Grain- and Grain Boundary-Featured Etching in Highly Crystalline MoS₂ Islands

This study systematically investigates the anisotropic etching behaviors and mechanisms of chemical vapor deposited (CVD) molybdenum disulfide (MoS₂) islands correlating with various crystal structures. The intra- and intergrain anisotropic oxygen etching in highly crystalline MoS₂ islands results in grain- and grain boundary (GB)-dependent etching characteristics. More importantly, time-resolved etching for these grains and GBs is conducted to investigate the etching process and dynamics of different types of crystal structural features. The tilt boundary (TB) structure is found to be the most reactive to oxygen, followed by vacancy and finally mirror twin boundary (MB), leading to the largest relative etching rate (v_e) of TB for both etching initialization ((3 ± 0.1)% min^-1) and a stabilization phase ((1.97 ± 0.31)% min^-1) based on a 100 μm² area. Density functional theory (DFT) calculations reveal that MB, vacancy, and TB structures exhibit reduced Mo-S bonding strength and increased chemical reactivity of oxygen absorption for etching, causing exclusive etching dynamic features at these inherent structural imperfections. These findings reveal significant anisotropic etching mechanisms of transition metal dichalcogenides (TMDs) correlating with various crystal micro/nanostructures derived from CVD growth, which would shed light on engineering functional applications in areas such as electrocatalysis and optoelectronics.