Hexagonal Wurtzite Structure Of ZnO Research Articles

Investigation of the effects of thickness on physical properties of AZO sol-gel films for photonic device applications

Aluminum doped zinc oxide (AZO) films are synthesized on glass substrates using different number of coatings (6, 12, 18 and 24) by the sol-gel dip-coating technique. The effects of thickness on structural, morphological, electrical optical and waveguiding properties of the prepared films are studied through X-ray diffraction (XRD), Raman spectroscopy (RS), scanning electron microscopy (SEM), atomic force microscopy (AFM), Hall effect measurements, UV–Visible–NIR spectrophotometer and m-lines spectroscopy (MLS). XRD analysis shows that all AZO films exhibit the hexagonal wurtzite crystal structure with a preferential growth orientation along the c-axis. The intensity of the (002) diffraction peak and crystallite size tend to increase with increase in the thickness indicating an improvement in the crystallinity of the films. Furthermore, the observation of the high-frequency E2 mode in the Raman spectra supports the hexagonal wurtzite structure of ZnO in the films. SEM and AFM images indicate that microstructure parameters, such as grain size and surface roughness depend sensitively on the thickness of the film. The electrical resistivity and carrier concentration are found to increase and decrease, respectively, as the film thickness increased. Optical transmittance spectra demonstrate that all the films exhibit high average transmittance (86.8–89.1%) in the visible spectral range. Moreover, the band gap energy shows a red-shift with the film thickness. MLS measurements at a 632.8 nm wavelength put into evidence that AZO planar waveguides support single and multi-well confined guided modes for both transverse electric (TE) and transverse magnetic (TM) polarizations. Both TE and TM refractive indexes are found to increase with thickness and reach saturation values when the number of coatings is greater than 18. Moreover, the thickest AZO planar waveguide demonstrates a propagation loss lower than 1 dB/cm.

Three-dimensional (3D) flower-like nanoarchitectures of ZnO-Au on MWCNTs for visible light photocatalytic applications

Nanoflower-like ZnO-Au (zinc oxide/gold) composites were grown on multiwall carbon nanotubes (MWCNT) for visible light functioning plasmonic photocatalysis via a simple wet chemical strategy. Pristine ZnO were also grown in the absence/presence of MWCNTs for comparative studies. The appearance of ZnO-Au and ZnO on MWCNTs were noted to be in flower form. The dimension of ZnO-Au nanoflowers on MWCNTs were measured to be around 350 nm by electron microscopy. Their structural studies revealed the hexagonal wurtzite structure of ZnO to be predominant. UV–vis absorbance measurements revealed a significant enhancement in the absorption of visible region photons in ZnO when composited with MWCNTs and Au. The photocatalytic studies carried out under sun light illumination demonstrated the enhanced activity of ZnO-Au systems grown on MWCNTs rather than that of ZnO or MWCNT/ZnO. The improved photocatalytic activity has been reasoned to the effective band alignment and transportation of electrons from MWCNTs and Au to the conduction band of ZnO.

Novel Magnetic Zinc Oxide Nanotubes for Phenol Adsorption: Mechanism Modeling.

Considering the great impact of a material’s surface area on adsorption processes, hollow nanotube magnetic zinc oxide with a favorable surface area of 78.39 m2/g was fabricated with the assistance of microwave technology in the presence of poly vinyl alcohol (PVA) as a stabilizing agent followed by sonic precipitation of magnetite nano-particles. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) micrographs identified the nanotubes’ morphology in the synthesized material with an average aspect ratio of 3. X-ray diffraction (XRD) analysis verified the combination of magnetite material with the hexagonal wurtzite structure of ZnO in the prepared material. The immobilization of magnetite nanoparticles on to ZnO was confirmed using vibrating sample magnetometry (VSM). The sorption affinity of the synthesized magnetic ZnO nanotube for phenolic compounds from aqueous solutions was examined as a function of various processing factors. The degree of acidity of the phenolic solution has great influence on the phenol sorption process on to magnetic ZnO. The calculated value of ΔH0 designated the endothermic nature of the phenol uptake process on to the magnetic ZnO nanotubes. Mathematical modeling indicated a combination of physical and chemical adsorption mechanisms of phenolic compounds on to the fabricated magnetic ZnO nanotubes. The kinetic process correlated better with the second-order rate model compared to the first-order rate model. This result indicates the predominance of the chemical adsorption process of phenol on to magnetic ZnO nanotubes.

