Bulk MoS 2 , a prototypical layered transition-metal dichalcogenide, is an indirect band gap semiconductor. Reducing its size to a monolayer, MoS 2 undergoes a transition to the direct band semiconductor. We support this experimental observation by first principles calculations and show that quantum confinement in layered d-electron dichalcogenides results in tuning the electronic structure at the nanoscale. We further studied the properties of related TmS 2 nanolayers (Tm = W, Nb, Re) and show that the isotopological WS 2 exhibits similar electronic properties, while NbS 2 and ReS 2 remain metallic independent on size.

Quantum conductance calculations on the mechanically deformed monolayers of MoS 2 and WS 2 were performed using the non-equlibrium Green's functions method combined with the Landauer-Büttiker approach for ballistic transport together with the density-functional based tight binding (DFTB) method. Tensile strain and compression causes significant changes in the electronic structure of TMD single layers and eventually the transition semiconductor-metal occurs for elongations as large as 11% for the 2D-isotropic deformations in the hexagonal structure. This transition enhances the electron transport in otherwise semiconducting materials.

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The stacking orders in layered hexagonal boron nitride bulk and bilayers are studied using high-level ab initio theory [local second-order Møller-Plesset perturbation theory (LMP2)]. Our results show that both electrostatic and London dispersion interactions are responsible for interlayer distance and stacking order, with AA' being the most stable one. The minimum energy sliding path includes only the AA' high-symmetry stacking, and the energy barrier is 3.4 meV per atom for the bilayer. State-of-the-art density functionals with and without London dispersion correction fail to correctly describe the interlayer energies with the exception of a Perdew-Burke-Ernzerhof functional intended for solid state and surface systems that agrees very well with our LMP2 results and experiment.

The emission and absorption of Cs2AgBiBr6 are dominated by the strong carriers–phonon coupling.