Hexagonal Nanocrystals Research Articles

Fabrication and characterization of zinc oxide nanoparticles by DC arc plasma

Zinc oxide nanoparticles were synthesized by DC arc plasma in pure oxygen atmosphere. High DC currents were passed through two approaching zinc rods in different oxygen pressures between 50 to 5000 mbars. Samples were characterized by TEM, SEM, XRD and electron diffraction analysis. The formation of zinc oxide nanoparticles were successfully proved by those analyses, although in some cases we had some unwanted materials. The XRD patterns show crystalline formation of zinc oxide, but in low pressures we have an excess peak belong to pure zinc. The height of this peak was reduced by increase in oxygen pressure and finally vanished completely at high pressure. The TEM and SEM images of samples show that in low pressures the samples mainly tend to have rod like shapes structures, but by increasing the oxygen pressure the structures steady revolve to the hexagonal nanocrystals. Upon the resulted data, the best conditions of arc plasma for desire product were inferred.

Structural and optical investigations of Nd-doped fluorozirconate-based glass ceramics for enhanced upconverted fluorescence

We found enhanced upconverted fluorescence in Nd3+-doped fluorozirconate glasses which were additionally doped with chlorine ions. Upon annealing between 240 and 290°C, hexagonal phase BaCl2 nanocrystals between 20 and 180nm in diameter were formed in the glass. During the thermal processing, some of the Nd3+ ions enter the nanocrystals leading to additional splitting of the infrared fluorescence spectra and to increased upconverted fluorescence intensities. The optimum value was found for the 270°C sample where the upconverted fluorescence intensity is approximately a factor of 60 stronger than the one found for the as-made glass.

Synthesis of Hexagonal-Shaped SnO2 Nanocrystals and SnO2@C Nanocomposites for Electrochemical Redox Supercapacitors

To realize a suitable supercapacitor nanomaterial, the recently developed technique of reaction under autogenic pressure at elevated temperature has been employed by us to synthesize SnO2 hexagonal nanocrystals and SnO2@C nanocomposites. The synthesis at different temperatures (viz. 500, 600, and 700 °C) yields three different composites. Characterization of these composites by various methods confirms the structural (XRD, Raman, FT-IR) and nanoparticulate (TEM, HRTEM) nature of the synthesized materials. TEM studies including HRTEM reveal that all the synthesized SnO2 and SnO2@C nanomaterials are highly crystalline with hexagonal shape. Cyclic voltammetric studies carried out to examine the capacitance of SnO2@C in 1 M H2SO4 show that the nanocomposite prepared at 700 °C has a maximum specific capacitance of 37.8 F/g at a scan rate of 5 mV/s.

Spherical hexagonal tellurium nanocrystals: fabrication and size-dependent structural phasetransition at high pressure

Single-crystalline spherical nearly monodisperse tellurium (Te) nanocrystals (NCs) withaverage diameters of 20 and 90 nm, respectively, have been fabricated for the first time by afacile solution sonochemistry process. The structural characterizations show thatthe as-synthesized Te NCs have pure hexagonal structure, as revealed by x-raydiffraction (XRD), selected-area electron diffraction (SAED), energy-dispersivex-ray (EDX) spectroscopy, and high-resolution transmission electron microscopy(HRTEM) methods. The size-dependent structural phase transition of Te NCs up tothe high pressure of 20 GPa has been investigated in a diamond anvil cell usingresistance measurement at room temperature, and compared with the behaviorof bulk Te under identical conditions. The experimental results indicate that20 nm Te NCs, 90 nm NCs, and bulk Te all undergo two phase transitions upto 20 GPa, their respective transition pressures being about 7.2 and 10.3 GPa,5.9 and 8.8 GPa, and 4.0 and 6.8 GPa. This indicates that the phase transitionpressures are higher for the smaller NCs. In this paper we discuss the size-dependentstructural phase transitions, the sluggishness of the phase transition process,and the fluctuating properties of the phase transition products at high pressure.The present work might open an avenue to real-time detection of the dynamicsof the phase transition in bulk and nanoscale materials at high pressure, andalso could serve as a guide to tailoring the microscopic properties of materials.

Facile Cd−Thiourea Complex Thermolysis Synthesis of Phase-Controlled CdS Nanocrystals for Photocatalytic Hydrogen Production under Visible Light

We describe a simple cadmium−thiourea complex thermolysis route for the formation of CdS nanocrystals with controlled dispersity, crystalline phase, composition, average grain size, and band gap. Visible-light-driven photocatalytic activities for hydrogen production over the different CdS products have been compared. Phase structure and composition of the obtained CdS nanocrystals has been optimized either by changing the ratio of thiourea to Cd or by changing the annealing temperature. Over a broad annealing temperature range of 150−500 °C, either cubic, a mixture of cubic and hexagonal, or hexagonal CdS nanocrystals are obtained at thiourea/Cd molar ratios of <1.0, 1.5−2.5, and 3.0−4.5, respectively. Nanocrystalline cubic CdS is stable at temperatures as high as 500 °C for 0.5 h, and is converted to hexagonal CdS for annealing time longer than 1 h. The phase transition from cubic to hexagonal CdS occurs at temperatures of 200−300 °C, and pure hexagonal CdS is formed at annealing temperatures higher than...

Structural and magnetic properties of MnAs/GaAs ferromagnetic semiconductor nanocomposite material

Self-organized (Ga,Mn)As nanoclusters, embedded in GaAs and formed during thermal annealing of Ga1-xMnxAs layer at 500 °C or 600 °C, were studied using Transmission Electron Microscopy (TEM) and Magnetic Force Microscopy (MFM). We found that 10–20 nm large NiAs-type hexagonal MnAs nanocrystals gave magnetic contrast in MFM images, whereas smaller zinc-blende nanoinclusions were not visible by means of this technique. Theoretical simulations showed that MFM contrasts reflect interaction between magnetic tip and many randomly distributed MnAs nanocrystals.

Structure and magnetism of MnAs nanocrystals embedded in GaAs as a function of post-growth annealing temperature

Self-organized Ga(Mn)As nanoclusters, embedded in GaAs, were formed during post-growth thermal annealing of Ga1−xMnxAs layers. Structural and magnetic properties of such composites were systematically studied as a function of the annealing temperature. Small (∼3 nm) Mn-rich zinc-blende Mn(Ga)As clusters, coherent with the GaAs matrix, were formed at the annealing temperature of 500 °C. An increase of the annealing temperature of up to 600 °C led to the creation of 10–20 nm large NiAs-type hexagonal MnAs nanocrystals. Magnetization measurements showed that the MnAs nanoprecipitates were superparamagnetic, with a distribution of blocking temperatures that depended on the MnAs cluster size. Some intermediate paramagnetic clusters (structurally disordered clusters) were also observed.

Atomic-scale studies on the growth of palladium and titanium on GaN(0 0 0 1)

We have used elevated-temperature scanning tunnelling microscopy (STM) to investigate the initial stages of growth of Pd and Ti on GaN(0 0 0 1). Deposition of Pd onto a Ga-rich GaN(0 0 0 1) surface followed by annealing at ∼600 °C leads to the formation of an ordered Pd/Ga alloy layer, followed by the nucleation and growth of hexagonal Pd nanocrystals. When Pd is deposited on a less Ga-rich surface, the Pd initially wets the surface. With increased Pd coverage the wetted 2D islands coalesce into 3D islands without a wetting layer. The 3D islands have hexagonal shapes with a flat (1 1 1) top surface, and STM images of the nanocrystal tops show that they are reconstructed. A separate series of experiments was performed investigating Ti deposition on GaN(0 0 0 1), where the Ti reacts with GaN to form TiN. Deposition of Ti onto GaN(0 0 0 1) followed by annealing in UHV results in the formation of 2D irregular TiN islands, which can be transformed into more regular triangular TiN nanocrystals upon annealing in an ammonia atmosphere. This study shows overall that the growth modes of Pd and Ti are strongly influenced by the stoichiometry and cleanliness of the GaN substrate. Therefore, to create reliable electrical contacts between metals and GaN, the atomic level order and stoichiometry of the GaN substrates needs to be controlled.

Solid-state metasynthesis and characterization of AlN nanocrystals

Abstract Hexagonal AlN nanocrystals were synthesized successfully from solid-state metathesis reactions by using Mg3N2 as the nitriding sources at the temperature of 500 °C. The samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), selected area electron diffractions, Fourier transformation infrared (FT-IR) spectroscopy and Raman spectroscopy respectively. The results show that the obtained samples are pure hexagonal AlN with a grain size less than 10 nm.

