Surface rumples and band gap reductions of cubic BaZrO3 （001） surface studied by means of first-principles calculations

Electronic properties of the （001） surface of cubic BaZrO3 with BaO and ZrO2 terminations have been studied using first-principles calculations. Surface structure, partial density of states, band structure and surface energy have been obtained. We find that the largest relaxation appears in the first layer of atoms, and the relaxation of the BaO-terminated surface is larger than that of the ZrO2-terminated surface. The surface rumpling of the BaO-terminated surface is also larger than that of the ZrO2-terminated surface. Results of surface energy calculations reveal that the BaZrO3 surface is likely to be more stable than the PbZrO3 surface.     

First Principles Study of Half Metallic Properties of VSb Surface and VSb/GaSb （001） Interface

The structural, electronic, and magnetic properties of VSb in zincblende, and NiAs phases, VSb （001） film surfaces and its interfaces with GaSh （001） have been investigated within the framework of the density functional theory using the FPLAPW＋lo approach. The NiAs structure is more stable than the ZB phase, ZB VSb is found to a half-metallic ferromagnetic. The V-terminated surfaces retain the half-metallic character, while the half-metallicity is destroyed for Sb-terminated surfaces due to surface states, which originate from p electrons. The phase diagram obtained through the ab-initio atomistic thermodynamics shows that the formation energy of ZB VSb is about 0.1 Ryd. The half-metallicity character is also preserved at VSb/GaSb （001） interface. The conduction band minimum （CBM） of VSb in the minority spin case lies about 0.47 eV above that of GaSb, suggesting that the majority spin can be injected into GaSb without being flipped to the conduction bands of the minority spin.     

Effect of （2 ×1） Surface Reconstruction on Elasticity of a Silicon Nano-Plate

A semi-continuum approach is developed to describe the effect of （2 ×1） surface reconstruction on the elastic modulus of the silicon nano-plate. Young＇s moduli of a （001） silicon nano-plate along the high-symmetry [100] direction are obtained with and without considering （2 × 1） surface reconstruction. The approach predicts that the nano-plate with unreconstructed （001） surface is elastically softer than the bulk while it exhibits the opposite behaviour with （2 × 1） reconstructed surface. On the （001） surface, the （2 × 1） reconstructed surface dominates the plate as the thickness of the plate scaling decreases to several tens of nanometre. Whether the nano-plate is softer or stiffer depends on bond loss, bond saturation and direction of bond alignment, which have major impacts on the mechanics of the nano-plate.     

Ab Initio Calculations for the BaTiO3 （001） Surface Structure

The ab initio method within the local density approximation is applied to calculate cubic BaTiO3 (001) surface relaxation and rumpling for two different terminations (BaO and TiO2 ). Our calculations demonstrate that cubic perovskite BaTiO3 crystals possess surface polarization, accompanied by the presence of the relevant electric field. We analyse their electronic structures (band structure, density of states and the electronic density redistribution with emphasis on the covalency effects). The results are also compared with that of the previous ab initio calculations. Considerable increases of Ti-O chemical bond covalency nearby the surface have been observed. The band gap reduces especially for the TiO2 termination.     

Structural and physical properties of BiFeO3 thin films epitaxially grown on SrTiO3（001） and polar（111） surfaces

The influence of surface polarity on the structural properties of BiFeO3 （BFO） thin films is investigated. BFO thin films are epitaxially grown on SrTiO3 （STO） （100） and polar （111） surfaces by oxygen plasma-assisted molecular beam epitaxy. It is shown that the crystal structure, surface morphology, and defect states of BFO films grown on STO substrates with nonpolar （001） or polar （111） surfaces perform very differently. BFO/STO （001）is a fully strained tetragonal phase with orientation relationship （001）[100]BFOII（001）[100]STO, while BFO/STO （111） is a rhombohedral phase. BFO/STO （111） has rougher surface morphology and less defect states, which results in reduced leakage current and lower dielectric loss. Moreover, BFO films on both STO （001） and STO （111） are direct band oxides with similar band gaps of 2.65 eV and 2.67 eV, respectively.     

Atomic-Scale Study of Ge-Induced Incommensurate Phases on Si（111）

Two Ge-induced incommensurate phases, γ and β, on Si（111） are observed and studied by {／it in situ} scanning tunneling microscopy. The γ phase consists of aligned triangular domains whose stacking sequence is faulted with respect to the Si（111）-1×1 surface. The β phase consists of two kinds of triangular domains whose stacking sequences are faulted and unfaulted with respect to the Si（111）-1×1 surface, respectively. In the β phase, two types of domain walls, zigzag＇＇ and face-to-face＇＇, form to release the strain. The triangular domains all exhibit a quasi-1×1 hexagonal close-packed structure. By studying the structural evolution from magic clusters to incommensurate structures, the structure models for γ and β phases are proposed.     

The structure and the analytical potential energy function of NH2 （X^2B1）

This paper reports that the equilibrium structure of NH2 has been optimized at the QCISD/6-311＋＋G （3df, 3pd） level. The ground-state NH2 has a bent （C2v, X^2B1） structure with an angle of 103.0582°. The geometrical structure is in good agreement with the other calculational and experimental results. The harmonic frequencies and the force constants have also been calculated. Based on the group theory and the principle of microscopic reversibility, the dissociation limits of NH2（C2v, X^2B1） have been derived. The potential energy surface of NH2（X^2B1） is reasonable. The contour lines are constructed, the structure and energy of NH2 reappear on the potential energy surface.     

Atomic Layer Deposition of Al2O3 on H-Passivated GeSi： Initial Surface Reaction Pathways with H/GeSi（100）-2 × 1

The reaction mechanisms of Al（CH3 ）3 （TMA） adsorption on H-passivated GeSi（100）-2 × 1 surface are investigated with density functional theory. The Si-Ge and Ge-Ge one-dimer cluster models are employed to represent the GeSi（100）-2 × 1 surface with different Ge compositions. For a Si-Ge dimer of a H-passivated SiGe surface, TMA adsorption on both Si-H^＊ and Ge-H^＊ sites is considered. The activation barrier of TMA with the Si-H^＊ site （1.2eV） is higher than that of TMA with the Ge-H^＊ site （0.91 eV）, which indicates that the reaction proceeds more slowly on the Si-H^＊ site than on the Ge-H^＊ site. In addition, adsorption of TMA is more energetically favorable on the Ge-Ge dimer than on the Si-Ge dimer of H-passivated SiGe.     

Surface rumpling of cubic CaTiO3 from density functional theory

In this paper, the structure of cubic CaTiO3 （001） surfaces with CaO and TiO2 terminations has been studied from density functional calculations. It has been found that the Ca atom has the largest relaxation for both kinds of terminations, and the rumpling of the CaO-terminated surface is much larger than that of TiO2-terminated surface. Also we have found that the metal atom relaxes much more prominently than the O atom does in each layer. The CaO-terminated surface is slightly more energetically favourahle than the TiO2-terminated surface from the analysis of the calculated surface energy.     

First-Principles Calculations of Atomic and Electronic Properties of T1 and In on Si（111）

The atomic and electronic structures of T1 and In on Si（111） surfaces are investigated using the firstprinciples total energy calculations. Total energy optimizations show that the energetically favored structure is 1/3 ML T1 adsorbed at the T4 sites on Si（111） surfaces. The adsorption energy difference of one T1 adatom between （√3 × √3） and （1 × 1） is less than that of each In adatom. The DOS indicates that TI 6p and Si 3p electrons play a very important role in the formation of the surface states. It is concluded that the bonding of TI adatoms on Si（111） surfaces is mainly polar covalent, which is weaker than that of In on Si（111）. So T1 atom is more easy to be migrated than In atom in the same external electric field and the structures of T1 on Si（111） is prone to switch between （√3 × √3） and （1 × 1）.     

Chemisorption of Fe on Au-passivated Si（001） surface

The chemisorption of one monolayer of Fe atoms on a Au-passivated Si(001) surface is studied by using the self-consistent tight-binding linear muffin-tin orbital method. The Fe adatom chemisorption on an ideal Si(001) surface is also considered for comparison. The chemisorption energy and layer projected density of states for a monolayer of Fe atoms on Au-passivated Si(001) surface are calculated and compared with that of the Fe atoms on an ideal Si(001) surface. The charge transfer is investigated. It is found that the most stable position is at the fourfold hollow site for the adsorbed Fe atoms, which might sit below the Au surface. Therefore there will be a Au－Fe mixed layer at the Fe/Au－Si(100) interface. It is found that the adsorbed Fe atoms cannot sit below the Si surface, indicating that a buffer layer of Au atoms may hinder the intermixing of Fe atoms and Si atoms at the Fe/Au－Si(001) interface effectively, which is in agreement with the experimental results.     

Structural Stabilities of Ordered Arrays of Nb4 Clusters on NaCI（IO0） Surface

Adsorption of ordered （2 × 2） arrays of Nb4 clusters on the insulating surface of NaCI（100） is studied by the first-principles calculations within the density functional theory. The calculations on the relaxed geometries and cohesive energies show that both the tetrahedron and quadrangle-Nb4 can be stably adsorbed on this substrate, which may have important applications. The adsorption of quadrangle-Nb4 on the NaCl（100） surface is more stable than that of tetrahedron-Nb4. Both the Nb4 clusters studied and a single Nb atom prefer the top site of the Cl atom in the NaCl（100） surface. Electronic structure analysis suggests that the interactions between the Nb4 clusters and the substrate are weak.     

Adsorption of L-Alanine on Cu（111） Studied by Scanning Tunnelling Microscopy

The adsorption of L-alanine on Cu（111） surface is studied by means of scanning tunnelling microscopy under ultra-high vacuum conditions. The results show that the adsorbates are chemisorbed on the surface, and can form a two-dimensional gas phase, chain phase and solid phase, depending on deposition rate and amount. The adsorbed molecules can be imaged as individual protrusions and parallel chains in gas and chain phases respectively. It is also found that alanine can form （2 × 2） superstructure on Cu（111） and copper step facet to （110） directions in solid phase. On the basis of our scanning tunnelling microscopic images, a model is proposed for the Cu（111）（2 ×2）-alanine superstructure. In the model, we point out the close link between （110）-direction hydrogen bond chains with the same direction copper step faceting.     

