U.S. patent application number 12/220580 was filed with the patent office on 2009-02-19 for scaffold-organized clusters and electronic devices made using such clusters.
This patent application is currently assigned to The State of Oregon acting by and through the State Board of Higher Education on behalf of the. Invention is credited to James E. Hutchison, Scott M. Reed, Martin N. Wybourne.
Application Number | 20090047753 12/220580 |
Document ID | / |
Family ID | 27392954 |
Filed Date | 2009-02-19 |
United States Patent
Application |
20090047753 |
Kind Code |
A1 |
Hutchison; James E. ; et
al. |
February 19, 2009 |
Scaffold-organized clusters and electronic devices made using such
clusters
Abstract
A method for forming arrays of metal, alloy, semiconductor or
magnetic clusters is described. The method comprises placing a
scaffold on a substrate, the scaffold comprising, for example,
polynucleotides and/or polypeptides, and coupling the clusters to
the scaffold. Methods of producing arrays in predetermined patterns
and electronic devices that incorporate such patterned arrays are
also described.
Inventors: |
Hutchison; James E.;
(Eugene, OR) ; Reed; Scott M.; (Eugene, OR)
; Wybourne; Martin N.; (Hanover, NH) |
Correspondence
Address: |
KLARQUIST SPARKMAN, LLP
121 SW SALMON STREET, SUITE 1600
PORTLAND
OR
97204
US
|
Assignee: |
The State of Oregon acting by and
through the State Board of Higher Education on behalf of
the
University of Oregon
|
Family ID: |
27392954 |
Appl. No.: |
12/220580 |
Filed: |
July 25, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11094049 |
Mar 29, 2005 |
7442573 |
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12220580 |
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10783515 |
Feb 19, 2004 |
6872971 |
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11094049 |
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09817708 |
Mar 26, 2001 |
6730537 |
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10783515 |
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60191814 |
Mar 24, 2000 |
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60226720 |
Aug 21, 2000 |
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60231193 |
Sep 7, 2000 |
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Current U.S.
Class: |
438/99 ;
257/E51.001 |
Current CPC
Class: |
G01N 33/54353 20130101;
H01L 51/0595 20130101; B82Y 30/00 20130101; H01L 51/0093 20130101;
B82Y 10/00 20130101 |
Class at
Publication: |
438/99 ;
257/E51.001 |
International
Class: |
H01L 51/40 20060101
H01L051/40 |
Goverment Interests
ACKNOWLEDGMENT OF GOVERNMENT SUPPORT
[0002] This invention was made in part using funds provided by (1)
the Department of Defense, Office of Naval Research, under contract
numbers N00014-93-0618 and N00014-93-1-1120, and (2) the National
Science Foundation, Grant No. DMR-9705343. The federal government
may have rights in this invention.
Claims
1. A method for forming arrays of metal, alloy, semiconductor
and/or magnetic clusters, comprising: placing a scaffold on a
substrate; and coupling monodispersed clusters, selected from the
group consisting of metal clusters, alloy clusters, semiconductor
clusters, magnetic clusters, and combinations thereof, to the
scaffold.
2. The method of claim 1, wherein coupling comprises contacting the
scaffold with clusters having plural exchangeable ligands, where at
least one of the ligands is exchanged for a functional group of the
scaffold.
3. The method of claim 1, wherein coupling comprises contacting the
scaffold with clusters having plural ligands, where at least one of
the ligands is charged and is electrostatically attracted to a
scaffold of opposite charge.
4. The method of claim 3, wherein the scaffold is positively
charged polylysine and the plural ligands of the clusters include
at least one having a negatively charged group.
5. The method of claim 4, wherein the negatively charged group is
selected from the group consisting of carboxylate, sulfonate, and
combinations thereof.
6. The method of claim 4, wherein the polylysine is
poly-L-lysine.
7. The method of claim 3, wherein the scaffold is a polynucleotide
having a negatively charged phosphate backbone and the plural
ligands of the clusters include at least one having a positively
charged group selected from the group consisting of protonated
amine groups, quaternary ammonium groups, and combinations
thereof.
8. The method of claim 1, wherein coupling comprises contacting the
scaffold with clusters having plural ligands, where at least one of
the ligands becomes associated with the scaffold through a
hydrophobic interaction.
9. The method of claim 8, wherein the scaffold is a polynucleotide
and the plural ligands of the clusters include at least one ligand
that intercalates into the polynucleotide.
10. The method of claim 9, wherein the polynucleotide is DNA.
11. The method of claim 1, wherein placing the scaffold on the
substrate comprises placing the scaffold on the substrate in a
predetermined pattern.
12. The method of claim 11, wherein placing the scaffold on the
substrate in a predetermined pattern comprises aligning the
scaffold in an electric field created between electrodes on the
substrate.
13. The method of claim 12, wherein the scaffold has an electric
dipole moment that causes the scaffold to align in the electric
field.
14. The method of claim 13, wherein the scaffold is a helical
polynucleotide.
15. The method of claim 13, wherein the scaffold is a helical
polypeptide.
16. The method of claim 15, wherein the helical polypeptide is in
the form of an .alpha.-helix.
17. The method of claim 16, wherein the polypeptide is
polylysine.
18. The method of claim 11, wherein placing the scaffold on the
substrate in a predetermined pattern comprises polymerizing
monomers, oligomers, or polypeptides into larger polypeptides to
form a scaffold between two electrodes on the surface of the
substrate.
19. The method of claim 11, wherein placing the scaffold on the
substrate in a predetermined pattern comprises anchoring the
scaffold and inducing alignment of the anchored scaffold in a
particular direction by fluid flow.
20. The method of claim 19, wherein the scaffold is attached to a
first electrode and aligned by fluid flow, substantially in the
direction of a second electrode.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a divisional of U.S. application Ser. No.
11/094,049, filed Mar. 29, 2005, which is a divisional application
of U.S. application Ser. No. 10/783,515, filed Feb. 19, 2004, which
is a divisional application of U.S. application Ser. No.
09/817,708, filed Mar. 26, 2001, which claims the benefit of prior
U.S. Provisional Application No. 60/191,814, filed Mar. 24, 2000,
entitled Scaffold-Organized Metal, Alloy, Semiconductor and/or
Magnetic Clusters and Electronic Devices Made Using Such Clusters;
U.S. Provisional Application No. 60/226,720, filed Aug. 21, 2000,
entitled Scaffold Organized Clusters; and U.S. Provisional
Application No. 60/231,193, filed Sep. 7, 2000, entitled Scaffold
Organized Clusters. These prior applications are incorporated
herein by reference.
FIELD
[0003] This invention concerns a method for forming organized
arrays of metal, alloy, semiconductor and/or magnetic clusters for
use in the manufacture of electronic devices, such as high-density
memory storage and nanoelectronic devices.
BACKGROUND
[0004] Fundamentally new technologies are required to continue
increasing device integration density and speed. Conventional
metal-oxide semiconductor-field-effect transistors soon will reach
fundamental density and speed limits as a result of quantum
mechanical tunneling. In order to scale electronic device sizes
down to nanometer dimensions, systems containing increasingly fewer
numbers of particles must be considered.
[0005] The ultimate limit is a system in which the transfer of a
single charge quanta corresponds to information transfer or some
type of logic operation. Such single-electron systems are presently
the focus of intense research activity. See, for example, Single
Charge Tunneling, Coulomb Blockade Phenomena in Nanostructure,
edited by H. Grabert and M. H. Devoret, NATO ASI Series B: Physics
Vol. 294 (1992). These systems have potential application to
nanoelectronic circuits that have integration densities far
exceeding those of present day semiconductor technology. See,
Quantum Transport in Ultrasmall Devices, edited by D. K. Ferry, H.
L. Grubin, C. Jacoboni, and A. Jauho, NATO ASI Series B: Physics
Vol. 342 (1995).
[0006] Single-electron transistors based on the concept of Coulomb
blockade are one proposed technology for realizing ultra-dense
circuits. K.K. Likharev, Single-electron Transistors: Electrostatic
Analogs of the DC SQUIDS," IEEE Trans. Magn. 23:1142 (1987); and
IBM J. Res. Dev. 32:144 (1988). Coulomb blockade is the suppression
of single-electron tunneling into metallic or semiconductor
islands. In order to achieve Coulomb blockade, the charging energy
of an island must greatly exceed the thermal energy. To reduce
quantum fluctuations the tunneling resistance to the island should
be greater than the resistance quantum h/e.sup.2. Coulomb blockade
itself may be the basis of conventional logic elements, such as
inverters. Id.
[0007] Equally promising is the fact that the Coulomb blockade
effect can be used to pump charges one-by-one through a chain of
dots to realize a frequency-controlled current source in which the
current is exactly equal to I=ef, where f is the clocking
frequency. See, L. J. Geerligs et al., Frequency-locked Turnstile
Device for Single-electrons, Phys. Rev. Lett., 64:2691 (1990); and
H. Pothier et al., Single-Electron Pump Based on Charging Effects,
Europhys. Lett. 17:249 (1992). Such turnstile devices are of
fundamental interest as highly accurate current standards.
[0008] The clocking of charge through an array is also one model of
information storage. It is possible that computation may be based
on switching of currents rather than charge which, due to the
extreme accuracy of single-electron current sources, may be more
robust towards unwanted fluctuations than single-electron
transistor-based circuits.
[0009] One of the most promising technologies for realizing
terabyte memories is founded on the principle of the Coulomb
blockade. Yano et al. have demonstrated room temperature operation
of single-electron devices based on silicon nanocrystals embedded
in SiO.sub.2. K. Yano et al., Room-Temperature Single-electron
Memory, IEEE Trans. Electron. Devices, 41:1628 (1994); and K. Yano
et al., Transport Characteristics of Polycrystalline-Silicon Wire
Influenced by Single-electron Charging at Room Temperature, Appl.
Phys. Lett., 67:828 (1995). Recently, a fully integrated 8.times.8
memory array using this technology has been reported. K. Yano et
al., Single-Electron-Memory Integrated Circuit for Giga-to-Tera Bit
Storage, IEEE International Solid State Circuits Conference, p.
266-267 (1996).
[0010] Microelectronic devices based on the principle of Coulomb
blockade have been proposed as a new approach to realizing
electronic circuits or memory densities that go beyond the
predicted scaling limit for present day semiconductor technology.
While the operation of Coulomb blockade devices has been
demonstrated, most operate only at greatly reduced temperatures and
require sophisticated nanofabrication procedures. The size scales
necessary for Coulomb blockade effects at such relatively elevated
temperatures of about room temperature impose limits on the number,
uniformity and connectivity of quantum dots. As a result,
alternative methodologies of nanofabrication need to be
investigated and developed.
SUMMARY
[0011] The present invention provides a new process for making
arrays comprising metal, alloy, semiconductor and/or magnetic
clusters. An "array" can be any arrangement of plural such clusters
that is useful for forming electronic devices. Three primary
examples of uses for such arrays are (1) electronic circuits, (2)
arrangements of computer memory elements, both of which can be in
one or several planes, and (3) sensors.
[0012] "Clusters" as used herein refers to more than one, and
typically three or more, metal, alloy, semiconductor or magnetic
atoms coupled to one another by metal-type bonds or ionic bonds.
Clusters are intermediate in size between single atoms and
colloidal materials. Clusters made in accordance with the present
invention also are referred to herein as "nanoparticles." This
indicates that the radius of each such cluster is on the order of
about one nanometer. A primary goal of the present invention is to
provide electronic devices that operate at or about room
temperature. This is possible if the cluster size is made small
enough to meet Coulomb blockade charging energy requirements at
room temperature. While cluster size itself is not dispositive of
whether the clusters are useful for forming devices operable at or
about room temperature, cluster size is nonetheless quite
important. It currently is believed that clusters having radii much
larger than about two nanometers likely will not be useful for
forming electronic devices that operate at or about room
temperature.
[0013] The metal, alloy, semiconductor and/or magnetic clusters are
bonded to "scaffolds" to organize the clusters into arrays.
"Scaffolds" are any molecules, including polymers, that can be
placed on a substrate in predetermined patterns, such as linear
bridges between electrodes, and to which clusters can be bonded to
provide organized cluster arrays. Without limitation, scaffolds
include biomolecules, such as polynucleotides, polypeptides, and
mixtures thereof. Polypeptides capable of forming .alpha.-helices
are particularly useful scaffold-forming molecules. Polypeptides
that are capable of forming other secondary structures, such as
3.sub.10-helices, .pi.-helices, and .beta.-sheets may in certain
embodiments serve as scaffolds. Polypeptides that are capable of
forming repetitive higher order structures (i.e., tertiary, and
quaternary structures) may also serve as scaffolds. One example is
the collagen helix. Double stranded DNA, Holliday junctions, and
RNA hairpins are non-limiting examples of polynucleotide
scaffolds.
[0014] One embodiment of a method for forming arrays of metal,
alloy, semiconductor and/or magnetic clusters involves placing a
scaffold on a substrate, in, for example, a predetermined pattern.
Arrays are formed by contacting the scaffold with plural,
monodispersed (clusters of substantially the same size)
ligand-stabilized metal, alloy, semiconductor and/or magnetic
clusters that couple to the scaffold. If the clusters are metal
clusters, then the metal may be selected from the group consisting
of Ag, Au, Pt, Pd and mixtures thereof. If gold is the metal, the
metal cluster may be Au.sub.55.
[0015] Clusters may be coupled to a scaffold by ligand exchange
reactions. Each cluster, prior to contacting the scaffold, includes
plural exchangeable ligands bonded thereto. The ligand-exchange
reactions involve exchanging functional groups of the scaffold for
at least one of the exchangeable ligands of the cluster that is
present prior to contacting the scaffold with the clusters.
Examples of exchangeable ligands suitable for forming metal
clusters in accordance with the invention may be selected from the
group consisting of thiols, thioethers (i.e., sulfides),
thioesters, disulfides, sulfur-containing heterocycles, 1.degree.,
2.degree. and perhaps 3.degree. amines, pyridines, phosphines,
carboxylates, nitriles, hydroxyl-bearing compounds, such as
alcohols, and mixtures thereof. Thiols are particularly useful
ligands for practicing the present invention.
