Surface structure of thin metallic films on mica as seen by scanning tunneling microscopy, scanning electron microscopy, and low-energy electron diffraction

Abstract

Silver, gold, palladium, and chromium films have been evaporated onto mica at substrate temperatures ranging from −150 °C to +400 °C. Scanning tunneling microscopy (STM) images were obtained under ambient conditions. Silver and gold films exhibit an increasing grain size together with a grain flattening as the substrate temperature increases. At 275 and 400 °C for silver and gold, respectively, terraces with dimensions of the order of more than 100 nm are formed. However, also characteristic holes on the scale of a few 10 nm were always present. In addition, the gold films exhibit characteristic holes on the nanometer scale. While the larger holes are also visible on scanning electron microscopy micrographs, the smaller are not. Low-energy electron diffraction patterns prove the (111) orientation of both, silver and gold films; however, the quality of the silver patterns is better, consistent with the more perfect terraces as seen by STM. Palladium, evaporated at temperatures up to 350 °C did not exhibit similarly large and flat terraces. An improvement is achieved if prior to the palladium, silver is evaporated at elevated temperatures. Chromium, contrary to the fcc metals Ag, Au, and Pd crystallizes bcc, and forms a stable surface oxide under ambient conditions. The reproducibility of its STM images under ambient conditions depends on the applied bias and can be attributed to the surface oxide. Films evaporated at 50 °C exhibit a grainy structure. For the 350 °C films areas with a columnar morphology have been observed. In conclusion, epitaxially grown Ag(111) films evaporated at 275 °C onto mica have been shown to exhibit large flat terraces suitable for nanoscale modifications and chemisorption studies with the STM.