1. Department of Chemistry, Hong Kong University of Science and Technology, Clear Water
Bay, Kowloon, Hong Kong, China
2. Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
3. Department of Physics and Organic and Carbon Electronics Laboratories (ORaCEL), North Carolina State University, Raleigh, North Carolina 27695, United States
4. Department of Mechanical Engineering, The University of Hong Kong, Pokfulam, Hong Kong, China