1. School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
2. Department of Materials Science and Engineering, Feng Chia University, Taichung 40724, Taiwan
3. Institute of Photonics and Optoelectronics and Department of Electrical Engineering, National Taiwan University, Taipei, 10617 Taiwan
4. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, China