We thank Miss SS Li for valuable discussions in sample fabrication.This work was financially supported by the National Natural Science Foundation of China (Grant Nos.51290271,11474364 and 51290273),the National Key Basic Research Program of China (Grant Nos.2013CB933601 and 2013YQ12034506) and the Guangdong Natural Science Funds for Distinguished Young Scholars (Grant No.2014A030306017).ZZ performed the theoretical calculations and nanoimaging experiments and analyzed the data.HF and JL helped fabricate the graphene nanostructures.TM and WR prepared the single-crystalline monolayer graphene samples.JC,JS,YZ and FL helped discuss the results.HC,SD and NX conceived the study,supervised the project,discussed and interpreted the results,and co-wrote the manuscript. （Grant Nos.51290271,11474364 and 51290273）