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NSF PR 01-105 - December 20, 2001
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Engineers Develop New Chemical Sensor Based on
Experimental Physics Breakthrough
For the first time, scientists have found evidence
of a long-suspected phenomenon, tiny electrical currents
produced when molecules interact with metal surfaces.
The discovery may usher in a new generation of chemical
detectors, and reveals details about catalytic processes
used to produce more than half of the chemicals manufactured
Investigators at the University of California, Santa
Barbara, funded by the National Science Foundation
(NSF), were searching for what they call "chemicurrent,"
- electrons excited by low-energy chemical reactions.
The team incorporated a pre-existing device called
a "Schottky" diode into a new chemical sensor, and
they describe the sensor and their findings in the
December 21st issue of Science.
Doctoral student Brian Gergen is lead author for the
findings. Says Eric McFarland, principal investigator
and the NSF grant awardee, "They (electrical phenomenon
and sensor) open up a new field of 'chemoelectronics,'
where there is a direct coupling of chemistry to electronics
using the chemically induced electrons produced in
A Schottky diode consists of a thin metal film nearly
one hundred-millionth of a meter thick, made of silver,
gold, platinum or another metal, sprayed onto a silicon
wafer. What the researchers found was that the diode
can function as a "species-specific" gas detector,
meaning that different kinds of molecules will produce
different signals, and different metals are better
for detecting particular molecules.
Since every detectable chemical produces a characteristic
signal, the sensor can differentiate common contaminants
such as water from useful gasses in a manufacturing
environment. Multiple sensors can also work together
as arrays. The arrays can detect a variety of species
and produce the types of systems used for "artificial
Previously, researchers thought that the energy liberated
when certain chemicals interact on a metal surface
was released as vibrational (heat) energy - at least
under common reaction conditions. But some theorized
that most of the energy might instead be transferred
to electrons, much as light beams excite electrons
in the photoelectric process.
McFarland and his colleagues showed that the latter
hypothesis is true; nearly all interactions between
molecules and solid metal surfaces produce energized
"The team has filled a substantial gap in our knowledge,"
says Geoffrey Prentice, NSF program director for kinetics,
catalysis, and molecular processes, the program that
funded the new study. "Prior to this work," says Prentice,
"there was no direct experimental evidence," for this
The Schottky sensor can capture the energized electrons,
producing a measurable electrical signal. In addition,
because the electrons are freed for a significant
time, they may interact with the chemicals adhering
to the metal surface, leading to new reactions.
Because so many chemicals - such as ammonia, sulfuric
acid and various hydrocarbons including gasoline -
are manufactured on solid catalyst surfaces, "and
we in general do not fully understand how," says McFarland,
the findings have "direct implications toward developing
a more complete understanding of these important reactions."
Other types of thin-metal sensors are in use. But,
they typically measure the presence of a chemical
indirectly, through changes in metal resistance or
another property. The signal in the chemicurrent sensor
is a direct manifestation of the detected molecule.
In addition, the Schottky detector can operate at
a wide range of temperatures, between 23 C to 150
C, is inexpensive to produce, and can be reactivated
simply by warming its surface.
The new findings, and the associated detector technology,
may one day find wide use in a variety of industrial
applications, and the group has already sold prototype
devices to a major electronics manufacturer for use
in semiconductor materials production.
Other co-authors for this study include Hermann Nienhaus,
now at Laboratorium für Festkörperphysik, Gerhard-Mercator-Universität,
Germany, and W. Henry Weinberg, now affiliated both
with the University of California, Santa Barbara and
Symyx Technologies in Santa Clara, California.
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of about $4.8 billion. NSF funds reach all 50 states,
through grants to about 1,800 universities and institutions
nationwide. Each year, NSF receives about 30,000 competitive
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and service contracts yearly.