Molecular Film on Liquid Mercury
Reveals New Properties
UPTON, NY -- A team of scientists from the U.S. Department of
Energy's Brookhaven National Laboratory, Harvard University, and
Bar-Ilan University in Israel have grown ultrathin films made of
organic molecules on the surface of liquid mercury. The results,
reported in the November 15, 2002, issue of Science, reveal a
series of new molecular structures that could lead to novel
applications in nanotechnology, which involves manipulating materials
at the atomic scale.
Growing molecular films on liquid surfaces is part of an ongoing
activity by Brookhaven scientists to create nanomaterials, which are a
few billionths of a meter in thickness. Ultrathin films are becoming
increasingly important for fast-developing applications, such as
faster and smaller electronic and magnetic devices, advanced
biotechnological membranes, and controlled drug release in the human
body. The Brookhaven team is a leader in the field of liquid
surface-supported film growth, with expertise gained over the past 20
years.
"When
you grow a film on a solid surface, the molecules of the film tend to
interlock with those of the underlying support," says Benjamin
Ocko, the Brookhaven physicist who participated in the study.
"But an underlying liquid surface is not ordered and provides an
ideal setting for studying ultrathin states of matter without the
complications of the solid support."
Ocko and his colleagues first filled a small tray with liquid
mercury and then deposited on the surface a nanometer-thin film of
stearic acid, an organic waxlike material that is a common component
of cell membranes. Since stearic acid is not soluble in mercury, it
floats on the surface.
To see how the molecules of the film organize on the surface, the
scientists measured how x-rays produced by the National
Synchrotron Light Source at Brookhaven scattered off the ultrathin
molecular film. Key to the study was a unique instrument used for
tilting the x-rays downward onto the liquid mercury surface, which was
developed by Peter Pershan, a physicist at Harvard and one of the
study’s authors, along with the Brookhaven team.
The scientists discovered that, as the number of molecules
deposited on the surface increased, they formed four distinct
patterns. "First, when a few molecules are deposited, they tend
to take as much space as they can, by lying on the surface,"
explains Henning Kraack, a physics Ph.D. student from Bar-Ilan and the
study's lead author. "When more molecules are added, a second
layer of molecules lies on top of the first one.
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This schematic drawing shows how the
stearic molecules of the film rearrange as they are added
onto the surface of the liquid mercury support.
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"Then, as even more molecules are
deposited," Kraack
continues, "they 'stand up' to leave more space to neighboring
molecules, allowing them to densely pack in one layer. But even then,
before standing up straight, the molecules are first tilted to the
side, and stand up completely only when they are 'squeezed' by other
molecules that 'elbow their way through.'"
These observations came as a surprise, since previous studies have
shown that, when stearic molecules are deposited on water -- the only
other liquid support studied so far -- they only stand up on the
surface. "Patterns in which molecules lie flat on a liquid
surface have never been observed before," Kraack says.
Moshe Deutsch, a physicist at Bar-Ilan and one of the authors of
the study, notes that because the liquid mercury does not seem to
influence too much the way the stearic molecules assemble, "growing
films on a liquid surface is like growing them without support at
all." It might be possible to choose a film pattern, he adds,
simply by selecting the appropriate molecular coverage.
"This work shows that without an underlying
lattice, we can
control film growth," Deutsch says. "By growing other
molecules on a liquid support, we will be able to control the size and
properties of other films, and thus tailor them for different
applications, in particular their use in nanoelectronics and
nanosensor technology."
This work was funded by the U.S. Department of
Energy, which
supports basic research in a variety of scientific fields, the
National Science Foundation, and the U.S.-Israel Binational Science
Foundation in Jerusalem, Israel.
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