U.S. patent application number 12/959749 was filed with the patent office on 2011-05-12 for nanoparticles.
Invention is credited to Steven Daniels, Paul O'Brien, Nigel Pickett.
Application Number | 20110108799 12/959749 |
Document ID | / |
Family ID | 35098225 |
Filed Date | 2011-05-12 |
United States Patent
Application |
20110108799 |
Kind Code |
A1 |
Pickett; Nigel ; et
al. |
May 12, 2011 |
NANOPARTICLES
Abstract
Method for producing a nanoparticle comprised of core, first
shell and second shell semiconductor materials. Effecting
conversion of a core precursor composition comprising separate
first and second precursor species to the core material and then
depositing said first and second shells. The conversion is effected
in the presence of a molecular cluster compound under conditions
permitting seeding and growth of the nanoparticle core.
Core/multishell nanoparticles in which at least two of the core,
first shell and second shell materials incorporate ions from groups
12 and 15, 14 and 16, or 11, 13 and 16 of the periodic table.
Core/multishell nanoparticles in which the second shell material
incorporates at least two different group 12 ions and group 16
ions. Core/multishell nanoparticles in which at least one of the
core, first and second semiconductor materials incorporates group
11, 13 and 16 ions and the other semiconductor material does not
incorporate group 11, 13 and 16 ions.
Inventors: |
Pickett; Nigel; (East
Croyden, GB) ; Daniels; Steven; (Manchester, GB)
; O'Brien; Paul; (High Peak, GB) |
Family ID: |
35098225 |
Appl. No.: |
12/959749 |
Filed: |
December 3, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11997973 |
Feb 5, 2008 |
7867557 |
|
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PCT/GB2006/003028 |
Aug 14, 2006 |
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12959749 |
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Current U.S.
Class: |
257/14 ;
257/E21.09; 257/E29.168; 438/478 |
Current CPC
Class: |
C01P 2002/84 20130101;
Y10T 428/2991 20150115; Y10T 428/2995 20150115; Y10T 428/2996
20150115; B82Y 30/00 20130101; C01B 19/007 20130101; Y10T 428/2982
20150115; C01P 2004/64 20130101; C01G 9/08 20130101; C09K 11/565
20130101; C30B 29/605 20130101; C09K 11/025 20130101; C30B 7/005
20130101; C09K 11/02 20130101; H01L 31/0296 20130101; Y10T 428/2993
20150115; C01P 2004/84 20130101; C09K 11/883 20130101; C30B 7/00
20130101 |
Class at
Publication: |
257/14 ; 438/478;
257/E29.168; 257/E21.09 |
International
Class: |
H01L 29/66 20060101
H01L029/66; H01L 21/20 20060101 H01L021/20 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 12, 2005 |
GB |
GB 0516598 |
Claims
1. A nanoparticle comprising a core comprising a core semiconductor
material, a first layer comprising a first semiconductor material
provided on said core and a second layer comprising a second
semiconductor material provided on said first layer, said core
semiconductor material being different to said first semiconductor
material and said first semiconductor material being different to
said second semiconductor material, wherein: a) at least two of the
core, first shell and second shell materials incorporate ions from
groups 12 and 15 of the periodic table, groups 14 and 16 of the
periodic table, or groups 11, 13 and 16 of the periodic table; or
b) the second shell material incorporates ions of at least two
different elements from group 12 of the periodic table and ions
from group 16 of the periodic table; or c) at least one of the
core, first and second semiconductor materials incorporates ions
from groups 11, 13 and 16 of the periodic table and at least one
other of the core, first and second semiconductor materials is a
semiconductor material not incorporating ions from groups 11, 13
and 16 of the periodic table.
2. A nanoparticle according to claim 1, wherein: at least two of
the core, first shell and second shell materials incorporate ions
from groups 12 and 15 of the periodic table, groups 14 and 16 of
the periodic table, or groups 11, 13 and 16 of the periodic table;
and the other of the core, first and second semiconductor materials
incorporates ions from the group consisting of: groups 12 and 15 of
the periodic table, groups 13 and 15 of the periodic table, groups
12 and 16 of the periodic table, groups 14 and 16 of the periodic
table, and groups 11, 13 and 16 of the periodic table.
3. A nanoparticle according to claim 1, wherein: the second shell
material incorporates ions of at least two different elements from
group 12 of the periodic table and ions from group 16 of the
periodic table; and said second semiconductor material has the
formula M.sub.xN.sub.1-xE, where M and N are the group 12 ions, E
is the group 16 ion, and 0<x<1.
4. A nanoparticle according to claim 3, wherein
0.1<x<0.9.
5. A nanoparticle according to claim 1, wherein: at least one of
the core, first and second semiconductor materials incorporates
ions from groups 11, 13 and 16 of the periodic table and at least
one other of the core, first and second semiconductor materials is
a semiconductor material not incorporating ions from groups 11, 13
and 16 of the periodic table; and said at least one other of the
core, first and second semiconductor materials not incorporating
ions from groups 11, 13 and 16 of the periodic table incorporates
ions from the group consisting of: groups 12 and 15 of the periodic
table, groups 13 and 15 of the periodic table, groups 12 and 16 of
the periodic table, and groups 14 and 16 of the periodic table.
6. A nanoparticle according to claim 1, wherein the nanoparticle
further comprises a third layer of a third semiconductor material
provided on said second layer.
7. A nanoparticle according to claim 6, wherein said third
semiconductor material is selected from the group consisting of: a
semiconductor material incorporating ions from groups 12 and 15 of
the periodic table, a semiconductor material incorporating ions
from groups 13 and 15 of the periodic table, a semiconductor
material incorporating ions from groups 12 and 16 of the periodic
table, a semiconductor material incorporating ions from groups 14
and 16 of the periodic table and a semiconductor material
incorporating ions from groups 11, 13 and 16 of the periodic
table.
8. A nanoparticle according to claim 1, wherein the group 12 ions
are selected from the group consisting of: zinc ions, cadmium ions,
and mercury ions.
9. A nanoparticle according to claim 1, wherein the group 15 ions
are selected from the group consisting of: nitride ions, phosphide
ions, arsenide ions, and antimonide ions.
10. A nanoparticle according to claim 1, wherein the group 14 ions
are selected from the group consisting of: lead ions, tin ions, and
germanium ions.
11. A nanoparticle according to claim 1, wherein the group 16 ions
are selected from the group consisting of: sulfide ions, selenide
ions, and telluride ions.
12. A nanoparticle according to claim 1, wherein the group 11 ions
are selected from the group consisting of: copper ions, silver
ions, and gold ions.
13. A nanoparticle according to claim 1, wherein the group 13 ions
are selected from the group consisting of: aluminium ions, indium
ions, and gallium ions.
14. A method for producing a nanoparticle comprising a core
comprising a core semiconductor material, a first layer comprising
a first semiconductor material provided on said core and a second
layer comprising a second semiconductor material provided on said
first layer, said core semiconductor material being different to
said first semiconductor material and said first semiconductor
material being different to said second semiconductor material,
wherein: a) at least two of the core, first shell and second shell
materials incorporate ions from groups 12 and 15 of the periodic
table, groups 14 and 16 of the periodic table, or groups 11, 13 and
16 of the periodic table; or b) the second shell material
incorporates ions of at least two different elements from group 12
of the periodic table and ions from group 16 of the periodic table;
or c) at least one of the core, first and second semiconductor
materials incorporates ions from groups 11, 13 and 16 of the
periodic table and at least one other of the core, first and second
semiconductor materials is a semiconductor material not
incorporating ions from groups 11, 13 and 16 of the periodic table,
the method comprising effecting conversion of a nanoparticle core
precursor composition to the material of the nanoparticle core,
depositing said first layer on said core and depositing said second
layer on said first layer.
15. A method according to claim 14, wherein said nanoparticle core
precursor composition comprises first and second core precursor
species containing the ions to be incorporated into the growing
nanoparticle core.
16. A method according to claim 15, wherein said first and second
core precursor species are separate entities contained in said core
precursor composition, and said conversion is effected in the
presence of a molecular cluster compound under conditions
permitting seeding and growth of the nanoparticle core.
17. A method according to claim 15, wherein said first and second
core precursor species are combined in a single entity contained in
said core precursor composition.
18. A method according to claim 14, wherein conversion of the core
precursor composition to the nanoparticle core is effected in a
reaction medium and said nanoparticle core is isolated from said
reaction medium prior to deposition of the first layer.
19. A method according to claim 14, wherein deposition of said
first layer comprises effecting conversion of a first semiconductor
material precursor composition to said first semiconductor
material.
20. A method according to claim 19, wherein said first
semiconductor material precursor composition comprises third and
fourth precursor species containing the ions to be incorporated
into the growing first layer of the nanoparticle.
21. A method according to claim 20, wherein said third and fourth
precursor species are separate entities contained in said first
semiconductor material precursor composition.
22. A method according to claim 20, wherein said third and fourth
precursor species are combined in a single entity contained in said
first semiconductor material precursor composition.
23. A method according to claim 14, wherein deposition of said
second layer comprises effecting conversion of a second
semiconductor material precursor composition to said second
semiconductor material.
24. A method according to claim 23, wherein said second
semiconductor material precursor composition comprises fifth and
sixth precursor species containing the ions to be incorporated into
the growing second layer of the nanoparticle.
25. A method according to claim 24, wherein said fifth and sixth
precursor species are separate entities contained in said second
semiconductor material precursor composition.
26. A method according to claim 24, wherein said fifth and sixth
precursor species are combined in a single entity contained in said
second semiconductor material precursor composition.
27. A nanoparticle comprising a core comprising a core
semiconductor material, a first layer comprising a first
semiconductor material provided on said core and a second layer
comprising a second semiconductor material provided on said first
layer, said core semiconductor material being different to said
first semiconductor material and said first semiconductor material
being different to said second semiconductor material, said
nanoparticle produced according to a method comprising: effecting
conversion of a nanoparticle core precursor composition to the
material of the nanoparticle core, depositing said first layer on
said core and depositing said second layer on said first layer,
said core precursor composition comprising a first precursor
species containing a first ion to be incorporated into the growing
nanoparticle core and a separate second precursor species
containing a second ion to be incorporated into the growing
nanoparticle core, said conversion being effected in the presence
of a molecular cluster compound under conditions permitting seeding
and growth of the nanoparticle core.
28. A nanoparticle according to claim 27, wherein: a. at least two
of the core, first shell and second shell materials incorporate
ions from groups 12 and 15 of the periodic table, groups 14 and 16
of the periodic table, or groups 11, 13 and 16 of the periodic
table; or b. the second shell material incorporates ions of at
least two different elements from group 12 of the periodic table
and ions from group 16 of the periodic table; or c. at least one of
the core, first and second semiconductor materials incorporates
ions from groups 11, 13 and 16 of the periodic table and at least
one other of the core, first and second semiconductor materials is
a semiconductor material not incorporating ions from groups 11, 13
and 16 of the periodic table.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/997,973, filed on Feb. 5, 2008, which is
the U.S. national stage application of International (PCT) Patent
Application Serial No. PCT/GB2006/003028, filed Aug. 14, 2006,
which claims the benefit of GB Application No. 0516598.0, filed
Aug. 12, 2005. The entire disclosures of these applications are
hereby incorporated by reference as if set forth at length herein
in their entirety.
BACKGROUND
[0002] There has been substantial interest in the preparation and
characterisation of compound semiconductors comprising of particles
with dimensions in the order of 2-100 nm, often referred to as
quantum dots and nanocrystals mainly because of their optical,
electronic or chemical properties. These interests have occurred
mainly due to their size-tuneable electronic, optical and chemical
properties and the need for the further miniaturization of both
optical and electronic devices that now range from commercial
applications as diverse as biological labelling, solar cells,
catalysis, biological imaging, light-emitting diodes amongst many
new and emerging applications.
[0003] Although some earlier examples appear in the literature,
recently methods have been developed from reproducible "bottom up"
techniques, whereby particles are prepared atom-by-atom, i.e. from
molecules to clusters to particles using "wet" chemical procedures.
Rather from "top down" techniques involving the milling of solids
to finer and finer powders.
[0004] To-date the most studied and prepared of nano-semiconductor
materials have been the chalcogenides II-VI materials namely ZnS,
ZnSe, CdS, CdSe, CdTe; most noticeably CdSe due to its tuneability
over the visible region of the spectrum. Semiconductor
nanoparticles are of academic and commercial interest due to their
differing and unique properties from those of the same material,
but in the macro crystalline bulk form. Two fundamental factors,
both related to the size of the individual nanoparticle, are
responsible for these unique properties.
[0005] The first is the large surface to volume ratio; as a
particle becomes smaller, the ratio of the number of surface atoms
to those in the interior increases. This leads to the surface
properties playing an important role in the overall properties of
the material.
[0006] The second factor is that, with semiconductor nanoparticles,
there is a change in the electronic properties of the material with
size, moreover, the band gap gradually becoming larger because of
quantum confinement effects as the size of the particles decreases.
This effect is a consequence of the confinement of an `electron in
a box` giving rise to discrete energy levels similar to those
observed in atoms and molecules, rather than a continuous band as
in the corresponding bulk semiconductor material. For a
semiconductor nanoparticle, because of the physical parameters, the
"electron and hole", produced by the absorption of electromagnetic
radiation, a photon, with energy greater then the first excitonic
transition, are closer together than in the corresponding
macrocrystalline material, so that the Coulombic interaction cannot
be neglected. This leads to a narrow bandwidth emission, which is
dependent upon the particle size and composition. Thus, quantum
dots have higher kinetic energy than the corresponding
macrocrystalline material and consequently the first excitonic
transition (band gap) increases in energy with decreasing particle
diameter.
[0007] The coordination about the final inorganic surface atoms in
any core, core-shell or core-multi shell nanoparticles is
incomplete, with highly reactive "dangling bonds" on the surface,
which can lead to particle agglomeration. This problem is overcome
by passivating (capping) the "bare" surface atoms with protecting
organic groups. The capping or passivating of particles not only
prevents particle agglomeration from occurring, it also protects
the particle from its surrounding chemical environment, along with
providing electronic stabilization (passivation) to the particles
in the case of core material.
[0008] The capping agent usually takes the form of a Lewis base
compound covalently bound to surface metal atoms of the outer most
inorganic layer of the particle, but more recently, so as to
incorporate the particle into a composite, an organic system or
biological system can take the form of, an organic polymer forming
a sheaf around the particle with chemical functional groups for
further chemical synthesis, or an organic group bonded directly to
the surface of the particle with chemical functional groups for
further chemical synthesis.
[0009] Single core nanoparticles, which consist of a single
semiconductor material along with an outer organic passivating
layer, tend to have relatively low quantum efficiencies due to
electron-hole recombination occurring at defects and dangling bonds
situated on the nanoparticle surface which lead to non-radiative
electron-hole recombinations.
[0010] One method to eliminate defects and dangling bonds is to
grow a second material, having a wider band-gap and small lattice
mismatch with the core material, epitaxially on the surface of the
core particle, (e.g. another II-VI material) to produce a
"core-shell particle". Core-shell particles separate any carriers
confined in the core from surface states that would otherwise act
as non-radiative recombination centres. One example is ZnS grown on
the surface of CdSe cores. The shell is generally a material with a
wider bandgap then the core material and with little lattice
mismatch to that of the core material, so that the interface
between the two materials has as little lattice strain as possible.
