In this study we use first principle methods to study the interaction of three different tip models with a CaF_2 (111) surface. The tip models are based on a realistic' approach to tip simulation and consist of a pure silicon tip, an oxygen terminated silicon tip and a water contaminated silica (SiO_2) tip. We find that each tip demonstrates characteristic features which should be observable in experiments and hence provide signatures which would aid in the identification of tip and surface species.
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