The authors thank A. R. Tanguay and A. Madhukar for discussions, and technical assistance by T. Aoki and N. Bozdin. J.R. acknowledges USC Viterbi School of Engineering Startup Funds and support from the Air Force Office of Scientific Research under award no. FA9550-16-1-0335. S.N. acknowledges Link Foundation Energy Fellowship. M.A.K. acknowledges support from the Office of Naval Research (grant no. N00014-16-1-2556). H.W. acknowledges support from the Army Research Office (grant no. W911NF-16-1-0435) and National Science Foundation (grant no. ECCS-1653870). Work at the University of Missouri (D.J.S.) was supported by the Department of Energy, Basic Energy Sciences through the Solid-State Solar Thermal Energy Conversion Center, an Energy Frontier Research Center, under award no. DE-SC0001299/DE-FG02-09ER46577. S.B.C. acknowledges support from the Department of Energy under award no. DE-FG02–07ER46376. The studies at Air Force Research Laboratory were supported by the Air Force Office of Scientific Research under award no. FA9550-15RXCOR198. The authors acknowledge the use of facilities at the Center for Electron Microscopy and Microanalysis at the University of Southern California and the Irvine Materials Research Institute at the University of California, Irvine.