Activation of methyl acetate on Pd(111)

The adsorption and activation of methyl acetate (CH₃COOCH₃), one of the simplest carboxylic esters, on Pd(111) have been studied using self-consistent periodic density functional theory calculations. Methyl acetate adsorbs weakly through the carbonyl oxygen. Its activation occurs via dehydrogenation, instead of direct C–O bond dissociation, on clean Pd(111): It is much more difficult to dissociate the C–O bonds (E_a ≈ 2.0 eV for the carbonyl and acetate–methyl bonds; E_a = 1.0 eV for the acetyl–methoxy bond) than to dissociate the C–H bonds to produce enolate (CH₂COOCH₃; E_a = 0.74 eV) or methylene acetate (CH₃COOCH₂; E_a = 0.82 eV). The barriers for C–H and C–O bond dissociation are directly calculated for enolate and methylene acetate, and estimated for further dehydrogenated derivatives (CH₃COOCH, CH₂COOCH₂, and CHCOOCH₃) based on the Brønsted–Evans–Polanyi linear energy relations formed by the calculated steps. The enolate pathway leads to successive dehydrogenation to CCOOCH₃, whereas methylene acetate readily dissociates to yield acetyl. The selectivity for dissociating the acyl–alkoxy C–O bond, which is desired for alcohol formation, is therefore fundamentally limited by the facility of dehydrogenation under vacuum/low-pressure conditions on Pd(111).     

Luminescent vacuum ultraviolet spectroscopy of Cr3+ ions in nanostructured aluminum oxide

Impurity Cr³⁺ centers in submicron and nanostructured Al₂O₃ crystals of different phase compositions at temperatures of 300 and 7.5 K were studied by a luminescent vacuum ultraviolet (VUV) spectroscopy method. Photoluminescence (PL) spectra and the energies of ²E, ⁴T₂, and ⁴T₁ excited states of Cr³⁺ ion depend on the type of crystalline samples phase. The PL excitation spectrum of R-line in α-Al₂O₃ nanoscale crystals is formed by intracenter transitions (2.5–5.5 eV region), by charge transfer band (6.9 eV) and by effective formation of impurity-bound excitons (9.0 eV region). Such impurity-bound excitons correspond to O₂p→Al₃s electron transition in surroundings of an impurity Cr³⁺ center. The efficiency of impurity-bound excitons formation decreases with the increase of the grain size above 100 nm. The size dependence is noticeably shown in PL excitation spectra in VUV region. Excitons bound to impurity centers do not appear in nanostructured δ+θ-Al₂O₃ crystals. The effect of the electron excitation multiplication is observed distinctly in nanostrucured α-Al₂O₃ at an excitation energy above 19 eV (more than 2E_g).     

Core hole and surface excitation correction parameter for XPS peak intensities

In XPS analysis, surface excitations and excitations originating from the static core hole created during the photoexcitation process are usually neglected. However, both effects significantly reduce the measured peak intensity. In this paper we have calculated these effects. Instead of considering the two effects separately, we introduce a new parameter, namely the Correction Parameter for XPS (or CP_XPS) defined as the change in probability for emission of a photoelectron caused by the presence of the surface and the core hole in comparison with the situation where the core hole is neglected and the electron travels the same distance in an infinite medium. The CP_XPS calculations are performed within the dielectric response theory by means of the QUEELS-XPS software determining the energy-differential inelastic electron scattering cross-sections for X-ray photoelectron spectroscopy (XPS) including surface and core hole effects. This study has been carried out for electron energies between 300 eV and 3400 eV, for angles to the surface normal between 0° and 60° and for various materials, especially metals, semiconductors and oxides. For geometries and energies normally used in XPS, i.e. for emission angle ≤ 60° and photoelectron energy ≤ 1500 eV, we find that CP_XPS values are significantly larger for oxides, (0.55 ? CP_XPS ? 0.75) than for metals and semiconductors (0.45 ? CP_XPS ? 0.6). We show that this behavior is due to the difference in the wave vector dispersion of the energy loss function. This dispersion has been determined from analysis of REELS and is found to be free electron like (α ? 1) for metals but is substantially smaller (α ≈ 0.02–0.05) for materials with a wide band gap. As a result, the group velocity of the valence electrons is very small for oxides with a large band gap. This leads to a reduction in the screening of the core-hole potential before the photoelectron has left the region of interaction and thereby to an increase in the intrinsic excitations caused by the core hole.     

