Brigida Alfano

Conductometric Gas Nanosensors

This paper presents a review of the current research activities in the field of gas nanosensors. Nanomaterials are characterized by physical and chemical properties that differ from their macroscopic counterparts and, in particular, by an enhanced chemical reactivity even at room temperature. This effect has stimulated the development of chemical sensors based on several different nanomaterials. Here we focus most attention on carbon nanotubes, silicon and metal oxide nanoparticles and metal nanowires. After introducing a few general definitions a discussion on the fundamental properties of the nanostate used in the sensor field is presented and several nanosensors, based on the aforementioned nanomaterials, are discussed. Finally, some personal conclusions will be drawn.

Palladium Nanowires Assembly by Dielectrophoresis Investigated as Hydrogen Sensors

Fabrication of hydrogen sensors based on array of palladium nanowires onto silicon substrate with interdigitated electrodes is reported. Two series of devices, where Pd-sensing elements are assembled by dielectrophoresis, are fabricated and electrically characterized under 4% hydrogen at room temperature. Their electrical responses are analyzed and compared to investigate the effect of nanowire thickness and surface morphology on sensitivity and response time. We have found thinner nanowires are more sensitive and their response time is shorter than thicker ones.

Inkjet printed graphene-based chemi-resistors for gas detection in environmental conditions

In this work, we report on the inkjet printing as potential technology to manufacture chemi-resistors based on liquid phase exfoliated graphene. With respect to the conventional solution-processable methods, the main IJP capability is related to the deposition of small ink volumes that entails a more controlled drying process. This specific potentiality of the IJP technique were exploited in order to investigate the reproducibility of the device performances upon NO2 and NH3 exposure and operating in environmental conditions.

Study of the correlation between sensing performance and surface morphology of inkjet-printed aqueous graphene-based chemiresistors for NO2 detection

The extremely high sensitivity to the external environment and the high specific surface area, as well as the absence of bulk phenomena that could interfere with the response signal, make graphene highly attractive for the applications in the field of sensing. Among the various methods for producing graphene over large areas, liquid phase exfoliation (LPE) appears to be very promising, especially if combined with inkjet printing (IJP), which offers several advantages, including the selective and controlled deposition of small ink volumes and the versatility of the exploitable inks and substrates. Herein we present a feasibility study of chemiresistive gas sensors inkjet-printed onto paper substrates, in which a LPE graphene suspension dispersed in a water/isopropanol (H2O/IPA) mixture is used as sensing ink. The device performances, in terms of relative conductance variations, upon exposure to NO2 at standard ambient temperature and pressure, are analysed. In addition, we examine the effect of the substrate morphology and, more specifically, of the ink/substrate interaction on the device performances, by comparing the response of different chemiresistors fabricated by dispensing the same suspension also onto Al2O3 and Si/SiO2 substrates and carrying out a supportive atomic force microscopy analysis. The results prove the possibility to produce sensor devices by means of a wholly environmentally friendly, low-cost process that meets the requests coming from the increasing field of paper-based electronics and paving the way towards a flexible, green-by-design mass production.

High sensitive gas sensors realized by a transfer-free process of CVD graphene

The work herein presented investigates the behavior of graphene-based gas sensors realized by using an innovative way to prepare graphene. The sensing layer was directly grown by chemical vapor deposition on pre-patterned CMOS compatible Mo catalyst and then it was eased on the underlying SiO2 through a completely transfer-free process. Devices with different geometries were designed and tested towards NO2 and NH3 in environmental conditions, i.e. room temperature and relative humidity set at 50%. Furthermore, these gas sensors were also calibrated, resulting in the ability to detect concentrations down to 240 ppb and 17 ppm of NO2 and NH3, respectively. These results are in agreement with the best performances reported in literature for graphene-based sensors. They not only confirm the successful devices fabrication through the transfer-free approach, but also pave the route for large-scale production of MEMS/NEMS sensors.

Graphene-like layers as promising chemiresistive sensing material for detection of alcohols at low concentration

In the manifold of materials for Volatile Organic Compound (VOC) sensing, graphene related materials (GRMs) gain special attention thanks to their versatility and overall chemico-physical tunability as a function of specific applications. In this work, the sensing performances of graphene-like (GL) layers, a new material belonging to the GRM family, are tested against ethanol and n-butanol. Two typologies of GL samples were produced by employing two different approaches and tested in view of their application as VOC sensors. The experiments were performed under atmospheric pressure, in dry air, and at room temperature and demonstrated that the sensing capabilities are related to the film surface features. The results indicated that GL films are promising candidates for the detection of low concentrations of VOCs at room temperature. The present investigation thus paves the way for VOC sensing optimization using cost-effective and easily scalable materials.

