High-Performance Nanofloating Gate Memory Based on Lead Halide

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24367−24376

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High-Performance Nanofloating Gate Memory Based on Lead Halide Perovskite Nanocrystals Tianhao Jiang,† Zhibin Shao,*,† Huan Fang,† Wei Wang,† Qiao Zhang,† Di Wu,‡ Xiujuan Zhang,*,† and Jiansheng Jie*,† †

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Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, P. R. China ‡ School of Physics and Engineering, and Key Laboratory of Material Physics, Ministry of Education, Zhengzhou University, Zhengzhou, Henan 450052, P. R. China S Supporting Information *

ABSTRACT: Lead halide perovskites have been extensively investigated in a host of optoelectronic devices, such as solar cells, light-emitting diodes, and photodetectors. The halogen vacancy defects arising from the halogen-poor growth environment are normally regarded as an unfavorable factor to restrict the device performance. Here, for the first time, we demonstrate the utilization of the vacancy defects in lead halide perovskite nanostructures for achieving high-performance nanofloating gate memories (NFGMs). CH3NH3PbBr3 nanocrystals (NCs) were uniformly decorated on the CdS nanoribbon (NR) surface via a facile dip-coating process, forming a CdS NR/CH3NH3PbBr3 NC core−shell structure. Significantly, owing to the existence of sufficient carrier trapping states in CH3NH3PbBr3 NCs, the hybrid device possessed an ultralarge memory window up to 77.4 V, a long retention time of 12 000 s, a high current ON/OFF ratio of 7 × 107, and a longterm air stability for 50 days. The memory window of the device is among the highest for the low-dimensional nanostructurebased NFGMs. Also, this strategy shows good universality and can be extended to other perovskite nanostructures for the construction of high-performance NFGMs. This work paves the way toward the fabrication of new-generation, high-capacity nonvolatile memories using lead halide perovskite nanostructures. KEYWORDS: nanofloating gate memories (NFGMs), perovskite nanocrystals, CH3NH3PbBr3, CdS nanoribbons (NRs), large memory window

1. INTRODUCTION Nanofloating gate memories (NFGMs) based on low-dimensional semiconductors have attracted considerable interest because of their large storage capacity, fast programming/ erasing speed, and stable memory performance.1−4 Compared to conventional floating gate memories, NFGMs normally introduce discrete metallic or semiconducting nanocrystals (NCs) as charge trapping layers. The charges are stored in the discrete NCs, thus eliminating the issues of cell-to-cell interference and leakage current in conventional floating gate memories.5,6 For NFGM devices, the memory window, that is, the difference in threshold voltages between programming and erasing states, is a key parameter that determines their charge storage capacities.7 The larger memory window denotes a higher accuracy during data reading as well as a better potential for multilevel storage in high-density memory devices.8 In order to achieve a large memory window, a viable route is to increase the charge capacities of the NCs. Metallic NCs exhibit a high charge storage capability because of the presence of a host of electronic states near the Fermi level that can trap charges. However, metallic NCs are usually prepared via high© 2019 American Chemical Society

vacuum physical vapor deposition techniques such as thermal evaporation and magnetron sputtering.9 During the hightemperature deposition process, metal atoms may penetrate into the tunneling layer, thus resulting in a large leakage current.10 Alternatively, the solution-processable semiconducting NCs have emerged as promising charge trapping media in NFGMs.11,12 In this case, the charges are stored in the conduction or valence band of semiconducting NCs.13,14 Nevertheless, the misalignment of energy levels between semiconducting NCs and channel semiconductor will induce a large energy barrier for charge tunneling, which restricts the charge storage capabilities of semiconductor NCs. In recent years, lead halide perovskites have been extensively studied in various optoelectronic devices, such as solar cells,15,16 light-emitting diodes,17−20 and photodetectors,21−25 owing to their high absorption efficiency, adjustable band gap, high photoluminescence quantum yields (PLQYs), and Received: February 25, 2019 Accepted: June 12, 2019 Published: June 12, 2019 24367

DOI: 10.1021/acsami.9b03474 ACS Appl. Mater. Interfaces 2019, 11, 24367−24376

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Low-magnification TEM image, (b) XRD pattern, (c) electron diffraction pattern, (d) HRTEM image, and (e) PL spectrum of the CH3NH3PbBr3 NCs. The inset in (d) is a corresponding FFT image of the HRTEM image. (f) Calculated energy band structures of CH3NH3PbBr3 without and with a bromine vacancy (VBr), respectively. Insets in (f) are atomic structures of the CH3NH3PbBr3 unit cells, where the green and blue spheres represent the Pb and Br atoms, respectively, and the central cluster represents a CH3NH3 group.

