Molecular Transport Junctions: An Introduction
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Molecular Transport Junctions: An Introduction
Cherie R. Kagan and Mark A. Ratner, Guest Editors Abstract This issue of MRS Bulletin on molecular transport junctions highlights the current experimental and theoretical understanding of molecular charge transport and its extension to the rapidly growing areas of molecular and carbon nanotube electronics. This introduction will outline the progress that has been made in understanding the mechanisms of molecular junction transport and the challenges and future directions in exploring charge transport on the molecular scale. In spite of the substantial challenges, molecular charge transport is of great interest for its intrinsic importance to potential single-molecule electronic, thin-film electronic, and optoelectronic applications. Keywords: carbon nanotubes, charge transport, molecular electronics, molecular junctions, optoelectronics, self-assembly.
Molecular Electronics: Definition and History Molecular electronics is most simply defined as electronics whose behavior is dictated by the chemical, physical, and electronic structures of molecules. This definition is very broad; it includes conductive polymers and even the insulating polymer layers on metal wiring. In common use, molecular electronics refers to molecular structures whose characteristic features are on the nanoscale and that contain between one and a few thousand molecules. The simplest challenge involves determining the structure/function relationships for electronic transport (intra- or intermolecular) through a junction containing one or a few molecules as the transport medium and with either two (source/drain) or three (source/gate/drain) electrodes. Such transport structures are a dominant theme of contemporary molecular electronics and are the focus of this issue of MRS Bulletin. The history of molecular electronics is brief. Some of the earliest work was performed at the laboratory of Hans Kuhn, working with Mann, Polymeropoulos, and Sagiv.1 Using organic adlayers on solid substrates, this group reported some of the earliest (1971) reproducible electrical transport measurements through organic
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molecules. They also developed some of the first effective self-assembly techniques for preparing structures in which molecules adhere to surfaces not by simple dispersion forces, but by molecular bond formation. Following Kuhn’s work, there were a number of important follow-up measurements.2 In addition, there were some visionary (if perhaps premature) papers3 suggesting the use of single molecules as rectifiers and extensive device ideas involving molecular logic structures.4 The great step forward came in 1983, with the development of scanning probe microscopy (SPM).5 The capability to both manipulate and measure molecular structures topologically and spectroscopically prompted a renewed focus on the possibilities of molecular electronics. Continuing advances in synthesizing organic molecules with possible device applicability, and in their assembly and measurement, have led to increasing interes
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