Excessive strain can further result in defects and non-radiative
electron-hole recombination resulting in low quantum
efficiencies.
Quantum Dot-Quantum Wells
[0011] Another approach which can further enhance the efficiencies
of semiconductor nanoparticles is to prepare a core-multi shell
structure where the "electron-hole" pair are completely confined to
a single shell such as a quantum dot-quantum well structure. Here,
the core is of a wide bandgap material, followed by a thin shell of
narrower bandgap material, and capped with a further wide bandgap
layer, such as CdS/HgS/CdS grown using a substitution of Hg for Cd
on the surface of the core nanocrystal to deposit just a few
monolayer of HgS. The resulting structures exhibited clear
confinement of photoexcited carriers in the HgS. Other known
Quantum dot quantum well (QDQW) structures include--ZnS/CdSe/ZnS,
CdS/CdSe/CdS and ZnS/CdS/ZnS. Colloidally grown QD-QW nanoparticles
are relatively new. The first and hence most studied systems were
of CdS/HgS/CdS grown by the substitution of cadmium for mercury on
the core surface to deposit one monolayer of HgS. A wet chemical
synthetic method for the preparation of spherical CdS/HgS/CdS
quantum wells was presented with a study of their unique optical
properties. The CdS/HgS/CdS particles emitted a red band-edge
emission originating from the HgS layer. Little et al. have grown
ZnS/CdS/ZnS QDQWs using a similar growth technique to that of
Eychmuller to show that these structure can be made despite the
large lattice mismatch (12%) between the two materials, ZnS and
CdS. Daniels et al produced a series of structures that include
ZnS/CdSe/ZnS, ZnS/CdS/CdSe/ZnS, ZnS/CdSe/CdS/ZnS,
ZnS/CdS/CdSe/CdS/ZnS. The aim of this work was to grow strained
nanocrystalline heterostructures and to correlate their optical
properties with modelling that suggested that there is relocation
of the carriers (hole/electron) from confinement in the ZnS core to
the CdSe shell. CdS/CdSe/CdS QDQW's, have also been produced by
Peng et al. although this structure is promising, the small CdS
band gap may not be sufficient to prevent the escape of electrons
to the surface.
[0012] Although there are now a number of methods for preparing
core-shell quantum dots, where it has been shown and reported for
the reaction solutions containing the quantum dots, core-shell
quantum dots can have quantum yields as high as 90%. However, it is
well known that once one tries to manipulate the freshly made
solutions of core-shell quantum dots such as isolating the
particles as dry powders, upon re-dissolving the particles quantum
yields can be substantially lower (sometimes as low as 1-5%).
[0013] According to a first aspect of the present invention there
is provided a method for producing a nanoparticle comprised of a
core comprising a core semiconductor material, a first layer
comprising a first semiconductor material provided on said core and
a second layer comprising a second semiconductor material provided
on said first layer, said core semiconductor material being
different to said first semiconductor material and said first
semiconductor material being different to said second semiconductor
material, wherein the method comprises effecting conversion of a
nanoparticle core precursor composition to the material of the
nanoparticle core, depositing said first layer on said core and
depositing said second layer on said first layer, said core
precursor composition comprising a first precursor species
containing a first ion to be incorporated into the growing
nanoparticle core and a separate second precursor species
containing a second ion to be incorporated into the growing
nanoparticle core, said conversion being effected in the presence
of a molecular cluster compound under conditions permitting seeding
and growth of the nanoparticle core.
[0014] This aspect of the present invention relates to a method of
producing core/multishell nanoparticles of any desirable form and
allows ready production of a monodisperse population of such
particles which are consequently of a high purity. It is envisaged
that the invention is suitable for producing nanoparticles of any
particular size, shape or chemical composition. A nanoparticle may
have a size falling within the range 2-100 nm. A sub-class of
nanoparticles of particular interest is that relating to compound
semiconductor particles, also known as quantum dots or
nanocrystals.
[0015] The current invention concerns the large scale synthesis of
nanoparticles by the reaction whereby a seeding molecular cluster
is placed in a dispersing medium or solvent (coordinating or
otherwise) in the presence of other precursors to initiate particle
growth. The invention uses a seeding molecular cluster as a
template to initiate particle growth from other precursors present
within the reaction medium. The molecular cluster to be used as the
seeding agent can either be prefabricated or produced in situ prior
to acting as a seeding agent.
[0016] Although manipulation of freshly made solutions of
core-shell quantum dots can substantially lower the particles'
quantum yields, by using a core-multishell architecture rather than
known core-shell structures, more stable nanoparticles (to both
chemical environment and photo effects) can be produced. It will be
appreciated that while the first aspect of the present invention
defines a method for producing nanoparticles having a core, and
first and second layers, the method forming the first aspect of the
present invention may be used to provide nanoparticles comprising
any desirable number of additional layers (e.g. third, fourth and
fifth layers provides on the second, third and fourth layers
respectively) of pure or doped semiconductor materials, materials
having a ternary or quaternary structure, alloyed materials,
metallic materials or non-metallic materials. The invention
addresses a number of problems, which include the difficulty of
producing high efficiency blue emitting dots.
[0017] The nanoparticle core, first and second semiconductor
materials may each possess any desirable number of ions of any
desirable element from the periodic table. Each of the core, first
and second semiconductor material is preferably separately selected
from the group consisting of a semiconductor material incorporating
ions from groups 12 and 15 of the periodic table, a semiconductor
material incorporating ions from groups 13 and 15 of the periodic
table, a semiconductor material incorporating ions from groups 12
and 16 of the periodic table, a semiconductor material
incorporating ions from groups 14 and 16 of the periodic table and
a semiconductor material incorporating ions from groups 11, 13 and
16 of the periodic table.
[0018] Thus, while at least one of the core, first and second
semiconductor materials may incorporate ions from groups 12 and 15
of the periodic table, the material(s) used in these layers may
include ions of one or more further elements, for example, more
than one element from group 12 and/or group 15 of the periodic
table and/or ions from at least one different group of the periodic
table. A preferred core/multishell architecture comprises at least
one layer incorporating two different types of group 12 ions (e.g.
Cd and Zn, or Cd and Hg) and group 16 ions (e.g. S, Se or Te).
[0019] In the nanoparticle of the present invention where at least
one of the core, first and second semiconductor materials is
selected from the group consisting of a semiconductor material
incorporating ions from groups 12 and 15 of the periodic table (a
`II-V` semiconductor material), a semiconductor material
incorporating ions from groups 14 and 16 of the periodic table (a
`IV-VI` semiconductor material) and a semiconductor material
incorporating ions from groups 11, 13 and 16 of the periodic table
(a `I-III-VI` semiconductor material), any other core, first or
second layers in a particular nanoparticle may comprise a II-V,
IV-VI or I-III-VI material. For example, where a nanoparticle in
accordance with the present invention has a core comprising a II-V
semiconductor material, the nanoparticle may possess a first layer
comprising any appropriate semiconductor material for example a
different II-V material (i.e. a II-V material in which the II ions
are ions of a different element of group 12 compared to the II ions
in the core material and/or the V ions are ions of a different
element compared to the group 15 ions in the core material), or a
IV-VI or I-III-VI semiconductor material. Furthermore, if the
nanoparticle in accordance with the present invention possess a
second layer comprising a I-III-VI semiconductor material, it may
possess a first layer comprising any suitable semiconductor
material including a different I-III-VI semiconductor material, or
a II-V or IV-VI material. It will be appreciated that when choosing
suitable semiconductor materials to place next to one another in a
particular nanoparticle (e.g. when choosing a suitable first layer
material for deposition on a core, or a suitable second layer
material for deposition on a first layer) consideration should be
given to matching the crystal phase and lattice constants of the
materials as closely as possible.
[0020] The method forming the first aspect of the present invention
may be used to produce a nanoparticle comprised of a core
comprising a core semiconductor material, a first layer comprising
a first semiconductor material provided on said core and a second
layer comprising a second semiconductor material provided on said
first layer, said core semiconductor material being different to
said first semiconductor material and said first semiconductor
material being different to said second semiconductor material,
wherein
a) at least two of the core, first shell and second shell materials
incorporate ions from groups 12 and 15 of the periodic table,
groups 14 and 16 of the periodic table, or groups 11, 13 and 16 of
the periodic table; b) the second shell material incorporates ions
of at least two different elements from group 12 of the periodic
table and ions from group 16 of the periodic table; c) at least one
of the core, first and second semiconductor materials incorporates
ions from groups 11, 13 and 16 of the periodic table and at least
one other of the core, first and second semiconductor materials is
a semiconductor material not incorporating ions from groups 11, 13
and 16 of the periodic table.
[0021] Preferably in set a) the other of the core, first and second
semiconductor materials incorporates ions from the group consisting
groups 12 and 15 of the periodic table, groups 13 and 15 of the
periodic table, groups 12 and 16 of the periodic table, groups 14
and 16 of the periodic table, and groups 11, 13 and 16 of the
periodic table.
[0022] It is preferred that in set b) said second semiconductor
material has the formula M.sub.xN.sub.1-xE, where M and N are the
group 12 ions, E is the group 16 ion, and 0<x<1. It is
preferred that 0.1<x<0.9, more preferably 0.2<x<0.8,
and most preferably 0.4<x<0.6. Particularly preferred
nanoparticles have the structure ZnS/CdSe/Cd.sub.xZn.sub.1-xS,
Cd.sub.xZn.sub.1-xS/CdSe/ZnS or
Cd.sub.xZn.sub.1-xS/CdSe/Cd.sub.xZn.sub.1-xS.
[0023] In a preferred embodiment of set c) said at least one other
of the core, first and second semiconductor materials not
incorporating ions from groups 11, 13 and 16 of the periodic table
incorporates ions from the group consisting of groups 12 and 15 of
the periodic table, groups 13 and 15 of the periodic table, groups
12 and 16 of the periodic table, and groups 14 and 16 of the
periodic table.
[0024] Preferably the nanoparticle formed using the method
according to the first aspect of the present invention further
comprises a third layer of a third semiconductor material provided
on said second layer. The nanoparticle may optionally comprise
still further layers of semiconductor material, such as fourth,
fifth, and sixth layers.
[0025] It is preferred that the third semiconductor material is
selected from the group consisting of a semiconductor material
incorporating ions from groups 12 and 15 of the periodic table, a
semiconductor material incorporating ions from groups 13 and 15 of
the periodic table, a semiconductor material incorporating ions
from groups 12 and 16 of the periodic table, a semiconductor
material incorporating ions from groups 14 and 16 of the periodic
table and a semiconductor material incorporating ions from groups
11, 13 and 16 of the periodic table.
[0026] Preferably the group 12 ions are selected from the group
consisting of zinc ions, cadmium ions and mercury ions. The group
15 ions are preferably selected from the group consisting of
nitride ions, phosphide ions, arsenide ions, and antimonide ions.
It is preferred that the group 14 ions are selected from the group
consisting of lead ions, tin ions and germanium ions. Preferably
the group 16 ions are selected from the group consisting of sulfide
ions, selenide ions and telluride ions. The group 11 ions are
preferably selected from the group consisting of copper ions,
silver ions and gold ions. In a preferred embodiment the group 13
ions are selected from the group consisting of aluminium ions,
indium ions and gallium ions.
[0027] The core, first and second semiconductor materials may
include ions in an approximate 1:1 ratio (i.e. having a
stoichiometry of 1:1). For example, the nanoparticle ZnS/CdTe/ZnS
contains a first layer of CdTe in which the ratio of cadmium to
telluride ions is approximately 1:1. The semiconductor materials
may possess different stroichiometries, for example the
nanoparticle ZnS/CuInS.sub.2/ZnS contains a first layer of
CuInS.sub.2 in which the ratio of copper to indium ions is
approximately 1:1 but the ratio of copper to sulfide ions is 1:2
and the ratio of indium to sulfide ions is 1:2. Moreover, the
semiconductor materials may possess non-empirical stoichiometries.
For example, the nanoparticle ZnS/CuInS.sub.2/Cd.sub.xZn.sub.1-xS
incorporates a second layer of Cd.sub.xZn.sub.1-xS where
0<x<1. The notation M.sub.xN.sub.1-xE is used herein to
denote a mixture of ions M, N and E (e.g. M=Cd, N.dbd.Zn, E=S)
contained in a semiconductor material. Where the notation
M.sub.xN.sub.1-xE is used it is preferred that 0<x<1,
preferably 0.1<x<0.9, more preferably 0.2<x<0.8, and
most preferably 0.4<x<0.6.
[0028] The temperature of the dispersing medium containing the
growing nanoparticles may be increased at any appropriate rate
depending upon the nature of the nanoparticle core precursor
composition and the molecular cluster compound being used.
Preferably the temperature of the dispersing medium is increased at
a rate in the range 0.05.degree. C./min to 1.degree. C./min, more
preferably at a rate in the range 0.1.degree. C./min to 1.degree.
C./min, and most preferably the temperature of the dispersing
medium containing the growing nanoparticles is increased at a rate
of approximately 0.2.degree. C./min.
[0029] Any suitable molar ratio of the molecular cluster compound
to first and second nanoparticle core precursors may be used
depending upon the structure, size and composition of the
nanoparticles being formed, as well as the nature and concentration
of the other reagents, such as the nanoparticle core precursor(s),
capping agent, size-directing compound and solvent. It has been
found that particularly useful ratios of the number of moles of
cluster compound compared to the total number of moles of the first
and second precursor species preferably lie in the range 0.0001-0.1
(no. moles of cluster compound): 1 (total no. moles of first and
second precursor species), more preferably 0.001-0.1:1, yet more
preferably 0.001-0.060:1. Further preferred ratios of the number of
moles of cluster compound compared to the total number of moles of
the first and second precursor species lie in the range
0.002-0.030:1, and more preferably 0.003-0.020:1. In particular, it
is preferred that the ratio of the number of moles of cluster
compound compared to the total number of moles of the first and
second precursor species lies in the range 0.0035-0.0045:1.
[0030] It is envisaged that any suitable molar ratio of the first
precursor species compared to the second precursor species may be
used. For example, the molar ratio of the first precursor species
compared to the second precursor species may lie in the range 100-1
(first precursor species): 1 (second precursor species), more
preferably 50-1:1. Further preferred ranges of the molar ratio of
the first precursor species compared to the second precursor
species lie in the range 40-5:1, more preferably 30-10:1. In
certain applications it is preferred that approximately equal molar
amounts of the first and second precursor species are used in the
method of the invention. The molar ratio of the first precursor
species compared to the second precursor species preferably lies in
the range 0.1-1.2:1, more preferably, 0.9-1.1:1, and most
preferably 1:1. In other applications, it may be appropriate to use
approximately twice the number of moles of one precursor species
compared to the other precursor species. Thus the molar ratio of
the first precursor species compared to the second precursor
species may lie in the range 0.4-0.6:1, more preferably the molar
ratio of the first precursor species compared to the second
precursor species is 0.5:1. It is to be understood that the above
precursor molar ratios may be reversed such that they relate to the
molar ratio of the second precursor species compared to the first
precursor species. Accordingly, the molar ratio of the second
precursor species compared to the first precursor species may lie
in the range 100-1 (second precursor species): 1 (first precursor
species), more preferably 50-1:1, 40-5:1, or 30-10:1. Furthermore,
the molar ratio of the second precursor species compared to the
first precursor species may lie in the range 0.1-1.2:1, 0.9-1.1:1,
0.4-0.6:1, or may be 0.5:1.
