ZnO Nanowire Arrays Research Articles

Controllable hydrothermal synthesis of bundled ZnO nanowires using cerium acetate hydrate precursors

ZnO nanowire arrays were grown by the hydrothermal decomposition of zinc acetate dehydrate in precursor solutions containing cerium acetate hydrate. The effects of the Ce precursor on the hydrothermal synthesis of ZnO nanowires were investigated as a function of the Ce concentration. It was found that all the samples exhibited preferential orientation toward the c-plane and the Ce precursor had no effect on the lattice constant in the ZnO nanowires. ZnO nanowires grown in solutions containing 1 and 2 at% Ce precursors were bent and bundled, suggesting that solutions containing adequate amounts of Ce precursors enable the top surfaces of the vertically aligned ZnO nanowires to be either positively or negatively charged, and that electrostatic interactions among these local charges causes bending and bundling of the ZnO nanowires. Thus, Ce precursors in the solutions can control the morphology of the ZnO nanowires, causing them to be bent and bundled. We suggest that the bundled ZnO is applicable to Dye-sensitized solar cells because it can be enhanced adsorbed dye amounts in devices.

N-ZnO/p-Si heterojunction nanodiodes based sensor for monitoring UV radiation

ZnO nanowire array on p-type Si (100) substrate based PN-heterojunction photodiode type UV radiation detector was fabricated using rf-magnetron sputtering. The detector exhibited visible blindness below the band gap of the material (3.2 eV) and ultra-fast response for ultraviolet radiation with a peak responsivity at around 380 nm. The temporal photocurrent response of the detector was measured to be 0.1 s which is much better than a commercially available UV-detector. The design, construction and working of the detector are discussed here in detail. The origin of selective and fast response has also been discussed with support of existing theoretical models. Accordingly, electron hole pairs created by the UV radiation at the p-type Si and n-type ZnO nanowire junctions and separated by junction barrier get immediately recombined to enhance the photoconductivity due to applied electric field. The barrier potential and the junction capacitor could be tuned and depend on the size of the nanowires and the band bending on surfaces of nanowires due to heterojunction. The steady state response of the detector studied by keeping it in a reverse biased condition and exposing to UV radiation shows very high stability of the device (around 4% decrease in the photocurrent in over five weeks for −3 V biasing and ∼50 μW/cm2 UV power). Other potential applications for this UV detector are in the field of medicine and in spectroscopy where UV sources are being used. It could also be used to take protective measures for eye damages and skin cancer by monitoring UV radiation levels.

A simple and transparent well-aligned ZnO nanowire array ultraviolet photodetector with high responsivity

Well-aligned zinc oxide (ZnO) nanowire arrays were grown on an interdigital patterned fluorine tin oxide (FTO)-coated glass substrate by a facile chemical bath deposition at low temperature. Morphology, crystalline structure, and optical properties of the ZnO nanowire arrays were analyzed in detail. The results revealed that the ZnO nanowires had wurtzite structure, typically ∼40–60 nm in diameter, and ∼700–800 nm in length, a great number of highly uniform and dense nanowires grew vertically on the substrate to form the well-aligned ZnO nanowire arrays, which had very high optical transmission (>86%) in the visible light region. In addition, the performance of ZnO nanowire arrays ultraviolet (UV) photodetector was systematically examined. The photosensitivity (S), responsivity (R), response and decay time of the photodetector were 703 at +0.2 V, 113 A/W at +5 V, 23 s and 73 s respectively. Also, the photoresponse mechanism of the UV photodetector was illuminated in terms of the oxygen adsorption-photodesorption process.

