Crystalline structure of a C60/C70 membrane

The crystalline structure of a C₆₀/C₇₀ membrane prepared by an original technique has been studied by x-ray diffraction and Raman spectroscopy. The effects of purification of the starting C₆₀/C₇₀ mixture to C₇₀ composition and of spatial separation of the C₆₀ and C₇₀ phases in the membrane have been revealed. The samples studied were established to have a composition gradient from C₆₀ to C₇₀. The main structure of the membrane is shown to be an fcc lattice with a=14.308 Å.     

Infrared multiple photon decomposition of CHBrF2 induced by a free-electron laser

2 . The laser generates an intense infrared macropulse with a duration of 17 μs; the macropulse consists of a train of 380 micropulses, each of which has a duration of a few picoseconds. The fluence of a macropulse was estimated to be about 16 Jcm^-2 at a beam waist. Peak wavelengths were set in the range of 9–10 μm. The macropulse induced the IRMPD of 1 and 5 Torr CHBrF₂; most of molecules in the focal region seemed to decompose at a wavelength of 9.3 μm. The mechanism is the initial decomposition of CHBrF₂ to CF₂ and HBr, followed by the dimerization of CF₂ to form C₂F₄. The decomposition was found to be isotopically selective at 9.7 μm; the final product C₂F₄ had a ¹³C atomic fraction of 6%. Th e addition of CO₂ to CHBrF₂ significantly decreased the yield of C₂F₄. vibrationally excited CHBrF₂ molecules produced by laser pulses were efficiently deactivated by CO₂ molecules. Received: 7 October 1996     

A high-voltage lithium-ion battery prepared using a Sn-decorated reduced graphene oxide anode and a LiNi<Subscript>0.5</Subscript>Mn<Subscript>1.5</Subscript>O<Subscript>4</Subscript> cathode

This paper describes the preparation and characterization of a high-voltage lithium-ion battery based on Sn-decorated reduced graphene oxide and LiNi_0.5Mn_1.5O₄ as the anode and cathode active materials, respectively. The Sn-decorated reduced graphene oxide is prepared using a microwave-assisted hydrothermal synthesis method followed by reduction at high temperature of a mixture of (C₆H₅)₂SnCl₂ and graphene oxide. The so-obtained anode material is characterized by thermogravimetric analysis, X-ray diffraction, scanning electron microscopy, and electron diffraction spectroscopy. The LiNi_0.5Mn_1.5O₄ is a commercially available product. The two materials are used to prepare composite electrodes, and their electrochemical properties are investigated by galvanostatic charge/discharge cycles at various current densities in lithium cells. The electrodes are then used to assemble a high-voltage lithium-ion cell, and the cell is tested to evaluate its performance as a function of discharge rate and cycle number.     

High pressure X-ray diffraction study of CdAl2Se4 and Raman study of AAl2Se4 (A=Hg, Zn) and CdAl2X4 (X=Se, S)

We report the results of an X-ray diffraction study of CdAl₂Se₄ and of Raman studies of HgAl₂Se₄ and ZnAl₂Se₄ at room temperature, and of CdAl₂S₄ and CdAl₂Se₄ at 80 K at high pressure. The ambient pressure phase of CdAl₂Se₄ is stable up to a pressure of 9.1 GPa above which a phase transition to a disordered rock salt phase is observed. A fit of the volume pressure data to a Birch-Murnaghan type equation of state yields a bulk modulus of 52.1 GPa. The relative volume change at the phase transition at ∼9 GPa is about 10%. The analysis of the Raman data of HgAl₂Se₄ and ZnAl₂Se₄ reveals a general trend observed for different defect chalcopyrite materials. The line widths of the Raman peaks change at intermediate pressures between 4 and 6 GPa as an indication of the pressure induced two stage order-disorder transition observed in these materials. In addition, we include results of a low temperature Raman study of CdAl₂S₄ and CdAl₂Se₄, which shows a very weak temperature dependence of the Raman-active phonon modes.     

Structure cristallographique et magnetique de Mn2N0.86 à basse temperature

The compound Mn₂N_0.86 which belongs to the crystallographic space group P6₃22 undergoes at 308K a paramagnetic-antiferromagnetic transition accompanied by a crystallographic distorsion. The magnetic structure is collinear with a magnetic moment of 1.7 μ_B per atom Mn. The Shubnikov group is Cp22'2'₁ (P_A2₁2₁2).     

