Underlying mechanisms for unique elastic properties of nanocrystalline Au

In order to get an insight into the grain boundaries (GBs) in nanocrystalline (n-) metal, we prepared the high-density n-Au with ρ/ρ₀>99% by the gas-deposition method and carried out the vibrating reed measurements, where ρ/ρ₀ is the relative density referring to the bulk density. The strain amplitude dependence (SAMD) of the resonant frequency (f) and the internal friction (Q⁻¹) was measured for the strain () amplitude between 10⁻⁶ and 2×10⁻³ and for temperature between 5 and 300 K. No plastic deformations are detected for the present strain range, where f decreases for up to 10⁻⁴ and then turns to increase, showing saturation for between 10⁻⁴ and 2×10⁻³. The low temperature irradiation by 2 MeV electrons or 20 MeV protons causes an increase in the Young’s modulus at 6 K, which is surmised to reflect a modification of the anelastic process in the GB regions. In contrast, the SAMD of f is hardly modified by irradiation, suggesting that it is indicative of a collective motion of atoms in n-Au.     

D.c. properties of ZnO thin films prepared by r.f. magnetron sputtering

In the development of ZnO-based varistors the electrical properties of ZnO/Bi₂O₃ junctions and of the two individual oxides are being investigated. Following our recent work on a.c. conductivity in Al---ZnO---Al sandwich structures we currently report d.c. measurements. The structures were prepared by r.f. magnetron sputtering in an argon/oxygen mixture in the ratio 4:1. Capacitance-voltage data confirm that the Al/ZnO interface does not form a Schottky barrier and measurements of the dependence of capacitance on film thickness indicate that the relative permittivity of the films is approximately 9.7. With increasing voltage the current density changed from an ohmic to a power-law dependence with exponent n≈3. Furthermore measurements of current density as a function of reciprocal temperature showed a linear dependence above about 240 K, with a very low activation energy below this temperature consistent with a hopping process. The higher temperature results may be explained assuming a room-temperature electron concentration n₀ and space-charge-limited conductivity, dominated by traps exponentially distributed with energy E below the conduction band edge according to N = N₀exp(−E/kT_t), where k is Boltzmann's constant. Typical derived values of these parameters are: n₀ = 7.2 × 10¹⁶ m⁻³, N₀ = 1.31 × 10⁴⁵ J⁻¹ m⁻³ and T_t = 623 K. The total trap concentration and the electron mobility were estimated to be 1.13 × 10²⁵ m⁻³ and (5.7−13.1) ×10⁻³m²V⁻¹s⁻¹ respectively.     

Charge trapping in polymer diodes

We report studies focusing on the nature of trap states present in single layer ITO/poly(phenylene vinylene)/Al light emitting diodes. At high applied bias the IV characteristics from 11 to 290 K can be successfully modelled by space charge limited current (SCLC) theory with an exponential trap distribution, giving a trap density H_t of 4(±2) × 10¹⁷ cm⁻³, μ_p, between 10⁻⁶ and 5 × 10⁻⁸ cm² V⁻¹ s⁻¹ and a characteristic energy E_t of 0.15 eV at high temperatures. The transient conductance follows a power-law relationship with time whose decay rate decreases with decreasing temperature. This can be directly related to the emptying of the trap distribution found in the SCLC analysis. Due to variations in structure, conformation and environment, the polymer LUMO and HOMO density of states form Gaussian distributions of chain sites. The deep sites in the tail of the distributions are the observed traps for both positive and negative carriers. The same sites dominate the photo- and electroluminescence emission. This implies that the emissive layer in organic LED's should be made as structurally disordered as possible.     

Thermal conductivity of BaWO4 single crystal

Barium tungstate (BaWO₄) single crystal has been grown using Czochralski technique. It belongs to the scheelite structure, forming the space group I4₁/a at room temperature and the primitive cell contains two formular units. The thermal expansion, specific heat and thermal diffusivity were measured, and then the thermal conductivity was calculated. These results show that BaWO₄ possesses large anisotropic thermal expansion and its thermal expansion coefficients are _a = 1.10 × 10⁻⁵/K, _b = 1.08 × 10⁻⁵/K, and _c = 3.51 × 10⁻⁵/K in the temperature range from 303 to 1423 K. However, its thermal conductivity shows small anisotropic in the temperature range from 297 to 563 K and even displays isotropic at about 428 K. The calculated thermal conductivities are 2.59 and 2.73 W m⁻¹ K⁻¹ at room temperature, along [1 0 0] and [0 0 1] directions, respectively.     

