Highly Dense ZnO Nanowires Grown on Graphene Foam for Ultraviolet Photodetection

Design and TCAD analysis of few-layer graphene/ZnO nanowires heterojunction-based photodetector in UV spectral region

Graphene and zinc oxide (ZnO) nanowires (NWs)-based photodetectors demonstrate excellent photodetection performance in the ultraviolet (UV) spectrum regime. This paper presents the design and analysis of a heterostructure model of p+-few-layer graphene (p+-FLG)/n--ZnO NWs-based UV photodetector. The design utilizes the unique properties of few-layer graphene to enhance light absorption and improve photodetector performance. The analysis under both self-biasing and conductive modes of operation reveals that the integrated electric field and the photovoltaic effect at the p⁺-FLG/n⁻-ZnO NWs hetero-interface create a rectifying behavior. The photodetector achieves an external photocurrent responsivity, external quantum efficiency, detectivity, and noise equivalent power of 0.12 A/W, 44.1%, 1.9 × 109 Jones, and 5.6 × 10-14 W, respectively, under UV illumination at 350 nm, 0 V bias, and 300 K. Additionally, the photodetector exhibits ultrafast photoswitching rise and fall times of 0.26 ns and a 3-dB cut-off frequency of 1.31 GHz. The comparative analysis with existing photodetectors demonstrates that the proposed model surpasses many in sensitivity, speed, and efficiency. The enhancement of charge collection with the applied reverse-biased voltage results in a response time of 0.16 ns, a peak photocurrent responsivity of 0.2 A/W, a maximum external quantum efficiency of 61%, a peak detectivity of 2.4 × 109 Jones, and minimum noise equivalent power of 4.4 × 10-14 W at - 0.5 V. The findings inspire the development of next-generation self-driving, highly efficient, broadband photodetectors, and other economically viable and multifunctional optoelectronic devices.

Defects in Nitrogen-Doped ZnO Nanoparticles and Their Effect on Light-Emitting Diodes

In this study, the effect of defects on the acceptor properties of nitrogen-doped ZnO nanoparticles (NPs) was investigated through the fabrication of light-emitting diodes (LEDs). Nitrogen-doped ZnO NPs were synthesized by an arc discharge in-gas evaporation method and post-annealed at 800 °C in an oxygen and nitrogen atmosphere. The annealed ZnO NPs were characterized by X-ray diffraction, scanning electron microscopy, Raman spectroscopy, and photoluminescence spectroscopy. It was found that the annealing of nitrogen-doped ZnO NPs in a nitrogen environment increased the number of zinc vacancies, while annealing in an oxygen environment increased the number of oxygen vacancies due to nitrogen desorption. The output characteristics of LEDs fabricated with oxygen-annealed NPs were degraded, while those with nitrogen-annealed NPs were significantly improved. From these results, the contribution of zinc vacancies to acceptor formation in ZnO NPs was confirmed for the first time in actual pn junction devices.

Suppression of Visible Emission in Low-Temperature Synthesized Cobalt-Doped ZnO Nanoparticles and Their Photosensing Applications

Cobalt (Co)-doped ZnO nanoparticles have been synthesized at 100 °C using a simple chemical technique, without post-deposition annealing. These nanoparticles are of excellent crystallinity and show a significant reduction in defect density upon Co-doping. By varying the Co solution concentration, it is observed that oxygen-vacancy-related defects are suppressed at lower Co-doping, while the defect density shows an increasing trend at higher doping densities. This suggests that mild doping can significantly suppress the defects in ZnO for electronic and optoelectronic applications. The effect of Co-doping is studied using X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), electrical conductivity, and Mott-Schottky plots. Photodetectors fabricated using pure and Co-doped ZnO nanoparticles show a noticeable reduction in the response time upon Co-doping, which again affirms the reduction in the defect density after Co-doping.

