Band Edge Ultraviolet Emission Research Articles

Double Layer Growth of ZnO Nanorods by a Low Temperature Solution Method: Synthesis and Photoluminescence Properties

In the present work, double layer Zinc Oxide (ZnO) nanorods (NRs) were fabricated hydrothermally and their photoluminescence (PL) properties were investigated. Two different recipes and their combination were used to obtain double layer vertically well-aligned ZnO NRs. These recipes include polyethylenimine (PEI) and citrate as additives in the growth solution resulting long NRs with a broad defect emission and relatively short NRs with a near band-edge ultraviolet (UV) emission, respectively. Double layer growth of long-long (LL), long-short (LS) and short-short (SS) ZnO NRs were considered. Grown samples were annealed in a forming gas atmosphere for a better quality NR structure. LL ZnO NRs (long ZnO NRs grown on a long ZnO NR layer) with a 65 µm thickness showed a broad yellow-orange PL emission and no any near band-edge UV emission was observed. LS ZnO NRs representing short ZnO NRs grown on a long ZnO NR layer (LS1= 36 µm, LS2= 48 µm and LS3= 44 µm) showed an enhanced near band-edge UV emission when compared to that of the long ZnO NRs. The UV intensity was found to decrease with the increase in thickness of the NRs in LS samples. Finally, SS ZnO NR sample, (short ZnO NRs grown on a short ZnO NR layer) which has a thickness of 33 µm, displayed a stronger near band-edge UV emission with a negligible broad emission than that of as-grown SS ZnO NRs (UV peak intensity ratio 59). This study should be important for applications where longer NRs with enhanced PL properties are strictly required.

Investigation of the threshold voltage instability in normally-off p-GaN/AlGaN/GaN HEMTs by optical analysis

A normally-off p-GaN/AlGaN/GaN high-electron-mobility transistor (p-GaN HEMTs) with a semitransparent gate electrode was investigated. Under forward gate bias, electroluminescence (EL) emission was observed which demonstrated the hole injection in the gate heterostructure. The hole injection emerged at a gate bias of 3.5 V and was significantly enhanced when the gate bias was larger than 6 V, the corresponding EL emission intensity was increased by approximately 100 times. It was also found that the electron injection and trapping in the p-GaN resulted in the threshold voltage (V TH) instability of the p-GaN HEMTs, while the hole injection resulted in the electron–hole recombination and the suppression of electron trapping. The carrier injection dynamics in the p-GaN gate region were quantitatively identified at different temperatures. Finally, the GaN band-edge ultraviolet emission with an energy of 3.4 eV was demonstrated as the most effective component in EL emission for V TH instability suppression in the p-GaN HEMTs.

Enhanced Optical and Photocatalytic Properties of Au/Ag Nanoparticle-decorated ZnO Films

Plasmonic Au- and Ag-decorated ZnO films on a glass substrate were obtained by a plasma electrolyte oxidation method with high-voltage discharge, followed by the decoration of Au nanoparticles (NPs) on their surface. With the increasing concentration of the Ag precursor, the thickness of the Ag-ZnO films decreased and the width of the bandgap increased, causing an enhancement in the near band-edge ultraviolet (UV) emission. With the decoration of the Au NPs, the photoluminescence of the Ag-ZnO films exhibited quenching of the band-edge UV emission, accompanied by the suppression of the visible emission in the photoluminescence spectra, and a significant enhancement in the photocatalytic efficiency for the photodegradation of Rhodamine B under solar simulator irradiation.

Reduction in point defects of sol–gel derived ZnO thin films with oxygen ambient

The influence of oxygen as an annealing ambient on the native point defects of ZnO thin films has been investigated. ZnO films were derived by sol-gel spin coating method. Films were annealed at 700°C in vacuum, air and with different oxygen flow rates from 1.5 to 5.0 Standard Cubic Centimetres per Minute (SCCM). Intensity of characteristic XRD peak (002) increased intensively with the increment in oxygen, which indicates the improvement in crystal stoichiometry. In photoluminescence (PL) measurements yellow-green emission reduced while near band edge ultraviolet emission enhanced gradually, which is a sign of reduction in native defects (VO/Zni) and improvement in optical quality of films. Resistivity of the films, measured in Van der Pauw geometry, increased significantly as the oxygen flow increased during the annealing. Ferromagnetism in the films was also observed, although at very low scale, which may be due to one or more types of native defects. It is found that saturated magnetic moment (Ms) reduced with the availability of oxygen during annealing.

