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experiment the input is latched into the central cell, which then acts as an input to the
top and bottom cells. When a clock is applied to the top and bottom cells the bit is
copied into both cells, and full signal strength is produced in these cells [
51
], con-
firming the operation of the circuit.
3.3
Molecular QCA
Molecules represent the smallest artificial structures that can be engineered by humans.
To form switchable QCA molecules, at least two charge centers are required that can
be reversibly occupied or unoccupied by an electron. The field of mixed-valence
chemistry [
52
] concerns itself with molecules that have at least two charge centers
connected by a bridging group through which tunneling can occur. Ongoing investi-
gation concerns the questions of what makes a good dot and what makes a good bridge.
Several early theoretical investigations used model electronic p-systems as dots
[
53
-
61
]. These molecules are often radical ions containing unpaired electrons and
would be very reactive and likely unstable in real systems. Their use was to establish the
fundamentals. Electrons in molecules can exhibit bistable switching and the perturba-
tion due to a similar molecule at a reasonable distance (such that the dots form a square)
is sufficient to switch the molecule. Energy levels are such that these effects survive
room temperature operation. Kink energies are large enough that molecular QCA is
robust against variations in position and orientation of molecules. Groups surrounding
the charge centers can effectively insulate them from conducting substrates but do not
screen the field. Applied electric fields which vary at a much larger length scale can
effectively clock molecules (with three appropriately arranged charge centers).
Molecular synthesis by the Fehlner [
62
-
66
] and Lapinte [
67
-
69
] groups have
succeeded in creating molecules that show the requisite bistability. These dots use Fe
and/or Ru charge centers. Electronic measurements of the Fehlner molecules attached
to a surface showed distinctive bistable behavior as the electron was switched by an
applied electric field. This demonstrated both the bistable character of the molecules,
and the potential for clocked control of the charge configuration by an applied (and
inhomogeneous) field. The Lapinte molecules have been imaged with STM by the
Kandel group [
67
-
70
] and show the desired charge localization on one end of a
symmetric double-dot molecule. Triple-dot molecules, of the sort required for clocked
control, have also been made and imaged. More recently ferrocene-base double dot
molecules have been made by the Henderson group and imaged by the Kandel group.
There is much chemistry yet to be understood in designing QCA molecules. One
issue is what makes the ideal dot. Transition metal atoms have the advantage of using
d-orbitals that participate less in bonding and so may be more isolated. Carbon-base
p-systems, on the other hand, can be chosen such that they involve anti-bonding
orbitals and may spread the charge out more and therefore yield a lower reorgani-
zation energy. The reorganization energy is the energy associated with the relaxation
of the surrounding atoms and may in some cases trap the charge and inhibit switching.
Creating appropriate bridging groups involves choosing a system that is either long
enough or opaque enough to be an effective barrier to through-bond tunneling.
Conjugated systems may be too conducting.
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