1. Division of Physics and Applied Physics School of Physical and Mathematical Sciences Nanyang Technological University 21 Nanyang Link Singapore 637371 Singapore
2. Institute of Fluid Physics CAEP P. O. Box 919‐106 Mianyang 621900 P. R. China
3. College of Physics and Information Engineering Institute of Micro‐Nano Devices and Solar Cells Fuzhou University Fuzhou 350108 P. R. China
4. School of Material Science and Engineering Kunming University of Science and Technology Kunming Yunnan 650093 P. R. China
5. Hwa Chong Institution (College) Singapore 269734 Singapore
6. Centre for Materials Science School of Chemistry and Physics Queensland University of Technology Brisbane Queensland 4001 Australia
7. Department of Materials Science and Engineering National University of Singapore Singapore 117575 Singapore
8. School of Electrical and Electronic Engineering Nanyang Technological University 50 Nanyang Ave Singapore 639798 Singapore