Broad Visible Emission Band Research Articles

Synthesis, optical and magnetic properties of pure and Co-doped ZnFe2O4 nanoparticles by microwave combustion method

Spinel zinc ferrite (Zn1−xCoxFe2O4) nanoparticles with various particle sizes were prepared by the microwave combustion method using urea as a fuel. The composites were prepared with the addition of cobalt at different molar ratios (x=0.0 to 0.5) to ZnFe2O4. The obtained spinel ferrites were characterized by X-ray powder diffraction (XRD) and their mean grain size and morphology were determined by the high resolution scanning electron microscopy (HR-SEM). The magnetic properties of the synthesized ferrites were investigated using room temperature vibrating sample magnetometer (VSM) and their hysteresis loops were obtained. The optical reflectance and photoluminescence (PL) emissions were determined by UV–visible diffuse reflectance spectra (DRS) and PL spectra respectively. The formation of single cubic spinel phase was confirmed by XRD and Rietveld analysis with an average crystallite size is in the range of 43–49nm. The broadband visible emission band is observed in the entire PL spectrum and the estimated energy band gap is about 2.1eV. The variation of saturation magnetization (Ms) value of the samples was studied. The prepared lower compositions (0.0, 0.1 and 0.2) show a superparamagnetic behavior and the higher compositions (0.3, 0.4 and 0.5) show a ferromagnetic behavior with hysteresis and that the Ms increases with increasing Co content to reach a maximum value of 65.20emu/g for Zn0.5Co0.5Fe2O4. The relatively high Ms of the samples suggests that this method is suitable for preparing high-quality nanocrystalline magnetic ferrites for practical applications. Different mechanisms to explain the obtained results and the correlation between magnetism and structure are discussed.

Broadband conversion of ultraviolet to visible and near-infrared emission in Gd3 +/Yb3 + codoped germanate glass

Gd3+/Yb3+ co-doped germanate glasses were fabricated using the melt-quenching method. Broadband visible emission was observed from the gadolinium doped germanate glasses under ultraviolet (UV) excitation, and white-light was obtained by modifying Yb3+ co-dopant concentration. Through this co-doped system, the white-light emission can be further converted into the near-infrared range which is matched well with the energy band gap of commercial silicon solar cell. The possible energy transfer processes involved are proposed and discussed according to the excitation, emission and fluorescence decay measurements. The effect of Yb3+ ion concentration with respect to fixed Gd3+ ions on the visible and near-infrared (NIR) emission has also been studied. The conversion efficiency from visible to near-infrared is as high as 69.3% in the GZN: 0.2Gd2O3–0.6Yb2O3 sample. The results indicate that this material is potential in various applications, such as white LED, enhancing silicon solar cell efficiency, and so on.

Comparative Study of Pure and Ni-Doped ZnFe<sub>2</sub>O<sub>4</sub> Nanoparticles for Structural, Optical and Magnetic Properties

Pure and Ni-doped ZnFe2O4 nanoparticles, Zn1-xNixFe2O4 (x = 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) were synthesized by microwave combustion method (MCM). The structural, morphological and magnetic properties of the products were determined by X-ray diffraction (XRD), high resolution scanning electron microscopy (HR-SEM) and vibrating sample magnetometer (VSM). X-ray analysis showed that all the compositions crystallize with cubic spinel structure. The broadband visible emission is observed in the entire photoluminescence (PL) spectrum and the estimated energy band gap is about 2.1 eV. VSM measurements shows superparamagnetic behavior for lower concentration of Ni (x ≤ 0.2), whereas for higher concentration (x ≥ 0.2), it becomes ferromagnetic. The saturation magnetization (Ms) varies considerably with Ni content to reach a maximum value for Ni0.5Zn0.5Fe2O4 composition, i.e. 57.89 emu/g.

