Extremely Low Operating Current Resistive Memory Based on

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Extremely Low Operating Current Resistive Memory Based on Exfoliated 2D Perovskite Single Crystals for Neuromorphic Computing He Tian, Lianfeng Zhao, Xuefeng Wang, Yao-Wen Yeh, Nan Yao, Barry P. Rand, and Tian-Ling Ren ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b05726 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 4, 2017

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Submitted Manuscript: Confidential Extremely Low Operating Current Resistive Memory Based on Exfoliated 2D Perovskite Single Crystals for Neuromorphic Computing He Tian†,⊥, Lianfeng Zhao‡,⊥, Xuefeng Wang†, Yao-Wen Yeh§, Nan Yao§, Barry P. Rand*,‡,∥, Tian-Ling Ren*,† †

Institute of Microelectronics and Tsinghua National Laboratory for Information Science and

Technology (TNList), Tsinghua University, Beijing 100084, China ‡

Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544,

United States §

Princeton Institute for Science and Technology of Materials, Princeton University, Princeton,

New Jersey 08544, United States ∥

Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey

08544, United States ⊥

*

These authors contributed equally to this work.

Corresponding author. Email: [email protected] (B.P.R.); [email protected] (T.-L.R.)

ABSTRACT Extremely low energy consumption neuromorphic computing is required to achieve massively parallel information processing on par with the human brain. To achieve this goal, resistive memories based on materials with ionic transport and extremely low operating current are required. Extremely low operating current allows for low power operation by minimizing the program, erase and read currents. However, materials currently used in resistive memories, such as defective HfOx, AlOx, TaOx, etc, cannot suppress electronic transport (i.e. leakage current) while allowing good ionic transport. Here, we show that 2D Ruddlesden-Popper phase hybrid 1 ACS Paragon Plus Environment

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Submitted Manuscript: Confidential lead bromide perovskite single crystals are promising materials for low operating current nanodevice applications because of their mixed electronic and ionic transport and ease of fabrication. Ionic transport in the exfoliated 2D perovskite layer is evident via the migration of bromide ions. Filaments with diameter of approximately 20 nm are visualized, and resistive memories with extremely low program current down to 10 pA are achieved, a value at least one order of magnitude lower than conventional materials. The ionic migration and diffusion as an artificial synapse is realized in the 2D layered perovskites at the pA level, which can enable extremely low energy neuromorphic computing.

KEYWORDS: organic-inorganic hybrid perovskites, 2D perovskites, perovskite single crystal, resistive memory, neuromorphic computing

Table of Contents Graphic

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Submitted Manuscript: Confidential The human brain performs massively parallel information processing with ultralow energy consumption of only ~1-100 fJ per synaptic event,1 more efficient than conventional von Neumann computing.2 A key information channel is the synapse, which enables short-term behavior for computations and long-term behavior for learning and memory.3 Recently, artificial synapses have been realized with resistive memories,4-7 with typical set/reset currents in the mA range,8, 9 with some examples down to the µA level.10 The set or program event is defined as the resistive switching from a high-resistance state (HRS) to a low-resistance state (LRS); reset or erase is defined as the resistive switching from LRS to HRS. In order to realize fJ per synaptic event in artificial systems and taking into account the operation speed mimicking bio synapses at ms timescales, lowering the operating current down to the pA range is required. In particular, lowering the program current is key to enabling a low operating current by confining the filament. However, resistive memories that can be programmed at the pA current level remain elusive because of the lack of materials that have sufficient ionic transport properties with limited electron transport properties. Previous resistive switching devices have introduced defects into high-κ materials, enabling defect-mediated oxygen ion movement. However, the leakage current is high in conventional resistive switching layers due to defect-mediated leakage paths, resulting in a high program current. Searching for defect-free, high quality resistive switching layers enabling low leakage current while providing sufficient ionic transport capabilities is required to enable ultralow program current. Organic-inorganic hybrid perovskites have emerged as a potential low cost, earthabundant semiconductor, and have achieved success in lab-scale optoelectronic devices such as photovoltaics and light emitting diodes.11-15 Organic-inorganic hybrid perovskites have a general formula of (L)2(SMX3)n+1MX4, where L, S, M, X represent long-chain organic cations, short-


