First-principles study on structural, thermal, mechanical and dynamic stability of T'-MoS2

Using first-principles density functional theory calculations, we investigate the structure, stability, optical modes and electronic band gap of a distorted tetragonal MoS2 monolayer (T'-MoS2). Our simulated scanning tunnel microscopy (STM) images of T'-MoS2 are dramatically similar to those STM images which were identified as K-x(H2O)(y)MoS2 from a previous experimental study. This similarity suggests that T'- MoS2 might have already been experimentally observed, but due to being unexpected was misidentified. Furthermore, we verify the stability of T'-MoS2 from the thermal, mechanical and dynamic aspects, by ab initio molecular dynamics simulation, elastic constants evaluation and phonon band structure calculation based on density functional perturbation theory, respectively. In addition, we calculate the eigenfrequencies and eigenvectors of the optical modes of T'-MoS2 at Gamma point and distinguish their Raman and infrared activity by pointing out their irreducible representations using group theory. At the same time, we compare the Raman modes of T'-MoS2 with those of II-MoS2 and T-MoS2. Our results provide useful guidance for further experimental identification and characterization of T'-MoS2.