Overview of Nanoelectronic Devices <s> C. Molecular Wires <s> Organic molecules, designed to have an asymmetrical double-well potential for two protons, were synthesized. The structure allows the switching of the position of the protons by an electric field. A monolayer of these molecules was chemically bound to a surface of gold. A scanning tunneling microscope was modified so that the current-voltage characteristics between the tip and a surface could be obtained rapidly, making it possible to observe the effects of single molecules. Whereas bare Au had usual tunneling characteristics, it was found that the surface with molecules had regions where the I-V was highly asymmetric. For negative voltages there was a switching threshold, beyond which the conductivity was much higher than when the tip was positive. A tentative explanation of the switching and rectification, based on the asymmetry of the molecule, is proposed. <s> BIB001 </s> Overview of Nanoelectronic Devices <s> C. Molecular Wires <s> when X = Y = oH' the pure monolayers a." *eti"d by water, but the mixed monolayers are less hydrophilic because nonpolar polymethylenecha insareexposedatt hesurface. whinX =cHr, Y = oH (n=2L,m= ll),averysharptr ansitionoccurs from a monolayer composed largely of the longer, methyl-terminated component to the shorter, hydroxyl-terminated component as the mole fraction of HS(CHz) r roH in-the adsorption solution is increased. From solutions .ontuining two thiols, adsorption of the thiol with the longer chain is preferred. Tiris preference is greater when the monolayers are adsorbed from ethanol than from isooctane' The mixed monolayers do not uit ", ideal two-dimensional solutions. T-he adsorption isotherms suggest a positive excess free energy of mixing of the two components in the monolayer. The compositions of the monolayers appear to be determined largely by thermodynamics, although in some cases there is also a kinetic contribution. The two components in the mixed monolayers do not phase-segregate into macroscopic islands (greater than a few tens of angstroms across) but are probably not randomly dispersed within the monolayer, ThL wettability of mixed, methyl-terminatea can be partially rationalized by the geometric m€an approximition, but a full description probably requires inclusion of the entropy of mixing at the monolayer-liquid interface. The hysteresis in the contact angle on these monolayers cannot be explained by theories of wetting based on macroscopic heterogeneity. Contact angles ur? -o.. sensitive than optical ellipsometry or X-ray photoelectron spectroscopy to certain types of changes in the coriposition and structure of these monolayers. <s> BIB002 </s> Overview of Nanoelectronic Devices <s> C. Molecular Wires <s> The paper reviews the current state of molecular electronics and considers the most interesting trends in its development. It is noted that microscopic dimensions in quantum processes are not the only advantage of molecular technology. It is also important that chemical synthesis of molecular systems ensures the identity of assembled functional elements, thereby providing reliability and efficiency of quantum processes in molecular electronic devices. We propose using the quantum effect of the Peierls instability, which is readily realized in one-dimensional molecular systems. Advantages of this approach are discussed. <s> BIB003 </s> Overview of Nanoelectronic Devices <s> C. Molecular Wires <s> We extend a model originally intended for the description of the scanning tunneling microscope (STM) current in molecular imaging of one‐dimensional systems, to encompass the more general process of electron transfer between two reservoirs of states. In the STM problem, the reservoirs are naturally associated with the metal density of states of the electrodes. In the molecular electron transfer problem, the identification of the reservoirs with the Franck–Condon weighted density of vibrational states allows a number of fruitful connections with the theory of nonadiabatic electron transfer (ET) in molecules to be established. In this article, we present an exact procedure, based on Lowdin’s partitioning technique, to determine the Green’s function and the T matrix, relevant to the transport process. We obtain compact expressions for the conductance and the density of states in the limit of small applied voltage and low temperature, and discuss the important case where the molecular wire is described by a t... <s> BIB004 </s> Overview of Nanoelectronic Devices <s> C. Molecular Wires <s> Properties of synthetic electroactive and photochromic molecular systems are described, with particular emphasis on microscopic processes controlling these properties. Both low-molecular weight and polymeric materials can be (and many have already been) used as conducting systems. The chemical flexibility of many molecular systems also allows for using them as information-processing materials, whose action is based on their