Molecular Electronics: Challenges and Opportunities

Integrating individual molecules into electrical circuits, often termed as " Molecular Electronics " , is currently a focus research area because it can not only meet the increasing technical demands of the miniaturization of traditional Si-based electronic devices, but also provide an ideal window of exploring the intrinsic properties of materials at the molecular level. Indeed, over the past decade we witnessed the significant progresses achieved in both experiments and theory for the purpose of constructing, measuring, and understanding the electronic and photonic responses of these conceptually simple molecular junctions in which molecular systems play an important role as pivotal elements [1,2]. These advances have attracted researchers from a variety of backgrounds and begun to move beyond simple descriptions of charge transport and branch out in different directions, reflecting the interdisciplinary realm. From the basic scientific point of view, the major question in the field of molecular electronics is " how do electrons move through molecules ". Understanding electron movements through a single molecule is central to the field, but presents a significant experimental and theoretical challenge. The first theoretical work in the history of molecular electronics was published in 1974 by Aviram and Ratner [3]. This important work discussed transport through a single molecule, suggesting that a single molecule when sandwiched between two electrodes could act as a device (such as a molecular rectifier). However, the most important problem was how to attach electrodes to a molecule. After over two decades, the first significant report attempting to measure single-molecule transport came from both Mark Reed's group and James Tour's group [4]. Their collaborative works fostered better understanding of how such measurements could be done and provided insights about charge transport properties of individual molecules. This was regarded as the true beginning of molecular electronics [5]. The success of these early demonstrations ignited broad interest worldwide, and led to an explosion of research in the field. So far, discrete approaches have been developed for molecular junction fabrication, including break junctions, scanning probe techniques, sandwich