Fast UV detection and hydrogen sensing by ZnO nanorod arrays grown on a flexible Kapton tape

ZnO nanorod arrays were grown on a flexible Kapton tape using microwave-assisted chemical bath deposition. High crystalline properties of the produced nanorods were proven by X-ray diffraction patterns and field emission scanning electron microscopy. Additionally, the photoluminescence spectrum showed higher UV peaks compared with visible peaks, which indicates that the ZnO nanorods had high quality and low number of defects. The metal-semiconductor-metal (MSM) configuration was used to fabricate UV and hydrogen gas detectors based on the ZnO nanorods grown on a flexible Kapton tape. Upon exposure to 395 nm UV light, the UV device exhibited fast response and decay times of 37 ms and 44 ms, respectively, at a bias voltage of 30 V. The relative sensitivities of the gas sensor made of the ZnO nanorod arrays, at hydrogen concentration of 2 %, at room temperature, 150 °C and 200 °C, are 0.42, 1.4 and 1.75 respectively.     

Enhanced sensitivity and response speed of graphene oxide/ZnO nanorods photodetector fabricated by introducing graphene oxide in seed layer

Graphene/ZnO nanocomposites photodetectors hold great potential for UV detection because of the combination of photosensitive ZnO and high electron-mobility graphene. In this paper, graphene oxide (GO)/ZnO nanorods photoconductive photodetector with seed layer of GO and ZnO nanocrystals (NCs) hybrids is fabricated via a low-cost solution process. Uniform and oriented GO/ZnO nanorods have been obtained due to the positive role of GO in the growth process of ZnO nanorods, which gives rise to less light scattering and thereby stronger absorption and enhanced photocurrent. When the growth time is 1 h, the optimum photocurrent of GO/ZnO nanorods is about 9.4 times than pure ZnO nanorods, meanwhile, the corresponding detectivity reaches 7.17?×?10¹¹ cm Hz^1/2 W^?1. In addition, owing to the high carrier mobility of graphene, the response time t ₉₀ of GO/ZnO photodetector beneficially decreases to ~1 s, which is much faster than many other GO/ZnO hybrid photodetectors.     

Optoelectronics behaviour of ZnO nanorods for UV detection

In this study ZnO nanorods have been synthesized by a chemical precipitation method. The room temperature UV–Vis absorption spectra of the ZnO nanorods indicated two absorption peaks in the UV region, one in the near UV region and the other attributed to the band gap of ZnO. The Photoluminescence spectra of ZnO nanorods show two emission bands, one ultraviolet emission band at 378 nm and the other in the defect related yellow emission band near 550 nm. The stimulated yellow luminescence of ZnO nanorods were affected by the synthesis time and annealing temperature. The same ZnO nanorods were deposited onto the ITO substrate to form a UV photoconductive detector. The ratio of the UV photogenerated current to dark current was as high as nine times under 3 V bias. Hence, these nanorods can be promising materials in the use of UV radiation detection.     

Vertically aligned and ordered ZnO/CdS nanowire arrays for self-powered UV–visible photosensing

Photodetectors based on photoconductivity effect are usually driven by an external power source. A self-powered photodetector can be powered by incident light using the photovoltaic effect. Here, photoelectrochemical cells with periodically aligned ZnO/CdS nanowire arrays as photoanodes were fabricated and investigated for detecting UV and visible light. At zero bias, this self-powered UV–visible photodetector showed high responsivities of 35.4 and 23.2 mA/W for UV and visible light, a fast rise time of 0.18 s, and a decay time of 0.32 s. The spectral responses of the self-powered photodetectors based on ZnO/CdS nanowire arrays exhibited superior photoresponse in both UV and visible regions in comparison with ZnO nanowire film and ZnO nanowire arrays. The high photosensing performance originates from the excellent light trapping ability at broadband wavelengths and the high charge collection efficiency of the highly ordered ZnO/CdS nanowire arrays. The results indicate that the ZnO/CdS heterojunctions with periodic nanostructures provide a facile frame for UV–visible detecting applications.     

Enhancing the performance of dye-sensitized solar cells by ZnO nanorods/ZnO nanoparticles composite photoanode

In this paper, ZnO nanorods (NRs) were prepared by a two-step solution phase reaction. A composite photoanode architecture is fabricated by adding 0–0.20 at.% ZnO NRs into ZnO nanoparticles (NPs). The scanning electron microscopy image shows that the average diameter and length of the ZnO NRs are about 50 nm and 2–5 µm, respectively, and the ZnO NRs are uniformly embedded into the ZnO NPs photoanode. The UV–vis spectrum analysis reveals that the amount of dye adsorption of the composite photoanode decreases with increasing ZnO NRs content. Meanwhile, the influence of ZnO NRs contents on the dye-sensitized solar cells (DSSCs) performance is systematically investigated. The photocurrent density–voltage (J–V) characteristics reveal that the device performance of DSSCs can be significantly enhanced by the composite photoanode. Typically, the DSSC with 0.15 at.% ZnO NRs obtains the optimal energy conversion efficiency of 3.8%, which is 28.4% higher than that of the pristine ZnO DSSCs. The electrochemical impedance spectroscopy (EIS) analysis shows that ZnO NRs can provide a direct pathway for accelerating electron transport, extending the electron lifetime, suppressing electron recombination and improving electron collection efficiency. These results indicate that the incorporation of ZnO NRs in the photoanode is an effective way to improve the performance of DSSCs.     

A crossbar-type high sensitivity ultraviolet photodetector array based on a one hole-one nanorod configuration via nanoimprint lithography

Single crystalline vertical ZnO nanorods were grown in a one hole-one rod configuration using a hydrothermal method with a patterned polymer template generated by nanoimprint lithography, allowing precise control over the position and density of the ZnO nanorods. An 8×8 ZnO nanorod-based ultraviolet photodetector array is demonstrated, in which a well-confined number of ZnO nanorods are sandwiched between crossbar-type platinum and indium tin oxide electrodes (e.g. 16 nanorods in a 2×2 μm2 area). A high photocurrent/dark current ratio of 3×10(3) at a reverse bias of 1.5 V under UV illumination at room temperature, a responsivity of 4381.4 A W(-1) at 365 nm, and an ultraviolet-to-visible rejection ratio of 83 are obtained and maintained, irrespective of pixel size. A uniform photoresponse is achieved in each of the pixels, indicating the scalability with this technique for fabricating an integrated UV photodetector array circuit.     

Flexible photodiodes constructed with CdTe nanoparticle thin films and single ZnO nanowires on plastics

We construct a flexible pn heterostructured photodiode using a CdTe nanoparticle thin film and a single ZnO nanowire (NW) on a plastic substrate. The photocurrent characteristics of the flexible photodiode are examined under illumination with 325 nm wavelength light and the photocurrent efficiencies at bias voltages of ± 2.5 V are estimated to be 8.0 and 2.1 μA W(-1) under forward and reverse bias conditions, respectively. The photocurrent generation of the pn heterostructured photodiode is dominantly associated with the transport of the photogenerated charge carriers in the single ZnO NW. Furthermore, the operations of our flexible photodiode are investigated in the upwardly and downwardly bent states, as well as in the flat state.     

Effects of Na-doping on the efficiency of ZnO nanorods-based dye sensitized solar cells

Na-doped ZnO nanorods (Zn_1?xNa_xO: x = 0.0, 0.02, 0.04) were grown by a chemical bath deposition method on ZnO seeded FTO substrates. The influence of Na-doping on the efficiency of ZnO nanorods-based dye-sensitized solar cells (DSSCs) was investigated. Undoped and Na-doped ZnO nanorods were used as photo-anodes for the fabricated DSSCs. X-ray diffraction measurements exhibited that all the samples had a wurtzite structure of ZnO with a preferred orientation of (002) plane. Scanning electron microscopy images of the samples revealed that all the samples displayed hexagonal shaped nanorods. It was observed from optical measurements that the band gap energy gradually decreased from 3.29 to 3.21 eV for undoped and 4 at.% Na-doped ZnO nanorods, respectively. Photoluminescence spectrum for undoped ZnO showed three peaks located at 379, 422, and 585 nm corresponding to UV emission, zinc vacancy, and deep level emission (DLE) peaks, respectively. When ZnO nanorods were doped with 2 at.% Na, the intensity of UV peak increased whereas the intensity of DLE peak decreased. The maximum conversion efficiency of DSSCs was found to be 0.22 % with a J_sc of 0.80 mA/cm², V_oc of 0.49 V, and fill factor of 0.523 as ZnO nanorods were doped with 2 at.% Na atoms.     

