Understanding a Bisferrocene Molecular QCA Wire - Field-Coupled Nanocomputing: Paradigms, Progress, and Perspectives - page 311

Differently from the previous molecules, this molecule implements a complete
QCA cell.

1.3 The Bisferrocene Molecule

In the continuous research for molecules suitable to be employed in QCA com-
puting, recently a new candidate molecule has been proposed and studied: the
bis-ferrocene molecule [ 18 - 23 ]. This molecule, shown in Fig. 9 is constituted by
two ferrocenes, functioning as redox centers, and a carbazole bridge which acts
as third dot for the NULL state. An alkyl chain is also present, which guar-
antees the attachment to a gold surface. The bis-ferrocene molecule is capable
of realizing a QCA half-cell, thus two molecules aligned are necessary for an
entire cell.

Ferrocene Ferrocene

Carbozole

Thiol

Fig. 9. The bis-ferrocene molecule: (A) structure and (B) scheme.

Its features have been demonstrated through theoretical and experimen-
tal analysis [ 18 , 19 ]. Three isomers of the molecule have been synthesized: two
diastereoisomers (the (S,S) and (R,R), chiral compounds) and the meso iso-
mer. Oxidation has also been performed chemically, with the introduction of
iodine atoms (I) as counterions in the solution, and electrically, through cyclic-
voltammetry (CV).
The possibility of binding bisferrocene molecules on a gold surface has also
been demonstrated, as proved by the results of STM imaging shown in [ 19 ]. STM
characterization has revealed the presence of structures with one and two lobes
with almost the same density; further analysis has allowed to associate them
with the (R,R and S,S) stereoisomers and (R,S) meso isomer, respectively.
The capacity of the bis-ferrocene molecule to correctly respond to a change in
local electric field has been verified on the (R,S) syn isomer in its oxidized form
[ 19 ]. In particular, after computing the electron density for an isolated molecule
(the driver) in its cationic state, the electron density of a second molecule (the
driven molecule) was deduced in presence of the point-charge electrostatic field
of the first molecule. As expected, the external field seems to move the internal
positive charge on the ferrocenes generating different charge configurations [ 19 ].