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We investigate, experimentally and theoretically, the initial stage of the formation of Ca/Si and Si/F structures that occurs during the adsorption of CaF${}_{2}$ molecules onto a bare Si(100) surface heated to 1000 K in a low-coverage regime (0.3 monolayer). A low-temperature (5 K) scanning tunneling microscope (STM) is used to observe the topographies and the electronic properties of the exposed silicon surfaces. Our atomic-scale study reveals that several chemical reactions arise during CaF${}_{2}$ deposition, such as dissociation of the CaF${}_{2}$ molecules and etching of the surface silicon dimers. The experimental and calculated STM topographies are compared using the density functional theory, and this comparison enables us to identify two types of reacted structures on the Si(100) surface. The first type of observed complex surface structure consists of large islands formed with a semiperiodic sequence of 3 \ifmmode\times\else\texttimes\fi{} 2 unit cells. The second one is made of isolated Ca adatoms adsorbed at specific sites on the Si(100)-2 \ifmmode\times\else\texttimes\fi{} 1 surface.

From an interplay between simultaneously recorded noncontact atomic force microscopy and scanning tunneling microscopy images and simulations based on density functional theory, we reveal the location of single hydrogen species in the surface and subsurface layers of rutile ${\mathrm{TiO}}_{2}$. Subsurface hydrogen atoms (${\mathrm{H}}_{\mathrm{sub}}$) are found to reside in a stable interstitial site as subsurface OH groups detectable in scanning tunneling microscopy as a characteristic electronic state but imperceptible to atomic force microscopy. The combined atomic force microscopy, scanning tunneling microscopy, and density functional theory study demonstrates a general scheme to reveal near surface defects and interstitials in poorly conducting materials.

The low-temperature (12 K) adsorption of single Au atoms on Si(100) is studied by scanning tunneling microscopy (STM). Comparison between experimental and calculated STM topographies as well as density-functional-theory calculations of the adsorption energies enable us to identify two adsorption configurations of Au atoms between Si-dimer rows (BDRs) and on top of Si-dimer rows (TDRs). In both adsorption configurations, the Au atoms are covalently bound to two Si atoms through a partial electron transfer from Si to Au. STM manipulation confirms that the TDR adsorption configuration is metastable, whereas the BDR one is the most stable configuration.

Starting from the theoretical prediction of the $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ structure using density-functional theory in the generalized gradient approximation, we have studied the (1 1 1), (0 0 1), (1 1 0), and (1 5 0) surfaces. The surface energies and their corresponding structures are computed and compared with predictions for (0 0 0 1) $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ and available experimental results for $\ensuremath{\gamma}$-alumina surfaces. (1 1 1) and (0 0 1) surfaces are predicted to be equally stable, but to show quite different structure and reactivity. Whereas a low coverage of highly reactive trigonal Al occurs on (1 1 1), (0 0 1) exhibits a more dense plane of both five-coordinate and tetrahedral Al. Microfaceting of a (1 1 0) surface into (1 1 1)-like planes is also observed. The implications for the structure of ultrathin dielectric films and for the surfaces of disordered transition aluminas are discussed.

We have used a quantum-chemical method developed for crystal calculations to investigate self-trapped hole polarons in technologically important BaTiO 3 perovskite-type crystal. The tetragonal structure of this material is considered in the present work, The computations are carried out in a self-consistent-field manner using the embedded molecular cluster model. The spatial configuration of a hole polaron and displacements of defect-surrounding atoms are obtained and analyzed. The probability of spontaneous hole self-trapping in a perfect crystal lattice is estimated by calculating the hole self-trapping energy as a difference of the atomic relaxation energy and the hole localization energy. This value is found to be negative, -0.87 eV, which demonstrates the preference of the self-trapped polaron state compared to a free hole state in the valence band. The computed polaron absorption energy, 0.4 eV is found to be in an excellent agreement with the available experimental data and independent estimations of the polaron theory.