Title: Optical studies of intercalated and strongly doped 2D materials
Description: This thesis describes optical microscopical and spectroscopical studies of 2D materials, including graphite/graphene and multilayer/single layer MoS2, under strong charge transfer doping. Under this conceptually unifying umbrella lie many aspects of materials behaviors unique to each of the systems. The strong chemical doping results from intercalation and surface adsorption, and changes the electronic properties of the host 2D materials drastically. Associated with the significant electronic change, aspects such as mass transport, surface reaction, and phase transformation are covered in the following chapters. The first chapter introduces representative members of the 2D materials family, graphene and molybdenum disulphide (MoS2). It briefly reviews the history, discovery and unique properties of each materials class. The other part of the introduction focuses on the main methods utilized in the study of these materials. A concise survey of Raman spectroscopy and optical reflectance contrast spectroscopy will be presented.

The second chapter investigates the intercalation process of Li into bulk graphite. This is a revisit of an extensively studied subject, with a new set of experiments and theories. Here we show that the daunting technical difficulties of disentangling complex electrochemical systems can be cleanly addressed with optical methods with well defined samples. Measuring and understanding the intrinsic transport of Li in graphite electrodes has been a difficult task. The challenge is well recognized to stem from a multitude of simultaneous electrochemical processes as well as systematic heterogeneities in the sample. We distinguish the Li intercalation process in graphite from all other processes, combining optical reflectance microscopy and Raman spectroscopy. The heterogeneity problem is circumvented by using lithography to tailor a single crystal into a defined geometry. We apply two levels of theoretical models to interpret the intrinsic information revealed in our data. Concentration dependent diffusion coefficients are measured, in agreement with theoretical results.

The effects of sample geometry and electrode reaction kinetics on the overall intercalation are elucidated. The third chapter presents the study of lithiation on single and few layer graphene. Raman spectroscopy reveals a high doping level similar in strength to that of the bulk intercalated compound. The optical reflectance imaging, however, shows a different observation from the bulk case. We directly visualize the surface film formation and associated strong doping. The lithiation in single and few layer graphene progresses differently from the bulk graphite, since certain stages of the intercalation compound cannot be sustained by a single or few layer sample. The realization of strong charge transfer doping in lithiated single and few layer graphene could lead to discoveries of interesting physics. The direct visualization of surface film formation could have important implications in the design of electrochemical energy storage systems. The fourth chapter explores the structural effect of strong charge transfer doping in bulk and multilayer MoS2 with optical methods.

MoS2, as a representative material of the transition metal dichalcogenide family, possesses different structural polymorphs. Strong charge transfer doping induces a structural phase change, which goes from the usual thermodynamically stable semiconducting 2H phase into the metallic 1T/1T' phase. The metallic 1T/1T' structure can remain a metastable phase without the stabilization of intercalants. We optically induce the 1T/1T' to 2H phase change and measure the temperature dependent kinetics of the structural phase transformation with in situ Raman spectroscopy. We demonstrate a photolithography technique, which efficiently patterns in-plane coherent heterojunctions between 1T/1T' and 2H MoS2. The fifth and final chapter describes the study of the structural change in single layer MoS2. More spectroscopic methods are employed for characterization, such as photoluminescence spectroscopy and second harmonic generation. The results indicate that the structural change occurs in single layer MoS2 after reaction with n-butyl lithium.

The structural change can be reversed by thermal and laser annealing, similar to the case of bulk and multilayer MoS2. The annealed MoS2 exhibits reduced crystallinity. Future directions to further this work are outlined in the last section.