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

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

10.2.2.5 Molecular Electrostatic Field Computers

In 1998, bulk semiconductor technology typically required ~16,000 electrons to transfer one bit of information.¹⁹⁸¹ SET and related techniques described above promise to reduce this to ~1 electron per bit. But a "traditional" molecular electronics nanocomputer with a mean gate density of ~1 nm^-3 operating at ~10 GHz and using single electrons to transport, indicate, fetch, or represent a binary digit would move ~1 coul/sec of electrons (~1 amp at ~1 volt, or ~1 watt) through a 1 micron³ CPU, producing an untenable ~10¹⁸ watt/m³ power density, compared to ~10¹² watts/m³ for the nanomechanical CPU described in Section 10.2.1.

In order to reclaim this factor of 10⁶, Tour and Seminario^1879,1981 conclude that a molecular CPU must operate by electrostatic interactions rather than by electron currents, using intermolecular "talk" to efficiently transfer a bit using only ~10^-6 electron.¹⁵¹⁷ This charge-density approach is already well-known in biology, where enzyme-ligand recognition is mediated by hydrogen bonding or van der Waals interactions, working through the electric fields of the molecules even though no formal charge transfer occurs. Similarly, in an electrostatic nanocomputer the input signal is initiated by bringing up a charge to one end of the molecular device, thus reshaping the local charge density. That molecular device, in turn, perturbs the electrostatic field of an adjacent molecular device, rearranging its electron density in <10^-15 sec,¹⁹⁸¹ and so the electrostatic field perturbation is propagated without charge transfer: "These movements of electrons (wave-particle entities) in the molecular orbitals (or circuits) are performed without any dissipation of energy (stationary states)."¹⁹⁸¹ Perturbation pulses may have an energy magnitude similar to those of van der Waals interactions (Section 3.5.1). The hypothetical electrostatic molecular logic device shown in Figure 10.10 serves as an OR gate if the output is active (as a potential) or as a NOR gate if the output is passive (as an impedance) if positive logic is used, or an AND or NAND gate if negative logic is used.¹⁸⁷⁹

Gates at input/output interfaces must still be able to receive or to drive signals from standard electronic circuits, but the vast majority of molecular gates would lie within the molecular CPU and could process information by controllably affecting the electrostatic potential of neighboring molecules. Such devices would likely shift the hardware/software equilibrium towards hardware, with massively wired-logic supplanting programmed-logic computing, and with large molecular arrays allowing computation to take place inside the CPU with minimal need for main or auxiliary memories.¹⁹⁸¹ Tour expects the first laboratory demonstration of electrostatic field information transfer by 2001-2003.¹⁵¹⁷ J.C. Ellenbogen [personal communication, 1998] notes that a charge density nanocomputer achieves low dissipation by operating very close to equilibrium at all times, but notes that clock speeds of at least 1-10 KHz should still be obtainable with this approach; Tour [personal communication, 1999] claims that 1-10 THz switching is possible. Current-free polarization gates have also been proposed³²¹⁶ and fabricated.³²¹⁷

Last updated on 24 February 2003