Oxygen species on the silver surface oxidized by MW-discharge: Study by photoelectron spectroscopy and DFT model calculations

A polycrystalline silver surface has been studied by synchrotron radiation photoelectron spectroscopy after deep oxidation by microwave discharge in an O₂ atmosphere. Oxidized structures with high oxygen content, AgO_x with x > 1, have been found on the silver surface after oxidation at 300–400 K. The line shapes observed in the O1s spectra were decomposed into five components and indicated that complex oxidized species were formed. An analysis of the oxidized structures with binding energies, Е_b(O1s), greater than 530 eV pointed to the presence of both Ag–O and O–O bonds. We have carried out a detailed experimental study of the valence band spectra in a wide spectral range (up to 35 eV), which has allowed us to register the multicomponent structure of spectra below Ag4d band. These features were assigned to the formation of Ag–O and O–O bonds composed of molecular (associative) oxygen species. DFT model calculations showed that saturation of the defect oxidized silver surface with oxygen leads to the formation of associative oxygen species, such as superoxides, with electrophilic properties and covalent bonding. The high stability of oxygen-rich silver structures, AgO_x, can be explained by the formation of small silver particles during the intensive MW oxidation, which can stabilize such oxygen species.     

Deep level transient spectroscopy (DLTS) study of defects introduced in antimony doped Ge by 2 MeV proton irradiation

Deep level transient spectroscopy (DLTS) and Laplace-DLTS have been used to investigate the defects created in Sb doped Ge after irradiation with 2 MeV protons having a fluence of 1×10¹³ protons/cm². The results show that proton irradiation resulted in primary hole traps at E_V +0.15 and E_V +0.30 eV and electron traps at E_C ?0.38, E_C ?0.32, E_C ?0.31, E_C ?0.22, E_C ?0.20, E_C ?0.17, E_C ?0.15 and E_C ?0.04 eV. Defects observed in this study are compared with those introduced in similar samples after MeV electron irradiation reported earlier. E_C ?0.31, E_C ?0.17 and E_C ?0.04, and E_V +0.15 eV were not observed previously in similar samples after high energy irradiation. Results from this study suggest that although similar defects are introduced by electron and proton irradiation, traps introduced by the latter are dose dependent.     

Correlation between phosphorus concentration and opto-electrical properties of doped a-Si:H films

Phosphorus doped hydrogenated amorphous silicon (a-Si:H) films were prepared by decomposition of silane using RF plasma glow discharge. Both DC dark conductivity measurements, and spectrophotometric optical measurements through the range 200–3000 nm were recorded for the prepared films. The DC conductivity activation energy E_a decreased from 0.8 eV for the undoped sample to 0.34 eV for the highest used doping value. The optical energy gap E_g decreased ranging from 1.66 eV to 1.60 eV. The refractive index n, the density of charge carriers N/m* and the plasma frequency ω_p showed an opposite behavior, i.e. an increase in value with doping. Fitting the dispersion values to Sellmeier equation led to the determination of the material natural frequency of oscillating particles. A correlation between the changes in these parameters with the doping has been attempted.     

Electronic core levels of hydroxyls at the surface of chromia related to their XPS O 1s signature: A DFT + U study

The Cr 2p and O 1s binding energy (BE) levels have been calculated by first principles methods for different models of hydroxylated (0001)-Cr₂O₃ surfaces. Several surface terminations have been considered. The calculations allow us to reproduce the O 1s shifts between O in oxide and OH groups. It is found that two main effects account for the OH binding energy shifts. On the one hand, the increased covalency of the O–H bond with respect to the Cr–O bond, lowers the electronic O (1s and 2p) energy, and in consequence the BE of the core levels (O 1s) are higher. On the other hand, the lower the OH coordination number, the higher the valence and core levels energy, and the lower the BE. Consequently, mono-coordinated hydroxyls have a binding energy near that of O^2? in the oxide (ΔBE_OH–O = ? 0.2–0.0 eV). Two-fold coordinated hydroxyls have a slightly higher BE (ΔBE_OH–O = + 0.3 eV). Three-fold coordinated OH groups have a higher binding energy (ΔBE_OH–O = + 0.6?0.7 eV), corresponding to that experimentally measured for OH groups. Finally, water adsorbed above OH groups exhibits a still higher BE (ΔBE_HOH–O = + 0.9–1.0 eV). The ΔBE are slightly under-estimated under the initial state approximation, and overestimated under the final state (Z + 1) approximation.     

Morphology of Cn thin films (50 ⩽ n < 60) on graphite: Inference of energy dissipation during hyperthermal deposition

Low energy cluster beam deposition, LECBD, under UHV conditions has been used to generate thin films comprising monodispersed non-IPR fullerenes, C_n, 50 ? n < 60, on pyrolithic graphite surfaces (HOPG). The morphology of the resulting C_n deposits has subsequently been studied by ex situ atomic force microscopy. Deposition experiments were carried out under nominally normal incidence and at hyperthermal incident kinetic energies, E₀, varied between 1 and 40 eV. Surface temperatures during deposition, T_s, were varied from 300 K to the desorption onset of ～700 K. Initial sticking of C_n cages is governed by the lateral density of step edges, which act as pinning and nucleation centres for migrating cages. Consequently in the early deposition stages, the surface exhibits large areas of empty terraces, while the step edges themselves are well-decorated. The terraces in turn become decorated by dendritic C_n islands in later deposition stages. Both, the mean size of these 2D islands and the mean distance between nearest islands, δ, scale with the size of the terraces. When increasing the primary kinetic energy, the fractal-like islands become smaller and less dendritic in shape. The mean initial sticking coefficient decays exponentially with increasing E₀. The island topography has also been found to depend sensitively on the deposition temperature. Instead of the dendritic/fractal islands generated at room temperature, densely packed islands terminated by smooth rims are observed upon deposition at elevated temperatures. We rationalize our findings in terms of a three step deposition process involving: (i) conversion of perpendicular E₀ into hyperthermal surface parallel gliding/sliding motion, (ii) friction–dissipation of this surface-parallel kinetic energy within an (unexpectedly large) mean free path Λ followed by (iii) thermal diffusion. Λ, is observed to scale with E₀ and T_s.     

Momentum-resolved lifetimes of image-potential states on Ag(001)

Binding energies and decay rates of image potential states at an Ag(001) surface have been investigated with time-resolved two-photon photoemission. For the first four image potential states the binding energies are determined to be 561, 170, 72 and 37 meV with respect to the vacuum level. Lifetimes of the first three states are extracted to be 57, 165 and 380 fs at k_∥ = 0. With increasing parallel momentum k_∥ the lifetime of the n = 1 state decreases such that the inverse lifetime scales proportional to the kinetic energy within the parallel motion with 34 meV/eV. As light source for the pump-probe photoemission experiments a novel all fiber based femtosecond laser system with a repetition rate of 1.5 MHz has been used.     

Spectroscopic study of nanocrystalline V2O5·nH2O films doped with Li ions

Optical properties of nanocrystalline, Li_xV₂O₅·nH₂O films (0<x<22 mol%), are explored in the present work. These films have been produced by the sol–gel technique (colloidal route), which was used for the preparation of high purity and homogeneity films. Optical measurements were carried out using a double-beam spectrophotometer. The optical constants such as refractive index n, the extinction coefficient k, absorption coefficient α, and optical band gap of the films material have been evaluated. The optical absorption coefficient was calculated from the measured normal reflectance, R, and transmittance, T, spectra. The optical spectra of all samples exhibited two distinct regions: at high energy, which suggests a direct forbidden transition with optical gap ranging from 1.75 to 2.0 eV and increases with increase in Li-content. On the other hand a second low-energy band suggests a direct allowed transition with optical gap ranging from 0.40 to 0.42 eV. The width of the localized states (band tail) E_e was also estimated for all samples. Additional calculations applying the real part of the optical dielectric function led to the evaluation of the charge carrier concentration and their effective mass.     