Structural, optical and gas sensing properties of vertically well-aligned ZnO nanowires grown on graphene/Si substrate by thermal evaporation method

We report catalyst-free vertical growth of high-density ZnO nanowire (NW) arrays on Si substrate with a graphene buffer layer by thermal evaporation method. Structural, surface morphology, chemical composition and optical properties of ZnO NWs were studied by Field emission scanning electron microscopy (FE-SEM), High-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), photoluminescence (PL) and Raman spectroscopy. The results indicate that the grown ZnO NWs are single-crystalline and exhibit the hexagonal wurtzite crystal structure with a preferred orientation along the [0001] direction; all of the ZnO NWs vertically grow on the graphene/Si substrate. The majority of the as-grown NWs have diameters in the range of 250–300 nm and length up to several tens of micrometers, whereas a small portion of NWs have a diameter of approximately of 9–12 nm. Room temperature PL spectrum of the ZnO NWs exhibits a sharp and strong ultraviolet emission at 380 nm and a weak visible emission at around 516 nm. The role of graphene buffer layer in the vertical-aligned ZnO NW arrays growth and its improved optical properties is demonstrated. Importantly, the gas sensor based on these ZnO NWs exhibits high sensitivity and rapid response/recovery characteristics to NO2 gas at 200 °C, and can detect NO2 concentration as low as 2, 6 and 10 ppm. The excellent sensing performance is mainly due to the combination of the large specific surface area and the presence of oxygen-vacancies at the surface of ZnO NWs, together with the creation of p-n heterojunctions between n-type ZnO NW and p-type graphene. Such a ZnO NW/graphene structure is very promising for various applications, such as photonic, optoelectronic and sensor devices. In addition, the possible growth mechanism and the NO2 sensing gas mechanism of the ZnO NWs-graphene structure have been discussed.

A Novel Nanowire Assembly Process for the Fabrication of CO Sensor.

Nanowires have been widely studied due to their outstanding mechanical and electrical properties; however, their practical applications are limited to the lack of an effective technique for controlled assembly. In the present work, zinc oxide (ZnO) nanowire arrays were assembled via a combing process using a makeup brush and the nanodevice was fabricated. The current–voltage (I–V) and ultraviolet (UV) characteristics of the device indicate stable and repeatable electrical properties. The carbon monoxide (CO) sensing properties were tested at operating temperatures of 200, 300 and 400 °C. It was found that ZnO based sensor exhibited the highest sensitivity to CO at 300 °C due to the change of dominant oxygen species. Comparing with others result, the sensitivity of the fabricated sensor exhibits higher sensing performance. The sensing mechanism of the CO sensor is also discussed.

Mapping Strain/Pressure with ZnO Nanowire Arrays By Piezotronic and Piezo-Phototronic Effect

