Palladium atoms and its dimers adsorbed on graphene: First-principles study

In this work we have studied the stabilty, electronic and magnetic properties of Pd adatoms and dimers adsorbed on graphene system using first-principles calculations. The adsorption energies for Pd adatom and its dimer have been found to range from −0.986 to −1.135 eV and −0.165 to −1.101 eV, respectively, which signify stable configuration and future utilization of this system in catalysis. A shift but no separation of π and π^? bands at the Dirac point has been observed in case of Pd dimer adsorption in perpendicular configuration, which can be accounted for the breaking of symmetry of the graphene structure due to adsorption. 64-68% spin polarization P(E_F) and 1.944-1.990 μ_B magnetic moment have been observed for Pd dimers adsorbed on graphene in perpendicular configuration for different sites. The unequal values of partial density of states for 4d and 5s orbitals of Pd dimers at Fermi level have been found to be responsible for the generation of high spin polarization.     

An x-ray diffraction study of krypton adsorbed on MgO(100) surfaces

Krypton in a triangular 2D solid phase was observed to form on the (100) surfaces of MgO crystallites. This system is unique because the physisorbed inert gas overlayer is not only incommensurate, but of a symmetry entirely unrelated to that of the substrate. Melting of the adsorbate occurred in the neighborhood of 70K for all coverages studied.     

CO adsorbed on MgO(100): a high resolution LEED study

The monolayer structure of CO molecules adsorbed on MgO(100) single crystal surfaces cleaved in situ has been analysed by LEED within the 30–56 K temperature range. At T ⩽ 40 K CO forms a 2 × 4 commensurate 2D solid phase. A sharp uniaxial transition occurs above this temperature, along the [10] surface direction which locks the monolayer into a new commensurate 2 × 3 phase stable over a temperature range of 8 K. Above 50 K, this second commensurate phase expands itself uniaxially in a sharp transition toward a solid with disorder increasing with temperature. This succession of transitions is an interesting illustration of the incomplete “devil's staircase”.     

SiO2-羟基表面上金属原子的第一性原理研究

采用第一性原理方法研究了SiO2-羟基表面上几种金属原子的吸附性质,发现In和Ga在SiO2-羟基表面上的结合很弱,而Fe,Co,Ni在该表面上与Si,O形成强的化学键.等势能面和扩散势垒计算表明In(Ga)的扩散激活能只有0.1-0.3 eV,表明这两种原子容易在表面上扩散.这些结果可以定性地解释纳米合成中的一些实验现象.     

First-principles calculations of carbon nanotubes adsorbed on Si(001)

We report first-principles calculations on the adsorption of a metallic (6,6) single-walled carbon nanotube (SWCN) on the Si(001) surface. We find stable geometries for the nanotube between two consecutive dimer rows where C-Si chemical bonds are formed. The binding energy in the most stable geometry is found to be 0.2 eV/A. Concerning the electronic properties, the most stable structure shows an increase in the density of states near the Fermi level due to the formation of C-Si bonds enhancing the metallic character of the nanotube by the contact with the surface. These properties may lead one to consider metallic SWCNs adsorbed on Si substrates for interconnections and contacts on future nanoscale devices. Finally, the nature of the nanotube-surface interaction for nanotubes of larger diameters is also discussed.     

SiO2-羟基表面上金属原子的第一性原理研究

采用第一性原理方法研究了SiO₂-羟基表面上几种金属原子的吸附性质，发现In和Ga在SiO₂-羟基表面上的结合很弱，而Fe，Co， Ni在该表面上与Si，O形成强的化学键.等势能面和扩散势垒计算表明In (Ga)的扩散激活能只有0.1—0.3 eV，表明这两种原子容易在表面上扩散.这些结果可以定性地解释纳米合成中的一些实验现象. 关键词： 第一性原理 表面扩散 结合能 金属原子     

