Realization of Self‐Rectifying and Self‐Powered Resistive Random‐Access Memory Memristor Using [001]‐Oriented NaNbO3 Film Deposited on Sr2Nb3O10 Nanosheet at Low Temperatures

[001]‐oriented NaNbO3 films are deposited on Sr2Nb3O10/TiN/SiO2/Si substrates at 300 °C. The Sr2Nb3O10 nanosheets are used as a template to form crystalline NaNbO3 films at low temperature. The NaNbO3 films deposited on one Sr2Nb3O10 monolayer exhibit a bipolar switching curve due to the construction and destruction of oxygen vacancy filaments. Because the Sr2Nb3O10 monolayer does not act as an insulating layer, the film does not exhibit self‐rectifying properties. Self‐rectifying properties are observed in the NaNbO3 memristor, which forms on two Sr2Nb3O10 monolayers that act as tunnel barriers in the memristor. The memristor exhibits extensive rectification and on/off ratios of 48 and 15.7, respectively. Tunneling is the current conduction mechanism of the device in the low‐resistance state, and Schottky emission and tunneling are responsible for the conduction mechanism in the high‐resistance state at low and high voltages, respectively. The piezoelectric nanogenerator produced using the [001]‐oriented NaNbO3 film generates high voltage (1.8 V) and power (3.2 μW). Furthermore, endurance of the resistive random‐access memory and nonlinear transmission characteristics of the biological synapse are accomplished in the NaNbO3 memristor powered by the NaNbO3 nanogenerator. Therefore, the [001]‐oriented crystalline NaNbO3 film formed at 300 °C may be utilized for self‐rectifying and self‐powered artificial synapses.


Introduction
[3][4][5] These systems have contributed to the DOI: 10.1002/aisy.202300634 [001]-oriented NaNbO 3 films are deposited on Sr 2 Nb 3 O 10 /TiN/SiO 2 /Si substrates at 300 °C.The Sr 2 Nb 3 O 10 nanosheets are used as a template to form crystalline NaNbO 3 films at low temperature.The NaNbO 3 films deposited on one Sr 2 Nb 3 O 10 monolayer exhibit a bipolar switching curve due to the construction and destruction of oxygen vacancy filaments.Because the Sr 2 Nb 3 O 10 monolayer does not act as an insulating layer, the film does not exhibit selfrectifying properties.Self-rectifying properties are observed in the NaNbO 3 memristor, which forms on two Sr 2 Nb 3 O 10 monolayers that act as tunnel barriers in the memristor.The memristor exhibits extensive rectification and on/off ratios of 48 and 15.7, respectively.Tunneling is the current conduction mechanism of the device in the low-resistance state, and Schottky emission and tunneling are responsible for the conduction mechanism in the high-resistance state at low and high voltages, respectively.The piezoelectric nanogenerator produced using the [001]-oriented NaNbO 3 film generates high voltage (1.8 V) and power (3.2 μW).Furthermore, endurance of the resistive random-access memory and nonlinear transmission characteristics of the biological synapse are accomplished in the NaNbO 3 memristor powered by the NaNbO 3 nanogenerator.Therefore, the [001]-oriented crystalline NaNbO 3 film formed at 300 °C may be utilized for self-rectifying and self-powered artificial synapses.replacement.[35] Hence, self-powered ReRAM devices (or artificial synapses) must be developed for use in biomedical and IoT devices.[41][42][43] However, because these materials contain PbO, they are not environmentally friendly and should ideally not be used in implantable biomedical devices.Instead, Pb-free piezoelectric thin films should be used.Recently, a nanogenerator fabricated using a NaNbO 3 crystalline film formed on a Sr 2 Nb 3 O 10 /Ni (SNi) substrate at low temperatures yielded high output voltage and power. [44]Such crystalline NaNbO 3 nanogenerators are expected to be used as permanent power sources for self-rectifying and self-powered ReRAM memristors.However, the self-rectifying ReRAM properties and artificial synaptic characteristics of NaNbO 3 films have not been investigated.
