The structure of SrTiO3(001)-2 × 1 surface analyzed by high-resolution medium energy ion scattering coupled with ab initio calculations

High-resolution medium energy ion scattering (MEIS) spectrometry coupled with photoelectron spectroscopy revealed unambiguously that the initial SrTiO3(001) surface chemically etched in a buffered NH4F-HF solution was perfectly terminated with a single-layer (SL) of TiO2(001) and annealing the surface at 600-800 [ordinal indicator, masculine]C in ultrahigh vacuum (UHV) led to a (2 × 1)-reconstructed surface terminated with a double-layer (DL) of TiO2(001). After annealing in UHV, rock-salt SrO(001) clusters with two atomic layer height grew epitaxially on the DL-TiO2(001)-2 × 1 surface with a coverage of 20%-30%. High-resolution MEIS in connection with ab initio calculations demonstrated the structure of the DL-TiO2(001)-2 × 1 surface close to that proposed by Erdman et al. [Nature (London) 419, 55 (2002)] rather than that predicted by Herger et al. [Phys. Rev. Lett. 98, 076102 (2007)]. Based on the MEIS analysis combined with the ab initio calculations, we propose the most probable (2 × 1) surface structure.

Control of the surface electronic structure of SrTiO3(001) by modulation of the density of oxygen vacancies

The influence of electron irradiation and the subsequent oxygen adsorption on the electronic structure of an SrTiO3(001) surface was investigated by ultraviolet photoemission spectroscopy (UPS). Electron irradiation induced an in-gap state (IGS) as observed by UPS keeping the surface 1 × 1, which is considered to originate from oxygen vacancies on the topmost surface due to the electron-stimulated desorption (ESD) of oxygen. Electron irradiation also caused a downward shift of the valence band maximum, indicating downward band bending and the formation of a conductive layer on the surface. Adsorption of oxygen on the electron-irradiated surface, on the other hand, reduced the intensity of the IGS along with yielding upward band bending, which points to disappearance of the conductive layer. The results show that ESD and oxygen adsorption can be used to control the surface electronic structure switching between semiconducting and metallic regimes by changing the density of the oxygen vacancies.

Atomic and Electronic Structure of the BaTiO(3)(001) (sqrt[5] × sqrt[5])R26.6° Surface Reconstruction

This contribution presents a study of the atomic and electronic structure of the (sqrt[5] × sqrt[5])R26.6° surface reconstruction on BaTiO(3) (001) formed by annealing in ultrahigh vacuum at 1300 K. Through density functional theory calculations in concert with thermodynamic analysis, we assess the stability of several BaTiO(3) surface reconstructions and construct a phase diagram as a function of the chemical potential of the constituent elements. Using both experimental scanning tunneling microscopy (STM) and scanning tunneling spectroscopy measurements, we were able to further narrow down the candidate structures, and conclude that the surface is either TiO(2)-Ti(3/5), TiO(2)-Ti(4/5), or some combination, where Ti adatoms occupy hollow sites of the TiO(2) surface. Density functional theory indicates that the defect states close to the valence band are from Ti adatom 3d orbitals (≈ 1.4 eV below the conduction band edge) in agreement with scanning tunneling spectroscopy measurements showing defect states 1.56 ± 0.11 eV below the conduction band minimum (1.03 ± 0.09 eV below the Fermi level). STM measurements show electronic contrast between empty and filled states' images. The calculated local density of states at the surface shows that Ti 3d states below and above the Fermi level explain the difference in electronic contrast in the experimental STM images by the presence of electronically distinctive arrangements of Ti adatoms. This work provides an interesting contrast with the related oxide SrTiO(3), for which the (001) surface (sqrt[5] × sqrt[5])R26.6° reconstruction is reported to be the TiO(2) surface with Sr adatoms.