Isomers Matter: Physicochemical Properties Dictated by the Position of S and Se in Single Layer MoSSe

Layered mixed dichalcogenides possess geometric isomerism when chalcogen atoms (for example, S and Se) occupy different positions in the lattice. Herein, we demonstrate the effect of geometric isomers in tuning physicochemical properties of single-layer hexagonal phase of MoSSe using first-principle calculations. Position of S and Se in the lattice of the molecular units, MoS₆ (MoA₂B₄, MoA₃B₃, MoA₄B₂: A = S, B = Se in trigonal prismatic geometry) leads to nine possible isomers and six different arrangements in a 2 × 2 unit of MoSSe single layer. Variation in the stability of molecular isomeric units is found to be translated to 2D layers of MoSSe. The reported internal strain in MoSSe layer is understood through bonding and angular parameters of the molecular units in MoSSe single layers which are found to deviate from respective pristine dichalcogenide layers. Large distortions are found at square faces rather than triangular faces. These deformations increase the fraction of d orbital population in Mo atoms and lead to changes in certain Mo–Mo distances in single layer MoSSe that contain different isomeric combinations. In the case of equal number of chalcogens in the MoX₆ unit (MoA₃B₃ molecular units), the Mo–Mo distances are found to be nearly the same throughout the lattice (3.245 ± 0.004 Å) while S- and Se-rich molecular configurations (MoA₂B₄, MoA₄B₂: A = S, B = Se) induce substantial elongation and reduction in Mo–Mo distances. All layers are found to be stable; however, electronic properties such as bandgap show variation depending on the isomers that constitute the lattice. The presence of S₂ and Se₂ pairs at bite distances increase electronic conductivity, while SSe increases the stability by enhancing the covalent nature of Mo–S bond. Experimentally synthesized materials may contain all the proposed isomeric combinations in different domains due to small energy differences/bandgap as predicted from calculations. This work reveals the presence and the role of various isomers in single layer MoSSe that may be responsible for the enhanced catalytic and electronic properties reported in the literature.