In this study, NaNbO 3 crystalline films were deposited on Sr 2 Nb 3 O 10 /TiN/SiO 2 /Si (STSS) substrates with various thicknesses of Sr 2 Nb 3 O 10 nanosheets at low temperature.Selfrectifying and artificial synaptic characteristics were observed in the NaNbO 3 films coated on two Sr 2 Nb 3 O 10 monolayers.Additionally, a PNG was produced using a 1.0 μm thick [001]oriented NaNbO 3 film and was used to operate the NaNbO 3 memristor.The endurance properties of the ReRAM device and nonlinear transmission characteristics of the biological synapse were accomplished in the NaNbO 3 artificial synapse powered by a NaNbO 3 nanogenerator.The results suggest that the [001]-oriented crystalline NaNbO 3 film coated at 300 °C may be utilized for self-powered and self-rectifying artificial synapses.1c.Peaks for the crystalline NaNbO 3 thin film are not detected in the films deposited at low temperatures (≤250 °C), implying that these films have an amorphous NaNbO 3 phase.However, a highintensity (001) peak is detected for the film deposited at 300 °C, and its intensity increases with increasing coating temperature.A (120) peak is also observed for these films; however, its intensity is low.Hence, the crystalline NaNbO 3 thin film with the preferred [001] orientation was deposited on the STSS substrate with a single Sr 2 Nb 3 O 10 monolayer at temperatures ≥300 °C.The crystalline NaNbO 3 phase is not observed in the films coated on the TiN/SiO 2 /Si substrate at temperatures ≤400 °C (Figure S1a Figure 2a-c shows the SEM images of the surface of the NaNbO 3 thin film deposited on the STSS substrate with a single Sr 2 Nb 3 O 10 monolayer at various temperatures.The amorphous phase is observed in the film deposited at 250 °C (Figure 2a).Grains with an average size of approximately 25 nm are observed in the film deposited at 300 °C (Figure 2b), suggesting the formation of a crystalline NaNbO 3 phase.The larger grains with an average size of 48 nm are detected from the film grown at 400 °C (Figure 2c).The SEM results confirm that the crystalline NaNbO 3 films are coated on the STSS substrate at temperatures ≥300 °C.

Results and Discussion
To determine the composition of the NaNbO 3 thin film deposited at 300 °C, an energy-dispersive X-ray spectrometer (EDX) study was performed (Figure 2d).The semiquantitative composition of the film is shown in the inset of Figure 2d.The atomic ratio of the Na þ1 ions was approximately 49.2%, which was similar to that of the Nb 5þ ions (50.8%).The composition of the film was the same as that of the NaNbO 3 ceramic used as the sputtering target (Figure S1b, Supporting Information).Therefore, the NaNbO 3 film is assumed to adequately coat the STSS substrate without the evaporation of Na 2 O at 300 °C.However, for the NaNbO 3 film deposited at 400 °C, the atomic percentage of the Na þ1 ions (42.1%) was less than that of the Nb þ5 ions (57.9%) (Figure S1c, Supporting Information).Hence, we speculate that a small quantity of Na 2 O evaporated during deposition at 400 °C, resulting in a Na-deficient NaNbO 3 film.Moreover, the appropriate coating temperature for the NaNbO 3 film on the STSS substrate might be 300 °C.Various thicknesses of the Sr 2 Nb 3 O 10 seed layers were deposited on the TiN/SiO 2 /Si substrate, and [001]-oriented crystalline NaNbO 3 films were adequately deposited on the STSS substrates at 300 °C by applying Sr 2 Nb 3 O 10 seed layers of different thicknesses (Figure S2a-d, Supporting Information).Hence, a crystalline NaNbO 3 film can be deposited at 300 °C without the vaporization of Na 2 O using Sr 2 Nb 3 O 10 seed layers.
Figure 3a shows the current versus voltage (I-V ) curves of the crystalline NaNbO 3 film deposited on the STSS with a single Sr 2 Nb 3 O 10 monolayer at 300 °C.Electroforming was used to obtain the I-V curves.The NaNbO 3 memristor exhibited normal bipolar switching curves with set (V set ) and reset voltages (V reset ) of approximately À1.7 and 1.9 V, respectively.The bipolar switching curves were maintained even after 300 cycles.Moreover, the NaNbO 3 memristor provides stable currents in the lowresistance (LRS) and high-resistance states (HRS) with good retention properties (Figure S3a,b, Supporting Information).Therefore, the NaNbO 3 memristor deposited on a single Sr 2 Nb 3 O 10 monolayer at 300 °C with the Pt top electrode provides good reliability.Space-charge-limited conduction is the mechanism of current conduction in the NaNbO 3 memristor in the HRS, and the metallic conductive properties can be attributed to the LRS (Figure S3c, Supporting Information).This NaNbO 3 memristor was deposited on the Sr 2 Nb 3 O 10 monolayer.We speculate that the development and destruction of oxygen vacancy filaments are responsible for the bipolar switching behavior of the NaNbO 3 memristor (Figure S3d, Supporting Information).
