1 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, USA
2 Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, China
3 School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332, USA
4 Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology, and Research), Singapore 117602, Singapore
5 School of Electrical and Electronic Engineering, Luminous! Center of Excellence for Semiconductor Lighting and Display, Nanyang Technology University, Singapore 639798, Singapore
§ These authors contributed equally to this work.