Silicidation in Ni/Si thin film system investigated by X-ray diffraction and Auger electron spectroscopy

Silicide formation induced by thermal annealing in Ni/Si thin film system has been investigated using glancing incidence X-ray diffraction (GIXRD) and Auger electron spectroscopy (AES). Silicide formation takes place at 870 K with Ni₂Si, NiSi and NiSi₂ phases co-existing with Ni. Complete conversion of intermediate silicide phases to the final NiSi₂ phase takes place at 1170 K. Atomic force microscopy measurements have revealed the coalescence of pillar-like structures to ridge-like structures upon silicidation. A comparison of the experimental results in terms of the evolution of various silicide phases is presented.     

Density functional study of hypophosphite adsorption on Ni (1 1 1) and Cu (1 1 1) surfaces

Surface structures and electronic properties of hypophosphite, H₂PO₂⁻, molecularly adsorbed on Ni(1 1 1) and Cu(1 1 1) surfaces are investigated in this work by density functional theory at B3LYP/6-31++g(d, p) level. We employ a four-metal-atom cluster as the simplified model for the surface and have fully optimized the geometry and orientation of H₂PO₂⁻ on the metal cluster. Six stable orientations have been discovered on both Ni (1 1 1) and Cu (1 1 1) surfaces. The most stable orientation of H₂PO₂⁻ was found to have its two oxygen atoms interact the surface with two PO bonds pointing downward. Results of the Mulliken population analysis showed that the back donation from 3d orbitals of the transition metal substrate to the unfilled 3d orbital of the phosphorus atom in H₂PO₂⁻ and 4s orbital's acceptance of electron donation from one lone pair of the oxygen atom in H₂PO₂⁻ play very important roles in the H₂PO₂⁻ adsorption on the transition metals. The averaged electron configuration of Ni in Ni₄ cluster is 4s^0.634p^0.023d^9.35 and that of Cu in Cu₄ cluster is 4s^1.004p^0.033d^9.97. Because of this subtle difference of electron configuration, the adsorption energy is larger on the Ni surface than on the Cu surface. The amount of charge transfers due to above two donations is larger from H₂PO₂⁻ to the Ni surface than to the Cu surface, leading to a more positively charged P atom in Ni_nH₂PO₂⁻ than in Cu_nH₂PO₂⁻. These results indicate that the phosphorus atom in Ni_nH₂PO₂⁻ complex is easier to be attacked by a nucleophile such as OH⁻ and subsequent oxidation of H₂PO₂⁻ can take place more favorably on Ni substrate than on Cu substrate.     

New feature in the Auger peak of adsorbed oxygen

Recent joint theoretical and experimental investigations of Auger core-core-valence spectra of alkali adatoms on simple metals have revealed that such technique is capable to ascertain contributions from different adsorption environments in the signal [M.I. Trioni, S. Caravati, G.P. Brivio, L. Floreano, F. Bruno, A. Morgante, Phys. Rev. Lett. 93 (2004) 206802]. Consequently, to verify if such an effect is present also for other chemical species, we study theoretically the KLV transition of oxygen either as a bulk impurity or as an adsorbate in/on Al and Ag (jellium-like). We make use of the Fermi golden rule in which the matrix elements of the interaction are calculated within DFT. We verify that the relevant physical quantity of this phenomenon is the excited local density of states (LDOS), calculated within a region centered on the core ionized atom. The Auger rate for oxygen in Ag bulk displays a single asymmetric peak, while for adsorbed oxygen a second smaller feature at lower energies, and very close to the first one, appears. This unexpected result follows from the removal of the degeneracy of the m quantum number of the 2p states of oxygen at the surface. It is only displayed on the electronically less dense metal (Ag), but not on Al.     

Raman tensor calculation for magnesium phthalocyanine

We present ab-initio density functional (DFT) calculations of the vibrational spectra of neutral magnesium phthalocyanine (MgPc) molecule and of its Raman scattering intensities.     

Segregation of Pt at clean surfaces of (Pt, Ni)3Al

Experimental evidence for surface segregation of Pt at (1 1 1) surfaces of ternary (Pt, Ni)₃Al alloys is presented, based upon Auger electron spectroscopy, low energy ion scattering, and angle-resolved X-ray photoelectron spectroscopy. Density functional calculations in the dilute limit confirm that Pt segregation is energetically favored.     

