Self-powered UV photosensor based on PEDOT:PSS/ZnO micro/nanowire with strain-modulated photoresponse

Boosting Charge Utilization in Self-Powered Photodetector for Real-Time High-Throughput Ultraviolet Communication

Ultraviolet (UV) communication is a cutting-edge technology in communication battlefields, and self-powered photodetectors as their optical receivers hold great potential. However, suboptimal charge utilization has largely limited the further performance enhancement of self-powered photodetectors for high-throughput communication application. Herein, a self-powered Ti3 C2 Tx -hybrid poly(3,4 ethylenedioxythiophene):poly-styrene sulfonate (PEDOT:PSS)/ZnO (TPZ) photodetector is designed, which aims to boost charge utilization for desirable applications. The device takes advantage of photothermal effect to intensify pyro-photoelectric effect as well as the increased conductivity of the PEDOT:PSS, which significantly facilitated charge separation, accelerated charge transport, and suppressed interface charge recombination. Consequently, the self-powered TPZ photodetector exhibits superior comprehensive performance with high responsivity of 12.3 mA W-1 and fast response time of 62.2 µs, together with outstanding reversible and stable cyclic operation. Furthermore, the TPZ photodetector has been successfully applied in an integrated UV communication system as the self-powered optical receiver capable of real-time high-throughput information transmission with ASCII code under 9600 baud rate. This work provides the design insight of highly performing self-powered photodetectors to achieve high-efficiency optical communication in the future.

Theoretical Nanoarchitectonics of GaN Nanowires for Ultraviolet Irradiation-Dependent Electromechanical Properties

In this paper, we propose a one-dimensional model that combines photoelectricity, piezoelectricity, and photothermal effects. The influence of ultraviolet light on the electromechanical coupling properties of GaN nanowires is investigated. It is shown that, since the ultraviolet photon energy is larger than the forbidden gap of GaN, the physical fields in a GaN nanowire are sensitive to ultraviolet. The light-induced polarization can change the magnitude and direction of a piezoelectric polarization field caused by a mechanical load. Moreover, a large number of photogenerated carriers under photoexcitation enhance the current density, whilst they shield the Schottky barrier and reduce rectifying characteristics. This provides a new theoretical nanoarchitectonics approach for the contactless performance regulation of nano-GaN devices such as photoelectric sensors and ultraviolet detectors, which can further release their great application potential.

Recent Progresses in Solution-Processed Tandem Organic and Quantum Dots Light-Emitting Diodes

Solution processes have promising advantages of low manufacturing cost and large-scale production, potentially applied for the fabrication of organic and quantum dot light-emitting diodes (OLEDs and QLEDs). To meet the expected lifespan of OLEDs/QLEDs in practical display and lighting applications, tandem architecture by connecting multiple light-emitting units (LEUs) through a feasible intermediate connection layer (ICL) is preferred. However, the combination of tandem architecture with solution processes is still limited by the choices of obtainable ICLs due to the unsettled challenges, such as orthogonal solubility, surface wettability, interfacial corrosion, and charge injection. This review focuses on the recent progresses of solution-processed tandem OLEDs and tandem QLEDs, covers the design and fabrication of various ICLs by solution process, and provides suggestions on the future challenges of corresponding materials and devices, which are anticipated to stimulate the exploitation of the emerging light technologies.

Ultraviolet Photodetector Based on Poly(3,4-Ethylenedioxyselenophene)/ZnO Core-Shell Nanorods p-n Heterojunction

In this work, we successfully assembled an organic-inorganic core-shell hybrid p-n heterojunction ultraviolet photodetector by the electropolymerization deposition of poly(3,4-ethylenedioxyselenophene) (PEDOS) on the surface of zinc oxide nanoarrays (ZnO NRs). The structures of composite were confirmed by FTIR, UV-Vis, XRD and XPS. Mott-Schottky analysis was used to study the p-n heterojunction structure. The photodetection properties of ZnO NRs/PEDOS heterojunction ultraviolet photodetector were systematically investigated current-voltage (I-V) and current-time (I-t) analysis under different bias voltages. The results showed that PEDOS films uniformly grew on ZnO NRs surface and core-shell structure was formed. The p-n heterojunction structure was formed with strong built-in electric field between ZnO NRs and PEDOS. Under the irradiation of UV light, the device showed a good rectification behavior. The responsivity, detection rate and the external quantum efficiency of the ultraviolet photodetector reached to 247.7 A/W, 3.41 × 10¹² Jones and 84,000% at 2 V bias, respectively. The rise time (τ_r) and fall time (τ_f) of ZnO NRs/PEDOS UV photodetector were obviously shortened compared to ZnO UV photodetector. The results show that the introduction of PEDOS effectively improves the performance of the UV photodetector.

Self-powered UV photodetectors and imaging arrays based on NiO/IGZO heterojunctions fabricated at room temperature

Self-powered UV photodetectors and imaging arrays based on p-type NiO/n-type InGaZnO (IGZO) heterojunctions are fabricated at room temperature by using ratio-frequency magnetron sputtering. The p-n heterojunction exhibits typical rectifying characteristics with a rectification ratio of 7.4×10⁴ at a ±4 V applied bias. A high photo-responsivity of 28.8 mA/W is observed under zero bias at a wavelength of 365 nm. The photodetector possesses a fast response time of 15 ms which is among the best in reported oxide-based p-n junction-based UV photodetectors. Finally, recognition of an "H" pattern is demonstrated by a 10×10 photodetector array at zero bias. The results indicate that the NiO/IGZO based photodetectors may have a great potential in constructing large-scale self-powered UV imaging systems.

