Scanning Tunnelling Microscopy Studies of Na Islands on Graphite, Impurity Interactions on Cu(111) and Low Temperature Structures of S/Cu(111)

Doktorsavhandling, 2000

A scanning tunnelling microscope (STM) can reveal the atomic order of a surface as well as details of its electronic structure. In the present work STM, in combination with core and valence photo-electron spectroscopy and low energy electron diffraction (LEED) was applied to three different systems: (i) simple metal quantum wells in the form of Na films and islands on highly oriented pyrolytic graphite (HOPG), (ii) low temperature S/Cu(111) structures discovered in the course of this work and (iii) impurities on Cu(111). Furthermore a procedure was developed to prepare the fine tip that is a key component of an STM. The STM investigations of Na/HOPG, show that Na metal can be prepared as islands on graphite that allow Na atoms to be resolved. The islands are regularly shaped, slightly buckled and have a height (9-30 Å) which depends on the temperature. Through low bias constant current images of (110) terraces the lateral Fermi wavelengths of electrons in the islands were determined. The results agree qualitatively with the photoemission data acquired from the same system. A study of S/Cu(111) reveals four previously unknown structures which condensate <230 K from species too mobile to be observed above the ordering temperature. The STM images indicate an order similar to that of a (0001) cleavage planes in CuS. One of the structures shows a honeycomb-like structure with open areas which support a surface state, characteristic of a clean Cu(111) surface. After heating of a Cu crystal, surface impurities are found at the (111) surface. The STM images indicate that the impurities are separated by distances which are multiples of half the Fermi wavelength for the surface state. This gives evidence for an interaction between the impurities mediated by the surface state. The quality of STM images depends critically on the tip scanned across the surface. In order to find suitable methods to remove unwanted oxide from etched W tips, we used transmission electron microscopy, field emission and ordinary STM imaging to evaluate the results of different preparation methods. The methods tried were: HF-treatment, sputtering, electron beam heating and self-sputtering. Self-sputtering and electron beam heating gave the best results.