1. Institute of Flexible Electronics, Northwestern Polytechnical University, Xi’an 710072, Shaanxi, China
2. Department of Physics, State Key Laboratory of Surface Physics and Key Laboratory of Micro and Nano Photonic Structures (Ministry of Education), Fudan University, Shanghai 200433, China
3. Institute of Optoelectronics, Fudan University, Shanghai 200433, China
4. Departamento de Física, Universidade Federal do Ceará, Caixa Postal 6030, 60455-760 Fortaleza, Ceará, Brazil
5. Department of Physics, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium