Synthesis and optical properties of ZnO nanorods on indium tin oxide substrate

ZnO nanorods with uniform diameter and length have been synthesized on an indium-tin oxide (ITO) substrate by using a simple thermal evaporation method which is suitable to larger scale production and without any catalyst or additives. The samples were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet-vis (UV-vis) absorption spectrum, photoluminescence (PL) spectrum and Raman spectrum. The single-phase ZnO nanorods grow well-oriented along the c-axis of its wurtzite structure on ITO substrate. The ZnO nanorods shows sharp and strong UV emission located at 380 nm without notable visible light emission in the PL spectrum, which suggests the good crystallinity of the nanorods, which was also testified by their Raman spectrum. The photodegradation of methylene orange (MO) in aqueous solution reveals that the well-arranged c-axis growth of ZnO nanorods possess evidently improved photocatalytic performance and these properties enable the ZnO nanorods potential application in UV laser.     

Effect of Mn doping on the microstructures and photoluminescence properties of CBD derived ZnO nanorods

Mn-doped ZnO nanorods were synthesized from aqueous solutions of zinc nitrate hexahydrate, manganese nitrate and methenamine by the chemical solution deposition method (CBD). Their microstructures, morphologies and optical properties were studied in detail. X-ray diffraction (XRD) results illustrated that all the diffraction peaks can be indexed to ZnO with the hexagonal wurtzite structure. Scanning electron microscope (SEM) results showed that the average diameter of Mn-doped ZnO nanorods was larger than that of the undoped one. Photoluminescence (PL) spectra indicated that manganese doping suppressed the emission intensity and caused the blue shift of UV emission position compared with the undoped ZnO nanorods. In the Raman spectrum of Mn-doped ZnO nanorods, an additional mode at about 525 cm⁻¹ appeared which was significantly enhanced and broadened with the increase of Mn doping concentration.     

Structural, morphological and photoluminescence properties of W-doped ZnO nanostructures

W-doped ZnO nanostructures were synthesized at substrate temperature of 600 °C by pulsed laser deposition (PLD), from different wt% of WO₃ and ZnO mixed together. The resulting nanostructures have been characterized by X-ray diffraction, scanning electron microscopy, atomic force microscopy and photoluminescence for structural, surface morphology and optical properties as function of W-doping. XRD results show that the films have preferred orientation along a c-axis (0 0 L) plane. We have observed nanorods on all samples, except that W-doped samples show perfectly aligned nanorods. The nanorods exhibit near-band-edge (NBE) ultraviolet (UV) and violet emissions with strong deep-level blue emissions and green emissions at room temperature.     

Structural and photoluminescent properties of Ni doped ZnO nanorod arrays prepared by hydrothermal method

Self-assembled Ni-doped zinc oxide (Zn_1−xNi_xO, x = 0.05, 0.10, 0.15, i.e., ZnNiO, nominal composition) nanorod arrays vertically grown on the ZnO seed layer covered glass along [0 0 1] direction were synthesized by hydrothermal method. Their images and structures have been characterized by scan electron microscope (SEM), X-ray diffraction (XRD) and Raman spectra, showing that Ni doping is beneficial to the formation of ZnO nanorods with hexagonal cross section and the enhancement of ZnO crystal quality. X-ray photoemission spectroscopy (XPS) study further demonstrated that Ni atoms were successfully doped into ZnO lattices. The photoluminescence (PL) spectra of ZnNiO samples show near bandedge emission (NBE) peaks at about 380 nm at a low excitation power and the NBE peak position redshifts while its intensity continuously increases with the increase of Ni doping concentration. With the excitation power increasing, the NBE peak redshifts from 380 nm to about 400 nm for ZnNiO nanorod arrays. The NBE mechanisms for ZnNiO nanorod arrays have been discussed, which is helpful for understanding their room temperature ferromagnetisms.     

Structural and optical properties of ZnO nanorods grown on MgxZn1−xO buffer layers

ZnO nanorod arrays were synthesized by chemical-liquid deposition techniques on Mg_xZn_1−xO (x = 0, 0.07 and 0.15) buffer layers. It is found that varying the Mg concentration could control the diameter, vertical alignment, crystallization, and density of the ZnO nanorods. The X-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) data show the ZnO nanorods prefer to grow in the (0 0 2) c-axis direction better with a larger Mg concentration. The photoluminescence (PL) spectra of ZnO nanorods exhibit that the ultraviolet (UV) emission becomes stronger and the defect emission becomes weaker by increasing the Mg concentration in Mg_xZn_1−xO buffer layers.     

