U.S. patent application number 10/854746 was filed with the patent office on 2005-08-04 for nanosubstrate with conductive zone and method for its selective preparation.
This patent application is currently assigned to YISSUM RESEARCH DEVELOPMENT COMPANY OF THE HEBREW UNIVERSITY OF JERUSALEM. Invention is credited to Banin, Uri, Mokari, Taleb.
Application Number | 20050167646 10/854746 |
Document ID | / |
Family ID | 34812092 |
Filed Date | 2005-08-04 |
United States Patent
Application |
20050167646 |
Kind Code |
A1 |
Banin, Uri ; et al. |
August 4, 2005 |
Nanosubstrate with conductive zone and method for its selective
preparation
Abstract
The present invention provides novel nanostructure composed of
at least one elongated structure element, an elongated structure
element of said nanostructure bearing an electrically conductive
zone selectively grown onto the elongated structure element. The
present invention further provides a selective method for forming
in a liquid medium, such nanostructures.
Inventors: |
Banin, Uri; (Mevasseret
Zion, IL) ; Mokari, Taleb; (Jerusalem, IL) |
Correspondence
Address: |
BROWDY AND NEIMARK, P.L.L.C.
624 NINTH STREET, NW
SUITE 300
WASHINGTON
DC
20001-5303
US
|
Assignee: |
YISSUM RESEARCH DEVELOPMENT COMPANY
OF THE HEBREW UNIVERSITY OF JERUSALEM
Jerusalem
IL
|
Family ID: |
34812092 |
Appl. No.: |
10/854746 |
Filed: |
May 27, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60541248 |
Feb 4, 2004 |
|
|
|
60554913 |
Mar 22, 2004 |
|
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Current U.S.
Class: |
257/14 |
Current CPC
Class: |
C30B 29/605 20130101;
H01L 29/0665 20130101; H01L 2924/0002 20130101; B82Y 10/00
20130101; H01L 29/0669 20130101; H01L 29/0673 20130101; B82Y 30/00
20130101; H01L 2924/0002 20130101; H01L 2924/00 20130101; Y10T
428/24917 20150115 |
Class at
Publication: |
257/014 |
International
Class: |
H01L 029/06; H01L
031/0328; H01L 031/0336 |
Claims
1. Nanostructure composed of at least one elongated structure
element and comprising a first material, an elongated structure
element of said nanostructure bearing an electrically conductive
zone made of a second material.
2. The nanostructure of claim 1, wherein said first material is
selected from semiconductor material, insulating material, metallic
material and mixtures thereof.
3. The nanostructure of claim 2 wherein said first material is a
semiconductor material.
4. The nanostructure of claim 3 wherein said semiconductor material
is selected from Group II-VI semiconductors, Group III-V
semiconductors, Group IV-VI semiconductors, Group IV
semiconductors, alloys made of these semiconductors, combinations
of the semiconductors in composite structures and core/shell
structures of the above semiconductors.
5. The nanostructure of claim 4 wherein said nanostructures are
made from Group II-VI semiconductors, alloys made from Group II-VI
semiconductors and core/shell structures made from Group II-VI
semiconductors.
6. The nanostructure of claim 1 wherein said second material is
selected from metal and metal alloy.
7. The nanostructure of claim 6 wherein said metal is a transition
metal.
8. The nanostructure of claim 7 wherein said transition metal is
selected from Cu, Ag, Au, Pt, Co, Pd, Ni, Ru, Rh, Mn, Cr, Fe and
Ti.
9. The nanostructure of claim 1 having an elongated shape selected
from rod, bipod, tripod and tetrapod.
10. A method for forming in a liquid medium, an electrically
conductive zone on a nanostructure having at least one elongated
structure element, the method comprising: contacting a solution
comprising nanostructures with a solution comprising a metal or
metal alloy source, to obtain upon isolation nanostructures bearing
at least one electrically conductive zone on said at least one
elongated structure thereof.
11. The method according to claim 10 wherein said nanostructure is
made of a material comprising semiconductor material, insulating
material, metallic materialor mixtures thereof.
12. The method according to claim 10 wherein said nanostructure is
made of semiconductor material.
13. The method according to claim 10 wherein said nanostructure has
an elongated shape.
14. The method according to claim 13 wherein said elongated shape
comprises a branched shape.
15. The method according to claim 14 wherein said branched shape
comprises rod, bipod, tripod and tetrapod.
16. The method according to claim 12 wherein said nanostructure is
made of a semiconductor material selected from Group II-VI
semiconductors, Group III-V semiconductors, Group IV-VI
semiconductors, Group IV semiconductors, alloys made of these
semiconductors, combinations of the semiconductors in composite
structures and core/shell structures of the above
semiconductors.
17. The method according to claim 16, wherein said nanostructures
are made from Group II-VI semiconductors, alloys made from Group
II-VI semiconductors and core/shell structures made from Group
II-VI semiconductors.
18. The method according to claim 11 wherein said nanostructure is
made of an insulating material selected from oxides and organic
polymers.