Growth and spectroscopy of Dy doped in ZnWO4 crystal

Single-crystal ZnWO₄:Dy³⁺ was grown by Czochralski technique. The XRD, absorption spectra as well as fluorescence spectrum are investigated and the Judd–Ofelt intensity parameters Ω₂, Ω₄, Ω₆ are obtained to be 7.76 × 10⁻²⁰ cm², 0.57 × 10⁻²⁰ cm², 0.31 × 10⁻²⁰ cm², respectively. Calculated radiative transition rate, branching ratios and radiative lifetime for different transition levels of ZnWO₄:Dy³⁺ crystals are presented. Fluorescence lifetime of ⁴F_9/2 level is 158 μs and quantum efficiency is 66%.The most intense fluorescence line at 575 nm correlative with transition ⁴F_9/2 → ⁶H_13/2 is potentially for application of yellow lasers.     

Enhanced fluoride sorption by mechanochemically activated kaolinites

This study investigated the surface modification of photocatalyst and photodecomposition of formaldehyde from indoor pollution source. This study explored the feasibility of the application of the ultraviolet light emitting diode (UVLED) instead of the traditional ultraviolet (UV) lamp to treat the formaldehyde. The photocatalytic decomposition of formaldehyde at various initial concentrations was elucidated according to the Langmuir–Hinshelwood model. The reaction rate constant (k) and adsorption equilibrium constant (K_L) over 0.334 g silver titanium oxide photocatalyst (Ag/TiO₂) coated on glass sticks with 254 nm ultraviolet lamp (UVC), 365 nm ultraviolet lamp (UVA), and UVLED are 650 ppmv min⁻¹ and 2 × 10⁻⁴ ppmv⁻¹, 500 ppmv min⁻¹ and 1.04 × 10⁻⁴ ppmv⁻¹, and 600 ppmv min⁻¹ and 2.52 × 10⁻⁵ ppmv⁻¹, respectively. A comparison of the simulation results with the experimental data was also made, indicating good agreement. The magnitudes of energy effectiveness (E_e) are in the order of UVLED (0.6942 mg kW⁻¹ h⁻¹) > UVA (0.007 mg kW⁻¹ h⁻¹) > UVC (0.0053 mg kW⁻¹ h⁻¹). The E_e of UVLED is 131 times larger than that of UVC. The UVLED can save a lot of energy in comparison with the traditional UV lamps. Thus, this study showed the feasible and potential use of UVLED in photocatalysis.     

Magnetic and electrical characterization of heavily boron-doped diamond

For a heavily boron-doped diamond (BDD) film, temperature variations of the electrical conductivity σ and magnetic susceptibility χ are reported. The room temperature σ 143 (Ω-cm)⁻¹ corresponds to a carrier concentration 10³ ppm, and its temperature variation yields an activation energy E_a 28 meV from 140 to 300 K and E_a0.88 meV from 40 to 80 K. It is argued that larger boron doping leads to lower magnitudes of E_a. The χ vs. T data (1.8–350 K) fits the Curie–Weiss law, with the concentration of paramagnetic species 120 ppm and a diamagnetic susceptibility −0.4×10⁻⁶ emu/g Oe. The results obtained from the measurements of σ and χ are discussed and compared.     

Deep levels in GaInAs grown by molecular beam epitaxy and their concentration reduction with annealing treatment

X-ray diffraction (XRD), current–voltage (IV), capacitance–voltage (CV), deep-level transient Fourier spectroscopy (DLTFS) and isothermal transient spectroscopy (ITS) techniques are used to investigate the thermal annealing behaviour of three deep levels in Ga_0.986In_0.014As heavily doped with Si (6.8 × 10¹⁷ cm⁻³) grown by molecular beam epitaxy (MBE). The thermal annealing was performed at 625 °C, 650 °C, 675 °C, 700 °C and 750 °C for 5 min. XRD study shows good structural quality of the samples and yields an In composition of 1.4%. Two main electron traps are detected by DLTFS and ITS around 280 K, with activation energies of 0.58 eV and 0.57 eV, capture cross sections of 9 × 10⁻¹⁵ cm² and 8.6 × 10⁻¹⁴ cm² and densities of 2.8 × 10¹⁶ cm⁻³ and 9.6 × 10¹⁵ cm⁻³, respectively. They appear overlapped and as a single peak, which divides into two smaller peaks after annealing at 625 °C for 5 min.

