Based on all-electron density-functional theory calculations using the generalized gradient approximation, we demonstrate the continuous tunability of the band gap and strength of the built-in electric field in GaN/AlN ͑0001͒ superlattices by control of the thickness of both the well ͑GaN͒ and barrier ͑AlN͒ regions. The effects of strain for these quantities are also studied. Calculations taking into account the self-interaction correction exhibit the same dependence on thickness. The calculated electric field strength values are in good agreement with recent experiments. Spontaneous polarization dominates the contribution to the electric field and the strain-induced piezoelectric polarization is estimated to contribute only about 5-10%.

Atom implantation in graphene or graphene nanoribbons offers a rich opportunity to tune the material structure and functional properties. In this study, zigzag graphene nanoribbons with Ti or Sn adatoms stabilised on a double carbon vacancy site are theoretically studied to investigate their sensitivity to sulfur-containing gases (H2S and SO2). Due to the abundance of oxygen in the atmosphere, we also consider the sensitivity of the structures in the presence of oxygen. Density functional theory calculations are performed to determine the adsorption geometry and energetics, and nonequilibrium Green's function method is employed to compute the current-voltage characteristics of the considered systems. Our results demonstrate the sensitivity of both Ti- and Sn-doped systems to H2S, and the mild sensitivity of Ti-doped sensor systems to SO2. The Ti-doped sensor structure exhibits sensitivity to H2S with or without oxidation, while oxidation of the Sn-doped sensor structure reduces its ability to adsorb H2S and SO2 molecules. Interestingly, oxygen dissociates on the Ti-doped sensor structure, but it does not affect the sensor's response to the H2S gas species. Oxidation prevents the dissociation of the H-S bond when H2S adsorbs on the Ti-doped structure, thus enhancing its reusability for this gas species. Our study suggests the potential of Ti- and Sn-doped graphene in selective gas sensing, irrespective of the sensing performance of the bulk oxides.