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.     

Density functional theory study of doping effects in monoclinic clinobisvanite BiVO4

The doping effects of several transition metal impurities for monoclinic BiVO₄ are studied by DFT calculations. The results indicated that transition metal doping could reduce the effective mass of holes on the top of valence band, except Zr doping. In particular, we found the “e” states of impurities have significant influence on the photophysical properties of BiV_1 − xM_xO₄ under visible-light irradiation.     

A density functional theory study of H2S decomposition on the (1 1 1) surfaces of model Pd-alloys

In this work, we report density functional theory calculations exploring H₂S dissociation on the (1 1 1) surfaces of Pd, Cu, Ag, Au, and various bimetallic surfaces consisting of those metals. To understand the contributions of lattice strain and electronic ligand effects, the thermodynamics of each elementary dissociation step were explored on model bimetallic surfaces, including PdMPd sandwiches and Pd pseudomorphic overlayers, as well as strained Pd(1 1 1) surfaces and homogeneous Pd₃M alloys. Sulfuric (H₂S, SH, and S) adsorption energies were found to correlate very well with lattice constant, which can be explained by the strong correlation of the lattice constant with d-band center, Fermi energy, and density of states at the Fermi level for strained Pd(1 1 1) surfaces. Compressing the Pd lattice shifts the d-band center away from the Fermi level, lowers the Fermi energy, and reduces the density of d-states at the Fermi level. All three effects likely contribute to the destabilization of sulfuric adsorption on Pd alloys. Introducing ligand effects was found to alter the distribution of the d-states and shift the Fermi level, which eliminates the correlation of the d-band center with the density of states at the Fermi level and the Fermi energy. As a result, the d-band center by itself is a poor metric of the H₂S reaction energetics for bimetallic surfaces. Furthermore, combining strain with ligand effects was found to lead to unpredictable alterations of the d-band. Therefore, adsorption of H₂S, SH, and S on PdMPd surfaces do not accurately predict adsorption on Pd₃M surfaces.     

Density functional theory study of AunMn(n=1-8) clusters

Equilibrium geometries, relative stabilities, and magnetic properties of small Au_nMn (n=1-8) clusters have been investigated using density functional theory at the PW91P86 level. It is found that Mn atoms in the ground state Au_nMn isomers tend to occupy the most highly coordinated position and the lowest energy structure of Au_nMn clusters with even n is similar to that of pure Au_n+1 clusters, except for n=2. The substitution of Au atom in Au_n+1 cluster by a Mn atom improves the stability of the host clusters. Maximum peaks are observed for Au_nMn clusters at n=2, 4 on the size dependence of second-order energy differences and fragmentation energies, implying that the two clusters possess relatively higher stability. The HOMO-LUMO energy gaps of the ground state Au_nMn clusters show a pronounced odd-even oscillation with the number of Au atoms, and the energy gap of Au₂Mn cluster is the biggest among all the clusters. The magnetism calculations indicate that the total magnetic moment of Au_nMn cluster, which has a very large magnetic moment in comparison to the pure Au_n+1 cluster, is mainly localized on Mn atom.     

Deoxygenation of IrO2(1 1 0) surface: Core-level spectroscopy and density functional theory calculation

Deoxygenation of the IrO₂(1 1 0) surface is investigated at 403-493 K, using the core-level spectroscopy and density functional theory (DFT) calculation. The Ir-4f_7/2 signals of 1f-cus-Ir with and without on-top oxygen (O_top) emerge as surface features of the baked-out surface, whose positive and negative shifts in binding energy are in line with the DFT computation results. Progressively increasing the reduction temperature, the 1f-cus-Ir feature quickly disappears and the signal of 2f-cus-Ir emerges at 403 K. Meanwhile the feature of 1f-cus-Ir + O_top diminishes but persists when the Ir metal signal is evident. The intriguing coexistence of 1f-cus-Ir + O_top and Ir metal at 433-443 K is elucidated in the theoretical pathway study. DFT calculation reveals that O₂ desorption via pairing two neighboring O_top atoms is the rate-determining step of surface deoxygenation. Under the UHV conditions, O_top is replenished via migration of the surface oxygen species, including the threefold coordinated oxygen (O_3f) of a reduced surface. Hence the O_top atom is an active and long-lived surface species, which does not vanish until O_3f is consumed and surface Ir begins to cluster. Under the realistic pressure conditions, O_top can also be refreshed via the dissociative adsorption of gas-phase oxygen. In either pathway, O_top is a critical intermediary of IrO₂(1 1 0) oxidation catalysis.     

