1. Division of Physics and Applied Physics; School of Physical and Mathematical Sciences; Nanyang Technological University; Singapore 637371 Singapore
2. State Key Laboratory of Silicon Materials; Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, 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