Band Edge UV Emission Research Articles

Argon plasma-enhanced UV light emission from ZnO submicrowires grown by hydrothermal method

In this work, the optical, morphological, and structural improvements of vertically aligned, hydrothermally grown ZnO submicrowires (SMWs) treated with a 250 W, 250 V RF argon plasma (Ar) during different exposure times were investigated by scanning and transmission electrons microscopy, X-ray diffraction, and micro-Raman and photoluminescence (PL) spectroscopies. In two steps, the SMWs were synthesized in an aqueous medium at low temperatures. The plasma-treated samples showed significantly improved room-temperature PL compared to untreated samples. All the treated samples exhibited a substantial increase of the near band edge UV emission intensity and a decrease of the deep level emission in the visible. The samples treated for 4 minutes presented the best UV/vis intensity ratio of ∼ 1568 (an increase of ⁓174-fold with respect to the untreated sample). The analysis of the UV band in terms of the first and second phonon replica of the excitonic emission indicated that the Ar plasma treatment favors the multiple phonon-exciton coupling, with partial correlation with the observed overall increase of the UV/visible intensity ratio. From the PL measurements at low temperatures, the presence of excitons bound to H donors in the ZnO structure was inferred. The possible reasons for the Ar plasma-induced enhancement of the UV emission from the treated SMWs are discussed in terms of previous work, the observed morphology and changes expected to occur at the SMW surfaces due to Ar ion impacts from the plasma.

Study of graphene incorporation into ZnO-PVA nanocomposites modified electrode for sensitive detection of cadmium

The presence of heavy metals often causes significant health risks, particularly cadmium, which is known for its high toxicity. In this study, a glassy carbon electrode was successfully modified by incorporating ZnO-PVA-Graphene nanocomposite, leveraging the excellent electrical properties and electron mobility of the material. Comprehensive material analysis, including XRD, confirmed that ZnO maintained its hexagonal wurtzite crystal structure despite the addition of graphene. Moreover, FESEM analysis showed that increasing graphene concentration led to a reduction in ZnO particle size by 85, 68, and 52 nm, respectively, accompanied by a decrease in band gap energy, as verified by UV–Vis measurements. Photoluminescence tests were also conducted and the result showed a noticeable blue shift in ZnO-PVA-Graphene nanocomposites compared to ZnO-PVA, specifically in the near band-edge (NBE) UV emission within the 374–379 nm wavelength range. Through I–V characterization, the optimal graphene concentration for cadmium detection was identified as 1.5 wt% in ZnO-PVA-Graphene nanocomposites, showing an approximate ohmic response. Meanwhile, square-wave voltammetry analysis of cadmium concentrations ranging from 0 to 80 ppm produced a coefficient of determination of 0.98926 and a Limit of Detection (LOD) of 9.88 ppm. These results showed the significant potential of ZnO-PVA-Graphene nanocomposites as a promising material for further development as an effective electrode modifier, enhancing the sensitivity of detection systems.

Simultaneous Growth Strategy of High-Optical-Efficiency GaN NWs on a Wide Range of Substrates by Pulsed Laser Deposition.

Here, we explore a catalyst-free single-step growth strategy that results in high-quality self-assembled single-crystal vertical GaN nanowires (NWs) grown on a wide range of common and novel substrates (including GaN, Ga2O3, and monolayer two-dimensional (2D) transition-metal dichalcogenide (TMD)) within the same chamber and thus under identical conditions by pulsed laser deposition. High-resolution transmission electron microscopy and scanning transmission electron microscopy (HR-STEM) and grazing incidence X-ray diffraction measurements confirm the single-crystalline nature of the obtained NWs, whereas advanced optical and cathodoluminescence measurements provide evidence of their high optical quality. Further analyses reveal that the growth is initiated by an in situ polycrystalline layer formed between the NWs and substrates during growth, while as its thickness increases, the growth mode transforms into single-crystalline NW nucleation. HR-STEM and corresponding energy-dispersive X-ray compositional analyses indicate possible growth mechanisms. All samples exhibit strong band edge UV emission (with a negligible defect band) dominated by radiative recombination with a high optical efficiency (∼65%). As all NWs have similar structural and optical qualities irrespective of the substrate used, this strategy will open new horizons for developing III-nitride-based devices.

