Low-cost fully transparent ultraviolet photodetectors based on electrospun ZnO-SnO2 heterojunction nanofibers

E-beam synthesized fast-switching TiO2/SnO2 type-II heterostructure photodetector

A fast-switching T i O 2/S n O 2 heterostructure thin-film (TF) photodetector synthesized by electron beam evaporation technique is analyzed in this study. The substrate utilized is n-type silicon (Si), while gold (Au) is employed as the top electrode. To assess sample morphology and confirm elemental composition, field emission scanning electron microscopy (FESEM), energy dispersive x-ray spectroscopy (EDS), and chemical mapping were conducted. Structural characteristics were determined using X-ray diffraction (XRD) analysis. The XRD analysis confirmed the presence of various phases of T i O 2 (anatase and rutile) and S n O 2 (rutile). UV-Vis spectroscopy revealed multiple absorption peaks, at 447 nm, 495 nm, 560 nm, and 673 nm, within the visible spectrum. The device demonstrates high detectivity (D ∗) of 1.737×109 Jones and a low noise equivalent power (NEP) of 0.765×10-10 W. Evaluation of the device's switching response through current-time characteristic (I-T) analysis indicates rapid switching with a rise time and fall time of 0.33 s and 0.36 s, respectively.

Flexible all-organic photodetectors via universal water-assisted transfer printing

Transfer printing of small-molecular organic semiconductors often faces challenges due to surface adhesion mismatch. Here, we developed a sacrificing-layer-assisted transfer printing technique for the deposition of small-molecular thin films. High-boiling-point ethylene glycol (EG) was doped in aqueous solution poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as the sacrificing layer to manipulate residual water in film, which allowed chlorobenzene solution to spontaneously spread and form uniform film. The residual water guaranteed film delamination from the stamp, allowing for its transfer onto various substrates and seeding layers. As a proof of concept, laterally conductive organic photodetectors using recyclable EG-PEDOT:PSS electrodes and a small-molecular active layer were consecutively fabricated via transfer printing in ambient air. The resulting device exhibited a high on/off ratio of 711 and a fast rise time of 0.5 ms. Notably, the polymer electrode and the bulk heterojunction demonstrated unique repairability and recyclability.

Piezo-phototronic effect regulated broadband photoresponse of a-Ga2O3/ZnO heterojunction

Amorphous Ga₂O₃ (a-Ga₂O₃) films have attracted considerable attention in the field of photodetectors due to their excellent optical absorption response and photoelectric properties. However, there are few studies that have utilized the piezo-phototronic effect to regulate the broadband photoresponse of Ga₂O₃-based photodetectors. Here, a flexible a-Ga₂O₃/ZnO heterojunction was constructed, which demonstrated a broadband response range from deep ultraviolet (265 nm) to the near-infrared (1060 nm) and realized a bidirectional adjustable photocurrent response via the piezo-phototronic effect. Under 265 nm illumination and 0.5 V bias, the responsivity and detectivity of the a-Ga₂O₃/ZnO heterojunction reached up to 12.19 A W^-1 and 4.71 × 10¹¹ Jones under 0.164% compressive strain, corresponding to enhancements of 67.7% and 66.8% compared to those under a strain-free state, respectively. Moreover, the broadband photoresponse of the a-Ga₂O₃/ZnO heterojunction beyond the bandgap limit was tunable under bidirectional strain. The working mechanism of photoresponse performance for the a-Ga₂O₃/ZnO heterojunction at different wavelengths was elucidated in detail. Oxygen vacancy-assisted carrier generation was found to be influenced by the wavelength of incident light, which mainly determined the broadband photoresponse of the heterojunction. The modulation of the a-Ga₂O₃/ZnO heterojunction photodetector was interpreted in light of the strain-induced regulation of the barrier height. This work represents an important step toward the development of adjustable broadband photodetectors based on a-Ga₂O₃ films.

Mode-tunable, micro/nanoscale electrohydrodynamic deposition techniques for optoelectronic device fabrication

The rapid development of fascinating new optoelectronic materials and devices calls for the innovative production of micro/nanostructures in a high-resolution, large-scale, low-cost fashion, preferably compatible with flexible/wearable applications. Powerful electrohydrodynamic (EHD) deposition techniques, which generate micro/nanostructures using high electrical forces, exhibit unique advantages in high printing resolution (<1 μm), tunable printing modes (electrospray for films, electrospinning for fibers and EHD jet printing for dots), and wide material applicability (viscosity 1-10 000 cps), making them attractive in the fabrication of high-density and high-tech optoelectronic devices. This review highlights recent advances related to EHD-deposited optoelectronics, ranging from solar cells, photodetectors, and light-emitting diodes, to transparent electrodes, with detailed descriptions of the EHD-based jetting mechanism, ink formulation requirements and corresponding jetting modes to obtain functional micro/nanostructures. Finally, a brief summary and an outlook on the future perspectives are proposed.

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.

Bridging 1D Inorganic and Organic Synthesis to Fabricate Ultrathin Bismuth-Based Nanotubes with Controllable Size as Anode Materials for Secondary Li Batteries

The growth of ultrathin 1D inorganic nanomaterials with controlled diameters remains challenging by current synthetic approaches. A polymer chain templated method is developed to synthesize ultrathin Bi₂ O₂ CO₃ nanotubes. This formation of nanotubes is a consequence of registry between the electrostatic absorption of functional groups on polymer template and the growth habit of Bi₂ O₂ CO₃ . The bulk bismuth precursor is broken into nanoparticles and anchored onto the polymer chain periodically. These nanoparticles react with the functional groups and gradually evolve into Bi₂ O₂ CO₃ nanotubes along the chain. 5.0 and 3.0 nm tubes with narrow diameter deviation are synthesized by using branched polyethyleneimine and polyvinylpyrrolidone as the templates, respectively. Such Bi₂ O₂ CO₃ nanotubes show a decent lithium-ion storage capacity of around 600 mA h g^-1 at 0.1 A g^-1 after 500 cycles, higher than other reported bismuth oxide anode materials. More interestingly, the Bi materials developed herein still show decent capacity at very low temperatures, that is, around 330 mA h g^-1 (-22 °C) and 170 mA h g^-1 (-35 °C) after 75 cycles at 0.1 A g^-1 , demonstrating their promising potential for practical application in extreme conditions.

