1. Division of Physics and Applied Physics School of Physical and Mathematical Sciences Nanyang Technological University 21 Nanyang Link Singapore 637371 Singapore
2. Department of Applied Physics The Hong Kong Polytechnic University Hung Hom Kowloon Hong Kong 999077 P. R. China
3. Department of Science School of Technology Pandit Deendayal Petroleum University Gandhinagar 382007 India
4. Energy Research Institute @ NTU (ERI@N) Interdisciplinary Graduate Programme Nanyang Technological University 50 Nanyang Avenue, Block S2‐B3a‐01 Singapore 639798 Singapore