Controlling the orientation of ZnO nanorod arrays using TiO<Subscript>2</Subscript> thin film templates dip-coated by sol–gel

The oriented ZnO nanorod arrays have been synthesized on a silicon wafer that coated with TiO₂ films by aqueous chemical method. The morphologies, phase structure and the photoluminescence (PL) properties of the as-obtained product were investigated by field-emission scanning electron microscopy (FE-SEM), X-ray diffractometer (XRD), transmission electron microscope (TEM) and PL spectrum. The nanorods were about 100 nm in diameter and more than 1 μm in length, which possessed wurtzite structure with a c axis growth direction. The room-temperature PL measurement of the nanorod arrays showed strong ultraviolet emission. The effect of the crystal structure and the thickness of TiO₂ films on the morphologies of ZnO nanostructures were investigated. It was found that the rutile TiO₂ films were appropriate to the oriented growth of ZnO nanorod arrays in comparison with anatase TiO₂ films. Moreover, flakelike ZnO nanostructures were obtained with increasing the thickness of anatase TiO₂ films.     

Effects of seed layer on the structure and property of zinc oxide thin films electrochemically deposited on ITO-coated glass

ZnO films with different morphologies were deposited on the ITO-coated glass substrate from zinc nitrate aqueous solution at 65 °C by a seed-layer assisted electrochemical deposition route. The seed layers were pre-deposited galvanostatically at different current densities (i_sl) ranging from −1.30 to −3.0 mA/cm², and the subsequent ZnO films had been done using the potentiostatic technique at the cathode potential of −1.0 V. Densities of nucleation centers in the seed layers varied with increasing the current density, and the ZnO films on them showed variable morphologies and optical properties. The uniform and compact nanocrystalline ZnO film with (0 0 2) preferential orientation was obtained on seed layer that was deposited under the current density (i_sl) of −1.68 mA/cm², which exhibited good optical performances.     

Zn-catalyzed growth processes and ferromagnetism of Mn-doped ZnO nanorods on Si substrate

Well-aligned ZnO nanorods and Mn-doped ZnO nanorods are fabricated on Si (1 0 0) substrate according to the contribution of Zn metal catalysts. Scanning electron microscopy and high-resolution transmission electron microscopy images indicate that the influence of Zn catalyst on the properties of ZnO can be excluded and the growth of ZnO nanorods follows a vapor-liquid-solid and self-catalyzed model. Mn-doped ZnO nanorods show a typical room temperature ferromagnetic characteristic with a saturation magnetization (M_S) of 0.273μ_B/Mn. Cathodoluminescence suggests that the ferromagnetism of Mn-doped ZnO nanorods originates from the Mn²⁺-Mn²⁺ ferromagnetic coupling mediated by oxygen vacancies. This technique provides exciting prospect for the integration of next generation Si-technology-based ZnO spintronic devices.     

ZnO nanorods with different morphologies and their field emission properties

ZnO nanorods with different morphologies were grown by changing the temperature of the process using the thermal vapor deposition method without a catalyst. The X-ray diffraction pattern of these nanorods showed a single-crystalline wurtzite structure and a c-axis orientation. The turn-on fields of the pencil-like and normal ZnO nanorods were 1.7 V/μm and 2.2 V/μm at a current density of 0.1 μA/cm², and the emission current density from the ZnO nanorods reached 1 mA/cm² at bias fields of 5.1 V/μm and 7.5 V/μm, respectively. The results indicated that ZnO nanorods could give sufficient brightness as a field emitter in a flat panel display.     