Optical properties of composite structure based on ZnO microneedles and Alq3 thin film

The composite material based on ZnO microneedles and Alq3 thin film has been obtained. The photoluminescence study shows a tenfold enhancement in the band-edge UV emission (390 nm) of ZnO microneedles and 2 × enhancement in visible emission of the hybrid composite, when excited by 266 nm laser beam. This enhancement can be explained by the interaction between ZnO and Alq3 molecules and the energy transfer from ZnO to Alq3 molecule. Discussed composite structures can find interesting applications as emitting layers in OLED devices.

Observation of monochromatic and coherent luminescence from nanocavities of GaN nanowall network

Scaling-down the size of semiconductor cavity lasers and engineering their electromagnetic environment in the Purcell regime can bring about spectacular advance in nanodevices fabrication. We report here an unprecedented observation of a coherent Cathodoluminescence from GaN nanocavities (20–100 nm). Incident lower energy (< 15 kV) electron beams excite the band edge UV emission from the walls of the network whereas for higher energies, the emitted photons are spontaneously down converted into NIR and preferentially emerge from the nanocavities. Non-centrosymmetric structure of GaN and its nanowall geometry together facilitate this unique observation which is substantiated by our numerical results. At cryogenic temperatures, an intense and narrow laser-like NIR beam emanates out of the nanocavities. The work promises the possibility of fabrication of very high density (over 108/cm2) cavity lasers that are addressable by simple deflection and tuning of incident electron beams.

Large area coverage and controlled photoluminescence from vertically aligned ZnO nanorods on Cu substrates

ZnO nanorods were deposited on large area Cu foil using cathodic electrodeposition technique. The growth time was used as a control parameter for tuning the crystallinity as well as the length of nanorods. It is observed that the crystallinity improves with deposition time and the nanorods grow with a preferred orientation of 〈002〉. A typical circular area of 4 cm diameter was uniformly coated with ZnO nanorods. The contact angle decreases gradually from 108° to 71° as deposition time increases from 3 to 15 min. This marks the enhanced wettability of the surface. Further, the room temperature Photoluminescence (PL) spectra of these rods reveal that the near band edge UV emission (~3.3 eV) increases at the cost of defects induced visible emission, signifying improved crystallinity. The color rendering index of the 10-minute deposited sample is 90 and it shows a near white color temperature of 9013 K due to optimum emission coverage over the entire visible wavelength range.

Influence of Ni and Sr on the structural, morphological and optical properties of ZnO synthesized by sol gel

ZnO nanoparticles doped and co-doped with Ni and Sr were synthesized by the sol gel method. Rietveld refinement indicated the single phase of ZnO for all the samples and suggests that the variations in structural parameters are dependent on the ions type. Fourier transform infrared studies showed the functional groups and chemical bonding of ZnO and confirmed the replacement of Zn by Ni and Sr ions. The micrographs showed particles with clusters shapes and different sizes. Photoluminescence measurements show a shift in near band edge UV emission, from 380 to 384 nm for doped and co-doped samples. Additionally, the Zn substitution by Ni and Sr ions minimizes the intrinsic defects in the ZnO structure. The optical study showed that the replacement of Zn by Ni decreases the band gap, whereas the substitution by Sr increases it. For co-doped samples, this value depends on the dopant concentration and the ions type. This work contributes to the understanding of optical properties for the ZnO nanoparticles co-doping with Ni and Sr, which may be important for future applications.

Suppression of the green emission, texturing, solute-atom diffusion and increased electron-phonon coupling induced by Ni in sol-gel ZnNiO thin films

Zn1−xNixO thin films (nominal x = 0, 0.01, 0.02, 0.04, 0.1 and 0.2) were synthesized on silicon substrates through a sol-gel/dip-coating technique. Samples were studied by X-ray diffraction, scanning electron microscopy, photoluminescence spectroscopy, Rutherford backscattering spectrometry and depth-profiling X-ray photoelectron spectroscopy. The results from X-ray diffraction show growth in the wurtzite crystal structure for all samples, with cubic NiO being detected as a secondary phase for x = 0.2. While for x = 0 (pure ZnO) no texture is present, for 0 ≤ x ≤ 0.1 strong preferential crystallization along the c-axis is observed. A tendency for Ni diffusion towards the film/Si substrate interface was observed. The formation of substitutional ZnxNi1−xO solid solution for 0.01 ≤ x ≤ 0.04 is suggested by the results. Photoluminescence spectra exhibit strong near band edge UV emission and suppression of deep defect-related emission in the visible upon Ni+2 incorporation into the ZnO lattice. As in pure ZnO, the UV emission in ZnNiO at room temperature is dominated by the first two phonon replica of the excitonic emission, however the LO phonon energy (ħωLO) is reduced by up to ∼15 meV in the 0 ≤ x ≤ 0.04 range. Due to this reduction of ħωLO, the exciton-phonon coupling increases, in consistency with a corresponding expected increase of the Fröhlich coupling constant with decreasing ħωLO.

Preparation and characterization of Ni, Co doped ZnO nanoparticles for photocatalytic applications

Ni, Co codoped ZnO nanoparticles have been synthesized by co-precipitation method. The structural and morphological properties of Ni, Co codoped ZnO nanoparticles were analyzed using X-ray diffraction, scanning/transmission electron microscopy and Energy-dispersive X-ray spectroscopy (EDX). Photoluminescence studies were also performed. PL spectra show a shift in near band edge (NBE) UV emission from 321 to 322 nm and a shift in red band (RB) emission from 620 to 631 nm which conforms the substitution of Ni into the ZnO lattice. XRD data revealed that Ni, Co codoped ZnO nanoparticles presented pure wurtzite phase. Their photocatalytic activity was investigated by the degradation of Rhodamine B (RhB) dye under visible light irradiation and compared with pure ZnO activity. The improved photocatalytic activity in degradation of rhodamine B (RhB) was observed at ∼0.2% at of both Ni and Co dopants. It seems that increasing doping concentrations of Ni, Co leads to a decrease of the photocatalytic efficiency. Further studies will target fine tuning of Ni and Co concentrations around the one with observed maximum activity and integration of the respective powder in a sensing system in order to verify also sensing ability of the material with best photocatalytic activity.

Effect of Gd doping on structure and photoluminescence properties of ZnO nanocrystals

Gd doped ZnO nanoparticles with varying dopant concentration are synthesized by simple wet chemical method. X-ray diffraction measurements confirm that the prepared samples are of hexagonal wurtzite structure with grain size around 50–60 nm. Gd doping induces lattice expansion in ZnO as the average crystallite size increases with the evolution of Gd2O3 phase after Gd doping. Electron microscopic studies show the presence of two types of particle distribution belonging to ZnO and Gd2O3 phases with an average size of 55 nm where the later surrounds the surface of ZnO. UV–VIS spectra show red-shift of absorption band of ZnO with Gd doping. A suppression of band edge UV emission and intensification of visible emission is observed in the photoluminescence spectra of doped ZnO. This has been explained on the basis of incorporation of impurity levels by the dopant along with intrinsic defect such as oxygen vacancies in the band gap of ZnO.

