Thermal behaviour of polystyrene/silica composites

The particle-size distribution in silica powder prepared by the sol–gel method has been determined by dynamic light scattering analysis. The average diameter of the particles was found to be 250?nm. Using a low-temperature nitrogen adsorption–desorption technique, it was found that the synthesised powder may be referred to as mesoporous materials. Polystyrene/silica composite films were fabricated by casting from o-xylene solutions. It was found, using thermogravimetry, that incorporation of silica leads to an increase in both the onset temperature of polymer degradation and in the temperature at which the maximum rate of weight loss occurs. Using differential scanning calorimetry, the phase transitions from the glassy state to the elastic one were studied for the polymeric materials. New data relating to the effect of silica on the glass-transition temperature, T_g, of composites with a low weight fraction of SiO₂ were found. Specifically, we found a non-monotonic concentration dependence of the value of T_g. The present results advocate for employing silica as an effective filler for producing polymer composites with enhanced thermal properties.     

Synthesis and characterization of nanocrystalline MoBi2Te5 thin films for photoelectrode applications

Molybdenum bismuth telluride thin films have been prepared on clean glass substrate using arrested precipitation technique which is based on self-organized growth process. As deposited MoBi₂Te₅ thin films were dried in constant temperature oven at 110°C and further characterized for their optical, structural, morphological, compositional, and electrical analysis. Optical absorption spectra recorded in the wavelength range 300–800?nm showed band gap (E _g) 1.44?eV. X-ray diffraction pattern and scanning electron microscopic images showed that MoBi₂Te₅ thin films are granular, nanocrystalline having rhombohedral structure. The compositional analysis showed close agreements in theoretical and experimental atomic percentages of Mo⁴⁺, Bi³⁺, and Te^2? suggest that chemical formula MoBi₂Te₅ assigned to as deposited molybdenum bismuth telluride new material is confirmed. The electrical conductivity and thermoelectric power measurement showed that the films are semiconducting with n-type conduction. The fill factor and conversion efficiency was characterized by photoelectrochemical (PEC) technique. In this article, we report the optostructural, morphological, compositional, and electrical characteristics of nanocrystalline MoBi₂Te₅ thin films to check its suitability as photoelectrode in PEC cell.     

Room temperature ferromagnetism and photoluminescence of nanocrystalline Zn1 − xCoxO thin films prepared by sol–gel MOD

Zn_1???xCo_xO (ZC) [x?=?0, 1, 3, 5, 7 and 9?mol%] thin films were prepared by sol–gel combined metallo-organic decomposition method. The films were deposited on the Si substrate with spin-coating technique and annealed at 600?°C for 3?h. X-ray diffraction pattern shows the formation hexagonal wurtzite phase and distortion (c/a) decreases with increasing Co concentration in ZnO. The average grain size is measured using Scherer relation. Atomic force microscopy is used to confirm the formation of nanograins resulted by the use of polyethylene glycol as surfactant. The photoluminescence was recorded by using He-Cd laser of excitation wavelength 325?nm in wavelength region of 350–650?nm which exhibits some influence of Co doping on the multiplication of defects such as O vacancies, Zn interstitials and grain boundary defects. All thin films show room temperature ferromagnetism except pure ZnO which is diamagnetic and 9?mol% of Co shows paramagnetism. This behaviour is interpreted as due to fluctuations in the magnetic ordering, depending on grain size and site location in grain boundaries or oxygen vacancies.     

Effect of mechanical stress on the absorption band tail of cubic boron nitride thin films synthesized by inductively coupled radio-frequency plasma chemical vapour deposition

Thin films of cubic boron nitride were deposited on silica and Si substrates by inductively coupled radio-frequency plasma chemical vapour deposition (IPCVD) technique using a B₂H₆?+?N₂?+?Ar gas mixture. Cubic phase formation was confirmed by glancing-angle X-ray diffraction studies, which showed reflections up to (311). Fourier-transform infrared (FTIR) spectra also indicated the predominantly cubic nature of the deposited films. The optical properties of the films were studied in the wavelength range 200–1000?nm. Both direct and indirect transitions were found to be present. Mechanical stress in the grain-boundary region of the films seems to contribute significantly to the optical absorption below the band gap. The intercrystalline barrier height (E _b) and the trap state density (Q _t ) were obtained from an analysis of the effects of grain boundaries on the optical properties of the samples.     

In situ simultaneous measurements of photodarkening and photoinduced defect creation in amorphous As2Se3 films

To understand the direct correlation between photodarkening (PD) and photoinduced defect creation (PDC) observed in amorphous chalcogenides, in situ simultaneous measurements of PD and photocurrent (PC) have been performed on amorphous As₂Se₃ films. The time evolution of PD and PDC during light excitations are empirically described by a stretched exponential function; 1 ??exp[?(t/τ) ^β ], where τ is the effective response time and β the dispersion parameter. The value of τ for the PDC is very much smaller than that for the PD, suggesting that there is no direct correlation between the two.     

Electrical properties and local structure of n-type conducting amorphous indium sulphide

An n-type amorphous chalcogenide, In₄₉S₅₁, having a band gap of 1.9eV, has been found. The conductivity in as-prepared films was ～10^?4?S?cm^?1, which increased to 1?×?10^?1?S?cm^?1 on post-annealing at 125°C in vacuum, accompanied by a reduction in the sulphur content of the films. TEM observations showed the amorphous nature of the films before and after annealing. Both Seebeck and Hall coefficients are negative, indicating that the major carriers are electrons. The Hall mobility can be as large as 26?cm²?V^?1?s^?1 at 300?K. No significant changes to the optical absorption were observed upon annealing. Analysis of the X-ray radial distribution function reveals that the sulphur atoms have four-fold coordination, making the structure more rigid than conventional amorphous chalcogenides in which the chalcogen is alloyed to elements of group IV or V of the periodic table. We tentatively associate the electron carrier generation with the formation of sulphur vacancies.     

An explanation for the low-temperature H evolution peak in hydrogenated amorphous silicon films deposited 'on the edge of crystallinity'

The puzzling existence of a sharp low-temperature (T = 400°C) H evolution peak in compact hydrogenated amorphous silicon (a-Si : H) films deposited 'on the edge of crystallinity' is examined. From infrared absorption and X-ray diffraction (XRD) measurements, we show that none of the standard methods used to explain the existence of this peak in a-Si : H materials is applicable to the present films. From the Si-H wag-mode peak frequency, we postulate the existence of very small Si crystallites contained within the amorphous matrix. While the crystallite volume fraction is too small to be detected by XRD in the as-grown films, crystallization is observed for this material at anneal temperatures as low as 500°C. It is proposed that these crystallites catalyse the crystallization of the remainder of the amorphous matrix upon moderate annealing, enabling H surface desorption and H₂ out-diffusion to the sample surface along newly formed grain boundaries at low anneal temperatures.     

Epitaxial Ni-Al thin films on NaCl using a Ag buffer layer

Epitaxial nanoscale [001] films of Ni _x Al_100-x (x = 62.5) have been prepared by physical vapour deposition on to a thin film of Ag [001] on NaCl (001) faces with occasional hillocks. The Ag film contains numerous dislocations and stacking faults and has a rms surface roughness of 2 nm. The Ni-Al film is ordered in the B2 structure and reveals many dislocations as well as antiphase boundaries between ordered domains. The formation of subgrains in the Ni-Al film results in severe height variations up to 30 nm across the surface. A cross-sectional model for the growth of both films is presented.     

Structural studies of ultrathin zirconia dielectrics

We present detailed structural and chemical studies on ultrathin zirconia films grown by ultraviolet oxidation and natural (no ultraviolet light) oxidation of Zr precursor metal layers on SiO2-passivated Si(100) wafers. Quantitative electron-energy-loss spectroscopy (EELS) was used to obtain the chemical composition and electronic structure on an atomic scale. The EELS OK near-edge fine structure was used as a fingerprint to study the stoichiometry and composition variation in the dielectric stacks. X-ray absorption spectroscopy studies were performed in order to aid interpretation of the EELS results. The electronic structure data are correlated with the electrical performance of the films.     

Electrical properties of thin films of a-Ga x Te100− x composed of nanoparticles

Thin films of Ga _x Te_{100? x} (x?=?3, 6, 9 and 12) have been synthesized by thermal evaporation. From SEM images, it is observed that all the films contain nanoparticles of sizes varying from 100 to 200?nm. The dc electrical conductivity of the as-deposited films of Ga _x Te_100?x nanoparticles is measured as a function of temperature range from 298 to 383?K, and increases exponentially with temperature. The value of the activation energy, calculated from the slope of ln?σ _dc versus 1000/T plots, is found to decrease with increase in the Ga content. On the basis of the value of the pre-exponential factor σ _o, it is suggested that the conduction is due to thermally assisted tunneling of carriers in localized states near the band edges. The optical measurements suggest an indirect optical band gap in this system. The value of the optical band gap decreases on increasing the Ga concentration.     

