Project VIII – Contact charging and triboluminescence

Contributors: A. Shluger, R. Bennewitz, E. Meyer

The charge transfer on the atomic scale (as seen in Scanning Probe Microscopies) and the charging observed macroscopically in contact charging, tribo-charging and erosion are strongly related to the mechanisms of friction. One can identify several areas where collaboration between theory and experiment proposed in this project could provide some new data and understanding: i) on mesoscopic scale, reliable determination of effective contact area at relative motion of two rough surfaces is needed to determine the absolute amount of charge transferred; ii) reliable data on the positions of surface defect states with respect to the vacuum level are missing, which prevents quantitative estimates of charge transfer; iii) the atomistic structure of metal/insulator interface is unknown in most important cases, which prevents identification of charge carriers; iv) there are no clear mechanisms of ionic transfer, especially in the case of metal/insulator contact; v) there are no reliable models of ion transfer during contact of two insulators, such as SFM tips and oxide surfaces.
The issues listed above are strongly linked to the effect of speed of relative motion of surfaces in contact or SFM tip velocity, especially at relatively high tip/surface velocities. One important concept, which is related to this effect and is often ignored, is the extent to which equilibrium is achieved in contact. Even in contact charging, rate-limiting steps (such as tunnelling to defects, or transport in one or more of the media in contact) can be so slow that the normal Fermi-level descriptions are insufficient. Yet another important issue is whether or not behaviour is determined by the average properties of the interface, or whether impurities, adsorbents or special sites (perhaps where a dislocation intersects the surface) are particularly important.

Triboluminescence is a well known macroscopic observation: Visible and ultraviolet light is emitted when surfaces of a variety of substances are scratched. The effect is closely related to contact charging, since the light emission is often initiated by an electric discharge in the breaking contact. The microscopic processes causing triboluminescence are poorly understood. It is planned to set up an instrument with nanonewton force sensitivity and single photon detection in order to study the light emission from nanometer-scale contacts. Questions to be answered are: What materials show triboluminescence? Is triboluminescence a collective phenomenon, or can the breaking of single contacts or even single bonds produce light emission? Is there triboluminescence without charge separation? The projects will profit from a close cooperation with experienced researchers in the field of contact charging and break junctions.