1. Division of Physics and Applied Physics; School of Physical and Mathematical Sciences; Nanyang Technological University; 21 Nanyang Link Singapore 637371 Singapore
2. Energy Research Institute @NTU (ERI@N); Research Techno Plaza; X-Frontier Block; Level 5, 50 Nanyang Drive Singapore 637553 Singapore
3. School of Materials Science and Engineering; Nanyang Technological University; Nanyang Avenue Singapore 639798 Singapore
4. Department of Materials Science and Engineering; University of California; Berkeley CA 94720 USA
5. Laboratory of Photonics and Interfaces; Department of Chemistry and Chemical Engineering; Swiss Federal Institute of Technology; Station 6 CH-1015 Lausanne Switzerland