U.S. patent application number 14/214587 was filed with the patent office on 2016-12-29 for continuous flow reactor for the synthesis of nanoparticles.
This patent application is currently assigned to Shoei Electronic Materials, Inc.. The applicant listed for this patent is Shoei Electronic Materials, Inc.. Invention is credited to Patrick M. Haben, Thomas E. Novet, Daniel A. Peterson, David M. Schut, George M. Williams.
Application Number | 20160375495 14/214587 |
Document ID | / |
Family ID | 51523492 |
Filed Date | 2016-12-29 |
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United States Patent
Application |
20160375495 |
Kind Code |
A9 |
Schut; David M. ; et
al. |
December 29, 2016 |
CONTINUOUS FLOW REACTOR FOR THE SYNTHESIS OF NANOPARTICLES
Abstract
A continuous flow reactor for the efficient synthesis of
nanoparticles with a high degree of crystallinity, uniform particle
size, and homogenous stoichiometry throughout the crystal is
described. Disclosed embodiments include a flow reactor with an
energy source for rapid nucleation of the procurors following by a
separate heating source for growing the nucleates. Segmented flow
may be provided to facilitate mixing and uniform energy absorption
of the precursors, and post production quality testing in
communication with a control system allow automatic real-time
adjustment of the production parameters. The nucleation energy
source can be monomodal, multimodal, or multivariable frequency
microwave energy and tuned to allow different precursors to
nucleate at substantially the same time thereby resulting in a
substantially homogenous nanoparticle. A shell application system
may also be provided to allow one or more shell layers to be formed
onto each nanoparticle.
Inventors: |
Schut; David M.; (Philomath,
OR) ; Haben; Patrick M.; (Corvallis, OR) ;
Novet; Thomas E.; (Corvallis, OR) ; Peterson; Daniel
A.; (Corvallis, OR) ; Williams; George M.;
(Beaverton, OR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Shoei Electronic Materials, Inc. |
Corvallis |
OR |
US |
|
|
Assignee: |
Shoei Electronic Materials,
Inc.
Corvallis
OR
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20140264171 A1 |
September 18, 2014 |
|
|
Family ID: |
51523492 |
Appl. No.: |
14/214587 |
Filed: |
March 14, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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61783753 |
Mar 14, 2013 |
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61783911 |
Mar 14, 2013 |
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61784107 |
Mar 14, 2013 |
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61784257 |
Mar 14, 2013 |
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61784306 |
Mar 14, 2013 |
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61784358 |
Mar 14, 2013 |
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61784183 |
Mar 14, 2013 |
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61949969 |
Mar 7, 2014 |
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Current U.S.
Class: |
252/301.4S ;
204/157.43; 252/301.4R; 420/441; 420/501; 422/186; 423/561.1;
427/553 |
Current CPC
Class: |
B01J 2219/00882
20130101; B01J 2219/0097 20130101; B22F 2999/00 20130101; B22F
1/0018 20130101; B01J 2219/0086 20130101; B01J 2219/00934 20130101;
C01P 2004/03 20130101; B01J 19/0093 20130101; B01J 2219/00903
20130101; B22F 2999/00 20130101; B22F 1/0062 20130101; B22F 2202/11
20130101; B22F 9/24 20130101; B01J 2219/00957 20130101; C01B 19/002
20130101; B22F 9/24 20130101; C01G 21/21 20130101; B01J 2219/00889
20130101; B01J 2219/00873 20130101; B22F 9/16 20130101; B01J
2219/00795 20130101; B22F 1/02 20130101; B01J 2219/00941 20130101;
B01J 2219/00869 20130101 |
International
Class: |
B22F 9/16 20060101
B22F009/16; C01G 21/21 20060101 C01G021/21 |
Claims
1. A method for producing uniformly sized nanoparticles comprising:
blending at least a first precursor and at least a second precursor
together to form a mixture of precursors that travels down a tube
in a continuous flow path; extending the continuous flow path
through a first energy source that applies microwave energy to the
mixture of precursors to uniformly activate the mixture of
precursors within the tube for a first duration time at a first
energy level thereby allowing uniform nucleation of the mixture of
precursors; extending the continuous flow path through a heating
source for a second duration time at a controlled temperature
allowing uniform thermodynamic growth around previously formed
nucleates to form desired core sized nanoparticles; and, quenching
the growth of the nanoparticles.
2. The method of claim 1, wherein the microwave energy is
monomodal.
3. The method of claim 1, wherein the microwave energy is
multimodal.
4. The method of claim 1, wherein the microwave energy is
multivariable frequency.
5. The method of claim 1, further including the step of providing
pressurized gas into the continuous flow path before the flow path
enters the first energy source so as to cause partitions of gas
within the tube positioned between segments of the mixture of
precursors thereby allowing the mixture of precursors within a
segment to mix as it travels within the tube down the flow
path.
6. The method of claim 5, wherein the gas is inert and non-reactive
to microwave energy, the precursors and the nanoparticles.
7. The method of claim 6, wherein the gas is selected from the
group consisting of nitrogen and argon.
8. The method of claim 1, wherein the first duration time is less
than or equal to 60 seconds.
9. The method of claim 1, wherein the first duration time is less
than or equal to 10 seconds.
10. The method of claim 1, wherein the first duration time is less
than or equal to 3 seconds.
11. The method of claim 1, wherein the first duration time is less
than or equal to 2 seconds.
12. The method of claim 1, further including a control system
operably connected to sensors, actuators and a computer system, the
sensors monitoring the quality of nanoparticles produced and the
control system modulating the actuators to adjust the first
duration time, first energy level, second duration time and
temperature in response to the detected quality of the
nanoparticles produced.
13. The method of claim 1, wherein a mixture of two or more
precursors from Group B and C and having different microwave
absorption cross sections, interact with the microwave in a manner
in which these precursors form nucleates with precursors of Group A
at substantially the same rate traveling through the energy
source.
14. The method of claim 13, wherein the nucleation of the first and
second precursors at the first duration time results in a
substantially homogeneous nanoparticle.
15. The method of claim 1, further including the steps of: using a
continuous process to expose the nanoparticles with a mixture of at
least a third precursor from Group A herein and a fourth precursor
from Group B or Group C herein; and, heating the exposed
nanoparticles in a heat source to form a first shell around the
nanoparticle.
16. The method of claim 15, further including the steps of: using a
continuous process to expose the nanoparticles with at least a
mixture of at least a fifth precursor and a sixth precursor; and,
heating the exposed nanoparticle within a heating source to form a
second shell having two layers around the nanoparticle.
17. The method of claim 16, further including the steps of: using a
continuous process to expose the nanoparticles with at least a
mixture of a fifth precursor to from Group A herein and a sixth
precursor from Group B or Group C herein; and, heating the exposed
nanoparticle with a supplemental heat source to form a shell having
two layers around the nanoparticle.
18. The method of claim 17, further including the steps of: using a
continuous process to expose the nanoparticles with at least a
mixture of a seventh precursor from Group A herein and a eighth
precursor from Group B or Group C herein; and, heating the exposed
nanoparticle within a heating source to form a shell having three
layers around the nanoparticle.
19. The method of claim 5, further including the step of separating
the gas from the nanoparticles following the step of quenching the
growth of the nanoparticle.
20. The method of claim 1, wherein the tube has an inner diameter
between 1/16 of an inch to 1 inch.
21. The method of claim 20, wherein the tube has an inner diameter
between 1/4 to 1/2 inch.
22. The method of claim 1 further comprising: exposing the
nanoparticles to a mixture of a third precursor and a fourth
precursor; and, applying heat to the mixture of the third and
fourth precursors to form a shell layer around the
nanoparticle.
23. The method of claim 1, wherein the first precursor is selected
from the group consisting of Group A means.
24. The method of claim 23, wherein the second precursor is
selected from the group consisting of Group B means and Group C
means
25. The method of claim 24, wherein the third precursor is selected
from the group consisting of Group A means and fourth precursor is
selected from the group consisting of Group B means and Group C
means.
26. The nanoparticle of claim 1, wherein the microwave energy
operates at frequencies that cause the first and second precursors
to nucleate at substantially the same time thereby producing
substantially homogenous nanoparticles.
27. A continuous flow cell reactor comprising: a mixer for mixing
at least a first and a second precursor together into a mixture and
extending the mixture down a continuous flow path; a gas inserter
for inserting an inert gas into the flow path to provide segmented
flow of the mixture along the flow path; an energy source for
nucleating the mixture as the flow path extends therethrough
thereby producing nucleates; a heating source unit for growing the
nucleates as the flow path extends therethrough thereby growing the
nucleates into nanoparticles; a quencher for cooling the
nanoparticles as the flow path extends therethrough thereby
stopping the grown of nanoparticles; and, a separator for
separating the inert gas from the nanoparticles.
28. The continuous flow cell reactor of claim 27, wherein the
energy source is a microwave.
29. The continuous flow cell reactor of claim 28, wherein the
microwave is a multimodal microwave.
30. The continuous flow cell reactor of claim 28, wherein the
microwave is a monomodal microwave.
31. The continuous flow cell reactor of claim 28, where the
microwave is a multivariable frequency microwave.
32. A nanoparticle formed by using the method of claim 1, wherein
the first precursor is selected from Group A means and the second
precursor is selected from the group consisting of Group B means
and Group C means.
33. The nanoparticle formed by using the method of claim 32,
wherein a core/shell structure is produced wherein the shell
consists of a third precursor selected from Group A means and a
fourth precursor selected from the group consisting of Group B
means and Group C means.
34. The nanoparticle formed by using the method of claim 33 wherein
a core/shell/shell structure is obtained wherein a second shell
consists of a fifth precursor selected from Group A means and a
sixth precursor selected from the group consisting of Group B means
and Group C means.
35. The nanoparticle formed by using the method of claim 34,
wherein a core/shell/shell/shell structure is obtained wherein a
third shell consisting of a seventh precursor selected from Group A
means and an eighth precursor selected from the group consisting of
Group B means and Group C means.
36. A group of nanoparticles produced in a production run, each
nanoparticle formed from nucleation of at least two precursors, the
group of nanoparticles having of at least one property selected
from the following properties: a less than 15 percent coefficient
of variance (COV) as determined by small angle x-ray scattering,
high resolution transmission electron microscopy, or dynamic light
scattering; a less than 50 nanometer (nm) full width half max
(FWHM) whereby the width of the absorbance or photoluminescence
peak is determined at half the height of the peak of interest as
determined by photoluminescent emission when excited at a first
exciton or higher energy for visibly emitting materials of between
400 nm to 700 nm, inclusive; a less than 150 nm FWHM as determined
by photoluminescent emission when excited at the first exciton or
higher energy for near-infrared emitting materials of between 700
nm to 2000 nm, inclusive; a less than 300 nm FWHM as determined by
photoluminescent emission when excited at the first exciton or
higher energy for mid-infrared emitting materials between 2000 nm
to 5000 nm, inclusive; a less than 300 nm FWHM as determined by
photoluminescent emission when excited at a first exciton or higher
energy for long-infrared emitting materials of between 5000 nm to
10000 nm, inclusive; a less than 50 nm FWHM of the surface plasmon
resonance emission when excited at the surface plasmon resonance
frequency of a metallic nanoparticle in the visible range of
between 400 nm to 700 nm, inclusive; a less than 150 nm FWHM of the
surface plasmon resonance emission when excited at the surface
plasmon resonance frequency of a metallic nanoparticle in the
near-infrared range of between 700 nm to 2000 nm, inclusive; and, a
less than 50 percent quantum yield when irradiated at a first
exciton band or higher energy.
37. The group of nanoparticles of claim 36, wherein each
nanoparticle in the group of nanoparticles is substantially
homogenous.
38. The group of nanoparticles of claim 36, wherein the production
run is a continuous flow production run through an energy source
for nucleation and a separate spaced apart heat source for growth
of the nanoparticles.
39. The group of nanoparticles of claim 36, wherein one precursor
of the at least two precursors is selected from the group
consisting of Group A means.
40. The group of nanoparticles of claim 39, wherein the other
precursor of the at least two precursors is selected from the group
consisting of Group B means and Group C means.
41. The group of nanoparticles of claim 36, wherein the energy
source is microwave energy.
