PhD Thesis Defence Presentations - ΧΡΗΣΤΟΣ ΤΣΑΚΩΝΑΣ

The isolation of graphene in 2004, was brought to the front a new category of materials with extraordinary properties that have since been called 2D Materials (2DMs). Their unique properties in terms of mechanical strength and flexibility as well as remarkable electrical and thermal conductivity give the perspective of advances in a variety of applications. But for further expansion and using the 2DMs in industrial applications, the main prerequisite is large-scale production in several forms (flakes, wafers, sheets etc.) and in high quality. Among the available production methods that have been employed to produce graphene or other 2DMs, chemical vapour deposition (CVD) seems to be the most ideal method in terms of scalability efficiency and automation.

CVD synthesis of graphene includes the dissociation and adsorption of gas-phase precursors on solid or liquid metal catalysts at high temperatures. Depending on the growth conditions and the specs of the used catalysts which also act as supporting substrates, the quality of the produced material is varied. More specifically, the conventional CVD synthesis of graphene on copper (Cu) foils is usually accompanied by crystalline defects, grain boundaries and surface roughness, which introduce severe imperfections to the growing crystal. In principle, a CVD synthesis is a multi-parameter process where variations in the gas flow, temperature profile and reactor geometry can lead to very dissimilar outcomes. In most cases, the research is focused on the finding of a recipe with specific steps, which leads to high quality graphene. Consequently, for industrial purposes and in order to avoid undesired results, the CVD graphene growth process demands the development of in situ metrology for real-time tailoring and dynamic control of the process which give the potential to adjust the parameters in real-time with respect to the assessment of produced quality. Moreover, the introduction of liquid metal catalysts (LMCats) could overcome some of the main issues in the produced quality. So, many of these defects which appeared with the use of solid metal catalysts are clearly not present in the liquid metals and therefore defect-free synthesis is endorsed. Generally, by taking advantage of the rheological properties of the liquid metal, the rotation and self-assembly of the 2D crystals can be effectively achieved. The weak bonding between graphene and the liquid phase promotes the removal of the grown crystal even at high temperatures.

The purpose of this thesis is the extensive study of production of CVD graphene growth on solid and liquid Cu with the use of in-situ characterization techniques which involves reflectance spectroscopy, Raman spectroscopy and optical microscopy. In Chapter 1 the state of the art of graphene, which is studied in the framework of the thesis, as well as the production methods and special the CVD process is presented. Following, in Chapter 2 the working principle and the instrumentation of the various techniques used, including the various CVD reactors and the used characterization techniques both in-situ and ex-situ, are presented. In Chapter 3 the results which were obtained on graphene growth on solid Cu foils with the reflectance spectroscopy either for in-situ characterization or in-situ kinetic studies are presented. The CVD graphene growth on liquid Cu and the results obtained from different configurations of in-situ Raman spectroscopy measurements, involving a movable probe geometry via fiber optics, an open microscope geometry and the uses of anti-Stokes Raman scattering are presented in Chapter 4. Moreover, the results from real time optical microscopy and how could be used for tailoring and controlling the whole process are included. Afterwards in Chapter 5, it is following the presentation of the detailed characterization of the produced samples in terms of their electrical, structural and mechanical properties as well as their surface morphology.

Exploring the physicochemical properties of CVD graphene on solid metal catalysts in comparison to those grown on liquid metal catalysts, gives the potential for better understanding of the CVD catalytic processes and the perspective for further expansion of the CVD production method towards automation and future commercialization.