photochromic properties. A relation between properties of individual molecules and those of macroscopic molecular systems allows one to envisage construction of molecular-scale devices. However, due to limitations associated with technological problems (construction of molecular-scale elements, their addressing, quantum-mechanical (size) limitations, etc.), the construction of such elements is still a problem that awaits solution. > <s> BIB005 </s> Overview of Nanoelectronic Devices <s> C. Molecular Wires <s> Conducting filaments of polyaniline have been prepared in the 3-nanometer-wide hexagonal channel system of the aluminosilicate MCM-41. Adsorption of aniline vapor into the dehydrated host, followed by reaction with peroxydisulfate, leads to encapsulated polyaniline filaments. Spectroscopic data show that the filaments are in the protonated emeraldine salt form, and chromatography indicates chain lengths of several hundred aniline rings. The filaments have significant conductivity while encapsulated in the channels, as measured by microwave absorption at 2.6 gigahertz. This demonstration of conjugated polymers with mobile charge carriers in nanometer channels represents a step toward the design of nanometer electronic devices. <s> BIB006 </s> Overview of Nanoelectronic Devices <s> C. Molecular Wires <s> Catalytically grown carbon fibers approximately 10 nm in diameter and several microns long were characterized by transmission electron microscopy and determined to be multiple- walled nanotubes, and a technique was developed to measure the conductivity of an individual nanotube. Nanotubes were dispersed in solvents and precipitated onto lithographically defined gold contacts to make a 'nano-wire' circuit. Non-contact AFM was used to image the nano- wires, and a resistance o_f LI.4 (+ 1.0 ) MC) was measured through a single nanotube at23o C. A resistivity of 9.5x10-) Q m was estimated for carbon conducting along the axis of a fiber. Local heating of nanotubes appeared to occur at high current densities. The nanotubes could sustain currents on the order of 10 pA per fiber, but application ofcurrents on the order of 100 pA per fiber resulted in rapid decomposition in air and breaking of the circuit. <s> BIB007 </s> Overview of Nanoelectronic Devices <s> C. Molecular Wires <s> We demonstrate experimentally with an STM operating in ultra-high vacuum and with model calculations that the current-voltage characteristics of a single C60 molecule are linear at low voltage. This is shown to prevail for tip-surface separations larger than the electrical contact point down to when the molecule is physically compressed by the tip. This linear I(V) relationship of a single molecule holds in general as long as its HOMO-LUMO gap in the junction exceeds the voltage used to define the junction electrical resistance and if the molecule is enough coupled to the electrodes. <s> BIB008 </s> Overview of Nanoelectronic Devices <s> C. Molecular Wires <s> Nanometer-size Au clusters deposited from a cluster beam onto a thin dithiol film were studied at room temperature using an ultrahigh vacuum scanning tunneling microscope (STM). The dithiol molecules tether the deposited Au clusters to the underlying gold substrate and repeatable STM scans of the Au clusters were achieved. Data of the tunneling current as a function of applied voltage yield reproducible evidence for single electron tunneling at room temperature. By fitting the measured I(V) data to a Coulomb blockade model, estimates for the electrical resistance of a single dithiol molecule are also obtained. <s> BIB009 </s> Overview of Nanoelectronic Devices <s> C. Molecular Wires <s> Abstract Molecular dynamics with realistic many-body atomic potentials was used to study the growth of carbon nanotubes. Analysis of the bond switching and ring migration processes has led to an identification of tube growth mechanisms. Wide tubes, initially open, were found to grow straight maintaining an all-hexagonal structure, while narrow tubes were found to develop permanent pentagonal rings that lead to tube closure upon further deposition. Continued deposition on the top of a closed tube yields a disordered cap structure, implying that open tubes are critical for defect-free growth. <s> BIB010 </s> Overview of Nanoelectronic Devices <s> C. Molecular Wires <s> Molecular wire candidates inserted into “nonconducting” n -dodecanethiol self-assembled monolayers on Au{111} were probed by scanning tunneling microscopy (STM) and microwave frequency alternating current STM at high tunnel junction impedance (100 gigohms) to assess their electrical properties. The inserted conjugated molecules, which were 4,4′-di(phenylene-ethynylene)benzenethiolate derivatives, formed single molecular wires that extended from the Au{111} substrate to about 7 angstroms above and had very high conductivity as compared with that of the alkanethiolate. <s> BIB011 </s> Overview of Nanoelectronic Devices <s> C. Molecular Wires <s> In