Molybdenum disulfide (MoS₂) can be an interesting two-dimensional material with uses, e.g., in electronics or photodetectors. For many applications, high-quality 2D crystals with very few defects are necessary. Chemical vapor deposition (CVD) is often used to prepare 2D transition metal dichalcogenides such as MoS₂. However, the products can suffer from defects— in the case of MoS₂, mostly vacancies where a sulfur atom should be. These defects can reduce the material’s performance, e.g., by reducing charge carrier mobility.

Bilu Liu, Tsinghua University, Shenzhen, China, and colleagues have used single-atom sulfur (S₁) as a highly reactive sulfur source that can prevent the formation of sulfur vacancy defects during the growth of MoS₂ and, thus, provide high-quality 2D crystals. The team used sulfurized polyacrylonitrile (SPAN) from lithium-sulfur battery waste as a source of this single-atom sulfur. They used SPAN as the sulfur precursor to grow monolayer MoS₂ via CVD, heating it to 400 °C to selectively release S₁.

The resulting monolayer MoS₂ showed an ultralow defect density and good optical properties with a narrow photoluminescence emission peak. Compared to two-dimensional MoS₂ grown using simple sulfur (i.e., S₈) powder, the crystals are of much higher quality. The team’s strategy might also be useful for the growth of other high-quality 2D transition metal dichalcogenides.

• Exclusive Generation of Single-Atom Sulfur for Ultrahigh Quality Monolayer MoS₂ Growth,
  Yunhao Zhang, Jingwei Wang, Yumo Chen, Xian Wu, Junyang Tan, Jiarong Liu, Huiyu Nong, Liqiong He, Qinke Wu, Guangmin Zhou, Xiaolong Zou, Bilu Liu,
  J. Am. Chem. Soc. 2024.
  https://doi.org/10.1021/jacs.4c10810