Synthesis and characterization of flower like ZnO nanorods for dye-sensitized solar cells

Flower like ZnO nanorods have been prepared by chemical bath deposition method. X-ray diffraction result shows that flower like ZnO nanorods exhibit hexagonal structure. Dye sensitized solar cells have been assembled by using ZnO nanorod film photoelectrode sensitized using natural dye extracted from daucus carota as sensitizer. The flower like ZnO nanorods have been used as photo-anode material to fabricate the dye sensitized solar cell which exhibited an overall light to electricity conversion efficiency of 0.78 % with a fill factor of 0.39, short-circuit current density of 3.70 mA/cm² and open-circuit voltage of 0.26 V.     

Flexible cellulose and ZnO hybrid nanocomposite and its UV sensing characteristics

Abstract

This paper reports the synthesis and UV sensing characteristics of a cellulose and ZnO hybrid nanocomposite (CEZOHN) prepared by exploiting the synergetic effects of ZnO functionality and the renewability of cellulose. Vertically aligned ZnO nanorods were grown well on a flexible cellulose film by direct ZnO seeding and hydrothermal growing processes. The ZnO nanorods have the wurtzite structure and an aspect ratio of 9 ~ 11. Photoresponse of the prepared CEZOHN was evaluated by measuring photocurrent under UV illumination. CEZOHN shows bi-directional, linear and fast photoresponse as a function of UV intensity. Electrode materials, light sources, repeatability, durability and flexibility of the prepared CEZOHN were tested and the photocurrent generation mechanism is discussed. The silver nanowire coating used for electrodes on CEZOHN is compatible with a transparent UV sensor. The prepared CEZOHN is flexible, transparent and biocompatible, and hence can be used for flexible and wearable UV sensors.     

Crabwise ZnO Nanowire UV Photodetector Prepared on ZnO : Ga/Glass Template

Vertical well-aligned and crabwise ZnO nanowires were prepared on patterned ZnO:Ga/glass substrates by reactive evaporation method under different growth conditions. The average length and diameter of vertical well-aligned ZnO nanowires were around 1 mum and 50-100 nm, respectively. In contrast, the average length and diameter of crabwise ZnO nanowires were around 5 mum and 30 nm, respectively. Upon illumination with UV light (lambda = 362 nm), it was found that measured responsivities were 0.015 and 0.03 A/W for the crabwise ZnO nanowire photodetector biased at 10 and 15 V, respectively. Furthermore, a rejection ratio of approximately 10 was obtained for the crabwise ZnO nanowire photodetector with an applied bias of 10 V.     

Transparent and flexible ultraviolet photodetectors based on colloidal ZnO quantum dot/graphene nanocomposites formed on poly(ethylene terephthalate) substrates

Transparent colloidal ZnO quantum-dot (QD)/graphene nanocomposites were formed on poly(ethylene terephthalate) (PET) substrates. Ultraviolet (UV)–visible absorption spectra showed a shoulder peak around 350 nm corresponding to the absorption of ZnO QDs. Optical transmittance of the ZnO QD/graphene/PET multilayer was approximately 80%. High-resolution transmission electron microscopy images showed that the ZnO QDs were distributed along the circumferences of the surfaces on the graphene layers. Current–voltage and current–time measurements on the UV photodetector after bending at 300 K exhibited the ON/OFF switching states and stability resulting from the light-induced conductivity of the flexible graphene layer.     