Light intensity effects on the sensitivity of ZnO:Cr gas sensor

ABSTRACTChromium-doped zinc oxide thin films (Cr: ZnO) with different Cr concentration were prepared using a homemade nebuliser spray pyrolysis system, and the influence of Cr doping level on the physical properties and the influence of light on the gas-sensing properties of ZnO films were investigated. The XRD results reveal that the hexagonal wurtzite structure of ZnO was not affected by the Cr doping. All the films have average transmittance about 85% in the visible region. The film shows good sensing behaviour against ammonia gas under dark condition than ambient (64 lx) and 150 W (5840 lx) incandescent lights. Of the tested samples, ZnO:Cr film with 2 at.-% of Cr doping shows the best response and recovery time for ammonia gas. The reasons for the enhanced sensing ability of the doped films were explained with the help of FESEM, AFM and optical results.

Spectroscopic and Morphological Investigation of ZnSn Thin Films Obtained by Radio Frequency Cosputtering

In this study, Zn, Sn, and ZnSn (containing 20% of Sn) thin films were fabricated by radio frequency magnetron cosputtering and their properties had been studied with various analytical techniques. The X‐ray photoelectron spectroscopy results showed that the oxidation states of the elements in the alloy thin films change in comparison with the monometallic thin films, namely, Zn was more oxidized wheras Sn was less oxidized. The crystallinity and the conductivity of ZnSn thin films improved compared to monometallic thin films, which were confirmed by X‐ray diffraction (XRD) and four‐point probe. In ZnSn thin films, some Sn thin film formed secondary phase according to the Hume Rothery rules and the other was substituted into Zn sites in the wurtzite hexagonal structure of ZnO. Due to the lattice mismatch, the residual stress was generated and the preferred crystal growth was observed to overcome the stress. This was confirmed by the atomic force microscope and scanning electron microscope.

Influence of Co loading on structural and morphological properties of Co-doped ZnO thin films grown by pulsed electron beam ablation

We report on the effect of cobalt loading in the target material on the properties of thin film nanocomposites of Co-doped zinc oxide (CZO) Zn1-xCoxO. The films have been deposited by pulsed electron beam ablation (PEBA) at 450°C on pre-annealed (1100°C) c-sapphire and Si (100) substrates. The experimental results, based on scanning electron and atomic force microscopies (SEM, AFM), indicate that Co doping significantly affects the morphology of the films. X-ray diffraction (XRD) measurements reveal that the films on both substrates exhibit a prominent ZnO hexagonal wurtzite structure for x = 0.05, whereas higher Co loading (x = 0.1, 0.2) results in a strong deformation of ZnO wurtzite lattice and in a higher content of hexagonal close-packed (hcp) metallic Co in CZO films. X-ray photoelectron spectroscopy (XPS) data confirm the presence of non-oxidized metallic Co (Co°) in the deposited films. Both XRD and XPS results indicate an increase in Co° content in the films as Co loading in the target is increased.

Structural and optical properties of zinc magnesium oxide nanoparticles synthesized by chemical co-precipitation

By chemical co-precipitation method a series of ZnMgO nanoparticles were synthesized with the particular composition (1-x)ZnO-xMgO (where 0 ≤ x ≤ 2, all are in mol%). XRD spectra recorded at room temperature of these samples have shown the typical peaks of the ZnO hexagonal wurtzite structure, such as (1 0 0), (0 0 2), (1 0 1) etc. With the increasing concentration of MgO, the blue shift is attained in the absorption band of ZnO. Due to the Moss-Bustein effect the optical band gap has been widened by shifting towards higher frequency side as the concentration of MgO added progressively in the ZnO. The IR spectra revealed structural modifications in the stretching vibrations of Zn−O units as a function of MgO.