Effect of Si substitution on electronic structure and magnetic properties of Heusler compounds Co2TiAl1−xSix

The electronic structures of Co-based Heusler compounds CoTiAl_1−xSi_x (x=0, 0.25, 0.5, 0.75 and 1) are calculated by first-principles using the full potential linearized augmented plane wave (FP-LAPW) method within GGA and LSDA+U scheme. Particular emphasis was put on the role of the main group elements. In recent years, the GGA calculations of Co₂TiAl (x=0) and Co₂TiSi (x=1) indicated that they are half-metallic, but the electronic structure of this compound with x=0.25, 0.5 and 0.75 has not been reported yet, neither theoretically nor experimentally. The calculated results reveal that these are half-metallic and exhibit an energy gap in the minority spin state and also show 100% spin polarization. The substitution of Al by Si leads to an increase in the number of valence electrons, with increasing x. Our calculated results clearly show that with the Si doping, the lattice parameter linearly decreases; bulk modulus increases, and the total magnetic moment increases. The calculated energy gap in the minority spin state, using GGA scheme, was smaller than that obtained by using LSDA+U scheme. The outcomes of this research also show that the Co-3d DOS and therefore, the magnetic properties of compounds are dependent on electron concentration of the main group elements and it will affect the degree of p-d orbital occupation.     

Structure and adhesion of MoSi2/Ni interfaces: Evaluation of MoSi2 as an alternative bond coat alloy

We use density functional theory to evaluate the stability of molybdenum disilicide coatings on a nickel substrate, as a possible bond coat alloy for high temperature coating applications. We consider the MoSi₂(0 0 1)/Ni(1 1 1), MoSi₂(1 0 0)/Ni(1 1 1), and MoSi₂(1 1 0)/Ni(1 1 1) interfaces and predict quite strong (3.5-3.8 J/m²) adhesion of this metal-silicide ceramic to nickel. The origin of this strong adhesion is elucidated by examining the geometric and electronic structure of the interfaces. We predict that Mo and Si atoms at the interface primarily occupy Ni 3-fold hollow sites, the typical adsorption site on Ni(1 1 1). Projected local densities of states and electron density difference plots reveal a mixture of localized, covalent Si-Ni bonds and more delocalized metallic Mo-Ni bonding, as the origin of the strong interfacial bonding. As emphasized in our earlier work, creation of strong covalent bonds at interfaces results in very strong adhesion. Such strong adhesion makes MoSi₂ a potential candidate for use in thermal barrier applications, in conjunction with a yttria-stabilized zirconia topcoat.     

Density functional study of small cobalt-platinum nanoalloy clusters

The structural, energetic, electronic and magnetic properties of small bimetallic Co_nPt_m (n+m≤5) nanoalloy clusters are investigated by density functional theory within the generalized gradient approximation. A plausible candidate for the ground state isomer and the other possible local minima, binding energies, relative stabilities, magnetic moments, the highest occupied and the lowest unoccupied molecular orbital energy gaps have been calculated. It is found as a general trend that average binding energies of Co-Pt bimetallic clusters increase with Pt doping. Planar structures of pure Co clusters become 3D with the addition of Pt atoms. CoPt₂, Co₂Pt₂, Co₃Pt₂, and CoPt₄ nanoalloys are identified as the most stable species since they have higher second finite difference in energy than the others. Pt doping decreases the total spin magnetic moment gradually. A rule for the prediction of the total spin moments of small Co-Pt bimetallic clusters is derived.     

