Determination of cadmium at ultra-trace levels by CPE–molecular fluorescence combined methodology

A highly sensitive micelle-mediated extraction methodology for the preconcentration and determination of trace levels of cadmium by molecular fluorescence has been developed. Metal was complexed with o-phenanthroline (o-phen) and eosin (eo) at pH 7.6 in buffer Tris medium and quantitatively extracted into a small volume of surfactant-rich phase of PONPE 7.5 after centrifugating. The chemical variables affecting cloud point extraction (CPE) were evaluated and optimized. The RSD for six replicates of cadmium determinations at 0.84 μg L⁻¹ level was 1.17%. The linearity range using the preconcentration system was between 2.79 × 10⁻³ μg L⁻¹ and 2.81 μg L⁻¹ with a correlation coefficient of 0.99. Under the optimal conditions, it obtained a LOD of 8.38 × 10⁻⁴ μg L⁻¹ and LOQ of 2.79 × 10⁻³ μg L⁻¹. The method presented good sensitivity and selectivity and was applied to the determination of trace amounts of cadmium in commercially bottled mineral water, tap water and water well samples with satisfactory results. The proposed method is an innovative application of CPE-luminescence to metal analysis comparable in sensitivity and accuracy with atomic spectroscopies.     

A study of very thin DLC foils as a gas barrier

Measurements of vacuum tightness and mechanical strength of diamond-like carbon (DLC) foils in the thickness range of 1–7 μg cm⁻² have been performed with a purpose to evaluate suitability of foils as a gas barrier. Hydrogen and argon at pressures from 10⁻² Pa to 20 kPa were used as test gases. The permeation rate specified as conductance density was found for the best sample of self-supporting foil to be around 1.5×10⁻³ l and 3.3×10⁻⁴ l s⁻¹ cm⁻² for H₂ and Ar, respectively. Conductance density of the same foils mounted on the frames with a mesh along the apertures as support was about twice higher than that for the self-supporting ones, likely due to the mechanical imperfections of the foil assemblies of the first ones. On the other hand, mesh-supported foils as thin as 3 μg cm⁻² and of 5 mm in diameter were withstanding the pressure of up to 18 kPa, while self-supporting foils of the same thickness ruptured at around 1.2 kPa. There was no observed relation between thickness of the foil and its mechanical properties and permeation rate. This suggests that rather tears and pinholes present in foils are the limiting factors of the foil–vacuum tightness and strength. Results obtained in the studies, presented in this work, demonstrate the ability of very thin DLC to isolate a high vacuum beam line from a gas cell in a variety of applications and ability to withstand the gas pressure relevant, in particular, to some gas-filled ionization chambers.     

Fast silicon p doped low temperature bolometer

The construction of a fast silicon p doped low temperature bolometer is described. It is a 5 × 5 × 0.3 mm Si <100 > n-type chip whose surface has been implanted with p doses of the order of 10¹⁸cm⁻³. The bolometer has a response time better than 1 μs, a responsivity of 10⁴VW⁻¹ and a NEP of .     

Electrical and optical properties of 12CaO·7Al2O3 electride doped indium tin oxide thin film deposited by RF magnetron co-sputtering

12CaO·7Al₂O₃ electride (C12A7:e⁻) doped indium tin oxide (ITO) (ITO:C12A7:e⁻) thin films were fabricated on a glass substrate by an RF magnetron co-sputtering system with increasing number of C12A7:e⁻ chips (from 1 to 7) and at various oxygen partial pressure ratios. The optical transmittance of the ITO:C12A7:e⁻ thin film was higher than 70% in the visible wavelength region. In the electrical properties of the thin film, a decrease of the carrier concentration from 2.6 × 10²⁰ cm^− 3 to 2.1 × 10¹⁸ cm^− 3 and increase of the resistivity from 1.4 × 10^− 3 Ω cm to 4.1 × 10^− 1 Ω cm were observed with increasing number of C12A7:e⁻ chips and oxygen partial pressure ratios. It was also observed that the Hall mobility was decreased from 17.27 cm²·V^− 1·s^− 1 to 5.13 cm²·V^− 1·s^− 1. The work function of the ITO thin film was reduced by doping it with C12A7:e⁻.     