solution processability.26−29 In particular, lead halide perovskites show great promise as next-generation, high-efficiency photovoltaic materials.30 Photoelectric conversion efficiencies of the perovskite solar cells have rapidly increased from 3.8 to 22.7% in the past few years.31 However, an undesirable offset of the open-circuit voltage is often observed for the current− voltage (I−V) curves under forward and reverse sweeps.32 Compared with other inorganic semiconductors, lead halide perovskites are prone to generate point defects during the synthesis process because of relatively lower thermal stability.33 Charge trapping in these defects is suggested to be responsible for the offset of open-circuit voltage in perovskite solar cells.34,35 In addition, these defects will enhance the recombination of photogenerated charges, leading to the degradation of device performance. To address this issue, many efforts have been devoted to eliminate or passivate the defects in lead halide perovskites.36−41 However, on the other hand, the defect states may offer sufficient charge storage sites for memory applications;42,43 the defect levels are easier to align with the energy level of the channel semiconductor, thus enabling a high-capacity charge storage. However, lead halide perovskite NC-based NFGMs have yet to be reported. Herein, we demonstrate, for the first time, the fabrication of NFGM devices based on the CH3NH3PbBr3 NC storage layer. The existence of abundant bromine vacancy defects in the CH3NH3PbBr3 NCs, which are generated due to the brominepoor growth environment, can provide sufficient charge storage sites for the NFGM devices. Significantly, CH3NH3PbBr3 NC-based NFGMs exhibit an ultralarge memory window of 77.4 V, which is among the highest for the low-dimensional nanostructure-based NFGMs (Table S1). The devices also possess excellent memory characteristics, including a long retention time of 12 000 s, a high current ON/OFF ratio of 7 × 107, and a long-term air stability for 50 days. This strategy shows good universality and could be extended to other perovskite materials such as CH3NH3PbI3 and inorganic CsPbBr3 NCs. Our work demonstrates the great potential of perovskite NCs as storage media for highperformance nonvolatile memory devices.

2. EXPERIMENTAL SECTION Materials: methylammonium bromide (CH3NH3Br, 99.5%), lead(II) bromide (PbBr2, 99.99%), lead(II) iodide (PbI2, 99.99%), and cesium bromide (CsBr, 99.9%) were purchased from Xi’an Polymer Light Technology Corp. and used without further purification. 2.1. Synthesis of CH3NH3PbBr3, CH3NH3PbI3, and CsPbBr3 NCs. In a typical synthesis of CH3NH3PbX3 (X = Br− or I−) NCs, noctylamine (30 μL) and oleic acid (250 μL) were added dropwise into 2 mL of dimethylformamide (DMF). The solution was put in a 70 °C water bath under magnetic stirring. After thermal equilibrium was established, a mixture of 0.16 mmol CH3NH3X and 0.2 mmol PbX2 was quickly dissolved in the solution. The mixed solution was allowed to react at 70 °C for 1 h. Then, 250 μL of the solution was added dropwise into 10 mL of toluene under vigorous stirring. After centrifugation at 7000 rpm for 5 min, the CH3NH3PbX3 NC colloidal solution was obtained by extracting the supernatant. In a typical synthesis of CsPbBr3 NCs, a mixture of 0.16 mmol PbBr2 and 0.2 mmol CsBr was dissolved in 5 mL of DMF containing 300 μL of oleylamine and 900 μL of oleic acid. The mixed solution was then stirred at 90 °C for 2 h. After that, the products were dispersed in a toluene solvent by a centrifugation process to obtain a CsPbBr3 NC colloidal solution. 2.2. Synthesis of CdS Nanoribbons and CH3NH3PbBr3 NCDecorated CdS Nanoribbons. The synthesis of CdS nanoribbons (NRs) was carried out in a horizontal corundum tube furnace by using a chemical vapor deposition (CVD) method. An alumina boat loading 0.5 g of CdS powder was placed at the center of the furnace tube, whereas a Si substrate covered with a thin layer of Au (5 nm) catalyst was placed at a position about 7 cm away from the CdS source. After the furnace tube was evacuated to a pressure of 0.02 mbar, argon gas mixed with 5% hydrogen was introduced into the furnace tube. The pressure in the furnace tube was maintained at 0.1 bar during the following CVD growth process. Then, the CdS powder was rapidly heated to 950 °C at a rate of 20 °C min−1. After a growth duration of 2 h, the yellow product was collected from the Si substrate. To decorate the CdS NRs, the substrate on which CdS NRs had been predispersed was immersed into the CH3NH3PbBr3 NC colloidal solution. CH3NH3PbBr3 NCs were then deposited uniformly on the CdS NRs via a dip-coating process by using a motorized positioning system (Longer Pump TS-1A) at a rate of 2 μm/s. 2.3. Material Characterizations. Transmission electron microscope (TEM, FEI Tecnai G2 F20 S-TWIN), optical microscope (Leica DM4000M), and atomic force microscope (AFM, Cypher S) were used to characterize the morphologies and structures of CH3NH3PbBr3 NCs as well as CH3NH3PbBr3 NC-decorated CdS 24368