Controllable Synthesis and Magnetic Properties of Cubic and Hexagonal Phase Nickel Nanocrystals

Controllable Synthesis and Magnetic Properties of Cubic and Hexagonal Phase Nickel Nanocrystals

Hydrothermal Synthesis and Characterization of Alkaline-Earth Metal Phenylphosphonate Nanostructures

A simple hydrothermal method has been designed for the synthesis of the nanorods of alkaline-earth metal phenylphosphonates Mg(O3PPh)·H2O and M(HO3PPh)2 (M = Ca or Sr) in the presence of surfactants cetyltrimethylammonium bromide or sodium dodecyl sulfate at pH 6.0. Rod-shaped nanocrystals, nanocubes, and hexagonal nanocrystals of the Ba(HO3PPh)2 have also been prepared separately. The possible growth mechanisms of these nanocrystals were elucidated in detail. Mesoporous nanorods of Ca2P2O7 possessing a large surface area and high thermal stability were obtained by removing the organic part of Ca(HO3PPh)2 nanorods at 700 °C. X-ray diffraction, transmission electron microscopy (TEM), high-resolution TEM, electron diffraction, and Brunauer−Emmett−Teller surface area were used to characterize these materials.

Self-Assembled ZnS Nanostructured Spheres: Controllable Crystal Phase and Morphology

We report a simple biomolecule-assisted synthesis of ZnS nanostructured spheres assembled from ZnS nanocrystals with the controllable crystal phase and morphology. l-Cysteine, a biomolecule, was used as the sulfur source and played a key role in the formation of ZnS nanostructured spheres. ZnS nanostructured spheres assembled from various ZnS nanocrystal building blocks, such as nanosheets, quantum dots, nanorods, and multimorphology nanocrystals, were successfully prepared by this simple method. The crystal phase of ZnS nanostructured spheres could be controlled by introducing ethanolamine or ethanediamine as a surface-modifying reagent in this synthetic system. The hexagonal ZnS nanocrystals were obtained in mixed solvents of water and ethanolamine at temperature as low as 95 °C. The growth mechanism of the nanostructured spheres assembled from nanorods was proposed. The optical properties of ZnS nanostructured spheres were investigated by ultraviolet−visible (UV−vis) absorption and photoluminescence (PL) spectra.

Bulk Modulus and Structural Phase Transitions of Wurtzite CoO Nanocrystals

High-pressure behaviors of wurtzite-type hexagonal CoO nanocrystals were investigated by in situ high-pressure synchrotron radiation X-ray diffraction measurements up to 57.4 GPa at ambient temperature. It is found that bulk modulus of the hexagonal CoO phase is about 115 GPa at zero pressure. During compression, the hexagonal CoO phase transfers into rocksalt-type cubic phase in the pressure range of 0.8−6.9 GPa. The volume collapse accompanied by the transition was estimated to be about 20%. This is irreversibly phase transformation; that is, the cubic CoO phase remains after pressure release. Based on the data of peak width vs pressure, a cubic-to-rhombohedral phase transition was detected for the nanocrystalline cubic CoO phase with the transition pressure of about 36 GPa, lower than 43 GPa for bulk cubic CoO phase. The bulk modulus of the nanocrystalline cubic CoO phase of about 258 GPa is larger than 180 GPa for the corresponding bulk cubic CoO phase.

Synthesis of Hexagonal‐Phase NaYF4:Yb,Er and NaYF4:Yb,Tm Nanocrystals with Efficient Up‐Conversion Fluorescence

AbstractIR‐to‐visible up‐conversion fluorescent nanocrystals of hexagonal‐phase NaYF4:20 %Yb,2 %Er and NaYF4:20 %Yb,2 %Tm have been synthesized by decomposition of multiprecursors of CF3COONa, (CF3COO)3Y, (CF3COO)3Yb, and (CF3COO)3Er/(CF3COO)3Tm in oleylamine at 330 °C. The average particle size is 10.5 ± 0.7 nm (from random measurements of 200 particles from five transmission electron microscopy images) and 11.1 ± 1.3 nm (from dynamic‐light‐scattering measurements). The up‐conversion fluorescence intensity of the hexagonal nanocrystals in this work is much higher than that of other cubic‐phase NaYF4:Yb,Er nanocrystals, including the ones in this work (by a factor of 7.5). Mechanisms for nucleation and growth of the hexagonal‐phase nanoparticles are proposed. These nanocrystals are easily dispersed in organic solvents, producing a transparent colloidal solution. The hydrophobic surfaces of the particles are made hydrophilic using a bipolar surfactant. These nanoparticles and their dispersions in various media have potential applications in optical nanodevices and bioprobes.