ZnO Film UV Photodetector with Enhanced Performance: Heterojunction with CdMoO4 Microplates and the Hot Electron Injection Effect of Au Nanoparticles

A novel CdMoO₄–ZnO composite film is prepared by spin‐coating CdMoO₄ microplates on ZnO film and is constructed as a heterojunction photodetector (PD). With an optimized loading amount of CdMoO₄ microplates, this composite film PD achieves a ≈18‐fold higher responsivity than pure ZnO film PD at 5 V bias under 350 nm (0.15 mW cm⁻²) UV light illumination, and its decay time shortens to half of the original value. Furthermore, Au nanoparticles are then deposited to modify the CdMoO₄–ZnO composite film, and the as‐constructed photodetector with an optimized deposition time of Au nanoparticles yields an approximately two‐fold higher photocurrent under the same condition, and the decay time reduces by half. The introduced CdMoO₄ microplates form type‐II heterojunctions with ZnO film and improve the photoelectric performance. The hot electrons from Au nanoparticles are injected into the CdMoO₄–ZnO composite film, leading to the increased photocurrent. When the light is off, the Schottky barriers formed between Au nanoparticles and CdMoO₄–ZnO composite film block the carrier transportation and accelerate the decay process of current. The study on Au‐nanoparticle‐modified CdMoO₄–ZnO composite film provides a facile method to construct ZnO film based PD with novel structure and high photoelectric performance.     

Fabrication and properties of sulfur (S)-doped ZnO nanorods

Highly-aligned sulfur (S)-doped ZnO nanorods have been grown using the hydrothermal approach at 90 °C for 2 h onto quartz substrate pre-coated with ZnO seed layer deposited by radio frequency magnetron sputtering system. The morphology, crystal structure, and transmittance of the S-doped ZnO nanorods grown with varied sulfur concentration have been investigated. The scanning electron microscope images showed that the S-doped ZnO nanorods dimension is affected by sulfur doping. The nanorods doped with sulfur concentration of ~1, 1.5, and 2 at.% found to show nanorods with an average diameter of ~130, 170, and 270 nm respectively. X-ray diffraction measurements revealed that the sulfur-doped ZnO nanorods have hexagonal-wurtzite crystal structure and grown vertically in the (002) plane along c-axis perpendicular to the substrate. The nanorods doped with 1 at.% sulfur showed ~70 % transmittance in the visible region while the nanorods doped with 2 at.% sulfur showed transmittance of ~77 % and exhibited blue shift in the fundamental absorption edge.     

A high-sensitivity,fast-response,rapid-recovery UV photodetector fabricated based on catalyst-free growth of ZnO nanowire networks on glass substrate

Here, we report for the first time the fabrication of metal–semiconductor–metal ultraviolet photodetector based on catalyst-free growth of ZnO nanowire networks on ITO seeds/glass substrates by thermal evaporation method. The morphological, structural, and optical properties of the sample were studied by using field emission scanning electron microscopy, X-ray diffraction, photoluminescence, and UV–Vis spectrophotometer. Upon exposure to 365 nm light (1.5 mW/cm²) at five-bias voltage, the device showed 2.32 × 10³ sensitivity. In addition, the photocurrent was 1.79 × 10⁻⁴ A, and the internal gain of the photodetector was 24.2. The response and the recovery times were calculated to be 3.9 and 2.6 s, respectively, upon illumination to a pulse UV light (365 nm, 1.5 mW/cm²) at five-bias voltage. All of these results demonstrate that this high-quality detector can be a promising candidate as a low-cost UV photodetector for commercially integrated photoelectronic applications.     

Fast response ultraviolet Ga-doped ZnO based photoconductive detector

A metal-semiconductor-metal photoconductive detector was fabricated using high quality Ga-doped ZnO film epitaxially grown onto alumina substrate by spray pyrolysis. The photocurrent increases linearly with incident power density for more than two orders of magnitude. Reflectance and photocurrent measurements were carried out to study optoelectronic properties of Ga-doped ZnO thin film. Both spectra are consistent with each other showing good response in UV than visible region. Peak responsivity of about 1187 A/W at 5 V bias for 365 nm light was obtained in UV region.     