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Figure 2. (a) Schematic illustration of the decoration process for CH3NH3PbBr3 NCs on the CdS NR surface via dip coating. (b) Photomicrograph of the CH3NH3PbBr3 NC-decorated CdS NR under UV light irradiation. (c,d) AFM images of the CdS NR before (left) and after (right) the NC decoration. (e) HRTEM image of the CH3NH3PbBr3 NC-decorated CdS NR. NRs. X-ray diffraction (XRD, PANalytical Empyrean) and electron diffraction were used to determine their crystal structures. PL (Varian model FLS920P) and Kelvin probe force microscope (KPFM, Cypher S) were used to detect the optical properties and the surface potential of CH3NH3PbBr3 NCs, respectively. 2.4. Device Fabrication and Characterizations. NFGM devices based on CH3NH3PbBr3 NC-decorated CdS NRs were constructed as follows: the as-synthesized CdS NRs were dispersed on SiO2 (300 nm)/n+-Si (resistivity < 0.02 Ω·cm) substrates via a contact printing technique.44 Then, indium (200 nm) electrodes with 25 μm spacing were defined on CdS NRs by ultraviolet (UV) photolithography (SUSS MicroTec MJB4), followed by metal evaporation (Kurt J. Lesker PVD 75) and a lift-off process. A positive photoresist (Allresist AR-P 5350) was used during the UV photolithography process. The Si substrate served as the global bottom gate electrode. Perovskite NCs were then deposited on the CdS NRs as storage media via a dip-coating method. The electrical memory characteristics of the CH3NH3PbBr3 NC-decorated CdS NRs were detected by a semiconductor characterization system (Keithley 4200-SCS). For comparison, the pure CdS NR and CH3NH3PbBr3 NC devices were investigated.

high-resolution TEM (HRTEM) image of a representative CH3NH3PbBr3 NC, along with the corresponding fast Fourier transform (FFT) image in the inset. The lattice fringes with a spacing of 0.29 nm and the one-dimensional reciprocal lattice in the FFT image corroborate the single-crystalline nature of CH3NH3PbBr3 NCs. The PL spectrum of the CH3NH3PbBr3 NCs was measured to investigate their energy band structure. In Figure 1e, a strong emission peak and a shoulder peak can be observed at 529 and 493 nm, respectively. As previously reported, the intrinsic PL peaks of CH3NH3PbBr3 NCs are normally located at 482−515 nm.45−47 Therefore, the luminescence peak at 493 nm (2.52 eV) can be attributed to the intrinsic luminescence of CH3 NH3 PbBr3 NCs. On the other hand, during the preparation of CH3NH3PbBr3 NCs, a large number of bromine vacancy defects could be generated because of the brominepoor growth environment, thus creating new defect energy levels in the forbidden band. As a result, a defect luminescence with longer wavelength can usually be observed, accompanying with the intrinsic luminescence.48,49 Hence, the luminescence peak at 529 nm (2.34 eV) in the PL spectrum should derive from the bromine vacancy defects in CH3NH3PbBr3 NCs. To gain more insight into the structure of CH3NH3PbBr3 NCs, we calculated the energy band structures of CH3NH3PbBr3 with and without bromine vacancies by first-principles density functional theory (Figure 1f). Obviously, the appearance of the bromine vacancy defect will introduce a new electronic state (red line) at about 0.19 eV below the conduction band of CH3NH3PbBr3. The theoretical energy difference of 0.19 eV between conduction band minimum and defect state is very close to the value (0.18 eV) extracted from the PL measurement, confirming that the shoulder emission peak should originate from the bromine vacancies in CH3NH3PbBr3 NCs. The strong defect emission in Figure 1e also implies the existence of a large number of defect states in CH3NH3PbBr3 NCs. Note that, for photovoltaic applications, the defect states are detrimental to the performance of photovoltaic devices

