems, the absorption features of the CdSe
are much more pronounced than in S@R systems, because of the
relatively large volume of the CdSe core rods compared to dots.
FIG. 18B shows the absorption and emission of 20 nm.times.2.5 nm
bare CdSe rods (inset) and of the same rods embedded in a 45
nm.times.5.2 nm CdS shell. As in the previous system, a red shift
is observed for both the emission peak (from 592 nm to 626 nm) and
the absorption excitonic peak (from 551 nm to 612 nm). Quantum
efficiency is increased from 2.3% for the bare cores to 38% for the
core/shell R@Rs (both excited at 510 nm). Comparing the absorption
spectra of both samples shows that the CdSe absorption features
become more apparent as the volume ratio between the core and the
shell decreases. In general, the quantum efficiencies of the
samples become lower (from .about.80 to .about.35%) as the length
of the core rod increases, consistent with the increased
interfacial region between the core and the shell, leading to
faster non-radiative decay rate, and with the reduced overlap
between the electron and hole wave functions, leading to a
decreased radiative decay rate.
[0335] One of the distinctive features of NRs is their linear
polarized emission which is strongly dependent on the aspect ratio
of the NRs [9,43,44,45]. In order to compare the degree of
polarization of different R@R and S@R systems, the excitation
photoselection (PS) method [46,47] was used, which does not depend
on external factors such as degree of arrangement or polymer
stretching, and only relies on the particles transition dipoles,
thus enabling the comparison between the polarizations of different
samples.
[0336] Within this method, an isotropic solution of randomly
oriented particles is excited with polarized light. The strength of
the dipole transition is proportional to , where is the dipole
moment and is the polarization of excitation field. As a
consequence, only particles whose absorption transition dipole has
a component parallel to the electric field vector of the excitation
are selectively excited, with a probability proportional to the
projection of the transition moments onto the light polarization
axis. The selective excitation results in a partially oriented
population of particles (photoselection) along the polarized
excitation light axis. The emitted light, which is polarized along
the emission transition dipole moment, is then collected and
separated to its components parallel (I.sub..parallel.) and
perpendicular (I.sub..perp.) to the polarization of the excitation.
The anisotropy of the sample, r, is calculated by Eq. 1:
r = I .parallel. - I .perp. I .parallel. + 2 I .perp. ( 1 )
##EQU00001##
[0337] while its polarization, p, is calculated by Eq. 2:
p = I .parallel. - I .perp. I .parallel. + I .perp. ( 2 )
##EQU00002##
[0338] For an isotropic solution of particles, the measured
anisotropy can range from r=0.4 for particles whose excitation
dipole moment is parallel to their emission dipole moment, to
r=-0.2 for particles whose excitation dipole moment is
perpendicular to their emission dipole moment. Particles which do
not have a defined excitation or emission dipole moments show no
anisotropy.
[0339] FIGS. 19A and 19B show the band-edge parallel and
perpendicular polarized emission components (normalized by the
parallel emission intensity) of CdSe/CdS core/shell and R@R
respectively, both excited near the band-edge. As a reference, we
chose the core/shell system (core diameter 3.5 nm, shell thickness
of 0.8 nm), whose polarization should be very low due to the
symmetric spherical shape of the particles [9]. As can be seen in
FIG. 19A, the two components are nearly similar, and the calculated
anisotropy is practically zero (.about.0.04). In comparison, for
the R@R2 system, a much higher difference between the polarization
components is observed, and anisotropy of 0.28 is obtained (FIG.
19B).
[0340] FIG. 19D summarizes the results obtained from photoselection
anisotropy measurements of several S@R and R@R systems with
different dimensions and ARs, excited using a vertically polarized
light at the band edge (purple), at 530 nm, at 470 nm and at 350
nm. The emission was always measured at the red side of the band
edge peak, and separated into the corrected vertical and horizontal
components, from which the anisotropy was obtained. In agreement to
previously published results showing polarized emission of single
particle [20] and of aligned ensembles of CdSe/CdS S@Rs [20,23], an
anisotropy of .about.0.2 was obtained for the S@R systems. In
general, the anisotropy obtained for R@R systems is 1.5 times
higher than the anisotropy obtained for S@Rs (with the exception of
the R@R1 system, whose anisotropy resembles that of S@Rs.
[0341] By analyzing the characteristics of the polarizations of the
absorption and emission for the single nanoparticle, it is possible
to perform a mapping from the single particle to the anisotropy of
a randomly distributed ensemble of particles in solution. Briefly,
for known absorption and emission polarization components of the
single particle in the particle's coordinate system, (shown in FIG.
19C), the anisotropy of the ensemble can be obtained by averaging
the projections of the single particle polarization in all possible
orientations on to the lab axes (shown in FIG. 19C). A full
description of the method for molecules appears elsewhere [47].
[0342] In rod shaped particles, the polarization components of both
absorption and emission are dictated by the cylindrical symmetry,
resulting in a z-component along the rod, and equal x and y
components which display a planar polarization perpendicular to the
rod. FIG. 20A shows a contour map of anisotropy obtained from
photoselection as function of the single particle polarization of
the absorption (horizontal axis) and of the emission (vertical
axis) under assumption of equal weights of x and y components. For
particles whose x, y and z components are similar (p=0) in both
absorption and emission, such as completely spherical quantum dots,
the obtained anisotropy is 0, while for a perfectly polarized
particle (p=1 in both absorption and emission) the obtained
anisotropy is 0.4. The bold lines on the contour describe the
obtained anisotropies for the R@R3 system excited at the band edge
and at 355 nm, and for S@R2 systems excited at the band edge and at
355 nm. For clarity only the results for these two samples are
shown in the graph, as representatives of the other samples.
[0343] For band edge excitation and emission, further
characterization of the polarization can be obtained by plausible
assumptions. As was shown theoretically for NRs [48,44], and also
experimentally for both NRs [9] and S@Rs [25], the lowest band-edge
state of the core in these systems has a strong polarization along
the main axis of the rod due to a predominant p.sub.z character of
the band-edge hole state. In the excitation, the electronic state
polarization is accompanied by an additional dielectric effect,
which strongly reduces the field polarized perpendicular to the
rod's main axis while hardly affecting the field parallel to the
main axis [49]. The combination of these two factors leads us to
the plausible assumption that the band-edge excitation is almost
completely polarized along the z-axis. The band edge emission is
expected to be also highly polarized along the z axis, but not to
the same extent as the absorption, because even though the
electronic polarization effect still plays an important role, there
is thermal occupation of higher emission states, which are not
polarized along the rod, and a much smaller dielectric effect.
[0344] Under these assumptions, for the R@R system, an emission
polarization of 0.82 (calculated according to Eq. 2) is obtained,
in agreement with measured values for single CdSe rods (0.85) [9]
and with theoretical calculations (0.86) [48,44]. Under the same
assumptions, for S@Rs, an emission polarization of 0.71 is
obtained, again slightly lower but still in good agreement with
results obtained in single particle polarization measurements
(0.75) [25]. The comparison to the single particle measurements
indeed supports the assumption of a highly polarized absorption in
these systems. The obtained emission polarization as a function of
the core aspect ratio for the different systems is depicted in FIG.
20B.
[0345] The existence of polarization in S@R systems, albeit their
emission emanates from the sphere core states, is attributed to the
crystal field effect and to the cylindrical symmetry exerted by the
CdS rod-shell on the electron and hole wave functions in the core
[20]. As mentioned before, sample R@R1, which has a low aspect
ratio shows a polarization similar to the S@R systems. This is in
agreement with the results obtained for CdSe NRs, where the steep
increase in polarization is obtained only at a specific AR [9], yet
in the seeded rods system, polarization is already obtained due to
the rod shell, thus the increase in polarization for samples with
larger AR (indicated in FIG. 20B by dashed line) is much smaller
than that seen in the transition from CdSe dots to rods, but it is
still noticeable.
[0346] The assumption that the excitation is completely polarized
along the z-axis does not hold for excitation above the band edge,
because in this case the number of possible electronic transitions
increases rapidly, and the electronic contribution to the
polarization of the absorption decreases. Yet, the emission still
occurs from the same band-edge states, and thus is expected to have
the same polarization obtained under the band-edge excitation.
