N-methylpyrrolidine-based precursors for chemical vapor deposition of GaNx particles

An attempt to prepare a metalorganic precursor of gallium with reactivity at low temperature in chemical vapor deposition (CVD) systems was done by reacting N-methylpyrrolidine with metal gallium or gallium nitrate under mild conditions. The precursors were bubbled into a CVD assemblage and then reacted with ammonia at temperatures between 400 and 700 °C. The depositions onto silicon substrates were pyramidal particles of 100 nm width at the base and up to 55 nm in height. The rise in growth temperature increased particle density from 0.9 to 27.1 particles per square micron, but reduced the height from 50 to 10 or 2 nm. XPS spectra showed the presence of gallium and nitrogen. The intensity of the gallium spectrum decreased as the process temperature increased indicating that GaN_x particles were deposited rather than the stoichiometric gallium nitride (GaN). An additional N1s band of impurities appeared whose intensity increases with the reaction temperature. The lower impurity content corresponded to the sample prepared with Ga(NO₃)₃ at 400 °C.     

Contact potential barriers and characterization of Ag-doped composite TiO2 nanotubes

Ag-doping TiO₂ composite nanotubes (Ag-TNTs) were synthesized by alkaline fusion followed by hydrothermal treatment. The microstructure and morphology of the materials were characterized by XRD, TEM, XPS, SPS (surface photovoltage spectroscopy), FISPS (electric field-induced surface photovoltage spectroscopy) and Raman spectroscopy. First-principles calculations based on density-functional theory (DFT) showed the formation of several impurity levels near the top of the valence band in the band gap (E_g) of rutile TiO₂ due to Ag doping. A “double junction” is proposed, involving a Schottky junction and p–n junction (denoted as “Ag-p–n junction”) occurring between the Ag particles and the nanotube surface, as well as forming inside TiO₂ nanotubes, respectively. The strongly built-in electric field of the junctions promotes the separation of photo-holes and photoelectrons, enhancing the photocatalytic efficiency. XRD results indicated that the composite Ag-TNTs exist as a mixture of anatase and rutile phases. XPS results showed that Ti⁴⁺ is the primary state of Ti. Raman spectral analysis of Ag-TNTs revealed the presence of a new peak at 271 cm⁻¹. The red-shift of the absorption light wavelength of Ag-TNTs was 0.16 eV (20 nm) due to a considerable narrowing of E_g by the existing impurity levels.     

XPS investigation of vacuum annealed vertically aligned ultralong ZnO nanowires

The surface properties of vertically aligned ZnO nanowires grown by chemical vapour deposition on GaN using a gold layer as a catalyst are investigated by X-ray Photoelectron Spectroscopy as a function of annealing temperature in ultra high vacuum (UHV). The nanowires are 8.5 μm long and 60 nm wide. 87% of the surface carbon content was removed after annealing at 500 °C in UHV. Analysis of the gold intensity suggests diffusion into the nanowires after annealing at 600 °C. Annealing at 300 °C removes surface water contamination and induces a 0.2 eV upward band bending, indicating that adsorbed water molecules act as surface electron donors. The contaminants re-adsorbed after 10 days in UHV and the surface band bending caused by the water removal was reversed. The UHV experiment also affected the nanowires arrangement causing them to bunch together. These results have clear implications for gas sensing applications with ZnO NWs.     

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.     

Characterization of Young’s modulus of silicon versus temperature using a “beam deflection” method with a four-point bending fixture

Young’s modulus (E) and Poisson’s ratio (ν) are dependent upon the direction on the silicon surface. In this work, E and ν of silicon have been calculated analytically for any crystallographic direction of silicon by using compliance coefficients (s₁₁, s₁₂, and s₄₄), and the values of E are confirmed experimentally by using a “beam deflection” method with a four-point bending fixture. Experimental results for E as a function of temperature from −150 °C to +150 °C are presented for (0 0 1) and (1 1 1) silicon wafers.     

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.     

