As-grown Crystals Research Articles

Growth and structure of Cd1–xDyxTe crystals

CdTe crystals doped with dysprosium were grown by the vertical Bridgman method to explore the possibility of obtaining Cd1−xDyxTe ternary compound. The growth was carried out from the Cd1−xDyxTe pre-synthesized feeds with the charge composition in the range of x=0.001–0.05. The as-grown crystals were studied by scanning electron microscopy, X-ray diffraction and optical spectroscopy. Experimental findings testify that the Cd1−xDyxTe crystals have the homogeneous cubic structure similar to CdTe in the narrow composition range of x≈0–0.01. At higher compositions x≥0.01, the crystals become a mixture of several phases such as CdTe host, Dy2Te3 textures and DyTe-rich inclusions.

Point-contact Andreev reflection spectroscopy on Bi2Se3 single crystals

To study how Andreev reflection (AR) occurs between a superconductor and a three-dimensional topological insulator (TI), we use superconducting Nb tips to perform point-contact AR spectroscopy at 4.2 K on as-grown single crystals of Bi2Se3. Scanning tunneling spectroscopy and scanning tunneling microscopy are also used to characterize the superconducting tip and both the doping level and surface condition of the TI sample. The point-contact measurements show clear spectral signatures of AR, as well as a depression of zero-bias conductance with decreasing junction impedance. The latter observation can be attributed to interfacial Rashba spin-orbit coupling, and the presence of bulk bands at the Fermi level in our samples suggests that bulk states of Bi2Se3 are involved in the observed AR.

Preparation and optical properties of LiInSe2 crystals

A red LiInSe2 crystal with 20mm in diameter and a yellow LiInSe2 crystal with 30mm in diameter were obtained by the vertical gradient freezing method in a two-zone furnace. The thermal annealing technologies in different atmospheres were investigated. The thermal and optical properties were characterized by simultaneous differential scanning calorimetric/thermo-gravimetric (DSC-TG) analysis and an infrared (IR) spectrophotometer. Four as-grown and annealed wafers whose colors changed from red to yellow were discussed in detail on the basis of IR spectra. The results demonstrate that the transparent range of all LiInSe2 samples is from 0.6μm to 13.5μm. The thermal annealing processes both in vacuum and in LiInSe2 polycrystalline can obviously reduce the optical losses of red crystals. The optical losses of the deep red annealed crystal is at the level of 0.1cm−1 in the range 2.0–8.5μm. The yellow as-grown crystals exhibit a better transmittance especially in near IR range by comparison with red crystals.

Growth of shape-controlled Ce:Y3Al5O12 scintillator crystal and their scintillation properties

A shape-controlled Ce doped Y3Al5O12 (Ce:YAG) single crystal has been grown by a micro-pulling-down method using an Ir crucible with a square die. The as-grown Ce:YAG single crystal has square-shape and flat surfaces with high transparency. The shape-controlled Ce:YAG single crystal showed more than 70% transmittance from 530nm to 900nm without any cutting and polishing. An emission peak around 540nm was observed in the radioluminescence spectrum under X-ray irradiation. In addition, the light yield and energy resolution under γ-ray irradiation of the as-grown crystal without any cutting and polishing were approximately 23,000pH/MeV and 16%, respectively.

Growth, structural, dielectric, ferroelectric, and mechanical properties of l-prolinium tartrate single crystal

Organic ferroelectric l-prolinium tartrate (LPT) single crystals have been successfully grown by slow evaporation solution technique. The as-grown LPT crystal has been analyzed by CHN analysis to find the elemental compositions. Single-crystal XRD reveals the monoclinic lattice with non-centrosymmetric space group (P21) as well as cell parameters (a = 5.0071(3) A, b = 17.7056(13) A, c = 6.5371(4) A, β = 100.525(6) °C, and V = 569.79(6) A3). The Hirshfeld surface analysis was carried out to understand various intermolecular interactions in the grown crystal. From temperature-dependent dielectric measurement, a ferroelectric to paraelectric phase transition was observed at 45 °C and a very low (0.05) dielectric loss for higher frequencies (2 MHz) was observed. The FTIR spectroscopy has been carried out in order to identify the different functional groups present in the grown crystal. In ferroelectric studies, P–E hysteresis loops were traced at different temperatures. A high value of piezoelectric charge coefficient (d 33 = 17 pC/N) was found. Vickers hardness test has been used to determine the mechanical strength of the grown crystals and the value of Meyer’s index was found to be n = 1.79 which suggested soft nature of crystals.

