All bulk and surface calculations employ periodic DFT calculations, and are carried out using using Quantum Espresso, an open source software package. Calculations are performed at the GGA level 34 using the PBE-GGA exchange correlation functional. The level of DFT that is used here has been shown to underestimate the bandgaps, but has been shown previously to capture the qualitative trends we are studying. All atoms are represented as ultrasoft GBRV-type pseudopotentials, and all calculations use a plane-wave cutoff of 40 Ry for the wavefunction and 320 Ry for the charge density. The convergence criterion for self-consistent relaxations was a maximum residual force of 5 meV/ Å per atom, and all atoms are allowed to relax during structural optimizations. The bulk cells are based on a