A room temperature solution-phase process to synthesize pure phase single-crystalline hexagonal cobalt hydroxide nanoplates

A simple room temperature solution-phase method has been developed to synthesize pure phase single-crystalline hexagonal β-Co(OH)₂ nanoplates. The chemical composition and morphology of the as-prepared β-Co(OH)₂ nanoplates were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area electron diffraction (SAED). The results indicated that the as-prepared β-Co(OH)₂ nanoplates were composed of pure brucite-like β-Co(OH)₂ phase with single-crystalline feature. The effect of polyethylene glycol (PEG), NH₄OH, and NaOH on the morphology and size of β-Co(OH)₂ nanocrystals were discussed in detail. The growth mechanism of the as-synthesized nanoplates was also discussed in detail based on the experimental results.     

Thermal-induced phase transition and assembly of hexagonal metastable In2O3 nanocrystals: A new approach to In2O3 functional materials

This paper reports on the thermal-induced performance of hexagonal metastable In₂O₃ nanocrystals involving in phase transition and assembly, with particular emphasis on the assembly for the preparation of functional materials. For In₂O₃ nanocrystals, the metastable phase was found to be thermally unstable and transform to cubic phase when temperature was higher than 600 °C, accompanied by assembly as well as evolution of optical properties, but the two polymorphs coexisted at the temperature ranging from 600 to 900 °C, during which the content of product phase and crystal size gradually increased upon increasing temperature. The assembly of In₂O₃ nanocrystals can be developed to fabricate In₂O₃ functional materials, such as various ceramic materials, or even desired nano- or micro-structures, by using metastable In₂O₃ nanocrystals as precursors or building blocks. The electrical resistivity of In₂O₃ conductive film fabricated by a hot-pressing route was as low as 3.72×10⁻³ Ω cm, close to that of In₂O₃ single crystal, which is important for In₂O₃ that is always used as conductive materials. The findings should be of importance for both the wide applications of In₂O₃ in optical and electronic devices and theoretical investigations on crystal structures.     

Crystal growth and spectroscopic properties of Yb:CaYAlO4 single crystal

A Yb³⁺-doped CaYAlO₄ laser crystal has been grown by the Czochralski technique. The segregation coefficient was measured by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). The cell parameters were analyzed with X-ray diffraction experiments. Color defects in Yb:CaYAlO₄ have been evidenced to be similar to those in undoped CaYAlO₄. The polarized absorption spectra and the fluorescence spectrum of the Yb:CaYAlO₄ crystal were measured at room temperature. The fluorescence decay time of the Yb³⁺ ion was investigated. The results show that Yb:CaYAlO₄ has potential as a laser gain medium for an ultrashort laser system.     

Growth and optical properties of self-frequency-doubling laser crystal Yb:LuAl3(BO3)4

Single crystal of Yb:LuAl₃(BO₃)₄(Yb:LuAB) was grown by the flux method for the first time. The cell parameters of the grown crystal were estimated by X-ray diffraction analysis. The result indicates the symmetry of trigonal space group R32, with lattice parameters a=b=9.26372 Å, c=7.21405 Å, V=536.14 Å³, and Z=4. The absorption and emission spectra of Yb:LuAB crystal at room temperature has also been studied. The fluorescence lifetime for Yb:LuAB crystal is about 1.48 ms. The heat capacity was measured from 25 to 500 °C. Its second harmonic generation efficiency in LuAl₃(BO₃)₄ crystal is 3–4 times that of KDP crystal. These results show that Yb:LuAB crystal would be a potential self-frequency-doubling laser crystal.     

