In-situ visualization of local fields at a sharp tungsten emitter using low-voltage scanning transmission electron microscope

In this paper, we demonstrate that the local electric field distribution at a sharp tip can be drawn as a contour map and it agrees well with numerical calculations based on finite element method (FEM). In the Rutherford scattering scheme, when the primary electron beam passes close to the tip apex, it is strongly deflected toward the anode by the local field and is interrupted at the orifice edge. Thus, the interruption generates a circular dark shadow in the STEM image (see Fig. 1(b)). In contrast, an electron beam scanned far from the tip apex is slightly deflected, and also reaches the orifice edge, producing the actual orifice shape, as shown in Fig. 2(a). Here, we define the impact factor b as the distance between the shadow boundary and the tip apex, and the deflection angle as the angle between those two electron-beam trajectories.