19. The method according to claim 10 wherein the metal or metal
alloy source solution further comprises a surfactant and/or a
stabilizer.
20. The method according to claim 19 wherein said surfactant is a
cationic surfactant.
21. The method according to claim 19 wherein said stabilizer
prevents aggregation of nanoparticles during the formation of an
electrically conductive zone on a nanostructure.
22. The method according to claim 20 wherein said stabilizer is
selected from ammonium salts, alkyl pyridinium alts and quaternary
ammonium salts.
23. The method according to claim 10 wherein said metal or metal
alloy source comprises a transition metal element.
24. The method according to claim 23 wherein said metal or metal
alloy source is a salt of a transition metal or transition metal
alloy.
25. The method according to claim 24 wherein said transition metal
is selected from Cu, Ag, Au, Pt, Co, Pd, Ni, Ru, Rh, Mn, Cr, Fe and
Ti.
26. The method according to claim 24 wherein said metal or metal
alloy salt is first dissolved in an organic solvent comprising a
surfactant and/or a stabilizer to give a mixture which is
subsequently added in a controllable manner to the nanostructures
solution.
27. The method according to claim 10 wherein said electron donor is
an organic compound.
28. The method according to claim 27 wherein said electron donor is
selected from aliphatic amine, hydride and ascorbic acid.
29. A method for forming in solution medium an electrically
conductive zone on a nanostructure having at least one elongated
structure element, the method comprising: contacting, an organic
solution comprising semiconductor nanostructures with an organic
solution comprising a metal or metal alloy source, a stabilizer
and/or surfactant and/or electron donor to obtain upon
precipitation semiconductor nanostructures bearing at least one
electrically conductive zone on said at least one elongated
structure thereof.
30. The method according to claim 29 wherein said nanostructures
are in the form of nanorods, bipods, tripods or tetrapods.
31. The method according to claim 29 wherein said semiconductor
nanostructures are made of a material comprising elements of Group
II-VI, alloys of such elements or core-shall layered structures
thereof.
32. The method according to claim 31 wherein said semiconductor
nanoparticles are made of a material comprising CdSe, CdS, CdTe,
alloys thereof, combinations thereof or core/shell
layered-structures thereof.
33. The method according to claim 29 wherein said electrically
conductive zone comprises a metal selected from Au, Ag, Cu, Pt, Co,
Pd, Ni, Ru, Rh, Mn, Cr, Fe, Ti or mixtures of such metals.
34. Article of manufacture comprising the nanostructure of claim
1.
35. An electronic device comprising the nanostructure of claim 1,
or into which the nanostructure of claim 1 is integrated.
36. An electrode comprising the nanostructure of claim 1.
37. An optical device comprising the nanostructure of claim 1, or
into which the nanostructure of claim 1 is integrated.
38. Self assembled construct comprising a plurality of
nanostructures according to claim 1, wherein each nanostructure is
linked to another nanostructure in the construct through its
conductive zone.
Description
FIELD OF THE INVENTION
[0001] This invention relates to the field of treatment of
semiconductor nanostructures.
LIST OF REFERENCES
[0002] The following references are considered to be pertinent for
the purpose of understanding the background of the present
invention:
[0003] 1. M. S. Gudiksen, L. J. Lauhon, J. Wang, D. Smith, and C.
M. Lieber, Nature 415, 617 (2002).
[0004] 2. Y. Wu, R. Fan, P. Yang, Nano Lett. 2, 83 (2002).
[0005] 3. D. V. Talapin, R. Koeppe, S. Goltzinger, A. Komowski, J.
M. Lupton, A. L. Rogach, 0. Benson, J. Feldmann, and H. Weller,
Nano Lett. 3, 1677 (2003).
[0006] 4. WO 03/097904
[0007] 5. WO 03/054953
[0008] 6. Y. Cui and C. M. Lieber, Science 291, 851 (2001).
[0009] 7. S. Heinze, J. Tersoff, R. Martel, V. Derycke, J.
Appenzeller, and Ph. Avouris, Phys. Rev. Lett. 89, 106801
(2002).
[0010] 8. A. Javey, J. Guo, Q. Wang, M. Lundstrom and H. Dai,
Nature 424, 654 (2003).
[0011] 9. Z. A. Peng, X.Peng, J. Am. Chem. Soc. 123, 1389
(2001).
[0012] 10. J. E.Cretier and G. A. Wiegers, Mat. Res. Bull. 8, 1427
(1973).
[0013] 11. W. W. Yu, Y. A. Wang, X.Peng, Chem. Mater. 15, 4300
(2003).
[0014] 12. D. Coucouvanis, Prog. Inorg. Chem. 11, 233 (1970). p0
13. U.S. Pat. No. 5,505,928.
[0015] 14. L. Manna, D. J. Milliron, A. Meisel, E. C. Scher, A. P.
Alivisatos, Nat. Mat. 2, 382 (2003).
[0016] 15. T. Mokari, U. Banin, Chem. Mater. 15, 3955 (2003).
[0017] 16. E. Nahum et al., Nano Lett. 4, 103 (2004).
