Overview of Nanoelectronic Devices <s> IV. MOLECULAR ELECTRONICS <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> BIB001 </s> Overview of Nanoelectronic Devices <s> IV. MOLECULAR ELECTRONICS <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> BIB002 </s> Overview of Nanoelectronic Devices <s> IV. MOLECULAR ELECTRONICS <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> IV. MOLECULAR ELECTRONICS <s> The following topics are discussed: the emergence of molecular electronics research; nanometer scale components; assembly techniques; synthetic organic molecules and biomolecules; the role of protein engineering; hierarchy of biological information processing; common infrastructure for molecular electronics, biosensor and molecular actuator technology; interfacing problems; molecular self-assembly and self-repair. > <s> BIB004 </s> Overview of Nanoelectronic Devices <s> IV. MOLECULAR ELECTRONICS <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> IV. MOLECULAR ELECTRONICS <s> A civilization often is named after the material which dominates in tool-making. Therefore historians speak of the Stone Age, Bronze Age, and Iron Age. Following these lines our present time would be called Silicon Age [1]. Without doubt, information technology has a decisive influence on all aspects of our everyday life and the material basis of information technology is silicon. Will there be a Gallium-Arsenide Age in the future? Or a Diamond Age, or an Organic Age, a Molecular Electronics Age? Diamond can be used as a semiconductor at higher temperatures than silicon, and diamond has a better thermal conductivity. Organics (phtalocyanines, conjugated polymers, dye-stuffs etc.) allow for a higher integration density than inorganic semiconductors. Will the emergence of diamond, polymers, and fullerenes as high-tech materials lead to a Carbon Age following the Silicon Age? Mankind has invested hundreds of thousands of man-years into research and development (R&D) of silicon technology and consequently silicon is the <s> BIB006 </s> Overview of Nanoelectronic Devices <s> IV. MOLECULAR ELECTRONICS <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> BIB007 </s> Overview of Nanoelectronic Devices <s> IV. MOLECULAR ELECTRONICS <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> BIB008 </s> Overview of Nanoelectronic Devices <s> IV. MOLECULAR ELECTRONICS <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> BIB009 </s> Overview of Nanoelectronic Devices <s> IV. MOLECULAR ELECTRONICS <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> BIB010 </s> Overview of Nanoelectronic Devices <s> IV. MOLECULAR ELECTRONICS <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> IV. MOLECULAR ELECTRONICS <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> BIB012 </s> Overview of Nanoelectronic Devices <s> IV. MOLECULAR ELECTRONICS <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> BIB013
Molecular electronics uses primarily covalently bonded molecular structures, electrically isolated from a bulk substrate BIB003 ,  ,  ,  , BIB004 -  ,  ,  ,  . Devices of this description, wires and switches composed of individual molecules and nanometer-scale supramolecular structures, sometimes are said to form the basis for an "intramolecular electronics" BIB003 . (This is to distinguish them from organic microscale transistors and other organic devices that use bulk materials and bulk-effect electron transport just like semiconductor devices. See, for example, BIB006 -  .) As indicated above, solids have the significant disadvantage that it is relatively difficult and expensive to fabricate or "sculpt" in them the many millions or billions of nearly identical nanometer-scale structures that will be needed in each ultra-dense computer chip. Individual molecules, natural nanometer-scale structures, easily can be made exactly the same by the trillions of billions. The great power and variety of organic chemistry also should offer more options for designing and fabricating nanometer-scale devices than are available in silicon  ,  ,  ,  , BIB004 , BIB005 . Increasingly, this is driving investigators to design, model, fabricate, and test individual molecules  ,  , BIB013 , BIB009 , BIB007 , BIB010 ,  ,  ,  , BIB011 , BIB001 and nanometer-scale supramolecular structures BIB012 , BIB008 that act as electrical switches and even exhibit some of the same properties as small solid-state transistors BIB013 . Molecular electronics does remain a more speculative research area than solid-state nanoelectronics, but it has achieved steady advances consistent with Aviram's strategy BIB002 for making molecular electronic circuits viable, inexpensive, and truly integrated on the nanometer scale.