Atomic structures of reduced SrTiO3(001) surfaces

The scanning tunneling microscope has been employed to study the reduced SrTiO₃(001) surface in ultrahigh vacuum. Topographic images show a change of structural ordering for the surface annealed under three different conditions. In addition, our STM images reveal row-like structures with spacings of 12 and 20 Å. The observations are discussed in terms of the formation of intergrowths of lamellae of Sr_{n + 1} Ti_nO_{3n + 1}; a result of accommodation of oxygen and titanium deficiencies.     

Atomic structure of InSb(001) and GaAs(001) surfaces imaged with noncontact atomic force microscopy

Noncontact atomic force microscopy (NC-AFM) has been used to study the c(8x2) InSb(001) and the c(8x2) GaAs(001) surfaces prepared by sputter cleaning and annealing. Atomically resolved tip-surface interaction maps display different characteristic patterns depending on the tip front atom type. It is shown that representative AFM maps can be interpreted consistently with the most recent structural model of A(III)B(V)(001) surface, as corresponding to the A(III) sublattice, to the B(V) sublattice, or to the combination of both sublattices.     

Atomic and electronic structures of carbon nanotubes on Si(001) stepped surfaces

We report first-principles total-energy calculations that provide energetics and electronic structures of adsorbed carbon nanotubes (CNTs) on stepped Si(001) surfaces. We find that adsorption energies strongly depend on the directions of CNTs, and that there are several metastable adsorption sites both on terraces and near step edges. We also find that the electronic structure of adsorbed metallic CNTs becomes semiconducting or remains metallic, depending on the adsorption site. Charge redistribution upon adsorption is prominent mainly at the CNT-surface interface.     

Atomic structures of gallium-rich GaAs(001)-4×2 and GaAs(001)-4×6 surfaces

Scanning tunneling microscopy is applied for the first time to an atomic-resolution investigation of the 4×2 and 4×6 phases on a gallium-rich GaAs(001) surface obtained by molecular-beam epitaxy and migration-enhanced epitaxy. A unified structural model is proposed with consideration of the results of experiments and first-principles calculations of the total energy. In this model the 4×2 phase consists of two Ga dimers in the top layer and a Ga dimer in the third layer, and the 4×6 phase is matched to periodically arranged Ga clusters at the corners of a 4×6 unit cell on top of the 4×2 phase. Zh. éksp. Teor. Fiz. 111, 1858–1868 (May 1997)     

Anisotropic interaction of Ag-induced missing dimer vacancies on Si(001) surfaces

Ag-induced missing dimer vacancies on Si(001)2 × 1 surfaces are shown to form vacancy lines when the vacancy concentration is larger than ∼10%. The formation of the vacancy lines is driven by the short-range attractive interaction between the vacancies in adjacent dimer rows and the long-range repulsive interaction between them in the same dimer row. The form and magnitudes of the interactions are derived from the thermally-excited wandering of the vacancy lines.     

Surface vibrations of Na(001) and K(001) surfaces

We present the results of a theoretical study of the dynamics of the atom motion of Na(001) and K(001) surfaces. The total electronic energy is calculated using a pseudopotential approach with a confined electron gas as unperturbed system. With this theory the dynamical matrix can he derived without resorting to empirical parametrizations. Surface phonon dispersion curves are reported for the high symmetry directions of the two-dimensional Brillouin zone for ideal and relaxed configurations. The calculated spectra are compared with the results of semi-empirical force constant calculations. The effects of single and multilayer relaxations on the location and the nature of the main surface bands are examined.     

Adsorption of water molecule on (001) and (110) surfaces of MgH2

First principle calculations have been performed to explore the adsorption characteristics of water molecule on (001) and (110) surfaces of magnesium hydride. The stable adsorption configurations of water molecule on the surfaces of MgH₂ were identified by comparing the total energies of different adsorption states. The (110) surface shows a higher reactivity with H₂O molecule owing to the larger adsorption energy than the (001) surface, and the adsorption mechanisms of water molecule on the two surfaces were clarified from electronic structures. For both (001) and (110) surface adsorptions, the O p orbitals overlapped with the Mg s and p orbitals leading to interactions between O and Mg atoms and weakening the O–H bonds in water molecule. Due to the difference of the bonding strength between O and Mg atoms in the (001) and (110) surfaces, the adsorption energies and configurations of water molecule on the two surfaces are significantly different.     