Gold atoms and clusters on MgO(100) films; an EPR and IRAS study

Single gold atoms deposited on single crystalline MgO(1 0 0) films grown on Mo(1 0 0) are characterized by electron paramagnetic resonance spectroscopy as well as IR spectroscopy using CO as probe molecules. In this article we describe the first angular dependent measurements to determine the principal hyperfine components of a secondary hyperfine interaction, namely, with ¹⁷O of the MgO. The values determined here are in perfect agreement with theoretical expectations and corroborate the previously reported binding mechanism of Au atoms on the oxygen anions of the MgO terrace. The temperature dependent EPR data reveal an onset of Au atom mobility at about 80 K while the formation of Au particles occurs only above 125 K. By an analysis of the EPR line width in combination with STM measurements it is possible to deduce an increase of the interatomic distance above 80 K. The Au/CO complexes show a somewhat smaller temperature stability as compared to the Au atoms. The observed thermal stability is in perfect agreement with theoretical predictions for CO desorption.     

Charging of metal atoms on ultrathin MgO/Mo(100) films

The chemical activity of supported metal nanoclusters is enhanced by electronic charging induced by the interaction with surface defects. We use density functional theory plane wave calculations to show that charging of metal atoms with high electron affinity like Au is possible also in the absence of defects by atom deposition on ultrathin MgO films (1 to 3 layers) grown on Mo(100). The Au 6s level falls below the Fermi level of Mo, leading to electron transfer from Mo to Au by direct tunneling through the insulating MgO film. The effect is not observed for Pd, whose 5s empty level falls just above the Fermi level of Mo, or for thicker MgO films.     

Theoretical study of AuCu nanoalloys adsorbed on MgO(001)

The structures of AuCu clusters adsorbed on the (001) face of MgO are searched for by a two-step methodology. In a first step, the relevant structural motifs are singled out by global optimization searches within an atomistic model. In a second step, the lowest energy structures of each motif are relaxed by density-functional calculations. Three different sizes (30, 40 and 50 atoms) are considered. For each size, three compositions are analyzed. For size 30, a competition between fcc pyramids and a new motif (the daisy structure) is found. For 40 and 50 atoms, icosahedral fragments prevail. The results are discussed in connection with experimental data related to clusters of larger sizes.     

Diffusivity of a two-dimensional lattice fluid: CH4 adsorbed on MgO(100)

The diffusive motions of a 0.8 layer of CH₄ adsorbed on MgO(100) are measured at 72, 88 and 97 K by quasi-elastic neutron scattering. It is shown that at 72 K the methane film is solid and its molecules perform an isotropic rotational motion. At 88 and 97 K, the adsorbed layer is in a two-dimensional fluid state in which the molecules jump between equidistant (4.21 Å) lattice sites of the MgO surface. The mean residence time has been determined ( ∼ 1 × 10 ⁻¹⁰ s at 88 K and ∼ 4 × 10⁻¹¹ s at 97 K). The corresponding translational diffusion coefficients are ∼ 5 × 10⁻⁶ cm² s⁻¹ at 88 K and 12 × 10⁻⁶ cm² s⁻¹ at 97 K. The diffusivity of this lattice fluid is compared to that of the same molecules adsorbed on graphite (0001) previously reported. The reduced mobility observed in the case of CH₄/MgO(100) is related to the important depth of the potential wells on the MgO(100) surface.     

Structural and electronic properties of carbon adsorbed on Fe(100)

Density functional theory within general gradient approximation (GGA) has been used to investigate sub-monolayer carbon atom adsorbed on Fe(100) as a function of coverage. The carbon atoms prefer to adsorb in the fourfold hollow site and bind strongly with the Fe surfaces. There is a substantial and strong coverage dependence of the carbon-induced expansion of the first interlayer spacing, reflecting a weakening of Fe–Fe bonds between the two outermost substrate layers. Some charge is found to transfer from substrate Fe to the adsorbate C atoms, which is responsible for the increase of work function. The density of states (DOS) analysis indicates the bonding of carbon with the first surface layer Fe atoms is primarily due to the interaction between Fe 3d_{x2-y2, xy} and C 2p_{x, y} orbitals, and the bonding of carbon with the second surface layer Fe atom that sits directly below the carbon atom is mainly from interaction between the minority spin Fe 3d_z2 and C 2p_z orbitals.     

