Atomic resolution by STM on ultra-thin films of alkali halides: experiment and local density calculations

Atomically resolved scanning tunneling microscopy (STM) of ultra-thin NaCl films on Al(111) and Al(100) demonstrates that only one atomic species of NaCl is imaged as a protrusion. By comparison of the constant current STM images with ab initio calculations of the local density of states by means of the full-potential linearized augmented plane wave method, the protrusions could be attributed to the anion Cl-. The calculations shows that a higher density of occupied states at the Cl-sites than for the Na-sites around the Fermi level causes the STM contrast between Cl and Na. With increasing number of NaCl layers the density of states in the band gap is reduced and the apparent height of additional NaCl layers decreases. The maximum film thickness allowing successful imaging by STM was found to be three layers. (C) 1999 Elsevier Science B.V. All rights reserved.