Optical properties of the Ba0.8Sr0.2TiO3/(Bi0.82, Nd0.02)FeO3 superlattice

A Ba_0.8Sr_0.2TiO₃/(Bi_0.82, Nd_0.02)FeO₃ multilayer heteroepitaxial structure is prepared by successively depositing 20 Ba_0.8Sr_0.2TiO₃ and (Bi_0.82, Nd_0.02)FeO₃ layers, each 4 nm thick, using the method of rf sputtering. The layers grow on a single-crystal MgO substrate following the layer-by-layer mechanism. The structural perfection of the layers is examined by X-ray diffraction. The optical transmission is studied in the wavelength interval 200–1100 nm. The spectra are processed using a dispersion formula for the permittivity represented as a superposition of two oscillators with regard to relaxation. Such an approach allows us to separate direct and indirect transitions. The absorption edge of the superlattice for direct transitions is 80 nm wide and is estimated to be 3.78 eV.     

Investigation of the chemical state of ultrathin Hf-Al-O films during high temperature annealing

Al₂O₃ incorporated HfO₂ films grown by atomic layer deposition (ALD) were investigated by high-resolution X-ray photoelectron spectroscopy (HRXPS). The core level energy state of a 15 Å thick film showed a shift to higher binding energy, as the result of a silicate formation and Al₂O₃ incorporation. The incorporation of Al₂O₃ into the HfO₂ film had no effect on silicate formation at the interface between the film and Si, while the ionic bonding characteristics and hybridization effects were enhanced compared to a pure HfO₂ film. The dissociation of the film in an ultrahigh vacuum (UHV) is effectively suppressed compared to a pure HfO₂ film, indicating an enhanced thermal stability of Hf-Al-O. Any dissociated Al₂O₃ on the film surface was completely removed into the vacuum by vacuum annealing treatment over 850 °C, while HfO₂ contributed to Hf silicide formation on the film surface.     

Autowave synthesis of cast aluminum oxynitrides with a high nitrogen content

Aluminum oxynitrides Al₂₂O₃₀N₂ and Al₂₃O₂₇N₅ and a number of Al-O-N solid solutions with a nitrogen content of 2.3 to 6.5 wt % were prepared from (CrO₃-Al-Al₂O₃-AlN) thermite-type mixtures with a high combustion temperature (2950–3200 K) by using the autowave synthesis method. The synthesis was conducted in a hermetically sealed reactor at an initial nitrogen pressure of 4.0 to 8.0 MPa. The aluminum nitride content was demonstrated to influence the nitrogen content in cast aluminum oxynitrides and on their microstructure and phase composition.     

Quantitative analysis of mixed self-assembled monolayers using ToF-SIMS

The spacing of chemical functional groups on self-assembled monolayers (SAMs) plays an important role in controlling the density of biomolecules in biochips and biosensors. In this sense, a mixed SAM made of two different terminal groups is a useful organic surface since spacing can be easily controlled by changing a relative mole fraction in a mixture solution. In this study, an acetylene-OCH₂O(EG)₃(CH₂)₁₁S-S(CH₂)₁₁(EG)₃OCH₂O-propene (Eneyne) SAM and mixed SAMs made by a mixture of (S(CH₂)₁₁(EG)₃OCH₂O-acetylene)₂ (Diyne) and (S(CH₂)₁₁(EG)₃OCH₂O-propene)₂ (Diene) were produced on gold substrates and measured by using ToF-SIMS. The secondary ion yield ratio of [Au·S(CH₂)₁₁(EG)₃OCH₂O-acetylene]⁻ to [Au·S(CH₂)₁₁(EG)₃OCH₂O-propene]⁻ was measured for each mixed SAM and plotted as a function of the mole fraction of Diyne to Diene in a SAM solution. The ion yield ratio of a mixed SAM produced from a solution with a mole fraction of 0.5 (i.e., 1:1 mixture) was 0.3, which corresponded well to the ion yield ratio measured from an Eneyne SAM. A time-dependent experiment of Eneyne SAM formation and immersion experiment of Eneyne SAM into Diyne solution or into Diene solution were performed. The relative ion yield ratio of 0.3 was due to a different secondary ion formation and not due to the difference in the amount of adsorbates on the surface, nor to the different binding strengths onto the gold surface. Our study shows that a mixed SAM with well-controlled spacing can be produced and quantified by using the ToF-SIMS technique.     

