## 1 Introduction
### Basic SEM Operation
#### 1.2.2 Imaging Process in the SEM  
SEM images are formed by using various signals (Figure 2) collected by the detectors present in the collection chamber. Each signal offers different types of imaging information for the sample [11]. Secondary electron imaging (SEI) is an extensively utilized imaging mode in SEM that produces images by detecting secondary electrons [54]. It provides topographic information such as surface texture, shapes, and features. Backscattered electron imaging (BSEI) is generated by detecting backscattered electrons originating from the surface of the sample due to interaction with the primary electron beam [55]. BSE images indicate compositional contrast with respect to the atomic number of the elements present in the sample (Figure 4). This atomic number contrast provides tremendous value in detecting elements in samples containing a variety of chemical compositions. Elements with higher atomic numbers appear brighter in BSE images, while a darker  
Figure 3: (**Top**): Ilmenite micrographs at various magnification levels (note: scale bars are correct, magnification values are incorrect), aimed at analyzing the existing (**a**) cracks, (**b**) furrows, and (**c**) particle shape in the sample [50]; CC-BY. (**Bottom**): Aspects of monazite crystals: (**a**) colloform, (**b**) acicular, (**c**) massive, and (**d**) as micrometric aggregates, where Mnz = monazite and Mag = magnetite [8]; CC BY-NC-ND with permission (5672760157749) from Elsevier. appearance represents an element with a lower atomic number [56; 57; 58; 59]. This feature is especially helpful for mineralologists and geologists because it allows the identification of various mineral phases present in a rock sample. BSE provides value in analyzing the sample surface topography by showing surface texture, morphology, and roughness, which may not be clear in secondary electron images. Certain features of the sample can be studied by BSE contrast enhancement, as it allows the characterization of subtle compositional variations. In Figure 4, the brightness of In\({}_{x}\)Ga\({}_{1-x}\)As layers (where \(0<x<0.5\)) are higher than those of the GaAs layers [57] since the atomic mass of In (114.818 u) is greater than that of Ga (69.723 u). The difference is faint, but still visually noticeable, and the pixelation of the image can be studied by software to assess the locations and widths of the various layers. Monte Carlo simulations and analytical models that consider single electron scattering and electron diffusion can be applied for BSE intensity-based compositional calculations [57]. Notably, the Pt layer on the top portion of the image has the highest brightness given Pt's large atomic mass (195.084 u). BSE imaging is also helpful in studying nonconductive elements, which is not the case with secondary electron images. BSE imaging can also be coupled with other diffraction and spectroscopy techniques, such as electron backscatter diffraction (EBD) for analyzing the crystallographic properties of materials at a granular level and with energy-dispersive X-ray spectroscopy (EDS) for quantitative mapping of elemental distributions. In Figure 4, this is exemplified by a BSE image of a rock sample that was correlated with EDS of mineral standards to produce the colored image showing in dark blue, blue, light blue, yellow, and red the regions composed of certain mean atomic numbers (\(Z\)) [56], including quartz (SiO\({}_{2}\); \(Z\) = 10.5; #4); cordierite ((Mg,Fe)\({}_{2}\)Al\({}_{4}\)Si\({}_{5}\)O\({}_{18}\); \(Z\) = 12.4; #8), almandine garnet (Fe\({}_{3}\)Al\({}_{2}\)Si\({}_{3}\)O\({}_{12}\); \(Z\) = 13.4; #7), Ti-rich ilmenite (\(Z\) = 17.6; #3, #6, #10), Fe-rich ilmenite (\(Z\) = 19; #2), and zircon (ZrSiO\({}_{4}\); \(Z\) = 23; #1, #5, #9), respectively. When high-energy electrons coincide with the sample, some materials have the tendency to emit light, and imaging that signal is considered cathodoluminescence imaging (CLI) [60]. The luminescent properties and defects of the sample material are revealed by CLI images.