[0018] The above references will be acknowledged in the text below
by indicating their numbers [in brackets] from the above list.
BACKGROUND OF THE INVENTION
[0019] Anisotropic growth of nanomaterials has led to the
development of complex and diverse nano-structures such as rods,
tetrapods, prisms, cubes and additional shapes. These architectures
display new properties and enrich the selection of nano-building
blocks for electrical, optical and sensorial device construction.
Even greater complexity and new function is introduced into the
nanostructure by anisotropic growth with compositional variations.
This has been elegantly realized by growing semiconductor
heterostructures such as p-n junctions and material junctions in
nanowires [1, 2], and in the case of colloidal nanocrystals, in
growth of a dot-rod of two different semiconductors [3] and in
complex branched growth. In these examples, anisotropic growth was
performed with the same material type (semiconductor).
[0020] A process for the preparation of nanocrystalline
semiconductors, having rod-like shape of controlled dimensions is
described in U.S. Pat. No. 5,505,928 [13] and in WO 03/097904 [4]
for especially Group III-V semiconductors, [4]. Nanocrystal
particles having core with first crystal structure, and at least
one arm with second crystal structure are described in WO 03/054953
[5].
[0021] Recently there have been several reports relating to
connectivity formation for micron-long quasi-one-dimensional
structures such as nanotubes and nanowires [6, 7, 8]. However,
wiring in solution, of shorter semiconductor nanoparticles such as
rods and tetrapods, with arm lengths of less then 100 nm is a
difficult open problem.
SUMMARY OF THE INVENTION
[0022] There is a need in the art for new nanostructures having
selective, well-defined anchor points (preferably conductive anchor
points) grown upon them for use in self-assembly in solution and
onto substrates. Such nanostructures and method for their
manufacture are not available to date.
[0023] Examples of desired nanostructures would be metal dots grown
onto the tips of nanoparticles, in a controllable and repeatable
manner that would also provide an electrical contact point. The
conductive zones grown onto the tips of nanoparticles would provide
well-defined anchor points onto which selective chemistries could
be used to generate self-assembled structures of controlled
arrangements.
[0024] The present invention thus provides in a first aspect new
nanoscale materials in which a metal tip (conductive zone) is
present on the edges of a nanostructure. The novel materials of the
invention are nanostructures having an elongated shape such as rod,
bipod, tripod and tetrapod. Excluded from the scope of the present
invention are nanotubes and nanowires bearing electrodes formed by
evaporation, such as those described in references [6-8] above.
[0025] The nanostructures of the invention are composed of at least
one elongated structure element and comprise a first material,
where an elongated structure element of the nanostructures bears an
electrically conductive zone made of a second material.
[0026] The first material mentioned above is selected from
semiconductor material, insulating material, metallic material and
mixtures thereof. More preferably, the first material is a
semiconductor material selected from Group II-VI semiconductors,
Group III-V semiconductors, Group IV-VI semiconductors, Group IV
semiconductors, alloys made of these semiconductors, combinations
of the semiconductors in composite structures and core/shell
structures of the above semiconductors. Even more preferably, the
nanostructures are made from Group II-VI semiconductors, alloys
made from Group II-VI semiconductors and core/shell structures made
from Group II-VI semiconductors. Specific examples of Group II-VI
semiconductors are CdSe, CdS, CdTe, alloys thereof, combinations
thereof and core/shell layered-structures thereof.
[0027] The second material mentioned above is a metal or metal
alloy. Preferably, the metal is a transition metal. Specific
examples of such transition metals are Cu, Ag, Au, Pt, Co, Pd, Ni,
Ru, Rh, Mn, Cr, Fe and Ti. In a preferred embodiment, the first
material is different than the second material.
[0028] The present invention provides, in another of its aspects, a
method for forming such an electrically conductive zone on a
nanostructure having at least one elongated structure portion. The
method of the present invention is carried out in liquid medium and
it comprises: contacting a solution comprising nanostructures with
a solution comprising a metal or metal alloy source, to obtain upon
isolation, nanostructures bearing an electrically conductive zone
on said at least one elongated portion thereof. The reaction is
carried out at a temperature between about -40.degree. C. to about
350.degree. C., preferably between about 10.degree. C. to about
80.degree. C., more preferably between about 20.degree. C. to about
30.degree. C. and even more preferably at room temperature.
[0029] According to a preferred embodiment the reaction is carried
out in the presence of at least one of the following agents in
addition to said nanostructures and metal source: electron donor,
surfactant and stabilizer.
[0030] The nanostructures used in the method of the invention have
an elongated shape, for example of rods, wires, tubes, or in
branched form. More preferably the nanostructures have an elongated
shape such as for example nanorods and branched shape such tripods,
tetrapods and the like. The term "nanorod" or "rod" as used herein
is meant to describe a nanoparticle with extended growth along the
first axis while maintaining the very small dimensions of the other
two axes, resulting in the growth of a rod-like shaped nanocrystal
of very small diameter in the range of about 1 nm to about 100 nm,
where the dimensions along the first axis may range from about
several nm to about 1 micrometer. The term "tetrapod" is meant to
describe a shape having a core from which four arms are protruding
at tetrahedral angles. In the case of nanorods, the resulting
structures after treating them with the metal or metal alloy source
are shaped as "nano-dumbbells".
