Vacuum annealing phenomena in ultrathin TiD y /Pd bi-layer films evaporated on Si(100) as studied by TEM and XPS

Using a combination of TEM and XPS, we made an analysis of the complex high-temperature annealing effect on ultrathin titanium deuteride (TiD _y ) films evaporated on a Si(100) substrate and covered by an ultrathin palladium layer. Both the preparation and annealing of the TiD _y /Pd bi-layer films were performed in situ under UHV conditions. It was found that the surface and bulk morphology of the bi-layer film as well as that of the Si substrate material undergo a microstructural and chemical conversion after annealing and annealing-induced deuterium evolution from the TiD _y phase. Energy-filtered TEM (EFTEM) mapping of cross-section images and argon ion sputter depth profiling XPS analysis revealed both a broad intermixing between the Ti and Pd layers and an extensive inter-diffusion of Si from the substrate into the film bulk area. Segregation of Ti at the Pd top layer surface was found to occur by means of angle-resolved XPS (ARXPS) and the EFTEM analyses. Selected area diffraction (SAD) and XPS provided evidence for the formation of a new PdTi₂ bimetallic phase within the top region of the annealed film. Moreover, these techniques allowed to detect the initial stages of TiSi phase formation within the film–substrate interlayer.     

Kinetic Monte Carlo study of submonolayer heteroepitaxial growth comparing Cu/Ni and Pt/Ni on Ni(100)

The surface patterns formed during submonolayer Cu/Ni and Pt/Ni heteroepitaxy upon a Ni(100) substrate have been investigated by kinetic Monte Carlo (KMC) simulations. The two-dimensional (2D) KMC simulations are based upon rate constants for a complete nearest-neighbor set of 729 uncorrelated Cu or Pt atoms and/or Ni site-to-site hopping mobilities. The rate constant activation energies are determined by classical-potential total-energy calculations using an embedded-atom method potential function from the literature. We find that diffusion of Cu atoms occurs at a faster rate and Pt atoms at a slower rate than that of Ni atoms on the flat Ni(100) surface, and the initial nucleation and growth patterns of 2D islands and the kinetic versus thermodynamic control of the growth vary as a consequence. In the temperature and deposition time regime in which we work, the Cu/Ni systems show less than random mixing, while the Pt/Ni systems show more than random mixing. The Cu/Ni system has bonding energies that result in a tendency to segregate toward subdomains of pure Ni and Cu, though kinetic effects in the epitaxy trap the development of the system at small subdomain sizes. The Pt/Ni system has bonding energies giving a tendency to intermix completely, while epitaxial kinetic effects modestly interfere with the complete mixing. The kinetically determined island morphologies under various Cu/Ni and Pt/Ni compositions and deposition rates differ substantially over time periods that are long on the deposition time scale, and therefore the island patterns can become frozen in place.     

Self-assembly of nanosize coordination cages on si(100) surfaces

Bottom-up fabrication of 3D organic nanostructures on Si(100) surfaces has been achieved by a two-step procedure. Tetradentate cavitand 1 was grafted on the Si surface together with 1-octene (Oct) as a spatial spectator by photochemical hydrosilylation. Ligand exchange between grafted cavitand 1 and self-assembled homocage 2, derived from cavitand 5 bearing a fluorescence marker, led to the formation of coordination cages on Si(100). Formation, quantification, and distribution of the nanoscale molecular containers on a silicon surface was assessed by using three complementary analytical techniques (AFM, XPS, and fluorescence) and validated by control experiments on cavitand-free silicon surfaces. Interestingly, the fluorescence of pyrene at approximately 4 nm above the Si(100) surface can be clearly observed.     

Grazing incidence X-ray absorption spectra of (Si/W) 100/Si multilayer

Tungsten (W) M_III X-ray absorption spectra of a periodic multilayer, (Si/W)₁₀₀/Si, were measured with the change of X-ray grazing angle using sample current method. Under not total reflection condition, the absorbance changed little except at W M_III absorption edge. While under the total reflection condition, the absorbance increased with the increase of the X-ray energy and the increment changed from low to high at the W M_III absorption edge. This result reflected the variation of the X-ray evanescent wavelength caused by the absorption effect of W.     

