Investigating Atomic Details of the CaF$_2$(111) Surface with a qPlus   Sensor
  The (111) surface of CaF$_2$ has been intensively studied with large-amplitude frequency-modulation atomic force microscopy and atomic contrast formation is now well understood. It has been shown that the apparent contrast patterns obtained with a polar tip strongly depend on the tip terminating ion and three sub-lattices of anions and cations can be imaged. Here, we study the details of atomic contrast formation on CaF$_2$(111) with small-amplitude force microscopy utilizing the qPlus sensor that has been shown to provide utmost resolution at high scanning stability. Step edges resulting from cleaving crystals in-situ in the ultra-high vacuum appear as very sharp structures and on flat terraces, the atomic corrugation is seen in high clarity even for large area scans. The atomic structure is also not lost when scanning across triple layer step edges. High resolution scans of small surface areas yield contrast features of anion- and cation sub-lattices with unprecedented resolution. These contrast patterns are related to previously reported theoretical results. 
Franz J. Giessibl and Michael Reichling