Unoccupied electronic states of 2D Si on Ag-3-Si(111).

Optimizing substrate characterization to grow 2D Si layers on surfaces is a major issue toward the development of synthesis techniques of the promising silicene. We have used inverse photoemission spectroscopy (IPES) to study the electronic band structure of an ordered 2D Si layer on the3×3-Ag/Si(111) surface (3-Ag). Exploiting the large upwards band bending of the3-Ag substrate, we could investigate the evolution of the unoccupied surface and interface states in most of the Si band gap. In particular, thek∥-dispersion of the3-Ag free-electron-likeS1surface state measured by IPES, is reported for the first time. Upon deposition of ∼1 ML Si on3-Ag maintained at ∼200 °C, the interface undergoes a metal-insulator transition with the complete disappearance of theS1state. The latter is replaced by a higher-lying stateU0with a minimum at 1.0 eV aboveEF. The origin of this new state is discussed in terms of various Si 2D structures including silicene.

Peculiar covalent bonding of C60/6H-SiC(0 0 0 1)-(3 × 3) probed by photoelectron spectroscopy.

High resolution photoemission with synchrotron radiation was used to study the interface formation of a thin layer of C60 on 6H-SiC(0 0 0 1)-(3 × 3), characterized by protruding Si-tetramers. The results show that C60 is chemisorbed by orbital hybridization between the highest-occupied molecular orbital (HOMO) and the p z orbital of Si adatom at the apex of the tetramers. The covalent nature of the bonding was inferred from core level as well as valence band spectra. The Si 2p spectra reveal that a large fraction (at least 45%) of the Si adatoms remain unbound despite the reactive character of the associated dangling bonds. This is consistent with a model in which each C60 is attached to the substrate through a single covalent C60-Si bond. A binding energy shift of the core levels associated with sub-surface Si or C atoms indicates a decrease of the SiC band bending caused by a charge transfer from the C60 molecules to the substrate via the formation of donor-like interface states.

Phase separation in potassium-doped ZnPc thin films.

In this study synchrotron radiation was used to investigate the electronic properties of a thin film of zinc-phthalocyanine (ZnPc) deposited on Si(001)-2x1 and progressively doped with K atoms. The molecular orientation was probed by angular-dependent x-ray absorption spectroscopy and the molecules were found to lie with the macrocycle plane roughly perpendicular to the surface. The evolution of the electronic properties of the film was then followed by measuring the photoemission spectra upon in situ evaporation of K atoms on the pristine ZnPc film. The results show that doping proceeds through charge donation from the K atoms to the molecular units whose lowest unoccupied molecular orbital (LUMO) becomes progressively filled. Despite the fact that the LUMO spectral weight increases as the stoichiometry x in the K(x)ZnPc compound varies from about 1 to 4 (as determined by core level photoemission), no detectable density of states was observed at the Fermi level, showing that the film remains insulating for all the investigated stoichiometries. On the other hand the C 1s spectra, which appear merely broadened at the earliest stages of doping (x approximately 1), clearly develop two distinct components when x exceeds 2, suggesting that the charge state is not the same for all the molecules. At the same time, the modification of the valence band points towards the coexistence of two distinct phases with x=2 and x=4.