1. Fujian Key Laboratory of Semiconductor Materials and Applications, Department of Physics, Xiamen University, Xiamen 361005, China
2. Program of Material Science and Engineering, University of Arkansas, Fayetteville, AR 72701, USA
3. Department of Environmental Dynamics, University of Arkansas, Fayetteville, AR 72701, USA
4. Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701, USA