Probing the Dynamics of the Metallic-to-Semiconducting Structural Phase Transformation in MoS2 Crystals

We have investigated the phase transformation of bulk MoS2 crystals from the metastable metallic 1T/1T′ phase to the thermodynamically stable semiconducting 2H phase. The metastable 1T/1T′ material was prepared by Li intercalation and deintercalation. The thermally driven kinetics of the phase transformation were studied with in situ Raman and optical reflection spectroscopies and yield an activation energy of 400 ± 60 meV (38 ± 6 kJ/mol). We calculate the expected minimum energy pathways for these transformations using DFT methods. The experimental activation energy corresponds approximately to the theoretical barrier for a single formula unit, suggesting that nucleation of the phase transformation is quite local. We also report that femtosecond laser writing converts 1T/1T′ to 2H in a single laser pass. The mechanisms for the phase transformation are discussed.