1. School of Physical Science and Technology Nanjing Normal University Nanjing 210023 China
2. School of Physics and Key Laboratory of MEMS of the Ministry of Education Southeast University Nanjing 211189 China
3. Institute of Materials Research and Engineering (IMRE) Agency for Science, Technology and Research (A*STAR) 2 Fusionopolis Way, Innovis, #08‐03 Singapore 138634 Singapore
4. School of Physical and Mathematical Sciences Nanyang Technological University Singapore 637371 Singapore
5. Department of Materials Science and Engineering National University of Singapore 9 Engineering Drive 1 Singapore 117575 Singapore
6. School of Electrical and Electronic Engineering Nanyang Technological University Singapore 639798 Singapore