Synthesis,growth mechanism,photoluminescence and field emission properties of metal–semiconductor Zn–ZnO core–shell microcactuses

Metal–semiconductor Zn–ZnO core–shell microcactuses have been synthesized on Si substrate by simple thermal evaporation and condensation route using NH₃ as carrier gas at 600 °C under ambient pressure. Microcactuses with average size of 65–75 μm are composed of hollow microspheres with high density single crystalline ZnO rods. The structure, composition and morphology of the product were characterized by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), scanning electron microscope (SEM), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). A vapor–liquid–solid (VLS) based growth mechanism was proposed for the formation of Zn–ZnO core–shell microcactuses. Room temperature photoluminescence (PL) investigations revealed a strong and broad blue emission band at 441 nm associated with a weak ultraviolet (UV) peak at 374 nm. This blue emission (BE) is different from usually reported green/yellow-green emission from Zn–ZnO or ZnO structures. The field emission (FE) measurements exhibited moderate values of turn-on and threshold fields compared with reported large field emissions for other materials. These studies indicate the promise of Zn–ZnO core–shell microcactuses for the applications in UV-blue light display and field emission microelectronic devices.     

Microstructural,Optical and Magnetic Study of Ni–Zn Nanoferrites

Journal of Superconductivity and Novel Magnetism - NixZn1-xFe2O4 (x?=?0.0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0) nanoferrites were synthesized using the citrate precursor method with...     

Au–ZnO hybrid nanostructures prepared by electro-exploding wire technique: Raman signal enhancement and photoluminescence emission quenching

Electro-exploding wire (EEW) technique was employed to prepare ZnO and Au–ZnO hybrid nanoparticles. Average size of the prepared ZnO nanoparticles is found to be 3.8 nm and uniform throughout. These ultrafine ZnO nanoparticles are found to agglomerate around the highly surface active Au nanoparticles. It also acts as a stabilizer for the Au nanoparticles by avoiding self agglomeration. The hybrid nanocrystals show strong crystallinity of face-centered cubic and hexagonal wurtzite structure of gold (Au) and zinc oxide (ZnO), respectively. Presence of Au₃Zn in pristine sample is a clear indication of a strong interaction between ZnO and Au systems. The hybrid system shows strong enhancement in the ZnO Raman signals and quenching in the visible Photoluminescence (PL) emission. Energy-dependent PL analysis shows the dominance of the surface defects over the bulk contribution in these ultrafine ZnO and Au–ZnO hybrid nanostructures.     

Molecular dynamics simulation of the formation of bimetallic core-shell nanostructures with binary Ni–Al nanoparticle quenching

Journal of Materials Science - Employing isothermal molecular dynamics, we simulated the self-assembly of core-shell nanostructures in the course of quenching binary Ni–Al nanoparticles (NPs)...     

Optical absorption and photoluminescence properties of Eu3+-doped ZnF2–PbO–TeO2 glasses

Several physical, optical absorption and photoluminescence properties of Eu3+-doped ZnF2–PbO–TeO2 glasses have been studied. From the measured intensities of various absorption bands of these glasses the Judd–Ofelt parameters 2, 4 and 6 have been computed. The Judd–Ofelt theory has been applied to characterize the photoluminescence spectra of these glasses. From this theory, various radiative properties, such as transition probability, A, branching ratio, r, and emission cross-section, EP, for various emission levels of these glasses, have been determined and reported. © 1998 Chapman & Hall.     

Field emission studies of carbon nanostructures synthesised over Ni–Cr catalyst layer

Current research on the carbon-based nanotechnology needs progressive methods to control the shape, location and size of the nanostructures. Here, we report significant progress by synthesising the density controlled carbon nanostructures (CNSs) using acetylene and hydrogen in microwave plasma enhanced chemical vapour deposition system. Thin films of Ni–Cr (80?:?20, 60?:?40 and 50?:?50) sputtered over silicon (1?0?0) were used as catalysts. Scanning electron microscopy images of plasma annealed Ni–Cr coated silicon substrates show distributed nanoparticles of varying compositions over plasma annealed substrates. Morphologically and structurally different CNS were obtained when plasma annealed substrates were exposed to carbon vapours present in plasma. Transmission electron microscopy images suggested that the length and tip of CNS were in the range 50–100?nm and 4–6?nm, respectively. High resolution transmission electron microscopy images of the samples confirmed the presence of graphite (0?0?2) and nickel (2?0?0) planes in CNS. The field emission studies and Kelvin probe measurements of CNS grown over 80?:?20 Ni–Cr substrate show turn-on field and corresponding work function as 1.4?V?µm^?1 and 4.4?eV, respectively. Preliminary results show that these nanostructures could act as stable field emitters.     