42. A method for producing uniformly sized nanoparticles
comprising: blending at least a first precursor and at least a
second precursor together to form a mixture of precursors that
travels down a tube in a continuous flow path; extending the
continuous flow path through a first energy source that applies
energy to the mixture of precursors to uniformly activate the
mixture of precursors within the tube for a first duration time
less than or equal to 60 seconds thereby allowing uniform
nucleation of the mixture of precursors; extending the continuous
flow path through a second heating unit for a second duration time
at a controlled temperature allowing uniform thermodynamic growth
around previously formed nucleates to form desired core sized
nanoparticles; and, quenching the growth of the nanoparticles.
43. The method of claim 42, wherein first duration time is less
than or equal to 10 seconds.
44. The method of claim 42, wherein the first duration time is less
than or equal to 3 seconds.
45. The method of claim 42, wherein the first time duration is less
than or equal to 2 seconds.
46. The method of claim 42, the first energy source applies
microwave energy.
47. The method of claim 42, the microwave energy is monomodal.
48. The method of claim 42, the microwave energy is multimodal.
49. The method of claim 42, the microwave energy is multivariable
frequency.
50. The method of claim 42, further including the step of providing
pressurized gas into the continuous flow path before the flow path
enters the first energy source so as to cause partitions of gas
within the tube positioned between segments of the mixture of
precursors thereby allowing the mixture of precursors within a
segment to mix as it travels within the tube down the flow
path.
51. The method of claim 50, wherein the gas is inert and
non-reactive to microwave energy, the precursors and the
nanoparticles
52. The method of claim 51, wherein the gas is selected from the
group consisting of nitrogen and argon.
53. The method of claim 42, further including a control system
operably connected to sensors, actuators and a computer system, the
sensors monitoring the quality of nanoparticles produced and the
control system modulating the actuators to adjust the first
duration time, first energy level, second duration time and
controlled temperature in response to the detected quality of the
nanoparticles produced.
54. The method of claim 46, wherein a mixture of two or more
precursors from Group B and C and having different microwave
absorption cross sections, interact with the microwave in a manner
in which these precursors form nucleates with precursors of Group A
at substantially the same rate traveling the first energy
source.
55. The method of claim 54, wherein the nucleation of the first and
second precursors at the first duration time results in a
substantially homogeneous nanoparticles.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. provisional patent
application Ser. Nos. 61/783,753; 61/783,911; 61/784,107;
61/784,257; 61/784,306; 61/784,358; and 61/784,183 filed on Mar.
14, 2013, all of the disclosures of which are hereby incorporated
by reference.
FIELD
[0002] The present disclosure relates to a system and method for
efficient and continuous production of uniformly-sized
nanoparticles which include metal nanoparticles and nanocrystalline
quantum dots.
BACKGROUND
[0003] This section provides background information related to the
present disclosure which is not necessarily prior art.
[0004] Nanoparticles, which can be classified as nanocrystalline
materials, nanocrystallites, nanocrystals, quantum dots, and
quantum dot materials are produced and used for a wide variety of
applications. For example, semiconductor nanocrystallites emit a
visible light over a narrow range of wavelengths and are used in
the fabrication of light emitting diodes and the like.
[0005] Basic steps, materials, and processes for producing
nanocrystalline quantum dot materials are described in U.S. Pat.
Nos. 6,179,912; 6,322,901; 6,833,019 8,101,021, and 8,420,155; U.S.
patent application publication No. US2012-0315391, and Japanese
patent application publication No. 2006-188666, the disclosures of
which are hereby incorporated by reference. These and other known
production and synthesis systems and methods for producing
nanoparticles give rise to several problems, including, but not
limited to, inefficient production, poor particle quality,
inconsistent particles sizes, and/or excessive waste of the raw
materials used to form the particles.
SUMMARY
[0006] This invention relates to the system and chemistries needed
for the production of nanoparticles. This invention also relates to
systems and chemistries suitable for production of nanocrystalline
quantum dots of a uniform and repeatable size and size distribution
on a large scale that is both economical and efficient.
Furthermore, this invention relates to the chemistries and
processes needed to place between one or more shells over the core
nanocrystalline quantum dots--to enhance the electronic and/or
optical properties of the nanocrystalline quantum dots and also to
improve durability of these materials.
DRAWINGS
[0007] The drawings described herein are for illustrative purposes
only of selected embodiments and not all possible implementations,
and are not intended to limit the scope of the present
disclosure.
[0008] FIG. 1 is an isometric schematic view of a four zone
continuous flow cell reactor for the production of nanoparticles in
accordance with a preferred embodiment of the present
invention.
[0009] FIG. 2 is a schematic view of a four zone continuous flow
cell reactor of FIG. 1 showing an optional shell fabricator for
fabricating shell layers on the nanoparticles.
[0010] FIG. 3 is a schematic view of zone 1 of the continuous flow
reactor of FIG. 1 showing introduction of precursors and a possible
segmented flow.
[0011] FIG. 4A is a schematic view of zone 3 of the continuous flow
reactor of FIG. 1 showing a nanoparticle growth phase using a
heating source allowing precise temperature control over multiple
lines entering therethrough.
[0012] FIG. 4B is a top view of a rack received within the heating
source of FIG. 4A allowing for flow lines, with possible variable
lengths that can be tailored to this system, for the growth phase
of the reaction to take place.
[0013] FIG. 4C is an isometric view of multiple racks of FIG. 4B
showing a possible stacking arrangement within the heating source
of FIG. 4A.
[0014] FIG. 5A is a schematic view of an alternative possible
heating system for introducing multiple lines into the growth phase
heat source of zone 3
[0015] FIG. 5B is an enlarged view of one line of the multiple
lines into the growth phase heat source of zone 3 in FIG. 5A.
[0016] FIG. 6 shows an enlarged cross-sectional view of the
continuous flow path in FIG. 1 showing a possible segmented flow of
precursors with a reactively inert gas such as nitrogen, argon, and
the like segmented therein.
[0017] FIG. 7 shows a schematic view of a separator for separating
the nanoparticles from the reactively inert gas following the
growth phase (Zone 3) and shell formation phase of the fabrication
process.
[0018] FIG. 8 shows a schematic view of a system layout with valves
placed at key locations so as to allow redundancy in the system
thereby allowing fabrication to continue if any one component in
the system fails.
[0019] FIG. 9 shows an exemplar reaction chart relating the
nucleation time and temperature or energy levels of two different
precursors.
[0020] FIG. 10 shows a schematic view of a possible baffle system
for redirecting microwave energy to a secondary growth heater
thereby improving energy efficiency of the system.
