Broad-band three dimensional nanocave ZnO thin film photodetectors enhanced by Au surface plasmon resonance

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

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

3D-ZnO Superstructure Decorated with Carbon-Based Material for Efficient Photoelectrochemical Water-Splitting under Visible-Light Irradiation

The depletion of fossil fuels is a worldwide problem that has led to the discovery of alternative energy sources. Solar energy is the focus of numerous studies due to its huge potential power and environmentally friendly nature. Furthermore, one such area of study is the production of hydrogen energy by engaging photocatalysts using the photoelectrochemical (PEC) method. 3-D ZnO superstructures are extensively explored, showing high solar light-harvesting efficiency, more reaction sites, great electron transportation, and low electron-hole recombination. However, further development requires the consideration of several aspects, including the morphological effects of 3D-ZnO on water-splitting performance. This study reviewed various 3D-ZnO superstructures fabricated through different synthesis methods and crystal growth modifiers, as well as their advantages and limitations. Additionally, a recent modification by carbon-based material for enhanced water-splitting efficiency has been discussed. Finally, the review provides some challenging issues and future perspectives on the improvement of vectorial charge carrier migration and separation between ZnO as well as carbon-based material, using rare earth metals, which appears to be exciting for water-splitting.

Hybrid Device Architecture Using Plasmonic Nanoparticles, Graphene Quantum Dots, and Titanium Dioxide for UV Photodetectors

In this work, a nanoscale device architecture is demonstrated for a UV photodetector application on sapphire (0001), incorporating the plasmonic hybrid nanoparticles (HNPs), graphene quantum dots (GQDs), and titanium oxide (TiO₂) for the first time. The hybrid GQDs/TiO₂/HNPs photodetector exhibits the photocurrent of 1.58 × 10^-5 A under the 1.64 mW/mm² of 275 nm illumination at 10 V, which is around two order increase from the bare TiO₂ device. The proposed architecture demonstrates a low dark current of ∼1 × 10^-10 A at 10 V and thus the device demonstrates an excellent photo to dark current ratio along with the improved rise and fall time on the order of several hundred millisecond. The enhanced performance of device architecture is attributed to the efficient utilization of localized surface plasmon resonance (LSPR) induced hot carriers as well as scattered photons from the plasmonic HNPs that are fully encapsulated by the photoactive TiO₂ layers. Furthermore, the addition of GQDs on the TiO₂ can offer an additional photon absorption pathway. The proposed hybrid architecture of GQDs/TiO₂/HNPs demonstrates the integration of the photon absorption and carrier transfer properties of plasmonic HNPs, GQDs, and TiO₂ for an enhanced ultraviolet (UV) photoresponse. The photocurrent enhancement mechanisms of the hybrid device architecture are thoroughly investigated based on the finite-difference time domain (FDTD) simulation along with the energy band analysis. This work demonstrates a great potential of the hybrid device architecture for high-performance UV photodetectors.

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

HIGHLIGHTS

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

ABSTRACT

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

ELECTRONIC SUPPLEMENTARY MATERIAL

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

Room-temperature gas sensors based on ZnO nanorod/Au hybrids: Visible-light-modulated dual selectivity to NO2 and NH3

Gas sensors play vital roles in air pollution monitoring. Despite considerable progress in improving the room-temperature gas sensing sensitivities and rates of materials, comparably less attention is paid to the sensor selectivity. Here, ultrathin ZnO nanorods (˜15 nm) were synthesized by a nanoseed-assisted wet chemical approach and subsequently functionalized by Au nanoparticles by a photoreduction method. The hybrid material exhibited visible-light-activity owing to the surface plasmon resonance (SPR) effects of Au nanoparticles. The ZnO/Au hybrids were assembled into a high-performance, optically-controlled gas sensor operating at room temperature, which was found to be more selective to NH₃ in dark but showed high selectivity to NO₂ under visible-light illumination (λ = 532 nm). Moreover, the sensors exhibited high response and short response and recovery times as well as excellent reversibility and selectivity at room temperature. Such visible-light-modulated dual gas selectivity could be mainly attributed to the opposite direction of electron transfer between ZnO and Au nanoparticles in dark and under visible-light illumination, which led to the different surface depletion characteristics of the ZnO nanorods. In addition, the ultrathin diameters of nanorods also synergistically contributed to the light-controlled dual gas selectivity. The presently developed light modulation strategy provides an alternative approach to highly-selective and dual-functional gas sensors operating at room temperature.

Solution-Based Micro- and Nanoscale Metal Oxide Structures Formed by Direct Patterning for Electro-Optical Applications

Due to their transparency and tunable electrical, optical, and magnetic properties, metal oxide thin films and structures have many applications in electro-optical devices. In recent years, solution processing combined with direct-patterning techniques such as micro-/nanomolding, inkjet printing, e-jet printing, e-beam writing, and photopatterning has drawn much attention because of the inexpensive and simple fabrication process that avoids using capital-intensive vacuum deposition systems and chemical etching. Furthermore, practical applications of solution direct-patterning techniques with metal oxide structures are demonstrated in thin-film transistors and biochemical sensors on a wide range of substrates. Since direct-patterning techniques enable low-cost fabrication of nanoscale metal oxide structures, these methods are expected to accelerate the development of nanoscale devices and systems based on metal oxide components in important application fields such as flexible electronics, the Internet of Things (IoT), and human health monitoring. Here, a review of the fabrication procedures, advantages, limitations, and applications of the main direct-patterning methods for making metal oxide structures is presented. The goal is to highlight the examples with the most promising perspective from the recent literature.