Facile synthesis and luminescence properties of octahedral YVO4:Eu microcrystals

Uniform octahedral YVO₄:Eu³⁺ microcrystals have been successfully prepared through a designed two-step hydrothermal conversion method. One-dimensional precursor Y₄O(OH)₉NO₃ was first prepared through a simple hydrothermal process without using any surfactant, catalyst or template. Subsequently, well-defined octahedral YVO₄ was synthesized at the expense of the precursor during a hydrothermal conversion process. XRD results demonstrate that the diffraction peaks of the final product can be well indexed to the pure tetragonal phase of YVO₄. The SEM and TEM images indicate that the as-prepared YVO₄ sample has regular octahedral shape with sharp corners and well-defined edges. The as-obtained YVO₄:Eu³⁺ phosphor shows strong red emission under ultraviolet excitation or low-voltage electron beam excitation. Furthermore, this facile and general conversion method may be of much significance in the synthesis of many other lanthanide compounds with uniform morphology.     

Growth and shape control of orthorhombic Fe5(PO4)4(OH)3·2H2O single crystalline dendrites

Orthorhombic Fe₅(PO₄)₄(OH)₃·2H₂O single crystalline dendritic nanostructures have been synthesized by a facile and reproducible hydrothermal method without the aid of any surfactants. The influences of synthetic parameters, such as reaction time, temperature, the amount of H₂O₂ solution, pH values, and types of iron precursors, on the crystal structures and morphologies of the resulting products have been investigated. The formation process of Fe₅(PO₄)₄(OH)₃·2H₂O dendritic nanostructures is time dependent: amorphous FePO₄·nH₂O nanoparticles are formed firstly, and then Fe₅(PO₄)₄(OH)₃·2H₂O dendrites are assembled via a crystallization-orientation attachment process, accompanying a color change from yellow to green. The shapes and sizes of Fe₅(PO₄)₄(OH)₃·2H₂O products can be controlled by adjusting the amount of H₂O₂ solution, pH values, and types of iron precursors in the reaction system.     

Simple hydrothermal preparation of -MnOOH nanowires and their low-temperature thermal conversion to β-MnO2 nanowires

Without the use of any extra surfactant or template, γ-MnOOH single crystalline nanowires were synthesized directly through the hydrothermal reaction between KMnO₄ and toluene in distilled water at 180 °C for 24 h; and β-MnO₂ single crystalline nanowires could be obtained just by calcination of the γ-MnOOH nanowires in air at 280 °C for 5 h. The as-prepared γ-MnOOH and β-MnO₂ nanowires were characterized by X-ray powder diffraction, atomic absorption spectroscopy, Fourier transformed infrared spectroscopy, scanning electron microscope, transmission electron microscope, high-resolution transmission electron microscope and selected area electron diffraction.     

Facile synthesis and assembly of CuS nano-flakes to novel hexagonal prism structures

Highly ordered hexagonal prism microstructures of copper sulfide (CuS) by assembling nano-flakes have been synthesized with high yield via a facile one-step route. We synthesized CuS microstructures using low cost beginning materials CuSO₄·5H₂O and Na₂S₂O₃·5H₂O under lower reaction temperature (60 °C). Hexamethylinetetramin (C₆H₁₂N₄, HMT) was introduced into the reaction system as a capped agent. The influence of reaction time and capping agent (HMT) on the final structure of products was studied systematically. The obtained products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopes (EDS), and transmission electron microscopy (TEM). The possible mechanism for the formation of the interesting highly ordered hexagonal prism microstructures CuS was also proposed.     

Synthesis,morphology and phase transition of the zinc molybdates ZnMoO4·0.8H2O/α-ZnMoO4/ZnMoO4 by hydrothermal method

ZnMoO₄ with a rhombus sheet or flower-like structure, α-ZnMoO₄ and needle-like ZnMoO₄·0.8 H₂O were successfully synthesized by simple hydrothermal crystallization processes with citric acid. ZnMoO₄·0.8 H₂O was easily synthesized in a shorter reaction time (2 h) at a higher reactant concentration. It gradually transformed into ZnMoO₄ with a monoclinic wolframite tungstate structure with an increased reaction time, and pure ZnMoO₄ was obtained with a longer reaction time (8 h). Citric acid (CA) played an important role in controlling the morphology of the as-obtained molybdates. The α-ZnMoO₄ and ZnMoO₄ were synthesized by heating ZnMoO₄·0.8 H₂O at 130 °C for 4 h and 8 h, respectively, under hydrothermal conditions. With transforming of ZnMoO₄·0.8 H₂O to α-ZnMoO₄ and further to ZnMoO₄, the needle-like crystals gradually disappeared and were transformed into crystals with rhombus sheet morphology and then further to pentacle or flower-like crystals that can be ascribed to continuous splitting and growing of the rhombus sheets.     

