1. Department of Physics; Yunnan University; Kunming Yunnan 650091 China
2. Yunnan Key Laboratory for Micro/Nano Materials and Technology; Yunnan University; Kunming Yunnan 650091 China
3. Department of Physics and Engineering Physics; University of Saskatchewan; Saskatoon Saskatchewan S7N 5E2 Canada
4. Department of Materials Science and Engineering; University of Toronto; Toronto Ontario M5S 3E4 Canada