Therefore, it is possible to assess also the polarization of the
absorption at shorter wavelengths. Interestingly, even for
absorption at short wavelengths such as 355 nm, where no
polarization is expected to be induced from the electronic states,
a noticeable anisotropy is obtained. This polarization is mainly
attributed to the dielectric effect, as was previously reported for
nanowires [49]. FIG. 20C presents the obtained absorption
polarizations for excitation at 530 nm, 470 nm, and 350 nm. In
general, as the excitation wavelength decreases, the excitation
polarization also decreases, in consistence with the decrease in
the electronic polarization contribution. However, the excitation
polarizations for all seeded rod samples have almost the same
value. For excitation at 530 nm and at 470 nm, the polarization
obtained is relatively high (0.70-0.75). However, for excitation at
350 nm, lower polarization is obtained (0.63). The higher
polarizations obtained at 530 nm and 470 nm can be attributed to
the fact that these excitations already include transitions to CdS
rod shell band-edge states, which are common to all seeded rods
samples, while the excitation at 350 nm already involves
transitions to much higher CdS states. Due to the rod structure,
the CdS band-edge states should also exhibit high polarization
along the main axis of the rod, and thus the transitions involving
these states exhibit relatively high polarization. However, for the
excitation at 350 nm, the electronic effect becomes negligible, and
the polarization obtained is mostly a result of the dielectric
effect. The polarization value obtained (.about.0.6) is in very
good agreement with theoretical values obtained for CdS rods (see
supporting information for further details). The ability to retain
the polarization even at lower excitation wavelength is unique to
such NR systems, and can be used for a wide range of applications
including bio-labeling and displays.
[0347] Seeded R@R particles combine the ease of synthesis, strong
emission quantum yields and good surface passivation which are
associated with seeded growth particles, along with properties
which are associated with NRs, including large absorbance
cross-sections and high linear polarization. The ability to tune
their polarization and emission wavelength by tuning the dimensions
of the rod seed make R@Rs interesting for a variety of optical and
optoelectronic usages and applications, and provide an example for
the ability to design and control the properties of nanostructures
through colloidal synthesis.
[0348] In order to compare the polarization of spheres in rods and
rod-in-rod samples, measurements were performed using the
photo-selection method. Within this method, a sample of SRs is
dissolved in hexane, and then excited using light with vertical
polarization. The emitted light from the particles is collected and
separated to its vertical and horizontal polarization components.
If the particles have a distinctive linear transition dipole
moment, particles oriented along the excitation polarization have
higher probability to get excited, and if the excitation
probability of different particles is similar, by analyzing their
emission it is possible to compare their polarization. Rod in a rod
exhibit absorption polarization parallel to the rod's main axis in
short wavelength (below 480 nm) due to their dielectric
confinement. Comparing their emission when excited at 470 nm and
355 nm shows that rods in rods exhibit higher emission polarization
than sphere in rods. This effect is even more pronounced when the
systems are excited to their band-edge, where rods in rods have
higher absorption polarization due to their electronic structure
which is induced by the rod shape. In the band edge, the emission
polarization of the rods in rods is .about.1.5 times higher than
that of spheres in rods, and it increases as the inner rod length
increase.
Example B2: InP Core Within ZnTe Rod used as Seed for ZnS Rod
Shell
[0349] Pre-synthesized InP cores were used as seeds for ZnTe rod
shaped shell. The resulting InP/ZnTe type II seeded rods are used
as seeds for a ZnS rod shaped shell for receiving rod in rod
particles (FIG. 11). ZnTe rod shaped shell is grown on top of the
pre-synthesized InP cores through the seeded growth method, in
which the InP cores are mixed with the Te precursor and injected
rapidly to a heated mixture of the Zn precursor and ligands (long
chained amines, phosphoric acids, phosphineoxides, etc.).
[0350] The ZnS closing shell is produced either by alternating
injections of the Zn and the S precursors (SILAR) or by a
continuous injection of a mixture of both precursors. Another
possible method seeded growth in which the ZnSe/ZnTe cores are
mixed with the S precursor and injected rapidly to a heated mixture
of the Zn precursor and ligands (long chained amines, phosphoric
acids, phosphineoxides, etc.).
* * * * *