Luminescence of GaN single crystals prepared by heating a Ga melt in Na–N2 atmosphere

Colorless transparent prismatic crystals (0.5‐2.0 mm long) and hopper crystals (1.0‐2.5 mm long) of GaN were prepared by heating a Ga melt at 800°C in Na vapor under N₂ pressures of 7.0 MPa for 300 h. The photoluminescence (PL) spectrum of a prismatic crystal at 4 K showed the emission peaks of neutral donor‐bound exciton (D⁰‐X) and free exciton (X_A) at 3.472 eV and 3.478 eV, respectively, in the near band edge region. The full‐width at half‐maximum (FWHM) of (D⁰‐X) peak was 1.9 meV. The emission peaks of a donor–acceptor pair transition (D⁰‐A⁰) and its phonon replicas were observed in a lower energy range (2.9‐3.3 eV). The emission peaks of the D⁰‐A⁰ and phonon replicas were also observed in the cathodoluminescence (CL) spectrum at 20 K. The (D⁰‐X) PL peak of a hopper crystal at 4 K was at 3.474 eV (2.1 meV higher), having a FWHM of 6.1 meV which was over 3 times larger than that of the prismatic crystal. A strong broad band with a maximum intensity around 1.96 eV was observed for the hopper crystals in the CL spectrum at room temperature. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth behaviour of bulk GaN single crystals grown with various flux ratios using solvent‐thermal method

Bulk GaN single crystals were grown using a solvent‐thermal method. They were grown for 200 h at 600 °C and 800 °C using 8 MPa of N₂ gas and 1–3 mm sized pyramid GaN single crystals. Pure Na, NaN₃ and Ca were used as the flux. The mole fraction of the [flux]/([flux] + [Ga]) was 0.30–0.67. The growth behavior differed according to the flux ratio. The quality of the bulk GaN single crystals was improved by increasing the flux ratio. The bulk GaN single crystals formed by spontaneous nucleation were deposited on the BN crucible wall and bottom during the first step of synthesis. The wurtzite structure of the GaN grown single crystal was confirmed by x‐ray diffration. The chemical composition was analyzed by electron probe microanalysis. The quality and optical properties of the GaN single crystal were examined by Raman spectroscopy and photoluminesence analysis. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Flux growth and characterization of Ti‐ and Ni‐doped forsterite single crystals

Forsterite monocrystals doped with Ti and Ni were grown by the flux growth technique. A suitable mixture of flux (MoO₃, V₂O₅, Li₂CO₃) and nutrient was slowly cooled down to 750 °C from 1250 °C or 1350 °C. The crystals were then characterized by powder and single‐crystal X‐ray diffraction, scanning electron microscopy and differential scanning calorimetry (DSC). Variations observed in crystal size were attributed by both the varying experimental conditions in which they had been obtained, and to the amount of Ni substituted for Mg in the structure. High abundances of doped forsterite required a cooling rate of 1.8 K h^‐1. These synthetic, well‐characterized Ti and Ni doped forsterite crystals may have potential for exploitation in industrial fields. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and dissolution of equally‐sized insulin crystals

A protocol for growing sets of nearly uniform size crystals was devised and tested experimentally. The experiments were centered on insulin because of its medical significance however the method is applicable to other substances as well (C.N. Nanev, V.D. Tonchev, F.V. Hodzhaoglu, Protocol for growing insulin crystals of uniform size, J. Cryst. Growth 375 (2013)10–15). Now, both growth and dissolution of equally‐sized crystals are described quantitatively by a common analytical model. In our model the emphasis is put on the dissolution case when crystals number and/or size are sufficiently large to secure reaching solubility, while some non‐dissolved crystalline substance is still remaining. Quantitative results are obtained for the relations between dimensionless values of crystal size, solution concentration and time elapsed, the assumption simplifying our calculations being that the crystals retain their shape during the entire dissolution process.     

Excitation intensity and temperature‐dependent photoluminescence and optical absorption in Tl4Ga3InSe8 layered crystals

Photoluminescence (PL) spectra of Tl₄Ga₃InSe₈ layered crystals grown by Bridgman method have been studied in the wavelength region of 600‐750 nm and in the temperature range of 17‐68 K. A broad PL band centered at 652 nm (1.90 eV) was observed at T = 17 K. Variations of emission band has been studied as a function of excitation laser intensity in the 0.13 to 55.73 mW cm^‐2 range. Radiative transitions from donor level located at 0.19 eV below the bottom of conduction band to shallow acceptor level located at 0.03 eV above the top of the valence band were suggested to be responsible for the observed PL band. From X‐ray powder diffraction and optical absorption study, the parameters of monoclinic unit cell and the energy of indirect band gap were determined, respectively. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural and photoluminescence properties of GaN/ZnO core‐shell nanowires with their shells sputtered

We have reported the preparation of ZnO‐coated GaN nanowires and investigated changes in the structural and photoluminescence (PL) properties by the application of a thermal annealing process. For fabricating the core‐shell nanowires, Zn target was used to sputter ZnO shell onto GaN core nanowires. X‐ray diffraction (XRD) analysis indicated that the annealed core‐shell nanowires clearly exhibited the ZnO as well as GaN phase. The transmissoin electron microscopy (TEM) investigation suggested that annealing has induced the crystallization of ZnO shell layer. We have carried out Gaussian deconvolution analysis for the measured PL spectra, revealing that the core GaN nanowires exhibited broad emission which consist of red, yellow, blue, and ultraviolet peaks. ZnO‐sputtering induced new peaks in the green region. Thermal annealing reduced the relative intensity of the green emission. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Layered crystals of apo‐ and holoferritin grown by alternating crystallization

We report on the use of alternating crystallization for deposition of layers of different (though closely related) proteins in a single crystal. Investigations were carried out with the unique protein couple consisting of two forms of ferritin, apoferritin and holoferritin from horse spleen, which, despite being of quite different molecular masses, still possess identical organic shells. Crystals of both proteins were used as substrates for subsequent contiguous growth of the partner protein in perfect alignment. We observed continuous growth of combined (onion‐like) single crystals; artificial structures of biological macromolecules can be designed in this way. The homoepitaxial layered growth shows in an unambiguous way that protein crystallization depends only on the surface protein conformation and amino‐acid composition, but not on the internal molecule structure. The limitations of protein crystal growth for designing layered structures of biological macromolecules were revealed by growing of heterogeneous protein crystals onto pre‐existing protein crystalline substrates. Tetragonal crystals of hen egg‐white lysozyme were grown onto cubic apoferritin crystals used as substrates. It was observed that the lysozyme crystals were not lattice‐matched to the ‘host’ apoferritin crystals; this led to mere aggregates of different crystals. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and characterization of pure and lithium doped L‐alanine single crystals for NLO devices

Single crystals of pure and lithium substituted L‐alanine are successfully grown by slow evaporation method at constant temperature of 32°C. The effect of lithium dopant on crystal properties has been studied. Powder and single crystal XRD analysis confirms the structure and change in lattice parameter values for the doped crystals. The crystals were characterized by solubility studies, density, melting point measurements, FTIR and UV‐Vis‐NIR techniques. Thermal and mechanical stability of crystals were tested by TGA/DTA and micro hardness analysis. NLO activity of the crystals is found to be increased in the presence of lithium ions. The dielectric constant and dielectric loss of the crystals were studied as a function of frequency. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of Nd:Sr3Ga2Ge4O14 crystals by a modified Bridgman method

Bridgman growth of Nd:SGG (Sr₃Ga₂Ge₄O₁₄) crystals has been investigated for the first time. Pt crucible of ∅︁25mm×250mm with a seed well of ∅︁10mm×80 mm is used, and seed is SGG crystal of ∅︁10mm×50mm grown by Bridgman method in advance. The growth parameters are optimized as the furnace temperature is set to 1450∼1500°C, temperature gradient in the crystal‐melt interface is less than 25 K/cm and growth rate is less than 0.5mm/h. The Nd:SGG crystals with 25mm in diameter and 60mm in length are grown successfully from 1.5 to 8at% Nd³⁺ doped stoichiometric Sr₃Ga₂Ge₄O₁₄ melt. The distribution coefficient and concentration of Nd³⁺ in Nd:SGG crystals are obviously higher than those of Nd:YAG crystal. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and crystal growth of sillenite phases in the Bi2O3 ‐ TiO2 ‐ Nb2O5 system

