Although numerous industrial processes exploit the surfaces of solid catalysts to produce a variety of chemicals, scientists are still somewhat unclear as to what exactly goes on at the surface. One theory posits that the energy freed when a chemical binds to the surface is released as heat energy in the form of tiny vibrations of the surface molecules. Another hypothesis holds that the liberated energy is transferred to the electrons of the surface molecules, raising them to a higher energy level or exciting them. Direct experimental evidence for the latter model, however, did not existuntil now.
Writing in the journal Science, Brian Gergen of the University of California, Santa Barbara, and colleagues describe a new chemical sensor made from a so-called Schottky diodea silicon wafer coated with a metal film only a one hundred-millionth of a meter thick. The researchers detected excited electrons (and the holes they left behind) produced by adsorption of chemicals onto the diode's surface. The sensor (right), in turn, captured the energized electrons and produced a measurable electrical signal, which the scientists deemed a chemicurrent.
The new set-up differs from other types of thin-metal sensors currently in use, the authors note, because it detects molecules directly instead of measuring indirect changes caused by the presence of a chemical. What is more, different metal substrates show varying affinities for detecting particular molecules. So a combination of sensors that operate over a wide range of temperatures and are relatively inexpensive to produce could feasibly detect a variety of contaminants in a manufacturing environment.