Template free-solvothermaly synthesized copper selenide (CuSe,Cu2−xSe, β-Cu2Se and Cu2Se) hexagonal nanoplates from different precursors at low temperature

Nonstoichiometric (Cu_2−xSe) and stoichiometric (CuSe, β-Cu₂Se and Cu₂Se) copper selenide hexagonal nanoplates have been synthesized using different general and convenient copper sources, e.g. copper chloride, copper sulphate, copper nitrate, copper acetate, elemental copper with elemental selenium, friendly ethylene glycol and hydrazine hydrate in a defined amount of water at 100 °C within 12 h adopting the solvothermal method. Phase analysis, purity and morphology of the product have been well studied by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray diffraction (EDAX) techniques. The structural and compositional analysis revealed that the products were of pure phase with corresponding atomic ratios. SEM, TEM and HRTEM analyses revealed that the nanoplates were in the range 200–450 nm and the as-prepared products were uniform and highly crystallized. The nanoplates consisted of {0 0 1} facets of top–bottom surfaces and {1 1 0} facets of the other six side surfaces. This new approach encompasses many advantages over the conventional solvothermal method in terms of product quality (better morphology control with high yield) and reaction conditions (lower temperatures). Copper selenide hexagonal nanoplates obtained by the described method could be potential building blocks to construct functional devices and solar cell. This work may open up a new rationale on designing the solution synthesis of nanostructures for materials possessing similar intrinsic crystal symmetry. On the basis of the carefully controlled experiments mentioned herein, a plausible formation mechanism of the hexagonal nanoplates was suggested and discussed. To the best of our knowledge, this is the first report on nonstoichiometric (Cu_2−xSe) as well as stoichiometric (CuSe, β-Cu₂Se and Cu₂Se) copper selenide hexagonal nanoplates with such full control of morphologies and phases by this method under mild conditions.     

Self-formation of dielectric layer containing CoSi2 nanocrystals by plasma-enhanced atomic layer deposition

Dielectric layer containing CoSi₂ nanocrystals was directly fabricated by plasma-enhanced atomic layer deposition using CoCp₂ and NH₃ plasma mixed with SiH₄ without annealing process. Synchrotron radiation X-ray diffraction and X-ray photoelectron spectroscopy results confirmed the formation of CoSi₂ nanocrystal. The gate stack composed of dielectric layer containing CoSi₂ nanocrystals with ALD HfO₂ capping layer together with Ru metal gate was analyzed by capacitance–voltage (C–V) measurement. Large hysteresis of C–V curves indicated charge trap effects of CoSi₂ nanocrystals. The current process provides simple route for the fabrication of nanocrystal memory compatible with the current Si device unit processes.     

Characterization of AlN single crystal fabricated by a novel growth technique, “pyrolytic transportation method”

Aluminum nitride single crystal fabricated by a novel growth technique “pyrolytic transportation method”, which is advantageous for industrial process because of using α-Al₂O₃ as a source material instead of aluminum nitride, was characterized. This growth technique shows high growth rate up to 1.6 mm/h. High crystalline quality was indicated by X-ray topogragh and X-ray rocking curve. Full width at half maximum of (0 0 0 2) and (1 0 −1 0) were excellent values of 90 and 148 arcsec, respectively. Homoepitaxial overgrowth by hydride vapor phase epitaxy was successfully conducted. No dislocation was observed at the interface between the substrate and overgrowth layer by transmission electron microscopy.     