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Submitted Manuscript: Confidential chain organic cations, a divalent metal cation, and a halide, respectively and n is the number of the MX4 monolayer sheets sandwiched between two long-chain organic layers.16 When n goes to infinity, the layered perovskite structure becomes the three-dimensional (3D) perovskite structure of SMX3. Finite n values represent the so-called Ruddlesden-Popper phases,16, 17 where the values of n describe the layered structure and, for X = I, Br and Cl, tunes the bandgap in the range of ~1.5 eV to ~3.6 eV.18, 19 It should be noted that conventional 2D materials (such as graphene) can be distinguished from their 3D bulk form on the grounds that they are singlylayered. In contrast, the terminology of 2D perovskites, which is widely used by the organicinorganic hybrid perovskite community, possess distinct crystal structures compared to their 3D perovskite counterparts, and are not thickness dependent. For a 2D perovskite, it has a layered crystal structure of (L)2(SMX3)n+1MX4 as described above, which is fundamentally different from a 3D perovskite with SMX3 crystal structure. Among the interesting properties of organic-inorganic hybrid perovskites is their status as a mixed ionic-electronic conductor. While ionic transport in these materials is not yet fully understood, for example in terms of whether or not it is an intrinsic property or if stoichiometry or grain boundaries dictate ion motion, certain properties have been revealed. For example, ion migration in 3D perovskites was observed in both vertical and lateral solar cell configurations, inducing switchable photovoltaic effects.20, 21 Memory effects have also been observed using 3D perovskite thin films.22 However, the leakage current is at least three orders of magnitude larger than that of state-of-the-art resistive memory devices because of a large intrinsic electronic current and parasitic leakage current from grain boundaries inherent to polycrystalline thin films. Two-dimensional (2D) perovskites with small n show anisotropic charge transport, suppressed in the out-of-plane direction, which is a promising property that can be exploited in 4 ACS Paragon Plus Environment

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Submitted Manuscript: Confidential resistive memories with extremely low program currents.17 Furthermore, the layered perovskite structure allows for the exfoliation of 2D perovskite single crystals into ultrathin films, which have intrinsic advantages over solution processed polycrystalline thin films, such as no grain boundaries and pin-holes, enabling ultra-low leakage currents when used in electronic devices. However, 2D perovskite single crystal based devices remain elusive so far. Here, we demonstrate an extremely low operating current resistive memory based on an exfoliated 2D layered perovskite single crystal. We also show a low-cost, facile approach to achieve

millimeter-sized

2D

(PEA)2PbBr4

perovskite

single

crystals

(PEA:

phenethylammonium), which exhibit a wide band gap of ~2.9 eV. Due to the layered structure, these perovskite single crystals can be exfoliated to thin layers with observable monolayer steps. Considering their unique properties, we believe that 2D perovskites can join the 2D material family as an important emerging member, especially the wide band gap 2D perovskites because, except for h-BN, most semiconducting 2D materials feature relatively narrow band gaps. Moreover, the wide band gap 2D perovskites provide a good platform to investigate ionic transport in perovskites owing to the low carrier density. In our investigations, we find that bromide ions accumulate at the anode interface and provide the basis for conducting channel (i.e. filament) formation under sufficient electric field. Finally, resistive memories based on exfoliated 2D perovskite single crystals were demonstrated with program current as low as 10 pA, which provides an approach toward extremely low energy neuromorphic computing. Results and Discussion Layered (PEA)2PbBr4 perovskite single crystals were synthesized at room temperature using a modified anti-solvent vapor-assisted crystallization method (Figure 1a). The detailed method is described in the supplementary information. Briefly, phenethylammonium bromide 5 ACS Paragon Plus Environment

