Long-Term Stability of Oxide Nanowire Sensors via Heavily Doped Oxide Contact

Metal-Organic Framework-Derived Ni-Doped Indium Oxide Nanorods for Parts per Billion-Level Nitrogen Dioxide Gas Sensing at High Humidity

Detecting parts per billion (ppb)-level nitrogen dioxide in high-moisture environments at room temperature without reducing sensing performance is a well-recognized significant challenge for metal oxide-based gas sensors. In this study, metal-organic framework-derived nickel-doped indium oxide (Ni-doped In2O3) mesoporous nanorods were prepared by a solvothermal method combined with the calcination process. The sensors prepared using the obtained Ni-doped In2O3 nanorods showcase an ultrahigh response, low detection limit, and excellent selectivity. Moreover, the abundant active sites triggered by nickel doping and the capillary enhancement effect caused by mesopores endow the sensor with ppb-level (20 ppb) NO2 detection capability in high-moisture environments (95% RH) at room temperature. With the increase in humidity, the carrier concentration of the sensor increases, and the nitric acid generated by nitrogen dioxide dissolved in water can be completely ionized in water and has high conductivity. Therefore, the gas response of the sensors increases with the increase in humidity. This study establishes a promising approach for the development of trace nitrogen dioxide-sensing devices that are resilient in high-humidity environments.

Gas Sensing Properties of Indium-Oxide-Based Field-Effect Transistor: A Review

Excellent stability, low cost, high response, and sensitivity of indium oxide (In2O3), a metal oxide semiconductor, have been verified in the field of gas sensing. Conventional In2O3 gas sensors employ simple and easy-to-manufacture resistive components as transducers. However, the swift advancement of the Internet of Things has raised higher requirements for gas sensors based on metal oxides, primarily including lowering operating temperatures, improving selectivity, and realizing integrability. In response to these three main concerns, field-effect transistor (FET) gas sensors have garnered growing interest over the past decade. When compared with other metal oxide semiconductors, In2O3 exhibits greater carrier concentration and mobility. The property is advantageous for manufacturing FETs with exceptional electrical performance, provided that the off-state current is controlled at a sufficiently low level. This review presents the significant progress made in In2O3 FET gas sensors during the last ten years, covering typical device designs, gas sensing performance indicators, optimization techniques, and strategies for the future development based on In2O3 FET gas sensors.

Bismuth Ferrite-Silver Nanowire Flexible Nanocomposites for Room-Temperature Nitrogen Dioxide Sensing

Nitrogen dioxide (NO2) is a major pollutant, causing acid rain, photochemical smog, and respiratory damage. The annual safe limit is 50 parts per billion (ppb), while concentrations exceeding 1 part per million (ppm) can result in respiratory ailments. Conventionally, n-type metal oxide semiconductors operating at elevated temperatures have been utilized for NO2 detection. Recently, p-type semiconductors with their hole accumulation layer, rapid recovery post-gas exposure, and good humidity tolerance are being investigated as potential NO2 sensors, once again working at elevated temperatures. In this work, a room-temperature (27 ± 2 °C) NO2 sensor is demonstrated by using a nanocomposite based on p-type bismuth ferrite (BFO) nanoparticles and silver nanowires (Ag NWs). This nanocomposite is capable of sensing a NO2 gas concentration of up to 0.2 ppm. The BFO nanoparticles are synthesized via a sol-gel route followed by sintering at 500 °C to form the crystalline phase. Nanocomposites are obtained by formulating a dispersion of the BFO nanoparticles and Ag NWs, followed by direct writing on both flexible and rigid substrates. The Ag NWs act as the conducting pathway, reducing the overall electrical resistance and thus enabling room-temperature operation. X-ray diffraction, scanning electron microscopy, and surface area studies provide phase information and surface morphology, and the porous nature of the film helps in room-temperature gas adsorption. The current-voltage and gas-sensing behavior are studied to obtain the optimized molar ratio (4:1 BFO/Ag NWs) for the sensor. The sensor deposited on poly(ethylene terephthalate) (PET) also works under a bent condition, indicating good flexibility. Rapid NO2 sensing was achieved in a BFO-Ag/PET device with response/recovery times of 7/8.5 s and 12/15 s in straight and bent geometries, respectively. Additionally, a good sensitivity of 30 to 60% was achieved for the BFO-Ag/PET device across 100 to 1000 ppb of NO2. The development of a nanocomposite combining an active sensing element (BFO) and a charge-transport element (Ag NWs) opens up a multitude of other application areas.