Porous Carbon Coated on Cadmium Sulfide-Decorated Zinc Oxide Nanorod Photocathodes for Photo-accelerated Zinc Ion Capacitors

The development of devices with dual solar energy-harvesting and storage functionalities has recently gained significant traction for off-grid power supply. In their most compact embodiment, these devices rely on the same electrode to harvest and store energy; however, in this approach, the development of energy-efficient photoelectrodes with intrinsic characteristics of good optical and electrochemical activities remains challenging. Here, we propose photoelectrodes with a porous carbon coated on a zinc oxide-cadmium sulfide heterostructure as an energy-efficient photocathode for photo-accelerated zinc ion capacitors (Photo-ZICs). The Photo-ZICs harvest light energy and store charge simultaneously, resulting in efficient charge storage performance under illumination compared to dark conditions (∼99% capacity enhancement at 500 mA g^-1 under illumination compared to dark conditions). The light absorption ability and charge separation efficiency achieved by the photocathodes meet the requirements for photo-ZIC applications. Moreover, Photo-ZICs display stable charge storage capacities over long-term cycling, that is, ∼1% capacity loss after 10,000 cycles.

3D Graphene Foam by Chemical Vapor Deposition: Synthesis, Properties, and Energy-Related Applications

In this review, we highlight recent advancements in 3D graphene foam synthesis by template-assisted chemical vapor deposition, as well as their potential energy storage and conversion applications. This method offers good control of the number of graphene layers and porosity, as well as continuous connection of the graphene sheets. The review covers all the substrate types, catalysts, and precursors used to synthesize 3D graphene by the CVD method, as well as their most viable energy-related applications.

Channel Scaling Dependent Photoresponse of Copper-Based Flexible Photodetectors Fabricated Using Laser-Induced Oxidation

Copper (Cu) oxide compounds (Cu_xO), which include cupric (CuO) and cuprous (Cu₂O) oxide, have been recognized as a promising p-channel material with useful photovoltaic properties and superior thermal conductivity. Typically, deposition methods or thermal oxidation can be used to obtain Cu_xO. However, these processes are difficult to apply to flexible substrates because plastics have a comparatively low glass transition temperature. Also, additional patterning steps are needed to fabricate applications. In this work, we fabricated a metal-semiconductor-metal photodetector using laser-induced oxidation of thin Cu films under ambient conditions. Raman spectroscopy, scanning electron microscopy-energy-dispersive X-ray spectroscopy, and atomic force microscopy were used to study the composition and morphology of our devices. Moreover, the photoresponse of this device is reported herein. We performed an in-depth analysis of the relationship between the channel size and number of carriers using scanning photocurrent microscopy. The carrier transport behaviors were identified; the photocurrent decreased as the length and width of the channel increased. Furthermore, we verified the suitability of the device as a flexible photodetector using a variety of bending tests. Our in-depth analysis of this Cu-based flexible photodetector could play an important role in understanding the mechanisms of other flexible photovoltaic applications.

Brief Review of Photocatalysis and Photoresponse Properties of ZnO–Graphene Nanocomposites

As a typical wide bandgap semiconductor, ZnO has received a great deal of attention from researchers because of its strong physicochemical characteristics. During the past few years, great progress has been made in the optoelectronic applications of ZnO, particularly in the photocatalysis and photodetection fields. To enable further improvements in the material’s optoelectronic performance, construction of a variety of ZnO-based composite structures will be essential. In this paper, we review recent progress in the growth of different ZnO–graphene nanocomposite structures. The related band structures and photocatalysis and photoresponse properties of these nanocomposites are discussed. Additionally, specific examples of the materials are included to provide an insight into the common general physical properties and carrier transport characteristics involved in these unique nanocomposite structures. Finally, further directions for the development of ZnO–graphene nanocomposite materials are forecasted.