Research and Progress of Transparent, Flexible Tin Oxide Ultraviolet Photodetector

Optical detection is of great significance in various fields such as industry, military, and medical treatment, especially ultraviolet (UV) photodetectors. Moreover, as the demand for wearable devices continues to increase, the UV photodetector, which is one of the most important sensors, has put forward higher requirements for bending resistance, durability, and transparency. Tin oxide (SnO2) has a wide band gap, high ultraviolet exciton gain, etc., and is considered to be an ideal material for preparing UV photodetectors. At present, SnO2-based UV photodetectors have a transparency of more than 70% in the visible light region and also have excellent flexibility of 160% tensile strain. Focusing on SnO2 nanostructures, the article mainly summarizes the progress of SnO2 UV photodetectors in flexibility and transparency in recent years and proposes feasible optimization directions and difficulties.

Improved Photoresponse Characteristics of a ZnO-Based UV Photodetector by the Formation of an Amorphous SnO2 Shell Layer

Although ZnO nanostructure-based photodetectors feature a well-established system, they still present difficulties when being used in practical situations due to their slow response time. In this study, we report on how forming an amorphous SnO₂ (a-SnO₂) shell layer on ZnO nanorods (NRs) enhances the photoresponse speed of a ZnO-based UV photodetector (UV PD). Our suggested UV PD, consisting of a ZnO/a-SnO₂ NRs core–shell structure, shows a rise time that is 26 times faster than a UV PD with bare ZnO NRs under 365 nm UV irradiation. In addition, the light responsivity of the ZnO/SnO₂ NRs PD simultaneously increases by 3.1 times, which can be attributed to the passivation effects of the coated a-SnO₂ shell layer. With a wide bandgap (~4.5 eV), the a-SnO₂ shell layer can successfully suppress the oxygen-mediated process on the ZnO NRs surface, improving the photoresponse properties. Therefore, with a fast photoresponse speed and a low fabrication temperature, our as-synthesized, a-SnO₂-coated ZnO core–shell structure qualifies as a candidate for ZnO-based PDs.

Low-voltage and fast-response SnO2nanotubes/perovskite heterostructure photodetector

One-dimensional metal-oxides (1D-MO) nanostructure has been regarded as one of the most promising candidates for high-performance photodetectors due to their outstanding electronic properties, low-cost and environmental stability. However, the current bottlenecks are high energy consumption and relatively low sensitivity. Here, Schottky junctions between nanotubes (NTs) and FTO were fabricated by electrospinning SnO₂NTs on FTO glass substrate, and the bias voltage of SnO₂NTs photodetectors was as low as ∼1.76 V, which can effectively reduce energy consumption. Additionally, for improving the response and recovery speed of SnO₂NTs photodetectors, the NTs were covered with organic/inorganic hybrid perovskite. SnO₂NTs/perovskite heterostructure photodetectors exhibit fast response/recovery speed (∼0.075/0.04 s), and a wide optical response range (∼220-800 nm). At the same time, the bias voltage of heterostructure photodetectors was further reduced to 0.42 V. The outstanding performance is mainly attributed to the formation of type-II heterojunctions between SnO₂NTs and perovskite, which can facilitate the separation of photogenerated carriers, as well as Schottky junction between SnO₂NTs and FTO, which reduce the bias voltage. All the results indicate that the rational design of 1D-MO/perovskite heterostructure is a facile and efficient way to achieve high-performance photodetectors.

Electrochemical synthesis of hydroxyl group-functionalized PProDOT/ZnO for an ultraviolet photodetector

Ultraviolet (UV) detectors based on zinc oxide (ZnO) nanorods (NRs) are ideal materials for UV radiation detection. However, owing to the surface effect of ZnO NRs, their speed of photoresponse and photosensitivity need to be improved. In this study, a UV photodetector was fabricated via electrochemical coating of poly(3,4-propylenedioxythiophene) grafted with functional groups (-OH) on a hydrothermally grown ZnO NRs. For comparison, poly(3,4-propylenedioxythiophene)/ZnO composites were synthesized using the same method. The structure of the composite film was characterized by Fourier transform infrared spectroscopy (FT-IR), UV-visible spectroscopy (UV-vis), X-ray diffraction (XRD), Raman spectroscopy (Raman), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). The effect of the polymer structure on the UV sensing ability of ZnO NRs was evaluated by fabricating a UV detector with a composite material. The structural results indicated that the PProDOT-type conductive polymer and ZnO composites were successfully synthesized. The UV photodetection results showed that the presence of functional groups (-OH) in polymer chains could enhance the responsivity of the material. The response time of the ZnO/PProDOT-OH composite was 15 s shorter than that of the ZnO/PProDOT composite. A rise in photocurrent induced an increase from 2.5 A W^-1 to 34.75 A W^-1 in the UV photoresponsivity of the ZnO/PProDOT-OH composite, compared with that of the pure ZnO NRs. The external quantum efficiency and detectivity significantly improved, the increases of which were attributed to the coupling of the polymer and ZnO NRs.

Preparation and Applications of Electrospun Optically Transparent Fibrous Membrane

The optically transparent electrospun fibrous membrane has been widely used in many fields due to its simple operation, flexible design, controllable structure, high specific surface area, high porosity, and unique excellent optical properties. This paper comprehensively summarizes the preparation methods and applications of an electrospun optically transparent fibrous membrane in view of the selection of raw materials and structure modulation during preparation. We start by the factors that affect transmittance among different materials and explain the light transmission mechanism of the fibrous membrane. This paper also provides an overview of the methods to fabricate a transparent nanofibrous membrane based on the electrospinning technology including direct electrospinning, solution treatment after electrospinning, heat treatment after electrospinning, and surface modification after electrospinning. It further summarizes the differences in the processes and mechanisms between different transparent fibrous membranes prepared by different methods. Additionally, we study the utilization of transparent as-spun membranes as flexible functional materials, namely alcohol dipstick, air purification, self-cleaning materials, biomedicine, sensors, energy and optoelectronics, oil-water separation, food packaging, anti-icing coating, and anti-corrosion materials. It demonstrates the high transparency of the nanofibers' effects on the applications as well as upgrades the product performance.

