The Challenges and Opportunities for TiO2 Nanostructures in Gas Sensing

Chemiresistive gas sensors based on metal oxides have been widely applied in industrial monitoring, medical diagnosis, environmental pollutant detection, and food safety. To further enhance the gas sensing performance, researchers have worked to modify the structure and function of the material so that it can adapt to different gas types and environmental conditions. Among the numerous gas-sensitive materials, n-type TiO2 semiconductors are a focus of attention for their high stability, excellent biosafety, controllable carrier concentration, and low manufacturing cost. This Perspective first introduces the sensing mechanism of TiO2 nanostructures and composite TiO2-based nanomaterials and then analyzes the relationship between their gas-sensitive properties and their structure and composition, focusing also on technical issues such as doping, heterojunctions, and functional applications. The applications and challenges of TiO2-based nanostructured gas sensors in food safety, medical diagnosis, environmental detection, and other fields are also summarized in detail. Finally, in the context of their practical application challenges, future development technologies and new sensing concepts are explored, providing new ideas and directions for the development of multifunctional intelligent gas sensors in various application fields.

Compositional Engineering of Cu-Doped SnO Film for Complementary Metal Oxide Semiconductor Technology

Metal oxide semiconductor (MOS)-based complementary thin-film transistor (TFT) circuits have broad application prospects in large-scale flexible electronics. To simplify circuit design and increase integration density, basic complementary circuits require both p- and n-channel transistors based on an individual semiconductor. However, until now, no MOSs that can simultaneously show p- and n-type conduction behavior have been reported. Herein, we demonstrate for the first time that Cu-doped SnO (Cu:SnO) with HfO2 capping can be employed for high-performance p- and n-channel TFTs. The interstitial Cu+ can induce an n-doping effect while restraining electron-electron scatterings by removing conduction band minimum degeneracy. As a result, the Cu3 atom %:SnO TFTs exhibit a record high electron mobility of 43.8 cm2 V-1 s-1. Meanwhile, the p-channel devices show an ultrahigh hole mobility of 2.4 cm2 V-1 s-1. Flexible complementary logics are then established, including an inverter, NAND gates, and NOR gates. Impressively, the inverter exhibits an ultrahigh gain of 302.4 and excellent operational stability and bending reliability.

Real-Time Monitoring of Air Discharge in a Switchgear by an Intelligent NO2 Sensor Module

An air-insulated power equipment adopts air as the insulating medium and is widely implemented in power systems. When discharge faults occur, the air produces decomposition products represented by NO2. The efficient NO2 sensor enables the identification of electrical equipment faults. However, single-sensor-dependent NO2 detection is vulnerable to interfering gases. Implementing the sensor array could reduce the interference and improve detection efficiency. In the field of NO2 detection, In2O3 sensors have exhibited tremendous advantages. In our work, four composites based on In2O3 are integrated into sensor arrays, which could detect 250 ppb of NO2 and exhibit excellent selectivity when simultaneously exposed to CO. To further reduce the impact of humidity on gas-sensing performance, a convolutional neural network and a long short-term memory model equipped with an attention mechanism are proposed to evaluate NO2 concentration within 1 ppm, and the detection error is 63.69 ppb. In addition, the NO2 concentration estimation platform based on a microgas sensor is established to detect air discharge faults. The average concentration of NO2 generated by 10 consecutive discharge faults at 15 kV is 726.58 ppb, which indicates severe discharge in the switchgear. Our NO2 estimation method has great potential for large-scale deployment in low- and medium-voltage switchgears.

Recent Progress in Spinel Ferrite (MFe2O4) Chemiresistive Based Gas Sensors

Gas-sensing technology has gained significant attention in recent years due to the increasing concern for environmental safety and human health caused by reactive gases. In particular, spinel ferrite (MFe2O4), a metal oxide semiconductor with a spinel structure, has emerged as a promising material for gas-sensing applications. This review article aims to provide an overview of the latest developments in spinel-ferrite-based gas sensors. It begins by discussing the gas-sensing mechanism of spinel ferrite sensors, which involves the interaction between the target gas molecules and the surface of the sensor material. The unique properties of spinel ferrite, such as its high surface area, tunable bandgap, and excellent stability, contribute to its gas-sensing capabilities. The article then delves into recent advancements in gas sensors based on spinel ferrite, focusing on various aspects such as microstructures, element doping, and heterostructure materials. The microstructure of spinel ferrite can be tailored to enhance the gas-sensing performance by controlling factors such as the grain size, porosity, and surface area. Element doping, such as incorporating transition metal ions, can further enhance the gas-sensing properties by modifying the electronic structure and surface chemistry of the sensor material. Additionally, the integration of spinel ferrite with other semiconductors in heterostructure configurations has shown potential for improving the selectivity and overall sensing performance. Furthermore, the article suggests that the combination of spinel ferrite and semiconductors can enhance the selectivity, stability, and sensing performance of gas sensors at room or low temperatures. This is particularly important for practical applications where real-time and accurate gas detection is crucial. In conclusion, this review highlights the potential of spinel-ferrite-based gas sensors and provides insights into the latest advancements in this field. The combination of spinel ferrite with other materials and the optimization of sensor parameters offer opportunities for the development of highly efficient and reliable gas-sensing devices for early detection and warning systems.