[0016] Clusters may also be coupled to the scaffold by
electrostatic interactions between the cluster and the scaffold.
For example, clusters may include plural ligands that possess a
charge or charges, either positive or negative, that serve to
attract the clusters to oppositely charged scaffolds. In one
embodiment, the cluster includes ligands having at least one
positive charge and the scaffold is a polynucleotide having plural
negative charges along its phosphate backbone. In a more particular
embodiment, the cluster includes ligands having quaternary ammonium
groups. In another embodiment, the cluster includes ligands with at
least one negative charge, such as ligands having carboxylate or
sulfonate group(s), and the scaffold is a polypeptide, such as
polylysine (PL), having plural positive charges. In a particular
disclosed embodiment, the scaffold is poly-L-lysine (PLL).
[0017] Clusters may be coupled to a scaffold through hydrophobic
interactions. In one embodiment, the cluster includes ligands with
a portion that can intercalate into DNA. For example the portion
may be an anthraquinone. Other examples of suitable intercalating
portions include planar cations such as acridine orange, ethidium,
and proflavin. In some embodiments, the portion facilitates
intercalation at particular, sequence-specific sites within a DNA
molecule. In other embodiments the clusters are coupled to a
scaffold through covalent bonds between the ligands of the cluster
and the scaffold.
[0018] There are several methods for placing a scaffold onto a
substrate in predetermined patterns. For example, one method
comprises aligning scaffold molecules in an electric field created
between electrodes on the substrate. It therefore will be
appreciated that the scaffold molecules advantageously have a
dipole moment sufficient to allow them to align between the
electrodes. This is one reason why polypeptides that form .alpha.
helices are particularly useful. The .alpha.-helix structure
imparts a sufficient dipole to the polypeptide molecules to allow
alignment of the molecules between the electrodes upon formation of
an electrical field. One example of a polypeptide useful for
forming scaffolds in accordance with the present invention is
polylysine. Similarly, polynucleotides, such as DNA, that assume
helical structures may be aligned by electric fields.
[0019] Another method of patterning scaffold molecules comprises
polymerizing monomers, oligomers (10 amino acids or nucleotides or
less), or small polynucleotides or polypeptides into longer
molecules on the surface of a substrate. For example, scaffold
molecules can be polymerized as a bridge between electrodes on a
substrate.
[0020] Yet another method of placing a scaffold onto a substrate in
a predetermined pattern is by anchoring the scaffold and inducing
alignment of the anchored scaffold in a particular direction by
fluid flow. For example, a scaffold may be aligned between two
electrodes by attaching the scaffold to a first electrode and using
fluid flow in the direction of a second electrode to align the
scaffold with the direction between the two electrodes. In a
particular embodiment, the substrate is mica, the scaffold is DNA,
and the DNA is attached to the first electrode using a thiol
linkage. Fluid-induced alignment is used to align the scaffold in
the direction of the second electrode, and the DNA scaffold is
bound to the mica substrate by Mg.sup.2+ ions, thereby holding the
DNA in its aligned position. Fluid-induced alignment may also be
subsequently used to align additional scaffolds so that they cross,
or intersect scaffolds already aligned on the substrate.
[0021] Other methods of placing a scaffold onto a substrate in a
predetermined pattern include positioning the scaffold on a
substrate using magnetic fields, optical tweezers, or laser traps.
Multiple scaffolds may be arranged on a substrate using any of the
above methods. Scaffolds may not only be aligned between electrodes
but may also be aligned such that they cross or otherwise contact
each other to form one-, two- or three-dimensional structures
useful as templates for forming electronic devices comprising
cluster arrays. Such cluster arrays may be used to provide high
density electronic or memory devices that operate on the principle
of Coulomb blockade at ambient temperatures.
[0022] The present invention also provides compositions that are
useful, for example, for forming metal, alloy, semiconductor and/or
magnetic cluster arrays. In a particular embodiment, the
composition comprises monodispersed, ligand-stabilized Au.sub.55
metal clusters coupled to a polypeptide in the shape of or capable
of forming an .alpha.-helix with the metal clusters bonded thereto.
In another embodiment, the composition comprises monodispersed,
ligand stabilized gold metal clusters coupled to a polynucleotide
capable of forming a helical structure. In particular embodiments,
the metal clusters have metal-cluster radii of from about 0.4 nm to
about 1.8 nm, such as from about 0.4 nm to about 1.0 nm.
[0023] In particular embodiments, the present invention includes
compositions comprising a polypeptide capable of forming
.alpha.-helix and plural monodispersed, ligand-stabilized metal
and/or semiconductor clusters, each cluster having plural ligands
that serve to couple the clusters to the polypeptide. In more
particular embodiments, the plural ligands of the clusters interact
with the polypeptide by an interaction selected from the group
consisting of ligand exchange reactions, electrostatic
interactions, hydrophobic interaction, and combinations thereof. In
other more particular embodiments, the metal and/or semiconductor
clusters have radii of from about 0.4 nm to about 1.8 nm, such as
between about 0.4 nm and about 1.0 nm. If the clusters comprise
metal clusters the metal may be selected from the group consisting
of Au, Ag, Pt, Pd and mixtures thereof, and in particular
embodiments the clusters may comprise Au.sub.55 metal clusters.
[0024] Compositions comprising polynucleotides capable of forming
helical structures and plural monodispersed, ligand-stabilized
metal and/or semiconductor clusters, where each cluster having
plural ligands serves to couple the clusters to the polynucleotide
are also provided by the invention. The plural ligands of the
clusters may serve to interact and couple the cluster to the
polynucleotide through interactions such as ligand exchange
reactions, electrostatic interactions, hydrophobic interactions,
intercalation reactions and combinations thereof.
[0025] In particular embodiments the invention provides organized
arrays of metal clusters comprising monodispersed,
ligand-stabilized metal clusters having metal-cluster radii of from
about 0.4 nm to about 1.8 nm, the metal being selected from the
group consisting of Ag, Au, Pt, Pd and mixtures thereof. Such
arrays include a scaffold and the metal clusters are coupled to the
scaffold to form the organized array.
[0026] The present invention further provides an electronic device
that operates at or about room temperature based on the Coulomb
blockade effect. Such electronic devices include a first cluster
(e.g. a cluster comprising a metal cluster core having a radius of
between about 0.4 nm and about 1.8 nm) and a second such cluster.
The clusters are physically spaced apart from each other at a
distance of less than about 5 nm by coupling the clusters to a
scaffold, such as a biomolecular scaffold, so that the physical
separation between the clusters is maintained. Electronic devices
according to the invention may also include pairs of biomolecular
scaffolds, each with coupled clusters, arranged so that the
scaffolds intersect to provide electric circuit elements, such as
single-electron transistors and electron turnstiles. Such elements
may be useful as components of chemical sensors or ultrasensitive
electrometers. Because of their unique architecture, electronic
devices according to the invention exhibit a linear increase in the
number of electrons passing between pairs of clusters as the
potential difference between the two clusters is increased above a
threshold value.
[0027] The present invention also provides methods of forming
monodispersed phosphine-stabilized gold nanoparticles that allow
the radii of nanoparticles to be controllably adjusted. In one
embodiment, the method comprises dissolving HAuCl.sub.4 and
PPh.sub.3 in a biphasic system (for example, a biphasic system
comprising a water phase, an organic phase, and a phase transfer
catalyst) and adding sodium borohydride to the biphasic system. In
particular embodiments, the biphasic system may comprise water and
an organic solvent selected from the group consisting of toluene,
xylenes, benzene, and mixtures thereof. The phase transfer catalyst
may be a quaternary ammonium salt, for example, tetraoctylammonium
bromide. Control of the nanoparticle size may be accomplished
through control of the rate at which sodium borohydride is added to
the biphasic system.
[0028] The invention also provides methods of preparing
thiol-stabilized gold nanoparticles. Thiol-stabilized gold
nanoparticles may be prepared by dissolving phosphine-stabilized
gold nanoparticles in an organic solvent and exchanging the
phosphine ligands of the phosphine-stabilized gold nanoparticle for
thiol ligands. Particles that are particularly useful for preparing
arrays according to the invention are prepared from thiol ligands
that comprise a group or groups of atoms that are capable of
coupling thiol-stabilized gold nanoparticle to scaffolds. Phosphine
and thiol ligands may be prepared in a single-phase system if the
thiol ligand is soluble in an organic solvent. However, if the
thiol ligand(s) is water soluble, it is still possible to exchange
thiol ligands for phosphine ligands at the interface between a
water-immiscible organic solvent containing the phosphine
stabilized gold nanoparticles and water containing the thiol
ligand(s).
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is a schematic diagram of an interdigitated electrode
array having saw-tooth edges.
[0030] FIG. 2 is a schematic representation of a poly-L-lysine
scaffold having thiophenolate-stabilized nanoparticles coupled
thereto.
[0031] FIG. 3 is a schematic representation of one method for
incorporating gate electrodes at the molecular level.
[0032] FIG. 4 shows UV-Vis spectra (in methylene chloride solution)
of gold clusters with the ligands (a) ODT, (b) Pth, and (c) MBP,
and where (d) is starting material and (e) is a sample of larger
ODT-stabilized clusters.
[0033] FIG. 5 is a TEM of ODT-stabilized clusters
(aerosol-deposited from methylene chloride solution onto a
carbon-coated copper grid).
[0034] FIG. 6 is an electron micrograph of a patterned gold cluster
structure.
[0035] FIG. 7 is a graph illustrating current-voltage (I-V)
characteristics of
Au.sub.55[P(C.sub.6H.sub.5).sub.3].sub.12Cl.sub.6 at temperatures
of 195K, 295K and 337K.
[0036] FIG. 8 is a graph illustrating observed current plateaus as
a function of the applied frequency at a temperature of 195K, with
the inset illustrating the plateau at f=0.626 MHz.
[0037] FIG. 9 is a graph illustrating current versus reduced
voltage at a temperature of 195K.
[0038] FIG. 10 is a graph illustrating current-voltage (I-V)
characteristics of a poly-L-lysine scaffold decorated with
11-mercaptoundeconic ligand-stabilized gold clusters.
[0039] FIG. 11 is a TEM image of a TEM grid having a poly-L-lysine
scaffold decorated with 11-mercaptoundeconic ligand-stabilized gold
clusters.
[0040] FIG. 12 is a representative TEM image showing nearly
monodispersed triphenylphosphine nanoparticle having a particle
size of 1.4 nm.+-.0.5 nm.
[0041] FIG. 13 is a background-subtracted graph of I-V
characteristics for PLL films decorated with gold
nanoparticles.
[0042] FIG. 14 is a conductance graph of the system of FIG. 13.
[0043] FIG. 15 is a 1.times.1 .mu.m area showing mercaptoundecanoic
acid-stabilized gold nanoparticle arrays formed on mica substrates
previously treated with PLL hydrobromide salt and soaked in dilute
sodium hydroxide solution until the PLL was no longer detectable by
AFM.
[0044] FIG. 16 is a graph of voltage sweeps versus threshold
voltage for a non-patterned sample versus a poly-L-lysine-patterned
sample.
DETAILED DESCRIPTION
Abbreviations and Definitions
[0045] The singular forms "a," "an," and "the" refer to one or more
unless the context clearly indicates otherwise.
[0046] PL--polylysine
[0047] PLL--poly-L-lysine
[0048] AFM--atomic force microscopy
[0049] TEM--transmission electron microscopy
[0050] SEM--scanning electron microscopy
[0051] XPS--x-ray photoelectron spectroscopy
[0052] ODT--octadecylthiol
[0053] TOABr--tetraoctylammonium bromide
[0054] Au.sub.55--refers to gold nanoparticles prepared by the
Schmid preparation, having a diameter of approximately 1.4 nm.
[0055] The general steps used to produce organized arrays
comprising metal, alloy, semiconductor and/or magnetic clusters in
accordance with the present invention include (1) attaching
molecular scaffolds to substrates in predetermined patterns, (2)
forming monodispersed, relatively small (i.e., nanoparticle size,
with room temperature Coulomb blockade applications typically using
nanoparticles where the diameter of the core, d.sub.core is less
than about 2 nm) ligand-stabilized metal, alloy, semiconductor
and/or magnetic clusters, (3) coupling the ligand-stabilized
clusters to the scaffolds to form organized arrays, (4) coupling
electrical contacts to the organized arrays, and (5) using such
constructs to form electronic, particularly nanoelectronic,
devices. The substrate generally is a metal, glass or semiconductor
material.
[0056] Most efforts have been directed to developing working
devices using metal clusters. Certain of the following passages
therefore focus on describing how to make and use devices based on
metal cluster arrays. It should be understood, however, that any
reference in this application to "metal clusters" or "clusters"
typically also refers to alloy clusters, semiconductor clusters,
magnetic clusters, and combinations thereof.
[0057] Important features of the present invention include the
small physical size of the metal clusters, the ligand exchange
chemistry and the nature of the ligand shell produced by the ligand
exchange chemistry. The small physical size of the metal clusters
provides a large Coulomb charging energy. The ligand-exchange
chemistry provides a means to tailor the ligand shell for a
particular purpose and immobilize the clusters on biomolecules.
And, the ligand shell offers a uniform and chemically adjustable
tunnel barrier between cluster cores.
[0058] The following paragraphs describe the present invention in
greater detail.
I. Forming Monodispersed Ligand-Stabilized Clusters
[0059] A feature of the present invention is the recognition that
monodispersed, relatively small metal clusters can be used to
develop electronic devices that operate at or about room
temperature based on the Coulomb blockade effect. "Monodispersed"
refers to the formation of a population of metal clusters of
substantially the same size, i.e., having substantially the same
radii (or diameters). In contrast, prior-art approaches typically
have used polydispersed metal clusters where the size of the metal
clusters is not substantially uniform. A completely monodispersed
population is one in which the size of the metal clusters is
identical. However, complete monodispersity is difficult, if not
impossible, to achieve. And complete monodispersity is not required
to produce devices operating at room temperature based on the
Coulomb blockade effect. Nevertheless, as the dispersity of the
cluster population proceeds from absolute monodispersity towards
polydispersity the likelihood that the device will operate reliably
at room temperature based on the Coulomb blockade effect
decreases.
[0060] Moreover, as the radius of the metal cluster decreases, the
intrinsic capacitance gets smaller. As capacitance gets smaller,
the charging energy of the cluster gets larger. Coulomb blockade
effects are observed when the charging energy exceeds the thermal
energy at room temperature. Prior approaches have used clusters
having radii generally larger than would be useful for forming
devices that operate at room temperature based on the Coulomb
blockade effect. In contrast, the present invention forms metal
"nanoparticles" having relatively small radii. The size requirement
for clusters made in accordance with the present invention can be
established in at least two ways, (1) by stating absolute radius
lengths, and (2) by comparing the radius of the cluster in question
to the radius of gold clusters having magic numbers (see the
discussion provided below) of gold atoms.
[0061] In terms of absolute numbers, "nanoparticle" is defined
herein as a cluster having a radius of from about 0.4 nm to about
1.8 nm (4 .ANG. to about 18 .ANG.), for example, from about 0.4 nm
to about 1.25 nm (4 .ANG. to about 12.5 .ANG.), such as from about
0.4 nm to about 1.0 nm (4 .ANG. to about 10 .ANG.). These radius
lengths refer solely to the radius of the metal cluster, and not
the radius of the metal cluster and ligand sphere.
[0062] With its insulating ligand shell, the diameter of the
ligand-stabilized metal cluster can vary. The size of the ligand
shell may influence the electron tunneling rate between clusters.
Tunneling rate is exponentially related to the thickness of the
ligand shell. As a result, the diameter of the ligand shell may be
tailored for a particular purpose. It currently is believed that
the diameters for ligand-stabilized clusters useful for practicing
the present invention should be from about 2.5 nm to about 5 nm.
The relatively large metal clusters made previously do not provide
a large Coulomb charging energy and do not operate at room
temperature, and instead generally only operate at temperatures of
from about 50 mK to about 10K.
[0063] "Bare" clusters, i.e., those without ligand shells, also may
be useful for practicing the present invention. For example, bare
clusters can be used to form electrical contacts.
[0064] Still another consideration is the distance between the
edges of metal cluster cores. It currently is believed that the
maximum distance between the edges of cluster cores for clusters
useful for practicing the present invention is about 5 nm (50
.ANG.), and ideally is on the order of from about 1 to about 2 nm
(10-20 .ANG..ANG.).
[0065] Originally it was believed that clusters in accordance with
the present invention generally should include numbers of atoms
that are based on the so-called "geometric magic numbers" of atoms
surrounded by a ligand shell. Geometric magic numbers result from
the most densely packed arrangement of atoms that form a "sphere."
Magic numbers are given by Formula 1 below
1 + n = 1 k ( 10 n 2 + 2 ) Formula 1 ##EQU00001##
where k is an integer that represents the number of shells of metal
atoms surrounding a central atom. Noble metal clusters with k=2, 4,
6, 7 and 8 have been synthesized and stabilized by a ligand shell.
While clusters having magic numbers of atoms will work to practice
the present invention, it has now been determined that magic
numbers of atoms likely are not required to provide clusters useful
for practicing the present invention.
[0066] Solely by way of example, metals used to form
ligand-stabilized metal clusters in accordance with the present
invention may be selected from the group consisting of silver (Ag),
gold (Au), platinum (Pt), palladium (Pd), and mixtures thereof.
"Mixtures thereof" refers to having more than one type of metal
cluster coupled to a particular scaffold, or different metal
clusters bonded to different scaffolds used to form a particular
electronic device. It also is possible that metal alloy clusters,
e.g., gold/palladium clusters, can be used to form cluster arrays
and electronic devices in accordance with the present
invention.
[0067] Gold is a particularly useful metal for forming
ligand-stabilized monodispersed metal clusters. This is because (1)
the ligand exchange chemistry for gold nanoparticles and the nature
of the ligand shell formed about gold is well understood, (2)
Au.sub.55 has a diameter of about 1.4 nm, which is particularly
useful for forming organized metal arrays that exhibit the Coulomb
blockade effect at or about room temperature, and (3) it is
possible to prepare nearly monodispersed gold clusters without
lengthy purification requirements, such as lengthy crystallization
processes.
[0068] Assuming that magic numbers do provide benefit, the magic
numbers of gold, palladium and platinum atoms for use with the
present invention are 13, 55, 147 and 309. The magic number 55 is a
particularly suitable magic number (represented as Au.sub.55,
Pd.sub.55 and Pt.sub.55). The magic number of silver atoms for
silver metal clusters useful for practicing the present invention
may be the same as for gold.
[0069] Semiconductor materials may also be useful for practicing
the present invention. Semiconductor materials that may be prepared
as nanoparticles and stabilized with ligand spheres include,
without limitation, cadmium selenide, zinc selenide, cadmium
sulfide, cadmium telluride, cadmium-mercury-telluride, zinc
telluride, gallium arsenide, indium arsenide and lead sulfide.
[0070] Magnetic particles also may be used to decorate scaffolds in
accordance with the present invention. An example, without
limitation, of such magnetic particles is iron oxide
(Fe.sub.2O.sub.3).
II. Ligands
A. Background
[0071] Once a suitable metal, alloy, semiconductor and/or magnetic
material is selected for forming nanoparticles, ligands for bonding
to the clusters also must be selected. The assembly of clusters
into Coulomb blockade structures requires molecular-scale
organization of the clusters while simultaneously maintaining the
insulating ligand sphere between individual clusters. The clusters
also should be coupled to the scaffold in a sufficiently robust
manner to allow for fabrication of devices incorporating cluster
arrays. This may be accomplished by ligand exchange reactions. The
selection of ligands for forming an insulating ligand layer about
the cluster and for undergoing ligand exchange reactions therefore
is a consideration. Criteria useful for selecting appropriate
ligands include, but are not limited to, (1) the ligands are
capable of coupling with the scaffold, such as through
ligand-exchange, acid-base or intercalation reactions (2) the
ligands increase the solubility of the ligand-metal cluster
complexes in organic solvents, thereby facilitating synthesis of
metal clusters and subsequent reactions, and (3) the ligands form
well ordered metal-ligand complexes having diameters that promote
room temperature Coulomb-blockade effects.
B. Classes of Ligands
[0072] Ligands suitable for forming metal clusters in accordance
with the present invention may be selected, without limitation,
from the group consisting of: thiols (RSH); thioethers (also known
as sulfides, R--S--R'); thioesters (RCOSR); disulfides
(R--S--S--R'); sulfur-containing heterocycles, such as thiophene;
1.degree., 2.degree. and 3.degree. amines (RNH.sub.2, R.sub.2NH and
R.sub.3N, respectively), particularly 1.degree. amines; pyridines;
phosphines (R.sub.3P); carboxylates (RCO.sub.2--); nitriles (RCN);
hydroxyl-bearing compounds, such as alcohols (ROH); and mixtures
thereof. Additional guidance concerning the selection of ligands
can be obtained from Michael Natan et al.'s Preparation and
Characterization of Au Colloid Monolayers, Anal. Chem., 67:735-743
(1995), which is incorporated herein by reference.
[0073] Organic sulfur-containing molecules (e.g., thiols,
thioethers, thioesters, disulfides, sulfur-containing heterocycles,
and mixtures thereof) are particularly useful class of ligands.
Thiols, for example, are a suitable type of sulfur-containing
ligand for several reasons. Thiols have an affinity for gold, which
may be formed into electrodes or electrode patterns. Moreover,
thiols have been shown to be good ligands for stabilizing gold
clusters. And, many thiol-based ligands are commercially available.
The thiols form ligand-stabilized metal clusters having a formula
M.sub.x(SR).sub.n wherein M is a metal, R is an alkyl chain or
aromatic group, x is a number of metal atoms that provide metal
clusters having the characteristics described above, and n is the
number of thiol ligands attached to the ligand-stabilized metal
clusters.
C. Organic Portion of Ligands
[0074] The organic portion of ligands useful for practicing the
present invention also can vary. For example, the length of the
alkyl chain can be varied to obtain particular features desired in
the ligand-stabilized metal clusters. These include the solubility
of the metal clusters in solvents used to carry out the present
invention, and the size and insulating characteristics of the
ligand-stabilized metal clusters. Alkyl chains having from about 2
carbon atoms to about 20 carbon atoms are particularly suitable for
practicing the present invention.
[0075] Aryl-type ligands, i.e., aromatic groups such as phenyl
rings, containing or having sulfur atoms coupled thereto also may
serve as ligands for forming ligand-stabilized metal clusters. For
example, mercaptobiphenyl (HS-phenyl-phenyl) has been used to form
ligand-stabilized gold clusters. The aromatic rings of such
compounds may further include functional groups capable of reacting
with the scaffold molecules. For example, the aromatic rings may
include acidic groups, such as carboxylic acids, for acid-base
reactions with functional groups of the scaffold molecules, such as
amines.
[0076] Aromatic ligands are quite useful for producing rigid
arrays, thereby stabilizing the electron transport properties. For
this reason, aryl ligands currently are particularly useful ligands
for practicing the present invention. In addition, small alkyl
groups, such as thioproprionic acid, also provide rigid ligand
systems.
[0077] Ligands that intercalate into DNA also may be used, since
this provides a convenient means for attaching clusters to DNA
molecules. Typically, the DNA intercalating ligands include rigid
.pi. systems. Examples of such DNA intercalating ligands include,
without limitation, anthraquinone, phenanthridinium, acridine
orange, proflavin, ethidium, combinations thereof and derivatives
thereof. DNA intercalating ligands may also be DNA-sequence
dependent. Thus, DNA having particular sequences can be used as a
scaffold that is intercalated at predetermined portions of the
scaffold. This provides a method for controlling and altering the
spacing between metal clusters.
[0078] The ligands also can be inter- and/or intra-molecularly
crosslinked. For example, intercalating ligands may be
photo-crosslinked to the scaffold to provide more rigid
systems.
[0079] Based on these considerations, a diverse family of
functionalized nanoparticles has been prepared using a 1.4 nm core
metal cluster [CORE]. Ligand exchange has used to prepare, from
Au.sub.55[P(C.sub.6H.sub.5).sub.3].sub.12Cl.sub.6, a wide variety
of ligand stabilized clusters of the general formulas
CORE-[PX.sub.3].sub.n, CORE-[S--X].sub.n, and CORE-[NHX].sub.n
where X serves to couple the cluster to a scaffold and n is at
least one. For example, X may include groups of atoms capable of
acid-base reactions with scaffolds, groups of atoms capable of
hydrophobic interactions with scaffolds, groups of atoms capable of
intercalating in nucleic acids (e.g. DNA), groups of atoms capable
of hydrogen bonding to scaffolds, groups capable of electrostatic
interactions with scaffolds, and groups capable of forming covalent
bonds with a scaffold. Groups of atoms that facilitate interaction
with scaffolds include, without limitation, alkyl groups from about
C.sub.3 to C.sub.18, aryl groups, carboxylic acid groups, sulfonic
acid groups, peptide groups, amine groups, and ammonium groups.
Other functional groups that may be part of X include aldehyde
groups and amide groups.
[0080] Charged species are especially useful for electrostatic
coupling of clusters to oppositely charged scaffolds. For example,
ligands having positively charged quaternary ammonium groups have
been made that interact strongly with anionic scaffolds, such as
phosphate backbone of DNA. Ligands having negatively charged
sulfonate groups have been made for interacting with positively
charged scaffolds, such as poly-L-lysine.
[0081] Specific examples of functionalized nanoparticles include:
phosphine-based clusters of the formula CORE-(PR.sub.3).sub.n,
where the R groups are independently selected from the group
consisting of phenyl, cyclohexyl and alkane, for example, octyl,
and n is at least one; amine-based clusters of the formula
CORE-(NHR).sub.n, where R is selected from alkyl groups having 20
or fewer carbon atoms, for example, pentadecyl and n is at least
one; and thiol-based clusters of the formula CORE-(SR), where the R
group is selected from the group consisting of phenyl, biphenyl,
alkyl groups having 20 or fewer carbon atoms, for example, propyl,
hexyl, nonyl, undecyl, hexadecyl and octadecyl, and n is at least
one. Yet other examples include clusters of the formula
CORE-[S--(CH.sub.2).sub.xCOOH].sub.n, where x is between about 2
and about 19 and n is at least one, for example, where x is equal
to 2, 5, or 10; clusters of the formula
CORE-[S--(CH.sub.2).sub.xOH].sub.n where x is between about 2 and
about 20 and n is at least one, for example, where x is equal to 2;
clusters of the formula CORE-[S--(CH.sub.2).sub.xNR.sub.2].sub.n
where R is independently selected from the group consisting of
hydrogen and C.sub.1-C.sub.4 alkyl, x is between about 2 and about
20 and n is at least one, for example, where x=2 and R is methyl;
clusters of the formula
CORE-[S--(CH.sub.2).sub.xNR.sub.3.sup.+].sub.n where R is
independently selected from the group consisting of C.sub.1-C.sub.4
alkyl, x is between about 2 and about 20, and n is at least one,
for example, where x=2, and R is methyl; clusters of the formula
CORE-[S--(CH.sub.2).sub.xSO.sub.3.sup.-].sub.n where x is between
about 2 and about 20, and n is at least one, for example, where
x=2; clusters of the formula
CORE-[S--(CH.sub.2).sub.xCONH(CH.sub.2).sub.yCH.sub.3].sub.n where
x+y is between 1 and about 20 and n is at least one, for example,
where x=2 and y=14; clusters with amino-acid containing ligands,
including glycine-based ligands, such as
CORE-[--S--(CH.sub.2).sub.2COGlyGlyOH].sub.n, and clusters having
intercalating ligands, such as shown below.
