Photoluminescence Spectra Of ZnO Research Articles

Interfacial engineering boosted narrow-band ultraviolet LED based on n-PtNPs@ZnO:Ga microwire/AlN/p-GaN heterojunction

Low-dimensional ZnO micro/nanocrystals having high optical gain, self-constructed microcavity and large-bandgap, is the largest potential constituent for constructing droop-free and high-brightness ultraviolet light sources. In this study, we reported a workable scheme to construct high-monochromatic ultraviolet electroluminescence devices including a Ga-doped ZnO microwire decoated with platinum nanoparticles (PtNPs@ZnO:Ga MW), an AlN electron blocking layer and p-GaN layer substrate. The device exhibits a robust narrow-band ultraviolet electroluminescence peaking at around 374.8 nm and a narrowing bandwidth ∼ 13.5 nm. The electroluminescence profile is comparable with photoluminescence spectrum of ZnO:Ga MW. The device properties can be interpreted by the working mechanism of plasmonic effects, heterojunction interface engineering and optimization strategies. The AlN intermediate layer appeared in the optimized device enables effectively to tune the paths of current traveling and carrier recombination, yielding the band-edge luminescence of ZnO:Ga MW. As the device is further modified using PtNPs with desired plasmons, the optimization of heterojunction interface is significantly boosted, like the observable enhancement of holes injection efficiency, electroluminescence efficiency and interface contact, etc. Thereby, the boosted band-edge luminescence of ZnO:Ga MW can contribute to the well-being of the enhanced ultraviolet electroluminescence in our constructed LED devices. The findings are looking forward to bringing new opportunities toward the implementation of ultra-bright and high-efficiency narrow-band ultraviolet light sources, especially for the achievement of electrically-pumped microlaser devices.

Magnetic and optical properties of printed ZnO:Co polycrystalline layers

The results of studies of printed ZnO:Co polycrystalline layers are first described. The layers with Co concentration of 0.78, 2.09 and 5.42 at.% were printed by a new stencil-free method developed by the authors and then were recrystallized at a temperature 1000–1100 °C. It was shown that the layers were polycrystalline with a typical grain size about 500 nm, were densely packed and had a rather uniform single-phase wurtzite structure. The influence of Co doping on the structure, composition, photoluminescence, optical absorption, confocal micro-Raman spectra and the magnetic properties of the layers was investigated. It was shown that Co ions substituted Zn2+ ions and were uniformly distributed in the ZnO lattice whereas in previous publications on ZnO:Co only the statistical average distribution of Co in ZnO films was reported. In was also found that, with increasing Co contamination, Raman spectra showed systematic frequency shift and broadening of the E2(high) and E2(low) phonon modes. The optical band gap of the layers decreased with increasing Co contamination and the lattice parameters a and c were found to depend slightly on the Co content. At cryogenic temperatures the donor bound excitons dominated in the photoluminescence spectra of ZnO:Co layers which indicated that the layers were of a good crystalline quality and had a n-type conductivity.Magnetization measurements made by the method of magnetic force microscopy (MFM) revealed a clear magnetic contrast in 2D magnetization maps, providing the evidence of in-plane ferromagnetic magnetization at room temperature. Magnetization was shown to be increased with increasing Co content. The direct correlation between the magnetic behavior of ZnO:Сo polycrystalline layers and their microstructure was identified. In particular, it was shown that secondary Co-related spinel phases, if any, were localized at the grain boundaries of the ZnO:Co microcrystallites.

