Monte Carlo simulation of secondary electron and backscattered electron images for a nanoparticle–matrix system

Abstract

Contrast simulation of scanning electron microscope (SEM) images for particle–matrix systems is important for understanding the contrast formation mechanism in such systems. Most of the previous works have been limited to backscattered electron (BSE) signals because of the difficulties involved in the simulation of the secondary electron (SE) signals. This limitation prevents some important information from being extracted from experimental SEM images. A comprehensive and accurate Monte Carlo model has been developed for simulation of electron–solid interactions, which enables the simulation of production and emission of cascade SEs and is then quite suitable for investigating both SE and BSE signals. In this paper, we have modified the simulation model for a particle–matrix system and performed a simulation of SE and BSE images for nano-size Pt particles in a carbon matrix. Since the size of the particles is comparable with the electron scattering mean free path, modification of the conventional Monte Carlo procedure for a solid is necessary in order to include the boundary correction due to different scattering mean free paths. SE and BSE line scans and two-dimensional images under various conditions (particle size, depth and beam incident angle) are obtained and discussed in detail. The results have shown that: (1) In the formation of Pt–C contrast, the material factor plays a dominant role. (2) Only those particles located on, or quite close to, the matrix surface can be clearly revealed in a SE image. The maximum visible depth in SE observation for a Pt particle embedded in carbon matrix is about three times the particle size, while it is twice deeper than that with BSE signals. (3) The maximum SE emission from Pt particles appears at the centre of, instead of the sides of, the particle when the size falls down to the nanometre region; (4) Both the SE and BSE contrast have maximum values at a certain primary energy for particles of a specific size. (5) For oblique incidence of the primary beam, the SE contrast is quite different for the particles placed on the surface from that for those embedded in the matrix. These simulation results on image contrast will be helpful in evaluating the fraction of catalyst particles on the matrix surface and the optimum operating condition for imaging.