Modeling realistic tip structures : scanning tunneling microscopy of NO adsorption on Rh(111)

We have performed a joint experimental and theoretical scanning tunneling microscopy (STM) study of NO adsorbed on Rh(111). The experimental STM images showed a strong sensitivity to the tip conditions that could be altered by dipping the tip into the sample or by application of voltage pulses. Only via STM simulations performed over an exhaustive range of tip-apex terminations, including several contaminants in different adsorption geometries and with different spatial orientations, we have been able to reproduce the rich variety of measured images. From the analysis, we are able to infer a realistic structural model for the ultimate tip-apex structure involving apex geometries considerably more complex than those typically employed in STM modeling.