Optical absorption and thermoluminescence of barytes single crystals irradiated with γ-rays

The optical absorption in the wavelength region 200 to 800 nm of barytes single crystals before and after -ray irradiation for different times has been measured. The thermoluminescence (TL) of the irradiated crystals has also been studied. -ray irradiation produces absorption bands at 590 and 370 nm possibly due to Ba⁺ and (SO₄)^– centres, respectively. Partial thermal bleaching experiments carried out on these absorption bands show that the absorption in both bands gradually decreases up to 200° C beyond which it falls rapidly. The -ray irradiated barytes exhibits TL peaks at 90, 180 and 208° C. An attempt is made to understand the results.     

Optical absorption,P NMR,and photoluminescence spectroscopy study of copper and tin co-doped barium–phosphate glasses

The optical and structural properties of 50P₂O₅:50BaO glasses prepared by melting have been investigated for additive concentrations of 10 and 1 mol% of CuO and SnO dopants. Absorption and photoluminescence spectroscopies were employed in the optical characterization, whereas structural properties were assessed by ³¹P nuclear magnetic resonance (NMR) spectroscopy. Residual Cu²⁺ was detectable by absorption spectroscopy for the highest concentration of CuO and SnO. More prominently, the optical data suggests contributions from both twofold-coordinated Sn centers and Cu⁺ ions to light absorption and emission in the glasses. The luminescence depends strongly on excitation wavelength for the highest concentration of dopants where a blue–white emission is observed under short-wavelength excitation (e.g., 260 nm) largely due to tin, while an orange luminescence is exhibited for longer excitation wavelengths (e.g., 360 nm) essentially due to Cu⁺ ions. On the other hand, dissimilar luminescent properties were observed in connection to Cu⁺ ions for the lowest concentration studied, as the copper ions were preferentially excited in a narrower range at shorter wavelengths near tin centers absorption. The structural analyses revealed the glass matrix to be composed essentially of Q² (two bridging oxygens) and Q¹ (one bridging oxygen) phosphate tetrahedra. A slight increase in the Q¹/Q² ratio reflected upon SnO doping alone suggests a major incorporation of tin into the glass network via P–O–Sn bonds, compatible with the 2-coordinated state attributed to the luminescent Sn centers. However, a significant increase in the Q¹/Q² ratio was indicated with the incorporation of copper at the highest concentration, consistent with a key role of the metal ions as network modifiers. Thus, the change in Cu⁺ optical properties concurs with different distributions of local environments around the ions induced by variation in metal ion concentration. Luminescence decay curve analyses were found in agreement with the presence of Cu⁺ ions in the glasses suggesting their existence in tetragonally-distorted octahedral sites.     

ZnO Additive Boosts Charging Speed and Cycling Stability of Electrolytic Zn–Mn Batteries

Electrolytic aqueous zinc-manganese(Zn–Mn) batteries have the advantage of high discharge voltage and high capacity due to two-electron reactions. However, the pitfall of electrolytic Zn–Mn batteries is the sluggish deposition reaction kinetics of manganese oxide during the charge process and short cycle life. We show that, incorporating ZnO electrolyte additive can form a neutral and highly viscous gel-like electrolyte and render a new form of electrolytic Zn–Mn batteries with significantly imp...     

Optical constants of thermally evaporated Se–Sb–Te films using only their transmission spectra

Amorphous Se_82 ? xTe₁₈Sb_x thin films with different compositions (x = 0, 3, 6 and 9 at.%) were deposited onto glass substrates by thermal evaporation. The transmission spectra, T(λ), of the films at normal incidence were obtained in the spectral region from 400 to 2500 nm. Based on the use of the maxima and minima of the interference fringes, a straightforward analysis proposed by Swanepoel has been applied to derive the optical constants and the film thickness. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple and DiDomenico model. Tauc relation for the allowed non-direct transition describes the optical transition in the studied films. With increasing antimony content the refractive index increases while the optical band gap decreases. The optical band gap decreases from 1.62 to 1.26 eV with increasing antimony content from 0 to 9 at.%. The chemical-bond approach has been applied successfully to interpret the decrease of the optical gap with increasing antimony content.     