Emulation of human senses via electronic means has long been a grand challenge in research of artificial intelligence as well as prosthetics, and is of pivotal importance for developing intelligently accessible and natural interfaces between human/environment and machine. Unlike other senses (seeing, hearing, smelling and tasting), capability of skin for touch sensing remains stubbornly difficult to be mimicked, which necessitates the development of large-scale pressure sensor arrays with high spatial-resolution, high-sensitivity and fast response. In this talk, we present a novel design of ZnO nanowire arrays, which can be used to directly record the strain distribution by piezotronic and piezo-phototronic effect. First, we have reported large-array three-dimensional (3D) circuitry integration of piezotronic transistors based on vertical zinc oxide nanowires as active taxel-addressable pressure/force-sensor matrix for tactile imaging with a high resolution of 100 μm1. Using the piezoelectric polarization charges created at metal-semiconductor interface under strain to gate/modulate transport process of local charge carriers, piezotronic effect has been applied to design independently addressable two-terminal transistor arrays, which convert mechanical stimuli applied on the devices into local electronic controlling signals. The device matrix has been demonstrated for achieving shape-adaptive high-resolution tactile imaging and self-powered, multi-dimensional active sensing. However, the signal of the piezotronic transistors array is the change of the resistance of each NWs, which can only be measured in a series way. That means such piezotronic transistors array can only mapping a static strain. Different with the electrical signal, the optical signal can be measured in a parallel way. And in our previous work, we have demonstrated how the piezo-phototronic effect can be effectively utilized to enhance the emission intensity of an n-ZnO/p-GaN NW LED. The emission light intensity and injection current at a fixed applied voltage has been enhanced by a factor of 17 and 4 after applying a 0.093% compressive strain, respectively. Here, we extend the single NW device to NW LEDs array, for pressure/force sensor arrays for mapping strain with a resolution as high as 2.7 μm. Such sensors are capable of recording spatial profiles of pressure distribution, and the tactile pixel area density of our device array is 6250000/cm2, which is much higher than the number of tactile sensors in recent reports (~ 6-27/cm2) and mechanoreceptors embedded in the human fingertip skins (~ 240/cm2). When the device is under pressure, the images unambiguously show that the change in LED intensity occurred apparently at the pixels that were being compressed by the molded pattern, while those were off the molded characters showed almost no change in LED intensity. Instead of using the cross-bar electrodes for sequential data output, the pressure image is read out in parallel for all of the pixels at a response and recovery time-resolution of 90 ms. This may be a major step toward digital imaging of mechanical signals by optical means, with potential applications in touch pad technology, personalized signatures, bio-imaging and optical MEMS. Furthermore, our recent studies achieve such piezo-phototronic effect induced strain mapping in a flexible n-ZnO NWs/p-polymer LEDs array system composed of PEDOT:PSS and patterned ZnO NWs with a spatial resolution of 7 μm for mapping of spatial pressure distributions. The emission intensity of the LED array sensor matrix is dominated by locally applied strains as indicated by piezo-phototronic effect. Therefore, spatial pressure distributions are immediately obtained by parallel-reading the illumination intensities of LED arrays based on electroluminescence working mechanism. A wide range of pressure measurements from 40 MPa to 100 MPa was achieved through controlling the growth conditions of ZnO nanowire array. Lastly, the piezo-phototronic effect was achieved on Si wafer based on a n-ZnO nanofilm/p-Si micropillar heterostructure (ZSH) LEDs array. White light emissions at room temperature, featuring with emission peaks in both visible and near-infrared regions were obtained. By applying a strain onto the top of the ZSH LEDs, the light emission intensity of ZSH LEDs array was enhanced as well by 120% under -0.05% compressive strains. A pressure map can be created by reading out in parallel the change of the electroluminescent intensities from all the pixels in the near future. This research not only introduce a novel approach to fabricate Si-based light-emitting components with high performances, but also may be a great step toward digital imaging of mechanical signals using optical means, having potential applications in artificial skin, touch pad technology, personalized signatures, bio-imaging and optical MEMS, and even and smart skin.

Growth C-Axis Zno Nanowires By Upturned Crystalline Growth Method on P-Si(111) Substrate

The using method a low temperature and low cost growth method of high quality active materials for optoelectronic devices. ZnO nanowire arrays growth on p-Si(111). The effects of thermal annealing on the optical properties of ZnO nanowires were prepared on sol-gel ZnO-seed-coated substrates. Atomic Force Microscopy (AFM) AFM and... AFM images were found at 130 °C well aligned vertically, and the well defined crystallographic planes, providing a strong evidence that the nanowire arrays orientate along the c-axis. The annealing temperature of the ZnO thin film plays an important role on the microstructure of the ZnO grains and then the growth of the ZnO nanowire arrays.From PL spectra, an evident ultraviolet near-band edge emission peak at 382 nm is observed. From (I-V) characteristic that the material behaves p-n junction diode, ideality factors >> 2.0, that was attributed to tunneling via deep levels in the forbidden gap. Impedancespectra shows the spectrum of the Impedance resistance that the curve does not represent a regular semicircle and this indicates that the structure of the material is not regulated granules but rather is in a different form which is the nanowires.

Temperature-dependent photoluminescence analysis of ZnO nanowire array annealed in air

ZnO nanowire arrays (NWAs) were prepared on transparent conducting fluorine doped tin oxide (FTO) substrates through a facile hydrothermal method, followed by a 500 °C annealing to improve their crystalline qualities and photoelectrochemical (PEC) activities. It was found that the annealing didn't change the morphology, but resulted in a significant reduction of the donor concentration. Temperature-dependent photoluminescence (PL) was carried out for a comprehensive analysis of the effect from annealing. Noteworthy, four dominant peaks were identified from the 10 K spectrum of a 500 °C annealed sample, and they were assigned to FX, D0X, (e, D0) and (e, D0) -1LO, respectively. Of them, the FX emission was only existed below 130 K, while the room-temperature (RT) PL spectrum was dominated by the D0X emission.

