Ab initio study of saddle-point excitons in monolayer SnS2

Abstract

Monolayer SnS2 has emerged as a promising visible-light photocatalyst for photoelectrochemical applications, owing to its strong optical absorption in the visible range and excellent chemical stability. Despite its reduced dimensionality - where excitonic effects are expected to be pronounced - comprehensive theoretical investigations of bound excitons in this material remain scarce. Notably, unlike most two-dimensional hexagonal crystals, monolayer SnS2 exhibits its lowest single-particle transition at the M point of the Brillouin zone (BZ). Here, the electronic valence bands form a saddle point while conduction states display a minimum with pronounced anisotropy, creating a distinctive band topology whose impact on optical excitations has not yet been systematically explored. In this work, we present a first-principles study of bound excitons in monolayer SnS2 based on state-of-the-art many-body perturbation theory, employing the GW approximation and the Bethe-Salpeter equation (BSE). We analyze how band symmetry and anisotropy shape the excitonic wavefunctions and transition dipole moments. By resolving the exciton dipoles in momentum space for different linear light polarizations, we demonstrate that linearly polarized light lifts the C3 rotational symmetry relating the three inequivalent M points, giving rise to three linearly independent excitonic states. This polarization-selective coupling, previously identified for saddle points in graphene, is achieved in SnS2 for bound excitons and provides a potential route toward state encoding schemes in valleytronics applications.

Figures (9)

• Figure 1: (a) Top and side views of the T-phase monolayer SnS$_2$. The gray line delineates the unit cell. Atomic species (color): Sn (blue) and S (yellow). (b) BZ of the hexagonal Bravais lattice and high-symmetry points used in the band-structure plots.
• Figure 2: Band structure of T-phase monolayer SnS$_2$ (a) and DFT-PBE PDOS (b). The arrows in (b) indicate the indirect and direct band gaps.
• Figure 3: GW@DFT-PBE band structure of T-phase monolayer SnS$_2$ along $\Gamma$M$\Gamma^\prime$ (a) and KMK$^\prime$ (b). The black solid (dashed) lines are the fitted parabolas used to estimate the hole (electron) effective masses $m^*$, which are given in terms of the rest mass of the electron $m_e$. Panel (c) reports the energetic landscape of the GW@DFT-PBE direct band gaps across the BZ, which shows the geometry of the lowest energy regions around $\Gamma$ and M.
• Figure 4: BSE optical absorption spectrum of monolayer SnS$_2$ for in-plane (x, y) electric-field polarization: red dots and bars indicate the optical excitation energies and associated oscillator strengths, the light-green curve is the imaginary part of the 2D polarizability with a Lorentzian broadening of 0.01 eV, and the dotted lines indicate the direct and indirect GW band gaps. The inset shows the symmetries of the excitons in terms of irreducible representations of the $D_{3d}$ point group.
• Figure 5: Single-particle contributions to the optical in Figure \ref{['SnS2-BSE']}: projected on the DFT band dispersion plot along $\Gamma$MK$\Gamma$ (a)-(d), and on the BZ (a1)-(d1) and (a2)-(d2).