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Several Na-S solid phases have been considered to be produced in low-temperature Na/S batteries during chargedischarge reactions. We study structural and electronic properties of ¡-S and Na 2 S x (x = 1, 2, 4, and 5) crystals using first-principles calculations. Calculated structural properties are in good agreement with experiments, and the van der Waals interactions have an essential role on the cohesive properties in ¡-S. The results of energetics and electronic structures show that all the calculated Na-S crystal phases are insulating and are stable relative to the elemental Na and S phases. We estimate voltage curves as a function of the Na concentration in the S-cathode of all-solid Na/S batteries, showing mainly three voltage regions at 2.

The band alignments of twin-plane superlattices in semiconductor nanowires are systematically investigated on the basis of density functional calculations. Our calculations demonstrate that for nanowires with small diameters the quantum-confinement effect is prominent within wurtzite structure regions and the energy gap in wurtzite-structured nanowires is remarkably larger than that including zinc-blende structure. This results in the straddling band alignment, in which both electrons and holes are confined in zinc-blende structure region. The analysis using a simple tight-binding methods also clarifies that the straddling band alignments can be realized when the diameters of nanowires are less than 4-8 nm, leading to full control of band alignments by varying the nanowire diameter. Our results provide the ability of band-alignment tuning and open new possibilities for band engineering.

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