Characterization, sintering and dielectric properties of nanocrystalline zinc oxide prepared by a citric acid-based combustion route

Nanosized zinc oxide has been synthesized through a novel single step solution combustion route using citric acid as fuel. The X-ray diffraction (XRD) analysis revealed that the synthesized ZnO nanopowder has the pure wurtzite structure. The phase purity of the nanopowder has been confirmed using differential thermal analysis (DTA), thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR). The morphology and crystalline size of the as-prepared nanopowder characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that the powder consisted of a mixture of nanoparticles and nanorods. The nanocrystalline ZnO could be sintered to ∼97% of the theoretical density at 1200 °C in 4 h. The dielectric constant (ε_r) and dielectric loss (ε_i) of sintered ZnO pellets at 5 MHz were 1.38 and 9×10⁻², respectively, at room temperature.     

Optical properties of ZnO and ZnO:Ce layers grown by spray pyrolysis

In this paper, zinc oxide (ZnO) and cerium-doped zinc oxide (ZnO:Ce) films were deposited by reactive chemical pulverization spray pyrolysis technique using zinc and cerium chlorides as precursors. The effects of Ce concentration on the structural and optical properties of ZnO thin films were investigated in detail. These films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence (PL) measurements. All deposited ZnO layers at the temperature 450 °C are polycrystalline and indicate highly c-axis oriented structure. The dimension of crystallites depends on incorporation of Ce atoms into the ZnO films. The photoluminescence spectra of the films have been studied as a function of the deposition parameters such as doping concentrations and post grows annealing. Photoluminescence spectra were measured at the temperature range from 13 K to 320 K.     

Effects of growth duration on the structural and optical properties of ZnO nanorods grown on seed-layer ZnO/polyethylene terephthalate substrates

Well-aligned single crystalline zinc oxide (ZnO) nanorods were successfully grown, by hydrothermal synthesis at a low temperature, on flexible polyethylene terephthalate (PET) substrates with a seed layer. Photoluminescence (PL), field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) measurements were used to analyze the optical and structural properties of ZnO nanorods grown for various durations from 0.5 h to 10 h. Regular and well-aligned ZnO nanorods with diameters ranging from 62 nm to 127 nm and lengths from 0.3 μm to 1.65 μm were formed after almost 5 h of growth. The growth rate of ZnO grown on PET substrates is lower than that grown on Si (1 0 0) substrates. Enlarged TEM images show that the tips of the ZnO nanorods grown for 6 h have a round shape, whereas the tips grown for 10 h are sharpened. The crystal properties of ZnO nanorods can be tuned by using the growth duration as a growth condition. The XRD and PL results indicate that the structural and optical properties of the ZnO nanorods are most improved after 5 h and 6 h of growth, respectively.     

Hydrothermal synthesis and characterization of ZnO films with different nanostructures

ZnO films with morphologies of nanorods, nanowires and nanosheets were grown on F-doped SnO₂ glass substrate, which may have potential application in solar cells. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to characterize the structures and morphologies of the as-synthesized samples. The photoluminescence (PL) and the photoelectrochemical properties of ZnO films were also measured. The results showed that ZnO nanorods preferentially oriented along the c-axis and had the largest photocurrent density which is as high as 60 μA/cm².     

Growth of ZnO nanorods by aqueous solution method with electrodeposited ZnO seed layers

ZnO nanorod arrays on ZnO-coated seed layers were fabricated by aqueous solution method using zinc nitrate and hexamethylenetetramine at low temperature. The seed layers were coated on ITO substrates by electrochemical deposition technique, and their textures were dominated by controlling the deposition parameters, such as deposition potential and electrolyte concentration. The effects of the electrodeposited seed layers and the growing parameters on the structures and properties of ZnO nanorod arrays were primarily discussed. The orientation and morphology of both the seed layer and successive nanorods were analyzed by using X-ray diffraction (XRD), SEM and TEM. The results show that the seed layer deposited at −700 mV has evenly distributed crystallites and (0 0 2) preferred orientation; the density of resultant nanorods is high and ZnO nanorods stand completely perpendicular onto substrates. Meanwhile, the size of nanorods quite also depends on the growth solution, and the higher concentration of growth solution primary leads to a large diameter of the ZnO nanorods.     

Thickness influence on surface morphology and ozone sensing properties of nanostructured ZnO transparent thin films grown by PLD

Transparent zinc oxide (ZnO) thin films with a thickness from 10 to 200 nm were prepared by the PLD technique onto silicon and Corning glass substrates at 350 °C, using an Excimer Laser XeCl (308 nm). Surface investigations carried out by atomic force microscopy (AFM) and X-ray diffraction (XRD) revealed a strong influence of thickness on film surface topography. Film roughness (RMS), grain shape and dimensions correlate with film thickness. For the 200 nm thick film, the RMS shows a maximum (13.9 nm) due to the presence of hexagonal shaped nanorods on the surface. XRD measurements proved that the films grown by PLD are c-axis textured. It was demonstrated that the gas sensing characteristics of ZnO films are strongly influenced and may be enhanced significantly by the control of film deposition parameters and surface characteristics, i.e. thickness and RMS, grain shape and dimension.     

