Detecting work-function differences in scanning tunneling microscopy

Variations of the work-function differences between tip and sample over dis tance have been investigated by a special procedure with a scanning tunneli ng microscope. This procedure allows the measurement of displacement curren t I-c dependencies on the voltage U. For a Pt tip on a Au surface and for a W tip on a Pt surface, work-function differences among 0, 2 and 0, 6 eV we re found. They increase with increasing distance. From the slope of the I-c (U) characteristics in different distances, the diameter of the tip is est imated. (C) 1999 American Institute of Physics. [S0003-6951(99)04220-5].