U.S. patent application number 12/196960 was filed with the patent office on 2009-06-18 for multicomponent nanorods.
This patent application is currently assigned to NORTHWESTERN UNIVERSITY. Invention is credited to Sung-Wook Chung, Ling Huang, Chad A. Mirkin, Sungho Park, Lidong Qin.
Application Number | 20090155587 12/196960 |
Document ID | / |
Family ID | 46325025 |
Filed Date | 2009-06-18 |
United States Patent
Application |
20090155587 |
Kind Code |
A1 |
Mirkin; Chad A. ; et
al. |
June 18, 2009 |
Multicomponent Nanorods
Abstract
Multicomponent nanorods having segments with differing
electronic and/or chemical properties are disclosed. The nanorods
can be tailored with high precision to create controlled gaps
within the nanorods or to produce diodes or resistors, based upon
the identities of the components making up the segments of the
nanorods. Macrostructural composites of these nanorods also are
disclosed.
Inventors: |
Mirkin; Chad A.; (Wilmette,
IL) ; Qin; Lidong; (Evanston, IL) ; Park;
Sungho; (Evanston, IL) ; Huang; Ling;
(Corning, NY) ; Chung; Sung-Wook; (Evanston,
IL) |
Correspondence
Address: |
MARSHALL, GERSTEIN & BORUN LLP
233 SOUTH WACKER DRIVE, 6300 SEARS TOWER
CHICAGO
IL
60606-6357
US
|
Assignee: |
NORTHWESTERN UNIVERSITY
Evanston
IL
|
Family ID: |
46325025 |
Appl. No.: |
12/196960 |
Filed: |
August 22, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11171894 |
Jun 30, 2005 |
7422696 |
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12196960 |
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11062983 |
Feb 22, 2005 |
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11171894 |
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60584702 |
Jun 30, 2004 |
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60661659 |
Mar 14, 2005 |
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60546641 |
Feb 20, 2004 |
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Current U.S.
Class: |
428/378 ;
216/24 |
Current CPC
Class: |
H01L 29/0673 20130101;
Y10T 428/2938 20150115; Y10S 977/856 20130101; H01L 29/861
20130101; H01L 29/068 20130101; G01N 33/531 20130101; H01C 17/06513
20130101; B82Y 5/00 20130101; Y10T 428/2982 20150115; Y10S 977/81
20130101; B82Y 20/00 20130101; Y10S 977/849 20130101; Y10S 977/857
20130101; Y10S 977/762 20130101; H01L 29/0665 20130101; B82Y 10/00
20130101 |
Class at
Publication: |
428/378 ;
216/24 |
International
Class: |
B32B 5/02 20060101
B32B005/02; B29D 11/00 20060101 B29D011/00 |
Goverment Interests
STATEMENT OF GOVERNMENTAL INTERESTS
[0002] This invention was made with government support under Air
Force Office of Scientific Research (AFOSR) grants F49620-00-1-0283
and F49620-02-1-0180, National Science Foundation grant
EEC-0118025, and Defense Advanced Research Projects Agency (DARPA)
grant DAAD-19-03-1-0065 and DARPA Spintronics grant
MDA972-03-1-0023 The government has certain rights in this
invention.
Claims
1-11. (canceled)
12. A lithographic process for producing a nanowire comprising the
steps of: a) providing a nanorod comprising alternating segments of
a metal and a sacrificial metal; b) depositing a coating onto one
side of the nanorod; and c) subjecting the coated nanorod to
etching to remove the sacrificial metal segments from the nanorod
and thereby produce a nanowire having gaps at positions previously
occupied by the sacrificial metal.
13-19. (canceled)
20. A nanowire prepared by the lithographic process of claim
12.
21. The nanowire of claim 20 wherein the metal is gold.
22. The nanowire of claim 20 wherein the coating is a gold/titanium
alloy or silicon dioxide.
23. The nanowire of claim 20 wherein the widths of the gaps
independently are about 2 nm to about 5 .mu.m.
24. The nanowire of claim 20 wherein the bridging material is
insulating or conducting.
25. The nanowire of claim 20 wherein the gaps are filled with a
filler.
26. The nanowire of claim 25 wherein the filler is a conducting
polymer.
27. A nanorod array comprising nanowires prepared by the
lithographic process of claim 12.
28. The nanorod array of claim 27 wherein the metal is gold.
29. The nanorod array of claim 27 wherein the thickness of the
array is about 20 nm to about 5 .mu.m.
30. The nanorod array of claim 27 wherein the space between the
nanorods comprising the nanorod array is about 5 nm to about 5
.mu.m.
31. A plasmon wire comprising a nanorod array comprising nanowires
produced from the lithographic process of claim 12.
32. A waveguide comprising a nanorod array comprising nanowires
produced from the lithographic process of claim 12.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 11/062,983, filed on Feb. 22, 2005, which
claims the benefit of U.S. Provisional Application Ser. No.
60/546,641, filed Feb. 20, 2004. This application also claims the
benefit of U.S. Provisional Application Ser. No. 60/584,702, filed
Jun. 30, 2004, and U.S. Provisional Application Ser. No.
60/661,659, filed Mar. 14, 2005.
FIELD OF THE INVENTION
[0003] The present invention relates to nanoscale compositions for
use as nanoresistors and nanodiodes, and to nanoscale compositions
and methods of forming nanogap wires and nanodisk/rod arrays. In
particular, the present invention relates to hybrid
organic/inorganic nanocompositions to form diodes and resistors,
and to inorganic compositions and on-wire lithographic methods of
forming nanoelectrodes, as well as nanodisk and nanorod arrays.
BACKGROUND OF THE INVENTION
[0004] There are two general challenges in nanoscience and
nanotechnology. One challenge is to fabricate nanoarchitechtures,
such as nano-electrodes, diodes, and resistors, in less than 10
nanometer (nm) resolution. The second challenge is to assemble
nanostructures into patterns, such as nanorod or nanodisk arrays,
in a regular arrangement, with precise control and high throughput
(Gates et al., Chem Rev, 105, 1171, 2005; Ieong, et al., Science,
306,2057, 2004).
