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Individual molecules have been used to create electrical
components like resistors, transistors, and diodes, that mimic the properties
of familiar semiconductors. But according to Nongjian (NJ) Tao, a researcher at
the Biodesign Institute at Arizona
State University,
unique properties inherent in single molecules may also allow clever designers
to produce novel devices whose behavior falls outside the performance observed
in conventional electronics.
In research appearing in of Nature Nanotechnology,
Tao describes a method for mechanically controlling the geometry of a single
molecule, situated in a junction between a pair of gold electrodes that form a
simple circuit. The manipulations produced over tenfold increase in
conductivity.
The unusual, often non-intuitive characteristics of single
molecules may eventually be introduced into a broad range of microelectronics,
suitable for applications including biological and chemical sensing; electronic
and mechanical devices.
Delicate molecular manipulations requiring patience and
finesse are routine for Tao, whose research at Biodesign’s Center for
Bioelectronics and Biosensors has included work on molecular diodes, graphene
behavior, and molecular imaging techniques. Nevertheless, he was surprised at
the outcome described in the current paper: “If you have a molecule
attached to electrodes, it can stretch like a rubber band,” he says.
“If it gets longer, most people tend to think that the conductivity will
decrease. A longer wire is less conductive than a shorter wire.”
Indeed, diminishing conductivity through a molecule is
commonly observed when the distance between the electrodes attached to its
surface is increased and the molecule becomes elongated. But according to Tao,
if you stretch the molecule enough, something unexpected happens: the
conductance goes up—by a huge amount. “We see at least 10 times greater
conductivity, simply by pulling the molecule.”
As Tao explains, the intriguing result is a byproduct of the
laws of quantum mechanics, which dictate the behavior of matter at the tiniest
scales: “The conductivity of a single molecule is not simply inversely
proportional to length. It depends on the energy level alignment.”
In the metal leads of the electrodes, electrons can move
about freely but when they come to an interface—in this case, a molecule that
sits in the junction between electrodes—they have to overcome an energy
barrier. The height of this energy barrier is critical to how readily electrons
can pass through the molecule. By applying a mechanical force to the molecule,
the barrier is lowered, improving conductance.
“Theoretically, people have thought of this as a
possibility, but this is a demonstration that it really happens,” Tao
says. “If you stretch the molecule and geometrically increase the length,
it energetically lowers the barrier so electrons can easily go through. If you
think in optical terms, it becomes more transparent to electrons.”
The reason for this has to do with a property known as
force-induced resonant tunneling. This occurs when the molecular energy moves
closer to the Fermi level of the electrodes—that is, toward the region of optimal
conductance. Thus, as the molecule is stretched, it causes a decrease in the
tunneling energy barrier.
For the experiments, Tao’s group used 1,4′-Benzenedithiol,
the most widely studied entity for molecular electronics. Further experiments
demonstrated that the transport of electrons through the molecule underwent a
corresponding decrease as the distance between the electrodes was reduced,
causing the molecule’s geometry to shift from a stretched condition to a
relaxed or squeezed state. “We have to do this thousands of times to be
sure the effect is robust and reproducible.”
In addition to the discovery’s practical importance, the new
data show close agreement with theoretical models of molecular conductance,
which had often been at variance with experimental values, by orders of
magnitude.
Tao stresses that single molecules are compelling candidates
for a new types of electronic devices, precisely because they can exhibit very
different properties from those observed in conventional semiconductors.
Microelectromechanical systems or MEMS are just one domain
where the versatile properties of single molecules are likely to make their
mark. These diminutive creations represent a $40 billion a year industry and
include such innovations as optical switches, gyroscopes for cars, lab-on-chip
biomedical applications and microelectronics for mobile devices.
“In the future, when people design devices using molecules,
they will have a new toolbox they can use.”
SOURCE – Arizona State University
