Y.L. acknowledges the financial support of this work in part by NSF (DMR-0847786) and faculty startup funds granted by the University of California, Santa Cruz. Y.L. thanks the UCSC MACS Facility at Ames Research Center in Silicon Valley. We thank Honghan Fei for XRD measurements. J.Z.Z. thanks the BES Division of the U.S. DOE (DE-FG02-ER46232) for financial support. J.R. thanks the UCSC Science Internship Program and The Harker School. C.C.W. thanks the NSFC (21034003) and the Shanghai Committee of Science and Technology, China (10XD1400500) for financial support.