Screen printed novel ZnO/MWCNTs nanocomposite thick films

Zinc oxide and multiwall carbon nanotubes (ZnO/MWCNT) nanocomposites thick films were prepared via sol-gel screen printing procedure and followed by sintering at 550 °C. Thus, the prepared films were characterized using X-ray diffraction (XRD), scanning electron microscope (SEM), Ultraviolet–visible (UV–vis), Photoluminescence (PL), Fourier transform infrared (FTIR), Raman spectroscopy and Two-probe method. XRD analysis revealed (101) orientation for both ZnO and ZnO/MWCNT thick films with wurtzite structure. SEM studies confirmed the porous nature of ZnO film while ZnO/MWCNT films showed ZnO particles trapped in the porous MWCNT network and free from cracks. Reflectance spectroscopy showed direct transition with decreasing band gap whereas refractive index and absorption index showed appreciable variation within the band gap regime related to the change in crystallite size. FTIR profile approved the Zn–O stretching and presence of carboxylic CDC group. The PL spectrum of ZnO and ZnO/MWCNT thick films shows red shift and exhibits UV, blue and green emissions confirmed from CIE diagram. Raman spectrum shows that Raman phonons are shifted and dominated due to doping of MWCNT in ZnO matrix. Electrical properties were investigated using 2-probe method and showed a reduction in resistance on MWCNT incorporation. The novelty of current-work is the fabrication of ZnO/MWCNT through a low-cost screen printing process for the first time and the results exhibits that the bandgap of the deposited film is decreased, which in turn, play a significant role in enhancement of conductivity and colour emission for fabrication of low cost optoelectronics devices (LEDs) as compared with the pure ZnO film.

Effect of fluorine and boron co-doping on ZnO thin films: structural, luminescence properties and Hall effect measurements

In this work, undoped and fluorine and boron co-doped ZnO (ZnO:B:F) thin films have been prepared at 400 °C by using Ultrasonic Spray Pyrolysis technique and optical, electrical, structural, surface and luminescence properties of the films have been investigated. Transmission spectra have been taken by UV–VIS spectrophotometer. It is found that ZnO:B:F films have high average optical transmittance. X-ray diffraction (XRD) patterns indicate that the obtained ZnO:B:F has a hexagonal wurtzite type structure with (002) preferential orientation. The crystallite size is calculated by Debye–Scherrer and Williamson–Hall method. Photoluminescence (PL) spectra of the films have been used at room temperature in order to obtain optical properties of the films. Green emission and UV emission band are observed in PL spectra of ZnO and ZnO:B:F. Surface morphologies of the films have been examined using Scanning Electron Microscope (SEM). Compositional analysis has been performed by EDX. The electrical resistivity, carrier concentration and Hall mobility have been measured. The highest Hall mobility of 13.22 cm2 v−1s and the lowest electrical resistivity of 3.13 × 10−4 Ω cm were obtained at the optimal boron-fluorine co-doping concentration of 5 at.%. All of the results have been appreciated in point of view of optoelectronic industry and photovoltaic solar cell applications and it has been concluded that B–F co-doping has a positively effect on electrical properties.

Plasmon mediated enhancement of visible light emission of Au-ZnO nanocomposites

We report on the enhancement of the visible luminescence of Au-ZnO nanocomposites synthesized via a simple photoreduction method. The attachment of Au nanoparticles on the surface of ZnO nanostructures has been observed using transmission electron microscopy. Raman analysis reveals an increase in disorderness via lattice defect formation due to Au attachment and thus results in the appearance of silent modes of ZnO. The photoluminescence spectra of ZnO is found to be dominated by defect related visible emission, which is enhanced with the inclusion of Au nanoparticles. At room temperature, the green emission for nanocomposites has been found to be enhanced by seven times as compared to the pristine ZnO. Temperature dependent photoluminescence of Au-ZnO nanocomposites has been investigated to elucidate the origin of green luminescence and the enhancement of intensity. The gradual dominance of the green emission is originated from the surface plasmon oscillation of Au followed by the electron capture at the singly ionized oxygen vacancies of ZnO. The observed enhancement of visible light emission of ZnO using Au nanoparticles can establish the platform to fabricate efficient ZnO based visible light emitting devices and display systems.

Dye-modified ZnO nanohybrids: optical properties of the potential solar cell nanocomposites

We report the hybridization of ZnO with natural dyes [Laali, Zobo] or synthetic dye [methyl red] forming ZnO–laali, ZnO–zobo and ZnO–methyl red nanocomposites in bright colours. The structural, optical and dye photosensitization influence of the hybrid nanocomposites were studied by X-ray diffraction (XRD), scanning electron microscope (SEM), UV–Visible absorption spectroscopy and photoluminescence (PL). The surface plasmon absorption band of ZnO–laali and ZnO–zobo shifts towards red and blue, respectively, with significantly enhanced absorption intensities, indicating the interaction and optical influence of the respective dyes in photosensitization. Optical and absorption character of ZnO methyl red and bare ZnO are similar indicating the insignificant effect of methyl red on photosensitization. PL spectra of ZnO–laali and ZnO–zobo display enhanced UV light emission due not only to the surface electron transfer from their respective inherent isoplumbagin and anthocyanin to ZnO but also to the extension of the Fermi energy level to the ZnO. Dyes adopted influence the optical band gaps of the evolved hybrid nanocomposites.

Manifestation of oxygen desorption in photoluminescence spectra of ZnO

The influence of oxygen desorption on the photoluminescence of zinc oxide has been investigated. Photodesorption and thermal desorption of oxygen have been controlled using mass spectrometry. It has been found that the removal of oxygen from the ZnO surface leads to a manifold increase in the exciton photoluminescence intensity. The process has a reversible character. The mechanism of increase in the exciton luminescence intensity of ZnO during the oxygen desorption has been discussed.

Effect of thermal annealing on properties of polycrystalline ZnO thin films

Electrical properties (density, carriers mobility, resistivity), optical absorption and photoluminescence spectra of ZnO, grown by MOCVD and hydrothermal methods, have been investigated depending on the annealing and treatment modes in a hydrogen plasma. It has been shown that the electrical and photoluminescent (PL) properties of ZnO are strongly dependent on gas atmosphere during annealing. The annealing in oxygen atmosphere causes a sharp drop of carrier mobility and films conductivity due to the absorption of oxygen on grain boundaries. The process of ZnO electrical properties recovery by the thermal annealing in inert atmosphere (nitrogen), in oil (2×10−2mbar) and oil-free (1×10−5mbar) vacuum has been investigated. The hydrogen plasma treatment influence on the intensity of near-band-gap emission (NBE) has been studied. The effect of annealing and subsequent plasma treatment on PL intensity depends on the gas atmosphere of preliminary thermal annealing.

Manifestation of strong d‐p hybridization in photoluminescence spectra of ZnO:Ni and ZnO:Co

AbstractFor the first time photoluminescence and photoluminescence excitation spectra for ZnO:Co and ZnO:Ni crystals were obtained at the excitation by the synchrotron radiation in the interval of energy of (3.7‐21) eV, T = 8 K. It is shown that these spectra are formed by intense radiative transitions through 3d charge transfer states in the band gap. This fact evidences about much stronger hybridization between d‐states of impurity and p‐states of anions in comparison with other II‐VI:3d compounds. It is very important for applications in optoelectronics, for visualization of ultraviolet radiation and understanding of photocatalytic activity of ZnO:3d nanocrystals in the visible region of the spectrum. From the comparison of PLE spectra for ZnO:Co and ZnO:Ni with the spectrum of calculated DOS for ZnO it is revealed a visible transformation of part of valence band states due to a strong d‐p hybridization. Obtained results allow us to consider ZnO:3d as a compounds with a strong correlations. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

UV-free red electroluminescence from the cross-connected p-ZnO:Cu nanobushes/n-GaN light emitting diode.

A p-ZnO:Cu/n-GaN heterojunction light emitting diode (LED) is fabricated by growing cross-connected p-ZnO:Cu nanobushes on n-GaN film using chemical vapor deposition under oxygen-rich condition. This LED emits stable UV-free red light of 677 nm and 745 nm. The electroluminescence (EL) light of this LED is tuned from ultraviolet (UV) of ZnO/GaN to UV-free red by the electronic interfacial transition from the conduction band of n-GaN to the deep acceptor levels of p-ZnO:Cu. Both room temperature and low temperature (5K) photoluminescence spectra of ZnO:Cu indicate that the UV emission of ZnO is suppressed and the green emission is enhanced, which implies the formation of Cu-related deep levels introduced by intentionally doping Cu in ZnO. These deep levels help the EL red emission in the LED device.

Photoluminescence engineering in polycrystalline ZnO and ZnO-based compounds

The results of the investigations of photoluminescence (PL) in ZnO and ZnO-based composite materials are presented. The PL and PL excitation (PLE) spectra of undoped and doped with I group elements or rear earth ions ZnO polycrystalline films, ZnO, Zn1–xMgxO and ZnMgO–TiO2 ceramics were studied. The structural properties of the samples were investigated with X-ray diffraction. Polycrystalline films were prepared by a screen-printing method and annealed at TS = 500–1000 °C. The films annealed at TS < 800 °C exhibited intense UV emission, whereas defect-related one appeared at 800 °C and enhanced with increasing TS. Improvement of the PL and structural characteristics of ZnO films due to Li-doping were achieved. The PL bands caused by Sm and Ho ions were observed under ZnO band-to-band excitation. In the PL spectra of ZnO and Zn1–xMgxO ceramics, two types of PL bands were separated: i) the bands, whose spectral positions were not influenced by the Mg content (green Cu-related as well as self-activated orange and red ones); ii) the bands, spectral positions of which exhibited some blueshift with increasing Mg content (orange Li- and Ag-related and self-activated green ones). In the PL spectra of ZnMgO–TiO2 composites, an intense red emission was found to appear in addition to the broad blue-orange band inherent in ZnMgO alloy. The red emission was ascribed to Mg2TiO4 inclusions in ZnMgO matrix.

The Fabrication of PSi/ZnO Nanostructures as Chemical Sensors for the Detection of Ethanol in Solution Using an Electrochemical Impedance Technique

In this work, porous silicon (PSi) was prepared by electrochemical etching and used as a template for ZnO nanostructures. ZnO nanostructures were grown using the catalytic immersion method at different molar ratio concentrations of the precursor and stabilizer. The ZnO nanostructures were analyzed using FESEM and photoluminescence (PL) spectrometry, before tested with ethanol solution. The population of the ZnO nanostructures on PSi increased with the concentration and followed the surface morphology of PSi. The photoluminescence spectra of ZnO show two dominant peaks in the UV and visible regions. When the concentration of the precursor increased, the PL peaks in the visible region (630 nm) shifted towards the blue region of the spectrum. The PSi/ZnO nanostructure chemical sensor has a large surface area, reversing sensor and fast response in ethanol. The performance of the sensor was affected by the morphology and defect structures of the ZnO nanostructures layer.

Emission of ZnO:Ag nanorods obtained by ultrasonic spray pyrolysis

Scanning electronic microscopy (SEM), X ray diffraction (XRD), photoluminescence (PL) and its temperature dependence have been studied in ZnO:Ag nanorods (NRs) prepared by the ultrasonic spray pyrolysis (USP) method. The time variation at the growth of ZnO:Ag films permits modifying the ZnO phase from the amorphous to crystalline, to change the size of ZnO:Ag NRs and to vary their emission spectra. PL spectra of ZnO:Ag NRs versus temperature has been investigated. This study reveals that the PL band related to the acceptor AgZn (LO phonon replicas of an acceptor bound exciton, ABE (2.877eV)), and its second-order diffraction peak (1.44eV) disappeared in the temperature range of 10–170K with the formation of free exciton (FX). The PL intensity of defect related PL bands decreases monotonously in the range 10–300K with the activation energy of 13meV. The PL band (3.22eV), related to the LO phonon replica of free exciton (FX-2LO) and its second-order diffraction peak (1.61eV) increase monotonously in the range 10–300K. FX related peak dominates in PL spectra at room temperature that testifies on the high quality of ZnO:Ag films prepared by the USP technology.

