Aligned ZnO Research Articles

Island nucleation, optical and ferromagnetic properties of vertically aligned secondary growth ZnO : Cu nanorod arrays

The paper reports an island nucleation and secondary growth of aligned ZnO : Cu nanorod arrays via thermal vapor phase transport. Results analysis indicates that the secondary segment is epitaxially grown on the ZnO : Cu nanorods with the radius strongly dependent on temperature and the concentration of zinc vapor. The modified characteristic radius (R(c)) model is used to explain the nucleation and secondary growth process. Temperature-dependent photoluminescence spectra indicate that the band gap emission of the secondary growth nanorods is greatly restrained. A controversial 3.31 eV emission (A line) and two different donor-acceptor pair (DAP) recombinations at 3.24 eV and 2.48 eV are observed at 13 K. The A line shows a different behavior from the two DAP emissions during the heat-up process. Intrinsic room temperature ferromagnetism (RTFM) is observed in the secondary growth ZnO : Cu nanorods and it can be explained by oxygen vacancy and copper defects related to bound magnetic polar (BMP) or double exchange mechanism.

ZnO薄膜的性质对水热生长ZnO纳米线阵列的影响

ZnO nanowire arrays with controlled diameter,density and orientation were prepared on the ZnO film coated substrates via hydrothermal method.The ZnO film were fabricated by atomic layer deposition(ALD) method and annealed at different temperatures.The annealing temperature has strong influences on the grain size,crystalline structure and defect property of the ZnO film.The diameter,density and orientation of ZnO nanowires depended on the features of the ZnO film.Field-emission scanning electron microscopy(SEM),X-ray diffraction(XRD) and photoluminescence(PL) were applied to analyze the ZnO film and ZnO nanowire arrays.The as-prepared vertical aligned ZnO nanowires are highly suitable for use in nanodevices,such as light-emitting diodes and solar cells.

Micromechanism and Kinetic Formulation of Vertically Aligned ZnO Nanorods Grown on Catalytic Bilayers

Vertically aligned ZnO nanorods were grown at 700°C for 2 h on sapphire substrates with catalysts in bilayer configurations of Sn (top)/Ni (bottom) and Sn/In, where the top layer is formed by sputtering and the bottom one is deposited by spin coating. The effects of bilayer catalysts on growth kinetics of nucleation and growth, growth micromechanism, and vertical alignment of growing ZnO nanorods have been investigated. The vertical alignment of the Sn/Ni‐catalyzing ZnO nanorods is determined at the initial nucleation stage, where the nuclei are formed as regular candlestick‐like platforms. The reason for the formation of the candlestick‐like nuclei is due to the contribution of strain energy built in the underlying catalyst bilayers. The variations of axial and radial dimensions with growth duration for the growth of ZnO rods were explained and data fitting with the aids of kinetic growth equations, which are based upon the well‐known ledge model for crystal growth from vapor and diffusion kinetics.

Photo-induced water–oil separation based on switchable superhydrophobicity–superhydrophilicity and underwater superoleophobicity of the aligned ZnO nanorod array-coated mesh films

Stimulus-responsive surface wettability, especially photoresponsive surface wettability, has been intensively studied. Meanwhile, multifunctional surfaces, especially for the treatment of oil contaminated water, have aroused worldwide attention. Recently, pH-responsive surfaces with controllable oil–water separation have also been reported. However, photoresponsive water–oil separation is still a challenge. Here we report photo-induced water–oil separation based on the switchable superhydrophobicity–superhydrophilicity and underwater superoleophobicity of aligned ZnO nanorod array-coated mesh films, which shows excellent controllability and high separation efficiency of different types of water–oil mixtures in an oil–water–solid three-phase system. The underwater superoleophobicity of the aligned ZnO nanorod array-coated stainless steel mesh film can effectively prevent the mesh film from being polluted by oils. This work is promising in photo-induced water–oil mixture treatments such as water-removal from a micro-reaction system and controllable filtration, and may also provide interesting insight into the design of novel functional devices based on controllable surface wettability.

On the origin of enhanced photoconduction and photoluminescence from Au and Ti nanoparticles decorated aligned ZnO nanowire heterostructures

We have investigated the mechanism of photoinduced charge transport and origin of enhanced PC and PL from Au and Ti NPs decorated vertically aligned ZnO NWs arrays. Uniform decoration of metal NPs on the surface of the ZnO NWs was confirmed by high resolution electron microscopy imaging. Absorption spectra also indicate the presence of metal NPs layer with different thicknesses. At a fixed bias, the dark current of the Au/ZnO heterostructures decreases with the increase of Au coverage, while the Ti/ZnO heterostructures show very high dark current. The photocurrent (PC) spectra show a strong peak at the UV region due to the band-edge absorption followed by generation of the photocarriers and two other peaks in the visible region. For the Au/ZnO heterostructures, the PC increases with increase of Au coverage up to a certain thickness and then decreased. On the other hand, the PC in Ti/ZnO heterostructures increases with the increase of the Ti coverage. The PL spectra for both the system are consistent with the respective PC spectra, which show significant improvement in the band-edge related UV emission and quenching of green emission. The Ti/ZnO heterostructures shows much faster photoresponse compared to the Au/ZnO heterostructures. The changes in the PC and PL spectra with the size of the metal NPs are studied systematically and explained. A model based on energy band alignment is proposed to explain the results.

