Cu Foil Pre-treatment for High-quality Graphene Synthesis and Low-Defect Transfer in Large Scale AsiaNANO 2016 2P-059 Dae Yool Jung1, Sang Yoon Yang1, Woonggi Hong1, and Sung-Yool Choi1 1School of Electrical Engineering and Graphene Research Center, KAIST, Korea ABSTRACT We optimized of Cu foil pre-treatment which resulted in roughness reducing of Cu foil into sub 10nm and unification of surface orientation of Cu foil into (111) single orientation in large scale (5x10 cm2). Pre-treatment including nitric acid treatment not only enhanced the cleanness and roughness of Cu foil, but also facilitated the surface orientation changing speed and grain size during thermal annealing. Besides, we demonstrated successful transfer of graphene without voids via direct delamination of graphene using polyvinyl alcohol (PVA) layer. I. Introduction III. Results & Analysis From the early stage of graphene research, Cu foil has been mostly used in CVD method as growth substrate, because of its good catalytic property and self-limiting growth of graphene. However, polycrystalline feature in commercial Cu foil results in imperfect graphene domain stitching, graphene quality variation. Besides, several-hundred-nm scale valleys in commercial Cu foil leads to lots of wrinkles, residues, and voids when the synthesized graphene is transferred onto flat surface [1-3]. Therefore, it is important to secure Cu substrate with extremely low roughness and the (111) oriented surface, which has been known as the ideal surface for the graphene growth [4,5]. References [1] J. Cho et al., ACS Nano 5, 3607-3613 (2011). [2] H. I. Rasool et al., Nano Lett. 11, 251-256 (2012). [3] A. W. Tsen et al., Science 336, 1143-1146 (2012). [4] Y. Ogawa et al., J. Phys. Chem. Lett. 3, 219-226 (2012). [5] L. Brown et al., Nano Lett. 14, 5706-5711 (2014). Bare foil Etching by HNO3 ECP HNO3 + ECP Before Annealing AFM Image (20μm x 20μm) Table 2. Roughness changes in pre-treated Cu foil before annealing process (a) (b) (c) (a) (b) Figure 4. EBSD texture mapping of Cu foil after annealing process (a) from bare Cu foil when x=100 (b) from HNO3 treated Cu foil when x=100 (c) from HNO3 treated Cu foil when x=140 (a) (b) Figure 1. OM images of (a) valleys in Cu foil due to rolling process (b) voids and supporting layer residues on transferred graphene along the valleys of Cu foil (growth substrate) II. Experiments Bare foil Etching by HNO3 ECP HNO3 + ECP Treatment condition None 5% HNO3, 30sec 2V, 60sec + 2V, 60sec Figure 5. (a) Raman spectra of synthesized graphene on Cu foil (b) SEM images of graphene domains obtained with 10sec of growth (a) (b) Bare foil HNO3 treated foil ※ Aqueous solution used in electrochemical polishing : DI water 1L + ortho-phosphoric acid 0.5L + ethanol 0.5L + IPA 1L + urea 10g Table 1. Cu foil pre-treatment conditions Figure 6. (a) EBSD texture mapping of 5x10 cm2 HNO3 treated Cu foil (b) AFM image of differently treated Cu foil after annealing process (a) (b) (a) (b) From Bare foil From HNO3 treated foil Figure 2. (a) Photograph of tube furnace with loaded Cu foil (b) Pre-annealing and graphene growth condition after solution based pre-treatment Figure 7. OM images of transferred graphene (on SiO2) via direct delamination using PVA layer from (a) bare Cu foil (b) HNO3 treated Cu foil IV. Conclusion In conclusion, we optimized of nitric acid treatment and annealing process to achieve flat and smooth (111) oriented Cu foil surface (Rq~5nm) in large scale (5x10cm2). Furthermore, we demonstrated the direct delamination of graphene using PVA layer from Cu foil in large scale without serious void defects with the aid of Cu foil pre-treatment. Acknowledgements Figure 3. Metal-etching-free transfer of graphene via direct delamination using PVA layer [6] We acknowledge the financial supports from IT R&D program of MOTIE/KEIT (10044412) and the Nano-Material Technology Development Program (2012M3A7B4049807). References [6] S. Y. Yang et al., Small 11, 175-181 (2015). MNDL (Molecular & Nano Device Lab.), School of Engineering and Graphene Research Center, KAIST 291 Daehak-ro, Yuseong-gu, Daejeon, 34141, Korea *E-mail : sungyool.choi@kaist.ac.kr Phone:+82-42-350-7627 Fax:+82-42-350-7283