1. Beijing Institute of Nanoenergy and Nanosystems; Chinese Academy of Sciences; National Center for Nanoscience and Technology (NCNST); Beijing 100083 China
2. School of Advanced Materials and Nanotechnology; Xidian University; Xi'an 710071 China
3. Microsystems and Terahertz Research Center; China Academy of Engineering Physics; Chengdu Sichuan 610200 China
4. Institute of Nanoscience and Nanotechnology; School of Physical Science and Technology; Lanzhou University; Gansu 730000 China
5. School of Material Science and Engineering; Georgia Institute of Technology; Atlanta GA 30332 USA