1. Innovative Center for Flexible Devices (iFLEX); School of Materials Science and Engineering Nanyang Technological University; 50 Nanyang Avenue 639798 Singapore
2. Key Laboratory of Graphene Technologies and Applications of Zhejiang Province; Ningbo Institute of Materials Technology and Engineering; Chinese Academy of Sciences; Ningbo 315201 Zhejiang P. R. China
3. School of Electronic Science and Engineering; Nanjing University; Nanjing 210093 Jiangsu P. R. China