1. Department of Materials Science and Engineering and California NanoSystems Institute University of California Los Angeles CA 90095 USA
2. College of Chemistry and Chemical Engineering Central South University Changsha 410083 China
3. National Laboratory of Solid State Microstructures School of Physics Collaborative Innovation Center of Advanced Microstructures Nanjing University Nanjing 210093 China
4. CAS Key Laboratory of Organic Solids CAS Research/Education Center for Excellence in Molecular Sciences Institute of Chemistry Chinese Academy of Sciences Beijing 100190 China
5. Department of Materials Science and Engineering National Chiao Tung University Hsinchu 30010 Taiwan
6. Advanced Light Source Lawrence Berkeley National Laboratory Berkeley CA 94720 USA