The synthesis of Bi₂O₃‐Nb₂O₅ sillenite phase (BNbO) and the solubility of this phase with Bi₁₂TiO₂₀ was investigated by solid‐state reaction synthesis and niobium doped Bi₁₂TiO₂₀ (BTO:Nb) crystals were grown by the Top Seeded Solution Growth (TSSG) technique. The structures of polycrystalline compounds were checked by X‐ray powder diffraction method at room temperature. The correct composition of the sillenite phase stabilized with niobium was determined as Bi₁₂[Nb_0.17Bi_0.83]O_19.7 (BNbO) with unit cell parameter a = 10.261(2) Å. The system BTO‐BNbO is poorly soluble, but niobium doped BTO crystals were grown from the liquid composition 10Bi₂O₃ : xTiO₂ : (1‐x)/2 Nb₂O₅, with x = 0.95 and 0.90. A niobium concentration limit in the liquid phase is established in order to grow BTO:Nb with good crystalline quality. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and electrical characterization of L‐arginine added KDP and ADP single crystals

Potassium dihydrogen orthophosphate (KDP) and ammonium dihydrogen orthophosphate (ADP) single crystals added with L‐arginine have been grown by the solution methods. DC and AC electrical measurements were carried out at various temperatures along both a‐ and c‐ directions. Results indicate an increase of the electrical parameters with the increase of temperature which can be attributed mainly to the increase of thermally generated hydrogen bond vacancies (L. defects). Also, the present study indicates that L‐arginine addition leads to reduction of electrical parameters of KDP and ADP single crystals which can be attributed mainly to the decrease of L‐defects due to creation of additional hydrogen bonds by the impurity in random directions. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and optical characterization of cerium and lead‐doped Bi12TiO20 sillenite single crystals

Bi₁₂TiO₂₀ (BTO) single crystals doped with PbO and CeO₂ were grown by the Top Seeded Solution Growth (TSSG) technique from the liquid phase with nominal compositions of 10Bi₂O₃ : (1–x)TiO₂ : x PbO and 10Bi₂O₃ : (1–x)TiO₂ : xCeO₂ with x = 0.25 and 0.10. No growth‐related difficulties were encountered other than those typical of sillenite crystals. Samples with good optical quality were obtained and were characterized by optical absorption, dark current, spectral photocurrent dependence, optical activity and electro‐optic coefficient measurements. A comparison is made of the results of the optical measurements of doped and undoped BTO crystals. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nano‐sized silver crystals and their dispersion over α‐alumina for ethylene epoxidation

Ethylene oxide catalyst is a high metal loading catalyst, in which silver crystals is impregnated on α‐Al₂O₃ support. In this type of catalyst, metal dispersion plays an important role on catalyst selectivity for desired products. In this work, silver nitrate and silver oxide together with oxalic and lactic acid as the raw materials were used with different impregnation techniques to make catalysts with high silver content and dispersion. It is also known that the use of promoters affect the metal dispersion on the catalyst support and for that cesium was used as the promoter to improve the silver crystal dispersion. Physical and chemical characteristics of the prepared catalysts, i.e., surface area, pore volume, silver content, nano‐sized silver crystals and their dispersion were measured using BET method, Atomic Absorption Spectroscopy, X‐ray diffraction and TEM. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of ZnO single crystals by an induced nucleation from a high temperature solution of the ZnO‐PbF2 system

To grow ZnO single crystals from a high temperature solution of the ZnO‐PbF₂ system, a gas cooling system was assembled at the bottom of the crucible to induce nucleation in the initial growth stage. The growth experiments were carried out in a homemade vertical Bridgman furnace and Pt crucible of 28 mm in diameter was used. The furnace temperature was set to 1100°C and the flow rate of the oxygen gas was optimized as 3.0 l/min. ZnO crystal up to 5∼8mm in the thickness was obtained with the lowering rate of 0.3 mm/h. XRD patterns showed that the as‐grown crystal was pure ZnO Wurtzite phase. The impurity ions were analyzed by the glow discharge mass spectroscopy (GDMS) as 390.0 ppm and 40.0 ppm for Pb²⁺ and F^‐, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of YCOB single crystals by flux technique and their characterization

Nonlinear optical single crystals of YCOB with good optical quality were grown by the flux technique for the first time. Polycrystalline YCOB samples were synthesized by solid state reaction method. The thermal analysis of the sample was performed with lithium carbonate flux in different weight proportions and the growth temperature was optimised. Single crystals of YCOB with dimensions 3 × 3 × 5 mm³ were obtained by the method of ‘slow‐cooling’. The grown crystals were characterized by XRD, UV‐VIS‐NIR, EDAX, FTIR and etching studies. The powder XRD pattern revealed the formation of YCOB compound. The lattice parameters were identified through single crystal XRD studies. The UV‐VIS‐NIR results showed that the crystal has a sharp cutoff at 220 nm and is nearly 55% transparent over a wide wavelength range enabling applications in the UV region. The EDAX measurement revealed the ‘flux‐free’ crystal formation. The presence of the functional groups belonging to the YCOB crystals was identified by the FTIR results. ‘Hillock‐like’ patterns are observed in the etching studies. The primary emphasis in this study is laid to describe ‘flux technique’ as an alternative method to grow YCOB crystals. The results are presented and discussed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis,structure and photoluminescence of a 3D pillared heterometallic coordination polymer containing 2D inorganic cadmium‐potassium‐oxide layer subunits

