Fractal patterns in Au–Ta multilayer films upon ion beam mixing

Metallic glass has been formed in Au₅₀Ta₅₀ multilayer films upon 200?keV xenon ion mixing at an irradiation dose of 1?×?10¹⁵?Xe⁺/cm². Electron diffraction and high-resolution transmission electron microscopy analysis confirm that the metallic glass consists of two amorphous phases, evolved from Au and Ta lattices, respectively, and also reveal the formation of a fractal morphology with a fractal dimension of 1.73?±?0.05 in the dual-phase glass features. In similar Au₆₅Ta₃₅ multilayer films, a fractal pattern is also observed at a dose of 1?×?10¹⁵?Xe⁺/cm², while the pattern, with a fractal dimension of 1.74?±?0.05, is composed in this case of a crystalline Au-based solid solution and a Ta-based amorphous phase. Interestingly, the fractal dimensions of the two irradiation-induced fractal patterns match quite well with that expected on a cluster diffusion-limited aggregation model.     

Effect of heat treatment on the NO2-sensing properties of sputter-deposited indium tin oxide thin films

Transparent conducting indium tin oxide (ITO) films were deposited onto glass substrates by radio-frequency magnetron sputtering at 648?K, under an oxygen partial pressure of 1?Pa. The effect of annealing on the electrical properties of the films was studied. Characterization of the coatings revealed an electrical resistivity below 6.5?×?10^??3?Ω?cm. The ITO films deposited at 648?K were amorphous, while the crystallinity improved after annealing at 700?K. The surface morphology examined by scanning electron microscopy appears to be uniform over the entire surface area after annealing. The NO₂-sensing properties of the ITO films were investigated and showed sensitivity at concentrations lower than 50?ppm, at a working temperature of 600?K.     

Epifluorescence optical microscopy: a sensitive tool for determining the spatial distribution of europium-dihalide precipitates in KCl : KBr : Eu2+ crystals

The spatial distribution of europium-dihalide-type precipitates in KCl?:?KBr?:Eu²⁺ single crystals annealed at 200°C for long periods of time was determined by epifluorescence optical microscopy. Laue-type patterns and optical spectroscopy were used to test the degree of long-range ordering of the specimens and to monitor the precipitation during annealing, respectively. Precipitates smaller than about 0.08?µm were found (2.84?×?10¹² precipitates cm^?3) all across the host, whereas larger precipitates of about 0.17?µm were found (2.9?×?10⁴ precipitates cm^?1) along certain linear structural singularities of the matrix. These singularities, identified as crystal dislocations, were observed to terminate always either at a triple node of singularities or at the crystal surface, forming a three-dimensional network of crystal singularities (2.0?×?10⁶ singularities cm^?2), identified as the Frank network. The presence of europium exhaustive matrix zones accompanying the observed europium-decorated paths indicates that impurity segregation processes, occurring during annealing, favour the precipitation phenomena.     

A new allotropic structure of silver nanocrystals nucleated and grown over planar polymer molecules

Polymer molecules of polyvinyl alcohol (PVA) serve as a good Ag⁺?→?Ag⁰ reducer as well as a template to produce Ag⁰ nanocrystals of anisotropic shapes. Thin Ag–PVA films (50–100?µm thick) were prepared by carrying out the reaction in aqueous PVA (under heating conditions) and then spin casting the sample in this shape. A topotactic Ag⁺?→?Ag⁰ reaction and in-situ Ag⁰ growth occur in support over the PVA molecules (as per the local structure and Ag⁰–PVA interactions) of elongated surfaces. The Ag⁰ state is confirmed with 3d _5/2 and 3d _3/2 XPS signals of 368.37 and 374.13?eV binding energies, respectively. As analyzed with X-ray diffraction, it is a new allotrope with an orthorhombic structure, lattice parameters a?=?0.8535, b?=?0.8410?nm and c?=?0.4119?nm, and density ρ?=?9.69?g?cm^?3. On heating in air, the well-known cubic structure develops, a?=?0.4082?nm (ρ?=?10.53?g?cm^?3) after 150°C, whereas a?=?0.4071?nm (ρ?= 10.61?g?cm^?3) after 300°C for 2?h. According to proposed models, it is a vacancy-stabilized allotrope, which evolves from multiply twinned particles.     

