Graphene-supported ultrathin two-dimensional materials and their applications

Abstract

The flexible and wearable technology has attracted tremendous attention from research field and commercial market in recent years. To ensure the users comfort, wearable devices are preferable to be lightweight and thin, and they are more likely to be built or attached to relatively soft and rough substrates, such as polymeric films, textiles and papers. Unfortunately, most of these flexible materials would deform under high temperature, be contaminated or damaged during chemical treatments. Therefore, two-dimensional (2D) transferrable and functional materials are important for building up the wearable electronics nowadays. Starting from 2004, there has been a number of reports on transferring mono-to-few layers graphene films on different electronic devices as flexible electrodes. Also, functional graphene surface with specific biological and chemical reactivity have been tailor-made via various modifications. This thesis focuses on describing several approaches to fabricate polymer-free transferable, ultrathin and flexible graphene-supported 2D materials. The statement of challenges and objectives of this research is presented in the beginning, followed by a comprehensive literature review on graphene properties and preparation methods, and the modifications that have been done on graphene as well as their applications. For the content about the experiments in this research project, Chapter 4 introduces the grafting of polymer brushes on graphene surface via non-covalent interaction and their application on surface wettability control and biomolecules immobilization. Chapter 5 states the graphene-assisted gold (Au) film transfer and detail characterization of the graphene-mediated grown gold layers. Chapter 6 describes the patterning and transfer processes of the Au@graphene (Au@G) films with their application on photovoltaic devices. Chapter 7 discusses the fabrication and transfer of graphene-supported metal oxide layer, and the micro-supercapacitor supported by the metal oxide@graphene bilayer. Finally, Chapter 8 provides conclusions and future outlooks. Firstly, by adopting a pyrene-group bearing initiator which interacts with graphene basal plane non-covalently through π-π interaction, four kinds of polymer brushes with distinct functionalities were grafted on the chemical vapor deposited (CVD) graphene via self-initiated atomic transfer radical polymerization (SI-ATRP). Copolymer designed with desired functional group with pyrene group as termini was also synthesized and successfully anchored on graphene surface. These fabricated polymer@graphene 2D objects are transparent, flexible, transferrable on various substrates with good stability, and are patternable into different structures. Secondly, ultrathin Au films with various thicknesses were thermally deposited on CVD graphene. The ultrathin Au@G films are transferable without protection and achieve relatively high smoothness and mechanical stability when compared with pure Au films. An unexpected hcp phase Au structure was also discovered from the graphene-mediated grown Au layer. These extraordinary behaviors of graphene-supported Au films were discussed with density functional theory (DFT) stimulation results. Moreover, Au@G meshes were fabricated in aids with photolithography. As the Au@G meshes could be patterned with high optical transmittance and possessed high flexibility, they were integrated in organic solar cells as the top electrode. Moreover, the growth and wet-transfer of metal oxide layers with graphene support were demonstrated. This is the first report on transferable silicon dioxide (SiO2) layer fabricated through plasma-enhanced chemical vapor deposition (PECVD) on CVD graphene. The resulted SiO2@graphene films were found to be highly transparent, thus making it an ideal ultra-thin insulating building block for transferable, transparent thin-film electronics. A planar micro-supercapacitor was built on these SiO2/graphene bilayers and was transferred on various substrates with good stability and flexibility. In conclusion, ultrathin graphene-supported functional organic layers, graphene-supported metal and metal oxide layers were fabricated. Given that these three kinds of graphene-supported ultrathin materials all have their own distinctive properties and functions, the work presented here is expected to have significant impact on physical, chemical, material, biological and engineering fields.