Scanning tunneling microscopy and density functional theory study of initial bilayer growth of Ag films on NiAl(110)

Abstract

Scanning tunneling microscopy (STM) studies of the deposition of Ag on bcc NiAl(110) in the temperature range from $200\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ reveal an initial bilayer growth mode. In this regime, which encompasses at least the first two levels of bilayer islands, the film appears to have an fcc Ag(110)-like structure. Selection of this structure reflects an almost perfect lateral match between the Ag(110) and NiAl(110) lattice constants. Density functional theory (DFT) analysis of supported Ag films with an ideal fcc(110) structure on NiAl(110) indicates that the bilayer growth mode is promoted by a quantum size effect. However, the system does not exhibit perfect Ag(110) film growth. STM analysis reveals that the tops of Ag islands are decorated by a ripple structure even in the initial levels of growth and also shows a deviation from Ag(110)-like bilayer growth to Ag(111)-like monolayer growth for thick films. DFT analysis is also applied to provide some insight into the observed deviations from perfect Ag(110) film structure.