next-generation memory devices such as ferroelectric random-access memory, magnetoresistive random-access memory, and resistive random-access memory (RRAM) have been intensively researched and developed. [4-12] Of these innovative nonvolatile memories, RRAM offers low power consumption, fast response speed, long retention time, high scalability, multistate data-storage capability, and a simple metalinsulator-metal structure. [9,13,14] Information storage in the RRAM is achieved by altering the resistance of its insulator. The adjustable resistance of the insulator is achieved by forming metallic or defect filaments with low resistance either by electrochemical metal dissolution or by changing the valence charge of the insulator. [15,16] Generally, the insulator used is either a transition metal oxide or perovskite oxide such as Sr 2 TiO 4 and Cr-doped SrZrO 3. [16,17] However, the synthesis of these materials often requires high temperatures and highly complex fabrication processes that are incompatible with flexible applications. [18,19] Furthermore, a high voltage is required to induce a soft breakdown of the oxide layer, which is detrimental to low-power operation. Lead-based perovskite has recently demonstrated great potential for the fabrication of RRAMs, because of its compact morphology, lowtemperature solution processability, and ionic-transport property. It functions analogously to the human synapse and offers flexible applications with a high on/off ratio, multistate-storage capability, and low set/reset voltage. [13-15,20-27] For example, Choi et al. fabricated an organo-lead halide perovskite RRAM with an on/off ratio of 10 6 , low set/reset voltage of 0.13 V, and four states of information storage. [25] Owing to the high absorption coefficient of perovskite, Zhou demonstrated optoelectronic devices with versatile functions such as photo-sensing, processing, and bit storage with the help of light pulses. [28] However, the structural stability of the perovskite in the air and the issue of its toxicity have led to uncertainty regarding its suitability as a commercial product. [29,30] To solve the issue of toxicity, Park et al. fabricated a zero-dimensional bismuth-based perovskite that had the crystal formula of A 3 Bi 2 I 9 and demonstrated a forming-free device by lowering the defect formation barrier with substitution of sodium into the Bi 3+ cation site. [31] Yang et al. reported forming-free resistive-switching devices Resistive switching devices based on halide perovskites exhibit promising potential in flexible resistive random-access memory (RRAM) owing to low fabrication cost and low processing temperature. However, the toxicity of these materials hinders their commercialization. Herein, bismuth iodide (BiI 3) is employed as an insulator in RRAM. A monolayer of graphene or hexagonal boron nitride (h-BN) is employed as a buffer layer to achieve van der Waals epitaxy of BiI 3 and meanwhile to prevent the intrusion of copper atoms. Thus, the film quality of the BiI 3 layer is greatly improved. Res...

The film quality of insulators significantly affects performance of resistive random‐access memories (RRAMs), particularly in current leakage and degradation. In this study, a facile and practical method is employed to achieve the van der Waals epitaxy of bismuth iodide (BiI3) on silicon by using a self‐assembled monolayer of octadecyltrichlorosilane (OTS) as a buffer layer. The BiI3 layer is compact and has high crystallinity, a pinhole‐free, and compact surface; every BiI3 crystal is horizontally aligned with OTS–Si substrate. The RRAMs with the Si++/OTS/BiI3/Au structure exhibit excellent resistive switching properties with a very high on/off ratio of 109, long‐term stability for data retention, high endurance in write–erase cycles, and multistate information storage capacity. The crystal orientation, anisotropic carrier transport, morphology, deposition rate, and roughness considerably influence the resistive switching results. These are thoroughly investigated by analyzing the current–voltage characteristics at various temperatures, scanning electron microscope, atomic force microscope, X‐ray photoemission spectroscopy, and X‐ray diffraction patterns. It is proposed that the resistive switching mechanisms is caused by the iodine ion migration, which changes the valence charge of bismuth. This leaves a partially formed conductive metallic bismuth filament in the BiI3 layer under the electrical field that enables multistate data storage.