In-Column Backscattered Electron Microscopy for a Rapid Identification of the Number of Layers in MoS₂ Nanosheets: Implications for Electronic and Optoelectronic Devices

Abstract

An accurate and rapid identification of the number of MoS 2 layers in a nondestructive manner is essential for implementing MoS 2 -based electronic and optoelectronic devices as their properties and performance depend sensitively on the layer number. This paper reports that scanning electron microscopy imaging with an in-column backscattered electron (BSE) detector coupled with a low acceleration voltage provides layer number-sensitive high contrast for MoS 2 on an insulating SiO 2 substrate, enabling the differentiation of MoS 2 with different layer numbers. The results show that, unlike graphene, where secondary electrons emitted from the SiO 2 substrate are attenuated by graphene, leading to reverse contrast with increasing the layer number, BSEs originating from the elastic scattering of incident primary electrons with Mo and S atoms give rise to a layer number-sensitive compositional contrast for MoS 2 at a low acceleration voltage. Contrary to low-energy secondary electrons, high-energy BSEs are less sensitive to the surface charging effect, which makes BSE imaging at a low acceleration voltage advantageous. This feature renders BSE imaging less noisy and more stable. A statistical analysis of the relationship between the MoS 2 layer number and BSE image contrast demonstrates that the unique relationship between the two allows for a quantitative estimation of the MoS 2 layer number up to four layers.