Surface coverage of ALE precursors on oxides

The work with single molecular layers on porous materials adds to the atomic level understanding of the film growth in ALE. In this paper the primary factors affecting the surface coverage of three different ALE precursors (TiCl4, Cr(acac)3 and hexamethyldisilazane (HMDS) on porous, high surface area SiO2 were studied by element determinations and FTIR spectroscopy. Also a preliminary low-energy ion scattering (LEIS) study on Ti/silica samples was carried out. First, the surface coverage of metal species achievable at a certain reaction temperature on an oxide is determined by the number of specific reactive sites for the precursor. TiCl4 utilizes both isolated and H-bonded OH groups while Cr(acac)3 and HMDS react preferentially with isolated OH groups. Second, the bonding mode affected the surface coverage of titanium species. The bifunctional reactivity of TiCl4, on the one hand, consumed two OH groups per one TiCl4 molecule lowering the surface coverage on silica preheated at elevated temperatures, and on the other hand, enabled the utilization of the H-bonded OH groups present on silica preheated at lower temperatures thereby increasing the surface coverage. Third, the steric hindrance between the precursor and the already chemisorbed metal complexes was shown to have an effect on the surface coverage of Cr(acac)3 because of the size of two remaining acac ligands of bound chromium complex. Hence, at saturating reaction conditions of compound precursors, where undecomposed, reproducible and selectively chemisorbed surface species are obtained, the full monolayer coverage on oxides is seldom achieved.