Optical properties and stability of new two-dimensional allotropes of PdS2, PdSe2 and PdSSe monolayers

In this work, we artificially create new metastable allotropes (called 1OT) of the two-dimensional (2D) Pd transition metal dichalcogenides (TMD) PdS2, PdSe2 and PdSSe belonging to the 2D monoclinic (oblique) structure but characterized by the same side view environment as the 1T phase. All three structures demonstrate a semimetallic behavior with a non zero density of states (DOS) at the Fermi level. Linear optical calculations suggest that they have a wider range of absorption spectra than the 1T systems and can emit or absorb within the infrared (IR) spectrum. They are also mechanically stable and should, in principle, possess a smaller thermal lattice conductivity (κL) than their 1T analogues. Molecular dynamics (MD) simulations reveal that the pristine 1OT structures are thermally stable at temperatures higher than 300 K whereas the Janus PdSSe system remains stable up to temperatures close to 600 K and is completely destroyed at 900 K. Therefore these structures should generally be experimentally synthesized at ambient temperatures.