Adsorption of thiophene on silica-supported Mo clusters

The adsorption/decomposition kinetics/dynamics of thiophene has been studied on silica-supported Mo and MoS_x clusters. Two-dimensional cluster formation at small Mo exposures and three-dimensional cluster growth at larger exposures would be consistent with the Auger electron spectroscopy (AES) data. Thermal desorption spectroscopy (TDS) indicates two reaction pathways. H₄C₄S desorbs molecularly at 190–400 K. Two TDS features were evident and could be assigned to molecularly on Mo sites, and S sites adsorbed thiophene. Assuming a standard preexponential factor (ν = 1 × 10¹³/s) for first-order kinetics, the binding energies for adsorption on Mo (sulfur) sites amount to 90 (65) kJ/mol for 0.4 ML Mo exposure and 76 (63) kJ/mol for 2 ML Mo. Thus, smaller clusters are more reactive than larger clusters for molecular adsorption of H₄C₄S. The second reaction pathway, the decomposition of thiophene, starts at 250 K. Utilizing multimass TDS, H₂, H₂S, and mostly alkynes are detected in the gas phase as decomposition products. H₄C₄S bond activation results in partially sulfided Mo clusters as well as S and C residuals on the surface. S and C poison the catalyst. As a result, with an increasing number of H₄C₄S adsorption/desorption cycles, the uptake of molecular thiophene decreases as well as the H₂ and H₂S production ceases. Thus, silica-supported sulfided Mo clusters are less reactive than metallic clusters. The poisoned catalyst can be partially reactivated by annealing in O₂. However, Mo oxides also appear to form, which passivate the catalyst further. On the other hand, while annealing a used catalyst in H/H₂, it is poisoned even more (i.e., the S AES signal increases). By means of adsorption transients, the initial adsorption probability, S₀, of C₄H₄S has been determined. At thermal impact energies (E_i = 0.04 eV), S₀ for molecular adsorption amounts to 0.43 ± 0.03 for a surface temperature of 200 K. S₀ increases with Mo cluster size, obeying the capture zone model. The temperature dependence of S₀(T_s) consists of two regions consistent with molecular adsorption of thiophene at low temperatures and its decomposition above 250 K. Fitting S₀(T_s) curves allows one to determine the bond activation energy for the first elementary decomposition step of C₄H₄S, which amounts to (79 ± 2) kJ/mol and (52 ± 4) kJ/mol for small and large Mo clusters, respectively. Thus, larger clusters are more active for decomposing C₄H₄S than are smaller clusters.     

Controllable oxidation of h-BN monolayer on Ir(111) studied by core-level spectroscopies

The effect of atomic oxygen adsorption on the structure and electronic properties of monolayer hexagonal boron nitride (h-BN) grown on Ir(111) has been studied using near edge X-ray absorption fine structure spectroscopy (NEXAFS), photoelectron spectroscopy (PES), and low-energy electron diffraction (LEED). It has been shown that the oxidation of the h-BN monolayer occurs through a gradual substitution of N by O in the h-BN lattice. This process leads to the formation of defect sites corresponding to three different types of the B atom environment (BN_3 ? xO_x with x = 1,2,3). The oxidation of the h-BN monolayer is very different from the case of graphene on Ir(111), where adsorption of atomic oxygen results mainly in the formation of epoxy groups [J. Phys. Chem. C. 115, 9568 (2011)]. A post-annealing of the h-BN monolayer after oxygen exposure results in further destruction of the B–N bonds and formation of a B₂O₃-like structure.     

Structure,magnetic and microwave properties of FeNi invar nanoparticles obtained by electrical explosion of wire in different preparation conditions

Magnetic nanoparticles (MNPs) of close to invar (Fe_0.635Ni_0.365) composition were prepared by the electrical explosion of wire using different conditions to insure different values of overheating rates. X-ray diffraction, transmission electron microscopy, low temperature nitrogen adsorption, magnetic and microwave measurements were used for the characterization of MNPs. Increase of the energy injected into the wire led to increase of the specific surface (S_sp) of the produced MNPs from 4.6 to 13.5 m²/g. The fabricated MNPs were spherical and weakly aggregated with the average weighted diameter in the range of 54–160 nm depending on the S_sp. The phase composition of FeNi MNPs consists of two solid solutions of Ni in α-phase and γ-phase lattices. The increase of the energy injected into the wire leads to increase of the α-phase from 5 to 10 wt% as the injected energy raised from 0.8 to 2.5 times the sublimation energies of the wire material. Comparative analysis of structure magnetic and microwave properties showed that the obtained MNPs are important magnetic materials with high saturation magnetization and significant zero field microwave absorption which can be expected to lead to important technological applications.     

