Engineers develop new chemical sensor based on experimental physics breakthrough

Image of the Schottky diode sensor. Courtesy of the University of California, Santa Barbara, Department of Chemical Engineering. Full size image available through contact

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 worldwide.

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 (electricalphenomenon 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 the metal."

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 noses."

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 electrons.

"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 phenomenon.

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.

NSF is an independent federal agency that supports fundamental research and education across all fields of science and engineering, with an annual budget 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 requests for funding, and makes about 10,000 new funding awards. NSF also awards over $200 million in professional and service contracts yearly.

Program contact:
Geoffrey Prentice
(703) 292-8371/gprentic@nsf.gov