Rapid fabrication technique for interpenetrated ZnO nanotetrapod networks for fast UV sensors

Enabling Broadband Solar-Blind UV Photodetection by a Rare-Earth Doped Oxyfluoride Transparent Glass-Ceramic

Oxyfluoride transparent glass-ceramics (GC) are widely used as the matrix for rare-earth (RE) ions due to their unique properties such as low phonon energy, high transmittance, and high solubility for RE ions. Tb3+ doped oxyfluoride glasses exhibit a large absorption cross section for ultraviolet (UV) excitation, high stability, high photoluminescence quantum efficiency, and sensitive spectral conversion characteristics, making them promising candidate materials for use as the spectral converter in UV photodetectors. Herein, a Tb3+ doped oxyfluoride GC is developed by using the melt-quenching method, and the microstructure and optical properties of the GC sample are carefully investigated. By combining with a Si-based photo-resistor,a solar-blind UV detector is fabricated, which exhibits a significant photoelectric response with a broad detection range from 188 to 400 nm. The results indicate that the designed UV photodetector is of great significance for the development of solar-blind UV detectors.

3D-printed wound dressing platform for protein administration based on alginate and zinc oxide tetrapods

Wound treatment requires a plethora of independent properties. Hydration, anti-bacterial properties, oxygenation and patient-specific drug delivery all contribute to the best possible wound healing. Three-dimensional (3D) printing has emerged as a set of techniques to realize individually adapted wound dressings with open porous structure from biomedically optimized materials. To include all the desired properties into the so-called bioinks is still challenging. In this work, a bioink system based on anti-bacterial zinc oxide tetrapods (t-ZnO) and biocompatible sodium alginate is presented. Additive manufacturing of these hydrogels with high t-ZnO content (up to 15 wt.%) could be realized. Additionally, protein adsorption on the t-ZnO particles was evaluated to test their suitability as carriers for active pharmaceutical ingredients (APIs). Open porous and closed cell printed wound dressings were tested for their cell and skin compatibility and anti-bacterial properties. In these categories, the open porous constructs exhibited protruding t-ZnO arms and proved to be anti-bacterial. Dermatological tests on ex vivo skin showed no negative influence of the alginate wound dressing on the skin, making this bioink an ideal carrier and evaluation platform for APIs in wound treatment and healing.

Photochromic Uranyl-Based Coordination Polymer for Quantitative and On-Site Detection of UV Radiation Dose

A highly sensitive detection of ultraviolet (UV) radiation is required in a broad range of scientific research, chemical industries, and health-related applications. Traditional UV photodetectors fabricated by direct wide-band-gap inorganic semiconductors often suffer from several disadvantages such as complicated manufacturing procedures, requiring multiple operations and high-cost instruments to obtain a readout. Searching for new materials or simple strategies to develop UV dosimeters for quantitative, accurate, and on-site detection of UV radiation dose is still highly desirable. Herein, a photochromic uranyl-based coordination polymer [(UO2)(PBPCA)·DMF]·DMF (PBPCA = pyridine-3,5-bis(phenyl-4-carboxylate), DMF = N,N'-dimethylformamide, denoted as SXU-1) with highly radiolytic and chemical stabilities was successfully synthesized via the solvothermal method at 100 °C. Surprisingly, the fresh samples of SXU-1 underwent an ultra-fast UV-induced (365 nm, 2 mW) color variation from yellow to orange in less than 1 s, and then the color changed further from orange to brick red after the subsequent irradiation, inspiring us to develop a colorimetric dosimeter based on red-green-blue (RGB) parameters. The mechanism of radical-induced photochromism was intensively investigated by UV-vis absorption spectra, EPR analysis, and SC-XRD data. Furthermore, SXU-1 was incorporated into an optoelectronic device to fabricate a novel dosimeter for convenient, quantitative, and on-site detection of UV radiation dose.

Probing size-dependent defects in zinc oxide using synchrotron techniques: impact on photocatalytic efficiency

In the present study, synchrotron-based X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and X-ray excited optical luminescence (XEOL) have been used to investigate the induced defect states in metal oxide nanomaterials. Specifically, two synthesis approaches have been followed to develop unique nano-sized peanut-shaped (N-ZnO) nanostructures and micron-sized hexagonal rods (M-ZnO). XANES analysis at the Zn K-edge revealed the presence of defect states with a divalent oxidation state of zinc (Zn2+) in a tetrahedral structure. Furthermore, XAS measurements performed at the Zn L3,2-edge and O K-edge confirm higher oxygen-related defects in M-ZnO, while N-ZnO appeared to have a higher concentration of surface defects due to size confinement. Moreover, the in-line XEOL and time dependent-XEOL measurements exposed the radiative excitonic recombination phenomena occurring in the band-tailing region as a function of absorption length, X-ray energy excitation, and time. Based on the chronology developed in the defect state improvement, a possible energy band diagram is proposed to accurately locate the defect states in the two systems. Furthermore, the increased absorption intensity at the Zn L3,2-edge and the O K-edge under the UV lamp suggests delayed recombination of electrons and holes, highlighting their potential use as photo catalysts. The photocatalytic activity degrading the rhodamine B dye established M-ZnO as a superior catalyst with a rapid degradation rate and significant mineralization. Overall, this work provides valuable insights into ZnO defect states and provides a foundation for efficient advanced materials for environmental or other optoelectronic applications.

An investigation into the hybrid architecture of Mn-Co nanoferrites incorporated into a polyaniline matrix for photoresponse studies

In this study, an enhanced photoresponse was observed in the Mn-Co Nanoferrites (MCFs)-Polyaniline (PANI) nanohybrid architecture due to the formation of interface between PANI and MCFs, which provided a conduction pathway for the movement of charge carriers, and these interfaces were observed in a high-resolution transmission electron micrograph (HR-TEM). X-ray photoelectron spectroscopy (XPS) suggests that the carbon (C 1s) of the MCF-PANI nanohybrid shows peaks at 287.80 eV for CO, 286.17 eV for C-O, 285.24 eV for C-N, 284.50 eV for the sp3 hybridized carbon (C-C/C-H) and 283.84 eV for the sp2 hybridized carbon (CC). Current-voltage (I-V) curves reveal an ohmic nature of the MCF-PANI nanohybrid photodetector device. The photoresponse measurements were analyzed using the trap depth concept, demonstrating that the conductive polymer increases the photoconduction mechanism efficiency of MCFs. The constructed photodetector device exhibits a high photoresponsivity of 22.69 A W-1, a remarkable detectivity of 1.36 × 1012 cm Hz1/2 W-1 and a fast rise/decay time of 0.7/0.8 s. The excellent performance of the as-fabricated photodetector device could be explained by the intimate interaction between MCFs and PANI at their interface.