Effect of Thickness on Micro-Structural and Optical Properties of Al-Doped ZnO Films Prepared by Sol-Gel Spin Coating

Aluminium (Al) doped Zinc oxide (ZnO) thin films of different thicknesses were prepared on glass substrates by sol-gel spin coating method. The effect of thicknesses on micro-structural and optical properties was investigated for transparent conducting oxide (TCO) application in solar cells and other optoelectronic applications. Grazing incidence x-ray diffraction (GIXRD) showed maximum orientation along (002) plane of c-axis. The variation of different structural parameters like crystallite size, micro-strain, c-axis strain, dislocation density as a function of film thickness was investigated. The FTIR spectra confirmed the formation of Al-doped ZnO film. FESEM images showed spherical shaped nanosized grains and formation of micro pores. The optical absorption increased and absorption peak shifted towards longer wavelength (red shift) with increase in the thickness of the film respectively. The optical transmittance of all the films has a transparency of more than 75% in the visible region. The optical band gap (Eg) decreased with increase in the film thickness. The diffused reflectance (DRS) showed very low reflectance in the region of 400-800 nm, but increased in the 800-900 nm region. Photoluminescence (PL) spectra of the prepared films showed intense band edge UV and low intense visible emissions respectively. The effect of thickness of Al-doped ZnO film on micro-structure, surface morphology, optical absorption and transmittance, diffused reflectance and PL have been investigated and the results are reported.

Effect of doping on the surface modification of nebulizer sprayed BaxZn1−xO nanocrystalline thin films

The influence of Ba doped zinc oxide films were investigated by nebulizer spray pyrolysis technique at 673 K. X-ray diffraction reveals the polycrystalline hexagonal (wurtzite) crystal structure with (0 0 2) preferential orientation. Energy dispersive spectroscopy confirms the presence of Ba, Zn and O elements in the films. Field emission scanning electron microscopy shows that the surface morphology of the nanocrystalline films were changed from spherical shape structure to flake net-like shape and sprout like spherical structure with average grain size is ~100 nm due to the critical doping concentration. PL spectra prominent peaks corresponding to near band edge UV emission and intrinsic defect of the visible blue light region and defect related deep level green emission regions were discussed. The films are highly transparent in the visible region with a transmittance higher than 74%, and have an optical band gap energy values are increased from 3.22 eV to 4.02 eV depending on the Ba doping concentration. Interparticle like grains, grain boundary effect of deposited films is studied by complex impedance spectroscopy.

Photocatalytic activity of Ag/ZnO core–shell nanoparticles with shell thickness as controlling parameter under green environment

Plasmonic Ag/ZnO core–shell nanoparticles have been synthesized via a simple two-step wet chemical method for application in Photocatalysis. The morphology, size, crystal structure, composition and optical properties of the nanoparticles are investigated by x-ray diffraction, transmission electron microscopy (TEM), FTIR spectroscopy, ultraviolet–visible (UV–Vis) absorption spectroscopy and photoluminescence (PL) spectroscopy. The shell thicknesses are varied by varying the concentration of zinc nitrate hexa-hydrate and triethanolamine. The ZnO shell coating over Ag core enhances the charge separation, whereas the larger shell thickness and increased refractive index of surrounding medium cause red shifts of surface Plasmon resonance (SPR) peak of Ag core. The photoluminescence (PL) spectra of Ag/ZnO core–shell show that the larger shell thickness quenches the near band edge UV emission of ZnO. The electrochemical impedance spectra (EIS) i.e. Nyquist plots also confirm the higher charge transfer efficiency of the Ag/ZnO core–shell nanoparticles. The Photocatalytic activities of Ag/ZnO core–shell nanoparticles are investigated by the degradation of methylene blue (MB) dye under direct sunlight irradiation. Compared to pure ZnO nanoparticles (NPs), Ag/ZnO core–shell NPs display efficient sunlight plasmonic photocatalytic activity because of the influence of SPR of Ag core and the electron sink effect. The photocatalytic activity of Ag/ZnO core–shell NPs is found to be enhanced with increase in shell thickness.