Solution-processed ultraviolet photodetectors based on colloidal ZnO nanoparticles

A "visible-blind" solution-processed UV photodetector is realized on the basis of colloidal ZnO nanoparticles. The devices exhibit low dark currents with a resistance >1 TOmega and high UV photocurrent efficiencies with a responsivity of 61 A/W at an average intensity of 1.06 mW/cm(2) illumination at 370 nm. The characteristic times for the rise and fall of the photocurrent are <0.1 s and about 1 s, respectively. The photocurrent of the device is associated with a light-induced desorption of oxygen from the nanoparticle surfaces, thus removing electron traps and increasing the free carrier density which in turn reduces the Schottky barrier between contacts and ZnO nanoparticles for electron injection. The devices are promising for use in large-area UV photodetector applications.     

Fabrication and characterization of X-ray array detectors based on polycrystalline PbI2 thick films

Polycrystalline PbI₂ thick films were grown by using close spaced vapor deposition method on glass substrate with a conducting indium–tin-oxide coating. The morphology shows a uniquely oriented film structure with hexagonal platelets accurately being upright on substrate surface. An array detector with 12 pixels was fabricated based on this structure of the thick film. It is shown that the dark current is lower than 3 nA at bias voltage below 500 V, and the dark resistivity is as high as 10¹¹ Ω cm. A mapping of dark current density of the sensitive area of the array detector exhibits a better uniformity in the central area than the fringe area. A quick photocurrent response to X-ray excitation was obtained. The photoresponse rise time about 250 μs was obtained from the detectors and the photocurrent decays in a few seconds. The values of photocurrent are higher about two orders of magnitude than the values of dark current. The distribution of photocurrent is more uniform than that of dark current in the sensitive area of the detector.     

Electroluminescence dependence on the organic thickness in ZnO nano rods/Alq3 heterostructure devices

ZnO nanorods are synthesised by a hydrothermal method on ITO glass. Their crystallization and morphology are detected by XRD and SEM, respectively. The results show that the ZnO nanorod array has grown primarily along a direction aligned perpendicular to the ITO substrate. The average height and diameter of the nanorods is about 130 nm and 30 nm, respectively. Then ZnO nano rods/Alq3 heterostructure LEDs are prepared by thermal evaporation of Alq3 molecules. The thicknesses of the Alq3 layers are 130 nm, 150 nm, 170 nm and 190 nm, respectively. The electroluminescence of the devices is detected under different DC bias voltages. The exciton emission of Alq3 is detected in all devices. When the thickness of Alq3 is 130 nm, the UV electroluminescence of ZnO is around 382 nm, and defect emissions around 670 nm and 740 nm are detected. Defect emissions of ZnO nanorods are prominent. When the thickness of Alq3 increases to over 170 nm, it is difficult to observe defect emissions from the ZnO nano rods. In such devices, the exciton emission of Alq3 is more prominent than other emissions under different bias voltage.     

Low-temperature growth of well-aligned ZnO nanowire arrays by chemical bath deposition for hybrid solar cell application

Well-aligned ZnO nanowire arrays were grown on indium tin oxide coated glass substrates by a facile chemical bath deposition technique. Morphologies, crystalline structure and optical transmission were investigated by field-emission scanning electron microscope, X-ray diffraction and UV–visible transmission spectrum, respectively. The results showed that ZnO nanowires were aligned in a dense array approximately perpendicular to substrate surface, they were wurtzite-structured (hexagonal) ZnO. In addition, the nanowire arrays exhibited high optical transmission (>85 %) in the visible region. Furthermore, an inverted inorganic/polymer hybrid solar cell was built using as-grown well-aligned ZnO nanowire arrays as inorganic layer, under the AM 1.5 illumination with a light intensity of 80 mW/cm², the device showed an open circuit voltage (V_oc) of 0.44 V, a short circuit current (J_sc) of 3.23 mA/cm², a fill-factor of 38 %, and a power conversion efficiency of 0.68 %.