[0021] FIG. 11A (PRIOR ART) shows a sketch of a crystalline
structure of a non-homogenous nanoparticle formed by different
precursors nucleating at different times. CuInSe.sub.2 material
produced using a thermal batch process resulting in Cu rich cores
around which In is later deposited, producing an inhomogeneous
nanoparticle of CuInSe.sub.2.
[0022] FIG. 11B shows a sketch possible homogenous crystalline
structure of a homogenous nanoparticle formed by different
precursors nucleating at the same time. CuInSe.sub.2 nanoparticles
produced using a microwave flow cell reactor in which the Cu and In
are nucleated at the same rate, producing homogeneous nanoparticles
having well-defined structure.
[0023] FIG. 12A (PRIOR ART) is an actual image obtained using
Transmission Electron Microscopy (TEM) of the crystalline structure
of FIG. 11A.
[0024] FIG. 12B is an actual image obtained using TEM of the
crystalline structure of FIG. 11B.
[0025] Corresponding reference numerals indicate corresponding
parts throughout the several views of the drawings.
DETAILED DESCRIPTION
[0026] Example embodiments will now be described more fully with
reference to the accompanying drawings. According to this disclosed
embodiment, the process is described in detail for synthesizing
nanoparticles such as quantum dots, among others.
[0027] A continuous flow cell reactor production system for the
production of quantum dots and its related components are shown in
FIGS. 1-10, 11B and 12B.
[0028] Process Overview
[0029] A conceptual diagram of the nanoparticle production system
20 is shown schematically in FIGS. 1 & 2 and provides a
specific example of the system. The system 20 includes a continuous
flow path 22 that passes through at least four zones (1-4,
respectively). In zone 1, two or more precursors 32, 34 are metered
and mixed together and passed through a tube 40 where a reactively
inert gas 42 such as nitrogen, argon, and the like may be inserted
therein to provide segmented flow of the mixture as it passes to
the next zone. For example, in one disclosed embodiment the two
precursors 32, 34 can be a reducing agent and one or more cationic
precursor(s). Alternatively, the two precursors 32, 34 can be one
or more anionic precursor(s) and one or more cationic
precursor(s).
[0030] In zone 2, the precursor mixture is rapidly energized by an
energy source 50, preferably by use of a microwave oven selected
from monomodal, multimodal or multivariable frequency where the
precursors 32, 34 are rapidly and uniformly nucleated. The flow of
the nucleated precursors then passes through to zone 3 where a heat
source 60 allows the nucleated precursors to enter a growth phase.
The process is quenched in zone 4, and the resulting nanoparticles
70 are separated from the reactively inert gas 42.
[0031] A nanoparticle quality testing system 72 may be provided
following the quenching that tests the quality of the nanoparticles
70 being continuously produced by the system 20. The quality
testing system 72 can be in communication with a control system 80
that is also operably connected to the precursor deliver system 82,
reactively inert gas delivery system 84, energy source 50 in zone
2, heat source 60 in zone 3 and the quenching system 86 to modulate
the volume of precursors 32, 34, gas injection, time, temperature,
energy level and flow rate through the energy source 50 and heat
source 60 as needed to automatically optimize nanoparticle quality
in real-time based on the detected quality of the particles being
produced.
[0032] As shown in FIG. 2, a shell fabrication system 100 may be
provided after zone 4 to allow one or more shell layers to be
formed over each nanoparticle 70.
[0033] A plurality of reductant elements of the production line may
be provided with the individual precursors and nanoparticle flow
paths interconnectable with valves or the like to allow redirection
of the flow path through alternative components should one
component become inoperable as shown in FIG. 8.
[0034] Each of these zones and their preferred related components
are discussed in greater detail below.
[0035] Zone 1--Precursor Metering and Mixing
[0036] In zone 1, at least one precursor 32 and preferably at least
two precursors 32, 34 are metered from their respective reservoirs
and metered in the proper proportion into the continuous flow path
22 that extends through zones 2-4. The flow path 22 is preferably a
tube 40 having an inner diameter between 1/16 of an inch to 1 inch,
and more preferably between 1/4 of and a 1/2 inch.
[0037] The desired concentration of precursors or a concentration
greater than that initially desired, is introduced into the flow
path 22 using a metering pump 110 (syringe pump, peristaltic pump,
diaphragm pump as examples) to dispense into the flow path the
desired quantity of material (precursor). In the case where the
concentration is higher than desired for the reaction, a dilution
can take place by metering the desired solvent to dissolve the
precursor, while in the line, to the desired concentration.
[0038] A mixing of the precursors/components for the reaction can
take place using a static mixer, such as a T-mixer, to ensure
complete mixing of the precursors/components of the reaction.
Alternatively, a different type of mixer, such as an active
stirrer, can be used such that a homogenous solution is formed
after exiting the mixing area of the precursors. The precursors are
preferably introduced into a tube 40 that defines a flow path 22
through the system 20 and then mixed with the mixer prior to
entering zone 2. Alternatively, the precursors can first be mixed
together and then passed to a tube that defines the flow path
through zone 2. The sum of the precursor flows establishes the
process flow rate. One or more additional precursors 35 (FIG. 2)
may also be provided.
[0039] Before advancing to zone 2 in the flow path 22, a plug of
reactively inert gas 42, such as nitrogen, argon or the like, that
is immiscible with the reactant fluid is introduced to create a
segmented flow 120 through the flow path 22 as best shown in FIGS.
1, 2 and 6. The introduction of the segmented flow has two
purposes: (1) turbulent mixing (the flow against the wall is slower
due to friction than the flow in the middle of the tube, creating
turbulence) is introduced within the precursor portion 122 of the
flow, and (2) reduction of material deposition on the sidewalls.
The latter event occurs through a combination of flow, turbulence
and surface tension to eliminate any deposition of material on the
sidewalls, which allows for sites of nucleation to occur (an
undesirable effect). If desired, mixing of the precursors can be
further enhanced by surface roughness on the interior surface of
the tube containing the mixture of precursors.
[0040] As shown in FIG. 3, a plurality of flow path 22 tubes 40 may
be used to increase the rate of production of nanoparticles.
Multiple lines are used to introduce two different precursors 32,
34 together, mix them, and then introduce the segmented flow 120 by
introducing nitrogen gas, argon gas or the like into the reaction
lines.
[0041] Zone 2--Nucleation
[0042] This zone through the flow path 22 is for the initiation of
nucleation. Rapidly increasing the energy level of the precursors
32, 34 with an energy source 50 quickly brings the solution and/or
precursors in the flow path tube 40 to the energy level (including
temperature) at which precursors will nucleate and undergo kinetic
growth within one minute, preferably being equal or less than ten
seconds, more preferably within less than 3 seconds and even more
preferably within less than 2 seconds.
[0043] This energy level increase may be done using several
different methods, such as microwave/dielectric heating,
sonication, thermal heating by induction or convection, or even
breaking of bonds through irradiation with lasers. This is the
rapid breakdown of precursors into reactive components forming a
nucleate that may be larger than the initial precursors, but much
smaller than a nanoparticle or quantum dot of interest. The
formation of nucleates is terminated rapidly in order to maintain
size control. The duration of the reaction in zone 2 of the flow
cell reactor is rapid, being less than one minute, preferably being
equal to or less than ten seconds, and more preferably within less
than 3 seconds and even more preferably within less than 2
seconds.
[0044] The need for rapid temperature equilibration of the flow
cell precursor material to a given temperature through application
of energy from the energy source is necessary in order to maintain
tight control of the resultant nanoparticle size. By having short
nucleation periods, which are typically conducted at temperatures
greater than the growth temperature, growth upon the resultant
nucleates can be conducted without the formation of new nucleates
during the growth (Zone 3) stage of the nanoparticle formation. If
the nucleation period is extended, nucleates are formed throughout
this period. Additionally, because growth may also occur at this
step (an undesirable feature at this point and is minimized through
the use of short nucleation times), nucleates that form immediately
in Zone 2 will be larger than those nucleates that form towards the
end of the Zone 2.
[0045] Preferably, the energy source 50 uniformly heats or excites
all precursors 32, 34 (or increases their energy levels) passing
through it. A microwave has proven to be particularly useful at
accomplishing uniform heating to nucleate the precursors. The
microwave can be implemented in three different fashions, depending
on implementation within the flow cell reactor. Monomodal waveforms
(where a high Q-factor is obtained through
deconstructive/constructive interference effects of a standing
wave) are good for producing high intensity (high energy density)
microwave peaks that rapidly heat the reaction solution within the
flow cell reactor. However, the limit to this technology is that
multiple lines cannot be effectively heated using this method. A
second method is the use of multi-modal microwaves where a single
wavelength of microwave is used (typically 915 MHz, 2.45 GHz, or
5.8 GHz), but no standing wave is created. Instead, a baffle system
is used to route the microwaves through multiple angles, allowing
uniform energy exposure throughout the flow cell reactor cavity.
This allows uniform heating of multiple flow cell tubes through the
reactor cavity. The drawback to this method is lower intensity
(lower energy density) microwave irradiation within the flow cell
reactor cavity. Finally, a multivariable frequency microwave may be
used where microwaves of different frequencies (typically between
5.8 GHz and 7.0 GHz) are rapidly cycled in small steps (0.01 to
0.1) to produce uniform heating through the microwave flow cell
cavity by creating multiple standing waves of a given
frequency.
[0046] The flow rate of the precursor thorough the microwave, the
concentration of the precursors, the diameter of the tubing
containing the precursor mixture, the length of tubing being
exposed to microwave energy and the power of the microwave being
used are selected to provide the optimal time and energy exposure
for nucleation of the mixture.
[0047] Moreover, the frequencies of the microwave can be oscillated
rapidly on the order of microseconds over a large range. When using
an oscillating multimodal or multivariable frequency microwave in a
heating cavity, an additive heating effect is produced. While there
are many dips and peaks in the wave pattern produced, the physical
window for processing becomes much larger than with a monomodal
microwave frequency. This allows for additional sample size and
latitude for placement of the cell in the flow cell reactor.
[0048] Moreover, because each transition in a molecule for
rotational, vibrational and bending is frequency dependent, using a
multivariable frequency microwave allows for excitation of a
plurality of modes of different molecules at once. This increases
the heating efficiency of the microwave as many molecules will have
very low capture cross-sections at certain frequencies.
[0049] In addition, the microwave can be selected or tuned to
excite two or more different precursors so that they nucleate at
the same time. As shown in FIG. 9, different precursors tend to
nucleate at a different time for a substantially constant
temperature. By modulating the frequencies and/or the applied power
at which the microwave engages the precursors, the nucleating time
between two different precursors can be substantially the same.
When two different precursors nucleate at the same time the
resulting nanoparticle produced is homogenous as shown by the
well-defined crystalline structure 400 in FIG. 11B. An actual image
of the homogenous structure obtained using Transmission Electron
Microscopy (TEM) is shown in FIG. 12B. The modulation of the
frequencies and/or applied power allows the nucleation time between
different precursors to be tuned and/or optimized so as to allow
them to nucleate at substantially the same time.
[0050] In contrast, if traditional constant heating methods are
used to nucleate the precursors, they will not consistently
nucleate at the same time as shown in broken lines for exemplar
precursor InP in FIG. 9 resulting in an irregularly-defined
crystalline structure 402 as shown in FIG. 11A (PRIOR ART). An
actual image of the irregularly-defined structure obtained using
TEM is shown in FIG. 12A (PRIOR ART).
[0051] As an example, CuInSe.sub.2, when produced by batch
processes or non-microwave initiated processes, produces material
which is inhomogeneous in nature (Cu rich or In rich regions within
a batch of materials or in a nanoparticle itself), such as shown in
FIGS. 11A (PRIOR ART) and 12A (PRIOR ART). When the microwave
conditions are set appropriately, because In has a larger d-orbital
system than Cu--and hence, more polarizable--it absorbs energy
faster than Cu does, increasing the rate of its reactivity,
enabling the resulting reaction to produce homogeneous materials
such as shown in FIGS. 11B and 12B.
[0052] This example provides an illustrative concept of an
embodiment of the invention, namely, the ability to create
nanoparticle materials having a high degree of crystallinity,
uniform particle size, homogeneous stoichiometry throughout the
crystal, batch-to-batch reproducibility as shown in FIG. 11B, and
the ability to produce such materials on a large scale as shown in
FIG. 2. These properties may be verified through several techniques
or the use of multiple techniques, such as:
[0053] Size Measurements: determination of the coefficient of
variance (COV) through the use of SAX (small angle x-ray
scattering), TEM (transmission electron microscopy), and XRD (x-ray
diffraction) using standard techniques. The COV is then defined as
being:
COV=((standard deviation of particle size)/(average of particle
size))*100%
[0054] Where a COV <15% within a single run demonstrates uniform
particle size, and a COV <15% from batch-to-batch demonstrates
reproducibility.
[0055] Degree of Crystallinity and Homogeneous Stoichiometry: The
degree of crystallinity or the purity of the crystalline phase (as
shown in FIG. 11A and FIG. 11B) can be determined by TEM using
diffraction scattering patterns and performing a fourier transform
analysis to determine the crystalline structure of the material.
Another technique that may be used to determine this is XRD, where
the resultant diffraction pattern can be matched to a library of
known crystal structures and verified as to being inhomogeneous
(multiple contributions from different crystals) or homogeneous
(one contributing pattern diffraction matching the desired crystal
structure). Lack of a diffraction pattern in either XRD and/or TEM
is indicative of an amorphous material, indicating poor or
non-existent crystal structure.
[0056] Homogeneous Stoichiometry and Uniform Particle Size: This
information is obtained using either absorbance spectrophotometry
or photoluminescent emission. The absorption and photoluminescence
characteristics of a nanoparticle are determined by the FWHM (full
width half max, where the width of the absorbance or
photoluminescence peak is determined at half the height of the peak
of interest) obtained through the spectrum. An increase in the FWHM
means that one of multiple effects could be taking place, such as:
large particle size distribution (COV>15%), insufficient degree
of crystallinity resulting in trap states that have different
energies than a highly crystalline nanoparticle, and inhomogeneity
of the material--giving rise to multiple excitations or emissions
from the various regions within the nanoparticle or batch of
nanoparticles. A nanoparticle having a high degree of
crystallinity, a homogeneous stoichiometry, and being monodisperse
will give rise to absorption and/or photoluminescence peaks of:
<50 nm FWHM from 400 nm-700 nm, <150 nm FWHM from 700 nm-2000
nm, <300 nm FWHM from 2000 nm-10000 nm.
[0057] In the case of metallic nanoparticles, instead of a first
exciton excitation and emission, a surface resonance plasmon can be
observed. Using the same arguments presented above, a metallic
nanoparticle having a high degree of crystallinity will have <50
nm FWHM when excited at the surface plasmon resonance frequency
when excited between 400 nm and 700, and <150 nm FWHM when
excited in the near-infrared range (700 nm-2000 nm) when exciting
at the surface plasmon resonance frequency.
[0058] The frequency or frequencies which the microwave operates
can also be selected to excite a particular material in the process
without exciting other materials such as binders or the like.
Microwave frequencies ranging from between 300 MHz (1.24 .mu.eV) to
300 GHz (1.24 meV), which are sufficiently low enough in energy
that they do not chemically change the substances by ionization.
These energies affect the rotational and bi-rotational energies of
molecules when absorbed by such species. These absorbances are
unique to each type of bending transition, rotation transition and
bi-rotational transition; hence, energies may be selected that
interact specifically with each transition. This property allows
the ability to select the desired microwave frequency to interact
with a specific reactant in a flow cell reactor, which allows
several capabilities.
[0059] For example, this allows temperature limitations associated
with the boiling point of solids to be overcome. By selectively
activating only the precursors associated with the synthesis of
nanoparticles, the solvent selection can be increased significantly
to allow for the solubility of precursors that would not normally
be used. Additionally, temperatures of the precursors can
effectively be much greater than the solvent, thereby allowing for
reactions that are not allowed through traditional heating of the
solvent.
[0060] The tube 40 carrying the precursors 32, 34 through the
energy source 50 can be configured with a cooling system, such as
tubes that encircle the tube and carry cooling liquid. This allows
the precursors within the tube to be heated, by microwaves or the
like, to high enough energy levels to promote nucleation without
overheating the tube itself and compromising its structural
limits.
[0061] Also, nanoparticles may be formed that are not feasible
using traditional colloidal nanoparticle synthetic techniques. For
example, the energy required for the formation of GaN nanoparticles
is great enough to surpass the boiling point of any solvent that is
available for synthetic techniques. Accordingly, the formation of
these nanoparticles is only done through high energy intensive and
expensive deposition systems such as Atomic Layer Deposition
("ALD"). This is done because only the precursors needed for the
formation of the GaN nanoparticles are heated in the microwave
initiated reaction of the present invention.
[0062] Moreover, in cases where one or more reaction pathways are
possible, the selective application of microwave frequencies allows
for the activation of a desirable reaction pathway. For example, if
a given reaction is thermodynamically dominated, the use of
selective microwave activating allows for the formation of the
kinetic product. The ability to selectively target which species
the reaction is going to absorb the microwave energy extends the
ability of the continuous flow cell reactor to deliver products
that would not normally be delivered at a cost that the microwave
continuous flow cell reactor is capable of delivering.
[0063] Another example of a possible benefit with selective
frequency microwaving involves the use of a polyol process to
synthesize nanoparticles of metallic salts. In this process, the
metallic (Ni, Co, Ag, and mixtures thereof) salts (acetate,
chloride, fluoride, nitrate) are dissolved at 1.0-3.0 mmol ethylene
glycol or polypropylene glycol (or similar polyol). At 2.45 GHz,
the solvent absorbs the microwave irradiation very strongly,
heating the solvent to the point where it then acts as the reducing
agent for the metallic precursor, allowing for the formation of
metallic nanoparticles. These types of reactions can be shown
symbolically as noted below:
Ni(O.sub.2CCH.sub.3).sub.2+propylene
glycol.fwdarw.Ni(0)nanoparticles
AgNO.sub.3+ethylene glycol.fwdarw.Ag(0)nanoparticles
[0064] Another example is the microwave absorption of precursors
for the synthesis of PbS nanoparticles. The synthesis of PbS may be
done in the following manner. 1.5 mmol of lead oleate is dissolved
in 1-octadecene with the addition of 3.0 mmol-12.0 mmol of oleic
acid. 1.4 mmol of bis(trimethylsilyl)sulfide (TMS.sub.2S) which was
previously dissolved in the 1-octadecene. The microwave frequency
of 2.45 GHz is chosen because both the oleic acid and the
1-octadecene have very low absorption cross-sections at this
frequency. On the other hand, both the TMS.sub.2S and the lead
oleate have a relatively large absorption cross-section at this
frequency, allowing the absorption by these materials and the
selective activation. This exemplar reaction can be shown
symbolically as noted below.