Optical simulation, optimized design and fabrication of (ZrO2)x-(Al2O3)1−x composite films with thin inserted TiO2 layers for ArF-line high transmission attenuated phase shift mask blank applications

The tunable optical constants of the stoichiometric (ZrO₂)_x-(Al₂O₃)_1−x composite films with thin inserted TiO₂ layers are simulated as π-phase shifters. The optimized composition range of the superlattices to be used as a high transmission attenuated phase shift mask (HT-APSM) blank is found. The absorption edge shifts to the longer wavelengths when the thickness fraction of the TiO₂ layer increases. The optimized film for ArF-line HT-APSM blank applications must have the lower inspection transmittance for the better inspection and the lower reflectance at the exposure wavelength for a better aerial image as π-phase shifters, and they will be easier to fabricate than a superlattice. In order to find such a film, (ZrO₂)_x-(Al₂O₃)_1−x composite films with various inserted TiO₂ layers are simulated. The optimal deposition processes of such a film are also determined. For example, a (ZrO₂)_0.187-(Al₂O₃)_0.813 composite film with two inserted TiO₂ thin layers is fabricated. The optical properties are as follows: a transmittance of 19.8%, a reflectance of 9.1%, a calculated phase shift of ∼181.5° at the exposure wavelength of 193 nm, and a transmittance of 18.9% at the inspection wavelength of 257 nm. Such a film should be used as an optimized HT-APSM blank.     

Influence of cooperativity on the frequency shift of the Ar–H stretch vibration in HArF complexes

CH₃Li–FArH–X (X?=?H₂, OC, N₂, P₂, CO₂, CO, BeH₂) trimers have been investigated using quantum chemical calculations at the QCISD/6-311++G(2d,2p) level. The results show that the lithium bonding has a prominent effect on the strength and properties of the hydrogen bonding. The hydrogen-bonding interaction energy is increased by 160–340% due to the presence of lithium bonding. The Ar–H stretch vibration shows a blue shift in the FArH–X (X?=?H₂, OC, N₂, CO₂, CO) dimer, but a red shift in the FArH–X (X?=?P₂, BeH₂) dimer. The red shift is increased in the corresponding trimer, while the blue shift shows a different change. The blue shift is also increased in CH₃Li–FArH–X (X?=?H₂, OC, N₂, CO₂) trimers, but it changes to a red shift in the CH₃Li–FArH–CO trimer. The shift change is consistent with the explanation given by Joseph and Jemmis.     

Photocatalytic activity of Nb2O5/SrNb2O6 heterojunction on the degradation of methyl orange

The pure SrNb₂O₆ powders were prepared at 1400 °C by a conventional solid-state method and characterized by X-ray powder diffraction and UV-vis diffuse reflection spectrum. The powders of Nb₂O₅ and SrNb₂O₆ were ball-milled together and annealed to form the Nb₂O₅/SrNb₂O₆ composite. Photocatalytic activities of the composites were investigated on the degradation of methyl orange. The results show that the proportion of Nb₂O₅ to SrNb₂O₆ and the annealing temperature greatly influence the photocatalytic activities of the composites. The best photocatalytic activity occurs when the weight proportion of Nb₂O₅ to SrNb₂O₆ is 30% and the annealing temperature is 600 °C. The tremendously enhanced photocatalytic activity of the Nb₂O₅/SrNb₂O₆ composite compared to Nb₂O₅ or SrNb₂O₆ is ascribed to the heterojunction effect taking place at the interface between particles of Nb₂O₅ and SrNb₂O₆. The powders also show a higher photocatalytic activity than commercial anatase TiO₂.     

X-Ray diffraction and Raman spectroscopy studies of superlattices BaTiO3/(Ba0.5,Sr0.5)TiO3/SrTiO3

The structural characteristics of the BaTiO₃/(Ba_0.5,Sr_0.5)TiO₃/SrTiO₃ superlattice on a (001) MgO substrate have been studied using X-ray diffraction. The modulation period and unit cell parameters of layers forming the superlattice have been measured. The sizes of coherent scattering regions and average microstrains in the direction perpendicular to the surface have been estimated. The obtained characteristics are compared to those of the two-layer BaTiO₃/(Ba_0.5,Sr_0.5)TiO₃ superlattice. The Raman spectra demonstrate a substantial shift of the soft E(TO) mode in the three-layer superlattice as compared to the position in the two-layer superlattice. The effects observed are associated with a substantial increase in the temperature of the phase transition of the three-layer superlattice to the paraelectric phase.     