Biosensor enhanced by glucose oxidase biomimetic membrane containing the platinum and silica nanoparticles

In this paper, glucose biosensor is fabricated with immobilization of glucose oxidase (GOx) in platinum and silica sol. The glucose biosensor combined with Pt and SiO₂ nanoparticles could make full use of the properties of nanoparticles. A set of experimental results indicates that the current response for the enzyme electrode containing platinum and silica nanoparticles increases from 0.32 µA cm^− 2 to 33 µA cm^− 2 in the solution of 10 mM β-D-glucose. The linear range is 3 × 10^− 5 to 3.8 × 10^− 3 M with a detection limit of 2 × 10^− 5 M at 3σ. The effects of the various volume ratios of Pt and SiO₂ sols with respect to the current response and the stability of the enzyme electrodes are studied.     

Carbon micro-ribbon and strip polarimeter targets for the AGS and RHIC

Current techniques used to produce carbon micro-ribbon targets 5 μm wide×3.7–4.5 μg/cm²×25-mm long will be described. Developmental emphasis was to provide nearly identical micro-ribbons with the minimum number of atoms per unit of length, and to position them within ±0.5 mm of the desired location on C-shaped frames.The foil strip targets to be described were 200–600 μm wide×3.7–4.5 μg/cm²×51 mm long. These were produced from 25-mm-wide carbon film deposits that were scribed using a jig prior to dissolving the betaine/sucrose release agent under ethanol.Both types of targets required methods and devices that differed significantly from those reported previously for substrate texturing, masking, vacuum deposition, releasing from the substrate, and mounting. Sets of 12–24 of the targets have been made for the 2006 run period at BNL.     

Three-dimensional measurement of ice crystals in frozen dilute solution

A Micro-Slicer Image Processing System (MSIPS) has been applied to observe the ice crystal structures formed in frozen dilute solutions. Several characteristic parameters were also proposed to investigate the three-dimensional (3-D) morphology and distribution of ice crystals, based on their reconstructed images obtained by multi-slicing a frozen sample with the thickness of 5 μm. The values of characteristic parameters were determined for the sample images with the dimension of 530×700×1000 μm. The 3-D morphology of ice crystals was found to be a bundle of continuous or dendrite columns at any freezing condition. The equivalent diameter of ice crystals were in the range of 73–169 μm, and decreased exponentially with increasing freezing rate at the copper cooling plate temperature of −20 to −80 °C. At the T_cp −40 °C, the volumes of ice crystals were in the range of 4.6×10⁴ μm³ to 3.3×10⁷ μm³, and 36 ice columns were counted in the 3-D image.     

Microcystins in groundwater wells and their accumulation in vegetable plants irrigated with contaminated waters in Saudi Arabia

The present study describes the presence of toxic cyanobacteria and microcystin (MCYST) concentrations in groundwater wells and tissues of vegetable plants irrigated with well waters in Asir region, southwest of Saudi Arabia. The results revealed the presence of cyanobacteria in all groundwater wells with a dominance of Oscillatoria limentica. This species was found to produce MCYSTs at a concentration of 336 μg g⁻¹ as determined by enzyme-linked immunosorbent assay (ELISA). HPLC chromatogram for the methanolic extract of this species showed one main peak corresponding to MCYST-YR. MCYSTs were also detected in well waters at concentrations (0.3–1.8 μg L⁻¹), exceeding the WHO guideline level (1 μg L⁻¹) in some wells. All vegetable plants collected during the present study were found to accumulate MCYSTs in their leaves and roots at concentrations ranged from 0.07 to 1.2 μg g⁻¹ fresh weight. The study suggests that ground waters and vegetable plants should be continuously monitored for the presence of MCYSTs to protect the public against the exposure to such potent hepatotoxins.     

