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Source: University Of Southern California
very small but hugely significant print, an interdisciplinary team of scientists at the
University of Southern California's Laboratory for Molecular Robotics has boldly placed
USC on the nanotechnology frontier.
Nanotechnology is the development of mechanical devices on a scale of nanometers; a
nanometer is a billionth of a meter. The USC scientists used a uniquely programmed atomic
force microscope as a robot to push gold particles 15 nanometers in size into precise
locations on a poly lysine-coated mica surface, spelling out the letters "USC."
The gold particles are about 500 times smaller than a red blood cell and are comparable in
size to some molecules.
Ari Requicha, Ph.D., professor of computer science and electrical engineering/systems and
leader of the USC team, says that the next frontier in miniaturization will be
nanoelectromechanical systems (NEMS).
"Control over the structure of matter at the atomic or molecular scale will
undoubtedly trigger a major revolution in man-made artifacts," Dr. Requicha says.
NEMS will be extremely small and fast, saving space and energy. Applications could include
cell repair, or ultrastrong materials derived from molecularly perfect prototypes, or
compact disc machines with a thousand times more capacity than current models.
Sales of currently used microelectromechanical systems (MEMS) devices such as pressure
sensors and accelerometers number in the millions, but MEMS will soon bump against the
size limits of the semiconductor technology used to fabricate them.
The USC team seeks to master the construction of NEMS by precisely positioning and
assembling molecular- sized components. The work is done at room temperature and normal
air pressure, and even in liquids, in contrast to some other projects that have used very
low temperatures and ultrahigh vacuums.
For now the scientists are using commercially available colloidal gold spheres, five to 30
nanometers in size. The balls are thinly coated with gold chloride, making them slightly
negatively charged to prevent clumping.
Requicha explains that at the nanometer level, material no longer behaves in a classical
manner. The scientists exploit the physical forces in play at the nanoscale with the
atomic force microscope (AFM).
The microscope has a sharp silicon tip and can sense the atomic forces between the tip and
the sample. Several piezoelectric actuators move the tip to and from the sample and
laterally to scan the surface with enough resolution to detect atomic-scale features.
When the tip is brought very close to the sample, say a nanometer or less, interatomic
repulsive forces keep it from penetrating. The same forces will also push a gold ball
across the mica substrate, in which case the AFM is acting as a robot.
Commercial AFMs are designed for imaging, not manipulation, and so the USC team had to
write software to direct the manipulation operation -- no minor task. With a standard AFM
that cannot act as a sensor and as a manipulator at the same time, the AFM operator would
have to fly blind using an image of the surface scanned previously. However, instrument
errors and thermal expansion and contraction cause the gold balls to drift with respect to
the AFM between the time they are scanned and the time they are to be moved.
The team has overcome these problems and now moves nanoparticles to precise locations
reliably, making increasingly elaborate two-dimensional patterns with the gold balls. The
team has begun experimenting with organic compounds called thiols, which could be used to
connect the balls into wires and more complex structures.
Requicha sees the assembly of three-dimensional structures as an important challenge and
said the invention of molecular grippers for picking up and placing nanoscale objects is
high on the research agenda.
"We can definitely do things that haven't been done," Requicha said. "But
these are small steps toward the production of useful NEMS. We still have a long way to
go. The first potential applications we are considering are nanoCDs, digital storage
devices analogous to compact discs but with very high densities. In the long run, one of
our main goals is to build nanomachines for biomedical applications."
Other members of the team are Bruce E. Koel, Ph.D., professor of chemistry and materials
science; Anupam Madhukar, Ph.D., professor of materials science and physics; Peter Will,
Ph.D., division leader at the Information Sciences Institute and research professor of
industrial and systems engineering; and postdoctoral researchers and graduate and
undergraduate students in computer science, materials science and chemistry.
The research was supported by a grant from the Z.A. Kaprielian Technology Innovation Fund.