[0031] In a preferred embodiment of the first aspect of the present
invention the molecular cluster compound and core precursor
composition are dispersed in a suitable dispersing medium at a
first temperature and the temperature of the dispersing medium
containing the cluster compound and core precursor composition is
then increased to a second temperature which is sufficient to
initiate seeding and growth of the nanoparticle cores on the
molecular clusters of said compound.
[0032] Preferably the first temperature is in the range 50.degree.
C. to 100.degree. C., more preferably in the range 70.degree. C. to
80.degree. C., and most preferably the first temperature is
approximately 75.degree. C.
[0033] The second temperature may be in the range 120.degree. C. to
280.degree. C. More preferably the second temperature is in the
range 150.degree. C. to 250.degree. C., and most preferably the
second temperature is approximately 200.degree. C.
[0034] The temperature of the dispersing medium containing the
cluster compound and core precursor composition may be increased
from the first temperature to the second temperature over a time
period of up to 48 hours, more preferably up to 24 hours, yet more
preferably 1 hour to 24 hours, and most preferably over a time
period in the range 1 hour to 8 hours.
[0035] In a further preferred embodiment of the first aspect of the
present invention the method comprises
[0036] a. dispersing the molecular cluster compound and an initial
portion of the nanoparticle core precursor composition which is
less than the total amount of the nanoparticle core precursor
composition to be used to produce said nanoparticle cores in a
suitable dispersing medium at a first temperature;
[0037] b. increasing the temperature of the dispersing medium
containing the cluster compound and core precursor composition to a
second temperature which is sufficient to initiate seeding and
growth of the nanoparticle cores on the molecular clusters of said
molecular cluster compound; and
[0038] c. adding one or more further portions of the nanoparticle
core precursor composition to the dispersing medium containing the
growing nanoparticle cores,
[0039] wherein the temperature of the dispersing medium containing
the growing nanoparticle cores is increased before, during and/or
after the addition of the or each further portion of the
nanoparticle core precursor composition.
[0040] In this preferred embodiment less than the total amount of
precursor to be used to produce the nanoparticle cores is present
in the dispersing medium with the cluster compound prior to the
initiation of nanoparticle growth and then as the reaction proceeds
and the temperature is increased, additional amounts of core
precursors are periodically added to the reaction mixture in the
dispersing medium. Preferably the additional core precursors are
added either dropwise as a solution or as a solid.
[0041] The temperature of the dispersing medium containing the
growing nanoparticle cores may be increased at any appropriate rate
depending upon the nature of the nanoparticle core precursor
composition and the molecular cluster compound being used.
Preferably the temperature of the dispersing medium is increased at
a rate in the range 0.05.degree. C./min to 1.degree. C./min, more
preferably at a rate in the range 0.1.degree. C./min to 1.degree.
C./min, and most preferably the temperature of the dispersing
medium containing the growing nanoparticle cores is increased at a
rate of approximately 0.2.degree. C./min.
[0042] While the first and second temperatures of the dispersing
medium may take any suitable value, in a preferred embodiment of
the present invention said first temperature is in the range
15.degree. C. to 60.degree. C. Said second temperature may be in
the range 90.degree. C. to 150.degree. C.
[0043] It is preferred that the or each further portion of the
nanoparticle core precursor composition is added dropwise to the
dispersing medium containing the growing nanoparticle cores.
[0044] The or each further portion of the nanoparticle core
precursor composition may be added to the dispersing medium
containing the growing nanoparticle cores at any desirable rate. It
is preferred that the core precursor composition is added to the
dispersing medium at a rate in the range 0.1 ml/min to 20 ml/min
per litre of dispersing medium, more preferably at a rate in the
range 1 ml/min to 15 ml/min per litre of dispersing medium, and
most preferably at a rate of around 5 ml/min per litre of
dispersing medium.
[0045] Preferably said initial portion of the nanoparticle core
precursor composition is less than or equal to approximately 90% of
the total amount of the nanoparticle core precursor composition to
be used to produce said nanoparticle cores. Said initial portion of
the nanoparticle core precursor composition may be less than or
equal to approximately 10% of the total amount of the nanoparticle
core precursor composition to be used to produce said nanoparticle
cores.
[0046] In a preferred embodiment where one further portion of the
nanoparticle core precursor composition is added to the dispersing
medium containing the growing nanoparticle cores said one further
portion is less than or equal to approximately 90% of the total
amount of the nanoparticle core precursor composition to be used to
produce said nanoparticle cores.
[0047] In a further preferred embodiment where more than one
further portion of the nanoparticle core precursor composition is
added to the dispersing medium containing the growing nanoparticle
cores, each of said further portions is less than or equal to
approximately 45% of the total amount of the nanoparticle core
precursor composition to be used to produce said nanoparticle
cores. Each of said further portions may be less than or equal to
approximately 10% of the total amount of the nanoparticle core
precursor composition to be used to produce said nanoparticle
cores.
[0048] It is preferred that formation of said molecular cluster
compound is effected in situ in said dispersing medium prior to
dispersing the molecular cluster compound and the initial portion
of the nanoparticle core precursor composition in said dispersing
medium.
[0049] In a preferred embodiment of the present invention said
process is subject to the proviso that the nanoparticle core
precursor composition does not contain Cd(CH.sub.3CO.sub.2).sub.2.
A further preferred embodiment provides that said process is
subject to the proviso that the nanoparticle core precursor
composition does not contain TOPSe. Said process may be subject to
the proviso that the nanoparticle core precursor composition does
not contain Cd(CH.sub.3CO.sub.2).sub.2 and TOPSe. In a still
further preferred embodiment said process is subject to the proviso
that the temperature of the dispersing medium containing the
growing nanoparticle cores is increased at a rate which is other
than 50.degree. C. over a period of 24 hours.
[0050] The conversion of the core precursor to the material of the
nanoparticles can be conducted in any suitable dispersing medium or
solvent. In the method of the present invention it is important to
maintain the integrity of the molecules of the cluster compound.
Consequently, when the cluster compound and nanoparticle core
precursor are introduced in to the dispersing medium or solvent the
temperature of the medium/solvent must be sufficiently high to
ensure satisfactory dissolution and mixing of the cluster compound
it is not necessary that the present compounds are fully dissolved
but desirable. It is most preferred that the temperature of the
dispersing medium containing the cluster and precursors should not
be so high as to disrupt the integrity of the cluster compound
molecules. Once the cluster compound and core precursor composition
are sufficiently well dissolved in the solvent the temperature of
the solution thus formed is raised to a temperature, or range of
temperatures, which is/are sufficiently high to initiate
nanoparticle core growth but not so high as to damage the integrity
of the cluster compound molecules. As the temperature is increased
further quantities of core precursor are added to the reaction,
preferably in a dropwise manner or as a solid. The temperature of
the solution can then be maintained at this temperature or within
this temperature range for as long as required to form nanoparticle
cores possessing the desired properties.
[0051] A wide range of appropriate dispersing media/solvents are
available. The particular dispersing medium used is usually at
least partly dependent upon the nature of the reacting species,
i.e. nanoparticle core precursor and/or cluster compound, and/or
the type of nanoparticles which are to be formed. Preferred
dispersing media include Lewis base type coordinating solvents,
such as a phosphine (e.g. TOP), a phosphine oxide (e.g. TOPO) or an
amine (e.g. HDA), or non-coordinating organic solvents, e.g.
alkanes and alkenes (e.g. octadecene). If a non-coordinating
solvent is used then it will usually be used in the presence of a
further coordinating agent to act as a capping agent for the
following reason.
[0052] If the nanoparticles being formed are intended to function
as quantum dots it is important that the surface atoms which are
not fully coordinated "dangling bonds" are capped to minimise
non-radiative electron-hole recombinations and inhibit particle
agglomeration which can lower quantum efficiencies or form
aggregates of nanoparticles. A number of different coordinating
solvents are known which can also act as capping or passivating
agents, e.g. TOP, TOPO, HDA or long chain organic acids such as
myristic acid. If a solvent is chosen which cannot act as a capping
agent then any desirable capping agent can be added to the reaction
mixture during nanoparticle growth. Such capping agents are
typically Lewis bases but a wide range of other agents are
available, such as oleic acid and organic polymers which form
protective sheaths around the nanoparticles.
[0053] The first aspect of the present invention comprises of a
method to produce nanoparticle materials using molecular clusters,
whereby the clusters are defined identical molecular entities, as
compared to ensembles of small nanoparticles, which inherently lack
the anonymous nature of molecular clusters. The invention consists
of the use of molecular clusters as templates to seed the growth of
nanoparticle cores, whereby other molecular sources, i.e. the
precursor compounds, or "molecular feedstocks" are consumed to
facilitate particle growth. The molecular sources (i.e. core
precursor composition) are periodically added to the reaction
solution so as to keep the concentration of free ions to a minimum
but also maintain a concentration of free ions to inhibit Oswald
ripening from occurring and defocusing of nanoparticle size range
from occurring.
[0054] A further preferred embodiment of the first aspect of the
present invention provides that the method comprises:
[0055] i. monitoring the average size of the nanoparticle cores
being grown; and
[0056] ii. terminating nanoparticle core growth when the average
nanoparticle size reaches a predetermined value.
[0057] It is preferred that the average size of the nanoparticle
cores being grown is monitored by UV-visible absorption
spectroscopy. The average size of the nanoparticle cores being
grown may be monitored by photoluminescence spectroscopy.
Preferably nanoparticle core growth is terminated by reducing the
temperature of the dispersing medium from the second temperature to
a third temperature.
[0058] Conveniently the method may comprise forming a precipitate
of the nanoparticle core material by the addition of a
precipitating reagent, which is preferably selected from the group
consisting of ethanol and acetone.
[0059] Preferably conversion of the core precursor composition to
the nanoparticle core is effected in a reaction medium and said
nanoparticle core is isolated from said reaction medium prior to
deposition of the first layer.
[0060] It is preferable that deposition of said first layer
comprises effecting conversion of a first semiconductor material
precursor composition to said first semiconductor material. The
first semiconductor material precursor composition preferably
comprises third and fourth precursor species containing the ions to
be incorporated into the growing first layer of the nanoparticle.
The third and fourth precursor species may be separate entities
contained in said first semiconductor material precursor
composition, or the third and fourth precursor species may be
combined in a single entity contained in the first semiconductor
material precursor composition.
[0061] Preferably deposition of said second layer comprises
effecting conversion of a second semiconductor material precursor
composition to said second semiconductor material. The second
semiconductor material precursor composition preferably comprises
fifth and sixth precursor species containing the ions to be
incorporated into the growing second layer of the nanoparticle. It
is preferred that the fifth and sixth precursor species are
separate entities contained in said second semiconductor material
precursor composition, alternatively the fifth and sixth precursor
species may be combined in a single entity contained in said second
semiconductor material precursor composition.
[0062] A second aspect of the present invention provides a
nanoparticle produced according to a method in accordance with the
first aspect of the present invention.
[0063] A third aspect of the present invention provides a
nanoparticle comprised of a core comprising a core semiconductor
material, a first layer comprising a first semiconductor material
provided on said core and a second layer comprising a second
semiconductor material provided on said first layer, said core
semiconductor material being different to said first semiconductor
material and said first semiconductor material being different to
said second semiconductor material, wherein
[0064] a) at least two of the core, first shell and second shell
materials incorporate ions from groups 12 and 15 of the periodic
table, groups 14 and 16 of the periodic table, or groups 11, 13 and
16 of the periodic table;
[0065] b) the second shell material incorporates ions of at least
two different elements from group 12 of the periodic table and ions
from group 16 of the periodic table;
[0066] c) at least one of the core, first and second semiconductor
materials incorporates ions from groups 11, 13 and 16 of the
periodic table and at least one other of the core, first and second
semiconductor materials is a semiconductor material not
incorporating ions from groups 11, 13 and 16 of the periodic
table.
[0067] Preferably in set a) the other of the core, first and second
semiconductor materials incorporates ions from the group consisting
groups 12 and 15 of the periodic table, groups 13 and 15 of the
periodic table, groups 12 and 16 of the periodic table, groups 14
and 16 of the periodic table, and groups 11, 13 and 16 of the
periodic table.
[0068] It is preferred that in set b) said second semiconductor
material has the formula M.sub.xN.sub.1-xE, where M and N are the
group 12 ions, E is the group 16 ion, and 0<x<1. It is
preferred that 0.1<x<0.9, more preferably 0.2<x<0.8,
and most preferably 0.4<x<0.6. Particularly preferred
nanoparticles have the structure ZnS/CdSe/Cd.sub.xZn.sub.1-xS,
Cd.sub.xZn.sub.1-xS/CdSe/ZnS or
Cd.sub.xZn.sub.1-xS/CdSe/Cd.sub.xZn.sub.1-xS.
[0069] In a preferred embodiment of set c) said at least one other
of the core, first and second semiconductor materials not
incorporating ions from groups 11, 13 and 16 of the periodic table
incorporates ions from the group consisting of groups 12 and 15 of
the periodic table, groups 13 and 15 of the periodic table, groups
12 and 16 of the periodic table, and groups 14 and 16 of the
periodic table.
[0070] Preferably the nanoparticle further comprises a third layer
of a third semiconductor material provided on said second layer.
The nanoparticle may optionally comprise still further layers of
semiconductor material, such as fourth, fifth, and sixth
layers.
[0071] Regarding the third aspect of the present invention it is
preferred that the third semiconductor material is selected from
the group consisting of a semiconductor material incorporating ions
from groups 12 and 15 of the periodic table, a semiconductor
material incorporating ions from groups 13 and 15 of the periodic
table, a semiconductor material incorporating ions from groups 12
and 16 of the periodic table, a semiconductor material
incorporating ions from groups 14 and 16 of the periodic table and
a semiconductor material incorporating ions from groups 11, 13 and
16 of the periodic table.
[0072] Preferably the group 12 ions are selected from the group
consisting of zinc ions, cadmium ions and mercury ions.
[0073] The group 15 ions are preferably selected from the group
consisting of nitride ions, phosphide ions, arsenide ions, and
antimonide ions.
[0074] It is preferred that the group 14 ions are selected from the
group consisting of lead ions, tin ions and germanium ions.
[0075] Preferably the group 16 ions are selected from the group
consisting of sulfide ions, selenide ions and telluride ions.
[0076] The group 11 ions are preferably selected from the group
consisting of copper ions, silver ions and gold ions.
[0077] In a preferred embodiment the group 13 ions are selected
from the group consisting of aluminium ions, indium ions and
gallium ions.
[0078] The current invention describes the design and preparation
methods of a number of unique quantum dot--quantum wells
nanoparticles including, ZnS/CuInS.sub.2/ZnS,
ZnS/CuInS.sub.2/Cd.sub.xZn.sub.1-xS,
Cd.sub.xZn.sub.1-xS/CuInS.sub.2/Cd.sub.xZn.sub.1-xS,
ZnS/CuGaS.sub.2/ZnS, ZnS/CuGaS.sub.2/Cd.sub.xZn.sub.1-xS,
Cd.sub.xZn.sub.1-xS/CuGaS.sub.2/Cd.sub.xZn.sub.1-xS,
ZnS/CuInSe.sub.2/ZnS, ZnS/CuInSe.sub.2/Cd.sub.xZn.sub.1-xS,
Cd.sub.xZn.sub.1-xS/CuInSe.sub.2/Cd.sub.xZn.sub.1-xS,
ZnS/CuGaSe.sub.2/ZnS, ZnS/CuGaSe.sub.2/Cd.sub.xZn.sub.1-xS and
Cd.sub.xZn.sub.1-xS/CuGaSe.sub.2/Cd.sub.xZn.sub.1-xS, where
0<x<1.
