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December 24, 2001
Volume 79, Number 52
CENEAR 79 52 p. 9
ISSN 0009-2347
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Surface electrical currents may form basis of future chemical detectors


Bucking conventional wisdom, researchers have shown that energy evolved during chemical reactions on surfaces may be channeled into electronic excitations--even in cases of very low energy reactions. The work may lead to new types of sensitive and selective chemical sensors.

THE SENSITIVE TYPE A nearly invisible nanometer-thick silver film deposited on a centimeter-sized silicon wafer (rectangle with rectangular windows) forms the basis of a Schottky diode used to detect tiny chemical currents.
As atoms and molecules impinge on metal surfaces, the species bond (adsorb) to the surface, releasing energy in the process. Depending on the energetics of the reaction, the system "chooses" between a number of pathways to dissipate the energy. One route stimulates collective vibrations of surface atoms (phonons); another excites surface electrons.

Researchers have long known that reactions that deposit a lot of energy into a surface excite electronic transitions that, in turn, can cause particles to be ejected from a surface. For example, scientists discovered a century ago that adsorbing certain gases on alkali metals causes negatively charged particles to be ejected. Other highly exothermic reactions cause chemiluminescence.

In cases where relatively little energy--less than 0.5 eV per molecule--needs to be dissipated, researchers have concluded that phonons are the dominant players. But recent studies at the University of California, Santa Barbara, show that an electronic excitation process that has generally been considered unimportant in low-energy reactions--electron-hole pair formation--is a key mechanism in energy transfer at surfaces [Science, 294, 2521 (2001)].

"Electron-hole pair" refers to an energetic surface-bound electron and the positively charged vacancy (hole) left behind as the electron is promoted to an excited state. Until now, these excited charge carriers have been difficult to detect directly. But the Santa Barbara group--which includes graduate student Brian Gergen, postdoctoral associate Hermann Nienhaus, and chemical engineering professors Eric W. McFarland and W. Henry Weinberg--has developed detectors based on a device known as a Schottky diode that directly measure tiny electrical currents caused by electron-hole pairs, thereby proving their importance in surface reactions.

By depositing an ultra-thin silver film on a silicon surface, the researchers made Schottky detectors that register species-specific chemical currents (chemicurrents) when beams of nitrogen oxides, alkanes, alkenes, noble gases, and other reactants adsorb on the silver film.

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