Influence of ambient gases on the morphology and photoluminescence of ZnO nanostructures synthesized with nickel oxide catalyst

The morphology and luminescence properties of ZnO nanowires synthesized using NiO catalyst in a chemical vapor deposition system under different growth ambient have been studied. ZnO nanostructures were prepared in nitrogen, ammonia and hydrogen ambient and characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and photoluminescence. Growth in nitrogen ambient yields ZnO nanoneedles while growth with ammonia and hydrogen ambient ends up with ZnO nanowires. Presence of the Ni tip at the end in either morphology indicated the involvement of vapor–liquid–solid growth mechanism. Enhanced green emission in ZnO nanowires implies the presence of a high density of oxygen vacancies. Influence of the ambient gases on the morphology and optical properties of ZnO nanostructures is discussed.     

Synthesis and optical properties of Ce-doped ZnO hexagonal nanoplatelets

Single crystalline Ce-doped ZnO hexagonal nanoplatelets are successfully synthesized. Zinc acetate, cerium nitrate, potassium hydroxide and poly vinyl alcohol were mixed together and transferred to a 100 mL Teflon-lined stainless steel autoclave kept at 150 °C for 24 h. The obtained precipitant is calcined at 600 °C. The morphology and microstructure were determined by field emission scanning electron microscopy (FE-SEM), X-ray diffraction transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX) and photoluminescence (PL) spectroscopy. The investigation confirmed that the products were of the wurtzite structure of ZnO. The doped hexagonal nanoplatelets have edge length 25 nm and thickness 11 nm. EDX result showed that the amount of Ce in the product is about 15%. Photoluminescence of these doped hexagonal nanoplatelets exhibits a blue shift and weak ultraviolet (UV) emission peak, compared with pure ZnO, which may be induced by Ce-doping. The growth mechanism of the doped hexagonal nanoplatelets was also discussed.     

Seed-mediated synthesis of uniform ZnO nanorods in the presence of polyethylene glycol

Large quantities of ZnO nanorods have been synthesized by the seed-mediated method in the presence of polyethylene glycol at 90 °C. The products are characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and high-resolution transmission electron microscopy. The as-grown ZnO nanorods are uniform with a diameter of 40–70 nm and length about 2 μm. The nanorods grew along the [0 0 1] direction. Possible roles of ZnO seeds and polymer in the growth of ZnO nanorods are also discussed.     

Effects of growth temperature on the properties of ZnO/GaAs prepared by metalorganic chemical vapor deposition

A series of ZnO films were grown on GaAs(0 0 1) substrates at different growth temperatures in the range 250–720°C by metalorganic chemical vapor depostion. Field emission scanning electron microscopy was utilized to investigate the surface morphology of ZnO films. The crystallinity of ZnO films was investigated by the double-crystal X-ray diffractometry. The optical and electrical properties of ZnO films were also investigated using room-temperature photoluminescence and Hall measurements. Arrhenius plots of the growth rate versus reciprocal temperature revealed the kinetically limited growth behavior depending on the growth temperature. It was found that the surface morphology, structural, optical and electrical properties of the films were improved with increasing growth temperature to 650°C. All the properties of the film grown at 720°C were degraded due to the decomposition of ZnO film.     

In-situ and ex-situ ZnO nanorod growth on ZnO homo-buffer layers

Vertically well-aligned ZnO nanorods were fabricated in-situ and ex-situ on ZnO homo-buffer layers using catalyst-free metal-organic chemical vapor deposition. Field-emission electron microscopy measurements demonstrated that the nanorods were well aligned and had a uniform diameter of 70–100 nm depending on the growth temperature, irrespective of growth conditions, in-situ and ex-situ. X-ray diffraction measurements demonstrated that the ZnO nanorods and the ZnO buffer layers had a wurtzite structure, and that the crystal quality of the nanorods grown on a smooth surface was better than that of the nanorods grown on a rough surface. Field-emission transmission electron microscopy measurements revealed the presence of a disordered layer at the interface of the nanorod and the buffer layer.     

Controlled synthesis of different morphologies of BaWO4 crystals via a surfactant-assisted method

BaWO₄ crystals with different morphologies, such as nanosheets, nanobelts, flower-like, quadrangled plates and sheaves of dendrite, have been successfully synthesized via PVP as templates. Our result shows that reaction parameters, such as the concentration of PVP aqueous solutions, pH value of the starting solution and molar ratio of [Ba²⁺]/[WO₄²⁻] played important roles in the formation of BaWO₄ crystal with different morphologies. It is obviously different between microwave irradiation heating and oil bath heating. The products were characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy and electron diffraction.     

Effect of oxygen partial pressure on the growth of zinc micro and nanostructures

Zinc micro and nanostructures were synthesized in vacuum by condensing evaporated zinc on Si substrate at different gas pressures. The morphology of the grown Zn structures was found to be dependent on the oxygen partial pressure. Depending on oxygen partial pressure it varied from two-dimensional microdisks to one-dimensional nanowire. The morphology and structural properties of the grown micro and nanostructures were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Transmission electron microscopy (TEM) studies on the grown Zn nanowires have shown that they exhibit core/shell-like structures, where a thin ZnO layer forms the shell. A possible growth mechanism behind the formation of different micro and nanostructures has been proposed. In addition, we have synthesized ZnO nanocanal-like structures by annealing Zn nanowires in vacuum at 350 °C for 30 min.     

Self-sacrificed template method for the synthesis of ZnO-tubular nanostructures:reaction kinetics and pathways

Using Zn nanowires as a self-sacrificed template, hierarchical tubes constructed by zinc oxide (ZnO) nanoflakes and ZnO nanotubes have been successfully fabricated by two different thermal-oxidation modes. The products were characterized by the X-ray powder diffraction, transmission electron microscopy and field-emission scanning electron microscopy. The experimental results show that the formation processes of ZnO nanostructures are sensitive to the growth temperature, which is lower or higher the melting point of Zn (419 °C). ZnO nanoflake tubes and ZnO nanotubes can be controlled through the variation of the heat-treatment process of Zn nanowires and their growth pathway can be described by two types of growth mechanism, in terms of Kirkendall effect and the sublimation of the Zn cores, respectively. Our method provides an easy and convenient way to prepare metal oxides tubular nanostructures with different morphologies through self-sacrificed template method via adjusting the heat-treatment process.     

Magnetization and structural studies of Mn doped ZnO nanoparticles: Prepared by reverse micelle method

Single phase Mn (2.5 at%) doped ZnO nanocrystalline samples were synthesized by reverse micelle method as characterized by Rietveld refinement analysis of X-ray diffraction data, high resolution transmission electron microscopy and selected area electron diffraction analyses. The X-ray photoelectron spectroscopy and electron paramagnetic resonance (EPR) studies indicated that manganese exist as Mn²⁺ in ZnO lattice. DC magnetization measurements as a function of field and temperature, of 2.5 at% Mn doped ZnO nanoparticles annealed at 675 K, showed room temperature ferromagnetism (RTF). This observation is further confirmed by the EPR spectrum of the sample, which shows a distinct ferromagnetic resonance signal at room temperature. These results indicate that the observed RTF in Mn-doped ZnO may be attributed to the substitutional incorporation of Mn at Zn sites.     

Synthesis and characterization of a micro scale zinc oxide–PVA composite by ultrasound irradiation and the effect of composite on the crystal growth of zinc oxide

Micro scale zinc oxide-polyvinyl alcohol (ZnO–PVA) composite has been synthesized by ultrasound irradiation. The properties of the as-prepared ZnO–PVA composite material are characterized by X-ray diffraction (XRD), thermo gravimetric analysis (TGA), transmission electron microscopy (TEM), and diffuse reflection spectroscopy (DRS). A band gap of 3.25 eV is estimated from DRS measurements. The controlled crystal growth of zinc oxide has been studied by using the as-prepared micro scale ZnO–PVA composite as seeds for the crystal growth of ZnO.     

