1. Department
of Electrical and Computer Engineering, University of Nebraska—Lincoln, Lincoln, Nebraska 68588-0511, United States
2. Wuhan
National Laboratory for Optoelectronics, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, China
3. Department
of Physics and Astronomy, Nebraska Center for Materials and Nanoscience, University of Nebraska—Lincoln, Lincoln, Nebraska 68588-0511, United States
4. School
of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China
5. Institut de Chimie de la Matière Condensée de Bordeaux, Avenue du Docteur Albert Schweitzer Pessac F-33608 Cedex, France