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Advances in renewable and sustainable energy technologies critically depend on our ability to design and realize materials with optimal properties. Materials discovery and design efforts ideally involve close coupling between materials prediction, synthesis and characterization. The increased use of computational tools, the generation of materials databases, and advances in experimental methods have substantially accelerated these activities. It is therefore an opportune time to consider future prospects for materials by design approaches. The purpose of this Roadmap is to present an overview of the current state of computational materials prediction, synthesis and characterization approaches, materials design needs for various technologies, and future challenges and opportunities that must be addressed. The various perspectives cover topics on computational techniques, validation, materials databases, materials informatics, high-throughput combinatorial methods, advanced characterization approaches, and materials design issues in thermoelectrics, photovoltaics, solid state lighting, catalysts, batteries, metal alloys, complex oxides and transparent conducting materials. It is our hope that this Roadmap will guide researchers and funding agencies in identifying new prospects for materials design.

The optical properties of GaBi x As 1-x (0.04 < x < 0.08) grown by molecular beam epitaxy have been studied by photomodulated reflectance spectroscopy. The alloys exhibit a strong reduction in the bandgap as well as an increase in the spin-orbit splitting energy with increasing Bi concentration. These observations are explained by a valence band anticrossing model, which shows that a restructuring of the valence band occurs as the result of an anticrossing interaction between the extended states of the GaAs valence band and the resonant T 2 states of the Bi atoms.

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The valence band anticrossing model (VBAC) is applied to explain the composition-dependent trends in dilute GaSb x As 1-x and GaBi x As 1-x alloys. Photomodulated reflectance spectroscopy (PR) of GaSb x As 1-x shows a reduction of the fundamental bandgap energy as well as an increase of the spin-orbit splitting energy with increasing x. The VBAC model predicts that these shifts are the result of a hybridization of the extended valence band states of GaAs with the localized states of the impurity atoms, which induce an upward movement of the valence band edge. Extrapolation of the model to other III-(Sb,Bi)-V alloys is also discussed.

We show that at dilute Sn concentrations ͑x Ͻ 10% ͒, the composition dependence of the direct band gap and spin-orbit splitting energies of Sn x Ge 1−x can be described by a valence band anticrossing model. Hybridization of the extended and localized p-like states of the Ge host matrix and the Sn minority atoms, respectively, leads to a restructuring of the valence band into E + and E − subbands. The notably large reduction in the band gap follows from an upward shift in the valence band edge by approximately 22 meV per x = 0.01. These results demonstrate that like III-V and II-VI compound semiconductors, group IV elements may form highly mismatched alloys in which the band anticrossing phenomenon is responsible for their unique properties.

We have investigated the effect of partial isovalent anion substitution in Ga1-xMnxAs on electrical transport and ferromagnetism. Substitution of only 2.4% of As by P induces a metal-insulator transition at a constant Mn doping of x=0.046 while the replacement of 0.4% As with N results in the crossover from metal to insulator for x=0.037. This remarkable behavior is consistent with a scenario in which holes located within an impurity band are scattered by alloy disorder in the anion sublattice. The shorter mean free path of holes, which mediate ferromagnetism, reduces the Curie temperature T_{C} from 113 to 60 K (100 to 65 K) upon the introduction of 3.1% P (1% N) into the As sublattice.

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