Porous Silicon as an Intermediate Buffer Layer for Zinc Oxide Nanorods

Zinc thin films were deposited onto porous silicon (PSi) substrates by dc sputtering using a Zn target. These films were then annealed under flowing (6 l/min) oxygen gas environment in the furnace at 600°C for 2 h. Porous silicon is used as an intermediate layer between silicon and ZnO films and it provides a large area composed of an array of voids. The PSi samples were prepared using photoelectrochemical method on n-type silicon wafer with (111) and (100) orientation. To prepare porous structures, the samples were dipped into a mixture of HF:ethanol (1:1) for 5 min with current densities of 50 mA/cm², and subjected to external illumination with a 500 W UV lamp. The surface morphology and the nanorod structure of the ZnO films were characterized by scanning electron microscope (SEM) and X-ray diffraction (XRD). We synthesized the ZnO nanorods with diameter of 80–100 nm without any catalysts or templates. The XRD pattern confirmed that the ZnO nanorods were of polycrystalline structure. The surface-related optical properties have been investigated by photoluminescence (PL) and Raman measurements at room temperature. Micro-Raman results showed that A₁(LO) of hexagonal ZnO/Si(111) and ZnO/Si(100) have been observed at 522 cm^–1 and 530 cm^–1, respectively. PL spectra peaks are clearly visible at 366 cm^–1 and 368 cm^–1 for ZnO film grown on porous Si(111) and Si(100) substrates, respectively. The PL spectral peak position in ZnO nanorods on porous silicon is blue-shifted with respect to that in unstrained ZnO (381 nm).     

Effect of growth time on the structure, Raman shift and photoluminescence of Al and Sb codoped ZnO nanorod ordered array thin films

Al and Sb codoped ZnO nanorod ordered array thin films have been deposited on glass substrate with a ZnO seed layer by hydrothermal method at different growth time. The effect of growth time on structure, Raman shift, and photoluminescence (PL) was studied. The thin films at growth time of 5 h consist of nanorods growth vertically oriented with ZnO seed layer, and the nanorods with an average diameter of 27.8 nm and a length of 1.02 μm consist of single crystalline wurtzite ZnO crystal and grow along [0 0 1] direction. Raman scattering analysis demonstrates that the thin films at the growth time of 5 h have great Raman shift of 15 cm⁻¹ to lower wavenumber and have low asymmetrical factor Г_a/Г_b of 1.17. Room temperature photoluminescence reveals that there is more donor-related PL in films with growth time of 5 h.     

An influence of deposition temperature on structural,optical and electrical properties of sprayed ZnO thin films of identical thickness

Nanocrystalline ZnO thin films were deposited at different temperatures (T_s = 325 °C–500 °C) by intermittent spray pyrolysis technique. The thickness (300 ± 10 nm) independent effect of T_s on physical properties was explored. X-Ray diffraction analysis revealed the growth of wurtzite type polycrystalline ZnO films with dominant c-axis orientation along [002] direction. The crystallite size increased (31 nm–60 nm) and optical band-gap energy decreased (3.272 eV–3.242 eV) due to rise in T_s. Scanning electron microscopic analysis of films deposited at 450 °C confirmed uniform growth of vertically aligned ZnO nanorods. The films deposited at higher T_s demonstrated increased hydrophobic behavior. These films exhibited high transmittance (>91%), low dark resistivity (～10^?2 Ω-cm), superior figure of merit (～10^?3 Ω^?1) and low sheet resistance (～10² Ω/□). The charge carrier concentration (η -/cm³) and mobility (μ – cm²V^?1s^?1) are primarily governed by crystallinity, grain boundary passivation and oxygen desorption effects.     

Shape alterations of ZnO nanocrystal arrays fabricated from NH3·H2O solutions

Well-aligned crystalline ZnO nanorod arrays were synthesized via an aqueous solution route with ammonia and zinc nitrate as inorganic precursors. ZnO crystalline seed films were firstly coated on ITO substrates for epitaxial growth of rods through sol-gel processing and heat treatment. SEM, TEM, SAED and XRD were utilized to characterize morphologies and structures of ZnO crystals. Heterogeneous nucleation is crucial for rod growth. A broad scope of pH favorable for heterogeneous nucleation was disclosed at zinc concentration from 0.04 to 0.1 M in the inorganic system due to the complex reaction of ammonia with Zn²⁺. Elevation of initial zinc concentration or pH promoted growth rate of rods and enlarged rod size. ZnO nanorods were transformed to nanotubes, nanosheets and rods with blanket-like shaped surface mainly by secondary pH adjustment. All ZnO nanocrystals are wurtzite structure preferentially oriented in c-axis direction.     