[0079] A fourth aspect of the present invention provides a method
for producing a nanoparticle according to the third aspect of the
present invention, wherein the method comprises effecting
conversion of a nanoparticle core precursor composition to the
material of the nanoparticle core, depositing said first layer on
said core and depositing said second layer on said first layer.
[0080] It will be evident to the skilled person how the method
forming the fourth aspect of the present invention may be put in to
effect by routine modification to the experimental details
disclosed herein and involving no undue experimentation for the
preparation of core/multishell nanoparticles in accordance with the
third aspect of the present invention.
[0081] Preferably said nanoparticle core precursor composition
comprises first and second core precursor species containing the
ions to be incorporated into the growing nanoparticle core. It is
preferred that the first and second core precursor species are
separate entities contained in the core precursor composition, and
the conversion is effected in the presence of a molecular cluster
compound under conditions permitting seeding and growth of the
nanoparticle core.
[0082] The first and second core precursor species may be combined
in a single entity contained in the core precursor composition.
[0083] Preferably conversion of the core precursor composition to
the nanoparticle core is effected in a reaction medium and said
nanoparticle core is isolated from said reaction medium prior to
deposition of the first layer.
[0084] In a preferred embodiment of the fourth aspect of the
present invention deposition of the first layer comprises effecting
conversion of a first semiconductor material precursor composition
to said first semiconductor material.
[0085] Preferably the first semiconductor material precursor
composition comprises third and fourth precursor species containing
the ions to be incorporated into the growing first layer of the
nanoparticle. The third and fourth precursor species may be
separate entities contained in the first semiconductor material
precursor composition (i.e. the precursor species may be provided
by a `multi source` or `dual source` precursor composition).
Alternatively or additionally the third and fourth precursor
species may be combined in a single entity contained in the first
semiconductor material precursor composition (i.e. the precursor
composition may contain a `single source` precursor comprising both
the third and fourth ions to be incorporated in to the first
layer).
[0086] Preferably deposition of the second layer comprises
effecting conversion of a second semiconductor material precursor
composition to said second semiconductor material.
[0087] Preferably the second semiconductor material precursor
composition comprises fifth and sixth precursor species containing
the ions to be incorporated into the growing second layer of the
nanoparticle. The fifth and sixth precursor species may be separate
entities contained in said second semiconductor material precursor
composition, and/or the fifth and sixth precursor species may be
combined in a single entity contained in said second semiconductor
material precursor composition.
[0088] The invention addresses a number of problems, which include
the difficulty of producing high efficiency blue emitting dots.
[0089] The most researched and hence best-characterized
semiconductor QD is CdSe, whose optical emission can be tuned
across the visible region of the spectrum. Green and red CdSe/ZnS
core-shell nanocrystals are the most widely available under
existing methodologies. CdSe nanoparticles with blue emission along
with narrow spectral widths and high luminescence quantum yields
are difficult to synthesize using the conventional high temperature
rapid injection "nucleation and growth" method. Using this
conventional method to make blue quantum dots is difficult as the
blue quantum dots are the smallest and are what is initially formed
but rapidly grow (about 3 seconds of reaction time) in to larger
does which have a green emission. There are also further problems
including difficulties in experimental work-up, processes and
overcoating with ZnS. Moreover, only small quantities of material
can be produced in a single batch due to the dilute reaction
solution necessary to keep the particle size small. Alternative
blue emitting semiconductor nanocrystals include ZnTe and CdS,
however, growing large (>4.5 nm diameter) ZnTe, needed for blue
emissions, with narrow size distributions has proved difficult.
[0090] CdS on the other hand has an appropriate band gap and has
been shown to emit in the 460-480 nm range with narrow size
distributions and good luminescence efficiency. Bare CdS cores tend
to emit white luminescence, attributed to deep trap emissions which
can be suppressed by overcoating by a wide band gap material such
as ZnS. These CdS/ZnS structures have shown recent promise as the
active material for blue QD LED's and blue QD lasers.
Quantum Dots Incorporating Lower Toxicity Elements
[0091] Another drive for designing and producing specific quantum
dot-quantum well structures in this invention is the current need
for quantum dots free of elements (e.g. cadmium and mercury) which
are deemed by national authorities to be toxic or potentially toxic
but which have similar optical and/or electronic properties to
those of CdSe--ZnS core-shell quantum dots. The current invention
includes the design and synthesis of a number of cadmium free QD-QW
structures based on II-VI/I-III-VI.sub.2/II-VI, III-V/II-VIII-V
materials such as but not restricted to ZnS/CuInS.sub.2/ZnS,
ZnS/CuGaS.sub.2/ZnS, ZnS/CuInSe.sub.2/ZnS,
ZnS/CuGaSe.sub.2/ZnS.
Current Synthetic Methods
[0092] Many synthetic methods for the preparation of semiconductor
nanoparticles have been reported, early routes applied conventional
colloidal aqueous chemistry, with more recent methods involving the
kinetically controlled precipitation of nanocrystallites, using
organometallic compounds.
[0093] Over the past six years the important issues have concerned
the synthesis of high quality semiconductor nanoparticles in terms
of uniform shape, size distribution and quantum efficiencies. This
has lead to a number of methods that can routinely produce
semiconductor nanoparticles, with monodispersity of <5% with
quantum yields >50%. Most of these methods are based on the
original "nucleation and growth" method described by Murray, Norris
and Bawendi, using organometallic precursors. Murray et at
originally used organometallic solutions of metal-alkyls (R.sub.2M)
M=Cd, Zn, Te; R=Me, Et and tri-n-octylphosphine sulfide/selenide
(TOPS/Se) dissolved in tri-n-octylphosphine (TOP). These precursor
solutions are injected into hot tri-n-octylphosphine oxide (TOPO)
in the temperature range 120-400.degree. C. depending on the size
of the particles required and the material being produced. This
produces TOPO coated/capped semiconductor nanoparticles of II-VI
material. The size of the particles is controlled by the
temperature, concentration of precursor used and length of time at
which the synthesis is undertaken, with larger particles being
obtained at higher temperatures, higher precursor concentrations
and prolonged reaction times.
[0094] This organometallic route has advantages over other
synthetic methods, including near monodispersity <5% and high
particle cystallinity. As mentioned, many variations of this method
have now appeared in the literature which routinely give high
quality core and core-shell nanoparticles with monodispersity of
<5% and quantum yield >50% (for core-shell particles of
as-prepared solutions), with many methods displaying a high degree
of size and shape control.
[0095] Recently attention has focused on the use of "greener"
precursors which are less exotic and less expensive but not
necessary more environmentally friendly. Some of these new
precursors include the oxides, CdO; carbonates MCO.sub.3 M=Cd, Zn;
acetates M(CH.sub.3CO.sub.2) M=Cd, Zn and acetylacetanates
[CH.sub.3COOCH.dbd.C(O.sup.-)CH.sub.3].sub.2 M=Cd, Zn; amongst
other. (The use of the term "greener" precursors in semiconductor
particle synthesis has generally taken on the meaning of cheaper,
readily available and easier to handle precursor starting
materials, than the originally used organometallics which are
volatile and air and moisture sensitive, and does not necessary
mean that "greener precursors" are any more environmentally
friendly).
[0096] Single-source precursors have also proved useful in the
synthesis of semiconductor nanoparticle materials of II-VI, as well
as other compound semiconductor nanoparticles.
Bis(dialkyldithio-/diseleno-carbamato)cadmium(II)/zinc(II)
compounds, M(E.sub.2CNR.sub.2).sub.2 (M=Zn or Cd, E=S or Se and
R=alkyl), have used a similar `one-pot` synthetic procedure, which
involved dissolving the precursor in tri-n-octylphosphine (TOP)
followed by rapid injection into hot tri-n-octylphosphine
oxide/tri-n-octylphosphine (TOPO/TOP) above 200.degree. C.
Single-source precursors have also been used to produce
I-III-VI.sub.2 materials i.e. CuInS.sub.2 using
(PPH.sub.3).sub.2CuIn(SEt).sub.4 dissolved in a mixture of
hexanethiol and dioctylphalate at 200.degree. C. to give
hexanethiol coated CuInS.sub.2.
[0097] I-III-VI.sub.2 nanoparticles have also been prepared from
multi-source precursors such as in the case of CuInSe.sub.2
prepared from CuCl dissolved in triethylene and elemental indium
and selenium. CuInTe.sub.2 was produce by a similar approach but
from using elemental tellurium.
[0098] For all the above methods, rapid particle nucleation
followed by slow particle growth is essential for a narrow particle
size distribution. All these synthetic methods are based on the
original organometallic "nucleation and growth" method by Murray et
al, which involves the rapid injection of the precursors into a hot
solution of a Lewis base coordinating solvent (capping agent) which
may also contain one of the precursors. The addition of the cooler
solution subsequently lowers the reaction temperature and assist
particle growth but inhibits further nucleation. The temperature is
then maintained for a period of time, with the size of the
resulting particles depending on reaction time, temperature and
ratio of capping agent to precursor used. The resulting solution is
cooled followed by the addition of an excess of a polar solvent
(methanol or ethanol or sometimes acetone) to produce a precipitate
of the particles that can be isolated by filtration or
centrifugation.
[0099] Preparation from single-source molecular clusters, Cooney
and co-workers used the cluster [S.sub.4Cd.sub.10(SPh).sub.16]
[Me.sub.3NH].sub.4 to produce nanoparticles of CdS via the
oxidation of surface-capping SPh.sup.- ligands by iodine. This
route followed the fragmentation of the majority of clusters into
ions which were consumed by the remaining
[0100] Another method whereby it is possible to produce large
volumes of quantum dots, eliminated the need for a high temperature
nucleation step. Moreover, conversion of the precursor composition
to the nanoparticles is affected in the presence of a molecular
cluster compound. Each identical molecule of a cluster compound
acts as a seed or nucleation point upon which nanoparticle growth
can be initiated. In this way, nanoparticle nucleation is not
necessary to initiate nanoparticle growth because suitable
nucleation sites are already provided in the system by the
molecular clusters. The molecules of the cluster compound act as a
template to direct nanoparticle growth. By providing nucleation
sites which are so much more well defined than the nucleation sites
employed in previous work the nanoparticles formed in this way
possess a significantly more well defined final structure than
those obtained using previous methods. A significant advantage of
this method is that it can be more easily scaled-up for use in
industry than conventional methods.
[0101] The particular solvent used is usually at least partly
dependent upon the nature of the reacting species, i.e.
nanoparticle precursor and/or cluster compound, and/or the type of
nanoparticles which are to be formed. Typical solvents include
Lewis base type coordinating solvents, such as a phosphine (e.g.
TOP), a phosphine oxide (e.g. TOPO) or an amine (e.g. HDA),
hexanethiol, or non-coordinating organic solvents, e.g. alkanes and
alkenes. If a non-coordinating solvent is used then it will usually
be used in the presence of a further coordinating agent to act as a
capping agent for the following reason.
[0102] If the nanoparticles are intended to function as quantum
dots an outer capping agent (e.g. an organic layer) must be
attached to stop particle agglomeration from occurring. A number of
different coordinating solvents are known which can also act as
capping or passivating agents, e.g. TOP, TOPO, alkylthiols or HDA.
If a solvent is chosen which cannot act as a capping agent then any
desirable capping agent can be added to the reaction mixture during
nanoparticle growth. Such capping agents are typically Lewis bases
but a wide range of other agents are available, such as oleic acid
and organic polymers which form protective sheaths around the
nanoparticles.
DESCRIPTION OF INVENTION
Type of System Covered by the Current Invention
[0103] The present invention is directed to the preparation of a
number of semiconductor nanoparticles which may be considered as
falling within the class of materials known as quantum dot-quantum
wells and includes materials within the size range 2-100 nm. The
present invention describes the architecture and the preparation of
a number of nanoparticles materials and includes a number of
compound semiconductor particles otherwise referred to as quantum
dots-quantum well, include material comprising of
ZnS/CuInS.sub.2/ZnS, ZnS/CuInS.sub.2/Cd.sub.xZn.sub.1-xS,
Cd:ZnS/CuInS.sub.2/Cd.sub.xZn.sub.1-xS, ZnS/CuGaS.sub.2/ZnS,
ZnS/CuGaS.sub.2/Cd.sub.xZn.sub.1-xS,
Cd.sub.xZn.sub.1-xS/CuGaS.sub.2/Cd.sub.xZn.sub.1-xS,
ZnS/CuInSe.sub.2/ZnS, ZnS/CuInSe.sub.2/Cd.sub.xZn.sub.1-xS,
Cd.sub.xZn.sub.1-xS/CuInSe.sub.2/Cd.sub.xZn.sub.1-xS,
ZnS/CuGaSe.sub.2/ZnS, ZnS/CuGaSe.sub.2/Cd.sub.xZn.sub.1-xS and
Cd.sub.xZn.sub.1-xS/CuGaSe.sub.2/Cd.sub.xZn.sub.1-xS, where
0<x<1.
II-VI/II-VI/II-VI Material
[0104] Comprising a core of a first element from group 12 of the
periodic table and a second element from group 16 of the periodic
table, a first layer of material comprising a shell of a first
element from group 12 of the periodic table and a second element
from group 16 of the periodic table and a second layer material
comprising a shell of a first element from group 12 of the periodic
table and a second element from group 16 of the periodic table and
also including ternary and quaternary materials and doped
materials. Nanoparticle materials include but are not restricted
to:--ZnS/CdSe/CdS/ZnS, ZnS/CdTe/ZnS, ZnS/CdHgS/ZnS, ZnS/HgSe/ZnS,
ZnS/HgTe/ZnS, ZnSe/CdSe/ZnSe, ZnSe/CdTe/ZnSe, ZnSe/HgS/ZnSe,
ZnS/HgSe/ZnS, ZnSe/HgTe/ZnSe, ZnTe/CdSe/ZnS, ZnTe/CdTe/ZnS,
ZnTe/CdHgS/ZnS, ZnTe/HgSe/ZnS, ZnTe/HgTe/ZnS, CdS/CdSe/ZnS,
CdS/CdTe/ZnS, CdS/CdHgS/ZnS, CdS/HgSe/ZnS, CdS/HgTe/ZnS,
CdSe/CdTe/ZnS, CdSe/CdHgS/ZnS, CdSe/HgSe/ZnS, CdSe/HgTe/ZnS,
CdTe/CdSe/ZnS, CdTe/CdHgS/ZnS, CdTe/HgSe/ZnS, CdTe/HgTe/ZnS,
HgS/CdSe/ZnS, HgS/CdTe/ZnS, HgS/CdHgS/ZnS, HgS/HgSe/ZnS,
HgS/HgTe/ZnS, HgSe/CdSe/ZnS, HgSe/CdTe/ZnS, HgSe/CdHgS/ZnS,
HgSe/HgTe/ZnS.
