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 US 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.
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. But some believed that most of the energy might instead be transferred to electrons.
McFarland and his colleagues showed that the latter hypothesis is true; nearly all interactions between molecules and solid metal surfaces produce energised electrons.
The Schottky sensor can reportedly capture the energised 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.
Many chemicals are manufactured on solid catalyst surfaces, but according to Eric McFarland, principal investigator and the NSF grant awardee, it isn’t fully understood how. He said 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.