CH4 dissociation on Ni surfaces: Density functional theory study

CH₄ dissociation on Ni surfaces, which is important in CH₄ reforming reactions, was discussed by using density functional theory. It was found that the CH_x species were changed to anions after chemisorption. The site preference of CH_x (x = 0-3) species on Ni(1 1 1), Ni(1 0 0) and Ni(1 1 0) was located on the basis of the computed chemisorption energies. Ni(1 0 0) is the most preferred surface for CH₄ dissociation, compared to Ni(1 1 0) and the widely investigated Ni(1 1 1).     

Density functional theory studies on stress stabilization of the Cu(1 1 0) striped phase

Under oxygen exposure, the Cu(1 1 0) surface shows a striped phase consisting of alternating bare (1 1 0) surface areas and added-row (2 × 1)O reconstructions. Density functional theory is used to show that the major origin for the formation of the striped phase is the elastic interaction between these areas. The difference between the surface stress of the bare surface and the (2 × 1)O covered surface is predicted to be 1.3 N/m in reasonable agreement with values derived from grazing incidence X-ray diffraction. Supercell calculations for periods of up to 62 Å confirm that the formation of the striped phase is favorable compared to an added-row (4 × 1)O reconstruction with the same coverage. But the predicted equilibrium period of roughly 30 Å is significantly smaller than in experiment. The calculations are impeded by the surface energy alternating with the number of layers in the slab. This behavior is related to a quantum well behavior of the Cu 4s-electrons. A simple model for this behavior is discussed and compared to ab initio results.     

Imidazole ligand effect on O2 interaction with metalloporphyrins

We investigate the imidazole ligand effect on O₂ interaction with metalloporphyrins (MPs) using ab initio density functional calculations. We select iron-porphyrin (FeP) and cobalt-porphyrin (CoP) as MPs, and compare MP with (imidazole)MP [(Im)MP] including their O₂ adducts. The O-O bond of (Im)MP-O₂ tends to be weaker than that of MP-O₂ due to more accessible back-donation to O₂, resulting from a strong electron push from the imidazole ligand, though the effect is not true for the interaction of excited singlet O₂.     

Theoretical study of magnetic ordering and electronic properties of AgxAl1−x N compounds

Ferromagnetic ordering of silver impurities in the AlN semiconductor is predicted by plane-wave ultrasoft pseudopotential and spin-polarized calculations based on density functional theory (DFT). It was found that an Ag impurity atom led to a ferromagnetic ground state in Ag_0.0625Al_0.9375N, with a net magnetic moment of 1.95 μ_B per supercell. The nitrogen neighbors at the basal plane in the AgN₄ tetrahedron are found to be the main contributors to the magnetization. This magnetic behavior is different from the ones previously reported on transition metal (TM) based dilute magnetic semiconductor (DMS), where the magnetic moment of the TM atom impurity is higher than those of the anions bonded to it. The calculated electronic structure band reveals that the Ag-doped AlN is p-type ferromagnetic semiconductor with a spin-polarized impurity band in the AlN band gap. In addition, the calculated density of states reveals that the ferromagnetic ground state originates from the strong hybridization between 4d-Ag and 2p-N states. This study shows that 4d transition metals such as silver may also be considered as candidates for ferromagnetic dopants in semiconductors.     

Interaction of SiHx precursors with hydrogen-covered Si surfaces: Impact dynamics and adsorption sites

SiH_x (x = 1, 2, 3) ions impact on Si(0 0 1)(1 × 1):H and Si(0 0 1)(2 × 1):H surfaces has been studied by classical molecular dynamic simulations, considering an energetic range for the impinging species from 5 to 15 eV. Despite the initial full H coverage a high sticking coefficient has been obtained for all the species under investigation. A considerable fraction of adsorption events causes H removal from the surface while for other simulations a soft landing mechanism of the ions has been observed. Few representative minima for SiH₂/Si(0 0 1)(2 × 1):H were re-converged by ab initio calculations in order to check the reliability of our results.     

