1. Center
for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States
2. Maryland
NanoCenter, University of Maryland, College Park, Maryland 20742, United States
3. Department
of Physics, South University of Science and Technology of China, Shenzhen 518055, China
4. Advanced
Materials Laboratory, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan