2, University of Minnesota, Minneapolis, Minnesota, United States
Chemical vapor deposition (CVD) growth of graphene has intrigued many research efforts as it enables high-quality graphene in very large scale with relatively low cost . However, the transition metal substrate (typically copper) on which the graphene is grown can still degrade the graphene quality in many ways [2, 3]. One imperfection on CVD graphene is the embedded ripple pattern caused by the relaxation of local compressive strain induced by thermal expansion mismatch between graphene and the transition metal [4, 5, 6]. In this work, we show that this ripple pattern remains after wet transfer of graphene onto a SiO2 substrate and is a major source of disorder. We analyze the non-idealities induced by the ripple pattern using atomic force microscopy, scanning electron microscopy, and Raman spectroscopy. Particularly, Raman mapping of transferred single-layer graphene shows spatially periodic patterns in the 2D and G peak positions. By using the decomposition method described in  we show that the ripple pattern creates periodic variations in the (hole) carrier concentration and strain levels and quantify their magnitudes. The potential variation induced by the doping disorder agrees with results from electrical characterization . We also show that thermal annealing of the transferred CVD graphene increases both the strain and the strain-induced disorder, while having little effect on the doping disorder. Finally, the effect of surface functionalization with 1-pyrene-methylamine on the ripple pattern is investigated. Raman mapping shows that the functionalization eliminates the ripple pattern, while also reducing the overall hole carrier concentration. This latter result is consistent with Dirac point shifts obtained from electrical measurements of graphene capacitor structures. These results provide valuable guidance for the optimization of performance and uniformity of a wide range of graphene-based nanodevices. The authors acknowledge funding from Boston Scientific Corporation. Portions of this work were also carried out in the University of Minnesota Characterization Facility, which received capital equipment funding from the University of Minnesota MRSEC under NSF Award DMR-1420013.
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