Atomic contrast on a point defect on CaF₂(111) imaged by non-contact atomic force microscopy

A point defect on a cleaved CaF₂(111) surface is imaged by non-contact atomic force microscopy in ultrahigh vacuum at room temperature to identify the nature of the point defect. First, atomic contrast formation on the point defect is examined using a negatively and a positively charged tip (i.e. F⁻- and Ca²⁺-terminated tips), which are prepared by modifying a Si tip with a CaF₂ cluster. Analysis of the two types of atomic contrast imaged by the F⁻- and Ca²⁺-terminated tips reveals that the point defect has a positive charge and the positive charge is located on a Ca²⁺ sublattice. Second, the lateral mobility of the point defect is investigated. We repeated imaging on the point defect by decreasing the tip–sample distance. At the smaller tip–sample distance, the point defect moved laterally to one of six neighbouring sites of the defect when the tip scanned the surface across the point defect. Taking into account (i) the atomic contrast on the point defect by the charged tips with a known terminating ion (i.e. F⁻ or Ca²⁺) and (ii) the lateral mobility of the point defect, we find a probable simple model of the point defect among six possible candidates.