[0005] Several routes for synthesizing nanostructures have been
developed. Many of these new structures have interesting
electronic, optical, and chemical sensor properties that derive
from their size, composition, and shape (Iijima et al., Nature,
363, 603, 1993; Thess et al., Science, 273, 483, 1996; Heath et al,
Chem Phys Lett, 208, 263, 1993; Morales et al, Science, 279, 208,
1998; Martin, Science, 266,1961, 1994; Martin, Acc Chem Res, 28,
61, 1995; Routkevitch et al, Chem Phys, 210, 343, 1996). However,
methods of synthesizing multicomponent materials made from both
organic and inorganic materials are few (Gudiksen et al, Nature,
415, 617, 2002; Lee et al, Angew Chem Int Ed, 43, 3048, 2004;
Kovtyukhova et al, J Phys Chem B, 105, 8762, 2001; Pena et al, J
Phys Chem B, 106, 7458, 2002; Park et al, Science, 303, 348, 2004;
Nicewarner-Pena et al; Science, 294, 137, 2001).
[0006] Porous templates offer an ability to routinely generate such
multicomponent materials through two distinct methods. Both rely on
the use of electrochemistry to generate an initial segment of metal
from a plating solution. However, one method utilizes
layer-by-layer chemisorption processes (Kovtyukhova et al, J Phys
Chem B, 105, 8762, 2001) to build organic segments on top of a
preformed metal segment, while the second method utilizes
conducting polymer monomers combined with an appropriately applied
potential to polymerize the monomer within the template at the
metal segment solution interface (Park et al, Science, 303, 348,
2004). An advantage of the latter approach is that it provides
excellent control over the segment length of the metal and organic
regions of the structure, simply by controlling the number of
Coulombs (C) that are passed in the experiment. This disclosure
provides another approach, based upon this synthetic strategy, for
preparing hybrid multicomponent (e.g., organic-inorganic or
metal-metal) nanorods having electronic properties derived from
their compositions, wherein the spatial distribution of the
different compositional segments can be precisely controlled.
[0007] Lithography is a powerful way of processing substrates for
use in many practical applications, including semiconductor and
optical industries. Many methods of printing structures on flat
substrates are known, and some methods for printing on large curved
architectures are also known (Melosh et al, Science, 300, 112,
2003; Chou et al, Science, 272, 85, 1996; Xia et al, Chem. Rev.,
99, 1823, 1999; Erhardt et al, Chem. Mater., 12, 3306, 2000).
Fabricating features on any of these substrates at the micron to
macroscopic length scale is now routine, and with advances in
nanotechnology, it is possible to print a limited set of structures
made from a variety of hard and soft materials with size control of
features down to ten nanometers (Crommie et al, Science, 262, 218,
1993 and Hua et al, Nano Lett., 4, 2467, 2004). Although they have
many attributes and capabilities, nanolithographic techniques, such
as electron beam lithography, dip-pen nanolithography (DPN),
focused ion-beam lithography, and nanoimprint lithography, are
limited with respect to throughput, materials compatibility,
resolution, and/or cost (Gates et al, Chem. Rev., 105, 1171, 2005).
For example, the field of nanoelectronics relies upon the ability
to fabricate and functionalize less than 20 nm, i.e., sub-20 nm,
electrode gaps for precise electrical measurements on
nanomaterials. Fabricating such structures is far from routine and
often involves low-yielding, imprecise, and difficult-to-control
procedures, such as break junction techniques and gap narrowing by
electroplating (Reed et al, Science, 278, 252, 1997; Park et al,
App Phys Lett, 75, 301, 1999; Li et al, App Phys Lett, 77, 3995,
2000; Xiang et al, Angew Chem Int Ed, 44, 1265, 2005). Other
methods of preparing nanorods having different segments are
disclosed in U.S. Patent Application Publication Nos. 2003/0209427,
2002/0104762 and 2004/0209376.
[0008] The present invention is directed to resistors and diodes
composed of nanorods produced in a high-throughput procedure that
allows for the systematic creation of large quantities of identical
nanorods in an aligned array. These nanorods are then facilely used
as diodes or resistors, depending upon the components of the
nanorod and their electronic properties. The present invention
provides a method of generating assembled nanorod structures with
control over both the length of, the distance, between, and the
electronic properties of rods, allowing for the formation of novel
resistors and diodes.
[0009] The present invention is also directed to a new, general,
and relatively high throughput procedure for lithographically
processing one-dimensional arrays of nanodisks in which the sizes
of the gaps between disks can be controlled down to the 5 nm length
scale. This procedure, termed on-wire lithography (OWL), combines
advances in template directed synthesis of nanowires with
electrochemical deposition and wet-chemical etching, and allows the
routine fabrication of architectures previously considered
difficult, if not impossible, to manufacture via any known
lithographic methodology. The present invention provides a method,
through OWL, of generating one-dimensionally assembled nanorod
structures and nanodisk arrays, with control over both the length
of, and the distance between, rods or disks respectively.
SUMMARY
[0010] The present invention relates to nanostructures having
electronic properties that can be assembled in a regular and
repeatable manner.
[0011] Therefore, one aspect of the present invention is to provide
multicomponent nanorods comprising discrete polymeric and metallic
segments, optionally further modified with segments comprising a
semiconductor or a metal with a low work function.
[0012] Another aspect of the invention is to provide nanostructures
having the electronic properties of diodes or resistors.
[0013] Yet another aspect of the invention is to provide of method
of manufacturing multicomponent nanorods in a controlled manner,
such that the nanorods can be produced with tailorable segments of
metal, polymer, and optionally, semiconductors or metals with low
work function.
[0014] Still another aspect of the invention is to provide
aggregates of multicomponent nanorods wherein the nanorod comprises
both hydrophobic and hydrophilic regions, and wherein the
characteristics and spatial distributions of the hydrophobic and
hydrophilic regions determine and control the structure of the
aggregate.