Structural and optical properties of (Al, K)-co-doped ZnO thin films deposited by a sol–gel technique

Thin films of Al-doped ZnO (AZO) and (Al, K)-co-doped ZnO (AKZO) were synthesized by sol–gel spin coating and their structural and optical properties were investigated. All the films had a preferential orientation in which the c-axis was perpendicular to the substrate. The optical bandgap increased after Al doping, but decreased after K doping at a given Al doping concentration. UV emission and a broad visible emission band were observed in photoluminescence (PL) spectra. The intensity of both emission bands decreased after Al and K co-doping. PL excitation (PLE) spectra of the blue emission band indicate that the initial state is possibly the same for all the samples and a similar case occurs for the orange–red emission band. The green emission can be attributed to electronic transitions involving oxygen vacancies. A possible process for the orange–red emission of the thin films is radiative recombination of an electron trapped in a zinc interstitial defect with a hole deeply trapped in interstitial oxygen.

Microwave combustion synthesis, structural, optical and magnetic properties of Zn1−xSrxFe2O4 nanoparticles

Abstract Pure and strontium doped zinc ferrite (Zn 1− x Sr x Fe 2 O 4 ) nanoparticles were prepared by the microwave combustion method using urea as the fuel. Rietveld refinements of X-ray diffraction pattern confirm the formation of single cubic spinel phase with an average crystallite size in the range of 25–42 nm. The broad visible emission band is observed in the entire photoluminescence spectrum. The estimated band gap energy is found to decrease with increasing Sr content, i.e. 2.1–1.72 eV. Magnetic measurements at room temperature revealed that at lower Sr concentration ( x ≤0.2), the system shows a superparamagnetic behavior, whereas at higher Sr concentration ( x ≥0.2), it becomes ferromagnetic. The relatively high saturation magnetization of the as-prepared Sr-doped ZnFe 2 O 4 nanoparticles suggest that this method is suitable for preparing high-quality nanocrystalline magnetic ferrites for practical applications. The mechanism for the formation of ZnFe 2 O 4 by the microwave combustion method is also discussed in the present study. Microwave combustion produced sufficient energy for the formation of ZnFe 2 O 4 , because of its homogeneous distribution within the raw materials. This results in the formation of nanoparticles and early phase formation within few minutes of time.

Cathodoluminescence and field emission from GaN/MgAl2O4grown by metalorganic chemical vapor deposition: substrate-orientation dependence

GaN films with single-crystalline and polycrystalline structures were deposited by metalorganic chemical vapor deposition (MOCVD) on (111) and (100) MgAl2O4 substrates modified by chemical etching and thermal passivation. The oriented GaN films on the as-processed (111) MgAl2O4 substrate revealed broad visible emission band bands at 3.2 eV and a sharp luminescence peak centered at 1.8 eV in room temperature cathodoluminescence measurements, which can be attributed to the recombination of donor–acceptor pairs and GaN-related emission, respectively. Field emission (FE) measurements demonstrated that the oriented nanostructured 150 nm-thick GaN film on the as-processed (111) MgAl2O4 substrate has an ultralow turn-on-field of 4.29 V μm−1 at 10 nA cm−2 and a stable emission of 0.028 mA cm−2 with 5% current density fluctuation for 22 h without any degradation. Compared to GaN films on as-processed (100) MgAl2O4 substrate, the mechanism for the improved performance in optical and FE properties of the GaN film on as-processed (111) MgAl2O4 substrate has been investigated in detail.

Structural, optical and magnetic properties of Zn1−xCuxFe2O4 nanoparticles prepared by microwave combustion method