Scale-up synthesis of zinc borate from the reaction of zinc oxide and boric acid in aqueous medium

Synthesis of zinc borate was conducted in a laboratory and a pilot scale batch reactor to see the influence of process variables on the reaction parameters and the final product, 2ZnO·3B₂O₃·3.5H₂O. Effects of stirring speed, presence of baffles, amount of seed, particle size and purity of zinc oxide, and mole ratio of H₃BO₃:ZnO on the zinc borate formation reaction were examined at a constant temperature of 85 °C in a laboratory (4 L) and a pilot scale (85 L) reactor. Products obtained from the reaction in both reactors were characterized by chemical analysis, X-ray diffraction, particle size distribution analysis, thermal gravimetric analysis and scanning electron microscopy. The kinetic data for the zinc borate production reaction was fit by using the logistic model. The results revealed that the specific reaction rate, a model parameter, decreases with increase in particle size of zinc oxide and the presence of baffles, but increases with increase in stirring speed and purity of zinc oxide; however, it is unaffected with the changes in the amount of seed and reactants ratio. The reaction completion time is unaffected by scaling-up.     

Inversion domains and parallel growth in ammonothermally grown GaN crystals

Macro-defects such as twins, inversion domains, crevices, and columnar growth occasionally appear in ammonothermally grown GaN crystals. Twinning mechanisms and parallel growth are proposed to explain the formation of these defects. As a polar crystal with wurtzite structure, GaN can have several different kinds of twins depending on the polarity arrangement of each individual twin. Inversion domains are formed in one of the twinning mechanisms. Parallel growth is used to explain the formation of pits on the nitrogen face and the columnar growth on the gallium face. Etching in hot H₃PO₄ is used to reveal the polarities and defects of GaN crystals when they are indistinguishable. Optical microscopy, scanning electron microscope, and cathodoluminescence are also employed to study these defects. In addition, seed quality, avoidance of macro-defect formation, and impurity effects are also discussed.     

Microwave-assisted synthesis of spheroidal vaterite CaCO3 in ethylene glycol–water mixed solvents without surfactants

Spheroidal vaterite CaCO₃ composed of irregular nanoparticals have been synthesized by a fast microwave-assisted method. The structures are fabricated by the reaction of Ca(CH₃COO)₂ with (NH₄)₂CO₃ at 90 °C in ethylene glycol–water mixed solvents without any surfactants. The diameters of the spheroidal vaterite CaCO₃ range from 1 to 2 μm, and the average size of the nanoparticals is about 70 nm. Bundle-shaped aragonite and rhombohedral calcite are also obtained by adjusting the experimental parameters. Our experiments show that the ratio of ethylene glycol to water, microwave power, reaction time, and two sources of ammonium ions and acetate anions are key parameters for the fabrication of spheroidal vaterite CaCO₃. A possible growth mechanism for the spheroidal structures has been proposed, which suggests that the spheroidal vaterite CaCO₃ is formed by an aggregation mechanism.     

Lu5Ir4Si10 whiskers: Morphology,crystal structure,superconducting and charge density wave transition studies

Highly perfect single crystal whiskers of Lu₅Ir₄Si₁₀ were successfully grown out of the melt. Details of the surface and morphology of the whiskers are presented. X-ray diffraction data confirmed that the whisker structure has the same tetragonal P4/mbm space group symmetry as bulk single crystals with lattice parameters a=12.484(1) and c=4.190(2) Å. By means of field emission scanning electron microscopy, the morphology of the whiskers has been studied. Using a 4-circle X-ray diffractometer we found that whiskers grow along the c-axis direction and all side faces are oriented along the [1 1 0] direction. The mosaicity has been measured and is found to be almost perfect: below 0.15° along the c-axis. According to our transport measurements performed along the c-axis, the whiskers present a sharp superconducting transition at T_c=4.1 K and show a charge density wave (CDW) transition at 77 K. From the hysteresis of the temperature dependance of the electrical resistivity study, the CDW transition is found to be of first order.     

