Engineering of multi-functional conductive thin films using graphene materials and their composites

Abstract

Graphene was successfully exfoliated in 2004 by A. Geim and K. Novoselov by using the mechanical scotch-tape technique and these inventors have awarded the Nobel Prize in 2010. Since then graphene has attracted enormous research interests due to its unique electrical, mechanical and optical properties which has potential applications across many sectors. The aim of this Ph.D, project is to explore the use of different graphene derivatives such as pristine graphene (pG), reduced graphene oxide (rGO) and graphene oxide (GO) and their composites for development of their inks and fabrication of highly conductive films that can be used for a broad range of applications such solar cells, supercapacitor electrodes, sensors, electromagnetic shielding etc. The first concept developed and explored in this thesis was to demonstrate a new method based on scanning atmospheric plasma for an ultrafast reduction of graphene oxide (P-rGO) and preparation of highly conductive graphene films and patterns. Outcomes of this study are development of a simple cold plasma approach for scalable preparation of graphene film based on atmospheric pressure plasma jets (APPJs). The atmospheric plasma provides not only high electron densities (>1013 cm(3)) and high energetic electrons (>20 eV), but also highly reactive gas species and radicals that react with the oxygen functional groups on GO films causing their ionization reduction and removal. This reduction process happens over very short time (~1 min), being able to covert nonconductive thin or thick GO films into rGO films throughout all layers (not only the top surface) on different substrates (glass, plastic and textile) with various shapes (circles, squares, lines, arrays, etc.) from micron to millimetre size patterns. The second concept explored in this thesis was to develop a new method to engineer graphene surfaces with continuous gradual change of multiple functions including structural, chemical, wettability, charge, surface energy, electrical and thermal conductivity at a large scale (cm) that are not achieved before. The developed fabrication protocol is based on conversion of GO films by non-uniform exposure of atmospheric pressure plasma beam (to gradually remove GO oxygen functional groups across the surface, thus to create rGO. The method is simple, low-cost, scalable, applicable on different surfaces (metals, plastic, textile, glass, curved, flexible) with the ability to create multifunctional surface gradients at a large scale, and used for solving many fundamental and engineering problems that cannot be addressed with the homogenous surfaces. The third topic explored in this Ph.D project was to develop of a facile method to improve both the stability and performance of silver nanowire (AgNW) film (conductivity and transparency). The AgNWs inks were synthesis and combined with pristine graphene (pG) inks with the goals to prove that the pG sheets can provide a barrier shielding to protect against AgNW oxidation and improving the connections between wires and stability of the films. The fabrication of these composite films was successfully demonstrated on wide range of substrates including glass, plastic, textile, and paper. A surface resistance of 18.23 Ω/sq and an optical transparency of 89% were obtained on the glass substrates, 50 Ω/sq and 88% transparency for poly(ethylene terephthalate) (PET), and 0.35 Ω/sq resistance on the textile substrate. The APPJ treatment was further used to enhance the performance of the film (i.e., glass), resulting in a significant reduction of 30.6% in sheet resistance (15.20 Ω/sq) and an improvement of transparency to 91%. The stability of AgNW/pG film under environmental conditions and higher temperatures was significantly improved due to the graphene acting as an oxidation barrier and dissipating heat. The test showed a minor sheet resistance increases after 30 days and further thermal stability to temperature up to 300 °C. In comparison, the control (AgNW film) showed a sharp sheet resistance increase after 8−10 days only and thermally stable until 150 °C as a result of Ag oxidation. Lastly, the thesis presented the development of a new method for the fabrication of highly conductive and transparent ultrathin nitrogen (N) doped graphene films from graphene inks by combining a microwave treatment, ultrasonic nebulizer coating and thermal annealing. This method involving in situ N-doping offers a promising environmentally-friendly, low-cost and scalable manufacture of high-quality conductive N-doped graphene films. The starting GO solution was mixed with poly(ionic liquids) (PIL) and treated with microwave (Mw) irradiation to prepare Mw-rGO@PIL inks, which is a gentle reduction of PIL attached rGO to not only mediate microwave irradiation and prevent disorder of the graphitic structure, but also repair the lattice defects and introduce nitrogen into the graphitic structure. The prepared films displayed a surface resistance of∼1.45×107 Ω/sq at a transparency of∼87%. A further thermal treatment was conducted to improve the conductivity of the prepared films by annealing at a high temperature (900 °C), which allowed complete reduction of oxygen containing groups, enhanced graphitization, and reordering of the basal graphene plane and N-doping of the carbon lattice (pyrolytic PIL). The resulting thin films significantly reduced the surface resistance in the range of 1.5×103 to 6.2×103 Ω/sq at a transparency ranging from 68 to 82%, respectively. By means of using physical strategies including cold plasma and microwave for treatment of advanced materials (graphene and AgNWs), the thin graphene-based films fabrication methods developed during this PhD research will provide considerable contribution to the field of high performing conductive films required for conventional electronic devices such as new wearable electronics, flexible displays, solar cells, supercapacitors, energy generations, sensors, electrothermal heaters, electromagnetic shielding and so on.