Effect of Cr doping on the structural and optical properties of ZnS nanoparticles

Zn_1−xCr_x S nanoparticles with x = 0.00, 0.005, 0.01, 0.02 and 0.03 were synthesized by chemical co‐precipitation method at room temperature for the first time. Ethylene diamine tetraacetic acid was used as stabilizer. Energy dispersive analysis of X‐rays confirmed the presence of Cr in the samples. The samples were characterized by scanning electron microscopy, X‐ray diffraction (XRD), transmission electron microscopy and optical absorption studies. XRD and selected area electron diffraction results showed that the samples of all compositions crystallized in cubic structure and the lattice parameters decreased linearly with increase in Cr content following Vegard's law indicating that the Cr ions have substituted for Zn in the ZnS lattice. The particle size estimated from XRD was in the range 6–10 nm.The optical absorption studies on the doped samples indicated that the absorption edge blue shifted with respect to those of bulk and nanocrystalline ZnS. The bandgap increased with increasing Cr concentration in a narrow range 3.81–4.03 eV. Photolumonesence studies showed blue emission with appreciable luminescence quenching with increasing Cr concentration. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of zinc doping on pentaerythritol tetranitrate single crystals

In this study, the effect of zinc impurity on the organic high explosive pentaerythritol tetranitrate (PETN) single crystal has been investigated with optical microscopy and ex situ atomic force microscopy (AFM). The optical images show that the crystal shape has a transition with a predictable trend from long crystal to compact one as the zinc concentration is increased. Also, the 2‐dimentional (2‐D) growth hillocks are observed clearly on (110) face with contact AFM. The crystal growth occurs on monomolecular steps generated by 2‐D nucleation and followed by layer‐by‐layer expansion, and the macro‐steps formed onto the surface before spreading laterally as step bunches. The zinc ions are incorporated in growth steps as the zinc concentration is increased. The mechanism of inorganic impurity on molecular crystallization growth is still unclear. However, the incorporation of impurities may significantly affect growth kinetics of defect structure, and the bulk properties of molecular crystals. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optimisation of chromium doping in LiNbO3 single crystals

Growth of undoped and Cr doped (0.1, 0.25 and 0.5 mol % Cr₂O₃) congruently‐ melting‐composition LiNbO₃ single crystals by Czochralski technique is reported. Chromium doping was optimised to get crystals with potential for an integrated, broadband, tunable laser in the 700‐1100 nm spectral range. Typical sizes of the grown crystals are 25‐30 mm in diameter and 30‐40 mm in length. Symmetrical and sharp conoscopy pictures confirm the optical homogeneity of the crystals. Optical transmission was recorded for both undoped and doped crystals. 70% transmittance was observed. The grown crystals have reasonably good laser damage threshold. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Studies on the vapour growth of ZnS,ZnSe and ZnTe single crystals

Large, highly perfect single crystals (up to 10 × 10 × 5 mm³) of ZnS, ZnSe and ZnTe have been grown from the vapour phase by dissociative sublimation and chemical transport with iodine. Good quality crystals were obtained when the growth rate was limited by diffusion of the vapours rather than by thermal convection or by reaction rates at the charge and or the growing surface. The presence of an inert gas and defined iodine concentrations increase the growth stability, especially for charges with slight deviations from stoichiometry. Electronmicrographs, X-ray topographs and etching experiments revealed low dislocation densities and relatively large inclusion-free regions.     

Domain structure investigations in triglycine sulphate single crystals

The domain structure of pure and irradiated TGS crystals was investigated by electron microscope decoration technique. Changes in the decoration pictures due to irradiation are shown and the way in which small lenticular domains disappear in the spontaneous ageing process is presented. Moreover, the thickness of the domain wall was assessed to be for good TGS crystals, grown in the paraelectric phase, of about 120 Å.     

Modeling of the defect structure in dislocation-free silicon single crystals

A mathematical model of the formation of primary grown-in microdefects on the basis of dissociation diffusion is presented. Cases of “vacancy-oxygen” (V + O) and “carbon-interstitial” (C + I) interaction near the crystallization front are considered for dislocation-free Si single crystals grown by the floating-zone and Czochralski methods. The approximate analytical expressions obtained by setting 1D and 2D temperature fields in a crystal are in good agreement with the heterogeneous mechanism of formation of grown-in microdefects.     

Defect structure in nickel single crystals in dependence on the cyclic deformation prehistory

The development of the dislocation structure in nickel single crystals push-pull fatigued at room temperature is investigated for different deformation types using electrical resistivity measurements and results of TEM observations. The dislocation density in the bundles of the matrix is higher after changing the strain amplitude from region A to region B (CD_AB-type) of the cyclic stress-strain curve (N_dB ≈︁ 1.6 ≈︁ 10¹⁵ m⁻²) than after virginal deformation in region B (VD_B-type) especially at higher strain amplitudes (N_dB ≈︁ 0.75 ≈︁ 10¹⁵ m⁻²). Increasing the strain amplitude within region B (CD_BB-type) does not change N_dB. The farther development of the matrix structure after reaching the v_H-minimum during the VD_B-type of deformation suggests the action of the „formation and dissolution”︁ – mechanism, whereas the CD_BB-type causes the „cord”︁-mechanism of the PSB nucleation.     

Refinement of the atomic structure of CdTe single crystals

A comparative refinement of the anharmonic and disordered models of the atomic structure was performed based on precision X-ray diffraction data sets collected from a CdTe single crystal at 295, 376, 491, and 583 K. Although both models provide the asymmetric contributions of Cd and Te to the structure factors, the anharmonic model seems to be more advantageous.     

Effect of the defect structure character in calcite single crystals on X-ray diffraction

The paper presents experimental results for the dependence of the integral intensity of anomalous transmission T_i and Laue reflection R_i of X-rays on the structural quality of calcite single crystals as obtained by a two-crystal spectrometer. The relation between X-ray dynamic and kinetic scattering as a function of the sample thickness, densities of perfect dislocations in perfect CaCO₃ crystals and of atom-vacancy complexes in calcite crystals with „background”︁ has been found. Distortions due to perfect grown-in dislocations in calcite are shown to exceed those produced in crystals with „background”︁. The latter appear to be more X-ray transparent. A comparison with results of twin layer thickening experiments on CaCO₃ crystals of different qualities again points to the different nature of these distortions. In CaCO₃ crystals with the dislocation density of up to 2 × × 10³ cm⁻², the X-ray scattering, in our experiments, was consistent with the dynamic diffraction theory, while at higher dislocation densities, the X-ray scattering markedly differs from it.     

Low-temperature growth and doping of InP:S single crystals from melt-solution in microgravity

The effect of microgravity on the growth of bulk InP:S single crystals from a melt with an initial equilibrium composition (84 at % In, 16 at % P, and ～2.2 × 10¹⁸ at cm^?3 of S) on board the Foton-11 satellite was investigated. The growth of crystals on board the satellite and on Earth (a reference crystal) was carried out by the traveling heater method. The samples of the grown crystals were investigated by metallography, double-crystal X-ray diffractometry, single-and double-crystal X-ray topography, and secondary-ion mass spectrometry. It is shown that the mass transfer in the melt in microgravity is similar to the diffusion mode. Hence, the mass transfer in the melt results in the following: the formation of a nonstationary boundary layer, depleted in phosphorus; the constitutional supercooling at the crystallization front accompanied with the development of a cellular substructure in the early growth stage; and the hypothetical phase structurization of the transition layer with the formation of In-based associates (clusters), which were found in the grown crystals in the form of spherical defects 10–20 μm in diameter. The coefficients of sulfur distribution k₀ = 0.274 and k_eff = 0.43, the sulfur diffusivity in the melt D_S = 4.2 × 10^?7 cm²/s, and the effective thickness of the transition layer δ = 0.07 cm in terrestrial gravity are determined. The data obtained are necessary to develop a mathematical model of crystallization in zero gravity.     

The influence of tartaric acid on the growth kinetics of silver single crystals

The influence of tartaric acid (TA) on both the growth kinetics and morphology of silver single crystals was investigated in situ during electrocrystallization. Using laser interferometry the dependence of the monoatomic step velocity vs. overvoltage at different concentrations of TA was studied. Measuring the normal growth rate R we calculated the current on the front crystal face. It is shown that the addition of TA changes its value in complicated way vs. overvoltage.     

The defect structure of ammonium sulphate single crystals

The defect structure of larger ammonium sulphate crystals, grown from aqueous solutions, has been studied by means of X-ray diffraction topography after Lang's method. Several types of dislocations were identified. Moreover it was found that no relationship exists between growth rate and dislocation density, which implies that crystal growth proceeds via two-dimensional nucleation. No difference was found in the defect structure of crystals grown from pure water solutions and from aqueous solutions with 10 – 20% glycerine as additive: In both cases neither growth bands nor sector boundaries were found. On the other hand, crystals grown in aqueous solutions contaminated with Mn²⁺ and Fe³⁺ revealed growth bands and for Fe³⁺ a mosaic-like structure.     