Bacterial biosurfactant in enhancing solubility and metabolism of petroleum hydrocarbons

Biosurfactant can make hydrocarbon complexes more mobile with the potential use in oil recovery, pumping of crude oil and in bioremediation of crude oil contaminant. In the investigation, bacterial isolates capable of utilizing poly-cyclic aromatic hydrocarbons like phenanthrene, pyrene and fluorene were used. A gradual decrease of the supplemented hydrocarbons in the culture medium was observed with corresponding increase in bacterial biomass and protein. The medium having the combined application of fluorine and phenanthrene caused better biosurfactant production (0.45 g l⁻¹) and (0.38 g l⁻¹) by Pseudomonas aeruginosa strains MTCC7815 and MTCC7814. The biosurfactant from MTCC7815 (41.0 μg ml⁻¹) and MTCC7812 (26 μg ml⁻¹) exhibited higher solubilization of pyrene; whereas, MTCC8165 caused higher solubilization of phenanthrene; and that of MTCC7812 (24.45 μg ml⁻¹) and MTCC8163 (24.49 μg ml⁻¹) caused more solubilzation of fluorene. Higher solubilization of pyrene and fluorene by the biosurfactant of MTCC7815 and MTCC7812, respectively enhanced their metabolism causing sustained growth. Biosurfactants were found to be lipopeptide and protein–starch–lipid complex in nature and they could reduce the surface tension of pure water (72 mN m⁻¹) to 35 mN m⁻¹. The critical micelle concentration (CMC) was also lower than the chemical surfactant sodium dodecyl sulphate (SDS). They differed in quantity and structure. The predominant rhamnolipids present in biosurfactants were Rha–C₈–C₁₀ and Rha–C₁₀–C₈.     

The bromination kinetics of phenolic compounds in aqueous solution

The purpose of this study was to investigate the bromination kinetics of selected phenolic compounds in aqueous solutions over the pH range of 5–11. The experiment results indicated that the reaction of hypobromous acid with the phenoxide ions controlled the overall reaction rate, whereas the reaction between hypobromite ion and the phenoxide ions and the reaction between hypobromous acid and the undissociated phenolic species were considered to be negligible respectively in the pH range of 7–9. The apparent second-order rate constants of the reaction of hypobromous acid with the phenoxide ions ranged from 7.9 × 10⁶ M⁻¹ s⁻¹ for 3-chlorophenol to 6.5 × 10⁸ M⁻¹ s⁻¹ for 3-methoxyphenol, respectively. The Hammett correlation could be successfully used to estimate the reactivity of bromine with substituted phenols and the linear regression was log(k₂) = −2.85б + 8.00. The rate constants of the reaction of bromine with phenol-like organic compounds were about three orders of magnitude higher than with chlorine and two to three orders of magnitude lower than with ozone.     

Meteorological variations of PM2.5/PM10 concentrations and particle-associated polycyclic aromatic hydrocarbons in the atmospheric environment of Zonguldak, Turkey

Airborne particulate matter (PM_2.5 and PM₁₀) concentrations were measured in Zonguldak, Turkey from January to December 2007, using dichotomous Partisol 2025 sampler. Collected particulate matter was analyzed for 14 selected polycyclic aromatic hydrocarbons (PAHs) by high-performance liquid chromatography with fluorescence detection (HPLC-FL). The seasonal variations of PM_2.5 and PM₁₀ concentrations were investigated together with their relationships with meteorological parameters. The maximum daily concentrations of PM_2.5 and PM₁₀ reached 83.3 μg m⁻³ and 116.7 μg m⁻³ in winter, whereas in summer, they reached 32.4 μg m⁻³ and 66.7 μg m⁻³, respectively. Total concentration of PM₁₀-associated PAHs reached 492.4 ng m⁻³ in winter and 26.0 ng m⁻³ in summer times. The multiple regression analysis was performed to predict total PM_2.5- and PM₁₀-associated PAHs and benzo(a)pyrene-equivalent (BaPE) concentrations with respect to meteorological parameters and particulate mass concentrations with the determination coefficients (R²) of 0.811, 0.805 and 0.778, respectively. The measured mean values of concentrations of total PM_2.5- and PM₁₀-associated PAHs were found to be 88.4 ng m⁻³ and 93.7 ng m⁻³ while their predicted mean values were found to be 92.5 ng m⁻³ and 98.2 ng m⁻³, respectively. In addition, observed and predicted mean concentration values of PM_2.5-BaPE were found to be 14.1 ng m⁻³ and 14.6 ng m⁻³. The close annual mean concentrations of measured and predicted total particulate related PAHs imply that the models can be reliably used for future predictions of particulate related PAHs in urban atmospheres especially where fossil fuels are mainly used for heating.     

Fluorescence and nonlinear optical properties of non-aggregating hexadeca-substituted phthalocyanine

Highly non-aggregating hexadeca-substituted phthalocyanine (Pc) complexes were prepared and their fluorescence and nonlinear optical properties were studied. Three visible fluorescence bands were observed when the Pc complexes were excited at 355 nm and found to be concentration dependent. They are attributed to the optical transitions S₂ → S₀ at 415 nm, T₂ → T₁ at 630 nm, and S₂ → S₁ at 755 nm. Nonlinear absorptive and refractive effects were measured with the help of Z-scan technique. Saturation absorption was observed at 632.8 nm where the nonlinear absorption coefficient is found to be very large (β = −2.8 × 10⁻² cm/W) and the refractive nonlinear coefficient γ = −9.5 × 10⁻¹¹ cm²/W. In the transparency domain at 532 nm, reverse absorption saturation is observed and β and γ are found to be 17.5 and 15.5 times smaller, respectively. Optical limiting performances are measured in the absorption and transparency domains. Purely refractive-based optical limiting at 632.8 nm is found to have a threshold of 0.16 kW/cm², lower than the reverse absorption saturation and refractive-based optical limiting of 0.90 kW/cm² at 532 nm.     

Growth, thermal and spectroscopic characteristics of LiNd(WO4)2 crystal

Crystal growth, thermal and optical characteristics of LiNd(WO₄)₂ crystal have been investigated. The LiNd(WO₄)₂ crystal up to Ø15 × 32 mm³ has been grown by Czochralski technique. The hardness is about 5.0 Mohs’ scale. The specific heat at 50 °C is 0.42 J g⁻¹ K⁻¹. The thermal expansion coefficient for c- and a- axes is 1.107 × 10⁻⁵ and 2.104 × 10⁻⁵ K⁻¹, respectively. The absorption and fluorescence spectra and the fluorescence decay curve of LiNd(WO₄)₂ crystal were measured at room temperature. Some spectroscopic parameters such as the intensity parameters, the spontaneous transition probabilities, the fluorescence branching ratios, the radiative lifetimes and emission cross sections were estimated.     

Microstructure and superplasticity of Mg–Al–Ca electromagnetic casting alloys after hot extrusion

Microstructure and superplastic properties of the plates extruded from the Ca containing Mg alloy (1 wt.% Ca–AZ31) billets fabricated by electromagnetic casting (EMC) without and with electromagnetic stirring (EMS) were examined. The linear intercept grain sizes of the extruded materials were 3.7 μm and 2.1 μm, respectively. The material extruded from the EMC + EMS billet exhibited good superplasticity at low temperatures as well as at high strain rates, including the tensile elongations of 370% at 1 × 10⁻³ s⁻¹, −523 K and 550% at 1 × 10⁻² s⁻¹, −673 K. These values largely exceeded those of the AZ31 alloys with the similar grain sizes. The superior superplasticity of the extruded EMC + EMS billet could be attributed to fine grains and high grain stability at elevated temperatures by the presence of finely dispersed particles of thermally stable (Al,Mg)₂Ca phase. The constitutive equations were developed for describing the high-temperature deformation behavior of the fine-grained 1 wt.% Ca–AZ31 alloys with different grain sizes in wide range of temperature and strain rate.     

Kapitza resistance and thermal conductivity of Kapton in superfluid helium

We have determined simultaneously the Kapitza resistance, R_K, and the thermal conductivity, κ, of Kapton HN sheets at superfluid helium temperature in the range of 1.4–2.0 K. Five sheets of Kapton with varying thickness from 14 to 130 μm, have been tested. Steady-state measurement of the temperature difference across each sheet as a function of heat flux is achieved. For small temperature difference (10–30 mK) and heat flux density smaller than 30 W m⁻², the total thermal resistance of the sheet is determined as a function of sheet thickness and bath temperature. Our method determines with good accuracy the Kapitza resistance, R_K=(10540±444)T⁻³×10⁻⁶ K m² W⁻¹, and the thermal conductivity, κ=[(2.28±0.54)+(2.40±0.32)×T]×10⁻³ W m⁻¹ K⁻¹. Result obtained for the thermal conductivity is in good agreement with data found in literature and the Kapitza resistance’s evolution with temperature follows the theoretical cubic law.     

Effect of the adsorbate (Bromacil) equilibrium concentration in water on its adsorption on powdered activated carbon. Part 1. Equilibrium parameters

This study was carried out to investigate the adsorption equilibrium and kinetics of a pesticide of the uracil group on powdered activated carbon (PAC). The experiments were conducted at a wide range of initial pesticide concentrations (5 μg L⁻¹ to 500 μg L⁻¹ at pH 7.8), corresponding to equilibrium concentrations of less than 0.1 μg L⁻¹ for the weakest, which is compatible with the tolerance limits of drinking water. Such a very broad range of initial solute concentrations resulting powdered activated carbon (PAC) concentrations (0.1–5 mg L⁻¹) is the main particularity of our study. The application of several monosolute equilibrium models (two, three or more parameters) has generally shown that Bromacil adsorption is probably effective on two types of sites. High reactivity sites (K_L  10³ L mg⁻¹) which are 10–20 less present in a carbon surface than lower reactivity sites (K_L  10 L mg⁻¹), according to the q_m values calculated by two- or three-parameter models. The maximum capacity of the studied powdered activated carbon (PAC), corresponding to monolayer adsorption, compared to the Bromacil molecule surface, would be between 170 mg g⁻¹ and 190 mg g⁻¹. This theoretical value is very close to the experimental q_m values obtained when using linearized forms of Langmuir, Tóth and Fritz–Schluender models.     