Wurtzite-to-Rocksalt Structural Transformation in Nanocrystalline CoO

Hexagonal CoO nanocrystals are coarsened under hydrothermal conditions to investigate the effect of particle size on phase transformation and stability property. Structural stability and phase transformation of the hexagonal CoO phase have been investigated by X-ray powder diffraction with Rietveld refinement, transmission electron microscopy, X-ray absorption fine structure, and differential scanning calorimeter. It is found that the hexagonal CoO phase is a metastable phase, which increases its grain size from 50 to 250 nm for refluxing times from 1 to 6 h at 200 degrees C. After 12 h, cubic-structured CoO grains with an average grain size of 20 nm are observed, which spread around big hexagonal CoO grains. After about 24 h, only the cubic CoO phase with an average grain size of 25 nm is detected. The onset temperature of hexagonal-to-cubic phase transformation in CoO is estimated to be 378 degrees C by DSC, using a heating rate of 20 deg/min. The results obtained indicate that the hexagonal-to-cubic phase transformation in nanocrystalline CoO is by nucleation and growth mechanism, starting from the surface to the center of the hexagonal grains.

Synthesis of complex rare earth fluoride nanocrystal phosphors

Complex rare earth fluoride (NaRF4, R = Ce,Y,Gd) nanocrystals of 30–50 nm were synthesized by hydrothermal and solvothermal techniques.The size, morphology and phase transition of the complex rare earth fluorides are discussedas the effects of different solvents, reaction time and temperature. Hexagonal phaseNaRF4 nanocrystals with good dispersibility can be prepared in thepresence of EDTA using ethanol as the solvent. After doping withYb3+,Er3+;Yb3+,Tm3+,and Eu3+, some typical up-conversion and down-conversion photoluminescence was characterizedand discussed.

Hexagonal zinc sulfide nanocrystals coated by esters with a well-defined exciton feature

Hexagonal zinc sulfide nanocrystals (ZnS NCs) were prepared under mild solvothermal condition at 170°C with the polymer ester used as ligand for their stability and effective passivation. Well-defined absorption peaks with the largest blueshift of 89nm were observed for the ZnS NCs based on the ultraviolet-visible (UV-vis) absorption spectra analyses compared with that of the bulk one, which are in correspondence with the apparent quantum confinement effects. Well excited states were obtained from UV-vis absorption and photoluminescent excitation (PLE) spectra for the ZnS NCs. PL emission spectra showed dominant band-edge luminescence.

Carbon structures in silicon carbide derived carbon

Carbide derived carbon (CDC) produced by etching SiC in halogens has been investigated using transmission electron microscopy (TEM). Depending on experimental conditions, CDC may contain sp 3- or sp 2-bonded carbon phases. Amorphous carbon, poorly ordered turbostratic carbon with lattice spacing exceeding values of 0.35 nm as well as highly ordered graphite were observed. sp 3-Bonded structures consist of mainly lonsdaleite and cubic diamond nanocrystals. Hexagonal diamond polytypes and n-diamond have been synthesized. During the annealing process, diamond nanocrystals eventually transform to sp 2-bonded carbons. The transformation process of hexagonal diamond nanocrystals produces spherical onion type structures. These structures appear either hollow or dense. The latter shows a decrease in lattice spacing compared to d/ n values for graphite. Graphitic structures include multi-wall nanotubes, polyhedral particles and ribbons. The graphitization during chlorination of samples may eventually lead to the formation of planar graphite.

Luminescence of CVD ZnO nanocrystals of different shapes

The effect of the shape and dimensions of zinc oxide nanocrystals on the threshold and directionality of UV lasing in the nanocrystals has been investigated. The results indicate that hexagonal columnar nanocrystals have the lowest optical threshold pump power at nanoresonator diameters of 100–200 nm, comparable to the absorption path length of the excitation light. The laser radiation is shown to have a mode structure, being directed predominantly along the prism axis. The lasing mechanism in prismatic nanocrystals differs from that in hexagonal-pyramidal nanocrystals. Electron microscopy results show that the ZnO nanocrystals are highly uniform in shape and size and that these parameters depend on the chemical vapor deposition conditions.

Shape Control of Monodisperse CdS Nanocrystals: Hexagon and Pyramid

The wurtzite CdS nanocrystals with hexagonal or pyramidal geometries were selectively synthesized by tuning the molar ratio of Cd and S precursors in the solution system. For hexagonal nanocrystals, a 2-D or 3-D superlattice assembly could be obtained due to the narrow particle size distribution. The pyramidal CdS nanocrystals were divided into two geometries: the hexagon-based pyramid and the triangle-based pyramid. The realization of the pyramidal geometries further extends the shape multiformity of wurtzite CdS nanocrystals, which may bring new opportunities for the development of CdS semiconductors. The room-temperature absorption spectra of CdS nanoparticles with hexagonal and pyramidal morphologies exhibited a discrepancy in peak positions, revealing the existence of a profound shape-property relationship for the CdS nanophase.