Overview of Nanoelectronic Devices <s> B. Brief Background on Molecular Electronics 1) History: <s> Surface microscopy using vacuum tunneling is demonstrated for the first time. Topographic pictures of surfaces on an atomic scale have been obtained. Examples of resolved monoatomic steps and surface reconstructions are shown for (110) surfaces of CaIrSn 4 and Au. <s> BIB001 </s> Overview of Nanoelectronic Devices <s> B. Brief Background on Molecular Electronics 1) History: <s> Limited slip differential employing a spring-loaded housing for transferring power to a driven axle shaft to allow limited slippage in the other axle shaft. Design eliminates side gears. <s> BIB002 </s> Overview of Nanoelectronic Devices <s> B. Brief Background on Molecular Electronics 1) History: <s> An electronic shift-register memory at the molecular level is described. The memory elements are based on a chain of electron-transfer molecules and the information is shifted by photoinduced electron-transfer reactions. This device integrates designed electronic molecules onto a very large scale integrated (silicon microelectronic) substrate, providing an example of a "molecular electronic device" that could actually be made. The design requirements for such a device and possible synthetic strategies are discussed. Devices along these lines should have lower energy usage and enhanced storage density. <s> BIB003 </s> Overview of Nanoelectronic Devices <s> B. Brief Background on Molecular Electronics 1) History: <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> BIB004 </s> Overview of Nanoelectronic Devices <s> B. Brief Background on Molecular Electronics 1) History: <s> The scanning tunneling microscope (STM) has proven to be an unsurpassed instrument for studying surfaces with sub-nanometer resolution in all three spatial dimensions. The same circumstances that permit the STM to image with atomic resolution also allow it to function as a highly localized probe capable of exerting human influence upon regions of only nanometers in extent. Demonstrations of the STM's ability to modify matter at the nanometer scale give evidence of its promise as a tool for nanofabrication applications. In addition to presenting some recent results of STM modifications of Au and graphite substrates, this paper introduces a system for classifying the various mechanisms by which an STM may induce modifications, and classifies published and new experimental observations within this framework. <s> BIB005 </s> Overview of Nanoelectronic Devices <s> B. Brief Background on Molecular Electronics 1) History: <s> Abstract : Described is an approach to orthogonally fused conjugated organic compounds that may act as molecular switching devices. Four thiophene trimers are added in a single operation to spiro-fused cores to afford the target molecules. A spiro-fused thiophene-based monomer system is converted to a spiro- fused heptamer that is 25 A long. The synthesis of a mixed phenylene-thiophene system is described that provides a spiro-fused octamer that is 30 long. In each case, alkyl substituents on the thiophenes afford soluble materials. Trimethylsilyl end groups flank each orthogonally fused system. Organophalladium- and organonickel-catalyzed procedures are used extensively for the synthesis of the orthogonally fused compounds. <s> BIB006 </s> Overview of Nanoelectronic Devices <s> B. Brief Background on Molecular Electronics 1) History: <s> Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication. <s> BIB007 </s> Overview of Nanoelectronic Devices <s> B. Brief Background on Molecular Electronics 1) History: <s> An approach to molecular nanotechnology is presented that utilizes branched DNA molecules to form stick figures. These branched molecules are directed to associate in a particular fashion by utilizing the addressability of DNA sticky ends. The sequences of molecules are designed using an algorithm that minimizes sequence symmetry. This system has been used to construct macrocycles, a DNA quadrilateral, a molecule with the connectivity of a cube and a single-stranded DNA knot. Possible uses for this system include the construction of macromolecular zeolites for structure determination by diffraction, the assembly of new catalysts, the solubilization and delivery of drugs, and the assembly of molecular electronic devices. <s> BIB008 </s> Overview of Nanoelectronic Devices <s> B. Brief Background on Molecular Electronics 1) History: <s> This paper describes the background of microelectronics science and technology that influenced the development of molecular electronics (ME) and includes a description of some thoughts on the development of molecular circuit elements. Three basic principles are proposed and exemplified by a particular example of a suitable molecule. It is pointed out that as the size of electronic devices approaches molecular dimensions, ME promises to have distinct advantages over other technologies because the component function can be an outcome of the electronic structure of the molecule. A plan that can lead to the development of the ME concept with the scientific and technological knowledge of the present day is proposed. <s> BIB009 </s> Overview of Nanoelectronic Devices <s> B. Brief Background on Molecular Electronics 1) History: <s> The authors review some experiments in molecular rectification and their implication for commercial uses of molecular electronic devices. Two of the cases involve rectification by single molecules which consist of an electron donor on one side, an electron acceptor on the other side, and a bridge in between, coupled to electrodes. The third case involves rectification at a graphite electrode derivatized with a Cu phthalocyanine