Comprehensive characterization of hydride VPE grown GaN layers and templates

GaN community has recently recognized that it is imperative that the extended, and point defects in GaN and related materials, and the mechanisms for their formation are understood. This is a first and an important step, which must be followed by defect reduction before full implementation of this material and its allied binaries/ternaries in devices. This review is based on a recent concerted effort to establish benchmarks as far as defects are concerned, and identify the basic issues involved. Samples were analyzed for extended defects by TEM and chemical etches, for polarity by electric force microscopy and convergent beam electron diffraction (CBED), for point defects by DLTS, for optical quality and deep defects by photoluminescence (PL), for vacancies by positron annihilation, for donor and acceptor like states within the gap by ODMR and EPR, and for carrier transport targeted for defects and impurities by variable temperature and magnetic field-dependent Hall measurements.Hydride VPE samples grown at Lincoln Laboratories with 1.5, 5.5 and 55 μm thicknesses were investigated for defects by TEM, and their polarity was found to be Ga-polarity, as expected, by CBDE combined with simulations. The density of misfit dislocations at the substrate/EPI interface was determined to be on the order of 10¹³ cm⁻² based on high-resolution electron microscopy images. The threading dislocation density decreased gradually with distance from the interface, reaching a value of about 10⁸ cm⁻² at the surface of a 55 μm film. A 200 μm thick laser separated and free-standing HVPE grown GaN template grown at Samsung was also characterized similarly. The free surface and substrate sides were confirmed to be Ga- and N-polarity, respectively. The density of dislocations near the N-face was determined to be, in order, (3±1)×10⁷ and (4±1)×10⁷ by cross-sectional TEM and plan-view TEM, respectively. Identical observations on the Ga-face revealed the defect concentration to be less than 1×10⁷ cm⁻² by plan-view TEM and 5×10⁵ cm⁻² by cross-sectional TEM.Defects in a 10 μm thick GaN layer grown by HVPE at Lincoln Laboratory have been investigated by photo electrochemical (PEC) etching, and by wet etching in hot H₃PO₄ acid and molten KOH. Threading vertical wires (i.e. whiskers) and hexagonal-shaped etch pits are formed on the etched sample surfaces by PEC and wet etching, respectively. Using atomic force microscopy, one finds the density of “whisker-like” features to be 2×10⁹ cm⁻², the same value found for the etch-pit density on samples etched with both H₃PO₄ and molten KOH. Values agree well with TEM results.A free standing GaN template has been characterized for its structural and optical properties using X-ray diffraction, defect delineation etch followed by imaging with atomic force microscopy (AFM). The Ga-face and the N-face of the c-plane GaN exhibited a wide variation in terms of the defect density. The defect concentrations on Ga- and N-faces were about 5×10⁵ cm⁻² for the former and about 1×10⁷ cm⁻² for the latter, again in good agreement with TEM results mentioned above.High resolution X-ray rocking curves (omega scans) were measured. The [0 0 2] symmetric and [1 0 4] asymmetric peaks in 10 μm thick HVPE films had FWHM values between 5.8 and 7.9 arcmin, and 3.9 and 5.2 arcmin, respectively. The Samsung template investigated had wide diffraction peaks (20.6 and 24 arcmin for [0 0 2] and [1 0 4] diffractions, respectively) on the Ga-face, similar for the N-face, when a 2 mm slit size was used. When the slit size was reduced to 0.02 mm, the Ga- and N-face [0 0 2] peaks reduced to 69 and 160 arcsec. A bowing radius of 1.2 m was calculated to account for increased broadening with wider slits.In the HVPE layer studied, SIMS investigations indicate that both O and Si concentrations drop rapidly away from the surface into the sample — mainly due in part to the artifact of the technique and in part due to condensates on the surface of the sample, down to about 10¹⁷ cm⁻³ for Si and high 10¹⁶ cm⁻³ for O. The Ga-face profile in the Samsung template indicated levels below mid-10¹⁶ cm⁻³ for all three of the impurities. The picture is different for the N-side, however, as it was juxtaposed to the substrate during growth and was mechanically polished after laser separation. The impurity concentration on this face, depending on the species, is some 1–3 orders of magnitude higher than the case for the Ga-face.Transport properties as a function of the layer thickness, ranging from about 1 to 68 μm, while all the other parameters being the same, as they evolve from the sapphire/GaN interface and up were determined in epitaxial layers. A strong dependence on distance from the interface was observed with the averaged mobility figures increasing from 190 cm²/V s in the 5 μm film to 740 cm²/V s in the 68 μm film. The preliminary differential Hall measurements indicate that the mobility at the surface of the thick layer is about 1200 cm²/V s. Electron mobilities in free-standing template were 1425 cm²/V s at T=273 K and 7385 cm²/V s at T=48.2 K. By using the most recent unscreened acoustic deformation potential and allowing only the acceptor concentration to vary (2.4×10¹⁵ cm⁻³ for the best fit), one obtains an excellent fit to the measured mobility in the temperature range of 30–273 K by using an iterative BTE method. In addition, an excellent fit for the temperature-dependent electron concentration was also obtained utilizing the acceptor concentration determined from the fit to the Hall data, and the charge balance equation. This led to a donor concentration of 1.6×10¹⁶ cm⁻³, and activation energy of 26 meV, the latter being the highest reported in the literature for GaN.In the free-standing template, the Γ₅ and Γ₆ free excitons were identified from emission measurements by utilizing polarization geometries where the E field is perpendicular to the c-axis, favoring the Γ₅ exciton, and E field parallel to the c-axis (incident beam from the edge of the wafer) favoring the Γ₆ exciton. Focusing on the defect region of the PL spectrum, the N-face of the sample exhibited the usual yellow line. However, the Ga-face exhibited a broad band encompassing both yellow and green bands. The yellow luminescence in the free-standing template is weak and can be easily saturated. In contrast, the green luminescence is dominant and is attributed to the isolated defect involving gallium vacancy, whereas the yellow luminescence is related to the same defect bound to dislocation or surface-bound structural defect.Deep centers have been characterized by DLTS in HVPE-grown GaN epilayers of different thickness and dislocation densities, and templates. The main deep centers, such as A₁, B, and D, show higher concentrations in thinner samples, which suggests a correlation to the high dislocation densities. Based on the anti-correlation between A₁ and B, which is observed in thin HVPE-GaN layers, the defect B was tentatively attributed to N_Ga. Centers A₁ and E₁ found in thin HVPE-GaN are very similar to centers A₂ and E induced by electron-irradiation, indicating their point-defect