Showing papers by "Craig S. Lent published in 2008"

Molecular quantum-dot cellular automata--from molecular structure to circuit dynamics

Abstract: We establish a method for exploring the dynamics of molecular quantum-dot cellular automata (QCA) devices by hierarchically combining the techniques of quantum chemistry with the nonequilibrium time-dependent coherence vector formalism. Single QCA molecules are characterized using ab initio quantum chemistry methods. We show how to construct a simple model Hamiltonian for each QCA cell based on parameters extracted from the ab initio calculation. The model Hamiltonian captures well the relevant switching behavior and can then be used to calculate the time-dependent coherence vector, including thermal and nonequilibrium behavior. This enables us to explore dynamic behavior and power dissipation for various QCA devices and circuits.

A metric for characterizing the bistability of molecular quantum-dot cellular automata.

Abstract: Much of molecular electronics involves trying to use molecules as (a) wires, (b) diodes or (c) field-effect transistors. In each case the criterion for determining good performance is well known: for wires it is conductance, for diodes it is conductance asymmetry, while for transistors it is high transconductance. Candidate molecules can be screened in terms of these criteria by calculating molecular conductivity in forward and reverse directions, and in the presence of a gating field. Hence so much theoretical work has focused on understanding molecular conductance. In contrast a molecule used as a quantum-dot cellular automata (QCA) cell conducts no current at all. The keys to QCA functionality are (a) charge localization, (b) bistable charge switching within the cell and (c) electric field coupling between one molecular cell and its neighbor. The combination of these effects can be examined using the cell–cell response function which relates the polarization of one cell to the induced polarization of a neighboring cell. The response function can be obtained by calculating the molecular electronic structure with ab initio quantum chemistry techniques. We present an analysis of molecular QCA performance that can be applied to any candidate molecule. From the full quantum chemistry, all-electron ab initio calculations we extract parameters for a reduced-state model which reproduces the cell–cell response function very well. Techniques from electron transfer theory are used to derive analytical models of the response function and can be employed on molecules too large for full ab initio treatment. A metric is derived which characterizes molecular QCA performance the way transconductance characterizes transistor performance. This metric can be assessed from absorption measurements of the electron transfer band or quantum chemistry calculations of appropriate sophistication.

Electronic quantum-dot cellular automata

Abstract: This paper presents an overview of the electronic implementation of quantum-dot cellular automata (QCA). QCA is a computing paradigm that encodes and processes information by the position of individual electrons. This opens the possibility of dense, ultra-low power devices. Resent results are presented from QCA cells implemented using metal-dots, as well as investigations toward molecular and silicon QCA devices.