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This hot filament chemical vapor deposition (HFCVD)
system is used for the hydrogen and deuterium termination of
diamond surfaces.
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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," appeared 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.
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