Pulsed Laser-Controlled Grain Boundary Engineering in MoS₂ for Enhanced NO₂ Gas Sensing

Precise control over defect structures in two-dimensional (2D) transition metal dichalcogenides (TMDCs) remains a critical challenge for high-performance gas sensors, as sensing characteristics are strongly governed by the density and distribution of adsorption-active sites. Among various defect types, grain boundary is of particular importance because they form interconnected networks of coordinatively unsaturated sites that enable both enhanced gas adsorption and efficient modulation of charge transport. However, practical strategies for controlling density of grain boundary in a simple and scalable manner remain limited, as it is primarily determined during nucleation and subsequent grain growth processes. Here, we report a pulsed laser-directed synthesis approach for controlling the density of grain boundary in MoS₂ films. The laser pulse repetition frequency serves as a key parameter that dictates the energy distribution per pulse, thereby tuning the balance between nucleation and grain growth in a single processing step. As a result, the average grain size is reduced from about 27 to 8 nm with increasing pulse repetition frequency, accompanied by a significant increase in the density of grain boundary. Consistent with these structural changes, the NO₂ gas sensing response is enhanced from about 10 to 48 %, demonstrating a clear correlation between the density of grain boundary and sensing performance. These results establish grain boundary as the dominant active sites governing gas adsorption and charge transfer in MoS₂-based sensors and provide a scalable strategy for defect engineering in 2D TMDCs-based sensing platforms.