Transient behavior of the thermal ion emission from KCl(0 0 1) upon polarity reversal of the electric extraction field

The technique of polarity reversal of the external electric extraction field (strength: 10² V/cm) was applied to study the relaxation of the thermal ion emission from the KCl(0 0 1) single crystal surface. Transient currents of the K⁺ and K₂Cl⁺ ions upon switching from the emission suppression to the ion extraction mode were recorded as a function of the evaporation time, the temperature, and the time of field reversal. The temperature dependence of the time constants of the K⁺ ions obtained from the exponential decreases of the emission currents to their steady-state emission resulted as logτ_h(s)=−(13.39±0.56)+(12.42±0.49)10³/T in a high temperature interval of 826–930 K after a prolonged heating period and as logτ_l(s)=−(20.65±1.04)+(16.77±0.81)10³/T in a low temperature interval of 750–801 K at the initial stage of evaporation, with corresponding activation energies of E_h(K⁺)=2.47±0.14 eV and E_l(K⁺)=3.32±0.16 eV, respectively. The transient currents can be interpreted by a partial adsorption of the suppressed ion currents at the kinks of the surface steps. The differences in the high- and low-temperature runs may be attributed to a strong coarsening of the surface at higher temperatures, which occurs as a bunching of monosteps to macrosteps and/or to an enrichment and segregation of divalent impurities at the surface. The transient behavior of the molecular K₂Cl⁺ ions seems to be strongly correlated with that of the K⁺ ions. This correlation is possibly caused by changes of the strength or the sign of the local electrical field connected with the excess charge at the kinks.     

Changes in OSL properties of quartz by preheating: an interpretation

A study of OSL variation with preheat temperature showed, in a majority of cases, that OSL recorded at room temperature, increases above 200°C before the normal drainage at higher temperature. To explain this behaviour, an alternative interpretation to the common ‘electronic thermal transfer’ mechanism is suggested, supported by a study of hydrothermally grown quartz crystals. This interpretation involves impurities in substitution of Si⁴⁺, specially Al³⁺, which are associated with species like, in the case of quartz, hydrogen (H⁺, in fact, OH⁻) and alkali ions (Li⁺, Na⁺, K⁺). These monovalent ions usually act as charge compensators and are mobile during heating. As a consequence of the mobility and a possible irreversible exchange between compensators, the number of radiative recombination centres associated with the OSL trap(s), observable within the detection spectral window used (250–400 nm), increases during preheating. This phenomenon could lead to a wrong ED determination.     

Effect of substrate bias on SE, XPS and XAES studies of diamond-like carbon films deposited by saddle field fast atom beam source

This paper reports the effect of positive substrate bias (V_s) varying from 0 to 180 V on the spectroscopic ellipsometry (SE), X-ray photoelectron spectroscopy (XPS) and X-ray Auger electron spectroscopy (XAES) studies of diamond-like carbon (DLC) films deposited using CH₄ gas as a feedstock into a saddle field fast atom beam (FAB) source. The values of optical constants like refractive index (n) and extinction coefficient (k) of the deposited DLC films were determined using a two phase model. The values of ‘n’ were found to fall in the range from 1.505 to 1.720 and ‘k’ from 0.03 to 0.125 by application of different values of V_s. Value of these optical constants were found to decrease with the increase of substrate bias up to 90 V and then increase beyond this value. Position of C 1s peak evaluated from XPS data was found to occur at 286.09±0.18 eV in DLC films deposited by application of different values of V_s. Observation of full width at half maximum (FWHM) (τ) value (1.928 eV at V_s=0 V, 2.0 eV at V_s=90 V and 1.89 eV at V_s=180 V) clearly hinted the existence of a point of inflection in the properties of DLC films deposited using FAB source this way. A parameter ‘D’ defined as the distance between the maximum of positive going excursion and the minimum of negative going excursion was calculated in the derivative XAES spectra. The values of ‘D’ evaluated from XAES data for DLC films were found to be 14.8, 14.5 and 15.2 at V_s=0, 90 and 180 V, respectively. The sp² percentage was calculated for samples deposited this way and was found to be low and lie approximately at 5.6, 2.8, 2.3, 5.7 and 11.5 for different values of V_s=0, 50, 90, 150 and 180 V. The sp³ content percentage and sp³/sp² ratio was found to be 94.4 and 16.7, 97.7 and 42.5 at V_s=0 and 90 V, respectively. Beyond V_s=90 V these values started decreasing. Mainly, a point of inflection in all the properties of DLC films studied over here at around 90 V of applied substrate bias has been observed, which has been explained on the basis of existing theories in the literature.     

