Layer-dependent properties ofSnS2andSnSe2two-dimensional materials

The layer dependent structural, electronic and vibrational properties of SnS2\nand SnSe2 are investigated using first-principles density functional theory\n(DFT). The in-plane lattice constants, interlayer distances and binding\nenergies are found to be layer-independent. Bulk SnS2 and SnSe2 are both\nindirect band gap semiconductors with Eg = 2.18 eV and 1.07 eV, respectively.\nFew-layer and monolayer 2D systems also possess an indirect band gap, which is\nincreased to 2.41 eV and 1.69 eV for single layers of SnS2 and SnSe2. The\neffective mass theory of 2D excitons, which takes into account the combined\neffect of the anisotropy, non-local 2D screening and layer-dependent 3D\nscreening, predicts strong excitonic effects. The binding energy of indirect\nexcitons in monolayer samples, Ex~0.9 eV, is substantially reduced to Ex = 0.14\neV in bulk SnS2 and Ex = 0.09 eV in bulk SnSe2. The layer-dependent Raman\nspectra display a strong decrease of intensities of the Raman active A1g mode\nupon decreasing the number of layers down to a monolayer, by a factor of 7 in\nthe case of SnS2 and a factor of 20 in the case of SnSe2 which can be used to\nidentify number of layers in a 2D sample.\n