Synthesis and characterization of Sm3+-doped ZnO nanoparticles via a sol–gel method and their photocatalytic application

Spherical ZnO nanoparticles doped by samarium ions were successfully synthesized via a simple sol–gel method. The structures, morphologies, optical properties and surface areas were investigated for all samples using specific characterization methods. The hexagonal wurtzite structure of ZnO and samarium-doped ZnO nanoparticles were determined. The results obtained showed that the sizes of samarium-doped ZnO nanoparticles decreased with increasing samarium ion concentration. It was noticed that in the presence of samarium ions, the band gap slightly changed from the 3.198 eV of ZnO to 3.288 eV for samarium-doped ZnO with enhanced absorption in the UV region. This can be attributed to the transition of electrons from the conduction band to the acceptor energy level of samarium. The XPS results of samarium-doped ZnO, showed that only one oxidation state of samarium, with good incorporation into the ZnO matrix, was presented, with no peak of samarium oxide. The surface areas analyses showed that higher surface areas were obtained for samarium-doped ZnO, which is attributed to the smaller size of the particles. The photocatalytic degradation of 2-chlorophenol was investigated under sunlight in presence of ZnO and samarium-doped ZnO nanoparticles. A higher performance of samarium-doped ZnO for photocatalytic degradation of 2-chlorophenol at 0.50 wt.% was observed, compared to pure ZnO nanoparticles under the same experimental conditions.

Optical, magnetic and structural properties of Cr-doped ZnO thin films by sol–gel dip-coating method

Cr-doped ZnO thin films are deposited by sol–gel dip coating. The effect of Cr on the structural, magnetic and optical parameters of zinc oxide films is studied. X-ray diffraction patterns of Cr-doped ZnO thin films are found to have a hexagonal wurtzite structure of ZnO. The bandgap energy of the Cr-doped ZnO thin films is calculated by the Tauq method. Crystallite size is evaluated by the Scherrer and Williamson–Hall relation. Both show that crystallite size enhances with an enhancement in Cr doping. The bandgap energy of the Cr-doped ZnO thin films is decreased by increasing the Cr dopant. The films are found to be ferromagnetic with a decrease in magnetic properties with an increase in Cr content.

Structure, morphology and photoluminescence attributes of Al/Ga co-doped ZnO nanofilms: Role of annealing time

The influence of annealing time on the structure, morphology and photoluminescence behavior (Al)/(Ga) co-doped ZnO (AGZO). nanofilms are grown on the p-type Si(100) substrate via combined sol-gel, spin coating annealed in air at 500°C at 0–3h. Samples are characterized using XRD, TEM, AFM, FESEM, EDX, (PL) and Raman measurements. XRD pattern confirmed the growth of highly poly-crystalline hexagonal wurtzite structure of ZnO with preferred orientation along (101) direction. At (3h) is found to cause lattice contraction and strain relaxation. TEM images revealed the nucleation of nanoparticles (NPs) and SAED pattern identified the lattice parameter. Raman spectra of AGZO exhibited optical and acoustic modes. FESEM displayed an increase in the particles size and number of nanoflakes with increasing annealing time. EDX detected right elemental traces. PL revealed an intense emission peak centered at 3.23eV, which is continuously shifted toward lower frequency with increasing time.

