1. Country State Key Laboratory for Superlattices and Microstructures; Institute of Semiconductors; Chinese Academy of Sciences; P.O. Box 912 Beijing 100083 China
2. Nano-Science Center & Department of Chemistry; University of Copenhagen; Universitetsparken 5 DK-2100 Copenhagen Ø Denmark
3. National Renewable Energy Laboratory; 1617 Cole Boulevard Golden CO 80401 USA