Very tiny wires, called nanowires, made from such metals as silver and gold,
may play a crucial role as electrical or mechanical switches in the development
of future-generation ultrasmall nanodevices.
Making nanodevices work will require a deep understanding of how these and
other nanostructures can be engineered and fabricated as well as their resultant
strengths and weaknesses. How mechanical properties change at the nanoscale
is of fundamental interest and may have implications for a variety of nanostructures
and nanodevices.
A major limiting factor to this understanding has been that experiments to
test how nanowires deform are many times slower than computer simulations can
go, resulting in more uncertainty in the simulation predictions than scientists
would like.
"Molecular dynamics simulations have been around for a long time,"
said Arthur Voter of the Theoretical Division at Los
Alamos National Laboratory. "But the simulations have never before
been able to mimic the atomistic tensile strength of nanowires at time scales
that even come close to experimental reality."
Using the "parallel-replica dynamics" method for reaching long time
scales that Voter developed, members of Voter's team adapted their computer
code to exploit the Roadrunner supercomputer's hybrid architecture, allowing
them to perform the first-ever simulation of a stretching silver nanowire over
a period of a millisecond, or one-thousandth of a second, a time that approaches
what can be tested experimentally.
"Bigger supercomputers have made it possible to perform simulations on
larger and larger systems, but they have not helped much with reaching longer
times -- the best we can do is still about a millionth of a second. However,
with the parallel-replica algorithm, we can utilize the large number of processors
to 'parallelize' time," said Voter. "Roadrunner is ideally suited
to this algorithm, so now we can do simulations thousands of times longer than
this."
With this new tool, scientists can better study what nanowires do under stress.
"At longer time scales we see interesting effects. When the wires are stretched
more slowly, their behavior changes -- the deformation and failure mechanisms
are very different than what we've seen at shorter time scales," said Voter.
Through these simulations, Voter and his team are developing a better understanding
of how materials behave when they are reduced to the size scale of a nanometer,
or one-billionth of a meter. "At this scale, the motion of just one single
atom can change the material's mechanical or electrical properties," said
Voter, "so it is really helpful to have a tool that can give us full atomic
resolution on realistic time scales, almost as if we are watching every atom
as the experiment proceeds."
Posted October 29th, 2009
