CH3Cl adsorption on a Si(100)2 × 1 surface modified by alkali metal overlayer studied by soft X-ray photoemission using synchrotron radiation

We present the first study of the effect of an alkali metal overlayer on the adsorption of an organic molecule, methylchloride, on a Si(100)2 × 1 surface. In strong contrast to the behavior of molecular oxygen or nitrogen which were found to react with the silicon substrate, there was no significant interaction between methylchloride and silicon, rather, the formation of alkali-chlorine bonds was observed. Core level and valence band spectroscopies using synchrotron radiation were used to study these systems. Sodium was found to exhibit the strongest interaction with mehtylchloride which was dissociated, while the effects produced by K and Cs were weaker.     

Characterisation of some gold clusters by X-ray photoelectron spectroscopy

An XPS characterisation of the electronic structures of some polynuclear gold compounds is presented. The photoelectron spectra of the core levels and the valence bands are discussed in terms of the differing geometries and coordination environments in the metal clusters. The XPS analysis of “centred” and “non-centred” gold clusters shows characteristic features which operate a new distinction among these two classes of compounds.     

The nature of the adsorption bond between graphite islands and iridium surface

To reveal the nature of adsorption bonds between two-dimensional graphite islands and iridium (111) and (100) faces, a study has been made of the adsorption of potassium and cesium atoms on the surface of these systems, using thermal desorption and Auger electron spectroscopy, as well as surface ionization and thermionic emission techniques. The graphite islands are shown to be weakly bound to the iridium substrate by Van der Waals forces. The unsaturated valence bonds at the periphery of the graphite islands are “lowered down” on to the metal. The recess between the graphite layer and the metal is filled by adsorbing particles through defects in the graphite layer. The atoms can penetrate into the recess in two ways: at T > 1000 K directly from the flux incident on the surface, and at T < 1000 K also by migration from the graphite island surface. The adsorption capacity of this state is ～ (2?3) × 10¹⁴cm^-2. Thermal destruction of the islands at T > 1900 K liberates the potassium and cesium atoms from under the graphite islands. Our study suggests that the reason for the “raised” position of the islands lies in the valence bonds of the graphite layer being saturated, the valence bonds of the metal and its crystallographic orientation being less significant. Therefore one may expect the graphite layer to be raised also above other metals as well. The filling by cesium of the recess between the graphite layer and iridium and of the adsorption phase on the graphite surface, does not change the general “graphitic” shape of the carbon Auger peak. This cesium results, however, in a pronounced splitting of the negative spike on the carbon peak (which provides information on its location relative to the graphite layer) indicating the appearance in the valence band of graphite near the Fermi level of two narrow (～ 2?3 eV) regions with an enhanced density of states originating from the presence of the alkali metal.     

