In Situ Observation of Atomic-Scale Growth of a NaCl Thin Crystal on Au(111) by Scanning Tunneling Microscopy

We have investigated the growth of the sodium chloride (NaCl) bilayer on Au(111) by scanning tunneling microscopy (STM). In the low-NaCl exposure regime, not only monocrystalline but also polycrystalline NaCl monolayer islands formed simultaneously on the Au surface at room temperature. With an increasing amount of NaCl exposure, the Au(111) surface is mostly covered by NaCl bilayer islands. The dynamic growth processes of the NaCl bilayer on Au(111) were revealed by sequential STM imaging. It was clarified that the two-dimensional growth of a second NaCl layer proceeds in an edge-advanced manner, the growth direction of which is strongly affected by the crystallographic orientation of NaCl monolayer islands. The edges and corners of NaCl islands become sharp and stabilized at the end of the bilayer formation. The results strongly suggest that the formation of the NaCl bilayer involves the migration of preadsorbed NaCl molecules from the Au surface onto the NaCl monolayer.