Light induced reduction of individual graphene oxide films

Abstract

Graphene oxide is a promising two-dimensional material with potential applications in many areas. It is basically a graphene basal plane decorated with various oxygen functional groups. By removing the functional groups through reduction, graphene oxide can be reduced to graphene-like films, hence it is an important precursor in industry for the mass fabrication of graphene products. Through reduction, a wide range of properties (optical, electrical, chemical and many others) of graphene oxide can be readily tuned, making it a highly versatile material in nanoscience and technology. This thesis reports original research work on laser-controlled reduction of individual graphene oxide films. Single flakes of graphene oxide film ranging from one to seven layers were reduced with controlled laser irradiation. Functional groups were removed by the heat generated from laser irradiation. Significant temperature gradients were built up across the atomic sheets of graphene oxide films irradiated by laser, as a result of the low thermal conductivity along the out-of-plane interlayer axis. Simulations suggest temperature gradients up to 100 K/nm were achieved under the illumination of modest laser powers of a few milliwatts. The reduction can be controlled under such a precise way that the film thickness can be accurately thinned with sub-nm steps. This allows tailor-making the properties of the films and custom-designing the functionality of nanodevices with on-demand properties. In this thesis, it is demonstrated that through controlled reduction, the work function of individual graphene oxide films can be tuned with unprecedented precision of only a few milli electronvolts. In a second strand research work, I investigated the adhesion and stability of thin gold films on highly oriented pyrolytic graphite (HOPG). Thin gold films of various thickness (5, 10 and 50 nm) were sputtered on HOPG and annealed at different temperatures (200-500 °C). The films were found to break apart and coalesce into clusters of different morphologies at different temperatures. When films were annealed at 500 °C, the top a few carbon sheets (monolayer and few-layer graphene films) were peeled off by the diffusion movement of coalescing gold clusters, forming terraced surface structures. It demonstrates that this approach could be a useful tool to experimentally characterise the adhesion force between atomic carbon sheets within HOPG, which will help to gain a better understanding of the fundamental science of graphene exfoliation and to devise better exfoliation techniques in the future.

Date of Award	Dec 2021
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Robert Pollard (Supervisor) & Fumin Huang (Supervisor)