Growth kinetics of vanadium pentoxide nanostructures under hydrothermal conditions

The work reported here involved a study of the growth kinetics of V₂O₅nH₂O nanostructures under hydrothermal conditions. The coarsening process of V₂O₅nH₂O nanoribbons was followed by subjecting the as-prepared suspensions to hydrothermal treatments at 80 °C for periods ranging from 0 to 7200 min. X-ray diffraction (XRD) confirms that the hydrothermal treatments at 80 °C caused no significant modification of the long-range order structure of samples subjected to different periods of hydrothermal treatment. Field emission scanning transmission electron microscope (FE-STEM) was used to analyze the morphology and width distribution of the nanostructures. The results indicated that the crystal growth mechanism in the [1 0 0] direction of vanadium pentoxide 1D nanostructure under hydrothermal conditions is well described by the oriented attachment (OA) mechanism. This evidence was supported by HRTEM images showing the existence of defects at the interface between nanostructures, which is characteristic of the oriented attachment (OA) mechanism.     

Crystallographic and dielectric properties of flux grown (B′B″: InNb, InTa, YbNb, YbTa and MgW) single crystals

Single crystals of PbIn_1/2Nb_1/2O₃ (PIN), PbIn_1/2Ta_1/2O₃ (PIT), PbYb_1/2Nb_1/2O₃ (PYN), PbYb_1/2Ta_1/2O₃ (PYT) and PbMg_1/2W_1/2O₃ (PMW) have been grown by the flux method. The PbO-based solvents were used. Transparent, light yellow and arrow like shaped PIN and PIT crystals of the perovskite structure were obtained. Small amounts of red and of octahedron habit PIN and PIT crystals of the pyrochlore type were simultaneously grown. In the case of PYN, PYT and PMW only the crystals of the perovskite structure have been grown. The transparent and brown PYN and PYT crystals of octahedron habit were obtained. The transparent, light yellow and of octahedron or truncated octahedron shape PMW crystals were grown. The crystals were characterised by X-ray and dielectric studies. They showed that as-grown PIN crystals are nearly disordered, exhibit the rhombohedral distortion of the pseudo-perovskite unit cell and reveal relaxor behaviour. The partially ordered PIT crystals show monoclinic distortion and undergo antiferroelectric–paraelectric phase transition. The PYN, PYT and PMW single crystals, characterised by chemical order in the B′/B″ ion sublattice, exhibit orthorhombic symmetry and undergo the first-order antiferroelectric–paraelectric phase transitions.     

Room-temperature template-free synthesis of dumbbell-like SrSO4 with hierarchical architecture

Novel dumbbell-like SrSO₄ with hierarchical architecture was fabricated with a facile template-free aqueous solution method at room temperature. The crystallographic morphology of SrSO₄ products depends mainly on the pH value of the reaction solution. The SrSO₄ products exhibit a dumbbell-like hierarchical architecture at pH=3 and 5, and have a tablet-like crystallographic morphology at pH=1 when keeping other reaction parameters unchanged. The dumbbell-like SrSO₄ synthesized at pH=3 has a length of 8–14 μm, and is composed of numerous well-aligned single crystalline nanoplates with an average width of 140 nm and a length of 0.7–1 μm. The Brunauer–Emmett–Teller (BET) surface area of the crystallized SrSO₄ products is about 2.8 m² g⁻¹. A formation mechanism is proposed for the evolution process of dumbbell-like SrSO₄ with hierarchical architecture.     

Cs2LiGdCl6 (Ce): New scintillation material

We investigated the scintillation properties of Cs₂LiGdCl₆:Ce³⁺ as a function of the Ce concentration. X-ray excited luminescence spectra of the scintillation material showed broad emission bands between 360 and 460 nm, with two overlapping peaks, due to the d→f transitions on Ce³⁺ ions. The samples provide good scintillation results. The energy resolution was found to be 5.0% (FWHM) at 662 keV for 10% Ce sample. Under γ-ray excitation, Cs₂LiGdCl₆:Ce³⁺ showed three exponential decay time components of about 130–200 ns decay time constant. The light output of the investigated samples was 20,000 photons/MeV for a 10% Ce concentration. The light output deviation from the linear response is within 7% between the energy range of 31 and 1333 keV. Overall, the scintillation properties confirm that Cs₂LiGdCl₆:Ce³⁺ single crystal is a promising candidate for medical imaging and radiation detection.