1. School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore
2. Division of Physics & Applied Physics, School of Physical and Mathematics Science, Nanyang Technological University Singapore, Singapore 639798, Singapore
3. College of Materials and Chemical Engineering, Key Laboratory of Inorganic Nonmetallic Crystalline and Energy Conversion Materials, China Three Gorges University, Yichang, Hubei 443002, P.R. China