1. Division of Physics and Applied Physics; School of Physical and Mathematical Sciences; Nanyang Technological University; Singapore 637371 Singapore
2. Department of Materials Science and Engineering; Peking University; 100871 Beijing China
3. National Center for Nanoscience and Technology; Chinese Academy of Sciences; Beijing 100190 China
4. Energy Research Institute @ NTU (ERI@N); Nanyang Technological University; 50 Nanyang Drive Singapore 637553 Singapore
5. Singapore-Berkeley Research Initiative for Sustainable Energy; 1 Create Way Singapore 138602 Singapore
6. NOVITAS; Nanoelectronics Centre of Excellence; School of Electrical and Electronic Engineering; Nanyang Technological University; Singapore 639798 Singapore