1. School of Semiconductor Science and Technology South China Normal University Foshan 528225 P. R. China
2. Guangdong Provincial Key Laboratory of Chip and Integration Technology Guangzhou 510631 P. R. China
3. Guangzhou Key Laboratory for Clean Energy and Materials School of Chemistry and Chemical Engineering Guangzhou University Guangzhou 510006 P. R. China
4. ICFO – Institut de Ciencies Fotoniques The Barcelona Institute of Science and Technology Castelldefels Barcelona 08860 Spain
5. ICREA – Institució Catalana de Recerca i Estudis Avançats Barcelona 08010 Spain