1. Department of Electrical and Computer Engineering, University of Wisconsin-Madison, 1415 Engineering Drive, Madison, Wisconsin 53706, USA
2. Department of Materials Science and Engineering, University of Wisconsin-Madison, 1509 University Avenue Madison, Wisconsin 53706, USA
3. Hunan Key Laboratory of Micro-Nano Energy Materials and Devices, Faculty of Materials, Optoelectronics and Physics, Xiangtan University, Hunan 411105, People's Republic of China