Introduction: MoSe₂ is an engaging member of the family of transition metal dichalcogenides (TMDCs), which has recently gained considerable attention for various applications in electrochemical, photocatalytic, and optoelectronic systems. However, in-depth studies on the structural stability of the various MoSe₂ polymorphs are still lacking.

Methods and Results: In the present work, the relative stability of different MoSe2 polymorphs are proposed and in-depth analysis of these polymorphs are carried out by employing first-principle calculations based on density functional theory (DFT). Structurally, MoSe2 can be regarded as strongly bonded two dimensional Se–Mo–Se layers or sandwiches which are loosely coupled to one another by relatively weak van der Waals-type forces. Within a single Se–Mo–Se sandwich, the Mo and Se atoms create two-dimensional hexagonal arrays. Depending on the relative alignment of the two Se-atom sheets within a single Se– Mo–Se sandwich, two distinct two-dimensional crystal polymorphs are obtained. From our simulation we observe (see Figure. 1) that the various polymorphs seem to have the same minimum energy, although with a varying range of volume. Which indicates that MoSe₂ can easily be found in any of these variants and 1T and 3Tpolymorphs were only found to be metastable phases and cannot be synthesized. Electronic properties of the involved phases were calculated by employing the hybrid functional of Heyd, Scuseria, and Erhzerhof (HSE06).  The mechanical and dynamical stability of all these phases also investigated by single crystal elastic constant and phonon studies, respectively.

Figure. 1 Calculated total energy as a function of the volume of the unit cell for the different polymorphs of MoSe₂. All the energy volumes are normalized to one formula unit (f.u.). In this abbreviated notation T, H, and R denote trigonal, hexagonal, and rhombohedral symmetries, respectively.  

Keywords: Relative stability, Electronic structure_, Density functional theory.