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Submitted Manuscript: Confidential (PEABr) and PbBr2 were dissolved in a solvent with high solubility and moderate coordination for the precursors (N,N’-dimethylformamide, DMF) and chlorobenzene was used as the antisolvent, in which the precursors are insoluble. Controlling the diffusion rate of the anti-solvent vapor, millimeter-sized 2D (PEA)2PbBr4 perovskite single crystals were synthesized (Figure 1b), and X-ray diffraction (XRD) patterns confirm the phase purity of the as-grown 2D Ruddlesden-Popper perovskite single crystals (Figure 1c). The absorbance of the 2D (PEA)2PbBr4 perovskite single crystals shows a band edge cutoff at approximately 428 nm, giving an optical bandgap of ~2.9 eV, consistent with the peak position in the photoluminescence (PL) spectrum (Figure 1d). As a comparison, 3D CH3NH3PbBr3 single crystals exhibit an absorption edge at approximately 569 nm (2.18 eV bandgap) (Figure S1). Figure 1e shows a structural illustration of the 2D layered (PEA)2PbBr4 perovskites. The weak van der Waals forces between the organic molecular layers in adjacent sheets (Figure 1e) allow for crystal exfoliation, and a scanning electron microscope (SEM) image confirms the layered morphology (Figure 1f). The exfoliated thin crystals can be dry transferred by a PDMS stamp, as shown in Figure 1g, which alludes to promising opportunities to integrate 2D perovskites with other 2D materials (graphene, MoS2, etc.). Notably, the AFM image (Figure 1g) also shows a monolayer step with 1 nm height, which indicates that the 2D films can be successfully exfoliated from the 2D single-crystals. The PL peak of the exfoliated layers (~40 nm) is slighted blue-shifted from 410 nm to 407 nm (Figure 1h), likely due to less selfabsorption. In order to investigate the ionic transport in 2D perovskite single crystals, a graphene/2D perovskite/Au vertical structure is built (Figure 2a). For electrical measurements, the graphene contact is grounded and voltage stress is applied at the Au electrode (similar electrical behavior


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Submitted Manuscript: Confidential is observed if the opposite polarity is applied, see Figure S2). The cross-sectional scanning transmission electron microscopy (STEM) is used to study the ionic migration. In order to prepare the cross-sectional STEM lamella samples, another Au protection layer is deposited on top of the graphene layer to avoid ion beam damage during sample preparation using focused ion beam (FIB). Corresponding elemental mapping of Au, C, Br and Pb by energy dispersive X-ray spectroscopy (EDS) is shown in Figure S3. Benefiting from the layer exfoliated from a crystal, we can avoid the influence of grain boundaries that complicate the study of polycrystalline films. When applying a constant voltage bias of 2 V, the corresponding current is extremely low (~0.1 pA). After applying a pulsed voltage stress (10 V, 60 ms), the elemental distributions of C, Pb and Au were analyzed using cross-sectional STEM imaging and EDS cross sectional profiles, as shown in Figure 2b and 2c. Based on the distribution of the C, Pb and Au, device layers of graphene, 2D perovskite and Au can be individually identified. As shown in the fresh sample in Figure 2b, the Br concentration in the perovskite layer is uniform, whereas the Br concentration is higher at the perovskite/Au interface after voltage stress (Figure 2c), due to the positive voltage at Au electrode driving Br- ions toward Au. This is direct evidence that Br- ion migration occurs under bias, even in 2D single crystals. A particular comparison of Br and Pb distribution is shown in Figure S4, confirming that the distribution of Pb and Br for the fresh device is very similar, while after the pulsed voltage stress, the distribution of Pb and Br at the perovskite/Au interface is altered. When applying a larger and longer pulsed voltage stress (12 V, 120 ms), an abrupt current jump up to 100 µA is observed (Figure 2d). Correspondingly, a filament region with circa 1 µm diameter is visualized by comparing the atomic force microscopy (AFM) images before and after voltage stress (Figure 2e and Figure 2f). The AFM profile analysis shows a


