Tungsten disulfide (WS2) is a promising two-dimensional material owing to its remarkable optical, electronic, and electrocatalytic behavior. However, morphology of this material varies significantly with growth conditions. In this work, we use salt-assisted low-pressure chemical vapor deposition (LP-CVD) to grow WS2 crystals of a few layers reaching over 50 μm in size on SiO2/Si substrates. We observe a transition from large, dendritic to triangular growth by systematically varying the amount of the NaCl promotor material as well as the presence of intermediate Wx+ states for low NaCl amounts. The transition from dendritic to triangular growth is discussed in the context of diffusion limited aggregation, with the transformation likely being the result of reduced formation energy, owing to increasing concentrations of transition metal oxyhalides for given precursor quantities. These results help to clarify the role of effects of the NaCl precursor in salt-assisted LP-CVD of WS2 and provide a new means to tune the morphology of this material.

Due to its unique electronic and optical properties, tungsten disulfide (WS₂) is a promising material for various device applications. However, achieving an efficient and cost-effective method for synthesizing large-area uniform WS₂ is still challenging. In this work, we demonstrate the synthesis of few-layer WS₂ crystallites by NaCl-assisted low-pressure chemical vapor deposition and study the effect of temperature and the carrier gas flow rate on the morphology, structure, and optical properties of the as-grown WS₂ films. We observe transitions between regular triangular to strongly disordered structures with sizes up to 50 µm through temperature and carrier gas flow rate tuning. As-grown samples were characterized by Raman spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy. The result of this work provides a path toward the optimization of growth conditions for obtaining WS₂ with desired morphologies for various applications.

NaCl has widely been used as a seeding promoter for chemical vapor deposition of large-scale 2D transition metal dichalcogenides. In this work, we report a study of the influence of NaCl on the growth and optical properties of layered CVD-grown WS2 using steady-state and time-resolved Kerr rotation measurements at room temperature. Strong photoluminescence signals from single flakes grown with a low NaCl content indicates direct band-gap emission, whereas flakes grown with higher amounts of NaCl exhibit red-shifted, weaker photoluminescence. Raman measurements from single flakes also indicate that WS2 grown with higher NaCl amounts result in multilayered structures, while lower NaCl quantities yield monolayer WS2. Ultrafast carrier decay measurements from single flakes also indicate a NaCl-dependent on the valley exchange interaction component (< 10 ps) and slower decay components (> 50 ps), attributed to a combination of phenomena, such as the band gap transitioning from direct to indirect and defect-related localized states. Our study provides insight into the influence of seeding promoters in layered CVD-grown WS2 in particular and 2D transition metal dichalcogenides in general.