Influence of Fe Ions on the Optical Properties of Fe–ZnO Inverse Opals

In this work we study the influence of Fe ions doping concentration on the optical properties of ZnO inverse opals. The ZnO inverse opals were obtained by impregnating the PMMA opal template with a zinc acetate solution. After the solidification of this solution in the void spaces of the synthetic PMMA and the thermal removal of the PMMA template, it remained a regular 3-D ordered porous ZnO solid which constitutes an inverse opal. The ZnO:Fe inverse opals were obtained following the same procedure but using zinc acetate and iron nitrate solutions instead. Scanning Electron Microscopy (SEM) images show the close-packed self-assembly of PMMA opals, the surface morphology of ZnO inverse opals formed by spherical void spaces of 295 nm diameter, and a discontinuous surface morphology of the ZnO:Fe inverse opals. The XRD diffractogram of ZnO inverse opals shows peaks characteristic of ZnO with wurtzite phase, and the micro-Raman spectrum shows phononic lines corresponding also to the same crystallographic structure. The energy band gap of ZnO and ZnO:Fe inverse opals were calculated from their absorption spectra giving the values of 3.2 and 2.4 eV, respectively.     

X-ray absorption near edge structure (XANES) and X-ray Kβ emission spectra in mixed valence manganese spinels

The X-ray MnKβ spectra and XANES of Mn₃O₄, LiMn₂O₄ and NiMn₂O₄ are reported. The chemical shifts (MnKβ_1.3 and MnK edge) and the X-ray absorption fine structure in the near edge region of NiMn₂O₄ compared with those of reference compounds are indicative of the charge distribution: Ni_x²⁺MN_1−x²⁺[NI_1−x²⁺MN_2x³⁺MN_2x³⁺MN_1−x⁴⁺]O₄ (O < x < 1)     

Electron–Phonon Interaction and Optical Spectra of Metals

Observed optical reflectivity in the infrared spectral region is compared with theoretical predictions in a strongly coupled electron–phonon system. Starting from a Fröhlich Hamiltonian, the spectral functions and their temperature dependence are derived. A full analysis including vertex corrections leads to an expression for the optical conductivity () that can be formulated in terms of the well-known optical conductivity for a quasi-isotropic system without vertex corrections. A numerical comparison between the full result and the so-called extended Drude formula, its weak coupling expansion, shows little difference over a wide range of coupling constants. Normal-state optical spectra for the high-T _c superconductors YBa₂Cu₃O₇ and La_{2 – x} Sr _x CuO₄ at optimal doping are compared with the results of model calculations. Taking the plasma frequency and from band structure calculations, the model has only one free parameter, the electron–phonon coupling constant . In both materials the overall behavior of the reflectivity can be well accounted for over a wide frequency range. Systematic differences exist only in the mid-infrared region. They become more pronounced with increasing frequency, which indicates that a detailed model for the optical response should include temperature-dependent mid-infrared bands.     

Synthesis and electrical properties of graphene–manganese oxide hybrid nanostructures

Journal of Materials Science: Materials in Electronics - In the present contribution, grapheme–manganese oxide hybrid nanostructures (G/MnO2) were synthesized via rapid and facile microwave...     

Solvent-dependent tuning of blue–green emission of chemically synthesized ZnO nanomaterials with high colour purity and electroluminescence efficiency