X-ray photoelectron spectroscopy (XPS) was performed on the TiN electrode to confirm the presence of oxygen vacancy filaments in the NaNbO 3 film.The XPS O1s spectra of the TiN electrode of the NaNbO 3 memristor in the HRS and LRS are shown in Figure 3b,c, respectively.The O1s spectra show two peaks corresponding to the Ti-O and Ti-N-O bonds at 530 and 531.4 eV, respectively.The O1s spectra of the NaNbO 3 memristor in the HRS exhibits high-intensity Ti-O and weak Ti-N-O bond peaks (Figure 3b).These results imply that the TiN electrode in the HRS contains a small quantity of oxygen ions.For the NaNbO 3 memristor in the LRS, the O1s spectra of the TiN electrode show an enhanced Ti-N-O peak (Figure 3c), implying the presence of numerous oxygen ions in the TiN electrode.Therefore, numerous oxygen ions drifted from the NaNbO 3 film to the TiN electrode during the set process, resulting in several oxygen vacancies in the NaNbO 3 film in the LRS.Moreover, the oxygen vacancies formed oxygen vacancy filaments in the NaNbO 3 film, imparting metallic properties to the LRS.Identical results were obtained for the XPS N1s spectrum of the bottom TiN electrode (Figure S4a,b, Supporting Information).
According to the I-V curve (Figure S3c, Supporting Information), the NaNbO 3 films in the HRS and LRS exhibit insulating and metallic behaviors, respectively.To confirm this result, the resistances of the NaNbO 3 films in the HRS and LRS were measured at different temperatures.The resistance of a metal is known to increase with increasing temperature and the resistance of an insulator decreases with increasing temperature. [45]Figure 3d shows the resistance for the NaNbO 3 film, which was deposited on a Sr 2 Nb 3 O 10 monolayer, in the LRS and HRS measured at various temperatures.The resistance of the film in the LRS increased with increasing temperature; thus, the NaNbO 3 film in the LRS has oxygen vacancy filaments connecting the top (Pt) and bottom (TiN) electrodes, resulting in the metallic behavior of the film.The single Sr 2 Nb 3 O 10 monolayer did not behave as an insulating layer, possibly because of its small thickness.The resistance of the NaNbO 3 film in the HRS decreased with increasing temperature, suggesting that the NaNbO 3 film in the HRS exhibits insulating characteristics.Oxygen vacancy filaments disconnected during the reset process owing to the drift of oxygen ions from TiN to the NaNbO 3 film, thereby imparting insulating properties.Therefore, the switching action of the NaNbO 3 memristor may be attributed to the development and destruction of oxygen vacancy filaments.
Because the Sr 2 Nb 3 O 10 monolayer did not behave as an insulating layer in the LRS, self-rectifying properties were not observed in the NaNbO 3 thin film deposited on the Sr 2 Nb 3 O 10 monolayer.Hence, to achieve self-rectifying properties, the NaNbO 3 films were deposited on the STSS substrate with two Sr 2 Nb 3 O 10 monolayers at 300 °C.
Figure 4a shows the I-V plots of the NaNbO 3 film.Selfrectifying properties were observed for the NaNbO 3 memristor.Furthermore, the NaNbO 3 memristors in the LRS and HRS exhibit insulating properties (Figure S5a, Supporting Information), suggesting that the two Sr 2 Nb 3 O 10 monolayers act as tunnel barriers in the NaNbO 3 memristor.The rectification and on/off ratios of the NaNbO 3 memristor deposited on the two Sr 2 Nb 3 O 10 monolayers were obtained at various reading voltages (Figure 4b).The largest rectification ratio is 57, and the on/off ratio is 17.9; both ratios were measured at À1.6 V, as indicated by the broken blue line in Figure 4b.Minimally reduced values are also obtained at À1.4 V (red broken line in Figure 4b).The retention properties of the NaNbO 3 memristor with two Sr 2 Nb 3 O 10 monolayers were studied at various reading voltages (Figure 4c).The NaNbO 3 memristor exhibits good retention characteristics at reading voltages >À1.5 V.However, when the reading voltage is À1.5 V, the current degrades after approximately 10 2 s (Figure 4c).Therefore, the reading voltage should be larger than À1.5 V for the NaNbO 3 memristor.The NaNbO 3 memristor provides large rectification and on/off ratios at À1.4 V (Figure 4b) and exhibits good endurance at reading voltages >À1.5 V (Figure S5b, Supporting Information).Therefore, the NaNbO 3 memristor deposited on two Sr 2 Nb 3 O 10 monolayers can be considered reliable.
