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In this work, nitrogen dioxide (NO2) gas sensors based on zinc oxide nanorods (ZnO NRs) decorated with gold nanoparticles (Au NPs) working under visible-light illumination with different wavelengths at room temperature are presented. The contribution of localized surface plasmon resonant (LSPR) by Au NPs attached to the ZnO NRs is demonstrated. According to our results, the presence of LSPR not only extends the functionality of ZnO NRs towards longer wavelengths (green light) but also increases the response at shorter wavelengths (blue light) by providing new inter-band gap energetic states. Finally, the sensing mechanism based on LSPR Au NPs is proposed.

A film of gas sensitive ZnO nanoparticles has been coupled with a low-power micro light plate (μLP) to achieve a NO2-parts-per-billion conductometric gas sensor operating at room temperature. In this μLP configuration, an InGaN-based LED (emitting at 455 nm) is integrated at a few hundred nanometers distance from the sensor material, leading to sensor photoactivation with well controlled, uniform, and high irradiance conditions, and very low electrical power needs. The response curves to different NO2 concentrations as a function of the irradiance displayed a bell-like shape. Responses of 20% to 25 ppb of NO2 were already observed at irradiances of 5 mWatts·cm-2 (applying an electrical power as low as 30 μW). In the optimum illumination conditions (around 60 mWatts·cm-2, or 200 μW of electric power), responses of 94% to 25 ppb were achieved, corresponding to a lower detection limit of 1 ppb of NO2. Higher irradiance values worsened the sensor response in the parts-per-billion range of NO2 concentrations. The responses to other gases such as NH3, CO, and CH4 were much smaller, showing a certain selectivity toward NO2. The effects of humidity on the sensor response are also discussed.

We report a miniaturized device integrating a photoactive material with a highly efficient Light Emitting Diode light source. This so-called micro light plate configuration allows for maximizing the irradiance impinging on the photoactive material, with a minimum power consumption, excellent uniformity, and accurate control of the illumination. We demonstrate these advantages with an example application: photoactivated gas sensors with a power consumption as low as 30 μW (this is 1000 times lower than the best figures reported to date). The letter also presents a quantitative model and a set of design rules to implement it in further integrated applications.

Inorganic conductometric gas sensors struggle to overcome limitations in high power consumption and poor selectivi-ty. Herein, recent advances in developing self-powered gas sensors with tunable selectivity are introduced. Alternative general approaches for powering gas sensors were realized via proper integration of complementary functionalities (namely; powering and sensing) in a singular heterostructure. These solar light driven gas sensors operating at room temperature without applying any additional external powering sources are comparatively discussed. The TYPE-1 gas sensor based on integration of pure inorganic interfaces (e.g. CdS/n-ZnO/p-Si) is capable of delivering a self-sustained sensing response, while it shows a non-selective interaction towards oxidizing and reducing gases. The structural and the optical merits of TYPE-1 sensor are investigated giving more insights into the role of light activation on the modu-lation of the self-powered sensing response. In the TYPE-2 sensor, the selectivity of inorganic materials is tailored through surface functionalization with self-assembled organic monolayers (SAMs). Such hybrid interfaces (e.g. SAMs/ZnO/p-Si) have specific surface interactions with target gases compared to the non-specific oxidation-reduction interactions governing the sensing mechanism of simple inorganic sensors. The theoretical modeling using density functional theory (DFT) has been used to simulate the sensing behavior of inorganic/organic/gas interfaces, revealing that the alignment of organic/gas frontier molecular orbitals with respect to the inorganic Fermi level is the key factor for tuning selectivity. These platforms open new avenues for developing advanced energy-neutral gas sensing devices and concepts.

The usual operation of a conductometric sensor device requires of an external energy source (i.e. an embedded heater). In the last years, the Joule effect in the sensing material, the so called self-heating effect, offered and alternative method to provide this energy: the probing current (or voltage) applied to measure the sensor signal also serves to heat up the sensor active film. Here, evidences of self-heating effects occurring on large arrays of nanostructures fabricated with low-cost methods are provided. The methodology is proven to be suitable to sense gases (humidity, NH3 and NO2) with low-powered heater-free devices.

Selectivity and low power consumption are major challenges in the development of sophisticated gas sensor devices. A sensor system is presented that unifies selective sensor-gas interactions and energy-harvesting properties, using defined organic-inorganic hybrid materials. Simulations of chemical-binding interactions and the consequent electronic surface modulation give more insight into the complex sensing mechanism of selective gas detection.

