1. Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon 999077, Hong Kong (SAR), China
2. Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon 999077, Hong Kong (SAR), China
3. Department of Physics and Organic and Carbon Electronics Lab (ORaCEL), North Carolina State University, Raleigh, North Carolina 27695, United States
4. Energy Institute, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon 999077, Hong Kong (SAR), China