First-principles studies on initial growth of Ni on MgO(0 0 1) surface

To elucidate the initial growth of metal on oxide surface, we studied adsorption of small nickel clusters, Ni_n (n = 1-5), on MgO(0 0 1) surface using first-principles method based on density-functional theory. It was found that the preferential adsorption site for an isolated Ni atom is directly above the surface oxygen atom. A strong covalent bond with partial ionic character is formed between the Ni adatom and the surface oxygen atom. Various structures were considered for the Ni_n isomers and 3D structures were found to be energetically more stable than 2D structures for clusters of more than two atoms. For the 2D clusters, metal-metal bonds prevail over metal-substrate bonds with increasing Ni coverage. The calculated work function and ionization energy were found to vary with Ni coverage which is attributed to the change of the surface dipole moment upon metal adsorption, while the evolution of Schottky barrier height at the initial growth stage is dominated by the adatom-induced gap states.     

Density functional theoretical study of Cun, Aln (n = 4-31) and copper doped aluminum clusters: Electronic properties and reactivity with atomic oxygen

A DFT study of the electronic properties of copper doped aluminum clusters and their reactivity with atomic oxygen is reported. Firstly we performed calculations for the pure Cu_n and Al_n (n = 4, 9, 10, 13, 25 and 31) clusters and we determined their atomization energy for some frozen conformations at the B3PW91 level. The calculated work functions and M-M (M = Cu, Al) bond energies of the largest clusters are comparable with experimental data. Secondly, we focused our attention on the change of the electronic properties of the systems upon the substitution of an Al atom by a Cu one. This latter stabilizes the system as the atomization energy of the 31-atoms cluster increases of 0.31 eV when the substitution is done on the surface and of 1.18 eV when it is done inside the cluster. We show that the electronic transfer from the Al cluster to the Cu atom located at the surface is large (equal to 0.7 e⁻) while it is negligible when Cu is inserted in the Al_n cluster. Moreover, the DOS of the Al₃₁ and Al₃₀Cu systems are compared. Finally, the chemisorption energies of atomic oxygen in threefold sites of the Al₃₁, Cu₃₁ and Al₃₀Cu clusters are calculated and discussed. We show that the chemisorption energy of O is decreasing on the bimetallic systems compared to the pure aluminum cluster.     

Ruthenium adsorption and diffusion on the GaN(0 0 0 1) surface

We report first principles calculations to analyze the ruthenium adsorption and diffusion on GaN(0 0 0 1) surface in a 2×2geometry. The calculations were performed using the generalized gradient approximation (GGA) with ultrasoft pseudopotential within the density functional theory (DFT). The surface is modeled using the repeated slabs approach. To study the most favorable ruthenium adsorption model we considered T₁, T₄ and H₃ special sites. We find that the most energetically favorable structure corresponds to the Ru- T₄ model or the ruthenium adatom located at the T₄ site, while the ruthenium adsorption on top of a gallium atom (T₁ position) is totally unfavorable. The ruthenium diffusion on surface shows an energy barrier of 0.612 eV. The resultant reconstruction of the ruthenium adsorption on GaN(0 0 0 1)- 2×2 surface presents a lateral relaxation of some hundredth of Å in the most stable site. The comparison of the density of states and band structure of the GaN(0 0 0 1) surface without ruthenium adatom and with ruthenium adatom is analyzed in detail.     

Surface structure and solidification morphology of aluminum nanoclusters

Classical molecular dynamics simulation with embedded atom method potential had been performed to investigate the surface structure and solidification morphology of aluminum nanoclusters Al_n (n=256, 604, 1220 and 2048). It is found that Al cluster surfaces are comprised of (1 1 1) and (0 0 1) crystal planes. (1 1 0) crystal plane is not found on Al cluster surfaces in our simulation. On the surfaces of smaller Al clusters (n=256 and 604), (1 1 1) crystal planes are dominant. On larger Al clusters (n=1220 and 2048), (1 1 1) planes are still dominant but (0 0 1) planes cannot be neglected. Atomic density on cluster (1 1 1)/(0 0 1) surface is smaller/larger than the corresponding value on bulk surface. Computational analysis on total surface area and surface energies indicates that the total surface energy of an ideal Al nanocluster has the minimum value when (0 0 1) planes occupy 25% of the total surface area. We predict that a melted Al cluster will be a truncated octahedron after equilibrium solidification.     

