All of the solids, liquids, and gases that we encounter in our daily life are classified as a common type of matter. In solids and liquids the distance between neighboring atoms is of the order of a few angstroms. In the case of the gases, an average distance between molecules is approximately 30 Å at room temperature under 1 atm. Solids are one of the major states of matter. On the basis of the atomic arrangement, solids are classified into three categories: crystalline, polycrystalline, and amorphous [1,2,3,4–5]. In the crystalline structure (or single crystal or monocrystalline), the periodicity of atoms (or molecules) extends throughout the material (e.g., diamond, quartz, etc.). The opposite of a single crystal is an amorphous structure where the atomic position is completely random. In between the two extremes exist polycrystalline structures, which are made up of a number of small crystals known as crystallites. The crystallites in polycrystalline structures are randomly oriented. The small crystallites are known as grains and the boundaries separating them as grain boundaries. The crystalline, polycrystalline, and amorphous structures are schematically illustrated in Fig. 1.1. Schematic illustration of three types of solids: (a) crystalline, (b) polycrystalline, and (c) amorphous. https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9781315364926/07fbaaad-83d9-4a7c-9c1c-e20cc2af4d5b/content/fig1_1_OB.tif"/>