Synthesis of In2S3 and Ga2S3 crystals for oxygen sensing and UV photodetection

Crystal growth, characterization, and sensing application of In2S3 and Ga2S3 are demonstrated herein. Single crystals of In2S3 and Ga2S3 were grown by chemical vapor transport method using ICl3 as the transport agent. The as-grown In2S3 crystals essentially show dark red and Ga2S3 displays transparent and light-yellow colored. X-ray diffraction measurements show tetragonal phase of the as-grown In2S3 crystals while Ga2S3 crystallizes in the monoclinic structure. The optical band edges of In2S3 and Ga2S3 were characterized by absorption and thermoreflectance measurements. The optical band gaps of In2S3 and Ga2S3 are 1.935eV and 3.052eV, respectively. The direct gaps of In2S3 and Ga2S3 lie in between red to violet visible region. Photoluminescence (PL) experiments characterize defect and surface-state emissions of the In2S3 and Ga2S3 crystals. The In2S3 can easily form surface oxide in environmental air with an In2O3 growth rate of ∼100nm/day (i.e. for oxygen sensing). The PL intensity of the In2O3 formed on In2S3 may be a sensing index for oxygen content detection. Besides, Ga2S3 may emit a broadened visible white light (near band edge) due to the existence of intrinsic defects inside the crystal. The Ga2S3 crystal also displays a high-sensitivity photoconductivity change illuminated by a 405nm laser. According to the experimental results, In2S3 can be an oxygen sensor with the formation of surface oxide (i.e. In2O3), while Ga2S3 may possess potential capability in fabrication of blue to UV photodetector.

Peculiar orientational disorder in 4-bromo-4'-nitrobiphenyl (BNBP) and 4-bromo-4'-cyanobiphenyl (BCNBP) leading to bipolar crystals.

180° orientational disorder of molecular building blocks can lead to a peculiar spatial distribution of polar properties in molecular crystals. Here we present two examples [4-bromo-4'-nitrobiphenyl (BNBP) and 4-bromo-4'-cyanobiphenyl (BCNBP)] which develop into a bipolar final growth state. This means orientational disorder taking place at the crystal/nutrient interface produces domains of opposite average polarity for as-grown crystals. The spatial inhomogeneous distribution of polarity was investigated by scanning pyroelectric microscopy (SPEM), phase-sensitive second harmonic microscopy (PS-SHM) and selected volume X-ray diffraction (SVXD). As a result, the acceptor groups (NO2 or CN) are predominantly present at crystal surfaces. However, the stochastic process of polarity formation can be influenced by adding a symmetrical biphenyl to a growing system. For this case, Monte Carlo simulations predict an inverted net polarity compared with the growth of pure BNBP and BCNBP. SPEM results clearly demonstrate that 4,4'-dibromobiphenyl (DBBP) can invert the polarity for both crystals. Phenomena reported in this paper belong to the most striking processes seen for molecular crystals, demonstrated by a stochastic process giving rise to symmetry breaking. We encounter here further examples supporting the general thesis that monodomain polar molecular crystals for fundamental reasons cannot exist.

Optically stimulated luminescence (OSL) from Ag-doped Li2B4O7 crystals

Optically stimulated luminescence (CW-OSL) is observed from Ag-doped lithium tetraborate (Li2B4O7) crystals. Photoluminescence, optical absorption, and electron paramagnetic resonance (EPR) are used to identify the defects participating in the OSL process. As-grown crystals have Ag+ ions substituting for Li+ ions. They also have Ag+ ions occupying interstitial sites. During a room-temperature exposure to ionizing radiation, holes are trapped at the Ag+ ions that replace Li+ ions and electrons are trapped at the interstitial Ag+ ions, i.e., the radiation forms Ag2+ (4d9) ions and Ag0 (4d105s1) atoms. These Ag2+ and Ag0 centers have characteristic EPR spectra. The Ag0 centers also have a broad optical absorption band peaking near 370nm. An OSL response is observed when the stimulation wavelength overlaps this absorption band. Specifically, stimulation with 400nm light produces an intense OSL response when emission is monitored near 270nm. Electrons optically released from the Ag0 centers recombine with holes trapped at Ag2+ ions to produce the ultraviolet emission. The OSL response is progressively smaller as the stimulation light is moved to longer wavelengths (i.e., away from the 370nm peak of the absorption band of the Ag0 electron traps). Oxygen vacancies are also present in the Ag-doped Li2B4O7 crystals, and their role in the OSL process as a secondary relatively short-lived electron trap is described.