Ultrahigh Responsivity UV Photodetector Based on Cu Nanostructure/ZnO QD Hybrid Architectures

Strong near-surface electromagnetic field formed by collective oscillation of electrons on Cu nanostructure a shows a strong dependence on geometry, offering a promising approach to boost the light absorption of ZnO photoactive layers with enhanced plasmon scattering. Here, a facile way to fabricate UV photodetectors with tunable configuration of the self-assembled Cu nanostructures on ZnO thin films is reported. The incident lights are effectively confined in ZnO photoactive layers with the existence of the uplayer Cu nanostructures, and the interdiffusion of Cu atoms during fabrication of the Cu nanostructures can improve the carrier transfer in ZnO thin films. The optical properties of the hybrid architectures are successfully tailored over a control of the geometric evolution of the Cu nanostructures, resulting in significantly enhanced photocurrent and responsivity of 2.26 mA and 234 A W^-1 under a UV light illumination of 0.62 mW cm^-2 at 10 V, respectively. The photodetectors also exhibit excellent reproducibility, stability, and UV-visible rejection ratio (R_{370 nm} /R_{500 nm} ) of ≈370, offering an approach of high-performance UV photodetectors for practical applications.

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 nanowire-based microfiber coupler for all-optical photodetection applications

The temporal response of conventional ZnO nanowire-based photodetectors suffers from the influence of carrier mobility and high electrical resistance. As a result, these devices can be prohibitively slow for some applications, generally with a response time on the order of 1 second. This study presents a novel ZnO nanowire-based microfiber coupler structure for all-optical photodetection without the demand for photocurrent generation. In this design, two waveguides are directly adsorbed by van der Waals forces or electrostatic forces. Compared with the conventional electrical bridge structure, the optical coupling architecture makes the device both compact and stable. In this configuration, resonance dips form in the transmission spectrum when the phase-matching conditions of the two waveguides are satisfied. The measurements of the resulting wavelength shift, low noise levels, and repetition time confirmed that the proposed optical device could operate as a photodetector. The device exhibits superior performance with a sensitivity of 1.657 nm (W cm-2)-1 and a response time of 0.43 ms. In addition, the detector features a simple fabrication as there is no need for extra modification of ZnO nanowires. The resulting photodetection capabilities could provide a new functionality for novel all-optical applications.

Broad-Band High-Sensitivity ZnO Colloidal Quantum Dots/Self-Assembled Au Nanoantennas Heterostructures Photodetectors

Tunable plasmonic resonance induced by the collective oscillation of the electrons on metallic nanostructures can excellently enhance the light response of ZnO films, which provides an effective way to break through the limitation of the performance of ZnO photodetectors. Here, broad-band high-performance ZnO/Au heterostructures photodetectors with various morphologies of self-assembled Au nanoantennas are fabricated via a facile approach under the spin-coated ZnO colloidal quantum dots films. With a systematic control on growth condition, the self-assembled Au nanoantennas undergo a drastic evolution from the corrugated nanomounds to the island-like nanostructures, and the light absorption of the resulting ZnO/Au heterostructures correspondingly exhibits a strongly morphological dependence on the Au nanoantennas. Meanwhile, the photoresponse of the ZnO-based photodetectors is significantly improved throughout a wide spectrum between UV and visible regions owing to the enhanced light absorption induced by the localized surface plasmon resonance. As a result, the optimal switch ratio of the ZnO/Au heterostructures photodetector increases by 1 order to ∼1.13 × 10⁵ than that of the pristine ZnO one because of the obviously increased photocurrent ( I_ph) and comparable dark current, thus leading to ∼9.1 and ∼4.9 times increases in the photoresponsivity and the normalized detectivity. Meanwhile, the significant increases in the I_ph of ∼5.2 and ∼9.7 times are likewise observed with the ZnO/Au heterostructures under 530 nm and white-light illumination. This work can offer a handy and effective approach for the fabrication of ultrasensitive ZnO-based photodetectors within a broad-band wavelength by utilizing the Au plasmonic nanostructures.

Graphene-Based Semiconductor Heterostructures for Photodetectors

Graphene transparent conductive electrodes are highly attractive for photodetector (PD) applications due to their excellent electrical and optical properties. The emergence of graphene/semiconductor hybrid heterostructures provides a platform useful for fabricating high-performance optoelectronic devices, thereby overcoming the inherent limitations of graphene. Here, we review the studies of PDs based on graphene/semiconductor hybrid heterostructures, including device physics/design, performance, and process technologies for the optimization of PDs. In the last section, existing technologies and future challenges for PD applications of graphene/semiconductor hybrid heterostructures are discussed.