Enhanced photoluminescence of ZnO nanorods via coupling with localized surface plasmon of Au nanoparticles

We demonstrated the synthesis of ZnO nanorods by using a facile solution route at low temperature. Au nanoparticles were deposited on the surface of ZnO nanorods by an in-situ solution method. The ZnO nanostructures had nanorod-like morphology in large quantity and possessed high crystallinity. An enhancement of ultraviolet emission of ZnO nanorods was observed as a result of Au nanoparticles decoration, while green emission was suppressed. This excellent photoluminescence performance is attributed to surface-plasmon-mediated quenching of defect emission and the enhancement of band-edge emission. The defect transition energy (green) of ZnO nanorods is in resonance with the localized surface plasmon energy of the Au nanoparticles, which led to the conversion of deep level green emission into band-edge ultraviolet emission.

A facile route to realize ultraviolet emission in a nano-engineered SnO2-based light-emitting diode

We reported a facile route to fabricate a tin dioxide (SnO2)-based light-emitting diode (LED) and obtain an electrically pumped band-edge ultraviolet (UV) emission. We first investigated the photoluminescence (PL) properties of the SnO2 thin films deposited on quartz substrates annealed at various temperatures. It was found that SnO2 nanocrystals were embedded in the SnO2 amorphous matrix after annealing at 400 °C to form a SnO2 nanoparticle/amorphous hybrid film; the band-edge UV emission was observed from the hybrid film due to the hybrid structure breaking the dipole-forbidden rule of bulk SnO2. This hybrid SnO2 film was then deposited on a p-type GaN substrate to form a SnO2 hybrid film-based LED and a band-edge UV electroluminescence (EL) was observed. Our results suggest that this easy and effective approach may find extensive application in the field of optoelectronics, displays and solid-state lighting.

Microstructural evolution of sputtered ZnO thin films with rapid thermal annealing

Zinc oxide thin films were deposited on silicon substrates by reactive RF magnetron sputtering technique. Post-deposition rapid thermal annealing of the sputtered thin films was carried out by varying temperatures, annealing duration and oxygen flow rate. The films, annealed at 1000 °C for 150 s in air ambient, have shown highest degree of crystallinity. The surface of the ZnO films, annealed for longer period, was greatly modulated with the evolution of porous surface. The films annealed in oxygen ambient have shown smoother morphology with the reduction in surface roughness. The characteristic absorption band of Zn–O became prominent due to the increase in Zn–O bond density during rapid thermal annealing process. A significant reduction of the deep level emission in the photoluminescence spectra was observed for annealed samples, whereas the near band edge ultraviolet emission was suppressed for the films annealed in oxygen ambient due to the oxygen adsorption at the film surface.

P-doping-free III-nitride high electron mobility light-emitting diodes and transistors

We report that a simple metal-AlGaN/GaN Schottky diode is capable of producing GaN band-edge ultraviolet emission at 3.4 eV at a small forward bias larger than ∼2 V at room temperature. Based on the surface states distribution of AlGaN, a mature impact-ionization-induced Fermi-level de-pinning model is proposed to explain the underlying mechanism of the electroluminescence (EL) process. By experimenting with different Schottky metals, Ni/Au and Pt/Au, we demonstrated that this EL phenomenon is a “universal” property of metal-AlGaN/GaN Schottky diodes. Since this light-emitting Schottky diode shares the same active structure and fabrication processes as the AlGaN/GaN high electron mobility transistors, straight-forward and seamless integration of photonic and electronic functional devices has been demonstrated on doping-free III-nitride heterostructures. Using a semitransparent Schottky drain electrode, an AlGaN/GaN high electron mobility light-emitting transistor is demonstrated.

Hydrothermal Growth of Vertically Aligned ZnO Nanorods Using a Biocomposite Seed Layer of ZnO Nanoparticles.

Well aligned ZnO nanorods have been prepared by a low temperature aqueous chemical growth method, using a biocomposite seed layer of ZnO nanoparticles prepared in starch and cellulose bio polymers. The effect of different concentrations of biocomposite seed layer on the alignment of ZnO nanorods has been investigated. ZnO nanorods grown on a gold-coated glass substrate have been characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) techniques. These techniques have shown that the ZnO nanorods are well aligned and perpendicular to the substrate, and grown with a high density and uniformity on the substrate. Moreover, ZnO nanorods can be grown with an orientation along the c-axis of the substrate and exhibit a wurtzite crystal structure with a dominant (002) peak in an XRD spectrum and possessed a high crystal quality. A photoluminescence (PL) spectroscopy study of the ZnO nanorods has revealed a conventional near band edge ultraviolet emission, along with emission in the visible part of the electromagnetic spectrum due to defect emission. This study provides an alternative method for the fabrication of well aligned ZnO nanorods. This method can be helpful in improving the performance of devices where alignment plays a significant role.

