Mixed-Dimensional Formamidinium Bismuth Iodides Featuring In-Situ Formed Type-I Band Structure for Convolution Neural Networks.

For valence change memory (VCM)-type synapses, a large number of vacancies help to achieve very linearly changed dynamic range, and also, the low activation energy of vacancies enables low-voltage operation. However, a large number of vacancies increases the current of artificial synapses by acting like dopants, which aggravates low-energy operation and device scalability. Here, mixed-dimensional formamidinium bismuth iodides featuring in-situ formed type-I band structure are reported for the VCM-type synapse. As compared to the pure 2D and 0D phases, the mixed phase increases defect density, which induces a better dynamic range and higher linearity. In addition, the mixed phase decreases conductivity for non-paths despite a large number of defects providing lots of conducting paths. Thus, the mixed phase-based memristor devices exhibit excellent potentiation/depression characteristics with asymmetricity of 3.15, 500 conductance states, a dynamic range of 15, pico ampere-scale current level, and energy consumption per spike of 61.08 aJ. A convolutional neural network (CNN) simulation with the Canadian Institute for Advanced Research-10 (CIFAR-10) dataset is also performed, confirming a maximum recognition rate of approximately 87%. This study is expected to lay the groundwork for future research on organic bismuth halide-based memristor synapses usable for a neuromorphic computing system.

Asymmetric carrier transport in flexible interface-type memristor enables artificial synapses with sub-femtojoule energy consumption.

Flexible and transparent artificial synapses with extremely low energy consumption have potential for use in brain-like neuromorphic electronics. However, most of the transparent materials for flexible memristive artificial synapses were reported to show picojoule-scale high energy consumption with kiloohm-scale low resistance, which limits the scalability for parallel operation. Here, we report on a flexible memristive artificial synapse based on Cs3Cu2I5 with energy consumption as low as 10.48 aJ (= 10.48 × 10-18 J) µm-2 and resistance as high as 243 MΩ for writing pulses. Interface-type resistive switching at the Schottky junction between p-type Cu3Cs2I5 and Au is verified, where migration of iodide vacancies and asymmetric carrier transport owing to the effective hole mass is three times heavier than effective electron mass are found to play critical roles in controlling the conductance, leading to high resistance. There was little difference in synaptic weight updates with high linearity and 250 states before and after bending the flexible device. Moreover, the MNIST-based recognition rate of over 90% is maintained upon bending, indicative of a promising candidate for highly efficient flexible artificial synapses.

A layered (n-C4H9NH3)2CsAgBiBr7 perovskite for bipolar resistive switching memory with a high ON/OFF ratio.

Lead-based halide perovskites have been proposed as potential candidates for resistive switching memristors due to the high ON/OFF ratio along with millivolt-level low operational voltage. However, lead-free perovskites with 3-dimensional structures, such as Cs2AgBiBr6, were reported to suffer from low ON/OFF ratios. We report here that reduction of dimensionality is an effective method to improve remarkably the ON/OFF ratio in lead-free perovskites. Introduction of butylammonium (BA) into the double perovskite Cs2AgBiBr6 forms 2-dimensional BA2CsAgBiBr7, which is confirmed by the well-developed (00l) peaks from powder X-ray diffraction. A 230 nm thick BA2CsAgBiBr7 film is sandwiched in between Ag and Pt electrodes, which demonstrates bipolar resistive switching behavior with a potential ON/OFF ratio up to 107. Reliable and reproducible SET and RESET processes occur at +0.13 V and -0.20 V, respectively. Endurance of 1000 cycles and a retention time of 2 × 104 s are measured. Multi-level storage capability is confirmed by controlling the compliance current. Schottky conduction at the high resistance state (HRS) and ohmic conduction at the low resistance state (LRS) are found to be responsible for resistive switching. The stability test at 85 °C or for 22 days under ambient conditions indicates that BA2CsAgBiBr7 is durably operable.

Roadmap on halide perovskite and related devices.

Since the first report on solid-state perovskite solar cells (PSCs) with ∼10% power conversion efficiency (PCE) and 500 h-stability in 2012, tremendous effort has been being devoted to develop PSCs with higher PCE, longer stability and recycling hazardous lead waste. As a result, PCE over 23% was recorded in 2018 and stability over 10 000 h was reported. Beyond photovoltaics, lead halide perovskite materials demonstrated superb properties when they were applied to flat-panel x-ray detectors and non-volatile resistive switching memory. In this review, the progress of the lead halide perovskite in photovoltaics, x-ray imaging and memristors is investigated. Pb-based PSCs and non-Pb-based PSCs are compared, where technologies of non-Pb-based PSCs are not matured for commercialization. Pb-based PSCs were found to be highly suitable for both terrestrial and space photovoltaics. Higher sensitivity under low dose rate observed from the lead halide perovskite suggests a bright future for perovskite x-ray imaging systems. Moreover, high on/off ratio and low energy consumption observed in resistive switching enables perovskite to be a promising candidate for high density memristors.

Effect of interlayer spacing in layered perovskites on resistive switching memory.

We report here the effect of interlayer spacing in 2-dimensional (2D) perovskites of [C6H5(CH2)nNH3]2PbI4 (anilinium (An) for n = 0, benzylammonium (BzA) for n = 1 and phenylethylammonium (PEA) for n = 2) on resistive switching properties. X-ray diffraction (XRD) reveals that the interlayer spacing of layered PbI2 is increased from 6.98 Å to 13.29 Å for (An)2PbI4, 14.20 Å for (BzA)2PbI4 and 15.92 Å for (PEA)2PbI4, which leads to a monolayer of organic cations with stacked benzene rings between inorganic PbI42- layers. All the samples in the device structure of Ag/PMMA (polymethyl methacrylate)/perovskite/Pt show bipolar switching behavior, where the SET voltage is near +0.2 V and the RESET voltage is less than -0.5 V. The ratio of LRS (low resistance state) to HRS (high resistance state), also called the ON/OFF ratio, is increased from 106 to 108 as interlayer spacing is increased due to the gradual increase in resistance in the HRS. Endurance is slightly improved from 1.3 × 102 for An to 2.2 × 102 for PEA, whereas substantial improvement in retention is observed from 2 × 103 to 5.5 × 103. This indicates that the enhanced 2D structure is beneficial to the kinetics of forming and rupturing the conducting filaments. The ohmic-like conduction mechanism in the LRS and the hopping mechanism in the HRS are observed for all three samples. This work finds that the resistive switching properties and conduction mechanism in the HRS depend on interlayer spacing in 2D perovskites.

The effect of compositional engineering of imidazolium lead iodide on the resistive switching properties.

We report here resistive switching memory characteristics of imidazolium lead iodide depending on the molar ratio of PbI2 to imidazolium iodide (ImI), that is, PbI2 : ImI = 1 : 0, 1 : 0.5, 1 : 1, 1 : 2, 1 : 3 and 0 : 1. X-ray diffraction confirms that the stoichiometric composition results in a hexagonal structure of (Im)PbI3, showing a one-dimensional face-sharing [PbI3-] chain. Bipolar resistive switching characteristics are observed regardless of the mixing ratio, where the forming process is required prior to SET and RESET processes at around +0.2 V and -0.2 V, respectively. The ON/OFF ratio is increased from 106 to 109 as the ImI content is increased due to the increased HRS associated with the pronounced insulating characteristics by ImI, whereas, the stoichiometric (Im)PbI3 exhibits 5 times longer endurance (103) and an order of magnitude longer retention time (104 s) as compared to other compositions. Multilevel data storage capability is confirmed by changing the compliance current. The low resistance state (LRS) and the high resistance state (HRS) are associated with Ohmic conduction and Schottky conduction, respectively. Density functional theory (DFT) calculation reveals that the defect formation energy of iodine vacancy is estimated to be low indicating that (Im)PbI3 has a sufficient concentration of iodide vacancy for filament formation. Further energy barrier calculations show that iodide migration preferentially occurs along the 1-dimensional [234] crystallographic direction rather than the interlayer [130] direction. A good performance of the (Im)PbI3-based memristor is thus related to the low defect formation energy of iodide vacancy and the preferential growth of the filament along the 1-dimensional chain.

Perovskite-related (CH3NH3)3Sb2Br9 for forming-free memristor and low-energy-consuming neuromorphic computing.

Organic-inorganic halide perovskite materials exhibit excellent memristive properties, such as a high on/off ratio and low switching voltage. However, most studies have focused on Pb-based perovskites. Here, we report on the resistive switching and neuromorphic computing properties of Pb-free perovskite-related MA3Sb2Br9 (MA = CH3NH3). The Ag/PMMA/MA3Sb2Br9/ITO devices show forming-free characteristics due to a self-formed conducting filament induced by metallic Sb present in the as-prepared MA3Sb2Br9 layer. An MA3Sb2Br9-based memristor exhibits a reliable on/off ratio (∼102), an endurance of 300 cycles, a retention time of ∼104 s and multilevel storage characteristics. Furthermore, synaptic characteristics, such as short-term potentiation, short-term depression and long-term potentiation, are revealed along with a low energy-consumption of 117.9 fJ µm-2, which indicates that MA3Sb2Br9 is a promising material for neuromorphic computing.

All-Inorganic Bismuth Halide Perovskite-Like Materials A3Bi2I9 and A3Bi1.8Na0.2I8.6 (A = Rb and Cs) for Low-Voltage Switching Resistive Memory.

