Seminars Archive

Surface/interface alloying properties of the bulk immiscible Au-Rh system in the presence of TiO₂ and hexagonal boron nitride

Abstract
While rhodium and gold are bulk immiscible metals, alloying in the topmost atomic layer was detected for Au deposits on Rh(111), detected by scanning tunneling microscopy (STM). Apart from random mixing, nano-range ordered (2×1) structures were also identified. As a next step, the structure of bimetallic clusters on TiO₂(110), formed via sequential deposition, was addressed. Low energy ion scattering spectroscopy (LEIS) revealed an efficient place exchange of the two metals already at room temperature, when Rh was evaporated on top of the TiO₂(110) surface precovered by gold nanoparticles. The driving force is the lower surface energy of Au compared to Rh. With the reverse sequence of deposition (i.e. Au on top of Rh/TiO₂(110)), Rh core – Au shell nanoparticles formed. Thermally induced migration of an ultrathin TiO_x layer onto Rh nanoparticles deposited on TiO₂, termed encapsulation or strong metal support interaction (SMSI) is a well-known phenomenon. In our work we addressed the effect of Au on this process. In another set of experiments, we prepared on Rh(111) a different type of ultrathin layer, a hexagonal boron nitride (h-BN) monolayer. On this substrate it is periodically corrugated (“nanomesh”) due to the lattice mismatch and the strong interaction between Rh and h-BN. On the other hand, flat h-BN layers are formed on coinage metals due to the weak interaction. Deposition of small amounts of Au on the h-BN/Rh(111) surface results in 1-2 atomic layer high clusters, but the growth mode is strongly three dimensional at larger gold doses. At higher temperatures Au intercalation sets in, and gold atoms dissolve in the topmost Rh layer. Interestingly, this phenomenon only slightly modifies the nanomesh structure up to a gold dose of 0.2 ML.