Ultraviolet Detectors Based on Wide Bandgap Semiconductor Nanowire: A Review

Ultraviolet (UV) detectors have attracted considerable attention in the past decade due to their extensive applications in the civil and military fields. Wide bandgap semiconductor-based UV detectors can detect UV light effectively, and nanowire structures can greatly improve the sensitivity of sensors with many quantum effects. This review summarizes recent developments in the classification and principles of UV detectors, i.e., photoconductive type, Schottky barrier type, metal-semiconductor-metal (MSM) type, p-n junction type and p-i-n junction type. The current state of the art in wide bandgap semiconductor materials suitable for producing nanowires for use in UV detectors, i.e., metallic oxide, III-nitride and SiC, during the last five years is also summarized. Finally, novel types of UV detectors such as hybrid nanostructure detectors, self-powered detectors and flexible detectors are introduced.

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.

Natural Biowaste-Cocoon-Derived Granular Activated Carbon-Coated ZnO Nanorods: A Simple Route To Synthesizing a Core-Shell Structure and Its Highly Enhanced UV and Hydrogen Sensing Properties

Granular activated carbon (GAC) materials were prepared via simple gas activation of silkworm cocoons and were coated on ZnO nanorods (ZNRs) by the facile hydrothermal method. The present combination of GAC and ZNRs shows a core-shell structure (where the GAC is coated on the surface of ZNRs) and is exposed by systematic material analysis. The as-prepared samples were then fabricated as dual-functional sensors and, most fascinatingly, the as-fabricated core-shell structure exhibits better UV and H₂ sensing properties than those of as-fabricated ZNRs and GAC. Thus, the present core-shell structure-based H₂ sensor exhibits fast responses of 11% (10 ppm) and 23.2% (200 ppm) with ultrafast response and recovery. However, the UV sensor offers an ultrahigh photoresponsivity of 57.9 A W^-1, which is superior to that of as-grown ZNRs (0.6 A W^-1). Besides this, switching photoresponse of GAC/ZNR core-shell structures exhibits a higher switching ratio (between dark and photocurrent) of 1585, with ultrafast response and recovery, than that of as-grown ZNRs (40). Because of the fast adsorption ability of GAC, it was observed that the finest distribution of GAC on ZNRs results in rapid electron transportation between the conduction bands of GAC and ZNRs while sensing H₂ and UV. Furthermore, the present core-shell structure-based UV and H₂ sensors also well-retained excellent sensitivity, repeatability, and long-term stability. Thus, the salient feature of this combination is that it provides a dual-functional sensor with biowaste cocoon and ZnO, which is ecological and inexpensive.

An Omnidirectionally Stretchable Photodetector Based on Organic-Inorganic Heterojunctions

Omnidirectionally stretchable photodetectors are limited by difficulties in designing material and fabrication processes that enable stretchability in multiaxial directions. Here, we propose a new approach involving an organic-inorganic p-n heterojunction photodetector comprised of free-standing ZnO nanorods grown on a poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate transport layer coated on a three-dimensional micropatterned stretchable substrate containing bumps and valleys. This structure allows for efficient absorption of stretching strain. This approach allows the device to accommodate large tensile strain in all of the directions. The device behaves as a photogated p-n heterojunction photodetector in which current modulation was obtained by sensing the mechanisms that rely on photovoltage and photogating effects. The device exhibits a high photoresponse to UV light and reliable electrical performance under applied stretching in uniaxial and omniaxial directions. Furthermore, the device can be easily and conformally attached to a human wrist. This allowed us to investigate the response of the device to UV light during human activity.

