Chemical analysis of PtxNi1−x alloy single crystal surfaces by scanning tunnelling microscopy

Two STM investigations are presented, in which irregular tip conditions enable direct access to chemical and structural information of a surface on an atomic scale, otherwise invisible for the STM. They allow a study of surface ordering of a Pt₂₅Ni₇₅(111) crystal by chemical contrast between the alloy components, and a study of carbon superstructures on a Pt₁₀Ni₉₀(100) surface by simultaneous imaging of substrate lattice and carbon atoms. All these images were obtained at very low tunnelling resistances and thus at small tip-sample distances. A chemical interaction between the probably adsorbate covered tip and the sample is proposed to explain these images.