[0015] Yet another aspect of the invention is to provide a
lithographic method of producing nanowires comprising impervious
metals and having gaps at positions previously containing a
sacrificial metal, wherein the lithographic method is compatible
with a high-through-put production and with a high level of
precision.
[0016] A further aspect of the invention is to provide nanowires
and nanoarrays produced from the lithographic process for use in
microelectrode circuitry.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1A is an optical microscope image of
gold-polypyrrole-gold (Au-Ppy-Au) nanorods;
[0018] FIG. 1B is an optical microscope image of
gold-polypyrrole-cadmium-gold (Au-Ppy-Cd--Au) nanorods with an
inset of a corresponding field emission electron scanning
microscopy (FESEM) image;
[0019] FIG. 2A is a current-voltage (I-V) measurement for Au
segments within a single nanorod, at room temperature, with an
inset showing the optical microscope image of a single Au-Ppy-Au
nanorod on prefabricated microelectrodes
[0020] FIG. 2B shows temperature dependent I-V curves for
measurements across electrodes 2 and 3 with an inset showing a plot
of log .sigma.(T) vs 1/T, wherein .sigma. is conductivity;
[0021] FIG. 3A is an energy dispersive X-ray (EDX) spectroscopy
mapping for the Au segment in a single Au-Ppy-Cd--Au nanorod;
[0022] FIG. 3B is an EDX spectroscopy mapping for the Cd segment in
a single Au-Ppy-Cd--Au nanorod;
[0023] FIG. 3C is a FESEM image of the Au-Ppy-Cd--Au nanorod;
[0024] FIG. 3D is a graph of the X-ray profile for the Cd (dark
trace) and Au (pale trace) segments over the dashed trace shown in
the image in FIG. 3C;
[0025] FIG. 3E is the I-V characteristics for a single
Au-Ppy-Cd--Au nanorod at room temperature;
[0026] FIG. 4 is a schematic of the method of synthesizing
multi-component inorganic-organic hybrid nanorod structures;
[0027] FIG. 5 is the FESEM image and corresponding EDX images for
each segment of a single four-segmented gold-cadmium
selenide-polyaniline-gold (Au--CdSe-PAN-Au) nanorod;
[0028] FIG. 6 is a schematic of the on-wire lithography (OWL)
process;
[0029] FIG. 7A is the FESEM image of a nanorod having Au and Ag
segments;
[0030] FIG. 7B is the FESEM image of a nanorod having Au and Ag
segments after etching;
[0031] FIG. 7C is the FESEM image of a nanowire having a 25 nm gap,
a 50 nm gap and a 100 nm gap;
[0032] FIG. 7D is a FESEM image of a nanorod structured assembly
having nickel (Ni) and Au segments;
[0033] FIG. 7E is a FESEM image of a nanowire structured assembly
having Ni and Au segments after coating with silica and subsequent
removal of the Ni segments;
[0034] FIG. 7F is a FESEM image of a 5 nm nanogap achieved using
OWL;
[0035] FIG. 8A shows the I-V characteristics of nanowires after
silica coating and etching with the inset describing the dip-pen
lithography (DPN) process;
[0036] FIG. 8B is a FESEM image of a nanowire created with a 13 nm
gap;
[0037] FIG. 8C is a FESEM image of a nanowire created with a 13 nm
gap, immobilized on microelectrodes and further modified using DPN
to deposit a mixture of polyethylene oxide and self-doped Ppy;
[0038] FIG. 9A is a bright-field optical image of metallic
nanowires dispersed on a glass slide;
[0039] FIG. 9B is a bright-field optical image of wire-shaped pits
after the wires are released;
[0040] FIG. 10 is a graph of length vs. charge passed during
electrochemical deposition of Ag, Au, and Ni;
[0041] FIG. 11 shows the EDX spectra of nanogap wires before and
after etching of Ni; and
[0042] FIG. 12 is an atomic force microscopy (AFM) topography image
of dot patterns composed of 1:1 (w/w) mixture of polyethylene
oxide: Ppy, generated by DPN.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0043] The present invention is directed to nanostructures having
electronic properties or applications. More particularly, the
present invention is directed (a) to organic/inorganic hybrid
nanostructures for use as diodes or resistors and (b) to
multicomponent nanowires and nanoarrays synthesized with high
accuracy and an exquisite control of gaps. Assembled
macrostructures of these nanowires also are disclosed.
[0044] Multicomponent rod-like structures containing segments of
metals and polymers can be systematically synthesized via
template-assisted in-situ electrochemical deposition, such that the
rod structures can be tailored through a choice of individual
segment compositions that exhibit desired electrical behavior in
the context of an integrated microelectrode device. The length of
the each segment can be controlled by monitoring the charge passed
during the electro-deposition process. FIG. 4 is a schematic
illustrating the present method of producing hybrid
organic/inorganic (e.g., polymer/metal) nanorod structures.
Nanorods comprising metal and polymer segments optionally can be
combined with a segment of an inorganic semiconductor or a metal
with a low work function. These optional segments further modify
the electronics of the nanorod to provide a wide range of practical
applications.
[0045] In the method of preparing hybrid organic-inorganic
nanorods, segmented metal-polymer nanorods are synthesized by
electrochemical deposition of a metal, e.g., Au, onto an alumina
template, followed by electrochemical polymerization of the organic
monomer (e.g., pyrrole). A second electrochemical deposition of Au
provides a resistor. An electrochemical deposition of a
semiconductor or low work function metal provides a diode. See FIG.
4. The length of each metal and polymer segment is controlled by
monitoring the charge passed during the electrochemical deposition
process.
[0046] As used herein, "nanostructure" and "nanorod," refer to
small structures that are less than 10 .mu.m, and preferably less
than 5 .mu.m, in any one dimension and that have a length to width
ratio greater than one
[0047] As used herein, "nanoarray" refers to groups of uniformly
spaced nanostructures.
[0048] As used herein, "multicomponent" refers to an entity that
comprises more than one type of material. For example, a
multicomponent nanorod refers to a nanorod having sections of
different materials, e.g., a nanorod with an Au segment and a Ppy
segment or a nanorod with an Au segment and a Ni segment.