X-Ray study of the defect structure of PbTe single crystals

PbTe single crystals grown by different methods were thinned electrolytically and examined by X-ray transmission topography. The transmission topographs, first obtained of this material, reveal substructure, slip lines and single dislocations. The samples exhibit different structural perfection in dependence on the growth method. In crystals of relatively high perfection, slip lines are the striking feature of defect structure, caused by surface damage during mechanical preparation. The most perfect sample has a dislocation density of 10² to 2 ṁ 10³ cm⁻². No slip occurs, probably due to small dopant contents.     

Preparation of HgJ2 single crystals and the effect of doping upon their physical properties

HgJ₂ single crystals both pure and doped were grown from an acetone solution using the method of the slow solvent evaporation. The iodides NaJ, KJ, SrJ₂, CdJ₂, NH₄J were used as doping materials in concentrations from 0.01 to 2.0 wt%. The grown crystals were studied with respect to the influence of impurity doping on the electric conductivity and the phase transitions at atmospheric pressure (the DTA method) and high pressure as well (with the help of the high pressure optical cell).     

A new piezoelectric single crystal obtained by Ge doping in the SiO2 structure

The interest of Si_1–xGe_xO₂ single crystals with alpha‐quartz structure is connected to improvement of electromechanical coefficients and rise of α – β phase transition of quartz one. Growth of an α‐Si_xGe_1–xO₂ crystal was realized by a hydrothermal method of temperature gradient in autoclaves, made from Cr–Ni alloys. Nutrient material was prepared from synthetic quartz as crashed rods and placed in the bottom of autoclaves. There was loaded GeO₂ powder additive in proportions to quartz nutrient. Single crystals were investigated by electron microprobe analysis, X‐ray diffraction and atomic force microscopy. The most important result, which was obtained during the investigations, is an experimental proof of growth of α‐Si_xGe_1–xO₂ single crystals under the hydrothermal conditions. The present results thus open the possibility to tune the piezoelectric properties of these materials by varying the chemical composition. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the influence of nonstoichiometry on luminescent properties of CdGa2S4 single crystals

Results of a study of photoluminescence and thermoluminescence of cadmium thiogallate single crystals grown from the melt are presented. Effect of nonstoichiometry on the properties of samples is studied. Some assumptions about the nature of centres in CdGa₂S₄ compound are made.     

Influence of doping on OH absorption in LiNbO3 crystals

Undoped, Cr doped and Mg, Cr codoped LiNbO₃ crystals were grown by conventional Czochralski technique. Comparative study was carried out using Fourier transform infrared (FTIR) and UV‐Visible spectroscopy. Infrared optical absorption for OH^– ion has been used to study the effect of dopants on the crystals. The peak position of OH^– shift to 3535 cm^‐1 for Mg, Cr codoped crystals compared to 3484 cm^‐1 for undoped and Cr doped crystals. Prominent absorption bands are found in the visible region centered at 480 nm (20833 cm^‐1) and 653 nm (15313 cm^‐1) in Cr doped crystals. Whereas in Mg, Cr codoped crystals these broad absorption bands are red shifted to 517 nm (19342 cm^‐1) and 678 nm (14749 cm^‐1). UV cutoff in Cr doped crystals shift towards higher wavelength compared to undoped LiNbO₃ crystals. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The influence of doping with tin on the structure of Cu0.70Si0.30 eutectic melt

The structural changes in liquid Cu_0.70Si_0.30 eutectic melt upon doping with tin (5, 10, 15 and 20 at.%) have been studied by means of the X-ray diffraction method and the obtained structure factors and pair correlation functions have been analyzed. Experimental total structure factors have also been used to calculate partial factors by means of the reverse Monte-Carlo method. It has been shown that Sn atoms exhibit a tendency to form clusters in the matrix of the liquid eutectic melt.     

The influence of OH−-ions on the mechanical properties in LiF and NaCl single crystals

The influence of the growing atmosphere on the mechanical properties was investigated on LiF and NaCl single crystals. The growing atmospheres were air or vacuum and argon, respectively. In a wide temperature range the air grown crystals have a higher flow stress and a smaller activation volume. Besides, the dislocations are less mobile. The differences arise probably from the OH⁻-ions, which were found in a considerable concentration in the air grown material. The OH⁻-ions may create lattice distortions by themselves or by the formation of different complexes with the divalent cation impurities. In this way the density of thermal and athermal obstacles for the dislocation motion increases.