Physisorption and reaction at silicon surfaces of a series of brominated organic molecules: bromoethane, 1,2-dibromoethane, 1-bromopropane, 1-bromobutane and 1-bromopentane were examined by Scanning Tunneling Microscopy (STM). On Si(111)-7×7, a widely-spaced “one-per-corner-hole” pattern was observed, formed by the physisorption and reaction of several alkyl bromides. This “one-per-corner-hole” pattern suggested long-range repulsion between the adsorbates. Density Functional Theory (DFT) calculations, performed by others in parallel with these experiments, showed that this long-range repulsion was due to lateral charge transfer in the Si(111)-7×7 surface consequent on the physisorption of an alkyl bromide or chemisorption of a Br atom. The reaction rate of bromine ‘abstraction’ (transfer of a Br-atom from the adsorbate to the silicon) was examined for two physisorbed states of 1 bromopentane on Si(111)-7×7, one vertical and one horizontal, each distinguishable by STM. The energy barrier was found to be significantly lower for abstraction of Br-atom from the vertical than for the horizontal 1 bromopentane, both for thermal and electron-induced reaction. This finding accords with previous DFT calculations for methyl bromide, for which theory exhibited a clear preference for a vertical transition state in the bromination of Si(111)-7×7. The effect of alkyl chain-length on the rate of thermally-induced dissociative attachment reactions was investigated for a series of primary bromo-alkanes (bromoethane, 1-bromopropane and 1-bromobutane) on a different face of silicon; Si(100)-c(4×2). These three bromo-alkanes all physisorbed exclusively ‘inter-row’, bridging the gap between Si dimer-rows of Si(100)-c(4×2). Thermal reaction was highly ‘localized’, i.e. the chemisorbed Br-atom was formed directly below the parent bromo-alkane. The thermal barrier heights were found experimentally to increase systematically with chain length. This trend was interpreted, on the basis of DFT calculations performed by the author, as being due to the extra energy required to lift the alkyl group in going from the initial physisorbed state to the more-nearly vertical transition state.

Physisorption and reaction at silicon surfaces of a series of brominated organic molecules: bromoethane, 1,2-dibromoethane, 1-bromopropane, 1-bromobutane and 1-bromopentane were examined by Scanning Tunneling Microscopy (STM). On Si(111)-7×7, a widely-spaced “one-per-corner-hole” pattern was observed, formed by the physisorption and reaction of several alkyl bromides. This “one-per-corner-hole” pattern suggested long-range repulsion between the adsorbates. Density Functional Theory (DFT) calculations, performed by others in parallel with these experiments, showed that this long-range repulsion was due to lateral charge transfer in the Si(111)-7×7 surface consequent on the physisorption of an alkyl bromide or chemisorption of a Br atom. The reaction rate of bromine ‘abstraction’ (transfer of a Br-atom from the adsorbate to the silicon) was examined for two physisorbed states of 1 bromopentane on Si(111)-7×7, one vertical and one horizontal, each distinguishable by STM. The energy barrier was found to be significantly lower for abstraction of Br-atom from the vertical than for the horizontal 1 bromopentane, both for thermal and electron-induced reaction. This finding accords with previous DFT calculations for methyl bromide, for which theory exhibited a clear preference for a vertical transition state in the bromination of Si(111)-7×7. The effect of alkyl chain-length on the rate of thermally-induced dissociative attachment reactions was investigated for a series of primary bromo-alkanes (bromoethane, 1-bromopropane and 1-bromobutane) on a different face of silicon; Si(100)-c(4×2). These three bromo-alkanes all physisorbed exclusively ‘inter-row’, bridging the gap between Si dimer-rows of Si(100)-c(4×2). Thermal reaction was highly ‘localized’, i.e. the chemisorbed Br-atom was formed directly below the parent bromo-alkane. The thermal barrier heights were found experimentally to increase systematically with chain length. This trend was interpreted, on the basis of DFT calculations performed by the author, as being due to the extra energy required to lift the alkyl group in going from the initial physisorbed state to the more-nearly vertical transition state.

O objetivo principal deste trabalho foi adicionar novos módulos (tanto eletrônicos quanto computacionais) necessários para efetuar medidas espectroscópicas com o microscópio de tunelamento construído há alguns anos no Laboratório de Novos Materiais Semicondutores do Instituto de Física da USP. Para checar a performance do novo sistema implementado, foram realizadas medidas sobre materiais condutores (grafite e ouro). Visando a análise topográfica e espectroscópica de amostras semicondutoras dos grupos III-V, estudamos alguns métodos para a preparação destas superfícies. Dentre eles, a passivação foi capaz de fornecer os resultados mais significativos. Finalmente, curvas da corrente de tunelamento em função da tensão aplicada à junção foram adquiridas sobre amostras de GaAs e pontos quânticos de InAs crescidos pela técnica de epitaxia por feixe molecular (MBE). / The goal of the present work was to upgrade the home-made Scanning Tunneling Microscope present in our group, adding the new hardware necessary to carry out spectroscopic measurements. A new software was also developed to control the new functions of the microscope. In order to check the performance of the whole system, several types of experiments where carried out on graphite and gold. A special care was taken to adequately prepare the samples of III-V semiconductors. The passivation of the sample yielded the best results both for topographic and spectroscopic measurements. Finally, I-V curves were taken on GaAs layers and InAs quantum dots grown by molecular beam epitaxy (MBE).

O objetivo principal deste trabalho foi adicionar novos módulos (tanto eletrônicos quanto computacionais) necessários para efetuar medidas espectroscópicas com o microscópio de tunelamento construído há alguns anos no Laboratório de Novos Materiais Semicondutores do Instituto de Física da USP. Para checar a performance do novo sistema implementado, foram realizadas medidas sobre materiais condutores (grafite e ouro). Visando a análise topográfica e espectroscópica de amostras semicondutoras dos grupos III-V, estudamos alguns métodos para a preparação destas superfícies. Dentre eles, a passivação foi capaz de fornecer os resultados mais significativos. Finalmente, curvas da corrente de tunelamento em função da tensão aplicada à junção foram adquiridas sobre amostras de GaAs e pontos quânticos de InAs crescidos pela técnica de epitaxia por feixe molecular (MBE). / The goal of the present work was to upgrade the home-made Scanning Tunneling Microscope present in our group, adding the new hardware necessary to carry out spectroscopic measurements. A new software was also developed to control the new functions of the microscope. In order to check the performance of the whole system, several types of experiments where carried out on graphite and gold. A special care was taken to adequately prepare the samples of III-V semiconductors. The passivation of the sample yielded the best results both for topographic and spectroscopic measurements. Finally, I-V curves were taken on GaAs layers and InAs quantum dots grown by molecular beam epitaxy (MBE).