Efficient room temperature hydrogen sensor based on UV-activated ZnO nano-network

Room temperature hydrogen sensors were fabricated from Au embedded ZnO nano-networks using a 30 mW GaN ultraviolet LED. The Au-decorated ZnO nano-networks were deposited on a SiO2/Si substrate by a chemical vapour deposition process. X-ray diffraction (XRD) spectrum analysis revealed a hexagonal wurtzite structure of ZnO and presence of Au. The ZnO nanoparticles were interconnected, forming nano-network structures. Au nanoparticles were uniformly distributed on ZnO surfaces, as confirmed by FESEM imaging. Interdigitated electrodes (IDEs) were fabricated on the ZnO nano-networks using optical lithography. Sensor performances were measured with and without UV illumination, at room temperate, with concentrations of hydrogen varying from 5 ppm to 1%. The sensor response was found to be ∼21.5% under UV illumination and 0% without UV at room temperature for low hydrogen concentration of 5 ppm. The UV-photoactivated mode enhanced the adsorption of photo-induced O− and O2− ions, and the d-band electron transition from the Au nanoparticles to ZnO—which increased the chemisorbed reaction between hydrogen and oxygen. The sensor response was also measured at 150 °C (without UV illumination) and found to be ∼18% at 5 ppm. Energy efficient low cost hydrogen sensors can be designed and fabricated with the combination of GaN UV LEDs and ZnO nanostructures.

Study of structural and electrical properties of ZnO thin film for Thin Film Transistor (TFT) applications

This work reports the room temperature deposition of undoped ZnO thin film using RF magnetron sputtering technique for thin film transistor (TFT) applications. RF sputtered ZnO thin film of thicknesses 100 and 200 nm were deposited over n-type silicon (Si) substrate using 99.9% pure ZnO target. For the deposition of ZnO thin films, a deposition pressure of 28 mTorr and RF power of 50 W was maintained. The gas flow rate was 5 and 20 sccm for oxygen and argon respectively. Structural and surface morphological characterization was done using X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) techniques. XRD analysis of the thin film gives a dominant X-ray diffraction peak corresponding to strong c-axis oriented phase of hexagonal wurtzite structure of ZnO. The XRD results confirm the polycrystalline nature of the thin film and shows crystallinity improvement with increase in film thickness. Further, FESEM results confirm that the grain size increases with the film thickness. The surface roughness is studied through AFM while surface elemental survey is done using XPS. To measure the resistivity of deposited films, four point probe method was used. The resistivity of undoped 100 nm thick ZnO film is found to be 4.47 kΩ cm, while the resistivity of undoped 200 nm thick ZnO film reduces to 724.13 Ω cm because of improved crystallinity.

Structural and optical properties of Cu doped ZnO aerogels synthesized in supercritical ethanol

Undoped and copper doped ZnO aerogels have been synthesized by the sol–gel method followed by supercritical drying in ethanol. The dopant contents in the aerogels were 1, 2, 3, 4 and 5 at.%. The as-obtained products were analyzed without any additional heat or chemical treatments. The XRD measurements revealed that aerogels have good polycrystalline ZnO hexagonal wurtzite structure with high crystalline quality. The data analysis showed that the crystallite sizes of ZnO and Cu-doped ZnO nanoparticles were within the range of 32–38 nm and the Cu dopant uniformly substitute for Zn sites. ATG measurements and EDAX studies showed that the aerogels have the extreme purity. SEM images revealed a snow-like morphology of the aerogels and the agglomeration state of the nanoparticles was not affected by Cu concentration. Optical absorption spectra showed a red shift of the absorption edge due to the merging of an impurity band into the conduction band. As the Cu content increases, the visible photoluminescence intensity decreases due to the scattering of the excitation radiation by surface-adsorbed dopant atoms and the non-radiative recombination process introduced by Cu impurity. The photoluminescence spectra of all aerogels showed one UV emission at 388 nm and a broad visible luminescence (490–650 nm) formed with two green (510 and 545 nm) and orange (612 nm) emissions. No shift observed with Cu concentration in emission positions in Cu-doped ZnO aerogels synthesized in supercritical ethanol.

