**3.1 Resistive switching (RS)**

Resistive switching is the one of most prevalent types of memristor devices that was first observed in TiO2-based devices. The RS mechanism is one of the most complicated and also conflicts between number of parameters, such as electrodes/ active layer interfaces, grain size, active area of the device, types of defects, and many more [12]. The basic characteristics of resistive switching are based on the movement of ions, such as oxygen vacancies [13–15], active metal cations [3, 16, 17], and anions, like halides [18, 19] and sulfurs [20]. The two most common type of resistive switching mechanism is the electrochemical mechanism (ECM) and valence charge mechanism (VCM) as discussed below:
