UV-activated ZnO films on a flexible substrate for room temperature O2 and H2O sensing

High-Performance Self-Powered Quantum Dot Infrared Photodetector with Azide Ion Solution Treated Electron Transport Layer

The demand for self-powered photodetectors (PDs) capable of NIR detection without external power is growing with the advancement of NIR technologies such as LIDAR and object recognition. Lead sulfide quantum dot-based photodetectors (PbS QPDs) excel in NIR detection; however, their self-powered operation is hindered by carrier traps induced by surface defects and unfavorable band alignment in the zinc oxide nanoparticle (ZnO NP) electron-transport layer (ETL). In this study, an effective azide-ion (N3 -) treatment is introduced on a ZnO NP ETL to reduce the number of traps and improve the band alignment in a PbS QPD. The ZnO NP ETL treated with azide ions exhibited notable improvements in carrier lifetime and mobility as well as an enhanced internal electric field within the thin-film heterojunction of the ZnO NPs and PbS QDs. The azide-ion-treated PbS QPD demonstrated a increase in short-circuit current density upon NIR illumination, marking a responsivity of 0.45 A W-1, specific detectivity of 4 × 1011 Jones at 950 nm, response time of 8.2 µs, and linear dynamic range of 112 dB.

Dual Light Temporal Coding Modes Enabled by Nanoparticle-Mediated Phototransistors via Gate Bias Modulation for Brain-Inspired Visual Perception

The core integration and cooperation of the retina, neurons, and synapses in the visual systems enable humans to effectively sense and process visual information with low power consumption. To mimic the human visual system, an artificial sensory nerve, along with optical sensing─a paired-pulse ratio (PPR) of the light pulse stimulated currents─and neural coding has been developed. For performing the artificial visual perception functions, we consistently reveal the positive and negative correlations between the PPR index and light pulse time interval by applying two consecutive light stimuli with gate voltages of -10 and 5 V, respectively, to a phototransistor. This phototransistor contains a heterostructured channel layer composed of zinc-oxide nanoparticles (ZnO NPs) interconnected with a solution-processed zinc-tin oxide (ZTO) film. The oxygen adsorption and desorption on the ZnO NP surface under light illumination are responsible for the positive-sloped PPR; the electron trapping effect at the ZnO NP/SiO2 interface is attributed to the negative-sloped PPR. The various accountable light power densities and number of surface trap states are considered to be directly realizing these spike-timing interval-dependent characteristics. The actual benefit of these characteristics is the dual temporal coding modes based on multiplicative operation using a ZTO/ZnO NP phototransistor realized via the active gate voltage modulation. The contrary tendency of the PPR index and temporal coding─a major biological neural coding─is well demonstrated by the potential of ZTO/ZnO NP phototransistors to be implemented in sensor networks for an artificial visual perception.

Machine Intelligence-Centered System for Automated Characterization of Functional Materials and Interfaces

Classic design of experiment relies on a time-intensive workflow that requires planning, data interpretation, and hypothesis building by experienced researchers. Here, we describe an integrated, machine-intelligent experimental system which enables simultaneous dynamic tests of electrical, optical, gravimetric, and viscoelastic properties of materials under a programmable dynamic environment. Specially designed software controls the experiment and performs on-the-fly extensive data analysis and dynamic modeling, real-time iterative feedback for dynamic control of experimental conditions, and rapid visualization of experimental results. The system operates with minimal human intervention and enables time-efficient characterization of complex dynamic multifunctional environmental responses of materials with simultaneous data processing and analytics. The system provides a viable platform for artificial intelligence (AI)-centered material characterization, which, when coupled with an AI-controlled synthesis system, could lead to accelerated discovery of multifunctional materials.