this paper, we present a method for computing the resistance of molecular wires and illustrate it with a systematic theoretical study of a particular class of organic molecules. These molecules consist of one or more benzene rings with a thiol~-SH! group at the ends. This end group can attach readily to metallic surfaces, thus allowing the molecule to function as a nanoelectronic interconnect. The conduction through these molecules at low bias occurs by tunneling, leading to resistances that are typically several tens of megaohms. The resistance goes up exponentially with the number of rings and is sensitive to the relative orientation of the rings and the bonding between them. The Green-function-based method presented here provides a powerful tool for accurate modeling of the semi-infinite contacts that are used to measure molecular resistance. <s> BIB012 </s> Overview of Nanoelectronic Devices <s> C. Molecular Wires <s> The electronic transparence of a single molecular wire connecting the two electrodes of a metal-insulating-metal nanojunction decreases exponentially with its length. The transparence attenuation can be quite small depending of the homo-lumo gap of the molecule and of electronic interaction of the wire ends with the electrodes. For a 10 nm long polyene connecting two nano-electrodes, a 100 mV bias voltage will lead to a tunnelling current intensity in the 10 pA range. <s> BIB013 </s> Overview of Nanoelectronic Devices <s> C. Molecular Wires <s> We have studied the current vs voltage curves (I–V characteristics) of a mesoscopic device consisting of two electrodes and a molecular wire. The wire Hamiltonian includes both electronic tunneling and Coulomb repulsion within a Hubbard model that is treated at the Hartree–Fock level. The inclusion of electron repulsion is an extension of our previous work that only considered the case of noninteracting electrons. We have found several important features in the calculated characteristics of the wire. These include (1) a staircaselike structure that strongly resembles that associated with Coulomb blockade in heterostructures and quantum dots, but that in the case of the wire is associated with the discrete nature of the molecular resonances; (2) regions of negative differential resistance associated with increased localization of the molecular resonances. Our theoretical model includes a consistent treatment of the conduction in the linear and nonlinear regimes which remains valid even when the device is o... <s> BIB014 </s> Overview of Nanoelectronic Devices <s> C. Molecular Wires <s> Double-ended aryl dithiols [alpha,alpha'-xylyldithiol (XYL) and 4,4'-biphenyldithiol] formed self-assembled monolayers (SAMs) on gold(111) substrates and were used to tether nanometer-sized gold clusters deposited from a cluster beam. An ultrahigh-vacuum scanning tunneling microscope was used to image these nanostructures and to measure their current-voltage characteristics as a function of the separation between the probe tip and the metal cluster. At room temperature, when the tip was positioned over a cluster bonded to the XYL SAM, the current-voltage data showed "Coulomb staircase" behavior. These data are in good agreement with semiclassical predictions for correlated single-electron tunneling and permit estimation of the electrical resistance of a single XYL molecule (approximately18 ± 12 megohms). <s> BIB015 </s> Overview of Nanoelectronic Devices <s> C. Molecular Wires <s> Historical Introduction. Carbon Materials. Structure of Fullerenes. Symmetry Considerations. Growth and Fragmentation Studies. Crystalline Structure of Fullerenes. Synthesis of Fullerene Molecules and Solids. Doping of Fullerenes. Structure of Doped Fullerenes and Fullerene Compounds. Fullerene Chemistry. Vibrational Modes. Thermal Properties. Electronic Structure. Optical Properties. Electrical and Thermal Properties. Superconductivity. Nuclear Magnetic Resonance Studies. Electron Paramagnetic Resonance. Surface Science Techniques on Fullerenes. Magnetic Properties. Fullerene-Related Tubules and Spherules. Scanning Tunnel Microscopy. Applications. <s> BIB016 </s> Overview of Nanoelectronic Devices <s> C. Molecular Wires <s> We describe the first single-molecule electromechanical amplifier, the active element of which is one fullerene molecule (C60) with a junction length and width of ∼ 1 nm. Forces in the nano-Newton range are generated by a metallic tip pressing on the molecule in its adsorbed state. This working nanodevice is based on the modulation of virtual resonance tunneling through the molecule by electromechanical deformation of the C60 cage. <s> BIB017
