ZnO nanodisk based UV detectors with printed electrodes

UV photodetectors based on W-doped ZnO thin films

W-doped ZnO thin films deposited on Si substrates with (100) orientation by sol-gel spin coating method at temperature 500 °C. W/Zn atomic ratio varies from 0% to 4%. Then, the UV detection performance analysis ofp-nheterojunction UV photodetectors based on W-doped ZnO/Si is analyzed. The current-voltage curves of W-doped ZnO/Si are investigated in dark and exhibit diode-like rectifying behavior. Among doped ZnO/Si, sample with atomic ratio of W/Zn = 2% is the best candidate to study photodetector characteristics in UV range. The resulting device exhibits a rectification ratioRRof 5587 at ±5 V, a higher responsivity of 3.84 A W-1and a photosensitivity value of 34 at 365 nm under 0.5 mW cm-2. The experimental findings reveal that the UV detection performance of the heterojunction-based photodetectors strongly dependent on the properties of metal oxide layer. The main goal of this work is to investigate the effect of W doping on the performance of ZnO/Si based photodetectors. Based on our results, it is observed that 2 at% of W dopant is the optimum amount of doping for high performance photodetector of ZnO:W/Si heterojunction thanks to the suppressed recombination ratio and enhanced carrier separation properties in the depletion zone.

Building Low-Cost, High-Performance Flexible Photodetector Based on Tetragonal Phase VO2 (A) Nanorod Networks

We present a straightforward and cost-effective method for the fabrication of flexible photodetectors, utilizing tetragonal phase VO2 (A) nanorod (NR) networks. The devices exhibit exceptional photosensitivity, reproducibility, and stability in ambient conditions. With a 2.0 V bias voltage, the device demonstrates a photocurrent switching gain of 1982% and 282% under irradiation with light at wavelengths of 532 nm and 980 nm, respectively. The devices show a fast photoelectric response with rise times of 1.8 s and 1.9 s and decay times of 1.2 s and 1.7 s for light at wavelengths of 532 nm and 980 nm, respectively. In addition, the device demonstrates exceptional flexibility across large-angle bending and maintains excellent mechanical stability, even after undergoing numerous extreme bending cycles. We discuss the electron transport process within the nanorod networks, and propose a mechanism for the modulation of the barrier height induced by light. These characteristics reveal that the fabricated devices hold the potential to serve as a high-performance flexible photodetector.

Out-of-Plane Electronics on Flexible Substrates Using Inorganic Nanowires Grown on High-Aspect-Ratio Printed Gold Micropillars

Out-of-plane or 3D electronics on flexible substrates are an interesting direction that can enable novel solutions such as efficient bioelectricity generation and artificial retina. However, the development of devices with such architectures is limited by the lack of suitable fabrication techniques. Additive manufacturing (AM) can but often fail to provide high-resolution, sub-micrometer 3D architectures. Herein, the optimization of a drop-on-demand (DoD), high-resolution electrohydrodynamic (EHD)-based jet printing method for generating 3D gold (Au) micropillars is reported. Libraries of Au micropillar electrode arrays (MEAs) reaching a maximum height of 196 µm and a maximum aspect ratio of 52 are printed. Further, by combining AM with the hydrothermal growth method, a seedless synthesis of zinc oxide (ZnO) nanowires (NWs) on the printed Au MEAs is demonstrated. The developed hybrid approach leads to hierarchical light-sensitive NW-connected networks exhibiting favorable ultraviolet (UV) sensing as demonstrated via fabricating flexible photodetectors (PDs). The 3D PDs exhibit an excellent omnidirectional light-absorption ability and thus, maintain high photocurrents over wide light incidence angles (±90°). Lastly, the PDs are tested under both concave and convex bending at 40 mm, showing excellent mechanical flexibility.

Liquid-metal based flexible a-IZTO ultrathin films for electrical and optical applications

Amorphous indium zinc tin oxide (a-IZTO) is a kind of transparent conductive oxide (TCO), which can be used in transparent electrodes, transistors, and flexible devices. At present, a key limitation of a-IZTO is the costly vacuum manufacturing technology, and its commercial production is also restricted by the complex raw material preparation process. In this article, we report a liquid metal-based van der Waals (vdW) exfoliation technique by which a-IZTO films with several nanometres thickness are fabricated. The a-IZTO films fabricated in ambient air have a size on the centimeter scale and an optical transmittance of 99.64%; they are also large-area flexible oxide films. In order to illustrate the capabilities of this technology, we fabricated thin film transistors (TFTs) and photodetectors based on a-IZTO films. An a-IZTO thin film transistor (TFT) has an on/off ratio of 10⁶. When the V_ds is 5 V, the responsivity, detectivity and external quantum efficiency of an a-IZTO photodetector are 7.57 × 10⁴ A W^-1, 4.00 × 10¹⁵ Jones and 3.68 × 10⁵%, respectively, exhibiting one of the top performances in this field.

On-substrate fabrication of a self-activated nanostructured ZnO gas sensor

Gaining rational control over bottom-up device fabrication processes is necessary to achieve high-performance devices and overcome technical obstacles. Among these is the need for activation of metal oxide gas sensors (GSs) by an external heating source, which limits their miniaturization and integration. A well-controlled, seedless, and position-selective hydrothermal method to fabricate high-performance self-activated zinc oxide (ZnO) nano-needle (ZNN) GSs directly on a substrate was developed. The morphology and position of the grown ZnO nanostructures were controlled by tuning the substrate coating and growth reaction parameters such as the growth solution concentration and the growth time, as well as introducing capping agents to the growth solution during the growth process. Furthermore, the efficiency of the fabricated device structure was improved and subsequently enhanced its performance substantially. Compared to other fabricated nanostructured ZnO GSs, the on-substrate fabricated bridging ZNN (BZNN) GS demonstrated superior sensitivity and self-activation, which were attributed to the reduction in the sensing material dimensions and ultrahigh surface-to-volume ratio, as well as the unique device structure with direct contact between ZnO and Au electrodes. This work paves the way for low cost, large scale, low temperature, seedless and position-selective fabrication of high-performance self-activated nanostructured ZnO GSs on flexible and transparent substrates.

Surface Engineering of Quantum Dots for Self-Powered Ultraviolet Photodetection and Information Encryption

We demonstrate fabrication of photodetectors in the UVC and UVA regions, based on surface engineering of Mn²⁺-doped ZnS Qdot. Mn²⁺-doped ZnS Qdot exhibited UVC detection with a responsivity of 0.3 ± 0.02 A·W^-1 and detectivity of 1.7 ± 0.2 10¹¹ Jones. Following this, the Qdot was surface modified with 8-hydroxyquinoline 5-sulfonic acid ligand, which resulted in the formation of a bluish green zinc quinolate complex (Zn(QS)₂) at the Qdot surface (defined as the quantum dot complex, QDC) exhibiting overall white photoluminescence. The detector developed with QDC as the photoactive material exhibited a responsivity of 0.2 ± 0.02 A·W^-1 and detectivity of 1.2 ± 0.2 10¹¹ Jones in the UVA band. This shift in the detection band from UVC in Qdot to UVA in QDC, through the surface complexation mechanism, is a new approach for tuning spectral detection featured in this work. Besides, the self-powered response of both the detectors exhibited attractive photoelectric characteristics. The detectors were incorporated in a portable prototype to show their potential application toward selective UVC and UVA spectral detection. Additionally, the dual-mode emission of the QDC was used for data encryption and decryption.

Interfacial modulation and plasmonic effect mediated high-brightness green light sources in a single Ga-doped ZnO microwire based heterojunction

Heterostructure manufacturing has been extensively studied as indispensable footstones in the progressive semiconductor optoelectronic devices due to their constituent materials, interfacial states and electronic transport capabilities, thus enabling competitive candidates...

Current Trends in Nanomaterials for Metal Oxide-Based Conductometric Gas Sensors: Advantages and Limitations. Part 1: 1D and 2D Nanostructures

This article discusses the main uses of 1D and 2D nanomaterials in the development of conductometric gas sensors based on metal oxides. It is shown that, along with the advantages of these materials, which can improve the parameters of gas sensors, there are a number of disadvantages that significantly limit their use in the development of devices designed for the sensor market.

State of the Art in Alcohol Sensing with 2D Materials

Since the discovery of graphene, the star among new materials, there has been a surge of attention focused on the monatomic and monomolecular sheets which can be obtained by exfoliation of layered compounds. Such materials are known as two-dimensional (2D) materials and offer enormous versatility and potential. The ultimate single atom, or molecule, thickness of the 2D materials sheets provides the highest surface to weight ratio of all the nanomaterials, which opens the door to the design of more sensitive and reliable chemical sensors. The variety of properties and the possibility of tuning the chemical and surface properties of the 2D materials increase their potential as selective sensors, targeting chemical species that were previously difficult to detect. The planar structure and the mechanical flexibility of the sheets allow new sensor designs and put 2D materials at the forefront of all the candidates for wearable applications. When developing sensors for alcohol, the response time is an essential factor for many industrial and forensic applications, particularly when it comes to hand-held devices. Here, we review recent developments in the applications of 2D materials in sensing alcohols along with a study on parameters that affect the sensing capabilities. The review also discusses the strategies used to develop the sensor along with their mechanisms of sensing and provides a critique of the current limitations of 2D materials-based alcohol sensors and an outlook for the future research required to overcome the challenges.

