1. Key Laboratory for UV Light-Emitting Materials and Technology, Northeast Normal University, Ministry of Education, 5268 Renmin Street, Changchun 130024, China
2. National Demonstration Center for Experimental Physics Education, Northeast Normal University, Changchun 130024, China
3. Dipartimento di Elettronica Informazione e Bioingegneria, Italian Universities Nanoelectronics Team Politecnico di Milano, Milan I-20133, Italy