A Novel Growth Mode of Mo on Au (111) from a Mo(CO)6 Precursor: An STM Study

The growth of a submonolayer of metallic Mo prepared by chemical vapor deposition (CVD) of Mo(CO)6 on Au(111) was studied by scanning tunneling microscopy (STM). At low coverage, nanoscale Mo clusters grow at the elbow sites and in the fcc regions of the reconstructed Au surface. When the coverage increases, rather than decorating uniformly all elbows as found in physical vapor deposition (PVD), the Mo clusters aggregate but do not coalesce, forming ramified cluster islands. Within the islands, the clusters preferentially aggregate along the fcc troughs and the domain boundaries. At step edges, Mo clusters are found to grow at both upper and lower step edges. Differences between CVD and PVD are attributed to differences in the mobility of the nascent Mo species leading to growth. We hypothesize that the difference in mobility for the CVD process is due to the presence of CO from the precursor molecules. Oxidation of the Mo islands leads to their spreading, proving that the Mo clusters are either three-dimensional above or embedded in the surface of the gold substrate.