Cu Foil Pre-treatment for High-quality Graphene Synthesis and Low-Defect Transfer in Large Scale

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 [1-3]. 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 [4,5]. Here, we report the optimization of Cu foil pre-treatment which results in roughness reducing of Cu foil into sub 10 nm and unification of surface orientation of Cu foil into (111) single orientation in large scale (5x10 cm2). We optimized the pre-treatment conditions including nitric acid treatment [6] and thermal annealing [5,7] before the CVD graphene synthesis. The nitric acid pre-treatment not only enhances the cleanness and roughness of Cu foil, but also facilitates the surface orientation changing speed and grain size during thermal annealing. Characterization of Cu foil before and after the CVD process was performed by using electron backscatter diffraction (EBSD) mapping and atomic force microscopy (AFM). Furthermore, both conventional wet transfer using metal etchant and metal-etching-free transfer through direct delamination using polyvinyl alcohol (PVA) layer [8] resulted in successful transfer without voids and little residue in large scale. Quality and uniformity of graphene were investigated through Raman spectroscopy, transmittance, Hall effect measurement, and electrical measurement of graphene field effect transistors. Applicability of our work in large scale is expected to contribute to the mass production of high-quality graphene, which is required to the realization of graphene industrialization.