A portion of this work was supported by the Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under contract DE-AC02-76SF0051 through the SLAC National Accelerator Laboratory, Laboratory Directed Research and Development funding, under contract DE-AC02-76SF00515 (J.N., M. F. T., Y.C.). Y.C. acknowledges support from a King Abdullah University of Science and Technology (KAUST) Investigator Award (No. KUS-l1-001-12). Y.Y. acknowledges financial support from the Stanford Graduate Fellowship (SGF). G.Z. acknowledges financial support from the Agency for Science, Technology and Research (A*STAR), Singapore. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences.