Improved Ordering of Quasi-Two-Dimensional MoS2 via an Amorphous-to-Crystal Transition Initiated from Amorphous Sulfur-Rich MoS2+x

The synthesis of stoichiometric two-dimensional (2D) transition-metal dichalcogenides (TMDC) over large areas remains challenging. Using a combination of X-ray diffraction and X-ray absorption spectroscopy, we demonstrate the advantages of using a thin amorphous layer of S-rich MoS2 (MoS4 in this paper) for the growth of well-ordered crystalline MoS2 films via annealing at 900 °C. In contrast to the crystallization of stoichiometric amorphous MoS2, the crystallization of the as-deposited amorphous MoS4 phase shows the strong preferred ordering of layered MoS2 on a Si/SiO x nontemplating substrate with the dominant (002) crystallographic plane and accompanying Kiessig fringes, which indicate the improved crystallinity of the MoS2 layers. A similar effect can only be achieved by the templated crystallization of an amorphous MoS2 thin film deposited on a c-plane sapphire substrate. We suggest that the crystal growth improvement originates from the lower coordination number (CN) of the Mo atoms in the MoS4 amorphous phase (CN = 4) in comparison with that of amorphous MoS2 (CN = 6) and the gradual release of free sulfur atoms from the thin film during crystallization.