1. School of Physical and Mathematical Sciences; Nanyang Technological University; Singapore 637371 Singapore
2. State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering; Zhejiang University; Hangzhou 310027 P. R. China
3. School of Materials Science and Engineering; Nanyang Technological University; Singapore 639798 Singapore
4. Energy Research Institute@NTU, Interdisciplinary Graduate School; Nanyang Technological University; Singapore 639798 Singapore