II-VI/I-III-VI.sub.2II-VI Material
[0105] Comprising a core of a first element from group 12 of the
periodic table and a second element from group 16 of the periodic
table, a first layer of material comprising of a shell of a first
element from group 11 of the periodic table and a second element
from group 13 of the periodic table a third element from group 16
of the periodic table and a second layer material comprising a
shell of a first element from group 12 of the periodic table and a
second element from group 16 of the periodic table and also
including ternary and quaternary materials and doped materials.
Nanoparticle materials include but are not restricted to:
ZnS/CuInS.sub.2/ZnS, ZnS/CuInS.sub.2/CdS/ZnS,
CdS/ZnS/CuInS.sub.2/CdS/ZnS, ZnS/CuGaS.sub.2/ZnS,
ZnS/CuGaS.sub.2/CdS/ZnS, CdS/ZnS/CuGaS.sub.2/CdS/ZnS,
ZnS/CuInSe.sub.2/ZnS, ZnS/CuInSe.sub.2/CdS/ZnS,
CdS/ZnS/CuInSe.sub.2/CdS/ZnS, ZnS/CuGaSe.sub.2/ZnS,
ZnS/CuGaSe.sub.2/CdS/ZnS, CdS/ZnS/CuGaSe.sub.2/CdS/ZnS.
II-V/II-V/II-V Material
[0106] Comprising a core first element from group 12 of the
periodic table and a second element from group 15 of the periodic
table, a first layer comprising a first element from group 12 of
the periodic table and a second element from group 15 of the
periodic table and a second layer of semiconductor material
comprising a first element from group 12 of the periodic table and
a second element from group 15 of the periodic table and also
including ternary and quaternary materials and doped materials.
Nanoparticle material includes but is not restricted
to:--Zn.sub.3P.sub.2/Zn.sub.3As.sub.2/Zn.sub.3P.sub.2,
Zn.sub.3P.sub.2/Cd.sub.3P.sub.2/Zn.sub.3P.sub.2,
Zn.sub.3P.sub.2/Cd.sub.3As.sub.2/Zn.sub.3P.sub.2,
Zn.sub.3P.sub.2/Cd.sub.3N.sub.2/Zn.sub.3P.sub.2,
Zn.sub.3P.sub.2/Zn.sub.3N.sub.2/Zn.sub.3P.sub.2,
Zn.sub.3As.sub.2/Zn.sub.3P.sub.2/Zn.sub.3As.sub.2,
Zn.sub.3As.sub.2/Cd.sub.3P.sub.2/Zn.sub.3As.sub.2,
Zn.sub.3As.sub.2/Cd.sub.3As.sub.2/Zn.sub.3As.sub.2,
Zn.sub.3As.sub.2/Cd.sub.3N.sub.2/Zn.sub.3As.sub.2,
Zn.sub.3As.sub.2/Zn.sub.3N.sub.2/Zn.sub.3As.sub.2,
Cd.sub.3P.sub.2/Zn.sub.3P.sub.2/Cd.sub.3P.sub.2,
Cd.sub.3P.sub.2/Zn.sub.3As.sub.2/Cd.sub.3P.sub.2,
Cd.sub.3P.sub.2/Cd.sub.3As.sub.2/Cd.sub.3P.sub.2,
Cd.sub.3P.sub.2/Cd.sub.3N.sub.2/Cd.sub.3P.sub.2,
Cd.sub.3P.sub.2/Zn.sub.3N.sub.2/Cd.sub.3P.sub.2,
Cd.sub.3As.sub.2/Zn.sub.3P.sub.2/Cd.sub.3As.sub.2,
Cd.sub.3As.sub.2/Zn.sub.3As.sub.2/Cd.sub.3As.sub.2,
Cd.sub.3As.sub.2/Cd.sub.3P.sub.2/Cd.sub.3As.sub.2,
Cd.sub.3As.sub.2/Cd.sub.3N.sub.2/Cd.sub.3As.sub.2,
Cd.sub.3As.sub.2/Zn.sub.3N.sub.2/Cd.sub.3As.sub.2,
Cd.sub.3N.sub.2/Zn.sub.3P.sub.2/Cd.sub.3N.sub.2,
Cd.sub.3N.sub.2/Zn.sub.3As.sub.2/Cd.sub.3N.sub.2,
Cd.sub.3N.sub.2/Cd.sub.3P.sub.2/Cd.sub.3N.sub.2,
Cd.sub.3N.sub.2/Cd.sub.3As.sub.2/Cd.sub.3N.sub.2,
Cd.sub.3N.sub.2/Zn.sub.3N.sub.2/Cd.sub.3N.sub.2,
Zn.sub.3N.sub.2/Zn.sub.3P.sub.2/Zn.sub.3N.sub.2,
Zn.sub.3N.sub.2/Zn.sub.3As.sub.2/Zn.sub.3N.sub.2,
Zn.sub.3N.sub.2/Cd.sub.3P.sub.2/Zn.sub.3N.sub.2,
Zn.sub.3N.sub.2/Cd.sub.3As.sub.2/Zn.sub.3N.sub.2,
Zn.sub.3N.sub.2/Cd.sub.3N.sub.2/Zn.sub.3N.sub.2.
III-V/III-V/III-V Material
[0107] Comprising a core of a first element from group 13 of the
periodic table and a second element from group 15 of the periodic
table, a first layer comprising of a first element from group 13 of
the periodic table and a second element from group 15 of the
periodic table and a second layer comprising of a first element
from group 13 of the periodic table and a second element from group
15 of the periodic table and also including ternary and quaternary
materials and doped materials. Nanoparticle materials include but
are not restricted to:--AlP/AlAs/AlP, AlP/AlSb/AlP, AlP/GaN/AlP,
AlP/GaP/AlP, AlP/GaAs/AlP, AlP/GaSb/AlP, AlP/InN/AlP, AlP/InP/AlP,
AlP/InAs/AlP, AlP/InSb/AlP, AlAs/AlP/AlAs, AlP/AlSb/AlP,
AlP/GaN/AlP, AlP/GaP/AlP, AlP/GaAs/AlP, AlP/GaSb/AlP, AlP/InN/AlP,
AlP/InP/AlP, AlP/InAs/AlP, AlP/InSb/AlP, AlSb/AlP/AlSb,
AlSb/AlAs/AlSb, AlSb/GaN/AlSb, AlSb/GaP/AlSb, AlSb/GaAs/AlSb,
AlSb/GaSb/AlSb, AlSb/InN/AlSb, AlSb/InP/AlSb, AlSb/InAs/AlSb,
AlSb/InSb/AlSb, GaN/AlP/GaN, GaN/AlAs/GaN, GaN/AlAs/GaN,
GaN/GaP/GaN, GaN/GaAs/GaN, GaN/GaSb/GaN, GaN/InN/GaN, GaN/InP/GaN,
GaN/InAs/GaN, GaN/InSb/GaN, GaP/AlP/GaP, GaP/AlAs/GaP,
GaP/AlSb/GaP, GaP/GaN/GaP, GaP/GaAs/GaP, GaP/GaSb/GaP, GaP/InNGaP,
GaP/InP/GaP, GaP/InAs/GaP, GaP/InSb/GaP, GaAs/AlP/GaAs,
GaAs/AlAs/GaAs, GaAs/AlSb/GaAs, GaAs/GaN/GaAs, GaAs/GaP/GaAs,
GaAs/GaSb/GaAs, GaAs/InN/GaAs, GaAs/InP/GaAs, GaAs/InAs/GaAs,
GaAs/InSb/GaAs, GaSb/AlP/GaSb, GaSb/AlAs/GaSb, GaSb/AlSb/GaSb,
GaSb/GaN/GaSb, GaSb/GaP/GaSb, GaSb/GaAs/GaSb, GaSb/InN/GaSb,
GaSb/InP/GaSb, GaSb/InAs/GaSb, GaSb/InSb/GaSb, InN/AlP/InN,
InN/AlAs/InN, InN/AlSb/InN, InN/GaN/InN, InN/GaP/InN, InN/GaAs/InN,
InN/GaSb/InN, InN/InP/InN, InN/InAs/InN, InN/InSb/InN, InP/AlP/InP,
InP/AlAs/InP, InP/AlSb/InP, InP/GaN/InP, InP/GaP/InP, InP/GaAs/InP,
InP/GaSb/InP, InP/InN/InP, InP/InAs/InP, InP/InSb/InP,
InAs/AlP/InAs, InAs/AlAs/InAs, InAs/AlSb/InAs, InAs/GaN/InAs,
InAs/GaP/InAs, InAs/GaAs/InAs, InAs/GaSb/InAs, InAs/InN/InAs,
InAs/InP/InAs, InAs/InSb/InAs, InSb/AlP/InSb, InSb/AlAs/InSb,
InSb/AlSb/InSb, InSb/GaN/InSb, InSb/GaP/InSb, InSb/GaAs/InSb,
InSb/GaSb/InSb, InSb/InN/InSb, InSb/InP/InSb, InSb/InAs/InSb.
IV-VI/IV-VI/IV-VI Material
[0108] Comprising a core semiconductor material comprising of a
first element from group 14 of the periodic table and a second
element from group 16 of the periodic table, a first layer
comprising of a first element from group 14 of the periodic table
and a second element from group 16 of the periodic table and a
second layer comprising of a first element from group 14 of the
periodic table and a second element from group 16 of the periodic
table and also including ternary and quaternary materials and doped
materials. Nanoparticle materials include but are not restricted
to:--PbS/PbSe/PbS, PbS/PbTe/PbS, PbS/Sb.sub.2Te.sub.3/PbS,
PbS/SnS/PbS, PbS/SnSe/PbS, PbS/SnTe/PbS, PbSe/PbS/PbSe,
PbSe/PbTe/PbSe, PbSe/Sb.sub.2Te.sub.3/PbSe, PbSe/SnS/PbSe,
PbSe/SnSe/PbSe, PbSe/SnTe/PbSe, PbTe/PbS/PbTe, PbTe/PbSe/PbTe,
PbTe/Sb.sub.2Te.sub.3/PbTe, PbTe/SnS/PbTe, PbTe/SnSe/PbTe,
PbTe/SnTe/PbTe, Sb.sub.2Te.sub.3/PbS/Sb.sub.2Te.sub.3,
Sb.sub.2Te.sub.3/PbSe/Sb.sub.2Te.sub.3,
Sb.sub.2Te.sub.3/PbTe/Sb.sub.2Te.sub.3,
Sb.sub.2Te.sub.3/SnS/Sb.sub.2Te.sub.3,
Sb.sub.2Te.sub.3/SnSe/Sb.sub.2Te.sub.3,
Sb.sub.2Te.sub.3/SnTe/Sb.sub.2Te.sub.3, SnS/PbS/SnS, SnS/PbSe/SnS,
SnS/PbTe/SnS, SnS/Sb.sub.2Te.sub.3/SnS, SnS/SnSe/SnS, SnS/SnTe/SnS,
SnSe/PbSe/SnSe, SnSe/PbS/SnSe, SnSe/PbTe/SnSe,
SnSe/Sb.sub.2Te.sub.3/SnSe, SnSe/SnS/SnSe, SnSe/SnTe/SnSe,
SnTe/PbS/SnTe, SnTe/PbSe/SnTe, SnTe/PbTe/SnTe,
SnTe/Sb.sub.2Te.sub.3/SnTe, SnTe/SnS/SnTe, SnTe/SnSe/SnTe.
DEFINITIONS RELATING TO THE INVENTION
Semiconductor Nanoparticle
[0109] Semiconductor nanoparticles are also known as nanocrystals
or quantum dots and generally possess a core surrounded by at least
one shell of semiconductor material. Nanoparticles comprising a
core and a plurality of shells are known as core/multi-shell
nanoparticles. An important class of core/multi-shell nanoparticles
are quantum dot-quantum wells which possess an architecture whereby
there is a central core of one material overlaid by another
material which is further over layered by another material in which
adjacent layers comprise different semiconductor materials.
Ternary Phase
[0110] By the term ternary phase nanoparticle for the purposes of
specifications and claims, refer to nanoparticles of the above but
having a core and/or at least one shell layer comprising a three
component material. The three components are usually compositions
of elements from the as mentioned groups, for example
(Zn.sub.xCd.sub.(1-x)mL.sub.n nanocrystal (where L is a capping
agent and 0<x<1).
Quaternary Phase
[0111] By the term quaternary phase nanoparticle for the purposes
of specifications and claims, refer to nanoparticles of the above
but having a core or at least one shell comprising a four-component
material. The four components are usually compositions of elements
from the as mentioned groups, example being
(Zn.sub.xCd.sub.x-1S.sub.ySe.sub.y-1)L nanocrystal (where L is a
capping agent, 0<x<1 and 0<y<1).
Solvothermal
[0112] By the term Solvothermal for the purposes of specifications
and claims, refer to heating the reaction solution so as to
initiate and sustain particle growth or to initiate a chemical
reaction between precursors to initiate particle growth and can
also take the meaning solvothermal, thermolysis, thermolsolvol,
solution-pyrolysis, lyothermal.
Core-Shell and Core/Multi Shell(Quantum Dot-Quantum Well)
Particles
[0113] The material used on any shell or subsequent numbers of
shells grown onto the core particle in most cases will be of a
similar lattice type material to the core material i.e. have close
lattice match to the core material so that it can be epitaxially
grown on to the core, but is not necessarily restricted to
materials of this compatibility. The material used on any shell or
subsequent numbers of shells grown on to the core present in most
cases will have a wider band-gap then the core material but is not
necessarily restricted to materials of this compatibility.
Capping Agent
[0114] The outer most layer (capping agent) of organic material or
sheath material is to inhibit particles aggregation and to protect
the nanoparticle from the surrounding chemical environment and to
provide a means of chemical linkage to other inorganic, organic or
biological material. The capping agent can be the solvent that the
nanoparticle preparation is undertaken in, and consists of a Lewis
base compound whereby there is a lone pair of electrons that are
capable of donor type coordination to the surface of the
nanoparticle and can include mono- or multi-dentate ligands of the
type but not restricted to:--phosphines (trioctylphosphine,
triphenolphosphine, t-butylphosphine), phosphine oxides
(trioctylphosphine oxide), alkyl-amine (hexadecylamine,
octylamine), ary-amines, pyridines, and thiophenes.
[0115] The outer most layer (capping agent) can consist of a
coordinated ligand that processes a functional group that can be
used as a chemical linkage to other inorganic, organic or
biological material such as but not restricted
to:--mercaptofunctionalized amines or mercaptocarboxylic acids.
[0116] The outer most layer (capping agent) can consist of a
coordinated ligand that processes a functional group that is
polymerisable and can be used to form a polymer around the
particle, polymerisable ligands such as but not limited to styrene
functionalized amine, phosphine or phosphine oxide ligand.
Nanoparticle Shape
[0117] The shape of the nanoparticle is not restricted to a sphere
and can consist of but not restricted to a rod, sphere, disk,
tetrapod or star. The control of the shape of the nanoparticle is
by the addition of a compound that will preferentially bind to a
specific lattice plane of the growing particle and subsequently
inhibit or slow particle growth in a specific direction. Example of
compounds that can be added but is not restricted to
include:--phosphonic acids (n-tetradecylphosphonic acid,
hexylphosphonic acid, 1-decanesulfonic acid, 12-hydroxydodecanoic
acid, n-octadecylphosphonic acid).
Description of Preparative Procedure
[0118] The current invention should lead to pure, monodispersed,
nanocrystalline particles of the materials as described above, that
are stabilized from particle aggregation and the surrounding
chemical environment by a capping agent, such as an organic
layer.
Synthetic Method Employed
[0119] The synthetic method employed to produce the initial core
and core-shell material can either be by the conventional method of
high temperature rapid injection "nucleation and growth" as in the
fourth aspect of the present invention or where larger quantities
of material is required by a seeding process using of a molecular
cluster with dual precursors in accordance with the first and
fourth aspects of the present invention.
[0120] Further consecutive treatment of the as formed nanoparticles
(ZnS and Cd.sub.xZn.sub.1-xS) to form core-shell and then quantum
dot-quantum well particles may be undertaken. Core-shell particle
preparation is undertaken either before or after nanoparticle
isolation, whereby the nanoparticles are isolated from the reaction
and redissolved in new (clean) capping agent/solvent, this can
result in a better quantum yield.
[0121] For II-VI material, a source for II and a source for VI
precursor are added to the reaction mixture and can be either in
the form of two separate precursors one containing I element, and
the other containing VI element or as a single-source precursor
that contains both II and VI within a single molecule to form a
core or shell layer of II-VI material (e.g. where II=Cd, Zn, VI=S,
Se).
[0122] For I-III-VI.sub.2 material, a source for I (group 11 of the
periodic table), a source for III and a source for VI element
precursor are added to the reaction mixture and can be either in
the form of three separate precursors one containing I element, one
containing III element and the other containing VI or as a
single-source precursor that contains both I and VI and III and VI
within a single molecules to form the I-III-VI.sub.2 layer (where
I.dbd.Cu and III=In, Ga and VI=S, Se), or a single-source precursor
which contains all three elements.
[0123] For II-V material, a source for II and a source for V
precursor are added to the reaction mixture and can be either in
the form of two separate precursors one containing a group II
element, and the other containing V element or as a single-source
precursor that contains both II and V within a single molecule to
form a core or shell layer of II-V material (where II=Zn, Cd, Hg
V.dbd.N, P, As, Sb, Bi).
[0124] For III-V material, a source for III and a source for V
precursor are added to the reaction mixture and can be either in
the form of two separate precursors one containing III, and the
other containing V or as a single-source precursor that contains
both III and V within a single molecules to form a core or shell
layer of III-V material (where III=In, Ga, Al, B, V.dbd.N, P, As,
Sb, Bi).
[0125] For IV-VI material, a source for IV and a source for VI
precursor are added to the reaction mixture and can be either in
the form of two separate precursors one containing IV element, and
the other containing VI element or as a single-source precursor
that contains both IV and VI within a single molecule to form a
core or shell layer of IV-VI material (where IV=Si, C, Ge, Sn, Pb
VI=S, Se, Te).
[0126] The process may be repeated with the appropriate element
precursors until the desired quantum dot-quantum well or
core/multi-shell material is formed. The nanoparticles size and
size distribution in an ensemble of particles is dependent on the
growth time, temperature and concentrations of reactants in
solution, with higher temperatures generally producing larger
nanoparticles.
Precursor Materials Used to Grow the Quantum Dot-Quantum Well
Structures
Core Material Source--Multi-Source Precursor Materials
Metal Ions
[0127] For a compound semiconductor nanoparticle comprising a core
semiconductor material of, for example, (ZnS)L or
(Cd.sub.xZn.sub.1-xS)L (where L is a ligand or capping agent) a
source for element Zn and Cd is further added to the reaction and
can consist of any Zn or Cd-containing compound that has the
ability to provide the growing particles with a source of Zn or Cd
ions. The precursor can comprise but is not restricted to an
organometallic compound, an inorganic salt, a coordination compound
or the element.
[0128] Examples for II-VI, for the first element include but are
not restricted to:--
[0129] Organometallic such as but not restricted to a MR.sub.2
where M=Mg R=alky or aryl group (Mg.sup.tBu.sub.2); MR.sub.2 where
M=Zn, Cd; R=alky or aryl group (Me.sub.2Zn, Et.sub.2Zn Me.sub.2Cd,
Et.sub.2Cd); MR.sub.3.
[0130] Coordination compound such as a carbonate or a
.beta.-diketonate or derivative thereof, such as acetylacetonate
(2,4-pentanedionate) [CH.sub.3COOCH.dbd.C(O--)CH.sub.3].sub.2 M=Zn,
Cd;
[0131] Inorganic salt such as but not restricted to a Oxides ZnO,
CdO, Nitrates Mg(NO.sub.3).sub.2, Cd(NO.sub.3).sub.2,
Zn(NO.sub.3).sub.2, M(CO.sub.3).sub.2 M=Zn, Cd;
M(CH.sub.3CO.sub.2).sub.2 M=Zn, Cd,
[0132] An element Zn, Cd,
Non-Metal Ions
[0133] For a compound semiconductor nanoparticle comprising, for
example, (ZnE).sub.nL.sub.m or
(Cd.sub.xZn.sub.(1-x)E).sub.nL.sub.m, a source of E ions, where E
is a non-metal, for example, sulfur or selenium, is further added
to the reaction and can consist of any E-containing compound that
has the ability to provide the growing particles with a source of E
ions. n and m are numerical values selected to provide the desired
compound. L is a ligand, such as a capping agent. The precursor can
comprise but is not restricted to an organometallic compound, an
inorganic salt, a coordination compound or an elemental source.
[0134] Examples for an II-VI, semiconductor where the second
elements include but are not restricted to:--
[0135] ER.sub.2 (E=S or Se; R=Me, Et, .sup.tBu, .sup.iBu etc.); HER
(E=S or Se; R=Me, Et, .sup.tBu, .sup.iBu, .sup.iPr, Ph etc);
thiourea S.dbd.C(NH.sub.2).sub.2.
[0136] An element S or Se. An elemental source can be used whereby
the element is directly added to the reaction or is coordinated to
a .sigma.-donor Lewis base compound (two electron pair donor); such
as elemental sulfur or selenium coordinating to TOP
(tri-octyl-phosphine) to form TOPS and TOPSe respectively or the
use of other Lewis bases such as phosphines, amines or phosphine
oxides but not restricted to, such as in the case of using
octylamine to coordinate sulfur.
Core Material Source--Single-Source Precursor Materials
[0137] For a compound semiconductor nanoparticle comprising, for
example, elements ZnS or Cd.sub.xZn.sub.(1-x)S a source for Zn or
Cd and S can be in the from of a single-source precursor, whereby
the precursor to be used contains both Zn or Cd and S within a
single molecule. This precursor can be an organometallic compound
and inorganic salt or a coordination compound,
(Zn.sub.aS.sub.b)L.sub.c or (Cd.sub.xZn.sub.(1-x)S).sub.nL.sub.m
Where Zn or Cd and S are the elements required within the
nanoparticles and L is the capping ligands.
[0138] Examples for an II-VI semiconductor where M=II and E=VI
element can be but is not restricted to
bis(dialkyldithio-carbamato)M, (II) complexes compounds of the
formula M(S.sub.2CNR.sub.2).sub.2 M=Zn, Cd; S.dbd.S, and R=alkyl or
aryl groups; CdS Cd[SSiMe.sub.3].sub.2,
Cd(SCNHNH.sub.2).sub.2Cl.sub.2, Cd(SOCR).sub.2.py;
[RME.sup.tBu].sub.5 M=Zn, Cd; E=S; R=Me, Et, Ph;
[X].sub.4[E.sub.4M.sub.10(SR).sub.16] E=S, M=Zn, Cd;
X=Me.sub.3NH.sup.+, Li.sup.+, Et.sub.3NH.sup.+R=Me, Et, Ph;
[Cd.sub.32S.sub.14(SPh).sub.36].L; [M.sub.4(SPh).sub.12].sup.+
[X].sub.2.sup.-M=Zn, Cd, X=Me.sub.4N.sup.+, Li.sup.+;
[Zn(SEt)Et].sub.10:
[MeMe.sup.iPr] M=Zn, Cd, E=S; [RCdSR'].sub.5 R.dbd.O(ClO.sub.3),
R'.dbd.PPh.sub.3, .sup.iPr;
[Cd.sub.10S.sub.4(S'Ph).sub.12(PR.sub.3).sub.4].
[(.sup.tBu)GaSe].sub.4; [.sup.tBuGaS].sub.7; [RInSe].sub.4
R=.sup.tBu, CMe.sub.2Et, Si(.sup.tBu).sub.3, C(SiMe.sub.3).sub.3;
[RInS].sub.4 R=.sup.tBu, CMe.sub.2Et; [RGaS].sub.4 R=.sup.tBu,
CMe.sub.2Et, CEt.sub.3; [SAlR'].sub.4 R.dbd.C(SMe.sub.3).sub.3,
CEtMe.sub.2; [(C(SiMe.sub.3).sub.3)GaS].sub.4; [.sup.tBuGaS].sub.6;
[RGaSe].sub.4 R=.sup.tBu, CMe.sub.2Et, CEt.sub.3,
C(SiMe.sub.3).sub.3, Cp*, [Cu.sub.12Se.sub.6(PR.sub.3).sub.8]
R=Et.sub.2Ph, .sup.nPr.sub.3, Cy.sub.3.
First Semiconductor Materials
For Use in First Layer
[0139] For a compound semiconductor quantum dot-quantum well
nanoparticle comprising a first layer of, for example,
I-III-VI.sub.2 or II-VI material, sources for element I, III, VI or
II are added to the reaction and can consist of any I, III, VI or
II-containing compound that has the ability to provide the growing
particles with a source of E ions. The precursor can consist of but
are not restricted to an organometallic compound, an inorganic
salt, a coordination compound or an elemental source. Examples
include but are not restricted to:--
Group I Source (e.g. Cu)
[0140] But is not restricted to:--CuX where X.dbd.Cl, Br, I;
Copper(II) acetate (CH.sub.3CO.sub.2).sub.2Cu, Copper(I) acetate
CH.sub.3CO.sub.2Cu, copper(II) acetylacetonate
[CH.sub.3COCH.dbd.C(O.sup.-)CH.sub.3].sub.2Cu and other
.beta.-diketonate, copper(I) butanethioate
CH.sub.3(CH.sub.2).sub.3SCu, Copper(II) nitrate Cu(NO.sub.3).sub.2,
CuO.
Group II Source (e.g. Mg)
[0141] Organometallic such as but not restricted to a MR.sub.2
where M=Mg R=alky or aryl group (Mg.sup.tBu.sub.2); MR.sub.2 where
M=Zn, Cd; R=alky or aryl group (Me.sub.2Zn, Et.sub.2Zn Me.sub.2Cd,
Et.sub.2Cd); MR.sub.3.
[0142] Coordination compound such as a carbonate or a
.beta.-diketonate or derivative thereof, such as acetylacetonate
(2,4-pentanedionate) [CH.sub.3COOCH.dbd.C(O.sup.-)CH.sub.3].sub.2
M=Zn, Cd;
[0143] Inorganic salt such as but not restricted to an Oxide, e.g
ZnO, CdO, a Nitrate, e.g. Mg(NO.sub.3).sub.2, Cd(NO.sub.3).sub.2,
Zn(NO.sub.3).sub.2, M(CO.sub.3).sub.2 M=Zn, Cd;
M(CH.sub.3CO.sub.2).sub.2 M=Zn, Cd,
[0144] An element Zn, Cd,
Group III Source (e.g. In and Ga)
[0145] But is not restricted to:--
[0146] MR.sub.3 Where M=Ga, In, Al, B; R=alky or aryl group
[AlR.sub.3, GaR.sub.3, InR.sub.3 (R=Me, Et, .sup.iPr)].
[0147] Coordination compound such as a .beta.-diketonate or
derivative thereof, such as
[CH.sub.3COOCH.dbd.C(O.sup.-)CH.sub.3].sub.2 M=Al, Ga, In.
[0148] Inorganic salt such as but not restricted to an Oxide, e.g.
In.sub.2O.sub.3, Ga.sub.2O.sub.3; a Nitrate, e.g.
In(NO.sub.3).sub.3, Ga(NO.sub.3).sub.3; M(CH.sub.3C).sub.3 M=Al,
Ga, In
[0149] An element Ga, In.
Group VI Source (S or Se)
[0150] MR.sub.2 (M=S, Se; R=Me, Et, .sup.tBu, .sup.iBu etc.); HMR
(M=S, Se; R=Me, Et, .sup.tBu, .sup.iBu, .sup.iPr, Ph etc); thiourea
S.dbd.C(NH.sub.2).sub.2; Se.dbd.C(NH.sub.2).sub.2.
[0151] An element S, Se. An elemental source can be used whereby
the element is directly added to the reaction or is coordinated to
a .sigma.-donor Lewis base compound (two electron pair donor); such
as elemental sulfur or selenium coordinating to TOP
(tri-octyl-phosphine) to form TOPS and TOPSe respectively or the
use of other Lewis bases such as phosphines, amines or phosphine
oxides but not restricted to, such as in the case of using
octylamine to coordinate sulfur.
First Semiconductor Materials--Single-Source Precursors
[0152] Examples for an II-VI semiconductor where M=II and E=VI
element can be but is not restricted to
bis(dialkyldithio-carbamato)M, (II) complexes compounds of the
formula M(S.sub.2CNR.sub.2).sub.2 M=Zn, Cd; S.dbd.S, and R=alkyl or
aryl groups; CdS Cd[SSiMe.sub.3].sub.2,
Cd(SCNHNH.sub.2).sub.2Cl.sub.2, Cd(SOCR).sub.2.py;
[RME.sup.tBu].sub.5 M=Zn, Cd; E=S.sub.5; R=Me, Et, Ph;
[X].sub.4[E.sub.4M.sub.10(SR).sub.16] E=S, M=Zn, Cd;
X=Me.sub.3NH.sup.+, Li.sup.+, Et.sub.3NH.sup.+R=Me, Et, Ph;
[Cd.sub.32S.sub.14(SPh).sub.36].L;
[M.sub.4(SPh).sub.12].sup.+[X].sub.2.sup.-M=Zn, Cd,
X=Me.sub.4N.sup.+, Li.sup.+; [Zn(SEt)Et].sub.10:
[MeMe.sup.iPr] M=Zn, Cd, E=S; [RCdSR'], R.dbd.O(ClO.sub.3),
R'.dbd.PPh.sub.3, .sup.iPr;
[Cd.sub.10S.sub.4(S'Ph).sub.12(PR.sub.3).sub.4].
[(.sup.tBu)GaSe].sub.4; [.sup.tBuGaS].sub.7; [RInSe].sub.4
R=.sup.tBu, CMe.sub.2Et, Si(.sup.tBu).sub.3, C(SiMe.sub.3).sub.3;
[RInS].sub.4 R=.sup.tBu, CMe.sub.2Et; [RGaS].sub.4 R=.sup.tBu,
CMe.sub.2Et, CEt.sub.3; [SAlR'].sub.4 R.dbd.C(SMe.sub.3).sub.3,
CEtMe.sub.2; [(C(SiMe.sub.3).sub.3)GaS].sub.4; [.sup.tBuGaS].sub.6;
[RGaSe].sub.4 R=.sup.tBu, CMe.sub.2Et, CEt.sub.3,
C(SiMe.sub.3).sub.3, Cp*, [Cu.sub.12Se.sub.6(PR.sub.3).sub.8]
R=Et.sub.2Ph, .sup.nPr.sub.3, Cy.sub.3.
Second Semiconductor Materials
For Use in Second, Outer or any Other Subsequent Layers
[0153] The precursor(s) used to provide the second semiconductor
material may be chosen from the same lists of materials set out
above in respect of the first semiconductor material.