A series of nanomaterials of ZnO have been synthesized via chemical co-precipitation method with fixed proportions of precursors and varied solvents. X-ray diffraction confirms wurtzite crystalline structure with nanometric crystallites (< 23 nm). Texture coefficient of crystallographic orientations show remarkable change for switching the solvents from water to alcohol. Morphological study reveals nanomaterials resembling prolate, sphere and oblate shaped structures for the solvents water, methanol and ethanol, respectively, with increasing particle size. All the nanomaterials show a similar absorption band in the UV region; though, more absorption covering a wider region in visible range is observed for nanomaterials prepared in alcoholic solutions. Red shifting in band gap of nanomaterials is correlated with band-tail effect. Variation in Urbach energy indicates that the nature of solvent plays a vital role in creating defects in ZnO, justifying enhanced absorptions in visible region. Photoluminescence (PL) spectra show various emission bands consisting of blue, green and yellow emissions corresponding to different intrinsic defects in nanomaterials. PL displays a tuning trend for blue–green emission by changing solvent from water to alcohol. However, overall enhanced PL intensity and particularly intense blue emission have been achieved by replacing water with alcohol. Tunability in emission colours and high colour purity is observed in the CIE chromaticity analysis. Theoretically, estimated electroluminescence of ZnO prepared in alcohol shows superiority compared to ZnO prepared in water. The mechanism of solvent-mediated defect creation and emission in ZnO will be beneficial for future QD LED applications.

     

Optical properties and electrical resistivity of boron-doped ZnO thin films grown by sol–gel dip-coating method

Sol–gel dip-coating was used to grow ZnO thin films doped with various concentrations of B ranging from 0 to 2.5 at.% on quartz substrates. The effects of B doping on the absorption coefficient (α), optical band gap (E_g), Urbach energy (E_U), refractive index (n), refractive index at infinite wavelength (n_∞), extinction coefficient (k), single-oscillator energy (E_o), dispersion energy (E_d), average oscillator strength (S_o), average oscillator wavelength (λ_o), moments M₋₁ and M₋₃, dielectric constant (ε), optical conductivity (σ), and electrical resistivity (ρ) of the BZO thin films were investigated. The transmittance spectra of the ZnO and BZO thin films show that the transmittance of the BZO thin films was significantly higher than that of the ZnO thin films in the visible region of the spectrum and that the absorption edge of the BZO thin films was blue-shifted. The BZO thin films exhibited higher E_g, E_U, and E_o and lower E_d, λ_o, M₋₁ and M₋₃ moments, S_o, n_∞, and ρ than the ZnO thin films.     

Magneto acoustical emission in nanocrystalline Mn–Zn ferrites

Mn_0.4Zn_0.6Fe₂O₄ powders were prepared by microwave hydrothermal method. The powders were characterized by X-ray diffraction, transmission electron microscope. The powders were sintered at different temperatures 400, 500, 600, 700, 800 and 900 °C/30 min using microwave sintering method. The grain size was estimated by scanning electron microscope. The room temperature dielectric and magnetic properties were studied in the frequency range (100 kHz–1.8 GHz). The magnetization properties were measured upto 1.5 T. The acoustic emission has been measured along the hysteresis loops from 80 K to Curie temperature. It is found that the magneto-acoustic emission (MAE) activity along hysteresis loop is proportional to the hysteresis losses during the same loop. This law has been verified on series of polycrystalline ferrites and found that the law is valid whatever the composition, the grain size and temperature. It is also found that the domain wall creation/or annihilation processes are the origin of the MAE.     

Formation and thermal stability of Ca–Mg–Zn and Ca–Mg–Zn–Cu bulk metallic glasses

Several Ca–Mg–Zn and Ca–Mg–Zn–Cu bulk metallic glasses were produced by copper mold casting method. The alloy compositions were selected using specific criteria recently identified by the authors. The glass transition temperature, crystallization temperature, temperature interval of the supercooled region, melting temperature as well as heats of crystallization, and melting are reported for these alloys.     

Microstructure,mechanical properties and machinability of Cu–Zn–Mg and Cu–Zn–Sb brass alloys

Lead-free alloys have attracted great attentions recently due to the toxic nature of lead for the human body. In this study, low amounts of Mg and Sb were added to the Cu65–Zn35 brass and microstructure, mechanical properties and machinability of samples were compared to Cu65–Zn35 brass. Both Mg and Sb led to the promotion of β′ phase as well as the formation of new ternary copper rich intermetallic particles. It was found that these particles had a significant role in the reduction of the ultimate tensile strength, toughness, work hardening and elongation while increasing the hardness of samples. Results of machinability evaluation of samples showed that the cutting forces were decreased significantly and morphology of chips were improved compared to Cu65–Zn35 brass sample.