1. Institute of Photonics and Optoelectronics and Department of Electrical Engineering, National Taiwan University, Taipei, 10617 Taiwan
2. School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States
3. Department of Materials Science and Engineering
4. Research Center for Energy Technology and Strategy
5. Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan, 701 Taiwan
6. Institute of Theoretical Physics, Lanzhou University, Lanzhou, Gansu 73000, People’s Republic of China