Layer-by-layer thinning of MoS₂ via laser irradiation

Layer-by-layer thinning of molybdenum disulfide (MoS₂) via laser irradiation was examined using Raman spectroscopy and atomic force microscopy. In particular, the effects of number of layers, laser conditions, and substrate were systematically identified. The results demonstrated the presence of nanoparticles on the MoS₂ at sufficient laser treatment conditions prior to layer-by-layer thinning. The volume of nanoparticles was found to increase and then decrease as the number of MoS₂ layers increased; the non-monotonic trend was ascribed to changes in the thermal conductivity of the film and interfacial thermal conductance between the film and substrate with number of layers. Moreover, the volume of nanoparticles was found to increase as the magnification of the objective lens decreased and as laser power and exposure time increased, which was attributed to changes in the power density with laser conditions. The effect of substrate on nanoparticle formation and layer-by-layer thinning was investigated through a comparison of freestanding and substrate-supported MoS₂ subjected to laser irradiation; it was illustrated that freestanding films were thinned at lower laser powers than substrate-supported films, which highlighted the function of the substrate as a heat sink. For conditions that elicited thinning, it was shown that the thinned areas exhibited triangular shapes, which suggested anisotropic etching behavior where the lattice of the basal plane was preferentially thinned along the zigzag direction terminated by an Mo- or S-edge. High-resolution transmission electron microscopy of freestanding MoS₂ revealed the presence of a 2 nm thick amorphous region around the laser-treated region, which suggested that the crystalline structure of laser-treated MoS₂ remained largely intact after the thinning process. In all, the conclusions from this work provide useful insight into the progression of laser thinning of MoS₂, thereby enabling more effective methods for the development of MoS₂ devices via laser irradiation.