Influence of cooling rate on the liquid-phase epitaxial growth of Zn3P2

We report the liquid-phase epitaxial growth of Zn₃P₂ on InP (1 0 0) substrates by conventional horizontal sliding boat system using 100% In solvent. Different cooling rates of 0.2–1.0 °C/min have been adopted and the influence of supercooling on the properties of the grown epilayers is analyzed. The crystal structure and quality of the grown epilayers have been studied by X-ray diffraction and high-resolution X-ray rocking measurements, which revealed a good lattice matching between the epilayers and the substrate. The supercooling-induced morphologies and composition of the epilayers were studied by scanning electron microscopy and energy dispersive X-ray analysis. The growth rate has been calculated and found that there exists a linear dependence between the growth rate and the cooling rate. Hall measurements showed that the grown layers are unintentionally doped p-type with a carrier mobility as high as 450 cm²/V s and a carrier concentration of 2.81×10¹⁸ cm⁻³ for the layers grown from 6 °C supercooled melt from the cooling rate of 0.4 °C/min.     

Morphology transition of one-dimensional ZnO grown by metal organic vapour phase epitaxy on (0 0 0 1)-ZnO substrate

Metal organic vapour phase epitaxy (MOVPE) has been used to successfully grow one-dimensional (1D) ZnO deposits on (0 0 0 1)-ZnO substrate. Dimethylzinc–triethylamine and nitrous oxide were used as zinc and oxygen sources, respectively, with nitrogen as the carrier gas. Vertically aligned 1D ZnO structures were observed along the c-axis by using lower VI/II mole ratio R_VI/II<2025 and/or high growth temperatures (Tg>800 °C). The diameter, length, density and the mechanism of formation could be controlled with the growth time. Scanning electron microscopy (SEM) shows different structures, i.e., sharp-top, flat-top and open-top with slim bottom and large-top one-dimensional ZnO. A good structural quality was revealed by X-ray diffraction rocking curve with a full-width at half-maximum (FWHM) varying from 40 to 92 arcsec with increasing growth time.     

Phase diagram of Si nanowire growth by disproportionation of SiO

Silicon nanowires have been grown in a horizontal tube furnace by disproportionation of silicon monoxide in combination with the vapor–liquid–solid mechanism. We present a phase diagram of the nanowire growth, indicating different morphologies for varying growth pressure and temperature. The morphology was characterized by scanning electron microscopy and detailed structural analysis was performed by transmission electron microscopy. A variety of morphologies is found and the optimum parameter range for the growth of straight and uniform nanowires consisting of crystalline silicon cores and amorphous SiO₂ shells is identified and discussed.     

Hydrothermal growth of ZnO nanorods on a-plane GaN/sapphire template

ZnO nanorod arrays are grown on a-plane GaN template/r-plane sapphire substrates by hydrothermal technique. Aqueous solutions of zinc nitrate hexahydrate and hexamethylenetetramine were employed as growth precursors. Electron microscopy and X-ray diffraction measurements were carried out for morphology, phase and growth orientation analysis. Single crystalline nanorods were found to have off-normal growth and showed well-defined in-plane epitaxial relationship with the GaN template. The 〈0 0 0 1〉 axis of the ZnO nanorods were observed to be parallel to the 〈1 0 1¯ 0〉 of the a-plane GaN layer. Optical property of the as-grown ZnO nanorods was analyzed by room temperature photoluminescence measurements.     

Synthesis of calcium carbonate polymorphs in the presence of polyacrylic acid

Calcium carbonate precipitates are prepared from a solution of CaCl₂ and K₂CO₃ in the presence of polyacrilic acid. The effect of polyacrilic acid incorporation in the [25–80 °C] temperature range on crystal morphologies and CaCO₃ precipitated polymorph concentrations are investigated using scanning electron microscopy and X-ray diffraction quantitative microstructural and phase analysis. Large changes in morphology and phase proportions are observed in the presence of polyacrylic acid, which strongly depend on the solution temperature. While crystallization of vaterite is favoured in the presence of polyacrilic acid up to 50 °C, it is largely destabilized at higher temperatures. Our process also enables the elaboration of particles in the range 10–20 nm.     