Synthesis and optical properties of flower-like ZnO nanorods by thermal evaporation method

Flower-like ZnO nanorods have been synthesized by heating a mixture of ZnO/graphite powders using the thermal evaporation and vapor transport on Si (1 0 0) substrates without any catalyst. The structures, morphologies and optical properties of the products were characterized in detail by using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and Raman spectroscopy. The synthesized products consisted of large quantities of flower-like ZnO nanostructures in the form of uniform nanorods. The flower-like ZnO nanorods had high purity and well crystallized wurtzite structure, whose high crystalline quality was proved by Raman spectroscopy. The as-synthesized flower-like ZnO nanorods showed a strong ultraviolet emission at 386 nm and a weak and broad yellow-green emission in visible spectrum in its room temperature photoluminescence (PL) spectrum. In addition, the growth mechanism of the flower-like ZnO nanorods was discussed based on the reaction conditions.     

The structural and optical properties of ZnO nanorod arrays

High quality vertical-aligned ZnO nanorod arrays were synthesized by a simple vapor transport process on Si (111) substrate at a low temperature of 520 °C. Field-emission scanning electron microscopy (FESEM) showed the nanorods have a uniform length of about 1 μm with diameters of 40-120 nm. X-ray diffraction (XRD) analysis confirmed that the nanorods are c-axis orientated. Selected area electron diffraction (SAED) analysis demonstrated the individual nanorod is single crystal. Photoluminescence (PL) measurements were adopted to analyze the optical properties of the nanorods both a strong UV emission and a weak deep-level emission were observed. The optical properties of the samples were also tested after annealing in oxygen atmosphere under different temperatures, deep-level related emission was found disappeared at 600 °C. The dependence of the optical properties on the annealing temperatures was also discussed.     

Temperature-dependent growth mode of W-doped ZnO nanostructures

We report on the effects of glass substrate temperature on the crystal structure and morphology of tungsten (W)-doped ZnO nanostructures synthesized by pulsed-laser deposition. X-ray diffraction analysis data shows that the W-doped ZnO thin films exhibit a strongly preferred orientation along a c-axis (0 0 0 L) plane, while scanning electron and atomic force microscopes reveal that well-aligned W-doped ZnO nanorods with unique shape were directly and successfully synthesized at substrate temperature of 550 °C and 600 °C without any underlying catalyst or template. Possible growth mechanism of these nanorods is suggested and discussed.     

Realization of p-ZnO thin films by GaP codoping

An attempt has been made to realize p-ZnO by directly doping (codoping) GaP into ZnO thin films. GaP codoped ZnO thin films of different concentrations (1, 2 and 4 mol%) have been grown by RF magnetron sputtering. The grown films on sapphire substrate have been characterized by X-ray diffraction (XRD), Hall measurement, Photoluminescence (PL) and Energy dispersive spectroscopy (EDS) to validate the p-type conduction. XRD result shows that all the films have been preferentially oriented along (0 0 2) orientation. The decrease of full-width at half maximum (FWHM) with increase in GaP doping depicts the decrease in native donor defects. Hall measurement shows that among the three films, 2 and 4 mol% GaP doped ZnO shows p-conductivity due to the sufficient amount of phosphorous incorporation. It has been found that low resistivity (2.17 Ωcm) and high hole concentration (1.8×10¹⁸ cm⁻³) for 2% GaP codoped ZnO films due to best codoping. The red shift in near-band-edge (NBE) emission and donar-acceptor-pair (DAP) and neutral acceptor bound recombination (A°X) observed by room temperature and low temperature (10 K) PL, respectively, well acknowledged the formation of p-ZnO. The incorporated phosphorous in the film has been also confirmed by EDS analysis.     

High-temperature ferromagnetism in cobalt-doped ZnO single-crystalline nanorods

Co-doped ZnO single-crystalline nanorods with 80–100 nm in diameter and 1.5–2 μm in length have been prepared in a simple solution route. X-ray diffraction data and selected area electron diffraction pattern of the diluted magnetic semiconductor nanorods confirm the single crystallinity of Zn_1−xCo_xO solid solution without impurities of metallic Co or other phases. Magnetic results show that the Zn_0.95Co_0.05O nanorods exhibit a ferromagnetic characteristic with Curie temperature higher than 380 K. The high-temperature ferromagnetic properties allow this Zn_1−xCo_xO nanorods potential applications in future spintronic devices.     

