Characterization of Ag adsorption on TiC（001） substrate： an ab initio study

Ag adsorptions at 0.25-3 monolayer （ML） coverage on a perfect TIC（001） surface and at 0.25 ML coverage on C vacancy are separately investigated by using the pseudopotential-based density functional theory. The preferential adsorption sites and the adsorption-induced modifications of electronic structures of both the substrate and adsorbate are analysed. Through the analyses of adsorption energy, ideal work of separation, interface distance, projected local density of states, and the difference electron density, the characteristic evolution of the adatom-surface bonding as a function of the amount of deposited silver is studied. The nature of the Ag/TiC bonding changes as the coverage increases from 0.25 to 3 MLs. Unlike physisorption in an Ag/MgO system, polar covalent component contributes to the Ag/TiC interfacial adhesion in most cases, however, for the case of 1-3 ML coverage, an additional electrostatic interaction between the absorption layer and the substrate should be taken into account. The value of ideal work of separation, 1.55 J/m^2, for a 3-ML-thick adlayer accords well with other calculations. The calculations predict that Ag does not wet TIC（001） surface and prefers a three-dimensional growth mode in the absence of kinetic factor. This work reports on a clear site and coverage dependence of the measurable physical parameters, which would benefit the understanding of Ag/TiC（001） interface and the analysis of experimental data.     

Methane adsorption on graphite（0001） films： A first-principles study

Using first-principles methods, we have systematically investigated the electronic density of states, work function, and adsorption energy of the methane molecule adsorbed on graphite(0001) films. The surface energy and the interlayer relaxation of the clean graphite(0001) as a function of the thickness of the film were also studied. The results show that the interlayer relaxation is small due to the weak interaction between the neighboring layers. The one-fold top site is found most favourable on substrate for methane with the adsorption energy of 133 meV. For the adsorption with different adsorption heights above the graphite film with four layers, the methane is found to prefer to appear at about 3.21 A above the graphite. We also noted that the adsorption energy does not dependent much on the thickness of the graphite films. The work function is enhanced slightly by adsorption of methane due to the slight charge transfer from the graphite surface to the methane molecule.     

Ar adsorptions on Al （111） and Ir （111） surfaces： a first-principles study

We investigate the adsorptions of Ar on Al (111) and Ir (111) surfaces at the four high symmetry sites,i.e.,top,bridge,fcc-and hcp-hollow sites at the coverage of 0.25 monolayer (ML) using the density functional theory within the generalized gradient approximation of Perdew,Burke and Ernzerhof functions.The geometric structures,the binding energies,the electronic properties of argon atoms adsorbed on Al (111) and Ir (111) surfaces,the difference in electron density between on the Al (111) surface and on the Ir (111) surface and the total density of states are calculated.Our studies indicate that the most stable adsorption site of Ar on the Al (111) surface is found to be the fcc-hollow site for the (2 × 2) structure.The corresponding binding energy of an argon atom at this site is 0.538 eV/Ar atom at a coverage of 0.25 ML.For the Ar adsorption on Ir (111) surface at the same coverage,the most favourable site is the hcp-hollow site,with a corresponding binding energy of 0.493 eV.The total density of states (TDOS) is analysed for Ar adsorption on Al (111) surface and it is concluded that the adsorption behaviour is dominated by the interaction between 3s,3p orbits of Ar atom and the 3p orbit of the base Al metal and the formation of sp hybrid orbital.For Ar adsorption on Ir (111) surface,the conclusion is that the main interaction in the process of Ar adsorption on Ir (111) surface comes from the 3s and 3p orbits of argon atom and 5d orbit of Ir atom.     

