Chemicurrents or the non-adiabatic dissipation of chemical energy at surfaces
CAMD/CINF Seminar by Hermann Nienhaus, University of Duiburg-Essen.
When photodiodes are illuminated internal photocurrents are observed. Likewise,
chemicurrents are detected with thin metal film electronic devices when the metal surface of the device is exposed to reactive gas particles. Heretofore used devices for chemicurrent detection are metal-semiconductor (Schottky) diodes, metal-insulatorsemiconductor (MIS) diodes and metal-insulator-metal (MIM) tunneling structures. The chemicurrent effect gives direct evidence that part of the released reaction energy is dissipated non-adiabatically by creation of hot charge carriers in the metal surface. Since the metal film thickness is in the nm range the chemically induced hot electrons and holes may travel ballistically from the surface to the interface and, in case of Schottky diodes, traverse the Schottky barrier if their energy exceeds the barrier height. The chemicurrent effect has been observed with a large variety of gas-surface reactions and may be considered as a general phenomenon. The current intensity is related to device properties like metal film thickness, barrier height and interface quality and to the exothermicity of the reaction. The time dependence of the current during exposure of the device represents directly the kinetics of the respective surface reaction. Hence, the method allows a novel way of studying surface reactions from ‘inside the bulk' and of extracting kinetic parameters with an electronic device.