Molecular staples on Si(001)-2 × 1: Dual-head primary amines

The present study aims at determining the bonding configurations of bifunctional primary amines, ethylenediamine (H 2N-[CH 2] 2-NH 2) and 1,4-diaminobutane (H 2N-[CH 2] 4-NH 2), on Si(001)-2 × 1. We carry out ? 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.