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.

Light-Activated Metal Oxide Gas Sensors: A Review

Conductometric gas sensors facilitated by photons have been investigated for decades. Light illumination may enhance device attributes including operational temperature, sensing sensitivity and selectivity. This paper aims to provide an overview on the progress of light-activated gas sensors, with a specific focus on sensors based on metal oxides. The material systems that have been studied include pure metal oxides, heterostructures of semiconductor-metal oxides and metal-metal oxides, and metal oxides with dopant. Other reported works on the use of different nanostructures such as one-dimensional and porous nanostructures, study of sensing mechanisms and the interplay between various factors are also summarized. Possible directions for further improvement of sensing properties, through optimizing the size of nanomaterials, film thickness, light intensity and wavelength are discussed. Finally, we point out that the main challenge faced by light-activated gas sensors is their low optical response, and we have analyzed the feasibility of using localized surface plasmon resonance to solve this drawback. This article should offer readers some key and instructive insights into the current and future development of light-activated gas sensors.

Electrochemical nanoimprint lithography: when nanoimprint lithography meets metal assisted chemical etching

The functional three dimensional micro-nanostructures (3D-MNS) play crucial roles in integrated and miniaturized systems because of the excellent physical, mechanical, electric and optical properties. Nanoimprint lithography (NIL) has been versatile in the fabrication of 3D-MNS by pressing thermoplastic and photocuring resists into the imprint mold. However, direct nanoimprint on the semiconductor wafer still remains a great challenge. On the other hand, considered as a competitive fabrication method for erect high-aspect 3D-MNS, metal assisted chemical etching (MacEtch) can remove the semiconductor by spontaneous corrosion reaction at the metal/semiconductor/electrolyte 3-phase interface. Moreover, it was difficult for MacEtch to fabricate multilevel or continuously curved 3D-MNS. The question of the consequences of NIL meeting the MacEtch is yet to be answered. By employing a platinum (Pt) metalized imprint mode, we demonstrated that using electrochemical nanoimprint lithography (ECNL) it was possible to fabricate not only erect 3D-MNS, but also complex 3D-MNS with multilevel stages with continuously curved surface profiles on a gallium arsenide (GaAs) wafer. A concave microlens array with an average diameter of 58.4 μm and height of 1.5 μm was obtained on a ∼1 cm²-area GaAs wafer. An 8-phase microlens array was fabricated with a minimum stage of 57 nm and machining accuracy of 2 nm, presenting an excellent optical diffraction property. Inheriting all the advantages of both NIL and MacEtch, ECNL has prospective applications in the micro/nano-fabrications of semiconductors.

Enhanced CMOS image sensor by flexible 3D nanocone anti-reflection film

Complementary metal oxide semiconductor (CMOS) image sensors (CIS) are being widely used in digital video cameras, web cameras, digital single lens reflex camera (DSLR), smart phones and so on, owing to their high level of integration, random accessibility, and low-power operation. It needs to be installed with the cover glass in practical applications to protect the sensor from damage, mechanical issues, and environmental conditions, which, however, limits the accuracy and usability of the sensor due to the reflection in the optical path from air-to-cover glass-to-air. In this work, the flexible 3D nanocone anti-reflection (AR) film with controlled aspect ratio was firstly employed to reduce the light reflection at air/cover glass/air interfaces by directly attaching onto the front and rear sides of the CIS cover glass. As both the front and rear sides of cover glass were coated by the AR film, the output image quality was found to be improved with external quantum efficiency increased by 7%, compared with that without AR film. The mean digital data value, root-mean-square contrast, and dynamic range are increased by 45.14%, 38.61% and 57, respectively, for the output image with AR films. These results provide a novel and facile pathway to improve the CIS performance and also could be extended to rational design of other image sensors and optoelectronic devices.

Low-Voltage High-Performance UV Photodetectors: An Interplay between Grain Boundaries and Debye Length

Accurate detection of UV light by wearable low-power devices has many important applications including environmental monitoring, space to space communication, and defense. Here, we report the structural engineering of ultraporous ZnO nanoparticle networks for fabrication of very low-voltage high-performance UV photodetectors. A record high photo- to dark-current ratio of 3.3 × 10⁵ and detectivity of 3.2 × 10¹² Jones at an ultralow operation bias of 2 mV and low UV-light intensity of 86 μW·cm^-2 are achieved by controlling the interplay between grain boundaries and surface depletion depth of ZnO nanoscale semiconductors. An optimal window of structural properties is determined by varying the particle size of ultraporous nanoparticle networks from 10 to 42 nm. We find that small electron-depleted nanoparticles (≤40 nm) are necessary to minimize the dark-current; however, the rise in photocurrent is tampered with decreasing particle size due to the increasing density of grain boundaries. These findings reveal that nanoparticles with a size close to twice their Debye length are required for high photo- to dark-current ratio and detectivity, while further decreasing their size decreases the photodetector performance.