##STR00001##
D. General Method for Producing Ligand-Stabilized Metal
Clusters
[0082] The general approach to making ligand-stabilized metal
clusters first comprises forming monodispersed metal clusters
having displaceable ligands. This can be accomplished by directly
forming monodispersed metal clusters having the appropriate ligands
attached thereto, but is more likely accomplished by first forming
monodispersed, ligand-stabilized metal clusters which act as
precursors for subsequent ligand-exchange reactions with ligands
that are more useful for coupling clusters to scaffolds.
[0083] One example, without limitation, of a monodispersed gold
cluster that has been produced and which is useful for subsequent
ligand-exchange reactions with the ligands listed above is
Au.sub.55[P(C.sub.6H.sub.5).sub.3].sub.12Cl.sub.6. A procedure for
making monodispersed
Au.sub.55[P(C.sub.6H.sub.5).sub.3].sub.12Cl.sub.6 nanoparticles is
provided by G. Schmid,
Hexachlorodecakis(triphenylphosphine)-pentapentacontagold,
Au.sub.55[P(C.sub.6H.sub.5).sub.3].sub.12Cl.sub.6, Inorg. Syn.,
27:214-218 (1990). Schmid's publication is incorporated herein by
reference. Schmid's synthesis involves the reduction of
AuCl[PPh.sub.3]. Example 1 below also discusses the synthesis of
Au.sub.55[P(C.sub.6H.sub.5).sub.3].sub.12Cl.sub.6. One advantage or
Schmid's synthesis is the relatively small size distribution of
clusters produced by the method, e.g., 1.4.+-.0.4 nm.
[0084] Once ligand-stabilized monodispersed metal clusters are
obtained, such clusters can be used for subsequent ligand-exchange
reactions, as long as the ligand-exchange reaction is readily
facile and produces monodispersed metal clusters. Prior to the
present invention it was not appreciated that the
Au.sub.55[P(C.sub.6H.sub.5).sub.3].sub.12Cl.sub.6 clusters could be
used to form nearly monodispersed derivatives by ligand-exchange
chemistry. In fact, some literature reports indicated that it was
difficult, if not impossible, to form linked metal clusters by
ligand-exchange reactions. See, for example, Andres et al.'s
Self-Assembly of a Two-Dimensional Supperlattice of Molecularly
Linked Metal Clusters, Science, 273:1690-1693 (1996).
[0085] To perform ligand-exchange reactions, a reaction mixture is
formed comprising the metal cluster having exchangeable ligands
attached thereto and the ligands to be attached to the metal
cluster, such as thiols. A precipitate generally forms upon solvent
removal, and this precipitate is then isolated by conventional
techniques. See Examples 2 and 3 for further details concerning the
synthesis of ligand-stabilized metals.
III. Molecular Scaffolds
A. Background
[0086] Metal clusters produced as stated above are coupled to
molecular scaffolds. "Coupling" as used herein refers to some
interaction between the scaffold and the ligand-stabilized metal
clusters such that the metal clusters become associated with the
scaffold. Associated may mean covalently bound, but also can
include other molecular associations, such as electrostatic
interactions (including dipole-dipole interactions, charge-dipole
interactions, and charge-charge interactions), and hydrophobic
interactions. "Coupling" includes attaching clusters to scaffolds
by (1) ligand exchange reactions where functional groups of the
scaffold molecules, such as sulfur-containing functional groups or
amines, exchange with the ligands of the metal-ligand cluster, (2)
acid-base type reactions between the ligands and molecules of the
scaffold, (3) intercalation of a ligand into a nucleic acid (e.g.,
DNA) helix, and (4) electrostatic interactions between charged
clusters and oppositely charged scaffolds.
B. Scaffolds Comprising Biomolecules
[0087] To form useful electronic devices, the scaffolds are
advantageously disposed on a substrate in predetermined patterns to
which electric contacts can be made. The scaffolds of the present
invention may comprise biomolecules, such as polynucleotides,
polypeptides and mixtures thereof, and hence may be referred to as
biomolecular scaffolds. There is some precedent for using
polynucleotides for forming molecular scaffolds. See, for example,
C. A. Mirkin et al., A DNA-Based Method for Rationally Assembling
Nanoparticles into Macroscopic Materials, Nature, 382:607 (1996);
and A. P. Alivisatos et al., Organization of "Nanocrystal
Molecules" using DNA, Nature, 382:609 (1996). Each of these
references is incorporated herein by reference. Polynucleotides
provide a different spacing between metal clusters than do
polypeptides. Thus, spacing between metal clusters can be varied by
changing the nature of the scaffold.
[0088] Polypeptides include polypeptides that form .alpha.-helical
secondary structures. Certain peptides, although attractive
candidates from the standpoint of being stabilizing ligands for the
metal clusters, do not form .alpha.-helices. However, many
polypeptides do form .alpha.-helices, and hence are good candidates
for forming scaffolds in accordance with the present invention.
[0089] It also should be appreciated that the polypeptide can be a
"homopolypeptide," defined herein to refer to polypeptides having
only one type of amino acid. One example of a homopolypeptide is
polylysine. The free base form of polylysine readily forms an
.alpha.-helix. Moreover, lysine provides a terminal amino group
that is oriented favorably in the .alpha.-helix for ligand exchange
reactions with the ligand-stabilized metal clusters.
Homopolypeptides generally have been used in the practice of the
present invention for several reasons. First, certain
homopolypeptides are commercially available, such as poly-L-lysine,
poly-D-lysine, and poly-DL-lysine (available from Sigma, St. Louis,
Mo.). Second, homopolypeptides provide more predictable
.alpha.-helix formation with the side chains oriented outwardly
from the a helix at known, predictable distances. This allows the
polypeptide to be designed for a particular purpose.
[0090] The peptide also may be a "heteropolypeptide" (having two or
more amino acids), or block copolymer-type polypeptides (formed
from plural different amino acids with identical amino acids being
organized in blocks in the amino acid sequence), as long as such
peptides contain groups that that facilitate coupling with metal
clusters.
[0091] Most amino acids can be used to form suitable homo- or
heteropolypeptides. Examples of particularly suitable amino acids
include, but are not limited to, naturally occurring amino acids
such as lysine, arginine, tyrosine, and methionine; and
nonnaturally occurring amino acids such as homolysine and
homocysteine.
IV. Placing Scaffolds on Substrates
A. General Discussion
[0092] The scaffold simply may be placed on the surface of the
substrate, in contrast to more tightly adhering the polypeptide to
the substrate such as through electrostatic or covalent bonds. As
used herein, the term "substrate" refers to any material, or
combination of materials, that might be used to form electronic
devices. For example, the substrate may be selected from the group
consisting of silicon, silicon nitride, glass, plastics, insulating
oxides, semiconductor materials, quartz, mica, metals, and
combinations thereof.
[0093] Simply placing the scaffold on the surface without
considering whether to electrostatically or covalently bind the
scaffold to the substrate simplifies the process for making working
devices. Placing the scaffold on the surface of the substrate can
be accomplished by (1) forming solutions containing the molecular
scaffold, (2) placing the solution containing the scaffold onto a
substrate, such as by spin coating the solution onto a substrate,
and (3) allowing the solvent to evaporate, thereby depositing the
solid molecular scaffold onto the substrate surface. In this
embodiment, the scaffold may adhere to the substrate by
physisorption or chemisorptions.
[0094] If simple deposition of the scaffold onto the substrate does
not produce a sufficiently robust device, then the scaffold might
be more tightly coupled to the substrate. One method for
accomplishing this is to use compounds that act as adhesives or
tethers between the substrate and the molecular scaffold. Which
compounds to use as adhesives or tethers depends on the nature of
the substrate and the metal cluster. For example, amino-silane
reagents may be used to attach molecular scaffolds to the
substrate. The silane functional group allows the tether to be
coupled to a silicon, glass or gold substrate. This provides a
tether having a terminal amino group that can be used to react with
the scaffold to tether the scaffold to the substrate. The terminal
amino group also can be used as an initiation site for the in situ
polymerization of polypeptides using activated amino acids. Another
class of tethers particularly useful for attaching polylysine to
substrates is the co-carboxyalkanethiols (H--O.sub.2C--R--SH). DNA
may be coupled to mica by the addition of Mg.sup.2+ ions.
B. Organization of Scaffolds on Substrates
[0095] There are many methods for forming organized molecular
arrays, particularly linear arrays, on the surface of substrates.
One method comprises depositing dilute solutions of scaffold
molecules onto substrates. A second method comprises aligning
biomolecular scaffolds between electrodes using an electric field.
Another comprises growing polypeptide chains between two or more
electrodes beginning from an initiation site placed on an
electrode. Yet another comprises flow-induced alignment of anchored
scaffolds. Each of these approaches is discussed below and/or in
the Examples that follow.
1. Deposition from Dilute Solutions
[0096] Isolated molecular scaffolds can be prepared by depositing
highly dilute solutions (i.e. dilute enough such that the scaffold
molecules are separated) onto substrate surfaces. Alternatively,
this can be accomplished by dilution of the molecular scaffold film
with an inert, .alpha.-helix polypeptide such as
poly-.gamma.-benzyl-L-glutamate. See,
Poly(.gamma.-Benzyl-L-Glutamate) and Other Glutamic Acid Containing
Polymers, H. Block (Gordon & Breach, NY) 1983.
2. Aligning Scaffolds in an Electrical Field
[0097] A practical method, for providing aligned scaffolds on a
substrate employ an electrical field produced between two
electrodes. FIG. 1 illustrates saw tooth electrodes 10 comprising
electrodes 12-20 that are placed on a substrate by conventional
methods, such as electron-beam lithography, UV-photolithography,
charged particle beam lithography, thermal evaporation, or lift-off
techniques. A solution comprising the scaffold molecules is first
formed and then applied to the surface of the substrate having the
electrode pattern placed thereon, such as a substrate having the
electrode pattern of FIG. 1. .alpha.-Helical polypeptides, for
example, self-align (pole) in the presence of an applied magnetic
field or electrical field (typically 20 Vcm.sup.-1). See, S. Itou,
Reorientation of Poly-.gamma.-benzyl-L-glutamate Liquid Crystals in
an Electric Field, Jpn. J. Appl. Phys., 24:1234 (1985). Presumably
this is due to their large diamagnetic anisotropy. See also, C. T.
O'Konski et al, Electric Properties of Macromolecules IV
Determination of Electric and Optical Parameters From Saturation of
Electric Birefringence in Solutions, J. Phys. Chem., 63:1558
(1959).
[0098] An electric field is generated between the electrodes, such
as the points of the saw tooth illustrated in FIG. 1. This local
field between the two points causes the scaffold to align between
the points. The solvent is evaporated to provide scaffolds oriented
between the electrodes.
[0099] Based on the above, it will be apparent that the dipole
moment of the scaffold influences whether the scaffold may be
oriented between the two electrodes, and the efficiency of the
orientation. This is one reason why .alpha.-helical polypeptides
are particularly useful polypeptides for forming scaffolds. The
hydrogen bonds formed in the .alpha.-helix all orient in the same
direction, thereby aligning the amide and carboxyl groups of the
peptide backbone and imparting an overall dipole to the secondary
.alpha. helical structure. It currently is believed that the dipole
is primarily the result of the .alpha. helix, and not the side
chains.
3. Growing Polypeptides Between Electrodes
[0100] In some instances, it may be desirable to use scaffolds to
bridge directly between two electrical contacts of interest. This
can be accomplished by first placing initiating sites on the
electrodes, and then "growing" polypeptides between the initiation
sites on the electrodes to form a bridge. One example of how this
would be accomplished is to attach a tether to an electrode, the
tether having a pendant functional group that is capable of forming
peptide bonds when reacted with an activated amino acid. The most
likely pendant functional group for this purpose is a 1.degree.
amine.
[0101] To provide a specific example to illustrate the procedure, a
tether comprising an alkyl chain having both a terminal amino group
and a terminal sulfhydryl group (i.e., an amino-thiol,
HS--R--NH.sub.2) is reacted with a gold electrode using
conventional chemistry. This covalently attaches the sulfhydryl
group of the tether to the metal (i.e., Au--S--R--NH.sub.2). The
terminal amino group is then used to initiate polymerization of a
polypeptide using activated amino acids, perhaps in the presence of
an applied field, between the two electrodes. The polymerization is
accomplished by supplying activated amino acids for reaction with
the primary amine in a chain-growing reaction that serially couples
amino acids to the end of the growing chain and regenerates the
primary amine for subsequent reaction with another activated amino
acid.
[0102] Activated amino acids are commercially available and are
described in the literature. Activated amino acids useful for
growing polypeptides include N-carboxyanhydride (NCA) amino acids.
NCA amino acids react with surface-bound initiator sites (e.g., the
primary amino groups) to begin a ring-opening polymerization of the
NCA-amino acid. See, J. K. Whitesell et al., Directionally Aligned
Helical Peptides on Surfaces, Science, 261:73 (1993). Whitesell's
publication is incorporated herein by reference.
[0103] When NCA polymerization is performed under the influence of
an electric field applied between two electrodes it is possible to
"grow" the polypeptide scaffolds from one electrode to another. One
specific example of an NCA amino acid that can be used for this
purpose is that derived from N-.di-elect
cons.-benzyloxycarbonyl-L-lysine. The amino acid side chains of
this compound can be deprotected using trimethylsilyl iodide.
Deprotection yields the poly-L-lysine scaffold.
[0104] Working embodiments of the present invention generally have
used polylysine as the polypeptide useful for forming the molecular
scaffold. Polylysine was chosen because it includes a hydrocarbon
chain that extends the amino functional group, which can undergo
ligand-displacement reactions with the ligand-stabilized metal
cluster, out and away from the polypeptide backbone. Thus, two
criteria that may be used to select polypeptides for use as
molecular scaffolds are (1) the ability of the polypeptide to form
.alpha. helices, and (2) the presence of side chains that provide
functional groups which are metal-cluster stabilizing and capable
of undergoing ligand-exchange reactions with the ligand-stabilized
metal clusters.
