challenge, however, in developing molecular electronics
is to better understand how electricity is conducted
between molecules and silicon contacts connecting
various devices in a circuit, said Geng-Chiau Liang,
a postdoctoral research assistant in Purdue's School
of Electrical and Computer Engineering.
Researchers will be able to use the new simulation tool to see precisely how
electrical conductivity changes depending on how molecules are connected to
silicon, information that is critical to properly design the devices.
"I believe we might be one of the first theorists who have created a tool to
show how electricity is conducted between molecules and silicon at the atomic
level," said Avik Ghosh, a research scientist in electrical and computer engineering
who worked with Liang.
Details about the simulation tool are appearing in the current issue (Aug.
12, Volume 95, Issue 7) of the journal Physical Review Letters. The paper was
written by Liang and Ghosh. The research has been funded through two national
centers based at Purdue's Discovery Park, the university's hub for interdisciplinary
Scientists and engineers are working to develop techniques for creating future
computers, sensors and other devices that use molecules, such as proteins and
DNA, instead of conventional electronic components. The concept may spawn new "biochips" that
will use proteins in sensors for detecting contaminants and pollutants in the
air and water and for analyzing the blood and biological samples.
"The idea is that molecules might be able to complement or supplement silicon," Ghosh
said. "All traditional research in molecular electronics has focused on combining
molecules with metal contacts, but we've been studying the interaction of molecules
and silicon instead of metals because the computer industry is built on semiconductors,
which is silicon."
Liang and Ghosh used the tool to show how current flows between silicon atoms
and molecules called buckminsterfullerenes, or "buckyballs."
Named after architect R. Buckminster Fuller, who designed the geodesic dome,
buckyballs are soccer-ball-shaped molecules containing 60 carbon atoms. A buckyball
has a width of about 1 nanometer, or one-billionth of a meter, which is roughly
10 atoms wide.
"This paper is a proof of concept showing that our theory is at a point that
we can actually look at experiments and explain them quantitatively," Liang said. "We
have shown how the conductance of electricity changes when you change the type
of bond connecting buckyballs to silicon."
The researchers used their computational model to predict how electricity flows
when buckyballs and silicon are connected in three ways. In one case, there
is no chemical bond - the buckyball is simply sitting on top of the silicon.
In another case, the molecule has been connected to silicon by annealing, or
heating, the silicon. And in the third case, the buckyball is resting inside
of a tiny pit, a natural defect existing in the silicon.
The model precisely plotted how conduction and voltage changed in the three
types of connections, and those predictions agreed with experimental data from
other researchers who measured the actual changes in current flow in laboratories.
"Because our predictions agreed with actual experimental data, we know they are
accurate," Ghosh said. "This means you can use the model to give theoretical
guidance to experiments instead of using strictly a trial-and-error approach."
Together with Supriyo Datta, the Thomas Duncan Distinguished Professor of Electrical
and Computer Engineering at Purdue, and his students, Liang and Ghosh developed
the mathematical theory on which the model is based. The researchers used a
Purdue supercomputer to develop and test the simulation.
The Purdue engineers used buckyballs in their simulation because the molecules
are well-known in the scientific community and data are readily available.
The tool, however, could be used to simulate conduction using any molecule
connected to silicon.
"Researchers want to know what kind of molecule can provide specific conduction
characteristics, and they can substitute other molecules for buckyballs," Liang
said. "What we can now do is theoretically explain the experiments in quantitative
detail, which is really important for any technology.
"To do this, you must have an atomistic understanding of current flow - basically,
how does electricity conduct at the atomic level."
The research is ongoing and has been supported by the Network for Computational
Nanotechnology, funded by the National Science Foundation and directed by Mark
Lundstrom, Purdue's Scifres Distinguished Professor of Electrical and Computer
Engineering; the Semiconductor Research Corporation; the Defense University
Research Initiative on Nanotechnology, which is supported by the U.S. Army
Research Office; and the Defense Advanced Research Projects Agency.
Writer: Emil Venere, (765) 494-4709, firstname.lastname@example.org
Sources: Geng-Chiau Liang, (765) 494-9050, email@example.com
Avik Ghosh, (765) 494-9023, (Cell Phone) (765) 532-0538, firstname.lastname@example.org
(After Aug. 25, use email@example.com)
Note to Journalists: An electronic copy of the research paper is available from