Adsorption of thiophene on Ni(1 0 0), Cu(1 0 0), and Pd(1 0 0) surfaces: ab initio periodic density functional study

Adsorption of thiophene on the (1 0 0) surfaces of Ni, Cu, and Pd has been investigated by the ab initio density functional theory method (periodic DMol³). Several parallel and perpendicular adsorption geometries are examined in detail. For Ni(1 0 0), both dissociative and molecular adsorption structures are found with small difference in energy. Thiophene adsorbs only molecularly on Cu(1 0 0) and Pd(1 0 0). The most stable molecular adsorption structures on all the surfaces are quite similar, where thiophene adsorbs on top of a 4-fold hollow with the symmetry axis rotated 45° from the metal rows. These stable structures arise from a good matching of the thiophene molecule to the metal surfaces. The calculated adsorption geometries are in reasonable agreement with XAFS experiments.     

Resonant Auger spectroscopy study of charge transfer phenomena in N 1s core-excited acetonitrile adsorbates on Si(0 0 1)-2 × 1

Acetonitrile (CH₃CN) adsorbs on Si(0 0 1)-2 × 1 at room temperature under two forms, a cycloaddition-like adduct (Si-CN-Si) and a pendent cyano (Si-CH₂-CN) resulting from the decomposition of the molecule. Resonant Auger spectroscopy has been used to study the excited-state-dependent electron transfer from the N 1s core-excited molecular adsorbate to the silicon substrate, using the core-hole lifetime (∼6 fs) as an internal clock. It is shown that the π_CN^∗ NEXAFS state lies within the silicon bandgap because of a core-excitonic effect. Therefore no charge transfer of the excited electron to the substrate is observed. On the other hand the π_CN^∗ NEXAFS state is placed within the silicon conduction band. Excitation to this orbital leads to valence/Auger spectra in which both resonant and normal Auger contributions are observed. Therefore there is evidence for a charge transfer from the pendent CN to the silicon surface, on a timescale estimated to tens of femtoseconds.     

Density functional theory investigation of CN on Cu(1 1 1), Ni(1 1 1) and Ni(1 0 0)

The adsorption of CN on Cu(1 1 1), Ni(1 1 1) and Ni(1 0 0) has been investigated using density functional theory (DFT). While experimental studies of CN on Cu(1 1 1) show the molecular axis to be essentially parallel to the surface, the normally-preferred DFT approach using the generalised gradient approximation (GGA) yields a lowest energy configuration with the C-N axis perpendicular to the surface, although calculations using the local density approximation (LDA) do indicate that the experimental geometry is energetically favoured. The same conclusions are found for CN on Ni(1 1 1); on both surfaces bonding through the N atom is always unfavourable, in contrast to some earlier published results of ab initio calculations for Ni(1 1 1)/CN and Ni(1 0 0)/CN. The different predictions of the GGA and LDA approaches may lie in subtly different relative energies of the CN 5σ and 1π orbitals, a situation somewhat similar to that for CO adsorbed on Pt(1 1 1) which has proved challenging for DFT calculations. On Ni(1 0 0) GGA calculations favour a lying-down species in a hollow site in a geometry rather similar to that found experimentally and in GGA calculations for CN on Ni(1 1 0).     

Spin-polarization in ccv Auger electron spectroscopy for transition metals

The contributions of core states and the valence band to spin-resolved ccv Auger electron spectra (AES) are discussed in terms of a relativistic, spin-polarized theory. The influence of the core-state spin polarization and of selecting particular valence bands via cross sections is shown for the surface layer of BCC Fe(1 0 0) as calculated by means of the Screened KKR ab-initio electronic structure method.     

X-ray photoelectron and X-ray Auger electron spectroscopy studies of heavy ion irradiated C60 films

The influence of 200 MeV Au ion irradiation on the surface properties of polycrystalline fullerene films has been investigated. The X-ray photoelectron and X-ray Auger electron spectroscopies are employed to study the ion-induced modification of the fullerene, near the surface region. The shift of C 1s core level and decrease in intensity of shake-up satellite were used to investigate the structural changes (like sp² to sp³ conversion) and reduction of π electrons, respectively, under heavy ion irradiation. Further, X-ray Auger electron spectroscopy was employed to investigate hybridization conversion qualitatively as a function of ion fluence.     

Structure of enantiopure and racemic alanine adlayers on Cu(1 1 0)

Plane wave density functional theory has been employed to analyze the structure of alanine adlayers on the Cu(1 1 0) surface. Alanine forms (3 × 2) adlayers on Cu(1 1 0) that are closely related to the structures of glycine on the same surface. There is essentially no energy difference between the most stable racemic and enantiopure alanine adlayers. This observation implies that adsorption of racemic alanine on Cu(1 1 0) will result in a pseudoracemate adlayer.     