Self‐assembly of Cd(NO₃)₂ with o ‐phthalic acid monopotassium salt (KHphth) in the presence of ethylenediamine (en) produced a new heterometallic coordination polymer [CdK₂(phth)₂(en)_0.5(H₂O)]_n ( 1 ). Single‐crystal X‐ray analyses reveal that it crystallizes in a monoclinic space group P 2₁/c. a = 11.6707(6) Å, b = 8.1019(4) Å, c = 20.9503(11) Å, β = 94.6640(10)^o. The complex displays an en‐pillared 3D framework, which is constructed from 2D [CdK₂(phth)₂(H₂O)]_n layers featuring uncommon inorganic cadmium‐potassium‐oxide layers containing potassium‐oxide layers. In the solid state, complex 1 shows photoluminescence with the maximum emission intensities at 355 nm upon excitation at 312 nm. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation of PbS nano‐microcrystals with different morphologies and their optical properties

PbS nano‐microcrystals were prepared from Pb(OAc)₂·3H₂O and sulfur in a solution without any surfactant using the solvothermal process. Different morphologies, mainly including polyhedron microcrystals and sphere‐like assemblies, were characterized using a scanning electron microscope (SEM) and a transmission electron microscope (TEM). PbS nano‐microcrystals with cubic crystal structure were detected using X‐ray diffraction (XRD), electron diffraction (ED) and high resolution transmission electron micrograph (HRTEM) techniques. The optical properties were investigated by ultraviolet‐visible (UV‐vis) spectroscopy, and photoluminescence spectroscopy (PL). The UV‐vis absorption peaks of PbS exhibited a large blue‐shift and the PL spectra had a strong and broad emission bands centered at 408 nm. The crystal growth mechanism of PbS was also discussed.     

Growth and characterization of 4‐(2‐hydroxyphenylamino)‐pent‐3‐en‐2‐one single crystals

4‐(2‐hydroxyphenylamino)‐pent‐3‐en‐2‐one (HPAP) was synthesized and single crystals were grown by the solution growth technique using methanol as a solvent. The crystals having orthorhombic symmetry were characterized by single crystal XRD, FTIR spectroscopy, NMR spectroscopy, TGA, DSC and dielectric studies. Very less variation in the value of dielectric constant is found for different frequencies of applied field. The crystals were exhibiting positive photoconductivity and poor NLO responses. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low‐temperature elastic and dielectric properties of K3Na(SO4)2 crystals

Elastic and dielectric properties of glaserite K₃Na(SO₄)₂ single crystal were examined using the method of composite oscillators, Brillouin light scattering methods and dielectric spectroscopy. Measurements were performed in the temperature range from 18 K to 300 K. Anomalies in the temperature dependencies of Brillouin shift and dielectric permittivity at about 70 K confirmed the earlier predicted phase transition at 75 ± 25 K. Temperature dependences of the resonance frequency of the vibrating composite oscillator, Brillouin shift measured in the [110] direction, components of dielectric permittivity tensor reveal an anomaly at about 50 K. Moreover, thermal hysteresis of the dielectric permittivity suggested the presence of an incommensurate state between T ₁ = 50 K and T ₂ = 70 K. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and characterization of magneto‐optical YFeO3 crystals

The floating zone growth of magneto‐optical crystal YFeO₃ has been investigated. The polycrystalline feed rod was prepared by a pressure of 250MPa and sintering at about 1500°C. A crack‐ free YFeO₃ single crystal has been successfully grown. The crystal preferred to crystallize along <100> direction with about 10° deviation. The X‐ray rocking curve of the crystal has a FWHM of 24 arcsec, confirming the high crystal quality of the sample. The (100) plane was etched by hot phosphoric acid and the dislocation density was about 10⁴/cm². A thin outer layer with Y₂O₃‐rich composition was found at the periphery of as‐grown crystals, which was attributed to the Fe₂O₃ evaporation during growth. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)