Adsorption and growth modes of Bi on Pt(111)

Abstract

The vapor deposition of Bi on Pt(111) at 110 and ∼600 K have been characterized by Auger electron spectroscopy (AES), thermal desorption mass spectroscopy (TDMS), low-energy electron diffraction (LEED), and changes in the work function (Δφ). At 110 K Bi growth follows a layer-by-layer mechanism. At ∼600 K Bi fills the first monolayer (θBi≂0.56) relatively uniformly, followed by 3D island growth. Bi desorption is characterized by a large, coverage-dependent desorption energy, Edes =(81−34.2 θBi ) kcal mol−1, in the first monolayer, and zero-order kinetics with constant activation energy (Edes =53–56 kcal mol−1) for the multilayer. Many LEED patterns are observed within the first monolayer for both cold and hot substrates. Structural models for these are proposed which are consistent with coverages obtained by AES. Annealed structures show continuous compression of hexagonal Bi overlayers with increasing coverage, subject to mild substrate constraints. At 110 K and θBi >0.33, uniaxial compression is instead seen, due to an unsurmounted energy barrier. Weakly repulsive lateral Bi–Bi interactions (due to dipole repulsions) dominate submonolayer growth. These results for the semimetal Bi are intermediate in behavior between alkali and transition metal overlayers on Pt(111). This is consistent with the relative strengths of the surface dipole of these adsorbed metals.