Fluorescence Quenching Microscopy for High-throughput Imaging of Graphene-based Sheets and Their New Application as Surfactant Sheets for Organic Photovoltaics

Graphene oxide (GO) is a chemical exfoliation product of graphite powder. Since its first synthesis in 19th century, GO has been largely viewed as hydrophilic due to its excellent colloidal stability in water. The research described in this dissertation includes studies of GO's interfacial activities, which demonstrate that it is an amphiphile with hydrophilic edges and a hydrophobic basal plane. Like other amphiphiles, GO can act as a surfactant, as evidenced by its capability to adsorb on interfaces and lower interfacial tensions. Due to the ionizable -COOH groups located on its edges, the amphiphilicity of GO is affected by its size and pH. Since the thickness of GO is of typical molecular dimensions while its lateral dimensions can reach up to micrometers, each GO sheet is a single molecule as well as a colloidal particle. Thus, GO can both behave as a molecular surfactant to disperse insoluble materials such as graphite or carbon nanotubes in water, and a colloidal surfactant to create Pickering emulsions. The ease of its conversion to chemically modified graphene renders GO a particularly attractive dispersing agent for electronic applications, as demonstrated by its uses for organic photovoltaic devices.;Meanwhile, being atomically thin, graphene-based sheets (GBS) are very challenging to image. A general visualization method for GBS is highly desirable as it could help advance our understanding on the processing-structure-property relationships. In this dissertation, a new imaging technique for GBS, namely fluorescence quenching microscopy (FQM), is developed based on the strong fluorescence quenching capability of graphitic materials. Vivid details of GBS, such as wrinkles, folds, and overlaps, become highly visible under fluorescent dye layers, as it quenches the emission from nearby dye molecules. FQM is highly versatile and can be used to image GBS deposited on arbitrary substrates or suspended in liquid phase. It can be applied to other 2D materials, as demonstrated with atomically thin MoS2 sheets. Lastly, a study on the materials chemistry of newly emerging inorganic 2D materials is described, with an example of spontaneous gold nanoparticle decoration onto MoS2 single layers for the electrocatalysis of hydrogen evolution reactions.