[1] Peking Univ, Frontiers Sci Ctr Nanooptoelectron, Sch Phys, State Key Lab Mesoscop Phys, Beijing, Peoples R China
[2] Ist Italiano Tecnol, Ctr Nanotechnol Innovat NEST, Pisa, Italy
[3] Nanjing Univ, Sch Elect Sci & Engn, Natl Lab Solid State Microstruct, Nanjing, Peoples R China
[4] Nanjing Univ, Collaborat Innovat Ctr Adv Microstruct, Nanjing, Peoples R China
[5] CNR, Ist Nanosci, NEST, Pisa, Italy
[6] Scuola Normale Super Pisa, Pisa, Italy
[7] Ist Italiano Tecnol, Graphene Labs, Genoa, Italy
[8] Chinese Acad Sci, Inst Phys, Songshan Lake Mat Lab, Dongguan, Peoples R China