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Size-Induced Phase Evolution of MoSe2 Nanoflakes Revealed by Density Functional Theory

The control of the relative stability between trigonal prismatic and octahedral structures in transition-metal dichalcogenides (TMDs) is an important step toward technological applications of 2D TMDs materials, where the electronic properties have a strong dependence on the structural phase and size effects. We report a density functional theory investigation of the size effect on the relative phase stability of stoichiometric (MoSe2)n nanoflakes with parallelogram shape for n = 15, 63, 108, 130, 154, 192. We found that the octahedral phase adopts a distorted configuration, which is driven by the Peierls transition mechanism, and, as expected, the Mo-terminated edges of the trigonal prismatic nanoflakes exhibit a strong reconstruction. Furthermore, for the smallest nanoflakes, the octahedral phase has the lowest energy, but with increasing the nanoflake size, the trigonal prismatic phase becomes the most stable. From our results and analyses, this transition is shown to be mainly caused by a difference in...