T. Polichetti

A calibrated graphene-based chemi-sensor for sub parts-per-million NO2 detection operating at room temperature

Here, we present a room temperature operating chemi-sensor based on a graphene film that shows sensitivity to NO2 up to a 50 parts-per-billion (ppb) with extremely limited interference from relative humidity and can be also calibrated in a sub-parts-per-million (ppm) range with a response and recovery time of few seconds. The device has been fabricated using as active material, a solution of graphene nanosheets suspended in N-methyl-pyrrolidone drop casted on an alumina substrate with gold interdigitated electrodes. The derivative of the device response is found to be univocally correlated to NO2 concentrations from 100 ppb up to 1000 ppb and the sensor can therefore be calibrated in this same range.

A study on the physicochemical properties of hydroalcoholic solutions to improve the direct exfoliation of natural graphite down to few-layers graphene

Straightforward methods to produce pristine graphene flakes in large quantities are based on the liquid-phase exfoliation processes. These one-step physical transformations of graphite into graphene offer many unique advantages. To date, a large number of liquids have been employed as exfoliation media exploiting their thermodynamic and chemical features as compared to those of graphene. Here, we pursued the goal of realizing water based mixtures to exfoliate graphite and disperse graphene without the aid of surfactants. To this aim, aqueous mixtures with suitable values of surface tension and Hansen solubility parameters (HSPs), were specifically designed and used. The very high water surface tension was decreased by the addition of solvents with lower surface tensions such as alcohols, obtaining, in this way, more favourable HSP distances. The specific role of each of these thermodynamic features was finally investigated. The results showed that the designed hydroalcoholic solutions were effective in both the graphite exfoliation and dispersion without the addition of any surfactants or other stabilizing agents. Stable graphene suspensions were obtained at concentration comparable to those produced with low-boiling solvents and water/surfactants.

A Simple Optical Model for the Swelling Evaluation in Polymer Nanocomposites

In the present study, we report on a simple optical method based on thin film interferometry for the swelling evaluation in polymer nanocomposite layers used for gas sensing applications. We show that white light interferometry can be profitably applied to characterize scattering materials such as polymer/carbon black nanocomposites. A properly adjusted experimental setup was implemented to monitor the swelling behavior of the sensitive films in real device operating conditions. In particular, the behavior of poly(2-hydroxyethyl methacrylate) (PHEMA) and of carbon black/PHEMA nanocomposite layers, used for volatile organic compounds (VOCs) detection, was investigated and measured under ethanol vapors exposure (max 1%). The method is very sensitive and the swelling in the range of only few nanometers can be measured. Interestingly, we have found that the nanocomposite undergoes a more pronounced swelling process with respect to pristine polymer. Ethanol diffusion coefficients in the nanocomposite were evaluated.

Reproducibility of the Performances of Graphene-Based Gas-Sensitive Chemiresistors

The potential of graphene as sensing layer relies on its two-dimensional nature that provides the greatest sensor area per unit volume. Thanks to this property, besides the highest mobility and the lowest resistivity values, graphene has put itself as the leader of the new discovered materials in every research field. Graphene can be produced by various approaches including micromechanical exfoliation of graphite, thermal dissociation of SiC, chemical vapor deposition, or by low-cost approaches such as chemical exfoliation methods. However, the sensor device development is still affected by several technological limitations mainly related to graphene preparation, introduction into device architectures, and reproducibility of the sensor performances. Regarding the last item, sensing performance may differ from device to device even though graphene materials come from the same batch and the same fabrication protocol. In this work, chemiresistive devices based on chemically exfoliated natural graphite are presented. Several parameters were taken into account: graphene preparation (including solvents, centrifugation speed, and batch), deposition, and conductance. Finally the device-to-device variation is addressed.

Exfoliation of Graphite and Dispersion of Graphene in Solutions of Low-Boiling-Point Solvents for Use in Gas Sensors

In this work we report on the development of an eco-friendly method for the chemical exfoliation of graphite in order to produce high-quality graphene for sensing applications. A mixture of low-boiling-point solvents, such as 1-butanol and 2-propanol, was employed for this purpose. The resulting colloidal suspension was a stable dispersion of few-layer flakes. This material was employed to fabricate chemiresistor devices that showed a remarkable variation of conductance when exposed to 350 ppb of NO2.

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.

Sub-PPM Nitrogen Dioxide Conductometric Response at Room Temperature by Graphene Flakes Based Layer

The two-dimensional nature of graphene, allowing a total exposure of all its atoms to the adsorbing gas molecules, provides the greatest sensor area per unit volume and outlines the possibility to employ this material as a powerful sensing layer. The synthesis and manipulation of graphene as well as the device fabrication are still challenging due to several technological limits. In the present work we report on a simple approach to fabricate chemiresistive sensors based on chemically exfoliated natural graphite. The devices show the ability to detect a toxic gas, such as NO2, down to few ppb at room temperature in controlled environments.

The effect of solvent on the morphology of ZnO nanostructure assembly by dielectrophoresis and its device applications

Different zinc oxide nanostructured morphologies were grown on photolithographically patterned silicon/silicon dioxide substrates by dielectrophoresis technique using different solvents, such as water and ethanol, obtaining rod‐like and net‐like nanostructures, respectively. The formation of continuous nanostructures was confirmed by scanning electron microscopic, atomic force microscopic images, and electrical characterizations. The rod‐like zinc oxide nanostructures were observed in the 10 μm gap between the fingers in the pattern, whereas net‐like nanostructures were formed independently of microgap. A qualitative study about the mechanism for the assembly of zinc oxide continuous nanostructures was presented. Devices were electrically characterized, at room temperature, in controlled environment to measure the conductance behavior in ultraviolet and humidity environment. Devices based on zinc oxide nanostructures grown in ethanol medium show better responses under both ultraviolet and humidity, because of the net‐like structure with high surface‐to‐volume ratio.