1. Department of Electrical and Computer Engineering, University of Wyoming, Laramie, Wyoming 82071, USA
2. Department of Physics and Astronomy, University of Wyoming, Laramie, Wyoming 82071, USA
3. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
4. Ministry of Education Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China