Electronic structure and fundamental absorption edges of KPb2Br5, K0.5Rb0.5Pb2Br5, and RbPb2Br5 single crystals

X-ray photoelectron core-level and valence-band spectra for pristine and Ar⁺-ion irradiated (001) surfaces of KPb₂Br₅, K_0.5Rb_0.5Pb₂Br₅, and RbPb₂Br₅ single crystals grown by the Bridgman method have been measured and fundamental absorption edges of the ternary bromides have been recorded in the polarized light at 300 K and 80 K. The present X-ray photoelectron spectroscopy (XPS) results reveal high chemical stability of (001) surfaces of K_xRb_1?xPb₂Br₅ (x=0, 0.5, and 1.0) single crystals. Substitution of potassium for rubidium in K_xRb_1?xPb₂Br₅ does not cause any changes of binding energy values and shapes of the XPS constituent element core-level spectra. Measurements of the fundamental absorption edges indicate that band gap energy, E_g, increases by about 0.14 and 0.19 eV when temperature decreases from 300 K to 80 K in KPb₂Br₅ and RbPb₂Br₅, respectively. Furthermore, there is no dependence of the E_g value for KPb₂Br₅ upon the light polarization, whilst the band gap energy value for RbPb₂Br₅ is bigger by 0.03–0.05 eV in the case of E6c compared to those in the cases of E6a and E6b.     

Intrinsic excitons in 12CaO·7Al2O3

Unusual crystal structure of 12CaO·7Al₂O₃ is composed by a framework of positively charged nanocages, which enable accommodation of various negative ions (and even electrons) inside these cages. Different filling of cages leads to significant changes in electronic structure and as the result in luminescence properties, as well. Luminescence was studied using time-resolved spectroscopy in VUV in the temperature range from 6 to 300 K. Electron loaded samples exhibit UV luminescence band peaked at ～5 eV. The excitation spectrum of this emission has the onset at the energy gap value of 6.8 eV, and its decay is well described with the sum of two exponential functions with life-times of τ₁ = 3.7 ns and τ₂ = 29 ns, respectively. Its thermal quenching is well approximated by the sum of two Mott-Seitz type curves with the activation energies of 34 meV and 70 meV. Experimental results indicate that this luminescence is possibly due to radiative decay of two singlet self-trapped exciton states, which hole components are localized on two non-equivalent framework oxygens.     

Surface and core hole effects in X-ray photoelectron spectroscopy

In XPS analysis, two effects, which significantly reduce the measured peak intensity, are usually neglected: the core hole left behind in an XPS process which causes “intrinsic” excitations and excitations as the photoelectron pass through the surface region. We have calculated these effects quantitatively for various energies, geometries, and materials. Instead of considering the two effects separately, we introduce a new parameter, namely the correction parameter for XPS or CP_XPS, which takes into account both effects. We define this CP_XPS as the change in probability for emission of a photoelectron caused by the presence of the surface and the core hole in comparison with the situation where the core hole is neglected and the electron travels the same distance in an infinite medium. The calculations are performed within the dielectric response theory by means of the QUEELS–XPS software determining the energy-differential inelastic electron scattering cross-sections for X-ray photoelectron spectroscopy (XPS) including surface and core hole effects. This study has been carried out for electron energies between 300 eV and 3400 eV, for angles to the surface normal between 0° and 60° and for various materials. We find that the absolute effect is a reduction by 35–45% in peak intensities but that the variation in CP_XPS with material, angle and energy are < ± 10% for emission angle ≤ 60° and photoelectron energy ≤ 1500 eV. This implies that when XPS analysis is done using relative intensities, the combined effect of the surface and of the core hole is typically less than ≈ ± 10% for geometries and energies normally used in XPS. In practice, it is however difficult to determine the bare peak intensity without the intrinsic electrons because the two overlap in energy.     