Self-heating operation, or the use of the resistance-probing signal to warm up and control the temperature of nanowire devices, has been the subject of research for more than a decade. In this review, we summarize the most relevant achievements reported to date in the specialized literature. The state-of-the-art shows that this approach is serving to lower the power demand in temperature-activated devices, especially in conductometric gas sensors, but the simplicity of eliminating the heating element comes with the complexity of integrating 1-dimensional nanomaterials in electronic devices. Results show however that this is feasible, and in some cases, even cost-effective. To contribute to the further development and optimization of the self-heating approach, we compile here a set of recommendations on how to increase the efficiency of the future devices. These suggestions aim at clarifying the impact on the power efficiency of factors like the nanowire cross-section, the electrical and thermal conductivities of the material, the thermal insulation characteristics, and the operating conditions. To facilitate the comparison of the performances obtained in past and future works, we also propose a figure of merit: the efficient self-heating coefficient (ESH), which accounts for the maximum temperature increase (in Kelvin) per microwatt of Joule power dissipated in the material. In this way, ESH values about 1 or above are indicative of highly efficient technologies, capable of raising the temperature over hundreds of degrees with less than a milliwatt of dissipated power.

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.

In this work we will prove that SiC-based MIS capacitors can work in environments with extremely high concentrations of water vapor and still be sensitive to hydrogen, CO and hydrocarbons, making these devices suitable for monitoring the exhaust gases of hydrogen or hydrocarbons based fuel cells. Under the harshest conditions (45% of water vapor by volume ratio to nitrogen), Pt/TaOx/SiO2/SiC MIS capacitors are able to detect the presence of 1 ppm of hydrogen, 2 ppm of CO, 100 ppm of ethane or 20 ppm of ethene, concentrations that are far below the legal permissible exposure limits.

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.

We report on how an inexpensive and very selective gas sensor can be implemented, simply combining colorimetric indicators casted on top of Scotch® tape, with a commercial microchip adapted here to measure optical reflectance. The system can be easily reproduced (leading to quantitatively consistent results), refreshed and reconfigured to sense different target gases, just replacing the colorimetric tape.

Sensor signal instability and drift are still unresolved challenges in conductometric gas sensors. Here, the use of self-heating effect to operate a gas sensor in a pulsed temperature modulation mode (pulsed self-heating operation) is presented as an effective method to enhance signal stability and reduce consumption figures down to a few μW. The sensor operation temperature was pulsed periodically between two levels, obtaining two different sensing states from one single device driven with self-heating, i.e. free of heater. The signal differences between both operating points correlated well with gas concentrations and displayed no drift. This methodology is exemplified with a thorough study of the response of carbon nanofibers to humidity. Specifically, after analyzing the influence of the pulse characteristics (i.e. temperature variation, pulse period and pulse duty cycle) on the sensor performance, thumb rules to select suitable pulsing conditions are provided. The methodology is successfully extended to other target gases, such as NO2 and NH3. Finally, its implementation in a real-time sensing system with low computational requirements is demonstrated and discussed in detail.

Localized growth of SnO2 nanowires on top of CMOS compatible micromembranes that incorporate a buried heater and prepatterned interdigitated electrodes has been achieved that presents the advantage that it allows to easily and directly integrate the advantageous properties of quasi-one dimensional structures in an advanced electronic device by a Vapor Liquid Solid (VLS) mechanism. A NWs based sensor of this type is characterized as a low power gas sensor towards NH3 at different temperatures. Stable and reproducible response is obtained, that allows detecting concentrations below the time-weighted average exposure limit for 8h.

The prevailing design approaches of semiconductor gas sensors struggle to overcome most of their current limitations such as poor selectivity, and high power consumption. Herein, a new sensing concept based on devices that are capable of detecting gases without the need of any external power sources required to activate interaction of gases with sensor or to generate the sensor read out signal. Based on the integration of complementary functionalities (namely; powering and sensing) in a singular nanostructure, self-sustained gas sensors will be demonstrated. Moreover, a rational methodology to design organic surface functionalization that provide high selectivity towards single gas species will also be discussed. Specifically, theoretical results, confirmed experimentally, indicate that precisely tuning of the sterical and electronic structure of sensor material/organic interfaces can lead to unprecedented selectivity values, comparable to those typical of bioselective processes. Finally, an integrated gas sensor that combine both the self-powering and selective detection strategies in one single device will also be presented. © 2015 Published by Elsevier Ltd.