[0049] As used herein, "polymer" means any material of repeating
units suitable for deposition using electrochemistry. In some
embodiments, the polymer is a polypyrrole. In other embodiments,
the polymer is a polyaniline. Examples of other polymers include,
but are not limited to, polythiophene,
poly(ethylenedioxy)thiophene, compounds of poly(heteraromic
vinylenes), polyvinylphosphate, and mixtures thereof. The monomers
comprising the above-listed polymers can be components of
copolymers as well. Optionally, the polymer can comprise an
acceptable salt, e.g., tetrafluoroborate, and/or be doped with
another polymer, e.g., poly(styrene p-sulfate). The polymer can be
modified with optional substituents on an aryl ring of the
corresponding monomer. Nonlimiting examples of such aryl
substituents include, but are not limited to, cyano, sulfate, and
nitro. Other suitable counterions, polymers for doping, or optional
aryl ring substituents are well known to those of skill in the
art.
[0050] The metal component of the nanorod can be any metal
compatible with in-situ electrochemical deposition. Examples of
such metals include, but are not limited to, indium-tin-oxide,
titanium, platinum, titanium tungstide, gold, silver, nickel,
copper, and mixtures thereof.
[0051] The optional inorganic semiconductor can be any material
displaying semiconducting properties. Examples of inorganic
semiconductors for use in the present disclosure include, but are
not limited to, cadmium selenide, zinc selenide, cadmium telluride,
zinc telluride, cadmium-tellurium selenide, copper-indium selenide,
copper oxide, copper sulfide, silicon, germanium, compounds and
alloys of silicon and germanium, gallium arsenide, gallium
phosphide, gallium nitride, cadmium sulfide, zinc sulfide, titanium
oxide, zinc oxide, tungsten oxide, molybdenum oxide, titanium
sulfide, or mixtures thereof.
[0052] As used herein, a metal with low work function is a metal,
such as, but not limited to, aluminum, magnesium, calcium, silver,
or cadmium.
[0053] Nanoarrays formed from multicomponent nanorods can be used
as multiple individually addressable microelectrodes. Multiple
individually addressable microelectrodes allow one to
electronically address the nanostructure at different points along
its long axis.
[0054] Multicomponent nanorods can be used as electronic components
as determined by the compositions of their segments. For example,
the Au portions of the nanorod exhibit linear I-V characteristics
and bulk metallic behavior at room temperature. Linear I-V plots
over a voltage range of -1 to +1 volts (V) demonstrate Ohmic
behavior. Such segments can be used as electrical contacts and
conductors. Significantly, I-V measurements across the Ppy segment
of the Au-Ppy-Au nanorod (FIG. 2B) also exhibit a linear response
at room temperature, but non-linear behavior at low temperatures
(<175 K), characteristic of a semiconductor (see FIG. 2B). Such
segments and arrangements of segments can be used as active
electronic components such as diodes. Therefore, the multicomponent
nanorods are contemplated for use as nano-scale semiconductors
and/or electronic components of microelectrodes.
[0055] Four segment nanorods (e.g., Au-Ppy-Cd--Au) can be prepared
via analogous procedures. The optical microscopy and field emission
electron scanning microscopy (FESEM) images of such rods exhibit
clear contrast between the three different chemical compositions
(bright Au ends, dark Ppy, and white Cd, see FIG. 1B). Other
nanorods containing more segments also are contemplated. In
particular, nanorods having five, six, seven, eight, and even
higher numbers of segments can be prepared using the method
disclosed herein. The end use of the resulting nanorod will dictate
the number of segments, the order of the segments, and the
composition of each segment. Each of these parameters easily can be
determined by a person of skill in the art, in combination with the
disclosure herein.
[0056] In accordance with the present invention, one can
systematically synthesize multicomponent nanorods that contain
metals, inorganic semiconductors, and polymers via
template-assisted in-situ electrochemical depositions, and that
such rod-structures can be tailored through choice of segment
composition to exhibit resistor or diode like behavior in the
context of an integrated microelectrode device. The approach can be
contrasted with the alternative layer-by-layer approach for
synthesizing multicomponent rod structures in two ways. First, the
electrochemical approach offers greater control over the
architectural parameters of the resulting structures (in particular
segment length). Second, the properties (e.g., turn on voltages) of
the resulting structures substantially differ, even when comparable
materials are used. It is theorized that this difference is
attributed to junctions formed in the layer-by-layer approach being
less well defined because the active materials are introduced as a
polymer particle dispersion with little control over where the
active interface is formed. In the present electrochemical
approach, only conducting materials can be deposited within the
pores. The present invention is a powerful method for producing
nanostructures having predetermined desirable electrical properties
by a straightforward synthetic procedure that offers a high degree
of reproducibility. The nanostructures can be used in a wide range
of electronic and sensor devices (Liu et al, Nano Lett, 4, 671,
2004).
[0057] Multicomponent nanorods produced from template-assisted
in-situ electrochemical depositions can be tailored such that
components of like polarity (hydrophobicity or hydrophilicity) are
deposited on one end of the nanorod while components of opposite
polarity are deposited on the other end of the nanorod. Such
multicomponent nanorods are suitable for assembly into aggregate
structures in a manner similar to the formation of liposomes or
micelles.
[0058] As used herein, "polarity" is an assessment of a component's
hydrophobic or hydrophilic character. Components aggregate or
associate with components of like polarity, i.e., polar components
with other polar components, and non-polar components with
non-polar components.
[0059] Aggregation of nanorods having both polar and non-polar
components grouped at opposite ends of the nanorod will occur via
the alignment of the polar components and alignment of the
non-polar components. This aggregation will result in a "fanning"
or micelle-like arrangement of multicomponent nanorods into
nano-disks, spheres, cylinders and other structures.