A density functional theory study on the H2S molecule adsorption onto small gold clusters

An all-electron scalar relativistic calculation on Au_nH₂S (n = 1-13) clusters has been performed by using density functional theory with the generalized gradient approximation at PW91 level. The small gold cluster would like to bond with sulfur in the same plane and the H₂S molecule prefers to occupy the on-top and single fold coordination site in the cluster. The Au_n structures and H₂S molecule in all Au_nH₂S clusters are only slightly perturbed and still maintain their structural integrity. After adsorption, the S-H, H-H bond-lengths and most Au-Au bond-lengths are elongated, only a few Au-Au bond-lengths far from H₂S molecule are shortened. The reactivity enhancement of H₂S molecule is obvious and the strong gold-sulfur bond is observed expectedly. The most favorable adsorption takes place in the case that the H₂S molecule is adsorbed by an even-numbered Au_n cluster and becomes Au_nH₂S cluster with even number of valence electrons. It is believed that the strong scalar relativistic effect is favorable to H₂S molecule adsorption onto small gold clusters and is also one of the important reasons for the strong gold-sulfur bond.     

Adsorption model determination of N2O/Ag(1 1 0) by theoretical studies of near-edge X-ray absorption fine structure

The nitrogen 1s near-edge X-ray absorption fine structure (NEXAFS) spectra of the N₂O adsorbed on Ag(1 1 0) have been studied by the multiple-scattering cluster (MSC) and self-consistent field (SCF) DV-Xα methods. Two adsorption models, in which the N₂O molecule attached to the Ag substrate through the central nitrogen (N_C) atom and the terminal nitrogen (N_T) atom, respectively, have been checked up thoroughly. The MSC calculation and the R-factor analysis show that the N₂O molecule is attached to the Ag substrate through the terminal nitrogen atom with the adsorption height h = 3.4 ± 0.1 Å. In the overlayer the N₂O molecules arrange themselves into a tilted chain due to the interaction between the cations and the anions in the molecules. The physical cause of the resonances in the NEXAFS spectra mentioned above has been discussed by the DV-Xα method, which confirms the MSC calculations.     

First-principles calculations of hydrogen molecule adsorption on Ti (0 0 0 1)-(2 × 1) surface

Adsorption of H₂ molecule on the Ti (0 0 0 1)-(2 × 1) surface was studied by density functional theory with generalized gradient approximation (GGA). The parallel and vertical absorption cases were investigated in detail by adsorption energy and electronic structure analysis, we obtained three stable configurations of FCC-FCC (the two H atoms adsorption on the two adjacent fcc sites of Ti (0 0 0 1) surface, respectively), HCP-HCP (the two H atoms adsorption on the two adjacent hcp sites of Ti (0 0 0 1) surface, respectively) and FCC-HCP (the one H atom adsorption on the fcc site and the other adsorption on the near hcp site) based on the six different parallel adsorption sites after the H₂ molecule dissociates. However, all the end configurations of four vertical adsorption sites were unstable, H₂ molecule was very easy to desorb from Ti surface. The H-H bond breaking and Ti-H bond forming result from the H₂ molecule dissociation. H-H bond breaking length ranges from 1.9 Å to 2.3 Å for different adsorption configurations due to the strong Ti-H bond forming. The H₂ dissociative approach and the end stable configurations formation in parallel adsorption processes are attributed to the quantum mechanics steering effects.     

First-principles studies of the oxygen adsorption on unreconstructed and reconstructed Ni(1 1 0) surfaces

First-principles calculations have been performed to investigate the adsorption of oxygen on unreconstructed and reconstructed Ni(1 1 0) surfaces. The energetics, structural, electronic and magnetic properties are given in detail. For oxygen adsorption on unreconstructed surface, (n×1)(n=2,3) substrate with oxygen atom on short-bridge site is found to be the most stable adsorption configuration. Whereas energetically most favorable adsorption phase of reconstructed surface is p(n×1) substrate with oxygen atom located at long-bridge site. Our calculations suggest that the surface reconstruction is induced by the oxygen adsorption. We also find there are redistributions of electronic structure and electron transfer from the substrate to adsorbate. Our calculations also indicate surface magnetic moment is enhanced on clean surfaces and oxygen atoms are magnetized weakly after oxygen adsorption. Interestingly, adsorption on unreconstructed surface does not change surface magnetic moment. However, adsorbate leads to reduction of surface magnetic moment in reconstructed system remarkably.     

