The research team of Leonhard Grill at Freie
Universität Berlin - in collaboration with the synthetic chemistry
group of Stefan Hecht from Humboldt University of Berlin and the theoretical
physics group of Christian Joachim of the CEMES-CNRS institute in Toulouse -
has succeeded in lifting single polymers from a gold surface, similar to chains,
and in measuring their electrical and mechanical properties during this process.
The scientists place one end of a polymer strand in contact with a metallic
tip, thereby inducing an electrical current through single molecular wires over
extraordinarily long distances during the pulling process. The results were
published in the most recent issue of Science.
A central vision of nanotechnology lies in the construction of electronic circuits
on the nanometer scale (1nm = 1 millionth millimeter). The development of such
fascinating devices, which would revolutionize many applications, requires molecular
"cables" and a detailed understanding of electrical transport through
such small wires. Thus, it is necessary to determine the electrical current
through a single molecular wire, contacted to two electrodes, as a function
of its length. Up to date, only relatively short wires with a fixed length have
been investigated, and most of the studies were based on statistical measurements,
making the exact characterization of a single wire impossible.
The molecular wires were constructed by connecting single molecules on a gold
surface to a polymer chain. After one end of the chain was contacted with the
tip, the other end remains on the metal surface and the distance between the
two electrodes (tip and surface) is varied continuously during the pulling of
the polymer. Using this method, it was possible for the first time to measure
the charge transfer through a single polymer for different lengths of up to
more than 20 nanometers. These experiments provide insight into the electrical
properties and also into the mechanical characteristics of single polymers,
which behave like macroscopic chains as one chain unit after another is detached
from the surface during the pulling process.
The electrical transport on the level of single molecular wires is of great
importance for any electronic application in molecular nanotechnology. In the
reported experiments it was possible for the first time to characterize the
dependence of the electrical conductance on the length of the molecular wire
and its mechanical properties. In the future, using this method, it should be
possible to optimize molecular wires with respect to their suitability for applications.
Posted February 26th, 2009