4. Forming Polynucleotide Scaffolds
[0105] DNA also is a useful material for forming scaffolds, and has
many advantages. For example, it is much easier to form long
polynucleotide chains than to form polypeptide chains. Furthermore,
DNA provides a more rigid material, and this is a beneficial
attribute of scaffold materials. Methods for providing
polynucleotide scaffolds also recently have been discovered. See,
for example, (1) E. Braun et al., "DNA Templated Assembly and
Electrode Attachment of a Conducting Silver Wire," Nature, p. 775
(1998); (2) N. Seeman, "DNA Components for Molecular Architecture,"
Accounts of Chemical Research, 30:357 (1997); Qi J., et al.
"Ligation of Triangles Built from Bulged 3-Arm DNA Branched
Junctions," J. Am. Chem. Soc., 18:6121 (1996); and C. Niemeyer et
al. "DNA as a Material for Nanotechnology," Angewandte Chemie,
International Edition in English, 36:585 (1997). Each of these
references is incorporated herein by reference. The Braun reference
provides a method for positioning a DNA molecule between electrodes
spaced by a particular distance, such as about 10 .mu.m. Double
stranded DNA, with single stranded sticky ends, and a pair of
electrodes that have single stranded DNA attached thereto that is
complementary to the sequence of the sticky ends of the DNA are
prepared. Annealing the sticky ends to the single-stranded primers
allows coupling of double stranded DNA between two electrodes
spaced by a known distance.
[0106] There are other methods for positioning DNA scaffolds on a
substrate. For example, and without limitation, DNA may be
manipulated by: electric fields between two electrodes; attaching
one end of a DNA strand to an electrode, and then using solution
flow toward another electrode to align the DNA between the two
electrodes; using optical tweezers or laser traps to place the DNA
in a particular alignment.
V. Decorating Scaffolds With Metal Clusters
[0107] To provide working electronic devices, clusters are coupled
to the scaffolds. FIG. 2 provides a schematic representation of a
poly-L-lysine that is "decorated" with metal clusters, i.e., the
clusters are coupled to the scaffold. A first consideration is
whether to decorate the scaffold with clusters prior to or
subsequent to placing the scaffold onto a substrate. Although both
of these approaches work, there are some disadvantages with
decorating the scaffold with the clusters prior to placing the
scaffold on the substrate. This approach places clusters on all
surfaces of the polypeptide, even those that come into contact with
the underlying substrate. This is undesirable for several reasons.
For example, such placement of the clusters might interfere with
fixing the decorated scaffold to the substrate. And, it places
clusters in locations in which they are not needed, and hence uses
more valuable monodispersed clusters than needed.
[0108] Based on the above, a method which first places the
scaffolds onto a substrate, and subsequently decorates the scaffold
with clusters is in most instances a more useful approach. This may
be accomplished by first forming a solution comprising the
ligand-stabilized monodispersed clusters using a solvent that does
not dissolve the scaffold. Solvents for this purpose include,
without limitation, dichloromethane and hexanes. The
ligand-stabilized clusters are then introduced onto the scaffold
and allowed to undergo reactions with the scaffold molecules, such
as ligand-exchange or acid-base type reactions, thereby coupling
the ligand-stabilized clusters to the scaffold. See Example 4 for
further details concerning decorating scaffolds with clusters.
[0109] The present approach to producing decorated scaffolds also
allows for good lateral definition, which is a key feature of the
present invention. "Lateral definition" refers to the width of an
array. Prior to the present invention, the state of technology was
capable of producing lines having a width of about 300 .ANG.. With
the present invention, lateral resolution is much improved, and is
on the order of about 10 .ANG.. In addition, branched polypeptides
offer the possibility of introducing control electrodes and
interconnects at the molecular level.
VI. Ultrafast, Ultrahigh Density Switching Devices
[0110] This section discusses the steps required to use the
decorated molecular scaffolds of the present invention to produce
ultrafast, ultrahigh density switching devices. First, a substrate
is selected and cleaned. One example of a substrate is a silicon
nitride chip or wafer. On top of this substrate would be placed
electrical contacts. This could be accomplished using known
technologies, such as lithography or thermal evaporation of a
metal, such as gold.
[0111] Once a substrate is obtained having the electrical contacts
placed thereon, a scaffold is then placed on the surface using the
techniques described above. Thereafter, the substrate with scaffold
is treated with monodispersed, ligand-stabilized clusters to attach
such clusters to the scaffold. The organization of scaffold likely
determines the particular device being made.
[0112] For a switching device, analogous to a transistor, saw tooth
electrical contacts, such as those shown in FIG. 1, are deposited
onto a substrate and a scaffold then oriented therebetween. This
provides two arms of a transistor. A capacitance contact required
to provide the third arm of a transistor is imbedded in the
substrate underneath the molecular scaffold. Direct electrical
contact with this "gate" imbedded in the substrate is not actually
required.
[0113] Alternatively, a third contact arm could be incorporated
into the template. FIG. 3 is a schematic representation of a
scaffold useful for this purpose. For example, a polypeptide of a
particular length, e.g., a 25-mer or 50-mer, could first be coupled
to an electrode. A branching portion of the scaffold could then be
attached, thereby forming an electrical arm, or plural such arms,
for further providing single or multiple gate electrodes to the
template. The scaffold is then coupled between two electrodes
subsequent to the formation of this contact arm, or arms.
[0114] The method of the present invention can be used to form a
variety of standard circuit components to implement Boolean logic
functions. These circuit components include, but are not limited
to, AND, NAND, NOR, OR and Exclusive OR gates. Additionally,
multiplexers and muliplexer-based circuits can be created and used
to implement Boolean logic functions.
VII. Production and Use of Phosphine-Stabilized Gold
Nanoparticles
[0115] The present method provides the first new route to producing
phosphine-stabilized gold nanoparticles since their first
description nearly twenty years ago. The described route is
substantially simpler and safer than the traditional route, which
involves the use of diborane gas (see Example 1, below). TEM, XPS
and ligand (thiol) exchange reactions respectively reveal that the
size, composition and reactivity of nanoparticles synthesized using
this new method are comparable to those produced by the traditional
route. Additionally, this simple route can produce large quantities
of gold nanoparticles capped by tricyclohexylphosphine or
trioctylphosphine, producing a novel class of trialkylphosphine
stabilized nanoparticles.
[0116] First described by Schmid in 1981, phosphine-stabilized gold
nanoparticles, commonly referred to as gold 55, paved the way for
investigating the properties of metal nanoparticles. The small size
and low dispersity of triphenylphosphine-passivated gold
nanoparticles continues to make them important tools in the field
of nanoelectronics, biological tagging, and structural studies.
Recently the ability to exchange thiol ligands onto
triphenylphosphine passivated nanoparticles was demonstrated, which
enabled the coupling of small size and low dispersity with the
stability of thiol-passivated gold. This has allowed investigation
of applications that require both high stability and small core
size, such as room temperature, Coulomb-blockade-based
nanoelectronics. One embodiment of the present method provides a
convenient gram-scale synthesis of 1.4 nm triphenylphosphine
stabilized nanoparticles that are comparable in both size and
reactivity to the traditional gold 55 nanoparticles (Example 8).
This route utilizes commercially available reagents and replaces a
hazardous reducing agent. The generality of this synthetic method
has been explored through the synthesis of previously unknown
aliphatic, phosphine-stabilized gold nanoparticles, particularly
trialkylphosphine stabilized nanoparticles.
[0117] A working embodiment of the synthesis is shown in Scheme
1.
##STR00002##
With reference to Scheme 1, "a" refers to reaction conditions,
including a toluene:water biphasic solvent system,
tetraoctylammonium bromide (see below), and a 5 hour reaction time,
and "b" refers to the empirical formula of the resulting product,
which is based upon size and atomic composition measurements.
[0118] Other suitable solvents for the method include benzene and
xylenes. Useful phase transfer catalysts include quaternary
ammonium salts of which tetraoctylammonium bromide is only and
example.
[0119] Phosphine-stabilized gold nanoparticles produced by the
method described herein can be used in any applications in which
traditionally synthesized gold nanoparticles are used. Such
applications include, of course, the construction of
scaffold-organized clusters and electronic devises including such
clusters described in the present application. In addition, the
aliphatic, phosphine-stabilized gold nanoparticles of this
invention can be used as biological tags for (e.g., in electron
microscopy or for the detection of positive associations on
biological microarrays such as cDNA microarrays). Gold particles
according to the invention can be used, for instance, to label
peptide molecules (Segond von Banchet and Heppelmann, Histochem.
Cytochem., 43, 821-827, 1995), proteins (for instance, antibodies
or fragments thereof as described in Hainfeld and Furuya, J.
Histochem. Cytochem., 40:177-184, 1992); or nucleic acid molecules
(such as hybridization probes), or liposomes (Hainfeld., Proc. An.
Mtg, Micros. Soc. Am., San Francisco Press, San Francisco, Calif.,
pp. 898-899, 1996).
[0120] In certain embodiments, the gold nanoparticles of the
invention can be used in combination with other labels, such as
fluorescent or luminescent labels, which provide different means of
detection, or other specific binding molecules, such as a member of
the biotin/(strept)avidin specific binding family (e.g., as
described in Hacker et al., Cell Vision, 4, 54-65, 1997).
VIII. EXAMPLES
[0121] The following examples are provided to illustrate certain
particular features of the present invention. These examples should
not be construed to limit the invention to the particular features
described.
Example 1
[0122] This example describes the syntheses of
Au.sub.55(PPh.sub.3).sub.12Cl.sub.6. Au[P(C.sub.6H.sub.5).sub.3]Cl
was obtained from Aldrich Chemical Company. This compound was
reduced using diborane (B.sub.2H.sub.6), which was produced in situ
by the reaction of sodium borohydride (NaBH.sub.4) and
borontriflouride etherate [BF.sub.3.O(C.sub.2H.sub.5)].
Au[P(C.sub.6H.sub.5).sub.3]Cl was combined with diborane in benzene
to form Au.sub.55(PPh.sub.3).sub.12Cl.sub.6.
Au.sub.55(PPh.sub.3).sub.12Cl.sub.6 was purified by dissolution in
methylene chloride followed by filtration through Celite. Pentane
was then added to the solution to precipitate a black solid. The
mixture was filtered and the solid was dried under reduced pressure
to provide Au.sub.55(PPh.sub.3).sub.12Cl.sub.6 in approximately 30%
yield.
Example 2
[0123] This example describes the synthesis of
Au.sub.55(SC.sub.18H.sub.37).sub.26. Dichloromethane (.about.10
ml), Au.sub.55(PPh.sub.3).sub.12Cl.sub.6 (20.9 mg) and
octadecylthiol (23.0 mg) were combined in a 25 ml round bottom. A
black solution was produced, and this solution was stirred under
nitrogen at room temperature for 36 hours. The solvent was then
removed under reduced pressure and replaced with acetone. This
resulted in the formation of a black powder suspension. The solid
was then isolated by vacuum filtration and washed with acetone
(10.times.5 ml). After the final wash, the solid was redissolved in
hot benzene. The benzene was removed under reduced pressure with
gentle heating to yield a dark brown solid.
[0124] The solid material was then subjected to UV-VIS
(CH.sub.2Cl.sub.2, 230-800 nm), .sup.1H NMR, .sup.13C NMR, X-ray
photoelectron spectroscopy (XPS) and atomic force spectroscopy.
These analytical tools were used to characterize the structure of
the compound produced, and such analysis indicated that the
structure of the metal-ligand complex was
Au.sub.55(SC.sub.18H.sub.37).sub.26.
[0125] X-ray photoelectron spectroscopy (XPS) data also was
collected concerning Au.sub.55(SC.sub.18H.sub.37).sub.26. This
involved irradiating molecules with high-energy photons of fixed
energy. When the energy of the photons is greater than the
ionization potential of an electron, the compound may eject the
electron, and the kinetic energy of the electron is equal to the
difference between the energy of the photons and the ionization
potential. The photoelectron spectrum has sharp peaks at energies
usually associated with ionization of electrons from particular
orbitals. X-ray radiation generally is used to eject core electrons
from materials being analyzed. Clifford E. Dykstra, Quantum
Chemistry & Molecular Spectroscopy, pp. 296-295 (Prentice Hall,
1992). Quantification of the data provided by XPS analysis of
Au.sub.55(SC.sub.18H.sub.37).sub.26 made according to this example
showed that Au 4f comprised about 67.38% and S 2p constituted about
28.01%+4.17%, which suggests a formula of
Au.sub.55(SC.sub.18H.sub.37).sub.26.
[0126] Quantification of XPS spectra gave a gold-to-sulfur ratio of
about 2.3:1.0 and shows a complete absence of phosphorus or
chlorine. As with Au.sub.55(PPh.sub.3).sub.12Cl.sub.6, a broad
doublet is observed for the Au 4f level. The binding energy of the
Au 4f 7/2 level is about 84.0-84.2 eV versus that of adventitious
carbon, 284.8 eV. This indicates absence of Au(I) and is similar to
binding energies obtained for clusters such as
Au.sub.55(PPh.sub.3).sub.12Cl.sub.6. The binding energy of the S 2p
3/2 peak ranges from 162.4 to 162.6 eV for the series of clusters.
These values are shifted to lower energy than those found for free
thiols (163.3-163.9 eV) and are close to the values reported for
thiolates bound to gold (162.0-162.4 eV). .sup.1H and .sup.13C NMR
unambiguously rules out the possibility that unattached thiols may
be present in the sample.
[0127] Thermal gravimetric analysis confirms the Au:S ratio
obtained from XPS. On heating to 600.degree. C., ODT-stabilized
clusters display a 40% mass loss, corresponding to 26 ODT ligands
on an assumed 55-atom gold cluster. This ratio alludes to the
retention of a small cluster size. A sample of the larger
hexadecanethiol-stabilized gold cluster has been shown to give a
33.5% mass loss, corresponding to from about 95 to about 126
ligands per cluster (diameter=2.4 nm).