Influence of Cu doping on structural, morphological, photoluminescence, and electrical properties of ZnO nanostructures synthesized by ice-bath assisted sonochemical method

Cu-doped ZnO (Zn1−xCuxO, x = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05) nanocrystals were synthesized by ice-bath assisted sonochemical method. X-ray diffraction (XRD) analysis of the undoped and Cu-doped ZnO nanostructures reveals the formation of ZnO hexagonal wurtzite structure for all samples which confirm the incorporation of Cu2+ ions into the ZnO lattice by substitution. Increasing the Cu concentration resulted in a contraction of the bond length and the unit cell volume, whereas, the crystallite size increases by the doping induced grain growth. High-resolution transmission electron microscopy (HRTEM) images show that the morphology of the pure and doped samples consists of mixtures of nanorods and nanosheets. The increase of Cu content leads to an improvement of a preferable growth direction and an increase of both the lengths and diameters of the nanorods. Analysis of the optical absorption spectra shows that incorporation of Cu ions into the ZnO lattice leads to a red shift of the optical band gap from 3.45 eV to 3.35 eV and the exciton peak from 3.35 to 3.24 eV. The photoluminescence (PL) spectrum of the undoped ZnO nanopowders at excitation wavelength (λex) = 325 nm, reveals near band edge UV emission and defect-related blue and green emission, whereas, the PL spectrum at λex = 380 nm exhibits only defect-related green and red emission. The incorporation of Cu leads to a decrease of the PL intensity due to the increase of nonradiative recombination centers. To explain the mechanism of the PL emission and to identify the different trapping and recombination levels in the ZnO nanopowders, energy band diagrams were suggested. Raman spectroscopy analysis show that the increase of Cu content leads to an increase in the number of defects in the ZnO lattice. The direct current (DC) electrical conductivity measurement reveals a presence of different shallow and deep trapping levels. In addition, an increase of the electrical conductivity with increasing the Cu content in ZnO lattice was observed and can be ascribed to the decrease in the activation energy.

Morphological evolution of hydrothermally derived ZnO nano and microstructures

ZnO nanorods with flowerlike morphologies were synthesized via hydrothermal route. The observations depicted that the as-synthesized flowerlike morphology of ZnO consisted of numerous nanorods. The length of nanorod is in the range of 2–3μm, and the diameter ∼ 150–200nm. The ZnO product was crystallized with hexagonal crystal structure. High resolution transmission microscopy image and electron diffraction pattern revealed a single crystalline nature altogether (0001) direction growth. The photoluminescence spectrum showed a near band edge UV emission peak and defect related blue emission peak in the visible region. Furthermore, the experimental results revealed that the extended reaction period and zinc precursor used in reaction mixture played an inevitable role in the morphological evolution of ZnO nano-/microstructures.

Enhanced UV–vis photoconductivity and photoluminescence by doping of samarium in ZnO nanostructures synthesized by solid state reaction method

In the present work, polycrystalline undoped and samarium doped zinc oxide (ZnO) nanostructures were successfully synthesized by solid state reaction method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–vis absorption spectroscopy, photoluminescence study and photoconductivity study. XRD patterns revels that the samples possess hexagonal wurtzite structure of ZnO without any secondary phase after samarium doping. The crystallite size is calculated by Debye–Scherer's formula. Williamson–Hall analysis reveals presence of strain in synthesized samples. SEM micrographs show spherical and oblate type shape of nanoparticles. UV–vis absorption study shows blue shift in absorption band edge in undoped as well as in samarium doped ZnO nanoparticles as compared to its bulk counterpart. Photoluminescence spectra of the samples show peaks corresponding to near band edge UV emission and defect related emissions in the visible region. In photoconductivity study, enhancement of photosensitivity has been observed in 1% Sm-doped ZnO sample while the maximum photocurrent is found in 3% Sm-doped sample. The variation of dark current and photocurrent with applied voltage exhibits super-linear nature. The rise and decay of photocurrent indicate anomalous behavior during steady state illumination.