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Submitted Manuscript: Confidential thickness decrease of approximately 10 nm (Figure 2g). This indicates that a conductive channel (filament) is formed in the perovskite layer in this region, leading to an abrupt increase in current and, in turn, the morphology and thickness change in the perovskite layer. In order to directly visualize the filament, a detailed analysis on this region is conducted by cross-sectional TEM imaging, as shown in Figure 2h, with corresponding elemental mapping of Au, C, Br and Pb by EDS shown in Figure S5. It is also noticed that the diameter of the filament close to the graphene side (~30 nm) is wider than that of the filament close to Au side (~15 nm). This indicates that ions could be driven from the graphene side to Au side. Since the voltage on the Au side is positive, the ions should be negatively charged Br-, which is consistent with the ion distribution results shown in Figure 2c. Moreover, the high current can generate a large amount of heat due to Joule heating in the local filament region. The high temperature spreads laterally through graphene and induces the 1 µm diameter and 10 nm depression in the filament area, an aspect supported by simulations presented in the next section. Figure 2j summarizes the Br- ion migration process in the 2D perovskite single crystal under electrical stress. Stage I: The positive potential applied at the Au electrode attracts Br- ions (Figure 2c). Since the Br- ions leave the crystal structure, the remaining Br- vacancies form a partial filament (Figure 2i). Due to the partially formed filament, the electric field in the nonfilament gap region is enhanced, promoting the formation of a complete filament (Figure 2h). Stage II: After one complete filament is formed, the electric field is reduced in the surrounding 2D perovskite layer and other partial filaments stop growing. Stage III: The filament is highly conductive, inducing high current, producing a temperature rise due to Joule heating, which could induce decomposition and lead to the thickness decrease (10 nm) in the filament region (Figure 2f).


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Submitted Manuscript: Confidential Considering the ion migration and low off current of 0.1 pA in the Br-based 2D perovskite single crystals, it is very suitable to be used for low energy applications, which can be operated at low program current and maintain a sufficient on/off ratio. Here we fabricated an array of graphene/2D perovskite/Au vertical devices for electrical characterization, as shown in the AFM image of Figure 3a, with Au electrodes of various widths (250 nm, 500 nm, 1 µm, 2 µm and 5 µm). Figure 3b shows a typical forming curve with a forming voltage of 7.6 V. The compliance current is set to 10 pA to confine the filament and prevent overshoot. Figure 3c shows that the forming voltage reduces with the perovskite layer thickness. A linear fit to the data indicates that the perovskite requires an electric field ~0.24 V/nm for forming. Typically, resistive memories require a higher initial voltage in order to form a complete filament, as can be observed in Figure 3b, where the current has an abrupt jump up to 10 pA at 7.6 V. The program current is thus 10 pA, at least one order of magnitude lower than conventional operation current in the range 10 mA ~ 0.1 nA.23-25 After the forming process, the set (reset) will only break (form) the partial filament instead of building the whole filament in the forming process, thus the set voltage should be lower than the forming voltage. As shown in Figure 3d, the set voltage is only 2.8 V, substantially lower than the initial forming voltage of 7.6 V. In the reset process, the negative sweep can reset the current from 3 pA to 0.1 pA (Figure 3d). Figure 3e shows the on and off current for switching at 10 pA program current up to 100 cycles, demonstrating reproducible switching behavior with an on/off ratio of ~10. More than 20 devices have been measured with behavior similar to those shown in Figure 3d. Figure 3f shows the HRS and LRS distribution showing that an average of 102 memory window can be obtained with the HRS ~1014 Ω and LRS ~1012 Ω. Furthermore, the 2D perovskite thickness dependence of set/reset voltage and on/off resistance was studied, as shown in Figure S6. The current level of HRS is increased


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Submitted Manuscript: Confidential with as 2D perovskite thickness decreases, which can be explained by the larger tunneling current due to the smaller gap distance between the edge of the filament and the top electrode for thinner perovskite layers. Since the device can be operated at very low program currents, multilevel storage can be realized by setting the device to different current levels (1 nA, 100 nA, 2 µA) as shown in Figure 3g. In order to prove that the filament dominates the response rather than surface effects, the factor dominating the on-state resistance is investigated through an array of 2D perovskite memory devices with different device areas (Figure 3a). If the on-resistance is dominated by filamentation, the on-resistance should be area-independent since the filament size is only ~20 nm diameter (Figure 2h), much smaller than the micron-scale device. In the plot of LRS and HRS vs. device area (Figure 3h), almost no change of the LRS resistance is seen for different device areas, providing convincing evidence that filament formation in the perovskite layer dominates the resistance switching, rather than surface or interface effects.26 Finally, in order to highlight the low program current in Br-based 2D perovskite memory, we compared our program current to previous work. As shown in Figure 3i, most of the previous resistive memory devices (such as devices based on TiO2,8, 9, 27 organic,28, 29 WOx,30 AlOx,24,31,32 TaOx,10, 33, 34 HfOx,35-37 MoS238) have program/set currents in the range from 10 mA to 1 µA. Our 2D perovskite-based resistive memory shows an extremely low set current down to 10 pA. Monte Carlo simulations based on a trap-assisted-tunneling (TAT) model (details provided in the methods section and a flow diagram of the simulation is shown in Figure S7) were performed to better understand the filament formation process. The model is established based on the assumption that Br- ions are responsible for the formation of conducting filaments (Figure 4a). The simulated forming curve (Figure 4b) shows the resistive switching at 6.1 V 10 ACS Paragon Plus Environment

