Variable-temperature scanning tunneling microscopy studies of atomic and molecular level surface phenomena on semiconductor and metal surfaces

Surface reconstructions, adsorption, and reactions have been investigated at an atomic and molecular level by variable-temperature scanning tunneling microscopy (VT-STM). On Si(111), dramatic changes to a quenched surface, brought on by the presence of a trace amount of nickel (∼3% in the near-surface region) were investigated. The higher coordination requirement of Ni forces the re-bonding of near surface Si and attracts additional Si adatoms to complete the coordination. Two different reconstructions were formed, √19 × √19 R23.4° and √7 × √7 R19.1°. The surface acts as a gettering site for trace metal contaminates that are frozen out as the surface is quenched. Surface gettering by this process is quite efficient; an estimate of the Ni surface concentration was ∼90% of the estimated amount of Ni in bulk Si prior to quenching.; The surface structural behavior of a self-assembled monolayer, decanethiol, on Au(111) as a function of coverage and temperature, was investigated. Decanethiol adopts a number of stable surface phases as a function of coverage up to a saturation monolayer. The phase behavior changes significantly with a change in temperature. Low-density solid phases become metastable with increasing coverage, and a high-density melt phase becomes stable over a larger exposure range. At T > ∼ 60°C, no solid phases other than the saturation phase were observed. Exceptions to this occur at gold “herringbone” ridges and at steps. At T < ∼ 30°C, low-density solid phases are stable over an increasingly large exposure range and the high density melt phase is metastable. These observations were mapped out in (1) a molecular area vs. temperature phase diagram, and (2) a schematic lateral pressure vs. temperature phase diagram.; The low coverage adsorption behavior of methyl nitrite on Ag(111) at 105 K and 45 K was investigated. At low exposures, small clusters ascribed to CH₃ONO are observed at 105 K, growing with increasing exposure, in agreement with prior TPD that suggest 2D islanding. At 45 K, methyl nitrite adsorbs preferentially at steps in 3D multilayer structures. The mechanism for cluster formation is believed to be a subtle dipole-dipole attraction.