Density functional study of oxygen adsorption on the Mg3Nd (0 0 1) surface

The adsorption of oxygen atoms on Mg₃Nd (0 0 1) surface was studied based on density function theory (DFT), in which the exchange-correlation potential was chosen as the generalized gradient approximation (GGA) in the Perdew and Wang (PW91). The most preferred adsorption position was at the top-hollow site. Upon the optimization on top-hollow site with different coverage, it was found that the adsorption energy decreased with oxygen coverage. The density of states analysis showed that obvious charge transfer took place between O atom and the nearest Nd atom and chemical bond formed between O atom and the nearest Nd atom after O adsorption. The result of surface energy as a function of chemical potential change of oxygen indicated the clean Mg₃Nd (0 0 1) surface was easy to adsorb oxygen and form 1.00 ML surface.     

Adsorption site preference of CO2 on the Pt(1 0 0) surface by ab initio calculations

This paper investigates the adsorption sites and electronic structure of the adsorption of CO₂ on the Pt(1 0 0) surface by ab initio periodic density functional theory (DFT) methods. Several parallel and vertical adsorption sites are examined in detail. The results show that the adsorption energy for the atop hollow horizontal (thh) site is 0.34 eV. However, other sites only have small binding energies and these values are very close. For an atop hollow horizontal site, the calculated elecronic interaction is contributed to not only the Pt-O atoms, but also Pt-C atoms. So the results indicate that the thh site is the most favorable and stable.     

Self-diffusion dynamic behavior of atomic clusters on Re(0 0 0 1) surface

Using molecular dynamics simulations and a modified analytic embedded atom potential, the self-diffusion dynamics of rhenium atomic clusters up to seven atoms on Re(0 0 0 1) surface have been studied in the temperature ranges from 600 K to 1900 K. The simulation time varies from 20 ns to 200 ns according to the cluster sizes and the temperature. The heptamer and trimer are more stable comparing to other neighboring non-compact clusters. The diffusion coefficients of clusters are derived from the mean square displacement of cluster's mass-center, and diffusion prefactors D₀ and activation energies E_a are derived from the Arrhenius relation. It is found that the Arrhenius relation of the adatom can be divided into two parts at different temperature range. The activation energy of clusters increases with the increasing of the atom number in clusters. The prefactor of the heptamer is 2-3 orders of magnitude higher than a usual prefactor because of a large number of nonequivalent diffusion processes. The trimer and heptamer are the nuclei at different temperature range according to the nucleation theory.     

Dynamics of molybdenum nano structure formation on the TiO2(1 1 0) surface: A kinetic Monte Carlo approach

The rutile TiO₂(1 1 0) surface is a highly anisotropic surface exhibiting “channels” delimited by oxygen rows. In previous experimental and theoretical DFT works we could identify the molybdenum adsorption sites. They are located inside the channels. Moreover, experimental studies have shown that during subsequent annealing after deposition, special molybdenum nano structures can be formed, especially two monolayer high pyramidal chains of atoms.In order to better understand the dynamics of nano structure formation, we present a kinetic Monte Carlo study on diffusion and adsorption of molybdenum atoms on a TiO₂(1 1 0) surface. A quasi one-dimensional lattice gas model has been used which describes the possible adsorption sites of a Mo atom in a single channel of the surface. The atomic positions of a 1.5 monolayer thick Mo film formed of pyramidal chains define the lattice sites of the model. Thereby the formation of three-dimensional clusters could be studied. Here we have studied the cluster formation as a function of parameters that can be controlled in a growth experiment by physical vapor deposition: deposition and annealing temperature, flux and total amount of deposited Mo. Good qualitative agreement with recent experiments is obtained.     