Nano-sized copper doped zinc ferrite powders, Zn1−xCuxFe2O4 (x=0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) were synthesized by microwave combustion method. The structural, morphological and magnetic properties of the products were determined and characterized in detail by X-ray diffraction (XRD), high resolution scanning electron microscopy (HR-SEM), energy dispersive X-ray spectroscopy (EDX) and vibrating sample magnetometer (VSM). X-ray analysis showed that all compositions crystallize with a cubic spinel-type structure. The lattice parameter decreased from 8.443 to 8.413Å with increasing Cu content. The average crystallite size was found in the range of 41.20–45.84nm. Magnetic measurements revealed that for lower Cu concentration (x⩽0.2), the system shows a superparamagnetic behavior whereas for higher concentration (x⩾0.2), it becomes ferromagnetic. It has been explained in terms of random distribution of Zn2+ and Fe3+ ions at tetrahedral [A] and octahedral [B] sites. The saturation magnetization (Ms) varies considerably with Cu content to reach a maximum value for Cu0.5Zn0.5Fe2O4 composition, i.e. 58.58emu/g. The high saturation magnetization of these samples suggests that this method is suitable for preparing high quality nanoparticles for magnetic applications. The broadband visible emission is observed in the entire photoluminescence (PL) spectrum and the estimated energy band gap is about 2.1eV. The composition with x=0.5 showed the highest intensity and was explained on the basis of disordered cluster model.

Origin of visible photoluminescence from arrays of vertically arranged Si-nanopillars decorated with Si-nanocrystals

Arrays of vertically aligned Si-nanopillars, with average diameters of 100 nm and 5 μm length, have been prepared by wet chemical etching of crystalline silicon in a special manner. Samples with smooth- and porous-walled nanopillars have been studied. In the case of the latter, Si-nanocrystals, passivated with SiOx, decorating the surface of the nanopillars are identified by the means of TEM and FTIR. When excited by UV–blue light, the porous-walled Si-nanopillars are found to have a strong broad visible emission band around 1.8 eV with a nearly perfect Gaussian shape, μs luminescence lifetimes, minor emission polarization and a non-monotonic temperature dependence of luminescence. The Si-nanocrystal surface is found to be responsible for the luminescence. The red-shift of the emission maximum and the luminescence quenching induced by oxidation in UV-ozone confirm this assumption. A model of luminescence involving UV photon absorption by Si-nanocrystals with subsequent exciton radiative recombination on defect sites in SiOx covering Si-nanocrystals has been proposed. Possible applications of the nanopillar arrays are discussed.

Effect of film thickness and annealing temperature on the structural and optical properties of ZnO thin films deposited on sapphire (0001) substrates by sol–gel

ZnO thin films were prepared on sapphire (0001) substrates by the dip-coating sol–gel technique. Then, ZnO thin films with thicknesses of 50nm, 150nm, 250nm, and 350nm were annealed at 800°C and with thickness of 250nm were annealed at 600°C, 700°C, 800°C, and 900°C, respectively. The effect of film thickness and annealing temperature on the structure and optical properties of sol–gel ZnO thin films on sapphire substrates were investigated by XRD, SEM, RT-PL. All the sol–gel ZnO thin films show polycrystalline hexagonal wurtzite structure and a high preferential c-axis orientation. The XRD and SEM results show that the better structural quality, high c-axis preferred orientation, uniform, compact sol–gel ZnO thin films 250nm thick were obtained when annealed at 700°C in air. Room-temperature PL spectrum of sol–gel ZnO thin films can be divided into the UV emission and the visible broad band emission. The UV emission can be attributed to the near band edge emission (NBE) and the visible broadband emission can be ascribed to the deep level emissions (DLE). The visible emission is suppressed with the annealing temperature of 600–700°C. By analyzing our experimental results, we concluded that the deep-level emission corresponds to oxygen vacancies (VO). The biggest ratio of the PL intensity of UV emission to that of visible emission (INBE/IDLE) is observed from sol–gel ZnO thin films 250nm thick annealed at 700°C. Therefore, we suggest that film thickness of 250nm and annealing temperature of 700°C are the most suitable conditions for obtaining high quality, high c-axis preferred orientation, uniform, compact sol–gel ZnO thin films with good luminescence performance.

