Layer-dependent properties of SnS2 and SnSe2 novel two-dimensional materials

TL;DR

This study uses first-principles calculations to explore how the structural, electronic, vibrational, and excitonic properties of SnS2 and SnSe2 change with layer thickness, revealing layer-dependent band gaps, exciton binding energies, and Raman spectra.

Contribution

It provides detailed first-principles insights into the layer-dependent properties of SnS2 and SnSe2, highlighting their potential for 2D material applications.

Findings

Bulk SnS2 and SnSe2 are indirect band gap semiconductors.

Monolayer SnS2 and SnSe2 have increased indirect band gaps of 2.41 eV and 1.69 eV.

Raman active mode intensities decrease significantly with fewer layers.

Abstract

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