ZnO Nanowire Arrays Research Articles

Unravelling Resistive Switching Mechanism in ZnO NW Arrays: The Role of the Polycrystalline Base Layer

The physical mechanism involved in resistive switching phenomena occurring in devices based on ZnO nanowire (NW) arrays may vary considerably, also depending on the structure of the switching layer. In particular, it is shown here that the formation of a ZnO base layer between the metallic catalyst substrate and the NW, which is typical of CVD-grown ZnO NW arrays, should not be neglected when explaining the switching physical mechanism. The structural and electrical properties of this layer are investigated after the mechanical removal of NWs. Electrical measurements were performed in the presence of NWs and, after their removal, showed that the base alone exhibits resistive switching properties. The proposed switching mechanism is based on the creation/rupture of an oxygen vacancies conductive path along grain boundaries of the polycrystalline base. The creation of the filament is facilitated by the high concentration of vacancies at the grain boundaries that are oriented perpendicularly to the electrode...

Fabrication of a P3HT-ZnO Nanowires Gas Sensor Detecting Ammonia Gas.

In this study, an organic-inorganic semiconductor gas sensor was fabricated to detect ammonia gas. An inorganic semiconductor was a zinc oxide (ZnO) nanowire array produced by atomic layer deposition (ALD) while an organic material was a p-type semiconductor, poly(3-hexylthiophene) (P3HT). P3HT was suitable for the gas sensing application due to its high hole mobility, good stability, and good electrical conductivity. In this work, P3HT was coated on the zinc oxide nanowires by the spin coating to form an organic-inorganic heterogeneous interface of the gas sensor for detecting ammonia gas. The thicknesses of the P3HT were around 462 nm, 397 nm, and 277 nm when the speeds of the spin coating were 4000 rpm, 5000 rpm, and 6000 rpm, respectively. The electrical properties and sensing characteristics of the gas sensing device at room temperature were evaluated by Hall effect measurement and the sensitivity of detecting ammonia gas. The results of Hall effect measurement for the P3HT-ZnO nanowires semiconductor with 462 nm P3HT film showed that the carrier concentration and the mobility were 2.7 × 1019 cm−3 and 24.7 cm2∙V−1∙s−1 respectively. The gas sensing device prepared by the P3HT-ZnO nanowires semiconductor had better sensitivity than the device composed of the ZnO film and P3HT film. Additionally, this gas sensing device could reach a maximum sensitivity around 11.58 per ppm.

Atomic Layer Deposition of Nickel on ZnO Nanowire Arrays for High-Performance Supercapacitors.

A novel hybrid core-shell structure of ZnO nanowires (NWs)/Ni as a pseudocapacitor electrode was successfully fabricated by atomic layer deposition of a nickel shell, and its capacitive performance was systemically investigated. Transmission electron microscopy and X-ray photoelectron spectroscopy results indicated that the NiO was formed at the interface between ZnO and Ni where the Ni was oxidized by ZnO during the ALD of the Ni layer. Electrochemical measurement results revealed that the Ti/ZnO NWs/Ni (1500 cycles) electrode with a 30 nm thick Ni-NiO shell layer had the best supercapacitor properties including ultrahigh specific capacitance (∼2440 F g-1), good rate capability (80.5%) under high current charge-discharge conditions, and a relatively better cycling stability (86.7% of the initial value remained after 750 cycles at 10 A g-1). These attractive capacitive behaviors are mainly attributed to the unique core-shell structure and the combined effect of ZnO NW arrays as short charge transfer pathways for ion diffusion and electron transfer as well as conductive Ni serving as channel for the fast electron transport to Ti substrate. This high-performance Ti/ZnO NWs/Ni hybrid structure is expected to be one of a promising electrodes for high-performance supercapacitor applications.

