1. SZU-NUS Collaborative Innovation Centre for Optoelectronic Science and Technology; International Collaborative Laboratory of 2D Materials for Optoelectronic Science and Technology of Ministry of Education; College of Optoelectronic Engineering; Shenzhen University; Shenzhen Guangdong 518060 P. R. China
2. Department of Physics; National University of Singapore; Singapore 117551 Singapore
3. Department of Chemistry; National University of Singapore; Singapore 117542 Singapore
4. Centre for Advanced 2D Materials; National University of Singapore; Science Drive 2 Singapore 117546 Singapore