1. State Key Laboratory of Crystal Materials; Shandong University; Jinan 250100 P. R. China
2. Beijing Institute of Nanoenergy and Nanosystems; Chinese Academy of Science; Beijing 100083 P. R. China
3. Catalonia Institute for Energy Research - IREC; Sant Adria del Besos; Barcelona 08930 Spain
4. Institució Catalana de Recerca i Estudis Avançats - ICREA; Barcelona 08010 Spain