Revealing the role of ripples in phonon modes for MoS2 and MoSe2: Insights from molecular dynamics and machine learning

Abstract

Interatomic potentials for single-layer MoS₂ and MoSe₂ were developed by training an Artificial Neural Network with a reference data set generated using Density Functional Theory. High accuracy was obtained for the phonon dispersion as well as a direct correspondence of the predicted Raman and IR active modes (between 100 cm^-1 and 600 cm^-1) with Molecular Dynamics results. The ability to perform simulations with thousands of atoms allowed our system to accommodate spontaneous fluctuations in height and ripples propagating in the material. As a result of these ripples, our simulations suggest the existence of IR activity in the E^' and A^"₂ modes with inverted polarization, as well as the presence of IR activity at frequencies close to E^" and A^'₁, which are generally considered IR-inactive. We propose that the rippling of the system is expected to be responsible for breaking some of the symmetries of the material, allowing non-trivial IR activity to occur.