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Fig. 3.8 Experimental
model of a molecular
rectifier
coordination (donor-acceptor) bond. In this complex a pair of electrons of the
nitrogen atom essentially fills the free orbital of the boron atom.
In the Aviram and Ratner model acceptor and donor groups are separated by a
hydrocarbon fragment that does not conduct electrons, and the whole system is
placed between two electrodes.
Let us investigate what causes unidirectional flow of electrons in the Aviram and
Ratner model. Initially it is assumed that in the absence of the electric field, the
highest filled level of the acceptor is located above the Fermi level of the nearest
electrode, while the lowest filled level of the donor below the Fermi level of another
electrode. Suppose that voltage is applied to the electrodes in the direction
acceptor-donor. If the voltage at the cathode is sufficiently large, the upper level
of the acceptor is lowered, facilitating electron capture by the acceptor from the
cathode. Likewise, a possibility arises for the donor to transfer the electron to the
anode. If the structure of the barrier between the acceptor and donor is such that
tunneling through the barrier between them is possible, the electrons will move
from the cathode to the anode. The situation changes radically if the voltage
between the electrodes is applied in the reverse direction. The arrangement of
levels then becomes such that the probability of electron transition through the
molecule becomes much smaller than in the previous case.
The work of Aviram and Ratner naturally stimulated experimentalist to test the
proposed model. In 1997, 23 years after the work of Aviram and Ratner was
published, an American physicist Metzger with his team discovered the effect of
electrical current rectification by a molecular film consisting of chemical com-
pounds of the donor-acceptor type. The structure of the molecule and the experi-
mental scheme are shown in Fig.
3.8
. The peculiarity of this work is that the object
of the study was not the conductivity of a single molecule, but rather of their
