© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

10.2.2.3 Field-Controlled Molecular Switching Devices

A second category of molecular electronics device that appears promising for the development of molecular nanoelectronic digital computers is the electric-field controlled molecular switching devices, including molecular quantum-effect devices. These are most closely descended from the solid-state microelectronics and nanoelectronic devices described earlier, and promise to be the fastest and most densely integrated among all of the current alternatives. A group at Purdue University has used self-assembly to fabricate and demonstrate functioning arrays of molecular electronic quantum confinement structures connected by molecular wires.^1811,1812 As another example, J.M. Tour proposes embedding a quantum well in a molecular wire by inserting pairs of barrier groups that break the sequence of conjugated porbitals (Figure 10.9), forming a two-terminal molecular RTD;¹⁷⁴⁷ other structures for three-terminal molecules have been suggested.¹⁸²⁵ (In February 1999, Tour reported the demonstration of RTD effects in the manner proposed above; at press time, the paper was still in review [J.M. Tour, personal communication, 1999].) Nondissipative (adiabatic) Thouless electron pumping has also been described at 0.33 K.³¹⁸⁹

Merkle and Drexler¹⁰⁹⁷ consider a hypothetical "helical logic" device in which a single electron can switch another single electron, with 1 or 0 represented as the presence or absence of individual electrons. The electrons are constrained to move along helical paths, driven by a rotating immersive electric field (normal to the helical axis) that confines each charge carrier to a fraction of a turn of a single helical loop, moving it like water in an Archimedean screw. A logic operation involves two helices, one of which splits into two "descendant" helices. At the point of divergence, differences in the electrostatic potential resulting from the presence or absence of a carrier in the adjacent helix control the direction taken by a carrier in the splitting helix; the sequence is reversible, allowing two initially distinct helical paths to merge into a single outgoing helical path without forcing a dissipative transition. Energy dissipation at ~10 GHz is ~10^-6 zJ/gate-cycle at an operating temperature of 1 K.

Last updated on 23 February 2003