When electrical devices are shrunk to a molecular scale, both electrical and mechanical properties of a given molecule become critical. Specific properties may be exploited, depending on the needs of the application. Here, a single molecule is attached at either end to a pair of gold electrodes, forming an electrical circuit, whose current can be measured. Credit: Arizona State Univ. |
In research appearing in Nature Nanotechnology, Nongjian “NJ”
Tao, a researcher at the Biodesign Institute at Arizona State Univ., has
demonstrated a clever way of controlling electrical conductance of a single
molecule, by exploiting the molecule’s mechanical properties.
Such control may eventually play a role in
the design of ultra-tiny electrical gadgets, created to perform myriad useful
tasks, from biological and chemical sensing to improving telecommunications and
computer memory.
Tao leads a research team used to dealing
with the challenges entailed in creating electrical devices of this size, where
quirky effects of the quantum world often dominate device behavior. As Tao
explains, one such issue is defining and controlling the electrical conductance
of a single molecule, attached to a pair of gold electrodes.
”Some molecules have unusual
electromechanical properties, which are unlike silicon-based materials. A
molecule can also recognize other molecules via specific interactions.” These
unique properties can offer tremendous functional flexibility to designers of
nanoscale devices.
In the current research, Tao examines the
electromechanical properties of single molecules sandwiched between conducting
electrodes. When a voltage is applied, a resulting flow of current can be
measured. A particular type of molecule, known as pentaphenylene, was used and its electrical conductance examined.
Atoms of a molecule (gray) are shown, with their accompanying pi orbitals (red). As the distance between electrodes is decreased, the pi orbitals can interact with the electron orbitals contained in the gold electrodes—a process known as lateral coupling. This effect increases electrical conductance through the molecule. Credit: Arizona State Univ. |
Tao’s group was able to vary the
conductance by as much as an order of magnitude, simply by changing the
orientation of the molecule with respect to the electrode surfaces.
Specifically, the molecule’s tilt angle was altered, with conductance rising as
the distance separating the electrodes decreased, and reaching a maximum when the
molecule was poised between the electrodes at 90 degrees.
The reason for the dramatic fluctuation in
conductance has to do with the so-called pi orbitals of the electrons making up
the molecules, and their interaction with electron orbitals in the attached
electrodes. As Tao notes, pi orbitals may be thought of as electron clouds,
protruding perpendicularly from either side of the plane of the molecule. When
the tilt angle of a molecule trapped between two electrodes is altered, these
pi orbitals can come in contact and blend with electron orbitals contained in
the gold electrode—a process known as lateral coupling. This lateral coupling
of orbitals has the effect of increasing conductance.
In the case of the pentaphenylene molecule, the lateral coupling effect was
pronounced, with conductance levels increasing up to 10 times as the lateral
coupling of orbitals came into greater play. In contrast, the tetraphenyl
molecule used as a control for the experiments did not exhibit lateral coupling
and conductance values remained constant, regardless of the tilt angle applied
to the molecule. Tao says that molecules can now be designed to either exploit
or minimize lateral coupling effects of orbitals, thereby permitting the
fine-tuning of conductance properties, based on an application’s specific
requirements.
A further self-check on the conductance
results was carried out using a modulation method. Here, the molecule’s
position was jiggled in three spatial directions and the conductance values
observed. Only when these rapid perturbations specifically changed the tilt
angle of the molecule relative to the electrode were conductance values
altered, indicating that lateral coupling of electron orbitals was indeed
responsible for the effect. Tao also suggests that this modulation technique
may be broadly applied as a new method for evaluating
conductance changes in molecular-scale systems.