As silicon-based metal oxide semiconductor field effect transistors get closer to their scaling limit, the importance of resistive random-access memory devices increases due to their low power consumption, high endurance and retention performance, scalability, and fast switching speed. In the last couple of years, organic-inorganic lead halide perovskites have been used for resistive switching applications, where they outperformed conventional metal oxides in terms of large on/off ratio and low power consumption. However, there were scarce reports on lead-free perovskites for such applications. In this report, we prepared lead-free Au/A3Bi2I9/Pt/Ti/SiO2/Si (A is either Cs+ or Rb+) devices and tested their resistive switching characteristics. They showed a forming step prior to repeating switching, low operating voltage (0.09 V for Rb3Bi2I9 and 0.1 V for Cs3Bi2I9), large on/off ratio (>107), relatively high endurance (200 cycles for Rb3Bi2I9 and 400 cycles for Cs3Bi2I9 cycles), and high retention (1000 s). Such low voltage could be explained by grain boundary-modulated ion drift. Difference in endurance was speculated to be due to the difference in the surface roughness of films because Cs3Bi2I9 films are smoother. To get rid of the forming step, 10% of the Bi3+ cations were substituted with Na+ cations. However, this method only worked on Rb-based structures. This phenomenon was explained by the defect formation energy, which can only be negative in a corner-sharing Rb3Bi2I9 structure compared to a face-sharing octahedral Cs3Bi2I9 structure. As a result, the forming step was removed, and 100 cycles endurance and 1000 s retention performance were obtained. Similarly, the lower endurance is suspected to be due to the poor surface quality of the film.

Correction: Wafer-scale reliable switching memory based on 2-dimensional layered organic-inorganic halide perovskite.

Correction for 'Wafer-scale reliable switching memory based on 2-dimensional layered organic-inorganic halide perovskite' by Ja-Young Seo, et al., Nanoscale, 2017, DOI: 10.1039/c7nr05582j.

Wafer-scale reliable switching memory based on 2-dimensional layered organic-inorganic halide perovskite.

Recently, organic-inorganic halide perovskite (OHP) has been suggested as an alternative to oxides or chalcogenides in resistive switching memory devices due to low operating voltage, high ON/OFF ratio, and flexibility. The most studied OHP is 3-dimensional (3D) MAPbI3. However, MAPbI3 often exhibits less reliable switching behavior probably due to the uncontrollable random formation of conducting filaments. Here, we report the resistive switching property of 2-dimensional (2D) OHP and compare switching characteristics depending on structural dimensionality. The dimensionality is controlled by changing the composition of BA2MAn-1PbnI3n+1 (BA = butylammonium, MA = methylammonium), where 2D is formed from n = 1, and 3D is formed from n = ∞. Quasi 2D compositions with n = 2 and 3 are also compared. Transition from a high resistance state (HRS) to a low resistance state (LRS) occurs at 0.25 × 106 V m-1 for 2D BA2PbI4 film, which is lower than those for quasi 2D and 3D. Upon reducing the dimensionality from 3D to 2D, the ON/OFF ratio significantly increases from 102 to 107, which is mainly due to the decreased HRS current. A higher Schottky barrier and thermal activation energy are responsible for the low HRS current. We demonstrate for the first time reliable resistive switching from 4 inch wafer-scale BA2PbI4 thin film working at both room temperature and a high temperature of 87 °C, which strongly suggests that 2D OHP is a promising candidate for resistive switching memory.

In-Situ Formed Type I Nanocrystalline Perovskite Film for Highly Efficient Light-Emitting Diode.

Excellent color purity with a tunable band gap renders organic-inorganic halide perovskite highly capable of performing as light-emitting diodes (LEDs). Perovskite nanocrystals show a photoluminescence quantum yield exceeding 90%, which, however, decreases to lower than 20% upon formation of a thin film. The limited photoluminescence quantum yield of a perovskite thin film has been a formidable obstacle for development of highly efficient perovskite LEDs. Here, we report a method for highly luminescent MAPbBr3 (MA = CH3NH3) nanocrystals formed in situ in a thin film based on nonstoichiometric adduct and solvent-vacuum drying approaches. Excess MABr with respect to PbBr2 in precursor solution plays a critical role in inhibiting crystal growth of MAPbBr3, thereby forming nanocrystals and creating type I band alignment with core MAPbBr3 by embedding MAPbBr3 nanocrystals in the unreacted wider band gap MABr. A solvent-vacuum drying process was developed to preserve nanocrystals in the film, which realizes a fast photoluminescence lifetime of 3.9 ns along with negligible trapping processes. Based on a highly luminescent nanocrystalline MAPbBr3 thin film, a highly efficient green LED with a maximum external quantum efficiency of 8.21% and a current efficiency of 34.46 cd/A was demonstrated.