Detection of n-Propanol Down to the Sub-ppm Level Using p-Type Delafossite AgCrO2 Nanoparticles

As an important biomarker of lung cancer, n-propanol at the sub-ppm level is still challenging to be detected for a simple and immediate early diagnosis. In this work, a new n-propanol gas sensor with an ultralow detection limit down to 100 ppb is presented using AgCrO₂ nanoparticles synthesized by a simple hydrothermal method. Compared with the congeneric CuCrO₂ and commercial SnO₂, AgCrO₂ exhibits prevailing performances, including a higher selectivity, dynamic response, and logarithmical linearity but lower working temperature. The first-principles calculation and the energy band theoretical analysis are combined to elucidate the sensing mechanism, in which the chemical adsorption of gaseous molecules to silver followed by the dehydrogenation on chromium on the surface of AgCrO₂ is responsible for the outstanding susceptibility toward n-propanol. The proposed metal oxide semiconductor gas sensor capable of sub-ppm n-propanol detection provides a route to design and optimize the sensitive material system for the advanced trace detection of the volatile organic compounds.

Metal Oxide Heterostructures for Improving Gas Sensing Properties: A Review

Metal oxide semiconductor gas sensors are widely used to detect toxic and inflammable gases in industrial production and daily life. The main research hotspot in this field is the synthesis of gas sensing materials. Previous studies have shown that incorporating two or more metal oxides to form a heterojunction interface can exhibit superior gas sensing performance in response and selectivity compared with single phase. This review focuses on mainly the synthesis methods and gas sensing mechanisms of metal oxide heterostructures. A significant number of heterostructures with different morphologies and shapes have been fabricated, which exhibit specific sensing performance toward a specific target gas. Among these synthesis methods, the hydrothermal method is noteworthy due to the fabrication of diverse structures, such as nanorod-like, nanoflower-like, and hollow sphere structures with enhanced sensing properties. In addition, it should be noted that the combination of different synthesis methods is also an efficient way to obtain metal oxide heterostructures with novel morphologies. Despite advanced methods in the metal oxide semiconductors and nanotechnology field, there are still some new issues which deserve further investigation, such as long-term chemical stability of sensing materials, reproducibility of the fabrication process, and selectivity toward homogeneous gases. Moreover, the gas sensing mechanism of metal oxide heterostructures is controversial. It should be clarified so as to further integrate laboratory theory research with practical exploitation.

Active-ion-gated room temperature acetone gas sensing of ZnO nanowires array

Reducing the high operation temperature of gas sensor to room temperature (RT) have attracted intense interests for its distinct preponderances, including energy-saving and super stability, which presents great prospects in commercial application. The exciting strategies for RT gas sensing, such as unique materials with activated surface or light activation, do not directly modulate the active ions for gas sensing, limiting the RT gas sensing performances. Here, an active-ion-gated strategy has been proposed for RT gas sensing with high performance and low power consumption, in which gas ions in triboelectric plasma are introduced into metal oxide semiconductor (MOS) film to act as both floating gate and active sensing ions. The active-ion-gated ZnO nanowires (NWs) array shows a sensitivity of 38.3% to 10 ppm acetone gas at RT, and the maximum power consumption is only 4.5 mW. At the same time, the gas sensor exhibits excellent selectivity to acetone. More importantly, the response (recovery) time of this sensor is as low as 11 s (25 s). It is found that OH-(H2O)4 ions in plasma are the key for realizing RT gas sensing ability, and an accompanied resistive switch is also observed. It is considered that the electron transfer between OH-(H2O)4 and ZnO NWs will forms a hydroxyl-like intermediate state (OH*) on the top of Zn2+, leading to the band bending of ZnO and activating the reactive O2 - ions on the oxygen vacancies. The active-ion-gated strategy proposed here present a novel exploration to achieving RT gas sensing performance of MOS by activating sensing properties at the scale of ions or atoms.

Organic/Inorganic-Based Flexible Membrane for a Room-Temperature Electronic Gas Sensor

A room temperature (RT) H₂S gas sensor based on organic–inorganic nanocomposites has been developed by incorporating zinc oxide (ZnO) nanoparticles (NPs) into a conductivity-controlled organic polymer matrix. A homogeneous solution containing poly (vinyl alcohol) (PVA) and ionic liquid (IL) and further doped with ZnO NPs was used for the fabrication of a flexible membrane (approx. 200 μm in thickness). The sensor was assessed for its performance against hazardous gases at RT (23 °C). The obtained sensor exhibited good sensitivity, with a detection limit of 15 ppm, and a fast time response (24 ± 3 s) toward H₂S gas. The sensor also showed excellent repeatability, long-term stability and selectivity toward H₂S gas among other test gases. Furthermore, the sensor depicted a high flexibility, low cost, easy fabrication and low power consumption, thus holding great promise for flexible electronic gas sensors.