[0031] The nanostructures have an elongated shape or even a
branched shape and serve as a template at the nanometer level for
the deposition of a conducting material, and as it will be
described and exemplified herein below, the deposition is
accomplished in a controllable manner on the edges of the elongated
portions of the nanostructures.
[0032] The nanostructures are made of a material comprising
semiconductor material, insulating material, metallic material or
mixtures thereof. Preferably, the nanostructures are made of
semiconductor material selected from Group II-VI semiconductors,
such as for example CdS, CdTe, ZnS, ZnSe, ZnO and alloys (e.g.
CdZnSe); Group III-V semiconductors such as InAs, InP, GaAs, GaP,
InN, GaN, InSb, GaSb and alloys (eg. InAsP); Group IV-VI
semiconductors such as PbSe and PbS and alloys; and Group IV
semiconductors such as Si and Ge and alloys. Additionally,
combinations of the above in composite structures consisting of
sections with different semiconductor materials, for example
CdSe/CdS or any other combinations, as well as core/shell
structures of different semiconductors such as for example CdSe/ZnS
core/shell nanorods, are also within the scope of the present
invention.
[0033] The nanostructures may also be made of an insulating
material such as for example oxides and organic materials or,
alternatively the nanostructures are made of metals. Examples of
oxides are silicon oxide, titanium dioxide, zirconia. Metals
include Au, Ag, Cu, Pt, Co, Ni, Mn and the like, and various
combinations and alloys thereof. Organic materials suitable for use
in the nanostructures are for example polymers.
[0034] The metal or metal alloy source used in the method of the
present invention preferably comprises a transition metal or
mixture of such metals. A variety of metals may be used. This
includes noble metals such as Cu, Ag, Au, or other transition metal
elements such as Pt, Co, Pd, Ni, Ru, Rh, Mn, Cr, Fe, Ti and the
like. The metal growth procedure is done by using a proper metal
salt source, for example AuCl.sub.3 for Au growth, Ag(CH.sub.3COO)
for silver growth, Cu(CH.sub.3COO).sub.2 for Cu growth, PtCl.sub.4
or Pt(acetylacetonate) for Pt growth, Ni(cyclooctadiene).sub.2 for
Ni growth, Co.sub.2(CO).sub.8 or CoCl.sub.2 for Co growth, and
Pd(NO.sub.3).sub.2 for Pd growth.
[0035] The metal salts are dissolved in a proper organic solvent
such as hydrocarbons, e.g. hexanes, cyclohexanes, etc., aromatic
solvents e.g. toluene, etc., using a proper surfactant and/or
stabilizer that stabilizes the nanostructures and the metal salt by
preventing aggregation. The organic solvent used in the method of
the present invention is one capable to solubilize both the
nanostructure and the metal source.
[0036] Examples of surfactants are cationic surfactants such as
ammonium salts, alkyl pyridinium and quaternary ammonium salts.
More specific examples are tetrabutylammonium borohydride (TBAB),
dodecyldimethylamonium bromide (DDAB), cetyltrimethylammonium
bromide (CTAB), and salts of quaternary ammonium with acetate group
ions such as acetate group ions, pivalate, glycolate, lactate and
the like.
[0037] Stabilizer compounds used in the method of the invention are
such compounds capable to coordinate to the nanostructure surface
and/or the metal particle surface and hence prevent aggregation of
the nanostructures. Examples of stabilizers are aliphatic amines,
e.g. hexadecylamine, dodecylamine, octylamine, alkylthiols, e.g.
hexane thiol, decylthiol, dodecylthiol, etc. and carboxylic
stabilizers such as myristic acid, palmitic acid and citrate.
[0038] The metal or metal alloy salt is first dissolved in an
organic solvent comprising a surfactant and a stabilizer to give a
solution which is subsequently added in a controllable manner and a
suitable temperature to the nanostructures solution.
[0039] When an electron source is desired in the method of the
invention, an electron donor compound may be used. Examples of
electron donors are organic compounds, such as aliphatic amines,
hydrides such as sodium borohydride and the like, ascorbic acid and
other reducing agents. According to another example the electron
source is obtained from an electron beam device. Alternatively, one
may use electromagnetic radiation in order to excite the
nanoparticles or the metal source.
[0040] More specifically, the present invention provides a method
for forming in solution medium an electrically conductive zone on a
nanostructure having at least one elongated structure, the method
comprising: contacting an organic solution comprising semiconductor
nanostructures with an organic solution comprising a metal or metal
alloy source, a stabilizer and/or surfactant to obtain upon
precipitation semiconductor nanostructures bearing at least one
electrically conductive zone on said at least one elongated
structure thereof. Preferably, the nanostructures used in the
method of the invention are in the form of nanorods, tetrapods or
any other branched structure and are made of elements of Group
II-VI, alloys of such elements or core-shell layered structures
thereof.