The Alexander-Anderson problem for two atoms adsorbed on graphene

The Green’s functions for the Alexander-Anderson problem have been obtained using the previously proposed model density of states for graphene. Both the ferromagnetic and antiferromagnetic dimers have been considered. It has been shown that, in order to describe the density of states of the dimer adatom, the density of states of the isolated adatom with two positions of the gravity center of the quasi-level shifted in opposite directions can be used. It has been demonstrated that the approximate method of obtaining the Green’s function of the dimer proposed by us previously and consisting in that the Green’s function of the adatom rather than that of the atom is taken as the seed function gives the same result as the Alexander-Anderson approach. The dependences of the indirect interaction of dimer adatoms on the problem parameters have been evaluated in the limit of low energies.     

Bonding character of lithium atoms adsorbed on a graphene layer

This work uses first-principles calculations to investigate the aspects of the bonding character of lithium atoms adsorbed on a graphene layer. The presented results are in contradiction to other results that have recently appeared in the specialized literature, although they confirm some previous claims. In particular, a discussion of the characteristics of the bonding between lithium and carbon atoms and whether they interact via an sp² or an sp³ hybridization is intended to clarify the problem. It is also found that the carbon-lithium bond is not purely covalent but instead presents a significant ionic character. The local geometry is governed by the π-acceptor character of lithium atoms which occupy reverse positions relative to the carbon atoms as compared to the positions of hydrogen in graphane.     

Polar oxide substrates for graphene growth: A first-principles investigation of graphene on MgO(111)

Given the recent excitement over the truly two-dimensional carbon “super” material – graphene, there is now much effort and focus on the various possibilities of engineering the band gap of graphene for its device applications. One possible and promising route will be to grow graphene directly on some non-metallic substrates. In this paper, we address the atomic and electronic structure of various graphene structures on the polar MgO(111) using first-principles density-functional theory (DFT) calculations. We find that graphene generally interacts strongly with the O-terminated polar oxide surface, forming strong chemical bonds, inferred from both energetics and detailed density-of-states analysis. We compare our theoretical findings with available experimental results, offering a possible direction for future band gap engineering of graphene on such oxide substrates.     

First-principles study of molecular NO dissociation on Ir(100) surface

The dissociation of NO on Ir(100) surface is investigated using density functional theory (DFT). The pathway and transition state (TS) of the dissociation of NO molecule are determined using climbing image nudge elastic band (CI-NEB). The prerequisite state of NO dissociation is determining the most stable sites of the reactant and products. We found that the most energetically stable sites are the hollow for N atom and the bridge for NO molecule as well as O atom. We found that the bending of NO is the first step of the dissociation reaction due to the increase of the back-donation from the d-band of Ir to 2π ^? orbital of NO, which causes the weakening of NO bond. The dissociation energy barrier of NO molecule on Ir(100) surface is 0.49 eV.     

Electronic excitations of CO adsorbed on MgO(001)

We report ab initio calculations of the quasiparticle band structure and the optical excitation spectrum of bulk MgO, the MgO(001) surface, and CO molecules adsorbed on MgO(001). Many-body exchange and correlation effects are included within the GW approximation of the electron self-energy operator and the corresponding electron–hole interaction. The excited electron–hole states are obtained from the Bethe–Salpeter equation. At the clean MgO(001) surface exciton states are found with binding energies that are significantly stronger than in the bulk. The exciton spectrum of the adsorbate system CO:MgO is dominated by charge-transfer excitons, which couple strongly to the molecular excitations of CO. PACS 73.20.At; 73.20.Hb; 34.70.+e