Properties Of ZnO Nanorod Arrays Research Articles

Effect of Cu additions on morphology and optical properties of ZnO nanorod arrays by two-step method

ZnO, Zn0.9Cu0.1O, Zn0.8Cu0.2O and Zn0.7Cu0.3O nanorod arrays are produced via a two-step process including an initial sol–gel method followed by hydrothermal growth. Effects of the Cu on microstructure, surface topography and optical properties of the nanorod arrays were studied and discussed. The results indicate that the nanorod arrays with a highly preferred orientation along the c axis were obtained on glass substrate. The Zn0.7Cu0.3O possesses the maximum density of nanorod arrays of 6.0 × 109 cm−2. The optical band-gap energy Eg decreases first and then increases with Cu additions increase. The broadening of the optical band-gap can be explained by Moss–Burstein effect. The Cu additions will effective enhance the violet emission and suppress the green emission. The violet and green emissions are likely due to the electron transition from the localized level below conduction band and oxygen vacancy to the valance band.

Defect-controlled ZnO nanorod arrays for enhanced photoelectrochemical performance

Abstract In this work, we demonstrated the effect of crystal defects on the photoelectrochemical properties of ZnO nanorod arrays (NRAs). Vertically aligned ZnO NRAs with different defect contents were grown on F doped SnO 2 (FTO) glass substrate via facile electrochemical process and annealing treatement.

Effect of KOH treatment on structural and photovoltaic properties of ZnO nanorod arrays

ZnO nanorod arrays (NRs) were synthesized on the fluorine-doped SnO2 transparent conductive glass (FTO) by a simple chemical bath deposition (CBD) method combined with alkali-etched method in potassium hydroxide (KOH) solution. X-ray diffraction (XRD), scanning electron microscopy (SEM) and current—voltage (I—V) curve were used to characterize the structure, morphologies and optoelectronic properties. The results demonstrated that ZnO NRs had wurtzite structures, the morphologies and photovoltaic properties of ZnO NRs were closely related to the concentration of KOH and etching time, well-aligned and uniformly distributed ZnO NRs were obtained after etching with 0.1 mol/L KOH for 1 h. ZnO NRs treated by KOH had been proved to have superior photovoltaic properties compared with high density ZnO NRs. When using ZnO NRs etched with 0.1 mol/L KOH for 1 h as the anode of solar cell, the conversion efficiency, short circuit current and open circuit voltage, compared with the unetched ZnO NRs, increased by 0.71%, 2.79 mA and 0.03 V, respectively.

Effect of Pulsed Electromagnetic Field on Growth and Structure Properties of ZnO Nanorod Arrays

Well-aligned ZnO nanorod arrays had been prepared by hydrothermal methods assisted with pulsed electromagnetic field (PEMF). The effects of pulsed electromagnetic field on growth and structure properties of ZnO nanorod arrays were studied in detail. XRD and SEM analysis showed ZnO nanorod arrays had bigger length to diameter ratio and better verticality on the substrate. And the Raman analysis showed well-aligned ZnO nanorod arrays have highly crystallized wurtzite structure with much fewer defects after a pulsed electromagnetic field was introduced. At last, a possible mechanism of pulsed electromagnetic field acted on nanorod arrays was proposed.

Controllable Low Temperature Vapor-Solid Growth and Hexagonal Disk Enhanced Field Emission Property of ZnO Nanorod Arrays and Hexagonal Nanodisk Networks

ZnO nanorod arrays and nanodisk networks were grown directly on Si substrate by thermal evaporation of ZnCl(2) powder and a mixture of ZnCl(2) and InCl(3)·4H(2)O at 450 °C in air, respectively. The ZnO nanorods with the diameters of 0.64 to 0.91 μm and length of about 5.1 μm are single crystalline with the hexagonal structure and grow along the [001] direction. The nanodisk has perfect hexagonal shape, grow mainly along the [0110] directions, and are enclosed by ±(0001) top and bottom surfaces. ZnO nanoparticle films oriented in the [001] direction formed first served as seeds, and grow into nanorod arrays via the vapor-solid (VS) process. However, when InCl(3)·4H(2)O was introduced into the reaction system ZnO thick nanosheet films are first formed because of the local segregation of the doping element of indium. The ZnO thick nanosheet films served as seeds, and grow into nanodisk networks via the V-S process. Photoluminescence and field emission properties of the as-obtained ZnO nanorod arrays and hexagonal nanodisk networks have been studied. It was found that the hexagonal nanodisk networks exhibit strong blue-green emissions originated from defect states and enhanced field emission property.