Highly Selective and Low-Power Carbon Monoxide Gas Sensor Based on the Chain Reaction of Oxygen and Carbon Monoxide to WO3

Carbon monoxide (CO) poisoning can easily occur in industrial and domestic settings, causing headaches, loss of consciousness, or death from overexposure. Commercially available CO gas sensors consume high power (typically 38 mW), whereas low-power gas sensors using nanostructured materials with catalysts lack reliability and uniformity. A low-power (1.8 mW @ 392 °C), sensitive, selective, reliable, and practical CO gas sensor is presented. The sensor adopts floated WO₃ film as a sensing material to utilize the unique reaction of lattice oxide of WO₃ with CO gas. The sensor locally modulates the electron concentration in the WO₃ film, allowing O₂ and CO gases to react primarily in different sensing areas. Electrons generated by the CO gas reaction can be consumed for O₂ gas adsorption in a remote area, and this promotes the additional reaction of CO gas, boosting sensitivity and selectivity. The proposed sensor exhibits a 39.5 times higher response than the conventional resistor-type gas sensor fabricated on the same wafer. As a proof of concept, sensors with In₂O₃ film are fabricated, and the proposed sensor platform shows no advantage in detecting CO gas. Fabrication of the proposed sensor is reproducible and inexpensive due to conventional silicon-based processes, making it attractive for practical applications.

Direct Detection of Free H2 Outgassing in Blisters Formed in Al2O3 Atomic Layers Deposited on Si and Methods of Its Prevention

The phenomenon of blistering, seen in atomic layer-deposited aluminum oxide layers caused by thermal treatment, represents a serious problem in the field of device fabrication. Determining its causes and controlling them have been a major task in this field. Various groups have so far confronted the challenge, with several mechanisms having been proposed, but it is still under investigation. This paper reports how we have systematically characterized and summarized the blistering phenomenon from the viewpoints of annealing temperature and Al₂O₃-Si interface conditions. In this study, we have succeeded in directly detecting hydrogen gas generation from the interface between Si and Al₂O₃ using blister-penetrating Raman spectroscopy. The results have enabled us to propose a mechanism for blister formation using a hydrogen outgassing model. Based on our model, we also propose a method of suppressing blister formation by applying surface treatment or passivation to eliminate the Si-H bonds. These discoveries and methods will provide important insights that are applicable to a wide range of applications such as electronic devices and nanostructured solar cells.

Light-Assisted Enhancement of Gas Sensing Property for Micro-Nanostructure Electronic Device: A Mini Review

In recent years, gas sensing electronic devices have always attracted wide attention in the field of environment, industry, aviation and others. In order to improve the gas sensing properties, many micro- and nano-fabrication technologies have been proposed and investigated to develop high-performance gas sensing devices. It is worth noting that light irradiation is an effective strategy to enhance gas sensitivity, shorten the response and recovery time, reduce operating temperature. In this review, firstly, the latest research advances of gas sensors based on different micro-nanostructure materials under UV light and visible light activation is introduced. Then, the gas sensing mechanism of light-assisted gas sensor is discussed in detail. Finally, this review describes the present application of gas sensors with improved properties under light activation assisted conditions and the perspective of their applications.

Research Progress of Gas Sensing Performance of 2D Hexagonal WO3

Metal oxide semiconductor gas sensing materials have attracted great research interest in the gas sensor field due to their outstanding physical and chemical properties, low cost, and easy preparation. Among them, two-dimensional hexagonal tungsten trioxide (2D h-WO₃) is especially interesting because of its high sensitivity and selectivity to some gases. We firstly introduce the characteristics of 2D h-WO₃ gas sensing materials and discuss the effects of microstructure, oxygen vacancy, and doping modification on the gas sensing properties of 2D h-WO₃ mainly. Finally, we explore the application of 2D h-WO₃ gas sensing materials and propose some research directions.

Multimode Operation of Organic-Inorganic Hybrid Thin-Film Transistors Based on Solution-Processed Indium Oxide Films

Solution-processed metal oxide (MO) thin films have been extensively studied for use in thin-film transistors (TFTs) due to their high optical transparency, simplicity of fabrication methods, and high electron mobility. Here, we report, for the first time, the improvement of the electronic properties of solution-processed indium oxide (InO_x) films by the subsequent addition of an organic p-type semiconductor material, here 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), yielding organic-inorganic hybrid TFTs. The addition of TIPS-pentacene not only improves the electron mobility by enhancing the charge carrier percolation pathways but also improves the electronic and temporal stability of the I_DS(V_G) characteristics as well as reduces the number of required spin-coating steps of the InO_x precursor solution. Very interestingly, the introduction of 10 nm TIPS-pentacene films on top of 15 nm InO_x layers allows the fabrication of either enhancement- or depletion-mode devices with only minimal changes to the fabrication process. Specifically, we find that when the TIPS-pentacene layer is added on top of the source/drain electrodes, resulting in devices with embedded source/drain electrodes [embedded electrode TFTs (EETFTs)], the devices exhibit an enhancement-mode behavior with an average mobility (μ) of 6.4 cm² V^-1 s^-1, a source-drain current ratio (I_on/I_off) of around 10⁵, and a near-zero threshold voltage (V_TH). When on the other hand the TIPS-pentacene layer is added before the source-drain electrodes, i.e., in top-contact electrode TFTs (TCETFTs), a very clear depletion mode behavior is observed with an average μ of 6.3 cm² V^-1 s^-1, an I_on/I_off ratio of over 10⁵, and a V_TH of -80.3 V. Furthermore, a logic inverter is fabricated combining the enhancement (EETFTs)- and depletion (TCETFTs)-mode transistors, which shows a potential for the construction of organic-inorganic hybrid electronics and circuits.

Gas-solid interfacial charge transfer in volatile organic compound detection by CuCrO2nanoparticles

Nanostructured metal oxide semiconductors have received great attention used as the chemiresistive layer of gas sensor to detect the volatile organic compound recently. As indispensable complementary parts for dominative n-type semiconductors, the p-type metal oxides based gas sensors fail to be studied sufficiently, which hampers their practical applications. In this work, the p-type delafossite CuCrO₂nanoparticles were synthesized, characterized, and tested for gas sensing, followed by the first principles calculations to simulate the generation of chemiresistive signal. The hydrothermal synthesis time of CuCrO₂nanoparticles is optimized as 24 h with a higher proportion of oxygen vacancies but a smaller size, which is confirmed by the microscopy and spectrum characterization and allows for a prevailing gas sensitivity. Meanwhile, this CuCrO₂gas sensor is proven to perform a higher selectivity to n-propanol and a low detection limit of 1 ppm. The adsorption sites and charge variations of dehydrogenation at the gas-solid interface predicted by the theoretical analysis are claimed to be crucial to such selectivity. It is an innovative approach to understand the chemiresistive gas sensing by evaluating the preference of charge transfer between the sensor and target gaseous molecule, which provides a new route to precisely design and develop the advanced sensing devices for the diverse applications.

The Key Role of Active Sites in the Development of Selective Metal Oxide Sensor Materials

Development of sensor materials based on metal oxide semiconductors (MOS) for selective gas sensors is challenging for the tasks of air quality monitoring, early fire detection, gas leaks search, breath analysis, etc. An extensive range of sensor materials has been elaborated, but no consistent guidelines can be found for choosing a material composition targeting the selective detection of specific gases. Fundamental relations between material composition and sensing behavior have not been unambiguously established. In the present review, we summarize our recent works on the research of active sites and gas sensing behavior of n-type semiconductor metal oxides with different composition (simple oxides ZnO, In₂O₃, SnO₂, WO₃; mixed-metal oxides BaSnO₃, Bi₂WO₆), and functionalized by catalytic noble metals (Ru, Pd, Au). The materials were variously characterized. The composition, metal-oxygen bonding, microstructure, active sites, sensing behavior, and interaction routes with gases (CO, NH₃, SO₂, VOC, NO₂) were examined. The key role of active sites in determining the selectivity of sensor materials is substantiated. It was shown that the metal-oxygen bond energy of the MOS correlates with the surface acidity and the concentration of surface oxygen species and oxygen vacancies, which control the adsorption and redox conversion of analyte gas molecules. The effects of cations in mixed-metal oxides on the sensitivity and selectivity of BaSnO₃ and Bi₂WO₆ to SO₂ and VOCs, respectively, are rationalized. The determining role of catalytic noble metals in oxidation of reducing analyte gases and the impact of acid sites of MOS to gas adsorption are demonstrated.

ZnO Metal Oxide Semiconductor in Surface Acoustic Wave Sensors: A Review

Surface acoustic wave (SAW) gas sensors are of continuous development interest to researchers due to their sensitivity, short detection time, and reliability. Among the most used materials to achieve the sensitive film of SAW sensors are metal oxide semiconductors, which are highlighted by thermal and chemical stability, by the presence on their surface of free electrons and also by the possibility of being used in different morphologies. For different types of gases, certain metal oxide semiconductors are used, and ZnO is an important representative for this category of materials in the field of sensors. Having a great potential for the development of SAW sensors, the discussion related to the development of the sensitivity of metal oxide semiconductors, especially ZnO, by the synthesis method or by obtaining new materials, is suitable and necessary to have an overview of the latest results in this domain.

