Filiberto Ricciardella

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.

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.

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.

Cross interference effects between water and NH 3 on a sensor based on graphene/silicon Schottky diode

The work herein presented investigates the sensing properties of a graphene-based Schottky diode, operating in real environmental conditions, that is at room temperature and in presence of humidity. The role of water in the field of gas sensors for air quality monitoring has always been a crucial point: the water is always present in the environment and in most cases acts as interfering molecule, by altering or masking the effects of the other analytes. In this work, reporting device employed for NH3 detection, we show that the adsorption kinetics of water takes place on completely different time scale so that the ammonia contribution can be distinguished. The hetero-junction consists of graphene sheets obtained by Liquid Phase Exfoliation and deposited onto a Si cathode by drop casting. Cross interference effect between water vapour and NH3 was studied by exposing the diode at several Relative Humidity values ranging from 20% up to 70%. All tests were performed in air at 500sccm total flow at room temperature. A current drift due to the water is mainly visible whereas the effect of the pollutant is revealed as an abrupt decrease of the diode current.

Easy recovery method for graphene-based chemi-resistors

In this work we report on the development of a simple and fast method for regenerating exhaust graphene based chemi-resistors for NO2 detection. A poisoning effect of the devices due to repeated exposure or to air exposure for > 48 h results in an overall worsening of the sensing response. The presented method consists in the dipping of exhaust devices into ultrapure water at 100 °C for 60 s. The device performances towards analyte are compared with those obtained after the restore by using the developed method.

Graphene-Si Schottky diode in environmental conditions at low NH3 ppm level

In this work, we present the behavior of a graphene/silicon Schottky diode exposed to NH3 flow of few tens of parts-per-million (ppm), at standard temperature and humidity conditions. Graphene was synthesized by Liquid Phase Exfoliation and transferred onto the Silicon substrate by drop casting. The Schottky barrier characterization towards NH3 was performed at a reverse bias of -3V in the range 10 ppm-200 ppm. Results show the effect on the device electric current of ammonia concentrations as low as 10 ppm, with a good repeatability of the voltamperometric response. The variations ΔφNH3, of the Schottky barrier, are reported as a function of the gas concentration. A spontaneous restoring is finally observed for the device.

UV Lithography On Graphene Flakes Produced By Highly Oriented Pyrolitic Graphite Exfoliation Through Polydimethylsiloxane Rubbing

Graphene is a promising candidate in sensing applications; indeed thanks to its two-dimensionality, it provides the highest surface volume ratio, allowing all its atoms to be totally exposed to the adsorbing gas molecules. Due to several technological limits in production and manipulation of graphene as well as the device fabrication, the synthesis of graphene is still far from a well-settled process. This work aims to illustrate an approach for the graphene preparation that is based on the mechanical exfoliation and circumvents some difficulties encountered in a graphene based nanodevice production. Relying on that fabrication technique a chemiresistive sensor device was prepared. Preliminary findings showed that the device responds to a toxic gas such as \(\hbox{NO}_2,\) up to a few ppm, and reducing gases, such as \(\hbox{NH}_3,\) to few hundred ppm, at room temperature in controlled environments.