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.

Effectiveness of Molecules for Quantum Cellular Automata as Computing Devices

“…However, according to the Lent theory [2], the best implementation of the QCA cell, in terms of performance and size, is the molecular cell, as discussed herein. The molecular implementation enables the production of uniformly sized devices [7] and it promises very low power consumption at very high frequency and ambient temperature [12].…”

“…Clarification: A similar relation has been proposed for a two-dot molecule in [7] following a probabilistic approach, using Two-State Approximation (TSA). The polarization of the MUT is evaluated analytically approximating the molecule as a two-state quantum system.…”

“…The proposed VVT can be linked to the methodology adopted in [4,7] enhancing the characterization of the molecule with a more geometry-independent version. The polarization can be linked to the input and output voltages using Equation (3).…”

“…Hence, the VVT can be obtained by solving Equation (3) for P 1 and P 2 and substituting the two results in the characteristics given in [7]. The parameters γ and β needs to be introduced to use the TSA theory, in particular, γ is the tunneling energy associated the two quantum states whereas β describes the bistable nature of the molecule [7].…”

“…Some analyses have been performed by Lent and coworkers to match the molecular device with the methodology used for standard QCA cells [4] by approximating a molecule as a two-state system [7], without considering the NULL dot, or a three-state system [8,9] also considering the presence of the third dot. These approximations are based on the principles of quantum mechanics and provide a good description of molecules for QCA paradigm.…”