Atomic contrast on a point defect onCaF2(111) imaged by non-contact atomic force microscopy

Abstract

A point defect on a cleaved CaF2(111) surface is imaged by non-contact atomic force microscopy in ultrahigh vacuum at roomtemperature to identify the nature of the point defect. First, atomic contrast formationon 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 aCaF2 cluster. Analysis of the two types of atomic contrast imaged by theF−- andCa2+-terminated tips reveals that the point defect has a positive charge and the positive charge is locatedon a Ca2+ sublattice. Second, the lateral mobility of the point defect is investigated. We repeatedimaging on the point defect by decreasing the tip–sample distance. At the smallertip–sample distance, the point defect moved laterally to one of six neighbouring sites of thedefect when the tip scanned the surface across the point defect. Taking into account (i) theatomic 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 thepoint defect among six possible candidates.