Pulsed laser deposition of Li–N dual acceptor in p-ZnO:(Li, N) thin film and the p-ZnO:(Li, N)/n-ZnO homojunctions on Si(100)

This article showed the p-type ZnO thin films which were co-doped with different components of Li and N by pulsed laser deposition (PLD) on Si(111) substrates. According to Hall-effect data, the Li-N co-doped ZnO:(Li, N) exhibited stable room-temperature p-type behavior. Combining the XRD, UV-vis transmittance spectrum, and Hall-effect data, the preferable preparation condition was proposed for growing high quality p-type ZnO:(Li, N) film with comparatively low resistivity of 0.09 Ω cm and relatively high carrier concentration of 2.64 × 1017 cm−3, which were obtained at 0.1 at. % lithium composition. Furthermore, compared with the photoluminescence spectrum of ZnO:(Li) with ZnO:(Li, N) thin film, the existence of Li–N dual acceptor, which activated the acceptor in ZnO:(Li, N) thin film, was confirmed. On the basis of these, the p-ZnO:(Li, N)/n-ZnO homojunctions on Si(100) substrate was realized by PLD. All these data proved that Li-N dual acceptor would be effective when activated by PLD doping and this approach was feasible to realize the physical device homojunctions on Si substrate.

Heteroepitaxial growth of non‐polar ZnO films on single crystal NdGaO3 (001) substrates by MOCVD

AbstractHigh‐quality non‐polar ZnO (11$ \bar 2 $0) films were epitaxially grown on single crystal NdGaO3 (NGO) (001) substrates. The non‐polar ZnO (11$ \bar 2 $0) films were grown by metal organic chemical vapour deposition using Zn(C5H7O2)2 and O2 gas as starting materials. X‐ray diffraction (XRD) peaks of ZnO (11$ \bar 2 $0) plane were observed in XRD patterns of 2θ‐ω scans in all films. Rotation domains were not found in XRD φ scan patterns of ZnO (11$ \bar 2 $0) films grown at a substrate temperature of 650 °C or less. XRD peaks of NGO (021) and ZnO (11$ \bar 2 $2) planes were observed at the same angle φ in the XRD φ scan patterns. The epitaxial relationship was ZnO (11$ \bar 2 $0)// NGO (001) and ZnO [0001] // NGO [010]. Lattice mismatch values along the ZnO [0001] direction and along the ZnO (1$ \bar 1 $00) direction were 5.36% and –3.70%, respectively. Growth rate was drastically decreased at the substrate temperature over 650 °C. Striped patterns along the ZnO [0001] direction were observed in atomic force microscopy images of ZnO (11$ \bar 2 $0) films grown at substrate temperatures of 600 and 650 °C. Emissions due to donor‐bound excitions were observed in photoluminescence spectra of ZnO (11$ \bar 2 $0) films measured at 10 K. It is thought that NGO (001) substrates are useful for growth of high‐quality non‐polar ZnO (11$ \bar 2 $0) films. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Synthesis, characterization and photoluminescence properties of Ce3+-doped ZnO-nanophosphors