Electron localization in conducting langmuir-blodgett films

The temperature dependence of the intracrystallite conductivity of Langmuir-Blodgett films of the (C₁₆H₃₃-TCNQ)_0.4(C₁₇H₃₅-DMTTF)_0.6 charge-transfer complex (CTC) is studied by measuring the surface acoustic wave attenuation in a piezoelectric delay line coated with such a film. (C₁₆H₃₃-TCNQ)_0.4(C₁₇H₃₅-DMTTF)_0.6 is a surface-active CTC made from a 1.5: 1 mixture of heptadecyl-dimethyltetrathiafulvalene (C₁₇H₃₅-DMTTF) and hexadecyl-tetracyanoquinodimethane (C₁₆H₃₃-TCNQ). The temperature dependence of the intracrystallite conductivity is found to have a maximum at T_MD=193.5 K. Above T_MD, the conductivity of the films is metallic (?σ/?T<0), while below this temperature it obeys a law that is close to the one-dimensional Mott law. The decrease in the conductivity with decreasing temperature at T<T_MD is shown to be related to the localization of electron states in the quasi-one-dimensional system under study and to be caused by the presence of impurities and defects in the TCNQ chains, along which a charge is transferred. The detected variation in the conductivity with temperature below T_MD is found to qualitatively and quantitatively agree with the model of localization in a weakly disordered quasi-one-dimensional system proposed earlier by Nakhmedov, Prigodin, and Samukhin. Fitting the experimental results to the theoretical dependences obtained in the framework of this model allows us to find the electron-phonon and electron-impurity scattering times. The structural parameters of the conducting layer are used to estimate the density of states at the Fermi level and the Fermi velocity in the films. With these values, the mean free path and the localization length in the films under study are determined.     

Zersetzung von CCl4 im HF-Niederdruckplasma für plasmachemisches Ätzen von Aluminium

After a brief summary of decomposition processes occuring in a plasma etching reactor investigations of CCl₄-decomposition by gaschromatography and time resolved emission spectroscopy are described. The decomposition takes place in a plasma tube with external r.f. electrodes. The qualitative identification of decomposition products of CCl₄ shows few main products only quite in contrast to plasma decomposition of hydrocarbons. These are CCl₄, Cl₂, C₂Cl₄, C₂Cl₆ and glow polymers. The dependence of the product distribution on power density and flow rate gives some hints for a rapid decomposition mechanism. The time resolved emission spectroscopy bases on a two channel method using a reference signal of an Ar line to eliminate the changing plasma conditions during the decomposition process. The limits of application of this method are discussed. Investigations of CCl-, Cl₂- and Cl-intensity-time-functions also reveal the existence of a the rapid decomposition mechanism for which a dissociative electron attachment is supported giving rate constants in the expected order of magnitude.     

Composition-dependent melting behaviour of NaxK55–x core–shell nanoalloys

Molecular dynamics simulations at constant temperature are performed to investigate melting-like transition in Na₁₃K₄₂, Na₁₉K₃₆ and Na₂₆K₂₉ nanoalloys using a second-moment-approximation tight-binding analytic potential to calculate the forces on the constituent atoms. A weighted histogram analysis method is employed to remove non-ergodicity issues due to the complex potential energy surface of these nanoalloys. The heat capacity shows three distinctive steps in melting for Na₁₃K₄₂, while Na₂₆K₂₉ and Na₁₉K₃₆ have two-step and one-step melting transition, respectively. The steepest descent method is used to quench the configurations in a given interval during the simulation and also study the isomerisation processes occurring at different temperatures. Analysing the configuration energies of quenched structures for the entire nanoalloy and the core atoms separately gives more details about the melting mechanism. The Lindemann parameter is also calculated at several temperatures during the simulation which shows a gradual increase for Na₁₃K₄₂ and Na₂₆K₂₉ while a sharp change is observed for Na₁₉K₃₆. These findings are in agreement with the multi-step nature of the phase transition in Na₁₃K₄₂ and Na₂₆K₂₉ and one-step melting of the Na₁₉K₃₆ magic composition.     

Facile liquid phase deposition of thick reflective GeO2 film for hollow waveguide delivery of CO2 laser radiation

A new idea of using LPD (liquid phase deposition) to prepare a GeO₂ thick reflective film for hollow waveguide delivery of CO₂ laser radiation was investigated in this work. The LPD process was achieved by designedly adding acid to GeO₂–aqueous ammonia. The addition of acid could induce the transformation of germanate ions into GeO₂ solutes, leading to the deposition of a GeO₂ ceramic film when the concentration of GeO₂ solute is higher than its saturation solubility. It was found that the highest film growth rate occurred at a pH value of 3, while a film with low surface roughness and good adhesion to the substrate was produced at a pH value of 2 and the film could be converted to a smooth, compact hexagonal GeO₂ film by heat treatment at 1120 °C for 30 min. Two abnormal dispersion bands within 7.6–9 μm and 9.6–11.2 μm were mainly caused by the silica glass substrate and the GeO₂ film, respectively. The film was thick enough to achieve the total reflectance of the CO₂ laser radiation. The use of this GeO₂ film in a hollow waveguide structure for CO₂ laser radiation delivery is discussed based on the transmission loss and the feasibility of the deposition of the GeO₂ film inside silica capillary tubes. The results show that the hollow waveguides with low transmission loss are most likely fabricated at a low cost using the LPD-derived GeO₂ reflective film. PACS 78.20.-e; 78.66.-w; 42.70.-a; 78.20.Ci; 78.40.-q     

Enhanced H2S gas-sensing properties of Pt-functionalized In2Ge2O7 nanowires

In₂Ge₂O₇ one-dimensional (1D) nanostructures were synthesized using an evaporation technique. The morphology, crystal structure, and sensing properties of the In₂Ge₂O₇ nanostructures functionalized with Pt to H₂S gas at 100 ^°C were examined. The diameters and lengths of the 1D nanostructures were a few tens of nanometers and up to a few hundreds of micrometers, respectively. Pt nanoparticles with diameters of a few tens of nanometers were distributed over the In₂Ge₂O₇ nanowires. Gas sensors fabricated from the multiple networked In₂Ge₂O₇ nanowires functionalized with Pt exhibited enhanced electrical responses to H₂S gas compared to the uncoated In₂Ge₂O₇ nanowires. The responses of the nanowires were improved 240, 234, and 225 fold at H₂S concentrations of 10, 25, and 50 ppm, respectively. The enhanced electrical response of the Pt-functionalized In₂Ge₂O₇ nanowire sensor to H₂S gas can be attributed to a combination of the spillover effect and the enhanced chemisorption, as well as the dissociation of the gas.     

Pressure effects in ferromagnetic manganites and carrier scattering by disordered magnetic rare earths

The influence of hydrostatic pressure on the transport properties of (La_1-xR_x)_0.67Ca_0.33MnO₃ (,Tb) ferromagnetic manganites is investigated. The enhancement of the Curie temperature T_C under pressure agrees with previous data. In the paramagnetic range, the resistivity can be represented by a Mott localisation law, with a characteristic temperature T₀ decreasing with pressure. The variation of T_C with pressure is compared to the effect induced by replacing La by a magnetic rare earth in (La_1-xR_x)_0.67Sr_0.33MnO₃ manganites (, ..., Tm). The main effect is not related to the decrease of the mean radius of the cation, but to an additional scattering by the magnetic moment of the rare earth. Received: 15 May 1998 / Revised: 6 July 1998 / Accepted: 16 July 1998     

Room temperature response of SnO2-electrode modified solid state electrochemical CO2 sensors

A thin film solid state electrochemical gas sensor has been investigated for CO₂ detection based on the cell reaction: Na⁺+OH+CO₂=NaHCO₃. The galvanic cell arrangement is Au | Na_xCoO_2−δ (ref.) | NASICON | Au, SnO₂ where the right hand electrode is in contact with CO₂ and O₂ in a humid atmosphere. The response has been compared to results obtained with a conventional pellet type sensor. Furthermore, both devices have been exposed to CO and humidity. Strong cross-sensitivities were observed leading to large changes in the emf in both cases. The response to moisture is reversible and fast with a response time of about 1 min according to a fast surface reaction of H₂O with SnO₂. The presence of CO leads to a signal change with a high response time and a very slow reverse reaction. However, the response to CO₂ is not influenced by the presence of CO or H₂O with regard to the signal height and response time. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996