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Active research in nanostructured materials aims to explore new paths for improving electronic device characteristics. In the field of gas sensors, those based on metal oxide single nanowires exhibit excellent sensitivity and can operate at extremely low power consumption, making them a highly promising candidate for a novel generation of portable devices. The mix of two different metal oxides on the same nanowire can further broaden the response of this kind of gas sensor, thus widening the range of detectable gases, without compromising the properties related to the active region miniaturization. In this paper, a first study on the synthesis, characterization and gas sensing performance of (GaxIn1-x)(2)O-3 nanowires (NWs) is reported. Carbothermal metal-assisted chemical vapor deposition was carried out with different mixtures of Ga2O3, In2O3 and graphite powders. Structural characterization of the NWs revealed that they have a crystalline structure close to that of In2O3 nanowires, with a small amount of Ga incorporation, which highly depends on the mass ratio between the two precursors. Dedicated gas nanosensors based on single NWs were fabricated and tested for both ethanol and nitrogen dioxide, demonstrating an improved performance compared to similar devices based on pure In2O3 or Ga2O3 NWs. Funding Agencies|Spanish Ministerio de Economia y Competitividad [TEC2013-48147-C6, TEC2016-79898-C6]; Ministerio de Economia e Innovacion [PID2019-107697RB-C41]

A methodology based on the use of Electron Beam Lithography for contacting individual nanowires on top of non-flat micromembranes and microhotplates has been implemented, and the practical details have been exhaustively described. The different fabrication steps have been adapted to the substrate’s topology, requiring specific holders and conditions. The methodology is demonstrated on individual SnO2 nanowires, which, after fabrication, have been characterized as functional resistive gas nanosensors towards NH3 and benchmarked against similar devices fabricated using more conventional Dual Beam Focused Ion Beam techniques, demonstrating the superior properties of the here presented methodology, which can be further extended to other non-conventional suspended substrates and nanomaterials.

We report the growth of micrometer-sized In2O3 octahedral structures, which are next aligned in chains using dielectrophoresis on top of microhotplates with prepatterned electrodes and integrated heater to work as chemoresistive gas sensors. The devices are relatively fast (180 s), highly sensitive (response up to ∼256%), and selective toward NO2 in humid environments, showing little response to O2 and ethanol, and being completely insensitive to CO and CH4. The here-presented fabrication method can be easily extended as a cost-effective post-process in CMOS-compatible microhotplate fabrication and, thus, represents a promising candidate for indoor and outdoor air quality monitoring devices. Funding: This work has been partially supported by the Spanish Ministeriode Economía y Competitividad, through projects TEC2013-48147-C6 and TEC2016-79898-C6 (AEI/FEDER, EU) and by the Spanish Agencia Estatal de Investigación (AEI), through projects PID2019-107697RB-C41/AEI/10.13039/501100011033 and PID2019-107697RB-C42/AEI/10.13039/501100011033.

The gas sensing mechanisms, response, and behaviour of a real and a virtual solid-state chemical gas sensor operating either in static or in dynamic mode have been compared. The analysis was done by exposing simultaneously both sensors to different concentrations of various volatile organic compounds diluted in dry, as well as humid, synthetic air. The results revealed similar responses and behaviours for both types of measurement modes when the sensors were exposed towards single gas compounds, but a sensitivity enhancement in measurements comprising mixtures of gases when the sensors were operated in dynamic mode. The method used is able to overcome surface saturation problems and is beneficial for applications where mixtures of gases diluted in relative humidity are present. Funding Agencies|EUEuropean Commission [814596]; Vinnova Innovation projectVinnova [2019-02095]

Gas sensitive iridium-gated field effect transistors based on silicon carbide were used to study the response towards formaldehyde, ammonia, carbon monoxide and nitrogen dioxide at concentrations ranging from parts per million to parts per billion diluted in dry synthetic air and under 50% of relative humidity. The sensor performance was studied using temperature cycled operation mode from 270 to 390 °C to investigate the capability of these devices to discriminate between the studied gases under different background conditions via pattern recognition algorithms.

A new method for the site-selective synthesis of nanowires has been developed to enable material growth with defined morphology and, at the same time, different composition on the same chip surface. The chemical vapor deposition approach for the growth of these nanowire-based resistive devices using micromembranes can be easily modified and represents a simple, adjustable fabrication process for the direct integration of nanowire meshes in multifunctional devices. This proof-of-concept study includes the deposition of SnO2, WO3, and Ge nanowires on the same chip. The individual resistors exhibit adequate gas sensing responses toward changing gas concentrations of CO, NO2, and humidity diluted in synthetic air. The data have been processed by principal component analysis with cluster responses that can be easily separated, and thus, the devices described herein are in principle suitable for environmental monitoring.

