The growth of magnesium chloride monolayer and multilayer structures on different transition metal (Pt, Pd, Rh) single crystals with varied orientations

Abstract

The growth of magnesium chloride (MgCl 2) monolayer and multilayer structures on a variety of transition metal single crystal surfaces [Pd(111) and (100), Pt(111) and (100)-hex and Rh(111)] has been studied. Low energy electron diffraction (LEED) has been employed as a probe of the long range order at both monolayer and multilayer coverages while temperature programmed desorption (TPD) provided information on the strength of adsorbate-substrate interactions. LEED results indicated that MgCl 2 growth at both monolayer and multilayer coverages is dependent upon the magnitude of the monolayer adsorbate-substrate interactions which are themselves a function of the underlying single crystal template. LEED results indicated that, on each transition metal surface studied, the MgCl 2 monolayer structures act as a template for subsequent multilayer structures. On Pt(111) and Pd(111), identical, well ordered LEED structures at both monolayer and multilayer coverages, were produced. The commensurate (4 × 4) and (√13 × √13)R13.9° monolayer structures were associated with expansions and contractions in the MgCl 2 unit cell dimension, respectively. These structural changes were correlated with the strength of the monolayer adsorbate-substrate interaction, which on these surfaces lead to distinct monolayer TPD features. In contrast, MgCl 2 growth on Pt(100)-hex and Pd(100) exhibited more complex LEED patterns associated with various degrees of ordered and rotationally disordered domains. This was attributed to a reduction in the magnitude of monolayer adsorbate-substrate interactions, correlated by TPD measurements. Experiments on Rh(111) indicate that the factors responsible for ordered MgCl 2 growth on this surface differ from those observed on Pt and Pd single crystals. In general, better matching between the substrate and adsorbate unit cell dimensions leads to enhanced adsorbate-substrate interactions which are responsible for the degree of ordering at monolayer and subsequent multilayer coverages.