[0041] The method of the present invention provides new
functionalities to the nanostructures, the most important of which
is the formation of anchor points for directed self assembly. The
selective tip growth of metal contacts provides the route to an
effective wiring scheme for soluble and chemically processable
nanostructures with branched shapes. This would allow to fully
realize the potential of miniaturization of devices using such
nano-building blocks, while employing the powerful principles of
self-assembly to connect them to the `outside` world.
BRIEF DESCRIPTION OF THE DRAWINGS
[0042] In order to understand the invention and to see how it may
be carried out in practice, preferred embodiments will now be
described, by way of non-limiting example only, with reference to
the accompanying drawings, in which:
[0043] FIG. 1 illustrates TEM (Transmission Electron Microscopy)
images A-D showing growth of Au onto CdSe quantum rods of
dimensions 29.times.4 nm (length.times.diameter). FIG. 1A shows the
rods before Au growth, FIG. 1B shows selective Au growth of about
2.2 nm; FIG. 1C shows selective Au growth of about 2.9 nm; and FIG.
1D shows selective Au growth of about 4 nm.
[0044] FIG. 2A illustrates the EDS (Energy dispersive X-ray
spectroscopy) spectrum of a goldenized CdSe rod sample. The
relative atom percentage of Au:Cd:Se is18%:42%:40%.
[0045] FIG. 2B illustrates a powder X-ray diffraction comparing
CdSe rods before (1), and after (2) Au growth.
[0046] FIGS. 2C and D illustrate HRTEM images of a single
nano-dumbbell and a nano-dumbbell tip, respectively. The CdSe
lattice for the rod in the center, and Au tips at the rod edges,
can be identified as marked.
[0047] FIG. 3 illustrates TEM images A-H showing growth of Au on
tips of various CdSe quantum rods and CdSe tetrapods. FIGS. 3A and
3B: 12.times.4 nm quantum rods before and after Au growth,
respectively. FIGS. 3C and 3D: 29.times.4 nm quantum rods before
and after Au growth, respectively. FIGS. 3E and 3F: 60.times.6 nm
quantum rods before and after Au growth, respectively. Au growth on
CdSe tetrapods showing a general view is presented in FIG. 3G and
higher magnification image for one tetrapod is presented in FIG.
3H.
[0048] FIG. 4A illustrates the absorption spectra for CdSe/ZnS
core/shell nanorod sample with varied Au tip size compared to the
original rod template, where Au tip size is indicated for each
trace. Spectra are offset vertically for clarity. Inset shows TEM
image of the sample after Au growth (scale bar is 50 nm).
[0049] FIG. 4B illustrates the photoluminescence (PL) spectra for
CdSe/ZnS core/shell nanorod sample with varied Au tip size compared
to the original rod template, where Au tip size is indicated for
each trace. Traces were multiplied by 25, 50 and 50 for the 2 nm,
3.2 nm, and 4.5 nm Au tips, respectively, for clarity. Inset shows
a plot of relative PL yield for template ((.PHI..sub.0) over Au-rod
(.PHI.), versus Au ball size. Measurements were performed for rod
solutions in a sealed cuvette under Ar using the 454 nm line of an
Ar-ion laser with intensity of 100 mW. Fluorescence was collected
using identical conditions for all solutions in a right angle
configuration with a spectrograph/CCD setup, with 500 ms
integration time.
[0050] FIG. 5 shows sizing histograms for goldenized rods shown in
FIG. 1. Histograms for rod diameter (FIG. 5A), length (FIG. 5B) and
Au diameter (FIG. 5C) are shown for the four samples: 1. Original
29.times.4 nm rods, 2. Rods (10 mg) after treatment with 4 mg
AuCl.sub.3, 25 mg DDAB and 40 mg dodecylamine. 3. Rods (10 mg)
after treatment with 8 mg AuCl.sub.3, 50 mg DDAB and 90 mg
dodecylamine. 4. Rods (10 mg) after treatment with 13.5 mg
AuCl.sub.3, 100 mg DDAB and 160 mg dodecylamine.
[0051] FIG. 6 illustrates TEM of product from mixture of rods with
AuCl.sub.3 and DDAB without dodecylamine, FIG. 6A. before exposure
to the TEM electron beam--aggregated rods are seen. FIG. 6B. after
exposure to the TEM electron beam--Au patches appear on the
rods.
[0052] FIG. 7 shows conductive atomic force microscopy current
image of single nano-dumbbell measured at a sample bias of 2V.
Higher conductance through the Au tips is observed, as seen also by
the current cut taken along the rod (along plotted line). In the
inset, a TEM image with the same scale of a Au-rod is shown for
comparison.
[0053] FIG. 8 shows the self assembly of nano-dumbbells into chains
formed by adding hexane dithiol bifunctional linker to a solution
of nano-dumbbells.
DETAILED DESCRIPTION OF THE INVENTION
[0054] The method is exemplified hereinbelow with reference to
selective growth of metal tips onto semiconductor nanorods and
tetrapods.