Mechanisms in photoluminescence enhancement of ZnO nanorod arrays by the localized surface plasmons of Ag nanoparticles

Photoluminescence properties of ZnO nanorod arrays decorated by Ag nanoparticles have been studied. It was found the enhancement of near-band-edge emission of ZnO was attributed to the giant local electronic fields of Ag nanoparticles, which was confirmed by annealing treatment and surface-enhanced Raman scattering spectra. The enhancement of visible emission band was due to the exciton-localized surface plasmons coupling effect.

Effect of Cu ions on the morphology, structure and luminescence properties of ZnO nanorod arrays prepared by hydrothermal method

Well-aligned ZnO nanorod arrays with different average diameters were grown by hydrothermal method in the presence of Cu ions in the deposition solution. Cu ions significantly inhibit diameter growth of ZnO nanorods and influence the luminescence properties. Quantitative analyses reveal that Cu ions were not incorporated into the synthesized nanorods. Photoluminescence spectra show that a weak ultraviolet emission and a strongly broad visible emission band in yellow region under ultraviolet light excitation for as-grown ZnO nanorods. The intensity of yellow emission bands decreases with Cu ions concentration increasing and the peak position has a blue-shift after low temperature thermal treatment. Photoluminescence spectra under different excitation energies and photoluminescence excitation spectra reveal that the yellow emission bands and green (blue) emission bands are associated with zinc interstitial related shallow donor level and oxygen vacancies related deep donor level as initial states, respectively. A plausible growth mechanism has been proposed to explain the narrowed diameter for ZnO nanorods in the presence of Cu ions based on crystallographic habits of wurtzite hexagonal.

Effects of Cu diffusion-doping on structural, optical, and magnetic properties of ZnO nanorod arrays grown by vapor phase transport method

Well-aligned ZnO nanorods were prepared by the vapor phase transport method on Si covered with a ZnO buffer layer. After the nanostructure growth, Cu was doped into the ZnO nanorods by diffusion at three different temperatures and for different times. Undoped and Cu diffusion-doped ZnO samples are highly textured, with the c axis of the wurtzite structure along the growth direction. The incorporation of Cu caused some slight changes in the nanorod alignment, although the wurtzite crystal structure was maintained. X-ray photoelectron spectroscopy measurements revealed that Cu ions were in a divalent state and substituted for the Zn2+ ions of the ZnO matrix. Photoluminescence results at 10 K indicate that the incorporation of copper leads to a relative increase of Cu-related structured green band deep level intensity. Magnetic measurements revealed that both undoped and Cu diffusion-doped ZnO samples exhibited room temperature ferromagnetism. It was also found that bound magnetic polarons play an important role in the appearance of room temperature ferromagnetism in Cu diffusion-doped ZnO nanorods.

Low temperature growth and properties of ZnO nanorod arrays

In this paper, well aligned ZnO nanorod arrays were synthesized by a simple hydrothermal route at a low temperature. The diameters of the as-synthesized products were 20–60 nm and the lengths were as much as several micrometers. The surfaces and tops of the nanorods were smooth. The as-grown nanorod arrays were investigated by x-ray powder diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), photoluminescence (PL) spectroscopy and contact angle (CA) analysis. The as-grown nanorods were single crystalline structures with a wurtzite phase, and grew along the [0001] direction. The PL spectrum with only one strong peak at 383 nm shows good intrinsic emission.

Effect of annealing on the structural and luminescent properties of ZnO nanorod arrays grown at low temperature

Vertically well-aligned ZnO nanorod arrays were synthesized on glass substrates by a two-step chemical bath deposition method. The structural and luminescent properties of as-grown and air annealed samples were investigated. Scanning electron microscopy, x-ray diffraction and Raman spectra demonstrate that ZnO nanorods are well oriented with c-axis perpendicular to the substrates. Photoluminescence spectra show a weak ultraviolet emission and an intense broad visible emission band for as-grown and air annealed samples. These visible emission bands exhibit dependences of post annealing temperatures and excitation energies: (1) as the annealing temperature increases, the visible emission band gradually red-shifts from yellow to orange-red; (2) the optimal excitation energy for yellow and orange-red emission band is near the band-gap energy; (3) green emission band can be excited only by the energies lower than the band-gap energy and this emission becomes weak after high temperature annealing. A depletion region model is presented to explain the origins and red-shift of the visible emission bands. The recombination of a delocalized electron in the interstitial zinc close to the conduction band with a deeply trapped hole in the single negatively charged interstitial oxygen center in the deletion region (in the bulk) is responsible for the yellow (orange-red) emission.

The effect of ZnO buffer layer on structural and optical properties of ZnO nanorods

AbstractA low‐temperature synthetic route was used to prepare oriented arrays of ZnO nanorods on ITO conducting glass substrate coated with buffer layer of ZnO seeds in an aqueous solution. The corresponding growth behavior and optical properties of ZnO nanorod arrays were studied. It was found that the nature of the buffer layer had effect on the microstructures and optical properties of the resultant ZnO nanorod arrays. X‐ray diffraction (XRD) results showed the nanorods were preferentially grown along (002) direction, but the diameter of the nanorods prepared with the buffer layer was much smaller than the without one, which can be clearly seen from the scanning electron microscopy (SEM) results. And it also found that the buffer layer was not only enhanced the density of overall coverage but also beneficial to grown the oriented arrays. Photoluminescence spectroscopy (PL) results indicated that the all the samples had the better optical behaviors. By computation, the relative PL intensity ratio of ultraviolet emission (IUV) to deep level emission (IDLE) of ZnO nanorods grown with the pure substrate was much higher than that of the sample with the buffer layer. The defects on the surface increased with the size reduction of nanorods caused by the buffer layer may be the main reason for it. And the small shift in the UV emission was caused by the rapid reduction in crystal size and compressive stress from Raman spectra results. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural and Antireflective Properties of ZnO Nanorods Synthesized Using the Sputtered ZnO Seed Layer for Solar Cell Applications

We report the structural and antireflective properties of ZnO nanorod arrays (NRAs) on silicon (Si) substrate by wet chemical growth using the sputtered ZnO seed layer for solar cell applications. The size, height, shape, and number of ZnO nanorods depend strongly on the ZnO seed layer thickness as well as the molar zinc nitrate concentration. Clearly, the ZnO nanorods are of wurzite crystal structure from the X-ray diffraction analysis. To achieve the low reflectance over a wide wavelength range, the ZnO seed layer thickness, molar concentration, and growth time are optimized. It is found that the specular reflection spectrum of ZnO NRAs is closely related to the ZnO seed layer thickness. The solar weighted reflectance, Rw, of ZnO NRAs as antireflection coatings for Si solar cells is estimated under AM1.5 g illumination. For ZnO NRAs with 50 nm ZnO seed layer in 10 mM aqueous solution for 12 hours, the low specular reflectance (i.e., <7%) is obtained at wavelengths of 300-1200 nm, indicating a low Rw of 3.86%.

Growth and Optical Properties of ZnO Nanorod Arrays

ZnO nanorod arrays were prepared using a chemical solution route on fluorine-doped SnO2 conductive glass pre-coated by a thin ZnO seed layer. Morphology, crystal structure and growth direction of the nanorod arrays were characterized by SEM and XRD respectively. The SEM results demonstrated that ordered arrays of ZnO nanorods grew vertically on the substrate. XRD measurement revealed the ZnO nanorods were grown along the [002] direction of the ZnO crystallites. Under optimal condition, ZnO nanorods with the average length about 6-7 μm and the diameters about 10nm at the tip and 150 nm at the base were obtained. The optical properties of ZnO nanorod arrays were characterized by transmittance spectra and photoluminescence measurement at room temperature.

