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US20020146742A1 - Scaffold-organized metal, alloy, semiconductor and/or magnetic clusters and electronic devices made using such clusters - Google Patents

Scaffold-organized metal, alloy, semiconductor and/or magnetic clusters and electronic devices made using such clusters Download PDF

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US20020146742A1
US20020146742A1 US10/013,334 US1333401A US2002146742A1 US 20020146742 A1 US20020146742 A1 US 20020146742A1 US 1333401 A US1333401 A US 1333401A US 2002146742 A1 US2002146742 A1 US 2002146742A1
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clusters
metal
scaffold
cluster
ligand
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Martin Wybourne
James Hutchison
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Priority to US10/816,603 priority patent/US7326954B2/en
Priority to US11/120,352 priority patent/US7626192B2/en
Priority to US11/960,528 priority patent/US20090155573A1/en
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    • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
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    • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
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    • B82—NANOTECHNOLOGY
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    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
    • H01F1/0036—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties showing low dimensional magnetism, i.e. spin rearrangements due to a restriction of dimensions, e.g. showing giant magnetoresistivity
    • H01F1/0045—Zero dimensional, e.g. nanoparticles, soft nanoparticles for medical/biological use
    • H01F1/0063—Zero dimensional, e.g. nanoparticles, soft nanoparticles for medical/biological use in a non-magnetic matrix, e.g. granular solids
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    • C—CHEMISTRY; METALLURGY
    • C40—COMBINATORIAL TECHNOLOGY
    • C40B—COMBINATORIAL CHEMISTRY; LIBRARIES, e.g. CHEMICAL LIBRARIES
    • C40B40/00—Libraries per se, e.g. arrays, mixtures
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S977/00—Nanotechnology
    • Y10S977/70—Nanostructure
    • Y10S977/701—Integrated with dissimilar structures on a common substrate
    • Y10S977/702—Integrated with dissimilar structures on a common substrate having biological material component
    • Y10S977/705—Protein or peptide
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S977/00—Nanotechnology
    • Y10S977/70—Nanostructure
    • Y10S977/778—Nanostructure within specified host or matrix material, e.g. nanocomposite films
    • Y10S977/779—Possessing nanosized particles, powders, flakes, or clusters other than simple atomic impurity doping
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S977/00—Nanotechnology
    • Y10S977/70—Nanostructure
    • Y10S977/778—Nanostructure within specified host or matrix material, e.g. nanocomposite films
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    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T428/00—Stock material or miscellaneous articles
    • Y10T428/25—Web or sheet containing structurally defined element or component and including a second component containing structurally defined particles
    • Y10T428/256—Heavy metal or aluminum or compound thereof

Definitions

  • 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.
  • the ultimate limit is a system in which the transfer of a single charge quanta corresponds to information transfer or some type of logical operation.
  • Such single electron systems are presently the focus of intense research activity. See, for example, Single Charge Tunneling, Coulomb Blockade Phenomena in Nanostructures , 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).
  • 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.
  • 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.
  • Two primary examples of arrays are (1) electronic circuits, and (2) arrangements of computer memory elements, both of which can be in one or several planes.
  • Clusters refers to more than one, and typically three or more, metal, alloy, semiconductor or magnetic atoms coupled to one another by metal-type 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 “nanoclusters.” This indicates that the radius of each such cluster preferably is from about 0.7 to about 1.0 nm.
  • 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 radiuses much larger than the maximum value stated above likely will not be useful for forming electronic devices that operate at or about room temperature.
  • the metal, alloy, semiconductor and/or magnetic clusters are bonded to “scaffolds” to organize the clusters into arrays.
  • “Scaffolds” are any molecules 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.
  • a preferred group of scaffolds comprise biomolecules, such as polynucleotides, polypeptides, and mixtures thereof. Polypeptides are currently preferred molecules for forming scaffolds, and polypeptides capable of forming ⁇ helices are particularly preferred scaffold-forming molecules.
  • One embodiment of a method for forming arrays of metal, alloy, semiconductor and/or magnetic clusters first involves placing the scaffold on a substrate, most likely in 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. If the clusters are metal clusters, then the metal preferably is selected from the group consisting of Ag, Au, Pt, Pd and mixtures thereof. A currently preferred metal is gold, and a currently preferred metal cluster is Au 55 .
  • Clusters generally are bonded to the 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 bonded to the cluster prior to contacting the scaffold with the clusters.
  • exchangeable ligands suitable for forming metal clusters in accordance with the invention can be selected from the group consisting of thiols, thioethers (i.e., sulfides), thioesters, disulfides, sulfur-containing heterocycles, 1°, 2° and perhaps 3° amines, pyridines, phosphines, carboxylates, nitrites, hydroxyl-bearing compounds, such as alcohols, and mixtures thereof.
  • Thiols currently are preferred ligands for practicing the present invention.
  • a first method comprises aligning scaffold molecules in an electrical field created between electrodes on the substrate. It therefore will be appreciated that the scaffold molecules used must have sufficient dipoles to allow them to align between the electrodes. This is one reason why polypeptides that form ⁇ helices are preferred. The ⁇ helix imparts a sufficient dipole to the polypeptide molecules to allow alignment of the molecules between the electrodes upon formation of an electrical field.
  • a polypeptide useful for forming scaffolds in accordance with the present invention is polylysine.
  • a second method 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.
  • scaffold molecules can be polymerized as a bridge between electrodes on a substrate.
  • the present invention also provides compositions, one use for which is in the formation of metal and/or semiconductor arrays.
  • a currently preferred embodiment of the composition comprises monodispersed, ligand-stabilized Au 55 metal clusters bonded to a polypeptide in the shape of or capable of forming an a helix with the metal clusters bonded thereto.
  • the metal clusters have metal-cluster radiuses of from about 0.7 nm to about 1.8 nm, and preferably from about 0.7 nm to about 1.0 nm.
  • An object of this invention is to provide methods for fabricating one-, two-, and three-dimensional, scaffold-organized metal cluster arrays.
  • An object of this invention is to provide high density electronic or memory devices that operate on the principle of Coulomb blockade at ambient temperatures.
  • FIG. 1 is a schematic diagram of an interdigitated electrode array having saw-tooth edges.
  • FIG. 2 is a schematic representation of a poly-L-lysine scaffold having thiophenolate-stabilized nanoclusters coupled thereto.
  • FIG. 3 is a schematic representation of one method for incorporating gate electrodes at the molecular level.
  • FIG. 4 is a UV-vis spectra (methylene chloride solution) of gold clusters with ligands (a) ODT, (b) Pth, and (c) MBP, and where (d) is starting material and (e) is a sample of larger ODT-stabilzed clusters.
  • FIG. 5 is a TEM of ODT-stabilized clusters (aerosol-deposited from methylene chloride solution onto a carbon-coated copper grid).
  • FIG. 6 is an electron micrograph of a patterned gold cluster structure.
  • FIG. 7 is a graph illustrating current-voltage (I-V) characteristics of AU 55 [P(C 6 H 5 ) 3 ] 12 Cl 6 at 195K, 295K and 337K.
  • FIG. 9 is a graph illustrating current versus reduced voltage at 195K.
  • 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.
  • FIG. 11 is a TEM image of a TEM grid having a poly-L-lysine scaffold decorated with 11-mercaptoundeconic ligand-stabilized gold clusters.
  • 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., nanocluster size) 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, particulary nanoelectronic, devices.
  • the substrate generally is a metal, glass or semiconductor material.
  • metal clusters typically also refers to alloy clusters, semiconductor clusters, magnetic clusters, and combinations thereof.
  • 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 taylor the ligand shell for a particular purpose and immobilize the clusters on biomolecules.
  • the ligand shell offers a uniform and chemically adjustable tunnel barrier between cluster cores.
  • 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 radiuses (or diameters).
  • 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.
  • complete monodispersity is difficult, if not impossible, to achieve.
  • 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.
  • the present invention forms metal “nanoclusters” having relatively small radiuses.
  • 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 made having magic numbers (see the discussion provided below) of gold atoms.
  • nanocluster is defined herein as a cluster having a radius of from about 0.7 nm to about 1.8 nm (7 ⁇ to about 18 ⁇ ), preferably from about 0.7 nm to about 1.25 nm (7 ⁇ to about 12.5 ⁇ ), and even more preferably from about 0.7 nm to less than or equal to 1.0 nm (7 ⁇ to less than or equal to 10 ⁇ ).
  • radius lengths refer solely to the radius of the metal cluster, and not the radius of the metal cluster and ligand sphere.
  • 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.
  • 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.
  • “Bare” clusters i.e., those without ligand shells, also may be useful for practicing the present invention.
  • bare clusters can be used to form electrical contacts.
  • the maximum distance between the edges of cluster cores for clusters useful for practicing the present invention is about 5 nm (50 ⁇ ), and ideally is on the order of from about 1 to about 2 nm (10-20 ⁇ ).
  • k is an integer that represents the number of shells of metal atoms surrounding a central atom.
  • the most likely metals to be used to form ligand-stabilized metal clusters in accordance with the present invention can 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.
  • Gold is the currently preferred metal for forming ligand-stabilized monodispersed metal clusters. This is because (1) the ligand exchange chemistry for gold nanoclusters and the nature of the ligand shell formed about gold is well understood, (2) Au 55 has a diameter of about 1.2 ⁇ m, which has proved ideal for forming organized metal arrays that exhibit the Coulomb 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.
  • the magic numbers of gold, palladium and platinum atoms for use with the present invention are 13, 39, 55, 147 and 309. 55 is the currently preferred magic number (represented as AU 55 , Pd 55 and Pt 55 ).
  • the magic number of silver atoms for silver metal clusters useful for practicing the present invention likely are the same as for gold, but this has not yet been verified.
  • semiconductor materials also likely are useful for practicing the present invention.
  • semiconductor materials that can be made into nanoclusters and stabilized with ligand spheres include, without limitation, cadmium selenide, zinc selenide, cadmium sulfide, cadmium tellurite, cadmium-mercury-tellurite, zinc tellurite, gallium arsenide, indium arsenide and lead sulfide.
  • Magnetic particles also can be used to decorate scaffolds in accordance with the present invention.
  • An example, without limitation, of such magnetic particles is iron oxide (Fe 2 O 3 ).
  • 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 must be coupled to the scaffold in a sufficiently robust manner to allow for fabrication of devices incorporating cluster arrays. This can 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 an important consideration.
  • a list of criteria useful for selecting appropriate ligands includes, but may not be limited to, (1) the ligands should be capable of undergoing reactions with the scaffold, such as ligand-exchange, acid-base or intercelation reactions (2) the ligands preferably increase the solubility of the ligand-metal cluster complexes in organic solvents, which helps synthesize metal clusters and perform subsequent reactions, and (3) the ligands selected preferably form well ordered metal-ligand complexes having diameters as stated above.
  • Ligands deemed most suitable for forming metal clusters in accordance with the present invention can be selected, without limitation, from the group consisting of: thiols (RSH); thioethers (also known as sulfides, R—S—R′); thioesters (R—S 2 H); disulfides (R—S—S—R′); sulfur-containing heterocycles, such as thiophene; 1°, 2° and perhaps 3° amines (RNH 2 , R 2 NH and R 3 N, respectively), particularly 1° amines; pyridines; phosphines (R 3 P); carboxylates (RCO 2 —); nitrites (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
  • Organic sulfur-containing molecules e.g., thiols, thioethers, thioesters, disulfides, sulfur-containing heterocycles, and mixtures thereof
  • Thiols are the currently preferred type of sulfur-containing ligand for several reasons. For example, thiols have an affinity for gold, which often is 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 x (SR) 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.
  • 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
  • n is the number of thiol ligands attached to the ligand-stabilized metal clusters.
  • the organic portion of ligands useful for practicing the present invention also can vary.
  • 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 deemed most suitable for practicing the present invention.
  • Aryl-type ligands i.e., aromatic groups such as phenyl rings, containing or having sulfur atoms coupled thereto also have been used as ligands for forming ligand-stabilized metal clusters.
  • aromatic groups i.e., aromatic groups such as phenyl rings, containing or having sulfur atoms coupled thereto also have been used as ligands for forming ligand-stabilized metal clusters.
  • mercaptobiphenyl HS-phenyl-phenyl
  • the aromatic rings of such compounds likely will be functionalized to include functional groups capable of reacting with the scaffold molecules.
  • the aromatic rings might include acidic groups, such as carboxylic acids, for acid-base reactions with functional groups of the scaffold molecules, such as amines.
  • Aromatic ligands are quite useful for producing rigid arrays, which helps stabilize the electron transport properties. For this reason, aryl ligands currently are considered preferred ligands for practicing the present invention. But, small alkyl groups, such as thioproprionic acid, also provide rigid ligand systems.
  • Ligands that intercalate into DNA also can be used. This allows a means for attaching the metal clusters to DNA molecules.
  • the DNA intercalating ligands include rigid ⁇ r systems. Examples of such DNA intercalating ligands include, without limitation, anthraquinone and phenanthridinium derivatives.
  • the DNA intercalating ligands also can be made DNA-sequence dependent. Thus, DNA having particular sequences can be used as a scaffold that is intercelated at predetermined portions of the scaffold. This provides a method for designing the spacing between metal clusters.
  • the intercalating ligands also can be photocrosslinked to provide a more rigid system.
  • 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 deemed more useful for practicing the present invention.
  • 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 55 [P(C 6 H 5 ) 3 ] 12 Cl 6 .
  • a procedure for making monodispersed AU 55 [P(C 6 H 5 ) 3 ] 12 Cl 6 nanoclusters is provided by G. Schmid's Hexachlorodecakis(triphenylphosphine)-pentapentacontagold, Au 55 [P(C 6 H 5 ) 3 ], 12 Cl 6 , Inorg. Svn., 27:214-218 (1990). Schmid's publication is incorporated herein by reference.
  • Schmid's synthesis involves the reduction of AuCl[Ph 3 ] 6 .
  • Example 1 also discusses the synthesis of Au 55 [P(C 6 H 5 ) 3 ] 12 Cl 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.
  • reaction mixture 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.
  • 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. “Coupling” most typically refers to attaching clusters to the scaffolds by either (1) ligand exchange reactions where functional groups of the scaffold molecules, such as sulfur-containing functional groups or amines, exchange with ligands forming the metal-ligand complex, (2) acid-base type reactions between the ligands and molecules of the scaffold, or (3) intercelation of a ligand into a DNA helix.
  • the scaffolds must be disposed on a substrate in predetermined patterns to which electric contacts can be made.
  • the scaffolds of the present invention can comprise biomolecules, such as polynucleotides, polypeptides and mixtures thereof, and hence are most appropriately referred to as biomolecular scaffolds.
  • biomolecules such as polynucleotides, polypeptides and mixtures thereof, and hence are most appropriately referred to as biomolecular scaffolds.
  • polynucleotides for forming molecular scaffolds. See, for example, C.A. Mirkin et al's A DNA - Based Method for Rationally Assembling Nanoparticles into Macroscopic Materials , Nature, 382:607 (1996); and A. P.
  • 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. Polypeptides may provide the best spacing for the formation of electronic devices operating at room temperature based on the Coulomb blockade effect.
  • Preferred polypeptides are those polypeptides that form ⁇ helical secondary structures. Certain peptides, although attractive candidates from the standpoint of being stabilizing ligands for the metal clusters, do not form a helices. However, many polypeptides do form ⁇ helices, and hence are good candidates for forming scaffolds in accordance with the present invention.
  • polypeptide can be a “homopolypeptide,” defined herein to refer to polypeptides having only one type of amino acid.
  • a homopolypeptide is poly-L-lysine.
  • the free base form of polylysine readily forms an ⁇ helix.
  • lysine provides a terminal amino group that is oriented favorably in the ⁇ 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 polylysine. Second, homopolypeptides provide more predictable ⁇ helix formation with the side chains oriented outwardly from the ⁇ helix at known, predictable distances. This allows the polypeptide to be designed for a particular purpose.
  • 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 (1) form ⁇ helices, and (2) provide functional groups positioned and capable of engaging in ligand exchange reactions with the monodispersed metal clusters.
  • amino acids can be used to form suitable homo- or heteropolypeptides.
  • suitable amino acids include, but are not limited to, naturally occurring amino acids such as arginine, tyrosine, and methionine; and nonnaturally occurring amino acids such as homolysine and homocysteine.
  • 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.
  • substrate refers to any material, or combination of materials, that might be used to form electronic devices.
  • the substrate might be selected from the group consisting of silicon, silicon nitride, ultraflat glass, metals, and combinations thereof.
  • 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.
  • 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.
  • 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 ⁇ -carboxyalkanethiols ( ⁇ O 2 C—R—SH).
  • the first comprises depositing dilute solutions of scaffold molecules onto substrates.
  • the second comprises aligning ⁇ -helical polypeptides between electrodes.
  • the third comprises growing polypeptide chains between two or more electrodes beginning from an initiation site placed on an electrode.
  • the fourth comprises forming DNA scaffolds between electrodes.
  • isolated molecular scaffolds can be prepared by depositing highly dilute solutions onto substrate surfaces. Alternatively, this can be accomplished by dilution of the molecular scaffold film with an inert, ⁇ -helix polypeptide such as poly- ⁇ -benzyl-L-glutamate. See, Poly( ⁇ - Benzyl - L - Glutamate ) and Other Glutamic Acid Containing Polymers , H. Block (Gordon & Breach, NY) 1983.
  • 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, 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.
  • ⁇ -Helical polypeptides self-align (pole) in the presence of an applied magnetic field or electrical field (typically 20 Vcm ⁇ 1 ). See, S.
  • the dipole moment of the scaffold influences whether the polypeptide can be oriented between the two electrodes, and the efficiency of the orientation.
  • ⁇ helical polypeptides are a currently preferred polypeptides for forming scaffolds.
  • the hydrogen bonds formed in the ⁇ helix all orient in the same direction, thereby imparting a dipole to the secondary ⁇ helical structure. It currently is believed that the dipole is primarily the result of the ⁇ helix, and not the side chains.
  • preferred polypeptides for practicing the present invention are those that form ⁇ helices.
  • scaffolds may be desirable to use 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° amine.
  • 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 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 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 which serially couples amino acids to the end of the growing chain and regenerates the primary amine for subsequent reaction with another activated amino acid.
  • Activated amino acids are commercially available and are described in the literature.
  • One example, without limitation, of an activated amino acid for formation of peptide bonds in this manner is 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.
  • surface-bound initiator sites e.g., the primary amino groups
  • 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 the other.
  • NCA amino acid that can be used for this purpose is that derived from N ⁇ -benzyloxycarbonyl-L-lysine.
  • the amino acid side chains of this compound can be deprotected using trimethylsilyl iodide. Deprotection yields the poly-L-lysine scaffold.
  • 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.
  • two important criteria for selecting polypeptides for use as molecular scaffolds are (1) does the polypeptide form ⁇ helices, and (2) do the amino acid side chains provide functional groups that are metal-cluster stabilizing and capable of undergoing ligand-exchange reactions with the ligand-stabilized metal clusters.
  • the Braun reference provides a method for positioning a DNA molecule between electrodes spaced by a particular distance, such as about 10 ⁇ 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.
  • 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.
  • 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.
  • a method which first places the scaffolds onto a substrate, and subsequently decorates the scaffold with clusters is a currently preferred approach. This can be accomplished by first forming a solution comprising the ligand-stabilized monodispersed clusters using a solvent that does not dissolve the scaffold.
  • a solvent that does not dissolve the scaffold 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.
  • 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 300A. With the present invention, lateral resolution is much improved, and is on the order of about 10 ⁇ .
  • branched polypeptides offer the possibility of introducing control electrodes and interconnects at the molecular level.
  • This section discusses the steps required to use the decorated molecular scaffolds of the present invention to produce ultrafast, ultrahigh density switching devices.
  • a substrate is selected and cleaned.
  • a substrate is a silicon nitride chip or wafer.
  • electrical contacts 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.
  • 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.
  • 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.
  • FIG. 3 is a schematic representation of a scaffold useful for this purpose.
  • a polypeptide of a particular length e.g., a 25-mer or 50-mer
  • 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.
  • 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.
  • This example describes the syntheses of AU 55 (PPh 3 ) 12 Cl 6 .
  • Au[P(C 6 H 5 ) 3 ]Cl was obtained from Aldrich Chemical Company. This compound was reduced using diborane (B 2 H 6 ), which was produced in situ by the reaction of sodium borohydride (NaBH 4 ) and borontriflouride etherate [BF 3 .O(C 2 H 5 )].
  • Au[P(C 6 H 5 ) 3 ]Cl was combined with diborane in benzene to form Au 55 (PPh 3 ) 12 Cl 6 .
  • Au 55 (pPh 3 ) 12 Cl 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 55 (PPh 3 ) 12 Cl 6 in approximately 30% yield.
  • This example describes the synthesis of AU 55 (SC 18 H 37 ) 26 .
  • Dichloromethane ( ⁇ 10 ml), AU 55 (PPh 3 ) 12 Cl 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 ⁇ 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.
  • the solid material was then subjected to UV-VIS (CH 2 Cl 2 , 230-800 nm), 1 HNMR (133 MHz), 13 CNMR, 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 55 (SC 18 H 37 ) 26 .
  • X-ray photoelectron spectroscopy (XPS) data also was collected concerning Au 55 (SC 18 H 37 ) 26 .
  • 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's Quantum Chemistry & Molecular Spectroscopy , pp.
  • Optical spectra of gold colloids and clusters exhibit a size-dependent surface plasmon resonance band at about 520 nm (See. FIG. 4).
  • absorption spectra of ligand-exchanged clusters produced as stated in this example the interband transition typically observed for small clusters including AU 55 (PPh 3 ) 12 Cl 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.
  • Quantitative size information can be obtained using transmission electron microscopy (TEM).
