High-quality CVD graphene synthesis for transparent/flexible OLED and graphene electrode with improved mechanical properties

Abstract

From the early stage of graphene research, copper (Cu) foil has been mostly used in chemical vapor deposition (CVD) method as growth substrate, because of its good catalytic property and self-limiting growth of graphene. However, several-hundred-nm scale valleys in commercial Cu foils, which are usually originated from the rolling process during the Cu foil production, result in lots of residues and voids when the synthesized graphene is transferred onto flat surface. These defects degrade the electrical or mechanical properties of graphene in real device applications. Besides, polycrystalline feature of commercial Cu foil leads to imperfect domain stitching and quality variation in large scale. Therefore, it is important to secure Cu substrate with extremely low roughness and the (111) orientation surface, which has been known as the ideal surface for the graphene growth. In this thesis, it analyzes the process of optimization of Cu foil pre-treatment including nitric acid and pre-annealing which result in roughness reducing of Cu foil down to several-nm scale and unification of surface orientation of Cu foil into (111) single orientation in large scale (5x10 cm$^2$). Besides, enhanced electrical characteristics and coverage after transfer process also included from the modified Cu foil surface. However, this CVD-grown graphene itself still has limitations, which means that graphene do not represent theoretical superior properties, when it is applied to electronic devices due to inherent/process defects. In order to improve this limitations, graphene electrodes which improve electrical characteristics were developed by applying graphene dyad stacking structure applied by using electroplating method, doping with organic acid, etc. to utilize the advantages of graphene with relatively low transmission loss. Furthermore, in light of the results of existing prior researchers, we evaluated whether the corresponding electrode structure is applicable to OLED electrodes. Graphene with electroplated defect healing has improved mechanical properties as a result of prior researchers, but there has been no evaluation of its micro-region. Therefore, force-displacement measurement (nanoindentation) via atomic force microscopy (AFM) was attempted to evaluate mechanical properties for the micro-region. In particular, for graphene to be applied to the devices, measurements and evaluations were conducted on graphene supported by substrate, as the structure supported on the substrate is realistic. In conclusion, nanoindentation method using AFM tips for the micro regions of graphene healed by electroplating was not suitable and we describe the difficulties of force-displacement measurement on graphene supported by substrates in this thesis. Of course, the mechanical properties of graphene will change depending on what material is stacked on the graphene layer, but the results and know-how in this thesis are expected to serve as a foundation for future graphene and two-dimensional material-based electronic device development and fusion.