Antibacterial Activity of Nanostructured Zinc Oxide Tetrapods

Zinc oxide (ZnO) tetrapods as microparticles with nanostructured surfaces show peculiar physical properties and anti-infective activities. The aim of this study was to investigate the antibacterial and bactericidal properties of ZnO tetrapods in comparison to spherical, unstructured ZnO particles. Additionally, killing rates of either methylene blue-treated or untreated tetrapods and spherical ZnO particles for Gram-negative and Gram-positive bacteria species were determined. ZnO tetrapods showed considerable bactericidal activity against Staphylococcus aureus, and Klebsiella pneumoniae isolates, including multi-resistant strains, while Pseudomonas aeruginosa and Enterococcus faecalis remained unaffected. Almost complete elimination was reached after 24 h for Staphylococcus aureus at 0.5 mg/mL and Klebsiella pneumoniae at 0.25 mg/mL. Surface modifications of spherical ZnO particles by treatment with methylene blue even improved the antibacterial activity against Staphylococcus aureus. Nanostructured surfaces of ZnO particles provide active and modifiable interfaces for the contact with and killing of bacteria. The application of solid state chemistry, i.e., the direct matter-to-matter interaction between active agent and bacterium, in the form of ZnO tetrapods and non-soluble ZnO particles, can add an additional principle to the spectrum of antibacterial mechanisms, which is, in contrast to soluble antibiotics, depending on the direct local contact with the microorganisms on tissue or material surfaces.

Hydrothermal synthesis of ZnZrO2/chitosan (ZnZrO2/CS) nanocomposite for highly sensitive detection of glucose and hydrogen peroxide

In this work, pure ZnZrO₂ and chitosan supported (ZnZrO₂/CS) nanocomposite have been synthesized at low coast by hydrothermal method. FT-IR, Micro Raman, PXRD, HR-SEM-EDAX, HR-TEM, AFM, BET and XPS were used to analyze the structural and morphological properties of the fabricated nanocomposites. The fabricated ZnZrO₂ and ZnZrO₂/CS nanocomposites were measured for their electrocatalytic activity towards glucose and hydrogen peroxide determinations. The ZnZrO₂/CS sensor exhibited wide detection range (5 μM to 5.85 mM), high sensitivity (6.78 μA mM^-1 cm^-2), LOD (2.31 μM), and long-term stability for glucose detection in alkaline solution. Also, as a multifunctional electrochemical sensor, ZnZrO₂/CS sensor exhibits excellent sensing ability towards hydrogen peroxide, with a wide dynamic range (20 μM to 6.85 mM), a high sensitivity (2.22 μA mM^-1 cm^-2), and a LOD (2.08 μM) (S/N = 3). The electrochemical measurement shows that the ZnZrO₂/CS sensor has excellent catalytic activity and a much LOD than ZnZrO₂. The modified electrode showed excellent anti interference nature. Furthermore, this ZnZrO₂/CS electrode was used to detection of glucose and H₂O₂ in human blood serum and HeLa cells respectively.

In Vitro Evaluation of Zinc Oxide Tetrapods as a New Material Component for Glaucoma Implants

In our previous study we were able to show that zinc oxide (ZnO) tetrapods inhibit wound healing processes. Therefore, the aim of this study was to test the antiproliferative effect of two types of porous polydimethylsiloxane (PDMS)/ tetrapodal zinc oxide (ZnO-T) materials, as well as their usability for glaucoma implants. To find the best implant material, two different porous PDMS/ZnO-T materials were examined. One consisted of 3D interconnected PDMS coarse-pored foams with protruding ZnO-T particles; the other consisted of fine-pored 3D interconnected ZnO-T networks homogeneously coated by a thin PDMS film in the nanometer range. Fibroblast cell viability was investigated for both materials via MTT dye, and some implant material samples were further processed for electron microscopy. Both PDMS/ZnO-T materials showed reduced cell viability in the MTT staining. Furthermore, the electron microscopy revealed barely any fibroblasts growing on the implant materials. At the surface of the fine-pored implant material, however, fibroblasts could not be observed in the etched control samples without ZnO-T. It was found that post-processing of the material to the final stent diameter was highly challenging and that the fabrication method, therefore, had to be adapted. In conclusion, we were able to demonstrate the antiproliferative potential of the two different PDMS/ZnO-T materials. Furthermore, smaller pore size (in the range of tens of micrometers) in the implant material seems to be preferable.

Zinc Oxide Tetrapods Modulate Wound Healing and Cytokine Release In Vitro—A New Antiproliferative Substance in Glaucoma Filtering Surgery

Glaucoma filtering surgery is applied to reduce intraocular pressure (IOP) in cases of uncontrolled glaucoma. However, postoperative fibrosis reduces the long-term success of both standard trabeculectomy and microstents. The aim of this study was to test the antiproliferative and anti-inflammatory potential of ZnO-tetrapods (ZnO-T) on human Tenon’s fibroblasts (HTFs) for glaucoma surgery. The toxicity of ZnO-T on HTFs was determined using an MTT test. For analysis of fibroblast proliferation, migration, and transdifferentiation, cultures were stained for Ki67, alpha-smooth muscle actin (α-SMA), and p-SMAD. A fully quantitative multiplex ELISA was used to determine the concentrations of different cytokines, platelet-derived growth factor (PDGF), and hepatocyte growth factor (HGF) in culture supernatants with and without previous ZnO-T treatment. Treatment with higher concentrations (10 and 20 µg/mL) was associated with HTF toxicity, as shown in the wound healing assay. Furthermore, the number of Ki67, α-SMA-positive, and pSMAD-positive cells, as well as IL-6 and HGF in supernatants, were significantly reduced following incubation with ZnO-T. In conclusion, we were able to show the antiproliferative and anti-inflammatory potentials of ZnO-T. Therefore, the use of ZnO-T may provide a new approach to reducing postoperative fibrosis in glaucoma filtering surgery.

Cs3Cu2I5/ZnO Heterostructure for Flexible Visible-Blind Ultraviolet Photodetection

Wearable, portable, and biocompatible optoelectronic devices made of all-green and abundant materials and fabricated by low-temperature solution method are the key point in the development of next generation of intelligent optoelectronics. However, this is usually limited by the weaknesses of mono-component materials, such as non-adjustable photoresponse region, high carrier recombination rate, high signal-to-noise ratio, as well as the weak mechanical flexibility of bulk films. In this work, the Cs₃Cu₂I₅/ZnO heterostructure flexible photodetectors were constructed by a low-temperature solution method combined with spin-coating technique. The heterostructure combines the low dark current and strong deep ultraviolet absorption of Cs₃Cu₂I₅ quantum dots with the high carrier mobility of ZnO quantum dots as well as the efficient charge separation of the vertical p-n junction, to improve the photodetection performance. The heterostructure shows enhanced light/dark current ratio and ultraviolet-to-visible rejection ratios. Under an illumination of 280 nm light, an optical detectivity as high as 1.26 × 10¹¹ Jones was obtained; the optical responsivity and response time are much better than those of control devices. After 300 times of 180° bending cycles, the photocurrent had no obvious change. The results demonstrate that the Cs₃Cu₂I₅/ZnO heterostructure has great potential in wearable and portable visible-blind ultraviolet optoelectronic devices.

Electrospun carbon nanofibre-assisted patterning of metal oxide nanostructures

This work establishes carbon nanofibre-mediated patterning of metal oxide nanostructures, through the combination of electrospinning and vapor-phase transport growth. Electrospinning of a suitable precursor with subsequent carbonization results in the patterning of catalyst gold nanoparticles embedded within carbon nanofibres. During vapor-phase transport growth, these nanofibres allow preferential growth of one-dimensional metal oxide nanostructures, which grow radially outward from the nanofibril axis, yielding a hairy caterpillar-like morphology. The synthesis of metal oxide caterpillars is demonstrated using zinc oxide, indium oxide, and tin oxide. Source and substrate temperatures play the most crucial role in determining the morphology of the metal oxide caterpillars, whereas the distribution of the nanofibres also has a significant impact on the overall morphology. Introducing the current methodology with near-field electrospinning further facilitates user-defined custom patterning of metal oxide caterpillar-like structures.

Low-Cost Naked-Eye UVB and UVC Dosimetry Based on 3,3',5,5'-Tetramethylbenzidine

Ultraviolet radiation (UVR) is both useful to human beings and can cause irreversible harm of varying degrees (UVA, UVB, and UVC). Especially, in areas with excessive sunlight, the appearance of UVB results in an increased risk of skin cancer. On the other hand, UV lamps (254 nm, UVC) are widely used in disinfection (air, water, and factory food) and hospital sterilization; the leakage of UVC is thus sometimes inevitable, which may cause fatal injuries to the related staff. Therefore, low-cost UV dosimetry-based personal protective equipment (PPE) and industrial monitoring devices are of great importance. Here, for the first time, we found that 3,3',5,5'-tetramethylbenzidine (TMB) could be rapidly oxidized upon UVB and UVC irradiation in a dose-dependent manner, in which TMB acts as a self-photosensitizer. Since TMB is a typical and widely used chromogenic substrate in enzyme-linked immunosorbent assay (ELISA), it is well-commercialized with low cost and vast availability worldwide, which permitted the development of low-cost naked-eye UVB and UVC dosimetry. A wearable bracelet mounted with TMB-loaded paper was developed for successful indication of whether the UVB exposure in the sunlight exceeded the minimum erythema dose (MED). In addition, we also developed a clock dial equipped with a TMB solution for unattended detection of UVC leakage from UVC disinfection lamps. The UVB- and UVC-selective coloration and low cost of TMB offered remarkable potential in facile detection of UVR in our daily life.

Effect of annealing environment on the photoelectrochemical water oxidation and electrochemical supercapacitor performance of SnO2 quantum dots

SnO₂ quantum dots (SQD) were prepared by utilizing the soft-chemical approach. The formed SQD's were annealed in two kinds of environments: air and nitrogen (N₂). Each annealing environment resulted in significant improvement in the performance of water oxidation and electrochemical supercapacitor performance. The specific capacitance of the SQD's under the N₂ annealing process (SQD-N₂) shows significantly better electrochemical performance. A specific capacitance of 79.13 F/g was achieved for SQD-N₂ sample by applying a current of 1 mA, which was approximately 1.5 times greater than that of the pristine SQD's. A cycle stability of 99.4% over 5000 cycles was achieved by SQD-N₂. The process of nitrogen annealing environment brings down the bandgap from 3.37 to 1.9 eV. The SQD-N₂ sample shows the highest photocurrent over SQD and SQD-Air samples. From the LSV study, SQD-N₂ shows the photocurrent density of 4.82 mA/cm², which is 1.43 times greater than pristine SQD sample. The nitrogen-annealing environment provides the optimal environment to tune the average crystallite size and crystallinity nature of SQD's to improve the optical properties like bandgap to enhance the water oxidation and also electrochemical performance.

Differential bioaccumulations and ecotoxicological impacts of metal-oxide nanoparticles, bulk materials, and metal-ions in cucumbers grown in sandy clay loam soil

Expanding applications of metal-oxide nanoparticles (NPs) and increased environmental deposition of NPs followed by their interactions with edible crops threaten yields. This study demonstrates the effects of aging (45 days in soil) of four NPs (ZnO, CuO, Al₂O₃, TiO₂; 3.9-34 nm) and their corresponding metal oxide bulk particles (BPs; 144-586 nm) on cucumbers (Cucumis sativus L.) cultivated in sandy-clay-loam field soil and compares these with the phytotoxic effects of readily soluble metal salts (Zn²⁺, Cu²⁺, and Al³⁺). Data revealed the cell-to-cell translocations of NPs, their attachments to outer and inner cell surfaces, nuclear membranes, and vacuoles, and their upward movements to aerial parts. Metal bioaccumulations in cucumbers were found in the order: (i) ZnO-NPs > ZnO-BPs > Zn²⁺, (ii) CuO-NPs > CuO-BPs > Cu²⁺, (iii) Al³⁺> Al₂O₃-NPs > Al₂O₃-BPs and (iv) TiO₂-NPs > TiO₂-BPs. Aging of NPs in soil for 45 days significantly enhanced metal uptake (P ≤ 0.05), for instance aged ZnO-NPs at 1 g kg^-1 increased the uptake by 20.7 % over non-aged ZnO-NPs. Metal uptakes inhibited root (RDW) and shoot (SDW) dry weight accumulations. For Cu species, maximum negative impact (%) was exhibited by Cu²⁺ (RDW:SDW = 94:65) followed by CuO-NPs (RDW:SDW = 78:34) and CuO-BPs (RDW:SDW = 27:22). Aging of NPs/BPs at 1-4 g kg^-1 further enhanced the toxic impact of tested materials on biomass accumulations and chlorophyll formation. NPs also induced membrane damage of root tissues and enhanced levels of antioxidant enzymes. The results of this study suggest that care is required when aged metal-oxide NPs of both essential (Zn and Cu) and non-essential (Al and Ti) metals interact with cucumber plants, especially, when they are used for agricultural purposes.

Strong Polarized Photoluminescence CsPbBr3 Nanowire Composite Films for UV Spectral Conversion Polarization Photodetector Enhancement

In this work, we proposed a fluorescence conversion layer with polarization characteristics to enhance UV polarization detection for the first time. To achieve this goal, the colloidal lead halide CsPbBr₃ nanowires (NWs) with appropriate lengths were synthesized by the method of ultrasonication synthesis assisted by the addition of hydrobromic acid (HBr) ligands. By adding HBr, the properties of synthesized NWs are improved, and due to the controllable perovskite-stretched NWs, polymer composite films were fabricated, which can generate photoluminescence (PL) with strong polarization. The optimized stretched composite film can achieve a polarization degree of 0.42 and dichroism ratio (I_∥/I_⊥) of 2.49 at 520 nm. Based on this film, an imaging system with polarization-selective properties and efficient UV spectral conversion was developed. The spectrum conversion of 266 to 520 nm luminescence wavelength was realized and sensitive to the polarization of incoming 266 nm UV light. The experimental results also showed that the response after spectral conversion is greatly improved, and different responsivities can correspond to different polarization states. This imaging system overcomes the insufficiency of the conventional charge coupled device (CCD), which makes it difficult to receive the optical signal for high-quality UV imaging. The use of light conversion films with polarization characteristics for polarized UV imaging is of great significance for improving the detection of solar-blind UV bands and the recognition of military targets.

UV-Responsive Screen-Printed Porous ZnO Nanostructures on Office Paper for Sustainable and Foldable Electronics

The fabrication of low-cost, flexible, and recyclable electronic devices has been the focus of many research groups, particularly for integration in wearable technology and the Internet of Things (IoT). In this work, porous zinc oxide (ZnO) nanostructures are incorporated as a UV sensing material into the composition of a sustainable water-based screen-printable ink composed of carboxymethyl cellulose (CMC). The formulated ink is used to fabricate flexible and foldable UV sensors on ubiquitous office paper. The screen-printed CMC/ZnO UV sensors operate under low voltage (≤2 V) and reveal a stable response over several on/off cycles of UV light exposure. The devices reach a response current of 1.34 ± 0.15 mA and a rise and fall time of 8.2 ± 1.0 and 22.0 ± 2.3 s, respectively. The responsivity of the sensor is 432 ± 48 mA W−1, which is the highest value reported in the literature for ZnO-based UV sensors on paper substrates. The UV-responsive devices display impressive mechanical endurance under folding, showing a decrease in responsivity of only 21% after being folded 1000 times. Their low-voltage operation and extreme folding stability indicate a bright future for low-cost and sustainable flexible electronics, showing potential for low-power wearable applications and smart packaging.

Photocurrent enhancement of AlxGa1-xN nanowire arrays photodetector based on coupling effects of pn junction and gradient component

Ultraviolet photodetector has a variety of applications in medical diagnosis, civilian testing and military security. The enhancement of photo response has far been a hot topic regrading to the performance improvement of the devices. In this study, we proposed a self-powered photodetector based on Al_xGa_1-xN nanowire arrays (NWAs) utilizing axial pn junction integrating with gradient Al component. The merit of the coupling structure is demonstrated by theoretical model and simulations. The photoelectric conversion model is built based on a continuity equation derived by its corresponding boundary conditions. The photocurrent for a single nanowire and NWAs are respectively obtained. According to the simulation results of a single nanowire, the optimal nanowire height is obtained with a photocurrent enhancement up to 330%. For NWAs, the aspect ratio of NWAs and incident angle of light synergistically determine the output photocurrent. The optimal aspect ratio for NWAs is 1:1 with an optimal incident angle of 57°. This study provides a reliable method for the design of photodetectors with micro-nano structures.

Oxide-based catalysis: tailoring surface structures via organic ligands and related interfacial charge carrier for environmental remediation

Hierarchical nanostructures and the effects of ligands on their structure formation were investigated. Morphological analysis showed the change in the morphology from nanospindles to hollow hexagonal nanodisks with the change in ligands. Structural analysis exhibited the formation of both hexagonal ZnO and monoclinic CuO structures in the composition. The elemental composition confirms the presence of CuO and ZnO in the composition. An ultra-fast degradation was achieved for the nanocomposites. The ZnO/CuO composite with ethylenediamine showed the best activity by degrading 98.77% of the methylene blue dye in 36 min. A possible photocatalytic mechanism is proposed.

Photo generated charge transport studies of defects-induced shuttlecock-shaped ZnO/Ag hybrid nanostructures

We report the stimulating effects of interfacial charge transfer process between spherical Ag nanoparticles and shuttlecock-shaped ZnO nanostructures observed by UV-visible spectroscopy and x-ray absorption spectroscopy. In specific, ZnO nanorods and shuttlecock-shaped ZnO/Ag nanostructures were developed using a simple chemical colloidal method and characterized for structural variations using XRD. The observed red shift in plasmonic peak and the increase in Urbach energy signify interfacial interactions and increased randomness in the hybrid ZnO/Ag nanostructures. Simultaneously, the enhanced intensity of deep-level emission in the ZnO/Ag hybrid suggests the increased recombination rate of electron-hole pairs. The red and blue emissions evolving with temperature subsequently suggests the presence of oxygen vacancies or zinc interstitials in the system. The decrease in intensities and emerging features in O K-edge and Zn L-edge indicates the charge transfer from Ag to ZnO at the interface of ZnO/Ag hybrids. Moreover, the differences in absorption edges with alternating light on/off conditions were analyzed for the exploitation of this ZnO-based system in various applications.

Solution processed transparent anatase TiO2 nanoparticles/MoO3 nanostructures heterojunction: high performance self-powered UV detector for low-power and low-light applications

Ultraviolet (UV) photodetectors are considered as the major players in energy saving technology of the future. Efforts are needed to further develop such devices, which are capable of operating efficiently at low driving potential as well as with weak illumination. Herein, we report an all-oxide, highly transparent TiO₂/MoO₃ bilayer film, with nanoparticulate anatase TiO₂ as the platform, fabricated by a simple solution based method and demonstrate its use in UV photodetection. Photoconductivity measurement with 352 nm light reveals the self-powered UV detection capability of the device due to the built-in potential at the bilayer interface. The device exhibits a high photoresponsivity (46.05 A W^-1), detectivity (2.84 × 10¹² Jones) and EQE (16 223%) even with a weak illumination of 76 μW cm^-2, at a low bias of only -1 V. The self-powered performance of the bilayer device is comparable to that of commercial Si photodetectors as well as other such UV detectors reported based on metal oxide heterojunctions. The improved and faster photoresponse shown by the device is due to the formation of an effective heterojunction, as evidenced by XPS, electrochemical and I-V studies. It can be further attributed to the better charge transport through the densely aligned nanostructures, reduced recombination and the better mobility of anatase TiO₂ nanoparticles. The performance is best-in-class and proves the potential of the transparent heterojunction to be used in highly responsive, self-powered UV detectors for low bias, low light applications.

Enhanced UV Photoresponsivity of ZnO Nanorods Decorated with Ag2S/ZnS Nanoparticles by Successive Ionic Layer Adsorption and Reaction Method

Recently, different kinds of energy band structures have been utilized to improve the photoelectric properties of zinc oxide (ZnO). In this work, ZnO nanorods were prepared by the hydrothermal method and then decorated with silver sulfide (Ag2S)/zinc sulfide (ZnS) via two-step successive ionic layer adsorption and reaction method. The photoelectric properties of nanocomposites are investigated. The results show that ZnO decorated with Ag2S/ZnS can improve the photocurrent of photodetectors from 0.34 to 0.56 A at bias of 9 V. With the immersion time increasing from 15 to 60 minutes, the photocurrent of photodetectors increases by 0.22 A. The holes in the valence band of ZnO can be transferred to the valence band of ZnS and Ag2S, which promotes the separation and suppresses the recombination of hole-electron pairs generated in ZnO. Moreover, electrons excited by ultraviolet (UV) light in Ag2S can also be injected into the conduction band of ZnO, which causes the photocurrent to increase more than the ZnO photodetector.

Broadband Detection Based on 2D Bi2Se3/ZnO Nanowire Heterojunction

The investigation of photodetectors with broadband response and high responsivity is essential. Zinc Oxide (ZnO) nanowire has the potential of application in photodetectors, owing to the great optoelectrical property and good stability in the atmosphere. However, due to a large number of nonradiative centers at interface and the capture of surface state electrons, the photocurrent of ZnO based photodetectors is still low. In this work, 2D Bi2Se3/ZnO NWAs heterojunction with type-I band alignment is established. This heterojunction device shows not only an enhanced photoresponsivity of 0.15 A/W at 377 nm three times of the bare ZnO nanowire (0.046 A/W), but also a broadband photoresponse from UV to near infrared region has been achieved. These results indicate that the Bi2Se3/ZnO NWAs type-I heterojunction is an ideal photodetector in broadband detection.

High-Performance Two-Dimensional Perovskite Ca2Nb3O10 UV Photodetectors

We first report two-dimensional (2D) perovskite Ca₂Nb₃O₁₀ ultraviolet photodetectors (UV PDs), which are prepared via a facile calcination-exfoliation method. The 2D Ca₂Nb₃O₁₀ PDs demonstrate high performance at 3 V at 280 nm, high responsivity (14.94 A W^-1), high detectivity (8.7 × 10¹³ Jones), high spectral selectivity (R₂₈₀/R₄₀₀ = 8.84 × 10³), fast speed (0.08/5.6 ms), and long-term stability, exceeding those of most reported UV PDs. Furthermore, the Ca₂Nb₃O₁₀ PDs integrated with poly(ethylene terephthalate) (PET) show excellent flexibility and have high linear dynamic range (96 dB). Our work provides a general strategy for searching new UV PDs based on numerous layered niobates. The Ca₂Nb₃O₁₀ nanosheets may be one of the optimum semiconductor materials for next-generation high-performance UV PDs.

Crystallization of ZnO thin films via thermal dissipation annealing method for high-performance UV photodetector with ultrahigh response speed

ZnO-based ultraviolet (UV) photodetector can be easily fabricated by using sol-gel spin-coating method, however, the crystallization of amorphous state ZnO thin films is necessary to fabricate high performance UV photodetector. Thus, we devised a thermal dissipation annealing (TDA) method in which the heat transfer to the ZnO thin films can be synchronized with the heat release from the substrate. It was found that sol-gel spin-coated ZnO thin films can be crystallized through the mobility difference of ZnO molecules placed at the surface of ZnO thin films. Also, UV photodetector based on ZnO thin films annealed with the TDA method exhibited faster rise and decay time constant (τ_r = 35 ms and τ_d = 73 ms, respectively), a higher on/off current ratio, and reproducible photocurrent characteristics compared to that of the ZnO thin films annealed by using furnace and IR lamp. Therefore, these results indicated that the TDA method is a feasible alternative route for the fabrication of ZnO based high performance optoelectronic devices.

Crystallization of ZnO thin films without polymer substrate deformation via thermal dissipation annealing method for next generation wearable devices

The synthesis method of transparent and flexible ZnO thin films is currently considered the most important factor for the fabrication of next generation wearable devices. To fabricate transparent and flexible devices by using sol–gel spin-coated ZnO thin films, an annealing step is necessary; however, annealing processes at high temperatures decompose polymer substrates due to their low melting temperature. It was found that sol–gel spin-coated ZnO thin films can be crystallized through the mobility difference of ZnO molecules placed at the surface of ZnO thin films. Especially, ZnO thin films can be annealed at high temperature (above 500 °C) by using a thermal dissipation annealing (TDA) method without the deformation of the polymer substrate. A transparent and flexible ultraviolet photodetector based on ZnO thin films annealed with the TDA method exhibited fast rise and decay time constants, a high on/off current ratio, and reproducible photocurrent characteristics. Thus, these results indicated that the TDA method is a feasible alternative route for the fabrication of next generation wearable devices.

Thermal dissipation annealing method is an effective way of fabricating transparent and flexible optoelectronics for next generation wearable devices.

Tuning optical and optoelectronic properties of gold nanoparticle and ZnO thin film hetero-structures

Thin film hetero-structures (TFHSs) involving metal oxide thin films and noble metal nanoparticles are very important for many optoelectronics based device applications. This work reports the growth, characterization, and tuning of photoluminescence and I–V properties of TFHSs involving zinc oxide (ZnO) and gold nanoparticles (GNPs). ZnO thin films and GNPs were respectively deposited by the Pulsed Laser Deposition (PLD) and DC sputtering with subsequent annealing. Three different TFHSs were prepared by varying the relative positions of ZnO and GNPs, namely Si-GNPs-ZnO, Si-ZnO-GNPs, and Si-ZnO-GNPs-ZnO. X-ray diffraction results confirmed the high crystallinity of the films, with single phase nature of the ZnO and GNPs. Scanning electron microscopy micrograph analysis confirmed that the morphology of structures containing both GNPs and ZnO is influenced by the bottom layer. Diffuse reflectance spectroscopy results also indicated that the position of GNPs relative to ZnO affects the plasmon resonance of GNPs as well as the overall optical properties of the TFHSs. Photoluminescence studies revealed that the presence of GNPs affects the defect concentration in the TFHSs. The I–V characteristics showed that the TFHSs where ZnO contains GNPs in embedded form are better suited for photodiode application. This study adds a new dimension to the research on optoelectronics devices.

Ionothermal synthesis of a photochromic inorganic-organic complex for colorimetric and portable UV index indication and UVB detection

Extended exposure to sunlight or artificial UV sources (particularly UVA and UVB) is a major cause of serious skin cancers and ocular diseases. A photochromic inorganic–organic complex was ionothermally synthesized via a decomposition-reassembly strategy, generated from a low-cost deep-eutectic solvent and a 4,4′-bipyridine system. Benefiting from the intrinsic synergy of the hydrogen bonding and π–π stacking interactions, the complex exhibited insensitivity towards visible light, outstanding color contrast from colorless to purple, rapid response time up to seconds, excellent reversibility and high thermal stability. UV index and UVB detection procedures indicated that the coloration performances of the complex exhibited a linear response towards UV index and UVB dose. Besides, the complex can be made to a portable test tablet, a freestanding mixed film with a cellulose paper and a mixed-matrix membrane with PVDF, which make it highly promising for portable and efficient visual UV index and detecting UVB dose.

A photochromic inorganic–organic complex was ionothermally synthesized via a decomposition-reassembly strategy, which is highly promising for the colorimetric and portable indication of UV index and quantification of UVB doses.

Controllable Heterogeneous Nucleation for Patterning High-Quality Vertical and Horizontal ZnO Microstructures toward Photodetectors

High-quality crystalline micro- and nanostructures based on inorganic semiconductors including zinc oxide (ZnO) have attracted considerable interest in electronic and optoelectronic applications due to their outstanding properties. ZnO micro- and nanocrystals can be fabricated by the moderate and high throughput hydrothermal synthesis. Yet it is restricted by patterning large-area ZnO crystals with high-quality and programmable geometries through the hydrothermal process for the optoelectronic integration. Here, a capillary-bridge manipulation approach is demonstrated to control the dewetting process of ZnO precursor solution for patterning precursor arrays. Based on precursor arrays, vertically aligned high-quality ZnO microrod arrays with homogeneous morphology and pure crystallographic orientation are fabricated via a hydrothermal epitaxial method. Statistical results and crystallization theories guide the experimental optimization and discussion of the crystallization mechanism, dominated by the competition between homogeneous nucleation and heterogeneous nucleation. High-quality ZnO microbelt arrays are achieved through a surfactant-mediated hydrothermal method after ZnO microrod arrays are transferred to a polydimethylsiloxane substrate. Photodetectors based on ZnO microbelts exhibit a high responsivity of 2.3 × 10⁴ A W^-1 , a light on-off ratio exceeding 10⁵ , and stable recyclability. It is anticipated that this work provides new insights into patterning inorganic high-quality micro- and nanostructures for multi-functional integrated devices.

Nanostructured cobalt antimonate: a fast responsive and highly stable sensing material for liquefied petroleum gas detection at room temperature

Herein, cobalt antimonate (CoSb2O6) nanospheres were fabricated via the sol-gel spin-coating process and employed as a functional liquefied petroleum gas (LPG) sensor at room temperature (25 °C). The microstructure of the fabricated CoSb2O6 thin films (thickness ∼ 250 nm) was analyzed via scanning electron microscopy, which revealed the growth of nanospheres having an average diameter of ∼45 nm. The XRD analysis demonstrated the crystalline nature of CoSb2O6 with a crystallite size of ∼27 nm. Finally, the fabricated thin films were investigated as sensors for LPG and carbon dioxide (CO2) at room temperature (25 °C) and 55% R.H. (relative humidity) with different concentrations in the range of 1000-5000 ppm. The sensing results demonstrated greater variations in the electrical properties of films for the incoming LPG than that of the CO2 gas adsorption. Furthermore, to ensure the long-term stability of fabricated sensors, they were tested periodically at 10 days interval, spanning a total duration of 60 days. In summary, our fabricated LPG sensor displayed high sensitivity (1.96), repeatability, quick response time (21 s) and high long-term stability (99%). Therefore, CoSb2O6 nanospheres can be functionalized as a potential LPG-sensitive material characterized by high sensitivity, reliability and stability at room temperature.

Collector Droplet Behavior during Formation of Nanowire Junctions

Formation of nanowire networks is an appealing strategy for demonstrating novel phenomena at the nanoscale, e.g., detection of Majorana Fermions, as well as an essential step in realizing complex nanowire-based architectures. However, a detailed description of mechanisms taking place during growth of such complex structures is lacking. Here, the experimental observations of gold-catalyzed germanium nanowire junction formation are explained utilizing phase field modeling corroborated with real-time in situ scanning electron microscopy. When the two nanowires collide head on during the growth, we observe two scenarios. (i) Two catalytic droplets merge into one, and the growth continues as a single nanowire. (ii) The droplets merge and subsequently split again, giving rise to the growth of two daughter nanowires. Both the experiments and modeling indicate the critical importance of the liquid-solid growth interface anisotropy and the growth kinetics in facilitating the structural transition during the nanowire merging process.

Strategy of All-Inorganic Cs3Cu2I5/Si-Core/Shell Nanowire Heterojunction for Stable and Ultraviolet-Enhanced Broadband Photodetectors with Imaging Capability

In this study, for the first time, the integration of nontoxic ternary copper halide Cs₃Cu₂I₅ with one-dimensional Si nanowires (NWs) was reported to achieve an ultraviolet (UV)-enhanced Si NW broadband photodetector. A compact and uniform coverage of Cs₃Cu₂I₅ on the top and sidewall of Si NWs formed a core/shell heterostructure, in which Si NWs served as the growth template and the electron-transport layer, and Cs₃Cu₂I₅ was employed as the UV photoactive material and the hole-transport layer. The as-fabricated Cs₃Cu₂I₅/Si-core/shell NW photodetector demonstrates a multiband photodetection from the deep UV to near-infrared region, a fast response speed of 92.5/189.2 μs (265 nm), and a high photoresponsivity of 130 mA/W, nearly 600 times as much as the reference device constructed using Si NWs. More importantly, the proposed photodetector exhibits an excellent stability in air ambient. Typically, it could endure a high temperature of 60 °C for 11 h consecutive working; after storage in air ambient for two weeks, its photodetection ability can almost be retained. Additionally, high-resolution UV imaging applications were presented by employing the proposed photodetector as sensing pixels. These obtained results verify the effectiveness of the Cs₃Cu₂I₅/Si-core/shell NW heterojunction strategy for UV-enhanced broadband photodetection, making such a device really possible for practical applications.

Conjuncted photo-thermoelectric effect in ZnO-graphene nanocomposite foam for self-powered simultaneous temperature and light sensing

The self-powered sensors are more and more important in current society. However, detecting both light and temperature signals simultaneously without energy waste and signal interference is still a challenge. Here, we report a ZnO/graphene nanocomposite foam-based self-powered sensor, which can realize the simultaneous detection of light and temperature by using the conjuncted photo-thermoelectric effect in ZnO-graphene nanocomposite foam sensor. The output current under light, heating and cooling of the device with the best ZnO/graphene ratio (8:1) for the foam can reach 1.75 µA, 1.02 µA and 0.70 µA, respectively, which are approximately three fold higher than them of devices with other ZnO/graphene ratios. The ZnO-graphene nanocomposite foam device also possesses excellent thermoelectric and photoelectric performances for conjuncted lighting and heating detection without mutual interference. The ZnO-graphene nanocomposite foam device exhibits a new designation on the road towards the fabrication of low cost and one-circuit-based multifunction sensors and systems.

Void-free 3D Bioprinting for In-situ Endothelialization and Microfluidic Perfusion

Two major challenges of 3D bioprinting are the retention of structural fidelity and efficient endothelialization for tissue vascularization. We address both of these issues by introducing a versatile 3D bioprinting strategy, in which a templating bioink is deposited layer-by-layer alongside a matrix bioink to establish void-free multimaterial structures. After crosslinking the matrix phase, the templating phase is sacrificed to create a well-defined 3D network of interconnected tubular channels. This void-free 3D printing (VF-3DP) approach circumvents the traditional concerns of structural collapse, deformation and oxygen inhibition, moreover, it can be readily used to print materials that are widely considered "unprintable". By pre-loading endothelial cells into the templating bioink, the inner surface of the channels can be efficiently cellularized with a confluent endothelial layer. This in-situ endothelialization method can be used to produce endothelium with a far greater uniformity than can be achieved using the conventional post-seeding approach. This VF-3DP approach can also be extended beyond tissue fabrication and towards customized hydrogel-based microfluidics and self-supported perfusable hydrogel constructs.