III-V compound semiconductor (001) surfaces

There has been renewed interest in the structure of III-V compound semiconductor (001) surfaces caused by recent experimental and theoretical findings, which indicate that geometries different from the seemingly well-established dimer models describe the surface ground state for specific preparation conditions. I review briefly the structure information available on the (001) surfaces of GaP, InP, GaAs and InAs. These data are complemented with first-principles total-energy calculations. The calculated surface phase diagrams are used to explain the experimental data and reveal that the stability of specific surface structures depends largely on the relative size of the surface constituents. Several structural models for the Ga-rich GaAs (001)(4×6) surface are discussed, but dismissed on energetic grounds. I discuss in some detail the electronic properties of the recently proposed cation-rich GaAs (001)ζ(4×2) geometry. Received: 18 May 2001 / Revised version: 23 July 2001 / Published online: 3 April 2002     

Structural phase transition on Si(001) and Ge(001) surfaces

Among a variety of solid surfaces, Si(001) and Ge(001) have been most extensively studied. Although they seem to be rather simple systems, there have been many conflicting arguments about the atomic structure on these surfaces. We first present experimental evidence indicating that the buckled dimer is the basic building block and that the structural phase transition between the low-temperature c(4x2) structure and the high-temperature (2x1) structure is of the order-disorder type. We then review recent theoretical work on this phase transition. The real system is mapped onto a model Ising-spin system and the interaction parameters are derived from total-energy calculations for different arrangements of buckled dimers. The calculated critical temperature agrees reasonably well with the experimental one. It is pointed out that the nature of the phase transition is crucially affected by a small amount of defects on the real surfaces.     

Atomic structure of the (001) surface of CuGaSe2

Hybrid exchange density functional theory is used to study the wide band gap chalcopyrite CuGaSe₂. The formation energies of the experimentally observed (4 × 1) and (1 × 1) atomic scale reconstructions on the CuGaSe₂ (001) surface are calculated for different environmental conditions. The results suggest that a Se-rich (1 × 1) reconstruction, and a Cu-poor, Se-rich (4 × 1) reconstruction, are the only stable surfaces under all the studied environmental conditions. Two complementary mechanisms for the stabilisation of CuGaSe₂ surfaces are proposed, and it is suggested that the presence of Na stabilises the (4 × 1) reconstructions, making them the stable terminations under Na-rich conditions.     

Structure of III-Sb(001) growth surfaces: the role of heterodimers

We have determined the structure of AlSb and GaSb (001) surfaces prepared by molecular beam epitaxy under typical Sb-rich device growth conditions. Within the range of flux and temperature where the diffraction pattern is nominally (1x3), we find that there are actually three distinct, stable (4x3) surface reconstructions. The three structures differ from any previously proposed for these growth conditions, with two of the reconstructions incorporating mixed III-V dimers within the Sb surface layer. These heterodimers appear to play an important role in island nucleation and growth.     

Atomic and electronic structures of GaAs (001) surface

The atomic and electronic structures of α, β, β₂, and ζ reconstructions for the Ga-terminated GaAs (001)-(4×2) surface are investigated in the framework of the pseudopotential approach. Total, surface, and local densities of electron states, electron-energy spectra, and relative surface energies of the structures under consideration are calculated. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 44–52, October, 2006.     

Dynamic ion pairs in the adsorption of isolated water molecules on alkaline-earth oxide (001) surfaces

Low coverage water adsorption on alkaline-earth oxides is studied by first principles methods and ab initio molecular dynamic simulations. On CaO and BaO(001), water dissociation is thermodynamically favored in contrast with the molecular adsorption found for MgO(001). On CaO(001), the barrier for water splitting is very small leading to a tight ion pair H-OH that exhibits a dynamic character with the hydroxyl group able to visit four equivalent adsorption sites while remaining attached to the proton. In contrast, ion pair separation is endothermic by 0.8 eV. These results are common to other basic surfaces such as BaO(001) and have important implications in the chemistry of partially hydroxylated oxide surfaces.