1. Department of Materials Science and Engineering California NanoSystems Institute University of California Los Angeles CA 90095 USA
2. Department of Materials Science and Engineering Center for Emergent Functional Matter Science National Chiao Tung University Hsinchu 30050 Taiwan
3. Department of Materials Science and Engineering College of Engineering Key Laboratory of Polymer Chemistry and Physics of Ministry of Education Peking University Beijing 100871 P. R. China
4. Laboratory of Advanced Optoelectronic Materials College of Chemistry Chemical Engineering and Materials Science Soochow University Suzhou 215123 P. R. China
5. Advanced Light Source Lawrence Berkeley National Laboratory Berkeley CA 94720 USA