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 be en 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 d ifferently oriented hexagonal domains on a single terrace was observed . Steps along the longer corrugation length of the surface reconstruct ion were found, on the average, to follow the [N 1] direction of the h exagonal Pt overlayer rather than any low-index direction of the squar e 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 fou nd to be initiated by heterogeneous nucleation, the nucleation centers being step edges and structural irregularities disrupting the hexagon al structure along a direction close to the hexagonal [1BAR, 5] direct ion. The growth of the (1 x 1) phase was highly anisotropic, i.e. the growth being much faster along the [011BAR] direction (the substrate d irection closer to the [N 1] direction of the hexagonal layer) than al ong the [011] direction. A higher density of atoms in the hexagonal th an in the (1 x 1) surface layer causes islands, a single Pt(100) latti ce spacing high, to form during the structural transformation. These i slands were found to be a mixture of isotropic islands with diameter 1 0-15 angstrom and islands highly elongated along the faster growth dir ection of the (1 x 1) phase.