Nb 3d and O 1s core levels and chemical bonding in niobates

A set of available experimental data on binding energies of Nb 3d_5/2 and O 1s core levels in niobates has been observed with using energy difference (O 1sNb 3d_5/2) as a robust parameter for compound characterization. An empirical relationship between (O 1sNb 3d_5/2) values measured with XPS for Nb⁵⁺-niobates and mean chemical bond length L(NbO) has been discussed. A range of (O 1sNb 3d_5/2) values possible in Nb⁵⁺-niobates has been defined. An energy gap ∼1.4–1.8 eV is found between (O 1sNb 3d_5/2) values reasonable for Nb⁵⁺ and Nb⁴⁺ states in niobates.     

AC susceptibilities and size effect in Ln0.7(Sr,Ca)0.3MnO3 CMR manganites

The low field susceptibility versus temperature has been studied for the series of colossal magnetoresistive (CMR) manganites Ln_0.7Ca_0.3−xSr_xMnO₃ (LnPr, x = 0.10, 0.05, 0; LnNd, x = 0). The effect of the average size of the interpolated cation upon T_c is confirmed for the larger sizes (LnPr, x = 0.10, 0.05) showing a classical ferromagnetic behavior at low temperature in agreement with the neutron diffraction studies. For a smaller size of the A-site cation (LnPr or Nd, x = 0) an original behavior is observed: the _χ′(T) curves show a spin-glass-like behavior with a cusp at T_cusp, whereas the neutron diffraction data in zero magneteic field evidence a ferromagnetic component starting around T_cusp.     

Growth and ordering of CuO chains on Ag(110) surface

Growth of CuO chains and their ordering on Ag(110) surface were monitored by scanning tunneling microscopy (STM). When Cu atoms were deposited on a (2 × 1)AgO/Ag(110) surface at room temperature, the AgO chains in the [001] direction diminished and new strings of CuO grew along the [110] direction and yield a (2 × 2) p2mg ordering. When a Ag(110) surface with coexisting AgO and CuO chains was exposed to CO at room temperature, the AgO chains were selectively reduced.     

Magnetic susceptibility of liquid Na1−xSbxalloys

The magnetic susceptibility of liquid NaSb alloys was measured in a temperature range between 600°C and 1100°C. The NaSb system is supposed to exhibit, as does the CsSb system, a metal to semiconductor transition dependent upon composition. A relatively strong diamagnetic minimum of x = ?26 × 10^?6 cm³/mol was found near the composition of the stoichiometric formula Na₃Sb. This minimal susceptibility can be relatively well described with an ionic type binding model.     

Mid-wave infrared liquid crystal shutter for breathalyzer applications

There exists a problem with an in situ diagnostics of contamination of ethyl alcohol in a human being exhaled air. When ethyl alcohol in a mouth blowing (in a gaseous state) exists, the characteristic CH stretch absorption bands in CH₃ and CH₂ functional groups in ethanol (CH₃CH₂OH) appear at a wavelength of λ = 3.42 μm. To investigate the presence of ethyl alcohol in exhaled human air, the light beam of λ = 3.42 μm is passing through an air sample. If one alternately measures the intensity of the investigated beam and the reference, a percentage of ethanol in the air sample can be estimated using a sensitive nondispersive infrared (NDIR) system with a stable operating flow mass detector. To eliminate a mechanical chopper and noise generating stepper motors, a photonic chopper as a liquid crystal shutter for λ = 3.42 μm has been designed. For this purpose, an innovative infrared nematic liquid crystal mixture was intentionally prepared. The working mixture was obtained by a selective removal of CH bonds and its exchange by heavier polar substituents, what ensures a lack of absorption band of CH bonds. The paper presents theory, concept and final experimental results of the infrared nematic liquid crystals mixture and the liquid crystal shutter for breathalyzer applications.     

Dimethyl methylphosphonate decomposition on fully oxidized and partially reduced ceria thin films

The thermal decomposition of dimethyl methylphosphonate (DMMP) on crystalline ceria thin films grown on Ru(0 0 0 1) was studied by temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and infrared absorption reflection spectroscopy (IRAS). TPD experiments show that methanol and formaldehyde desorb as the two main products at 575 K, while water, formaldehyde and CO are produced above 800 K. IRAS studies demonstrate that DMMP adsorbs via the phosphoryl oxygen at 200 K, but the PO bond converts to a bridging OPO species at 300 K. DMMP decomposition initially occurs via POCH₃ bond scission to form methyl methylphosphonate (MMP) and methyl phosphonate (MP) between 300 and 500 K; XPS and IRAS data are consistent with a methoxy intermediate on the surface at these temperatures. The more stable PCH₃ bonds remain intact up to 700 K, and the only surface intermediate at higher temperatures is believed to be PO_x. Although the presence of PO_x decreases activity for DMMP decomposition, some activity on the ceria surface remains even after 7 cycles of adsorption and reaction. The ceria films become reduced by multiple DMMP adsorption-reaction cycles, with the Ce⁺⁴ content dropping to 30% after seven cycles. Investigations of DMMP reaction on reduced ceria surfaces show that CO and H₂ are produced in addition to methanol and formaldehyde. Furthermore, DMMP decomposition activity on the reduced ceria films is almost completely inhibited after only 3 adsorption-reaction cycles. Similarities between DMMP and methanol chemistry on the ceria films suggest that methoxy is a key surface intermediate in both reactions.     

Structural characteristics of mixed oxides MOx/SiO2 affecting photocatalytic decomposition of methylene blue

A series of photocatalysts based on silica (nanoparticulate) supported titania, ceria, and ceria/zirconia were synthesized and characterized by a variety of techniques including surface area measurements, X-ray diffraction, Fourier transform infrared spectroscopy, zeta potential, surface charge density, and photocatalytic behavior toward methylene blue decomposition. Thermal treatment at 600 °C increases the anatase content of the titania based catalysts detected by XRD. Changes in the infrared spectra before and after thermal treatment indicate that at low temperature there are more SiOTi bonds than at high temperature. As these bonds break upon heating the SiO₂ and TiO₂ separate, allowing the TiO₂ anatase phase to form. This results in an increased catalytic activity for the thermally treated samples. Nearly all titania based samples exhibit a negative surface charge density at pH 7 (initial pH of photocatalytic studies) which aids adsorption of methylene blue. The crystallinity of ceria and ceria/zirconia based catalysts are in some cases limited, and in others non-existent. Even though the energy band gap (E_g) can be lower for these catalysts than for the titania based catalysts, their photocatalytic properties are inferior.     

Oxygen chemisorption as a tool for atom discrimination in the SiGe termination layer

This Letter describes a novel method of employing the phenomenon of oxygen chemisorption for atom discrimination in the SiGe surface termination layer. Formation of SiO species on clean Si(100) gives rise to peaks at 7 and 10.2 eV in He I UPS and a peak at 532.3 eV in O 1s XPS. Whereas GeO species on a Ge(100) surface exhibits a single peak at 5.2 eV in He I UPS and a peak at 531.3 eV in O 1s XPS. These signature spectra of SiO and GeO species have been effectively employed for atom discrimination in the termination layer of SiGe surfaces. Upon dosing at room temperature, on a sample prepared by depositing 5ÅGe on Si(100) at 550°C, oxygen bonds with Ge atoms forming GeO, exclusively. This indicates termination entirely by Ge atoms. Oxygen adsorption at room temperature, on a sample prepared by codeposition of Ge and Si (total 5Å) onto Si(100) at 550°C, forms a mixture of SiO and GeO species suggesting a surface termination by both Ge and Si atoms.     

Adsorption and oxidation of sulfur dioxide on the yttria-stabilized zirconia surface: ab initio atomistic thermodynamics study

The interaction of SO₂ with the yttria-stabilized zirconia (YSZ) (111) and oxygen-enriched YSZ (111) (YSZ+O) surface is investigated using the first-principles method based density functional theory (DFT). It is found that SO₂ is adsorbed either as a molecule or forms SO²⁻₃ species with new SO bonds to a surface oxygen on the YSZ (111) surface. In addition, there exist other species, e.g., SO₃ and SO²⁻₄ on the very active YSZ+O (111) surface. Using the ab initio atomistic thermodynamics method, we present a detailed analysis on the stability of the SO₂YSZ/YSZ+O system as a function of the ambient conditions.     

