Understanding a Bisferrocene Molecular QCA Wire - Field-Coupled Nanocomputing: Paradigms, Progress, and Perspectives - page 305

Fig. 1. Molecular implementation: (A) molecule sketch; (B)-(D) logic encoding and
switching in case of oxidized molecule.
A simplified scheme of an oxidized molecule is shown in Fig. 1 : an electron
is free to move along the tunneling path and localize into one of the two redox
sites available (the circles in this simplified representation). Encoding of binary
information is related to the charge configuration of the cell: for example, when
the positive charge is in the redox site on the left (the circle with internal pattern
in this simplified representation, Fig. 1 (B)), the cell is, for example, in a logic 1
state. When the positive charge is located on the right site (patterned circle on
the right, Fig. 1 (D)), then the molecule is in the 0 state. These charge configu-
rations are represented in Fig. 1 , along with the switching state corresponding
to the free charge moving along the tunneling path from one dot to the other
(Fig. 1 (C)). The intermediate symbol (stretched hexagon) represents a part of
the molecule slightly active from the electrostatic point of view, but acting like a
channel to favor charge movement and as separator to favor charge localization
in one of the two dots.
" NULL "
+e
Fig. 2. State encoding of a 3-dot molecule.
The encoding scheme chosen in the previous example is ostensibly arbitrary,
since other choices are possible. No choice can be made, instead, on the num-
ber of different possible configurations of charge within a given molecule, and
correspondingly on the number of encoded states. The motivation for having
more charge configurations in a single molecule resides in the mode of switch-
ing adopted in circuits with many QCA cells. A mode of switching consists in
a procedure with several steps which allow the QCA cells to switch their state