Electronic Noses for Composites Surface Contamination Detection in Aerospace Industry

The full exploitation of Composite Fiber Reinforced Polymers (CFRP) in so-called green aircrafts design is still limited by the lack of adequate quality assurance procedures for checking the adhesive bonding assembly, especially in load-critical primary structures. In this respect, contamination of the CFRP panel surface is of significant concern since it may severely affect the bonding and the mechanical properties of the joint. During the last years, the authors have developed and tested an electronic nose as a non-destructive tool for pre-bonding surface inspection for contaminants detection, identification and quantification. Several sensors and sampling architectures have been screened in view of the high Technology Readiness Level (TRL) scenarios requirements. Ad-hoc pattern recognition systems have also been devised to ensure a fast and reliable assessment of the contamination status, by combining real time classifiers and the implementation of a suitable rejection option. Results show that e-noses could be used as first line low cost Non Destructive Test (NDT) tool in aerospace CFRP assembly and maintenance scenarios.

CVD transfer-free graphene for sensing applications

The sp2 carbon-based allotropes have been extensively exploited for the realization of gas sensors in the recent years because of their high conductivity and large specific surface area. A study on graphene that was synthetized by means of a novel transfer-free fabrication approach and is employed as sensing material is herein presented. Multilayer graphene was deposited by chemical vapour deposition (CVD) mediated by CMOS-compatible Mo. The utilized technique takes advantage of the absence of damage or contamination of the synthesized graphene, because there is no need for the transfer onto a substrate. Moreover, a proper pre-patterning of the Mo catalyst allows one to obtain graphene films with different shapes and dimensions. The sensing properties of the material have been investigated by exposing the devices to NO2, NH3 and CO, which have been selected because they are well-known hazardous substances. The concentration ranges have been chosen according to the conventional monitoring of these gases. The measurements have been carried out in humid N2 environment, setting the flow rate at 500 sccm, the temperature at 25 °C and the relative humidity (RH) at 50%. An increase of the conductance response has been recorded upon exposure towards NO2, whereas a decrease of the signal has been detected towards NH3. The material appears totally insensitive towards CO. Finally, the sensing selectivity has been proven by evaluating and comparing the degree of adsorption and the interaction energies for NO2 and NH3 on graphene. The direct-growth approach for the synthesis of graphene opens a promising path towards diverse applicative scenarios, including the straightforward integration in electronic devices.

Graphene Decoration for Gas Detection

A comparison among the gas sensing properties of pristine graphene and graphene decorated with noble metal nanoparticles is herein investigated. Pristine graphene sheets are realized using the Liquid Phase Exfoliation method; noble metal decoration (namely platinum and palladium) is performed by a facile one-step chemical procedure which relies on the reduction of metal precursor salts directly onto graphene surface. All the materials have been employed as chemical sensing layer in a conductometric structure and tested towards some key analytes of interest for environmental monitoring, namely NO2, NH3 and H2. The device based on pristine graphene exhibits a specific response to NO2, whereas the device based on palladium-decorated graphene is more sensitive towards hydrogen. The third typology of device, based on graphene functionalized with platinum, shows a poorly selective behaviour. Unexpectedly, thanks to the remarkable stability of the material, this apparent drawback can be profitably exploited and overcome by integrating the sensing devices into an array, which enables to discriminate hydrogen from ammonia.

Inkjet Printed Graphene-Based Chemiresistive Sensors to NO 2

In this work, the possibility of manufacturing chemiresistive gas sensing devices by inkjet printing different LPE (Liquid Phase Exfoliation) graphene suspensions, formulated in standard organic solvents or aqueous mixtures, on rigid and flexible substrates has been studied. The sensing film has been obtained by printing a different number of graphene layers, depending on the specific ink/substrate system. The device performances have been investigated upon exposure to different concentrations of NO2 at ambient pressure and temperature, addressing the device-to-device variation as function of the number of printed layers and the base conductance.