1. Department of Chemistry, Nebraska Center for Materials and Nanoscience, University of Nebraska−Lincoln, Lincoln, Nebraska 68588, United States
2. School of Physics, Southeast University, Nanjing 211189, China
3. Department of Physics and Astronomy, Nebraska Center for Materials and Nanoscience, University of Nebraska−Lincoln, Lincoln, Nebraska 68588, United States
4. Department of Mechanical & Materials Engineering, University of Nebraska−Lincoln, Lincoln, Nebraska 68588, United States