A theoretical study of atomic oxygen chemisorption on the Ni(100) and Ni(111) surfaces

Atomic oxygen chemisorption has been studied for the fourfold hollow site of the Ni(100) surface and for the threefold hollow site of the Ni(111) surface. To model the Ni(100) surface, 10 different clusters in the range Ni₅ to Ni₄₁ were used, and for the Ni(111) surface, 11 different clusters in the range Ni₁₃ to Ni₄₃ were used. A detailed analysis of the orbital occupations of the cluster with and without oxygen for the different clusters shows that there are three different possible bonding mechanisms. In two of these, the basic feature is that a₁ electrons of the cluster are kicked out by the oxygen 2p_z orbital and moved to holes in the 2p_{x, y} orbitals. A picture where the oxygen electrons fit into the electronic structure of the cluster is emphasized. The third mechanism, which is applicable for only one cluster, can be described as the formation of two covalent bonds of E symmetry. The final best estimate of the oxygen chemisorption energy for the Ni(100) surface is about 130 kcal/mol, and for the Ni(111) surface, about 115 kcal/mol. In particular for the Ni(111) surface, an excited oxygen state with radical character is identified, which might be a catalytically important species. The excitation energy to reach this state should be on the order of 10 kcal/mol for the Ni(111) surface.     

Electronic excited states of Si(100) and organic molecules adsorbed on Si(100)

The electronically excited states of the Si(100) surface and acetylene, benzene, and 9,10-phenanthrenequinone adsorbed on Si(100) are studied with time-dependent density functional theory. The computational cost of these calculations can be reduced through truncation of the single excitation space. This allows larger cluster models of the surface in conjunction with large adsorbates to be studied. On clean Si(100), the low-lying excitations correspond to transitions between the pi orbitals of the silicon-silicon dimers. These excitations are predicted to occur in the range 0.4-2 eV. When organic molecules are adsorbed on the surface, surface --> molecule, molecule --> surface, and electronic excitations localized within the adsorbate are also observed at higher energies. For acetylene and benzene, the remaining pipi* excitations are found to lie at lower energies than in the corresponding gas-phase species. Even though the aromaticity of 9,10-phenanthrenequinone is retained, significant shifts in the pipi* excitations of the aromatic rings are predicted. This is in part due to structural changes that occur upon adsorption.     

Nanoporosity of Si (100) bars

Si(100) samples cut from a typical bar (100 mm in diameter) prepared using industrial technology are studied. Measurements of the electron work function (EWF) show that the size effects in these samples (a reduction in thickness along with a sample’s area and the EWF) detected earlier were due to nanostructure porosity that was buried by the technological treatment of a bar’s surface. This hidden nanoporosity is assumed to be a manifestation of the secondary crystal structure.     

Density-functional study of the cycloaddition of acrylonitrile on the Si(100) surface

Using a density functional approach, we have explored the cycloaddition of acrylonitrile on the Si(100) surface. The buckling of the surface dimers characteristic for the (2x1) reconstructed surface is shown to favor structures with a dipolar moment such as the resonant form of acrylonitrile with cumulative double bonds. The bond of acrylonitrile via a single C atom is a possible intermediate leading to the nitrile structure of the adsorbed molecule.     

Passive and active oxidation of Si(100) by atomic oxygen: a theoretical study of possible reaction mechanisms

Reaction mechanisms for oxidation of the Si(100) surface by atomic oxygen were studied with high-level quantum mechanical methods in combination with a hybrid QM/MM (Quantum mechanics/Molecular Mechanics) method. Consistent with previous experimental and theoretical results, three structures, "back-bond", "on-dimer", and "dimer-bridge", are found to be the most stable initial surface products for O adsorption (and in the formation of SiO(2) films, i.e., passive oxidation). All of these structures have significant diradical character. In particular, the "dimer-bridge" is a singlet diradical. Although the ground state of the separated reactants, O+Si(100), is a triplet, once the O atom makes a chemical bond with the surface, the singlet potential energy surface is the ground state. With mild activation energy, these three surface products can be interconverted, illustrating the possibility of the thermal redistribution among the initial surface products. Two channels for SiO desorption (leading to etching, i.e., active oxidation) have been found, both of which start from the back-bond structure. These are referred to as the silicon-first (SF) and oxygen-first (OF) mechanisms. Both mechanisms require an 89.8 kcal/mol desorption barrier, in good agreement with the experimental estimates of 80-90 kcal/mol. "Secondary etching" channels occurring after initial etching may account for other lower experimental desorption barriers. The calculated 52.2 kcal/mol desorption barrier for one such secondary etching channel suggests that the great variation in reported experimental barriers for active oxidation may be due to these different active oxidation channels.     

