The authors thank L. Tan, F. Zheng, E. J. Mele, J. B. Baxter and I. Grinberg for discussions. J.E.S. acknowledges the support of the US Army Research Office through grant no. W911NF-14-1-0500. A.M.R. acknowledges the support of the Department of Energy through grant no. DE-FG02-07ER46431. Y.Q. and C.J.H. acknowledge the support of the Office of Naval Research through grant no. N00014-14-1-0761. S.M.Y. was supported by a National Research Council Research Associateship Award at the US Naval Research Laboratory. The authors acknowledge core materials characterization facilities at Drexel for access to electron and focused ion beam microscopy, including instrumentation supported by the National Science Foundation under grant no. DMR 0722845. The authors also acknowledge additional support from the National Science Foundation and the Semiconductor Research Corporation under the Nanoelectronics in 2020 and Beyond Program under grant no. DMR 1124696.