Metal-Oxide Nanomaterials Synthesis and Applications in Flexible and Wearable Sensors

Metal-oxide nanomaterials (MONs) have gained considerable interest in the construction of flexible/wearable sensors due to their tunable band gap, low cost, large specific area, and ease of manufacturing. Furthermore, MONs are in high demand for applications, such as gas leakage alarms, environmental protection, health tracking, and smart devices integrated with another system. In this Review, we introduce a comprehensive investigation of factors to boost the sensitivity of MON-based sensors in environmental indicators and health monitoring. Finally, the challenges and perspectives of MON-based flexible/wearable sensors are considered.

Impact of Lateral SnO2 Nanofilm Channel Geometry on a 1024 Crossbar Chemical Sensor Array

We propose a rational strategy to fabricate thermally robust, highly integrated molecular and gas sensors utilizing a lateral SnO₂ nanofilm channel geometry on a 1024 crossbar sensor array. The proposed lateral channel geometry substantially suppresses the detrimental effects of parasitic interconnect wire resistances compared with those of a conventional vertical sandwich-type crossbar array because of its excellent resistance controllability. A conductive oxide top-contact electrode on the lateral SnO₂ nanofilm channel enhances the thermal stability at temperatures of up to 500 °C in ambient air. Integrating this lateral SnO₂ nanofilm geometry with analog circuits enables the operation of a 1024 crossbar sensor array without selector devices to avoid sneak currents. The developed 1024 crossbar sensor array system detects the local spatial distribution of the molecular gas concentration. The spatial data of molecular concentrations include molecule-specific data to distinguish various volatile molecules based on their vapor pressures. Thus, this integrated crossbar sensor array system using lateral nanofilm geometry offers a platform for robust, reliable, highly integrated molecular and gas sensors.

Nanowire-based sensor electronics for chemical and biological applications

Detection and recognition of chemical and biological species via sensor electronics are important not only for various sensing applications but also for fundamental scientific understanding. In the past two decades, sensor devices using one-dimensional (1D) nanowires have emerged as promising and powerful platforms for electrical detection of chemical species and biologically relevant molecules due to their superior sensing performance, long-term stability, and ultra-low power consumption. This paper presents a comprehensive overview of the recent progress and achievements in 1D nanowire synthesis, working principles of nanowire-based sensors, and the applications of nanowire-based sensor electronics in chemical and biological analytes detection and recognition. In addition, some critical issues that hinder the practical applications of 1D nanowire-based sensor electronics, including device reproducibility and selectivity, stability, and power consumption, will be highlighted. Finally, challenges, perspectives, and opportunities for developing advanced and innovative nanowire-based sensor electronics in chemical and biological applications are featured.

Single Nanowire Gas Sensor Able to Distinguish Fish and Meat and Evaluate Their Degree of Freshness

A non-invasive, small, and fast device is needed for food freshness monitoring, as current techniques do not meet these criteria. In this study, a resistive sensor composed of a single semiconductor nanowire was used at different temperatures, combining the responses and processing them with multivariate statistical analysis techniques. The sensor, very sensitive to ammonia and total volatile basic nitrogen, proved to be able to distinguish samples of fish (marble trout, Salmo trutta marmoratus) and meat (pork, Sus scrofa domesticus), both stored at room temperature and 4 °C in the refrigerator. Once separated, the fish and meat samples were classified by the degree of freshness/degradation with two different classifiers. The sensor classified the samples (trout and pork) correctly in 95.2% of cases. The degree of freshness was correctly assessed in 90.5% of cases. Considering only the errors with repercussions (when a fresh sample was evaluated as degraded, or a degraded sample was evaluated as edible) the accuracy increased to 95.2%. Considering the size (less than a square millimeter) and the speed (less than a minute), this type of sensor could be used to monitor food production and distribution chains.

