Chemical sensor based on experimental physics

Wednesday, 19 December, 2001

Scientists have found evidence of the long-suspected phenomenon that tiny electrical currents are 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 50% of the chemicals manufactured worldwide.

Investigators at the University of California, Santa Barbara, 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 chemical sensor.

"[The findings] 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," said Eric McFarland, principal investigator.

A Schottky diode consists of a thin metal film nearly one hundred-millionth of a metre 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 theorised 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 energised electrons.

The 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.

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