Why do ‘dirty’ tips produce higher-resolution images when graphite is scanned in a scanning tunnelling microscope?

Abstract

The puzzling observation that scanning tunnelling microscope (STM) graphite surface topographies obtained with conventionally treated tips show much higher lateral resolution than those obtained with high-quality pure metallic tips can be well understood by examining the electronic structures of samples and tips and their inter-relations. Conventionally treated tips are grained and etched and in the course of operations are covered with carbon or polycrystalline graphite mini-tips. These ‘dirty’ interfaces exhibit an electronic structure similar to that seen on the surface of the graphite sample. As a result the tunnelling processes are carried out by electrons, which usually produces high-resolution scans. On the other hand, on using pure metallic tips with no common transverse momentum in the tip and the sample Fermi surface, tunnelling becomes very complex and the topographic information is lost. As a consequence, tips for graphite surface investigations should be coated with carbon (crystalline graphite) to obtain scans with high lateral resolution. © 1988 IOP Publishing Ltd.