Recombination luminescence of CsI(Tl) under electron pulse irradiation

The luminescence kinetics of CsI(Tl) exposed to an electron pulse irradiation (E_e = 250 keV, t_1/2 = 10 ns, j = 2 ÷ 160 mJ/cm²) has been studied. It has been discovered that the slow emission rise is due to hole V_k–Tl⁰ recombination luminescence at temperature from 100 to 160 K and electron–V_kA recombination, where electrons released from single Tl⁰ at temperature from 180 to 300 K. The effect of Tl concentration on both processes has been investigated.     

Exploring the role of samarium in the modification of rhodium catalysts through surface science approach

In this paper we review the preparation and reaction properties of ordered SmRh surface alloys and SmO_x/Rh(1 0 0) model catalyst which have been systematically investigated by low energy electron diffraction (LEED), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS) and temperature desorption spectroscopy (TDS). The growth of Sm on Rh(1 0 0) at room temperature follows the Stranski-Krastanov mode. Thermal treatment of the Sm films on Rh(1 0 0) leads to the formation of ordered SmRh surface alloys. An “inverse” SmO_x/Rh(1 0 0) model catalyst is produced under controlled oxidation of the SmRh surface alloy. CO adsorption on the SmRh alloy and SmO_x/Rh(1 0 0) surfaces gives rise to five TDS characteristic features originating from different adsorption sites. Both the site blocking of SmO_x and the electron transfer between SmO_x and Rh substrate significantly affect the CO adsorption. Acetate decomposition on both Rh(1 0 0) and the SmO_x/Rh(1 0 0) surfaces are found to undergo two competitive pathways that yields either (i) CO(a) and O(a) or (ii) CO₂(g) and H₂(g) at high temperature. The reactive desorption of acetic acid on SmO_x/Rh(1 0 0) is dramatically different from that on Rh(1 0 0). A rapid decomposition of acetic acid to produce CO(g) and CO₂(g) can be observed only on SmO_x/Rh(1 0 0), where CO(g) becomes the predominant product at 225 K, indicating a strong promotional effect of SmO_x in the selective decomposition of acetate. Finally, we briefly describe the future outlook of research on rare earth oxide/metal model catalysts.     

Structural and photoluminescence investigation on the hot-wire assisted plasma enhanced chemical vapor deposition growth silicon nanowires

High density of silicon nanowires (SiNWs) were synthesized by a hot-wire assisted plasma enhanced chemical vapor deposition technique. The structural and optical properties of the as-grown SiNWs prepared at different rf power of 40 and 80 W were analyzed in this study. The SiNWs prepared at rf power of 40 W exhibited highly crystalline structure with a high crystal volume fraction, X_C of ～82% and are surrounded by a thin layer of SiO_x. The NWs show high absorption in the high energy region (E>1.8 eV) and strong photoluminescence at 1.73 to 2.05 eV (red–orange region) with a weak shoulder at 1.65 to 1.73 eV (near IR region). An increase in rf power to 80 W reduced the X_C to ～65% and led to the formation of nanocrystalline Si structures with a crystallite size of <4 nm within the SiNWs. These NWs are covered by a mixture of uncatalyzed amorphous Si layer. The SiNWs prepared at 80 W exhibited a high optical absorption ability above 99% in the broadband range between 220 and ～1500 nm and red emission between 1.65 and 1.95 eV. The interesting light absorption and photoluminescence properties from both SiNWs are discussed in the text.     

Studies on structural and dielectric properties of Na1/2Dy1/2TiO3 ceramic

The polycrystalline sample of Na_1/2Dy_1/2TiO₃ ceramic was prepared by a standard high-temperature solid-state reaction technique. X-ray structural analysis confirmed the formation of single-phase (with minor secondary phase) compound in the orthorhombic (distorted tetragonal) crystal system at room temperature. Study of surface morphology by scanning electron microscope exhibits uniform distribution of rectangular/cubical grains with less voids. The elemental composition of the prepared compound was confirmed by energy dispersive X-ray spectroscopy microanalysis. Detailed studies of dielectric properties exhibit a dielectric anomaly at 94 °C suggesting a possible ferroelectric–paraelectric phase transition in the compound. The activation energy (E_a), calculated from the temperature dependence of ac conductivity plot, was found to be small (∼0.1 eV) in low temperature and large (∼0.5 eV) in high temperature region.