Comparative study of adsorption characteristics of Cs on the GaN （0001） and GaN （0001） surfaces

The adsorption characteristics of Cs on GaN （0001） and GaN （0001） surfaces with a coverage from 1/4 to 1 monolayer have been investigated using the density functional theory with a plane-wave uttrasoft pseudopotential method based on first-principles calculations. The results show that the most stable position of the Cs adatom on the GaN （0001） surface is at the N-bridge site for 1/4 monolayer coverage. As the coverage of Cs atoms at the N-bridge site is increased, the adsorption energy reduces. As the Cs atoms achieve saturation, the adsorption is no longer stable when the coverage is 3/4 monolayer. The work function achieves its minimum value when the Cs adatom coverage is 2/4 monolayer, and then rises with Cs atomic coverage. The most stable position of Cs adatoms on the GaN （000i） surface is at H3 site for 1/4 monolayer coverage. As the Cs atomic coverage at H3 site is increased, the adsorption energy reduces, and the adsorption is still stable when the Cs adatom coverage is 1 monolayer. The work function reduces persistently, and does not rise with the increase of Cs coverage.     

A density functional theory study on the adsorption of CO and O2 on Cu-terminated Cu2O （111） surface

The adsorptions of CO and 02 molecules individually on the stoichiometric Cu-terminatcd Cu20 （111） surface are investigated by first-principles calculations on the basis of the density functional theory. The calculated results indicate that the CO molecule preferably coordinates to the Cu2 site through its C atom with an adsorption energy of-1.69 eV, whereas the 02 molecule is most stably adsorbed in a tilt type with one O atom coordinating to the Cu2 site and the other O atom coordinating to the Cul site, and has an adsorption energy of -1.97 eV. From the analysis of density of states, it is observed that Cu 3d transfers electrons to 2π orbital of the CO molecule and the highest occupied 5σ orbital of the CO molecule transfers electrons to the substrate. The sharp band of Cu 4s is delocalized when compared to that before the CO molecule adsorption, and overlaps substantially with bands of the adsorbed CO molecule. There is a broadening of the 2π orbital of the 02 molecule because of its overlapping with the Cu 3d orbital, indicating that strong 3d-2π interactions are involved in the chemisorption of the 02 molecule on the surface.     

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）.     

Pits and adatoms at the interface of 1-ML C84/Ag （111）

We prepare a well-defined C84 monolayer on the surface of Ag （111） and study the geometric structure by scanning tunneling microscopy （STM）. The C84 molecules form a nearly close-packed incommensurate R30° lattice. The lattice is long-distance ordered with numerous local disorders. The monolayer exhibits complex bright/dim contrast; the largest height difference between the molecules can be greater than 0.4 nm. Annealing the monolayer at 380 ℃ can desorb part of the molecules, but more than sixty percent molecules stay on the Ag （111） surface even after the sample has been annealed at 650 ℃. Our analyses reveal that the 7-atom pits form beneath many molecules. Some other molecules sit at the 1-atom pits. Ag adatoms （those removed substrate atoms, accompanying the pit formation） play a very important role in this system. The adatoms can either stabilize or destabilize the monolayer, depending on the distribution manner of the adatoms at the interface. The distribution manner is determined by the co-play of the following factors： the dimension of the interstitial regions of the C84 overlayer, the number of the adatoms, and the long-distance migration of part adatoms.     

Effect of the stoichiometry on the electronic structure of the Ni（111）/α-Al2O3（0001） interface： a first-principles investigation

In this paper first-principles calculations of Ni（111）/α-Al2O3（0001） interfaces have been performed, and are compared with the preceding results of the Cu （111）/α-Al2O3（0001） interface [2004 Phil. Mag. Left. 84 425]. The AI- terminated and O-terminated interfaces have quite different adhesion mechanisms, which are similar to the Cu（111）/α Al2O3（0001） interface. For the O-terminated interface, the adhesion is caused by the strong O-2p/Ni-3d orbital hybridization and ionic interactions. On the other hand, the adhesion nature of the Al-terminated interface is the image-like electrostatic and Ni-Al hybridization interactions, the latter is substantial and cannot be neglected. Charge transfer occurs from Al2O3 to Ni, which is opposite to that in the O=terminated interface. The charge transfer direction for the Al-terminated and O-terminated Ni（111）/α-A1203（0001） interfaces is similar to that in the corresponding Cu（111）/α- Al2O3（0001） interface, but there exist the larger charge transfer quantity and consequent stronger adhesion nature, respectively.     

