A single monolayer of hexagonal boron nitride (h-BN) forms a remarkable nanomesh with 2 nm wide pores, both on Rh(111) [M. Corso et al, Science 303 (2004) 217], and Ru(0001) [A. Goriachko et al, Langmuir Letters, web-published on 8/Feb/2007] substrates. The nanomesh pores are proved to act as traps where Au, Pd, or Co atoms can assemble to form nanoparticles just a couple of nm in diameter.
We have investigated the h-BN/Ru(0001) system in terms of its resilience to etching by molecular oxygen, which is a prerequisite for Me/h-BN/Ru(0001) application as oxidation nanocatalist. Our investigations were performed with ultra-high vacuum (UHV) based scanning tunneling microscopy (STM) and low energy electron microscopy (LEEM). At 600 C substrate temperature and 1E-6 mbar oxygen pressure, the nanoscale openings in the h-BN layer appear and grow until the entire h-BN is consumed, as observed by STM. Such etching process is proceeding simultaneously over the entire surface. In contrast to this, the nucleation and growth of macroscopic h-BN free areas were observed by LEEM at 750 C and 5E-8 mbar. The nucleation takes place at macroscopic defects (e.g. scratches on the surface) and the h-BN free area is growing radially from the nucleation site. A sharp moving "front-line" separates the h-BN covered and h-BN free regions, the latter being depleted of oxygen in the immediate vicinity of the front-line. The oxygen depleted region forms due to its consumption in the etching process. Far away from the front-line, the h-BN free region consists of a nicely ordered O-2x2/Ru(0001) phase. Both diffraction and reflection modes of LEEM could easily distinguish the h-BN covered, the oxygen depleted Ru(0001), and the O-2x2/Ru(0001) regions. In real time LEEM, the etching front-line is promptly followed by the oxygen diffusion front, due to oxygen transport from oxygen-rich into oxygen-depleted regions.
We have covered the h-BN/Ru(0001) nanomesh with up to 1 ML of Au or Pd, thus producing well defined metal nanoparticles, as demonstrated by STM. Covering nanomesh with up to 1 ML of Au has extend the range of its thermal stability up to 900 C under 1E-6 mbar oxygen pressure. On the contrary, covering nanomesh with Pd did not produce any kind of protection against etching by oxygen. |