Characterization of Al/n-ZnO/p-Si/Al structure with low-cost solution-grown ZnO layer

We report the fabrication of Al/n-ZnO/p-Si/Al diode structures with a flower-like ZnO layer. The average grain size, microstrain and dislocation density in the ZnO layer were determined as 25?nm, 1.55?×?10^?3 and 3.23?×?10¹³?cm^?2, respectively. From absorption spectra, the optical band gap was found to be ～3.17?eV. A red shift was attributed to non-stoichiometry arising from Zn⁺² ions substituting for oxygen vacancies. The ideality factor was determined as 1.55. The barrier height was calculated as 0.71?eV from I–V characteristics and 0.73?eV using the Norde plots.     

Ni-based fully amorphous metallic coating with high corrosion resistance

A Ni₅₃Nb₂₀Ti₁₀Zr₈Co₆Cu₃ fully amorphous metallic coating has been deposited by means of kinetic metallization (KM) using gas atomized powders. As the thickness of the amorphous metallic coating is increased to 400?µm, it attains the excellent corrosion resistance of the amorphous alloy, as indicated by the extremely low passive current density and wide passive region in 1?kmol/m³ HCl aqueous solution. The corrosion rate is as low as 10^–3?mm/year in the extremely corrosive environment of 6?kmol/m³ HCl aqueous solution.     

Surface tension measurement of undercooled liquid Ni-based multicomponent alloys

The surface tensions of liquid ternary Ni–5%Cu–5%Fe, quaternary Ni–5%Cu–5%Fe–5%Sn and quinary Ni–5%Cu–5%Fe–5%Sn–5%Ge alloys were determined as a function of temperature by the electromagnetic levitation oscillating drop method. The maximum undercoolings obtained in the experiments are 272 (0.15T _L), 349 (0.21T _L) and 363?K (0.22T _L), respectively. For all the three alloys, the surface tension decreases linearly with the rise of temperature. The surface tension values are 1.799, 1.546 and 1.357?N/m at their liquidus temperatures of 1719, 1644 and 1641?K. Their temperature coefficients are ?4.972?×?10^–4, ?5.057?×?10^?4 and ?5.385?×?10^?4?N/m/K. It is revealed that Sn and Ge are much more efficient than Cu and Fe in reducing the surface tension of Ni-based alloys. The addition of Sn can significantly enlarge the maximum undercooling at the same experimental condition. The viscosity of the three undercooled liquid alloys was also derived from the surface tension data.     

Interpretation of the low-temperature photoconductivity in a-Si

Abstract

The paper is concerned with the interpretation of steady-state photoconductivity results on undoped a-Si at temperatures of 50K and below which lead to an essentially constant value of the (photogeneration efficiency x mobility x lifetime) product νμτ?10^?11 cm² V^?1. Measurements on p⁺-i-n⁺ junctions and Cr-i-n⁺ barriers were carried out to determine the above parameters separately: (i) steadystate reverse saturation currents gave a generation efficiency of ν? 5×10-² below 50K, suggesting that geminate recombination limits the generation process. (ii) the electron drift mobility μ_e through the tail states and the charge extracted from the absorption region of the incident light were investigated by transient experiments, these showed that μ_eτ_a is limited to about 3×10^?10cm²V^?1 at low T. The independent results account for the observed νμτ values and suggest that, contrary to the interpretation of Hoheisel, Carius and Fuhs (1984), the main contribution to the low-temperature photoconductivity arises from transport in tail states.     

Pressure dependence of the electrical resistivity of natural cassiterite SnO2 and of undoped and Co-doped nanocrystalline SnO2

The pressure dependence of the electrical resistivity of three different samples of cassiterite, namely natural cassiterite SnO₂, synthetic nanocrystalline SnO₂ (with crystallite size 46?nm) and nanocrystalline Co-doped SnO₂ (with crystallite size 32?nm), has been measured up to 7?GPa at room temperature. The resistivity of natural cassiterite SnO₂ decreases from 2.5?×?10⁴?Ωm at normal pressure and temperature to 1.7?×?10⁴?Ωm at 7.0?GPa. The nanocrystalline SnO₂ has a high resistivity 6.0?×?10⁵?Ωm at normal pressure and temperature and decreases with pressure reaching a value of 2.98?×?10⁵?Ωm at 7?GPa. The activation energy of the electrical conduction of the studied samples were found to be 0.32?eV for the natural SnO₂, 0.40?eV for the nanocrystalline SnO₂ sample and 0.28?eV for the nanocrystalline Co-doped SnO₂. Measurements of the pressure dependence of the electrical resistivity of the Co-doped SnO₂ showed a decrease from 3.60?×?10⁵ to 5.4?×?10⁴?Ωm at 7.0?GPa. We did not observe any pressure-induced phase transition in SnO₂ up to 7?GPa. This study of the high-pressure phase stability of cassiterite corroborates the experimental findings of SnO₂ nanoinclusions in diamonds.     

