1. Department of Chemical Engineering Columbia University New York NY 10027 USA
2. National Synchrotron Radiation Laboratory CAS Center for Excellence in Nanoscience University of Science and Technology of China Hefei Anhui 230029 P. R. China
3. School of Materials Science and Engineering Nanyang Technological University Singapore 639798 Singapore
4. Chemistry Division Brookhaven National Laboratory Upton NY 11973 USA
5. Center for Functional Nanomaterials Brookhaven National Laboratory Upton NY 11973 USA
6. Department of Physics Florida A and M University Tallahassee FL 32307 USA