Dimensional-Hybrid Structures of 2D Materials with ZnO Nanostructures via pH-Mediated Hydrothermal Growth for Flexible UV Photodetectors

Complementary combination of heterostructures is a crucial factor for the development of 2D materials-based optoelectronic devices. Herein, an appropriate solution for fabricating complementary dimensional-hybrid nanostructures comprising structurally tailored ZnO nanostructures and 2D materials such as graphene and MoS₂ is suggested. Structural features of ZnO nanostructures hydrothermally grown on graphene and MoS₂ are deliberately manipulated by adjusting the pH value of the growing solution, which will result in the formation of ZnO nanowires, nanostars, and nanoflowers. The detailed growth mechanism is further explored for the structurally tailored ZnO nanostructures on the 2D materials. Furthermore, a UV photodetector based on the dimensional-hybrid nanostructures is fabricated, which demonstrates their excellent photocurrent and mechanical durability. This can be understood by the existence of oxygen vacancies and oxygen-vacancies-induced band narrowing in the ZnO nanostructures, which is a decisive factor for determining their photoelectrical properties in the hybrid system.

All-Printable ZnO Quantum Dots/Graphene van der Waals Heterostructures for Ultrasensitive Detection of Ultraviolet Light

In ZnO quantum dot/graphene heterojunction photodetectors, fabricated by printing quantum dots (QDs) directly on the graphene field-effect transistor (GFET) channel, the combination of the strong quantum confinement in ZnO QDs and the high charge mobility in graphene allows extraordinary quantum efficiency (or photoconductive gain) in visible-blind ultraviolet (UV) detection. Key to the high performance is a clean van der Waals interface to facilitate an efficient charge transfer from ZnO QDs to graphene upon UV illumination. Here, we report a robust ZnO QD surface activation process and demonstrate that a transition from zero to extraordinarily high photoresponsivity of 9.9 × 10⁸ A/W and a photoconductive gain of 3.6 × 10⁹ can be obtained in ZnO QDs/GFET heterojunction photodetectors, as the ZnO QDs surface is systematically engineered using this process. The high figure-of-merit UV detectivity D* in exceeding 1 × 10¹⁴ Jones represents more than 1 order of magnitude improvement over the best reported previously on ZnO nanostructure-based UV detectors. This result not only sheds light on the critical role of the van der Waals interface in affecting the optoelectronic process in ZnO QDs/GFET heterojunction photodetectors but also demonstrates the viability of printing quantum devices of high performance and low cost.

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.

Synthesis of graphene-transition metal oxide hybrid nanoparticles and their application in various fields

Single layer graphite, known as graphene, is an important material because of its unique two-dimensional structure, high conductivity, excellent electron mobility and high surface area. To explore the more prospective properties of graphene, graphene hybrids have been synthesised, where graphene has been integrated with other important nanoparticles (NPs). These graphene-NP hybrid structures are particularly interesting because after hybridisation they not only display the individual properties of graphene and the NPs, but also they exhibit further synergistic properties. Reduced graphene oxide (rGO), a graphene-like material, can be easily prepared by reduction of graphene oxide (GO) and therefore offers the possibility to fabricate a large variety of graphene-transition metal oxide (TMO) NP hybrids. These hybrid materials are promising alternatives to reduce the drawbacks of using only TMO NPs in various applications, such as anode materials in lithium ion batteries (LIBs), sensors, photocatalysts, removal of organic pollutants, etc. Recent studies have shown that a single graphene sheet (GS) has extraordinary electronic transport properties. One possible route to connecting those properties for application in electronics would be to prepare graphene-wrapped TMO NPs. In this critical review, we discuss the development of graphene-TMO hybrids with the detailed account of their synthesis. In addition, attention is given to the wide range of applications. This review covers the details of graphene-TMO hybrid materials and ends with a summary where an outlook on future perspectives to improve the properties of the hybrid materials in view of applications are outlined.

Highly Wavelength-Selective Enhancement of Responsivity in Ag Nanoparticle-Modified ZnO UV Photodetector

We proposed and demonstrated Ag nanoparticles (NPs)-decorated ZnO photodetectors for UV light sensing. After decoration of their surface with random Ag NPs, the dark current density of ZnO UV photodetectors decreases obviously. Moreover, the device exhibits an obvious increase in peak responsivity at around 380 nm, which can be attributed to the narrow-band quadrupole plasmon resonance of Ag NPs in the UV range. Meanwhile, the responsivity at the other wavelengths decreases a lot. As a result, the response peak becomes more significant, and the response of the devices presents an excellent wavelength selectivity after covering with Ag NPs. The detailed mechanism for this phenomenon was explained. We believe that our findings would open a way to harness the high-order plasmon modes in the field of UV optoelectronic devices.