Physical properties of ZnO nanoparticles doped with Mn and Fe

This study reports the effect of magnetic ions doping on the structural, optical and magnetic properties of ZnO nanoparticles. Samples of Zn0.97-xMn0.03FexO, 0.0 ≤ x ≤ 0.1 wt. %, nanoparticles are synthesized by Co-Precipitation route. The synthesized samples are characterized by X-ray diffraction analysis (XRD), Transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectrometer and UV spectroscopy. XRD results reveal the hexagonal wurtzite structure of ZnO with the formation of secondary phase in the Fe-doped samples corresponding to Zinc Iron Oxide. The crystallite size is calculated by X-ray peak broadening using Debye-Scherer’s formula, and the results are in good agreement with TEM. The FTIR reveals two major peaks that are shifted toward a lower value when the concentration of Fe ions becomes higher than that of Mn ions. The UV results indicate a shift in the band-gap energy toward lower value upon increasing Fe-content. The study of magnetization hysteresis loop measurements infers that the samples of Zn0.97-xMn0.03FexO show a well-defined hysteresis loop, reflecting the paramagnetic behavior.

Structural, bandgap tuning and electrical properties of Cu doped ZnO nanoparticles synthesized by mechanical alloying

Cu doped ZnO nanoparticles abbreviated as Zn1−xCuxO (x = 0, 0.01 and 0.03) were synthesized by mechanical alloying. The change in structure, morphology, band gap and dielectric properties of the synthesized nanoparticles were investigated by XRD, FE-SEM, FTIR, UV–Vis and impedance analyzer respectively. The incorporation of the dopant Cu into ZnO hexagonal wurtzite structure has been verified by X-ray diffraction (XRD) and the Cu doping on the structural bonding of ZnO has been verified by fourier transformation infrared spectra (FTIR).The XRD spectra shows that all the synthesized nanoparticles are single phase, hexagonal wurtzite structure and belong to the space group of P63mc.Crystallite size of Cu doped ZnO (15 nm) nanoparticles is smaller than pure ZnO (18 nm) and peak broadening exists in the system. FE-SEM analysis indicates that Cu doping affects the surface morphology of ZnO. The band gap (Eg) of ZnO decreases with Cu doping which can be attributed to sp-d exchange interaction between the ZnO band electrons and localized d electrons of Cu2+ ions. The dielectric constant of ZnO decreases with Cu doping.

LUMINESCENCE INVESTIGATION OF POWDER ZNO NANOPARTICLES DOPED WITH SM AND LI IONS PREPARED BY COPRECIPITATION METHOD

The present study focuses on the structural and optical properties of ZnO doped with Sm and Li at very dilute concentrations, with compounds Zn1-x-ySmxLiyO (x = 0, 0.03 and y =0, 0.01, 0.03, 0.05). These compounds have been synthesized by a chemical coprecipitation method and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and Raman microscopy. The XRD pattern of all the samples showed ZnO hexagonal wurtzite structure, sharp and intense peaks with small change in lattice parameters due to doping in ZnO, indicating the substitution of Sm and Li ions for Zn sites. For Raman spectra, the strongest peak centered at about 435-437 cm-1 can be observed in all the samples, and the intensity of this peak decreases with Sm doping and increases with Li doping. The optical properties of the samples were systematically studied by ultraviolet-visible spectroscopy (UV-V) and photoluminescence (PL) spectroscopy. The maximum absorption of ZnO and Zn0.97Sm0.03O samples appears around 370 and 374 nm respectively near the absorption edge. Doping with Li affects the absorption edge and make it shift toward ultra violet and that the absorption increase with increasing Li content. The PL spectra show a broad and strong visible emission at 564 nm emission covering a large part of the visible region indicating the presence of intrinsic defects like vacancies.

