Surface sensitivity of atomic-resolution secondary electron imaging

The surface sensitivity of high-resolution secondary electron (SE) imaging is examined using twisted bilayers of MoS2 stacked at an angle of 30°. High-resolution SE images of the twisted bilayer MoS2 show a honeycomb structure composed of Mo and S atoms, elucidating the monolayer structure of MoS2. Simultaneously captured annular dark-field scanning transmission electron microscope images from the same region show the projected structure of the two layers. That is, the SE images from the bilayer MoS2 selectively visualize the surface monolayer. It is noted that the SE yields from the surface monolayer are approximately three times higher than those from the second monolayer, likely attributable to attenuation when SEs emitted from the second layer traverse the surface layer. The surface sensitivity of high-resolution SE imaging is examined using twisted bilayers of MoS2 stacked at an angle of 30°. It was found that the SE images of the MoS2 bilayer visualize the surface monolayer approximately three times more intensely than the second monolayer.