Probing graphene grain boundaries with optical microscopy

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Abstract

Grain boundaries in graphene are formed by the joining of islands during the initial growth stage, and these boundaries govern transport properties and related device performance1,2. Although information on the atomic rearrangement at graphene grain boundaries can be obtained using transmission electron microscopy3,4 and scanning tunnelling microscopy2,5,6,7,8, large-scale information regarding the distribution of graphene grain boundaries is not easily accessible. Here we use optical microscopy to observe the grain boundaries of large-area graphene (grown on copper foil) directly, without transfer of the graphene. This imaging technique was realized by selectively oxidizing the underlying copper foil through graphene grain boundaries functionalized with O and OH radicals generated by ultraviolet irradiation under moisture-rich ambient conditions: selective diffusion of oxygen radicals through OH-functionalized defect sites was demonstrated by density functional calculations. The sheet resistance of large-area graphene decreased as the graphene grain sizes increased, but no strong correlation with the grain size of the copper was revealed, in contrast to a previous report9. Furthermore, the influence of graphene grain boundaries on crack propagation (initialized by bending) and termination was clearly visualized using our technique. Our approach can be used as a simple protocol for evaluating the grain boundaries of other two-dimensional layered structures, such as boron nitride and exfoliated clays.

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Figure 1: Observation of graphene grain boundaries (GGBs) after ultraviolet exposure under moisture-rich ambient conditions.
Figure 2: Confocal Raman mapping of GGBs.
Figure 3: Height profiles of various topological GGBs, and the oxidation mechanism.
Figure 4: The correlation between GGBs and sheet resistance.

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Acknowledgements

This work was supported by the Star Faculty programme (2010-0029653), the International Research and Development programme (2011-00242) and the WCU programme (R31-2008-10029) of the NRF of Korea funded by MEST.

Author information

D.L.D. and G.H.H. contributed equally to this work in experiment planning, experiment measurements, data analysis and manuscript preparation. S.M.L. performed the theoretical calculations. F.G. prepared the samples for TEM measurements. The copper grain size was characterized by H.K. SEM and EDS were performed by E.S.K. and J.W.J. S.H.C. and S.C.L. designed the ultraviolet chamber and humidity controller. S.T.K. and S.C.L. performed conductance AFM. K.P.S. designed and performed the bending test. Graphene on nickel was prepared by S.J.C. S.J.Y. performed recovery of sheet resistance after ultraviolet exposure. Q.H.T. prepared the graphene samples for all the experiments. The TEM images were taken by M.H.P. S.M.L. and J.Y.C. contributed to the manuscript preparation. Y.H.L. contributed to experiment planning, data analysis and manuscript preparation.

Correspondence to Young Hee Lee.

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Duong, D., Han, G., Lee, S. et al. Probing graphene grain boundaries with optical microscopy. Nature 490, 235–239 (2012) doi:10.1038/nature11562

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