The conduction and switching mechanisms of the NaNbO 3 memristor deposited on two Sr 2 Nb 3 O 10 monolayers were investigated.Figure 4d shows the ln( J/T 2 ) versus E 1/2 curve of the NaNbO 3 memristor in the HRS, in which J, T, and E are the current density, temperature, and applied electric field, respectively; this curve was measured at a low voltage (>À0.85V).The dielectric constant at the optical range (ε op ) of the NaNbO 3 film can be calculated using the slope of the ln( J/T 2 ) versus E 1/2 plot and the ε op is identical to the square of the index of refraction of the film (n 2 ). [31,44,46]If the calculated n (or ε op 1/2 ) value is the same as that of the detected n value of the NaNbO 3 film, then the leakage current mechanism of the film is assumed to be the Schottky emission. [31,34,44,47]The calculated n value for the NaNbO 3 film is 1.23 (Figure 4d).The n value of the NaNbO 3 film, measured using an ellipsometer, is approximately 1.25 in the optical frequency range (inset of Figure 4d).Hence, the calculated n value is similar to the detected n value.Therefore, the leakage current in the NaNbO 3 film in the HRS at low voltages may be attributed to Schottky emission.However, the tunneling mechanism, which consists of direct tunneling and Fowler-Nordheim (FN) tunneling, is responsible for the current in the NaNbO 3 memristor in the HRS at high voltages.Direct tunneling occurs at a low supply voltage (<barrier height), whereas FN tunneling occurs at a high supply voltage (>barrier height). [31,32,46,48]The tunneling current (I) can be expressed in terms of the applied voltage (V ) using Equation (S1) and (S2), Supporting Information, which can be shortened as follows: at V<V t for direct tunneling (1) where V t is the transition voltage from direct to FN tunneling. [49,50]igure 4e shows the ln (I/V 2 ) versus (1/V ) curve measured with the NaNbO 3 memristor in the HRS at high voltages and the ln (I/V 2 ) versus (1/V ) curve calculated using Equation ( 1) and (2), respectively.The detected curve coincides with the theoretical curve with a V t of 1.65 V. Therefore, the current of the NaNbO 3 memristor in the HRS at a high voltage (<À0.85V) may be attributed to the tunneling mechanism.Hence, the current conduction in the HRS for the NaNbO 3 film deposited on two Sr 2 Nb 3 O 10 monolayers can be explained using Schottky emission at low voltage and the tunneling mechanism at high voltage.The ln (I/V 2 ) versus (1/V ) plots measured for the NaNbO 3 memristor in the LRS and calculated using Equation ( 1) and ( 2) are shown in Figure 4f.The detected ln (I/V 2 ) versus (1/V ) curve corresponds well with the theoretical ln (I/V 2 ) versus (1/V ) curve with a V t of 0.95 V. Hence, tunneling also explains the current of the NaNbO 3 memristor in the LRS.
The switching behavior of the NaNbO 3 memristor deposited on two Sr 2 Nb 3 O 10 monolayers was investigated using the conduction mechanisms of the NaNbO 3 memristor in the LRS and HRS.The NaNbO 3 memristor was in the HRS under a small negative voltage, and the current conduction was controlled by Schottky emission.When the applied voltage increased, numerous oxygen vacancies were produced in the NaNbO 3 film owing to the drift of oxygen ions toward TiN.Hence, the NaNbO 3 film became conductive, focusing the applied voltage on the two Sr 2 Nb 3 O 10 monolayers.Because the supplied voltage was lower than À0.85 V, direct tunneling occurred, and changed to FN tunneling when the applied voltage was close to V set .Finally, the NaNbO 3 memristor changed from the HRS to the LRS.The NaNbO 3 film in the LRS had metallic properties owing to the numerous oxygen vacancies; thus, the applied voltage was also focused on the Sr 2 Nb 3 O 10 monolayers.Therefore, direct tunneling occurs at a small positive voltage, and FN tunneling occurs when the applied voltage exceeds V t .Moreover, because the number of oxygen vacancies decreases with the application of a positive voltage to the Pt electrode, the NaNbO 3 film transforms from the LRS to the HRS when the applied voltage approaches V reset .Therefore, the switching behavior of the NaNbO 3 memristor is considerably influenced by the number of oxygen vacancies in the NaNbO 3 film.