Annealing at higher temperatures further reduces the trap concentrations. A secondary electron trap is found at 150 K with an activation energy of 0.274 eV, a capture cross section of 8.64 × 10⁻¹⁵ cm² and a density of 1.38 × 10¹⁵ cm⁻³. The concentration of this trap level is also decreased by thermal annealing.     

Ideal 4H-SiC pn junction and its characteristic shunt

The results of a study of forward current–voltage characteristics of 4H-SiC pn structures produced by various methods are presented in this paper. In all pn structure types a forward current was identified which is consistent with the model of recombination in the space charge region via a deep level: J=J_o exp(qU/nkT), where J_o=J_o^* exp[−E_a/(kT)] (or J=J_o^* exp[(qU−2E_a)/nkT)]) with the ideality factor n=2. The values of the other parameters are as follows: J_o(293 K)(10⁻²⁴–10⁻²⁵) A cm⁻², E_a≈1.75 eV, J_o^*(10⁵–10⁶) A cm⁻². In the absence of other leaks, this current represents a natural (intrinsic) shunt for the diffusion current at low biases and determines the value of the injection coefficient of a pn junction. In many of the pn structures the forward current, while showing an exponential dependence on voltage and temperature, was significantly higher than the current described above. The ideality factor, in some instances, was close to unity. A situation with such characteristics can be considered as an ideal extrinsic (due to nonuniformities) barrier type shunt. The potential barrier height of this shunt is ≈1.4 eV. This sort of shunt is characteristic of SiC; earlier a similar object, presumably of the same origin, was identified in 6H-SiC pn structures.     

Influence of Si doping level on the Raman and IR reflectivity spectra and optical absorption spectrum of GaN

The optical absorption (hν) and Raman and Infra Red (IR) spectra of Si doped GaN layers deposited on sapphire through buffer layers have been recorded for electron concentrations from 5×10¹⁷ to 5×10¹⁹ cm⁻³. The (hν) values deduced from photothermal deflection spectroscopy (0.5–3.5 eV) and IR absorption (0.15–0.5 eV) vary between 50 and 10⁴ cm⁻¹ showing doping dependant free electron absorption at low energy, doping independant band gap at high energy, and slowly doping dependant defect absorption in the medium energy range. In our micro Raman geometry, maxima appear or can be deduced near the frequency expected for either the A₁(LO⁻) or the A₁(LO⁺) modes split from the A₁(LO) mode by plasmon phonon interaction. There is a large systematic evolution in the expected way for the IR reflectivity.     

Anelasticity of nanocrystalline metals

Nanocrystalline (n-) materials posses a characteristic ultrafine structure where small crystallites less than 100 nm in diameter are connected by highly disordered grain boundaries (GBs). Although the recent mechanical tests on the high-density n-metals suggest that the dynamic modulus is close to the polycrystalline metals, we have found a large anelastic strain comparable with the elastic strain in the quasi-static tests. The estimated activation energy as low as 0.2 eV suggests that certain cooperative motions of many atoms in the GBs are responsible for this large anelastic strain. The strain amplitude dependence (SAMD) in the resonant frequency, f, was measured by the vibrating reed technique, where f decreased by about 1% with increasing strain, , up to 10⁻⁴ and then turned to increase showing the saturation at =10⁻⁴–5×10⁻³. This strange SAMD in f showed no change by the low-temperature irradiation, in contrast, f showed a large increase due to the accumulation of the point defects probably in the GBs during the irradiation. We surmise that the (an)elastic properties of n-metals are governed by the several processes not only in the GBs but also in the crystallites.     