Structure and Magnetic Properties of （In,Mn）As Based Core-Shell Nanowires Grown on Si（111） by Molecular-Beam Epitaxy

We report the structure and magnetic properties of （In,Mn）As based core-shell nanowires grown on Si （111） by molecular-beam epitaxy. Compared to the core InAs nanowire with a flat side facet and consistent diameter, the core-shell nanowire shows a rough sidewall and an inverse tapered geometry. X-ray diffraction, transmission electron microscopy and energy-dispersive x-ray spectroscopy show that （In,Mn）As is formed on the side facets of In As nanowires with a mixture ofwurtzite and zinc-blende structures. Two ferromagnetic transition temperatures of （In,Mn）As from magnetic measurement data are observed： one is less than 25 K, which could be attributed to the magnetic phase with diluted Mn atoms in the InAs matrix, and the other is at ～300 K, which may originate from the undetectable secondary phases such as MnAs nanoclusters. The synthesis of （In,Mn）As based core-shell nanowires provides valuable information to exploit a new type of spintronic nano-materials.     

A density functional theory study on the adsorption of CO and O₂ on Cu-terminated Cu₂O（111） surface

The adsorptions of CO and O2 molecules individually on the stoichiometric Cu-terminated Cu2O(111) surface are investigated by first-principles calculations on the basis of the density functional theory.The calculated results indicate that the CO molecule preferably coordinates to the Cu2 site through its C atom with an adsorption energy of -1.69 eV,whereas the O2 molecule is most stably adsorbed in a tilt type with one O atom coordinating to the Cu2 site and the other O atom coordinating to the Cu1 site,and has an adsorption energy of -1.97 eV.From the analysis of density of states,it is observed that Cu 3d transfers electrons to 2π orbital of the CO molecule and the highest occupied 5σ orbital of the CO molecule transfers electrons to the substrate.The sharp band of Cu 4s is delocalized when compared to that before the CO molecule adsorption,and overlaps substantially with bands of the adsorbed CO molecule.There is a broadening of the 2π orbital of the O2 molecule because of its overlapping with the Cu 3d orbital,indicating that strong 3d-2π interactions are involved in the chemisorption of the O2 molecule on the surface.     

First Principles Study of Adsorption and Reaction of CO on SrTiO3 （100） Surface： the Role of Surface Oxygen Vacancies

The adsorption and reaction of CO on SrTiO3 （100） surface with and without surface oxygen vacancy are investigated by the first-principles calculation based on the density functional theory. The calculated results reveal that the oxygen vacancy site prefers to the activation of the C-O bond. The adsorption energies increase to 1.0855 and 0.3245eV for defect-CO and defect-OC orientations, respectively. Particularly the C-O bond is elongated by about 0.1285 ？ in the defect-OC orientation compared with that in the Ti-OC one without surface oxygen vacancies. There is predominantly a chemisorption mechanism between the CO molecule and the surface in the defect-CO orientation.     

News on PHOTOS Monte Carlo： γ＊→π^＋π^-（γ） and K^±→π^＋π^-e^±υ（γ）

PHOTOS Monte Carlo is widely used for simulating QED effects in decay of intermediate particles and resonances. It can be easily connected to other main process generators. In this paper we consider decaying processes γ＊→π^＋π^-（γ） and K^±→π^＋π^-e^±υ（γ） in the framework of Scalar QED. These two processes are interesting not only for the technical aspect of PHOTOS Monte Carlo, but also for precision measurement of αED（Mz）, g--2, as well as ππ scattering lengths.