Triethylamine on Si(001)-(2 x 1) at 300 K: Molecular Adsorption and Site Configurations Leading to Dissociation
Résumé
We provide a comprehensive investigation of the adsorption of a tertiary amine molecule, triethylamine (TEA, N(CH2CH3)(3)), on the Si(001)-(2 x 1) surface at room temperature (RT), using real-time synchrotron radiation X-ray photoemission (XPS) and scanning tunneling microscopy (STM), in combination with density functional theory (DFT). calculations. Real-time XPS measurements point to two consecutive reactions, the first one leading to the formation of a dative adduct, the second one to the conversion of the latter into a dissociated adduct via N C bond cleavage (to give Si-N(CH2CH3)(2) and Si-CH2CH3 moieties). The kinetic model, that fits the experimental data, distinguishes unreactive dative bond molecules (forming a reservoir) from reactive ones, exchanged in a two-way reaction. At low coverage, the STM images show that the adsorption of a dative-bond TEA molecule induces a static buckling of the Si dimers, leading to the c(4 X 2) templating of the silicon surface. Approaching saturation coverage, dative bonded TEA molecules self-organize into c(4 x 2) domains, with an occupancy of one molecule per two dimers, and exhibit a ternary symmetry. While DFT points to two possible molecular conformations (staggered and eclipsed) with almost the same adsorption energy, only the eclipsed conformer is observed at RT. STM helps also in identifying sites and experimental conditions (pressure) that make the dissociation of TEA possible. At high dilution, dissociated molecules are systematically sitting as pairs on two adjacent dimers in a row. Remarkably, dissociated adducts do not induce static buckling, in stark contrast with the case of dative bonding. Close to surface saturation, the dative-to-dissociated conversion starts from the boundaries of dative-bond self-assembled c(4 x 2) domains, emphasizing the role of these domains as reservoirs of stable molecular adducts. Our data show also that the conversion reaction needs the presence of the gas phase and is accelerated by an increase of pressure.