UHV-STM studies of cobalt deposited on clean, oxygen- and sulfur-covered molybdenum(110).

摘要

Metal deposition and growth are critical to a number of technologies including, particle size control in heterogeneous catalysis and deposition of interconnects in semiconductor processing. As the demand increases for new materials that are more selective in reactivity and more uniform at nanometer dimensions, it is important to understand the effect of the particle-substrate interface on thin film morphology.;Herein, we examine the surface structure and morphology of cobalt deposited on clean, sulfur- and oxygen-covered Mo(110) using scanning tunneling microscopy and low energy electron diffraction. We find that the morphology of the Co deposited at 400 K is strongly dependent on the nature of the interface. Furthermore, the morphology is not predicted for all cases by bulk thermodynamic quantities, which have been used in the literature to explain the growth modes of metals on metal oxides. We find that two-dimensional islands of cobalt grow on clean and S-covered Mo(110). Three-dimensional growth would be predicted on S-covered Mo(110) based on bulk heats of formation of Co- and Mo-sulfides. Three-dimensional growth occurs on oxygen-covered Mo(110), in accord with predictions based on heats of formation of bulk oxides. The difference between the growth mode of the cobalt on sulfur- and oxygen-covered Mo(110) is explained in terms of differences in microscopic bonding at the interface.;The thermodynamic stability of the Co islands deposited on modified Mo(110) was examined by heating to 760 K. The islands in the Co/S/Mo(110) system coalesced upon heating forming two-dimensional islands which are greater than an order of magnitude larger than in the unannealed case. In addition, sulfur appears to migrate from the Mo(110) to the Co islands upon heating, based on the fact that oxygen no longer adsorbs on the islands indicating site-passivation. In contrast, upon heating the Co/O/Mo(110) system, surface oxygen diffuses into the bulk Mo(110) creating more Mo-like sites. Consequently, the three-dimensional Co islands decrease in height and become more anisotropic, indicative of the more "wetting"-like behavior found in the Co/Mo(110) and Co/S/Mo(110) systems.