Force microscopy of cleaved and electron-irradiated CaF2(111) surfaces in ultra-high vacuum

We present scanning force micrographs of as-cleaved and electron-irradiated CaF2(lll) surfaces taken in ultra-high vacuum at room temperature. Among the forces acting on the tip, the electrostatic force was found to make an important contribution. This allows us to study the ionic conductivity of the crystals. Freshly cleaved surfaces can be imaged in contact mode with high resolution exhibiting the hexagonal structure of the (lll) surface. For electron-irradiated surfaces, noncontact mode is required for imaging radiation-induced stoichiometric changes. Strong adhesive forces between the tip and metal-enriched areas are found to be a severe obstacle for contact mode imaging. Weak irradiation with 850 eV electrons results in the formation of 10 nm-wide holes with surrounding elevations as the first stages of metallization. (C) 1997 Elsevier Science B.V.