1. Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, USA
2. Applied Physics Program, University of Michigan, Ann Arbor, Michigan 48109, USA
3. Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109, USA
4. School of Materials Science and Engineering and Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072, People's Republic of China
5. Department of Physics, Organic, and Carbon Electronics Laboratories, North Carolina State University, Raleigh, North Carolina 27695, USA
6. Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA