X-ray Photoelectron Spectroscopy (XPS) is an analytical technique that provides information on the chemical composition, elemental distribution, and chemical bonding state of the surface of a material. It is based on the photoelectric effect, which is a fundamental principle in quantum mechanics that explains the interaction between photons and matter. When a photon of sufficient energy is absorbed by an atom in a material, an electron is ejected from that atom, resulting in the emission of a photoelectron. The energy of the photoelectron is equal to the difference between the energy of the photon and the binding energy of the electron to the atom. The binding energy is dependent on the chemical environment of the atom and is therefore a unique signature of the element and its chemical bonding state.