Molecular Electronics: Theory and Device Prospects
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Molecular Electronics: Theory and Device Prospects
A.W. Ghosh, P.S. Damle, S. Datta, and A. Nitzan Abstract 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. Keywords: molecular electronics, nanoscale devices, transport.
One of the principal driving forces behind the semiconductor microelectronics industry has been miniaturization, motivated by a large device density per chip and high operational speeds. State-of-the-art transistors in industry are currently at the 90 nm node,1 while transistors with gate lengths of 6 nm,2 comprising just a few dozen atoms, have been demonstrated. Although this represents a technological tour de force, it will be progressively difficult to continue downscaling at this rate, as quantum tunneling, interconnect delays, gate oxide reliability, and excessive power dissipation, among other factors, start hampering the performance of such devices.3 While some of these issues can, in principle, be handled by improving device design, packaging, processing, and channel mobilities,4 the rapidly increasing cost of fabrication motivates exploration of entirely new paradigms, such as novel architectures and new channel materials. One promising direction involves replacing the “top-down” lithographic approach with a “bottom-up” synthetic chemical approach of assembling nanodevices and circuits directly from their molecular constituents. Molecules are naturally small, and their abilities of selective recognition and bindMRS BULLETIN/JUNE 2004
ing can lead to cheap fabrication using self-assembly. In addition, they offer tunability through synthetic chemistry and control of their transport properties due to their conformational flexibility. Remarkable progress in this field has been made in the last few years, as researchers have developed ways of growing, addressing, imaging, manipulating, and measuring small groups of molecules connecting metal leads. Several prototype devices such as conducting wires, insulating linkages, rectifiers, switches, and transistors have been demonstrated.5 In parallel, there has been significant theoretical activity toward developing the description of nonequilibrium transport through
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