Synthesis of graphene/ZnO nanowire arrays/graphene foam and its application for determination of folic acid

Graphene foam (GF) was synthesized by chemical vapor deposition (CVD) using nickel foam as the template. Then, ZnO nanowire arrays (ZnO NWAs) were grown on the GF by hydrothermal synthesis. Finally, graphene (Gr) was deposited on the ZnO NWAs by CVD to obtain the hybrid of Gr/ZnO NWAs/GF. Due to large specific surface area and outstanding electric conductivity, the hybrid can be used for the determination of folic acid (FA) by cyclic voltammetry and differential pulse voltammetry. The results show that ZnO NWAs are uniformly and vertically grown on the GF and Gr is deposited on the ZnO NWAs. The sensitivity and the measured detection limit of the hybrid for FA in the range of 0–60μM are 0.18μAμM−1 and 1μM, respectively. The hybrid can accurately detect FA in the presence of uric acid, and the hybrid also shows good reproducibility and stability.

Near band edge photoluminescence of ZnO nanowires: Optimization via surface engineering

Zinc oxide (ZnO) nanowire arrays have potential applications for various devices such as ultra-violet light emitting diodes and lasers, where photoluminescence of intense near band edge emission without defect emissions is usually desired. Here, we demonstrate, counter-intuitively, that the near band edge emission may become dominant by introducing certain surface defects to ZnO nanowires via surface engineering. Specifically, near band edge emission (NBE) is effectively enhanced after a low pressure O2 plasma treatment that sputters off surface oxygen species to produce a reduced and oxygen vacancy-rich surface. The effect is attributed to the lowered surface valence band maximum of the reduced ZnO surface that creates an accumulative band bending, which screens the photo-generated minority carriers (holes) from reaching or being trapped by the surface defects.

Photodetectors: Extreme Carrier Depletion and Superlinear Photoconductivity in Ultrathin Parallel‐Aligned ZnO Nanowire Array Photodetectors Fabricated by Infiltration Synthesis (Advanced Optical Materials 24/2017)

Photodetectors: Extreme Carrier Depletion and Superlinear Photoconductivity in Ultrathin Parallel‐Aligned ZnO Nanowire Array Photodetectors Fabricated by Infiltration Synthesis (Advanced Optical Materials 24/2017)

In-situ growth of ZnO nanowire arrays on the sensing electrode via a facile hydrothermal route for high-performance NO2 sensor

ZnO nanowire (ZNW) arrays were in-situ grown on the sensing electrode via a facile hydrothermal route for NO2 sensing application. ZNW arrays were prepared by a seed layer deposition on the surface of the sensing electrode using a dipping process in a Zn(CH3COO)2·2H2O ethanol solution followed by a seed growth using a hydrothermal route in the Zn(NO3)2·6H2O-HMTA (C6H12N4) system. The microstructural characterizations of the ZNW arrays by means of XRD, FESEM, TEM, FTIR and XPS showed that ZnO nanowires with the diameters of 80–90 nm and the lengths of 0.6–1 μm had a single crystal hexagonal wurtzite structure. Gas sensing properties demonstrated the response of the sensor based on the ZNW arrays was linearly proportional to the NO2 concentration in the range of 1–30 ppm with good reproducibility and selectivity. The maximum sensor response to NO2 was obtained at an operating temperature of 250 °C. The response and recovery times reduced rapidly with increasing the operating temperature. The growth mechanism and sensing mechanism of the ZNW arrays were discussed in accordance with the deposition of the seed layer and the modulation of the depletion layer, respectively.

Enhanced absorption and cavity effects of three-photon pumped ZnO nanowires

Semiconductor nanowire (NW) lasers attract a lot of attention as potential elements of nanophotonic circuits and lab-on-a chip devices. Here, we report on the experimental investigation of stimulated near ultraviolet (NUV) emission, pumped by three-photon absorption from near infrared femtosecond laser pulses, from ZnO NW arrays of different morphologies and compare it to the bulk. The spectrally and temporally resolved measurements of the NUV emission show both strong enhancements in the absorption and emission properties of the nanowire arrays compared to bulk samples. Thus, we determine a many times higher three-photon absorption in the nanostructure morphology compared to the bulk material. Furthermore, the threshold pumping intensity for stimulated emission in a vertically oriented nanowire array is twice lower and the emission onset time is shorter than in randomly oriented arrays, revealing strong influence of the macroscopic nanowire arrangement.

Extreme Carrier Depletion and Superlinear Photoconductivity in Ultrathin Parallel‐Aligned ZnO Nanowire Array Photodetectors Fabricated by Infiltration Synthesis

AbstractUltrathin semiconductor nanowires enable high‐performance chemical sensors and photodetectors, but their synthesis and device integration by standard complementary metal‐oxide‐semiconductor (CMOS)‐compatible processes remain persistent challenges. This work demonstrates fully CMOS‐compatible synthesis and integration of parallel‐aligned polycrystalline ZnO nanowire arrays into ultraviolet photodetectors via infiltration synthesis, material hybridization technique derived from atomic layer deposition. The nanowire photodetector features unique, high device performances originating from extreme charge carrier depletion, achieving photoconductive on–off ratios of >6 decades, blindness to visible light, and ultralow dark currents as low as 1 fA, the lowest reported for nanostructure‐based photoconductive photodetectors. Surprisingly, the low dark current is invariant with increasing number of nanowires and the photodetector shows unusual superlinear photoconductivity, observed for the first time in nanowires, leading to increasing detector responsivity and other parameters for higher incident light powers. Temperature‐dependent carrier concentration and mobility reveal the photoelectrochemical‐thermionic emission process at grain boundaries, responsible for the observed unique photodetector performances and superlinear photoconductivity. The results elucidate fundamental processes responsible for photogain in polycrystalline nanostructures, providing useful guidelines for developing nanostructure‐based detectors and sensors. The developed fully CMOS‐compatible nanowire synthesis and device fabrication methods also have potentials for scalable integration of nanowire sensor devices and circuitries.

Facile preparation of branched hierarchical ZnO nanowire arrays with enhanced photocatalytic activity: A photodegradation kinetic model

Branched hierarchical zinc oxide nanowires (BH-ZnO NWs) were fabricated successfully by a facile and rapid synthesis using two-step growth process. Initially, ZnO NWs have been prepared by anodizing zinc foil at room temperature and followed by annealing treatment. Then, the BH- ZnO NWs were grown on the ZnO NWs by a solution based method at very low temperature (31 oC). The BH- ZnO NWs with different aspect ratio were obtained by varying reaction time (0.5, 2, 5, 10 h). Photocatalytic activity of the samples was studied under both UV and visible light. The results indicated that the optimized BH-ZnO NWs (5 h) as a photocatalyst exhibited the highest photoactivity with about 3 times higher than the ZnO NWs under UV light. In addition, it was also determined that photodegradation rate constant (k) for the BH- ZnO NWs surface obeys a linear function with the branch length (l) and their correlation was described by using a proposed kinetic model.

Controlled facile fabrication of plasmonic enhanced Au-decorated ZnO nanowire arrays dye-sensitized solar cells

Facile controlled formation of well-dispersed Au-nanoparticles (Au-NPs) decorated on high aspect ratio ZnO nanowires (NWs) was demonstrated. Au-NPs decorated ZnO NWs obtained exhibited a significant enhancement of 75% in power conversion efficiency (PCE) of dye-sensitized solar cells (DSSCs) performance. Superior PCE of 1.01% achieved was attributed to the surface plasmon resonance (SPR) effect of Au-NPs near 520nm which promoted improved dye light absorption resulting in more electrons generation as indicated in the significant increment of the short circuit current density, JSC. SPR phenomenon was further supported by a distinctive extinction spectra obtained.