[0128] Optical spectra of gold colloids and clusters exhibit a
size-dependent surface plasmon resonance band at about 520 nm (See
FIG. 4). In absorption spectra of ligand-exchanged clusters
produced as stated in this example, the interband transition
typically observed for small clusters including
Au.sub.55(PPh.sub.3).sub.12Cl.sub.6 was observed. Little or no
plasmon resonance was observed, consistent with a cluster size of
about 1.7 nm or less. For the ODT-passivated cluster, no plasmon
resonance was observed.
[0129] Quantitative size information can be obtained using
transmission electron microscopy (TEM). The core size obtained from
TEM images of the ODT-stabilized cluster (FIG. 5) is found to be
1.7.+-.0.5 nm and is in good agreement with that obtained from
atomic force microscope images.
[0130] Atomic force microscopy (AFM) also was performed on the
Au.sub.55(SC.sub.18H.sub.37).sub.26 produced according to this
example. The analysis produced a topographical representation of
the metal complex. AFM probes the surface of a sample with a sharp
tip located at the free end of a cantilever. Forces between the tip
and the sample surface cause the cantilever to bend or deflect. The
measured cantilever deflections allow a computer to generate a map
of surface topography. Rebecca Howland et al., A Practical Guide to
Scanning Probe Microscopy, p. 5, (Park Scientific Instruments,
1993). The AFM data showed heights of 1.5 nm for single clusters
and aggregates subjected to high force. This corresponds to the
size of the gold core clusters. This helped establish that the gold
clusters of this example were close to the correct size for forming
devices in accordance with the present invention.
[0131] In a manner similar to that described above for Example 2,
thiol stabilized structures also have been made using
1-propanethiol.
Example 3
[0132] This example describes the synthesis of
Au.sub.55(SPh-Ph).sub.25. Dichloromethane (.about.10 ml),
Au(PPh.sub.3).sub.12Cl.sub.6 (25.2 mg) and 4-mercaptobiphenyl (9.60
mg) were combined in a 25 ml round bottom. A black solution was
produced, and this solution was stirred under nitrogen at room
temperature for 36 hours. The solvent was removed under reduced
pressure and replaced with acetone. This resulted in the formation
of a black powder suspension. The solid was isolated by vacuum
filtration and washed with acetone (6.times.5 ml). The solvent was
then removed under reduced pressure to yield 16.8 mg of a dark
brown solid.
[0133] The solid material was subjected to UV-Vis
(CH.sub.2Cl.sub.2, 230-800 nm), .sup.1H NMR, .sup.13C NMR, X-ray
photoelectron spectroscopy (XPS) and atomic force spectroscopy as
in Example 2. This data confirmed the structure and purity of the
metal complex, and further showed complete ligand exchange. For
example, quantification of the XPS data made according to this
example showed that Au 4f comprised about 71.02% and S 2p
constituted about 28.98%, which suggests a formula of
Au.sub.55(S-biphenyl).sub.25.
[0134] AFM analysis showed isolated metal clusters having measuring
about 2.5 nm which correlates to the expected size of the gold core
with a slightly extended sphere.
[0135] Thiol-stabilized clusters as produced above display
remarkable stability relative to
Au.sub.55(PPh.sub.3).sub.12Cl.sub.6, which undergoes decomposition
in solution at room temperature to give bulk gold and
AuCl[PPh.sub.3]. No decomposition for the thiol-stabilized clusters
was observed, despite the fact that some samples were deliberately
stored in solution for weeks. In other tests, the mercaptobiphenyl
and octadecylthiol-stabilized clusters (in the absence of free
thiol) were heated to 75.degree. C. for periods of more than 9
hours in dilute 1,2-dichloroethane solution with no resultant
degradation. Under identical conditions,
Au.sub.55(PPh.sub.3).sub.12Cl.sub.6 is observed to decompose to
Au(O) and AuCl[PPh.sub.3] within 2 hours.
Example 4
[0136] This example describes the electron transfer properties of
organometallic structures formed by electron-beam irradiation of
Au.sub.55[P(C.sub.6H.sub.5).sub.3].sub.12Cl.sub.6. This compound
was produced as stated above in Example 1. A solution of the gold
cluster was made by dissolving 22 mg of the solid in 0.25 mL of
CH.sub.2Cl.sub.2 and 0.25 mL of 1,2-dichloroethane. A supernatant
solution was spin coated onto a Si.sub.3N.sub.4 coated Si wafer at
1500 rpm for 25 seconds immediately after preparation. The film was
patterned by exposure to a 40 kV electron beam at a line dosage of
100 nC/cm. The areas of the film exposed to the electron beam
adhered to the surface and a CH.sub.2Cl.sub.2 rinse removed the
excess film. This procedure produced well defined structures. See
FIG. 6. These structures appeared to be smooth and continuous under
SEM inspection. Attempts were made to pattern the material using
254 nm UV lithography, but it was found to be insensitive to this
wavelength. The defined structures had dimensions as small as 0.1
.mu.m and AFM inspection measured the film thickness to be 50
nm.
[0137] The organometallic samples were spin-coated with PMMA which
was electron-beam exposed and developed to define contact regions.
Contacts were fabricated using thermal evaporation of 100 nm of
gold and conventional liftoff procedures.
[0138] DC current-voltage (I-V) measurements of several samples
were taken. A shielded chamber, submerged in an oil bath, contained
the sample mounted on a clean teflon stage. Rigid triaxial
connections were used to connect the sample to a constant DC
voltage source and electrometer. The oil bath temperature was
controlled from 195 to 350K. Thermal equilibrium was achieved with
a 10 Torr partial pressure of He in the chamber. Before electrical
measurements the chamber was evacuated to a pressure
.about.10.sup.-5 Torr. The data showed little temperature drift
over a typical four hour measurement sweep. The intrinsic leakage
current of the system was measured using a control sample which had
the same substrate and contact pad arrangement as the actual
samples, but did not have the organometallic between the pads. At
room temperature, the leakage current was almost linearly dependent
on bias over the range -100 to 100V, and had a maximum value
.ltoreq.100 fA. While the ultimate resolution of the current
measurement was 10 fA, the leakage current set the minimum resolved
conductance .about.10.sup.-15.OMEGA..sup.-1. Constant amplitude RF
signals with frequencies, f, from 0.1 to 5 MHz, were applied to the
samples through a dipole antenna at 195K. No attempt was made to
optimize the coupling between the RF signal and the sample.
[0139] Without RF, the I-V characteristics for one sample at
several temperatures are shown in FIG. 7. As the temperature was
reduced, the low voltage portion of the curve flattened out and the
current became indistinguishable from the leakage current. Above an
applied voltage magnitude of 6.7.+-.0.6 V, the current increased
abruptly. The data illustrated in FIG. 7 establishes that the
monodispersed gold clusters can produce devices that operate on the
basis of the Coulomb blockage effect. This can be determined from
FIG. 7 because one of the curves has zero slope, indicating no
current at the applied voltage, i.e., the cluster is blockaded at
the particular temperature tested.
[0140] The application of the RF signal introduced steps in the I-V
characteristic, as shown in the inset to FIG. 8. FIG. 8 establishes
that an applied external varying signal (the frequency of which is
provided by the X axis) actually controls the rate at which
electrons move through metal clusters made in accordance with the
present invention. The current at which these steps occurred was
found to be proportional to the applied signal frequency, as shown
in FIG. 8. A least squares analysis of the linear current-frequency
relationship for the highest current step shown gives a slope
1.59.+-.0.04.times.10.sup.-19 C.
[0141] The introduction of plateaus in the patterned sample I-V
characteristics is similar to the RF response reported in other
Coulomb blockade systems. This effect has been attributed to phase
locking of single-electron tunneling events by the external RF
signal. When the nth harmonic of the applied frequency corresponds
to the mth harmonic of the frequency of tunneling in the system,
mile, the current becomes locked to a value I=(n/m)ef. The results
obtained suggest that correlated tunneling is present in the
samples.
[0142] The patterned samples had stable I-V characteristics with
time and temperature. Furthermore, as the temperature was raised
above about 250K the I-V characteristics developed almost linear
behavior up to V.sub.T. The conductance below V.sub.T was
activated, with activation energies E.sub.A in the range 30-70 meV.
One method to estimate the charging energy from the activation
energy is to use the argument that the charging energy for one
island in a infinite two-dimensional array, E.sub.C.about.4E.sub.A.
Assuming current suppression requires E.sub.C.gtoreq.10 kT, the
sample with the largest activation energy should develop a Coulomb
gap below .about.300 K. This value is within a factor of 2 of the
measured temperature at which clear blockade behavior occurs in the
patterned samples. Given the accuracy to which E.sub.C is known,
the temperature dependence of the conductance within the Coulomb
gap is consistent with the observation of blockade behavior. Using
this value of E.sub.C, the effective capacitance of a metal core in
the patterned array is
3.times.10.sup.-19F<C<7.times.10.sup.-19F. These values are
close, but larger than the classical geometric capacitance of an
isolated Au55 cluster C=4.pi..di-elect cons..di-elect
cons..sub.0r.about.2.times.10.sup.-19F, where the dielectric
constant, .di-elect cons., of the surrounding ligand shell is
expected to be .about.3. The agreement between the two estimates of
capacitance supports the notion that the current suppression in the
metal cluster arrays is due to charging of individual Au.sub.55
clusters.
[0143] The non-linear I-V characteristic is similar to that of
either a forward biased diode or one-/two-dimensional arrays of
ultra small metal islands or tunnel junctions. However, the
dependence of the I-V characteristic on the applied RF signal is
not consistent with straightforward diode behavior. Therefore, the
data has been analyzed in the context of an array of ultra small
metal islands.
[0144] Several reports have discussed the transport in ordered
arrays of tunnel junctions that have tunneling resistances greater
than the quantum resistance h/e.sup.2 and a charging energy
significantly above the thermal energy. In this case Coulomb
blockade effects introduce a threshold voltage below which current
through the array is suppressed. As the applied voltage is
increased well beyond threshold, the current-voltage characteristic
approaches a linear asymptote with a slope related to the tunnel
resistance. With the same temperature and tunnel resistance
constraints, Middleton and Wingreen have discussed one- and
two-dimensional arrays of maximally disordered normal metal islands
where disorder is introduced as random offset charges on each dot.
These authors predict current suppression below a threshold voltage
and high bias current I.about.(V/V.sub.T-1).sup..gamma.. Here, the
threshold voltage V.sub.T scales with the number of junctions N
along the current direction. Analytically .gamma.=1 for
one-dimensional systems and 5/3 for infinite two-dimensional
systems. Numerical simulations of a finite two-dimensional array
gave .gamma.=2.0.+-.0.2.
[0145] While no effort was made to order samples, the data was
analyzed using both the ordered and the disordered models. The only
consistent analysis was found to be given by the disordered model.
In particular, the high bias data did not have the linear asymptote
predicted for an ordered system, but did scale as expected for a
disordered system, as shown in FIG. 9. FIG. 9 also shows that a
two-dimensional array so that sample is propagating through the
sample tested along plural parallel paths. Such an arrangement is
important for developing memory storage devices. The exponent
.gamma..about.1.6 which is closest to the analytical prediction for
an infinite, disordered two-dimensional array. From the analysis
the magnitude of V.sub.T.about.6.+-.1 V which is in good agreement
with that estimated directly from the I-V data.
[0146] The introduction of steps in the I-V characteristics by a RF
field is similar to the RF response reported in other systems. This
effect has been attributed to phase locking of single-electron
tunneling events by the external RF signal. If the applied
frequency corresponds to a rational fraction multiple of the
frequency of tunneling in the system, I/e, then the current is
locked to a value I=(n/m)ef, where n and m are integers. Therefore,
the linear relationships shown in FIG. 6 between f and I suggests
that correlated tunneling is present in the samples. The lowest
slope observed is best described with n/m=1/5. For frequencies up
to 3 MHz, the current resolution is insufficient to distinguish
between the 1/5 and 1/4 harmonics. However, at higher frequencies
where it should have been possible to distinguish between 1/5 and
1/4, the 1/4 step was not observed.
[0147] At temperatures above about 250K, the I-V characteristic was
almost linear up to V.sub.T. In this regime the conductance was
activated, with activation energies E.sub.A in the range 30 to 70
meV for the samples studied. Similar activated behavior has been
reported for tunnel junction systems. It was argued that for an
infinite 2D array the charging energy for one island
E.sub.C.apprxeq.4E.sub.A. Applying this argument to the present
system, and assuming current suppression requires E.sub.C.gtoreq.10
kT, the sample with the largest activation energy should develop a
Coulomb gap below about 300 K. This estimate is within a factor of
two of the measured temperature at which clear blockage behavior is
seen. Thus, the temperature dependence of the observed current
within the Coulomb gap is consistent with the observation of
blockade behavior. From the threshold voltage, V.sub.T=.alpha.Ne/C,
and this estimate of E.sub.C, .alpha.N is approximately 10. The
energy E.sub.C can also be estimated if the capacitance of an
island is known. The capacitance of an isolated Au.sub.55 cluster
is C=4.pi..di-elect cons..di-elect cons..sub.o.tau., where .tau. is
the radius of the cluster and .di-elect cons. is the dielectric
constant of the surrounding medium. The radius of a Au.sub.55 is
0.7 nm and the ligand shell is expected to have .di-elect
cons..apprxeq.3, which C.apprxeq.2.times.10.sup.-19F. The Coulomb
charging energy, E.sub.C=e.sup.2/2C.apprxeq.340 meV which is within
twenty percent of the maximum value of 4E.sub.A found from the
activation data. This result suggests that the current suppression
is due to charging of individual Au.sub.55 clusters.
[0148] Given the constraint that steps in the I-V characteristics
are only found when f<0.1/(R.sub.TC), the fact that steps are
seen up to f=5 MHz gives the upper limit
R.sub.T<1.times.10.sup.11.OMEGA.. The differential resistance
obtained from the I-V characteristic well above threshold is
anticipated to be R.sub.diff.apprxeq.(N/M)R.sub.T, where M is the
number of parallel channels. This estimate yields N/M.gtoreq.30.