Synthesis and characterization of silanized-SiO2/povidone nanocomposite as a gate insulator: The influence of Si semiconductor film type on the interface traps by deconvolution of Si2s

The polymer nanocomposite as a gate dielectric film was prepared via sol-gel method. The formation of cross-linked structure among nanofillers and polymer matrix was proved by Fourier transform infrared spectroscopy (FT-IR). Differential thermal analysis (DTA) results showed significant increase in the thermal stability of the nanocomposite with respect to that of pure polymer. The nanocomposite films deposited on the p- and n-type Si substrates formed very smooth surface with rms roughness of 0.045 and 0.058 nm respectively. Deconvoluted Si_2s spectra revealed the domination of the SiOH hydrogen bonds and SiOSi covalence bonds in the structure of the nanocomposite film deposited on the p- and n-type Si semiconductor layers respectively. The fabricated n-channel field-effect-transistor (FET) showed the low threshold voltage and leakage currents because of the stronger connection between the nanocomposite and n-type Si substrate. Whereas, dominated hydroxyl groups in the nanocomposite dielectric film deposited on the p-type Si substrate increased trap states in the interface, led to the drop of FET operation.     

Extrinsic fluorescence as a sensitive method for studying photo-degradation of high density polyethylene part I

Artificial ageing of high density polyethylene (HDPE) exposed to ultra-violet irradiation has been investigated by fluorescence spectroscopy. In the case of HDPE, fluorescence spectroscopy requires the addition of fluorophore, (rhodamine 101 laser dye). The spectral features of fluorescence are very sensitive to the interaction between the exited molecules and the polymer matrix. Under wavelength close to 254 nm, the diffusion of oxygen can introduce groups such as CO, CO and C(O)O into the molecular chains. Thus entail structural and chemical modifications such as polymeric chain breaking, cross linking and oxidation. In the present work, we clearly illustrate the fact that fluorescence spectroscopy is very worthwhile in particular to follow the early stages of photo-degradation.     

IR wavelength-selective laser desorption via OH and CH stretching modes

We present new results on wavelength-selective desorption of solid samples using the resonant interaction between the laser beam and the bulk. The experimental set-up is based on the coupling of three techniques: laser desorption in the near-infrared (IR) (2.7–4 μm) with a tunable IR optical parametric oscillator (OPO), UV multi-photon ionization, and reflectron time-of-flight mass spectrometry. The resonant character of the laser desorption process has been investigated for an ice/polycyclic aromatic hydrocarbon (PAH) mixture, by excitation of the OH and CH stretching modes. Highly preferential desorption has been evidenced, with exclusive desorption of water and PAH molecules at the OH and CH resonances, respectively. Potential analytical (e.g. selective analysis of complex samples) and technological (e.g. dry laser cleaning, DLC) applications are discussed.     

Infrared diode laser spectroscopy of N212C18O2

The high-resolution infrared spectrum of N₂¹²C¹⁸O₂ has been observed in the ν₃ band (2314 cm^?1) region of ¹²C¹⁸O₂ with diode laser absorption spectroscopy of pulsed molecular beam. The geometry of N₂¹²C¹⁸O₂ is similar to N₂¹²C¹⁶O₂, a T-shaped structure with the nitrogen molecular axis pointing towards the carbon atom. The geometrical parameters of the T-shaped ground-state structure are determined as R_NcmC = 3.7285(5) Å and (90?Θ_NcmCO) = 6.85(3)°. The vibrational band origin of N₂¹²C¹⁸O₂ corresponding to the ν₃ mode of ¹²C¹⁸O₂ shows a shift of 0.52499(10) cm^?1 with respect to that of ¹²C¹⁸O₂.     

Thermal activation and reaction of allyl alcohol on Ni(100)

The thermal chemistry of allyl alcohol (CH₂CHCH₂OH) on a Ni(100) single-crystal surface was studied by the temperature programmed desorption (TPD) and the X-ray photoelectron spectroscopy (XPS). The allyl alcohol adsorbs molecularly on the metal surface at 100 K. Intact molecular desorption from the surface occurs at temperatures around 180 K, but some molecules exhibit chemical reactivity on the surface: activation of the OH, CC, and CO bonds produces η¹(O)-allyloxy CH₂CHCH₂O_(a), η²(C, C) allyl alcohol (C_(a)H₂C_(a)HCH₂OH), and η³(C, C, O)-alkoxide (C_(a)H₂C_(a)CH₂ O_(a)) intermediates. Further thermal activation of allyl alcohol on the surface yields propylene (CH₂CHCH₃), 1-propanol (CH₃CH₂CH₂OH), propanal (CH₃CH₂CHO), and combustion and dehydrogenation products (H₂O, H₂, and CO). Propylene desorbs from the surface at temperatures of around 270 K. Hydrogenation to the η³(C, C, O)-alkoxide intermediate leads to the production of propanal which desorbs from the surface around 320 K, while hydrogenation of the η²(C, C) allyl alcohol intermediate produces 1-propanol, which desorbs at around 310 K. The co-adsorption of hydrogen atoms on the surface enhances the formation of the saturated alcohol, while co-adsorption of oxygen enhances the formation of both the saturated alcohol and the saturated aldehydes.     

Nonstoichiometry of the perovskite-type solid solution La0.9Ca0.1Cr1−yAlyO3−δ

Nonstoichiometry of the perovskite-type solid solutions La_0.9Ca_0.1Cr_1−yAl_yO_3−δ was studied by high-temperature gravimetry under controlled P(O₂) atmospheres of 1–10^− 23 bar at 1073–1273 K. The observed data were described by a regular solution-like model for the randomly distributed defects of V_O¨, Cr_Cr., Ca_La,, and Al_Cr^X. With the increase in y, V_O¨ formation becomes much easier. For y > 0.8, some fraction of Ca_La, becomes surrounded only by Al_Cr^X and V_O¨ remains around such Ca_La, up to high P(O₂) to reduce the maximum oxygen content below 3.000.     

Crystal growth and characterization of an NLO organic crystal: N′-[(Z)-(4-methylphenyl)methylidene]-4-nitrobenzohydrazide

In this paper, we report the synthesis, growth and characterization of a new organic NLO single crystal of NMPMN (N′-[(Z)-(4-methylphenyl)methylidene]-4-nitrobenzohydrazide), for the first time. The single crystal was grown by slow evaporation method at room temperature. The cell dimensions obtained by single crystal XRD studies reveal that the crystal belongs to triclinic system. It was characterized by different techniques like powder XRD, UV, FTIR, TGA and DSC. The Knoop microhardness test was carried out to measure the mechanical strength of the crystal. Its refractive index was determined by the Brewster’s angle method. The laser damage threshold studies have been carried out for the crystal using a Q-switched Nd:YAG laser of ns pulses at a wavelength of 532 nm. The Kurtz Powder Second Harmonic Generation revealed that the SHG efficiency of the grown crystal is about 50% that of KDP and is found to be phase matchable. The intermolecular OH⋯O, OH⋯N and CH⋯O hydrogen bonds and a π–π stacking interaction between the nitrobenzene and tolyl rings helps to create a delicate balance between the molecular and supramolecular charge transfer processes by creating a noncentrosymmetric structure.     

Mechanism of sonochemical reduction of permanganate to manganese dioxide in aqueous alcohol solutions: Reactivities of reducing species formed by alcohol sonolysis

The sonochemical reduction of MnO₄⁻ to MnO₂ in aqueous solutions was investigated as a function of alcohol concentration under Ar. The rate of MnO₄⁻ reduction initially decreased with increasing alcohol concentration, and then increased when the alcohol concentration was increased further. The concentrations at which the reduction rates were minimum depended on the hydrophobic properties of the added alcohols under ultrasonic irradiation. At low concentrations, the alcohols acted as OH radical scavengers; at high concentrations, they acted as reductant precursors: R_ab, formed by abstraction reactions of the alcohols with sonochemically formed OH radicals or H atoms, and R_py, formed by alcohol pyrolysis under ultrasonic irradiation. The results suggest that the reactivity order of the sonochemically formed reducing species with MnO₄⁻ at pH 7–9 is the sum of H₂O₂ and H > R_py > R_ab. The peak wavelengths of MnO₂ colloidal solutions formed at high 1-butanol concentrations shifted to shorter wavelengths, suggesting the formation of small particles at high 1-butanol concentrations. The rates of sonochemical reduction of MnO₂ to Mn²⁺ in the presence of 1-butanol were slower than that in the absence of 1-butanol, because the sonochemical formation of H₂O₂ and H, which act as reductants, was suppressed by 1-butanol in aqueous solutions.     

A density functional theory study of CF3CH2I adsorption and reaction on Ag(111)

The adsorption and reaction behaviors of CF₃CH₂I on Ag(111) were systematically studied by density functional theory (DFT) calculations. Physical adsorption of CF₃CH₂I on Ag(111) occurs due to the weak interactions between surface Ag atoms and iodine atom of CF₃CH₂I; while strong chemisorption occurs for CF₃CH₂ fragment on Ag(111). Electronic analysis indicates that the singly occupied molecular orbital (SOMO) of CF₃CH₂ strongly interacts with the surface Ag atoms. It is very interesting to find that the most stable structures of CF₃CH₂ on Ag(111) locate at the top site, instead of the hollow sites. This might be attributed to the facts that CF₃CH₂ adsorbed at the top site will maximize the sp³-type hybridization, and the possible weak interaction between the fluorine lone pair electrons of p orbitals for CF₃CH₂ and surface Ag(111) occurs, which is supported by the charge density difference (CDD) analysis with a low isosurface value. We propose that the charge density difference (CDD) analysis with a high or low isosurface value can be widely applied to analyze the strong or weak electronic interactions upon adsorption. Transition state calculations suggested that the energy barrier of CF bond rupture for CF₃CH₂I on Ag(111) (1.44 eV) is much higher than that of CI bond breakage for CF₃CH₂I (0.43 eV); and the activation energy of the CF bond dissociation for CF₃CH₂(a) is 0.67 eV.     

Reflection absorption infrared spectroscopy analysis of the evolution of ErSb on InSb

We discuss an ex-situ monitoring technique based on glancing-angle infrared-absorption used to determine small amounts of erbium antimonide (ErSb) deposited on an indium antimonide (InSb) layer epitaxially grown on an InSb (100) substrate by low pressure metal organic chemical vapor deposition (MOCVD). Infrared absorption from the indium–hydrogen (InH) stretching mode at 1754.5 cm^? 1 associated with a top most surface of an epitaxial InSb layer was used to compare varying levels of surface coverage with ErSb. Among four samples of varying coverage of ErSb deposition (7.2 to 21.5 monolayers), detected infrared absorption peaks distinct to InH weakened as ErSb surface coverage increased. In the early stage of ErSb deposition, our study suggests that outermost indium atoms in the InSb buffer layer are replaced by Er resulting in increase in absorption associated with the InH mode. Using this simple ex-situ technique, we show that it is possible to calibrate the amount of ErSb deposited atop each individual InSb substrate for depositions of few to tens of monolayers.     

Time-evolution of thermal oxidation on high-index silicon surfaces: Real-time photoemission spectroscopic study with synchrotron radiation

The initial oxidation on high-index silicon surfaces with (113) and (120) orientations at 820 K has been investigated by real-time X-ray photoemission spectroscopy (Si 2p and O 1s) using 687 eV photons. The time evolutions of the Siⁿ⁺ (n = 1–4) components in the Si 2p spectrum indicate that the Si^2 + state is suppressed on high-index surfaces compared with Si(001). The O 1s state consists of two components, a low-binding-energy component (LBC) and a high-binding-energy component (HBC). It is suggested that the O atom in strained SiOSi contributes to the LBC component. The reaction rates are slower on high-index surfaces compared with that on Si(001).