STM studies of clean, CO- and O 2-exposed Pt(100)-hex-R0.7°

Abstract

The structure of the Pt(100)-hex-R0.7° reconstructed phase has been studied by scanning tunneling microscopy (STM). The hexagonal first layer superstructure of this surface, written in matrix notation as N 1 −1 5 , N = 12−14, was imaged with atomic resolution. The fivefold periodicity of this reconstruction was confirmed. The coexistence of differently oriented hexagonal domains on a single terrace was observed. Steps along the longer corrugation length of the surface reconstruction were found, on the average, to follow the [ N 1] direction of the hexagonal Pt overlayer rather than any low-index direction of the square substrate. The transition from the N 1 −1 5 reconstruction to the Pt(100)−(1 × 1) surface structure due to exposure to CO and O 2 was found to be initiated by heterogeneous nucleation, the nucleation centers being step edges and structural irregularities disrupting the hexagonal structure along a direction close to the hexagonal [1̄ 5] direction. The growth of the (1 × 1) phase was highly anisotropic, i.e. the growth being much faster along the [01̄1] direction (the substrate direction closer to the [ N 1] direction of the hexagonal layer) than along the [011] direction. A higher density of atoms in the hexagonal than in the (1 × 1) surface layer causes islands, a single Pt(100) lattice spacing high, to form during the structural transformation. These islands were found to be a mixture of isotropic islands with diameter 10–15 Å and islands highly elongated along the faster growth direction of the (1 × 1) phase.