A stable and highly sensitive room-temperature liquefied petroleum gas sensor based on nano-cubes/cuboids of zinc antimonate

Trirutile zinc antimonate (ZnSb2O6) nano-cubes/cuboids have been fabricated by a sol-gel spin-coating method using polyethylene glycol (PEG) as the structure-directing agent. The fabricated films were characterized for surface morphology, along with structural, FT-IR and thermal analysis. The crystallite size of ZnSb2O6 is found to be 35 nm. The fabricated films have been tested for the detection of liquefied petroleum gas (LPG) and carbon dioxide (CO2) gas leakage at room temperature (27 °C). They exhibit fairly high sensitivity (1.73), low response and recovery times (∼41 and 95 s, respectively), and good reproducibility and stability (99.2%) at room temperature for the detection of LPG leakage. Based on these observations, the fabricated film has the potential to be used as a LPG sensor at room temperature.

Self-Assembled Al Nanostructure/ZnO Quantum Dot Heterostructures for High Responsivity and Fast UV Photodetector

HIGHLIGHTS

High performance Al nanostructures/ZnO quantum dots heterostructure photodetectors with a controllable geometry of the Al nanostructures are demonstrated. Light utilization of the photoactive layers is significantly boosted with the Al nanostructures. The light confinement effect is inherently determined by the geometries of the Al nanostructures.

ABSTRACT

Light confinement induced by spontaneous near-surface resonance is inherently determined by the location and geometry of metallic nanostructures (NSs), offering a facile and effective approach to break through the limitation of the light-mater interaction within the photoactive layers. Here, we demonstrate high-performance Al NS/ZnO quantum dots (Al/ZnO) heterostructure UV photodetectors with controllable morphologies of the self-assembled Al NSs. The Al/ZnO heterostructures exhibit a superior light utilization than the ZnO/Al heterostructures, and a strong morphological dependence of the Al NSs on the optical properties of the heterostructures. The inter-diffusion of Al atoms into ZnO matrixes is of a great benefit for the carrier transportation. Consequently, the optimal photocurrent of the Al/ZnO heterostructure photodetectors is significantly increased by 275 times to ~ 1.065 mA compared to that of the pristine ZnO device, and an outstanding photoresponsivity of 11.98 A W⁻¹ is correspondingly achieved under 6.9 mW cm⁻² UV light illumination at 10 V bias. In addition, a relatively fast response is similarly witnessed with the Al/ZnO devices, paving a path to fabricate the high-performance UV photodetectors for applications. [Image: see text]

ELECTRONIC SUPPLEMENTARY MATERIAL

The online version of this article (10.1007/s40820-020-00455-9) contains supplementary material, which is available to authorized users.

Ultrasensitive Fiber-Based ZnO Nanowire Network Ultraviolet Photodetector Enabled by the Synergism between Interface and Surface Gating Effects

A flexible UV photodetector with a high on/off ratio is extremely important for environmental sensing, optical communication, and flexible optoelectronic devices. In this work, a flexible fiber-based UV photodetector with an ultrahigh on/off ratio is developed by utilizing the synergism between interface and surface gating effects on a ZnO nanowire network structure. The synergism between two gating effects is realized by the interplay between surface band bending and the Fermi level through the nanowire network structure, which is proved through the control experiments between the ZnO micro/nanowire photodetector and micro/nanowire junction photodetector, and the corresponding Kelvin probe force microscopy (KPFM) measurements. The on/off ratio of the fiber-based ZnO nanowire network UV photodetector reaches 1.98 × 10⁸ when illuminated by 1.0 mW cm^-2 UV light, which is 20 times larger than the largest reported result under the same UV illumination. This new UV sensor also has a high resolution to UV light intensity change in the nW cm^-2 range. Furthermore, when the fiber-based photodetector is curved, it still shows excellent performance as above. This work gives a new effective route for the development of a high-performance UV photodetector or other optoelectronic detection devices.

Polyoxometalate–viologen photochromic hybrids for rapid solar ultraviolet light detection, photoluminescence-based UV probing and inkless and erasable printing

Polyoxometalate–viologen photochromic hybrids showed efficient and fast low intensity ultraviolet light detection, potential fluorescence based UV probing and inkless and erasable printing.

Synthesis and characterization of high-purity SnO2(ZnO:Sn)m superlattice nanowire arrays with broad-spectrum emissions

High purity SnO₂(ZnO:Sn)_m nanowire arrays growth has been realized for the first time since it was discovered in 2012, displaying an interesting white light emission in the range of around 380–570 nm though having major structural frame of ZnO.

Microscale electrohydrodynamic printing of in situ reactive features for patterned ZnO nanorods

Patterning of zinc oxide (ZnO) nanorods has attracted considerable interests to enhance the performance of ZnO-based functional devices. Most of the existing techniques for patterned ZnO nanorods are based on conventional microfabrication methods that commonly require cleanroom environment, high-cost equipment and complicated processes. In this study, electrohydrodynamic (EHD) printing strategy was accommodated to fabricate microscale ZnO nanorods patterns based on in situ reactive inks. Smaller working voltage and larger nozzle-to-collector distance facilitated the formation of thinner PEO-Zn(NO₃)₂ filaments, which were decomposed into ZnO nanoparticles to serve as the seeding template for the hydrothermal growth of ZnO nanorods. The resultant ZnO nanorods can be flexibly tuned by the EHD printed patterns. The effect of growth time on the size and morphology of ZnO nanorods was investigated. Compared with the spin-coating method, the photoelectrochemical property of patterned ZnO nanorods was well controlled and showed enhanced photoelectrochemical stability. The presented method provides a flexible and rapid way to customize patterned ZnO nanorods that can be potentially used in the fields of optical detectors, biosensors or solar-driven devices.

Redox-Inactive CO2 Determines Atmospheric Stability of Electrical Properties of ZnO Nanowire Devices through a Room-Temperature Surface Reaction

Emerging interactive electronics for the Internet of Things era inherently require the long-term stability of semiconductor devices exposed to air. Nanostructured metal oxides are promising options for such atmospherically stable semiconductor devices owing to their inherent stability in air. Among various oxide nanostructures, ZnO nanowires have been the most intensively studied for electrical and optical device applications. Here, we demonstrate a strategy for achieving the atmospheric electrical stability of ZnO nanowire devices. Although the chemically active oxygen and water in air are strong candidates for affecting the electrical stability of nanoscale metal oxides, we found that the ppm-level redox-inactive CO₂ in air critically determines the atmospheric electrical stability of hydrothermally grown single-crystalline ZnO nanowires. A series of analyses using atmosphere-controlled electrical characterization of single nanowire devices, Fourier transform infrared spectroscopy, scanning transmission electron microscopy, and X-ray photoelectron spectroscopy consistently revealed that atmospheric CO₂ reacts substantially with the ZnO nanowire surfaces, even at room temperature, to form an electrically insulative zinc carbonate thin layer. The formation of this layer essentially limits the atmospheric electrical stability of the ZnO nanowire devices. Based on this surface carbonation mechanism, we propose a strategy to suppress the detrimental surface reaction, which is based on (1) reducing the density of surface hydroxyl groups and (2) improving the nanowire crystallinity by thermal pretreatment. This approach improves the atmospheric electrical stability to at least 40 days in air.

