Surface Reactions of μ2-η2-(tBu-acetylene)dicobalthexacarbonyl with Oxidized and H-terminated Si(111) Surfaces

The reactions of a gas-phase metal carbonyl mu(2)-eta(2)-(Bu-t-acetylene)dicobalthexacarbonyl (CCTBA) with oxidized (SiO2) and H-terminated (H/Si(111)) silicon surfaces at decarbonylation temperature (140 degrees C) are presented. Upon the first CCTBA exposure, the surface reactions strongly depend on the nature of the surfaces. On H/Si(111), metallic Si-Co bonds are formed after similar to 80% of the Si-H bonds are consumed, and the carbonyl groups of the ligand form a stable structure of mixed Co-2(CO)(6) and semibridging CO. In contrast, Si-O-Co bonds are formed on SiO2 at the outset, as confirmed by both IR absorbance spectra and Co 2p core-level XPS. Furthermore, carbonyls are not observed on SiO2 after a completion of the first reaction, because of a weakening of their bond by the change of the Co oxidation state from Co-0 in CCTBA to Co2+ in Si-O-Co. On both substrates, the surface reaction of CCTBA gives rise to hydrosilylation, converting the alkyne group of the ligand into a C=C bond at the surface, which is very likely catalyzed by the presence of the cobalt carbonyl CCTBA itself. A cyclic CCTBA exposure of 2 s at 140 degrees C, followed by a 300 degrees C anneal, leads to metallic Co deposition on both H/Si(111) and SiO2 surfaces, with Co thicknesses of 5 angstrom and 14 angstrom, respectively, after 15 cycles. After a few cycles, the surface chemistry changes, leading predominantly to the formation of enones together with (semi)bridging carbonyls.