1. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences; Nanyang Technological University; Singapore 637371 Singapore
2. Energy Research Institute @ NTU (ERI@N); 50 Nanyang Drive Singapore 637553 Singapore
3. School of Materials Science and Engineering; Nanyang Technological University; Singapore 639798 Singapore
4. NOVITAS, Nanoelectronics Centre of Excellence, School of Electrical and Electronic Engineering; Nanyang Technological University; Singapore 639798
5. Department of Materials Chemistry; Huzhou University; Huzhou 313000 P. R. China