Dielectric properties, ac conductivity and thermal behaviour of flux grown cadmium titanate crystals

The dependence of loss tangent (tan δ) and both real and imaginary parts of the dielectric constant (′ and ″) on temperature in the range 298–923 K and frequency in the range 10³–10⁶ Hz for flux grown CdTiO₃ single crystals is reported. The ln σ_ac versus T plots suggest the conduction mechanism to be ionic hopping conduction. From ln σ_ac versus frequency curves, it can be seen that the slope decreases with the rise in temperature, suggesting that the ionic hopping conduction diminishes with the rise in temperature. The activation energy at various fixed frequencies is calculated from the slope of the graph between ln σ_ac versus 1/T (×10³ K⁻¹). Thermal behaviour of flux grown CdTiO₃ crystals using thermoanalytical techniques including TG, DTA and DTG is discussed. Thermal analysis suggests decomposition of CdTiO₃ in the temperature interval of 1386–1693 K leading to the formation of TiO₂ as the final product. Results obtained on application of TG based models viz. Horowitz–Metzger, Coats–Redfern and Piloyan–Novikova are reported. The results of kinetics of thermal decomposition suggest contracting cylinder model as the one that is relevant to the decomposition of CdTiO₃. The kinetic parameters viz. the order of reaction, activation energy, frequency factor, and entropy of activation using the above mentioned models are computed.     

A kinetic model for the degradation of benzothiazole by Fe3+-photo-assisted Fenton process in a completely mixed batch reactor

The degradation of benzothiazole in aqueous solution by a photo-assisted Fenton reaction has been studied in a batch reactor in the pH range 2.0–3.2 and for H₂O₂ and Fe(III) concentrations respectively between 1.0×10⁻³–1.5×10⁻¹ and 1.0×10⁻⁶–4.0×10⁻⁶ M.

A kinetic model has been developed to predict the decay of benzothiazole at varying reaction conditions. The use of kinetic constants from the literature in the model allows to simulate the system behavior by taking into account the influence of pH, hydrogen peroxide, Fe(III) and sulfate concentrations and the ionic strength.     

Highly conducting indium tin oxide (ITO) thin films deposited by pulsed laser ablation

Highly conducting and transparent indium tin oxide (ITO) thin films were prepared on SiO₂ glass and silicon substrates by pulsed laser ablation (PLA) from a 90 wt.% In₂O₃-10 wt.% SnO₂ sintered ceramic target. The growths of ITO films under different oxygen pressures (P_O2) ranging from 1×10⁻⁴–5×10⁻² Torr at low substrate temperatures (T_s) between room temperature (RT) and 200°C were investigated. The opto-electrical properties of the films were found to be strongly dependent on the P_O2 during the film deposition. Under a P_O2 of 1×10⁻² Torr, ITO films with low resistivity of 5.35×10⁻⁴ and 1.75×10⁻⁴ Ω cm were obtained at RT (25°C) and 200°C, respectively. The films exhibited high carrier density and reasonably high Hall mobility at the optimal P_O2 region of 1×10⁻² to 1.5×10⁻² Torr. Optical transmittance in excess of 87% in the visible region of the solar spectrum was displayed by the films deposited at Po₂≥1×10⁻² Torr and it was significantly reduced as the P_O2 decreases.     

Elastic stiffness and thermal expansion coefficients of various refractory silicides and silicon nitride films

The elestic stiffness parameter E_f/(1−ν_f) and the thermal expansion coefficient _f were obtained for four different silicides (TiSi₂, TaSi₂, MoSi₂ and WSi₂) and for two different nitrides (chemically vapor-deposited Nitrox Si₃N₄ and r.f. plasma SiN) from stress-temperature measurements on identical films deposited on two different substrate materials. The values determined for _f and E_f/(1−ν_f) were quite similar for all silicides and averaged 15 ppm °C⁻¹ and 1.1 × 10¹² dyncm⁻² respectively. The thermal mismatch of these silicides is such that, once safely formed, the silicide film should be able to withstand high temperature processing steps without cracking. For the nitrides the values were essentially the same (approximately 1.5 ppm°C^-1), although the larger value of E_f/(1−ν_f) chemically vapor-deposited Si₃N₄ film (3.7 × 10¹² as opposed to 1.1 × 10¹² dyn cm^-2) indicates that it is somewhat stiffer than the SiN film.     

