Investigation of dendritic structures forming during chemical vapour deposition growth of graphene Istanbul Technical University, Department of Physics, Maslak, 34469, Istanbul, TURKEY Umut Kamber, Cem Kıncal, Elif Peksu, B. Gamze Arslan, Dilek Yıldız and Oğuzhan Gürlü* http://nanobees.fizik.itu.edu.tr (http://www.nanobees.web.tr) GRAPHENE CHEMICAL VAPOUR DEPOSITION Graphene is a single sheet of carbon atoms with hexagonal lattice. It was first studied theoretically as a two dimensional case of graphite [1]. Graphene has large electron mobility even under ambient conditions with a peculiar band structure. In addition to that, high optical transparency and mechanical strength makes graphene significant for future technologies [2, 3]. Ar & H2 + CH4 T ≈ 1000°C Quartz Tube Copper Foil Quartz Plate A common method to produce graphene is the Chemical Vapour Deposition (CVD) of graphene on metal surfaces. With this method, producing relatively large area single layer graphene flakes is possible [4, 5]. We built a CVD setup. Copper foils are heated up to a temperature of 1000°C and annealed at this temperature for crystallization in Hydrogen-Argon atmosphere. After that, graphene is grown by decomposition of methane at high temperature on metal substrate. Figure 1: Honeycomb lattice of graphene OXIDIZATION OF COPPER DENDRITIC STRUCTURES 100 µm a b Low H2 Contribution Growth High H2 Contribution Growth 2 µm 1 µm 40 µm 1 µm 10 µm c 50 µm Figure 2: (a) Optical images of annealed copper foil without graphene and (b) graphene on copper, (c) EBSD IPF-Z measurement of annealed copper foil Copper is immediately oxidized under ambient conditions. However, Cu surfaces with graphene on it is distinctively and fully protected from oxidization. When copper was annealed, it is crystalizing variously. The amount of oxidization is clearly dependent on the type of the Cu surface . Also, different orientations can affect the uniformity of graphene. Figure 4: Scanning Electron Micrographs of dendritic structures on graphene/Cu samples. Figure 5: Scanning Electron Micrographs of dendritic structures on graphene/Cu samples. 20 µm These exciting structures were obtained by varying the deposition parameters. Their shape and coverage strongly depend on the amount of H2 used during the CVD process. Surface is fully covered with dendrimers when graphene was grown under high H2 contribution. 1 µm a 1 µm b c 1 µm 1 µm g Figure 3: Raman spectrum of G/SiO2 LOW METHANE CONTRIBUTION 100 µm 10 µm a b 2 µm d e 2 µm f 500 nm Figure 8: (a) Optical image and (b) Scanning Electron Micrographs of graphene/Cu samples which were grown under low methane contribution. 500 nm There should be a minimum amount of carbon to start forming graphene on surface. When methane contribution is not enough to crystalizing, bulk carbon is filled some surface orientations and protect from oxidization. Transferred Figure 6: (a, d) Scanning Electron Micrographs and (b, e) AFM topography and (c) phase image of dendritic structures on graphene/Cu sample. (f) Line profile of dendritic structure along with green line. (g) Scanning Electron Micrographs of graphene/Cu surface without dendrimes. CONCLUSION Graphene layer on the copper prevents it from oxidizing. Because of the various crystalization of copper surface after annealing, graphene is not grown uniformly on some surface orientations. Some surface orientations are more favourable for carbon. We showed that there should be a minimum value of amount of carbon to start forming graphene on surface. We observed dendritic structures on graphene/Cu samples. Their shape and coverage strongly depend on the amount of H2 used during the CVD process. Figure 7: Scanning Electron Micrographs of dendritic structures on graphene/SiO2 samples. We also observed dendritic structures with Atomic Force Microscopy. These structures can be transferred on dielectric surfaces together with graphene. FUTURE WORK REFERENCES ACKNOWLEDGEMENTS Clarify what these dendritic structures are Improve CVD system and find best conditions for growing single layer large area graphene sheets Determine the minimum value of amount of carbon to start forming graphene during CVD   [1] Wallace P. R., Phy. Rev., vol.71, no.9, p.622-634, (1947). [2] Geim A. K., Novoselov K. S., Nature Materials, vol.6, p.183-191, (2007). [3] Vlassiouk I., et al, Carbon, vol. 54, p.58–67, (2013) [4] Tao L., et al, ACS Nano, vol.6, no.3, p.2319-2325, (2012). [5] Gao L., et al, App. Phy. Lett., 97, 183109, (2010). We thank to Assoc. Prof. Özgür Birer and Dr. Barış Yağcı from KOÇ University for SEM and Raman measurements. Also, we thank to Yzb. Rafet Sayar, Ebubekir Erdoğan and Ünal Küçükel from Kuleli Military High School for their helps in building our CVD setup. Contact: kamberu@itu.edu.tr kincal@itu.edu.tr gurlu@itu.edu.tr*