1. Department of Physics and Astronomy and Nebraska Center of Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0299, USA
2. Department of Physics, University at Buffalo, The State University of New York, Buffalo, New York 14260-1900, USA
3. CAS Key Lab of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China
4. Department of Electrical Engineering, University at Buffalo, The State University of New York, Buffalo, New York 14260-1900, USA
5. Department of Physics, University of Nebraska at Omaha, 60th and Dodge Streets, Omaha, Nebraska 68182-0266, USA