**4.3 Hybrid materials**

Hybrid materials are combinations of organic and inorganic components, for instance, graphene oxides and composites, polymer-oxide composites, hybrid perovskites, etc. The organic and inorganic components, when merged, can create some desirable property material with enhanced quality and diminished defects. These materials are quite popular for non-volatile memory applications having low power consumption and high speed. The first hybrid perovskite (MA)PbX3 (MA = methylammonium and X = Cl, Br, I) was first reported by Dieter Weber in 1978 [65]. Perovskites have a general chemical formula ABX3, where A occupies eight corners of the cubic unit cell, it is a large monovalent cation, such as methylammonium (MA+ ) and formamidium (FA+ ). B is represented by a divalent metal cation, occupying body central position in the cubic unit, for example, Pb2+, Sn2+, Eu2+, Cu2+, etc. X is the halide anion, such as I− , Br− , and Cl− , six of which surround the

**Figure 4.**

*Illustration of basic structure of (a) perovskite, (b) Ruddlesden-Propper phase, and (c) Dion-Jacobson phase.*

B cation in the octahedral geometry. Making [BX6] 4− octahedron [66]. The structure of perovskite is depicted in **Figure 4**.

Hybrid organic–inorganic perovskites (HOIPs) are considered a recent family in the perovskites class. Due to the number of possible combinations of the A, B, and X, there are many perovskite family members reported, also they can form 3D, 2D, 1D, and 0D structures as well as having the same unit cell. The structural and chemical diversity offered by HOIPs give rise to tuning to achieve desirable properties and opening doors to many potential applications [67]. HOIPs have certain exclusive properties, such as tunable band gap, wide range of light absorption, ambipolar charge transport, long electron–hole diffusion length, and optical absorption, making them apt for devices such as solar cells, light emitting diodes (LEDs), transistors, and memristors. In 2016, the first time reported the resistive switching phenomenon and synaptic properties in three-dimensional (3D) HOIP (MAPbX3, X = Br and I) devices, due to the presence of ion migration. HOIPs containing organic-based cations are hydrophilic in nature, thermally unstable, and immediately degrade in ambient air. Furthermore, inorganic materials are mixed with organic materials to reduce operating voltage. By decreasing dimensionality from three dimension (3D) to two dimension (2D), increasing quantity of organic insulating cations, which increase the activation energy of halide ions in perovskite layer. As a result, 2D has a high amount of insulating cations with the ability of lowest magnitude of energy consumption. Mainly, memristors operated due to the ion migration has reported, but several studies also claim that metallic filament growth plays an important role in resistive switching properties. For synthesizing stable materials, low-dimensional perovskite phases, namely defined as Ruddlesden-Propper (RP) and Dion-Jacobson (DJ) perovskites, which distinguished by the change in their interlayer spacer cation alignment (**Figure 4b** and **c**) [4, 68, 69].