Metal–Oxide Nanowire Molecular Sensors and Their Promises

During the past two decades, one–dimensional (1D) metal–oxide nanowire (NW)-based molecular sensors have been witnessed as promising candidates to electrically detect volatile organic compounds (VOCs) due to their high surface to volume ratio, single crystallinity, and well-defined crystal orientations. Furthermore, these unique physical/chemical features allow the integrated sensor electronics to work with a long-term stability, ultra-low power consumption, and miniature device size, which promote the fast development of “trillion sensor electronics” for Internet of things (IoT) applications. This review gives a comprehensive overview of the recent studies and achievements in 1D metal–oxide nanowire synthesis, sensor device fabrication, sensing material functionalization, and sensing mechanisms. In addition, some critical issues that impede the practical application of the 1D metal–oxide nanowire-based sensor electronics, including selectivity, long-term stability, and low power consumption, will be highlighted. Finally, we give a prospective account of the remaining issues toward the laboratory-to-market transformation of the 1D nanostructure-based sensor electronics.

Electrically Transduced Gas Sensors Based on Semiconducting Metal Oxide Nanowires

Semiconducting metal oxide-based nanowires (SMO-NWs) for gas sensors have been extensively studied for their extraordinary surface-to-volume ratio, high chemical and thermal stabilities, high sensitivity, and unique electronic, photonic and mechanical properties. In addition to improving the sensor response, vast developments have recently focused on the fundamental sensing mechanism, low power consumption, as well as novel applications. Herein, this review provides a state-of-art overview of electrically transduced gas sensors based on SMO-NWs. We first discuss the advanced synthesis and assembly techniques for high-quality SMO-NWs, the detailed sensor architectures, as well as the important gas-sensing performance. Relationships between the NWs structure and gas sensing performance are established by understanding general sensitization models related to size and shape, crystal defect, doped and loaded additive, and contact parameters. Moreover, major strategies for low-power gas sensors are proposed, including integrating NWs into microhotplates, self-heating operation, and designing room-temperature gas sensors. Emerging application areas of SMO-NWs-based gas sensors in disease diagnosis, environmental engineering, safety and security, flexible and wearable technology have also been studied. In the end, some insights into new challenges and future prospects for commercialization are highlighted.

Oxygen-Induced Reversible Sn-Dopant Deactivation between Indium Tin Oxide and Single-Crystalline Oxide Nanowire Leading to Interfacial Switching

An impurity doping in semiconductors is an important irreversible process of manipulating the electrical properties of advanced electron devices. Here, we report an unusual reversible dopant activation/deactivation phenomenon, which emerges at an interface between indium tin oxide (ITO) and single-crystalline oxide channel. We found that the interface electrical resistance between ITO electrodes and single-crystalline oxide nanowire channel can be repeatedly switched between a metallic state and a near-insulative state by applying thermal treatments in air or vacuum. Interestingly, this electrical switching phenomenon disappears when the oxide nanowire changes from the single-crystalline structure to the lithography-defined polycrystalline structure. Atmosphere-controlled annealing experiments reveal that atmospheric oxygen induces repeatable change in the interfacial electrical resistance. Systematic investigations on metal cation species and channel crystallinity demonstrate that the observed electrical switching is related to an interface-specific reversible Sn-dopant activation/deactivation of ITO electrode in contact with a single-crystalline oxide channel.

