1. Key Laboratory of Graphene Technologies and Applications of Zhejiang Province & Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences; Ningbo 315201 China
2. Nano Science and Technology Institute, University of Science and Technology of China; Suzhou 215123 China
3. Department of Physics, Government College University Faisalabad; Allama Iqbal Road 38000 Faisalabad Pakistan
4. State Key Laboratory of Luminescent Materials and Devices, South China University of Technology; Guangzhou 510640 China
5. Department of Physics, National Sun Yat-Sen University; Kaohsiung 80424 Taiwan