[0055] In a method for selective growth of contacts made of gold,
AuCl.sub.3 was dissolved in toluene by use of
dodecyldimethylamonium bromide (DDAB) and dodecylamine, and the
resulting solution was added to a toluene solution comprising of
colloidal grown nanorods or tetrapods. The method is exemplified
for the prototypical CdSe nanocrystal system that is highly
developed synthetically and widely studied for its size and shape
dependent properties.
[0056] CdSe rods and tetrapods of different dimensions (see below),
were prepared by high temperature pyrolisys of suitable precursors,
in a coordinating solvent containing a mixture of
trioctylphosphineoxide (TOPO), and of phosphonic acids [9]. In a
typical Au growth reaction, a gold solution was prepared containing
12 mg AuCl.sub.3 (0.04 mmol), 40 mg of DDAB (0.08 mmol) and 70 mg
(0.37 mmol) of dodecylamine in 3 ml of toluene and sonnicated for 5
minutes at room temperature. The solution changed color from dark
orange to light yellow. 20 mg of CdSe quantum rods of the required
dimensions were dissolved in 4 ml toluene in a three neck flask
under argon. The gold solution was added drop-wise over a period of
three minutes. During the addition, carried out at room
temperature, the color gradually changed to dark brown. Following
the reaction, the rods were precipitated by addition of methanol
and separated by centrifugation. The purified product could then be
redissolved in toluene for further studies.
[0057] FIG. 1 presents transmission electron microscopy (TEM)
images showing growth of Au onto CdSe quantum rods of dimensions
29.times.4 nm (length.times.diameter). FIG. 1A shows the rods
before Au growth, while in FIGS. 1B-D, selective Au growth onto the
rod tips is clearly identified as the appearance of points with
enhanced contrast afforded by the higher electron density of the Au
compared with CdSe. The rods now appear as `nano-dumbbells`.
Moreover, by controlling the amount of initial Au precursor, it is
possible to control the size of the Au tips on the nano-dumbbell
edges, from .about.2.2 nm in FIG. 1B, to .about.2.9 nm in FIG. 1C,
to .about.4.0 nm in FIG. 1D as summarized in Table 1. The procedure
clearly leads to the growth of natural contact points on the tips
of the rods.
1TABLE 1 NC's.sup.1 HDA.sup.2 DDAB.sup.3 AuCl.sub.3 Rods size Gold
ball Sample amount(mg) amount(mg) amount(mg) amount(mg) (L .times.
D)nm size (nm) 1 -- -- -- -- 29 .times. 4 nm (original rod) 2 10 mg
40 mg 25 mg 4 mg 25.6 .times. 3.3 nm 2.22 nm 3 10 mg 90 mg 50 mg 8
mg 23.9 .times. 3.4 nm 2.9 nm 4 10 mg 160 mg 100 mg 13.5 mg 20.8
.times. 3.2 nm 4 nm .sup.1NC--nanocrystals
.sup.2HDA--hexadecylamine .sup.3DDAB--dodecyldimethylamonium
bromide
[0058] An additional observation from the analysis of .about.200
particles per sample is that the overall rod length becomes shorter
upon Au growth, and there is also a decrease in the average
diameter of the rods, (Table 1 and FIG. 5 for the complete sizing
histograms). Control experiments with the DDAB and dodecylamine
without AuCl.sub.3 were carried out and also in that case the
average rod dimensions decreased, implying that reduction of rod
size is perhaps related to dissolution of rods in the presence of
DDAB and not to the Au growth.
[0059] Several structural and chemical characterization methods
have been carried out in order to verify the material content and
structure of the gold on the rod tips. FIG. 2A shows EDS analysis
of a micron area of rods after growth and the appearance of Au in
the goldenized and purified rod sample is clear. The powder
Xray-diffraction pattern for the 29.times.4 nm rod sample comparing
the rods before and after gold growth is shown in FIG. 2B. The
appearance of the Au (111), (200) and (220) peaks following Au
growth is evident, demonstrating crystalline Au is formed on the
tips.
[0060] Further evidence for Au growth onto single rods, is provided
by HRTEM (high resolution TEM) studies of the nano-dumbbells. FIG.
2C shows a HRTEM image of a single rod after gold treatment. The
lattice image for the rod part composed of CdSe corresponds to
growth of rods along the CdSe <001>axis. The Au is discerned
once again as the region at the edge with enhanced contrast and the
gold lattice is also shown in FIG. 2D.
[0061] Relating to the interface at the Au--CdSe, it is suggested
that Au--Se bonds are formed, analogous to the known AuSe material
[10]. This means that the interface is formed with covalent
chemical bonds between the metal and the semiconductor and hence
can be expected to provide good electrical connectivity.
[0062] The method for selective Au growth could be easily expanded
and applied to rods of arbitrary dimensions, and to tetrapods, as
well as to growth of other metals and to rods made of various
semiconductor materials.