[0060] Another embodiment of the present process of
segment-by-segment formation of nanostructures is the formation of
nanogap wires and nanodisk and nanorod arrays. These wires, disks,
and arrays have electronic properties that can be tailored from
their compositional components (i.e., the identities of the metals
forming the nanocompositions). The use of metals having different
chemical and electrical properties allows the creation of gaps in
these nanowires where the nanowire is treated with a solution that
dissolves a certain metal but not the other metal. These nanogaps
allow the formation of facing electrodes with controlled gaps,
which is an important goal of nanoelectronics. In particular, this
method allows for the facile and controlled formation of arrays of
such facing electrodes. This technique of selectively stripping
out, or etching, one metal segment type (i.e., the sacrifical metal
segment) in the presence of different metal segment types to form
gaps has been named on-wire lithography (OWL).
[0061] As used herein, the term "nanowire" refers to the product of
on-wire lithography, comprising coated nanorods that have been
subjected to etching to dissolve a sacrificial metal, leaving gaps
where the sacrificial metal segments were positioned prior to
etching.
[0062] As used herein, the term "sacrificial metal" refers to a
metal that can be dissolved under the proper chemical conditions.
Examples of sacrificial metals include, but are not limited to,
nickel which is dissolved by nitric acid, and silver which is
dissolved by a methanol/ammonia/hydrogen peroxide mixture.
[0063] As used herein, the term "etching" refers to a process of
dissolving a sacrificial metal segment using conditions suitable
for dissolving or removing the metal comprising the sacrificial
segment. As mentioned above, such etching solutions include, but
are not limited to, nitric acid and a methanol/ammonia/hydrogen
peroxide mixture.
[0064] As used herein, "coating" refers to a material that is
positioned to contact one side of a multicomponent nanorod. The
purpose of the coating is to provide a bridging substrate to hold
segments of the etched nanorod together after removal of the
intervening sacrificial metal segments in the etching process.
Nonlimiting examples of coatings used in this invention include a
gold/titanium alloy and silica.
[0065] A method of preparing nanogap wires of the present
disclosure is summarized in FIG. 6. OWL is based upon manufacturing
segmented nanowires comprising at least two materials, one that is
susceptible to, and one that is resistant to, wet chemical etching.
There are a variety of material pairs that can be used. Au--Ag and
Au--Ni are two such examples of metal pairs of differing chemical
properties. The sacrificial metal in these pairs are Ag and Ni,
respectively. However, any combination of metals having contrasting
susceptibility to chemical etching conditions can be used.
[0066] Using the OWL procedure, nanowires having desired gaps of 5,
25, 40, 50, 70, 100, 140, and 210 nm, for example, can be prepared
(FIG. 7A-7F). Other desired gap sizes of about 5 nm to about 250 nm
can be prepared. The gap size of the nanowire is directly
correlated to the segment size of the sacrificial metal segment.
Sacrificial metal segment size is controlled by monitoring the
current passed during its deposition (FIG. 10). A nanowire can have
sacrificial metal segments of different size, as determined by the
desired end use of the nanowire.
[0067] A significant issue involving the characterization and
utility of gaps fabricated by OWL pertains to their transport
properties. For nanowires having a silica bridging substrate, the
micrometer scale metal ends can be used as electrically isolated
electrode leads that can be interfaced with larger microelectrode
electronic circuitry. Nanowires of this type have been
characterized by current versus voltage measurements and exhibit
insulating behavior (FIG. 8A, horizontal trace). Therefore, the
nanowires of the present invention are contemplated for use in
microelectrode applications.
[0068] The gaps within the nanowires can be functionalized with
many materials in a site-specific manner using, for example, DPN
(Piner et al, Science, 283, 661, 1999 and Ginger et al, Angew.
Chem. Int. Ed., 43, 30, 2004). By using DPN, one can monitor the
device architecture in the active region, measure the topography of
the nanowires, and, simultaneously, functionalize the nanowire gaps
with molecule-based materials. Therefore, modified nanowires
produced using OWL are contemplated. Modification at site-specific
points using DPN is particularly contemplated, as is deposition of
conducting polymers using DPN.
[0069] The present novel lithographic process allows one to
generate designed gap structures on nanowire templates. The process
is remarkably controllable, high yielding, easy to implement, does
not require sophisticated and expensive instrumentation and
facilities, and allows manipulation of an important class of
structures that are not easily manipulated with conventional
lithographic tools. The ability to make gap or notched structures
with nanowires via OWL and relatively inexpensive instrumentation
will facilitate the study of the electronic properties of
nanomaterials and open avenues to the preparation of novel disk
array structures, which could be designed to have unusual optical
properties as a function of gap and metal segment size (e.g.,
plasmon waveguides as disclosed in Maier et al, Nature Mater., 2,
229, 2003 or in surface enhanced raman scattering as disclosed in
Kniepp et al, Chem. Rev., 99, 2957, 1999). The present nanowires
also can be assembled into arrays of disks and/or rods, depending
on the length of the segment compared to its diameter. In
accordance with an important feature of the present invention, all
the remaining segments, either as arrays of rods or disks, are
aligned linearly, which has not been achieved consistently in the
field of nanoassemblies prior to this disclosure.
[0070] Therefore, a further aspect of this invention is to provide
plasmon wires. Plasmon wires are nanodisk structures of nanowires,
wherein the gaps or notches of the nanowires are aligned and the
metal segments of the nanowires are aligned. This alignment of the
gaps in the nanowires allows for the plasmon wires to propagate
light through coupled plasmons. Such light propagation is discussed
in detail in Schuck et al., Phys Rev Lett, 94, 017402, 2005; Maier
et al., Phys Rev B, 65, 193408, 2002; and Rechberger et al., Optics
Comm, 220, 137, 2003. Also called photonic crystals, these plasmon
wires are important developments in new electronic and optical
devices.
[0071] Plasmon wires are structures that restrict the propagation
of particular wavelengths of electromagnetic radiation by the use
of destructive interference and can be designed for very complex
routing of light and other such electromagnetic radiation.
Furthermore, the gaps between segments of a plamon wire can act as
resonant cavities. The high electromagnetic field strengths that
can be generated by the excitation of such cavities can induce
novel and useful non-linear effects in materials contained within
these gaps. Both the diffractive and resonant forms of such plasmon
wire devices find utility in lasers, filters, communications,
sensors, and similar applications.