Vibrational spectroscopy study of H2O on Pd/MgO(100) films

A study of water on Pd/MgO(100) films prepared in situ upon Mo(100) single crystals has been performed by using low-energy electron diffraction, high-resolution electron energy-loss spectroscopy, and ultraviolet photoelectron spectroscopy in an ultrahigh vacuum system. The results show that the adsorption of water on MgO(100) film surfaces is enhanced significantly owing to the presence of Pd particles. The formation of a hydroxyl group suggests a dissociation of the water. The size and density of Pd particles on the substrate of MgO(100) films play an important part in the adsorption of water.     

Geometric and Electronic Structures of Pyrazine Molecule Chemisorbed on Si(100) Surface by XPS and NEXAFS Spectroscopy

The geometric and electronic structures of several possible adsorption configurations of the pyrazine ({C\begin{document}$ _{4} $\end{document}}{H\begin{document}$ _{4} $\end{document}}{N\begin{document}$ _{2} $\end{document}}) molecule covalently attached to Si(100) surface, which is of vital importance in fabricating functional nano-devices, have been investigated using X-ray spectroscopies. The Carbon K-shell (1s) X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy of predicted adsorbed structures have been simulated by density functional theory with cluster model calculations. Both XPS and NEXAFS spectra demonstrate the structural dependence on different adsorption configurations. In contrast to the XPS spectra, it is found that the NEXAFS spectra exhibiting conspicuous dependence on the structures of all the studied pyrazine/Si(100) systems can be well utilized for structural identification. In addition, according to the classification of carbon atoms, the spectral components of carbon atoms in different chemical environments have been investigated in the NEXAFS spectra as well.     

Si 2p XPS spectrum of the hydrogen‐terminated (100) surface of device‐quality silicon

An experimental study of the Si 2p XPS spectrum at different take‐off angles of atomically flat, hydrogen‐terminated 1 × 1 Si(100) is reported. The observed spectrum can be described accurately by considering three additional contributions to the spectrum of elemental silicon. Each contribution is attributed to a chemical state of silicon on the basis of its chemical shift with respect to elemental silicon and the depth of the region where it was originated. Copyright © 2003 John Wiley & Sons, Ltd.     

载Ni超稳Y型沸石的XPS研究

本文用XPS研究了载Ni超稳Y型沸石上Ni~(2+)的还原作用.结果表明,于450℃用氢还原仍有明显数量的Ni~(2+)没有被还原.当试样经氢还原或热处理后,NiHUSY沸石的表面(Ni/Si)_s值增加.表面金属Ni与异辛烷临氢裂化及其产物分布有密切关系,随着Ni~(2+)还原程度提高,异辛烷临氢裂化活性显著增加,二次裂化反应减少,“焦”的前躯产物(不饱和烃)含量大大降低.     

A theoretical study of hydrogen surface coverage over Ni(100)

We have used the MINDO/SR molecular orbital method in order to model the migration of hydrogen atoms over a Ni(100) surface. The present calculations indicate the existence of two different states for adsorbed hydrogen, a result which is in agreement with experimental thermal desorption data and LEED . A detailed analysis of the electronic factors involved in this process is presented.     

Cyanide-bridged NiCr and alternate NiFe-NiCr magnetic ultrathin films on functionalized Si(100) surface

Sequential growth in solution (SGS) was performed for the magnetic cyanide-bridged network obtained from the reaction of Ni(H(2)O)(2+) and Cr(CN)(6)(3-) (referred to as NiCr) on a Si(100) wafer already functionalized by a Ni(II) complex. The growth process led to isolated dots and a low coverage of the surface. We used the NiFe network as a template to improve the growth of the magnetic network. We elaborated alternate NiFe (paramagnetic)-NiCr (ferromagnetic) ultrathin films around 6 nm thick. The magnetic behaviour confirmed the alternate structure with the ferromagnetic zones isolated between the paramagnetic ones since the evolution of the blocking temperature is consistent with the evolution of the layers' thickness expected from the SGS process.     

Electrocatalytic oxidation of formaldehyde and methanol on Ni(OH)2/Ni electrode

A nickel hydroxide-modified nickel electrode (Ni(OH)₂/Ni) was successfully prepared by the cyclic voltammetry (CV) method and the electrocatalytic properties of the electrode for formaldehyde and methanol oxidation have been investigated respectively. The Ni(OH)₂/Ni electrode exhibits high electrocatalytic activity in the reaction. A new method has been developed for formaldehyde determination at the nickel hydroxide-modified nickel electrode and the experimental parameters were optimized. The oxidation peak current is linearly proportional to the concentration of formaldehyde in the range of 7.0 × 10^?5 to 1.6 × 10^?2 M with a detection limit of 2.0 × 10^?5 M. Recoveries of artificial samples are between 93.3 and 103.5%. The effect of scan rate and methanol concentration on the electrochemical behavior of methanol were investigated respectively.     

QM/QM study of the coverage effects on the adsorption of amino-cyclopentene at the Si(100) surface

In this work, we have tested 30 different adsorption situations in several coverage scenarios for the 1-amino-3-cyclopentene (ACP) molecule on the Si(100) surface. We have used a five-spot testing zone inserted in the high-level part of a quantum-mechanical/quantum-mechanical study performed in a big cluster. By defining several different scenarios, each one with a typical adsorption energy, we were able to understand in detail the process of surface functionalization. We are able to justify why the functionalization of this silicon surface achieves only a coverage of approximately 0.5 ML (half monolayer) and why the completely covered surface should be thermodynamically impossible to obtain.     

Grafting cavitands on the Si(100) surface

Cavitand molecules having double bond terminated alkyl chains and different bridging groups at the upper rim have been grafted on H-terminated Si(100) surface via photochemical hydrosilylation of the double bonds. Pure and mixed monolayers have been obtained from mesitylene solutions of either pure cavitand or cavitand/1-octene mixtures. Angle resolved high-resolution X-ray photoelectron spectroscopy has been used as the main tool for the monolayer characterization. The cavitand decorated surface consists of Si-C bonded layers with the upper rim at the top of the layer. Grafting of pure cavitands leads to not-well-packed layers, which are not able to efficiently passivate the Si(100) surface. By contrast, monolayers obtained from cavitand/1-octene mixtures consist of well-packed layers since they prevent silicon oxidation after aging. AFM measurements showed that these monolayers have a structured topography, with objects protruding from the Si(100) surface with average heights compatible with the expected ones for cavitand molecules.     

In situ surface oxidation study of a planar Co/SiO2/Si(100) model catalyst with nanosized cobalt crystallites under model Fischer-Tropsch synthesis conditions

The oxidation of nanosized metallic cobalt to cobalt oxide during Fischer-Tropsch synthesis (FTS) has long been postulated as a major deactivation mechanism. In this study a planar Co/SiO(2)/Si(100) model catalyst with well-defined cobalt crystallites, close to the threshold value reported for oxidation in the literature (4-10 nm), was prepared by the spin coating method. The planar Co/SiO(2)/Si(100) model catalyst was characterized with atomic force microscopy, X-ray photoelectron spectroscopy, and Rutherford backscattering. The surface oxidation behavior of the nanosized metallic cobalt crystallites of 4-5 nm was studied using in situ near-edge X-ray absorption fine structure under model FTS conditions, i.e., H(2)/H(2)O = 1, P(Total) = 0.4 mbar, and 150-450 degrees C. No surface oxidation of metallic cobalt was observed under these model FTS conditions over a wide temperature range, i.e., 150-400 degrees C.     

Ab initio study of epitaxial growth on stepped Si(100) surface

Diffusion of a Si adatom over the reconstructed Si(100) surface with a single-height step on it is studied using the pseudopotential total energy method. The S_B rebonded step is shown to act as a good sink for adatoms descending onto the lower ledge. This is due to the presence of deep traps on the lower terrace and to the negative Ehrlich-Schwoebel barrier (the activation barrier for descent from the edge is 0.23 eV lower than for the motion on a flat surface). The diffusion characteristics of the adatom on both terraces are virtually unaffected by the presence of the step. However, the dimer buckling sequence on a lower terrace is strongly dependent on the position of the adatom along the diffusion path. © 1997 John Wiley & Sons, Inc.