Direct growth of flower-like ZnO nanostructures on porous silicon substrate using a facile low-temperature technique

Flower-like zinc oxide (ZnO) nanostructures were successfully grown on seed layer-free porous silicon (PSi) substrate using a facile, efficient, and low-temperature technique. An electrical behavior was studied using an electrochemical impedance spectroscopy. Field emission scanning electron microscopy results showed that rough nanoporous surface play an important role in the formation of ZnO flower-like nanostructures. The strong intensity and narrow width of X-ray diffraction peaks indicated good crystallinity of the as-grown ZnO nanostructures. Data from photoluminescence measurements showed a UV band-edge emission with a peak at about 381nm accompanied by a larger and broader visible emission in a 500–800nm wavelength range that indicated the presence of high structural defect levels. The large value of interfacial charge transfer resistance observed in the ZnO/PSi nanostructures (2.25MΩ) can be attributed to the formation of an oxide layer on the Psi surface that possessed a smaller resistance (0.982MΩ). The present flower-like ZnO/PSi nanostructures are envisaged to have promising applications in novel nanoscale chemical solution sensors.

Evolution of room temperature ferromagnetism with increasing 1D growth in Ni-doped ZnO nanostructures

Zn1−xNixO (x = 0, 0.03, and 0.5) 1D nanostructures showing room temperature ferromagnetism (RT FM) with high moment have been synthesized by a solvothermal route. X-ray diffraction, transmission electron microscopy (TEM), energy dispersive X-ray spectrum (EDS) and X-ray photoelectron spectroscopy (XPS) analysis reveal the growth of single phase wurtzite structure Zn1−xNixO NRs of diameter 60–70 nm and length of 0.4–0.6 μm with the successful incorporation of Ni ions inside ZnO matrix. High resolution TEM lattice images show that all the NRs are single crystalline with a d-spacing of 2.57 Å with c-axis growth. Room temperature magnetic measurements exhibit strong ferromagnetic characteristic with magnetic moment of 1.13 emu/g, coercivity of 150 G. Photoluminescence (PL) spectra exhibit near band edge UV emission as well as defect related visible emission which is expected to play a significant role in the FM ordering, also PL spectra reveal slight band edge modification due to doping effect. Systemic structural, magnetic, and optical properties reveal that both the nature of the defects as well as Ni2+ ions are significant ingredients to attain FM characteristics with high moment and ordering temperature in the 1-dimensional ZnO NRs. Magnetic interaction is analysed using a bound magnetic polaron model and expected to arise from the intrinsic exchange interaction of Ni ions, Zn Vacancy and O iterstitial related defects.

Structural, morphological characteristics and optical properties of Y doped ZnO thin films by sol–gel spin coating method

Un-doped and Y doped ZnO thin films were deposited successfully by sol–gel spin coating method with different Y concentrations. The X-ray diffraction spectra revealed that all the films have polycrystalline of hexagonal wurtzite structure. SEM images of the films show different micro structure with nano clusters and show a reduction in surface roughness. The EDX spectrum confirms the presence of Zn, O and Y elements in the prepared films. The optical transmittance spectrum indicates the average transmittance of the films is increased from 78% to 83%. The optical band gaps of the ZnO films were changed from 3.33eV to 3.43eV with increasing Y doping. PL spectra show the blue shift in near band edge (NBE) UV emission. The presence of functional groups and the chemical bonding is confirmed by FTIR spectra. The Hall measurements show that the films have n-type conductivity.

Structural Analysis By Rietveld Method And Its Correlation With Optical Propertis Of Nanocrystalline Zinc Oxide

The correlation between structural and optical properties of nanocrystalline ZnO synthesized by the citrate precursor method has been investigated. The Rietveld refinement of X-ray diffraction pattern confirms the P63mc space group and formation of single phase hexagonal wurtzite structure with presence of tensile strain at the lattice site. The presence of Raman active optical phonon mode at 436 cm -1 which is a significant character of ZnO with hexagonal wurtzite structure supports the XRD result. FE-SEM result shows that the size of the particle is about 20 nm with nearly spherical shapes. The optical band gap energy at room temperature has been calculated as 3.28 eV using the Tauc plot technique. The UV-Vis sub-gap absorption curve supports the presence of strain inside the crystal. The photoluminescence spectrum indicates the dominancy of the defect related deep level or trap state emissions over the near band edge UV emissions using an excitation wavelength of 320 nm.