**4.1 Inorganic materials**

Inorganic materials consist of metal oxides, halides, chalcogenides, 2D materials, etc. The main advantage of inorganic materials is their strong environmental stability in varied conditions. The first ever memristor was prepared by Hewlett-Packard (HP), composed of two inorganic layers: stoichiometric titanium oxide and oxygen-deficient non-stoichiometric TiO2-x layers [6]. The conductivity was achieved by the migration of the oxygen vacancies when the external electric field is applied. Later, many other inorganic oxides, such as HfO2, CuO [37], NiO [38], and TiO2 [39]. attracted the attention of researchers. Many 2D materials were also used as the active layer, such as nitrides, transition-metal dichalcogenides (MoS2 and WS2) [40], InSe [41], black phosphorus (BP) [42], MXenes [43], bismuthene [44], and tellurene [45]. Few nanorod structures explored consist of ZnO [46], TiO2 [47], HfO2 [48], etc. **Table 1** summarizes some of the advancements in the materials used till date (**Figure 3**).