Ho:LaF3 single crystal as potential material for 2 μm and 2.9 μm lasers

Optical properties of a Ho-doped LaF3 single crystal have been detailed investigated as a promising material for 2μm and 2.9μm lasers for the first time. Judd–Ofelt theory was applied to analyze the absorption spectrum to determine the J–O intensity parameters Ωt(t=2,4,6), based on which the emission probabilities, branching ratio and radiative lifetime for the as-grown crystal were all calculated. The stimulated emission cross-sections of the 5I7→5I8 and 5I6→5I7 transitions were obtained by using the Fuchtbauer–Ladenburg method. The gain cross-section for 2μm emission becomes positive once the population inversion level reaches 30%. The Ho:LaF3 crystal shows long fluorescence lifetime of 5I7 manifold (25.81ms) as well as 5I6 manifold (10.37ms) compared with other Ho3+-doped crystals. It can be proposed that the Ho:LaF3 crystal may be a promising material for 2μm and 2.9μm laser applications.

Low-temperature crystal and magnetic structure ofα−RuCl3

Single crystals of the Kitaev spin-liquid candidate $\alpha$-RuCl$_3$ have been studied to determine low-temperature bulk properties, structure and the magnetic ground state. Refinements of x-ray diffraction data show that the low temperature crystal structure is described by space group $C2/m$ with a nearly-perfect honeycomb lattice exhibiting less than 0.2 \% in-plane distortion. The as-grown single crystals exhibit only one sharp magnetic transition at $T_{N}$ = 7~K. The magnetic order below this temperature exhibits a propagation vector of $k$ = (0, 1, 1/3), which coincides with a 3-layer stacking of the $C2/m$ unit cells. Magnetic transitions at higher temperatures up to 14~K can be introduced by deformations of the crystal that result in regions in the crystal with a 2-layer stacking sequence. The best fit symmetry allowed magnetic structure of the as-grown crystals shows that the spins lie in the $ac$-plane, with a zigzag configuration in each honeycomb layer. The three layer repeat out-of-plane structure can be refined as a 120$^o$ spiral order or a collinear structure with spin direction 35$^o$ away from the $a$-axis. The collinear spin configuration yields a slightly better fit and also is physically preferred. The average ordered moment in either structure is less than 0.45(5) $\mu_B$ per Ru$^{3+}$ ion.

Exploring the potential of laser assisted flow deposition grown ZnO for photovoltaic applications

Zinc oxide (ZnO) is a widely studied wide band gap semiconductor with applications in several fields, namely to enhance solar cells efficiency. Its ability to be grown in a wide variety of nanostructured morphologies, allowing the designing of the surface area architecture constitutes an important advantage over other semiconductors. Laser assisted flow deposition (LAFD) is a recently developed growth method, based on a vapour-solid mechanism, which proved to be a powerful approach in the production of ZnO micro/nanostructures with different morphologies as well as high crystallinity and optical quality. In the present work we report the use of the LAFD technique to grow functional ZnO nanostructures (nanoparticles and tetrapods) working as nano templates to improve the dye-sensitized solar cells (DSSCs) efficiency. The structural and morphological characterization of the as-grown ZnO crystals were performed by X-ray diffraction and electron microscopy, respectively, and the optical quality was assessed by photoluminescence spectroscopy. DSSCs were produced using a combination of these nanostructures, which were subsequently sensitized with N719 dye. An efficiency of ∼3% was achieved under simulated AM 1.5 illumination conditions for a dye loading time of 1 h.

Nonstoichiometry and luminescent properties of ZnSe crystals grown from the melt at high pressures

50mm diameter ZnSe crystals have been grown from the melt by a vertical Bridgman technique at 100atm argon pressure in a graphite crucible. 3D impurities concentration and nonstoichiometry mappings of the grown crystals have been defined by ICP-MS and a direct physic-chemical method, correspondingly. Photoluminescence mapping of the analyzed crystal has been done. It was found out that along the crystal height the nonstoichiometry changed from Se excess over stoichiometrical composition in the cone (bottom) part to Zn excess in the tail (upper) part passing through the stoichiometrical composition in the cylindrical part of the crystal. Metal impurities concentrated in the upper part of the crystal. The gas-forming impurities (H, C, O, N, F) had stochastic distribution but Cl impurity concentrated in the crystal peripheral part (near the crucible walls). It was found out that the as-grown crystal had a single wide PL peal with maximum of 583nm. A proposal about complex structure luminescent center based on Cl dopant an overstoichiometric Se has been made.