[0072] A plasmon wire may be used for a specific application which
requires design control of the bandgap, those wavelengths that are
"forbidden" (do not pass through the structure) and/or the narrow
band of transmitted wavelengths within the "forbidden" range of
wavelengths.
[0073] Plasmon wires comprising nanowires of the present invention
are one dimensional periodic structures. The periodicity of the
gaps or notches produced via OWL allow for tailoring of desired end
properties of the plasmon wire, such as suppression or enhancement
of various wavelengths of interest.
EXAMPLES
Synthesis of Multicomponent Metal-Conducting Polymer-Metal
Nanorods
[0074] In a typical experiment, a thin layer of silver (200 nm) was
evaporated on one side of an alumina filter (Whatman International
Ltd, d=13 mm, pore size--20 nm; the pore diameter in the central
region of the filter is substantially larger than the quoted 20 nm)
and served as a cathode in a three electrode electrochemical cell
after making physical contact with aluminum foil. Platinum wire was
used as a counter electrode, and silver/silver chloride (Ag/AgCl)
was used as the reference electrode. The nanopores were filled by
the electrochemical deposition of Ag (Technic ACR silver RTU
solution from Technic, Inc.) at a constant potential, -0.9 V vs.
Ag/AgCl, by passing 1.5 C/cm.sup.2 for 30 minutes. An Au segment
then was electroplated from Orotemp 24 RTU solution (Technic, Inc.)
at -0.9 volts (V) vs. Ag/AgCl followed by a conducting polymer
segment from various plating solutions at positive potentials. For
Ppy segments, the solution was a mixture of 0.5 M pyrrole and 0.2 M
tetraethylammonium tetrafluoroborate in acetonitrile, and the
applied potential was 1.0 V vs. Ag/AgCl. For polyaniline segments,
an aqueous solution containing 0.5 M aniline and 0.2 M perchloric
acid was used for polymerization. The polymerization potential was
1.0 V vs. Ag/AgCl. The Cd segment was deposited from an aqueous
solution of 0.3 M cadmium sulfate and 0.25 M sulfuric acid at -0.8
V vs. Ag/AgCl.
[0075] The procedure involving Au was repeated to form the final
capping segment. Each segment length was controlled by monitoring
the charge passed through the membrane. The first 2.6 .mu.m
(.+-.0.2) long segment of Au was generated by passing 2.3 coulombs.
The 1 .mu.m (.+-.0.4) segment of Ppy required 0.1 C, and the final
2.4 .mu.m (.+-.0.2) Au segment required 2.3 C (the exposed membrane
surface area is about 1 cm.sup.2). The Ag backing and alumina
membrane were dissolved with concentrated nitric acid and 3M sodium
hydroxide solutions, respectively. The rods were rinsed repeatedly
with distilled water until the pH of the solution was 7.
Preparation of Microelectrodes and Electrical Transport
Measurements of Single Nanorod Devices
[0076] Electrodes were fabricated via two separate steps. First,
contact pads of thermally evaporated layers of 45 nm Au and 5 nm
chromium (Cr) were patterned using photolithography onto a silicon
(Si) wafer with a 5000 .ANG. thermally grown oxide. Next, electron
beam lithography (EBL) was utilized to define an inner electrode
pattern (25 nm-thick Au on 7 nm-thick Cr) that was connected to
contact pads. EBL was performed using a Hitachi S4500 SEM equipped
with a Nabity Pattern Generation System (NPGS, JC Nabity
Lithography Systems, Bozeman, Mont.) at 30 kV acceleration voltage
and 15 pA beam current. One drop of the aqueous solution of
nanorods was deposited on a chip with prefabricated electrodes and
the chip was dried in vacuum. After identifying an area where only
a single nanorod bridged the prefabricated electrodes, the
underlying electrodes were wire-bonded to a chip carrier using a
wedge wirebonder (K&S 4526 wirebonder, Kulick & Soffa,
Willow Grove, Pa.). The current-voltage (I-V) characteristics of a
majority of the devices were obtained using a shielded, temperature
controlled cryostat (OptistatCF--continuous flow, exchange gas
cryostat, Oxford Instruments, England) equipped with coaxial
connections. All the measurements were made in the absence of
light. For all measurements, a 16-bit digital acquisition board
(DAQ, National Instruments, DL Instrument, Ithaca, N.Y.) and the
output voltage of the preamplifier was measured using either an
analog-to-digital input on the 16-bit DAQ board, or with an
electrometer (Model 6430, Keithley Electronics, Cleveland,
Ohio).
Analysis of Multicomponent Nanorods
[0077] An Au-Ppy-Au nanorod produced via the above-disclosed
procedure was observed using Field Emission Scanning Electron
Microscopy (FESEM). Optical microscopy images show a dark Ppy
domain sandwiched between two bright segments of Au (see FIG. 1A).
Single nanorod devices were prepared from these Au-Ppy-Au nanorods
at different temperatures by depositing multicomponent Au-Ppy-Au
nanorods on top of a microelectrode array, These nanorod devices
were assessed for their electronic properties (see FIG. 2A
inset).
[0078] Analysis of the I-V curves and the corresponding electrical
conductivities of the Au-Ppy-Au nanorods provides two important
observations. First, the room temperature conductivity of the
polymer segment (about 3 mS/cm) is six orders of magnitude lower
than the metallic segments, and all data are consistent with Ohmic
contact between the Ppy and Au junctions. Indeed, the inner polymer
segments dictate the electric properties of the hybrid
three-component system, and the two Au segments function simply as
electrical leads to the microscopic circuitry. Second, the I-V
response for the Au-Ppy-Au nanorod becomes slightly nonlinear as
the temperature decreases (see FIG. 2B). Such nanorods exhibit an
Arrhenius-type temperature dependence with respect to conductivity,
characteristic of thermally activated charge transport within the
segment of Ppy (see FIG. 2B insert). The semiconducting behavior of
the Au-Ppy-Au nanorods is reminiscent of that observed for
electrochemically polymerized bulk Ppy films. The experimentally
determined activations energy (E.sub.a=about 0.07 eV) of the
Au-Ppy-Au nanorod is in good agreement with the values reported for
a moderately doped bulk Ppy film and a nanotubular thin film
structure (see FIG. 2B insert) (Watanabe et al, Macromolecules, 22,
4231-4235, 1989; Park et al, Thin Solid Films, 438, 118, 2003).