AZO Nanoparticles-Decorated CNTs for UV Light Sensing: A Structural, Chemical, and Electro-Optical Investigation

Nanocomposites formed by aluminum-doped zinc oxide nanoparticles (AZO-NP) and multiwall carbon nanotubes (CNT) are proposed here as a promising material for UV light sensing applications, with the great advantage of operating in air, at room temperature, and at low voltage. Nanocomposite layers were prepared with different AZO:CNT weight ratios by a simple methodology at room temperature. They were characterized by means of UV-Vis spectroscopy, scanning and transmission electron microscopies (SEM and TEM), and X-ray photoelectron spectroscopy (XPS). The interaction between the two nanomaterials was demonstrated by comparing the properties of the nanocomposite with the ones shown by the AZO-NPs. Dense AZO-CNT nanocomposite layers were deposited between two metal electrodes on a SiO₂/Si substrate, and the electrical properties were investigated in dark condition and under UV light irradiation. The electrical response to the UV light was a sudden current increase that reduced when the light was switched off. Several UV on/off cycles were performed, showing good repeatability and stability of the response. The mechanisms involved in the electrical response are discussed and compared to the ones previously reported for ZnO-CNT nanocomposites.

Recent Progress on Flexible Room-Temperature Gas Sensors Based on Metal Oxide Semiconductor

With the rapid development of the Internet of Things, there is a great demand for portable gas sensors. Metal oxide semiconductors (MOS) are one of the most traditional and well-studied gas sensing materials and have been widely used to prepare various commercial gas sensors. However, it is limited by high operating temperature. The current research works are directed towards fabricating high-performance flexible room-temperature (FRT) gas sensors, which are effective in simplifying the structure of MOS-based sensors, reducing power consumption, and expanding the application of portable devices. This article presents the recent research progress of MOS-based FRT gas sensors in terms of sensing mechanism, performance, flexibility characteristics, and applications. This review comprehensively summarizes and discusses five types of MOS-based FRT gas sensors, including pristine MOS, noble metal nanoparticles modified MOS, organic polymers modified MOS, carbon-based materials (carbon nanotubes and graphene derivatives) modified MOS, and two-dimensional transition metal dichalcogenides materials modified MOS. The effect of light-illuminated to improve gas sensing performance is further discussed. Furthermore, the applications and future perspectives of FRT gas sensors are also discussed.

Development of Tetrapod Zinc Oxide-Based UV Sensor for Precision Livestock Farming and Productivity

In order to ensure the health and welfare of livestock, there has been an emphasis on precision farming of ruminant animals. Monitoring the life index of ruminant animals is of importance for intelligent farming. Here, a wearable sensor for monitoring ultraviolet (UV) radiation is demonstrated to understand the effect of primary and secondary photosensitization on dairy animals. Thin films of wide bandgap semiconductor zinc oxide (ZnO) comprising multilevel of nanostructures from microparticles (MP) to nanoparticles (NP), and tetrapod (T-ZnO), were prepared as the UV sensing active materials. The sensitivity was evaluated by exposing the films to various radiation sources, i.e., 365 nm (UV A), 302 nm (UV B), and 254 nm (UV C), and measuring the electrical resistance change. T-ZnO is found to exhibit higher sensitivity and stable response (on/off) upon exposure to UV A and UV B radiation, which is attributed to their higher surface area, aspect ratio, porosity, and interconnective networks inducing a high density of chemical interaction sites and consequently improved photocurrent generation. A wearable sensor using T-ZnO is packaged and attached to a collar for dynamic monitoring of UV response on ruminant animals (e.g., sheep in this study). The excellent performance of T-ZnO wearable sensors for ruminant animals also holds the potential for a wider range of applications such as residential buildings and public spaces.

A Transparent Electrode Based on Solution-Processed ZnO for Organic Optoelectronic Devices

Achieving high-efficiency indium tin oxide (ITO)-free organic optoelectronic devices requires the development of high-conductivity and high-transparency materials for being used as the front electrode. Herein, sol-gel-grown zinc oxide (ZnO) films with high conductivity (460 S cm⁻¹) and low optical absorption losses in both visible and near-infrared (NIR) spectral regions are realized utilizing the persistent photoinduced doping effect. The origin of the increased conductivity after photo-doping is ascribed to selective trapping of photogenerated holes by oxygen vacancies at the surface of the ZnO film. Then, the conductivity of the sol-gel-grown ZnO is further increased by stacking the ZnO using a newly developed sequential deposition strategy. Finally, the stacked ZnO is used as the cathode to construct ITO-free organic solar cells, photodetectors, and light emitting diodes: The devices based on ZnO outperform those based on ITO, owing to the reduced surface recombination losses at the cathode/active layer interface, and the reduced parasitic absorption losses in the electrodes of the ZnO based devices.

