## 1 Introduction
(a) Low-angle scattering: very little energy loss is experienced by the incident electrons and they scatter to the next layer of atoms; (b) high-angle (or back) scattering; (c) emission of characteristic X-rays and a secondary electron; (d) emission of an Auger electron and a secondary electron. depth examination of the specimen compared to OM. Surface smoothness affects the quality of micrographs taken with OM, as high-magnification OM possesses a very low depth of field. SEM, on the other hand, has a large depth of field that benefits simultaneous focus on the specimen surface, irrespective of surface roughness [23]. SEM has the possibility to go beyond analyzing the surface topography [24], providing information about the chemical composition [25], crystal structure [26], and electrical properties [27]. Confidence in the analysis can be further gained by switching between different imaging techniques, which enables cross-correlation of the acquired information. SEM is also beneficial over TEM in several analytical scenarios. SEM can cater to larger-sized samples (wafers of 200 mm diameter, while specially adapted SEMs can go further up), in comparison to TEM, which can analyze to only 2.3 mm or 3 mm [22]. SEM is a nondestructive analytical technique [28], while the specimen preparation process of TEM makes it a destructive technique [29]. The time needed for preparing the sample for SEM is also less when compared to the TEM technique. SEM can be further classified into three types, i.e., conventional SEM (CSEM), low vacuum SEM (LVSEM), and environmental SEM (ESEM) [30; 31; 32]. CSEM usually possesses a high vacuum (10\({}^{-6}\) Torr) condition for interaction of the electron beam and specimen. This allows the emission of low-energy secondary electrons from the specimen, resulting in minimum collisions with gas molecules present in the chamber. CSEM reinforces those analyses where dehydration and cracking of the sample (due to high vacuum) is not a problem, such as the identification of alkali-silica reactivity in concrete [33]. The second type, LVSEM, is similar to CSEM, with adaptation of elevated pressure (0.2 to 1 Torr) operations as well. The LVSEM environment slowly dissipates any liquid water present in the sample; therefore, crack propagation in the sample moves very slowly. For nonconductive samples, it is important to add a conductive coating to avoid any charging effects. The third type, ESEM, permits imaging of the sample at high humidity and therefore is considered \"wet mode.\" ESEM has a relatively high-pressure environment, i.e., 0.2 to 20 Torr, which reduces or eliminates dehydration. The elevated pressure reinforces the ionization of gas molecules due to the emission of surface charges, thereby reducing the need for conductive coatings [34]. The strength of the electron signal increases with the ionization of gas molecules, providing better results. ESEM supports coherent imaging but has limited ability in X-ray microanalysis, as frequent collisions lead to defocusing and scattering of the electron beam, which makes the position of the beam on the specimen uncertain. Field emission gun SEM (FEG SEM) typically falls under the category of CSEM, as its operation includes high vacuum conditions. In conclusion, the type of SEM should be chosen based on the nature of the specimen and the needed analysis.