Hydrothermal synthesis of Bi2Te3 nanowires through the solid-state interdiffusion of Bi and Te atoms on the surface of Te nanowires

Polycrystalline Bi₂Te₃ nanowires were prepared by a hydrothermal method that involved inducing the nucleation of Bi atoms reduced from BiCl₃ on the surface of Te nanowires, which served as sacrificial templates. A Bi–Te alloy is formed by the interdiffusion of Bi and Te atoms at the boundary between the two metals. The Bi₂Te₃ nanowires synthesized in this study had a length of 3–5 μm, which is the same length as that of the Te nanowires, and a diameter of 300–500 nm, which is greater than that of the Te nanowires. The experimental results indicated that volume expansion of the Bi₂Te₃ nanowires was a result of the interdiffusion of Bi and Te atoms when Bi was alloyed on the surface of the Te nanowires. The morphologies of Bi₂Te₃ are strongly dependent on the reaction conditions such as the temperature and the type and concentration of the reducing agent. The morphologies, crystalline structure and physical properties of the product were analyzed by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and X-ray photoelectron spectroscopy (XPS).     

Synthesis and optical properties of high-purity CoO nanowires prepared by an environmentally friendly molten salt route

CoO nanowires with diameters of 50^_80 nm, and lengths of up to more than 5 μm have been successfully synthesized by a simple environmentally friendly molten salt route, in which the precursor CoCO₃ nanoparticles are decomposed to form high-purity CoO nanowires in NaCl flux. The structure features and morphology of the as-prepared CoO nanowires were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and selected area electron diffraction (SAED). The chemical composition and oxidation state of the prepared nanowires were systemically studied by X-ray photoelectron spectra (XPS) and laser Raman spectroscopy. The results indicated that the as-prepared CoO nanowires were composed of pure cubic CoO phase. The growth mechanism of the synthesized nanowires was also discussed in detail based on the experimental results.     

Low temperature deposition of zinc oxide nanoparticles via zinc-rich vapor phase transport and condensation

ZnO nanoparticles as small as 80 nm were successfully synthesized using a modified vapor phase transport (VPT) process at substrate temperatures as low as 222 °C. Particle size distribution and morphology were characterized by scanning electron microscopy and atomic force microscopy. Energy dispersive X-ray spectroscopy and X-ray diffraction indicate the synthesis of high quality crystalline ZnO structures. Low temperature (4.2 K) photoluminescence (PL) spectroscopy was used to characterize the optical quality of the nanoparticles. Ultraviolet emission and a nanostructure specific feature at 3.366 eV are strong in the PL spectra. The 3.366 eV feature is observed to predominate the spectrum with decrease in particle size. This size effect corroborates the luminescence as a nanostructure-specific surface related exciton feature as previously speculated in the literature. In addition, self-assembled ZnO mesoparticles (>100 nm) were realized by increasing the growth time. Low growth temperatures of the particles allow for their potential utilization in flexible organic hybrid optoelectronics. However, this work focuses mainly on the modified synthesis and optical characterization of nanoparticles.     

Reactions at the liquid silicon/silica glass interface

Experiments have been carried out to determine the nature and origin of the spots growing on silica glass surfaces in contact with liquid silicon during CZ–Si crystal growth. Silica glass ampoules were filled with silicon and tempered between 5 min and 40 h at a temperature (1693 K) slightly above the melting point of silicon. Cross sections of the ampoules with solidified silicon have been examined by scanning electron microscopy and optical polarization microscopy. In addition cross sections from commercial silica glass crucibles used in the Czochralski process or dipped into the silicon melt were investigated with the same methods. At the silicon/silica glass interface different reaction zone morphologies were detected. A solution-precipitation mechanism is suggested for the fast lateral growth of the reaction zone, which is proposed to consist of small cristobalite crystals embedded in a silica glass matrix.     

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.