Phenomenal Ultraviolet Photoresponsivity and Detectivity of Graphene Dots Immobilized on Zinc Oxide Nanorods

A combination of dimensionally reduced graphene quantum dots (GQDs) having edge effects and the vertically aligned ZnO nanorods shows highly selective visible-blind ultraviolet (UV) sensing. The GQD immobilized ZnO nanorod heterostructure shows remarkable responsivity of ∼6.62 × 10⁴ A/W and detectivity of ∼1.78 × 10¹⁵ Jones under 365 nm (10 μW) incident light and 2 V bias potential with high stability of at least 5 cycles, fast response time of 2.14 s, and recovery time of 0.91 s. The grain boundary assisted electron transport across GQDs was calculated from the normalized absorption below bandgap. The highest UV responsivity and detectivity were found to be proportional to the lowest trap state density at the grain boundaries (Q_t) and minimum grain boundary potential (E_b). For the best GQD, Q_t and E_b were found to be ∼4 × 10¹³ cm^-2 and 0.4 meV, respectively. The phenomenal performance of ZnO-GQD heterostructure is attributed to the efficient immobilization of GQDs on ZnO nanorods and the idea of employing GQDs as photosensitizers than solely as electron transporting medium. The efficiency of GQDs is superior to carbon quantum dots (CQDs) containing minimal graphitic domains, and graphene oxide (GO) or reduced graphene oxide (rGO) having larger dimensions preventing their immobilization on ZnO nanorods.

Photo-Response of Functionalized Self-Assembled Graphene Oxide on Zinc Oxide Heterostructure to UV Illumination

Convective assembly technique which is a simple and scalable method was used for coating uniform graphene oxide (GO) nanosheets on zinc oxide (ZnO) thin films. Upon UV irradiation, an enhancement in the on-off ratio was observed after functionalizing ZnO films by GO nanosheets. The calculations of on-off ratio, the device responsivity, and the external quantum efficiency were investigated and implied that the GO layer provides a stable pathway for electron transport. Structural investigations of the assembled GO and the heterostructure of GO on ZnO were performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The covered GO layer has a wide continuous area, with wrinkles and folds at the edges. In addition, the phonon lattice vibrations were investigated by Raman analysis. For GO and the heterostructure, a little change in the ratio between the D-band and G-band was found which means that no additional defects were formed within the heterostructure.

Conjugated assembly of colloidal zinc oxide quantum dots and multiwalled carbon nanotubes for an excellent photosensitive ultraviolet photodetector

Conjugation of highly dense colloidal zinc oxide quantum dots (ZnO QDs) on multiwalled carbon nanotubes (ZnO QDs@MWCNTs) is achieved for high performance ultraviolet (UV) photodetection. Significant improvement in the photoresponse of the ZnO QDs@MWCNTs photodetector (PD) is established as compared to a pristine ZnO QDs PD. The conjugation of two constituents allows the direct transfer of photoinduced charge carriers in ZnO QDs to MWCNTs for an efficient electrical path that considerably reduces charge recombination during UV exposure. Linearity in the response current with both the UV illumination intensity as well as external bias voltage reveals the photoelastic behavior of the ZnO QDs@MWCNTs PD. Moreover, the PD displays faster response and recovery times of 1.6 s and 1.9 s, respectively, than the most conventional PDs. In addition, spectral photoresponse analysis of the PD presents visible-blind behavior. Overall, conjugation of the hybrid heterostructure presented excellent photoelastic, high performance and visible-blind UV photodetection.

Development of Solution-Processed ZnO Nanorod Arrays Based Photodetectors and the Improvement of UV Photoresponse via AZO Seed Layers

Designing a rational structure and developing an efficient fabrication technique for bottom-up devices offer a promising opportunity for achieving high-performance devices. In this work, we studied how Al-doped ZnO (AZO) seed layer films influence the morphology and optical and electrical properties for ZnO aligned nanorod arrays (NRs) and then the performance of ZnO NRs based ultraviolet photodetectors (UV PDs) with Au/ZnO NRs Schottky junctions and p-CuSCN/n-ZnO NRs heterojunctions. The PD with AZO thin film with 0.5 at. % Al doping (named as AZO (0.5%)) exhibited more excellent photoresponse properties than that with pristine ZnO and AZO (1%) thin films. This phenomenon can be ascribed to the good light transmission of the AZO layer, increased density of the NRs, and improved crystallinity of ZnO NRs. The PDs based on CuSCN/ZnO NRs heterojunctions showed good rectification characteristics in the dark and self-powered UV photoresponse properties with excellent stability and reproducibility under low-intensity illumination conditions. A large responsivity located at 365 nm of 22.5 mA/W was achieved for the PD with AZO (0.5%) thin film without applied bias. The internal electric field originated from p-CuSCN/n-ZnO NRs heterojunctions can separate photogenerated carriers in ZnO NRs and drift toward the corresponding electrode.

A Simple Route to Reduced Graphene Oxide-Draped Nanocomposites with Markedly Enhanced Visible-Light Photocatalytic Performance

Nanocomposites (denoted RGO/ZnONRA) comprising reduced graphene oxide (RGO) draped over the surface of zinc oxide nanorod array (ZnONRA) were produced via a simple low-temperature route, dispensing with the need for hydrothermal growth, electrochemical deposition or other complex treatments. The amount of deposited RGO can be readily tuned by controlling the concentration of graphene oxide (GO). Interestingly, the addition of Sn(2+) not only enables the reduction of GO, but also functions as a bridge that connects the resulting RGO and ZnONRA. Remarkably, the incorporation of RGO improves the visible-light absorption and reduces the bandgap of ZnO, thereby leading to the markedly improved visible-light photocatalytic performance. Moreover, RGO/ZnONRA nanocomposites exhibit a superior stability as a result of the surface protection of ZnONRA by RGO. The mechanism on the improved photocatalytic performance based on the cophotosensitizations under the visible-light irradiation has been proposed. This simple yet effective route to the RGO-decorated semiconductor nanocomposites renders the better visible-light utilization, which may offer great potential for use in photocatalytic degradation of organic pollutants, solar cells, and optoelectronic materials and devices.

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.

Effect of Magnetic Field on Photoresponse of Cobalt Integrated Zinc Oxide Nanorods

Cobalt integrated zinc oxide nanorod (Co-ZnO NR) array is presented as a novel heterostructure for ultraviolet (UV) photodetector (PD). Defect states in Co-ZnO NRs surface induces an enhancement in photocurrent as compared to pristine ZnO NRs PD. Presented Co-ZnO NRs PD is highly sensitive to external magnetic field that demonstrated 185.7% enhancement in response current. It is concluded that the opposite polarizations of electron and holes in the presence of external magnetic field contribute to effective separation of electron-hole pairs that have drifted upon UV illumination. Moreover, Co-ZnO NRs PD shows a faster photodetection speed (1.2 s response time and 7.4 s recovery time) as compared to the pristine ZnO NRs where the response and recovery times are observed as 38 and 195 s, respectively.

Ultrahigh sensitivity in the amorphous ZnSnO UV photodetector

An ultraviolet photodetector based on α-ZTO exhibits ultrahigh sensitivity and good performance due to the TFTs structure.

Giant UV-sensitivity of ion beam irradiated nanocrystalline CdS thin films

A highly sensitive UV-detector is devised for the first time from ion beam irradiated nanocrystalline CdS thin films. The sensor exhibits improvements in the responsivity, photosensitivity, and efficiency as a function of ion fluence.

One-step in situ synthesis of single aligned graphene–ZnO nanofiber for UV sensing

Schematic of the microfabrication of gold electrodes, electrospinning with collector as pre patterned electrode, UV sensing with single aligned Gr–ZnO nanofiber device.

ZnO quantum dots and graphene based heterostructure for excellent photoelastic and highly sensitive ultraviolet photodetector

Heterostructures comprised of zinc oxide quantum dots (ZnO QDs) and graphene are presented for ultraviolet photodetectors (UV PD).