Efficient ultraviolet emission of ZnS nanospheres: Co doping enhancement

High-quality ultraviolet (UV) emission materials are essential for making short-wavelength light-emitting diodes and diode lasers. We report here that efficient ultraviolet luminescence at room-temperature can be obtained in ZnS nanospheres synthesized with an economical and simple chemical bath deposition method. The homogeneity, crystallinity and size of cubic ZnS nanospheres lead to a narrow and strong UV emission centered at 345nm. Interestingly, Co doping dramatically enhances the near band edge UV emission and suppress the deep-level (DL) emission due to the improvement of crystallinity and the decrease of the DL defects. This work suggests that Co-doped ZnS is a promising candidate for ultraviolet functional material.

Structural, optical, FTIR and photoluminescence properties of Zn0.96−xCo0.04CuxO (x=0.03, 0.04 and 0.05) nanopowders

Un-hydrogenated and hydrogenated Cu, Co co-doped ZnO (Zn0.96−xCo0.04CuxO, x=0.03, 0.04 and 0.05) nanopowders have been synthesized by co-precipitation method. The synthesized samples have been characterized by powder X-ray diffraction, energy dispersive X-ray spectra, UV–Visible spectrophotometer and Fourier transform infrared spectroscopy. The calculated average crystalline size increases from 37.3 to 50.6nm for un-hydrogenated samples from x=0.03 to 0.05 and it changes from 29.4 to 34.9nm for hydrogenated samples. The change in lattice parameters, micro-strain, a small shift of X-ray diffraction peaks towards lower angles and reduction in energy gap reveal the substitution of Cu2+ ions into Zn–Co–O lattice. The hydrogenation effect reduces the particle size and induces the more uniform distribution of particles than the un-hydrogenated samples which is confirmed by SEM micrographs. Photoluminescence spectra of Zn0.96−xCo0.04CuxO system shows that red shift in near band edge ultraviolet emission from 393 to 403nm with suppressing intensity and a blue shift in green band emission from 537 to 529nm with enhancing intensity confirms the substitution of Cu into the Zn–Co–O lattice.

Uniaxial tensile strain and exciton–phonon coupling in bent ZnO nanowires

We investigate the optical property of bent ZnO nanowires (NWs) obtained by low energy argon ion milling. At room temperature, the bent NWs demonstrates an enhanced near band edge ultraviolet emission, while the deep level green emission is totally suppressed. Temperature dependent photoluminescence measurements were carried out between 10 and 300 K for both the as-grown and the bent ZnO NWs. It is found that the emission peak energy of the bent NWs systematically shifts to lower energy compared to the as-grown NWs in the whole measured temperature range. Our results indicate that the redshift in the emission peak is related to the uniaxial tensile strain and the enhanced exciton–phonon coupling strength in the bent ZnO NWs.

Growth and optical properties of tetrapod-like indium-doped ZnO nanorods with a layer-structured surface

We report tetrapod-like indium-doped ZnO nanorods with a layer-structured surface, synthesized by a thermal vapor transport method. The high crystalline quality of the nanostructure sample obtained was confirmed by using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. We carried out photoluminescence and photoluminescence excitation measurements to investigate the possible application of this indium-doped ZnO nanostructure in the field of optical devices. The photoluminescence exhibited an ultra strong visible wavelength emission with a peak intensity 300 times stronger than the band edge ultraviolet emission. In addition, a very slow decay of visible wavelength emission was revealed by time-resolved photoluminescence spectroscopy.

Investigations on the structural and optical properties of Li, N and (Li, N) co-doped ZnO thin films prepared by sol–gel technique

Pure and doped zinc oxide thin films have been deposited on sapphire substrates by using the sol–gel method and spin coating technique. The X-ray diffraction pattern showed that the deposited films exhibit hexagonal zinc oxide structure. Room temperature photoluminescence measurements show the presence of two emission bands. The predominant near band edge ultraviolet emission is at 3.28 eV and a suppressed broad band of deep level emission in the range of 2.1–2.5 eV. The incorporation of nitrogen is indicative of p-type behavior as observed from X-ray photoelectron spectrum of nitrogen in the doped samples. The p-type conduction of Li, N:ZnO may be attributed to the formation of a LiZn–N complex acceptor.

