Self-heating in pulsed mode for signal quality improvement: application to carbon nanostructures-based sensors

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 W. 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

Revisiting Colorimetric Gas Sensors: Compact, Versatile and Cost-Effective

We report on an inexpensive and very selective gas sensor implemented by simply combining colorimetric indicators casted on top of acetate-based transparent tape, with a commercial microchip adapted here to measure optical reflectance. This sensor can be easily reproduced (leading to quantitatively consistent results), refreshed and reconfigured to sense different target gases replacing only the colorimetric tape. The device may either work as sensor (CO2 and NH3) or dosimeter (Formaldehyde) depending on the targeted gas

Visible-Light-Driven Room Temperature NO2 Gas Sensor Based on Localized Surface Plasmon Resonance: The Case of Gold Nanoparticle Decorated Zinc Oxide Nanorods (ZnO NRs)

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

Electrical properties of individual tin oxide nanowires contacted to platinum electrodes

A simple and useful experimental alternative to field-effect transistors for measuring electrical properties free electron concentration nd, electrical mobility , and conductivity in individual nanowires has been developed. A combined model involving thermionic emission and tunneling through interface states is proposed to describe the electrical conduction through the platinum-nanowire contacts, fabricated by focused ion beam techniques. Current-voltage I-V plots of single nanowires measured in both two- and four-probe configurations revealed high contact resistances and rectifying characteristics. The observed electrical behavior was modeled using an equivalent circuit constituted by a resistance placed between two back-to-back Schottky barriers, arising from the metal-semiconductor-metal M-S-M junctions. Temperature-dependent I-V measurements revealed effective Schottky barrier heights up to BE= 0.4 eV

How to implement a selective colorimetric gas sensor with off the shelf components?

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

A review on efficient self-heating in nanowire sensors: Prospects for very-low power devices

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

A low-cost approach to low-power gas sensors based on self-heating effects in large arrays of nanostructures

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

Visible Light‐Driven p-Type Semiconductor Gas Sensors Based on CaFe2O4 Nanoparticles

In this work, we present conductometric gas sensors based on p-type calcium iron oxide (CaFe2O4) nanoparticles. CaFe2O4 is a metal oxide (MOx) with a bandgap around 1.9 eV making it a suitable candidate for visible light-activated gas sensors. Our gas sensors were tested under a reducing gas (i.e., ethanol) by illuminating them with different light-emitting diode (LED) wavelengths (i.e., 465-640 nm). Regardless of their inferior response compared to the thermally activated counterparts, the developed sensors have shown their ability to detect ethanol down to 100 ppm in a reversible way and solely with the energy provided by an LED. The highest response was reached using a blue LED (465 nm) activation. Despite some responses found even in dark conditions, it was demonstrated that upon illumination the recovery after the ethanol exposure was improved, showing that the energy provided by the LEDs is sufficient to activate the desorption process between the ethanol and the CaFe2O4 surface

Colorimetric sensor for bad odor detection using automated color correction

Colorimetric sensors based on color-changing dyes offer a convenient approach for the quantitative measurement of gases. An integrated, mobile colorimetric sensor can be particularly helpful for occasional gas measurements, such as informal air quality checks for bad odors. In these situations, the main requirement is high availability, easy usage, and high specificity towards one single chemical compound, combined with cost-efficient production. In this contribution, we show how a well stablished colorimetric method can be adapted for easy operation and readout, making it suitable for the untrained end user. As an example, we present the use of pH indicators for the selective and reversible detection of NH3 in air (one relevant gas contributing to bad odors) using gas-sensitive layers dip coated on glass substrates. Our results show that the method can be adapted to detect NH3 concentrations lower than 1 ppm, with measure-to-result times in the range of a few minutes. We demonstrate that the color measurements can be carried out with the optical signals of RGB sensors, without losing quantitative performance

Micro light plates for low-power photoactivated (gas) sensors

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