Plasma Functionalised GNP for Volume Printed Flexible Electronics

GNP (Graphene Nano Platelets) have the potential to create cost-effective electronic inks for a wide range of volume printed applications. However, in common with other nano carbons these are inherently inert making it difficult to bond and disperse. While some naturally occurring functional groups, such as oxygen, ethers, carboxyls or hydroxyls, are attached to the raw GNP, adding to or modifying these chemical groups, in a controlled fashion can exfoliate sheets, improve consistency and enable tailored interactions. It also enhances particle separation and improves dispersion, which are key factors for the manufacture of functional inks. This also enables the formulation of functional inks for applications such as printed electronics, sensors, energy storage, smart packaging and wearable technology. A low temperature, dry plasma process based on tumbling nano materials through a plasma has the capability to add a range of functional groups in a high volume scalable process. The functional groups added depend on the plasma gas with the quantity dependent on the processing time. It has been postulated that the functional groups attach to the edge of the graphene sheets and at defect sites. In order to gain insight into the location and the nature of the attachment of the chemical groups, Scanning Transmitting Electron Microscope (STEM) was used to study the atomic structures of the functionalised GNP's. This confirmed that the various functional groups had attached to the edge of the graphene sheets. The number of layers towards the centre of the GNP meant it was not possible to identify whether there were also loosely bonded groups attached to the surface or chemically bonded to defects. Further insight into the effect of the functionalisation was gained by formulating inks to study the rheology of the ink and printability. Model inks were created with GNPs with different functional groups. A combination of rheological measures, including constant shear, Small Amplitude Oscillatory Shear (SAOS) and Controlled Stress Parallel Superposition (CSPS), were used to establish the relationships between carbon loading, functionalisation and printability. These techniques also have potential applications for ink quality assurance and formulation. Inks with properties ranging from flexible conductive inks to pressure sensing inks were made by adding carbon black and graphite. These can be printed over large areas using conventional processes such screen printing, flexography and roller coating. The performance was dependent on the blend of nano carbons and the other components in the ink. Triple roll milling was used to homogenise the ink. The high shear forces this induces causes further exfoliation of some of the GNP's, leading to inks containing a mix of graphene, FLG and GNP's. Screen printable conductive and pressure sensor inks have been created. The latter has unique properties as it is sensitive in one direction only which enables the creation of very large area pressure sensor arrays.