Diameter-Controlled Vapor−Solid Epitaxial Growth and Properties of Aligned ZnO Nanowire Arrays

A facile, template-free method was used to grow large areas of well-aligned ZnO nanowire arrays on amorphous SiO2 substrates. The arrays are composed of vertically aligned, single-crystalline, wurtzitic [001] ZnO nanowires whose diameters were easily controlled by growth temperature, adjusted by changing the distance between the substrate and the precursor material in the growth chamber. A vapor−solid epitaxial growth mechanism is proposed by which ZnO nanocrystals, nucleated on a NiO catalytic film, seed the growth of the ZnO nanowires. Photoluminescence spectra indicate broad visible wavelength emission, likely caused by near surface traps, whose intensity relative to band edge ultraviolet emission grows as nanowire radii decrease. UV photoconductivity measured for individual ZnO nanowire devices demonstrates their potential as a UV light nanosensor.

Phosphorus doped ZnO light emitting diodes fabricated via pulsed laser deposition

ZnO-based light emitting diodes were fabricated on c-plane sapphire using ZnO:P∕Zn0.9Mg0.1O∕ZnO∕Zn0.9Mg0.1O∕ZnO:Ga p-i-n heterostructures. The p-i-n heterojunction diodes are rectifying and show light emission under forward bias. The electroluminescence spectra shows deep level emission at low bias, but near band edge ultraviolet emission at high voltage bias. A decrease in leakage currents in as-fabricated structures was achieved via low temperature oxygen annealing.

Photoluminescence of ZnO quantum dots produced by a sol–gel process

We report synthesis and spectroscopic characterization of sol–gel derived zinc oxide (ZnO) quantum dots (Qdots) of 3–8 nm size. The Qdots exhibited a broad and strong visible emission peak centered near 520 nm with a quantum efficiency of approximately 5%. A weak ultraviolet (UV) emission peak was also observed near the band gap at 370 nm. The integrated area under the curve for the visible emission band was 35 times greater than that of the band-edge UV emission band of ZnO Qdots at room temperature. The photophysical characteristics (luminescence wavelength and intensity) of ethanol-dispersed Qdots were greatly affected over time (aging), suggesting the continuation of particle growth. In contrast, no such spectroscopic changes were noticed for air-dried ZnO Qdots. Time-resolved emission spectra of Qdots recorded at 520 nm emission were fitted with two sequential exponential decay curves with lifetimes of 14 ns and 77 ns. Visible photoluminescence of ZnO at low temperature (80 K) exhibited 150% higher intensity than that at room temperature. This was discussed based on the standard model of competitive radiative and non-radiative relaxation processes at various temperatures.

Li doped and undoped ZnO nanocrystalline thin films: a comparative study of structural and optical properties

Thin films of ZnO were grown by the sol–gel method using spin-coating technique on (0001) sapphire substrates. The effect of doping after annealing on the structural and optical properties has been investigated by means of X-ray diffraction (XRD), cathodoluminescence (CL) spectrum, scanning electron microscopy (SEM) and atomic force microscopy (AFM). The films that were dried at 623 K and then post annealed at 873 K showed (0002) as the predominant orientation. Two emission bands have been observed from CL spectrum. Lithium doped film shows shift in the near band edge UltraViolet emission peak and suppressed defect level emission peak in the visible range. SEM analysis of the films exhibits many spherical shaped nanoparticles. Roughness of the films determined using atomic force microscopy.

Photoluminescence of hydrothermally epitaxied ZnO films

In this study, epitaxial ZnO films were grown hydrothermally on (1 1 1)-oriented single crystal MgAl 2O 4 substrates at 150 °C from aqueous precursor solutions. It was observed that the film morphology varied with the pH value of the precursor solution, giving pitted films at higher pH and smooth films at lower pH. The photoluminescence spectra of these ZnO films showed a strong near band-edge ultraviolet emission together with deep level emission bands comprised of green and orange-red luminescence. The green band centred around 500 nm was attributed to the presence of Zn vacancies, whereas the orange-red band centred around 650 nm could be related to the presence of oxygen interstitials.

Time-Resolved Investigation of Bright Visible Wavelength Luminescence from Sulfur-Doped ZnO Nanowires and Micropowders

Sulfur-doped zinc oxide (ZnO) nanowires grown on gold-coated silicon substrates inside a horizontal tube furnace exhibit remarkably strong visible wavelength emission with a quantum efficiency of 30%, an integrated intensity 1600 times stronger than band edge ultraviolet emission, and a spectral distribution that closely matches the dark-adapted human eye response. By comparatively studying sulfur-doped and undoped ZnO micropowders, we clarify how sulfur doping and nanostructuring affect the visible luminescence and the underlying energy transfer mechanisms.