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Fig. 3.11 Experimental model of a molecular switch
In order to test his ideas experimentally Aviram made two attempts to create
molecular switches. In particular, Aviram, Seiden, and Ratner used chemical
reactions to bind semiquinone molecules to the surface of metallic aluminum. In
semiquinones hydrogen atoms can be in one of two positions and can move from
one position to another (Fig.
3.11
). It was shown that they are situated perpendicular
to the surface and that their absorption spectra differ at different positions of
hydrogen atoms relative to the skeleton of the molecule (Fig.
3.11
). This allowed
for integral (i.e., for the entire set of molecules on the surface) observation of the
transitions of these atoms under the influence of an electric field. Later Aviram,
Joachim, and Pomerantz synthesized a similar system and demonstrated the feasi-
bility of controlling molecular states using a scanning tunneling microscope.
The development of molecular electronics in the 1980s was strongly influenced
by the theoretical studies of Forrest Carter who treated in detail various molecular
mechanisms that could be used for creating molecular electronic devices.
Forrest Carter's two main areas of research were:
The use of the tunnel mechanism of electron conductivity in a system with
consecutive potential barriers and the control of this mechanism by shifting the
levels in one of the potential wells between the two barriers (the concept of
control groups)
The use of the soliton mechanism (see below) of signal transmission to change
the electronic structure of molecular systems and thereby to transfer molecules
from one stable state to another (the concept of switching molecules)
Electronic conductivity of extended molecular systems, i.e., the process of
electron transfer from an electron-donating to an electron-accepting group along
the chain of atoms, attracted the attention of many researchers in the postwar years.
Of particular interest are conjugated (polyene, polyacene, etc.) systems in which
electrons form extended molecular orbitals upon overlap. These molecules repre-
sent quasi one-dimensional systems whose properties are substantially different
from the conventional three-dimensional systems.
Without going into details of the theory of electronic conductivity of such
molecules, let us only consider the mechanism of electron passage through a