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Submitted Manuscript: Confidential and program current at 10 pA, which are similar to the experimental results shown in Figure 3b. The temperature distribution upon filament formation reaches approximately 450 K (Figure 4c) and is responsible for the thickness reduction of the 2D perovskite observed by AFM (Figure 2f). Three stages (1), (2) and (3) are shown as representative images marked in Figure 4b for the filament growth process (Figure 4d). In stage (1), Br- ions accumulate at the Au/perovskite interface while Br- vacancies form, in agreement with our experimental observations in Figure 2c. With increasing voltage, stage (2) shows the growth of a partial filament. After voltage further increases to 6.1 V, a complete filament is formed and there is also a partial filament formed nearby, in agreement with the TEM images shown in Figure 2h and Figure 2i. The related potential and electric field distribution for these three stages are shown Figure 4e and Figure 4f. The electric field in stage (2) in Figure 4f shows that the electric field peaks near the top of the partial filament, further promoting filament growth. After filament formation at stage (3) in Figure 4e, the potential in the filament drops and the other partial filaments stop growing. To further verify the TAT model, we have compared the Monte Carlo simulation and experimental results of HRS and LRS at 300 K, 320 K and 340 K, which are shown in Figure S8. It is shown that, at higher temperatures, the resistance in HRS decreases, which indicates the validation of the TAT model. The ionic transport in 2D Br-based perovskite is analogous to biological synapses that show Ca2+ ion release and recycling processes (Figure 5a). As shown in Figure 5b, the peak current is 40 pA under a 1 V, 10 ms pulse. We assume that energy consumption is governed by ∙ ∙ , where I is the current, V is the voltage pulse amplitude, and t is the pulse duration. The calculated energy consumption is only 400 fJ/spike, very close to that of biological synapses of 1~100 fJ/spike and much lower than previous artificial synapses with energy consumption 11 ACS Paragon Plus Environment

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Submitted Manuscript: Confidential above the pJ/spike level. Further increasing the pulse amplitude can increase the peak current, as shown in Figure 5b. The 3 V, 10 ms pulse can induce a weight change (the percentage of current change/initial current) of the post-synaptic current (PSC) as short-term potentiation (Figure 5c). Positive pulses drive Br- ions toward the Au electrode, resulting in the formation of partial filaments by Br- vacancies that enhance the conductance. Negative pulses are capable of driving Br- ions back to the Br- vacancies, recovering the conductance to the initial lower current level (Figure 5d), which can be regarded as short-term depression. Pulse trains with two -2 V pulses and two 2 V pulses are also applied to the device (Figure 5e). The first pulse of -2 V resets the PSC to the lower level while the second -2 V pulse does not have a pronounced effect because the Br- ions have already migrated as a result of the first pulse. Then the following two pulses of 2 V can induce short-term potentiation. Such short-term potentiation relies on Br- ion motion influenced by the electric field (increasing the PSC instantly) followed by the back diffusion of Br- ions (recovery time of PSC in 1 s). Short-term potentiation (STP) is shown as a first round of six input pulses with gradually increasing peak current (Figure 5f). Additional scans are shown in Figure S9 to prove the paired pulse facilitation (PPF) effect. Moreover, a second round of six pulses can induce short-term transition to long-term potentiation at approximately 1 nA (Figure 5g). This is due to complete filament formation after more pulses applied. The long-term behavior is stable and can be sustained for more than 1000 s. As shown in Table 1, comparing with previous perovskite and metal-oxide memories,9,22,24,39-43 our 2D perovskite memory shows the lowest power consumption for both set and reset operations. Previously, perovskite based memories mainly used 3D perovskite polycrystalline films prepared via spin coating, which can induce a lot of grain boundaries and defects resulting in larger leakage current, operation current and power consumption.22,41-43 In