Pb(oleate).sub.2+TMS.sub.2S.fwdarw.PbS(oleate)nanoparticles
[0065] Zone 3--Growth
[0066] This is the growth zone. At this point, the nucleates
undergo one of two processes: (1) combination with other nucleates
to form nanoparticles/quantum dots of the correct core size, or (2)
combination with unreacted precursors to form an epitaxial growth
system allowing for the formation of the nanoparticles/quantum dots
at a very controlled pace. The material is allowed to remain in the
growth zone for a period necessary for them to grow to the specific
desired core size, after which, the material is moved through Zone
4.
[0067] In general, in the growth phase, the nucleates are
preferably heated in a heat source 60 over a longer period of time,
such as greater than 100 seconds, at a lower energy level than what
they faced during nucleation. This allows thermodynamic growth and
Ostwald Ripening. This heating may be done using several different
systems, including, but not limited to, sand baths, convection
ovens, forced air heating, induction ovens, oil baths and column
heaters. Preferably, this heat source 60 is spaced apart from the
energy source 50 used in nucleation and is custom-tailored to
provide optimal growth of the nucleates. The length of the flow
path tube 40 extending through the heat source, diameter of the
tube, temperature of the heat source, uniform distribution of heat
within the tube, and nucleate flow rate though the heat source are
selected to optimize growth of the nucleates during this phase (as
shown in FIGS. 4A-C and 5A-B) thereby providing uniform morphology
and size among the nanoparticles produced.
[0068] Referring to FIGS. 4A-4C, the flow tube 40 may be arranged
in a serpentine arrangement within a rack 41 that is receivable
within the heat source 60. A plurality of racks may be stacked on
top of each other as shown in FIG. 4C thereby allowing effective
heat distribution to the flow tubes 40 while optimizing space
within the heat source 60. An alternative possible arrangement is
shown in FIGS. 5A & 5B where individual flow tubes are coiled
to define a heat transfer coil 43 with a plurality of heat transfer
coils received within the heat source 60
[0069] Zone 4--Quenching
[0070] The flow path continues past zone 3 to zone 4, where the
reaction is immediately terminated through a temperature reduction
using a quenching system 86 such as a quenching bath or the like.
After quenching the growth of the nanoparticle 70, the segmentation
is removed through a degassing step 150 (FIG. 7) to allow for
introduction of more material for shell growth and for ease of
in-line analysis to be performed.
[0071] If needed, increasing the pressure in the flow path 22 can
increase the boiling point of a solvent used in the process,
thereby allowing the system to operate at higher temperatures and
energy levels. One possible way to increase the pressure in the
flow path involves inserting a restrictive flow valve into the flow
path downstream of the quenching stage. The flow through the valve
can be adjusted so as to increase the pressure in the tube upstream
of the valve, thereby increasing the pressure in the tube through
zones 2 and 3, where the precursor and nucleates are activated and
grown.
[0072] Preferably and as best shown in FIG. 7, the reactively inert
gas 42 is also separated from the nanoparticles 70. The flow path
22 extends into a chamber 160 where the nucleates drop downward and
exit from below while the gas escapes and is collected from a vent
162 above. Alternatively, the reactively inert gas can be separated
at a future point downstream in the flow path as needed.
[0073] Real-Time Quality Testing and System Optimization
[0074] As shown in FIG. 1, a testing system 72 can be provided
following nanoparticle production that tests the quality of the
nanoparticles produced. For example, Dynamic Light Scattering
("DLS") can be used to test the properties of the particles
produced. Other possible in-line testing systems include
spectrophotometry including UV, VIS and IR spectra, fluorometry,
and measurement of refractive index.
[0075] The testing structure can be in communication with a control
system 80 that monitors the results from the testing system 72 and
can modulate, preferably in real-time, components in zones 1-4 as
needed to optimize the quality of the nanoparticles produced. For
example, the flow of the individual precursors, the time and
temperature-heating-excitation energy applied through zone 2 and 3
and the amount of reactively inert gas segmented into the flow path
in zone 1 can be adjusted by the control system as needed to
optimize detected quality of the nanoparticles produced.
[0076] Depending on how many shells are introduced onto the surface
of the core material (which is produced in Zones 1-4), Zones 1, 3
and 4 can be repeated using a different set of materials
(precursors/components) to form core/shell, core/shell/shell and
core/shell/shell/shell type structures.
[0077] Shell Fabrication System
[0078] A post-production shell application system 100 may be
provided following the production of the nanoparticles as shown in
FIG. 1. As shown in FIG. 2, the shell fabrication system may
include structures for supplying one or more additional precursors
(here precursors 170, 172, 174, and 176 are shown) and a
supplemental heat source 61 for heating downstream therefrom. A
continuous flow loop 180 may be provided where any combination of
the precursors can be applied to any given shell layer and passed
through the heat source 61, thereby allowing multiple shell layers
to be formed on each nanoparticle. A second quality testing system
72' may be provided following each shell layer application. With
this testing system 72' and the components of the shell fabrication
system in operable communication with the control system 80, the
control system 80 can provide real-time modulation of the shell
fabrication systems as needed to optimize quality of the shell
layer on each nanoparticle produced.
[0079] The purpose for the shell architecture surrounding the core
nanoparticle material is two-fold. First, by matching the lattice
parameters closely of the core material, a first shell can be added
which increases the quantum yield of the resultant nanoparticle
upon exposure to light. This is done by passivating the
nanoparticle core surface and eliminating dangling bonds which
contribute to non-radiative recombination events. Also, by lattice
matching the materials of the nanoparticle core and the first
shell, strain effects are reduced, which also causes an increase in
the quantum yield of the resultant nanoparticle.