[0154] For a quantum dot-quantum well with the second or outer most
layer comprising, for example, (ZnS).sub.nL.sub.m or
(Cd.sub.xZn.sub.(1-x)S).sub.nL.sub.m a source for element Zn and Cd
is further added to the reaction and can consist of any Zn or
Cd-containing compound that has the ability to provide the growing
particles with a source of Zn or Cd ions. The precursor can consist
of but are not restricted to an organometallic compound, an
inorganic salt, a coordination compound or the element.
[0155] Examples for II-VI, for the first element include but are
not restricted to:--
[0156] Organometallic such as but not restricted to a MR.sub.2
where M=Mg R=alky or aryl group (Mg.sup.tBu.sub.2); MR.sub.2 where
M=Zn, Cd; R=alky or aryl group (Me.sub.2Zn, Et.sub.2Zn Me.sub.2Cd,
Et.sub.2Cd); MR.sub.3.
[0157] Coordination compound such as a carbonate or a
.beta.-diketonate or derivative thereof, such as acetylacetonate
(2,4-pentanedionate) [CH.sub.3COOCH.dbd.C(O.sup.-)CH.sub.3].sub.2
M=Zn, Cd;
[0158] Inorganic salt such as but not restricted to a Oxides ZnO,
CdO, Nitrates Mg(NO.sub.3).sub.2, Cd(NO.sub.3).sub.2,
Zn(NO.sub.3).sub.2, M(CO.sub.3).sub.2 M=Zn, Cd;
M(CH.sub.3CO.sub.2).sub.2 M=Zn, Cd,
[0159] An element Zn, Cd.
Non-Metal Ions
[0160] For a compound semiconductor nanoparticle comprising, for
example (ZnS).sub.nL.sub.m or (Cd:ZnS).sub.nL.sub.m a source for
non-metal ions, E, e.g. sulfur is further added to the reaction and
can consist of any E-containing compound that has the ability to
provide the growing particles with a source of E ions. The
precursor can consist of but are not restricted to an
organometallic compound, an inorganic salt, a coordination compound
or an elemental source. Examples for an II-VI, semiconductor where
the second elements include but are not restricted to:--
[0161] MR.sub.2 (M=S; R=Me, Et, .sup.tBu, .sup.iBu etc.); HMR (M=S;
R=Me, Et, .sup.tBu, .sup.iBu, .sup.iPr, Ph etc); thiourea
S.dbd.C(NH.sub.2).sub.2.
[0162] An element S or Se. An elemental source can be used whereby
the element is directly added to the reaction or is coordinated to
a .sigma.-donor Lewis base compound (two electron pair donor); such
as elemental sulfur or selenium coordinating to TOP
(tri-octyl-phosphine) to form TOPS and TOPSe respectively or the
use of other Lewis bases such as phosphines, amines or phosphine
oxides but not restricted to, such as in the case of using
octylamine to coordinate sulfur.
Second Semiconductor Materials--Single-Source Precursors
[0163] For a compound semiconductor nanoparticle comprising of
elements ZnS or Cd.sub.xZn.sub.(1-x)S a source for Zn or Cd and S
source can also be in the from of a single-source precursor,
whereby the precursor to be used contains both Zn or Cd and S
within the single molecule. This precursor can be an organometallic
compound and inorganic salt or a coordination compound,
(Zn.sub.aS.sub.b)L.sub.c or (Cd.sub.xZn.sub.(1-x)S).sub.nL.sub.m
Where Zn or Cd and S are the elements required within the
nanoparticles and L is the capping ligands.
[0164] Examples for an II-VI semiconductor were M=II and E=VI
element can be but is not restricted to
bis(dialkyldithio-carbamato)M, (II) complexes compounds of the
formula M(S.sub.2CNR.sub.2).sub.2 M=Zn, Cd; S.dbd.S, and R=alkyl or
ary groups; CdS Cd[SSiMe.sub.3].sub.2,
Cd(SCNHNH.sub.2).sub.2Cl.sub.2, Cd(SOCR).sub.2.py;
[RME.sup.tBu].sub.5 M=Zn, Cd; E=S; R=Me, Et, Ph:
[X].sub.4[E.sub.4M.sub.10(SR).sub.16] E=S, M=Zn, Cd;
X=Me.sub.3NH.sup.+, Li.sup.+, Et.sub.3NH.sup.+:
[Cd.sub.32S.sub.14(SPh).sub.36].L:
[M.sub.4(SPh).sub.12].sup.+[X].sub.2.sup.-M=Zn, Cd;
X=Me.sub.4N.sup.+, Li.sup.+: [Zn(SEt)Et].sub.10: [MeMe.sup.iPr]
M=Zn, Cd; E=S: [RCdSR'].sub.5 R.dbd.O(ClO.sub.3), R'.dbd.PPh.sub.3,
.sup.iPr: [Cd.sub.10S.sub.4(S'Ph).sub.12(PR.sub.3).sub.4]
Detailed Discussion
[0165] The synthesis of quantum dot-quantum wells is preferably a
three-step process, optionally involving isolation of the product
of a step prior to further modification to provide the next layer
of the nanoparticle structure. By way of example, for the
nanoparticle, ZnS/CdSe/Cd.sub.xZn.sub.1-xS, the cores are
synthesized and isolated from a growth solution and the first shell
is grown onto the cores in a separate reaction and isolated once
again. Finally an outer Cd.sub.xZn.sub.1-xS shell layer is grown
onto the core-shell structure to produce the
ZnS/CdSe/Cd.sub.xZn.sub.1-xS quantum dot-quantum well.
Synthesis of ZnS Cores
[0166] Zinc sulfide (or cadmium/zinc sulphide) particles were
synthesized by a number of methods when a small quantity was needed
by decomposing [Et.sub.3NH].sub.4[Zn.sub.10S.sub.4(SPh).sub.16]
clusters in HDA at 180.degree. C. and heating to 250.degree. C. or
300.degree. C. to produce 2 nm or 5.5 nm diameter ZnS
particles.
Synthesis of ZnS/CdSe Core Shell Dots
[0167] Either a combination of two precursors was used such as
Me.sub.2Cd and TOPSe or a single-source precursor such as
[Et.sub.3NH].sub.4[Cd.sub.10Se.sub.4(SPh).sub.16] was used as
precursors for the formation of the CdSe layer. The precursors
decompose onto the ZnS cores enabled the synthesis of multi-gram
quantities of ZnS/CdSe core-shell particles.
Quantum Well Modifications ZnS/CdSe/Cd.sub.xZn.sub.1-xS
[0168] Growth of the Cd.sub.xZn.sub.1-xS shell is performed at a
low temperature and added very slowly to prevent thick shell growth
and renucleation of CdSe nanoparticles. The likelihood of alloying
is minimal at this growth temperature. The ZnS/CdSe core-shell
nanocrystals exhibit quantum efficiencies of about 3%. The growth
of the outer Cd.sub.xZn.sub.1-xS also shifts the emission and the
first absorption feature by about 2 nm. Again, similar shifts in
the emission/absorption are common with CdSe or CdS overcoated with
ZnS.
Cadmium-Free Quantum Dot-Quantum Wells
[0169] There is also a great need for quantum dots that perform
similarly to CdSe--ZnS core-shell quantum or quantum dot-quantum
wells that are cadmium free. Nanoparticles in accordance with the
present invention may therefore be produced which include a layer
of cadmium-free semiconductor material in place of a
cadmium-containing layer. For example, the nanoparticles
ZnS/CuInS.sub.2/ZnS and ZnS/CuInSe.sub.2/ZnS can be produced in
accordance with the method of the present invention and used in
place of ZnS/CdS/ZnS and ZnS/CdSe/ZnS.
ZnS/CuInS.sub.2 Core/Shell Structure
[0170] This was achieved by using either a combination of
precursors each containing just one element required within the
final composite nanoparticle or by the use of single-source
precursors which contain all or more than one element required
within the final composite.
Multi-Source Precursors
[0171] ZnS core particles were dissolved in warm capping
agent/solvent such as HDA-hexanethiol or TOPO-hexanethiol followed
by the addition of a copper source, an indium source and a sulfur
source such as CuI dissolved in an amine, InI.sub.3 dissolved in an
amine and sulfur coordinated to TOP to give TOPS. The growth of the
CuInS.sub.2 shell onto the ZnS cores is achieved by the addition of
the above precursors to the HDA-hexanethiol solution while
increasing the temperature between 150.degree. and 300.degree. C.
The solution was then cooled to 150.degree. C. before further
precursor additions, this being repeated until the desired emission
wavelength was achieved. The particles-containing solution was then
cooled and the particles isolated using excess methanol.
Single-Source Precursors
[0172] Single-source precursors may be used such as
(Ph.sub.3P).sub.2CuIn(SEt).sub.4 or a combination of single-source
precursors such as In(S.sub.2CNEt.sub.2).sub.3 and
Cu(S.sub.2CNEt.sub.2).sub.2.
ZnS/CuInSe.sub.2 Core/Shell Structure
[0173] This was achieved by using either a combination of
precursors each containing just one element required within the
final composite nanoparticle or by the use of single-source
precursors which contain all or more than one element required
within the final composite.
Multi-Source Precursors
[0174] ZnS core particles were dissolved in warm capping
agent/solvent such as HDA or TOPO-hexanethiol mix followed by the
addition of a copper source an indium source and a selenium source
such as CuI dissolved in an amine, InI.sub.3 dissolved in an amine
and selenium coordinated to TOP to give TOPSe. The growth of the
CuInSe.sub.2 shell onto the ZnS cores is achieved by the addition
of the above precursors to the HDA-hexanethiol solution while
increasing the temperature between 150.degree. and 300.degree. C.
The solution was then cooled to 150.degree. C. before further
additions, this being repeated until the desired emission
wavelength was achieved. The particles containing solution was then
cooled and the particles isolated using excess methanol.
Single-Source Precursors
[0175] Single-source precursors may be used such as
(Ph.sub.3P).sub.2CuIn(SeEt).sub.4 or a combination of single-source
precursors such as In(Se.sub.2CNEt.sub.2).sub.3 and
Cu(Se.sub.2CNEt.sub.2).sub.2.
ZnS/CuInS.sub.2/ZnS and ZnS/CuInSe.sub.2/ZnS Core/Multishell
Nanoparticles
[0176] The amount of zinc and sulfur precursor used was varied
depending on the thickness of the outer ZnS shell required.
ZnS/CuInS.sub.2 or ZnS/CuInSe.sub.2 particles were added to
degassed HDA at 70.degree. C. and heated to 180-200.degree. C.
Me.sub.2Zn and sulfur solutions were used to grow the outer ZnS
layers by dropwise addition until the desired ZnS shell thickness
was reached.
[0177] By the use of an in situ optical probe, moreover, an Ocean
Optics USB2000 spectrometer, the progressive formation/growth of
the core, core-shell or quantum-well particle can be followed by
the maximum of the photoluminescence emission peak or the maximum
of the absorption spectra, when the required the photoluminescence
emission was achieved the reaction was stopped by cooling the
reaction solution.
[0178] The present invention is illustrated with reference to the
following figures and non-limiting Example and Reference Examples,
in which:
[0179] FIG. 1 is an illustration of a) Core nano-particle
comprising of a ZnS core and HDA as an organic capping agent, b)
core-shell particle comprising of a ZnS core a CdSe shell and HDA
as an organic capping agent, c) quantum dot-quantum well organic
capped particle comprising of a ZnS core a CdSe shell followed by a
Cd.sub.xZn.sub.1-xS shell with a HDA capping agent;
[0180] FIG. 2 is an illustration of a) Core nano-particle
comprising of a ZnS core and HDA as an organic capping agent, b)
core-shell particle comprising of a ZnS core a CdSe shell and HDA
as an organic capping agent, c) quantum dot-quantum well organic
capped particle comprising of a ZnS core a CdSe shell followed by a
ZnS shell with a HDA capping agent d) quantum dot-multi quantum
well comprising of a ZnS core a CdSe shell followed by a shell of
CdS followed by another shell of ZnS with a HDA capping agent;
[0181] FIG. 3 is a diagram of a) core particle comprising of a ZnS
core and HDA as an organic capping agent, b) core-shell particle
comprising of a ZnS core a CuInS.sub.2 shell and HDA as an organic
capping agent, c) quantum dot-quantum well organic capped particle
comprising of a ZnS core a CuInS.sub.2 central layer followed by a
ZnS shell with a HDA capping agent;
[0182] FIG. 4 illustrates properties of ZnS core quantum dots a)
excitation (to the left) and emission spectra of 5.5 nm ZnS
nanocrystals. (b) Powder x-Ray diffraction pattern of 5.5 nm. (c)
Transmission electron micrograph (TEM) image of 5.5 nm ZnS core.
Inset shows a high-resolution image of a single ZnS particle;
[0183] FIG. 5 shows absorption and photoluminescence spectra for a
core-shell ZnS--CdSe quantum dots with an outer capping layer of
hexadecylamine (HDA), with the absorption maximum at 440 nm and the
emission maximum at 460 nm;
[0184] FIG. 6 shows absorption and PL spectra of
ZnS/CdSe/Cd.sub.xZn.sub.(1-x)S quantum well nanocrystals. The
longest wavelength adsorption feature occurs at .lamda.=453 nm and
the maximum emission peak is at .lamda.=472 nm;
[0185] FIG. 7 shows absorption and PL spectra of ZnS cores;
[0186] FIG. 8 shows absorption and PL spectra of ZnSe cores;
[0187] FIGS. 9A and 9B show absorption and PL spectra of ZnS/InP
core/shell nanocrystals respectively;
[0188] FIGS. 10A and 10B show absorption and PL spectra of ZnS/InP
core/shell nanocrystals respectively in which the ZnS cores are
larger than those shown in FIGS. 9A and 9B;
[0189] FIGS. 11A and 11B show PL and absorption spectra of
ZnS/InP/ZnS quantum well nanocrystals; and
[0190] FIG. 12 shows a PL spectrum for the growth of ZnSe quantum
dots.
EXAMPLES
[0191] All syntheses and manipulations were carried out under a dry
oxygen-free argon or nitrogen atmosphere using standard Schlenk or
glove box techniques. All solvents were distilled from appropriate
drying agents prior to use (Na/K-benzophenone for THF, Et.sub.2O,
toluene, hexanes and pentane). HDA, octylamine, hexanethiol,
dioctylphalate, TOP, Cd(CH.sub.3CO.sub.2).sub.2, sulfur, selenium
powder, CdO.sub.2, CdCO.sub.3, InI, CuI (Adrich) were procured
commercially and used without further purification.
[0192] UV-vis absorption spectra were measured on a
He.lamda.ios.beta. Thermospectronic. Photoluminescence (PL) spectra
were measured with a Fluorolog-3 (FL3-22) photospectrometer at the
excitation wavelength 380 nm. Powder X-Ray diffraction (PXRD)
measurements were preformed on a Bruker AXS D8 diffractometer using
monochromated Cu-K.sub..alpha. radiation.