Synthesis, growth, structure and characterization of chalcone crystal: A novel organic NLO material

Single crystals of a chalcone, (E)-3-(4-bromophenyl)-1-(2-hydroxyphenyl)prop-2-en-1-one (BHP), were grown by the slow evaporation solution growth technique. The structure is elucidated by single-crystal X-ray diffraction analysis and the crystal belongs to the monoclinic system with noncentrosymmetric space group P21. Optical studies reveal that the absorption is minimum in the visible region and the cut-off wavelength is at ∼468 nm. The band-gap energy was estimated by the application of the Kubelka–Munk algorithm. The powder X-ray diffraction pattern reveals the good crystallinity of the as-grown specimen. The vibrational patterns in FT–IR are used to identify the functional groups and thermal studies indicate the stability of the material. The second harmonic generation efficiency (SHG), as estimated by Kurtz and Perry powder technique, reveals the superior nonlinear optical character of this material. Hirshfeld surface analysis is done to quantify the intermolecular interactions, responsible for developing a nonlinear atmosphere. As-grown crystals were further characterized by SEM, NMR, mass spectrometry and elemental analysis.

Critical Current Density and Vortex Dynamics in Fe(Te,Se) Annealed in Various Atmosphere

As-grown crystals of Fe(Te,Se) do not show bulk superconductivity due to the presence of excess irons. Annealing in an appropriate atmosphere can remove harmful excess irons and make them bulk superconductors. We report the effects of annealing on Fe1+y(Te0.6Se0.4) crystal in chalcogen (O2, S, Se, Te) and pnictogen (P, As, Sb) atmospheres. It is found that the optimal molar ratios of agents to the total irons are different for different agents, indicating that different mechanisms for the removal of excess irons are operative. The normal state and superconducting properties of optimally annealed crystal are carefully investigated. Critical current density and its relaxation measurements clarified that the vortex dynamics in Fe1+y(Te0.6Se0.4) is described by collective creep theory. Detailed analyses of the temporal evolution of the critical current at various temperatures clarified that the pinning of vortices in Fe1+y(Te0.6Se0.4) is a combination of $\Delta T_{\mathrm{c}}$ and $\Delta l$ pinnings.

Mechanism of Surface Cracking in a Ca12Al14O33 Crystal during the Cooling Process

Bulk Ca12Al14O33 (C12A7) crystals were successfully grown by the Czochralski (Cz) method. However, the surface exhibited severe cracking to a depth of about 1 mm during cooling of the as-grown crystals to room temperature. To determine the mechanism of the surface cracking we measured powder crystal X-ray diffraction, Raman spectroscopy, thermal gravimetry, and infrared spectroscopy of the OH-stretching band. It was found that the lattice parameters of the cracked crystal surface and the core were different. A weight increase in the TGA curve was observed during the cooling process. Furthermore, the OH– ion content of the crystal increased after annealing experiments. It can be concluded that incorporation of hydroxide ions caused the surface cracking of the as-grown C12A7 crystals.

Melt growth, structure and properties of (ZrO2)1−x(Sc2O3)x solid solution crystals (x=0.035−0.11)

Crystals of (ZrO2)1−x(Sc2O3)x solid solutions with x=0.035, 0.06, 0.09 and 0.11 have been grown for the first time using the directional crystallization technique. Analysis of the scandium distribution along the crystal showed that the composition of all specimens is homogeneous, and the Sc2O3 concentration is almost identical to its content in the charge. All specimens exhibit a little decline in the scandium concentration along the crystal, this indicating that the effective distribution coefficient Sc is slightly greater than 1. The structure of as-grown crystals has been studied as a function of the Sc2O3 stabilising oxide concentration by X-ray diffraction, transmission electron microscopy and Raman spectroscopy. Crystals containing 3.5mol% Sc2O3 are a mixture of the monoclinic and tetragonal phases, the crystals containing 6mol% Sc2O3 have a tetragonal structure, those with 9mol% Sc2O3 have the tetragonal phase with inclusions of the rhombohedral one and the specimens with 11mol% Sc2O3 represent the rhombohedral phase with inclusions of the cubic phase. The electrical conductivity was measured as a function of temperature by electrochemical impedance spectroscopy. The conductivity of the scandia stabilized crystals, in spite of their inhomogeneity, presence of stresses and low fracture toughness, is comparable with that of the yttria stabilized zirconia crystals.

Polar Order and Frustrated Antiferromagnetism in Perovskite Pb2MnWO6 Single Crystals.

