The Interfacial Physics Group

Carnegie Mellon University, Department of Physics

The wetting characteristics and diffusive growth of precursing films in single and multi component metallic systms

by Jaehyun Moon



In this thesis, we both address two longstanding questions in the wetting literature and reveal new quantitative features of surface diffusion in pure and alloyed metallic systems. By studying equilibrium wetting and precursing film growth kinetics experimentally in Pb, Bi and a Pb-Bi alloy on Cu(111), and by atomistic computer simulation for Ag on Ni(100), we have given the first clear recognition of pseudo-partial wetting - a thin film in equilibrium with a macroscopic body having a non-zero contact angle - in the literature and show that the mass transport mechanism during the film growth is diffusive. In addition, our detailed, spatially and temporally resolved compositional studies of the films, allows us to reconstruct the diffusion coefficient as a function of concentration for the pure Pb and Bi on Cu(111) systems. We connect those concentration dependence variations with surface structure. In the case of Pb-Bi on Cu(111), such reconstruction is not yet possible, but we observe the clear impact of the interaction between the species on the diffusive transport in the film.
The UHV apparatus and our sample geometries provide us with unique capabilities to study wetting and diffusion. It affords us both a clean environment and electron beam based diagnostic tools. The cleanliness of the UHV environment allows us to study wetting phenomena without the complexities of contact angle hysteresis and of solutal Maragoni flows that likely occur due to contamination in an ambient environment. Due to the exceedingly low vapor pressure of our systems, evaporation is negligible, simplifying both the film and contact angle behaviors. In our Pb-Bi alloy system, the low vapor pressures also remove the possibility of differential evaporation of various components which can cause instabilities in contact lines or dominate transport in precursing films. The Auger electron based spectroscopy characterization tools allow measurement of composition and density within the precursing film at submicron resolution. Thus, we can characterize the film growth by tracking the full compositional profile as it evolves with time. Finally, the nonzero contact angle of the systems investigated here ensures that the advancing contact line of a spreading drop is not influencing the film growth.