CATALYST–FREE GROWTH OF WELL–ALIGNED ZnO NANOWIRES ON GRAPHENE/Si SUBSTRATEBY THERMAL EVAPORATION

Vertically well–aligned ZnO nanowire (NW) arrays with high density were directly synthesized on graphene/Si substrate by thermal evaporation of zinc powder without catalysts or additives. The ZnO NWs were characterized by field emission scanning electron microscopy (FE–SEM), high resolution transmission electron microscopy (HRTEM), X–ray diffraction (XRD), photoluminescence (PL), and Raman spectroscopy. The results showed that the obtained ZnO NWs have diameters in the range of 300–350 nm with lengths of several tens micrometers. The prepared ZnO NWs are of a single crystal, which have a hexagonal wurtzite crystal structure with c–axis (002) orientation growth perpendicular to the substrate surface. The NW arrays had a good crystal quality with excellent optical properties, indicating a sharp and strong ultraviolet emission at 380 nm, and a weak visible emission at around 516 nm.

Evidence of a strong electron–hole separation effect in ZnO@TiO2 core/shell nanowires

In this work, a strong electron-hole separation effect in ZnO@TiO2 core/shell nanowire arrays was investigated. On the surface of well-aligned ZnO nanowires, a high-quality TiO2 shell was grown in a range of thicknesses through the deposition of TiO2 nanoparticles on ZnO nanowire arrays by pulsed laser deposition (PLD). In the photoluminescence (PL) spectra of the ZnO@TiO2 core/shell nanowires, a strong and broad visible emission band from 450 to 650 nm appears, which is related to surface defects; meanwhile, the ultraviolet (UV) emission due to exciton recombination simultaneously disappears. These changes in the PL spectra originate from a highly efficient electron-hole separation effect. This work presents evidence of this strong electron-hole separation effect in these ZnO@TiO2 core/shell nanowire arrays, which makes them promising candidates for use in optoelectronic devices for dye-sensitized solar cells and ultraviolet photodetectors.

Highly stable field emission from ZnO nanowire field emitters controlled by an amorphous indium–gallium–zinc-oxide thin film transistor

Lowering the driving voltage and improving the stability of nanowire field emitters are essential for them to be applied in devices. In this study the characteristics of zinc oxide (ZnO) nanowire field emitter arrays (FEAs) controlled by an amorphous indium–gallium–zinc-oxide thin film transistor (a-IGZO TFT) were studied. A low driving voltage along with stabilization of the field emission current were achieved. Modulation of field emission currents up to three orders of magnitude was achieved at a gate voltage of 0–32 V for a constant anode voltage. Additionally, a-IGZO TFT control can dramatically reduce the emission current fluctuation (i.e., from 46.11 to 1.79% at an emission current of ∼3.7 µA). Both the a-IGZO TFT and ZnO nanowire FEAs were prepared on glass substrates in our research, demonstrating the feasibility of realizing large area a-IGZO TFT-controlled ZnO nanowire FEAs.

Earth abundant ZnO/CdS/CuSbS2 core-shell nanowire arrays as highly efficient photoanode for hydrogen evolution

CuSbS2 is regarded as a promising photo-absorber for solar energy conversion due to its proper bandgap, high absorptivity and earth abundance. Herein, novel low-cost ZnO/CdS/CuSbS2 core-shell nanowire arrays were constructed as photoanode for hydrogen evolution through non-aqueous electrodeposition and cation exchange. The nanowire demonstrates even and compact multilayer structure. The optimal ZnO/CdS/CuSbS2 photoanode achieves a photocurrent of 6.48 mA/cm2 at 0 V versus Ag/AgCl and the remarkable IPCE value with approximately 52% at 480 nm in an electrolyte solution containing 0.35 mol/L Na2SO3 and 0.25 mol/L Na2S. It also exhibited a good stability, maintained 87.9% of the initial current after 1 h measurement. The high performance benefits from well-crystalline and compact multilayer structure, high absorptivity of CuSbS2 and CdS, p-n junction formed between CdS and CuSbS2 which promotes the electron-hole separation and ZnO nanowire array as three dimensional scaffolds for electron percolation pathway. This work suggests the potential applications of low-cost p-n junction core-shell nanowire arrays as a highly efficient photoanode for hydrogen evolution in oil-field waste water with reductive sulfide.

CdTe quantum-dot-modified ZnO nanowire heterostructure

The effect of CdTe quantum-dot (QD) decoration on the photoluminescence (PL) behaviour of ZnO nanowire (NW) array is presented in the present work. Highly crystalline and vertically 40–50 nm diameter range and 1 µm in length aligned ZnO NWs are synthesized using low-cost method. The crystallinity and morphology of the NWs are studied by scanning electron microscopy and X-ray powder diffraction methods.Optical properties of the nanowires are studied using photo-response and PL spectroscopy. CdTe QDs are successfully synthesized on ZnO nanowire surface by dip-coating method. ZnO NWs are sensitized with CdTe QDs characterized by transmission electron microscopy, energy-dispersive X-ray spectroscopy, and PL spectroscopy. The highly quenched PL intensity indicates the charge transfer at interface between CdTe QDs and ZnO NWs and is due to the formation of type-II heterostructure between QDs and NWs. Photo-response behaviour of heterostructure of the film is also been incorporated in the present work.

Influence of the morphology of ZnO nanowires on the photoelectrochemical water splitting efficiency

Defect-free ZnO nanowire arrays were synthesized and assessed as photoanodes in photoelectrochemical cells. Several tens of samples classified into five different average diameters in the range 40–260 nm were prepared to explore the role of morphology and polar surfaces. The photoelectrochemical performance of the NWs was studied in basic aqueous electrolytes (pH 12.7). A non-monotonic behavior of the performance was demonstrated, which maximizes for nanowires with diameter ∼120 nm. The maximum applied bias photoconversion efficiency is ∼6.3% upon irradiation with 11.5 mW at 365 nm. The photoanodes exhibit a rather stable performance for ∼10 h, while they stabilize at ∼60% of the initial photocurrent after 20 h of continuous bias illumination. The degradation of their performance was attributed to partial detachment of the NWs from the supporting conductive film. The enhanced stability is attributed to the decrease of the pH at the electrochemical interface to values that inhibit the dissolution of ZnO.

Core–shell solar cell fabrication using heterostructure of ZnO-nanowires arrays decorated with sputtered CdTe-nanoparticles

Core–shell heterostructures of ZnO-NWs/CdTe-NPs were fabricated through covering ZnO-NWs arrays using CdTe-NPs and the room temperature RF magnetron sputtering method. The influence of different CdTe-NPs deposition time (5, 20, 40 and 60 min) on the physical properties of core–shell heterostructures were investigated. In order to achieve the highest coverage level and a wide range of optical absorption at a visible range for a ZnO-NWs/CdTe-NPs (60 min) array, FTO/ZnO-NWs/CdTe-NPs (60 min)/Ni/Au core–shell solar cells were used. Solar cell fabrication was performed by soaking the samples in a saturated CdCl2 solution in methanol and a post-annealing treatment at 400 °C for 1 h in air which led to grain growth, the passivation of deep level defects, and the decrease of stacking faults. Short-circuit current and power conversion efficiency of the fabricated cell under illumination with visible light AM1.5 (100 mW cm−2) were 13.3 mA cm−2 and 3.41%, respectively. It was found that introducing a thin interfacial layer of CdSe to the configuration (FTO/ZnO-NWs/CdSe (10 nm)/CdTe-NPs (60 min)/Ni/Au) led to a 5.58% enhancement of photovoltaic performance of the solar cell (20.9 mA cm−2), which is 63.6% more than that of the same configuration without CdSe.