Comparative PEEM and AES study of surface morphology and composition of Cu films on Si and 3C–SiC substrates

We have conducted photoemission electron microscope (PEEM) and Auger electron spectroscopy (AES) studies on the Cu(30 nm)/3C–SiC(1 0 0) and Cu(30 nm)/Si(1 0 0) samples annealed successively up to 850 °C. With PEEM, lateral diffusion of Cu atoms on the 3C–SiC substrate was observed at 400 °C while no lateral diffusion was seen for the Cu/Si(1 0 0) samples up to 850 °C. The 30 nm Cu thin film on 3C–SiC began to agglomerate at 550 °C, similar to the case for the Cu/Si(1 0 0) system. No further spread of the lateral diffusion region was found in subsequent annealing up to 850 °C for Cu/3C–SiC while separated regular-sized dot structures were found at 850 °C for Cu/Si(1 0 0). AES studies of Cu/Si(1 0 0) system showed partial interface reaction during Cu deposition onto the Si(1 0 0) substrate and oxidation of the resultant Cu₃Si to form SiO₂ on the specimen surface at room temperature in air. Surface segregation of Si and C was observed after annealing at 300 °C for Cu/Si(1 0 0) and at 850 °C for the Cu/3C–SiC system. We have successfully elucidated the observed phenomena by combining PEEM and AES considering diffusion of the constituent elements and/or reaction at interfaces.     

DFT study of carbon monoxide adsorption on α-Al2O3(0001)

The adsorption of CO on α-Al₂O₃(0001) was studied using the DFT-GGA computational method and on α-Al₂O₃ powder experimentally by Infra red spectroscopy. The core and valence level regions of α-Al₂O₃(0001) single crystal surface were also studied experimentally. Ar ions sputtering of the surface results in a slight but reproducible decrease in the XPS O2p lines in the valence band regions due to preferential removal of surface (and near surface) O atoms. Core level XPS O1s and Al2p further confirmed oxygen depletion with an associated surface stoichiometry close to Al₂O_2.9. The adsorption energy of CO was computed and found equal to 0.52 eV for θ = 0.25, it decreased to 0.42 eV at θ = 1. The IR frequency of νCO was also computed and in all cases it was blue shifted with respect to gas phase CO. The shift, Δν, decreased with increasing coverage where it was found equal to 56 cm^? 1 for θ = 0.25 and decreased to 30 cm^? 1 for θ = 1. Structural analyses indicated that the change in the adsorption energy and the associated frequency shift is due to surface relaxation upon adsorption. Experimentally the adsorption of CO gave rise to one main IR peak at 2154 cm^? 1 at 0.3 Torr and above. Two far smaller peaks are also seen at lower pressures of 0.03–0.2 Torr at 2189 and 2178 cm^? 1. The isosteric heat of adsorption was computed for the IR band at 2154 cm^? 1 and was found equal to 0.2 eV which did not change with coverage in the investigated range up to θ = 0.6.     

Electret properties of ethylene–propylene random co-polymer

Thermally stimulated current (TSC) spectra were examined for ethylene–propylene (EP) random co-polymer at different charging voltages V_p with positive and negative polarities. Observed TSC spectra showed two well-separated TSC bands, B_L and B_H, which respectively appeared in the temperature regions below and above 100 °C. Observed V_p dependence of B_L was quite different from that of typical polypropylene homo-polymer: As V_p increased, B_L band grew keeping its peak position same at 65 °C, and the band shape unchanged, as if the traps responsible for the B_L band are a single set of traps with the same trap depth and capture cross section. The trap depth of B_L was about 1.9 eV and 1.7 eV for positively charged EP and talc-containing EP samples, respectively. EP samples also showed unique TSC bands above 100 °C: one is a narrow TSC band peaked at 120 °C and the other is an unusual TSC band which was non-vanishing even at 165 °C just before destruction of samples by their melting. Consequently, the utmost stable charge density in EP co-polymer above 100 °C was found to be 3.5 × 10^?4 C/m² and 6.0 × 10 ^?4 C/m² for positively and negatively charged samples, respectively. These equivalent surface charge densities are much larger than those of usual polypropylene homo-polymer.     

Surface stoichiometry of La0.7Sr0.3MnO3 during in vacuo preparation; A synchrotron photoemission study

We present a study of the surface stoichiometry and contamination of La_0.7Sr_0.3MnO₃ thin films following exposure to air and subsequent in vacuo preparation. Samples were studied using both soft X-ray synchrotron photoemission (hν = 150 to 350 eV) and traditional Mg-Kα XPS (hν = 1253.6 eV) whilst annealing incrementally to ≈ 510°C in low pressures of O₂. In all cases, a Mn depleted and Sr rich surface oxide layer is observed, it is of reduced crystalline quality and is charge depleted. This surface layer is weakly affected by subsequent annealing, and is partially reversed by annealing in higher O₂ pressure. Surface carbon contamination is incrementally removed by annealing at increased temperatures, and at 270 °C, it is reduced to ≈ 0.4% of the topmost unit cell. The modification of the surface stoichiometry and electronic properties is consistent with the reported loss of magnetic properties in thin LSMO films.     

Theoretical study of optoelectronic properties of GaAs1−xBix alloys using valence band anticrossing model

The (12 × 12) and (14 × 14) valence band anticrossing (V-BAC) models were applied to calculate the electronic band structure of GaAs_1−xBi_x dilute alloys along Δ-, Λ- and Σ-directions at room temperature. A comparative study based on these models was performed in terms of energy levels, optical transitions, spin–orbit splitting and effective mass. We found a significant reduction of the band-gap energy E_g by roughly 81 meV/%Bi accompanied by an increase in the spin–orbit splitting Δ_so+ by about 56 meV/%Bi. Furthermore, Δ_so+ does come into resonance with E_g at ∼12%Bi for resonance energy equal to 0.73 eV. An excellent agreement has occurred between the (14 × 14) V-BAC model predictions and experimental results reported in the literature. In addition, we have investigated the Bi composition and k-directions dependence of the effective mass at Γ point. A slight increase of the holes effective mass with x can affect the holes transport properties of GaAsBi. The intrinsic carrier density increases with both x and the temperature T, but it remains below 10¹⁰ cm⁻³ for x ⩽ 5% and T ⩽ 300 K.     

Thermoluminescence,infrared reflectivity and electron paramagnetic resonance properties of hemimorphite

Silicate mineral hemimorphite has been investigated concerning its TL, IR and EPR properties. A broad TL peak around 180 °C and a weaker and narrower peak around 360 °C were found in a sample annealed at 600 °C for 1 h and then irradiated. The deconvolution using the CGCD method revealed peaks around 132, 169, 222 and 367 °C. The reflectivity measurements showed several bands in the NIR region due to H₂O, OH and Al–OH complexes. No band was observed in the visible region. The thermal treatments were carried out from ∼110 to 940 °C and dehydration was observed, first causing a diminishing optical absorption in general and the disappearance of water and hydroxyl absorption bands. The EPR spectrum of natural hemimorphite, presented Cu²⁺ signals at g = 2.4 and g = 2.1 plus E₁′ signal superposed to Fe³⁺ signal around g = 2.0.     

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).     

Thin films of molecule-based charge transfer complex cobalt tetracyanoethylene: In situ X-ray photoemission study

Thin films of molecule-based charge transfer magnet, cobalt tetracyanoethylene [Co(TCNE)_x, x ~ 2] consisting of the transition metal Co, and an organic molecule viz. tetracyanoethylene (TCNE) have been deposited by using physical vapor deposition method under ultra-high vacuum conditions at room temperature. X-ray photoelectron spectroscopy (XPS) technique has been used extensively to investigate the electronic properties of the Co(TCNE)_x thin films. The XPS measurements show that the prepared Co(TCNE)_x films are clean, and oxygen free. The stoichiometries of the films, based on atomic sensitive factors, are obtained, and yields a ~ 1:2 ratio between metal Co and TCNE for all films. Interestingly, the positive shift of binding energy position for Co(2p), and negative shifts for C(1s) and N(1s) peaks suggest a charge-transfer from Co to TCNE, and cobalt is assigned to its Co(II) valence state. In the valence band investigation, the highest occupied molecular orbital (HOMO) of Co(TCNE)_x is found to be at ~ 2.4 eV with respect to the Fermi level, and it is derived either from the TCNE^? singly occupied molecular orbital (SOMO) or Co(3d) states. The peaks located at ~ 6.8 eV and ~ 8.8 eV are due to TCNE derived electronic states. The obtained core level and valence band results of Co(TCNE)_x, films are compared with those of V(TCNE)_x thin film magnet: a well known system of M(TCNE)_x type of organic magnet, and important points regarding their electronic properties have been brought out.     

Structural,XPS and impedance analysis of LixCoVO4 (x = 0.8, 1.0, 1.2)

Lithium cobalt vanadate Li_xCoVO₄ (x = 0.8; 1.0; 1.2) has been prepared by a solid state reaction method. The XRD analysis confirms the formation of the sample. A new peak has been observed for Li_1.0CoVO₄ and for Li_1.2CoVO₄ indicating the formation of a new phase. The XPS analysis indicates the reduction in the oxidation of vanadium and oxygen with the addition of Li in Li_xCoVO₄ (x = 0.8, 1.0, 1.2). The impedance analysis gives the conductivity value as 2.46 × 10^− 5, 6.16 × 10^− 5, 9 × 10^− 5 Ω^− 1 cm^− 1 for Li_xCoVO₄ (x = 0.8; 1.0; 1.2), all at 623 K. The similarity in the bulk activation energy (E_a) and the activation enthalpy for migration of ions (E_ω) indicate that the conduction in Li_1.2CoVO₄ has been due to hopping mechanism.     

Reduction of thin-film ceria on Pt(111) by supported Pd nanoparticles probed with resonant photoemission

Local defects present in CeO_2 ? x films result in a mixture of Ce³⁺ and Ce⁴⁺ oxidation states. Previous studies of the Ce 3d region with XPS have shown that depositing metal nanoparticles on ceria films causes further reduction, with an increase in Ce³⁺ concentration. Here, we compare the use of XPS and resonant photoemission spectroscopy (RESPES) to estimate the concentration of Ce³⁺ and Ce⁴⁺ in CeO_2 ? x films grown on Pt (111), and the variation of this concentration as a function of Pd deposition. Due to the nature of the electronic structure of CeO_2 ? x, resonant peaks are observed for the 4d–4f transitions when the photon energy matches the resonant energy; (hν = 121.0 eV) for Ce³⁺ and (hν = 124.5 eV) for Ce⁴⁺. This results in two discrete resonant photoemission peaks in valence band spectra. The ratio of the difference of these peaks with off-resonance scans gives an indication of the relative contribution of Ce³⁺. Results from RESPES indicate reduction of CeO_2 ? x on deposition of Pd, confirming earlier findings from XPS studies.     

Annealing temperature dependent on structural,optical and electrical properties of indium oxide thin films deposited by electron beam evaporation method

In the present work the influence of annealing temperature on the structural and optical properties of the In₂O₃ films deposited by electron beam evaporation technique in the presence of oxygen was studied. The deposited films were annealed from 350 to 550 °C in air. The chemical compositions of In₂O₃ films were carried out by X-ray photoelectron spectroscopy (XPS). The film structure and surface morphologies were investigated as a function of annealing temperature by X-ray diffraction (XRD) and atomic force microscopy (AFM). The structural studies by XRD reveal that films exhibit preferential orientation along (2 2 2) plane. The refractive index (n), packing density and porosity (%) of films were arrived from transmittance spectral data obtained in the range 250–1000 nm by UV–vis-spectrometer. The optical band gap of In₂O₃ film was observed and found to be varying from 3.67 to 3.85 eV with the annealing temperature.     

Effect of Li2CO3 addition on the structural,optical, ferroelectric,and electric-field-induced strain of lead-free BNKT-based ceramics

In this work, we reported the effect of Li₂CO₃ addition on the structural, optical, ferroelectric properties and electric-field-induced strain of Bi_0.5(Na,K)_0.5TiO₃ (BNKT) solid solution with CaZrO₃ ceramics. Both rhombohedral and tetragonal structures were distorted after adding Lithium (Li). The band gap values decreased from 2.91 to 2.69 eV for 5 mol% Li-addition. The maximum polarization and remanent polarization decreased from 49.66 μC/cm² to 27.11 μC/cm² and from 22.93 μC/cm² to 5.35 μC/cm² for un-doped and 5 mol% Li- addition BNKT ceramics, respectively. The maximum S_max/E_max value was 567 pm/V at 2 mol% Li₂CO₃ access. We expected this work will help to understand the role of A-site dopant in lead-free ferroelectric BNKT materials.     

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.