First application of calorimetric low-temperature detectors in accelerator mass spectrometry

For the first time, calorimetric low-temperature detectors were applied in accelerator mass spectrometry, a well-known method for determination of very small isotope ratios with high sensitivity. The aim of the experiment was to determine with high accuracy the isotope ratio of ²³⁶U/²³⁸U for several samples of natural uranium, ²³⁶U being known as a sensitive monitor for neutron flux. Measurements were performed at the VERA tandem accelerator at Vienna, Austria. The detectors consist of sapphire absorbers and superconducting transition edge thermometers operated at T≈ 1.5 K. The relative energy resolution obtained for 17.39 MeV ²³⁸U is ΔE/E=4–9×10⁻³, depending on the experimental conditions. This performance enabled to substantially reduce background from neighbouring isotopes and to increase the detection efficiency. Due to the high sensitivity achieved, a value of ²³⁶U/²³⁸U=6.5×10⁻¹² could be obtained, representing the smallest ²³⁶U/²³⁸U ratio measured until now.     

The optical and electrical properties of copper indium di-selenide thin films

Thin films of copper indium di-selenide (CIS) with a wide range of compositions near stoichiometry have been formed on glass substrates in vacuum by the stacked elemental layer (SEL) deposition technique. The compositional and optical properties of the films have been measured by proton-induced X-ray emission (PIXE) and spectrophotometry (photon wavelength range of 300–2500 nm), respectively. Electrical conductivity (σ), charge-carrier concentration (n), and Hall mobility (μ_H) were measured at temperatures ranging from 143 to 400 K. It was found that more indium-rich films have higher energy gaps than less indium-rich ones while more Cu-rich films have lower energy gaps than less Cu-rich films. The sub-bandgap absorption of photons is minimum in the samples having Cu/In ≈ 1 and it again decreases, as Cu/In ratio becomes less than 0.60. Indium-rich films show n-type conductivities while near-stoichiometric and copper-rich films have p-type conductivities. At 300 K σ, n and μ_H of the films vary from 2.15 × 10⁻³ to 1.60 × 10⁻¹ (Ω cm)⁻¹, 2.28 × 10¹⁵ to 5.74 × 10¹⁷ cm⁻³ and 1.74 to 5.88 cm² (V s)⁻¹, respectively, and are dependent on the composition of the films. All the films were found to be non-degenerate. The ionization energies for acceptors and donors vary between 12 and 24, and 3 and 8 meV, respectively, and they are correlated well with the Cu/In ratios. The crystallites of the films were found to be partially depleted in charge carriers.     

Dielectric properties of AlN films prepared by laser-induced chemical vapour deposition

The dielectric properties and electrical conductivity of AlN films deposited by laser-induced chemical vapour deposition (LCVD) are studied for a range of growth conditions. The static dielectric constant is 8.0 ± 0.2 over the frequency range 10²−10⁷ Hz and breakdown electric fields better than 10⁶ V cm⁻¹ are found for all films grown at temperatures above 130°C. The resistivity of the films grown under optimum conditions (substrate temperature above 170°C, NH₃/TMA flow rate ratio greater than 300 and a deposition pressure of 1–2 Torr) is about 10¹⁴ Ω cm and two conduction mechanisms can be identified. At low fields, F < 5 × 10⁵ V cm⁻¹ and conductivity is ohmic with a temperature dependence showing a thermal activation energy of 50–100 meV, compatible with the presumed shallow donor-like states. At high fields, F > 1 × 10⁶ V cm⁻¹, a Poole-Frenkel (field-induced emission) process dominates, with electrons activated from traps at about 0.7–1.2 eV below the conduction band edge. A trap in this depth region is well-known in AlN. At fields between 4 and 7 × 10⁵ V cm⁻¹ both conduction paths contribute significantly. The degradation of properties under non-ideal growth conditions of low temperature or low precursor V/III ratio is described.     

Doping effects on the optical properties of evaporated a-Si:H films

Thin films of a-Si:H are deposited on substrates at 300°C by a conventional thermal evaporation technique. The electrical conductivity of these films is modified by the addition of antimony giving n-type films. The optical properties of the films are investigated using spectrophotometric measurements of the transmittance and reflectance in the wavelength range 200–3000 nm. Both the refractive index n and the absorption coefficient increase when the Sb content is increased. The absorption edge shifts to lower energies for doped films. The optical gap E_g is evaluated using three different plots for comparison, namely; (hν)^1/2, (/hν)^1/2 and (hν)^1/3. The value of E_g decreases with doping for the three expressions. The Urbach parameter E₀ is calculated and found to increase with doping from 74 meV for the undoped film to 183 meV for concentrations of 9.4 at.% Sb.