[0080] This first shell may also have the added benefit of
providing a barrier against environmental degradation effects, such
as photo-bleaching and/or oxidation of the core material, which
will result in either a blue-shifting of emitted light, or provide
multiple trap sites for reduction of effective and desirable
electronic properties. However, in the event that this is not
provided by the first shell, a second and/or third shell may be
provided that will enhance the operational lifetime of nanoparticle
materials when used in applications. These second and third shells
do not necessarily have to be lattice matched to enhance optical
properties unless they interact with the wave function associated
with the nanoparticle in the excited state. The primary purpose of
the second and third shell are to provide increased operational
lifetime by providing protection to the nanoparticle core/shell
from environmental effects, which include, but are not limited to:
oxidation, photobleaching and temperature extremes.
[0081] The first shell integrity can be verified by measuring the
quantum yield of the nanoparticle after the first shell has been
placed onto the core of the nanoparticle. Poor coverage by the
first shell, or poor lattice matching by the first shell will
result in low quantum yields (<50%), whereas good coverage by
the first shell and good lattice matching between the first shell
and the core material will result in large quantum yields
(>50%).
[0082] The lifetime of the materials can be evaluated by exposure
to light, preferably between 250 nm and 700 nm, and measuring the
photoluminescent response as a function of time. Increased
operational lifetime and enhancement of the stability of these
nanoparticles by inclusion of a second and, perhaps, a third shell,
will show less than 5% photodegration of a 10 wt % material in
solvent exposed to a minimum of 5 mW light source over the period
of two weeks upon continuous exposure in standard atmospheric
conditions.
[0083] System Redundancy and Redirectable Flow Paths
[0084] As shown in FIG. 8, a plurality of reductant elements of the
production line, such as two energy sources 50 and two heat sources
60 may be provided with redundant sets of the individual precursors
32, 24 and nanoparticle flow 22 paths interconnectable with valves
300 or the like to allow redirection of the flow path 22 through
alternative components should one component before inoperable.
[0085] Conservation of Excess Microwave Energy
[0086] As shown schematically in FIG. 10, in cases where the energy
source 50 in zone 2 is a microwave oven, excess microwave energy
may be directed to assist with warming the growth area heating
source 60 in zone 3. For example, a series of mirrors or the like
can be directed to a heat sink such as rubber or the like that
collects the excess microwaves and coverts them to heat.
[0087] The microwave energy entering the growth chamber can be
controlled through an insertable and movable baffle 310 which can
attenuate the amount of microwave energy entering the growth area
heating source. The temperature of the growth area heating source
can be monitored by the control system 72 which modulates the
baffle position as needed to maintain a desired temperature in the
growth chamber.
[0088] Exemplar precursor combinations that have may work
particularly well in this flow cell reactor include first
precursors selected from those found in "Group A" below with the
second precursor is selected from "Group B" or "Group C" below
using conventional periodic table nomenclature.
[0089] Group A--Precursors
H.sub.2X [0090] Where X=O, S, Se, Te
[0090] R.sub.3P=X [0091] Where [0092] R=--H,
--(CH.sub.2).sub.n--CH.sub.3, --C.sub.6H.sub.5,
--C.sub.6H.sub.4--R' [0093] n=3-18 [0094]
R'=--(CH.sub.2).sub.m--CH.sub.3, --CH(CH.sub.3).sub.2,
--C(CH.sub.3).sub.3 [0095] m=0-17 [0096] X=Se, Te
[0096] R.sub.3N=X [0097] Where [0098] R=--H,
--(CH.sub.2).sub.n--CH.sub.3, --Si(CH.sub.3).sub.3 [0099] n=0-4
[0100] X=S, Se, Te
[0100] ((CH.sub.3).sub.3Si).sub.2X [0101] Where X=S, Se, Te
[0101] (((CH.sub.3).sub.3Si).sub.2N).sub.2X [0102] Where X=S, Se,
Te
[0102] H--X--(CH.sub.2).sub.n--CH.sub.3 [0103] Where [0104] X=O, S,
Se, Te [0105] n=1-18, preferably n=4-12, more preferably n=8-10
[0105] HO--CH.sub.2--(CH(OH)).sub.n--CH.sub.3 [0106] n=1-50,
preferably n=1-25, more preferably n=1-5
[0106] HO--CH.sub.2--(CH(OH)).sub.n--CH.sub.2--OH [0107] n=1-50,
preferably n=1-25, more preferably n=1-5
[0107] H.sub.2NNH.sub.2
NaBH.sub.4
NaCNBH.sub.3
[0108] and mixtures thereof
[0109] including anionic precursors and/or reducing agents
[0110] Group B--Precursors
M(ligand).sub.y [0111] When y=1, M=Tl, Ag, Cu [0112] When y=2,
M=Zn, Cd, Hg, Cu, Pb, Ni [0113] When y=3, M=Al, Ga, B, In, Bi, Fe
[0114] Ligand=--(O.sub.2C--(CH.sub.2).sub.n--CH.sub.3),
--(O.sub.2C--(CH.sub.2).sub.m-- [0115]
CH=CH--(CH.sub.2).sub.o--CH3), --S--(CH.sub.2).sub.n--CH.sub.3,
--PR.sub.3, --OPR.sub.3 [0116] n=2-24, preferably n=8-20, more
preferably n=12-16 [0117] m and o=1-15, preferably n and o=12-16,
more preferably n and o=7-9 [0118] R=--(CH.sub.2).sub.pCH.sub.3,
--C.sub.6H.sub.5, --C.sub.6H.sub.4--R' [0119] p=0-18 [0120]
R'=--(CH.sub.2).sub.p--CH.sub.3, --CH(CH.sub.3).sub.2,
--C(CH.sub.3).sub.3
[0121] Or mixtures thereof.
[0122] Group C--Precursors
M(ligand).sub.y [0123] When y=1, M=Na, K, Rb, Cs, Ag, Cu [0124]
When y=2, M=Mg, Ca, Sr, Ba, Pd, Pt, Cu, Ni [0125] When y=3, M=La,
Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Au [0126] When
y=4, M=Ti, Zr, Hf, Pt, Pd [0127] ligand=--O.sub.2C--CH.sub.3, --Cl,
--F, --NO.sub.3
[0128] or mixtures thereof.
[0129] The invention is disclosed above and in the accompanying
figures with reference to a variety of configurations. The purpose
served by the disclosure, however, is to provide an example of the
various features and concepts related to the invention, not to
limit the scope of the invention. One skilled in the relevant art
will recognize that numerous variations and modifications may be
made to the configurations described above without departing from
the scope of the present invention, as defined by the appended
claims.
* * * * *