1. Department of Materials Science and Engineering, University of California, Los Angeles, Los Angeles, CA 90095, USA.
2. California NanoSystems Institute, University of California, Los Angeles, Los Angeles, CA 90095, USA.
3. Department of Chemistry, University of Washington, Box 351700, Seattle, WA 98195–1700, USA.
4. Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195, USA.
5. Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Luoyu Road 1037, Wuhan 430074, P.R. China.