Oxygen vacancy injection-induced resistive switching in combined mobile and static gradient doped tin oxide nanorods

Resistive switching devices offer a great potential for advanced computing and data storage, including neuromorphic networks and random-access memory. State-of-the-art memristors are mostly realized by a three-layer structure, which is comprised of an active metal oxide layer sandwiched between two metal electrodes. Thus, there is always an interface involving two materials differing strongly in crystallographic and electronic properties. In this study, we present a resistive switching nanorod device based on a metal oxide sandwiched between two transparent conductive oxide electrodes. Thus, the system is characterized by a different, smooth interface offering new possibilities for increased energy efficiency and transparent electronics. Antimony-doped tin oxide (ATO) is used as an electrode material. The heavily doped ATO nanorods, exhibiting a good conductivity, are produced by a templated electrochemical deposition approach of alloy particles with subsequent thermal oxidation. The process enables precise control of the doping level within the nanorods and the formation of a doping level gradient. Electrical characterization reveals that a stronger gradient between heavily doped and undoped tin oxide within the nanorods results in a more rectifying character of the junction. Three-domain nanorods consisting of an undoped tin oxide segment in between two ATO segments are utilized to introduce memristive properties into the nanorod device. The resistive switching of these nanorods can be attributed to an oxygen vacancy doping gradient introduced during thermal oxidation. These vacancies are mobile within the tin oxide host structure and their injection from the ATO segment into the undoped tin oxide segment results in altered conductivity of the device, when an external bias is applied.

Sensitive Cross-Linked SnO2:NiO Networks for MEMS Compatible Ethanol Gas Sensors

Nowadays, it is still technologically challenging to prepare highly sensitive sensing films using microelectrical mechanical system (MEMS) compatible methods for miniaturized sensors with low power consumption and high yield. Here, sensitive cross-linked SnO2:NiO networks were successfully fabricated by sputtering SnO2:NiO target onto the etched self-assembled triangle polystyrene (PS) microsphere arrays and then ultrasonically removing the PS microsphere templates in acetone. The optimum line width (~ 600 nm) and film thickness (~ 50 nm) of SnO2:NiO networks were obtained by varying the plasma etching time and the sputtering time. Then, thermal annealing at 500 °C in H2 was implemented to activate and reorganize the as-deposited amorphous SnO2:NiO thin films. Compared with continuous SnO2:NiO thin film counterparts, these cross-linked films show the highest response of ~ 9 to 50 ppm ethanol, low detection limits (< 5 ppm) at 300 °C, and also high selectivity against NO2, SO2, NH3, C7H8, and acetone. The gas-sensing enhancement could be mainly attributed to the creating of more active adsorption sites by increased stepped surface in cross-linked SnO2:NiO network. Furthermore, this method is MEMS compatible and of generality to effectively fabricate other cross-linked sensing films, showing the promising potency in the production of low energy consumption and wafer-scale MEMS gas sensors.

Redox-Inactive CO2 Determines Atmospheric Stability of Electrical Properties of ZnO Nanowire Devices through a Room-Temperature Surface Reaction

Emerging interactive electronics for the Internet of Things era inherently require the long-term stability of semiconductor devices exposed to air. Nanostructured metal oxides are promising options for such atmospherically stable semiconductor devices owing to their inherent stability in air. Among various oxide nanostructures, ZnO nanowires have been the most intensively studied for electrical and optical device applications. Here, we demonstrate a strategy for achieving the atmospheric electrical stability of ZnO nanowire devices. Although the chemically active oxygen and water in air are strong candidates for affecting the electrical stability of nanoscale metal oxides, we found that the ppm-level redox-inactive CO₂ in air critically determines the atmospheric electrical stability of hydrothermally grown single-crystalline ZnO nanowires. A series of analyses using atmosphere-controlled electrical characterization of single nanowire devices, Fourier transform infrared spectroscopy, scanning transmission electron microscopy, and X-ray photoelectron spectroscopy consistently revealed that atmospheric CO₂ reacts substantially with the ZnO nanowire surfaces, even at room temperature, to form an electrically insulative zinc carbonate thin layer. The formation of this layer essentially limits the atmospheric electrical stability of the ZnO nanowire devices. Based on this surface carbonation mechanism, we propose a strategy to suppress the detrimental surface reaction, which is based on (1) reducing the density of surface hydroxyl groups and (2) improving the nanowire crystallinity by thermal pretreatment. This approach improves the atmospheric electrical stability to at least 40 days in air.