Understanding scanning tunneling microscopy contrast mechanisms on metal oxides: a case study.

A comprehensive analysis of contrast formation mechanismsinscanningtunnelingmicroscopy(STM)experimentson a metal oxide surface is presented with the oxygen-induced (2 √ 2� √ 2)R45� missing row reconstruction of the Cu(100) surface as a model system. Density functional theory and electronic transport calculations were combined to simulate the STM imaging behaviorofpureandoxygen-contaminatedmetaltipswithstructur- ally and chemically different apexes while systematically varying bias voltage and tipsample distance. The resulting multipara- meter database of computed images was used to conduct an extensive comparison with experimental data. Excellent agreement was attained for a large numberofcases,suggestingthattheassumedmodeltipsreproducemostofthecommonlyencounteredcontrast-determining effects.Specifically,we find that depending on the bias voltage polarity, copper-terminated tips allow selective imaging of two structurally distinct surface Cu sites, while oxygen- terminated tips show complex contrasts with pronounced asymmetry and tipsample distance dependence. Considering the structural and chemical stability of the tips reveals that the copper-terminated apexes tend to react with surface oxygen at small tipsample distances. In contrast, oxygen- terminated tips are considerably more stable, allowing exclusive surface oxygen imaging at small tipsample distances. Our results provide a conclusive understanding of fundamental STM imaging mechanisms, thereby providing guidelines for experimentalists to achieve chemically selective imaging by properly selecting imaging parameters.