1. Department
of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3255, United States
2. School
of Chemistry and Biochemistry and Center for Organic Photonics and
Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
3. National Center for Nanoscience and Technology, Beijing, 100190, People’s Republic of China
4. Department
of Physics, Wake Forest University, Winston-Salem, North Carolina 27019-7507, United States
5. School
of Materials Science and Engineering, Nanyang Technological University, Singapore, Singapore 639798
6. Department
of Chemistry, Wake Forest University, Winston-Salem, North Carolina 27109, United States