The temperature dependence of the optical dispersion parameters in Si and Ge

Abstract

In Si and Ge, the optical dispersion parameters (single-oscillator energy E_o , dispersion energy E_d and bond energy gap E_g developed by Wemple and DiDomenico, and Phillips) have been analysed in the temperature range 100-300 K using data obtained by Icenogle et al. E_o and E_g exhibit a very small temperature dependence in both materials. The thermal coefficients of the dispersion energy, dE_d/dT, have opposite signs (Si, –41·9 × 10^?4eVK^?1; Ge, +37·7 × 10^?4eVK^?1).     

Thermophysical properties of substantially undercooled liquid Ti-Al-Nb ternary alloy measured by electromagnetic levitation

The thermophysical properties of undercooled liquid alloys at high temperature are usually difficult to measure by experiment. Here, we report the specific heat of liquid Ti₄₅Al₄₅Nb₁₀ ternary alloy in the undercooled state. By using electromagnetic levitation technique, a maximum undercooling of 287?K (0.15 T _L) is achieved for this alloy. Its specific heat is determined to be 32.72?±?2.51?J?mol^?1 K^?1 over a broad temperature range of 1578–2010?K.     

Giant electrocaloric effect of highly (1 0 0)-oriented 0.68PbMg1/3Nb2/3O3–0.32PbTiO3 thin film

PbZr_0.95Ti_0.05O₃ thin film has the highest electrocaloric properties of all the oxide thin films (0.48?K?V^?1). Here, it is shown giant electrocaloric properties in 200?nm (1?0?0)-oriented PMN–PT 68/32 film near the ferroelectric Curie temperature of 146?°C. The results indicate the significance of this system to achieve electrocaloric entropy change and temperature change, up to 32.21?J/kg?°C and 14?K, respectively, in 12?V (i.e. 1.155?K?V^?1) near the Curie point. This exceeds the previous best results obtained in PbZr_0.95Ti_0.05O₃ thin film.     

Effects of annealing temperature on structural,optical, and electrical properties of antimony-doped tin oxide thin films

Antimony-doped tin oxide (ATO) films, approximately 320 nm in thickness, have been prepared by electron beam evaporation onto glass substrates. The films were annealed at temperatures between 400°C and 550°C in air and their structure and surface morphologies were observed by X-ray diffraction (XRD) and atomic force microscopy (AFM) after the different annealing treatments. XRD patterns of the ATO thin films as-deposited and annealed at 400°C showed that they were amorphous, but annealing beyond 400°C caused the films to become polycrystalline with tetragonal structure and orientated in the (1 1 0) direction. The grain size in the annealed films, obtained from the XRD analysis, was in the range 146–256 Å and this increased with the annealing temperature. The dislocation density, cell volume and strain were found to decrease gradually with increasing annealing temperature. Photoluminescence spectra revealed an intensive blue/violet peak at 420 nm, which increased gradually in height with annealing. It is suggested that an increase in the population of Sb⁺⁵ ions might be the reason for the enhancement of the blue/violet emission. The optical properties of the films were also investigated in the UV-visible-NIR region (300–1000 nm). The optical constants, namely the refractive index n and the extinction coefficient k in the visible region were calculated. The optical energy band gap, as determined by the dependence of the absorption coefficient on the photon energy at short wavelengths, was found to increase from 3.59 to 3.76 eV with annealing temperature.     

Recrystallization of amorphous ion-implanted silicon carbide after thermal annealing

Single crystals of 6H–SiC were implanted at room temperature with 4-MeV Au ions to a fluence of 10^15?cm^?2. Raman spectra showed that full amorphization was achieved. The recrystallization process was studied by micro-Raman spectrometry after isochronal thermal annealing between 700 and 1500°C. The spectra permitted the evolution upon annealing of Si–C bonds, and also of Si–Si and C–C bonds, to be followed. Amorphous phase relaxation takes place below 700°C; then recrystallization of the 6H polytype sets in at 700°C. At 900°C crystallites with different crystalline states are formed. Moreover, Raman spectra provide evidence of graphitic nanocluster formation at 1500°C.     

