STM STUDIES OF CLEAN, CO-EXPOSED AND O2-EXPOSED PT(100)-HEX-R0.7-DEGREES

The structure of the Pt(100)-hex-R0.7-degrees constructed 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 x 1) surface structure due to exposure to CO and O2 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 [1BAR, 5] direction. The growth of the (1 x 1) phase was highly anisotropic, i.e. the growth being much faster along the [011BAR] 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 x 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 angstrom and islands highly elongated along the faster growth direction of the (1 x 1) phase.