Comparison of ‘blue’ and ‘infrared’ emission bands in thermoluminescence of alkali feldspars

Dating quaternary sediments by thermoluminescence (TL) or optically stimulated luminescence (OSL) calls for a detailed knowledge of the luminescence of feldspars. TL of the various alkali feldspars ((K, Na) Si₃AlO₈) display many common features, and some of these cause great difficulties for dating. After long storage following ionizing irradiation, the TL of most alkali feldspars is known to fade away by “anomalous fading”, which is incompatible with dating. This effect had been attributed to tunnel recombination. Following irradiation, a very intense tunnelling afterglow is observed at temperatures down to liquid nitrogen, in accordance with the observed rate of fading. This emission has a Gaussian spectrum entirely in the infrared (IR) with a maximum at 1.7 eV. It displays an important thermal quenching from 77 to 300 K. Its intensity is related with the ‘disorder’ of the crystal lattice. At higher temperatures, in TL proper, two emission bands can be separated. One is the well-studied complex visible emission, distributed over the spectral region from UV to orange, but mostly ‘blue’. The other is the ‘infrared’ band already observed at lower temperatures, which is attributed to Fe³⁺ ions. These two bands are clearly separated, with the spectral maxima, respectively, below and above 600 nm. They have also different kinetics, the glow peaks temperatures are different. But these two different bands are also coupled in many ways, they have parallel growth and fading. With ‘disordered’ feldspars, the ‘blue’ emission displays anomalous fading, which is stronger than that of the ‘infrared’. The infrared emission is more stable, which may be interesting for the purpose of dating.     

Secondary ion emission from clean and K-covered Ni surfaces near threshold impact energies

Clean and K-covered Ni surfaces are bombarded with low energy (10–500 eV) beams of He⁺, Ne⁺, Ar²⁺, and Kr⁺ ions, and the emitted ion yield is measured as a function of beam energy. The apparent threshold energies for K⁺ and Ni⁺ emission are proportional to the ionic binding energies of K⁺ and Ni⁺ to the Ni surface. From comparison of the ion and neutral yield curves, it is suggested that these ions are emitted via momentum transfer collisions similar to neutral sputtering.     

Determination of the K(1800) spin parity

A spherical harmonic moment analysis of the reactions K⁻p → K⁻π⁺n and K⁺p → K⁺π⁻Δ⁺⁺ at 13 GeV/c demonstrates the existence of a broad K^* state with mass in the vicinity of 1800 MeV and spin parity 3⁻.     

The application of high speed photography and white light speckle to the study of dynamic fracture

A white light speckle technique is combined with high speed photography to study the dynamic fracture of a sample of PMMA. A series of single exposure speckle patterns are photographically recorded at the rate of 2 × 10⁵ frames s^-1 during fracture of the sample. The displacement field around the tip of the propagating crack may then be determined by superimposing pairs of individual frames to form a ‘double-exposure’ speckle pattern. It is shown that the method gives values for the stress intensity factor K which are consistent with those obtained from other techniques.     

Structural and electrical properties of crystalline (1−x)Ta2O5−xTiO2 thin films fabricated by metalorganic decomposition

Polycrystalline (1−x)Ta₂O₅−xTiO₂ thin films were formed on Si by metalorganic decomposition (MOD) and annealed at various temperatures. As-deposited films were in the amorphous state and were completely transformed to crystalline after annealing above 600 °C. During crystallization, a thin interfacial SiO₂ layer was formed at the (1−x)Ta₂O₅−xTiO₂/Si interface. Thin films with 0.92Ta₂O₅–0.08TiO₂ composition exhibited superior insulating properties. The measured dielectric constant and dissipation factor at 1 MHz were 9 and 0.015, respectively, for films annealed at 900 °C. The interface trap density was 2.5×10¹¹ cm⁻² eV⁻¹, and flatband voltage was −0.38 V. A charge storage density of 22.8 fC/μm² was obtained at an applied electric field of 3 MV/cm. The leakage current density was lower than 4×10⁻⁹ A/cm² up to an applied electric field of 6 MV/cm.     

Emission properties of Ta-centres in KCl

Electrolytic colouration at about 600°C and 350 V/cm, for KCl crystal containing Pb²⁺ and for KCl:Pb²⁺ crystals co-doped with Li⁺, Na⁺ and Rb⁺ has been undertaken. Several absorption bands were observed in both doped and co-doped crystals in the visible-UV region. Excitation into these bands gives rise to the same 0.86 eV emission band except for the Li⁺-co-doped crystal which gives rise to a 0.80 eV emission band. These absorption bands are due to the same T_a-centre related to Pb⁻. The observed infrared emission intensity of the crystal with F-centres is higher than of without F-centres.     

AFM observations of latent fission tracks on surfaces: amorphous SiO2 and quartz

Preliminary results of a systematic AFM experimental investigation of the surface ‘track’ effects produced by the passage of fission fragments from a californium (²⁵²Cf) source into amorphous SiO₂ and quartz are described. Fission fragments from the source were collimated using a 10 μm thick aluminum foil and comprised fragments with the usual binary distribution of energies—light and heavy—79.4 and 103.8 MeV. Irradiations and AFM measurements were carried out in air at normal room temperature and pressure. Remarkably high sputtering yields/fragment were discovered, and in the case of crystalline quartz the ejecta was found to be arranged in an ordered manner. A brief discussion is given of a part likely to be played by electronic energy loss induced Coulomb explosion of target atoms for each point of fragment entry.     

Conductivity relaxation parameters of some H and K conducting in the polycrystalline compound KDyHP3O10

Polycrystalline samples of KDyHP₃O₁₀ was obtained by heating for 12 hour at 553 K a mixture containing K₂CO₃, Dy₂O₃ and H₃PO₄. Samples were characterized through X-ray diffraction, examined by IR vibrational spectroscopy and impedance and modulus spectroscopy techniques. The conductivity relaxation parameters of some H⁺ and K⁺ conducting in this compound have been determined from an analysis of ac conductivity data measured in a wide temperature range. Transport properties in this material appears as due to H⁺ and K⁺ ions hopping mechanism. The stretched exponential function exp[−(t/τ_σ)^β] has been used to describe the conductivity relaxation. The relaxation parameters have been investigated as a function of the nature of mobile ions. The results obtained are shown to be in good agreement with the predictions of the Ngai coupling model.     

Optical characterization of InGaN/AlGaN/GaN diode grown on silicon carbide

Electroluminescence (EL) properties of In_xGa_1−xN/Al_yGa_1−yN/GaN/SiC diode were studied. The spectral range for which EL spectra were recorded is 1–3.5 eV. Room temperature EL was obtained for forward bias (3.18 V, 220 μA) at 446.067 nm (blue luminescence band), 606.98 nm (yellow luminescence band) and 893.84 nm (Infrared luminescence band). The EL temperature dependence shows that, BL band is mostly given by e–h recombination corresponding to indium composition equal to 0.17 ± 0.01 and 0.14 ± 0.02 obtained theoretically and experimentally, respectively. The yellow band is generally weak and absent at low temperature. The IRL band is more consistent with the DAP recombination and could be explained by the thermal activation of Mg states. The luminescence bands shift to lower energies is due probably to the larger potential fluctuations effect.     

Far-infrared and millimeter wave spectroscopy of superionic copper conductors

Far-infrared and millimeter wave spectra of copper ion conducting crystal RbCu₄Cl_3+xI_2−x, which has the same structure as the room temperature silver ion conductor RbAg₄I₅, were investigated. Broad absorption peaks observed around 40, 80, and 110–200 cm⁻¹ at room temperature show doublet structures at low temperature; this may be attributed to the difference of local structure by chlorine and iodine ion. The 110–200 cm⁻¹ bands seem to be symmetric breathing modes of CuX₄ (X = Cl or I) tetrahedron and the frequency shift coincides with the square root of the mass ratio of conduction ions. The 80 cm⁻¹ band seems to be Rb-X vibration in RbX₆ octahedron. The 40 cm⁻¹ band seems to be the attempt mode which is an outward motion of the mobile ion in halogen cage. The increase of the absorption intensity at the low energy side with temperature corresponds to an increase of the DC conductivity. Plasmon fitting in energy loss function spectra was attempted.     

Effect of heat-pretreatment of the graphite rod on the quality of SWCNTs by arc discharge

Single-walled carbon nanotubes (SWCNTs) have been synthesized in high yield by the dc arc discharge technique under heat-pretreatment of the graphite rod conditions. Before executing arc discharge, the graphite rods containing the catalysts were heat treated at 600, 700, 800 and 900 °C for 1–3 h, respectively. Effects of heat-pretreatment of the graphite rod on the quality of SWCNTs by arc discharge were investigated. The heat-treatment temperature and time were found to be crucial for a high yield of high-purity SWCNTs. Optimum parameter was found to be at the heat-treatment temperature of 800 °C for 2 h. The SWCNTs synthesized under the optimum condition have better field-emission characteristics. The turn-on field needed to produce a current density of 10 μA/cm² is found to be 1.9 V/μm and the threshold field where current density reaches 10 mA/cm² is 3.9 V/μm.     

Positive electrode materials for lithium batteries based on VOPO4

A detailed electrochemical study of Li insertion in the -form of VOPO₄ and the optimization of the cycling performance are presented. The redox process occurs in one step close to 3.76 V, along with a phase transition. In order to improve the intercalation kinetics, the electronic conductivity was optimized by introducing a mixed valency, and the ionic conductivity was favored by introducing ‘pillaring’ molecules or ions in the interlayer space. In this way, the electrochemical behaviors of -VOPO₄·2H₂O, -Na_xVOPO₄, -K_xVOPO₄ and -Mg_xVOPO₄ (0≤x≤1) have been studied: the hydrate compound shows a good specific capacity (100 mA h/g at a C/5 regime), but a poor cyclability was mainly because of water oxidation. The inserted large alkaline ions improved the cyclability up to 80 cycles (Na⁺) or over 100 cycles (K⁺). Enhancements of the VOPO₄ specific capacity have been obtained as well by mechanical ball-millings.     

Conductivity of two-dimensional crystals over liquid helium for different holding fields

The results are presented of the experimental study of electron crystals over liquid helium with surface electron density of 3.2×10⁸ and 6.4×10⁸ cm⁻² (melting temperatures 0.4 and 0.58 K) at temperature 83 mK and for holding electric fields 300–1200 V/cm. The measurements are performed in the frequency range 1–14 MHz where the coupled phonon–ripplon resonances are observed in the experimental cell. The real and imaginary parts of the complex conductivity of the crystal have been obtained as a result of analysis of an electron layer response on exciting ac voltage with frequency corresponding to the mode (0,1) of the coupled phonon–ripplon oscillations. An analysis of the results allows us to suppose that structural defects of the electron crystal play an essential role in dissipation processes.     

Radiation-induced processes and defects in alkali and alkaline-earth borate crystals

The paper presents the results of a study of the radiation-induced processes and defects in nonlinear optical crystals Li₂B₄O₇ (LTB), LiB₃O₅ (LBO), CsLiB₆O₁₀, KB₅O₈·4H₂O, β-BaB₂O₄. It was revealed that a pulsed electron beam irradiation at 290 K forms the radiation-induced pairs of the ‘vacancy—interstitial atom’ defects in the cation sublattice of these crystals. This gives rise to a creation of metastable electronic (interstitial atom) and hole (small-radius polaron near the cation vacancy) centers in high concentrations. Optical hole-transitions from the local level of the trapped hole centers to the valence band states are responsible for the transient optical absorptions (TOA) of borates in the visible and UV spectral ranges. A sublattice of the weakly bound mobile lithium cations in LTB and LBO favors a spatial separation of the radiation-induced pair defects ‘hole polaron near Li-vacancy—mobile interstitial Li⁰ atom’. Their decay rated by the electron–hole nonradiative tunnel recombination determines a peculiar feature of the TOA decay kinetics in LTB and LBO.     

电场条件下氧化锌结晶特性及极化产物的拉曼光谱分析

开展高压电场调控纳米材料结构形貌和性能研究在功能材料领域具有重要的理论和实际意义.本文在高压电场条件下合成了氧化锌纳米粉体,并对粉末试片进行了后期电场极化处理,研究了电场对氧化锌的结构形貌、点缺陷、拉曼光谱的影响.以X射线衍射仪(XRD)、扫描电镜(SEM)和拉曼光谱仪对产物的结构形貌、拉曼位移、缺陷分布等进行了表征.结果表明,高压电场条件下氧化锌的完全晶化时间和温度比未施加电场时明显延长和升高,直流电场能够显著促进前驱物中氧化锌的形核,并降低晶化速度.不同电场强度下氧化锌具有不同的显微形貌.纳米氧化锌粉末试片在直流电场中极化后,其阴极面和阳极面的拉曼光谱表现出明显的差异.有明显漏电电流的情况下,阳极面在1050 cm~(–1)处的二级光学声子模A_1(LO)的强度显著提高,且拉曼强度I_1=438 cm~(–1)和I_2=1050 cm~(–1)的比值与极化电场的场强呈线性关系.当调转试片正反面进行二次极化时,原来在阳极面尖锐的1050 cm~(–1)峰经过阴极极化而消失.阳极面1050 cm~(–1)拉曼峰的锐化与氧化锌晶粒内的缺陷重新分布和双肖脱基势垒有关.     

Relationship between electronic and crystal structure in Cu–Ni–Co–Mn–O spinels: Part B: Binding energy anomaly in Cu photoemission spectrum

In very rare circumstances, X-ray photoemission spectra of copper in spinel oxides exhibit a “negative binding energy shift”. The origin of such an anomalous XPS chemical shift was investigated. A metastable Ni_0.48Co_0.24Cu_0.6+xMn_1.68−xO₄ (0 < x < 0.6) spinel was fabricated at 600 °C using a low-temperature solution technique. The binding energy of the 2p_3/2 level of copper (930.8 eV) is found 1.9 eV lower than that of Cu⁰ (932.7 eV). XPS and EXAFS studies revealed that the post-thermal annealing between 600 and 800 °C undergoes an irreversible cubic-to-tetragonal phase transformation through oxidation–reduction reaction Cu¹⁺ + Mn⁴⁺  Cu²⁺ + Mn³⁺, and only tetrahedral Cu¹⁺ species in the cubic spinel shows this anomalous chemical shift. The negative shift of the core levels was correlated to an equal shift of the Cu 3d valence band levels. XPS valence bands from the samples annealed at different temperatures were compared to DOS calculations. The DOS computations were performed with FEFF-8.1 code using experimental crystal parameters established by the EXAFS analysis. It was found that the tetrahedral Cu¹⁺ in the 600 °C annealed sample exhibits localization of the 3d orbitals showing behavior characteristic to zinc. The completely filled and isolated 3d electron shell appears as a false valence band edge in the XPS spectrum. The position of the Cu 3d, and other core levels, is established by oxygen pinning the Cu valence band levels and by the fixed value of the p–d gap characteristic to the tetrahedral copper environment in this spinel.     

Feasibility of finding malignant diseases with the Ar 501.7 nm laser line using a new blood-analysis method

Utilizing a new blood-analysis method which uses the interaction between an Ar⁺ laser beam and a blood sample, it can be shown that there is a significant difference in the reflectance variation δR between the malignant and benign blood, at 501.7 nm of the Ar⁺ laser, under a dc magnetic field, for both human and mouse blood. For the malignant mouse-blood with leukaemia, δR tends to be positive with the magnetic field, while for the benign blood it is negative. This is also true for the human blood. This fact is confirmed by the inverted variation of the higher order Fourier pattern obtained by the Fourier pattern analyser from the transmitted beam through the diluted sample, for the malignant and benign mouse-blood under the magnetic field.