Fig. 1. Schematic operation principle of an electrochemical metallization cell in conjunction with the typical I–V curve with the SET (A–D) and RESET processes (E). Reproduced with permission.^[51] Copyright 2011, IOP Publishing.

Fig. 2. The filament geometry in the device of Ag/Ag₂S/Pt, including the density and diameter of the dendritic branches, can be tuned independently through controlling the compliance current level (I_C) and number of stimulus pulses, respectively. Two implementing examples of LTP (cases 1 and 2) and STP (cases 3 and 4). Reproduced with permission.^[62] Copyright 2015, American Chemical Society.

Fig. 3. In situ TEM observation of the planar structured Au/SiO_xN_y:Ag/Au device with threshold switching behavior, suggesting that the interfacial energy minimization induces the rapid contraction of filament after removing the external electric field. Reproduced with permission.^[73] Copyright 2017, Springer Nature.

Fig. 4. Atomic structure of the conductive filaments with Magnéli structures observed in VCM cells. (a) High-resolution TEM image of a Ti₄O₇ nanofilament in the device of Pt/TiO₂/Pt with unipolar resistive switching behavior. Reproduced with permission.^[87] Copyright 2010, Springer Nature. (b) Electron diffraction pattern, dark field imaging of the Ti₄O₇ Magnéli crystallite and the physical model for bipolar resistive switching in the Pt/TiO₂/Pt memristive devices. Reproduced with permission.^[88] Copyright 2010, Wiley. (c) Structure evolution in the forming process of the Pt/WO₃/Pt device, illustrating that a conductive filament with Magnéli phase is formed during the forming process. Reproduced with permission.^[89] Copyright 2016, Wiley.

Fig. 5. Amorphous conductive filaments in VCM cells based on oxides. (a) TEM, EELS, and the physical switching mechanism for the Au/Ta₂O₅/Au device. Reproduced with permission.^[91] Copyright 2015, Wiley. (b) Electron holography graph of the HfO₂-based memristive device and the schematic of the switching mechanism. Reproduced with permission.^[83] Copyright 2017, Wiley.

Fig. 6. Asymmetric structured memristive device with a reservoir layer of V_O and the switching mechanism of the Pt/Ta₂O_5−x/TaO_2−x/Pt device. (a) TEM image of the device. Reproduced with permission.^[92] Copyright 2011, Springer Nature. (b) Schematic diagrams of formation and annihilation of nanoscale TaO_1−x filaments during switching. Reproduced with permission.^[93] Copyright 2013, Springer Nature.

Fig. 7. Switching polarity determined by the dynamic oxygen migration process. (a) Two types of switching properties in the Au/Sr₂TiO₄/Nb:SrTiO₃ (STO) device; the device is schematically shown in the inset. Reproduced with permission.^[32] Copyright 2010, Wiley. (b) Schematic of the switching process induced by the internal oxygen migration in the oxides. Reproduced with permission.^[95] Copyright 2009, Springer Nature. (c) Schematic of switching in Pt/STO/Nb:STO device, where oxygen migrates between the top Pt electrode and the Pt/STO interface. Reproduced with permission.^[102] Copyright 2017, Wiley.

Fig. 8. Resistive switching induced by electron trapping/detrapping process. (a) I–V curve of the Pt/Nb:SrTiO₃/Al devics, which is set to LRS by a forward bias. Reproduced with permission.^[131] Copyright 2014, Springer Nature. (b) Resistance state retention characteristics, illustrating that the electrons might escape from the trapped sites.^[131] Copyright 2014, Springer Nature. (c) Resistive switching I–V curves of the Pt/Ta₂O₅/HfO_2−x/TiO_x/Ti sample with self-rectification. (d) Retention of the LRS and HRS at different temperatures. Reproduced with permission.^[125] Copyright 2015, Wiley. (e) I–V characteristics of SiO₂:0.2Pt atomic mixtures with various thicknesses. Reproduced with permission.^[118] Copyright 2011, Springer Nature. (f) Good retention performance of SiO₂:0.2Pt device at 85 °C subsequent to switching. Reproduced with permission.^[43] Copyright 2011, Wiley.