Fig. 1

Filament-free switching memristor structures and their switching mechanisms. a A typical simple two-terminal memristor structure consisting of electrode/switching layer/electrode. b–g Illustrations of the switching principles for diverse types of filament-free switching memristors. b Ion migration. The switching is mainly governed by the electric field-driven migration of the ions. c Intercalation. The electric field-driven intercalation of ions into lattices is responsible for the main switching behavior. d Electron trapping/detrapping. The electronic (de)trapping processes at trap sites depending on the external electric field generates the switching events. e Ferroelectric polarization. The ferroelectric polarization switching results in the conductance switching. f Phase transition. A lower conductance phase (insulator or semiconductor) transforms into a higher conductance phase and vice versa. g Filling-controlled Mott transition. Mott–Hubbard gap increases or decreases corresponding to electron density in the transition metal orbitals