Exploring Screw Dislocations in Layered Transition Metal Dichalcogenides

When a screw dislocation occurs during the growth process, it generates three-dimensional continuous spiral steps in the van der Waals layered transition metal dichalcogenides (TMDs). Besides their two-dimensional monolayers, these dislocation-driven layered TMDs are also being explored as potential materials for nonlinear optics and quantum electronics. Understanding the growth kinetics of these materials is crucial for the development of lab-on-chip devices. In this study, we report the growth of layered WS₂, one of the popular TMDs, driven by screw dislocations using the NaCl-promoted atmospheric pressure chemical vapor deposition technique on SiO₂/Si substrate. The growth process was systematically explored by varying the crystal growth temperature, nucleation time, catalyst concentration, and argon carrier gas flow rates. Atomic force microscopy images reveal crystallographic step edges with heights ranging from a single monolayer (~0.7 nm) to several layers (~3 nm) between surface terminations, observed in hexagonal and triangular shaped screw dislocation patterns. Sharp dominant Raman spectra were detected at 419 cm^-1 in the center of the patterns and at 349 cm^-1 near the edges. Powder x-ray diffraction shows the strongest peak at =14.40, corresponding to the (002) plane. We discuss the potential for controlling the formation of screw dislocations in TMDs.