Density functional theory calculation of the molecular structure of three benzenedithiols sandwiched between two gold surfaces. Click here for more photos.
A large supercomputer at ORNL is being used to learn more about the best ways to design
electronic device components on a very small scale.
The successor to the silicon chip may be a nanoscale device—a self-assembled
monolayer of organic molecules of benzene (a ring of six carbon atoms bonded with four
hydrogen atoms) attached to sulfur atoms at each end. Because sulfur (thiol) has an affinity
for gold, a single layer of these benzenedithiol molecules can be sandwiched between thin
gold contacts. Scientists at Rice and Yale universities have induced self-assembly of such
a device by dipping a gold surface into a beaker of benzenedithiol molecules. In large
numbers these molecules attached themselves to the gold surface.
The scientists added nitrogen-containing (nitro) groups to the molecule’s center benzene
ring. The resulting perturbed electron cloud made the asymmetric molecule twist when an
electric field was set up by applying a voltage between the gold contacts. When the
molecule twisted, current flow through the “molecular wire” was blocked. When the
voltage was removed, the molecule adopted its original shape, allowing current to flow
Such a device, if fabricated on a large scale, could be
used as an ultrafast on-off switch, a key to creating
ultrasmall, highly dense computer circuits required to
make computers fast and powerful enough to mimic the
human brain. Or the device could be used to make
superior computer memory elements. A charge can be
stored on the nitro group to prevent electrical conduction
(binary 0), or the group can have no charge, allowing
conduction (binary 1). Such a molecular memory cell
retains a stored bit for nearly 10 minutes. By comparison,
today’s silicon-dynamic, random-access memories must
be refreshed by an electrical current every 20
milliseconds. The new type of memory would save energy, allowing laptop computer
batteries to last 100 times longer.
Such a concept is being modeled computationally on ORNL’s IBM supercom-puter,
dubbed Eagle, in a Laboratory Directed Research and Development project. ORNL’s
David Dean, Bill Butler (now at the University of Alabama), Peter Cummings (an
ORNL-UT Distinguished Scientist), Predrag Krstic, David Schultz, Mike Strayer, Jack
Wells, and Xiaoguang Zhang are running the calculations using a modified version of
NWChem, a computational chemistry code.
“Using ab initio methods, we modeled the self-assembly
and electrical conductivity of five benzenethiol (BT) and
benzenedithiol (BDT) molecules on a gold surface,” says
Dean. In the example shown in the illustration, two gold
lattices are shown on the top and bottom. Three BDT
molecules are seen in the middle area. This particular
configuration has 70 atoms and 590 active electrons. A
single calculation of this type requires 46.67 hours on 80
nodes of Eagle, or 14,930 processor hours. The
single-particle wave functions resulting from this
calculation will be used in a conductance calculation to
determine the current-voltage characteristics of this
The researchers have also performed preliminary molecular simulations of
self-assembled monolayers composed of BT molecules on the  surface of gold. They
included state-of-the-art force fields generated through electronic structure calculations.
Both molecular dynamics (MD) and Monte Carlo (MC) simulations are being used. Gibbs
ensemble MC simulation is being used to establish the equilibrium between adsorbed
monolayers of BT and BDT with a low-density solution. MD is then being used to
equilibrate the structures thus found. “The structure of the adsorbed monolayers appears
to be consistent with available experimental results,” Dean says.
To produce a useful device, self-assembly must be combined with fabrication methods
such as photolithography. The ORNL scientists will model how best to assemble these
molecules and align them with the gold contacts to optimize electrical conductivity.
Device Design and Performance
Marco Buongiorno Nardelli, who holds a joint
position with ORNL and North Carolina State
University (NCSU, one of UT-Battelle’s core
universities), has been exploring the feasibility of
using carbon nanotubes in nanoscale electronic
devices. He is currently using Eagle to run his own
suite of codes simulating electron transport in carbon
nanotubes in contact with other materials.
In one project in which he provided computer
modeling, experiments at the University of North
Carolina (UNC) at Chapel Hill have shown that it is
possible to build a nano-rheostat, similar to a dimmer
light switch. In such a device, a carbon nanotube—a
cylinder resembling rolled-up chicken wire because its
carbon atoms are arranged in a hexagonal
configuration—is placed on a sheet of graphite whose
carbon atoms also have a hexagonal arrangement.
“If you place the carbon cylinder on the graphite sheet
so that the carbon atoms of both are aligned, a current
will flow at the interface,” Buongiorno Nardelli says.
“As you rotate the carbon cylinder on the graphite
sheet, changing the angle between the atoms in the
system, you get increased electrical resistance and
reduced current flow. As the atoms become aligned, you get low resistance and high
Computational simulations by Buongiorno Nardelli verified that the interface between a
carbon nanotube and graphite gives tunable resistance (as in a dimmer switch). His
theoretical predictions on the feasibility of a nano-rheostat agreed with the UNC
experimental results. The work was published in Science magazine in 2000.
If carbon nano-tubes are to be used as nanowires or
other components in nanoscale devices, electrons
must flow between these nanotubes and metal
contacts in the device. In some experimental
devices, high resistance at the tube-contact interface
can make the mechanism of electron transfer quite
inefficient. Buongiorno Nardelli and his NCSU
colleagues have used computer modeling to address
the question of why some nanodevices have better
performance than others.
“In some devices,” Buongiorno Nardelli says,
“electrons in the carbon nanotube stay in the tube
and electrons in the aluminum stay in the metal. Our
simulations suggest that contacts can be improved by
mechanical deformations. For example, if a carbon
nanotube sandwiched between two aluminum
contacts is squeezed and deformed, new bonds form
between the carbon and aluminum atoms, increasing
electron flow at the tube-contact interface.”
The strength of carbon nanotubes is also of interest to Buongiorno Nardelli. Of all
materials, carbon nanotubes have the highest tensile strength. They are 100 times stronger
than steel but have one-sixth its weight. Scientists propose using carbon nanotubes as
fibers in a polymer composite to form stronger structural materials for aircraft, spacecraft,
and suspension bridges.
Computational simulations by Buongiorno Nardelli and his colleagues have shown that
the geometry—the arrangement of the carbon hexagons along the nanotube—influences
tube strength. “Our simulations,” he says, “predicted that whether a nanotube is brittle or
ductile depends on the temperature at which it was deformed, the orientation of the
hexagons with respect to the tube’s axis, and the amount it is stretched—that is, strain.”
Carbon nanotubes are very small, but simulations of their behavior in nanoscale
electronic devices require a large amount of computer capacity.
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