nature. Centers A, C, and D are not affected by 1 MeV electron-irradiation, thus ruling out the possibility of these centers being identical to any EI-induced centers; however, their nature remains unknown. As the layer thickness decreases, an increase of deep centers, both in species and concentrations, was clearly observed, which is believed to be closely associated with the significant increase of threading dislocations as revealed by TEM. Based on a comparison with EI-induced centers and an observation of anti-correlation, A₁ is tentatively assigned to N_I, and B to N_Ga. The template exhibited a new trap B′, with parameters E_T=0.53 eV and σ_T=1.5×10⁻¹⁵ cm² on the Ga-face, in addition to the four traps commonly observed in various epitaxial GaN layers. For the N-face, an N vacancy-related trap E₁, with E_T=0.18 eV and σ_T=4×10⁻¹⁷ cm², was observed. On the other hand, the Ga-face sample contained trap C, with E_T=0.35 eV and σ_T=1.6×10⁻¹⁵ cm². This trap may be related to surface damage caused by the RIE process employed.Electron beam and optical depth-profiling of thick (5.5–68 μm) n-type HVPE-GaN samples have been carried out using electron beam-induced current (EBIC) and micro-PL to determine the minority carrier diffusion length, L, and minority carrier lifetime. The minority carrier diffusion length increased linearly from 0.25 μm, at a distance of about 5 μm from the GaN/sapphire interface, to 0.63 μm at the GaN surface for a 36 μm thick sample. The increase in L was accompanied by a corresponding increase in PL band-to-band radiative transition intensity as a function of distance from the GaN/sapphire interface. These observations in PL intensity and minority carrier diffusion length have been attributed to a reduced carrier mobility and lifetime at the interface and to scattering at threading dislocations.Positron annihilation experiments have been conducted in HVPE films with varying thicknesses from 1 to 68 μm. Mg-doped samples and bulk GaN platelets have also been investigated and the behavior of positron annihilation in Mg-doped samples established. Unlike the Mg-doped samples, the positron lifetime in the HVPE samples increased with decreasing lattice temperature. This was interpreted as acceptors in these n-type samples being due to Ga vacancies as opposed to relatively shallow acceptor impurities. The similarities in the behavior of these samples and those investigated previously where the III/V ratio was changed also lend support to the Ga vacancy argument. Previous investigations established that as the III/V is lowered by increasing the ammonia flow during the growth, the Ga vacancy concentration increased. Using Mg-doped samples as a standard, the vacancy concentration was determined to be about 10¹⁷ cm⁻³ near the surface for the layer with a thickness greater than 30 μm. Assuming that the growth parameters in the set of layers with varying thicknesses that were investigated are the same, the Ga vacancy concentration increases to mid-10¹⁹ cm⁻³ near the interface. Since the interfacial region is n-type and highly conductive, this region must also contain even larger concentrations of O and/or N vacancies which lead to n-type material. SIMS results already indicate mid-10¹⁹ cm⁻³ levels of O being present in this region. This has been attributed to O out-diffusion from sapphire as previously reported.FTIR, ODMR and EPR measurements have been performed in GaN layers and templates. In FTIR measurements, two absorption bands corresponding to binding energies of 30.9 (Si) and 33.9 meV were found. Splitting of the binding energies with magnetic field is consistent with an effective mass of 0.22m₀. Angular rotation studies were performed with the magnetic field oriented perpendicular and parallel to the c-axis to provide symmetry information. The ODMR on the 2.2 eV peak in a 5–10 μm thick GaN layer, the notorious yellow emission, showed signatures of shallow donor (effective mass like) and deep defect centers with g-values of 1.95 and 1.99, respectively. The 3.27 eV peak with resolved LO phonon replicas, which is the blue peak observed in many GaN films grown by a variety of methods, is attributed to transitions involving shallow acceptors with g2.1 and g2.0. ODMR on the 2.4 eV “green” PL band in the free-standing template also revealed evidence for shallow donors with a g-value of 1.95 and other deeper centers. The larger line width of the shallow donor signal from the template, relative to that found for the epitaxial layers, is indicative of a lower concentration of this center, which leads to an increased hyperfine interaction. EPR studies confirmed the notable difference between the epilayers and the template, particularly the larger line widths in the template due to the lower concentration of shallow donors. Specifically, the free-standing sample has about 6×10¹⁵ cm⁻³ uncompensated donors while the epilayers have a concentration about a factor of four higher.Calculations indicate that incorporation of Si has a negligible effect on the lattice constant, but O and Mg can lead to an observable expansion of the lattice. Since values of the GaN lattice constant have often been based on bulk crystals that are now known to contain large concentrations of oxygen, the “true” GaN lattice constant is actually smaller than what has been measured for such crystals. Boron is an unintentional impurity that can be introduced during MBE growth. There has been speculation about whether B might act as an acceptor in GaN; this would require it to be incorporated on the nitrogen site. Computations indicate that incorporating B on the N site is energetically unfavorable. Even if it did incorporate there, it would act as a deep, rather than a shallow acceptor. Formation energies of H in AlN and GaN have also been calculated. The behavior of H in AlN is very similar to GaN: H⁺ dominates in p-type, H⁻ in n-type. Surprisingly, H in InN behaves exclusively as a donor, i.e. it is not amphoteric as in GaN and AlN, but actually contributes to the n-type conductivity of the material.Scanning thermal microscopy (SThM) has been applied to measure the local thermal conductivity of epitaxial GaN as it is affected to a large extent by phonon scattering, and a closer to the true value of this parameter can be obtained by a local measurement in areas of lower defect concentration such as those found in the wing regions of lateral epitaxially grown GaN. The method relies on a thermo-resistive tip forming one quadrant of a Winston bridge. The bridge is balanced with the tip heated followed by bringing the tip in contact with the sample under test which cools down due to thermal dissipation. However, the feedback circuit attempts to keep the thermo-resistance and thus the tip temperature the same. The square of the feedback voltage necessary for this is proportional to the thermal conductivity. Accurate values can be obtained with calibration using known substrates such as GaSb, GaAs, InP, Si and Al metal. Using SThM, thermal conductivity, κ, values of 2.0–2.1 W/cm K in the wing regions of lateral epitaxially grown GaN, 1.70–1.75 W/cm K in HVPE grown GaN, and 3.0–3.3 W/cm K for free-standing AlN have been measured.     