[0063] FIG. 3 shows TEM images for three rod samples of dimensions
12.times.4 nm (FIG. 3A, B), 29.times.4 nm (FIG. 3C, D), and
60.times.6 nm (FIG. 3E, F), before and after Au treatment. The
presence of the high-contrast tips on the treated rods, forming
nano-dumbbells, is evident in all cases. Highly selective tip
growth is discerned and demonstrated for three rod sizes and could
easily be applied to arbitrary rod sizes. In addition the method
was applied to a CdSe tetrapod sample, as can be seen in FIG. 2G
showing several tetrapods, and in FIG. 2H showing an enlargement of
one tetrapod, following the Au growth process. In this case, the
growth occurs selectively on all the tips of the tetrapods leading
to a tetrahedral arrangement for the Au tips, and once again
providing the natural contact points for this unique structure, for
further self-assembly and for electrical connections.
[0064] In another example, CdTe nanostructures served as the
template for growing various metals on its edges. The synthesis of
the CdTe in different shapes is known [14]. In a typical synthesis
of CdTe rods, a mixture of 1 mmol of CdO dissolved in 1.125 gr
oleic acid and 2.5 gr of 1-octadecene is heated in three neck flask
to 300.degree. C. to obtain a clear colorless solution. In the
glove box, a solution of Te (0.5 mmol of Te is dissolved in 1 ml of
TOP) is prepared and brought out in a vial sealed with septum to
the injection. After the injection of the Te solution into the
mixture in the three necked flask, the mixture is cooled to
260.degree. C. for growth. Modification of this procedure in terms
of the temperature or precursor concentration results in size and
shape changes. The oleic acid is used as a ligand and it dissolves
the CdO in the octadecene.
[0065] Another specific semiconductor material that may be used is
CdS, which is controllable in size and shape. The synthesis is
based on the same principle which is injection precursor to hot
solution, the Cd and S precursor in this case is
Cd(S.sub.2CNEt).sub.2 that could be synthesized according to known
literature method [12]. In typical synthesis of CdS nanorods, a
warm solution of Cd(S.sub.2CNEt).sub.2 (50 mg dissolved in about
0.3 g of hexadecylamine (HDA) at about 70.degree. C.) is injected
into hot solution of HDA and after 1 hr is cooled to 70.degree. C.
and treated with ethanol and separated by centrifuging. Controlling
the shape of the nanocrystals is done by changing the growth
temperature of the synthesis from 300.degree. C. (rods) to
120.degree. C. (tetrapods).
[0066] Metal tips by the method described above have also been
grown onto CdSe/ZnS core/shell nanorods (29.times.4 nm rods with 2
monolayer ZnS shell) with initial emission quantum yield of 2%
[15]. Treatment of these rods with DDAB and dodecylamine without Au
led to an increased quantum yield of 4%, likely because of the
effect of the excess amine. Several Au sizes were grown from about
2 nm to about 4.5 nm Au at the tips of the rods.
[0067] The metalized structures (in the case of Au growth the
formed structures are termed herein "goldenized structures")
exhibit new and different electronic, electrical and optical
properties as compared to the original rods, due to the strong
effect of the metal on the semiconductor properties. Absorption and
photoluminescence (PL) measurements were carried out to study the
effect of Au growth on the rod optical properties as shown in FIG.
4. Absorption spectra (FIG. 4A) for the small Au tips on the rods
still shows the excitonic structure but with increased absorbance
in the visible and the appearance of a tail to the red of the
particle gap. Upon increased Au size, the features of the
absorption of the rods are washed out and the tail to the red
becomes more prominent. The spectra should contain in principle
contributions from the semiconductor part and the plasmon resonance
associated with the Au tips. However, attempts to add spectra of
the rod template and Au nanocrystals did not reproduce the observed
absorption and we suggest that the spectra are not a simple sum of
both components because of the modified electronic structure of the
Au-rod nano-dumbbell system. The strong mixing of the semiconductor
and metal electronic states leads to modified density of states
exhibiting broadened levels and a reduced band-gap.
[0068] The significant coupling of the Au is also observed for the
PL (FIG. 4B) that undergoes considerable quenching with increased
Au ball size, by a factor of about 100 initially for the smaller Au
balls (about 2 nm), and gradually down to a factor of about 500 for
the large Au balls (about 4.5 nm). Quenching of the emission by the
metal edges is expected via the new non-radiative pathways created
by the proximity of metals, likely leading to electron transfer to
the Au. Moreover, a systematic dependence of quenching on Au size
is seen as shown in the Stem-Volmer type plot (inset of FIG. 4B).
Both absorption and emission spectra exemplify the significant
effect of the Au on the semiconductor rod properties in this new
system, further proving the strong bonding of the Au to the CdSe
rod.
[0069] The selective tip growth of Au onto the rods and tetrapods
not only provides important developments for enabling electrical
connectivity and new paths for self-assembly for such
nanostructures. It is also an interesting and novel chemical
reaction route with clear selectivity and anisotropic character.
The reaction mechanism for the gold growth entails a reduction of
Au. Examining by TEM the Au solution with DDAB and dodecylamine,
already reveals the formation of Au particles. When the reaction is
carried out without dodecylamine, considerable aggregation of the
CdSe rods was seen (FIG. 6A). Additionally, without the amine,
growth of Au on rods was not apparent initially and only after the
irradiation under the electron beam of the TEM we could observe
some Au growth (FIG. 6B).