High-performance perovskite photodetectors based on CH3NH3PbBr3 quantum dot/TiO2 heterojunction

Organo-lead halide perovskite materials have opened up a great opportunity to develop high performance photodetectors because of their superior optoelectronic properties. The main issue with perovskite-only photodetector is severe carrier recombination. Integration of perovskite with high-conductive materials such as graphene or transition metal sulfides certainly improved the photoresponsivity. However, achieving high overall performance remains a challenge. Here, an improved photodetector is constructed by perovskite quantum dots (QDs) and atomic layer deposited ultrathin TiO2 films. The designed CH3NH3PbBr3 QD/TiO2 bilayer device displays inclusive performance with on/off ratio of 6.3 × 102, responsivity of 85 A W-1, and rise/decay time of 0.09/0.11 s. Furthermore, we demonstrate that interface plays a crucial role in determining the device current and enhance the overall performance of heterostructure photodetector through interface engineering. We believe that this work can provide a strategy to accelerate development of high-performance solution-processed perovskite photodetectors.

Boosting the performance of ZnO microrod metal-semiconductor-metal photodetectors via surface capping of thin amorphous Al2O3 shell layer

1D ZnO nanostructures have been widely explored due to their potential applications in ultraviolet (UV) region photodetectors because of their unique structural and optoelectronic properties. However, a large number of surface defect states leading to a noticeable dark current hinders their practical applications in UV photodetection. In this work, we have shown improved ZnO/Al₂O₃ core-shell microrod photodetectors, whose performance is significantly enhanced by defect passivation and the introduction of trap states by atomic layer deposition grown thin amorphous Al₂O₃ shell layer, as evidenced by steady-state and transient photoluminescence investigations. The photodetectors demonstrated suppressed dark current and increased photocurrent after capping the Al₂O₃ layer. Specifically, the ZnO/Al₂O₃ core-shell microrod photodetector exhibited a photoresponsivity as high as 0.019 A/(W cm^-2) with the dark current as low as ∼1 × 10^-11 A, and a high I _light/I _dark ratio of ∼10⁴ under relatively weak light illumination (∼10 μW cm^-2). The results presented in this work provide valuable pathways to boost the performance of 1D ZnO microrod-based photodetectors for future practical applications.

Artificial Carbon Graphdiyne: Status and Challenges in Nonlinear Photonic and Optoelectronic Applications

The creative integration of sp-hybridized carbon atoms into artificial carbon graphdiyne has led to graphdiyne with superior properties in terms of uniformly distributed pores, ambipolar carrier transport, natural bandgap, and broadband absorption. Consequently, graphdiyne, regarded as a promising carbon material, has garnered particular attention in light-matter interactions. Light-matter interactions play an important role in optical information technology and meet the increasing demand for various energy sources. Herein, the status and challenges in nonlinear photonic and optoelectronic applications of graphdiyne, which are still in the infancy stage, are summarized. Furthermore, the bottleneck and perspective of graphdiyne in these aspects are discussed. It is therefore anticipated that this review could promote the development of graphdiyne in photonic and optoelectronic fields.

Enhancement in the Detection Ability of Metal Oxide Sensors Using Defect-Rich Polycrystalline Nanofiber Devices

The development of SnO2 and TiO2 polycrystalline nanofiber devices (PNFDs) has been widely researched as a method of protecting humans from household air pollution. PNFDs have three significant advantages. The nanofibers before the annealing process are polymer-rich materials, which can be used as particulate material (PM) filters. The multiporous nanofibers fabricated by the annealing process have numerous defects that can serve as generation-recombination centers for electron-hole pairs, enabling the PNFDs to serve as multiple-wavelength light (from 365 to 940 nm) detectors. Lastly, the numerous surface/interface defects can drastically enhance the toxic gas detection ability. The toxic gas detection range of PNFDs for CO(g) and NO(g) is from 400 to 50 ppm and 400 to 50 ppb, respectively. Quick response times and recovery properties are key parameters for commercial applications. The recovery time of NO(g) detection can be improved from 1 ks to 40 s and the PNFD operating temperature lowered to 50 °C. These results indicate that SnO2 and TiO2 PNFDs have good potential for commercialization and use as toxic gas and photon sensors in daily lives.

Ultrasensitive Flexible Solar-Blind Photodetectors Based on Graphene/Amorphous Ga2O3 van der Waals Heterojunctions

Flexible photodetectors (PDs) have become the latest research interest owing to their potential applications in future implantable sensors and foldable/wearable optoelectronics. Ga₂O₃, an emerging ultrawide band gap semiconductor, is considered as the native photosensitive material for solar-blind PDs. The reported fabrication temperature of Ga₂O₃ films is usually above 600 °C, which hinders its practical application for flexible devices. In this work, flexible PDs based on graphene/amorphous Ga₂O₃ van der Waals heterojunctions are fabricated, which demonstrate promising photoresponse to solar-blind ultraviolet light. The device yields a high photo-to-dark current ratio (∼10⁵) and large responsivity (22.75 A/W) under 254 nm light illumination, which could be ascribed to the efficient photogenerated electron-hole pair separation by the strong built-in field. Moreover, flexible PDs also show long-term environmental stability and outstanding mechanical flexibility without any encapsulation. Our work provides a new potential candidate for realizing cost-effective high-performance flexible optoelectronic applications.

Graphene Quantum Dot-Sensitized ZnO-Nanorod/GaN-Nanotower Heterostructure-Based High-Performance UV Photodetectors

The fabrication of a superior-performance ultraviolet (UV) photodetector utilizing graphene quantum dots (GQDs) as a sensitization agent on a ZnO-nanorod/GaN-nanotower heterostructure has been realized. GQD sensitization displays substantial impact on the electrical as well as the optical performance of a heterojunction UV photodetector. The GQD sensitization stimulates charge carriers in both ZnO and GaN and allows energy band alignment, which is realized by a spontaneous time-correlated transient response. The fabricated device demonstrates an excellent responsivity of 3.2 × 10³ A/W at -6 V and displays an enhancement of ∼265% compared to its bare counterpart. In addition, the fabricated heterostructure UV photodetector exhibits a very high external quantum efficiency of 1.2 × 10⁶%, better switching speed, and signal detection capability as low as ∼50 fW.

Recent Advances of Spatial Self-Phase Modulation in 2D Materials and Passive Photonic Device Applications

Optical nonlinearity in 2D materials excited by spatial Gaussian laser beam is a novel and peculiar optical phenomenon, which exhibits many novel and interesting applications in optical nonlinear devices. Passive photonic devices, such as optical switches, optical logical gates, photonic diodes, and optical modulators, are the key compositions in the future all-optical signal-processing technologies. Passive photonic devices using 2D materials to achieve the device functionality have attracted widespread concern in the past decade. In this Review, an overview of the spatial self-phase modulation (SSPM) in 2D materials is summarized, including the operating mechanism, optical parameter measurement, and tuning for 2D materials, and applications in photonic devices. Moreover, some current challenges are also proposed to solve, and some possible applications of SSPM method are predicted for the future. Therefore, it is anticipated that this summary can contribute to the application of 2D material-based spatial effect in all-optical signal-processing technologies.

Modulating electrical and photoelectrical properties of one-step electrospun one-dimensional SnO2 arrays

One-dimensional nanostructured SnO₂ has attracted intense research interest due to its advantageous properties, including a large surface-to-volume ratio, high optical transparency and typical n-type properties. However, how to fabricate high-performance and multifunctional electronic devices based on 1D nanostructured SnO₂ via low-cost and efficient preparation techniques is still a huge challenge. In this work, a low-cost, one-step electrospun technology was employed to synthesize the SnO₂ nanofiber (NF) and nanotube (NT) arrays. The electrical and photoelectrical parameters of SnO₂ NTs-based devices were effectively controlled through simple changes to the amount of Sn in the precursor solution. The optimal 0.2 SnO₂ NTs-based field effect transistors (FETs) with 0.2 g SnCl₂*4H₂O per 5 ml in the precursor solution exhibit a high saturation current (∼9 × 10^-5 A) and a large on/off ratio exceeding 2.4 × 10⁶. Additionally, 0.2 SnO₂ NTs-based FET also exhibit a narrowband deep-UV photodetectivity (240-320 nm), including an ultra-high photocurrent of 307 μA, a high photosensitivity of 2003, responsibility of 214 A W^-1 and detectivity of 2.19 × 10¹³ Jones. Furthermore, the SnO₂ NTs-based transparent photodetectors were as well be integrated with fluorine-doped tin oxide glass and demonstrated a high optical transparency and photosensitivity (∼199). All these results elucidate the significant advantages of these electrospun SnO₂ NTs for next-generation multifunctional electronics and transparent photonics.

Photogating-controlled ZnO photodetector response for visible to near-infrared light

In recent years, as a direct wide band gap semiconductor, zinc oxide (ZnO) nanomaterial has attracted a lot of attention. However, the widely investigated ZnO materials are strongly limited in fast-response and broadband photodetectors due to their inherent weaknesses, so an effective structure or mechanism of ZnO nanostructure photodetector is greatly needed. In this work, a photogating-controlled photodetector based on a ZnO nanosheet-HfO₂-lightly doped Si architecture is demonstrated. Its performance was significantly improved by the photogating-controlled local field at the Si and HfO₂ interfaces compared to the findings in other published works on ZnO. Consequently, the photodetector not only effectively balances the responsivity (as high as 5.6 A W^-1) and response time (400 µs), but also broadens the wavelength response of the ZnO-based photodetectors from visible to near-infrared light range (~1200 nm). Additionally, the photogating-controlled ZnO photodetector enables high-resolution imaging both in the visible and near-infrared bands. Our photogating-controlled ZnO photodetectors not only exemplify the controllability of the gate electrode in high mobility materials but also provide a basis for the development of fast speed and high responsivity detection of high mobility materials.

Self-Powered and Broadband Lead-Free Inorganic Perovskite Photodetector with High Stability

Metal halide perovskite materials have opened up a great opportunity for high-performance optoelectronic devices owing to their extraordinary optoelectronic properties. More than lead halide ones, stable and nontoxic bismuth halide perovskites exhibit more promise in their future commercialization. In this work, we developed for the first time photodetectors based on full-inorganic Cs₃Bi₂I_9-xBr_x perovskites and modulate their performance by varying x in the composition systematically. Among those self-powered photodetectors, those based on Cs₃Bi₂I₆Br₃ shows the best performance with excellent photosensitivity of 4.1 × 10⁴ at zero bias as well as the responsivity and detectivity reaching 15 mA/W and 4.6 × 10¹¹ Jones, respectively. More strikingly, the full-inorganic perovskite photodetectors exhibit excellent stability in the ambient environment and can maintain over 96% of the initial value after 100 days owing to the high stability of the core perovskite film. The paper definitely paves an alternative and promising strategy for the design of future commercial photodetectors that are self-powered, stable, nontoxic, etc.

Controllable growth of three-dimensional CdS nanoparticles on TiO2 nanotubes to enhance photocatalytic activity

Exploiting photocatalysts with characteristics of low cost, high reactivity and good recyclability is a great significance for environmental remediation and energy conversion. Herein, hollow TiO2 nanotubes were fabricated by a novel and efficient method via electrospinning and an impregnation calcination method. With the hydrothermal method, the CdS nanoparticles were modified on the surface and in walls of the TiO2 nanotubes. By changing the reaction conditions, the morphology of CdS nanoparticles presents a controllable three-dimensional (3D) structure. The morphology of the samples was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The structure and components of samples were characterized by X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS). The light absorption efficiency was detected using UV-vis diffuse reflectance spectroscopy (DRS) and photoluminescence (PL). The photocatalytic properties were evaluated by degradation of methyl orange (MO) and photocatalytic hydrogen evolution under visible light irradiation. From the results, the TiO2/CdS nanotubes exhibit better photocatalytic activity than the pure TiO2. The synthetic mechanism of TiO2/CdS heterostructures and a possible photocatalytic mechanism based on the experimental results were proposed.

Enhancement of UV Photodetection Properties of Hierarchical Core-Shell Heterostructures of a Natural Sericin Biopolymer with the Addition of ZnO Fabricated on Ultra-Nanocrystalline Diamond Layers

A novel self-assembled hierarchical heterostructure is derived from cocoon-derived sericin biopolymer (CSP) biowaste with ZnO deposited on ultra-nanocrystalline diamond (UNCD) substrates using a scalable chemical deposition technique. Then, high-performance long-life UV photodetectors are fabricated using this hybrid sericin, diamond, and ZnO (SDZ) nanostructure. The microstructural analysis reveals a several nanometer-thick CSP shell coated with a highly uniform ZnO nanorod (ZNR) array grown on the UNCD substrate. The CSP shell also contains columnar nanograins on top of the ZNR as well as vertical sidewalls with unique alignments. The hierarchical core-shell SDZ heterostructures reveal superior UV diode performance, with an ultrahigh UV switching ratio of 1.1 × 10⁵ at 5 V, an increase of up to 49 900% greater than that of as-grown ZNRs (220). High UV responsivity is observed around 3.6 A W^-1 under 365 nm UV light illumination. The perfect distribution of the sericin in the ZNRs on the UNCD substrates resulted in the ultrafast electron-hole recombination. The sericin dopants and the UNCD interlayer enabled the device to reach new energy levels in the conduction band, with the reduced barrier height allowing for improved charge carrier transportation during UV light illumination. It is believed that the sericin dopants and the UNCD layer increased the UV adsorptivity and the amount of conducting carbon dopants within the ZNRs was sufficient for s0tability. These noteworthy features make the SDZ heterostructures promising candidates for the fabrication of cost-efficient biopolymers and UNCD hybrid-based UV photodetectors.

High-Performance Transparent Ultraviolet Photodetectors Based on InGaZnO Superlattice Nanowire Arrays

Due to the efficient photocarrier separation and collection coming from their distinctive band structures, superlattice nanowires (NWs) have great potential as active materials for high-performance optoelectronic devices. In this work, InGaZnO NWs with superlattice structure and controllable stoichiometry are obtained by ambient-pressure chemical vapor deposition. Along the NW axial direction, perfect alternately stacking of InGaO(ZnO)₄⁺ blocks and InO₂^- layers is observed to form a periodic layered structure. Strikingly, when configured into individual NW photodetectors, the Ga concentration is found to significantly influence the amount of oxygen vacancies and oxygen molecules adsorbed on the NW surface, which dictate the photoconducting properties of the NW channels. Based on the optimized Ga concentration (i.e., In_1.8Ga_1.8Zn_2.4O₇), the individual NW device exhibits an excellent responsivity of 1.95 × 10⁵ A/W and external quantum efficiency of as high as 9.28 × 10⁷% together with a rise time of 0.93 s and a decay time of 0.2 s for the ultraviolet (UV) photodetection. Besides, the obtained NWs can be fabricated into large-scale parallel arrays on glass substrates as well to achieve fully transparent UV photodetectors, where the performance is on the same level or even better than many transparent photodetectors with high performance. All the results discussed above demonstrate the great potential of InGaZnO superlattice NWs for next-generation advanced optoelectronic devices.

Flexible Visible-Blind Ultraviolet Photodetectors Based on ZnAl-Layered Double Hydroxide Nanosheet Scroll

Visible-blind ultraviolet (UV) photodetectors have received a great deal of attention for realizing Internet of Things technologies as well as for monitoring the level of UV exposure to humans. Realizing next-generation flexible and visible-blind UV photodetectors requires development of new functional material systems with easy fabrication, selectively strong UV light absorption, environmental friendliness, and high stability regardless of ambient conditions. Herein, flexible visible-blind UV photodetectors are successfully fabricated on the basis of two-dimensional ZnAl-layered double hydroxide (LDH) nanosheets with scroll structures grown on flexible substrates. The ZnAl-LDH nanosheet scrolls exhibit highly resistive semiconducting properties with a band gap of 3.2 eV and work function of 3.64 eV. The photodetector based on the ZnAl-LDH shows photoresponse in the UV spectral range below 420 nm, indicating visible-blind spectral response. In addition, the UV photodetector shows a maximum responsivity of 17 mA/W under illumination with 365 nm light. Moreover, the flexible photodetector shows reproducible photoresponse even after 1000 bending cycles, which indicates the acceptable stability of the ZnAl-LDH nanosheet scrolls.

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.

Zinc oxide ultraviolet photodetectors: rapid progress from conventional to self-powered photodetectors

Currently, the development of ultraviolet (UV) photodetectors (PDs) has attracted the attention of the research community because of the vast range of applications of photodetectors in modern society. A variety of wide-band gap nanomaterials have been utilized for UV detection to achieve higher photosensitivity. Specifically, zinc oxide (ZnO) nanomaterials have attracted significant attention primarily due to their additional properties such as piezo-phototronic and pyro-phototronic effects, which allow the fabrication of high-performance and low power consumption-based UV PDs. This article primarily focuses on the recent development of ZnO nanostructure-based UV PDs ranging from nanomaterials to architectural device design. A brief overview of the photoresponse characteristics of UV PDs and potential ZnO nanostructures is presented. Moreover, the recent development in self-powered PDs and implementation of the piezo-phototronic effect, plasmonic effect and pyro-phototronic effect for performance enhancement is highlighted. Finally, the research perspectives and future research direction related to ZnO nanostructures for next-generation UV PDs are summarized.

A ZnO/porous GaN heterojunction and its application as a humidity sensor

A heterojunction of ZnO/porous GaN (ZnO/PGAN) was fabricated and directly applied to a diode-type humidity sensor. ZnO disks were loaded onto PGAN using a spraying process. The structure and surface morphology of the ZnO/PGAN were characterized using X-ray diffraction and scanning electron microscopy. The heterojunction displayed an excellent diode nature, which was investigated using photoluminescence spectra and I-V characteristics. The excellent transport capability of ZnO/PGAN contributes to enhanced electron transfer, and hence results in high sensitivity and quick response/recovery properties under different relative humidity (RH) levels. In the range of 12-96% RH, a fast sensing response time as low as 7 s and a recovery time of 13 s can be achieved. The simple design of a ZnO/PGAN based humidity sensor highlights its potential in various applications.

Self-Powered Dual-Color UV-Green Photodetectors Based on SnO2 Millimeter Wire and Microwires/CsPbBr3 Particle Heterojunctions

Multiband detection has always been a challenge and has drawn much attention in the development of photodetectors (PDs). Herein, we present controllable synthesis of SnO₂ wires with different sizes via chemical vapor deposition and formed composites with CsPbBr₃ particles to realize dual spectral response. We constructed PDs based on a single SnO₂ millimeter wire decorated with CsPbBr₃ particles (SnO₂ MMW/CsPbBr₃), which showed a stepped spectrum, fast response speed, and self-powered function. Meanwhile, SnO₂ microwires/CsPbBr₃ composites (SnO₂ MWs/CsPbBr₃) were also utilized to fabricate PDs. It is noteworthy that detection occurred in two different wavelength bands (320 and 520 nm) with equivalent intensity at a bias of 0 V. The self-powered feature of this device comes from the built-in electric field at the interface of SnO₂/CsPbBr₃, and the dual-color response originates from asymmetric junction barriers between conduction bands of SnO₂ and CsPbBr₃. This work demonstrated promising self-powered PDs that are capable of multiband detection.

High responsivity ultraviolet detector based on novel SnO2 nanoarrays

In this work, tin oxide (SnO₂) nanoarrays (NAs) with a novel morphology were fabricated by a hydrothermal method.

PbS Quantum Dots/2D Nonlayered CdS xSe1- x Nanosheet Hybrid Nanostructure for High-Performance Broadband Photodetectors

Two-dimensional (2D) nonlayered nanomaterials have attracted extensive attention for electronic and optoelectronic applications recently because of their distinct properties. In this work, we first employed a facile one-step method to synthesize 2D nonlayered cadmium sulfide selenide (CdS _xSe_{1- x}, x = 0.33) nanosheets with a highly crystalline structure and then we introduced a generic spin-coating approach to fabricate hybrid nanomaterials composed of PbS quantum dots (QDs) and 2D CdS _xSe_{1- x} nanosheets and demonstrated their potential for high-performance broadband photodetectors. Compared with pure 2D CdS _xSe_{1- x} nanosheet photodetectors, the photoelectric performance of the PbS/CdS _xSe_{1- x} hybrid nanostructure is enhanced by 3 orders of magnitude under near-infrared (NIR) light illumination and maintains its performance in the visible (Vis) range. The photodetector exhibits a broadband response range from Vis to NIR with an ultrahigh light-to-dark current ratio (3.45 × 10⁶), a high spectral responsivity (1.45 × 10³ A/W), and high detectivity (1.05 × 10¹⁵ Jones). The proposed QDs/2D nonlayered hybrid nanostructure-based photodetector paves a promising way for next-generation high-performance broadband optoelectronic devices.

Solution Processed Trilayer Structure for High-Performance Perovskite Photodetector

Due to their outstanding performance, low cost, ease of fabrication, diverse photonic, and optoelectronic applications, metal halides perovskite have attracted extensive interest in photodetector applications. Currently, devices made by metal oxides, metal sulfides, and 2D materials had achieved good responsivity, but suffered from high dark current, slow response speed, small on-off ratio, and poor stability. Whole performances of these photodetectors are not satisfactory. Here, a lateral perovskite (CH₃NH₃PbBr₃)/Ethanolamine/TiO₂ (in ethanol) trilayer photodetector is designed for achieving high performance. EA treatment enhances electron extraction and reduces undesired recombination. This trilayer device shows good performances with low dark current of 1.5 × 10^-11 A, high on-off ratio of 2700, high photodetectivity of 1.51 × 10¹² Jones, high responsivity of 0.13 A W^-1, and high stability, comparative to conventional single layer devices. This work provides the way to improve the performance of metal halide perovskite photodetectors.

One-dimensional nanostructures of II-VI ternary alloys: synthesis, optical properties, and applications

One-dimensional (1D) nanostructures of II-VI ternary alloys are of prime interest due to their compatible features of both 1D nanostructures and semiconducting alloys. These features can facilitate materials with tunable bandgaps, which are crucial to the performance of photoelectrical devices. Herein, we present a comprehensive review summarizing the recent research progress pertinent to the diverse synthesis, optical fundamentals and applications of 1D nanostructures of II-VI ternary alloys. Considering multifunctional applications, the different growth mechanisms of the rational design and synthesis techniques are highlighted. Investigations of the fundamentals of the optical and photoelectrical properties of ternary alloyed II-VI semiconductors via the corresponding characterization techniques are also particularly discussed. Furthermore, we present the versatile potential practical applications of these materials. Finally, we extend the discussion to the most recent research advances on quaternary alloys, which provides a possible prospective forecast for the sustained development of alloyed 1D nanostructures.

Ferroelectric Materials: A Novel Pathway for Efficient Solar Water Splitting

Over the past few decades, solar water splitting has evolved into one of the most promising techniques for harvesting hydrogen using solar energy. Despite the high potential of this process for hydrogen production, many research groups have encountered significant challenges in the quest to achieve a high solar-to-hydrogen conversion efficiency. Recently, ferroelectric materials have attracted much attention as promising candidate materials for water splitting. These materials are among the best candidates for achieving water oxidation using solar energy. Moreover, their characteristics are changeable by atom substitute doping or the fabrication of a new complex structure. In this review, we describe solar water splitting technology via the solar-to-hydrogen conversion process. We will examine the challenges associated with this technology whereby ferroelectric materials are exploited to achieve a high solar-to-hydrogen conversion efficiency.

High-Performance Ultraviolet Photodetector Based on Graphene Quantum Dots Decorated ZnO Nanorods/GaN Film Isotype Heterojunctions

A novel isotype heterojunction ultraviolet photodetector was fabricated by growing n-ZnO nanorod arrays on n-GaN thin films and then spin-coated with graphene quantum dots (GQDs). Exposed to UV illumination with a wavelength of 365 nm, the time-dependent photoresponse of the hybrid detectors manifests high sensitivity and consistent transients with a rise time of 100 ms and a decay time of 120 ms. Meanwhile, an ultra-high specific detectivity (up to ~ 10¹² Jones) and high photoresponsivity (up to 34 mA W^-1) are obtained at 10 V bias. Compared to the bare heterojunction detectors, the excellent performance of the GQDs decorated n-ZnO/n-GaN heterostructure is attributed to the efficient immobilization of GQDs on the ZnO nanorod arrays. GQDs were exploited as a light absorber and act like an electron donor to effectively improve the effective carrier concentration in interfacial junction. Moreover, appropriate energy band alignment in GQDs decorated ZnO/GaN hybrids can also be a potential factor in facilitating the UV-induced photocurrent and response speed.

Electrospun one-dimensional nanostructures: a new horizon for gas sensing materials

Electrospun one-dimensional (1D) nanostructures are rapidly emerging as key enabling components in gas sensing due to their unique electrical, optical, magnetic, thermal, mechanical and chemical properties. 1D nanostructures have found applications in numerous areas, including healthcare, energy storage, biotechnology, environmental monitoring, and defence/security. Their enhanced specific surface area, superior mechanical properties, nanoporosity and improved surface characteristics (in particular, uniformity and stability) have made them important active materials for gas sensing applications. Such highly sensitive and selective elements can be embedded in sensor nodes for internet-of-things applications or in mobile systems for continuous monitoring of air pollutants and greenhouse gases as well as for monitoring the well-being and health in everyday life. Herein, we review recent developments of gas sensors based on electrospun 1D nanostructures in different sensing platforms, including optical, conductometric and acoustic resonators. After explaining the principle of electrospinning, we classify sensors based on the type of materials used as an active sensing layer, including polymers, metal oxide semiconductors, graphene, and their composites or their functionalized forms. The material properties of these electrospun fibers and their sensing performance toward different analytes are explained in detail and correlated to the benefits and limitations for every approach.

Low-voltage and high-performance field-effect transistors based on ZnxSn1-xO nanofibers with a ZrOx dielectric

One-dimensional (1D) nanofibers have been considered to be important building blocks for nano-electronics due to their superior physical and chemical properties. In this report, high-performance zinc tin oxide (ZnSnO) nanofibers with various composition ratios were prepared by electrospinning. The surface morphology, crystallinity, grain size distribution, and chemical composition of the nanofibers were investigated. Meanwhile, field-effect transistors (FETs) based on ZnSnO nanofiber networks (NFNs) with various composition ratios were integrated and investigated. For optimized Zn0.3Sn0.7O NFNs FETs, the device based on an SiO2 dielectric exhibited a high electrical performance, including a high on/off current ratio (Ion/off) of 2 × 107 and a field-effect mobility (μFE) of 0.17 cm2 V-1 s-1. When a high-permittivity (κ) ZrOx thin film was employed as the dielectric in Zn0.3Sn0.7O NFNs FETs, the operating voltage was substantially reduced and a high μFE of 7.8 cm2 V-1 s-1 was achieved. These results indicate that the Zn0.3Sn0.7O NFNs/ZrOx FETs exhibit great potency in low-cost and low-voltage devices.

Transparent, Flexible Silicon Nanostructured Wire Networks with Seamless Junctions for High-Performance Photodetector Applications

Optically transparent photodetectors are crucial in next-generation optoelectronic applications including smart windows and transparent image sensors. Designing photodetectors with high transparency, photoresponsivity, and robust mechanical flexibility remains a significant challenge, as is managing the inevitable trade-off between high transparency and strong photoresponse. Here we report a scalable method to produce flexible crystalline Si nanostructured wire (NW) networks fabricated from silicon-on-insulator (SOI) with seamless junctions and highly responsive porous Si segments that combine to deliver exceptional performance. These networks show high transparency (∼92% at 550 nm), broadband photodetection (350 to 950 nm) with excellent responsivity (25 A/W), optical response time (0.58 ms), and mechanical flexibility (1000 cycles). Temperature-dependent photocurrent measurements indicate the presence of localized electronic states in the porous Si segments, which play a crucial role in light harvesting and photocarrier generation. The scalable low-cost approach based on SOI has the potential to deliver new classes of flexible optoelectronic devices, including next-generation photodetectors and solar cells.

Large-Scale Direct-Writing of Aligned Nanofibers for Flexible Electronics

Nanofibers/nanowires usually exhibit exceptionally low flexural rigidities and remarkable tolerance against mechanical bending, showing superior advantages in flexible electronics applications. Electrospinning is regarded as a powerful process for this 1D nanostructure; however, it can only be able to produce chaotic fibers that are incompatible with the well-patterned microstructures in flexible electronics. Electro-hydrodynamic (EHD) direct-writing technology enables large-scale deposition of highly aligned nanofibers in an additive, noncontact, real-time adjustment, and individual control manner on rigid or flexible, planar or curved substrates, making it rather attractive in the fabrication of flexible electronics. In this Review, the ground-breaking research progress in the field of EHD direct-writing technology is summarized, including a brief chronology of EHD direct-writing techniques, basic principles and alignment strategies, and applications in flexible electronics. Finally, future prospects are suggested to advance flexible electronics based on orderly arranged EHD direct-written fibers. This technology overcomes the limitations of the resolution of fabrication and viscosity of ink of conventional inkjet printing, and represents major advances in manufacturing of flexible electronics.

A new 'sticking' coating method for the in situ formation of nanofiber networks on micrometer to millimeter-sized surfaces

A simple versatile method to form a nanofiber coating in situ on micrometer to millimeter-sized surfaces is developed. A fiber-filled porous sheet is designed by electrospinning a dense polymer solution on a patterned PET/aluminum alloy collector. By sticking the small area surface onto a fiber-filled porous sheet, a nanofiber-coated small area surface is obtained, which overcomes conventional nanofiber coating difficulties.

Novel Transparent and Self-Powered UV Photodetector Based on Crossed ZnO Nanofiber Array Homojunction

A novel self-powered UV photodetector based on electrospun ZnO nanofiber arrays is introduced. Aligned pure ZnO nanofibers and Ag-doped p-type ZnO nanofibers are processed perpendicular to each other, and p-n junction arrays of ZnO nanofibers are fabricated as a result. Owing to the intrinsic intervals between nanofibers, the device is fully transparent on quartz substrate. Various characterization methods including TEM, XRD, and XPS are used to testify the existence form of Ag element in ZnO nanofibers, and a field effect transistor is constructed to judge their conductivity. It is discovered that the Ag doping process not only transforms ZnO to p-type conductivity, making it possible to build this self-powered photodetector, but also forms Ag nanoparticles in ZnO nanofibers and thus helps reduce the response time. Benefiting from the abovementioned dual effects, this UV detector is found to have an enhanced performance, with the on-off ratio up to 10⁴ at zero bias and a rather short rise/decay time of 3.90 s/4.71 s.

Flexible Transparent and Free-Standing SiC Nanowires Fabric: Stretchable UV Absorber and Fast-Response UV-A Detector

Transparent and flexible materials are desired for the construction of photoelectric multifunctional integrated devices and portable electronics. Herein, 2H-SiC nanowires are assembled into a flexible, transparent, self-standing nanowire fabric (FTS-NWsF). The as-synthesized ultralong nanowires form high-quality crystals with a few stacking faults. The optical transmission spectra reveal that FTS-NWsF absorbs most incident 200-400 nm light, but remains transparent to visible light. A polydimethylsiloxane (PDMS)-SiC fabric-PDMS sandwich film device exhibits stable electrical output even when repeatedly stretched by up to 50%. Unlike previous SiC nanowires in which stacking faults are prevalent, the transparent, stretchable SiC fabric shows considerable photoelectric activity and exhibits a rapid photoresponse (rise and decay time < 30 ms) to 340-400 nm light, covering most of the UV-A spectral region. These advances represent significant progress in the design of functional optoelectronic SiC nanowires and transparent and stretchable optoelectronic systems.

Doping controlled pyro-phototronic effect in self-powered zinc oxide photodetector for enhancement of photoresponse

The pyro-phototronic effect can be used in pyroelectric semiconductor materials to significantly contribute in enhancing the self-powered photoresponse of photodetectors (PDs) via modulation of the photogenerated charge density. The pyro-phototronic effect in zinc oxide (ZnO) nanorods (NRs) was exploited thoroughly by doping with halogen elements, such as fluorine, chlorine (Cl), bromine and iodine. Cl-doped ZnO NRs (Cl : ZnO NRs) induces a large number of free charge carriers to enhance the self-powered photoresponse behavior (nearly 333% enhancement in response current) due to the pyro-phototronic effect as compared to pristine ZnO NRs. Moreover, 405% enhancement in pyrocurrent was measured for the Cl : ZnO NRs PD under a ultraviolet illumination intensity of 3 mW cm^-2, as compared to 0.3 mW cm^-2, in the absence of external bias voltage. Furthermore, other photoresponse parameters such as responsivity, external quantum efficiency and specific detectivity are measured to be higher due to the pyro-phototronic effect. Therefore, this study reveals the direct use of the pyro-phototronic effect to enhance the self-powered photoresponse.

Fabrication of Bi19S27I3 nanorod cluster films for enhanced photodetection performance

Bi₁₉S₂₇I₃ nanorod cluster films are directly grown on rigid substrates for potential application in wide range photodetectors.

Size-Controlled Graphene Nanodot Arrays/ZnO Hybrids for High-Performance UV Photodetectors

Graphene nanodots (GNDs) are one of the most attractive graphene nanostructures due to their tunable optoelectronic properties. Fabricated by polystyrene-nanosphere lithography, uniformly sized graphene nanodots array (GNDA) is constructed as an ultraviolet photodetector (PD) with ZnO nanofilm spin coated on it. The size of GNDA can be well controlled from 45 to 20 nm varying the etching time. It is revealed in the study that the photoelectric properties of ZnO/GNDA PD are highly GNDA size-dependent. The highest responsivity (R) and external quantum efficiency of ZnO/GNDA (20 nm) PD are 22.55 mA W^-1 and 9.32%, almost twofold of that of ZnO PD. Both ZnO/GNDA (20 nm) PD and ZnO/GNDA (30 nm) PD exhibit much faster response speed under on/off switching light and have shorter rise/decay time compared with ZnO PD. However, as the size of GNDA increase to 45 nm, the PD appears poor performance. The size-dependent phenomenon can be explained by the energy band alignments in ZnO/GNDA hybrids. These efforts reveal the enhancement of GNDs on traditional photodetectors with tunable optoelectronic properties and hold great potential to pave a new way to explore the various remarkable photodetection performances by controlling the size of the nanostructures.