Iron Ion Sensor Based on Functionalized ZnO Nanorods

AbstractIn this work, we are presenting an iron ion (Fe3+) potentiometric sensor based on functionalized ZnO nanorods with selective ionophore (18 crown 6). Zinc oxide nanorods with a diameter of about 100 to 150 nm and 1 µm in length were grown on gold coated glass. The selective Fe3+ ionophore sensor with highly aligned ZnO nanorods showed high sensitivity, acceptable selectivity, reproducibility and a stable signal response for detecting Fe3+. The potentiometric response of the Fe3+ sensor with functionalized ZnO nanorods versus a Ag/AgCl reference electrode was observed to be linear over a logarithmic concentration range from 10−5 M to 10−2 M. The detection limit of the proposed sensor was about 5 µM, which is lower than the normal blood concentration of Fe3+ which is about 10 µM and can be up to 30 µM. The sensitivity of the proposed Fe3+ sensor was found to be 70.2±2.81 mV/decade with a regression coefficient R2=0.99 and a response time less than 5 s. The functionalized ZnO nanorods sensor with selective iron ionophore has a life time greater than one month and has shown insignificant interference with other ions usually present in the human blood serum. The proposed sensor was used as an indicator electrode for potentiometric titration.

Influence of silver dopant on the morphology and ultraviolet emission in aligned ZnO nanostructures

AbstractSilver‐doped ZnO nanostructures were prepared by a chemical vapor deposition (CVD) method. Compared with pure ZnO, the doped samples have a lower crystallinity and a shift of the (002) diffraction peak to lower angle. X‐ray photoelectron spectroscopy (XPS) results testify the incorporation of silver (Ag) into the ZnO nanostructures with the ratio of Ag/Zn = 1.22%. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) micrographs show that the introduction of Ag leads to morphological changes, and that the prepared nanostructures are single crystalline and grow along the (0002) direction. The detailed growth mechanism of the nanostructures is proposed and discussed. The room‐temperature photoluminescence (PL) spectra indicate that the introduction of Ag can improve the intensity of ultraviolet (UV) emission, suggesting the great application prospect in UV optoelectronic devices. The temperature‐dependent PL spectra reveal that the nature of UV emission in Ag‐doped ZnO nanostructures should be related with the free‐exciton (FX) longitudinal optical (LO) phonon replicas, and the enhanced UV emission could be attributed to the increasing concentration of excitons caused by Ag doping.

Enhancement of zinc interstitials in ZnO nanotubes grown on glass substrate by the hydrothermal method

In this study, high density well aligned ZnO nanotubes were grown on glass via a two-step growth-then-etching by simple and template-free hydrothermal method. We used etching procedure to introduce additional zinc interstitial defects in the ZnO nanotubes. The optical properties of the ZnO nanotubes have been investigated by depth-resolved cathodluminescence spectroscopy (DRCLS) which provides information about the physical origin and growth dependence of optically active defects together with their spatial distribution. The DRCLS study gives clear evidence about the enhancement of zinc interstitial defects which are responsible for the violet and decrease of the DL emission in ZnO nanotubes when compared to the as grown ZnO nanorods. We observed a variation in the zinc interstitials along the nanotube depth.

Growth of aligned ZnO nanorods on transparent electrodes by hybrid methods

The fabrication of ZnO (80 nm) thin film was achieved by hybrid atomic layer deposition (ALD) to prevent the reaction between the reactants and conductive layer of the substrates. ZnO nanorods (ZnO-NRs) growth over the substrates was performed by wet chemical procedure in which Zn(NO3)2 and hexamethylenetetramine were used as the precursors. HR-TEM, SAED, FE-SEM, X-ray diffraction (XRD), and UV–Vis spectroscopy were employed to characterize the ZnO-NRs samples on the substrates. XRD and HR-TEM analyses confirmed that the ZnO nanorod structure is hexagonal wurtzite type with growth in the [0001] direction. Length and thickness of the ZnO-NRs ranged between 45 and 90 nm and 480 and 600 nm, respectively. It was observed that the growth rate of NRs in [0001] direction is 10 times higher than in [1000] direction. The growth mechanism and resulted dimensions of nanorods are function of the synthesis parameters (in hybrid ALD process) such as reaction time, temperature, precursor molar ratio, and thickness of ZnO film.

Light confinement-induced antireflection of ZnO nanocones

The antireflective features of aperiodic vertical aligned ZnO nanocones on Si wafer were studied both experimentally and theoretically through comparison with planar ZnO films on Si substrates and bare Si substrates. The measured diffuse reflectance spectra show that the nanocone-based texture reduces the light reflection in a broad spectral range and is much more effective than the planar textures. The numerical simulations exhibit a good agreement with the experimental data and suggest that the light confinement inside nanocones by controlling the diameters can bring further improvement of light absorption into Si.

Photoluminescence study of aligned ZnO nanorods grown using chemical bath deposition

The photoluminescence study of self-assembled ZnO nanorods grown on a pre-treated Si substrate by a simple chemical bath deposition method at a temperature of 80°C is hereby reported. By annealing in O2 environment the UV emission is enhanced with diminishing deep level emission suggesting that most of the deep level emission is due to oxygen vacancies. The photoluminescence was investigated from 10K to room temperature. The low temperature photoluminescence spectrum is dominated by donor-bound exciton. The activation energy and binding energy of shallow donors giving rise to bound exciton emission were calculated to be around 13.2meV, 46meV, respectively. Depending on these energy values and nature of growth environment, hydrogen is suggested to be the possible contaminating element acting as a donor.

Resonant exciton-phonon coupling in ZnO nanorods at room temperature

Vibronic and optoelectronic properties, along with detailed studies of exciton-phonon coupling at room temperature (RT) for random and aligned ZnO nanorods are reported. Excitation energy dependent Raman studies are performed for detailed analysis of multi-phonon processes in the nanorods. We report here the origin of coupling between free exciton and its associated phonon replicas, including its higher order modes, in the photoluminescence spectra at RT. Resonance of excitonic electron and resonating first order zone center LO phonon, invoked strongly by Frolich interaction, are made responsible for the observed phenomenon.

Carbon-assisted nucleation and vertical growth of high-quality ZnO nanowire arrays

We developed a carbon-assisted physical-vapor-deposition method for the growth of highly aligned ZnO nanowire arrays on any flat substrates in large area. Amorphous carbon (a-C) films acted as the preferential nucleation sites to facilitate the growth of high-quality ZnO nanowire array patterns. The ultrathin a-C films can effectively retard the inclined growth of ZnO nanowires at the edge of the a-C patterns. The investigations of the nanowire structures, photoluminescence and electrical transport properties have shown that the ZnO nanowires were well crystallized and the formation of defects in the nanowires was largely suppressed.

Light-emitting diodes composed of n-ZnO and p-Si nanowires constructed on plastic substrates by dielectrophoresis

In this study, light-emitting diodes (LEDs) composed of n-ZnO and p-Si nanowires (NWs) are constructed on flexible plastic substrates and their electroluminescence (EL) emission is characterized and analyzed in the flat and bent states. For each of the LEDs, ZnO NWs are aligned between the electrodes and crossed with a single Si NW by means of a dielectrophoresis (DEP). The number of aligned ZnO NWs determines the current levels and EL intensities of the LEDs. The EL spectra taken from the LEDs exhibit two types of emission, strong UV peaks at 385 nm and weak broad bands centered at 510 nm. The bending test of a single ZnO NW LED under a strain of 0.65% in the upwardly or downwardly bent states reveals that the turn-on voltage remains unchanged at 3 V and that the EL emission characteristics are stable. In addition, the performance of the LED is recovered when the device is returned to the flat state again after undergoing bending test.

Influence of Sb on a controlled-growth of aligned ZnO nanowires in nanoparticle-assisted pulsed-laser deposition

Single-crystalline ZnO nanowires on a sapphire substrate have been synthesized by a nanoparticle-assisted pulsed-laser deposition (NAPLD) using a pure and Sb2O3 doped ZnO target. Low density and vertically well-aligned ZnO nanowires were grown on hexagonal cone-shape ZnO cores by introduction of a ZnO buffer layer. More than 90% of the ZnO cores of the Sb-induced ZnO nanowires are formed in the same size of 400 nm. The ZnO nanowires consist of single-crystalline wurtzite ZnO crystal and grow along [0001] direction. The room-temperature photoluminescence spectrum exhibited a strong ultraviolet emission at around 380 nm and a relatively low broad band emission in the visible region, indicating a low concentration of structural defect in the nanowires. Sb can be used as one of the effective additives to control the morphology and alignment of ZnO nanowires synthesized by NAPLD.

Surfactant mediated growth of ZnO nanostructures and their dye sensitized solar cell properties

Single crystalline and highly aligned ZnO nanorods, faceted microrods, nanoneedles and nanotowers were grown onto glass substrates by a facile aqueous chemical method at relatively low temperature (90 °C). Various structure directing agents or organic surfactants such as diaminopropane (DAP), polyacrylic acid (PAA) and polyethylenimine (PEI) were used to modify the surface morphology. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and optical absorption. It was found that, vertically aligned ZnO nanorods formation takes place with preferential orientation along (002) plane. The organic surfactants play an important role in modifying the morphology. The samples were further used to fabricate dye sensitized solar cells. The highest photocurrent (670 μA) and efficiency were observed for the ZnO:PEI sample.

Aligned Growth of Zinc Oxide Nanorods on Catalyst-Seeded Si Substrate by Aqueous-Solution Immersion Method

We report an optimisation of reaction conditions leading to successful growth of aligned ZnO nanorods by a simple aqueous immersion method. Si substrates were immersed in aqueous Zn2+ solution at various temperatures ranging between 50 - 90°C and at various immersion times. Degree of crystallisation, thermal decomposition, surface morphology and crystallite size were analysed by FT-IR, TGA, SEM and XRD. Seeded noble metals like gold, platinum and nickel on Si substrate were found to improve the vertical alignment of rods’ growth and reduce the diameter of ZnO nanorods.

Growth of Aligned Hexagonal ZnO Nanorods on FTO Substrate for Dye-Sensitized Solar Cells (DSSCs) Application

This article reports a facile growth of well-crystalline aligned hexagonal ZnO nanorods on fluorine-doped tin-oxide (FTO) substrate via non-catalytic thermal evaporation process. The morphological investigations done by field-emission scanning electron microscope (FESEM) and transmission electron microscopy (TEM) reveal that the grown products are aligned hexagonal ZnO nanorods which are grown in a very high density over the whole substrate surface. The detailed structural properties observed by high-resolution TEM equipped with selected area electron diffraction (SAED) and X-ray diffraction (XRD) pattern confirmed that the synthesized nanorods are well-crystalline possessing wurtzite hexagonal phase and preferentially grown along the c-axis direction. A sharp and strong UV emission at 381 nm in room-temperature photoluminescence (PL) spectrum showed that the as-grown ZnO nanorods possess excellent optical properties. The as-grown nanorods were used as photo-anode for the fabrication of dye-sensitized solar cells (DSSCs) which exhibits an overall light-to-electricity conversion efficiency (ECE) of 0.7% with V(oc) of 0.571 V, J(sc) of 2.02 mA/cm2 and FF of 0.58.

ZnO nanocoral reef grown on porous silicon substrates without catalyst

Porous silicon (PS) technology is utilized to grow coral reef-like ZnO nanostructures on the surface of Si substrates with rough morphology. Flower-like aligned ZnO nanorods are also fabricated directly onto the silicon substrates through zinc powder evaporation using a simple thermal evaporation method without a catalyst for comparison. The characteristics of these nanostructures are investigated using field-emission scanning electron microscopy, grazing-angle X-ray diffraction (XRD), and photoluminescence (PL) measurements of structures grown on both Si and porous Si substrates. The texture coefficient obtained from the XRD spectra indicates that the coral reef-like nanostructures are highly oriented on the porous silicon substrate with decreasing nanorods length and diameter from 800–900 nm to 3.5–5.5 μm and from 217–229 nm to 0.6–0.7 μm, respectively. The PL spectra show that for ZnO nanocoral reefs the UV emission shifts slightly towards lower frequency and the intensity increase with the improvement of ZnO crystallization. This non-catalyst growth technique on the rough surface of substrates may have potential applications in the fabrication of nanoelectronic and nanooptical devices.

A template-free sol–gel technique for controlled growth of ZnO nanorod arrays

The growth of ZnO nanorod arrays via a template-free sol–gel process was investigated. The nanorod is single-crystalline wurtzite structure with [0 0 0 1] growth direction determined by the transmission electron microscope. The aligned ZnO arrays were obtained directly on the glass substrates by adjusting the temperatures and the withdrawal speeds, without seed-layer or template assistant. A thicker oriented ZnO nanorod arrays was obtained at proper experimental conditions by adding dip-coating layers. Room temperature photoluminescence spectrum exhibits an intensive UV emission with a weak broad green emission as well as a blue double-peak emission located at 451 and 468 nm, respectively. Further investigation results show that the difference in the alignment of nanorods ascribes to the different orientations of the nanoparticles-packed film formed prior to nanorods on the substrate. Well ordered ZnO nanorods are formed from this film with good c-axis orientation. Our study is expected to pave a way for direct growth of oriented nanorods by low-cost solution approaches.