RHEED detection of near surface crystalline phases on a Pd 80Si 20 metallic glass surface with adsorbates present

Abstract

Phases which are thicker than two-dimensional surface phases may be called near surface phases. Surface phases may be related to the underlying bulk thermodynamically by the surface phase diagram and structurally by a relaxation and/or reconstruction of the top layer of the bulk. In contrast, near surface phases may form as a result of surface-biased transformation of a non-equilibrium bulk phase, such as crystallization on a Pd-Si metallic glass. Oxides and graphitic overlayers formed on the Pd-Si metallic glasses are near surface phases which have structures related to their own bulk structures, and not to the underlying amorphous bulk. Crystallization of Pd-Si metallic glass alloy has been detected by RHEED, when TEM of the bulk showed no crystallization. In addition, atom-milled thin sections which were milled from one side to preserve the surface also showed no crystallization. These results show a near surface crystalline phase between 10 and 50 Å formed on the amorphous bulk. The presence of SiO 2 and graphite has also been detected by RHEED and not by TEM. These near surface phases were formed while the glasses were acting as catalysts in the methanation reaction. Removal of the surface adsorbates via sputtering performed prior to the RHEED analysis would have removed the near surface phases also. Auger depth profiling showed a 25–100 Å carbonaceous layer on amorphous Pd-Si metallic glass through which RHEED could detect near surface phases on unsputtered samples.