Structural, optical and electrical properties of ZnO/Zn2GeO4 porous-like thin film and wires

Zinc oxide/zinc germanium oxide (ZnO/Zn₂GeO₄) porous-like thin film and wires has been fabricated by simple thermal evaporation method at temperature about 1120 °C for 2.5 h. The structural and optical properties of the porous-like-thin film and wires have been investigated by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy. Metal semiconductor metal (MSM) photodetector structure was used to evaluate the electrical characteristics by using current-voltage (I-V) measurements. Room temperature photoluminescence spectrum of the sample shows one prominent ultraviolet peak at 378 nm and a shoulder at 370 nm. In addition, broad visible blue emission peak at wavelength 480 nm and green emission peak at 500 nm are also observed. Strong photoelectric properties of the MSM in the UV demonstrated that the porous-like-thin film and wires contribute to its photosensitivity and therefore making ZnO/Zn₂GeO₄ wires potential photodetector in the shorter wavelength applications.     

Low-temperature growth of ZnO nanorods on PET fabrics with two-step hydrothermal method

An effective low-temperature growth method to fabricate hexagonally oriented ZnO nanorod arrays onto PET fabrics is reported. The effect of substrate pre-treatment and C₆H₁₂N₄ concentration on the structure of ZnO nanorod arrays were investigated in details by X-ray diffraction (XRD), FE-SEM and ultraviolet protection factor (UPF). The results show that substrate pre-treatment, C₆H₁₂N₄ concentration indeed have great influence on the growth of ZnO nanorod arrays. It is indispensable to introduce a ZnO seed layer on the substrate and under growth condition of n(C₆H₁₂N₄):n[Zn(NO₃)₂] = 1:1, T = 90 °C, t = 3 h, the well-aligned ZnO nanorod arrays with 40-50 nm in diameter and 300-400 nm in length were achieved on the pre-treated PET fabrics. The ZnO nanorods grown on PET fabrics possessed an ultrahigh ultraviolet protection factor of 480.52 in this study, indicating an excellent protection against ultraviolet radiation in comparison with the untreated PET fabrics.     

Effect of surface preparation on the morphology of ZnO nanorods

The effects of Si substrate orientation and surface treatment on the morphology and density of Zinc oxide (ZnO) nanorods were investigated. The size and density of ZnO nanorods were influenced by Si substrate orientation and surface preparation. ZnO nanorods synthesized on the ideally H-terminated Si(1 1 1) prepared with an NH₄F solution resulted in the biggest size and the lowest density. It is suggested that the smoother surface of the Si substrate and lattice shape match with a larger atomic distance result in the increase of the ZnO seedlayer's grain size, which in turn enhances the size of ZnO nanorods grown on it. The optical properties of the ZnO nanorods were affected by their size and crystallinity. The smallest ZnO nanorods with a preferential c-axis orientation synthesized on the HF-treated Si(1 1 1) surface showed the highest intensity ratio of UV to visible emission, and the biggest ZnO nanorods synthesized on the N₂-sparged NH₄F-treated Si(1 1 1) surface showed the lowest intensity ratio of UV to visible emission. Therefore, it can be concluded that Si substrate orientation and surface preparation significantly affect the optical properties of ZnO nanorods.     

Studies of luminescence properties of ZnO and ZnO:Zn nanorods prepared by solution growth technique

Pyramidal ZnO nanorods with hexagonal structure having c-axis preferred orientation are grown over large area silica substrates by a simple aqueous solution growth technique. The as-grown nanorods were studied using XRD, SEM and UV-vis photoluminescence (PL) spectroscopy for their structural, morphological and optical properties, respectively. Further, the samples have also been annealed under different atmospheric conditions (air, O₂, N₂ and Zn) to study the defect formation in nanorods. The PL spectra of the as-grown nanorods show narrow-band excitonic emission at 3.03 eV and a broad-band deep-level emission (DLE) related to the defect centers at 2.24 eV. After some mild air annealing at 200 °C, fine structures with peaks having energy separation of ∼100 meV were observed in the DLE band and the same have been attributed to the longitudinal optical (LO) phonon-assisted transitions. However, the annealing of the samples under mild reducing atmospheres of N₂ or zinc at 550 °C resulted in significant modifications in the DLE band wherein high intensity green emission with two closely spaced peaks with maxima at 2.5 and 2.7 eV were observed which have been attributed to the V_O and Zn_i defect centers, respectively. The V-I characteristic of the ZnO:Zn nanorods shows enhancement in n-type conductivity compared to other samples. The studies thus suggest that the green emitting ZnO:Zn nanorods can be used as low voltage field emission display (FED) phosphors with nanometer scale resolution.