1. Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China
2. Chemical & Materials Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, MS K8-88, Richland, Washington 99352, United States and Department of Physics, Washington State University, 2710 University Drive, Richland, Washington 99354, United States