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.

Using scanning tunnelling microscopy ͑STM͒, photoelectron and photoabsorption spectroscopies, we have examined how acrylonitrile ͑H 2 C v CH-C w N͒ reacts with the Si͑001͒-2 ϫ 1 surface for coverages ranging from ϳ10 12 molecules/ cm 2 to ϳ 10 14 molecules/ cm 2 . At 300 K, in the very low coverage regime ͑below 10 13 molecules/ cm 2 ͒, filled-and empty-state STM images show that the molecule bridges, via its ␤ carbon and nitrogen ends, two silicon dangling bonds, across the trench separating two dimer rows. A cumulative-doublebond unit ͑C v C v N͒ is formed. The 300 K STM image results from the dynamic flipping of the molecule between two equivalent equilibrium positions, which can be seen when the molecular motion is slowed down at 80 K. For coverages larger than 10 13 molecules/ cm 2 , for which STM does not show ordered adsorption any more, the adsorption kinetics were observed in real-time using valence band photoemission and resonant Auger yield, associated with N 1s x-ray absorption spectroscopy ͑NEXAFS͒. At 300 K, these techniques point to a situation more complex than the one explored by STM at very low coverage. Three species ͑cyano-bonded, vinyl-bonded, and cumulative-double-bond species͒ are detected. Their distribution does not vary with increasing coverage. All dimerization-related surface states are quenched at saturation. The uptake rates versus coverage relationship points to the presence of a mobile precursor. Finally, the paper discusses a possible mechanism leading to the formation of cross-trench C v C v N unit at low coverage, and the reasons why the product branching ratio changes with increasing coverage.

Using a combination of local -- scanning tunneling microscopy -- and spatially integrated, but chemically sensitive probes -- X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure spectroscopy -- we have examined how 3-butenenitrile reacts with the Si(001)-2 x 1 surface at room temperature. Electron spectroscopies indicate three different nitrogen chemical bonds: a Si-C=N-Si bond, a C=C=N cumulative double bond, and a CN moiety datively bonded to a silicon atom. All molecular imprints detected by scanning tunneling microscopy (STM) involve two adjacent silicon dimers in the same row. The three geometries we propose -- a double di-sigma bonding via the CN and the C=C, a cumulative double bond formation associated with alphaC-H bond dissociation, and a di-sigma vinyl bonding plus a CN datively bonded to a silicon atom -- are all compatible with electron spectroscopies and data. Real-time Auger yield kinetic measurements show that the double di-sigma bonding geometry is unstable when exposed to a continuous flux of 3-butenenitrile molecules, as the Si-C=N-Si unit transforms into a CN moiety. A model is proposed to explain this observation.

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