The subject of molecular wires is of primary significance, even in an overview of nanoelectronic switching devices. Before one can seriously discuss such electronic devices embedded in single molecules, one must deal with the question of whether a small single molecule can conduct appreciable current. The answer to this question was in doubt, because very narrow wire-like structures often exhibit high resistance, even if they are made from substances that conduct electricity when they are present in bulk. However, a series of very difficult and sensitive experiments BIB001 , BIB008 , BIB011 and theoretical investigations BIB012 ,  , BIB013 , BIB004 - BIB014 over the past few years have answered this question affirmatively. Extensive experimental work on buckyballs by Joachim and his collaborators established conclusively that one such molecule conducts a current BIB008 . In very recently reported results, Joachim and Gimzewski have even shown resonant tunneling through a quantum well in a single buckyball, although the switching required deforming the well electromechanically with an STM BIB017 . Fig. 7 depicts the structure of a molecular wire invented by Tour  that was used recently to demonstrate conductance in a single molecule BIB011 . In that experiment BIB011 , one end of the conducting molecule was adsorbed to a gold surface, but an STM tip was used as the other electrode in the circuit. However, in more recent work by Reed and Tour, each end of a conducting molecule was adsorbed to a different gold electrode mounted on a surface in order to complete a circuit  , and even to demonstrate quantum-device effects  . Wires of this general type also were used in the Purdue self-assembled molecular electronic circuit array mentioned above, in which the characteristic "staircase" pattern of Coulomb blockade [cf. Fig. 5(d) ] was observed at room temperature in the plot of current versus bias voltage BIB015 , BIB009 . The thiol (-SH) functional groups at either end of the molecular wire structure in Fig. 7 adsorb well to gold surfaces and act as "alligator clips" for attaching molecular BIB011 . electronic units to metal substrates  , BIB002 . Such molecular wires also have the desirable property that they can be made quite long, if necessary, because they can be lengthened systematically using chemosynthetic methods  . Wires like the one illustrated in Fig. 7 are characterized by extended repeating structures-here a sequence of benzene-like rings connected by acetylene linkages-each part of which is linked to the next by bonds including many -electrons above and below the plane of the structure. These orbitals or clouds of -electrons  conjugate with each other, or interact, to form a single large orbital throughout the length of the wire to permit mobile electrons to flow BIB005 . There has been much study of other molecular nanowires, as well. Bein and coworkers BIB006 used substrates with nanometer-scale pores  as templates to create carbonbased conducting polymer wires 3 nm in diameter. A similar method developed by Martin and coworkers  also polymerizes wires inside small channels. These nanowires show high conductivity compared to bulk polymers, suggesting that the wires are not amorphous but have regular structure. Tour and his collaborators  , BIB011 have more recently shown other potential molecular wires that could be self-assembled onto a gold surface. Another new type of nanostructure, termed a "buckytube" because of its structural similarity to carbon "buckyballs" BIB016 , also presents possibilities for chemically synthesizing nanowires  ,  . Buckytubes are cylindrical carbon nanotubes  . These hollow tubes might be used as support for molecular circuit elements-e.g., filled with conducting metal atoms to create among the structurally strongest nanowires chemically possible. The structure of the nanotube derives its strength from the carbon-carbon bonds. The carbon atoms are bonded in virtually flawless hexagonal arrays Simulations of carbon nanotubes have shown that isolated flaws migrate to the ends of the tube and are eliminated, a phenomenon termed "self-healing" BIB010 . Alternatively, the tubes might be used to conduct current themselves, as they have been shown to do. A measurement of the conductivity of carbon nanotubes has shown that a tube 10 nm in diameter can carry currents of approximately 10 microamps per molecular strand or "fiber" BIB007 . More recent work by two research teams shows that, in theory, carbon nanotubes could even be made to behave like electronic switches  . Unfortunately, the commonly studied nested nanotubes show large and uncontrollable variations in electrical properties from one tube to the next  . Single-layer nanotubes now starting to be fabricated in quantity offer hope for more reproducible properties and may be useful in molecular electronic devices. Historically, the field of molecular electronics has focused much attention on understanding and demonstrating the conductance of molecular wires BIB003 , BIB005 , BIB012 , BIB014 , BIB011 . Also, to manipulate and fabricate tiny molecular switching devices it is convenient to embed them in longer wire-like structures. Thus, much of today's work on molecular electronic devices is intimately tied to the study of the electrical properties of molecular wires  .