Because the polymer segments for these nanostructures were
generated by oxidative polymerization, they are p-type.
Preparation and Characterization of Multicomponent Nanorods with a
Semiconductor Segment.
[0079] Gold-polypyrrole-cadmium-gold nanorods can be prepared via
the above-disclosed process. The chemical composition of each
inorganic segment was confirmed by energy dispersive X-ray
spectroscopy (EDX) elemental mapping experiments (see FIGS. 3A-3D).
The FESEM of an individual rod clearly shows two interfaces (from
left to right) between the Ppy and Cd segments, and between the Cd
and Au segments, see FIG. 3C. The EDX analysis of the dotted region
in FIG. 3C exhibits characteristic elemental signatures for Cd and
Au, see FIG. 3D. All data are consistent with the asymmetric
junction structure within the single nanorod.
[0080] Current vs. voltage measurements on devices constructed from
single Au-Ppy-Cd--Au rods exhibit "diode" behavior at room
temperature, see FIG. 3E. The typical response is asymmetric and
non-Ohmic. In the forward bias, there is a positive voltage on the
Au segment interfaced with the Cd segment. Therefore, holes move
from the Ppy segment to the Cd segment during the forward bias. In
reverse bias, current does not flow until the bias overcomes the
breakdown potential (-0.61 V). The turn-on voltage for these diode
nanorods is about 0.15 V, almost 1 V lower than rods prepared to
date via the layer-by-layer assembly method (Kovtyukhova et al, J
Phys Chem B, 105, 8762, 2001). The rectifying ratio (i.e., forward
bias current/reverse bias current) is 200 at .+-.0.6 V. The I-V
characteristics of the Au-Ppy-Cd--Au nanorods at room temperature
suggest that a Schottky-like junction is formed at the Ppy/Cd
interface due to the difference in work function for the two
materials (Abthagir et al, J Appl Polym Sci, 81, 2127, 2001;
Watanabe et al, Macromolecules, 22, 4231, 1989; Sze, Physics of
Semiconductor Devices, 2.sup.nd ed., Wiley: New York, 1981). The
difference in work function (.DELTA..PHI.) between a metal such as
Cd and a moderately doped p-type semiconducting Ppy is about 0.68
eV, which is larger than that for Au and Ppy (.DELTA..PHI.(Au-Ppy)
about 0.1 eV) based upon the assumption that the work function of
an electrochemically polymerized Ppy film with a similar doping
level is about 4.9 to about 5.1 eV as reported in Abthagir et al, J
Appl Polym Sci, 81,2127,2001 and Watanabe et al, Macromolecules,
22, 4231, 1989. By fitting the experimental I-V responses to the
model for metal semiconductor Schottky junctions (Abthagir et al, J
Appl Polym Sci, 81, 2127, 2001; Sze, Physics of Semiconductor
Devices, 2.sup.nd ed., Wiley: New York, 1981), a barrier height
(.PHI..sub.BH) for the Ppy/Cd junction was determined to be about
0.68 eV, and is in good agreement with the reported values of
electrochemically polymerized bulk Ppy/indium junctions (Watanabe
et al, Macromolecules, 22, 4231, 1989).
On-Wire Lithography Process
[0081] The materials can be electrochemically deposited in porous
alumina templates in a controlled fashion from suitable plating
solutions via well-established methods (see FIG. 6) (Martin,
Science, 266, 1961, 1994; Routkevitch, et al, J. Phys. Chem., 100,
14037, 1996; Nicewarner-Pena et al, Science, 294, 137, 2001;
Kowtyukhova et al, Chem. Eur. J., 8, 4354, 2002). The length of
each segment is tailored by controlling the charge passed during
the electrodeposition process (see FIG. 10). The resulting
multi-metallic wires then are released from the template by
dissolution of the template via known procedures (Park et al,
Science, 303, 348, 2004). In one example, an aqueous suspension of
Au--Ni nanorods is cast upon a glass microscope slide pre-treated
with a piranha solution which makes the slide more hydrophilic.
After drying, a layer of silicon dioxide was deposited on the
nanorods, using plasma enhanced chemical vapor deposition (PECVD)
(see FIG. 9A). This results in one side of the nanorod being coated
with silicon dioxide while the other side, which is protected by
the microscope slide substrate, remains uncoated. Sonication of the
substrate leads to the release of the coated nanorods into solution
(FIG. 9B). The final step of the OWL process involves the selective
wet-chemical etching of the sacrificial segments. Nickel segments
can be removed from the rods by treating the rods with concentrated
nitric acid for 1 hr. This results in the generation of nanowire
structures with gaps precisely controlled by the length of the
original Ni segments (FIG. 6). The Au segments remaining after
removal of the Ni segments are held in place by the stripe of
silicon dioxide. As silicon dioxide is an insulator, the Au
segments can be electrically connected to one another if the
nanowires are coated with Au/Ti rather than silicon dioxide in this
process. In another alternative, the nanorods are comprised of Au
and Ag segments, the sacrificial Ag segments are removed by
treating the coated nanorods with an etching solution containing
methanol, 30% ammonium hydroxide, and 30% hydrogen peroxide (4:1:1
v/v/v) for 1 hour. Numerous other combinations of materials and
etchants can likewise be used for such purposes depending upon the
intended use of the structures formed.
[0082] The physical dimensions and segment compositions of the
nanowires, before and after etching, were determined by FESEM and
EDX (see FIG. 7A-7F and FIG. 11). Structures made of Ag and Au
before coating with Au/Ti and wet-chemical etching exhibit a bright
contrast for the Au regions and a dark contrast for the Ag regions
(FIG. 7A). After etching, the notched structures are clearly
visible (FIG. 7B). The EDX spectra before etching shows Ni present,
and after etching the Ni peak has disappeared. The Si and oxygen
peaks correspond to the presence of the silica coating (FIG. 11).