Optimization of electrolyte to significantly improve photoelectrochemical water splitting performance of ZnO nanowire arrays

In this work, a simple and facile strategy has been applied to improve the photoelectrochemical (PEC) water splitting performance of ZnO nanowire arrays by the optimization of Na2SO4 and Na2SO3 electrolytes. The ZnO nanowire array electrode exhibits significant PEC water splitting enhancement in Na2SO3 electrolyte solution compared to in Na2SO4 electrolyte solution. The photocurrent density of ZnO nanowire array photoanode in Na2SO3 electrolyte solution is 6 times higher than that in Na2SO4 electrolyte solution at 0.5 V vs. Ag/AgCl. Mott-Schottky (MS) and electrochemical impedance spectroscopy (EIS) studies of ZnO nanowire array electrodes in Na2SO4 and Na2SO3 electrolyte solutions reveal that SO32− anions consume the holes and efficiently improve the separation of photogenerated electron-hole pairs, leading to significant improvement for PEC water splitting performance in Na2SO3 electrolyte solution. This work provides a promising strategy to improve the PEC water splitting performance of photoanode materials by the optimization of electrolytes.

High aspect-ratio semiconducting ZnO nanowires formed by anodic oxidation of Zn foil and thermal treatment

Arrays of zinc oxide (ZnO) nanowires with diameter of about 200nm and length of >20µm were successfully obtained by simple anodic oxidation of Zn foil in sodium bicarbonate electrolyte and thermal post-treatment. The as formed anodic film consists of overlapping nanowire bundles forming a flower-like or grass-like structure. Thermal annealing in air at temperatures higher than 150°C results in the conversion of initially formed hydroxycarbonate precursor to crystalline wurtzite ZnO. The crystallinity of ZnO nanowires increases with increasing annealing temperature. On the other hand, optical band-gap energy of obtained semiconducting nanowires was found to be independent of the temperature applied during the thermal treatment. It is expected that the described method can be further scaled up and offers a great potential even for technological applications.

Formation of ZnO nanowires during anodic oxidation of zinc in bicarbonate electrolytes

Dense arrays of ZnO nanowires were obtained by one-step anodic oxidation of metallic Zn in bicarbonate electrolytes followed by 2h of annealing in air at 200°C. A detailed inspection of the anode morphology at the initial stages of anodizations was performed. It was confirmed that nanowire bundles are initially formed both inside and outside the pits created during the anodic dissolution of the metallic substrate. In consequence, root-like structures are formed inside the cavities. However, no roots are observed for nanowire bundles formed outside the pits, directly on the surface of Zn. The effect of anodizing conditions on the growth of nanowires was studied. It was found that applying higher anodizing potentials and more concentrated electrolytes result in not only the significantly faster nanowires growth, but also the formation of denser NW films. In addition, the electrolyte agitation can also increase the rate of nanowires formation. Finally, we proved for the first time that the previous usage of electrolyte and the orientation of electrodes, can significantly affect the morphology of as-formed anodic films. Thermal treatment of as-anodized samples resulted in the successful conversion of the carbonate-containing precursor to crystalline wurtzite ZnO without the loss of nanowire morphology.

Surface morphology evolution with fabrication parameters of ZnO nanowires toward emission properties enhancement

ZnO nanowire (NW) arrays were successfully grown on pre-seeded indium Tin oxide (ITO)-coated glass substrate using hydrothermal synthesis. Herein, the effects of ZnO seed layer density on the performance of ZnO NWs were investigated in details. The orientation and the dimension of ZnO NWs were found to depend on seeded substrate density as shown by scanning electron microscopy (SEM) micrographs which revealed that the typical morphology with the most uniform size can be obtained. From the X-ray diffraction (XRD) measurement, it can be seen that hexagonal c -axis oriented NWs were grown. The resonant Raman scattering was also investigated in details. It confirmed the wurtzite structure of the NW arrays and expected for good optical properties. The optical band gaps of synthesized ZnO NWs were found to decrease comparing to bulk ZnO and as function of seed layer. The photoluminescence (PL) spectra at room temperature have shown strong UV excitonic emission and weak deep-level emissions which reveal that the as-grown NW arrays have good optical properties with limited deep-level defects.