Dramatic Responsivity Enhancement Through Concentrated H2 SO4 Treatment on PEDOT:PSS/TiO2 Heterojunction Fibrous Photodetectors

In order to satisfy the growing requirements of wearable electronic devices, 1D fiber-shaped devices with outstanding sensitivity, flexibility, and stability are urgently needed. In this study, a novel inorganic-organic heterojunction fibrous photodetector (FPD) based on poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and highly ordered TiO₂ nanotube array is fabricated, which endows a high responsivity, large external quantum efficiency, and fast response speed at 3 V bias. To further ameliorate its performance in the self-powered mode, a facile acid treatment is adopted and the assembled H-PEDOT:PSS/TiO₂ FPD demonstrates outstanding self-powered properties with ≈3000% responsivity enhancement (161 mA W^-1 at 0 V under 365 nm irradiation, photocurrent enhancement of ≈50 times) compared with the untreated device. It is found that the concentrated H₂ SO₄ post-treatment helps decrease the tube wall thickness of TiO₂ and partially removes the insulated PSS component in PEDOT:PSS, leading to enhanced conductivity and facilitated charge transportation, and thereby superb responsivity/photocurrent enhancement of self-powered H-PEDOT:PSS/TiO₂ FPD. This low-cost and high-performance self-powered FPD shows high potential for applications in wearable electronic devices.

Flexible ultraviolet photodetector based on single ZnO microwire/polyaniline heterojunctions

Flexible ultraviolet (UV) photodetectors are considered as potential building blocks for future-oriented photoelectric applications such as flexible optical communication, image sensors, wearable devices and so on. In this work, high-performance UV photodetector was fabricated via a facile combination of single ZnO microwire (MW) and p-type polyaniline. Due to the formation of effective organic/inorganic p-n junction, the as-prepared flexible UV photodetector based on ZnO MW/polyaniline hybrid heterojunction exhibits high performance (responsivity ∼ 60 mA/W and detectivity ∼ 2.0 ×10¹¹ Jones) at the reverse bias of -1 V under the UV illumination. The ZnO MW/polyaniline photodetector displays short response/recovery times (∼ 0.44 s/∼ 0.42 s), which is less than that of most reported UV photodetectors based on ZnO/polymer heterojunction. The fast response speed and recovery speed can be attributed to the high crystallinity of ZnO MW, built-in electric field in space-charge region and the passivation of oxygen traps on the surface. Further, the photodetector using ZnO MW/polyaniline junctions shows excellent flexibility and stability under bent conditions. This work opens a new way to design next-generation high-performance, low-cost and flexible optoelectronic devices for lab-on-a-chip applications.

A flower-inspired divergent light-trapping structure with quasi-spherical symmetry towards a high-performance flexible photodetector

Molybdenum disulfide (MoS2) has received widespread attention in recent years due to its exciting properties. However, the practical applications of MoS2 in optoelectronic devices are impeded by the power supply problem, the lack of flexibility, and the low light absorption for planar nanosheets and nanosheet arrays. Inspired by the elaborate architecture of the flower Tagetes erecta L., in this work, a self-assembled divergent MoS2 nanoflower (MoS2_F) with quasi-spherical symmetry is successfully synthesized by a facile one-step hydrothermal method. It is of significance that coupled with asymmetric silver electrodes and packaged by polymethyl methacrylate (PMMA), a self-powered flexible photodetector (PD) based on MoS2_F is actualized and shows an excellent flexible photoresponse performance at zero bias voltage. The divergent structure with quasi-spherical symmetry enables the MoS2_F to achieve strong broadband and omnidirectional absorption (92.7%) and ensures that the MoS2_F maintains the same physical contact on a different bending degree. Intriguingly, excellent flexibility and stability have been achieved as MoS2_F PD retains 91.4% of the initial efficiency even when bent to 151° and retains 92.5% of the initial efficiency even after 1000 bending cycles. Therefore, by a low-cost process, this work demonstrates an innovative avenue to fabricate a self-powered flexible photodetector with excellent light absorption, broadband response, flexibility, and stability, which is of great practical significance for optoelectronic applications in various environments.

Device Architecture for Visible and Near-Infrared Photodetectors Based on Two-Dimensional SnSe2 and MoS2: A Review

While band gap and absorption coefficients are intrinsic properties of a material and determine its spectral range, response time is mainly controlled by the architecture of the device and electron/hole mobility. Further, 2D-layered materials such as transition metal dichalogenides (TMDCs) possess inherent and intriguing properties such as a layer-dependent band gap and are envisaged as alternative materials to replace conventional silicon (Si) and indium gallium arsenide (InGaAs) infrared photodetectors. The most researched 2D material is graphene with a response time between 50 and 100 ps and a responsivity of <10 mA/W across all wavelengths. Conventional Si photodiodes have a response time of about 50 ps with maximum responsivity of about 500 mA/W at 880 nm. Although the responsivity of TMDCs can reach beyond 10⁴ A/W, response times fall short by 3–6 orders of magnitude compared to graphene, commercial Si, and InGaAs photodiodes. Slow response times limit their application in devices requiring high frequency. Here, we highlight some of the recent developments made with visible and near-infrared photodetectors based on two dimensional SnSe₂ and MoS₂ materials and their performance with the main emphasis on the role played by the mobility of the constituency semiconductors to response/recovery times associated with the hetero-structures.