Nanotransducers on printed circuit boards by rational design of high-density, long, thin and untapered ZnO nanowires

Nanotransducers can offer crucial advantages in comparison with conventional sensors and actuators. However, interfacing and packaging nanostructures into complete electronic systems is very complex. Here we describe a wet chemical method for cointegrating arrays of ZnO nanowires into systems on printed circuit boards (PCBs). First, we deposit on the PCB a MnOOH layer for reproducibly increasing the nanowires density. Afterwards, we numerically demonstrate that the ligand ethylenediamine, at the isoelectric point of the ZnO nanowires tips, can effectively control, at very low concentrations, both zinc speciation and supersaturation in the nutrient solution. Accordingly, we combine potassium chloride and ethylenediamine, produced in situ from a safer precursor (ethanolamine), for concurrently thinning the nanowires (top width around 60nm) and stabilizing their top faces. Our synergic approach enables the solution growth of ZnO nanowires which are untapered and have high densities (> 8/μm2) and record length (> 15µm) and aspect ratio (> 200) for plastic substrates which may not withstand high temperatures. These characteristics permit to simultaneously add the top electrode, package the nanowires, improve their mechanical robustness and connect to the electronic interface by a simple flip chip adhesive bonding procedure. As proofs of concept we describe flexible PCB heaters (electrical to thermal energy conversion) with power densities comparable with state of the art flexible heaters and a wearable piezotronic heartbeat detector comprising both ZnO nanowires (mechanical to electric energy conversion) and electronics on a single flexible PCB. Our results open the way to the cointegration of nanotransducers and electronics on conventional PCBs.

Al doping influences on fabricating ZnO nanowire arrays: Enhanced field emission property

Increase of free electrons concentration and decrease of defects density are the most desirable solution for stimulating the field emission property in semiconductor nanostructures. To implement this, herein we study the field emission efficiency of Al-doped ZnO nanowire arrays with different Al doping concentration, which were prepared by a simple facile hydrothermal method. The Al doping concentration plays a very important role on the structure, morphology, photoluminescence and field emission properties of the as-assembled samples. The results indicate that Al-doped ZnO nanowire arrays with Al doping concentration of 7at% exhibit the highest quality crystalline structure and lowest defect density relative to others samples thanks to the suitable modification of Al doping, and this led to the increase of free electrons concentration and decrease of defects density, which have an excellent field emission performance with the lower turn on field of 1.03V/μm and higher field enhancement factor of 20658. This remarkable field emission performances of the Al-doped ZnO nanowire arrays may provide promising applications for different field emission devices.

Enhancement of the Sub-Band-Gap Photoconductivity in ZnO Nanowires through Surface Functionalization with Carbon Nanodots

We report on the surface functionalization of ZnO nanowire (NW) arrays by attachment of carbon nanodots (C-dots) stabilized by polyethylenimine. The photoconductive properties of the ZnO NWs/C-dots devices were investigated under photoexcitation with photon energies below and above the ZnO band gap. The results indicate an increased photoresponse of the functionalized devices in the visible spectral range, as well as enhanced UV photoconductivity. This is attributed to the fast injection of photoexcited electrons from the C-dots into the conduction band of the ZnO NWs, and the subsequent slower desorption of molecular species from the NW surface, which reduces the surface depletion region in the NWs. The surface functionalization of the ZnO NWs with carbon nanodots also impacts the dynamics of the photocurrent decay, inducing a slower relaxation of the photogenerated charge carriers.

ZnO nanowire arrays decorated with PtO nanowires for efficient solar water splitting

Vertically-oriented PtO nanowires have been selectively grown on ZnO nanowire arrays for the first time by a light-controlled method and were utilized as highly efficient OER cocatalysts for remarkably enhancing the PEC performance of water splitting.

Enhanced field emission of ZnO nanowire arrays by the control of their structures

Four kinds of ZnO nanostructrues, i.e. nanorods, nanopencils, nanotowers, and nanoneedles, were successfully synthesized by chemical vapor deposition method, and their field emission properties were comparatively investigated. The results showed that field emission performance was strongly affected by nanowire morphology, and the nanotower-like ZnO exhibited the lowest turn-on field of 4.36 V/μm. The classical F-N theory was employed to analyze the enhancement mechanism. It was found that only the optimized combination of field enhancement factors and geometrical factors could assure excellent field emission performance.