Single crystals of the multiferroic double-perovskite Pb2MnWO6 have been synthesized and their structural, thermal, magnetic and dielectric properties studied in detail. Pure perovskite-phase formation and stoichiometric chemical composition of the as-grown crystals are confirmed by X-ray single-crystal and powder diffraction techniques as well as energy-dispersive X-ray and inductively coupled plasma mass spectrometry. Detailed structural analyses reveal that the crystals experience a structural phase transition from the cubic space group (s.g.) Fm3̅m to an orthorhombic structure in s.g. Pn21a at about 460 K. Dielectric data suggest that a ferrielectric phase transition takes place at that same temperature, in contrast to earlier results on polycrystalline samples, which reported a transition to s.g. Pnma and an antiferroelectric low-temperature phase. Magnetic susceptibility measurements indicate that a frustrated antiferromagnetic phase emerges below 8 K. Density functional theory based calculations confirm that the cationic order between Mn and W is favorable. The lowest total energy was found for an antiferromagnetically ordered state. However, analyses of the calculated exchange parameters revealed strongly competing antiferromagnetic interactions. The large distance between the magnetic atoms, together with magnetic frustration, is shown to be the main reason for the low value of the ordering temperature observed experimentally. We discuss the structure-property relationships in Pb2MnWO6 and compare these observations to reported results on related Pb2BWO6 perovskites with different B cations.

Low-Temperature Growth of Two-Dimensional Layered Chalcogenide Crystals on Liquid.

The growth of high-quality two-dimensional (2D) layered chalcogenide crystals is highly important for practical applications in future electronics, optoelectronics, and photonics. Current route for the synthesis of 2D chalcogenide crystals by vapor deposition method mainly involves an energy intensive high-temperature growth process on solid substrates, often suffering from inhomogeneous nucleation density and grain size distribution. Here, we first demonstrate a facile vapor-phase synthesis of large-area high-quality 2D layered chalcogenide crystals on liquid metal surface with relatively low surface energy at a growth temperature as low as ∼100 °C. Uniform and large-domain-sized 2D crystals of GaSe and GaxIn1-xSe were grown on liquid metal surface even supported on a polyimide film. As-grown 2D GaSe crystals have been fabricated to flexible photodetectors, showing high photoresponse and excellent flexibility. Our strategy of energy-sustainable low-temperature growth on liquid metal surface may open a route to the synthesis of high-quality 2D crystals of Ga-, In-, Bi-, Hg-, Pb-, or Sn-based chalcogenides and halides.

Microstructure and shape memory behavior of [111]-oriented NiTiHfPd alloys

The relationship between the microstructure and shape memory properties of [111]-oriented Ni45.3Ti29.7Hf20Pd5 (at%) single crystals was explored. In this precipitation-strengthened alloy, the size and volume fraction of precipitates and interparticle distances govern the martensite morphology and the ensuing shape memory responses. Aging of the solution-treated material, leading to a microstructure of fine, closely spaced precipitates, resulted in a material capable of a shape memory strain of 2.15% at 1000 MPa in compression. Larger precipitates formed after aging the as-grown single crystals (without a prior solution treatment) resulting in a shape memory strain of 2.5% at this same stress level in constant-stress thermal cycling experiments. Superelastic strains of 4% in compression without any residual strain were possible under various microstructural conditions and the stress hysteresis could be varied between nearly 500 and 1000 MPa depending on the microstructure.

Existence of Large Antiferromagnetic Spin Fluctuations in Ce-Doped T′-Cuprate Superconductors

For the understanding of Ce-doped high-Tc cuprates with the so-called T′-structure, we have performed 63,65Cu and 139La NMR measurements in T′-Pr1.3−xLa0.7CexCuO4+δ single crystals with x = 0.10 and 0.15. The Neel temperature (TN) was evaluated from the temperature (T) dependence of the 139La NMR line width. For x = 0.10, TN decreased from 190 K in the as-grown crystal to 60 K in the oxygen-reduced crystal annealed at 800 °C. For x = 0.15, TN of 170 K in the as-grown crystal decreased to zero through the same heat treatment. The oxygen reduction strongly suppressed the antiferromagnetic (AF) order. 63,65Cu NMR spectra had a narrow center line and broad satellites with a small electric field gradient in the paramagnetic state. In the AF state, in contrast, 63,65Cu NMR spectra had a large electric field gradient. The T dependence of the 63Cu nuclear spin–lattice relaxation rate (1/T1) showed the existence of large spin fluctuations in the paramagnetic state. The spin fluctuations seem to play a key role in ...