Heavier hydrogen on the atomic scale reduces friction
ARGONNE, Ill. (Nov. 2, 2007) — Scientists may be one step closer to understanding
the atomic forces that cause friction, thanks to a recently published study
by researchers from the University
of Pennsylvania, the University
of Houston and the
U.S. Department of Energy's Argonne National Laboratory.
The research, led by Robert Carpick of the University of Pennsylvania, found
a significant difference in friction exhibited by diamond surfaces that had
been coated with different isotopes of hydrogen and then rubbed against a small
carbon-coated tip.
Scientists lack a comprehensive model of friction on the nanoscale and only
generally grasp its atomic-level causes, which range from local chemical reactions
to electronic interactions to phononic, or vibrational, resonances.
To investigate
the latter, Argonne scientist Anirudha Sumant and his colleagues used single-crystal
diamond surfaces coated with layers of either atomic hydrogen or deuterium,
a hydrogen atom with an extra neutron. The deuterium-terminated diamonds had
lower friction forces because of their lower vibrational frequencies, an observation
that Sumant attributed to that isotope's larger mass. They have also observed
same trend on a silicon substrate, which is structurally similar to that of
diamond.
Previous attempts to make hydrogen-terminated diamond surfaces relied on the
use of plasmas, which tended to etch the material.
"When you're looking at such a small isotopic effect, an objectively
tiny change in the mass, you have to be absolutely sure that there are no other
complicating effects caused by chemical or electronic interferences or by small
topographic variations," Sumant said. "The nanoscale roughening of
the diamond surface from the ion bombardment during the hydrogen or deuterium
termination process, even though it was at very low level, remained one of
our principal concerns."
Sumant and his collaborators had looked at a number of other ways to try to
avoid etching, even going to such lengths as to soak the films in olive oil
before applying the hydrogen layers. However, no method had provided a smooth,
defect-free hydrogen layer with good coverage that would avoid generating background
noise, he said.
However, while performing work at the University
of Wisconsin-Madison, Sumant
developed a system for depositing diamond thin films. The technique, called
hot filament chemical vapor deposition, involves the heating of a tungsten
filament (like those found in incandescent light bulbs) to over 2000 degrees
Celsius.
If the diamond film is exposed to a flow of molecular hydrogen while
sitting within a centimeter of the hot filament, the heat will cause the
molecular hydrogen to break down into atomic hydrogen, which will react with
the film's surface to create a perfectly smooth layer. Since this method does
not require the use of plasma, there is no danger of ion-induced etching.
"We've proved that this is a gentler method of terminating a diamond
surface," Sumant said.
Sumant said that he hopes to use the knowledge gained from the experiment
to eventually discover a way to manipulate the friction of surfaces on the
atomic level. Such a result would prove immensely valuable to the development
of nanoelectromechanical systems, or NEMS, based on diamonds, one of Sumant's
primary research interests at Argonne's Center
for Nanoscale Materials.
The paper, "Nanoscale
Friction Varied by Isotopic Shifting of Surface Vibrational Frequencies," appears in the November 2 issue of Science.
The research was supported by the National
Science Foundation, an NSF Graduate
Research Fellowship, the Air
Force Office of Scientific Research and the Department
of Energy's Office of Science, Office of Basic
Energy Sciences.
About The Center for Nanoscale Materials
The Center for Nanoscale Materials at Argonne National Laboratory is a joint
partnership between the U.S. Department of Energy (DOE) and the State of Illinois, as part of DOE'S Nanoscale Science Research Center program. The CNM serves
as a user-based center, providing tools and infrastructure for nanoscience
and nanotechnology research. The CNM's mission includes supporting basic research
and the development of advanced instrumentation that will help generate new
scientific insights and create new materials with novel properties. The existence
of the CNM, with its centralized facilities, controlled environments, technical
support, and scientific staff, enabled researchers to excel and significantly
extend their reach.
Argonne National Laboratory seeks solutions to pressing national problems in science and technology.
The nation's first national laboratory, Argonne conducts leading-edge basic
and applied scientific research in virtually every scientific discipline. Argonne
researchers work closely with researchers from hundreds of companies, universities,
and federal, state and municipal agencies to help them solve their specific
problems, advance America 's scientific leadership and prepare the nation for
a better future. With employees from more than 60 nations, Argonne is managed
by UChicago
Argonne, LLC for
the U.S.
Department of Energy's Office
of Science.
By Jared Sagoff.
For more information, please
contact Steve McGregor (630/252-5580 or media@anl.gov)
at Argonne.
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