3. RESULTS AND DISCUSSION In this work, CH3NH3PbBr3 NCs were prepared according to a previously reported ligand-assisted reprecipitation method.45 Figure 1a shows a low-magnification TEM image of the assynthesized CH3NH3PbBr3 NCs. The spherical CH3NH3PbBr3 NCs possess a relatively narrow diameter distribution of 4.8− 10.2 nm. The XRD measurement was performed to characterize the crystal structure of the CH3NH3PbBr3 NCs (Figure 1b). The diffraction peaks at 15.4°, 21.6°, 30.6°, 33.7°, 37.0°, 43.6°, and 46.3° can be indexed to the (100), (110), (200), (210), (211), (220), and (300) lattice planes of the cubic CH3NH3PbBr3, respectively. Also, the cubic crystal structure of CH3NH3PbBr3 NCs can be confirmed by the electron diffraction measurement (Figure 1c). The electron diffraction pattern obtained from the entire region in Figure 1a displays a series of diffraction rings that correspond to (200), (210), and (300) lattice planes of cubic CH3NH3PbBr3. Figure 1d shows a 24369


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Figure 3. (a) Schematic illustration of the NFGM based on the CH3NH3PbBr3 NC-decorated CdS NR. (b) Output characteristic curves of a representative NFGM device measured with VG ranging from 0 to 60 V. (c) Electrical transfer characteristics of the NFGM device recorded after every sweeping cycle. The sweeping directions are indicated by the arrows. A fixed VDS of 1 V was applied. (d) Switching characteristics of the device measured by switching VG between −60 and +60 V.

∼118.7 nm, along with an increase of surface roughness of the CdS surface from 0.49 to 3.24 nm. The height difference of ∼10.6 nm implies that 2−3 layers of NCs were closely deposited on the surface of the CdS NR. HRTEM characterization was performed to verify the core−shell structure of the CH3NH3PbBr3 NC-decorated CdS NR. In Figure 2e, the CdS NR core shows a single-crystalline wurtzite structure with a growth direction of [001], whereas the NCs are found to tightly wrap on the surface of the CdS NR, forming the shell layer. The NCs present several sets of fringes with different lattice spacings of 0.26 and 0.29 nm, corresponding to (210) and (200) lattice planes of cubic CH3NH3PbBr3, respectively. Note that some CH3NH3PbBr3 NCs will fall off from the CdS NR surface during the TEM sample preparation; hence, the thickness of the NC layer (∼3.5 nm) observed in HRTEM is thinner than the thickness (∼10 nm) measured in AFM. The closely packed CH3NH3PbBr3 NCs will serve storage media to improve the storage capacity of the core−shell structure-based floating-gate memory. Figure 3a illustrates the NFGM device based on the CH3NH3PbBr3 NC-decorated CdS NR on the SiO2 (300 nm)/n+-Si substrate. The indium (In) source and drain electrodes, which exhibit excellent ohmic contact with the CdS NR,53,54 were defined by UV photolithography, whereas the highly conductive Si substrate served as a bottom gate electrode. The uniform CH3NH3PbBr3 NC layer was deposited on the CdS NR as a charge trapping layer. Figure 3b depicts the output characteristics of a representative device measured by increasing the gate voltage (VG) from 0 to 60 V. Note that the conductance of the device increases monotonically with increasing VG, revealing a typical behavior for the nchannel field-effect transistor. Also, at different gate voltages, source−drain currents (IDS) are always proportional to the source−drain voltage (VDS). The linear curves demonstrate the excellent ohmic contact between CdS NR and In electrodes. Electrical transfer characteristics of the NFGM were measured by applying a sweeping voltage of ±60 V (Figure 3c).

