Atomare Mechanismen des Wachstums von Edelgasschichten auf Platinoberflächen untersucht mittels temperaturvariabler Rastertunnelmikroskopie

The initial stages of the growth of thin rare gas films on platinum surfaces have been studied on an atomic scale. For this purpose, a variable temperature scanning tunnelling microscope (STM) was developed, which can be operated over the full range of sample temperature from 10K up to 400 K. The possibility of obtaining atomically resolved images of a growing rare gas layer from the very first adsorbed atoms up to completion of the first monolayer allowed us to investigate the atomic mechanisms of growth, which turned out to be quite complex. After a summary of the present understanding of the model systems of rare gases on metal surfaces, the construction of the ultrahigh vacuum STM system is described in detail: A special sample manipulator was designed which allows atomic STM resolution at variable sample temperature and variable coverage. Furthermore, with this design it is possible to use quasi-simultaneously other techniques like LEED, AES or TDS on the sample without changing the sample temperature. Prior to the main results obtained on Pt(111), some results concerning the bare and the xenon-covered Pt(110) face are presented. Due to the strong anisotropy of the reconstructed Pt(110) face xenon grows in a quasi one-dimensional mode; the nucleation and growth preferentially occur along the close-packed rows of the Pt(110) face until the first layer is completed. On the more isotropic Pt(111) surface, however, xenon grows in a more complicated way. The very first atoms preferentially adsorb at the low coordination upper edge of the preexisting platinum steps, thus forming chains of xenon atoms decorating the step edges. These chains do not serve as nuclei for further growth but remain monatomically thin during further adsorption until just before completion of the monolayer. This is indicative of a local repulsive interaction between the xenon atoms in the chains and xenon atoms approaching the chains from the upper terrace. Instead of contributing to the thickening of the xenon chains, these further xenon atoms rather nucleate as two-dimensional (2D) islands on the platinum terraces and at the lower platinum step edges. Depending on the adsorption temperature, xenon either forms numerous small 2D islands or large compact xenon areas. A comparison of this behaviour to the ripening processes of a xenon sub-monolayer adsorbed at low temperature and annealed in steps of increasing temperature shows that two different processes determine the morphology of a xenon sub-monolayer: On the one hand, the diffusion of single xenon atoms on the bare Pt(111) face sets in at about 20K, on the other hand, the evaporation of xenon atoms from the small xenon islands becomes observable at about 27 K. Annealing a sub-monolayer of xenon above 27 K therefore leads to a mass transport of xenon towards the lower edges of the platinum steps. Xenon can overcome the barrier at the step edge and "jump downwards " by crossing the xenon chains at the upper step edge. The exact nature of this process, however, is still unclear. The xenon chains decorate the step edges even at elevated temperature thus confirming the very strong binding of xenon atoms at these low coordination sites. Interestingly, a reversal of the direction of the interlayer mass transport (i. e. xenon atoms "jumping upwards") is observed during adsorption.Increasing the coverage eventually leads to the formation of the well-known Novaco-McTague-rotated phase of the completed xenon monolayer. The long-range height modulation of this structure was imaged with atomic resolution . Due to the presenceof atomic site defects in the xenon film the structure is slightly distorted.Krypton and argon on Pt(lll) were found to exhibit a similar behaviour as xenonon Pt(111) . In the case of argon, difficulties in obtaining stable STM images from thisspecies and the very low temperatures required do not yet allow a clear statement onthe precise structure of the argon