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Using Si 2p core-level X-ray photoelectron spectroscopy we have measured the upward band bending at the surface of n + -doped water-saturated Si(001)-2 × 1 and inferred the macroscopic negative surface charge density of the surface. These macroscopic results are in excellent accord with the microscopic view provided by dual-bias scanning tunneling microscopy showing that the isolated silicon dangling bonds (∼1.2 × 10−2 defects per Si atom) bear indeed a negative charge. Noting the structural analogy between isolated dangling bonds on water-saturated Si(001) and H-terminated Si(001), in the final, prospective section of the paper, we raise the question of the possible role that these defects could play in radical chain reactions with π-bonded molecules, in relationship with the hydride/hydroxyl patterns that are resolved in the scanning tunneling images.

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The present study aims at determining the bonding configurations of bifunctional primary amines, ethylenediamine (H2N-[CH2]2-NH2) and 1,4-diaminobutane (H2N-[CH2]4-NH2), on Si(001)-2 × 1. We carry out Δ Kohn−Sham ionization potential calculations of various plausible geometries and compare the calculation outputs to synchrotron radiation core-level photoemission (XPS) data. Besides the primary motivation of chemical identification, the DFT calculations point to interesting and unexpected issues, related to the propagation of energy shifts throughout the molecular chain, or to the influence of intramolecular hydrogen bonds on ionization energies. Thanks to the theoretical/experimental combined approach, we can determine that a majority of ethylenediamine molecules adopts a dual-head dissociated geometry at room temperature and high coverage. In the very low coverage limit, complementary STM experiments indicate that ethylenediamine bridges two Si dimers over the trench possibly in a dual dative bond configuration. Such dative bonds are only detected by XPS after molecular adsorption at low temperature. Despite an aliphatic spacer length longer than that of ethylenediamine, 1,4-diaminobutane also adopts a dual-head dissociative geometry at room temperature.