1. Jiangsu Key Laboratory of Thin Films, College of Physics, Optoelectronics and Energy, Soochow University, Suzhou 215006, People's Republic of China
2. School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, USA
3. Department of Electrical and Computer Engineering, Texas A&M University, College Station, Texas 77843-3128, USA
4. College of Science, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, People's Republic of China