Full-color photoluminescence of ZnO nanorod arrays based on annealing processes

Vertically well-aligned ZnO nanorod arrays with highly crystalline wurtzite structures were synthesized on ITO glass substrates by a two-step hydrothermal method, and then, annealed at different temperatures and under different atmospheres. Photoluminescence spectra showed a near-band-edge (NBE) emission peak and an intense broad visible emission band for as-grown and annealed samples. These visible emission bands exhibited dependences on post-annealing conditions and excitation energies. Emission shifting in the visible region was investigated and explained in terms of the presence of defects. Based on these findings, we demonstrate that the visible emission in these materials can be tailored by controlling the defects in ZnO. Under different annealing conditions, it is possible to manipulate the emission in the visible region, which has important implications for the production of optoelectronic devices.

Synthesize and Characterization of Ca2CuO3 Nanostructures via a Modified Sol–Gel Method Assisted by Hydrothermal Process

A sol–gel-hydrothermal method was used to prepare the highly homogeneous nanopowder of Ca2CuO3 ceramic with an average diameter of about 20–30 nm. In comparison with the conventional sol–gel process, this method caused the more purity product with smaller size of particle due to a high pressure and long reaction time. Photoluminescence measurement indicated that for Ca2CuO3 nanoparticles there are two emission peaks: one sharp band at about 400 nm and one broad visible emission band. The band gap energy of Ca2CuO3 nanoparticles has been estimated to be 3.09 eV that is far above the values reported earlier. The results showed that the morphology and particle size of the synthesized samples can be obviously affected by the reaction temperature and molar ratio (w) between citric acid and calcium nitrate. In this procedure, diethylene glycol monobutyl ether was used as both the solvent and the modest surfactant.

Structural and optoelectronic characterization of prepared and Sb doped ZnO nanoparticles

This paper briefly reports the structural and optoelectronics properties of prepared pure and Sb doped ZnO nanoparticles. Doping with suitable elements offers an efficient method to control and enhance the optical properties of ZnO nanoparticles, which is essential for various optoelectronics applications. Sb doped ZnO nanoparticles have significant concern due to their unique and unusual electrical and optical properties. In the present work, we report the synthesis of Sb doped ZnO successfully with average particle size range from 26 to 29 nm via direct precipitation method. The phase purity and crystallite size of synthesized ZnO and Sb doped nano-sized particles were characterized and examined via X-ray diffraction (XRD) and scanning electron microscopy (SEM). The elemental analyses of undoped and doped ZnO nanoparticles were examined by using energy-dispersive X-ray spectroscopy (EDAX).We investigated and measured the optoelectronics properties of synthesized ZnO and Sb doped ZnO nanoparticles by employing photoluminescence and UV–Visible spectroscopy. The influence of Sb doping on photoluminescence (PL) spectra of ZnO nanoparticles, which consists of UV emission and broad visible emission band, is found to be strongly dependent upon the Sb concentration for all the Sb doped ZnO nanoparticles samples under investigation. The UV–Visible absorption study shows an increase in band gap energy as Sb is incorporated on the ZnO nanoparticles.

Effect of growth conditions on zinc oxide nanowire array synthesized on Si (100) without catalyst

A uniformly distributed ZnO nanowire array has been grown on silicon (100) substrates by catalyst-free chemical vapor transport and condensation. The effect of growth conditions including source heating temperature, substrate temperature, and gas flow rate on growth properties of ZnO nanowire arrays are studied. Scanning electron microscopy, X-ray diffraction, and room temperature photoluminescence are employed to study the structural features and optical properties of the samples. The results show a correlation among experimental growth parameters. There is a zone for substrate temperature, by controlling gas flow rate, that uniformly distributed and well aligned ZnO nanowire arrays can be grown. Also, experiments indicate that ZnO nanowire arrays with different diameter along their length have been formed under various growth conditions in the same distance from source material. It is found that supersaturation is a crucial parameter determining the growth behavior of ZnO nanowire arrays. The growth mechanism of ZnO nanowires is discussed. The room temperature photoluminescence spectrums of ZnO nanowire array show two emission bands. One is the exciton emission band (centered at 380nm) and the other is a broad visible emission band centered at around 490nm. As the substrate temperature decreases, the intensity of UV emission increases while the intensity of visible emission peak decreases.

Synthesis and Optical Properties of Single-Crystalline Silicon Nitride Nanowires with Controlled Dimensionality

Single-crystalline silicon nitride nanowires with high purity, controlled dimensionality have been prepared via nitriding the nanocrystalline silicon powders at 1300°C~1400°C. The nanocrystalline silicon powders with average particle size of 20-80nm were obtained by cryomilling with the liquid nitrogen as the medium. Scanning electron microscopy, high resolution transmitted electron microscope, X-ray diffraction and UV-lamp microzone Raman spectrometer were used to characterize the as-synthesized nanowires. The effects of nitridation process (reaction temperature and holding time) and the particle size of nanocrystalline silicon powders on the phase and microstructure of the silicon nitride nanowires were analyzed. The obtained results show that the diameter of the nanowires can be controlled in the range of 40~100nm, and the length of 10~80 μm. The formation of the nanowires can be explained by the vapor-solid growth mechanism. The room temperature photoluminescence spectra show that the silicon nitride nanowires exhibit a broad visible emission band which ranges from 370 nm to 700 nm.

Effect of Cu ions on the morphology, structure and luminescence properties of ZnO nanorod arrays prepared by hydrothermal method

Well-aligned ZnO nanorod arrays with different average diameters were grown by hydrothermal method in the presence of Cu ions in the deposition solution. Cu ions significantly inhibit diameter growth of ZnO nanorods and influence the luminescence properties. Quantitative analyses reveal that Cu ions were not incorporated into the synthesized nanorods. Photoluminescence spectra show that a weak ultraviolet emission and a strongly broad visible emission band in yellow region under ultraviolet light excitation for as-grown ZnO nanorods. The intensity of yellow emission bands decreases with Cu ions concentration increasing and the peak position has a blue-shift after low temperature thermal treatment. Photoluminescence spectra under different excitation energies and photoluminescence excitation spectra reveal that the yellow emission bands and green (blue) emission bands are associated with zinc interstitial related shallow donor level and oxygen vacancies related deep donor level as initial states, respectively. A plausible growth mechanism has been proposed to explain the narrowed diameter for ZnO nanorods in the presence of Cu ions based on crystallographic habits of wurtzite hexagonal.

Citrate–nitrate Derived Sr0.5Ba0.5Ta2O6 Tetragonal Tungsten Bronze Nanorods: Investigation Of Their Optical And Dielectric Properties

First time novel complex strontium barium tantalate, Sr0.5Ba0.5Ta2O6 was successfully synthesized in 1D structure (nanorods) by citrate-nitrate gel route. Their structural properties were examined via X-ray diffractometry (XRD) and revealed the formation of tetragonal tungsten bronze (TTB) structure at as low as 1100oC. Also, FT-IR, FT-Raman, UV-vis, SEM, TEM and PL were used to identify the structure and properties of powders. Well isolated nanorods of the average diameter of ~200nm can be fabricated by this route. PL spectrum showed strong and broad visible emission band around 439nm due to particles with little surface defects. The frequency dependent dielectric dispersion of SBT50 powders sintered at 1300°C/4 h was investigated in a frequency range from 1 kHz-1MHz and at different temperatures (25°C- 450°C). It is observed that: (i) the dielectric constant (e') and loss tangent (tan δ) are dependent on frequency, (ii) the temperature of dielectric constant maximum shift toward lower temperature side and the maximum dielectric constant (e) was observed to be 2400 at 1 kHz. The Tc was found to be ~444- 449°C and ferroelectric relaxor or diffuse phase transition like behavior was observed at around 449°C. Copyright © 2012 VBRI Press.

