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.

In conventional solid-state electronic devices, junctions and interfaces play a significant if not dominant role in controlling charge transport. Although the emerging field of molecular electronics often focuses on the properties of the molecule in the design and understanding of device behavior, the effects of interfaces and junctions are often of comparable importance. This article explores recent work in the study of metal–molecule–metal and semiconductor–molecule–metal junctions. Specific issues include the mixing of discrete molecular levels with the metal continuum, charge transfer between molecules and semiconductors, electron-stimulated desorption, and resonant tunneling. By acknowledging the consequences of junction/interface effects, realistic prospects and limitations can be identified for molecular electronic devices.

Understanding current flow through molecular conductors involves simulating the contact surface physics, the molecular chemistry, the device electrostatics, and the quantum kinetics of nonequilibrium transport, along with more sophisticated processes such as scattering and many-body effects.We summarize our current theoretical understanding of transport through such nanoscale devices. Our approach is based on self-consistently combining the nonequilibrium Green's function (NEGF) formulation of transport with an electronic structure calculation of the molecule.We identify the essential ingredients that go into such a simulation. While experimental data for many of the inputs required for quantitative simulation are still evolving, the general framework laid down in this treatment should still be applicable.We use these concepts to examine a few prototype molecular devices, such as wires, transistors, and resonant-tunneling diodes.

The design of future molecular electronic devices requires a firm understanding of the conduction mechanisms that determine their electrical characteristics. Progress toward this goal has been hindered by complications in controlling the exact configuration and makeup of fabricated molecular junctions, thus limiting the availability of quantitative experimental data for developing cohesive theories to model and predict molecular transport. This article summarizes recent research aimed at developing well-controlled systems for comparing molecular conduction and vibrational spectra using crossed-wire and in-wire metal–molecule–metal junctions. Systematic variations in molecular structure and metal–molecule contacts show strong quantitative agreement in device properties, while spectroscopic data provide evidence that the properties are due to the molecular junction. Further investigations using these and other molecular junction test beds will provide the needed experimental data to advance fundamental understanding of molecular transport and facilitate future molecular electronics applications.

Carbon nanotubes (CNTs) are one-dimensional nanostructures with unique properties. This article discusses why CNTs provide an ideal basis for a future carbonbased nanoelectronic technology, focusing specifically on single-carbon-nanotube fieldeffect transistors (CNT-FETs). Results of transport experiments and theoretical modeling will be used to address such issues as the nature of the switching mechanism, the role of the metal contacts, the role of the environment, the FET scaling properties, and the use of these findings to produce high-performance p-type, n-type, and ambipolar CNT-FETs and simple intra-nanotube circuits. CNTs are also direct-gap nanostructures that show promise in the field of optoelectronics. This article briefly reviews their optical behavior and presents results that show that ambipolar CNT-FETs can be used to produce electrically controlled light sources based on radiative electron–hole recombination. The reverse process—that is, the generation of photocurrents by the irradiation of single CNT—FETs—and photoconductivity spectra of individual CNTs are also demonstrated.