Modelling of dissolution kinetics of thin amorphous chalcogenide films

A theoretical relationship for the dissolution kinetics of thin chalcogenide films is proposed. The influence of light irradiation on the film dissolution is taken into account. The theoretical curves are in good agreement with the experimental dissolution results obtained for As₂S₃ thin amorphous films.     

Interfacial defects in YBa2Cu3O7-δ/SrTiO3(0 0 1) heterostructures studied by aberration-corrected ultrahigh-resolution electron microscopy

The microstructure of interfacial defects in YBa₂Cu₃O_7-δ/SrTiO₃(0?0?1) heterostructures has been investigated by aberration-corrected ultrahigh-resolution electron microscopy. We determine that c-axis-oriented YBa₂Cu₃O_7-δ thin films epitaxially grow on SrTiO₃(0?0?1) with two types of interface structure. The coalescence of nucleation sites with different types of interface structure leads to the formation of antiphase domain boundaries in YBa₂Cu₃O_7-δ thin films, which terminate at planar faults with different configurations near the interface. Stand-off misfit dislocations are observed and the dislocation core structure is explored. Based on the interface structure and interfacial defects, the initial growth mode of YBa₂Cu₃O_7-δ thin films on SrTiO₃(0?0?1) is discussed.     

Pulsed electrodeposition of soft magnetic CoNiFe films under centrifugal forces

A new technique, pulsed electrodeposition under a centrifugal force, is introduced for the preparation of soft magnetic CoNiFe films. The films have enhanced properties such as a high B _s (magnetic saturation) and a low H _c (coercivity), which are desirable for soft magnetic materials. They are also smoother than films deposited under non-pulsed conditions. It is suggested that the technique could be used to prepare various other materials with desirable properties.     

Nanocomposite-like structure in an epitaxial CaCu3Ti4O12 film on LaAlO3 (001)

We report the detailed microstructural study of a CaCu₃Ti₄O₁₂ (CCTO) thin film using transmission electron microscopy (TEM). The CCTO thin film studied in this work was deposited on a (001)-oriented LaAlO₃ (LAO) substrate by pulsed-laser ablation and has a high dielectric constant of about 10⁴ at 1?MHz at room temperature; however, the mechanism for such a dielectric property is not yet understood. Plan-view TEM studies show that the CCTO samples have orthogonal domain structures with the edge nearly parallel to either the [100] or the [010] direction of the CCTO. A minor anatase TiO₂ phase was found at the domain boundaries. The CCTO and the TiO₂ phases are separated by an amorphous-like layer that has a thickness of several nanometres. Cross-sectional TEM studies reveal that both CCTO and TiO₂ in the films are c axis oriented with a very sharp interface to the LAO-(001) substrate and possess a unique crystallographic orientation relationship of (001) _CCTO //(001)_{TiO 2} //(001) _LAO and [100] _CCTO //[100] _{TiO 2} //[100] _LAO. The Rutherford back-scattering ion chanelling studies suggest a composition of Ca_1.5Cu₃Ti_5.5O₁₆ for the film, in which the extra calcium, titanium and oxygen form the anatase TiO₂ phase and amorphous calcium oxide layer that separates the CCTO and TiO₂ phases. Such nanocomposite-like structures may provide an important clue to the mechanism of the dielectric property of these films.     

Modification of the optical band gap of polyethylene by irradiation with electrons and gamma rays

Irradiation of polymers has become an important technique in producing special materials for electronic applications. In this study, the effects of electron and gamma irradiation on ultra-high-molecular-weight polyethylene (UHMWPE) samples have been studied using UV–Vis absorption spectroscopy. The UHMWPE samples were irradiated with electron and gamma rays with doses ranging from 25 kGy up to 500 kGy. An increase in the optical absorption and a shifting from the UV–Vis towards longer wavelengths were observed. Concomitantly, a pronounced decrease in the optical band gap was recorded as a function of dose level. The optical band gap E _g varied from 2.86 eV for a pristine sample down to ～1.98 eV and ～2 eV for samples irradiated with gamma rays and electrons at the highest doses, respectively. A correlation between E _g and the number of carbon atoms N in a formed cluster is discussed in terms of a modified Tauc equation.     

Roughening kinetics of reactively sputter-deposited Ti-Al-N films on Si(100)

The roughening kinetics of Ti_{1? x} Al _x N (0?x? 1) films 600 nm thick synthesized by reactive dc magnetron sputtering on Si(100) substrates has been investigated by atomic force microscopy (AFM). The quantification of surface roughening was achieved by calculation of both vertical root-mean-square roughness and lateral correlation lengths of the film surface using the height-height correction functions of measured AFM images. For all the Ti_{1? x} Al _x N films, a steady roughness exponent α = 0.94 ± 0.03 was determined. The evolution of the surface topography as a function of Al concentration is discussed in terms of the competition between surface diffusion and shadowing instability during sputter deposition.     

Energetic effects of dopants on the eutectoid decomposition of Nb–Si in situ composites

The influences of doping elements (Ti, Zr, Hf, V, Ta, Cr, Mo, W and Al) on the heats of formation of Nb, Nb₅Si₃ and Nb₃Si have been studied using first-principles pseudopotential plane-wave method based on density functional theory. Site preferences of dopants in Nb₅Si₃ and Nb₃Si intermetallics are first determined by comparing the heats of formation of the systems with different site occupations of the doping atoms. The partitionings of dopants between the equilibrium phases Nb and Nb₅Si₃ of Nb–Si in situ composites are then discussed and compared with experimental results. Phase partitioning behaviours of the alloying elements are found to depend strongly on the number of their valence electrons. By calculating the enthalpy of reaction regarding the eutectoid decomposition of Nb₃Si phase, we conclude that Ta is a Nb₃Si stabiliser while other dopants destabilise Nb₃Si at the investigated concentration.     

Structural phase transitions in epitaxial SrRuO3 thin films

The microstructural evolution of epitaxial SrRuO₃ thin films from ambient temperature (about 293K) to about 900K has been studied by in-situ transmission electron microscopy. Upon heating from the ambient temperature, the intensities of h, k, 2n+1 and h, -h+2n+1, 0 reflections in selected-area electron diffraction patterns decrease with increasing temperature. Two structural phase transitions were observed at about 673and about 783K, revealed by the vanishing of the h, k, 2n+1 and h,-h+2n+1, 0 reflections respectively. The examination of electron diffraction patterns along several different zone axes, taking into account the possible tilting configurations of RuO₆ octahedra, indicates that, upon heating, the orthorhombic structure of SrRuO₃ transforms into a tetragonal structure at about 673K, and further transforms into a cubic structure at about 783K. Possible structural models for the high-temperature phases are considered.     

Structural modification of Ge–Se amorphous films with the addition of Sb

Extended X-ray absorption fine-structure (EXAFS) measurements were undertaken to understand the structural modification of amorphous Ge–Se films that occurs with the addition of Sb. We found that Ge–Ge bonds exist along with Ge–Se bonds in Ge-rich Ge–Se films. These bonds disappear with increasing Se content, to the extent that Ge–Se bonds predominate in stoichiometric GeSe₂ films. When Ge is replaced by Sb, stable Sb–Se bonds are formed; concurrently, the proportion of Ge–Ge bonds is decreased at the expense of Ge–Se bonds. As a result, GeSbSe glasses have a more stable and stronger network structure.     

Dynamics of upconversion photoluminescence in Ge–Ga–S: Er3+: application of quadrature frequency resolved spectroscopy

Quadrature frequency resolved spectroscopy (QFRS) on green (~550 nm) upconversion photoluminescence (UCPL) of Ge–Ga–S: Er³⁺ glasses pumped at 975 nm revealed double-peaked spectra with long lifetime τ₁ ranging from 0.2 to 2 ms and short lifetime τ₂ ≈ several tens of μs. The results are analysed in terms of the transfer function derived by linearizing rate equations for a three-level model. The τ₁ component is attributed to the relaxation at the intermediate level ⁴I_11/2 of Er³⁺ ions and the τ₂ component, to that at the upper ⁴S_3/2 manifold. The dominance of energy transfer upconversion over excited state absorption in the UCPL manifest itself as a greater dependence of τ₁ on the pump power and a lower ratio of the components of τ₂ to τ₁ in the QFRS spectra of a heavily Er-doped sample.