Improving Photocatalytic Performance in Degradation of Methylene Blue using Magnetite Fe-doped ZnO/Montmorillonite Nanocomposite

In this work, we have successfully synthesized Fe:ZnO/Montmorillonite (MMT) nanocomposite with various loading of magnetite (Fe3O4) using co-precipitation method. The as-prepared samples were characterized by some measurements such as X-ray Diffraction (XRD), Burneur-Emment-Teller (BET) surface area analysis, and Fourier Transform Infrared Spectroscopy (FTIR). The XRD result shows that the diffraction pattern of nanocomposites exhibit characteristic from hexagonal wurtzite structure of ZnO and cubical spinel structure of Fe3O4. However, the diffraction pattern from MMT could not be detected using XRD measurement. The presence of MMT in the nanocomposite was further confirmed in the FTIR spectra. Furthermore, the photocatalytic performance of the samples was also checked for degrading methylene blue (MB) under UV light irradiation. The result shows that incorporation of Fe3O4 in Fe:ZnO/MMT has better photocatalytic efficiency than Fe:ZnO/MMT alone. Stability of the nanocomposites was also monitored after several cycle processes.

Synthesis of flower shaped ZnO thin films for resistive sensing of NO2 gas

A soft chemical synthetic method is used to obtain highly crystalline ZnO flower thin films. Hexagonal wurtzite ZnO structure is observed from X-ray diffraction studies. A large-area jasmine flower-shaped morphology of ZnO is uniformly maintained at all deposition temperatures. The films were characterized by energy dispersive X-ray spectroscopy, Raman spectroscopy and field emission scanning electron microscopy. Elemental analysis showed the presence of Zn and O elements without any other impurity. Raman spectroscopy spectroscopy in combination with elemental and resistivity analysis indicated wurtzite ZnO structure and the presence of oxygen vacancies. The films deposited at 338 K were studied with respect to their gas sensing capability at operating temperatures of 473 K. They show a fast response (65 s) and recovery time (54 s) for NO2 gas at a concentration as low as 10 ppm.

Adsorption and sonocatalytic performance of magnetite ZnO/CuO with NGP variation

A series of Fe3O4/ZnO/CuO/nanographene platelets (Fe3O4/ZnO/CuO/NGP) nanocomposites with various NGP weight percents were studied as catalysts for methylene blue removal under adsorption followed by sonocatalytic process. Weight percents (wt.%) of NGP in the nanocomposites were varied (5, 10, and 15 wt.%). The physicochemical properties of the samples were characterized using X-Ray diffraction (XRD), ultraviolet-visible (UV-VIS) spectroscopy, Brunauer–Emmett–Teller (BET) surface area analysis, and a vibrating sample magnetometer (VSM). The heterogeneous structure of all samples consisted of the cubic spinel structure of Fe3O4, hexagonal wurtzite structure of ZnO, monoclinic structure of CuO, and graphite-like structure of NGP. With increasing NGP weight percent, sample surface area increased from 14 m2/g to 23 m2/g. Adsorption and sonocatalytic activity were examined on degradation of methylene blue in alkaline conditions. The results show that the adsorption ability of samples increased with increasing NGP weight percent. However, in the sonocatalytic process, Fe3O4/ZnO/CuO/NGP with 10 wt.% NGP exhibited the maximum degradation. The effect of addition different radical scavenger was also examined to understand the sonocatalytic mechanism.

Deposition, characterization and gas sensors application of RF magnetron-sputtered terbium-doped ZnO films

Influence of terbium (Tb) doping on structural, optical and morphological properties of radio frequency magnetron-sputtered ZnO films has been investigated using various characterization techniques. Investigations revealed the formation of hexagonal wurtzite structure of ZnO with preferential c-axis orientation, presence of oxygen vacancies, modification of surface morphology and surface roughness, and confirmation of oxidation states of elements present in ZnO and Tb-doped ZnO films. Subsequently, films deposited with varied Tb concentrations have been tested for ethanol vapor sensing. Doped films exhibited reduction in optimum operable temperature, enhancement in ethanol sensing response and improvement in response/recovery time in comparison with pristine ZnO. The observed improvement in doped samples has been attributed to modifications in surface properties such as morphology, surface roughness, basicity, structural disorder and oxygen vacancies introduced due to dopant incorporation.