1. Department of Physics, University of Illinois, Urbana, Illinois 61801, USA
2. Frederick Seitz Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA
3. Institute of Physics, Vietnam Academy of Science and Technology, 10 Dao Tan, Hanoi, Vietnam
4. Department of Materials Science, University of Illinois, Urbana, Illinois 61801, USA
5. Department of Chemistry, University of Illinois, Urbana, Illinois 61801, USA