Rapid and Large-Area Visualization of Grain Boundaries in MoS₂ on SiO₂ Using Vapor Hydrofluoric Acid

Grain boundaries in two-dimensional (2D) material layers have an impact on their electrical, optoelectronic, and mechanical properties. Therefore, the availability of simple large-area characterization approaches that can directly visualize grains and grain boundaries in 2D materials such as molybdenum disulfide (MoS₂) is critical. Previous approaches for visualizing grains and grain boundaries in MoS₂ are typically based on atomic resolution microscopy or optical imaging techniques (i.e., Raman spectroscopy or photoluminescence), which are complex or limited to the characterization of small, micrometer-sized areas. Here, we show a simple approach for an efficient large-area visualization of the grain boundaries in continuous chemical vapor-deposited films and domains of MoS₂ that are grown on a silicon dioxide (SiO₂) substrate. In our approach, the MoS₂ layer on a SiO₂/Si substrate is exposed to vapor hydrofluoric acid (VHF), resulting in the differential etching of SiO₂ at the MoS₂ grain boundaries and SiO₂ underneath the MoS₂ grains as a result of VHF diffusing through the defects in the MoS₂ layer at the grain boundaries. The location of the grain boundaries can be seen by the resulting SiO₂ pattern using optical microscopy, scanning electron microscopy, or Raman spectroscopy. This method allows for a simple and rapid evaluation of grain sizes in 2D material films over large areas, thereby potentially facilitating the optimization of synthesis processes and advancing applications of 2D materials in science and technology.