60439By adding a non-linear core correction to the well established Dual Space Gaussian type pseudopotentials for the chemical elements up to the third period, we construct improved pseudopotentials for the Perdew Burke Ernzerhof (PBE) [J. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77, 3865 (1996)] functional and demonstrate that they exhibit excellent accuracy. Our benchmarks for the G2-1 test set show average atomization energy errors of only half a kcal/mol. The pseudopotentials also remain highly reliable for high pressure phases of crystalline solids. When supplemented by empirical dispersion corrections [S. Grimme, J. Comput. Chem. 27, 1787 (2006); S. Grimme, J. Antony, S. Ehrlich, and H. Krieg, J. Chem. Phys. 132, 154104 (2010)] the average error in the interaction energy between molecules is also about half a kcal/mol. The accuracy that can be obtained by these pseudopotentials in combination with a systematic basis set is well superior to the accuracy that can be obtained by commonly used medium size Gaussian basis sets in all-electron calculations.

Thin layers of inorganic halide perovskites A n+1M n X3n+1 (n = 1–6, A= Cs, M = Pb and Sn, and X = Cl, Br, and I) have been studied in orthorhombic and cubic phases along with layers of monoclinic CsSnCl3. It is found that one-unit-cell-thick layers have low stability except the monoclinic  CsSnCl3 for which formation energy is slighly less than the bulk value. However, Cs2PbI4 is unstable in both cubic and orthorhombic phases. The formation energy for n > 3 becomes comparable to bulk, but the inclusion of spin–orbit coupling is found to be important for the stabilization particularly for layers with Pb. Importantly, layers of environmentally friendly Sn-based systems have similar values of formation energy in the orthorhombic and cubic phases as well as similar band gaps which make them good materials for solar cell applications as temperature range changes during their operation. The studied 66 cubic and orthorhombic nanosystems have direct band gap (0.6–2.9 eV) using generalized gradient approximation for the exchange-correlation functional, but the use of the HSE06 method increases the band gap. The reduced dimensionality leads to elongation (contraction) of MX6 octahedra perpendicular (parallel) to the plane of the layers and an increase in the band gap. The presence of surface makes the hybridization between M s and X p orbitals near the valence band maximum stronger than in bulk which is good for light absorption. The effective mass of the electrons and holes is very light which augers well for the transport properties. Lead-based systems have larger band gap, and these can be useful in applications such as light-emitting diodes.

Ab initio calculations on one, two, and three layers of SnS and SnSe compound semiconductors show that they all have indirect band gap similar to bulk and it varies in the range of ∼0.5–1.6 eV within the generalized gradient approximation due to quantum confinement as well as structural relaxations. In two‐dimensional structures, the difference between the direct and the indirect band gap is very low and in the case of a SnS bilayer, this difference is minimum. Further, the band gaps calculated with HSE06 functional are in good agreement with the experimental results available for bulk and single layer. The total and projected densities of states show that the top of the valence band arises from the hybridization of Sn 5s and chalcogen p valence orbitals while the bottom of the conduction band has predominantly Sn 5p character. The effective mass of electrons and holes are found to be small. These features are similar to the recently discovered perovskite materials for photovoltaics with high efficiency and suggest that SnS and SnSe layered materials are also promising for photovoltaics. Further calculations on bulk and layers of GeS and GeSe are reported and the results are compared with those of SnS and SnSe structures.

Ab initio calculations on CsSnX3 perovskites and mixed halides CsSn(XxY 1−x)3, X and Y = I, Cl, and Br, show that all of them have a direct band gap of ∼1 eV which can be tuned by varying the compositions of X and Y. The optimized supercells are tetragonal, orthorhombic or monoclinic. The top of the valence band arises from hybridization of Sn 4s and halogen p valence orbitals while the bottom of the conduction band has predominantly Sn p character. Similar to organo-metallic lead halides this is expected to facilitate p-p optical transitions that are highly favourable for photoabsorption. Our results suggest that these inorganic perovskites have the desired features to achieve high efficiency of photo-response with appropriate combination of halogens.

Numerical modeling is used to explain the origin of the large ON/OFF ratios, ultra-low leakage, and high ON current densities exhibited by BEOL-friendly Access Devices (AD) based on Cucontaining MIEC materials [1-5]. Motion of large populations of copper ions and vacancies leads to exponential increases in hole current, with a turn-ON voltage that depends on material bandgap. Device simulations match experimental observations as a function of temperature, electrode aspect-ratio, thickness, and device CD.