derivative, and probed with a Pt/Ir scanning tunneling microscope tip. Some potential applications of molecular devices are in high-density memory storage of holographic memory devices, neural networks, cellular automata, and chemical and biochemical sensors. <s> BIB010 </s> Overview of Nanoelectronic Devices <s> B. Brief Background on Molecular Electronics 1) History: <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> BIB011 </s> Overview of Nanoelectronic Devices <s> B. Brief Background on Molecular Electronics 1) History: <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> BIB012 </s> Overview of Nanoelectronic Devices <s> B. Brief Background on Molecular Electronics 1) History: <s> Edited by the inventor of polymerase chain reaction (PCR) and the 1993 Nobel Prize winner in Chemistry, Kary Mullis, as well as two experts in the field, this handbook provides up-to-date methodological protocols from the world's leading laboratories, in addition to new techniques and enhanced applications not yet available in book form. Nearly 40 chapters inform the novice and experienced PCR user on how to optimize their results. In the chapters on applications, researchers provide not only protocols, but also descriptions of how PCR has revolutionized their particular field. Future enhancements of PCR, as well as new potential uses, are discussed. Readers can learn how PCR has changed the face of diagnostic testing, cancer research, genetics, forensics, plant biology, DNA sequencing and gene therapy. Special sections include the latest on QPCR, non-isotopic detection, genetic analysis, and PCR and the world of business, which includes a behind-the-scenes look at the legal battles between biotech giants Cetus and DuPont. It also provides insights into the origins of PCR and the history of nucleic acid research. <s> BIB013 </s> Overview of Nanoelectronic Devices <s> B. Brief Background on Molecular Electronics 1) History: <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> BIB014 </s> Overview of Nanoelectronic Devices <s> B. Brief Background on Molecular Electronics 1) History: <s> A molecule is usually understood to be a stable collection of atoms connected bv a continuous network of covalent bonds. The develipment of methods for constructing these networks has been a central occupation of organic chemistry, and the success of these methods has made possible the power, elegance, and utility of modern organic synthesis. The preparations of vitamin B12,l palytoxin,2 calicheamicin,3 and other complex secondary metabolites illustrate the extraordinary sophistication of this field. This type of synthesis-which we refer to as covalent synthesis, in the absence of a better termcontinues to expand its capabilities, but it may be understandably difficult to provide very large and structurally complex molecules quickly and economically by using it.4 Organic chemistry has always taken much of its inspiration and motivation from Nature. As biological molecules-especially large molecules having complex tertiary structures such as proteins, DNA, and RNA-have become central concerns of organic chemistry, noncovalent interactions have moved toward the center of attention. Although biological macromolecules are largely composed of Covalent bonds, the networks of these bonds are not always continuous, and many important structures-including multimeric proteins and DNA itself-are "aggregates" and not simply "molecules". Many biological molecules and aggregates derive much of their unique structure and function from noncovalent interactions: that is, from <s> BIB015 </s> Overview of Nanoelectronic Devices <s> B. Brief Background on Molecular Electronics 1) History: <s> Two Micro-Scanning Tunneling Microscopes (Micro -STMs) have been fabricated and characterized. The integrated Micro-STMs are fabricated from Single Crystal Silicon (SCS) using the high-aspect-ratio SCREAM process. Each Micro-STM includes integrated xy comb drive actuators and a torsional z actuator with integrated cantilever and tunneling or atomic force tip. One Micro-STM measures approximately 200 /spl mu/m on-a-side and is an example of an STM element for an array architecture. Another, larger Micro-STM/AFM measures 2 mm on-a-side including the I mm long cantilever with a 20 nm diameter tip. We demonstrate the operation of this larger STM by STM-imaging a 300 nm metal conductor on a silicon chip. <s> BIB016 </s> Overview of Nanoelectronic Devices <s> B. Brief Background on Molecular Electronics 1) History: <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> BIB017 </s> Overview of Nanoelectronic Devices <s> B. Brief Background on Molecular Electronics 1) History: <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> BIB018 </s> Overview of Nanoelectronic Devices <s> B. Brief Background on Molecular Electronics 1) History: <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> BIB019 </s> Overview of Nanoelectronic Devices <s> B. Brief Background on Molecular Electronics 1) History: <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> BIB020 </s> Overview of Nanoelectronic Devices <s> B. Brief Background on Molecular Electronics 1) History: <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> BIB021 </s> Overview of Nanoelectronic Devices <s> B. Brief Background on Molecular Electronics 1) History: <s> A brief review of scanned probe nanofabrication is presented followed by an in-depth discussion of ultrahigh vacuum (UHV) scanning tunneling microscope (STM) nanofabrication on hydrogen passivated silicon surfaces. In this latter case the UHV STM functions as a nanolithography tool by selectively desorbing hydrogen from silicon surfaces. Patterns of clean Si, down to atomic