LEED and DFT Study of the Quasihexagonal TiO2 Structure on Cu(001)

Reactive deposition of Ti on the Cu(001) surface saturated with chemisorbed oxygen results in the formation of an ordered ultrathin film with a slightly distorted hexagonal (quasihexagonal) unit cell. We find that, under appropriate substrate temperature, evaporation rates, and O2 pressure, it is possible to cover the whole substrate surface with the TiO2 quasihexagonal phase. An O-Ti-O trilayer model for the (quasi) hexagonal structure is consistent with angle scanned XPD data relative to the Ti 2p signal and with the LEED I-V experimental intensity curves. From a careful inspection of the LEED pattern, we derive a distortion of the hexagonal unit cell corresponding to a p(2 × 7) coincidence mesh with the substrate. The p(2 × 7) coincidence was used to model XPD data. For the analysis of LEED-IV data and for DFT calculation, the p(2 × 7) was approximated by a c(2 × 6) coincidence mesh. The c(2 × 6) approximation is very close to a p(2 × 7) in terms of lattice parameters but contains a reduced number of nonequivalent atoms. From LEED-IV analysis and DFT calculations, we derive the registry between the c(2 × 6) titanium oxide mesh and the substrate. We find that interfacial O atoms located at the corners and at the center of the unit mesh sit on top of the Cu atoms at the interface. The comparison of the DFT calculations performed for the free-standing O-Ti-O trilayer with a regular hexagonal unit cell and for the trilayer on Cu with a c(2 × 6) coincidence mesh indicates that the strain energy is more than compensated by the interaction with the substrate. The values of the interlayer spacing and the registry of the oxide film derived from the DFT calculations are in good agreement with the experimental results. The DFT calculations indicate that the bonding at the interface is characterized by a direct Cu-O interaction with overlap between the O 2p and the Cu 4sp states. The calculations indicate that, because of an upshift of the O 2p bands and a downshift of the Ti 4s states, the TiO2 film loses its insulator character and the electron states close to the Fermi level are dominated by the contribution from the Ti 3d orbitals.