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Submitted Manuscript: Confidential this work, since the 2D perovskite thin single-crystal film via mechanical exfoliation from a bulk 2D perovskite single crystal is used, the leakage pathway is blocked and the off-current can be extremely low (0.1 pA), which enables low operation current (10 pA) and power consumption. Moreover, for synaptic applications, the energy consumption is only 400 fJ/spike, which is much lower than previous perovskite synapses.44,45 The energy consumption reported in this work is very close to the energy consumption in biological synapses (1-100 fJ/spike), an aspect that is very intriguing for neuromorphic electronics applications. Conclusions In conclusion, we have synthesized 2D (PEA)2PbBr4 perovskite single crystals with a wide band gap of ~2.9 eV. These single crystals were exfoliated to thin layers and incorporated within a graphene/2D perovskite/Au vertical structure to investigate their electrical properties in resistive memory devices. After a set operation, a ~20 nm diameter filament inside the 2D perovskite was directly observed by TEM, and elemental density profiling support a mechanism of Br- ion motion as being responsible for the filament. Furthermore, Monte Carlo simulations based on Br- ion and vacancy movement to form filaments supported the experimental observations. Due to suppressed charge transport in the out-of-plane direction of the 2D perovskite films, the off current can be as low as 0.1 pA, which enabled the resistive memory to work at extremely low operation currents of 10 pA with an on/off ratio of 100. To the best of our best knowledge, the 10 pA operation current in 2D perovskite resistive memory is the lowest operation current ever reported for resistive memory devices. Because of the extremely low program current, the resistive memory demonstrated 400 fJ/spike synaptic operation, which is very close to the energy consumption of biological synapses. Short-term potentiation and longterm potentiation have been demonstrated in 2D perovskite synaptic devices, which show the 13 ACS Paragon Plus Environment

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Submitted Manuscript: Confidential potential of using 2D perovskites as building blocks of neuromorphic circuits and systems. In the future, extremely energy efficient backpropagation neural networks can be built based on an array of 2D perovskite resistive memories, which is fundamental for high performance, lowenergy applications. Methods Materials synthesis. PEABr was synthesized by mixing phenethylamine (Sigma-Aldrich) with HBr (Sigma-Aldrich) in a 1:1 molar ratio at 0 °C with constant stirring under N2. The PEABr was washed with an ethanol/ether mixture and rotovaped several times for purification and then recrystallized from an isopropyl alcohol/toluene mixture. PbBr2 was purchased from Alfa Aesar, and used as received. PEA2PbBr4 2D perovskite single crystals were synthesized using an antisolvent vapor-assisted crystallization method. PEABr and PbBr2 with a molar ration of 2:1 were dissolved in DMF (Sigma-Aldrich, 99.8% anhydrous) and stored in an atmosphere of chlorobenzene vapor. After several days, PEA2PbBr4 2D perovskite single crystals were obtained with a size of a few millimeters. Material characterization. XRD measurements were performed with a Bruker D8 Discover Xray diffractometer with a conventional Cu target X-ray tube set to 40 kV and 40 mA. The SEM measurements were conducted with an FEI Verios 460 XHR (extreme high resolution) SEM. Cross-sectional TEM lamella samples of devices were prepared by an FEI Helios DualBeam microscope. A voltage/current as small as 5 kV/41 pA was used for final polishing in order to effectively minimize ion-beam-induced sample damage. STEM images and EDS measurements were carried out in an FEI Talos (S)TEM at 200 kV. The AFM images were captured using a Bruker Dimension-Icon FastScan system. Absorption spectra were measured using a Cary 5000


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Submitted Manuscript: Confidential UV-Vis-NIR system (Agilent). PL spectra were measured with a Horiba Raman Spectrometer with an excitation wavelength of 325 nm. Device fabrication and characterization. 2D perovskite layers were exfoliated by the wellknown tape method from bulk crystals. A PDMS stamp was used to pick up the flakes and transfer them onto the Au electrode. Following transfer of the perovskite layer, graphene was transferred on top. Electrical characterizations were carried out using an Agilent B1500A parameter analyzer in a Lakeshore cryogenic probe station. All measurements were performed in vacuum (

Extremely Low Operating Current Resistive Memory Based on

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