From the sample dimensions and the size of an individual cluster, a
close packed array would have N/M.about.5. This disparity between
the expected and experimentally derived values of the N/M suggests
that the full width of the sample is not involved in transport. One
explanation for the discrepancy in N/M may be that many of the gold
cores coalesce during sample fabrication so that transport is
dominated by individual clusters between larger regions of
gold.
Example 5
[0149] This example describes a method for making cluster arrays
using poly-L-lysine as the scaffold and 11-mercaptoundeconic
ligand-stabilized metal clusters. Prefabricated electrodes were
drop-cast with a 2.2.times.10.sup.-5 mol/l solution of 56,000 amu
poly-L-Lysine.HBr in H.sub.2O/CH.sub.3OH. After a 20-hour soak in
1% NaOH in nanopure water and a nanopure water rinse, the
current-voltage characteristics of the sample were found to be
comparable with that of a bare electrode. The polylysine coated
electrode was then exposed to a drop of 11-mercaptoundeconic
ligand-stabilized gold clusters in DMSO (about 8 mg/1 ml). After
about 20 minutes, the sample was subjected to a thorough rinse with
DMSO followed by another rinse in methylene chloride. After
correcting for the leakage current of the bare electrode, the
current-voltage characteristic of the sample were measured, as
shown in FIG. 10.
[0150] A TEM grid was prepared as well using the polylysine
scaffold and the 11-mercaptoundeconic ligand-stabilized gold
clusters in DMSO. The polylysine solution was drop cast onto TEM
grids. A 20-hour soak in 1% NaOH was followed by a nanopure water
rinse. The dry TEM grids were then exposed to a drop of
11-mercaptoundeconic ligand-stabilized gold clusters in DMSO. After
about twenty minutes, the grids were thoroughly rinsed, first using
DMSO and then using methylene chloride. Lines of clusters can be
seen in FIG. 11.
Example 6
[0151] This example describes how to make electrical connections to
metal cluster arrays. Saw tooth interdigitated array (IDA) gold
electrodes are used and are made using electron beam lithography.
The gap between saw tooth points in the array will be approximately
200-300 Angstroms. An omega-amino alkanethiol will be chemisorbed
to the gold surface and subsequently electrochemically desorbed
from one set of the IDA fingers. An omega-NHS-ester alkylthiol will
be attached to the bare set of fingers. A precursor to
poly-L-lysine will be polymerized from the amino-modified fingers
toward the NHS-ester fingers where the growing end will be
captured. The side chains of the poly-L-lysine chain will be
deprotected and treated with carboxy-terminated gold nanoparticles
to form the desired one-dimensional array. Gates will be
incorporated either under the substrate or as an additional
electrode near (above) the surface of the device.
Example 7
[0152] This example describes a method for making
phosphine-stabilized gold nanoparticles, particularly 1.4 nm
(.+-.0.5 nm) phosphine-stabilized gold nanoparticles. Traditional
methods for making such molecules are known, and are, for instance,
described by G. Schmid (Inorg. Syn., 27:214-218, 1990) and in
Example 1).
[0153] Scheme 1 (above) illustrates a convenient one-pot, biphasic
reaction in which the nanoparticles can be synthesized and purified
in less than a day from commercially available materials. Hydrogen
tetrachlorolaurate trihydrate (1.11 g, mmol) and tetraoctyl
ammonium bromide (1.8 g, mmol) were dissolved in a nitrogen sparged
water/toluene mixture (100 mL each). Triphenylphosphine (2.88 g,
11.0 mmol) was added, the solution stirred for five minutes until
the gold color disappeared, and aqueous sodium borohydride (2.0 g,
41.0 mmol, dissolved in 5 mL water immediately prior to use) was
rapidly added resulting in a dark purple color (this addition
results in vigorous bubbling and should be performed cautiously).
The mixture was stirred for three hours under nitrogen, the toluene
layer was washed with water (5.times.100 mL) to remove the
tetraoctylammonium bromide and borate salts and the solvent removed
in vacuo to yield 1.3 g of crude product.
[0154] To effect further purification, the resulting solid was
suspended in hexanes, filtered on a glass frit, and washed with
hexanes (300 mL) to remove excess triphenylphosphine. Washing with
a 50:50 mixture of methanol and water (300 mL) removed
triphenylphosphine oxide. Each of these washes was monitored by TLC
and the identity of the collected material was confirmed by .sup.1H
and .sup.31P NMR. Pure samples were obtained by precipitation from
chloroform by the slow addition of pentane (to remove gold salts,
as monitored by UV-Vis and NMR). After purification, this procedure
yielded 644 mg of purified nanoparticle product from 1.35 g of
hydrogen tetrachlorolaurate. In contrast, the traditional synthesis
yields about 300 mg of purified nanoparticle product per 2g
hydrogen tetrachlorolaurate.
[0155] For comparison of these nanoparticles to the products of the
traditional synthesis the newly synthesized nanoparticles were
analyzed to determine size, atomic composition, and reactivity as
described below. The small size of the nanoparticles, which allows
for examination of Coulomb blockade phenomena at room temperature,
is an important consideration for evaluating the effectiveness of
the synthesis.
[0156] Direct evidence of nanoparticle size and dispersity is
provided by transmission electron microscopy (TEM). TEM was
performed on a Philips CM-12 microscope operating at a 100 kV
accelerating voltage. Samples were prepared by drop casting dilute
methylene chloride solutions onto 400-mesh nickel grids coated with
carbon. Images were recorded as photographic negatives, scanned,
and processed using NIH image software. A total of 1628 particles
were examined from two separate synthetic runs, for the
triphenylphosphine nanoparticles. Background noise and agglomerated
nanoparticles were removed from the measurements by removing core
sizes of <0.5 nm and >3 nm from the analysis.
[0157] A representative TEM (FIG. 12) shows nearly monodisperse
triphenylphosphine nanoparticles with a size of 1.4 nm.+-.0.5 nm.
The FIG. 12 insert is a bar graph showing particle size
distribution on this TEM. The x axis of the inset is the size of
the particles (measured in nm, starting at 0.75 nm and increasing
by increments of 0.25 nm to 3 nm). The y axis of the inset
represents the number of particles observed in each size (beginning
at zero and increasing by increments of 50 to 350 particles). The
size measurements in this TEM compare well with the traditional
synthesis, which yields 1.4 nm.+-.0.4 nm particles.
[0158] UV/Vis spectroscopy, a technique that is representative of
the bulk material, was used to confirm TEM size determinations.
UV-visible spectroscopy was performed on a Hewlett-Packard HP 8453
diode array instrument with a fixed slit width of 1 nm using 1 cm
quartz cuvettes. The absence of a significant surface plasmon
resonance at .about.520 nm indicates gold nanoparticles that are
<2 nm diameter. UV/Vis spectra of newly synthesized
nanoparticles are dominated by an interband transition, with no
significant plasmon resonance at 520 nm. This indicates that there
is no substantial population of nanoparticles greater than 2 nm in
size.
[0159] Atomic composition of the nanoparticles was determined using
the complementary techniques of x-ray photoelectron spectroscopy
(XPS) and thermogravimetric analysis (TGA) allowing further
comparison to traditionally prepared nanoparticles. TGA was
performed under a nitrogen flow with a scan rate of 5.degree. C.
per minute. XPS was performed on a Kratos Hsi operating at a base
pressure of 10.sup.-8 torr. Samples were prepared by drop-casting a
dilute organic solution of the nanoparticles onto a clean glass
slide. Charge neutralization was used to reduce surface charging
effects. Multiplexes of carbon, sulfur, and phosphorus were
obtained by 30 scans each. Binding energies are referenced to
adventitious carbon at 284.4 eV. Data was recorded with a pass
energy of 20 eV. XPS spectra provides an average composition of 71%
gold, 26% carbon, 2.6% phosphine, and 0.7% chlorine, corresponding
to molar ratios of 18 Au: 108 C:4.3 P:1 Cl. TGA indicates a mass
ratio of 71% gold to 29% ligand, independently confirming the
ligand-to-ratio determined by XPS. For direct comparison with the
nanoparticles made by traditional methods, an average empirical
formula was generated by assuming a core size of 55 gold atoms.
Based on this assumption, an average particle produced by the
method of the present invention was formulated as
Au.sub.101(PPh.sub.3).sub.12.5Cl.sub.3, in comparison with the
Au.sub.55(PPh.sub.3).sub.12Cl.sub.6 reported by Schmid. While the
gold-to-phosphorus ratio matches that of the Schmid nanoparticles,
the phosphorus-to-chlorine ratio of 4:1 is double that of the
Schmid:nanoparticles (2:1). This may be a reflection of the
phosphorus-to-chlorine ratio in the starting materials of each
reaction.
[0160] The reactivity of the nanoparticles to thiol ligand exchange
further confirms their similarities to traditional
triphenylphosphine stabilized nanoparticles. Using previously
reported methods, a number of straight-chain alkanethiol and
charged .omega.-Functionalized alkanethiol ligands have been
exchanged onto these nanoparticles. In each thiol-for-phosphine
ligand exchange reaction, there is little change in the surface
plasmon resonance of the UV/Vis spectra, indicating negligible size
changes during the thiol for phosphine ligand exchange. Thus, the
newly synthesized nanoparticles are similar in size, atomic
composition, and reactivity to the Schmid preparation.
[0161] The disclosed methods have enabled the facile exploration of
various phosphine ligands that have previously not been explored.
Substitution of PR.sub.3 for PPh.sub.3, and slight modification of
the work-up, allows for isolation of alkyl-stabilized nanoparticles
in good yield. Trioctylphosphine and tricyclohexylphosphine
stabilized gold nanoparticles have been successfully synthesized,
which appear to be substantially larger by both UV-Vis. This
approach apparently is the first reported synthesis of
alkyl-phosphine stabilized gold nanoparticles. Further
investigations regarding the effect of the Lewis basicity of the
ligand and the steric bulk of the ligand are underway.
[0162] This synthesis allows for the expansion of
phosphine-stabilized nanoparticle materials. Large amounts of
nanoparticle material can be made in a single step using
borohydride in place of diborane. Second, this synthesis allows for
flexibility in the choice of phosphine ligand that was previously
unknown. Variation of ligand-to-gold ratios using the disclosed
embodiments of the present invention can be used to achieve
unprecedented size control of phosphine-stabilized gold
nanoparticles.
Example 8
[0163] This example describes a method for determining the size of
the nanoparticles made using a process similar to that described in
Example 8. Controlling the rate at which the reducing agent, such
as sodium borohydride, is added to the reaction mixture, can be
used to make nanoparticles materials having desired core diameters,
such as a gold core diameter (d.sub.core<2 nm). The synthesis is
the same in every respect as that stated in Example 8 except for
the addition rate of the reducing agent (NaBH.sub.4). In Example 8,
NaBH.sub.4 was added rapidly. In this preparation the same quantity
of reducing agent was added slowly (over a period of 10 minutes)
from a dropping funnel fitted with a ground glass joint and Teflon
stopcock. The resultant nanoparticles were shown by UV-visible
spectroscopy to have an average diameter of larger than 2 nm.
Example 9
[0164] This example describes the formation of gold nanoparticle
networks fabricated between the fingers of gold, interdigitated
array electrodes having a 15 (or 1.5) or 2 .mu.m gap by
electrostatic assembly of carboxylic-acid-modified, gold
nanonparticles onto the amino side chains of the biopolymer
poly-L-lysine (PLL). The samples were prepared as follows. First, a
2.2.times.10.sup.-5 mol/l solution of poly-L-lysine-hydrobromide
complex (54,000 amu) in 10/90% water/methanol was drop cast onto
the electrodes. The electrodes were pre-cleaned using a UV/ozone
dry process followed by a rinse in nanopure water. The hydrobromide
was removed from the amine side chains of the biopolymer by
submerging the cast film in a solution of 1% sodium hydroxide in
water for about 20 hours. The
11-mercapto-undecanoic-acid-stabilized, gold nanoparticles were
synthesized from Schmid-Au.sub.55 nanoparticles [see, G. Schmid,
Inorg. Synth., 27, 214 (1990)] using ligand exchange. See L. O.
Brown and J. E. Hutchison, J. Am. Chem. Soc., 119, 12, 384 (1997).
Nanoparticle decoration of the biopolymer was accomplished by
placing a concentrated solution of the nanoparticles in
dimethylsulfoxide onto the poly-L-lysine film for about 20 minutes,
after which it was rinsed in dimethylsulfoxide and then
dichloromethane. From the molecular weight, the average length of
the poly-L-lysine was determined to be about 30 nm. Therefore, each
polymer accommodated about seven or eight nanoparticles.
[0165] Current-voltage (I-V) measurements were performed at room
temperature with the samples in an electrically shielded vacuum
chamber. See, L. Clarke, M. N. Wybourne, M. Yan, S. X. Cai, and J.
F. W. Keana, Appl. Phys. Lett., 71, 617 (1997). Control
measurements were made on the bare electrodes and again after the
PLL had been deposited and deprotonated. The I-V characteristics of
the deprotonated PLL and the bare surface were linear (Ohmic)
without any structure. Importantly, these two sets of control data
were indistinguishable, which shows that to within experimental
uncertainty the surface conductance of the glass substrate was
unaffected by the deprotonated PLL. In contrast, when decorated
with nanoparticles, the samples exhibited pronounced non-linear I-V
characteristics. After subtraction of the linear I-V behavior
measured before PLL decoration, to within the measurement accuracy
the electrical characteristics showed a region of zero conductance
at low voltages. See FIG. 13. The onset of current is characterized
by a threshold voltage, V.sub.T, that was found to be different for
different samples. Above the threshold, the current increases and
the scaling I .alpha.(V/V.sub.T-1).sup..gamma. describes all sets
of data with .gamma.=1.2.+-.0.2. Here the error includes the
uncertainty in the current measurement and the spread between
different data sets. At voltages above threshold, structure of
period DV was observed in the I-V curves of most samples, with the
ratio .DELTA.V/V.sub.T.about.2. This is most easily seen in the
conductance, as shown in FIG. 14. For this data the measured
threshold voltage is V.sub.T=12.+-.1 V, and the period of the
oscillations is .DELTA.V=25.+-.3 V.