On-chip grown ZnO nanosheet-array with interconnected nanojunction interfaces for enhanced optoelectronic NO2 gas sensing at room temperature

Herein, we demonstrated the on-chip growth of nanostructured ZnO films with abundant nanojunctions for the fabrication of high-performance optoelectronic NO₂ sensors. A fast solution approach allowed the controllable growth of ZnO nanorod- and nanosheet-arrays directly on flexible substrates, which were endowed with abundant nanojunctions. Electron microscopy observations revealed the existence of two types of the nanojunction interfaces, i.e., the attached and interconnected interfaces within the nanostructure networks. Compared with the attached nanorods, the optoelectronic NO₂ sensors based on interconnected ZnO nanosheets showed higher responses and faster response/recovery rates under UV illumination at room temperature. The responses of the nanosheet-based sensor ranged from 28% to 610% toward NO₂ concentrations of 10 ppb to 1000 ppb. Moreover, the optoelectronic sensors exhibited excellent reversibility, and mechanical and long-term stabilities along with low detection limits. The enhanced optoelectronic NO₂ sensing properties of the interconnected ZnO nanosheets could be attributed to different types of nanojunction interfaces, which played a key role in modulating the interfacial potential barrier heights of the nanojunctions according to the surface depletion model. The presently developed strategy of nanojunction interface engineering is expected to have wide interest for semiconductor-based sensor applications.

Growth Mechanism of Sea Urchin ZnO Nanostructures in Aqueous Solutions and Their Photocatalytic Activity for the Degradation of Organic Dyes

This work reports the development of a fast and simple route for the synthesis of ZnO sea urchin (SU) nanostructures by the formation and assembly of ZnO nanorods under favorable growth conditions in an aqueous solution. The thermal treatment of a basic zinc acetate solution in ethanol results in the formation of aggregated seed clusters consisting of small ZnO nanorods, which were then grown in a precursor solution containing Zn(NO₃)₂·6H₂O and hexamethylenetetramine to assemble the SU structures from the anisotropic ZnO nanorods on the surface of the seed clusters. Each ZnO nanoparticle in the aggregated seed clusters grew sequentially into a ZnO nanorod, and the nanorods were concentric to the core of the clusters yielding the unique SU-like shape. In the presence of a capping agent such as cetyl trimethyl ammonium bromide (CTAB), the aggregated seed clusters were not formed, and the growth of the CTAB-capped ZnO nanorods resulted in separated rods with average aspect ratios of ∼10. The SU ZnO nanostructures exhibit a hexagonal wurtzite crystal structure and higher specific surface area (26.9 m²/g) than the CTAB-capped nanorods (17.7 m²/g). The SU ZnO nanostructures show superior photocatalytic efficiency for the degradation of three common organic dyes compared to the ZnO nanorods. The removal efficiencies of indigo carmine, methylene blue, and rhodamine B by the SU nanostructures were 99, 86, and 96%, respectively, after 1 h of UV irradiation. Therefore, the ZnO SU structures have the potential to be a versatile photocatalyst for the photodegradation of organic dyes in industrial wastewater.

Intermediate phase-assisted solution preparation of two dimensional CsPbCl3 perovskite for efficient ultraviolet photodetection

Fully-inorganic halide perovskites (HPs) have realized respectable progress in multiple optoelectronic applications. However, Cl-based fully-inorganic HPs that are ideal for ultraviolet (UV) photodetection applications in high demand still remain rarely explored mainly due to the poor solution processability compared with other counterparts. Here we propose a facile solution method to fabricate CsPbCl₃ with not only high crystallinity but also a two dimensional (2D) morphology for efficient UV photodetection. 2D Ruddlesden-Popper perovskites (RPPs) are firstly prepared as the intermediate phase, which habitually grow into microplates owing to an intrinsic 2D structure. Then Cs⁺ was introduced in the form of highly soluble cesium acetate to exchange with the organic cations in the RPPs to produce 2D CsPbCl₃ with preserved morphology and micron scale size. By this chemical route, the poor solubility issue can be addressed. All the procedures are conducted at room temperature in open air. The perfect band gap, high crystallinity and 2D morphology promise superior UV light sensing capability, one of the best overall performances featuring high responsivity, fast response speed, low driving voltages and good stability is obtained. This work is believed to fill in the "Cl-gap" for this promising class of material.

High-Performance Flexible Ultraviolet Photodetectors Based on AZO/ZnO/PVK/PEDOT:PSS Heterostructures Integrated on Human Hair

Flexible optoelectronics is an emerging research field that has attracted a great deal of interest in recent years due to the special functions and potential applications of these devices in flexible image sensors, optical computing, energy conversion devices, the Internet of Things, and other technologies. Here, we examine the high-performance ultraviolet (UV) photodetectors using AZO/ZnO nanorods/PVK/PEDOT:PSS heterostructures integrated on human hair. Due to the precise interfacial energy-level alignment among all layers and superior mechanical characteristics of human hair, the as-obtained photodetector shows a fast response time, high photoresponsivity, and excellent flexibility. According to integrate 7 heterostructures as 7 display pixels, the flexible UV-image sensor has superior device performance and outstanding flexibility and can produce vivid and accurate images of Arabic numerals from 0 to 9. Different combinations of the two heterostructures can also be used to achieve flexible photon-triggered logic functions, including AND, OR, and NAND gates. Our findings indicate the possibility of using human hair as a fiber-shaped flexible substrate and will allow the use of hair-based hierarchical heterostructures as building blocks to create exciting opportunities for next-generation high-performance, multifunctional, low-cost, and flexible optoelectronic devices.

Transfer Printing Technology as a Straightforward Method to Fabricate Chemical Sensors Based on Tin Dioxide Nanowires

Metal oxide multi-nanowire-based chemical gas sensors were manufactured by a fast and simple transfer printing technology. A two-step method employing spray pyrolysis deposition and a thermal annealing process was used for SnO2 nanowires fabrication. A polydimethylsiloxane stamp was used to transfer the SnO2 nanowires on two different gas sensing devices—Si-based substrates and microhotplate-based platform chips. Both contained a metallic inter-digital electrode structure (IDES), on which the SnO2 nanowires were transferred for realization of multi-NW gas sensor devices. The gas sensor devices show a very high response towards H2S down to the 10 ppb range. Furthermore, a good response towards CO has been achieved, where in particular the microhotplate-based devices exhibit almost no cross sensitivity to humidity.