Concept and chemical structure of periodically twisted molecular wires. Credit:
Osaka University.
Researchers
at Osaka University synthesized twisted molecular wires just one molecule thick
that can conduct electricity with less resistance compared with previous
devices. This work may lead to carbon-based electronic devices that require
fewer toxic materials or harsh processing methods.
Organic
conductors, which are carbon-based materials that can conduct electricity, are
an exciting new technology. Compared with conventional silicon electronics,
organic conductors can be synthesized more easily, and can even be made into
molecular wires. However, these structures suffer from reduced electrical
conductivity, which prevents them from being used in consumer devices. Now, a
team of researchers from The Institute of Scientific and Industrial Research
and the Graduate School of Engineering Science at Osaka University has
developed a new kind of molecular wire made from oligothiophene molecules with
periodic twists that can carry electric current with less resistance.
Molecular
wires are composed by several-nanometer-scale long molecules that have
alternating single and double chemical bonds. Orbitals, which are states that
electrons can occupy around an atom or molecule, can be localized or extended
in space. In this case, the pi orbitals from individual atoms overlap to form
large "islands" that electrons can hop between. Because electrons can
hop most efficiently between levels that are close in energy, fluctuations in
the polymer chain can create energy barriers. "The mobility of charges,
and thus the overall conductivity of the molecular wire, can be improved if the
charge mobility can be improved by suppressing such fluctuations," first
author Yutaka Ie says.
The
overlap of pi orbitals is very sensitive to the rotation of the molecule.
Adjacent segments of the molecule that are aligned in the same plane form one
large hopping site. By purposely adding twists to the chain, the molecule is
broken into nanometer-sized sites, but because they are close in energy, the
electrons can hop easily between them. This was accomplished by inserting a
3,3'-dihexyl-2,2'-bithiophene unit after every stretch of 6 or 8 oligothiophene
units.
The team
found that, overall, creating smaller islands that are closer in energy
maximized the conductivity. They also measured how temperature affects the
conductivity, and showed that it was indeed based on electron hopping.
"Our work is applicable to single-molecule wires, as well as organic
electronics in general," senior author Yoshikazu Tada says. This research
may lead to improvements in conductivity that will allow nanowires to become
incorporated into a wide array of electronics, such as tablets or computers.