[0166] Thus, FIGS. 13 and 14 establish stable Coulomb blockade
behavior at room temperature. With reference to FIG. 13 and the I-V
characteristics of the disclosed systems, above the threshold
voltage the current is linear. Moreover, the conductance
oscillations show that the systems are defect tolerant.
[0167] The value of the scaling exponent .gamma. is indicative of
the electronic degrees of freedom in the sample. The values
obtained for the tested materials are consistent with
single-electron transport in-one-dimensional systems where it is
predicted that .gamma..about.1. These predictions are supported by
measurements of the low-temperature transport in one-dimensional
chains of lithographically-defined tunnel junctions that found
.gamma.=1.36.+-.0.1. Further, the almost linear scaling is distinct
from the quadratic scaling reported for thin films containing gold
nanoparticles. The current-voltage scaling, threshold behavior and
periodic structure are all reminiscent of single-electron behavior
in one-dimensional systems, with the region of zero conductance
resulting from a Coulomb-gap at the Fermi level. These are
remarkable results given the simple method of sample fabrication
and the fact that the measurements were made at 300 K. One
intriguing feature is the voltage scale of the conductance
structure, which is considerably larger than commonly found in
other single-electron systems.
Example 10
[0168] This example concerns the morphology of
nanoparticle/poly-L-lysine (PLL) assemblies. Samples for
morphological studies were prepared on mica substantially as
described above in Example 10. The assemblies were imaged using
tapping mode AFM. The initial, dried PLL.HBr films were found to be
smooth with voids probably due to film contraction while drying.
During the deprotonation step, PLL is removed and the film becomes
more porous, leading to a submonolayer lattice of PLL aggregate.
Upon decoration with functionalized nanoparticles, extended,
chain-like assemblies were observed. See FIG. 15. Thus, by this
method, low dimensional nanoparticle arrays can be made, which
allows production of a system having useful electrical properties
as opposed to systems comprising monolayers of material.
[0169] FIG. 15 also raises the issue of the effects of disorder and
defects. There are two main types of disorder experienced with the
disclosed systems, positional disorder and particle size
dispersion. FIG. 15 shows that the illustrated embodiment has
nanoparticles that are not evenly spaced one from another. This is
referred to herein as positional disorder. In traditional
semiconductor structures, there is no tolerance for unequal spacing
of the metal islands. However, with the small dimensions of the
disclosed systems, the electrical properties do not depend on the
spacing between cluster. Another potential disorder is particle
size dispersion, which can adversely affect the useful electrical
properties of the described systems.
[0170] The described wet chemical fabrication method produces
quasi, one-dimensional structures consistent with the morphology
suggested by the current scaling above threshold. The surface
coverage of these structures is low, far below that required for a
continuous path to be formed between the electrodes. This
observation rules out the possibility that bottleneck regions, or a
single pathway dominate the electrical behavior. Individual
nanoparticles also are found on the surface after chemical
fabrication. Their low area density gives an average separation
considerably larger than the distance between the nanoparticles
forming the extended chains. Thus, the isolated nanoparticles are
unlikely to contribute to the overall electrical behavior. The
electrical properties suggest single-electron effects in
one-dimensional structures and the AFM images show that the
fabrication method is capable of producing such structures.
However, there is an apparent discrepancy between the disordered
nature of the sample seen by AFM (a collection of randomly sized,
placed and oriented nanoparticle arrays) and the clearly defined,
periodic conductance features in the electrical characteristics
that suggest an ordered system. The electrical behavior of randomly
oriented nanoparticle arrays that contain defects has been
calculated. Periodic conductance features occur despite the
presence of defects and that surface conduction in conjunction with
conduction through the array explains the large voltage scale found
in the data.
[0171] Single-electron charging effects are governed by the
capacitance between adjacent nanoparticles and the capacitance of
each nanoparticle to a ground plane. The nanoparticles can be
treated as identical metal spheres of radius 0.7 nm surrounded by a
homogeneous ligand shell with a dielectric constant of 3. Including
the ligand shell, the minimum center-to-center separation is 4.2
nm. Calculating the capacitance matrix for a row of nanoparticles
the interparticle capacitance was determined to be Cdd.apprxeq.0.04
aF and a capacitance to ground C.sub.g.apprxeq.0.17 aF. Thus, the
dimensions of these nanoparticle building blocks result in a regime
where C.sub.g>C.sub.dd, which is opposite to that studied in
most lithographically defined systems. The capacitance values imply
that the total capacitance of a nanoparticle is dominated by
C.sub.g and the calculated value shows that the 5 electrostatic
charging energy e.sup.2/2C.sub.g is more than an order of magnitude
larger than k.sub.BT at 300 K, consistent with Coulomb blockade
effects at room temperature.
[0172] Numerical simulations of perfect chains confirm that
threshold behavior, linear scaling above threshold and a Coulomb
staircase can all be expected at room temperature. To simulate the
number of conductance peaks observed, a minimum of four particles
is required in a chain. While the expected and experimental values
of the ratio .DELTA.V/V.sub.T agree, there is a discrepancy in the
absolute voltage values for the threshold and the periodicity. The
anticipated value of V.sub.T.apprxeq.e/2C.sub.g=0.47 V is more than
a factor of twenty smaller than the measured value. Reducing
C.sub.g will increase V.sub.T. However, assuming that at very small
dimensions the capacitance still can be estimated from the geometry
of a particle, the reduction in C.sub.g necessary to explain the
data would require nanoparticles with unphysically small radii.
From this argument it appears that the conduction path must include
potential drops that may be the result of contact resistance
between the electrodes and the nanoparticle system, surface
conduction, weak links within the network itself, or a combination
of all three.
[0173] The presence of radio frequency (RF) signals and other
phenomena, such as quantum size effects and the physical motion of
nanoparticles in a field (the shuttle mechanism) also can introduce
conductance features. RF signals applied to the sample had no
perceivable affect on the conductance structure. Quantum size
effects are weak at room temperature and the energy level structure
is highly dependent on the structure of the nanoparticles, the
ligands and the coupling between particles. Thus, it seems unlikely
that resonant tunneling through discrete electronic levels is the
cause of the regularly spaced structure we observe. A shuttle
mechanism is ruled out because it predicts structure equally spaced
in current rather than in voltage as found with the present
systems. For the I-V characteristics measured, this mechanism also
would require vibrational frequencies that are much lower than is
reasonable for the properties of the ligand.
[0174] Given the preparation method and the large area (.about.3
mm.sup.2) sampled by the IDA electrodes, disorder and spatial
averaging are expected in the samples. The types of disorder
expected to have the greatest influence on the electrical
properties are variations in core size that influence C.sub.g and
the particle-particle spacing (positional disorder) that affects
Cdd. In addition, the effects of particle chain length and chain
orientation must be considered. Numerical simulations were used to
explore these effects individually and in combination. Chains of
between four and nine particles whose core radii were randomly
dispersed by up to .+-.30% (the measured value) showed conductance
structure that was periodic to within the measurement uncertainty
(12%). For chains that contain ten or more particles, the
uncertainty in the periodicity was much larger than measured.
Similarly, when the radius dispersion was increased to .+-.50% the
position of the conductance peaks was found to change significantly
and the ratio .DELTA.V/V.sub.T deviated markedly from a value of
two. Dispersion in C.sub.dd due to a distribution of
particle-particle spacings was found to have little effect on the
features. This is not surprising for a system in which
C.sub.g>C.sub.dd since the conductance is relatively insensitive
to the inter-particle capacitance. From this analysis it appears
that individual, one-dimensional chains containing less than ten
particles with .+-.30% radius variation can support Coulomb
staircase behavior.
[0175] When many chains are in parallel the periodicity is
maintained provided the chain lengths have a narrow distribution,
implying that the samples contain chains of a well-defined length.
This length may arise from individual PLL chains that, based on
their molecular weight, can accommodate seven or eight
nanoparticles. Given length uniformity, angular averaging over all
possible orientations of a perfect chain between the electrodes
does not remove the conductance peaks, but does broaden them and
increases the conductance in the valleys between peaks. When core
size dispersion (+30%) and orientation averaging are combined, the
simulations still predict periodicity in the conductance. For these
simulations 756 chains each having a different randomized set of
capacitances and a different orientation were used. Interestingly,
even with this degree of averaging, residual conductance
periodicity is still found. While a direct comparison with the
measured data cannot be made, it appears as if the amount of
disorder used in the simulations overestimates the actual degree of
disorder in the samples.
[0176] Finally, note that the nature of the current path and the
fact that the measured voltage scale of the Coulomb blockade
structure disagrees with the value determined from the capacitance.
The conduction process must involve both the chains and the surface
of the substrate. The origin of the surface conductance is likely a
thin water layer, which is known to have Ohmic behavior and is
expected given the wet chemical preparation method. The surface
conductance is the background that is removed from the data and is
the means by which chains, arranged randomly on the surface, are
electrically connected. Once the potential drop across a chain
reaches the threshold value, the chain will come out of blockade
and become part of the conduction path. Given that the chains are
short compared to the inter-electrode spacing and that there does
not appear to be a continuous path between the electrodes, the
point at which a chain begins to conduct is a particular fraction
of the applied voltage: that is, the surface conductance behaves in
the manner of a potential divider which provides an explanation for
the difference between the predicted and observed scales. It is
known that the interparticle spacing in ordered arrays of
nanoparticles plays a role in the nature of the electrical
transport. In certain embodiments of the arrays described herein
the ligands provide a core separation that suggests electron
hopping is the process responsible for charge transfer. In this
case, transport will be dominated through chains that have the
lowest potential barriers between nanoparticles. Defects are
expected to increase the potential barrier. Hence, chains that have
the fewest missing or misplaced nanoparticles (defects) will govern
the transport properties.
[0177] The wet chemical process has been used to produce extended
nanoparticle arrays on biopolymer templates between electrode
pairs. The I-V characteristics show clear evidence for
single-electron charging effects in transport that is limited to
one-dimension. From the computed capacitance values and numerical
simulations the chains likely contain between four and nine
nanoparticles and that the I-V behavior of an ensemble of chains
interconnected by the surface conduction of the substrate is
tolerant toward variations of chain orientation, core size and
inter-particle spacing. The measurements reported here used
indirect electrical contact to an ensemble of nanoparticle arrays.
This suggests that similar contact techniques which avoid alignment
between electrodes and nanoparticles will be useful in their future
electrical characterization and application.
Example 11
[0178] This example concerns using DNA as a scaffold for receiving
nanoparticles. Thin macroscopic gold or silver pads are deposited
onto freshly cleaved mica through a shadow mask that defines
contacts. Vacuum annealing is used, where necessary, to produce
extremely flat metal surfaces. Silver is preferred because it does
not interfere with the detection of gold particles on the surface
by XPS. Purified lambda DNA (a Hind III digest from New England
BioLabs, Inc consists of eight fragments of defined length, ranging
from 42-7,800 nm) is deposited onto the mica substrate in the
presence of Mg.sup.2+ that serves to bind the DNA to the mica such
that the DNA double strands are extended along the surface. Some of
these strands rest partially on the gold pads and partially on the
mica surface. These samples, after rinsing and drying, are used for
control experiments and as templates for assembling the gold
nanoparticles. Individual undecorated double-stranded DNA chains
are identified by AFM. A survey of the periphery of the electrode
contact pads reveals the number of appropriate strands on the
surface and aids in optimizing the deposition conditions.
[0179] Functionalized nanoparticles for assembly on the DNA
templates are prepared as described herein. For example, one
embodiment of such a method was used to make functionalized, 1.5 nm
diameter gold nanoparticles. The reaction conditions were: a)
two-phase mixture of toluene and water, one equivalent
tetraoctylammonium bromide, 10 equivalents, sodium borohydride and
b) either a single-phase ligand exchange with an organic solution
of a thiol or a two-phase exchange between Au-TPP in organic
solvent and a water-soluble thiol in aqueous solution.
Example 12
[0180] This example describes a method for making an intentionally
crossed junction of DNA-templated, one-dimensional, nanoparticle
assemblies. DNA is first attached to an electrode, such as by using
a thiol linkage. The DNA is then correctly aligned by flow-induced
alignment of the DNA strand. The DNA strand is bound to the mica
surface, such as by using Mg.sup.2+. Cationic nanoparticles are
deposited onto the DNA template, and the DNA is attached to the
adjacent electrode. A second DNA strand is aligned by flow-induced
alignment orthogonal to the first DNA strand. The second DNA strand
binds to the cationic nanoparticles on the first DNA strand.
Additional cationic nanoparticles may be deposited onto the new DNA
strand to form an intentionally crossed junction of DNA-templated,
one-dimensional nanoparticle assemblies.
[0181] In another embodiment, complex DNA architectures, such as
Holliday junctions are used as the scaffold for creating patterns
of nanoparticles between electrodes. Assembly of branched
structures on electrode patterned surfaces provide a method for
assembling gates of electronic dimensions. For example crossed
strands of nanoparticles, where the two strands are produced from
nanoparticles of differing radii, may be used to produce a
molecular-scale gate for the strand having the smaller radius
nanoparticles.
Example 13
[0182] FIG. 16 illustrates that the poly-L-lysine templated sample
has a stable and reproducible voltage response, and that the
response of the system does not decay over time. In contrast, when
a template is not used and a non-patterned system formed, the
response decays. Thus, the template stabilizes the voltage
response, likely because the particles are in fixed positions, and
hence such systems are electrically more stable than systems that
are not patterned.
[0183] The disclosed embodiments provide a novel approach to
providing structures having well defined electrical properties.
Coulomb blockade at room temperature is routinely observed in these
systems, and the Coulomb blockade response is stabilized using
biopolymer templating. And, single-electron charging effects in
one-dimensional pathways are remarkably tolerant of defects and
disorder.
[0184] The present invention has been described with reference to
preferred embodiments. Other embodiments of the invention will be
apparent to those skilled in the art from a consideration of this
specification or practice of the invention disclosed herein. It is
intended that the specification and examples be considered as
exemplary only, with the true scope and spirit of the invention
being indicated by the following claims.
* * * * *