The electronic structures of the core and valence ion states of metal oxides

SCF-Xα SW MO calculations on metal core ion hole states and X-ray emission (XES) and X-ray photoelectron (XPS) transition states of the non- transition metal oxidic clusters MgO₆^10?, AlO₄^5? and SiO₄^4? show relative valence orbital energies to be virtually unaffected by the creation of valence orbital or metal core orbital holes. Accordingly, valence orbital energies derived from XPS and XES are directly comparable and may be correlated to generate empirical MO diagrams. In addition, charge relaxation about the metal core hole is small and valence orbital compositions are little changed in the core hole state. On the other hand, for the transition metal oxidic clusters FeO₆^10?, CrO₆^9? and TiO₆^8? relative valence orbital energies are sharply changed by a metal core orbital or crystal field orbital hole, the energy lowering of an orbital increasing with its degree of metal character. Consequently O 2p nonbonding → M 3d-O 2p antibonding (crystal field) energies are reduced, while M 3d bonding → O 2p nonbonding and M 3d-O 2p antibonding → M 4s,p-O 2p antibonding (conduction band) energies increase. Charge relaxation about the core hole is virtually complete in the transition metal oxides and substantial changes are observed in the composition of those valence orbitals with appreciable M 3d character. This change in composition is greater for e _g than for t_2g orbitals and increases as the separation of the e_g crystal field (CF) orbitals and the O 2p nonbonding orbital set decreases. Based on the hole state MO diagrams the higher energy XPS satellite in TiO₂ (at about 13 eV) is assigned to a valence → conduction band transition. The UV PES satellites at 8.2 eV in Cr₂O₃ and 9.3 eV in FeO are tentatively assigned to similar transitions to conduction band orbitals, although the closeness in energy of the crystal field and O 2p nonbonding orbitals in the valence orbital hole state prevents a definite assignment on energy criteria alone. However the calculations do clearly show that charge transfer transitions of the e_g bonding → e_g crystal field orbital type would generally occur at lower energy than is consistent with observed satellite structure.A core electron hole has little effect upon relative orbital energies and is only slightly neutralized by valence electron redistribution for MgO and SiO₂. For the transition metal oxides a core hole lowers the relative energies of M3d containing orbitals by large amounts, reducing O → M charge transfer and increasing M 3d crystal field → conduction band energies. Large and sometimes overcomplete neutralization of the core hole is observed, increasing from CrO₆^9? to FeO₆^10? to TiO₆^8?. as the O → M charge transfer energy declines.High energy XPS satellites in TiO₂ may be assigned to O 2p nonbonding → conduction band transitions while lower energy UV PES satellites in FeO and Cr₂O₃ arise from crystal field or O 2p nonbonding → conduction band excitations. Our “shake-up” assignment for FeO₆^10?, CrO₆^9? and TiO₆^8? are less than definitive because no procedure has yet been developed to calculate “shake-up” intensities resulting from transitions of the type described. However the results do allow a critical evaluation of earlier qualitative predictions of core and valence hole effects. First, we find that the comparison of hole or valence state ionic systems with equilibrium distance systems of higher nuclear and/or cation charge (e.g. the comparison of the FeO₆^10? Fe 2p core hole state to Co₃O₄) is dangerous. For example, larger MO distances in the ion states substantially reduce crystal field splittings. Second, core and CF orbital holes sharply reduce O → M charge transfer energies, giving 2e_g → 3e_g energy separations which are generally too small to match observed satellite energies. Third, highest occupied CF-conduction band energies are only about 4–5 eV in the ground states, but increase to about 7–11 eV in the core and valence hole states of the transition metal oxides studied. The energetic arguments presented thus support the idea of CF and/or O 2p nonbonding → conduction band excitations as assignments for “shake-up” satellites, at least in oxides of metals near the beginning of the transition series.     

FDTD simulation study of size/gap and substrate-dependent SERS activity study of Au@SiO_2 nanoparticles

We use Au@SiO_2 nanoparticles(NPs) to systematically and comprehensively study the relationship between nanostructure and activity for surface-enhanced Raman scattering. Calculation simulation using the finite different time domain method verifies the experiment results and further reveals that the particle size and the distance between the NPs play vital roles in the surface-enhanced Raman scattering(SERS). Furthermore, in order to better simulate the real experiment, a Au@SiO_2 nanosphere dimer is placed on the silicon substrate and Au substrate, separately. The simulation results show that the large EM field coupling is due to the "hot spots" transferred from the NP–NP gaps to NP–surface of metal gaps,meanwhile, more "hot spots" occur. We also find that the signal intensity strongly depends on the position of the probe molecule. This work provides a better understanding of EM field enhancement.     

Epitaxial SiC formation induced by medium energy ions on Si(1 1 1) at room temperature

In the search for silicon technology compatible substrate for III-nitride epitaxy, we present a proof-of-concept for forming epitaxial SiC layer on Si(1 1 1). A C/Si interface formed by ion sputtering is exposed to 100-1500 eV Ar⁺ ions, inducing a chemical reaction to form SiC, as observed by core-level X-ray photoelectron spectroscopy (XPS). Angle dependent XPS studies shows forward scattering feature that manifest the epitaxial SiC layer formation, while the valence band depicts the metal to insulator phase change.     

On the mechanism of carbon nanotube formation in electrochemical processes

Quantum-chemical methods are used to analyze the mechanism of carbon nanotube formation in the electrochemical bath, where tiny fragments of graphene planes are in the environment of atoms and ions of alkali metals and halogens. In the optimal configuration, alkali metal atoms move toward the edge of a graphene fragment, whereas halogen atoms remain at the sites of their initial attachment. When the graphene fragments “burdened” by alkali metal and halogen atoms interact with each other, the overall graphene configuration twists in a natural way into a nanotube-like open-end structure.     

