1. State Key Laboratory of Solidification Processing Center for Nano Energy Materials School of Materials Science and Engineering Northwestern Polytechnical University and Shaanxi Joint Laboratory of Graphene (NPU) Xian 710072 P. R. China
2. Chongqing Innovation Center Northwestern Polytechnical University Chongqing 401135 P. R. China
3. School of Microelectronics State Key Discipline Lab of Wide Band Gap Semiconductor Technology Shaanxi Joint Key Lab of Graphene Advanced Interdisciplinary Research Center for Flexible Electronics Xidian University Xian 710071 P. R. China