Recently, organometallic and all-inorganic halide perovskites (HPs) have become promising materials for resistive switching (RS) nonvolatile memory devices with low power consumption because they show current–voltage hysteresis caused by fast ion migration. However, the toxicity and environmental pollution potential of lead, a common constituent of HPs, has limited the commercial applications of HP-based devices. Here, RS memory devices based on lead-free all-inorganic cesium tin iodide (CsSnI3) perovskites with temperature tolerance are successfully fabricated. The devices exhibit reproducible and reliable bipolar RS characteristics in both Ag and Au top electrodes (TEs) with different switching mechanisms. The Ag TE devices show filamentary RS behavior with ultralow operating voltages (<0.15 V). In contrast, the Au TE devices have interface-type RS behavior with gradual resistance changes. This suggests that the RS characteristics are attributed to either the formation of metal filaments or the ion migration of defects in HPs under applied electric fields. These distinct mechanisms may permit the opportunity to design devices for specific purposes. This work will pave the way for lead-free all-inorganic HP-based nonvolatile memory for commercial application in HP-based devices.

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Neuromorphic computing, which mimics biological neural networks, can overcome the high-power and large-throughput problems of current von Neumann computing. Two-terminal memristors are regarded as promising candidates for artificial synapses, which are the fundamental functional units of neuromorphic computing systems. All-inorganic CsPbI 3 perovskite-based memristors are feasible to use in resistive switching memory and artificial synapses due to their fast ion migration. However, the ideal perovskite phase α-CsPbI 3 is structurally unstable at ambient temperature and rapidly degrades to a non-perovskite δ-CsPbI 3 phase. Here, dual-phase (Cs 3 Bi 2 I 9 ) 0.4 −(CsPbI 3 ) 0.6 is successfully fabricated to achieve improved air stability and surface morphology compared to each single phase. Notably, the Ag/polymethylmethacrylate/(Cs 3 Bi 2 I 9 ) 0.4 −(CsPbI 3 ) 0.6 /Pt device exhibits non-volatile memory functions with an endurance of ≈10 3 cycles and retention of ≈10 4 s with low operation voltages. Moreover, the device successfully emulates synaptic behavior such as long-term potentiation/depression and spike timing/widthdependent plasticity. This study will contribute to improving the structural and mechanical stability of all-inorganic halide perovskites (IHPs) via the formation of dual phase. In addition, it proves the great potential of IHPs for use in low-power non-volatile memory devices and electronic synapses.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201906686.femto-Joule scale. [1,2] Neuromorphic computing systems that emulate the human brain are considered promising candidates for overcoming the energy and throughput limitations of conventional von Neumann computing systems. Inspired by the human brain, neuromorphic computing systems are composed of electronic neurons and synapses, and achieve a high degree of parallelism with the physically united memory and information processing unit. Artificial synapses are fundamental functional units of neuromorphic architectures, where electrical stimuli from a pre-neuron are transmitted to a post-neuron, thereby generating updated synaptic weight (i.e., causing a conductance change in an electronic device). Notably, resistive switching (RS) memory has been actively investigated for synaptic devices due to its scalability, simple structure, fast operation speed, and low-energy consumption, which are the most important requirements for neuromorphic computing. [3,4] In the last few years, halide perovskites (HPs) with the chemical formula ABX 3 [where A is an organic or inorganic (Cs or Rb) cation, B is a divalent metal cation (Pb or Sn), and X is a halide anion (I, Cl, or Br)] have been widely investigated for RS memory and artificial synapses because of their tunable bandgaps and fast ion migration. [5][6][7] To date, organolead HPs (OHPs, A = methylammonium (MA), B = Pb, and X = I, Cl, or Br) have been successfully utilized as the active layer of memristor-based artificial synapses, emulat...

In photoelectrochemical (PEC) water splitting, BiVO4 is considered the most promising photoanode material among metal oxide semiconductors because of its relatively narrow optical bandgap and suitable band structure for water oxidation. Nevertheless, until now, the solar-to-hydrogen conversion efficiency of BiVO4 has shown significant limitations for commercialization because of its poor charge transport. Various strategies, including the formation of a heterojunction and doping of electron donors, have been implemented to enhance the charge transport efficiency; however, fundamental approaches are required for further enhancement. In this regard, we report the fundamental approach for BiVO4 thin film photoanodes by fabricating epitaxial oxide thin films with different crystallographic orientations for PEC water splitting. The crystalline anisotropy generally reveals distinct physical phenomena along different crystallographic orientations. In the same vein, in terms of the anisotropic properties of BiVO4, the electrical conductivity of BiVO4 is greater along the ab-plane than along the c-axis. Consequently, as the crystallographic orientation of the BiVO4 thin film changes from (001) to (010), the charge transport properties in the epitaxial BiVO4 thin film are significantly enhanced. Thus, at 1.23 VRHE, the photocurrent density of the epitaxial BiVO4 (010) thin film (2.29 mA cm–2) is much higher than that of the epitaxial BiVO4 (001) thin film (0.74 mA cm–2) because of significant enhancement in charge transport properties even for undoped BiVO4. These results strongly suggest that the growth of epitaxial BiVO4 thin films with specific crystallographic orientations has great potential to considerably improve the charge transport efficiency of photoanodes for solar water splitting.

As the BO6 octahedral structure in perovskite oxide is strongly linked with electronic behavior, it is actively studied for various fields such as metal–insulator transition, superconductivity, and so on. However, the research about the relationship between water-splitting activity and BO6 structure is largely lacking. Here, we report the oxygen evolution reaction (OER) of LaNiO3 (LNO) by changing the NiO6 structure using compositional change and strain. The 5 atom % La deficiency in LNO resulted in an increase of the NiONi bond angle and an expansion of bandwidth, enhancing the charge transfer ability. In-plane compressive strain derives the higher d z 2 orbital occupancy, leading to suitable metal–oxygen bond strength for OER. Because of the synergistic effect of A-site deficiency and compressive strain, the overpotential (η) of compressively strained L0.95NO film is reduced to 130 mV at j = 30 μA/cm2 compared with nonstrained LNO (η = 280 mV), indicating a significant enhancement in OER.