Visible photoluminescence from ZnO/diamond-like carbon thin films

ZnO/diamond-like carbon (DLC) thin films are deposited by pulsed laser deposition (PLD) on Si (111) wafer. Visible room-temperature photoluminescence (PL) is observed from ZnO/DLC thin films by fluorescence spectrophotometer. The Gaussian curve fitting of PL spectra reveals that the broadband visible emission contains three components with =508 nm, 554 nm and 698 nm. The origin and possible mechanism of the visible PL are discussed, and they can be attributed to the PL recombination of ZnO and DLC thin films.

Defect-related visible luminescence of ZnO nanorods annealed in oxygen ambient

ZnO nanorods prepared by a solution-phase method are annealed at different temperatures in oxygen ambient. The luminescence properties of the samples are investigated. In the same excitation condition, the photoluminescence (PL) spectra of all samples show an ultraviolet (UV) emission and a broad strong visible emission band. The asymmetric visible emission band of annealed samples has a red-shift as the annealing temperature increasing from 200 °C to 600 °C and it can be deconvoluted into two subband emissions centered at 535 nm (green emission) and 611 nm (orange-red emission) by Gaussian-fitting analysis. Analyses of PL excitation (PLE) spectra and PL spectra at different excitation wavelengths reveal that the green emission and the orange-red emission have a uniform initial state, which can be attributed to the electron transition from Zn interstitial (Zni) to oxygen vacancy (Vo) and oxygen interstitial (Oi), respectively.

Visible Luminescence Mechanism of ZnO Nanoparticles Synthesized by Sol–Gel Method

We presented our investigations on the absorption and emission properties of the nanocrystalline ZnO particles of different particle sizes (2 nm-5 nm) by sol-gel method. In the room temperature PL spectra, three emission bands, ultraviolet (UV), blue and green were observed. With increasing the particle sizes, both the UV and the visible emission bands shifted to lower energies progressively. From the size-dependency, there was a linear relationship between the energetic maxima of the UV and the green emission bands with a slope of about 0.26, which indicated that the green luminescence of ZnO was produced by the transitions of electrons from deep level to the valence band (or shallow acceptor level). A linear dependence was also found between the energetic maxima of the UV and the blue emissions with a slope of 0.15, the origin this blue emission band is not clear at present. While in van Dijken et al.'s paper, however, they identified only two emission bands in the emission spectra, an UV and a broad visible emission band, and the linear fit between the energetic maxima of these two bands in particles of different sizes has a slope of 0.6, so they proposed that the visible emission in ZnO was originated from the recombination of a shallowly trapped electron with a deeply trapped hole. We attributed this divergence to the fact that the broad visible band is actually composed of two separate emission bands originated from two different recombination processes, and should not had been treated as one emission band.

Effect of annealing temperature and annealing atmosphere on the structure and optical properties of ZnO thin films on sapphire (0 0 0 1) substrates by magnetron sputtering

ZnO thin films were epitaxially grown on sapphire (0001) substrates by radio frequency magnetron sputtering. ZnO thin films were then annealed at different temperatures in air and in various atmospheres at 800°C, respectively. The effect of the annealing temperature and annealing atmosphere on the structure and optical properties of ZnO thin films are investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL). A strong (002) diffraction peak of all ZnO thin films shows a polycrystalline hexagonal wurtzite structure and high preferential c-axis orientation. XRD and AFM results reveal that the better structural quality, relatively smaller tensile stress, smooth, uniform of ZnO thin films were obtained when annealed at 800°C in N2. Room temperature PL spectrum can be divided into the UV emission and the Visible broad band emission. The UV emission can be attributed to the near band edge emission (NBE) and the Visible broad band emission can be ascribed to the deep level emissions (DLE). By analyzing our experimental results, we recommend that the deep-level emission correspond to oxygen vacancy (VO) and interstitial oxygen (Oi). The biggest ratio of the PL intensity of UV emission to that of visible emission (INBE/IDLE) is observed from ZnO thin films annealed at 800°C in N2. Therefore, we suggest that annealing temperature of 800°C and annealing atmosphere of N2 are the most suitable annealing conditions for obtaining high quality ZnO thin films with good luminescence performance.