1. Department of Materials and Optoelectronic Science/Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan
2. Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China
3. Paul-Drude-Institut für Festkörperelektronik, 10117 Berlin, Germany