Deposition dynamics and chemical properties of size-selected Ir clusters on TiO2

We report a study of Ir_n/TiO₂ samples prepared by size and energy-selected deposition of Ir_n⁺ (n=1, 2, 5, 10, 15) on rutile TiO₂(1 1 0) at room temperatures. The Ir clusters are found to be formally in the zero oxidation state, and there are no significant shifts in Ir 4f binding energy with cluster size. Over a wide range of impact energies, both Ir XPS intensity and peak position are constant, indicating constant sticking coefficient, and no impact-driven redox chemistry. Low energy ion scattering spectroscopy (ISS) suggests that the deposited Ir clusters remain largely intact, neither fragmenting nor agglomerating, and retaining 3-D structures for the larger sizes. For impact energies above 10 eV/atom, comparison of ISS and XPS data show that the Ir clusters are penetrating into the TiO₂ surface, with the extent of penetration increasing with both per atom energy and cluster size. Temperature programmed desorption (TPD) of CO is used to further characterize the deposited Ir_n. This system shows pronounced substrate-mediated adsorption (SMA) in low CO exposures, with strong dependence on cluster size. ISS and sputtering experiments indicate that CO adsorbed via SMA is bound differently than CO adsorbed in high dose experiments. In experiments with sequential C¹⁶O and C¹⁸O doses, facile C¹⁶O → C¹⁸O exchange is observed for Ir₅ and larger clusters, but not for Ir₂. The peak CO desorption temperature is found to decrease with cluster size. The cycle of CO adsorption and heating comprising a TPD experiment have a dramatic effect on the sample morphology, leading to encapsulation of Ir by a thin TiO_x layer.     

A first-principles surface phase diagram study for Si-adsorption processes on GaAs(1 1 1)A surfaces

The adsorption processes of an Si atom on GaAs(1 1 1)A surfaces under growth conditions are investigated on the basis of first-principles surface phase diagrams, in which adsorption-desorption behavior is described by comparing the calculated adsorption energy obtained by total-energy electronic-structure calculations with vapor-phase chemical potential estimated by quantum statistical mechanics. The calculated surface phase diagram as functions of temperature and As₂ pressure demonstrates that both Ga and As atoms are adsorbed on the Ga-vacancy site of GaAs(1 1 1)A-(2×2) surface under low As-pressure conditions, resulting in the formation of (2×2) surface with an As adatom. The surface phase diagrams as functions of temperature and Si pressure also reveal that an Si atom can be adsorbed on the (2×2) surface with an As adatom for temperatures less than ∼1160 K and this Si atom can occupy one of As-lattice sites after the incorporation of another As atom, leading to p-type conductivity. In contrast, the (2×2) surface with an As trimer is found to be stabilized under high As-pressure conditions. The surface phase diagram for Si incorporation clarify that an Si atom can be adsorbed at one of Ga-lattice sites of the (2×2) surface with an As trimer for temperatures less than ∼870 K. These calculated results provide one of possible explanations for the formation of p-type and n-type GaAs on GaAs(1 1 1)A surfaces under low and high As-pressure conditions, respectively.     

First-principles study on the catalytic role of Ag in the oxygen adsorption of LaMnO3(0 0 1) surface

In the present paper, the catalytic role of Ag in the oxygen adsorption of LaMnO₃(0 0 1) surface has been theoretically investigated using first-principles calculations based on the density functional theory (DFT) and pseudopotential method. The O₂ adsorption energy is larger for the vertical adsorption and the covalent bond was formed between O₂ molecule and surface Mn. The calculation of electronic properties of interaction between Ag atom and LaMnO₃(0 0 1) surface demonstrates that the most stable position for Ag adsorption is hollow site. The O₂ adsorption energy dramatically increased from 0.298 eV to 1.108 eV due to Ag pre-adsorbed. It is Ag pre-adsorbed that facilitates O₂ adsorption on surface. The bond length and bond population of O₂ molecule indicate that Ag atom facilitates O₂ molecule dissociative adsorption. The Ag atom strengthens LaMnO₃(0 0 1) substrate activity and activity center was formed on surface, which enhances the electrocatalytic activity of LaMnO₃ as solid oxide fuel cells cathode material at low temperature.     

Structures of Pt clusters on graphene by first-principles calculations

The interactions between Pt_n clusters (n?13) and a graphene sheet have been investigated by first-principles calculations based on density functional theory. For single Pt-atom and Pt₂-dimer adsorptions, the stable adsorption sites are bridge sites between neighboring carbon atoms. When the number of Pt atoms in a cluster increases, the Pt-C interaction energy per contacting Pt atom becomes smaller. For smaller clusters (3?n?7), the adsorption as a vertical planar cluster is more stable than that as parallel planar or three-dimensional (3D) clusters, due to the stability of a planar configuration itself and the stronger planar-edge/graphene interaction, while the adsorption as a parallel planer cluster becomes stable for larger cluster (n?7) via the deformation of the planar configuration so as to attain the planar-edge/graphene contact. For much larger clusters (n?10), the adsorption as a 3D cluster becomes the most stable due to the stability of the 3D configuration itself as well as substantial Pt-C interactions of edge or corner Pt atoms. The interfacial interaction between a Pt cluster and graphene seriously depends on the shape and size of a cluster and the manner of contact on a graphene sheet.     

