1. School of Electrical and Electronic Engineering; Nanyang Technological University; Singapore 639798 Singapore
2. State Key Laboratory of Precision Spectroscopy; School of Physics and Material Science; East China Normal University; Shanghai 200062 P. R. China
3. School of Material Science and Engineering; Nanyang Technological University; Singapore 639798 Singapore
4. School of Energy and Chemical Engineering; Ulsan National Institute of Science and Technology (UNIST); Ulsan 44919 Republic of Korea