  • TEM transmission electron microscopy
  • 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.
  • Atomic force microscopy also was performed on the AU 55 (SC 18 H 37 ) 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's 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.
  • This example describes the synthesis of Au 55 (SPh-Ph) x .
  • Dichloromethane ( ⁇ 10 ml), Au(PPh 3 ) 12 Cl 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 ⁇ 5 ml). The solvent was then removed under reduced pressure to yield 16.8 mg of a dark brown solid.
  • the solid material was subjected to UV-VIS (CH 2 Cl 2 , 230-800 nm), 1 HNMR (133 MHz), 13 CNMR, X-ray photoelectron spectroscopy (XPS) and atomic force spectroscopy as in Example 2.
  • XPS X-ray photoelectron spectroscopy
  • This data confirmed the structure and purity of the metal complex, and further showed complete ligand exchange.
  • 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 55 (S-biphenyl) 25 .
  • 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.
  • Thiol-stabilized clusters as produced above display remarkable stability relative to Au 55 (PPh 3 ) 12 Cl 6 , which undergoes decomposition in solution at room temperature to give bulk gold and AuCl[PPh 3 ].
  • No decomposition for the thiol-stabilized clusters was observed, despite the fact that some samples were deliberately stored in solution for weeks.
  • the mercaptobiphenyl and octadecylthiol-stabilized clusters (in the absence of free thiol) were heated to 75° C. for periods of more than 9 hours in dilute 1,2-dichloroethane solution with no resultant degradation.
  • Au 55 (PPh 3 ) 12 Cl 6 is observed to decompose to Au(O) and AuCl[PPh 3 ] within 2 hours.
  • polypeptide molecular templates can be used to organize small, monodisperse nanoclusters into linear arrays of molecular dimension.
  • a 1 ⁇ 10 ⁇ 8 M solution of poly-L-lysine is mixed with a large excess (1,000-fold) of “normal” gold 55 in methanol solution.
  • the amino sidechains of the poly-L-lysine replace some of the labile triphenylphosphine ligands on the gold cluster and thus bind the cluster to the template.
  • the decorated clusters precipitate out of solution onto a TEM grid. Single gold clusters that become non-specifically adsorbed on the grid will be removed by rinsing with benzene. Transmission electron micrography (TEM) analysis will show the gold cores of the cluster and will indicate the extent to which the cluster have aggregated into low-dimensional arrays due to template-induced organization.
  • TEM Transmission electron micrography
  • This example describes the electron transfer properties of organometallic structures formed by electron-beam irradiation of Au 55 [P(C 6 H 5 ) 3 ] 12 Cl 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 2 Cl 2 and 0.25 mL of CH 2 ClCH 2 Cl.
  • a supernatant solution was spin coated onto a Si 3 N 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 2 Cl 2 rinse removed the excess film.
  • 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.
  • the leakage current was almost linearly dependent on bias over the range ⁇ 100 to 100V, and had a maximum value ⁇ 100 fA. While the ultimate resolution of the current measurement was 10 fA, the leakage current set the miN/Mum resolved conductance ⁇ 10 ⁇ 15 ⁇ ⁇ 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.
  • 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 ⁇ 10 ⁇ 19 C.
  • 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 VT.
  • the conductance below V T was activated, with activation energies EA 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 C ⁇ 4E A . Assuming current suppression requires E c >10 kT, the sample with the largest activation energy should develop a Coulomb gap below ⁇ 300 K. This value is within a factor of 2 of the measured temperature at which clear blockade behavior occurs in the patterned samples.
  • the temperature dependence of the conductance within the Coulomb gap is consistent with the observation of blockade behavior.
  • 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 55 clusters.
  • 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.
  • 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.
  • FIG. 9 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 ⁇ 1.6 which is closest to the analytical prediction for an infinite, disordered two-dimensional array. From the analysis the magnitude of V T ⁇ 6 ⁇ 1 V which is in good agreement with that estimated directly from the I-V data.
  • the energy E C can also be estimated if the capacitance of an island is known.
  • the Coulomb charging energy, E C e 2 /2C ⁇ 340 meV which is within twenty percent of the maximum value of 4E A found from the activation data. This result suggests that the current suppression is due to charging of individual AU 55 clusters.
  • 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 ⁇ 10 ⁇ 5 mol/l solution of 56,000 amu poly-L-Lysine•HBr in H 2 O/CH 3 OH. 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 be 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/l 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.
  • 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.
  • 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 lithiography.
  • 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.

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US20040203074A1 (en) 2004-10-14

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