Surface and grain-boundary energies as well as surface mass transport in polycrystalline CeO2

The multiphase equilibration technique for the determination of the equilibrium angles that develop at the interphase boundaries of a solid–liquid–vapor system, has been used to calculate the surface and interfacial energies in polycrystalline CeO₂ and CeO₂/Cu system in argon atmosphere at the temperature range 1473–1773 K. Linear temperature functions were obtained by extrapolation, for the surface energy γ_sv (J/m²) = 2.465–0.563 × 10⁻³ T and the grain-boundary energy γ_ss (J/m²) = 1.687–0.391 × 10⁻³ T of the ceramic, as well as for the interfacial energy γ_sl (J/m²) = 2.623–1.389 × 10⁻³(T −1356 K) of the CeO₂/Cu system. Grain-boundary grooving studied on polished surfaces of CeO₂ annealed in argon atmosphere at the same temperature range has shown that surface diffusion was the dominant mechanism for the mass transport. The surface diffusion coefficient can be expressed according to the equation D_s (m²/s) = 3.82 × 10⁻⁴ exp(−308,250/RT).     

Electrochemical determination of cadmium(II) at platinum electrode modified with kaolin by square wave voltammetry

In this work, determination of cadmium(II) using square wave voltammetry (SWV) was described. The method is based on accumulation of these metal ions on kaolin platinum electrode (K/Pt). The K/Pt performance was optimized with respect to the surface modification and operating conditions. The optimized conditions were obtained in pH of 5.0 and accumulation time of 25 min. Under the optimal conditions, the relationship between the peak current versus concentration was linear over the range of 9 × 10⁻⁸ to 8.3 × 10⁻⁶ mol L⁻¹. The detection limit (DL, 3σ) was 5.4 × 10⁻⁹ mol L⁻¹. The analytical methodology was successfully applied to monitor the Cd(II) content in natural water. Interferences were also evaluated.