Growth and investigation of antifungal properties of ZnO nanorod arrays on the glass

In this study, we have investigated the antifungal activity of ZnO nanorods prepared by the chemical solution method against Candida albicans. In the study, Zinc oxide nanorods have been deposited on glass substrates using the chemical solution method. The as-grown samples are characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). X-ray diffraction (XRD) showed zinc oxide nanorods grown in (0 0 2) orientation. The antifungal results indicated that ZnO nanorod arrays exhibit stable properties after two months and play an important role in the growth inhibitory of Candida albicans.

Effect of hydrothermal reaction temperature on growth, photoluminescence and photoelectrochemical properties of ZnO nanorod arrays

Well-aligned ZnO nanorod arrays have been successfully prepared by using low-temperature hydrothermal methods. The influence of the reaction temperature on the hydrothermal growth, photoluminescence and photoelectrochemical properties of the ZnO nanorod arrays was studied in detail. The ultraviolet emission peak increases and shows a slightly red shift, while the photoelectrochemical performance was improved with the growth temperature increasing. The overall conversion efficiency of 0.31% is obtained for the dye-sensitized solar cells based on the ZnO nanorod arrays as the photoelectrode under the AM1.5 simulated solar light.

Electrical and gas sensing properties of ZnO nanorod arrays directly grown on a four-probe electrode system

Abstract A method for measuring the electrical characteristics of aligned ZnO nanorod arrays (NRAs) directly grown on a pre-patterned four-point probe system in solution was proposed. This four-probe method enabled us to perform electrical measurements directly on the as-grown ZnO nanorod arrays without any additional processing. The location, shape and length of the rods directly grown on the four-probe electrodes were well controlled. The current–voltage characteristics showed a low turn-on voltage and a high saturation current. These ZnO NRAs devices were implemented as gas sensors for detecting hydrogen. The sensitivity increased with the concentration of H 2 and the operating temperature.

Morphological effects on the plasma-induced emission properties of large area ZnO nanorod arrays

Large area well-aligned ZnO nanorod arrays with different morphologies were fabricated by hydrothermal methods. The plasma-induced emission properties of ZnO nanorod arrays with different morphologies under a pulsed electric field were investigated. When the aspect ratios of nanorods increased, the emission currents of arrays were enhanced remarkably. As for the synthesized high density ZnO nanorod arrays, the emission currents of the nanorod arrays decreased with the increase in the nanorod densities owing to the screening effect. The plasma-induced emission properties of the ZnO nanorod arrays were improved by controlling the array morphologies. Under the pulsed electric field of 7–8 V µm−1, the highest emission current density of the ZnO nanorod arrays reached 91.16 A cm−2. Moreover, the surface plasmas on the ZnO nanorod arrays with different morphologies were always distributed uniformly. The results of this study provide a design strategy for ZnO nanorod arrays, which are used as plasma-flashover cathode emitters.

Growth and electrical properties of ZnO nanorod arrays prepared by chemical spray pyrolysis

Abstract Chemical spray pyrolysis was applied to grow ZnO nanorod arrays from zinc chloride solutions with pH=2 and 5 on glass/ITO substrate at 480 and 550 °C. The obtained structures were characterized by their morphological, electrical and PL properties. According to SEM, deposition of acidic solutions retards coalescence of the growing crystals. The charge carrier density in ZnO nanorods was determined from the C – V characteristics of ZnO/Hg Schottky barrier. Carrier densities ∼10 15 cm −3 and slightly above 10 16 cm −3 were recorded for ZnO deposited at 550 and 480 °C, respectively. According to PL studies, intense UV-emission is characteristic of ZnO independent of growth temperature, the concentration of oxygen vacancy related defects is lower in ZnO nanorods deposited at 550 °C. Solution pH has no influence on carrier density and PL properties.

Site-controlled Growth and Field Emission Properties of ZnO Nanorod Arrays

ZnO nanorod arrays with different structures were grown on Si substrates at different temperatures by using a chemical vapor transport process. The growth sites and population of ZnO nanorods were controlled by the primarily grown micropyramids. The density and size of these nanorods were greatly affected by the growth temperature. A growth mechanism is proposed which attributes the overgrowth of ZnO nanorods to the orientation adhesion and to the difference in the saturation pressure at different temperatures. The field emission (FE) measurements show that the high density arrays of small nanorods exhibit excellent FE properties.

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.