Further insights into the Na₂WO₄-assisted synthesis method for WS₂

Two-dimensional (2D) materials have become a hot topic in materials science, electronics, optoelectronics, and other fields. However, the practical applications of 2D materials rely heavily on the reliable synthesis of large-area, high-quality materials, which still poses a significant challenge. In this study, we present a detailed investigation into the Na₂WO₄-assisted synthesis of WS₂. Our findings reveal that the substrate temperature and the sequence and duration of introducing S vapor are critical factors in manipulating the morphology of the WS₂ products. Monolayer, thick film, and one-dimensional nanostructures can be obtained by varying the substrate temperature and the introduction sequence of S vapor. Furthermore, the introduction sequence and duration of S vapor can significantly impact the monolayer films' optical and electrical properties. Films synthesized with the introduction of S vapor before the evaporation of the W source exhibited strong photoluminescence (PL) emission, with a greater contribution from excitons. In contrast, films synthesized with the introduction of S vapor after the evaporation of the W source showed reduced PL emission, with a greater contribution from trions. Additionally, field effect transistors based on films synthesized with the introduction of S vapor before the evaporation of the W source displayed a larger threshold voltage and higher electron mobility. These findings suggest that the Na₂WO₄-assisted synthesis method for WS₂ is highly controllable and pave the way for utilizing these monolayer WS₂ materials for technological applications.