Light-Matter Interactions in Two-Dimensional Transition Metal Dichalcogenides: Dominant Excitonic Transitions in Mono- and Few-Layer MoX$_2$ and Band Nesting

Abstract

We report ab initio calculations of the dielectric function of six mono and bilayer molybdenum dichalcogenides based in a Bethe Salpeter equation+G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> W <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> (BSE@G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> W <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ) ansatz, focussing on the excitonic transitions dominating the absorption spectrum up to an excitation energy of 3.2 eV. Our calculations suggest that switching chalcogen atoms and the strength of interlayer interactions should affect the detailed composition of the high “C” peaks in an experimental optical spectra of molybdenum dichalcogenides and cause a significant spin-orbit-splitting of the contributing excitonic transitions in monolayer MoSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and MoTe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . This can be explained through changes in the electronic dispersion around the Fermi energy along the chalcogen series S→Se→Te that move the van-Hove singularities in the density of states of the two-dimensional materials along the Γ-K line in the Brillouin zone. Further, we confirm the distinct interlayer character of the “C” peak transition in few-layer MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> that was predicted before from experimental data and show that a similar behavior can be expected for MoSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and MoTe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> as well.