Watching graphene grow: The growth of graphene on metal surfaces can be catalyzed by mobile surface metal atoms. Patera et al. used a high-speed scanning tunneling microscope to image the growth of graphene islands on a nickel surface. High temperatures caused carbon to diffuse to the surface, where mobile nickel atoms catalyzed graphene growth on the edges of islands. Molecular dynamics and density functional theory calculations provide mechanistic insights into the reaction steps. Science , this issue p. 1243

We measure electron localization in different materials by means of a “localization tensor”, based on Berry phases and related quantities. We analyze its properties, and we actually compute such tensor from first principles for several tetrahedrally coordinated semiconductors. We discuss the trends in our calculated quantity, and we relate our findings to recent work by other authors. We also address the “hermaphrodite orbitals”, which are localized (Wannier-like) in a given direction, and delocalized (Bloch-like) in the two orthogonal directions: our tensor is related to the optimal localization of these orbitals. We also prove numerically that the decay of the optimally localized hermaphrodite orbitals is exponential.

The local Zn/Se relative concentration at the interface in ZnSe-GaAs(001) heterostructures synthesized by molecular beam epitaxy was found to be controlled by the Zn/Se flux ratio employed during the early growth stage of ZnSe on GaAs. Correspondingly, the valence band discontinuity varies from 1.20 eV (Zn-rich interface) to 0.58 eV (Se-rich interface). Comparison with the results of first-principles calculations suggests that the observed trend in band offsets is related to the establishment of neutral interfaces with different atomic configurations.