Although most molecular electronics schemes focus on charge transfer through a molecule, charge transfer within a molecule offers an alternative physical basis for computation. A purely Coulombic mechanism for information transmission and processing has been extensively studied in theoretical work on quantum-dot cellular automata (QCA). This work envisions arrays of cells built from quantum dots or (on a molecular scale) from individual redox centers, in which charges move within the cells in response to external electric fields. According to this scheme, there is no need to let charges flow through the cells, computation is a ground-state phenomenon and contacts need only be made to some cells at the edges of the array (minimal interconnect).
Theoretical work on QCA has been experimentally verified in nearly a dozen devices in which the quantum dots are lithographically fabricated and joined by tunneling junctions. Among the devices that have been successfully demonstrated are QCA cells, wires, and logic elements (AND/OR gates), clocked QCA cells, a QCA memory cell, and a shift register. Power gain of two was measured for the last device.
Recent efforts in QCA have focused on implementing QCA at the molecular size scale, in which case each "quantum dot" would consist of a single redox center and the "tunneling junction" would be an organic or inorganic bridging group. Many mixed-valence molecules would be suitable as QCA cells, and ought to function well at room temperature. Recent ab initio calculations  show that the Coulombic forces between adjacent molecules are strong enough to transmit signals for QCA operation. This talk will introduce the challenges of implementing cellular automata at the molecular level, and will highlight developments in molecular QCA in the areas of measurement, patterning, and architecture.
 C.S. Lent and P.D. Tougaw, A device architecture for computing with quantum dots, Proc. of the IEEE 85, 541-557 (1997).
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