AbstractThe present study involves the synthesis of Ce3+ doped ZnO nanophosphors by the zinc nitrate and cerium nitrate co-precipitation method. The synthesized nanophosphors were characterized with respect to their crystal structure, crystal morphology, particle size and photoluminescence (PL) properties using X-ray diffraction (XRD), scanning electron microscopy (SEM)/energy dispersive X-ray (EDX), transmission electron microscopy (TEM)/Energy-dispersive X-ray spectroscopy (EDS) and PL-spectroscopy respectively. XRD results revealed that ZnO nanophosphors are single phase and cubic type structures. Further, PL spectra of ZnO:Ce3+ nanophosphors showed green emission because of the charge transfer at single occupied oxygen vacancies with ZnO holes and red emission due to the cerium ion transitions. Intensity and fine structure of the Ce3+ luminescence and its temperature dependence are strongly influenced by the doping conditions. The formation of ZnO:Ce3+ nanophosphors was confirmed by Fourier transform infrared (FTIR) and XRD spectra.

Analysis of the Urbach tails in absorption spectra of undoped ZnO thin films

We report the analysis of the Urbach effect in the absorption spectra of the undoped ZnO thin films. The absorption coefficients of the ZnO thin films show the exponential rise, also known as the Urbach tails, just below the free exciton peak. Fitting of the steepness parameter of the Urbach tails yields the phonon energy to be ℏωp=76 ± 4 meV, consistent with ℏωp=72 meV measured from the photoluminescence spectra of ZnO. The temperature dependence of the Urbach energy, the steepness parameter, and the energy gap strongly suggests that the observed Urbach effect is a result of the cumulative effect of impurities, structural disorders, and electron-phonon interaction in the absorption processes.

Deep ultraviolet detecting property of AlxZn1−xO alloy thin film as photocathode layer

The deep ultraviolet detecting property of AlxZn1−xO (AZO) alloy thin film as photocathode layer was investigated. This AZO thin film was deposited via RF magnetron sputtering using a ceramic target, in which the concentration of Al was about 30at%. UV/dark contrast nearly three orders of magnitude has been achieved by using it in a simply fabricated photodetector, under 254nm UV illumination with power of 0.5mW/cm2 and bias of 80V. The photocurrent can be generated even without external electric field after illumination and come to saturation when the bias reaches 80V. Conventional UV emission peak around 380nm in photoluminescence spectra of ZnO is absent in the AZO thin film. Optical band gap of the AZO thin film is estimated to be 4.41eV and corresponding cutoff wavelength is 281nm. Negative surface potential of this thin film that enables high emitting efficiency is detected using Kelvin probe microscope, due to the elimination of surface barrier.

Recombination luminescence and trap levels in undoped and Al-doped ZnO thin films on quartz and GaSe (0 0 0 1) substrates

Photoluminescence spectra of ZnO and ZnO:Al (1.00, 2.00 and 5.00at.%) films on GaSe (0001) lamellas and amorphous quartz substrates, obtained by annealing, at 700K, of undoped and Al-doped metal films, are investigated. For all samples, the nonequilibrium charge carriers recombine by radiative band-to-band transitions with energy of 3.27eV, via recombination levels created by the monoionized oxygen atoms, forming the impurity band laying in the region 2.00−2.70eV. Al doping induces an additional recombination level at 1.13eV above the top of the valence band of ZnO films on GaSe substrates. As a result of thermal diffusion of Zn and Al into the GaSe interface layer from ZnO:Al/GaSe heterojunction, electron trap levels located at 0.22eV and 0.26eV below the conduction band edge of GaSe, as well as a deep recombination level, responsible for the luminescent emission in the region 1.10−1.40eV, are created.

SYNTHESIS AND CHARACTERIZATION OF DYE (PHENOSAFRANINE) SENSITIZED FLOWER-TYPE ZnO NANORODS

Flower-type ZnO nanorods were synthesized by two-step low temperature solution growth and dye-sensitized with phenosafranine for the first time. The scanning electron microscope result shows that the diameter and the length of a single rod of the flower-type nanostructures depend on the method of synthesis. Optical absorption analysis shows a visible absorption at 517 nm, which was otherwise absent in nano ZnO . The photoluminescence spectra of ZnO and dye-sensitized ZnO nano flowers were also analyzed.