Density functional theory study into the adsorption of CO2, H and CHx (x = 0-3) as well as C2H4 on α-Mo2C(0 0 0 1)

The structures and energetics of the chemisorbed CO₂, CH_x species and H as well as C₂H₄ on the α-Mo₂C(0 0 0 1) surface have been computed at the GGA-RPBE level of density functional theory. It is found that CO₂ adsorbs dissociately into CO and O, in agreement with the experimental finding. The adsorbed O, CH_x and H species prefer the site of three surface molybdenum atoms over a second layer carbon atom (V_C site). On the basis of the calculated adsorption energies of CH_x and H, the sequential dehydrogenation of CH₄ and the C/C coupling reaction of CH_x have been discussed.     

利用Gupta势结合遗传算法研究ConCu55－n(n=0—55)混合团簇的结构演化及基态能量

采用半经验的Gupta多体势结合遗传算法对Co_nCu_55－n(n=0—55)混合团簇的基态结构和能量进行了研究,发现这些混合团簇的基态结构是在Co₅₅,Cu₅₅单质团簇(Mackay二十面体)的基础之上发生的畸变;从n=0(Cu₅₅)开始,Co原子从中心到表面,从棱到顶点依次、连续替换Cu原子;基态结构与键能较大键的数目及其平均键长有关;Co₁₃Cu₄₂具有最稳定的结构,13个Co原子全部位于团簇内部形成Mackay二十面体对整个团簇的稳定性有显著影响. 关键词： 团簇 结构和能量 Gupta势 遗传算法     

DFT study of structural, electronic and vibrational properties of pure (Al2O3)n (n = 9, 10, 12, 15) and Ni-doped (Al2O3)n (n = 9, 10) clusters

The geometrical, electronic and vibrational properties of pure (Al₂O₃)_n (n = 9, 10, 12, 15) clusters and Ni-doped (Al₂O₃)_9-10 clusters are investigated by density functional theory. There are four different Ni-doped (Al₂O₃)₉ clusters and one Ni-doped (Al₂O₃)₁₀ cluster taken into account. Compared with the pure clusters, the Ni-doped (Al₂O₃)_9-10 clusters have narrower HOMO-LUMO energy gaps. The results indicate that the impurity of Ni atom is mainly responsible for the reduction of the HOMO-LUMO energy gap. One characteristic vibration band at about 1030 cm⁻¹ is found in the vibrational frequencies of the Ni-doped (Al₂O₃)_9-10 clusters, which is caused by the asymmetric Al-O-Al stretching vibration. Another band at around 826 cm⁻¹ involving the characteristic vibration of Ni-O bond is in good agreement with experimental results.     

A density-functional study on the atomic geometry and adsorption of the Cu(1 0 0) c(2 × 2)/N Surface

In this work we have performed total-energy calculations on the geometric structure and adsorption properties of Cu(1 0 0) c(2 × 2)/N surface by using the density-functional theory and the projector-augmented wave method. It is concluded that nitrogen atom was adsorbed on a FFH site with a vertical distance of 0.2 Å towards from surface Cu layer. The bond length of the shortest Cu-N bonding is calculated to be 1.83 Å. Geometry optimization calculations exclude out the possibilities of adsorbate induced reconstruction mode suggested by Driver and Woodruff and the atop structural model. The calculated workfunction for this absorbate-adsorbent system is 4.63 eV which is quite close to that of a clean Cu(1 0 0) surface. The total-energy calculations showed that the average adsorption energy per nitrogen in the case of Cu(1 0 0) c(2 × 2)-N is about 4.88 eV with respect to an isolated N atom. The absorption of nitrogen on Cu(1 0 0) surface yields the hybridization between surface Cu atoms and N, and generates the localized surface states at −1.0 eV relative to Fermi energy E_F. The stretch mode of the adsorbed nitrogen at FFH site is about 30.8 meV. The present study provides a strong criterion to account for the local surface geometry in Cu(1 0 0) c(2 × 2)/N surface.