1. Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada
2. Department of Physics, University of Toronto, Toronto, Ontario M5S 1A7, Canada
3. Department of Physics and Material Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong, China
4. World Premier International (WPI) Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan