1. State Key Discipline Laboratory of Wide Band Gap Semiconductor Tecchnology Shaanxi Joint Key Laboratory of Graphene Advanced Interdisciplinary Research Center for Flexible Electronics School of Microelectronics Xidian University 2 South Taibai Road Xi'an 710071 China
2. Department of Applied Physical Sciences University of North Carolina at Chapel Hill Chapel Hill NC 27599 USA
3. State Key Laboratory of Solidification Processing Northwestern Polytechnical University Xi'an Shaanxi 710072 P. R. China