D-sigma-A unimolecular rectifiers

Unimolecular devices can be defined as devices which manipulate the energy levels or the conformations of either a single molecule or a very small cluster of molecules. When made practical, these devices, the ''advanced guard'' of molecular electronics, should leapfrog conventional inorganic microelectronics, which should be limited to ''design rules'' of components no closer than 10 nm. Fast organic rectifiers and transistors should be addressable by scanning tunneling microscopy. The quest for the proposed Aviram-Ratner D-sigma-A unimolecular rectifier is reviewed, where D is a good one-electron donor, sigma is a covalent, saturated ''sigma'' bridge and A is a good one-electron acceptor. Under gentle bias, D-sigma-A should more easily form the zwitterion D+-sigma-A(-) than the zwitterion D--sigma-A(+) (by fast intramolecular downhill tunneling, the state D+-sigma-A(-) reverts quickly to D-sigma-A). Many D-sigma-A molecules form Langmuir-Blodgett films. Sambles' group has observed Schottky barrier rectification in D-sigma-A molecules and in D+-pi-A(-) zwitterions; the groups of Kuhn, Sugi and Roth and von Klitzing have observed rectification by Langmuir-Blodgett multilayers. Intramolecular electron tunneling through a single D-sigma-A molecule has not yet been confirmed.