1. Department of Chemistry; National University of Singapore; 3 Science Drive 3 117543 Singapore
2. Centre for Advanced 2D Materials and Graphene Research Centre; National University of Singapore; 6 Science Drive 2 117546 Singapore
3. Department of Physics; National University of Singapore; 2 Science Drive 3 117542 Singapore
4. SZU-NUS Collaborative Innovation Center for Optoelectronic Science and Technology; Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province; College of Optoelectronic Engineering; Shenzhen University; Shenzhen 518060 P. R. China