A double-sided radiating flat-panel X-ray source using ZnO nanowire field emitters

A double-sided radiating flat-panel X-ray source device has been developed. The device has a diode configuration composed of an anode target of a tungsten thin film and field-emitter arrays of ZnO nanowires prepared on glass substrates. The X-ray radiation comes from both transmission and reflection of X-rays induced by electron bombardment. When operated at 24 kV with an anode current of 3.16 mA, radiation dose rates of ∼200 mGy/min and ∼210 mGy/min were recorded positioning at 4 cm away from the front and back sides of the vacuum-sealed flat-panel X-ray source, respectively. Clear X-ray images with high contrast were taken by the radiation from both sides of the device. The reported device has great potential in making compact X-ray radiography and irradiation systems.

Enhanced photoresponse performance of self-powered UV–visible photodetectors based on ZnO/Cu2O/electrolyte heterojunctions via graphene incorporation

A photoelectrochemical cell can be used as a self-powered photodetector based on the photovoltaic effect. Here, we report an effective way to enhance the self-powered photoresponse performance of the ZnO nanowire arrays (NAs)/Cu2O photoanodes by introducing graphene interlayers. The self-powered photodetectors based on ZnO NAs/graphene (G)/Cu2O have large responsivities of 21.2 mA/W and 17.1 mA/W for ultraviolet (UV) and visible light, respectively, and fast rise and decay times (0.6 ms). Introduction of the Cu2O film improves the visible light-harvesting ability, and the step-like band alignment between ZnO and Cu2O benefits the separation of photoexcited carriers. Furthermore, the graphene interlayers also contribute to the carrier separation and transfer process because of their excellent charge collection capacity. The results demonstrate the ZnO/G/Cu2O heterojunction can be a promising configuration for self-powered UV–visible photodetecting.

Synthesizing vertical porous ZnO nanowires arrays on Si/ITO substrate for enhanced photocatalysis

In this work, porous ZnO nanowires arrays (NWAs) which are perpendicular to the substrate were synthesized using facile and simple hydrothermal process approaches. The surface of ZnO nanowires (NWs) was increased by chemical electroless etching many macropores on them, which results in better photocatalytic activity, which is 1.73 times in terms of the rate constant for methyl orange (MO) degradation for porous ZnO NWAs compared with ZnO NWAs. The absorption of the vertical porous ZnO NWAs was considerably enhanced in the visible region. The optical band-gap became slightly narrower from 3.24eV (ZnO NWAs) to 3.22eV (porous ZnO NWAs). Thus, the vertical porous ZnO NWAs can be employed for various applications, such as gas sensor, photocatalyst, and solar cell.

Disordered photonics coupled with embedded nano-Au plasmonics inducing efficient photocurrent enhancement

Spatial order used to be considered as a benefit for photonics; but recently the study of disorder has broken into people’s horizons for its strong random scattering of light. In this work, disordered photonics coupled with plasmonics for efficiently enhanced photocurrent was first investigated using Au-ZnO nanowire array as a model. The embedded Au-ZnO nanowire array was facilely prepared using a template-free electrodeposition method. On the optimal plasmonic substrate, the photocurrent of disorder-enhanced Au-ZnO nanowire array is about 20-fold that of ZnO nanowire array. Both the plasmonic effect of Au NPs such as localized surface plasmons, surface plasmon polarizations and the disorder-enhanced photonics in the hybrid structure are available to improve the photoelectric conversion efficiency by enhancing the trapping of the simulated sunlight and the collection of charge carriers. Herein, disordered photonics was coupled with plasmonics to explain for the enhanced photocurrent. This work also provided a facile fabricating avenue for plasmonic noble metal embedded in semiconductor devices.

Enhanced field emission of ZnO nanoneedle arrays via solution etching at room temperature

ZnO nanoneedle arrays (ZnO nns) were synthesized by a facile two-step solution-phase method based on the etching of pre-synthesized ZnO nanowire arrays (ZnO nws) with flat ends at room temperature. Field emission measurement results showed that the turn-on electronic fields of ZnO nns and nws were 2.7 and 5.3Vμm−1 at a current density of 10μAcm−2, and the field enhancement factors were 4939.3 for ZnO nns and 1423.6 for ZnO nws. The enhanced field emission properties in ZnO nns were ascribed to the sharp tip geometry.