Calculation of the surface binding energy for ion sputtered particles

In this paper, we consider a simple model, based on the electronegativity concept, which makes it possible to calculate the surface binding energy of any particles, sputtered from a solid surface by ion bombardment. The model contains empirical equations for calculation of “ionic” and “covalent” parts of the binding energy happened between a surface atom and its close neighbors. The model is strongly desirable in the case of the ion sputtering of any strong electronegative (C, N, O, F, Cl, etc.) or electropositive elements (alkali metals, rare earth elements) from semiconductor surfaces, when the strong ionic binding between these elements and the surface atoms can be estimated.     

Alkali metal intercalation into SnS2

The intercalation of sodium and potassium into the layered semiconductor SnS₂ has been investigated by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and ion scattering spectroscopy (ISS). After deposition of the alkali metals onto (0001) cleavage planes of SnS₂ in ultra high vacuum (UHV), semiconducting intercalation phases were formed. They seem to be homogeneous and disordered under the given experimental conditions. The valence electrons of the alkali metals are transferred into electronic states of the host lattice, whose valence band density of states changes significantly during intercalation. The underlying changes of the binding properties of the host lattice are discussed. The course of intercalation can be separated into three phases. During an induction period the concentration of the alkali metal on the surface remains very small, the electronic states of the substrate are shifted by band bending. During an intercalation period the topotactic reaction proceeds. After reaching saturation compositions of the intercalation phase at the surface, the alkali metal diffuses into the bulk. Crystal or surface defects seem to have a significant influence on the kinetics of intercalation and on the stoichiometry of the intercalation compounds.     

硅烯饱和吸附碱金属原子的第一性原理研究

基于密度泛函理论的第一性原理计算, 研究了硅烯饱和吸附碱金属元素原子的稳定性、微观几何结构和电子性质, 并与纯硅烯及其饱和氢化结构进行了对比分析. 研究发现复合物SiX(X=Li, Na, K, Rb)的形成能都是负的, 相对于纯硅烯来说可以稳定存在. Bader电荷分析表明, 电荷从碱金属原子转移至硅原子. 从成键方式来看, 硅烯与氢原子形成共价键, 而与碱金属原子之间形成的键主要是离子性成键, 但还存在部分共价关联成分. 能带计算表明, 锂原子饱和吸附在硅烯形成的复合物SiLi是直接带隙的半导体, 带隙大小为0.34 eV. 其他碱金属饱和吸附在硅烯上形成的复合物都表现为金属性.     

Excitons in Si nanocrystals

The states of electron-hole pairs in spherical silicon nanocrystals are theoretically studied using the “multiband” effective-mass approximation in the limit of an infinitely high potential barrier at the boundary. The degeneracy of the states at the top of the valence band is taken into account in the spherical approximation, and the ellipsoidal character of the electronic spectrum in the conduction band is allowed for. Coulomb interaction-induced corrections to the energy of an electron-hole pair are found.     

X-ray and UV photoelectron-spectroscopic studies of pyridine adsorbed on evaporated nickel and palladium in the temperature range 140–385 k

The states of pyridine adsorbed on evaporated nickel and palladium films have been investigated as a function of temperature in the range 140–385 K by means of X-ray and UV photoelectron spectroscopy. At ～ 140 K, pyridine “N-bonded” on the metal surfaces gives C 1s and N 1s peaks whose binding energies are very close to those for condensed pyridine and “N-bonded” pyridine on pre-oxidized nickel. The high-lying valence orbitals, 2b₁ (π) and 1a₂ (π) + 7a₁ (n), of pyridine show shifts similar to those for the “N-bonded” molecule on pre-oxidized nickel. At ～ 290 K, “π-bonded” pyridine shows large shifts in the C 1s and N 1s peaks and in the high-lying valence orbitals, as observed for “π-bonded” benzene on nickel. The assignments of the adsorbed states are supported by work-function change data. A large proportion of pyridine converts from the “N-bonded” to the “π-bonded” form between 220 and 290 K. Formation of “α-pyridyl” is suggested at ～ 375 K on nickel.     

Coverage dependent organic–metal interaction studied by high-resolution core level spectroscopy: SnPc (sub)monolayers on Ag(1 1 1)

We study the electronic structure of tin-phthalocyanine (SnPc) molecules adsorbed on a Ag(1 1 1) surface by high-resolution photoelectron spectroscopy. We particularly address the effect of different SnPc coverages on the interaction and charge transfer at the interface. The results give evidence for a covalent molecule–substrate interaction, which is temperature and coverage dependent. The valence and core level spectra as well as the work function measurements allow us monitoring subtle differences in the strength of the interface interaction, thus demonstrating the sensitivity of the methods. The results consistently show the effect of charge exchange between substrate and molecules which obviously leads to a net charge transfer into the SnPc molecules, and which is increased with decreasing coverage. Surprisingly, the Sn3d core levels are neither effected by variations of charge transfer and interaction strength, nor by a possible “Sn-up” or “Sn-down” orientation, which have been observed for sub-monolayers.     

A novel natural surface-enhanced fluorescence system based on reed leaf as substrate for crystal violet trace detection

The preparation of surface-enhanced fluorescence (SEF) substrates is often influenced by experimental strategies and factors such as the morphology and size of the nanostructures. In this study, using the natural reed leaves (RLs) without any special pretreatment as the substrate, metal silver is modified by magnetron sputtering technology to prepare a stable and efficient SEF system. The abundant “hedgehog-like”protrusions on the RL substrate surface can generate high-density “hot spots”, thus enhancement factor (EF) is enhanced up to 3345 times. The stability and reproducibility are verified in many measurements. The contribution of the intervention of silver nanostructure to the radiation attenuation process of fluorescent molecules is analyzed with the aid of Jablonski diagrams. Three-dimensional (3D) finite difference time domain (FDTD) simulates the spatial electric field and “hot spots”distribution of the substrate. The “hedgehog-like”protrusion structure generates multiple “hot spots”, which produce an excellent local surface plasmon resonance (LSPR) effect and provide higher fluorescence signal. Finally, RL/Ag-35 substrate is used to detect crystal violet (CV), and the detection limit is as low as 10^-13 M. This “hedgehog-like”SEF substrate provides a new strategy for the trace detection of CV, which has a good practical application value.     

Surface plasmon satellites in x-ray photoemission from core states of adsorbed atoms

It is shown that the kinetic energy spectrum of electrons emitted from core levels of atoms adsorbed on metal surfaces should exhibit surface plasmon satellites. At sufficiently high energies, the satellites arise only via the so-called “intrinsic” mechanism, namely, screening of a suddenly created hole by the metal electrons. Their observation for a suitable adsorption system would thus constitute the first unequivocal evidence for the “intrinsic” effect.     

Selective valence electron transfer in alkali atom-alkali ion collisions

Charge exchange reactions between alkali atoms and ions are studied. It is shown that the use of spin-polarised alkali atoms allows the experimental separation of valence electron and core electron transfer. Parameters are defined which are exactly independent of any core contributions. It is discussed in detail how these parameters, which allow an unambigious comparison between theory and experiment, can be measured.     

Soft X-ray appearance potential spectroscopy of C,N, O,and S chemisorbed on some 3d transition metals

The 1s level soft X-ray appearance potential spectra of C, N, O, and S adsorbed on polycrystalline 3d transition metal surfaces (e.g. Ti, Cr, Fe, and Ni) are presented. In the chemisorption regime of adsorption the adsorbate Is level spectra are characterized by a single peak above the excitation threshold. The peak is thought to be related to an enhanced local density of unfilled valence states on the adatoms. The peak widths correlate with the widths of the unfilled portion of substrate d-band in a way consistent with a mixing of the adsorbate valence states and the substrate d-states.     

On the semiclassical approach to the ionization process during sputtering

The semiclassical “rate equation” approach to the ionization process during sputtering is shown to be the correct one in the limit of high temperatures. Specifically we show that simple “ rate equation” represents a high temperature asymptotic form of the equation of motion for the occupation number 〈n_a(t)〉 of relevant valence level of the sputtered particle.