Investigation of novel catalytically active surfaces requires a comprehensive experimental method for the identification and rapid characterization of prospective catalytically active sites. The powerful method of three-dimensional atomic force microscopy (3D-AFM) in non-contact mode has been combined with scanning tunneling microscopy (STM) to study the oxygen-terminated copper (100) surface. Complex 3D data sets, obtained by simultaneously recording the tunneling current and the AFM frequency shift, allow for site specific quantification of forces and tunneling currents. The wealth of information obtained is promising for future applications, but the interpretation of the wide range of contrast modes requires a thorough characterization of the sources of contrast in AFM and STM imaging.
We combine DFT total-energy calculations with Non-equilibrium Green's Function (NEGF) methods for electronic transport to determine the tip-surface interaction and tunnelling current for a large set of tip models in order to clarify the different contrast modes obtained in the experiments. At the outset, we obtained a stable Cu(100) (2√2x√2)R45?-O surface reconstruction model. Surface features were found to be in good agreement with experimental data, and the analysis of surface electronic properties enabled us to identify prospective reactive sites. The effect of tip changes on contrast modes was explored by considering tips of different reactivity. Our simulations, in comparison with AFM experimental images, identified a contaminated tip with a Cu-terminated experimental configuration. Charge density and current calculations further helped to investigate the STM imaging mode and explain the lateral shift between surface features identified separately in AFM and STM images. Consideration of different atom defect species and geometries helped us to understand detailed STM image features. The combination of conductance calculations with total energy methods provides insight into (1) the fundamentals of contrast formation in this novel experimental technique and (2) into the correlation between tip-sample forces and local chemical reactivity, factors that are essential for the further development and application of this approach to characterise catalytic activity.
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