COMPUTING WITH QUANTUMDOT CELLULAR AUTOMATA - Bio-Inspired and Nanoscale Integrated Computing - page 144

electron forming the neutral radical can be transferred from one end group to
the other without significant change to the molecular geometry. The sigma bridge
separating the allyl groups provides a potential barrier and enables the measur-
able bistability of the molecule. The application of an electric field will cause the
molecule to exhibit a change in the electronic configuration from donor-bridge-
acceptor to acceptor-bridge-donor. The change in configuration can result from
either electron or hole transfer in the molecule.
In order to construct a molecular implementation of a QCA cell, a molecule
that exhibits bistability and charge localization is required. Lent et al. [33]
analyzed the performance of the Aviram molecule (1,4-diallyl butane radical
cation shown in Fig. 4.36) as a test bed for molecular QCA. Ab initio calculations
were performed with Gaussian 98 and the unrestricted Hartree-Fock theory
(UHF) and the Slater Type Orbitals simulated by the 3 Gaussians (STO-3G) basis
set. Figure 4.36 shows the highest occupied molecular orbitals (HOMOs) and the
isopotential surfaces for two neighboring molecules. Lent et al. also demonstrated
that these molecules could be used to implement majority gates and wires.
Although the molecule was found to exhibit nonlinear bistability, charge localiza-
tion, and molecule-molecule interactions necessary for information transmission,
other necessary features such as a mechanism for I/O and a functional group for
attachment to a substrate were not designed into this molecule.
Molecular QCA was also shown to exhibit a second level of nonlinearity as a
result of the coupling between electronic and nuclear degrees of freedom. This
Figure 4.36. The HOMO levels (a) and (c) and isopotential sufraces (b) and (d) for
the two stable states of a pair of QCA molecules studied in [33]. Reprinted with
permission from [33]. (r 2003 American Chemical Society)