Gas nanosensors, comprised of arrays of nanoelectrodes with finger-widths of ∼100 nm developed by electron beam lithography and aerosol assisted chemical vapor deposited non-functionalized and Pt-functionalized tungsten oxide nanowires (

The gas sensing properties of germanium (Ge) monocrystalline nanowires (NWs) at temperatures up to 100 °C have been demonstrated for the first time. The devices have been fabricated based on an energy efficient and site-specific vapor-liquid-solid growth of NW meshes on top of microhotplates, which contain a buried heater and top electrodes. The devices have been investigated for the detection of oxygen, nitrogen dioxide and carbon monoxide gases, showing the important effect played by pre-adsorbed surface oxygen in the response to the different gases. The Ge NW-based devices exhibit p-type conductivity and show high selectivity in their response towards water vapor. Water vapor interaction is not dependent on the presence of oxygen and the adsorption leads to electron donation in the Ge nanowires. TEM analysis of the NWs proves that they are covered by a thin, outer germanium oxide layer, which is stable and does not grow upon exposure to these gases and operation temperatures up to 100 °C. The presence of this oxide layer plays a key role in the sensing mechanisms.

Indium oxide nanostructures have been prepared by carbothermal reduction method. From them, nanowires have been structurally and optically characterized using X-ray diffraction, scanning and transmission electron microscopy and photoluminescence. The indium oxide nanowires have been removed from the substrate and contacted by Focused Electron- and Focused Ion-Beam Induced Deposition techniques to interdigitates on suspended microhotplates, which allow heating up to 300 °C with a low power consumption of only 8 mW. The gas response of the tested devices towards ethanol, carbon monoxide and nitrogen dioxide diluted in dry synthetic air at different concentrations and temperatures has been carried out showing a selectivity towards ethanol with responses up to 50% at temperatures between 200 and 300 °C, while only small response to high concentrations of carbon monoxide and nitrogen dioxide is observed.

Nanowires of different materials have been grown site-selectively and integrated in one single chip using a new methodology. The chemical vapor deposition approach performed to grow these nanowire-based devices can be easily tuned, representing a simple, fast and adjustable fabrication process for the direct integration of nanowire meshes in multifunctional devices. This proof-of-concept study focuses on the deposition of SnO 2 , WO 3 and Ge nanowires on the same chip. The prepared sensors have been tested under varying concentrations of CO, NO 2 and humidity diluted in synthetic air, exhibiting satisfactory gas sensing responses. A principal component analysis has been performed with the collected data and responses towards different gases can be easily classified in clusters. Therefore, the devices here presented are suitable for environmental monitoring.

Ge NWs have been selectively grown directly on top of the suspended microhotplates, with heater and interdigitated electrodes, via Au catalyzed Vapor Liquid Solid (VLS) mechanism through a low-power consumption CVD-like process. The so-fabricated devices, with a p-type semiconductors behavior, have been characterized towards water vapor in synthetic air, keeping the temperature at 100°C in order to achieve a stable oxide. The results show reproducible responses towards H 2 O.

SnO2 NWs networks on heated micromembranes have been characterized as ammonia sensors. The approach allows achieving reproducible growth and stable and long-lasting ammonia sensors with site- specific grown SnO2 NWs. The devices have been tested both in dry and humid conditions showing response time down to two minutes. Sensors have been tested up to 1 month, only presenting variation of the base resistance with full retention of the response towards the gaseous analytes. Different concur- rent sensing mechanisms have been identified relating the determined sensing kinetics with previous theoretical calculations. Specifically, oxygen dissociation seems to play a key role in the overall ammonia sensing sequence. In humid conditions, moisture reduces the response to ammonia but also lowers the activation energy of the reaction process.

Self-heating of metal oxide nanowires when a measuring current flows through them allows simultaneously heating the metal oxide, which is required for correct gas sensing operation, and measuring the nanowire resistance change, which is achieved from the ratio between the voltage drop at its edges and the current injected by the source measurement unit. In this way a drastic reduction of the power consumption of the gas sensor down to some μW is obtained and, additionally, it simplifies the practical operation of the devices, but the required control electronics that assures the correct and stable current flow through the device becomes much more complex. In this work the degree of maturity of this almost zero-power consuming gas detection systems based on nanowires will be shown and some recent advances in the use of nanowires mats or carbon nanofibers will be presented.