Chemical Vapor Deposition Graphene Sensors for Scale Detection
Authors: Younes Hammad , Kaddour Souhila , Al Ghaferi Amal , Saadat Irfan , Tizani Lina 
Published: February 2020

Scale is the creation of inorganic soluble salts from aqueous brines during oil and gas production, which deposits at the internal surface of the pipelines under the supersaturating conditions. Calcium carbonate (CaCO3) and barium Sulphate (BaSO4) are the most insoluble types of inorganic scale formed during the extraction of oil. The deposition can be seen not only in the pipelines but also in water handling equipment, pumps, valves and any other parts that interact with water. The deposition of the scale leads to unavoidable damage of the equipment parts. Therefore, suspension of oil operations becomes essential in order to replace the damaged parts. In the petroleum industry, such interruptions are escorted by extremely high costs.

The unique properties of CVD graphene has made it an ideal candidate for a new class of sensor systems, which were not possible before. This is due to its 2-d nature (inert, conductive where the conductivity can be modulated when exposed to various ions, molecules and gases) , mechanically very strong yet flexible so that it can be applied on various shapes and forms; in addition to its tolerance to high temperatures (stable till 700 ᵒC). In addition, the electrical resistance of the CVD graphene changes when chemicals in the surrounding covalently or non-covalently interact with the CVD graphene. The absorbed molecules can act as dopants that causes shifting on the fermi energy of the CVD graphene. Moreover, the bonds formed between absorbed chemicals and the CVD graphene change the band-structure of the tube.

The goal of this work is to fabricate FET (field effect transistor) like sensor with a CVD graphene mat as a channel deposited between metal source and drain. The gate turns the transistor on and off with an electric field through the oxide. This structure is a back gate structure in the meaning that the gate will be at the back of the structure, and the sensing martials (CVD graphene) are at the front. Such a sensor will utilize the advantage of gate biasing to affect sensor recovery and reproducibility. There are plenty of reports regarding fabrication of ChemFETs for sensing applications, with gas detection prevailing other types of analytes. Although synthesis of carbon nanomaterials and integration methods vary from one research to another, the sensor configuration is confined within a FET utilizing Si as a back gate. A role of a gate bias to recover CNT based sensor was demonstrated by other researchers. They fabricated SWNT FET sensor to detect NH3 and NO2 gases. Applying negative gate bias helped to recover the sensor after exposure to ammonia, while positive gate voltage was required to reset sensor exposed to NO2.

In our study we implement CVD graphene sensor device based on FET structure to detect NaCl salt in water solution. The successful operation of such a sensor opens a path to utilizing CVD graphene based devices for monitoring other scale, including CaCO3 and BaSO4 which are common for scale precipitation in oil industry. CVD graphene is grown in our lab on a copper and used as a sensing material and exfoliation technique was selected to deposit it on a SiO2 substrate. The electrical response of the as-fabricated sensor was studied by means of measuring changes in the drain current, i.e. the current in the channel between source and drain. The future work will be reported in the conference which include the characterization for other salts like carbonates and sulfates along with the response due to various sizes.