Nucleation of He bubbles in amorphous FeBSi alloy irradiated with He ions

It is interesting to investigate the formation of He bubbles in amorphous alloys because point defects do not exist in amorphous materials. In the present study, the microstructural evolution of amorphous Fe₇₉B₁₆Si₅ alloy, either irradiated with 5?keV He⁺ ions or implanted with 150?eV He⁺ ions without causing displacement damage, and then annealed at a high temperature, was investigated using transmission electron microscopy (TEM). Vacancy-type defects were formed in the amorphous alloy after irradiation with 5?keV He⁺ ions, and He bubbles formed during annealing the irradiated samples at high temperature. On the other hand, for samples implanted with 150?eV He⁺ ions, although He atoms are also trapped in the free volume, no He bubbles were observed during annealing the samples even up to 873?K. In conclusion, the formation of He bubbles is related to the formation and migration of vacancy-type defects even in amorphous alloys.     

Strain rate response of a Ni–Ti shape memory alloy after hydrogen charging

In this work, we investigate the susceptibility of Ni–Ti superelastic wires to the strain rates during tensile testing after hydrogen charging. Cathodic hydrogen charging is performed at a current density of 10?A/m² during 2–12?h in 0.9% NaCl solution and aged for 24?h at room temperature. Specimens underwent one cycle of loading-unloading reaching a stress value of 700 MPa. During loading, strain rates from 10^?6 to 5?×?10^?2?^?s^?1 have been achieved. After 8?h of hydrogen charging, an embrittlement has been detected in the tensile strain rate range of 10^?6 to 10^?4?s^?1. In contrast, no embrittlement has been detected for strain rates of 10^?3?s^?1 and higher. However, after 12?h of hydrogen charging and 24?h of annealing at room temperature, the embrittlement occurs in the beginning of the austenite-martensite transformation for all the studied strain rate values. These results show that for a range of critical amounts of diffused hydrogen, the embrittlement of the Ni–Ti superelastic alloy strongly depends on the strain rate during the tensile test. Moreover, it has been shown that this embrittlement occurs for low values of strain rates rather than the higher ones. This behaviour is attributed to the interaction between the diffused hydrogen and growth of the martensitic domain.     

High-efficiency single-layer organic light-emitting diode based on green fluorescent protein

We have fabricated a molecular organic light-emitting device comprising indium–tin oxide/a molecular organic layer/aluminum in which the organic layer is a green fluorescent protein. The device exhibits peak external quantum and power efficiencies of 8?±?0.2% and 13?±?0.7?lm?W?^??1 at a current of J?=?1.5?A?m^?2, respectively. In addition, the turn-on voltage is 2.5?V for 1?cd?m^?2 and the maximum luminance achieved is 1275?cd?m^?2. This good performance can be explained by the presence of singlet-excited states, leading to a high internal efficiency.     

Analytical derivation of the effective temperature for field-dependent hopping conductivity

The electric-field enhancement of hopping conductivity in amorphous solids can be described in terms of an effective temperature T _eff given by for hopping, at a temperature T and field F, within a uniform distribution of electronic states with localization length γ^?1. We have derived this expression analytically and find at low fields a value for C of 1/2^1/2, which is consistent with the F-independent value C = 0.67 previously obtained numerically for band-tail hopping. With increasing electric field, the decrease in C(F) matches the hopping transport characteristics in amorphous semiconductors (hydrogenated amorphous silicon and hydrogenated amorphous carbon nitride) better than previous numerical simulations.     

Dynamic properties of an ultrafine-grained Mg–Zn–Zr alloy

The effect of ultrafine-grained structure formation in Mg–Zn–Zr alloy ZK60 on its mechanical response was investigated at strain rates ranging from quasi-static to dynamic regimes. The study demonstrated that the strength characteristics of the material rise significantly with increasing strain rate, while its ductility is reduced. These effects are particularly pronounced in the dynamic loading regime, at strain rates in the (1?5)?×?10²?s^?1 range. In the ultrafine-grained alloy ZK60, the energy absorption per unit volume, W, is enhanced by grain refinement by a factor as high as eight for the highest strain rate of 5?×?10²?s^?1 investigated. The analysis is focused on the microstructure features that bring about the observed improvement of the tensile characteristics, as well as the deformation and fracture modes prevalent at different strain rates. The results obtained contribute to the exploration and understanding of dynamic behaviour of magnesium alloys.     

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.