because of the enhanced recombination of photogenerated carriers at defect states.50 However, from the viewpoint of NFGM device applications, these electronic states in CH3NH3PbBr3 NCs can provide sufficient charge storage sites, making CH3NH3PbBr3 NCs an ideal candidate as storage media in floating-gate memories. Next, CH3NH3PbBr3 NCs were uniformly decorated on the CdS NR surface via a facile dip-coating method, forming a CdS NR/CH3NH3PbBr3 NC core−shell structure. The CdS NRs possess a single-crystalline wurtzite structure with a growth orientation along the [001] direction. The width of the NRs is 1−7 μm, whereas the length of the NRs is up to several hundreds of micrometers (Figure S1). As illustrated in Figure 2a, the SiO2 (300 nm)/n+-Si substrate with CdS NRs on it was immersed into a suspension of CH3NH3PbBr3 NCs (1.8 mmol/L) and then pulled out at a rate of 2 μm/s. The suspension near the substrate formed a meniscus at the air/ suspension interface on the substrate under the combined effect of viscous drag and gravity. As the solvent evaporated, CH3NH3PbBr3 NCs precipitated at the meniscus interface and assembled into a NC film.51 However, in addition to viscous drag and gravity, the suspension around the CdS NR was also subjected to capillary force between the CdS NR and the substrate. Therefore, during the dip-coating process, the CdS NR would serve as a “pinner” to pin the suspension droplets.52 This droplet-pinned effect led to the steady deposition of CH3NH3PbBr3 NCs around the CdS NR, eventually forming a closely packed NC film that wrapped on the CdS NR surface. Figure 2b shows a photomicrograph of the CH3NH3PbBr3 NC-decorated CdS NR under UV light irradiation. Because CH3NH3PbBr3 NCs can emit green fluorescence under UV light irradiation, we can observe that more green-emitting CH3NH3PbBr3 NCs have been successfully decorated on both the top surface and the sides of the CdS NR. Thickness and surface roughness of the CdS NR before and after decoration of NCs were further characterized by AFM (Figure 2c,d). After decoration, the height of the CdS NR increases from ∼108.1 to 24370


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Figure 4. (a) Transfer characteristic curves of the NFGM device based on the CH3NH3PbBr3 NC-decorated CdS NR under different sweeping voltage ranges from ±60 to ±10 V. (b) KPFM image of the CH3NH3PbBr3 NC-decorated CdS NR. The inset is the line profile of the Kelvin potential extracted from the KPFM image. (c) Energy band diagrams of the NFGM device during the charge storage process.

This result clearly demonstrates the existence of a large number of defects in the NCs synthesized under a brominepoor environment.56,57 Furthermore, NFGM devices were fabricated based on the CH3NH3PbBr3 NCs with different defect densities. From the electrical transfer characteristics in Figure S4b, it can be seen that the hysteresis windows of NFGM devices become narrower with decreasing the defect density in CH3NH3PbBr3 NCs. This result also demonstrates that the storage sites in the NFGM devices are derived from the bromine vacancy defects in CH3NH3PbBr3 NCs. To investigate the charge storage process, transfer characteristics of the NFGM device based on the CH3NH3PbBr3 NCdecorated CdS NR were measured under different sweeping voltage ranges. As shown in Figure 4a, six sets of sweeping gate voltages from ±10 to ±60 V are applied to the device. As the absolute value of sweeping gate voltage increases, the transfer characteristic curves under forward and reverse sweeps are separated wider to each other, indicating that the CH3NH3PbBr3 NCs can offer sufficient trapping sites for electron storage. Notably, the device still exhibits a hysteresis window of 6.2 V at a sweeping voltage of ±10 V because of the high charge storage efficiency of the device, indicating that the memory function of the device can be implemented with a relatively small operation voltage. In addition, surface potentials of CH3NH3PbBr3 NCs on the surface and the periphery of the CdS NR were compared by using a KPFM, as shown in Figure 4b. It is observed that the Kelvin potential of the NCs on the CdS surface (422.7 mV) is 42.2 mV lower than that of the NCs around the CdS NR (464.9 mV), which is an evidence of electron transfer from CH3NH3PbBr3 NCs to the CdS NR. As a result, the memory device exhibits a lowresistance status (Figure S5), confirming the increase of electron concentration in the CdS NR. Figure 4c displays the energy band diagrams of the NFGM device based on the CH3NH3PbBr3 NC-decorated CdS NR during the charge storage process. When a positive gate voltage (VG > 0 V) is applied, the electron concentration in the CdS NR is remarkably improved, so that the CdS NR transistor is switched on. Driven by the gate electric field, electrons are thus injected from the CdS NR into the trap states in the

Significantly, the device exhibits an ultralarge hysteresis window of 77.4 V, which is superior to the previously reported low-dimensional nanostructure-based NFGMs (Table S1). Such a large memory window confirms the high charge storage capacity of the CH3NH3PbBr3 NCs. After multiple sweeping cycles, the hysteresis window remains essentially unchanged, indicating the good repeatability of memory performance. In addition, the resistivities at VG = 0 in positive and negative sweeps are calculated to be 0.38 and 4.2 × 106 Ω cm, respectively, corresponding to the low- and highimpedance states (i.e., bistable states) of the device. Figure 3d shows the dynamic switching characteristics of the CH3NH3PbBr3 NC-decorated CdS NR-based NFGM. The current ON/OFF ratio of the device can still be maintained at 107 after 500 switches, manifesting the high stability and durability of the device. To shed light on the storage medium in our NFGM, various comparative experiments were conducted. First, the pure CH3NH3PbBr3 NCs and pure CdS NR-based devices were investigated. Figure S2 shows the I−V curve of the In/ CH3NH3PbBr3 NC film/In device, revealing the ultrahigh resistance of the NC film. Such a high resistance may be attributed to the insulating long-chain ligands wrapped on the NC surface. Therefore, no storage function can be observed for the pure CH3NH3PbBr3 NC-based device. On the other hand, we had also measured the electrical transfer characteristics of CdS NR-based transistors without CH3NH3PbBr3 NC decoration (Figure S3). Only a small hysteresis window (∼7 V) is observed because of the weak charge storage capability of the pure CdS NR. In this case, the storage medium mainly comes from the storage sites introduced by oxygen adsorption on the surface of the CdS NR.55 Second, NFGM devices based on CH3NH3PbBr3 NCs with different defect densities were also investigated. The density of bromine vacancy defects can be adjusted by controlling the ratio of lead to bromine precursors. Figure S4a shows the PLQYs of the CH3NH3PbBr3 NCs synthesized by changing the ratio between Pb and Br precursors. It is obvious that the PLQY of the CH3NH3PbBr3 NCs decreases with increasing the Pb/Br ratio; the PLQY decreases to ∼27% for a relatively higher Pb/Br ratio of 1/2.8. 24371


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Figure 5. (a) Retentivity and (b) endurance measurements of the NFGM device based on the CH3NH3PbBr3 NC-decorated CdS NR. In (a), the ON/OFF current was recorded at a reading voltage of 0 V after applying a programming/erasing pulse of −60/60 V for 2 s. In (b), the current (bottom) was recorded at the reading voltage of 0 V after applying a periodic programming/erasing pulses of −60/60 V (top). (c) Statistical histogram of the hysteresis windows for 30 memory devices. (d) Air stability of the memory device. ΔVTH is the difference between the threshold voltages in positive sweep and negative sweep, that is, the hysteresis window. The inset shows transfer characteristic curves of the memory device exposed to air for 0 and 50 days.

CH3NH3PbBr3 NCs via Fowler−Nordheim tunneling,58 which corresponds to an erasing process of the device. Notably, after the removal of the gate voltage, electrons could still be trapped in the NCs because of the insulating nature of alkyl chains wrapped on the NC surface (Figure S6). During the reading process of the device (VG = 0 V), the negatively charged NCs can serve as floating gates to switch off the CdS NR transistor (i.e., OFF state). When a negative gate voltage (VG < 0 V) is applied, the electrons in the CdS NR are depleted. Meanwhile, electrons in the CH3NH3PbBr3 NCs will be pumped back into the CdS NR for device programming. After removing the negative gate voltage, the device can return to the low impedance state (i.e., ON state). To assess the reliable nonvolatile storage characteristics of memory devices based on CH3NH3PbBr3 NC-decorated CdS NRs, the retentivity, endurance, reproducibility, and air stability of the devices were further investigated. Figure 5a shows the retention characteristics of the floating-gate memory after applying a programming/erasing pulse. During the continuous reading of 12 000 s, both ON and OFF currents of the device remain essentially constant, maintaining the current ON/OFF ratio greater than 7 × 107. Notably, after applying the programming/erasing pulse cyclically, the dualresistance states of the device can also be accurately repeated, indicating a good endurance of the device (Figure 5b). To maintain the high ON/OFF ratio, programming/erasing pulses of 0.7 s (switching time) were applied during the endurance measurements. We note that the switching time of our NFGM device is comparable to those of other low-dimensional semiconductor-based NFGMs (0.1−5 s, Table S1). For comparison, the retention and endurance characteristics of the pure CdS NR-based device were also investigated (Figure S7). It is observed that the pure CdS NR-based device shows much poorer retention and endurance characteristics. This result unambiguously demonstrates the importance of the decoration of perovskite NCs in improving the device

performance. In order to study the reproducibility of the memory devices, 30 devices were fabricated and analyzed (Figure S8). Figure 5c shows a statistical histogram of the hysteresis windows of these devices. The hysteresis windows of memory devices are distributed in the range of 46.2−77.4 V, with an average value of 58 V. The change in the hysteresis windows is mainly attributed to the fluctuation of the dipcoating process for the deposition of CH3NH3PbBr3 NC shells. Furthermore, the stability of the device in the atmosphere was investigated (Figure 5d). After 50 days of exposure to the atmosphere, the current ON/OFF ratio changes little, remaining larger than 107, although the hysteresis window decreases slightly from 77.4 to 61.3 V. This result demonstrates the excellent air stability of the device. The slight decline in the hysteresis window may be caused by the cleavage reaction of a small portion of CH3NH3PbBr3 NCs in the air.59 In recent years, lead halide perovskite-based resistance random access memory (Re-RAM) devices have attracted much research interests because of their simple configuration, low operating voltage, and fast speed.60−69 Table S2 compares the performances of the NFGM device and the lead halide perovskite-based Re-RAM devices in terms of switching speed, retentivity, endurance, ON/OFF current, ON/OFF ratio, and power consumption. Note that the Re-RAM devices have faster switching speeds (100 ns to 0.2 s) than our NFGM device (0.7 s) because the switching mechanism of formation and rupture of conducting filaments determines the fast switching characteristics of Re-RAMs. In the case of stability, both the NFGM and Re-RAM devices based on perovskite materials exhibit long retention times (103 to 104 s) and excellent endurance characteristics (102 to 103). Nevertheless, the ON/OFF ratio of our NFGM device (>7 × 107) is higher than those of the Re-RAM devices (1 to 107). This might be attributed to the lower leakage current of the NFGM device. In the Re-RAM devices, the use of the lead halide perovskite 24372


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Figure 6. (a) Electrical transfer characteristics of the NFGM device measured by applying different sweeping voltages of from +60 to −60 V (black line), from −20 to +60 V (blue line), from −40 to +60 V (green line), and from −60 to +60 V (red line). VDS was fixed at 1 V. (b) Conductivities extracted at VG = 0 V from four sweeps in (a). (c,d) Pulsed gate voltages applied in retention measurement and corresponding retention characteristics of the device in four states. The current was recorded at a reading voltage of 0 V after applying an erasing pulse (+60 V) for 2 s and a programming pulse (0, −20, −40, or −60 V) for 2 s.

Figure 7. (a,b) TEM images of the CH3NH3PbI3 and CsPbBr3 NCs, respectively. Insets are their corresponding HRTEM images. (c,d) Transfer characteristic curves of the NFGM devices based on the CH3NH3PbI3 and CsPbBr3 NC-decorated CdS NRs, respectively.

dielectric layer will generate a considerable leakage current (OFF current, >1 nA), whereas conducting filaments in the ON state will also cause a large ON current (∼1 mA). The large ON/OFF current will increase the power consumption of the device in the active/standby mode. As a result, the power consumptions of Re-RAM devices, especially in the standby mode, are higher than those of our NFGM device. The high ON/OFF ratio and large memory window are beneficial for multilevel storage. In our NFGM device, the multilevel storage can be achieved by applying the different

programming and erasing voltages (Figure 6). In Figure 6a, electrical transfer characteristics were measured by applying the sweeping voltages of from +60 to −60 V (black line), from −20 to +60 V (blue line), from −40 to +60 V (green line), and from −60 to +60 V (red line). Interestingly, the device exhibits four states at VG = 0 V in four different sweeps. Their conductivities can be calculated to be 1.7 × 10−5, 60.3, 123.9, and 209.5 S m−1 (Figure 6b). Notably, the conductivity increases with decreasing the programming voltage, revealing that fewer electrons are retained in the CH3NH3PbBr3 NCs at 24373