Electrochemical-Based Biosensors on Different Zinc Oxide Nanostructures: A Review

Electrochemical biosensors have shown great potential in the medical diagnosis field. The performance of electrochemical biosensors depends on the sensing materials used. ZnO nanostructures play important roles as the active sites where biological events occur, subsequently defining the sensitivity and stability of the device. ZnO nanostructures have been synthesized into four different dimensional formations, which are zero dimensional (nanoparticles and quantum dots), one dimensional (nanorods, nanotubes, nanofibers, and nanowires), two dimensional (nanosheets, nanoflakes, nanodiscs, and nanowalls) and three dimensional (hollow spheres and nanoflowers). The zero-dimensional nanostructures could be utilized for creating more active sites with a larger surface area. Meanwhile, one-dimensional nanostructures provide a direct and stable pathway for rapid electron transport. Two-dimensional nanostructures possess a unique polar surface for enhancing the immobilization process. Finally, three-dimensional nanostructures create extra surface area because of their geometric volume. The sensing performance of each of these morphologies toward the bio-analyte level makes ZnO nanostructures a suitable candidate to be applied as active sites in electrochemical biosensors for medical diagnostic purposes. This review highlights recent advances in various dimensions of ZnO nanostructures towards electrochemical biosensor 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.

Fast response UV detection based on waveguide characteristics of vertically grown ZnO nanorods partially embedded in anodic alumina template

Zinc oxide (ZnO)-based ultraviolet (UV) detector has been fabricated and its photoresponse is studied in an out-of-plane contact configuration. Porous anodic aluminum oxide (AAO) template-based deposition method is adopted for the aligned and well-separated growth of ZnO nanorods (NRs). Through-hole in silicon (Si) by modified metal assisted chemical etching is used as a window for the electrochemical deposition of ZnO in the template and for out-of-plane electrical contacts during device analysis. The fabricated photodetector shows a fast response under UV (365 nm) light illumination, with rise and decay times of 31 ± 2 ms and 85 ± 3 ms, respectively. This fast response is analysed in terms of vertical growth and the waveguide nature of ZnO NRs embedded in anodic alumina. These results are further supported by a simulation comparing the electric field distribution of ZnO NR embedded in AAO with that of bare ZnO NR.

Ultrahigh photosensitivity of the polar surfaces of single crystalline ZnO nanoplates

Single crystalline ZnO nanoplatelet structures were synthesized via a hydrothermal process on the surface of GaN microparticles. Growth of ZnO seeded on the GaN surface promoted faster growth along the directions within the basal plane of the ZnO crystal structure, resulting in the formation of 2-dimensional nanoplates with a thickness less than a few tens of nanometers at most. Electrical conduction across an individual nanoplate was measured and found to be extremely sensitive to UV illumination and the surrounding atmospheric environment. Such electrical behaviors of the nanoplates were attributed to the dominance of the polar (0001) surfaces and the adsorption and desorption of the ambient gas molecules on these surfaces. Their coupling with conduction electrons near the surface is the critical factor responsible for the highly sensitive electrical properties of the nanoplate. Virtually the entire volume of the nanoplates is under the influence of the surface adsorbed molecules, which changes the electrical properties of the nanoplates extensively, depending on their environmental conditions. Combining the very high photocurrent to dark current ratio and the high effective resistance of the ZnO nanoplates reported in the present study, ultrasensitive photo-devices operating at very low power can be expected with the use of 2-dimensional nanoplates.

Ultraviolet sensing using a TiO2 nanotube integrated high resolution planar microwave resonator device

This paper presents a unique integrated UV light sensing concept and introduces a device with a detection limit of 1.96 nW cm^-2. The combination of a high quality factor, a microwave planar resonator (Q ∼ 50 000) with a semiconducting nanomaterial enables a revolutionary potential paradigm for photodetection of low light intensities and small form factors. The presenting device employs a high-resolution microwave microstrip resonator as the signal transducer to convert the variant dielectric properties (permittivity and conductivity) of the nanotube membrane into electrical signals such as the resonant frequency, quality factor and resonant amplitude. The microwave resonator has an active feedback loop to improve the initial quality factor of the resonator from 200 to 50 000 and leads to boosting of the sensing resolution by orders of magnitude. Anatase TiO₂ nanotubes are assembled on the surface of the microwave resonator. Upon exposure to UV light, electron-hole pair generation, trapping and recombination in the nanotubes are exploited as a unique signature to quantify the UV light intensity. The change of dielectric properties of the nanotube membrane is monitored using the underlying active microwave resonator. The proposed concept enables the detection and monitoring of UV light at high resolution, with very small exposure power and integrated form factors.

Versatile NiO/mesoporous carbon nanodisks: controlled synthesis from hexagon shaped heterobimetallic metal-organic frameworks

Hexagonal NiO/mesoporous carbon nanodisks (NiO/MCN) are facilely and controllably synthesized via constructing nickel-zinc trimesic acid heterobimetallic metal-organic framework (HMOF) disks before pyrolysis at 910 °C. Tailoring the Ni/(Zn + Ni) feed ratio and the reaction time during the HMOF synthesis creates a well-defined hexagonal carbon nanodisk with properly populated NiO nanocrystals while maintaining high porosity and conductivity. Such an elaborately fabricated NiO/MCN is highly stable, and exhibits the largest specific capacitance of 261 F g^-1 and the highest specific activity factor of 1.93 s^-1 g^-1 of any composite nanodisk during the capacitive test and 4-nitrophenol reduction, respectively.

Three-dimensional nano-heterojunction networks: a highly performing structure for fast visible-blind UV photodetectors

Visible-blind ultraviolet photodetectors are a promising emerging technology for the development of wide bandgap optoelectronic devices with greatly reduced power consumption and size requirements. A standing challenge is to improve the slow response time of these nanostructured devices. Here, we present a three-dimensional nanoscale heterojunction architecture for fast-responsive visible-blind UV photodetectors. The device layout consists of p-type NiO clusters densely packed on the surface of an ultraporous network of electron-depleted n-type ZnO nanoparticles. This 3D structure can detect very low UV light densities while operating with a near-zero power consumption of ca. 4 × 10^-11 watts and a low bias of 0.2 mV. Most notably, heterojunction formation decreases the device rise and decay times by 26 and 20 times, respectively. These drastic enhancements in photoresponse dynamics are attributed to the stronger surface band bending and improved electron-hole separation of the nanoscale NiO/ZnO interface. These findings demonstrate a superior structural design and a simple, low-cost CMOS-compatible process for the engineering of high-performance wearable photodetectors.

On-chip fabrication of high performance nanostructured ZnO UV detectors

Developing rationally controlled bottom-up device fabrication processes is essential for the achievement of high performance optimal devices. We report a controlled, seedless and site-selective hydrothermal technique to fabricate high-performance nanostructured ZnO UV-detectors directly on-chip. We demonstrate that by controlling the nanowire growth process, via tuning the experimental parameters such as the concentration of reactants and the growth time, and by introducing a refresh of the growth solution, the device structure efficiency can be enhanced to significantly improve its performance. The on-chip fabricated bridging nanosyringe ultraviolet detector demonstrates improved sensitivity (~10(5)), nanowatts detectability, and ultrafast response-time (90 ms) and recovery-time (210 ms). The improvement in response-time and recovery-time is attributed to the unique nanowire-nanowire junction barrier dominated resistance and the direct contact between ZnO and Au electrodes. Furthermore, the enhanced sensitivity and nanowatts detectability of the bridging nanosyringe device are due to the reduction in dimensionality and ultrahigh surface-to-volume ratio. This work paves the way toward low cost, large scale, low temperature, seedless and site-selective fabrication of high performance ZnO nanowire sensors on flexible and transparent substrates.

Facile synthesis of core/shell ZnO/ZnS nanofibers by electrospinning and gas-phase sulfidation for biosensor applications

This study describes a new method of passivating ZnO nanofiber-based devices with a ZnS layer.

A model for the impact of the nanostructure size on its gas sensing properties

The size of a metal oxide nanostructure plays a key role in its performance as a gas sensor.

Spontaneous hyper-branching in ZnO nanostructures: morphology dependent electron emission and light detection

The structure and intrinsic defect-induced electron field emission and photodetection are monitored in ZnO nanoforms with assorted morphology prepared in ambient conditions via a facile wet chemical approach.

Hexagonal ZnO nanorings: synthesis, formation mechanism and trimethylamine sensing properties

ZnO nanorings are synthesized by the Ostwald ripening of ZnO nanoplates and the porous film formed by ZnO nanorings shows an excellent trimethylamine-sensing property.

Hierarchically designed ZnO nanostructure based high performance gas sensors

Enhanced gas sensing properties of ZnO were achieved by designing hierarchical nanostructures with high surface-to-volume ratios and more exposed polar facets.