High Quality Graphene For Sensing And Automotive Applications

Graphene, a single layer of graphite, is an extremely promising material for a wide range of future applications. It presents remarkable electrical and thermal conductivity, optical transparency, impermeability, and striking tribological properties: it is stronger than steel while retaining a high flexibility. For all these reasons, graphene has recently become a material of interest for automotive applications: an ideal candidate to realize composites with enhanced properties, e.g., improved rubbers for tires, and highly performing sensors. In this thesis, the adoption of graphene for the realization of devices (i.e., magnetic and visible light sensors) and components (e.g., tires) in automotive industry, is studied. In the first part of the thesis, we demonstrate an approach to transfer highly-crystalline graphene originally grown on Cu foil via chemical vapor deposition (CVD) to a target substrate while maintaining a high level of cleanliness of the material. This is a relevant step which subsequently allows for the realization of high-performance devices and fundamental investigations focusing on interfacing graphene with rubbers. To this end, a specific two-step cleaning approach for obtaining ultra-clean graphene with high mobility and low residual carrier density is demonstrated. Indeed, enhancing the performance of graphene-based devices in terms of carrier density, mobility and stability is a primary factor to be considered before integrating it in different technological applications. After developing an approach to obtain high quality CVD graphene on target substrates with technological relevance, such as SiO2/Si, we move towards its inclusion in ambient-stable magnetic-sensing applications. Stable Hall sensors are magnetic sensors which nowadays are extensively used for proximity sensing, accurate positioning, switching, angular sensing, speed detection, and current sensing, thus being of crucial importance in automotive, aeronautics, consumer electronics, Internet of Things (IoT) and robotic applications. Here we demonstrate a facile and scalable approach to realize polymer-encapsulated graphene Hall sensors based on CVD graphene with remarkably high magnetic sensitivity and air stability. These magnetic probes are promising alternatives to current semiconductor-based Hall sensing technology. In the following part of the thesis, we focus on the realization of graphene-based photodetectors. Graphene offers interesting properties for light sensing applications, especially for visible light detection. We demonstrated an easy, stable and scalable approach to create PMMA capped graphene based pn-junctions, which can be employed as photodetectors and in particular as visible light sensors. These graphene based pn-junctions work on the principle of photovoltaic effect where the photogenerated carriers are collected from graphene by using source-drain contacts. The fabricated pn-junction in graphene provides slow recombination of charge carriers due to the built-in potential at the junction. These pn-junction based photodetectors exhibit a responsivity of around 18 μA/W, a response time of approximately 200 ms when subjected to illumination with a 532 nm wavelength, and are expected to have improved stability over time, indicating electron-beam irradiation as an interesting technique for the large-scale fabrication of p-n junction graphene photodetectors. Finally, the last part of the thesis focuses on investigating the interface between graphene and rubbers. Indeed, besides using graphene in sensing applications, its derivatives i.e., graphene inks, could also be adopted for developing tires in automotive industry. It is expected that the overall performance of tires including heat dissipation, conductivity, grip, wear and weight reduction, could be improved by including graphene in rubber blends. However, preliminary tests evidenced a limited electrical conductivity in graphene-enhanced tires, probably due to limitations in the dispersion of graphene-based material in rubber blends. This issue calls for the development of graphene functionalization techniques to ensure acceptable electrical discharge properties in graphene-enhanced tires. For this purpose, we have investigated the impact of rubber on CVD graphene electrical properties. Furthermore, both CVD graphene and graphene ink functionalization with eco-friendly molecules such as pyrroles and tetrazoles is investigated. By thoroughly characterizing the electrical properties of graphene interfaced with rubbers and by exploring functionalization paths to improve the electrical conductivity in graphene-enhanced rubbers blends, this study provides relevant information for the adoption of graphene in tires.