Generic Approach to Boost the Sensitivity of Metal Oxide Sensors by Decoupling the Surface Charge Exchange and Resistance Reading Process

As one of the bottleneck parameters for practical applications of metal oxide semiconductor-based gas sensors, sensitivity enhancement has attracted significant attention in the past few decades. In this work, alternative to conventional strategies for designing sensitive surfaces via morphology/defect/heterojunction control (then operating at an optimized isothermal temperature with a maximal response), a facile enhancement approach by decoupling surface charge exchange and resistance reading process (possessing different temperature-dependent behaviors) through pulsed temperature modulation (PTM) is reported. Substantially magnifying electrical responses of a generic metal oxide (e.g., WO₃) micro-electromechanical systems sensor toward diverse analyte molecules are demonstrated. Under the optimal PTM condition, the response toward 10 ppm NO₂ can be boosted from (isothermal) 99.7 to 842.7, and the response toward 100 ppm acetone is increased from (isothermal) 2.7 to 425, which are comparable to or even better than most of the state-of-the-art WO₃-based sensors. In comparison to conventional (isothermal) operation, PTM allows to sequentially manipulate the physisorption/chemisorption of analyte molecules, generation of surface reactive oxygen species, and sensor resistance reading and thus provides additional opportunities in boosting the electrical response of oxide sensors for advanced health and/or environment monitoring in future.

ZnO-CuO Core-Hollow Cube Nanostructures for Highly Sensitive Acetone Gas Sensors at the ppb Level

This paper presents a ZnO-CuO p-n heterojunction chemiresistive sensor that comprises CuO hollow nanocubes attached to ZnO spherical cores as active materials. These ZnO-CuO core-hollow cube nanostructures exhibit a remarkable response of 11.14 at 1 ppm acetone and 200 °C, which is a superior result to those reported by other metal-oxide-based sensors. The response can be measured up to 40 ppb, and the limit of detection is estimated as 9 ppb. ZnO-CuO core-hollow cube nanostructures also present high selectivity toward acetone against other volatile organic compounds and demonstrate excellent stability for up to 40 days. The outstanding gas-sensing performance of the developed nanocubes is attributed to their uniform and unique morphology. Their core-shell-like structures allow the main charge transfer pathways to pass the interparticle p-p junctions, and the p-n junctions in each particle increase the sensitivity of the reactions to gas molecules. The small grain size and high surface area of each domain also enhance the surface gas adsorption.

Recent Advances of Solution-Processed Heterojunction Oxide Thin-Film Transistors

Thin-film transistors (TFTs) made of metal oxide semiconductors are now increasingly used in flat-panel displays. Metal oxides are mainly fabricated via vacuum-based technologies, but solution approaches are of great interest due to the advantages of low-cost and high-throughput manufacturing. Unfortunately, solution-processed oxide TFTs suffer from relatively poor electrical performance, hindering further development. Recent studies suggest that this issue could be solved by introducing a novel heterojunction strategy. This article reviews the recent advances in solution-processed heterojunction oxide TFTs, with a specific focus on the latest developments over the past five years. Two of the most prominent advantages of heterostructure oxide TFTs are discussed, namely electrical-property modulation and mobility enhancement by forming 2D electron gas. It is expected that this review will manifest the strong potential of solution-based heterojunction oxide TFTs towards high performance and large-scale electronics.

Cuprous Oxide Based Chemiresistive Electronic Nose for Discrimination of Volatile Organic Compounds

This paper reports a facile functionalization method on a metal-oxide semiconductor and a cuprous oxide (Cu₂O) based chemiresistive electronic nose for the detection of volatile organic compounds (VOCs). A library of functionalized Cu₂O nanospheres was developed through silanization using chemically diverse organosilanes. An electronic nose was fabricated with unmodified Cu₂O nanospheres and five types of functionalized Cu₂O nanospheres as the sensing elements. The electronic nose showed stable and rapid resistance responses to 25-200 ppm model VOCs, with the operating temperature of 180 °C. Single VOCs and ternary VOC mixtures could be discriminated by the electronic nose, and six types of tea leaves were also proved to be distinguishable as an illustration of the application of the electronic nose. We expected that the silanization could provide a simple approach for material diversification and the electronic nose would have further application in identification and discrimination of complex gas samples.

Developing a Low-Cost Wearable Personal Exposure Monitor for Studying Respiratory Diseases Using Metal-Oxide Sensors

Global industrialization and urbanization have led to increased levels of air pollution. Those with respiratory diseases, such as asthma, are at the highest risk for adverse health effects and reduced quality of life. Studying the relationship between pollutants and symptoms is usually achieved with data from government air quality monitoring stations, but these fail to report the spatial and temporal resolution required to track a person's true exposure, especially when the majority of their time is spent indoors. We develop and build eight wrist-worn wearable devices, weighing only 64 g, to measure known asthma symptom triggers: ozone, total volatile organic compounds, temperature, humidity, and activity level. The devices use commercial off-the-shelf components, costing under $150 each to build. This report focuses on the design, calibration, and testing of the devices. Emphasis is placed on the calibration of a metal-oxide-semiconductor gas sensor for detecting ozone, which is a difficult task because of the large variations in ambient temperature and humidity found when using a wearable device. Examples of testing the devices in four real environments are also discussed: 11 days inside a well-ventilated laboratory, ten days outdoors during the summer, alternating the devices between indoor and outdoor environments to examine their response to quickly changing environments, and a field test where scripted activities are performed for a full day. The work demonstrates a wearable device for environmental health studies and addresses the challenges of existing sensors for real-world applications.

