A Systematic Scanning Electron Microscopy Study on MoS2 Layers Using In-lens Detector and Everhart-Thornley Detector

A systematic SEM imaging of partially covered CVD-grown monolayer MoS2 on SiO2/Si substrates and the exfoliated MoS2 on Si substrates by both the in-lens detector and the Everhart-Thornley detector under varying acceleration voltage and working distance has been demonstrated. Superior contrast of MoS2 layers can be feasibly realized with the in-lens detector even at high acceleration voltage of 10 kV and large working distance of 20 mm. In the in-lens images, MoS2 layers appear consistently darker than the bare SiO2/Si substrate, which is a nice example of mass-thickness contrast. For Everhart-Thornley images, a contrast reversal of MoS2 layers was observed. Specifically, at working distance≤10 mm, MoS2 layers are brighter than the bare SiO2/Si substrates, which belong to atomic number contrast; at working distance≥15 mm, MoS2 layers appear darker than the bare SiO2/Si substrates. The underlying mechanism of contrast reversal was revealed. The dependence of MoS2 contrast in in-lens images and Everhart-Thornley images on acceleration voltage and working distance have been discussed by analyzing different responses of three types SE. The findings reported here will have important implications for effective SEM characterizations of MoS2, and other transition metal dichalcogenides.