A highly conductive and transparent electrode is essential to achieving a high efficiency in indium tin oxide-free optoelectronic devices. Here, the authors strategically prepare sol-gel-grown zinc oxide films based on photoinduced doping effect and demonstrate enhanced performance of devices.

Room-Temperature Gas Sensors Under Photoactivation: From Metal Oxides to 2D Materials

Room-temperature gas sensors have aroused great attention in current gas sensor technology because of deemed demand of cheap, low power consumption and portable sensors for rapidly growing Internet of things applications. As an important approach, light illumination has been exploited for room-temperature operation with improving gas sensor's attributes including sensitivity, speed and selectivity. This review provides an overview of the utilization of photoactivated nanomaterials in gas sensing field. First, recent advances in gas sensing of some exciting different nanostructures and hybrids of metal oxide semiconductors under light illumination are highlighted. Later, excellent gas sensing performance of emerging two-dimensional materials-based sensors under light illumination is discussed in details with proposed gas sensing mechanism. Originated impressive features from the interaction of photons with sensing materials are elucidated in the context of modulating sensing characteristics. Finally, the review concludes with key and constructive insights into current and future perspectives in the light-activated nanomaterials for optoelectronic gas sensor applications.

Highly Sensitive UV Photodiode Composed of β-Polyfluorene/YZnO Nanorod Organic-Inorganic Hybrid Heterostructure

The highly sensitive ultra-violet (UV) photodiode was demonstrated on the organic-inorganic hybrid heterostructure of β-phase p-type polyfluorene (PFO)/n-type yttrium-doped zinc oxide nanorods (YZO-NRs). The device was fabricated through a simple fabrication technique of β-phase PFO coating onto YZO-NRs that had been directly grown on graphene by the hydrothermal synthesis method. Under UV illumination (λ = 365 nm), the device clearly showed excellent photoresponse characteristics (e.g., high quantum efficiency ~690%, high photodetectivity ~3.34 × 1012 cm·Hz1/2·W-1, and fast response time ~0.17 s). Furthermore, the ratio of the photo current-to-dark current exceeds 103 even under UV illumination with a small optical power density of 0.6 mW/cm2. We attribute such superb photoresponse characteristics to both Y incorporation into YZO-NRs and conformation of β-phase PFO. Namely, Y dopants could effectively reduce surface states at YZO-NRs, and β-phase PFO might increase the photocarrier conductivity in PFO. The results suggest that the β-phase p-PFO/n-YZO-NR hybrid heterostructure holds promise for high-performance UV photodetectors.

Amorphous ZnO/PbS Quantum Dots Heterojunction for Efficient Responsivity Broadband Photodetectors

The integration of lead sulfide quantum dots (QDs) with a high-conductivity material that is compatible with a scalable fabrication is an important route for the applications of QD-based photodetectors. Herein, we first developed a broadband photodetector by combining amorphous ZnO and PbS QDs, forming a heterojunction structure. The photodetector showed detectivity up to 7.9 × 10¹² and 4.1 × 10¹¹ jones under 640 and 1310 nm illumination, respectively. The role of the oxygen background pressure in the electronic structure of ZnO films grown by pulsed laser deposition was systematically studied, and it was found to play an important role in the conductivity associated with the variation of the oxygen vacancy concentration. By increasing the oxygen vacancy concentration, the electron mobility of amorphous ZnO layers dramatically increased and the work function decreased, which were beneficial for the photocurrent enhancement of ZnO/PbS QD photodetectors. Our results provide a simple and highly scalable approach to develop broadband photodetectors with high performance.

Sensing of Oxygen Partial Pressure in Air with ZnO Nanoparticles

The demand for sensors in response to oxygen partial pressure in air is increasingly high in recent years and small-size sensors on a micrometer scale and even a nanometer scale are particularly desirable. In this paper, the sensing of oxygen partial pressure in air was realized by a solution-processed ZnO nanoparticle (NP). Thin-film ZnO NP was prepared by spin-coating and a highly sensitive sensor was then fabricated. The oxygen sensing performance was characterized in air and compared with that in nitrogen, which showed an increase in electrical conductance by more than 100 times as a result of decreasing oxygen partial pressure from 10³ mBar to 10⁻⁵ mBar. Moreover, higher sensitivity was achieved by increasing the annealing temperature and the effect of thermal annealing was also investigated. Furthermore, ZnO NP lines with 7 μm in width were successfully patterned with low cost by a mould-guided drying technique from ZnO NP dispersion, which makes ZnO NP extremely promising for miniaturized and integrated sensing applications.

Scratch to sensitize: scratch-induced sensitivity enhancement in semiconductor thin-film sensors

Semiconductor gas sensors are advantageous in miniaturization and can be used in a wide range of applications, yet consume large power due to high operating temperature. Here we demonstrated the ability of nanoscale scratches produced with mechanical abrasion to enhance the chemical sensitivity of thin-film-type semiconductor sensors. Well-aligned arrays of scratches parallel to the electrical current direction between the source and drain electrodes were made, using typical polishing machines with diamond suspensions, on semiconductor thin films produced with various deposition methods such as atomic layer deposition (ALD), sputtering, and the sol-gel technique. Processing with sharp diamond microparticles left nano-grooves on the surface, together with changes in chemical composition. For all of the tested metal oxide thin films, the introduction of scratches yielded increased quantities of oxygen vacancies and metallic components. Scratched ZnO devices exhibited superior performance even at room temperature, as predicted by a computational simulation that showed increased binding energy of gas molecules on defects. The scratch technique shown in the present study may be used to produce dense arrays of nanometer-scale, chemically functionalized line patterns on substrates larger than a few tens of centimeters with minimum cost, which in turn may be used in a variety of applications including massive arrays of sensors displaying high sensitivity.

Zinc Titanate Nanoarrays with Superior Optoelectrochemical Properties for Chemical Sensing

In this report, the gas sensing performance of zinc titanate (ZnTiO₃) nanoarrays (NAs) synthesized by coating hydrothermally formed zinc oxide (ZnO) NAs with TiO₂ using low-temperature chemical vapor deposition is presented. By controlling the annealing temperature, diffusion of ZnO into TiO₂ forms a mixed oxide of ZnTiO₃ NAs. The uniformity and the electrical properties of ZnTiO₃ NAs made them ideal for light-activated acetone gas sensing applications for which such materials are not well studied. The acetone sensing performance of the ZnTiO₃ NAs is tested by biasing the sensor with voltages from 0.1 to 9 V dc in an amperometric mode. An increase in the applied bias was found to increase the sensitivity of the device toward acetone under photoinduced and nonphotoinduced (dark) conditions. When illuminated with 365 nm UV light, the sensitivity was observed to increase by 3.4 times toward 12.5 ppm acetone at 350 °C with an applied bias of 9 V, as compared to dark conditions. The sensor was also observed to have significantly reduced the adsorption time, desorption time, and limit of detection (LoD) when excited by the light source. For example, LoD of the sensor in the dark and under UV light at 350 °C with a 9 V bias is found to be 80 and 10 ppb, respectively. The described approach also enabled acetone sensing at an operating temperature down to 45 °C with a repeatability of >99% and a LoD of 90 ppb when operated under light, thus indicating that the ZnTiO₃ NAs are a promising material for low concentration acetone gas sensing applications.

Investigation of ZnO-decorated CNTs for UV Light Detection Applications

Multi-walled carbon nanotubes (CNTs) decorated with zinc oxide nanoparticles (ZnO NPs) were prepared in isopropanol solution by a simple, room-temperature process and characterized from structural, morphological, electronic, and optical points of view. A strong interaction between ZnO and CNTs is fully confirmed by all the characterization techniques. ZnO-CNTs nanocomposites, with different weight ratios, were deposited as a dense layer between two electrodes, in order to investigate the electrical behaviour. In particular, the electrical response of the nanocomposite layers to UV light irradiation was recorded for a fixed voltage: As the device is exposed to the UV lamp, a sharp current drop takes place and then an increase is observed as the irradiation is stopped. The effect can be explained by adsorption and desorption phenomena taking place on the ZnO nanoparticle surface under irradiation and by charge transfer between ZnO and CNTs, thanks to the strong interaction between the two nanomaterials. The nanocomposite material shows good sensitivity and fast response to UV irradiation. Room temperature and low-cost processes used for the device preparation combined with room temperature and low voltage operational conditions make this methodology very promising for large scale UV detectors applications.

Magnetron Sputtering for ZnO:Ga Scintillation Film Production and Its Application Research Status in Nuclear Detection

As a wide band-gap and direct transition semiconductor material, ZnO has good scintillation performance and strong radiation resistance, but it also has a serious self-absorption phenomenon that affects its light output. After being doped with Ga, it can be used for the scintillator of ultra-fast scintillating detectors to detect X-ray, gamma, neutron, and charged particles with extremely fast response and high light output. Firstly, the basic properties, defects, and scintillation mechanism of ZnO crystals are introduced. Thereafter, magnetron sputtering, one of the most attractive production methods for producing ZnO:Ga film, is introduced including the principle of magnetron sputtering and its technical parameters’ influence on the performance of ZnO:Ga. Finally, ZnO:Ga film’s application research status is presented as a scintillation material in the field of radiation detection, and it is concluded that some problems need to be urgently solved for its wider application.

The Effect of UV Illumination on the Room Temperature Detection of Vaporized Ammonium Nitrate by a ZnO Coated Nanospring-Based Sensor

The effect of UV illumination on the room temperature electrical detection of ammonium nitrate vapor was examined. The sensor consists of a self-assembled ensemble of silica nanosprings coated with zinc oxide. UV illumination mitigates the baseline drift of the resistance relative to operation under dark conditions. It also lowers the baseline resistance of the sensor by 25% compared to dark conditions. At high ammonium nitrate concentrations (120 ppm), the recovery time after exposure is virtually identical with or without UV illumination. At low ammonium nitrate concentrations (20 ppm), UV illumination assists with refreshing of the sensor by stimulating analyte desorption, thereby enabling the sensor to return to its baseline resistance. Under dark conditions and low ammonium nitrate concentrations, residual analyte builds up with each exposure, which inhibits the sensor from returning to its original baseline resistance and subsequently impedes sensing due to permanent occupation of absorption sites.

Quantitative comparison of closed-loop and dual harmonic Kelvin probe force microscopy techniques

Kelvin probe force microscopy (KPFM) is a widely used technique to map surface potentials at the nanometer scale. In traditional KPFM, a feedback loop regulates the DC bias applied between a sharp conductive probe and a sample to nullify the electrostatic force (closed-loop operation). In comparison, open-loop techniques such as dual harmonic KPFM (DH-KPFM) are simpler to implement, are less sensitive to artefacts, offer the unique ability to probe voltage sensitive materials, and operate in liquid environments. Here, we directly compare the two techniques in terms of their bandwidth and sensitivity to instrumentation artefacts. Furthermore, we introduce a new correction for traditional KPFM termed "setpoint correction," which allows us to obtain agreement between open and closed-loop techniques within 1%. Quantitative validation of DH-KPFM may lead to a wider adoption of open-loop KPFM techniques by the scanning probe community.

Multi-modal, ultrasensitive, wide-range humidity sensing with Ti3C2 film

Gravimetric, direct-current electrical, and electrical impedance sensing modes were used to measure response of high surface area 2D Ti3C2 MXene film to water vapor pressures spanning 3 orders of magnitude (20 mTorr-20 Torr). The Ti3C2 film exhibited reproducible reversible response in 0.1%-95% relative humidity (RH) range with a detection limit of <20 mTorr H2O partial pressure (<0.1% RH). DC electrical current-based sensing with 3 mV operating voltage and 0.8 pW power consumption was demonstrated. The highest normalized sensitivity was shown for gravimetric sensing modalities which scale with the overtone number, reaching highest sensitivity of about 12 Hz/% RH at the 9th crystal overtone (45 MHz oscillation).

High performance and low temperature coal mine gas sensor activated by UV-irradiation

In this work, well-aligned vertical ZnO nanorod (ZnO NRs) on p-type Si substrate was fabricated by a microwave-assisted hydrothermal reactor to study the coal mine methane (CMM) gas sensing properties. The XRD diffraction peaks and Raman spectra of the ZnO NRs confirmed the hexagonal wurtzite structure with strong preferential orientation along the c axis and well crystal quality. SEM analysis showed NRs with 100 nm average diameter and ~600 nm length. The variations of the sensor electrical resistance in the presence of CMM were investigated at different gas concentrations and various temperatures in the dark and under UV light. The selectivity and response time of the sensor to CMM gas were improved under UV irradiation. The optimal operating temperatures were 225 °C and 100 °C in dark and exposing UV-irradiation, respectively. Also the response of ZnO NRs sensor under UV excitation in humid condition was higher. The sensor was more selective to CMM than CO₂. The sensor stability was considered by repeating CMM detection for 90 days.

Two-Dimensional Nanomaterials for Gas Sensing Applications: The Role of Theoretical Calculations

Two-dimensional (2D) nanomaterials have attracted a large amount of attention regarding gas sensing applications, because of their high surface-to-volume ratio and unique chemical or physical gas adsorption capabilities. As an important research method, theoretical calculations have been massively applied in predicting the potentially excellent gas sensing properties of these 2D nanomaterials. In this review, we discuss the contributions of theoretical calculations in the study of the gas sensing properties of 2D nanomaterials. Firstly, we elaborate on the gas sensing mechanisms of 2D layered nanomaterials, such as the traditional charge transfer mechanism, and a standard for distinguishing between physical and chemical adsorption, from the perspective of theoretical calculations. Then, we describe how to conduct a theoretical analysis to explain or predict the gas sensing properties of 2D nanomaterials. Thirdly, we discuss three important methods that have been applied in order to improve the gas sensing properties, that is, defect functionalization (vacancy, edge, grain boundary, and doping), heterojunctions, and electric fields. Among these strategies, theoretical calculations play a very important role in explaining the mechanisms underlying the enhanced gas sensing properties. Finally, we summarize both the advantages and limitations of the theoretical calculations, and present perspectives for further research on the 2D nanomaterials-based gas sensors.

Light-Activated Hybrid Nanocomposite Film for Water and Oxygen Sensing

Oxygen and water vapor sensing properties are investigated in metal-oxide-hybrid polymer nanocomposite thin films generated by infiltration synthesis, which incorporates molecular ZnO into the matrix of SU-8 polymer, a common negative-tone photoresist. The hybrid thin films display 20-fold higher gravimetric responses to oxygen and water vapor than those of control ZnO thin films in the dark. An additional 50-500% enhanced responses are detected under UV irradiation. The overall enhanced gravimetric response in the hybrid film is attributed to the ZnO molecules distributed in the polymer matrix, whereas the UV enhancement is explained by the light-induced, reversible generation of hydrophilic fluoroantimonic acid from triarylsulfonium hexafluoroantimonate photoacids, which leads to the increased surface potential and adsorption energies for oxygen and water. A gravimetric sensor based on a series of ZnO-infiltrated SU-8 films under UV excitation enables 96% accurate classification of water and oxygen environment with sub 10 mTorr detection limits. The results demonstrate UV-induced fully reversible surface hydrophilicity of ZnO/SU-8 hybrid nanocomposites.

ZnO Nanowire Application in Chemoresistive Sensing: A Review

This article provides an overview of the recent development of ZnO nanowires (NWs) for chemoresistive sensing. Working mechanisms of chemoresistive sensors are unified for gas, ultraviolet (UV) and bio sensor types: single nanowire and nanowire junction sensors are described, giving the overview for a simple sensor manufacture by multiple nanowire junctions. ZnO NW surface functionalization is discussed, and how this effects the sensing is explained. Further, novel approaches for sensing, using ZnO NW functionalization with other materials such as metal nanoparticles or heterojunctions, are explained, and limiting factors and possible improvements are discussed. The review concludes with the insights and recommendations for the future improvement of the ZnO NW chemoresistive sensing.