1. Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, CA 90095, USA.
2. Physics Department, Lancaster University, Lancaster LA1 4YB, UK.
3. Stratingh Institute for Chemistry and Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, Groningen 9747 AG, Netherlands.
4. Department of Materials Science and Engineering, University of California, Los Angeles, Los Angeles, CA 90095, USA.
5. Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education, School of Physics and Electronics, Hunan University, Changsha 410082, China.
6. California NanoSystems Institute, University of California, Los Angeles, Los Angeles, CA 90095, USA.