dimensions, are achieved as well as detailed information about the H-desorption mechanisms. At higher sample voltages direct electron stimulated desorption occurs, whereas, at lower voltages, vibrational heating of the Si-H bond leads to desorption. The chemical contrast between clean and H-passivated silicon enables a wide variety of spatially selective nanoscale chemical reactions. Results are presented in which these templates are used for selective oxidation, nitridation, and metallization by chemical vapor deposition. An unexpected byproduct of this research was the discovery that deuterium is about two orders of magnitude more difficult to desorb from silicon than hydrogen. This served as the basis for the idea of treating CMOS transistors with deuterium to reduce their susceptibility to hot carrier degradation effects. Tests have now shown that the lifetimes of CMOS transistors increase by factors of 10 to 50 when deuterium treatment is substituted for the traditional hydrogen processing. <s> BIB022 </s> Overview of Nanoelectronic Devices <s> B. Brief Background on Molecular Electronics 1) History: <s> Application of a scanning tunneling microscopy (STM) and an atomic force microscopy (AFM) to electron devices and an optical device are introduced in this paper. Using STM tip/AFM cantilever as a cathode, surfaces of a metal or a semiconductor are oxidized to form a few tens of nanometers-wide oxidized metal line or an oxidized semiconductor line, which works as an energy barrier for an electron. A single-electron transistor (SET), a photoconductive switch, and a high-electron mobility transistor (HEMT) are fabricated using this fabrication process. The fabricated SET operates even at high room temperatures and shows the large Coulomb gap and staircase of 200-mV periods and the large Coulomb oscillation periods of 406 mV. The fabricated photoconductive switch shows a ultra-fast response time, i.e., a full-width at half-maximum response of 380 fs at a bias voltage of 10 V. The drain current of HEMT was controlled by the oxidized semiconductor wire on the channel region formed by this fabrication process. <s> BIB023
The search for individual molecules that would behave as electrical switches began in 1974, with the pioneering work of Aviram and Ratner, who proposed a theory on molecular rectification  . Research on molecular electronics was stimulated in the early 1980's by such visionaries as the late Forrest Carter  ,  , [95] and by some notable research efforts later in the decade BIB003 . Aviram's further work in the late 1980's and early 1990's  , BIB009 helped enlist a new cadre of investigators and establish a plan for the development of the field. Finally, in the 1990's, interest in the field has grown rapidly. Tour et al., have synthesized the spiro-switch proposed by Aviram  , BIB006 , and different variants of the molecular rectifier have been made BIB010 ,  . Much work has been done to measure BIB004 , BIB014 ,  ,  , BIB017 , BIB011 the conductance and other electrical properties of individual molecules or to model BIB018 ,  - BIB019 , BIB012 - BIB020 them. This growth has been driven by recognition of the need for ultra-miniaturization of electronics, and it has been catalyzed by the wide availability of sensitive new methods for imaging, manipulating, and fabricating molecular and supramolecular structures. 2) Role of New Methods for Nanomanipulation and Nanofabrication: Although the structure and workings of molecular electronic devices are emphasized in this overview, no discussion of molecular electronics can ignore the exciting new methods for nanofabrication that have made research on molecular electronics feasible and important. Especially significant are the methods for mechanosynthesis  , BIB005 and chemosynthesis BIB015 of nanometer-scale structures. Mechanosynthesis is the fabrication of nanostructures molecule by molecule using nanoprobes  , such as the scanning-tunneling electron microscope (STM), the atomic force microscope (AFM), and the new microelectromechanical systems (MEMS) chips that contain arrays of these STM's and AFM's [73] , BIB016 . These sensitive new tools, invented in the 1980's BIB001 , BIB002 , have opened a plethora of new experimental possibilities with molecules. Nanoprobes also have provided realtime visual and tactile feedback and an increased sense of contact with the behavior of the molecular-scale experimental systems that are essential for progress in molecular electronics. By providing a means to image and manipulate individual atoms and molecules, STM's and AFM's have given much impetus to research on molecular electronics. The topic of nanoprobes is discussed more thoroughly in other papers in this issue BIB022 , BIB023 . Chemosynthesis includes the growing study of the chemical "self-assembly" of nanostructures  , BIB007 , which also is having considerable impact on the fabrication of solid-state circuit elements. It also includes the application of methods borrowed from biochemistry and molecular genetics BIB008 - BIB013 , as well as creative and elegant organic syntheses of molecular electronic devices in individual organic molecules  ,  ,  ,  , BIB006 . As one example of the application of chemosynthetic selfassembly to molecular electronics, we note that Martin et al., used a self-assembled Langmuir-Blodgett film  ,  to demonstrate molecular rectification of the type first suggested in Aviram's and Ratner's theory. Also, in very promising recent work, an interdisciplinary group at Purdue University, West Lafayette, IN, has used self assembly to fabricate and demonstrate functioning arrays of molecular electronic quantum confinement structures connected by molecular wires BIB021 ,  .