Highly Sensitive p + n Metal Oxide Sensor Array for Low-Concentration Gas Detection

Nowadays, despite the easy fabrication and low cost of metal oxide gas sensors, it is still challenging for them to detect gases at low concentrations. In this study, resistance-matched p-type Cu₂O and n-type Ga-doped ZnO, as well as p-type CdO/LaFeO₃ and n-type CdO/Sn-doped ZnO sensors were prepared and integrated into p + n sensor arrays to enhance their gas-sensing performance. The materials were characterized by scanning electron microscopy, transmittance electron microscopy, and X-ray diffractometry, and gas-sensing properties were measured using ethanol and acetone as probes. The results showed that compared with individual gas sensors, the response of the sensor array was greatly enhanced and similar to the gas response product of the p- and n-type gas sensors. Specifically, the highly sensitive CdO/LaFeO₃ and CdO/Sn-ZnO sensor array had a high response of 21 to 1 ppm ethanol and 14 to 1 ppm acetone, with detection limits of <0.1 ppm. The results show the effect of sensor array integration by matching the two sensor resistances, facilitating the detection of gas at a low concentration.

Silicon Cations Intermixed Indium Zinc Oxide Interface for High-Performance Thin-Film Transistors Using a Solution Process

Solution-processed amorphous metal-oxide thin-film transistors (TFTs) utilizing an intermixed interface between a metal-oxide semiconductor and a dielectric layer are proposed. In-depth physical characterizations are carried out to verify the existence of the intermixed interface that is inevitably formed by interdiffusion of cations originated from a thermal process. In particular, when indium zinc oxide (IZO) semiconductor and silicon dioxide (SiO₂) dielectric layer are in contact and thermally processed, a Si⁴⁺ intermixed IZO (Si/IZO) interface is created. On the basis of this concept, a high-performance Si/IZO TFT having both a field-effect mobility exceeding 10 cm² V^-1 s^-1 and a on/off current ratio over 10⁷ is successfully demonstrated.

Oxide Charge Engineering of Atomic Layer Deposited AlOxNy/Al2O3 Gate Dielectrics: A Path to Enhancement Mode GaN Devices

Nitrogen incorporation to produce negative fixed charge in Al2O3 gate insulator layers is investigated as a path to achieve enhancement mode GaN device operation. A uniform distribution of nitrogen across the resulting AlOxNy films is obtained using N2 plasma enhanced atomic layer deposition (ALD). The flat band voltage (Vfb) increases to a significantly more positive value with increasing nitrogen concentration. Insertion of a 2 nm thick Al2O3 interlayer greatly decreases the trap density of the insulator/GaN interface, and reduces the voltage hysteresis and frequency dispersion of gate capacitance compared to single-layer AlOxNy gate insulators in GaN MOSCAPs.

Wafer-Scale Microwire Transistor Array Fabricated via Evaporative Assembly

One-dimensional (1D) nano/microwires have attracted significant attention as promising building blocks for various electronic and optical device applications. The integration of these elements into functional device networks with controlled alignment and density presents a significant challenge for practical device applications. Here, we demonstrated the fabrication of wafer-scale microwire field-effect transistor (FET) arrays based on well-aligned inorganic semiconductor microwires (indium-gallium-zinc-oxide (IGZO)) and organic polymeric insulator microwires fabricated via a simple and large-area evaporative assembly technique. This microwire fabrication method offers a facile approach to precisely manipulating the channel dimensions of the FETs. The resulting solution-processed monolithic IGZO microwire FETs exhibited a maximum electron mobility of 1.02 cm(2) V(-1) s(-1) and an on/off current ratio of 1 × 10(6). The appropriate choice of the polymeric microwires used to define the channel lengths enabled fine control over the threshold voltages of the devices, which were employed to fabricate high-performance depletion-load inverters. Low-voltage-operated microwire FETs were successfully fabricated on a plastic substrate using a high-capacitance ion gel gate dielectric. The microwire fabrication technique involving evaporative assembly provided a facile, effective, and reliable method for preparing flexible large-area electronics.

Spatial Separation of Charge Carriers in In2O3-x(OH)y Nanocrystal Superstructures for Enhanced Gas-Phase Photocatalytic Activity

The development of strategies for increasing the lifetime of photoexcited charge carriers in nanostructured metal oxide semiconductors is important for enhancing their photocatalytic activity. Intensive efforts have been made in tailoring the properties of the nanostructured photocatalysts through different ways, mainly including band-structure engineering, doping, catalyst-support interaction, and loading cocatalysts. In liquid-phase photocatalytic dye degradation and water splitting, it was recently found that nanocrystal superstructure based semiconductors exhibited improved spatial separation of photoexcited charge carriers and enhanced photocatalytic performance. Nevertheless, it remains unknown whether this strategy is applicable in gas-phase photocatalysis. Using porous indium oxide nanorods in catalyzing the reverse water-gas shift reaction as a model system, we demonstrate here that assembling semiconductor nanocrystals into superstructures can also promote gas-phase photocatalytic processes. Transient absorption studies prove that the improved activity is a result of prolonged photoexcited charge carrier lifetimes due to the charge transfer within the nanocrystal network comprising the nanorods. Our study reveals that the spatial charge separation within the nanocrystal networks could also benefit gas-phase photocatalysis and sheds light on the design principles of efficient nanocrystal superstructure based photocatalysts.

Microscale Soft Patterning for Solution Processable Metal Oxide Thin Film Transistors

We introduce a microscale soft pattering (MSP) route utilizing contact printing of chemically inert sub-nanometer thick low molecular weight (LMW) poly(dimethylsiloxane) (PDMS) layers. These PDMS layers serve as a release agent layer between the n-type Ohmic metal and metal oxide semiconductors (MOSs) and provide a layer that protects the MOS from water in the surrounding environment. The feasibility of our MSP route was experimentally demonstrated by fabricating solution processable In2O3, IZO, and IGZO TFTs with aluminum (Al), a typical n-type Ohmic metal. We have demonstrated patterning gaps as small as 13 μm. The TFTs fabricated using MSP showed higher field-effect-mobility and lower hysteresis in comparison with those made using conventional photolithography.

Printable Ultrathin Metal Oxide Semiconductor-Based Conformal Biosensors

Conformal bioelectronics enable wearable, noninvasive, and health-monitoring platforms. We demonstrate a simple and straightforward method for producing thin, sensitive In2O3-based conformal biosensors based on field-effect transistors using facile solution-based processing. One-step coating via aqueous In2O3 solution resulted in ultrathin (3.5 nm), high-density, uniform films over large areas. Conformal In2O3-based biosensors on ultrathin polyimide films displayed good device performance, low mechanical stress, and highly conformal contact determined using polydimethylsiloxane artificial skin having complex curvilinear surfaces or an artificial eye. Immobilized In2O3 field-effect transistors with self-assembled monolayers of NH2-terminated silanes functioned as pH sensors. Functionalization with glucose oxidase enabled d-glucose detection at physiologically relevant levels. The conformal ultrathin field-effect transistor biosensors developed here offer new opportunities for future wearable human technologies.

Facile Routes To Improve Performance of Solution-Processed Amorphous Metal Oxide Thin Film Transistors by Water Vapor Annealing

Here, we report on a simple and high-rate oxidization method for producing solution-based compound mixtures of indium zinc oxide (IZO) and indium gallium zinc oxide (IGZO) metal-oxide semiconductors (MOS) for thin-film transistor (TFT) applications. One of the issues for solution-based MOS fabrication is how to sufficiently oxidize the precursor in order to achieve high performance. As the oxidation rate of solution processing is lower than vacuum-based deposition such as sputtering, devices using solution-processed MOS exhibit relatively poorer performance. Therefore, we propose a method to prepare the metal-oxide precursor upon exposure to saturated water vapor in a closed volume for increasing the oxidization efficiency without requiring additional oxidizing agent. We found that the hydroxide rate of the MOS film exposed to water vapor is lower than when unexposed (≤18%). Hence, we successfully fabricated oxide TFTs with high electron mobility (27.9 cm(2)/V·s) and established a rapid process (annealing at 400 °C for 5 min) that is much shorter than the conventional as-deposited long-duration annealing (at 400 °C for 1 h) whose corresponding mobility is even lower (19.2 cm(2)/V·s).

Interface Trap Density Reduction for Al2O3/GaN (0001) Interfaces by Oxidizing Surface Preparation prior to Atomic Layer Deposition

We correlate interfacial defect state densities with the chemical composition of the Al2O3/GaN interface in metal-oxide-semiconductor (MOS) structures using synchrotron photoelectron emission spectroscopy (PES), cathodoluminescence and high-temperature capacitance-voltage measurements. The influence of the wet chemical pretreatments involving (1) HCl+HF etching or (2) NH4OH(aq) exposure prior to atomic layer deposition (ALD) of Al2O3 were investigated on n-type GaN (0001) substrates. Prior to ALD, PES analysis of the NH4OH(aq) treated surface shows a greater Ga2O3 component compared to either HCl+HF treated or as-received surfaces. The lowest surface concentration of oxygen species is detected on the acid etched surface, whereas the NH4OH treated sample reveals the lowest carbon surface concentration. Both surface pretreatments improve electrical characteristics of MOS capacitors compared to untreated samples by reducing the Al2O3/GaN interface state density. The lowest interfacial trap density at energies in the upper band gap is detected for samples pretreated with NH4OH. These results are consistent with cathodoluminescence data indicating that the NH4OH treated samples show the strongest band edge emission compared to as-received and acid etched samples. PES results indicate that the combination of reduced carbon contamination while maintaining a Ga2O3 interfacial layer by NH4OH(aq) exposure prior to ALD results in fewer interface traps after Al2O3 deposition on the GaN substrate.

Direct light pattern integration of low-temperature solution-processed all-oxide flexible electronics

The rise of solution-processed electronics, together with their processing methods and materials, provides unique opportunities to achieve low-cost and low-temperature roll-to-roll printing of non-Si-based devices. Here, we demonstrate a wafer-scale direct light-patterned, fully transparent, all-solution-processed, and layer-by-layer-integrated electronic device. The deep ultraviolet irradiation of specially designed metal oxide gel films can generate fine-patterned shapes of ∼3 μm, which easily manifest their intrinsic properties at low-temperature annealing. This direct light patterning can be easily applied to the 4 in. wafer scale and diverse pattern shapes and provides feasibility for integrated circuit applications through the penetration of the deep ultraviolet range on the quartz mask. With this approach, we successfully fabricate all-oxide-based high-performance transparent thin-film transistors on flexible polymer substrates.

Towards point of care testing for C. difficile infection by volatile profiling, using the combination of a short multi-capillary gas chromatography column with metal oxide sensor detection

Rapid volatile profiling of stool sample headspace was achieved using a combination of short multi-capillary chromatography column (SMCC), highly sensitive heated metal oxide semiconductor (MOS) sensor and artificial neural network (ANN) software. For direct analysis of biological samples this prototype offers alternatives to conventional GC detectors and electronic nose technology. The performance was compared to an identical instrument incorporating a long single capillary column (LSCC). The ability of the prototypes to separate complex mixtures was assessed using gas standards and homogenised in house 'standard' stool samples, with both capable of detecting more than 24 peaks per sample. The elution time was considerably faster with the SMCC resulting in a run time of 10 minutes compared to 30 minutes for the LSCC. The diagnostic potential of the prototypes was assessed using 50 C. difficile positive and 50 negative samples. The prototypes demonstrated similar capability of discriminating between positive and negative samples with sensitivity and specificity of 85% and 80% respectively. C. difficile is an important cause of hospital acquired diarrhoea, with significant morbidity and mortality around the world. A device capable of rapidly diagnosing the disease at the point of care would reduce cases, deaths and financial burden.

On-line monitoring of food fermentation processes using electronic noses and electronic tongues: a review

Fermentation processes are often sensitive to even slight changes of conditions that may result in unacceptable end-product quality. Thus, close follow-up of this type of processes is critical for detecting unfavorable deviations as early as possible in order to save downtime, materials and resources. Nevertheless the use of traditional analytical techniques is often hindered by the need for expensive instrumentation and experienced operators and complex sample preparation. In this sense, one of the most promising ways of developing rapid and relatively inexpensive methods for quality control in fermentation processes is the use of chemical multisensor systems. In this work we present an overview of the most important contributions dealing with the monitoring of fermentation processes using electronic noses and electronic tongues. After a brief description of the fundamentals of both types of devices, the different approaches are critically commented, their strengths and weaknesses being highlighted. Finally, future trends in this field are also mentioned in the last section of the article.

Sb-mediated Ge quantum dots in Ti-oxide-Si diode: negative differential capacitance

The negative differential capacitance (NDC) effect is observed on a titanium-oxide-silicon structure, formed on n-type silicon with embedded germanium quantum dots (QDs). The Ge QDs were grown by an Sb-mediated technique. The NDC effect was observed for temperatures below 200 K. We found that approximately six to eight electrons can be trapped in the valence band states of Ge QDs. We explain the NDC effect in terms of the emission of electrons from valence band states in the very narrow QD layer under reverse bias.

A comprehensive review of one-dimensional metal-oxide nanostructure photodetectors

One-dimensional (1D) metal-oxide nanostructures are ideal systems for exploring a large number of novel phenomena at the nanoscale and investigating size and dimensionality dependence of nanostructure properties for potential applications. The construction and integration of photodetectors or optical switches based on such nanostructures with tailored geometries have rapidly advanced in recent years. Active 1D nanostructure photodetector elements can be configured either as resistors whose conductions are altered by a charge-transfer process or as field-effect transistors (FET) whose properties can be controlled by applying appropriate potentials onto the gates. Functionalizing the structure surfaces offers another avenue for expanding the sensor capabilities. This article provides a comprehensive review on the state-of-the-art research activities in the photodetector field. It mainly focuses on the metal oxide 1D nanostructures such as ZnO, SnO₂, Cu₂O, Ga₂O₃, Fe₂O₃, In₂O₃, CdO, CeO₂, and their photoresponses. The review begins with a survey of quasi 1D metal-oxide semiconductor nanostructures and the photodetector principle, then shows the recent progresses on several kinds of important metal-oxide nanostructures and their photoresponses and briefly presents some additional prospective metal-oxide 1D nanomaterials. Finally, the review is concluded with some perspectives and outlook on the future developments in this area.