Growth of Carbon Dot-Decorated ZnO Nanorods on a Graphite-Coated Paper Substrate to Fabricate a Flexible and Self-Powered Schottky Diode for UV Detection

The fabrication of flexible as well as self-powered optoelectronic devices is a growing and challenging area of research. Some scientists have reported the fabrication of either flexible or self-powered photodetectors recently. However, most of the literature studies fail to report the fabrication of self-powered as well as flexible photodetectors. This study reports the fabrication of self-powered, carbon dot (CD)-enhanced, flexible ZnO/graphite heterojunction-based UV detector where cellulose paper has been used as the substrate. A detailed study on the crystallinity and the defects of the ZnO nanorods has been done with appropriate characterizations. The CD-enhanced ZnO/graphite heterojunction showed Schottky characteristics. The Schottky parameters such as the barrier height, ideality factor, and the series resistance have also been calculated using the Cheung-Cheung method. The observed values of barrier height, ideality factor, and the series resistance are 0.74 eV, 3.74, and 503 kΩ, respectively. The transient response at self-powered condition has been demonstrated. The response time and the recovery time at self-powered condition have also been calculated with the help of the transient response, and those values are ∼2 and ∼3.2 s, respectively. The responsivity and the specific detectivity of the fabricated UV detector have been calculated as 9.57 mA/W and 4.27×10⁸ Jones, respectively, at 330 nm wavelength, which is quite comparable with literature-reported values, considering a self-powered photodetector.

Schottky Contacts on Polarity-Controlled Vertical ZnO Nanorods

Polarity-controlled growth of ZnO by chemical bath deposition provides a method for controlling the crystal orientation of vertical nanorod arrays. The ability to define the morphology and structure of the nanorods is essential to maximizing the performance of optical and electrical devices such as piezoelectric nanogenerators; however, well-defined Schottky contacts to the polar facets of the structures have yet to be explored. In this work, we demonstrate a process to fabricate metal-semiconductor-metal device structures from vertical arrays with Au contacts on the uppermost polar facets of the nanorods and show that the O-polar nanorods (∼0.44 eV) have a greater effective barrier height than the Zn-polar nanorods (∼0.37 eV). Oxygen plasma treatment is shown by cathodoluminescence spectroscopy to affect midgap defects associated with radiative emissions, which improves the Schottky contacts from weakly rectifying to strongly rectifying. Interestingly, the plasma treatment is shown to have a much greater effect in reducing the number of carriers in O-polar nanorods through quenching of the donor-type substitutional hydrogen on oxygen sites (H_O) when compared to the zinc-vacancy-related hydrogen defect complexes (V_Zn-nH) in Zn-polar nanorods that evolve to lower-coordinated complexes. The effect on H_O in the O-polar nanorods coincides with a large reduction in the visible-range defects, producing a lower conductivity and creating the larger effective barrier heights. This combination can allow radiative losses and charge leakage to be controlled, enhancing devices such as dynamic photodetectors, strain sensors, and light-emitting diodes while showing that the O-polar nanorods can outperform Zn-polar nanorods in such applications.

High-Performance Ultraviolet Photodetector Based on a Zinc Oxide Nanoparticle@Single-Walled Carbon Nanotube Heterojunction Hybrid Film

A hybrid film consisting of zinc oxide nanoparticles (ZnO NPs) and carbon nanotubes (CNTs) is formed on a glass substrate using a simple and swift spin coating process for the use in ultraviolet photodetectors (UV PDs). The incorporation of various types of CNTs into ZnO NPs (ZnO@CNT) enhances the performance of UV PDs with respect to sensitivity, photoresponse, and long-term operation stability when compared with pristine ZnO NP films. In particular, the introduction of single-walled CNTs (SWNTs) exhibits a superior performance when compared with the multiwalled CNTs (MWNTs) because SWNTs can not only facilitate the stability of free electrons generated by the O₂ desorption on ZnO under UV irradiation owing to the built-in potential between ZnO and SWNT heterojunctions, but also allow facile and efficient transport pathways for electrons through SWNTs with high aspect ratio and low defect density. Furthermore, among the various SWNTs (arc-discharged (A-SWNT), Hipco (H-SWNT), and CoMoCat (C-SWNT) SWNTs), we demonstrate the ZnO@A-SWNT hybrid film exhibits the best performance because of higher conductivity and aspect ratio in A-SWNTs when compared with those of other types of SWNTs. At the optimized conditions for the ZnO@A-SWNT film (ratio of A-SWNTs and ZnO NPs and electrode distance), ZnO@A-SWNT displays a sensitivity of 4.9 × 10³ % with an on/off current ratio of ~10⁴ at the bias of 2 V under the UV wavelength of 365 nm (0.47 mW/cm²). In addition, the stability in long-term operation and photoresponse time are significantly improved by the introduction of A-SWNTs into the ZnO NP film when compared with the bare ZnO NPs film.

Recent advances in solution-processed photodetectors based on inorganic and hybrid photo-active materials

Due to their excellent and tailorable optoelectronic performance, low cost, facile fabrication, and compatibility with flexible substrates, solution-processed inorganic and hybrid photo-active materials have attracted extensive interest for next-generation photodetector applications. This review gives a comprehensive compilation of solution-processed photodetectors. The basic structures of the device and important parameters of photodetectors will be firstly summarized. Then the development of various solution processing technologies containing solution synthesis and liquid phase film-forming processes for the preparation of semiconductor films is described. From the materials science point of view, we give a comprehensive overview about the current status of solution processed semiconductor materials including inorganic and hybrid photo-active materials for the application of photodetectors. Moreover, challenges and future trends in the field of solution-processed photodetectors are proposed.

An ultrahigh responsivity self-powered solar-blind photodetector based on a centimeter-sized β-Ga2O3/polyaniline heterojunction

Wide band gap semiconductors are promising UV photodetector materials due to their suitable bandgap, high crystal quality, strong absorption and large carrier mobility. Up to now, deep UV photodetectors are mainly based on epitaxial thin films, which have some undesired properties such as p-type doping difficulty. Lattice mismatch hinders the further development of these devices. Here, a high performance self-powered solar-blind UV photodetector was realized by a facile combination of a centimeter-sized single crystal β-Ga2O3 microwire and polyaniline. Owing to the excellent organic/inorganic hybrid p-n junction, the device shows an ultrahigh responsivity of 21 mA W-1 at 246 nm with a sharp cut-off wavelength of 272 nm without an external power supply. Moreover, the dark current is 0.08 pA, which is smaller than those of almost all the previous metallic oxide based solar-blind UV photodetectors. The photodetector also shows a high UV/visible rejection ratio (102) at zero bias voltage. Finally, a physical model of the self-powered photodetector is also proposed. This work provides a simple, low-cost, and effective method for preparing high performance self-powered solar-blind UV photodetectors based on organic/inorganic heterojunctions.

Photodetection Characteristics of Gold Coated AFM Tips and n-Silicon Substrate nano-Schottky Interfaces

Silicon (Si)-based photodetectors are appealing candidates due to their low cost and compatibility with the complementary metal oxide semiconductor (CMOS) technology. The nanoscale devices based on Si can contribute efficiently in the field of photodetectors. In this report, we investigate the photodetection capability of nano-Schottky junctions using gold (Au) coated conductive atomic force microscope (C-AFM) tips, and highly cleaned n-Si substrate interface. The Au nanotip/n-Si interface forms the proposed structure of a nano Schottky diode based photodetector. The electrical characteristics measured at the nanoscale junction with different Au nanotip radii show that the tunneling current increases with decreasing the tip radius. Moreover, the tunneling process and photodetection effects are discussed in terms of barrier width/height decrease at the tip-semiconductor interface due to the applied electric field as well as the generation of plasmon-induced hot-electron at the nanoparticle (i.e. C-AFM tip)/n-Si interface. Furthermore, the photodetection sensitivity is investigated and it is found to be higher for C-AFM tips with smaller radii. Moreover, this research will open a new path for the miniaturization of photodetectors with high sensitivity based on nano-Schottky interfaces.

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.

Toward a Fast and Highly Responsive SnSe2-Based Photodiode by Exploiting the Mobility of the Counter Semiconductor

In photodetection, the response time is mainly controlled by the device architecture and electron/hole mobility, while the absorption coefficient and the effective separation of the electrons/holes are the key parameters for high responsivity. Here, we report an approach toward the fast and highly responsive infrared photodetection using an n-type SnSe₂ thin film on a p-Si(100) substrate keeping the overall performance of the device. The I- V characteristics of the device show a rectification ratio of ∼147 at ±5 V and enhanced optoelectronic properties under 1064 nm radiation. The responsivity is 0.12 A/W at 5 V, and the response/recovery time constants were estimated as ∼57 ± 25/34 ± 15 μs, respectively. Overall, the response times are shown to be controlled by the mobility of the constituent semiconductors of a photodiode. Further, our findings suggest that n-SnSe₂ can be integrated with well-established Si technology with enhanced optoelectronic properties and also pave the way in the design of fast response photodetectors for other wavelengths as well.

Piezo-phototronic mediated enhanced photodetection characteristics of plasmonic Au-g-C3N4/CdS/ZnO based hybrid heterojunctions on a flexible platform

We have studied the piezo-phototronic induced enhancement in the photo-response of CdS/ZnO heterojunctions attached with plasmonic Au nanoparticle loaded 2D-graphitic carbon nitride (g-C3N4). The hybrid g-C3N4/CdS/ZnO heterojunction favours the charge carrier separation through the formation of a step-like band alignment. Furthermore, the integration of plasmonic Au loaded g-C3N4 nanosheets on the conventional CdS/ZnO heterojunction facilitates improved visible light absorption properties. The heterojunction device on a flexible platform under the application of a strain (∼0.017%) exhibits ∼102 times higher photoresponse over the control sample at a constant bias of ∼2 V. The variation in the photo-response under different bending conditions has been explained in terms of the improved charge transport through the modified energy bands at the interface of ZnO. The improved piezo-phototronic properties originated from the plasmonic properties of Au loaded g-C3N4 and the piezoelectric characteristics of c-axis oriented ZnO films may be used for future flexible photonic devices.

Spectral Sensor Inspired by Cone Photoreceptors and Ion Channels without External Power

In this study, we focused the mimicking of the cone photoreceptor along with ion channel system, which is similar to real optical system. By realizing the ion channel and photoreceptor based on photoresponsive material and photoelectric film, we achieved the wavelength-selective sensor with self-power ability. For the first time, we combined the photoreceptor and ion channel system without external power. For this, we used the channel membrane with pores coated with light-responsive material. We measured the sensing signals without any external power, because photoelectric film assists the sensitive operation of the ion channel system. As a result, we demonstrated a spectral ion channel sensor triggered by the photoelectric effect. Mimicking the slow and fast responses typically found in cone photoreceptor, when induced light causes photovoltaic effect from the pyrrole-coated polyvinylidene fluoride film, this helps to normally operate the ion channel system with slow and fast responses to the light wavelengths. Consequently, this research opens the new scientific fields to realize the photosensor very similar to the real vision sensory organ.

Enhanced photoresponse of a high-performance self-powered UV photodetector based on ZnO nanorods and a novel electrolyte by the piezo-phototronic effect

A flexible self-powered ultraviolet (UV) photodetector based on ZnO nanorods (NRs) and a novel iodine-free quasi solid-state electrolyte was fabricated. The obtained device has a fast and high response to UV light illumination at zero bias and also shows long-term stability. The responsivity is 50.5 mA W⁻¹ and the response time is less than 0.2 s. Strain-induced piezo-phototronic potential within wurtzite-structured ZnO can optimize the performance of corresponding optoelectronic devices since it could effectively tune the charge carriers' separation and transport. The photoresponse performances of the photodetector under different upward angles (tensile strain) and downward angles (compressive strain) at 0 V bias were studied in detail. A 163% change of responsivity was obtained when the downward angle reached 60°. The enhancement could be interpreted by the piezo-phototronic effect. The piezoelectric potential (piezopotential) at the ZnO NRs/electrolyte interface can expand the built-in field, and as a result, it is easier for charge carriers to separate and transport.

A flexible UV detector exhibits high performance. The photoresponse of the device under different upward angles (tensile strain) and downward angles (compressive strain) were studied. A 163% change in responsivity was obtained when the downward angle reached 60°.

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.

Tunable WSe2-CdS mixed-dimensional van der Waals heterojunction with a piezo-phototronic effect for an enhanced flexible photodetector

Due to the absence of bond fracture and atomic reconstruction under strain, vdWs structures hold great promise in flexible electronic/optoelectronic applications. Besides all-2D heterojunctions, the dangling-bond-free surfaces of 2D materials also enable vdWs interaction with other materials of different dimensionalities, forming mixed-dimensional vdWs heterostructures. Such structures allow a much broader selection of materials and may provide a promising approach to compensate for the intrinsic weakness of 2D crystals before realizing their full potential. In this study, we present the fabrication of a WSe2-CdS mixed-dimensional vdWs p-n heterojunction for flexible photodetection. A strain-tunable vdWs interface was demonstrated and the photoresponse was dramatically enhanced with the piezo-phototronic effect. The photocurrent can be increased by ∼110% under a compressive strain of -0.73% and the corresponding photoresponsivity reaches up to 33.4 A W-1. The enhancement originates from realigned local energy-band tilting at the WSe2-CdS interface by strain-induced piezopolarization, which promotes the transport process of photoexcited carriers. Our work provides a new route to a tunable vdWs interface other than with electrostatic gating, which may inspire the development of novel flexible vdWs optoelectronics.

High-Performance Self-Powered UV Detector Based on SnO2-TiO2 Nanomace Arrays

Photoelectrochemical cell-typed self-powered UV detectors have attracted intensive research interest due to their low cost, simple fabrication process, and fast response. In this paper, SnO₂-TiO₂ nanomace arrays composed of SnO₂ nanotube trunk and TiO₂ nanobranches were prepared using soft chemical methods, and an environment-friendly self-powered UV photodetector using this nanostructure as the photoanode was assembled. Due to the synergistic effect of greatly accelerated electron-hole separation, enhanced surface area, and reduced charge recombination provided by SnO₂-TiO₂ nanomace array, the nanostructured detector displays an excellent performance over that based on bare SnO₂ arrays. The impact of the growing time of TiO₂ branches on the performance of UV photodetector was systematically studied. The device based on optimized SnO₂-TiO₂ nanomace arrays exhibits a high responsivity of 0.145 A/W at 365 nm, a fast rising time of 0.037 s, and a decay time of 0.015 s, as well as excellent spectral selectivity. This self-powered photodetector is a promising candidate for high-sensitivity, high-speed UV-detecting application.

1D Piezoelectric Material Based Nanogenerators: Methods, Materials and Property Optimization

Due to the enhanced piezoelectric properties, excellent mechanical properties and tunable electric properties, one-dimensional (1D) piezoelectric materials have shown their promising applications in nanogenerators (NG), sensors, actuators, electronic devices etc. To present a clear view about 1D piezoelectric materials, this review mainly focuses on the characterization and optimization of the piezoelectric properties of 1D nanomaterials, including semiconducting nanowires (NWs) with wurtzite and/or zinc blend phases, perovskite NWs and 1D polymers. Specifically, the piezoelectric coefficients, performance of single NW-based NG and structure-dependent electromechanical properties of 1D nanostructured materials can be respectively investigated through piezoresponse force microscopy, atomic force microscopy and the in-situ scanning/transmission electron microcopy. Along with the introduction of the mechanism and piezoelectric properties of 1D semiconductor, perovskite materials and polymers, their performance improvement strategies are summarized from the view of microstructures, including size-effect, crystal structure, orientation and defects. Finally, the extension of 1D piezoelectric materials in field effect transistors and optoelectronic devices are simply introduced.

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.

Regulation of Charge Carrier Dynamics in ZnO Microarchitecture-Based UV/Visible Photodetector via Photonic-Strain Induced Effects

A feasible, morphological influence on photoresponse behavior of ZnO microarchitectures such as microwire (MW), coral-like microstrip (CMS), fibril-like clustered microwire (F-MW) grown by one-step carrier gas/metal catalyst "free" vapor transport technique is reported. Among them, ZnO F-MW exhibits higher photocurrent (I_Ph ) response, i.e., I_{Ph/ZnO F-MW} > I_{Ph/ZnO CMS} > I_{Ph/ZnO MW} . The unique structural alignment of ZnO F-MW has enhanced the I_Ph from 14.2 to 186, 221, 290 µA upon various light intensities such as 0 to 6, 11, 17 mW cm^-2 at λ_{405 nm} . Herein, the nature of the as-fabricated ZnO photodetector (PD) is also demonstrated modulated by tuning the inner crystals piezoelectric potential through the piezo-phototronic effect. The I_Ph response of PD decreases monotonically by introducing compressive strain along the length of the device, which is due to the synergistic effect between the induced piezoelectric polarization and photogenerated charge carriers across the metal-semiconductor interface. The current behavior observed at the two interfaces acting as the source (S) and drain (D) is carefully investigated by analyzing the Schottky barrier heights (Φ_SB ). This work can pave the way for the development of geometrically modified strain induced performances of PD to promote next generation self-powered optoelectronic integrated devices and switches.

Metal oxide decorated layered silicate magadiite for enhanced properties: insight from ZnO and CuO decoration

Surface modification of a layered silicate magadiite (mag) was accomplished using cetyltrimethyl ammonium bromide (CTAB), copper oxide (CuO) and zinc oxide (ZnO). All the ions were intercalated using an ion-exchange reaction, and CuO and ZnO are prepared by a precipitation method. A precursor of CTAB modified mag enables good dispersion of the metal oxide on the surface of the lamellas. Characterization techniques (XRD, XRF, TGA, FTIR and XPS spectroscopy) confirmed that CuO and ZnO are homogeneously immobilized onto the silicate sheets of mag. The porosity of the products was explored by BET analysis. SEM and TEM provided the visualization of CuO/ZnO, which were in a good morphology and uniformly distributed on the platelets of the modified mag. Consequently, hybrids with well dispersed metal oxides showed unique properties in photoluminescence and magnetism compared to the metal oxide in the bulk state. The formation mechanism of the synthesis of CuO and ZnO decorated mag was further studied, which can give suggestion for fabricating other metal oxide decorated layered silicates. The significance of this current work is the fact that it provides a new method to prepare well dispersed metal oxides using an easy and flexible precipitation method that can be applied to other layered materials and metal oxides.

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.

Enhancing performance of Ag–ZnO–Ag UV photodetector by piezo-phototronic effect

In this work, an ultraviolet (UV) photodetector based on a ZnO nanowires (NWs) array with metal–semiconductor–metal Schottky junction structure was successfully fabricated on a flexible polyester fibre substrate by a low-temperature hydrothermal method. Subjected to a 0.2% tensile strain at −1 V, the I_light and sensitivity of the as-prepared UV photodetector are lifted by 82% and 130%, respectively. Furthermore, the response speed and recovery speed are significantly raised under the same tensile strain. The working principle can be explained as that the Schottky barrier height (SBH) is effectively improved by the negative strain-induced polarization at the metal–ZnO interface which is favorable for the separation of photogenerated electron–hole pairs. This work not only provides a facile and promising means to optimize the performance of a ZnO based MSM photodetector by applying a tensile strain but also opens up the way for fabrication and integration of ZnO photodetectors on flexible polyester fiber substrates.

An ultraviolet photodetector based on a ZnO nanowires with metal–semiconductor–metal Schottky structure was fabricated on a flexible polyester fibre substrate.

High-Performance and Self-Powered Deep UV Photodetectors Based on High Quality 2D Boron Nitride Nanosheets

High-quality two-dimensional (2D) crystalline boron nitride nanosheets (BNNSs) were grown on silicon wafers by using pulsed plasma beam deposition techniques. Self-powered deep ultraviolet (DUV) photodetectors (PDs) based on BNNSs with Schottky contact structures are designed and fabricated. By connecting the fabricated DUV photodetector to an ammeter, the response strength, response time and recovery time to different DUV wavelengths at different intensities have been characterized using the output short circuit photocurrent without a power supply. Furthermore, effects of temperature and plasma treatment on the induced photocurrent response of detectors have also been investigated. The experimental data clearly indicate that plasma treatment would significantly improve both induced photocurrent and response time. The BNNS-based DUV photodetector is demonstrated to possess excellent performance at a temperature up to 400 °C, including high sensitivity, high signal-to-noise ratio, high spectral selectivity, high speed, and high stability, which is better than almost all reported semiconducting nanomaterial-based self-powered photodetectors.

Self-Powered Ultraviolet Photodetectors Driven by Built-In Electric Field

Self-powered ultraviolet (UV) photodetectors, which have vast applications in the military and for civilian purposes, have become particularly attractive in recent years due to their advantages of high sensitivity, ultrasmall size, and low power consumption. In particular, self-powered UV photodetectors driven by a built-in electric field cannot only detect UV signals but also be powered by the incident signals instead of external power. In this concept, the key issues and most recent developments on photovoltaic type UV photodetectors driven by p-n homojunction, heterojunction, and Schottky junction are surveyed. This should generate extensive interest in this field and encourage more researchers to engage in and tackle the scientific challenges.

Self-Powered Nanoscale Photodetectors

Novel self-powered nanoscale photodetectors that can work without an external power source, which have great application potential in next-generation nanodevices that operate wirelessly and independently, are being widely studied. This review aims to give a comprehensive summary of the state-of-the-art research results on self-powered nanoscale photodetectors. An introduction of recent progress on Schottky junction photodetectors is provided. Two types of Schottky junctions are discussed in detail: metal-semiconductor and semiconductor-graphene junctions. Next, recent developments of p-n junction photodetectors are highlighted, including homojunction and heterojunction photodetectors. Then, piezo-phototronic effect enhanced photodetection performances of Schottky junctions and p-n junctions are discussed. Then, significant results on the photoelectrochemical-cell-based photodetector and integrated self-powered nanosystem are presented, followed by a systematic comparison of different types of photodetectors. Finally, a summary of the previous results is given, and the major challenges that need to be addressed in the future are outlined. The hope is that this review can provide valuable insights into the current status of self-powered photodetectors and spur new structure and device designs to further enhance photodetection performance.

Flexible Photodetectors Based on Novel Functional Materials

Flexible photodetectors have attracted a great deal of research interest in recent years due to their great possibilities for application in a variety of emerging areas such as flexible, stretchable, implantable, portable, wearable and printed electronics and optoelectronics. Novel functional materials, including materials with zero-dimensional (0D) and one-dimensional (1D) inorganic nanostructures, two-dimensional (2D) layered materials, organic semiconductors and perovskite materials, exhibit appealing electrical and optoelectrical properties, as well as outstanding mechanical flexibility, and have been widely studied as building blocks in cost-effective flexible photodetection. Here, we comprehensively review the outstanding performance of flexible photodetectors made from these novel functional materials reported in recent years. The photoresponse characteristics and flexibility of the devices will be discussed systematically. Summaries and challenges are provided to guide future directions of this vital research field.

Indium-doped SnO2 nanobelts for high-performance transparent and flexible photosensors by a facile assembly

Flexible and transparent electronics is the emerging future technology for optoelectronic devices. Recently, it has attracted considerable attention from the research community due to its prodigious commercial applications. Herein, we report highly flexible and transparent ultraviolet photosensors based on indium-doped tin oxide nanobelts with enhanced simultaneous photosensitivity and recovery speed, compared to pure SnO₂ nanobelts. Optoelectronic properties of the nanobelt photosensors have been found to be strongly related to the indium doping concentration and grain size of the nanobelts. A facile assembly method has been used to prepare the well-aligned nanobelt device for UV photosensors. Excellent flexible properties of the nanobelts have been explored, which show a superior response during bending tests and almost maintain their properties after 300 bending cycles. The enhanced photosensitivity of about 70 times that of undoped SnO₂ nanobelts along with a highly enhanced recovery speed of less than 1.75 s have been achieved by the precise doping of In³⁺ into SnO₂ lattice nanobelts. All these results show that our prepared photosensors demonstrate superior mechanical, electrical, and optical properties for their use in flexible and transparent electronics.

Surface photo-charge effect in doped-ZnO nanorods for high-performance self-powered ultraviolet photodetectors

The unique photo-charge characteristics of chlorine-doped zinc oxide nanorods (Cl-ZnO NRs) are explored for the first time in ultraviolet (UV) photodetector (PD) that offers an outstanding self-powered photoresponse towards low UV illumination signals. A self-powered Cl-ZnO NRs PD exhibits superior photon detection speed of the order of a few ms with high sensitivity and photoelasticity. Therefore, the presented PD opens up a novel route to fabricate highly efficient self-powered PDs on a large scale without employing complex multilayer systems.

Graphene Quantum Dot-Sensitized ZnO Nanorod/Polymer Schottky Junction UV Detector with Superior External Quantum Efficiency, Detectivity, and Responsivity

Graphene quantum dot (GQD)-sensitized ZnO nanorods/poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) Schottky junction has been fabricated for visible-blind ultraviolet (UV) photodetector applications. Schottky diode parameters such as ideality factor, effective work function, and series resistance have been studied for GQD-modified and pristine ZnO nanorod-based devices. Under illumination of broadband light of intensity 80 mW/cm², GQD-sensitized samples showed 11 times higher photocurrent value compared to pristine ZnO at -0.75 V external bias. GQD-modified detector demonstrated maximum photocurrent at UV region (wavelength ∼340 nm) for all reverse bias voltages. ZnO nanorods/polymer Schottky junction UV detectors revealed high external quantum efficiency (EQE) more than 100%. Interestingly, GQD sensitized nanorod-based device demonstrated high EQE value of 13,161% at -1 V bias (wavelength ∼340 nm), which is eight times higher than pristine ZnO NR-based detector. GQD-modified detectors also showed superior responsivity (36 A/W), detectivity (1.3 × 10¹² Hz^1/2/W) at -1 V bias under incident of light of wavelength 340 nm. Even at very low intensity of UV light (0.07 mW/cm²), GQD-modified UV detectors showed high photocurrent (∼7.0 mA/cm²).

Interface Engineering To Boost Photoresponse Performance of Self-Powered, Broad-Bandwidth PEDOT:PSS/Si Heterojunction Photodetector

Organic-inorganic hybrid heterojunctions are poised to push toward novel optoelectronics applications, such as photodetectors, but significant challenges complicating practical use remain. Although all organic based photodetectors have been reported with great success, their potential in high-speed, broadband, self-powered photodetectors have not been fully explored. Herein, a self-powered, broad bandwidth of photodetector based on

PEDOT

PSS/Si heterojunction is built by a facial low temperature spin-coating method. By interface engineering of heterojunction with optimal band alignment and heteromicrostructures, enhanced photoresponse performances are obtained. The bandwidth of the hybrid photodetector could be broadened by 10 kHz after interfacial passivation by a methyl group. Further manipulating the geometrical structure of the hybrid heterojunction with silicon nanowire, a broad spectrum response from 300 to 1100 nm, with bandwidth as high as 40.6 kHz, fast response speed of 2.03 μs and high detection of 4.1 × 10(11) Jones under zero bias was achieved. Meanwhile, the close dependence between the photoresponse performance of heterojunctions and Si nanowire length is observed in the top-coverage configuration. Finally, a coverage effects model is proposed based on the competition of Si bulk and surface recombination, which is also confirmed by the designed bottom-coverage experiment. The mechanisms behind the enhanced photoresponse of the hybrid photodetector is attributed to the optimum band alignment, as well as the optimum balance of carrier dissociation and recombination of heterojunction. The scalable and low temperature method would be of great convenience for large-scale fabrication of the

PEDOT

PSS/Si hybrid photodetector.

Energy-Efficient Hydrogenated Zinc Oxide Nanoflakes for High-Performance Self-Powered Ultraviolet Photodetector

Light absorption efficiency and doping induced charge carrier density play a vital role in self-powered optoelectronic devices. Unique vanadium-doped zinc oxide nanoflake array (VZnO NFs) is fabricated for self-powered ultraviolet (UV) photodetection. The light harvesting efficiency drastically improved from 84% in ZnO NRs to 98% in VZnO NFs. Moreover, the hydrogenation of as-synthesized VZnO (H:VZnO) NFs displayed an outstanding increase in response current as compared to pristine structures. The H:VZnO NFs device presents an extraordinary photoelastic behavior with faster photodetection speed in the order of ms under a low UV illumination signal. Excellent responsivity and external quantum efficiency with larger value of specific detectivity of H:VZnO NFs device promises an outstanding sensitivity for UV signal and self-powered high-performance visible-blind photodetector.

Enhancing performance of ZnO/NiO UV photodetector by piezo-phototronic effect

The performance of the ZnO/NiO UV photodetector is enhanced by piezo-phototronic effect.

Temperature Dependence of the Piezotronic and Piezophototronic Effects in a-axis GaN Nanobelts

The temperature dependence of the piezotronic and piezophototronic effects in a-axis GaN nanobelts from 77 to 300 K is investigated. The piezotronic effect is enhanced by over 440% under lower temp-eratures. Two independent processes are discovered to form a competing mechanism through the investigation of the temperature dependence of the piezophototronic effect in a-axis GaN nanobelts.

Stretchable and Tunable Microtectonic ZnO-Based Sensors and Photonics

The concept of realizing electronic applications on elastically stretchable "skins" that conform to irregularly shaped surfaces is revolutionizing fundamental research into mechanics and materials that can enable high performance stretchable devices. The ability to operate electronic devices under various mechanically stressed states can provide a set of unique functionalities that are beyond the capabilities of conventional rigid electronics. Here, a distinctive microtectonic effect enabled oxygen-deficient, nanopatterned zinc oxide (ZnO) thin films on an elastomeric substrate are introduced to realize large area, stretchable, transparent, and ultraportable sensors. The unique surface structures are exploited to create stretchable gas and ultraviolet light sensors, where the functional oxide itself is stretchable, both of which outperform their rigid counterparts under room temperature conditions. Nanoscale ZnO features are embedded in an elastomeric matrix function as tunable diffraction gratings, capable of sensing displacements with nanometre accuracy. These devices and the microtectonic oxide thin film approach show promise in enabling functional, transparent, and wearable electronics.

Self Powered Highly Enhanced Dual Wavelength ZnO@CdS Core-Shell Nanorod Arrays Photodetector: An Intelligent Pair

On the face of the impending energy crisis, developing low-energy or even zero-energy photoelectronic devices is extremely important. A multispectral photosensitivity feature of a self-powered device provides an additional powerful tool. We have developed an unprecedented high performance dual wavelength self-powered ZnO@CdS/PEDOT:PSS core-shell nanorods array photodetector through a simple aqueous chemical method wherein a suitable band alignment between an intelligent material pair, i.e. ZnO and CdS, has been utilized. Besides a noteworthy advantage of the devices being that they show a very sharp and prominent dual wavelength photosensitivity, both the ultraviolet and visible light sensitivity (ratio of current under illumination (Iphoto)/current under dark (Idark)) of the device are two orders of higher magnitude than those of pristine ZnO, attaining values of 2.8 × 10(3) and 1.07 × 10(3), respectively. At the same time, temporal responses faster than 20 ms could be achieved with these solution-processed photodetectors. The present study provides a very important direction to engineer core-shell nanostructured devices for dual wavelength high photosensitivity.

One-Dimensional Au-ZnO Heteronanostructures for Ultraviolet Light Detectors by a Two-Step Dielectrophoretic Assembly Method

One-dimensional ZnO decorated with metal nanoparticles has received much attention in the field of ultraviolet light detection because of its high photosensitivity and fast response, while how to form effective metal-ZnO heterostructures cost efficiently is still in development. We report an efficient and well-controlled method to form Au-ZnO heterostructures by two-step dielectrophoretic assembly. First, ZnO nanowires dispersed in deionized water were assembled dielectrophoretically in a planar microelectrode system. To control the number and position of assembled ZnO nanowires, a planar triangle-shaped microelectrode pair was imposed with a high-frequency ac voltage signal in this assembly process. Then a droplet of Au nanoparticle suspension was applied to decorate the preformed ZnO nanowire by another dielectrophoretic assembly process. The near-field dielectrophoretic force induced by the existence of ZnO nanowire spanning the electrode gap attracts Au nanoparticles onto the surface of ZnO nanowires and forms effective Au-ZnO heterostructures. After the adsorption of Au nanoparticles, the performances of Au-ZnO heteronanostructures in UV detection were studied. Experimental results indicate that the ratio of the photo-to-dark current of the Au-ZnO heteronanostucture-based detector was improved significantly, and the photoresponse was accelerated considerably. This kind of enhancement in performance can be attributed to the localized Schottky junctions on the surface of ZnO nanowire which improves the surface band bending.

Enhanced performance of ZnO piezotronic pressure sensor through electron-tunneling modulation of MgO nanolayer

Piezoelectric materials can be applied into electromechanical conversion and attract extensive attention with potential applications in various sensors. Here, we present two types of piezotronic pressure sensors based on ZnO nanoarrays. By introducing an insulating MgO (i-MgO) nanolayer, the "on/off" current ratio of the sensor is significantly improved up to 10(5). Furthermore, the sensor shows a high sensitivity of 7.1 × 10(4) gf(-1), a fast response time of 128 ms. The excellent properties are attributed to the combination of piezoelectric effect of ZnO nanoarrays and electron-tunneling modulation of MgO nanolayer, and the reversible potential barrier height controlled by piezoelectric potential. We further investigate the service behavior of the sensor, which can detect force varying from 3.2 to 27.2 gf. Our research provides a promising approach to boost the performance of nanodevices.

Self-powered ultrafast broadband photodetector based on p-n heterojunctions of CuO/Si nanowire array

A new self-powered broadband photodetector was fabricated by coating an n-silicon nanowire (n-Si NW) array with a layer of p-cupric oxide (CuO) nanoflakes through a new simple solution synthesis method. The p-n heterojunction shows excellent rectification characteristics in the dark and distinctive photovoltaic behavior under broadband light illumination. The photoresponse of the detector at zero bias voltage shows that this self-powered photodetector is highly sensitive to visible and near-infrared light illuminations, with excellent stability and reproducibility. Ultrafast response rise and recovery times of 60 and 80 μs, respectively, are shown by the CuO based nanophotodetector. In addition, the broadband photodetector can also provide a rapid binary response, with current changing from positive to negative upon illumination under a small bias. The binary response arises from the photovoltaic behavior and the low turn-on voltage of the CuO/Si NW device. These properties make the CuO/Si NW broadband photodetector suitable for applications that require high response speeds and self-sufficient functionality.