[0070] One of the benefits of the method of the present invention
is its specificity leading to selective tip growth. This results
from the preferential adsorption of the metal, e.g. Au complex
formed in the Au solution by adding Au salt to DDAB and
dodecylamine onto the nanostructures edges. The tips are more
reactive due to the increased surface energy and also possibly due
to imperfect passivation of the ligands on these faces, which also
leads to preferential growth along the <001>axis of CdSe
rods. Once Au nucleates on the edge, it is preferential for
additional Au to adhere and grow on that seed. This gains support
from controlling the extent of Au growth on the rod tips by using
increased concentration of Au in the gold solution as was shown in
FIG. 1. Moreover, early Au growth as shown in FIG. 1B reveals that
in some rods preferential early growth occurs on one tip, in
agreement with the surfactant-controlled growth model suggested for
CdSe rods [9].
[0071] It is important to note that in some cases Au growth was
identified on branching and defect points, but at slower rate
compared to the distinctive tip growth discussed above. This can be
seen in FIGS. 3E and 3G, where weak dark Au spots appear also in
some positions other then the tips of the long rods and tetrapods.
This growth can be controlled by the amounts of Au added to the
rods. At such defect points, such as points where the diameter of
the structure changes, there is also increased reactivity due to
the imperfect chemical bonding and increased surface energy. This
leads to Au adhesion and growth in agreement with the mechanism for
tip growth. It is emphasized that the tip growth occurs more
readily and rapidly then growth on the defects and hence can be
controlled to achieve contact points.
[0072] The method may easily be expanded to additional
semiconductor nanocrystal systems and to additional metals, to
tailor the metal tip contacts as desired and the semiconductor
element as well.
[0073] One application for the metal tips is in serving as
electrical contact points. The role to be played by the Au tips as
contact points for wiring the rods is exemplified by conductive
atomic force microscopy (C-AFM) measurements carried out on
goldenized 60.times.6 nm rods. Rods were deposited onto a
conducting highly ordered pyrolitic graphite substrate, and
embedded in a thin layer of poly methyl methacrylate (PMMA) to
avoid dragging by the tip as reported earlier for regular rods
[16]. The current image of a single rod measured by this method
reveals that already at a bias of 1.5-2 V, small tunneling current
is flowing through the tips which are composed of Au, while the
central part of the rod consisting of the semiconductor is
non-conductive at these conditions (see FIG. 7). The small
tunneling current is determined by the tunneling barriers at
tip-nanocrystal and nanocrystal-substrate junctions, dominated
primarily by the PMMA. This measurement reveals the significantly
higher conductance of the Au tips which would be critical for using
them as electrical contact points.
[0074] Several strategies can be employed to realize such contacts.
It is possible to form the metallized nanorods or other branched
structures onto a substrate, identify their position, and then
write by electron-beam lithography electrodes to overlap with the
Au tips. In a different approach, it is also possible to deposit
the metalized rods onto pre-existing electrode structures, with or
without electrostatic trapping by an applied electric field. Since
the metal tipped nanostructures enable the connectivity to
electrode structures, this clearly opens the path for using them as
transistors, in sensing applications, andin light emitting or light
detecting devices.
[0075] The metal edges can also impart the rods with advantageous
and novel optical properties. They exhibit enhanced linear and
non-linear optical properties. The polarizibility of such a
structure may obviously be significantly increased compared with
that of the regular rods. For example, enhancement in second
harmonic generation and also the observation of novel plasmon
resonances related to highly controlled distances that could be
tailored for the metal tips on rods.
[0076] Additionally, is possible to apply the powerful approach of
self assembly by using for example, biological templates e.g. DNA,
for creating the connections to the metal tips of nanorods or of
branched structures, or bifunctional ligands such as dithiols or
diamines for binding preferentially to the Au tips. In such
applications the metal tips serve as selective anchor points for
ligands and chemistries preferential for the Au surface. Such self
assembly could for example be done in solution or onto surfaces. In
solution, examples include formation of AAAA chains where A
represents rods of one type. This is done by adding bifunctional
ligands such as dithiols, for example hexane dithiol, to a solution
with goldenized nanorods. The preferential binding of thiols to the
Au tips leads to chain formation as can be seen in FIG. 8. Another
example is the formation of ABAB chains where A represents one rod
type and B another rod type. Here biochemical linkers such as
avidin-biotin chemistry or DNA linking can be used to make
selective ABAB chains. In another approach, combining tetrapods
with rods on tip to tip basis may lead to formation of propeller
structures.
[0077] The same chemistries can be used to self-assemble rods and
tetrapods with Au tips onto patterned or non-patterned substrates.
For example, a gold or silicon substrate is used together with a
bifunctional ligand that binds with one function to the substrate
and with the second function to the Au tip on the
nanostructure.
[0078] Metal tipped structures also provide selective metal growth
points for additional materials via a seeded growth
solution-liquid-solid mechanism.
* * * * *