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lower programming voltage. These four levels can be defined as binary “00”, “01”, “10”, and “11” states. Furthermore, the retention characteristics of the device in four states were investigated by applying pulsed programming voltages (0, −20, −40, and −60 V) and reading voltage of 0 V (Figure 6c). Before applying the programming voltage, the device was reset into the high-resistance state by applying an erasing voltage of +60 V. From Figure 6d, it is observed that the device possesses a retention time over 5000 s, revealing the high stability of the four states. In order to demonstrate the universality of our strategy, we have also constructed the NFGM devices by using other perovskite materials as storage media, including CH3NH3PbI3 and inorganic CsPbBr3 NCs. Similar to CH3NH3PbBr3 NCs, both the CH3NH3PbI3 and CsPbBr3 NCs were synthesized via the ligand-assisted reprecipitation method.45 Figure 7a,b shows the TEM images of CH3NH3PbI3 and CsPbBr3 NCs, respectively. The diameter of spherical CH3NH3PbI3 NCs is distributed in the range of 5.7−9.6 nm. The lattice fringes with a spacing of 0.28 nm can be indexed to the (210) crystal plane of cubic CH3NH3PbI3. In addition, the cubic CsPbBr3 NCs have a particle size of about 10 nm and the lattice fringes with a spacing of 0.41 nm correspond to the (110) lattice plane of cubic CsPbBr3. From the transfer characteristics in Figure 7c,d, the NFGM devices based on CH3NH3PbI3 and CsPbBr3 NCdecorated CdS NRs possess hysteresis windows of 26.4 and 45.8 V, respectively. The defect states inside the CH3NH3PbI3 and CsPbBr3 NCs have been intensively studied,70,71 which can also provide sufficient trapping sites for charge storage in NFGM devices. Therefore, these memory devices exhibit storage behavior similar to the NFGMs based on the CH3NH3PbBr3 NC-decorated CdS NR. In contrast to the CH3NH3PbBr3 and CsPbBr3 NCs, the device based on CH3NH3PbI3 NCs shows a relatively smaller hysteresis window. The main reason is that energy levels for the defects in CH3NH3PbI3 (i.e., iodine vacancy defects) are located above the conduction band minimum of CH3NH3PbI371 and are higher than the conduction band of CdS, leading to a large energy level misalignment (Figure S9). Charge transfer from the CdS NR to CH3NH3PbI3 NCs is thus hindered by the energy barrier between them. However, the energy levels for bromine vacancy defects are located in the band gaps of CH3NH3PbBr3 and CsPbBr3. The energy level alignments allow the efficient charge transfer and storage.

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b03474. Comparison of device performances of low-dimensional semiconductor-based NFGMs, comparison of device performances between NFGM and Re-RAM devices based on lead halide perovskites, characterizations of CdS NRs, Fourier transform infrared spectroscopy of CH3NH3PbBr3 NCs, electrical properties of pure CH3NH3PbBr3 NCs and pure CdS NR, stability measurements of pure CdS NR device, memory performances of the CH3NH3PbBr3 NC-decorated CdS NR-based NFGM devices with different defect densities in the CH3NH3PbBr3 NCs, I−V curve of the our NFGM device measured under ON status, transfer characteristic curves of 30 NFGM devices, and energy level diagram of CdS and different perovskite materials (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.S.). *E-mail: [email protected] (X.Z.). *E-mail: [email protected] (J.J.). ORCID

Zhibin Shao: 0000-0003-0803-4785 Qiao Zhang: 0000-0001-9682-3295 Di Wu: 0000-0003-3266-0612 Jiansheng Jie: 0000-0002-2230-4289 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research P r o g r a m of Ch i n a ( n o s . 2 01 6 Y F A 0 2 02 4 0 0 a n d 2016YFB0401002), the National Natural Science Foundation of China (nos. 51401138, 51672180, 51622306, 51821002, 91833303, and 21673151), China Postdoctoral Science Foundation (nos. 2016M601880, 2017T100396), Qing Lan Project, 111 project, Collaborative Innovation Center of Suzhou Nano Science and Technology (NANO-CIC), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

4. CONCLUSIONS In summary, for the first time, a perovskite NFGM device has been successfully fabricated by using CH3NH3PbBr3 NCs as the charge trapping media. CH3NH3PbBr3 NCs possess a large number of bromine vacancy defects, which can supply sufficient sites for charge storage, thus leading to a pronounced memory effect of the device. Significantly, the CH3NH3PbBr3 NC-decorated CdS NR shows a large memory window of 77.4 V, which has surpassed those of the previously reported lowdimensional nanostructure-based NFGMs. The device also exhibits excellent memory characteristics in terms of a long retention time of 12 000 s, a high current ON/OFF ratio of 7 × 107, and a long-term air stability for 50 days. Moreover, other lead halide perovskite materials, including CH3NH3PbI3 and inorganic CsPbBr3 NCs, can also serve as storage media for NFGMs. These findings open a new avenue for the fabrication of high-capacity nonvolatile memory devices by using perovskite nanostructures.

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