Growth of Mn5Ge3 ultrathin film on Ge（111）

The growth of Mn5Ge3 ultrathin films with different thicknesses, prepared by solid phase epitaxy, is studied. The results of scanning tunnelling microscopy and low energy electron diffraction studies show that the film can be formed and it is terminated with a （√3 × √3） R30° surface reconstruction when the thickness of Mn exceeds 3 monolayers. The magnetic properties show that the Curie temperature is about 300 K and the T^2-dependent behaviour is observed to remain up to 220 K.     

First-principles study of the （001） surface of cubic PbHfO3 and BaHfO3

Using first-principles techniques,we investigate the(001) surfaces of cubic PbHfO3(PHO) and BaHfO3(BHO) terminated with both AO(A=Pb and Ba) and HfO2.Surface structure,partial density of states,band structure,and surface energy are obtained.The BaO surface is found to be similar to its counterpart in BHO.For the HfO2-terminated surface of cubic PHO,the largest relaxation appears on the second-layer atoms but not on the first-layer ones.The analysis of the structure relaxation parameters reveals that the rumpling of the(001) surface for PHO is stronger than that for BHO.The surface thermodynamic stability is explored,and it is found that both the PbO-and the BaO-terminated surfaces are more stable than the HfO2-terminated surfaces for PHO and BHO,respectively.The surface energy calculations show that the(001) surface of PHO is more easily constructed than that of BHO.     

Atomic and Electronic Structures of Cd0.96Zn0.04Te（l10） Surface

X-Ray diffraction is used to analyse the lattice structure of Cdo.96Zno.o4 Te （CZT）, and the lattice constant is measured to be 0.647nm. The atomic structure of the clean CZT（110） surface obtained by Ar^＋ etching in vacuum is observed by low-energy electron diffraction, where no surface reconstruction is discovered. Angleresolved photoemission spectroscopy was used to characterize the surface state of the clean CZT （110） surface, by which we find a 1.5-eV-wide surface band with the peak at 0.9eV below the Fermi energy containing about 6.9 × 10^14 electrons/cm^2, approximately one electron per surface atom.     

Bi- and Au-Induced Reconstructions on GaAs（001）-2 × 4 Surface

Submonolayer Bi and Au adsorptions on the GaAs（001）-2× 4 surface are investigated by scanning tunnelling microscopy, low energy electron diffraction and first-principles calculations. The 1 ×4 and 3 × 4 reconstructed surface induced by Bi and Au, respectively, are revealed and their structural models are proposed based on experiments and first-principles calculations. Moreover, the validity of the recently proposed generalized electron counting （GEC） model [Phys. Rev. Lett. 97 （2006） 126103] is examined in detail by using the two surfaces. The GEC model perfectly explains the structural features, such Bi-1 × 4 surface and the 3x arrangement of four-atom Au as the characteristic short double-line structure in the clusters.     

Density Functional Calculation of the 0.5ML-Terminated Allyl Mercaptan/Si（100）-（2 × 1） Surface

The structural and electronic properties of the 0.5 ML-terminated allyl mercaptan （ALM）/Si（IO0）-（2 x 1） surface are studied using the density functional method. The calculated absorption energy of the ALM molecule on the 0.5 ML-terminated ALM/Si（IO0）-（2 x 1） surface is 3.36eV, implying that adsorption is strongly favorable. The electronic structure calculations show that the ALM/Si（IO0）-（2 x 1）, the clean Si（100）-（2 x 1）, and the fully-terminated H/Si（IO0）-（2 ~ 1） surfaces have the nature of an indirect band gap semiconductor. The highest occupied molecular orbital is dominated by the ALM, confirming the mechanism proposed by Hossain for its chain reaction.     

Size Driven Ferroelectric—paraelectric Phase Transition from the Surface Energy Viewpoint

The size driven ferroelectric-paraelectric phase transition in a ferroelectric of small size is studied within the framework of Landau phase transition theory.The transition is a consequence of the competition between decreasing the volume free energy by polarization and increasing the surface energy of the ferroelectric phase,which has a surface energy density higher than that in the paraelectric phase.A simple expression for the ferroelectric critical size as a function of the Landau free energy coefficients and the surface energy density is derived.     

Initial Processes of Sulfur Adsorption on Si（100） Surface

The adsorption of one monolayer S atoms on ideal Si（100） surface is studied by using the self-consistent tight binding linear muffon-tin orbital method. Energies of adsorption systems ors atoms on different sites are calculated. It is found that the adsorbed S atoms are more favorable on B1 site （bridge site） with a distance 0.131 nm above the Si surface. The .S, Si mixed layer might exist at S/Si（100） interface. The layer projected density of states are calculated and compared with that of the clean surface. The charge transfers are also investigated.