The average length of the wires is 4.5.+-.0.25 .mu.m, and each wire
exhibits two 210.+-.10 nm, two 140.+-.8 nm, and two 70.+-.5 nm
notches (FIG. 7B). The diameter of each wire is 360.+-.20 nm.
Certain views show the Au/Ti backing, which bridges the notched
regions on these structures (FIG. 7B, inset). Structures with
sub-100 nm gap sizes can routinely be generated via OWL. To
demonstrate this capability, OWL was used to prepare wires having
25, 50, and 100 nm gaps (FIG. 7C).
[0083] Multi-segment nanorods composed of Ni and Au segments were
synthesized using electrochemical deposition into a porous alumina
membrane. A thin layer of Ag (200 nm) was evaporated on one side of
an alumina filter (Whatman International Ltd, d=13 mm, pore size=20
nm; the pore diameter in the central region of the filter is
substantially larger than the quoted 20 nm) and served as a cathode
in a three electrode chemical cell after making physical contact
with aluminum foil. Platinum wire was used as a counter electrode,
and Ag/AgCl was used as the reference electrode. The nanopores were
partially filled with Ag, leaving headroom to accommodate the
growth of additional domains (Technic ACR silver RTU solution from
Technic, Inc.) at a constant potential, -0.9 V vs. Ag/AgCl, by
passing 1.5 C/cm.sup.2 for 30 min. An Au segment then was
electroplated from Orotemp 24 RTU solution (Technic, Inc.) at -0.9
V vs. Ag/AgCl followed by a Ni segment from nickel sulfamate RTU
solution (Technic, Inc.) at -0.9 V vs. Ag/AgCl. The procedure
involving Au was repeated to form a second Au segment. Each segment
length was controlled by monitoring the charge passed through the
membrane. The first 1.4 .mu.m (.+-.0.2) long segment of Au was
generated by passing 1.3 C. The Ag backing and the alumina membrane
then were dissolved with concentrated nitric acid and 3 M sodium
hydroxide solutions, respectively. The rods were repeatedly rinsed
with nanopure water until the solution reached at pH of 7. Nanorods
containing more than three segments are prepared by repeating the
above steps until the desired number of segments have been
constructed. These added segments may be constructed of the same or
different materials than used in the construction of the initial
three segments, by appropriate selection of the plating materials
and conditions in the manner known to those of skill in the
art.
[0084] To demonstrate the capabilities of OWL in the preparation of
repeating nanostructures having regular 40 nm gaps, nanowire
structures with twenty-two 40 nm Ni segments and twenty-three 40 nm
Au segments were made (FIG. 7D). Silicon dioxide was used as the
bridging material. After etching, the removal of the Ni segments
was confirmed by EDX spectroscopy of the nanowires (FIG. 11). The
Ni peak is present before etching, and is absent after. The Si and
oxygen peaks in the after etching spectrum are due to the presence
of silica as the coating material. Face-to-face disk arrays with 40
nM gaps were generated (FIG. 7E). The statistical variation of gap
size generally increases with decreasing gap size, but is typically
less than 10%. Note that in some images the variation looks
greater, but this is due to mechanical stress on the wire
structures, which results in a "fanning" effect with respect to the
gaps. The smallest gap structures generated via OWL are 5 nm (FIG.
7E), but with appropriate electrochemical control, smaller gaps can
be generated, such as down to 2 nm and preferably 1 nm.
[0085] To test the properties of these structures and their
suitability for making transport measurements on small amounts of
materials containing within the gaps, a droplet of a suspension
containing wires with 13 nm gaps was evaporated on a microelectrode
array fabricated by conventional photolithography (FIGS. 8B and
8C). The electrodes were 3 .mu.m wide and separated by 2 .mu.m.
Some of the wires end up bridging the microelectrodes, allowing for
easy electrical measurements of the structures.
Further Modified Nanowires with Photosensitivity
[0086] A mixture of 1:1 (w/w) polyethylene oxide (PEO) and Ppy was
deposited into the gaps of a nanowire using DPN. The mixture
consisted of 3 mL of PEO (0.05 g, M.sub.v.apprxeq.100,000, Aldrich)
in acetonitrile and 1 mL of 5% aqueous self-doped Ppy (Aldrich).
Contact times between tip and substrate were 2, 1, and 0.5 sec for
the large, medium and small sized dots, respectively (FIG. 12, top
to bottom). The chamber humidity was 80% and the tip substrate
contact force was 1.0 nN. The DPN patterning was done with a
ThermoMicroscopes CP AFM interfaced with customized software.
Nanoink, type S-1, AFM probes having a spring constant of 0.041 N/m
were used for these depositions.
[0087] A SEM image of nanowires having polymer deposited in the
gaps of the nanowire shows the clear contrast between the clean Au
surface and the polymer covered area including the gap (FIG. 8C,
inset). Current vs. voltage measurements after deposition of the
polymer shows a linear response from -1.0 V to +1.0 V,
characteristic of a conducting polymer (FIG. 8A). The measured
conductance of 1.1 nS is similar to the value of 9.6 nS determined
by functionalizing 60 nm conventional nanoelectrode gaps fabricated
by EBL. There is no noticeable I-V hysterisis between the forward
(from -1.0 V to +1.0 V) and backward (from +1.0 V to -1.0 V) scans,
and they are highly linear at room temperature, as expected for a
structure with an Ohmic-like contact in a symmetric device
configuration. To prove that the response is indeed due to the
polymer within the gap, the I-V response as a photoexcitation was
studied using a Xe lamp (150 W). The I-V response for the polymer
filled nanowire becomes slightly more conductive upon Xe light
exposure. During the backward scan, the device was irradiated with
the Xe lamp starting at -0.1 V (FIG. 8A, grey arrows), and a change
in slope in the current voltage response was observed. The
transient conductance change between 1.1 nS in the dark to 1.6 nS
when irradiated is consistent with an increase in charge carrier
density expected if the gap was filled with the p-type Ppy.
* * * * *