Structural, optical and electrical properties of Al-N codoped ZnO films by RF-assisted MOCVD method

N-doped ZnO films were produced using N₂ as N source by metal-organic chemical vapor deposition (MOCVD) system which has been improved with radio-frequency (RF)-assisted equipments. The data of secondary ion mass spectroscopy (SIMS) indicate that the concentration of N in N-doped ZnO films is around 5 × 10²⁰ cm⁻³, implying that sufficient incorporation of N into ZnO can be obtained by RF-assisted equipment. On this basis, the structural, optical and electrical properties of Al-N codoped ZnO films were studied. Then, the effect of RF power on crystal quality, surface morphologies, optical properties was analyzed using X-ray diffraction, atomic force microscopy and photo-luminescence methods. The results illustrate that the RF plasma is the key factor for the improvement of crystal quality. Then the observation of A⁰X recombination associated with N_O acceptor in low-temperature PL spectrum proved that some N atoms have occupied the positions of O atoms in ZnO films. Hall measurements shown that p-type ZnO film deposited on quartz glasses was obtained when RF power was 150 W for the Al-N codoped ZnO films, while the resistivity of N-doped ZnO films was rather high. Compared with the Al-doped ZnO film, the obviously increased resistivity of codoped films indicates that the formation of N_O acceptors compensate some donors in ZnO films effectively.     

Effect of Co-doping content on hydrothermal derived ZnO array films

Cobalt doped ZnO films are synthesised using a hydrothermal process. The effect of Co²⁺ concentration on morphology, phase composition, crystallisation and spectroscopic characteristics of ZnO films is investigated. The results indicate that both the structure and morphology of the ZnO films evolve with the concentration of cobalt ions incorporated into the lattice. In the presence of a small amount of Co²⁺ ions, films are formed that comprise hexagonal ZnO nanorods, oriented with the c-axis perpendicular to the substrate. With increasing amount of Co²⁺, cracks in the ZnO nanorods can be observed and growth in the [0 0 1] direction is significantly inhibited. When the Co²⁺ concentration exceeds 0.010 M, ZnO rods with the typical hexagonal structure are no longer observed and instead, ZnO films comprising close-packed grains with an irregular polygonal structure are formed. The epitaxial growth of ZnO films is nearly completely inhibited when the concentration of Co²⁺ is increased above 0.050 M. This behaviour can be explained by the selective adsorption of the organic substances in the solution onto the (0 0 1) ZnO crystal face, thus inhibiting growth in the [0 0 1] direction and disrupting the crystallisation of ZnO films. Increasing the Co content deteriorates the crystallisation of ZnO rods and increases tensile stresses present in the ZnO films.     

Effect of the reaction conditions on the formation of the ZnO nanostructures

ZnO nanorods were synthesized through a simple chemical method by reacting Zn(C₂H₃O₂)₂·2H₂O and NaOH at low temperature and the effects of changing the order of addition of reactants on the morphological evolution of ZnO nanorods were investigated. The samples were characterized by using XRD, SEM, EDX, TEM, BET and Raman techniques. Optical properties of the ZnO nanostructures were too investigated by UV–Vis spectroscopy at room temperature.The hexagonal wurtzite phase of ZnO was confirmed by X-ray diffraction (XRD) for all the samples. SEM and TEM analysis indicated that different morphologies were obtained by changing the order of addition of reactants.     

Nanoporous ZnO nanostructures for photocatalytic degradation of organic pollutants

ZnO porous bamboo-leave-like nanorods and nanoporous networks were prepared by thermal conversion from Zn₂CO₃(OH)₂?H₂O bamboo-leave-like nanorods, Zn(OH)₂ nanoparticle networks and Zn(OH)₂ long nanostrand networks, respectively. Among them, the ZnO nanoporous networks prepared from Zn(OH)₂ nanostrands had the highest surface area of 78.57 m²/g and presented the best photocatalytic decomposition of organics. The morphologies of the Zn(OH)₂ nanostructures significantly depended on the solvent used for the precursors of aminoethanol and Zn(NO₃)₂ and then determined the corresponding structures and properties of the final ZnO nanostructures. The ethanol/water mixture solvent dramatically increased the stability of Zn(OH)₂ nanostrands. This is very beneficial for the collection and application of Zn(OH)₂ nanostrands.     

