1. Department of Physics, Imperial College London, London SW7 2AZ, United Kingdom
2. Department of Chemistry, University of Duisburg-Essen, Universitätsstrasse 5, 45141 Essen, Germany
3. Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore, Singapore 117583
4. Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China
5. Shenzhen Research Institute, The Hong Kong Polytechnic University, Shenzhen, Guangdong, China
6. SZU-NUS Collaborative Innovation Center for Optoelectronic Science & Technology, Shenzhen, Guangdong 518060, China