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.