Intermediate range order in concentrated aqueous solutions of copper nitrate. X-ray diffraction and Raman investigations

Concentrated aqueous solutions of copper nitrate of different concentrations (0.66 mol dm⁻³ up to 4.84 mol dm⁻³) have been investigated by X-ray diffraction at room temperature. In these solutions a maximum of intensity (prepeak) is observed at low values of Q (approx. 0.6 to 1 Å⁻¹) suggesting the existence of an intermediate range order in their structure, in agreement with previous investigations on different electrolytes. In order to get information on the solvation shell of the copper cation, Raman spectroscopy experiments have been performed for the more concentrated solution. Polarization observations, isotopic substitution of the solvent and the comparison with a Raman spectrum of a concentrated aqueous solution of aluminum nitrate were performed in order to improve the interpretation of the obtained results with these convergent observations.     

Correlation effects in Auger spectra of Ni and Cu nanoclusters

Results of experimental research of exciton-like two-hole states in nanoclusters of narrow-band metals (Ni, Cu) on surface of high-oriented pyrolitic graphite by X-ray photoelectron and Auger electron spectroscopy are presented. It was found that the evolution of the electronic structure in Ni nanoclusters with the decreasing of their sizes can lead to appearance of long-living two-hole states in the valence band. One-particle and two-particle density of states are analyzed, and the Auger-electron spectra confirming the presence of the bound and localized states are obtained.     

Temperature-induced phase transition in phlogopite revealed by Raman spectroscopy

Raman study of a natural hydrous phlogopite was carried out at temperatures up to 500 °C for the first time. Evolution of four well-resolved Raman modes at wavenumbers 196, 278, 322, and 682 cm⁻¹ was followed in detail with temperature increase. The analysis of data reveals linear decrease of vibrational wavenumbers in the studied temperature range, with small but experimentally significant discontinuities occurring at a temperature of 365±15 °C. Although the overall appearance of Raman spectra remains intact on crossing this temperature, the presence of discontinuities, as well as a marked difference between Gruneisen parameters calculated for high- and low-temperature ranges, signifies the presence of a temperature-induced phase transformation. By combining and correlating the results of the present Raman study with the high-temperature X-ray work performed by Tutti et al. [High-temperature study and thermal expansion of phlogopite, Phys. Chem. Miner. 27 (2000) 599-603] we arrive at the interpretation of a temperature-induced structural phase transformation in phlogopite without a significant symmetry change, with an underlying microscopic mechanism involving deformation of Mg octahedra and rotation of tetrahedral grid from ditrigonal toward hexagonal at the transition temperature.     

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.     

对巯基苯胺分子构象和拉曼光谱的密度泛函理论研究

本文采用密度泛函方法B3LYP/6-311+G**计算了对巯基苯胺分子(PATP)的平衡结构和振动拉曼光谱。结果表明该分子有两个稳定构象,反式构象能更低。利用振动模分析所得的势能分布我们对该分子振动基频进行了归属,并计算了低能构象的标度振动频率,其结果与实验值相吻合。     

Geometrical characterization of adenine and guanine on Cu(1 1 0) by NEXAFS, XPS, and DFT calculation

Adsorption of purine DNA bases (guanine and adenine) on Cu(1 1 0) was studied by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine-structure spectroscopy (NEXAFS), and density-functional theory (DFT) calculation. At coverages near 0.2 monolayers, Angular-resolved NEXAFS analysis revealed that adenine adsorbates lie almost flat and that guanine adsorbates are tilted up on the surface with the purine ring parallel to the atom rows of Cu(1 1 0). Referring to the previous studies on pyrimidine DNA bases [M. Furukawa, H. Fujisawa, S. Katano, H. Ogasawara, Y. Kim, T. Komeda, A. Nilsson, M. Kawai, Surf. Sci. 532-535 (2003) 261], the isomerization of DNA bases on Cu(1 1 0) was found to play an important role in the adsorption geometry. Guanine, thymine and cytosine adsorption have an amine-type nitrogen next to a carbonyl group, which is dehydrogenated into imine nitrogen on Cu(1 1 0). These bases are bonded by the inherent portion of -NH-CO- altered by conversion into enolic form and dehydrogenation. Adenine contains no CO group and is bonded to Cu(1 1 0) by participation of the inherent amine parts, resulting in nearly flatly-lying position.     

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.