1. Center of Super-Diamond and Advanced Films (COSDAF), Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, China, School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, China, and Nano-organic Photoelectronic Laboratory, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, China