Synergy study on charge transport dynamics in hybrid organic solar cell: Photocurrent mapping and performance analysis under local spectrum

Charge transport dynamics in ZnO based inverted organic solar cell (IOSC) has been characterized with transient photocurrent spectroscopy and localised photocurrent mapping-atomic force microscopy. The value of maximum exciton generation rate was found to vary from 2.6 × 10²⁷ m⁻³s⁻¹ (J_sat = 79.7 A m⁻²) to 2.9 × 10²⁷ m⁻³s⁻¹ (J_sat = 90.8 A m⁻²) for devices with power conversion efficiency ranging from 2.03 to 2.51%. These results suggest that nanorods served as an excellent electron transporting layer that provides efficient charge transport and enhances IOSC device performance. The photovoltaic performance of OSCs with various growth times of ZnO nanorods have been analysed for a comparison between AM1.5G spectrum and local solar spectrum. The simulated PCE of all devices operating under local spectrum exhibited extensive improvement with the gain of 13.3–13.7% in which the ZnO nanorods grown at 15 min possess the highest PCE under local solar with the value of 2.82%.     

Morphology transition of ZnO films with DMZn flow rate in MOCVD process

We used a metal-organic chemical vapor deposition (MOCVD) method to grow ZnO films on MgAl₂O₄ (1 1 1) substrate, and succeeded in preparing films with microstructures from well-aligned ZnO nanorods to continuous and dense films by adjusting the ratio of the input rates of oxygen and zinc sources (VI/II). At the growth temperature of 350 °C, the ZnO nanorods were formed under a low flow rate of a zinc precursor. On the other hand, continuous and dense ZnO films were formed under a high flow rate of the zinc precursor. There is a transition zone at medium zinc precursor flow rate, where nanorods transform to dense films. We proved that the height of ZnO nanorods and the thickness of ZnO dense films both increase with zinc flow rate, and are consistent with the mass-transport mechanism for ZnO growth. The XRD spectra of the sample in the transition zone show both (0 0 2) and (1 0 1) peaks, where (1 0 1) peaks are formed only in the transition zone. We consider that there are (0 0 2) and (1 0 1) ZnO grains in the early growth stage of dense ZnO films.     

Effect of substrate temperature on the structural and optical properties of ZnO and Al-doped ZnO thin films prepared by dc magnetron sputtering

ZnO and Al-doped ZnO(ZAO) thin films have been prepared on glass substrates by direct current (dc) magnetron sputtering from 99.99% pure Zn metallic and ZnO:3 wt%Al₂O₃ ceramic targets, the effects of substrate temperature on the crystallization behavior and optical properties of the films have been studied. It shows that the surface morphologies of ZAO films exhibit difference from that of ZnO films, while their preferential crystalline growth orientation revealed by X-ray diffraction remains always the (0 0 2). The optical transmittance and photoluminescence (PL) spectra of both ZnO and ZAO films are obviously influenced by the substrate temperature. All films exhibit a transmittance higher than 86% in the visible region, while the optical transmittance of ZAO films is slightly smaller than that of ZnO films. More significantly, Al-doping leads to a larger optical band gap (E_g) of the films. It is found from the PL measurement that near-band-edge (NBE) emission and deep-level (DL) emission are observed in pure ZnO thin films. However, when Al was doped into thin films, the DL emission of the thin films is depressed. As the substrate temperature increases, the peak of NBE emission has a blueshift to region of higher photon energy, which shows a trend similar to the E_g in optical transmittance measurement.     

Synthesis and Characterization of ZnO Nanoflowers Grown on AlN Films by Solution Deposition

ZnO nanoflowers are synthesized on AlN films by solution method. The synthesized nanoflowers are composed of nanorods, which are pyramidal and grow from a central point, thus forming structures that are flower-shaped as a whole. The nanoflowers have two typical morphologies： plate-like and bush-like. The XRD spectrum corresponds to the side planes of the ZnO nanorods made up of the nanoflowers. The micro-Raman spectrum of the ZnO nanoflowers exhibits the E2 （high） mode and the second order multiple-phonon mode. The photoluminescence spectrum of the ZnO nanoflowers exhibits ultraviolet emission centred at 375nm and a broad green emission centred at 526 nm.     

Properties of ZnO thin films deposited on (glass,ITO and ZnO:Al) substrates

ZnO thin films were deposited on glass, ITO (In₂O₃; Sn) and on ZnO:Al coated glass by spray pyrolysis. The substrates were heated at 350 °C. Structural characterization by X-ray diffraction (XRD) measurements shows that films crystallize in hexagonal structure with a preferential orientation along (0 0 2) direction. XRD peak-shift analysis revealed that films deposited on glass substrate (−0.173) were compressive, however, films deposited onto ITO (0.691) and on ZnO:Al (0.345) were tensile. Scanning electron microscopies (SEM) show that the morphologies of surface are porous in the form of nanopillars. The transmittance spectra indicated that the films of ZnO/ITO/glass and ZnO/ZnO:Al/glass exhibit a transmittance around 80% in the visible region. An empirical relationship modeled by theoretical numerical models has been presented for estimating refractive indices (n) relative to energy gap. All models indicate that the refractive index deceases with increasing energy band gap (E_g).     

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.     

Direct comparison of the electrical,optical, and structural phase transitions of VO2 on ZnO nanostructures

We examined the temperature-dependent electrical, optical, and structural properties of VO₂ on ZnO nanorods with different lengths in the temperature range from 30 to 100 °C. ZnO nanorods with a uniform length were grown on Al₂O₃ substrates using a metal organic chemical vapor deposition, and subsequently, VO₂ was ex-situ deposited on ZnO nanorods/Al₂O₃ templates using a sputtering deposition. The optical properties of the VO₂/ZnO nanorods were measured simultaneously with direct current (DC) resistance using the reflectivity of an infrared (IR) laser beam with a wavelength of 790 nm. The local structural properties around V atoms of VO₂/ZnO nanorods were simultaneously measured with the DC resistance using x-ray absorption fine structure at the V K edge. Direct comparison of the temperature-dependent resistance, IR reflectivity, and local structure reveals that an optical phase transition first occurs, a structural phase transition follows, and an insulator-to-metal transition finally appears during heating.     

掺银氧化锌纳米棒的水热法制备研究

利用水热法在直流磁控溅射制备的掺铝氧化锌 (AZO) 种子层上制备了不同形貌和光学性能的掺银ZnO纳米棒, 并采用XRD、扫描电镜、透射谱、光发射谱和EDS谱详细研究了Ag离子与Zn离子的摩尔百分比 (R_Ag/Zn) 及AZO种子层对掺银ZnO纳米棒的结构和光学性质的影响. 随着R_Ag/Zn的增加, 掺银ZnO 纳米棒的微结构和光学性质的变化与银掺杂诱导的纳米棒的端面尺寸变化有关. 平均端面尺寸的变化归结于种子层颗粒大小和颗粒数密度不同导致掺入的Ag离子的相对比例不同. 溅射15 min的AZO种子层上生长的ZnO纳米棒由于缺陷增多导致在可见光区的发光峰明显强于溅射10 min 的AZO种子层上、相同R_Ag/Zn 条件下生长的ZnO纳米棒. Ag掺杂产生的点缺陷增多导致可见光区PL波包较宽. 纯ZnO纳米棒的微结构与种子层厚度导致的结晶度和颗粒大小有关. 关键词： ZnO纳米棒 水热法 Ag掺杂 直流磁控溅射