Effectiveness of in situ NH3 annealing treatments for the removal of oxygen from GaN surfaces

An in situ NH₃ annealing procedure for the cleaning of GaN(0 0 0 1) is studied in detail using density functional theory (DFT), microkinetic modeling and X-ray photoelectron spectroscopy (XPS). The microkinetic model was calibrated and tested against published H₂ and NH₃ temperature programmed desorption (TPD) experiments on GaN(0 0 0 1). We find that an NH₃ treatment is efficient for the removal of carbon contaminants, but a complete removal of oxygen contaminants cannot be achieved. The remaining oxygen coverage after the treatment was estimated from XPS measurements to be 0.92 ML. In contrast, our microkinetic model based on DFT derived parameters predicts complete removal of OH species and a final oxygen coverage of 0.19 ML. We assign the difference between model and experiments to the formation of a surface oxide phase, which is not included in the model. DFT results also indicate strong adsorbate-adsorbate interactions for H, N, NH, NH₂, O, and OH on the GaN(0 0 0 1) surface which were incorporated into the microkinetic model to a first approximation. XPS experiments and microkinetic modeling demonstrate that the final surface composition shows little dependence on process parameters such as temperature or the time the sample is kept at an elevated temperature. Furthermore, the microkinetic model suggests that complete removal of OH from the surface can also be achieved using a NH₃/H₂ mixture, or even pure H₂ as a hydrogen source. The amount of H₂ present in the feed changes the coverage of NH_x species, but a certain amount of adsorbed oxygen is always left on the surface.     

Density functional theory study of 3R- and 2H-CuAlO2 in tensile stress

We report a direct computational result of a phase transformation from the 3R phase to the 2H phase in CuAlO₂ with the application of tensile stress using the first-principles density functional theory calculations. The calculations of enthalpy variation with tensile stress indicates the 3R-to-2H phase transformation is expected to occur around −26.0 GPa. As the applied tensile stress increases, the independent elastic constants of 3R- and 2H-CuAlO₂ show the presences of mechanical instability at −27.5 and −27.6 GPa, which are possibly related with the ideal tensile strength.     

Density functional theory analysis of the Ni(1 1 0)c(2 × 2)-CN surface phase

Density functional theory slab calculations have been used to investigate the structure of the Ni(1 1 0)c(2 × 2)-CN adsorption phase. The results show excellent agreement with experimental quantitative determinations of this structure by photoelectron diffraction and low energy electron diffraction. In particular, they show that a lying-down orientation with the C–N axis along [0 0 1] perpendicular to the close-packed Ni rows on the surface is strongly favoured over end-on adsorption (with the C–N axis perpendicular to the surface). This geometry is also favoured over a lying-down geometry with the C–N axis aligned along the azimuth, as originally proposed for this system and supported by cluster calculations.     

Structure and dynamics of the missing-row reconstruction on O/Cu(0 0 1) and O/Ag(0 0 1)

The geometry and the vibrational properties of missing row reconstructed O/Cu(0 0 1) and O/Ag(0 0 1) surfaces are investigated by means of density functional theory and density functional perturbation theory, using the local density and the generalized-gradient approximations. Our results predict very similar structural and vibrational properties for the two reconstructed surfaces. In the case of copper our calculations reproduce quite accurately the experimental results, while for the missing row reconstructed O/Ag(0 0 1) surface the agreement between theory and experiment is less satisfactory.     

The effect of surface symmetry on the adsorption energetics of SCH3 on gold surfaces studied using Density Functional Theory

Adsorption of methanethiol onto the three, high symmetry gold surfaces has been studied at the density functional level using a linear combination of atomic orbitals approach. In all three cases the bond energy between the thiolate radical and surface is typical of a covalent bond, and is of the order of 40 kcal mol⁻¹. For the (1 1 1) surface the fcc hollow site is slightly more stable than the bridge site. For the (1 0 0) surfaces the four-fold hollow is clearly the most stable, and for the reconstructed (1 1 0) surface the bridge/edge sites either side of the first layer atoms are preferred. The calculated differences in binding energy between the three surfaces indicate that the thiolate will preferentially bind to the Au(1 1 0) or (1 0 0) before (1 1 1) surface, by about 10 kcal mol⁻¹. The (1 1 0) surface is slightly more favourable than the (1 0 0), although the energy difference is only 3 kcal mol⁻¹. The results suggest the possibility of selectively functionalising the different facets offered by a gold nanoparticle.