Cluster Preparation
Preparation of [HNEt.sub.3].sub.2[Zn.sub.4(SPh).sub.10]
[0193] To a stirred methanol (360 ml) solution of benzenethiol (168
ml, 1.636 mmol) and triethylamine (229 ml, 1.64 mmol) was added
dropwise Zn(NO.sub.3).sub.2.6H.sub.2O (189 g, 0.635 mol) that had
previously been dissolved in methanol (630 ml). The solution was
then allowed to stir while warming until the precipitate had
completely dissolved to leave a clear solution. This was then place
at 5.degree. C. for 24 h in which time large colourless crystals of
[HNEt.sub.3].sub.2[Zn.sub.4(SPh).sub.10] had formed (169 g).
Preparation of [HNEt.sub.3].sub.4[Zn.sub.10S.sub.4(SPh).sub.16]
[0194] To a stirred acetonitrile (100 ml) solution of
[HNEt.sub.3].sub.2[Zn.sub.4(SPh).sub.10] (168.9 g, 0.1086 mol) was
added 3.47 g (0.1084 mmol) of sulfur powder, the resulting slurry
was left to stirrer for 10 minutes. A further 750 ml of
acetonitrile was added and the solution warmed to 75.degree. C. to
give a clear pale yellow solution which was allowed to cool to
5.degree. C., yielding large colourless crystals (74.5 g). The
crystals were washed in hexane to give 71.3 g of
[HNEt.sub.3].sub.4[Zn.sub.10S.sub.4(SPh).sub.16].
Preparation of Quantum Dot Cores (ZnS or Cd.sub.xZn.sub.(1-x)S)
[0195] Method 1--Preparation of ZnS Nanoparticles from
[Et.sub.3NH].sub.4[Zn.sub.10S.sub.4(SPh).sub.16]/TOPS/Me.sub.2Zn in
HDA by Dropwise Addition of Me.sub.2Zn.TOP
[0196] HDA was placed in a three-neck round bottomed flask and
dried and degassed by heating to 120.degree. C. under a dynamic
vacuum for >1 hour. The solution was then cooled to 60.degree.
C. To this was added
[HNEt.sub.3].sub.4[Zn.sub.10S.sub.4(SPh).sub.16]. Initially 4 mmol
of TOPS and 4 mmols of Me.sub.2Zn.TOP were added to the reaction at
room temperature and the temperature increased and allowed to stir
for 2 hours. The temperature was progressively increased at a rate
of .about.1.degree. C./5 min with equimolar amounts of TOPS and
Me.sub.2Zn.TOP being added dropwise as the temperature was steadily
increased. The reaction was stopped when the PL emission maximum
had reached the required emission, by cooling to 60.degree. C.
followed by addition of 300 ml of dry ethanol or acetone. This
produced was isolated by filtration. The resulting ZnS particles
which were recrystallized by re-dissolving in toluene followed by
filtering through Celite followed by re-precipitation from warm
ethanol to remove any excess HDA, selenium or cadmium present.
Method 2 (for Reference Purposes Only)
[0197] 2 nm cores were prepared in 250 g hexadecylamine (HDA) which
was previously degassed at 120.degree. C. for one hour then, under
nitrogen, [Et.sub.3NH].sub.4[Zn.sub.10S.sub.4(SPh).sub.16] (4.75 g,
1.64 mmol) was added and the solution was heated to 250.degree. C.
for 30 minutes which resulted in the nucleation and growth of ZnS
nanoparticles. The resulting solution was then cooled to 65.degree.
C. and the particles were isolated by the addition of 400 ml dry
methanol giving 1.1 g ZnS particles with approximately 20% w/w of
ZnS. To grow 5.5 nm ZnS, the above-mentioned procedure was repeated
at 300.degree. C. growth temperature for 30 minutes giving 0.69 g
ZnS particles with approximately 33% w/w of ZnS.
Synthesis of ZnS/CdSe Composite Quantum Dots
Method 1
[0198] In a typical synthesis, 0.35 g ZnS cores (or approximately
4.9.times.10.sup.7 particles) were added to 100 g of degassed HDA
at 70.degree. C., the solution was then heated to 150.degree. C.
The growth of the CdSe layer onto the ZnS core is achieved by a
successive addition of the cluster
[Et.sub.3NH].sub.4[Cd.sub.10Se.sub.4(SPh).sub.16] to the ZnS-HDA
solution, between 150 to 300.degree. C. for two hours. The solution
was cooled to 150.degree. C. before the further addition of
precursor. The ZnS/CdSe particles were then cooled and isolated
with excess methanol.
Method 2
[0199] In a typical synthesis, ZnS cores were added to degassed and
moisture-free HDA at 70.degree. C., the solution was then heated to
150.degree. C. The growth of the CdSe layer onto the ZnS core is
achieved by a successive addition of Me.sub.2Cd.TOP and TOPSe to
the ZnS-HDA solution, between 150 to 300.degree. C. for two hours.
The solution was then cooled to 150.degree. C. before additional
Me.sub.2Zn'TOP and TOPS were added, this was repeated until the
desired emission wavelength was achieved.
Synthesis of ZnS/CdSe/Cd.sub.xZn.sub.1-xS
[0200] The amount of zinc, cadmium and sulfur precursor used was
varied depending on the thickness of the outer Cd.sub.xZn.sub.1-xS
shell required. The synthesis of ZnS/CdSe/Cd.sub.xZn.sub.1-xS 2.5
ml Me.sub.2Cd (0.05M), 2.5 ml Me.sub.2Zn (0.05M) solutions along
with 5.0 ml 0.05M sulfur solution was added to the ZnS/CdSe cores
to produce ZnS/CdSe/Cd.sub.xZn.sub.1-xS nanoparticles.
Reference Examples
Preparation of ZnS/InP/ZnS and ZnSe/InP/ZnSe Quantum Dot-Quantum
Wells
Preparation of Core ZnS
[0201] HDA(250 g) was placed in a three neck flask and degassed at
120.degree. C. under vacuum for one hour. At 100.degree. C.
[Et.sub.3NH.sub.4][Zn.sub.10S.sub.4(SPh).sub.16] (10 g) was added
and the solution was then heated to 300.degree. C. for 30 minutes.
After 30 minutes, the solution was cooled to 200.degree. C. and the
reaction mixture was annealed for one hour. The reaction mixture
was left to cool overnight to room temperature.
Particles of HDA coated ZnS were isolated by the addition of warm
dry methanol (250 ml). The precipitation of white particles
occurred these were isolated by centrifugation, washed with acetone
and left to dry under nitrogen. Mass of product=1.7442 g. UV-vis
and PL spectra of the ZnS cores are shown in FIG. 7.
Preparation of Core ZnSe
[0202] HDA(150 g) was placed in a three neck flask, dried and
degassed at 120.degree. C. for one hour. After one hour the mixture
was cooled to 60.degree. C.
[Zn.sub.10Se.sub.4(SPh).sub.16][Et.sub.3NH.sub.4] (5 g) was added
to the HDA under nitrogen at 90.degree. C. and left to stir for 5
mins before adding TOPSe (3.53 ml). The reaction mixture changed
colour from colorless to pale yellow. The temperature was increased
to 120.degree. C. The temperature of the reaction mixture was then
increased gradually to 280.degree. C. After 280.degree. C. the
reaction was left to cool. Once the temperature had decreased to
65.degree. C., the particles were isolated by addition of methanol
(250 ml) followed by centrifuged, washed with acetone and left to
dry under nitrogen. Mass of product=1.2443 g. UV-vis and PL spectra
of the ZnSe cores are shown in FIG. 8.
Preparation of Core-Shell ZnS/InP
Method (a)
[0203] Dibutyl ester (50 ml) and stearic acid (5.65 g) were
dried/degassed by heating to between 65-100.degree. C. under vacuum
for 1 hour. The temperature was then increased to 180.degree. C.
followed by the addition of InMe.sub.3 (1.125 ml), (TMS).sub.3P
(1.125 ml) and ZnS particles (0.235 g) and left to stir for 10
mins. The reaction mixture turned pale yellow after 5 mins of
addition. When the reaction temperature had reached 200.degree. C.,
further quantities of InMe.sub.3 (2.25 ml) and (TMS).sub.3P (2.25
ml) were added dropwise which resulted in the colour changing from
pale yellow to clear bright orange, the temperature was
subsequently increased to 220.degree. C. This was followed by
further addition of InMe.sub.3 (3.375 ml) and (TMS).sub.3P (3.375
ml) resulting in a dark red solution colour.
[0204] The reaction mixture was then left to anneal for 1 hour at
220.degree. C. and then allowed to cool to room temperature. This
was followed by isolation; by adding 100 ml of dry warm ethanol
which produced a precipitate of orange/red particles which were
isolated via centrifugation, washed with acetone and left to dry.
Mass of product=2.29 g. UV-vis spectrum of the ZnS/InP core/shell
particles is shown in FIG. 9A. PL spectrum of the ZnS/InP
core/shell particles is shown in FIG. 9B.
Preparation of Core-Shell ZnS/InP
Method (b) (Using Larger Sized ZnS Core Particles)
[0205] Dibutyl ester (50 ml) and stearic acid (5.65 g) were
dried/degassed by heating to between 65-100.degree. C. under vacuum
for 1 hour. The temperature was then increased to 180.degree. C.
and ZnS particles (0.5 g) along with InMe.sub.3 (1.125 ml) and
(TMS).sub.3P (1.125 ml) were added dropwise under N.sub.2 to the
reaction solution this was left to stir for 10 mins, in which time
the reaction mixture turned pale yellow. When the reaction
temperature had reached 200.degree. C., further addition of
InMe.sub.3 (2.25 ml) and (TMS).sub.3P (2.25 ml) was made which
resulted in the colour changing from pale yellow to clear bright
orange. The temperature was then increased to 220.degree. C., with
further addition of InMe.sub.3 (3.375 ml) and (TMS).sub.3P (3.375
ml) resulting in the reaction solution turning a dark red solution
colour.
[0206] The reaction mixture was then left to anneal for 1 hour at
220.degree. C. followed by cooling to room temperature. 100 ml of
dry warm ethanol was then added to gave a precipitate of orange/red
particles, these particles were isolated by centrifugation, washed
with acetone and left to dry. Mass of product=3.2844 g. UV-vis
spectrum of the ZnS/InP core/shell particles is shown in FIG. 10A.
PL spectrum of the ZnS/InP core/shell particles is shown in FIG.
10B.
Preparation of Core-Shell ZnSe/InP
[0207] Dibutyl ester (50 ml) and stearic acid (5.65 g) were placed
in a three neck flask and dried and degassed for one hour at a
temperature of 90.degree. C. The temperature was increased to
180.degree. C. with addition of ZnSe particles (0.5 g),
(TMS).sub.3P (1.125 ml) and InMe.sub.3 (1.125 ml). The solution was
left at 180.degree. C. for 10 mins followed by increasing the
temperature to 200.degree. C. At 200.degree. C. a further addition
of (TMS).sub.3P (2.25 ml) and InMe.sub.3 (2.25 ml) was made. The
temperature was then increased to 220.degree. C. followed by a
final addition of (TMS).sub.3P (3.375 ml) and InMe.sub.3 (3.375
ml). The reaction mixture changed colour from orange/yellow to dark
red and was left to anneal for one hour at 220.degree. C. before
cooling to room temperature. 100 ml of dry warm ethanol was then
added to the reaction solution to give a precipitate of orange/red
particles, which were isolated by centrifugation, washed with
acetone and left to dry. Mass of product=3.33 g.
Final Shelling
Preparation of ZnS/InP/ZnS
[0208] HDA (150 g) was placed in a 3 neck flask and dried and
degassed for one hour the temperature was then increased to
200.degree. C. In a separate flask core-shell particles of ZnS/InP
(with an orange emission) (2.6343 g) were dissolved in Dibutyl
ester (5 ml) and placed under vacuum for 20 mins this was followed
by sonication for 5 mins, this was followed by the addition of
(TMS).sub.3S (3.75 ml). This solution was then added to the HDA
solution dropwise followed by the addition of Zn(Et.sub.2)
dissolved TOP (7.50 ml). The reaction mixture was left at
200.degree. C. for 26 hours. After 26 hours some luminescence was
observed. The temperature was then decreased to room temperature
followed by the addition of chloroform. The reaction solution was
then filtered through Celite. The QD-QW's were then isolated under
nitrogen by addition of warm dry methanol followed by
centrifugation. UV-vis spectrum of the ZnS/InP/ZnS core/shell/shell
particles is shown in FIG. 11A. PL spectrum of the ZnS/InP/ZnS
core/shell/shell particles is shown in FIG. 11B.
Preparation of ZnSe Quantum Dots
[0209] Alternative methods are set out below for preparing ZnSe
quantum dots which can be further modified for use as cores in the
preparation of core/multishell quantum dot-quantum wells as
described above.
Molecular Cluster Method
[0210] [Et.sub.3NH].sub.4[Zn.sub.10Se.sub.4(SPh).sub.16] (2.5 g)
and 5 mmol TOP-Se were added to a stirred solution of HDA (55 g)
under N.sub.2 while at 100.degree. C. using standard airless
techniques. The temperature was then increased to 250.degree. C.
this was left to stir for 2 hours, the initial PL peak of ZnSe was
at 385 nm. Zn(Et).sub.2 and further quantities of and TOP-Se
precursors were added to the reaction solution while the
temperature was slowly increased to 290.degree. C. Further
quantities of Zn(Et).sub.2 and TOP-Se were added while the
temperature was kept at 290.degree. C. The growth of ZnSe was
followed by monitoring the evolution of UV-Vis absorption and PL
emission.
[0211] 1. 1 ml TOP-Se (0.5M) and 1 ml Zn(Et).sub.2 (0.5M) was
slowly injected into the above reaction solution at 290.degree. C.,
and then kept at 290.degree. C. for 30 mins. The obtained PL is 393
nm.
[0212] 2. 2 ml TOP-Se (0.5M) and 2 ml Zn(Et).sub.2 (0.5M) was added
into the reaction solution at 290.degree. C. and then kept at
290.degree. C. for 60 mins. The obtained PL is 403 nm.
[0213] 3. Additional of 2 ml, 2 ml, 3 ml and 3 ml etc of the same
stock solution was dropwise injected into reaction solution by the
same reaction condition.
[0214] 4. The PL peak will be the red-shift with the
multi-injection of Zn(Et).sub.2 and TOP-Se precursors and the
longer annealing time. The maximum finial PL peak can reach to 435
nm (See FIG. 12).
[0215] 5. Total 20 mmol TOP-Se and Zn(Et).sub.2 were used to make
ZnSe nanoparticles.
[0216] 6. The final ZnSe nanoparticle was collected by size
selective precipitation with hot butanol (70.degree. C.),
centrifugation and then redispersed in octane. Excess HDA was
completely removed by repeating those previous steps. The particles
were re-dispersed in toluene, hexane, heptane and octane, resulting
in clear nanoparticle solution.
[0217] The PL peak width of ZnSe product by this method is as
narrow as 16 nm with a QY of 1020%.
Preparation of ZnSe Quantum Dots
Dual Source Precursor Method
[0218] ZnSe quantum dots were prepared by using the injection of 5
ml Zn(Et).sub.2 (0.5M) and 5 ml TOP-Se (0.5M) into ODA at
345.degree. C.
[0219] After obtaining the ZnSe quantum dots, the multi-injection
of Zn(Et).sub.2 and TOP-Se precursors for the growth of larger ZnSe
nanoparticles was analogous to the above Cluster Method for the
production of ZnSe quantum dots.
[0220] The PL peak width of ZnSe product by this method is as
narrow as 20 nm with a QY of 10.about.30%.
* * * * *