The artificial synaptic characteristics of the NaNbO 3 films deposited on two Sr 2 Nb 3 O 10 monolayers were investigated.Figure 5a shows the I-V curves of the NaNbO 3 memristor with five negative voltage swings supplied between 0 and À1.4 V.As the voltage swing increases, the current of the NaNbO 3 memristor increases negatively.By contrast, when five positive voltage swings are applied between 0 and 1.5 V, the current in the NaNbO 3 memristor decreases as the voltage swing increases.Figure 5b shows the changes in the current with the continuous application of 100 negative pulses (P-spikes).The size and period of the P-spikes are À1.7 V and 4.0 ms, respectively.The current of the memristor is approximately 630 μA when one P-spike is applied and increases to 690 μA for 100 P-spikes.In total, 100 positive pulses (D-spikes) were continuously supplied after the 100 P-spikes.The size and period of the D-spikes are 1.8 V and 4.0 ms, respectively.The current reduces to approximately 630 μA after the application of 100 D-spikes.The current increases with the supply of the P-spikes and decreases with the supply of the D-spikes, and the current is equivalent to the synaptic weight.Therefore, the change in current in the memristor resulting from direct current (DC) biases and consecutive pulses is identical to the nonlinear transmission characteristics of a biological synapse.Therefore, NaNbO 3 memristors can emulate the nonlinear transmission properties of biological synapses.
Synaptic plasticity typically increases with an increase in the P-spike number.However, the increase in synaptic plasticity depends on the P-spike interval, which corresponds to spike-rate-dependent plasticity (SRDP).The increase in synaptic plasticity of the NaNbO 3 memristor was negligible when the P-spike interval was large (≥80 μs); however, the plasticity increased considerably with a small P-spike interval (≤40 μs) (Figure 5c).Therefore, the NaNbO 3 memristor deposited on two Sr 2 Nb 3 O 10 monolayers emulates the SRDP of biological synapses.The spike-time-dependent plasticity (STDP) of biological synapses was also realized in a NaNbO 3 memristor deposited on two Sr 2 Nb 3 O 10 monolayers.When a P-spike was applied to the top electrode (pre-neuron) before the bottom electrode (postneuron), which corresponds to potentiation, the time difference between the two P-spikes (Δt) is defined as positive.The Δt is negative when the P-spike is applied to the postneuron prior to the preneuron (depression).To measure the synaptic weight change (Δw), various net spikes obtained from the pre-and postspikes were used (Figure S6a-c, Supporting Information).The variation in Δw as a function of Δt for the NaNbO 3 memristor deposited on two Sr 2 Nb 3 O 10 monolayers is shown in Figure 5d.Large depression and potentiation values were obtained for a small absolute value of Δt, whereas they were close to zero for a large absolute value of Δt.Equation (S3), Supporting Information, which represents the STDP of a biological synapse, was used to calculate Δw as indicated by the red lines in Figure 5d.The measured STDP results agree well with the calculated values, indicating that the NaNbO 3 memristor exhibits the STDP characteristics of a biological synapse.Therefore, the NaNbO 3 memristor deposited on the two Sr 2 Nb 3 O 10 monolayers exhibits both ReRAM and biological synaptic characteristics.Moreover, it exhibits self-rectifying properties.Thus, a NaNbO 3 memristor with two Sr 2 Nb 3 O 10 monolayers can be used as a cross-point array structure.In addition, the NaNbO 3 memristor deposited on a single Sr 2 Nb 3 O 10 monolayer exhibits good artificial synaptic properties (Figure S7a-g, Supporting Information).
The development of a self-powered memristor consisting of a PNG and ReRAM memristor is necessary to overcome the complicated wiring and replacement problems of batteries.The power of a PNG is affected by the d 33 Â g 33 value of the piezoceramic because the nanogenerator generally operates at an off-resonance frequency, where d 33 and g 33 are the piezoelectric charge and voltage constants, respectively. [51,52]The g 33 is identical to d 33 /(ε r ε 0 ) where ε r and ε 0 are the relative dielectric constant and the vacuum dielectric constant, respectively. [51,52]urthermore, g 33 is related to the output voltage of the nanogenerator, which is typically used to operate memristors.Thus, the g 33 (or d 33 /(ε r ε 0 )) value is also important for the operation of the memristor and thus the piezoelectric film for the nanogenerator should possess small ε r and large d 33 values.
Figure 6a shows the ε r value of the NaNbO 3 film deposited on the STSS substrate with a single Sr 2 Nb 3 O 10 monolayer at different temperatures.For the films deposited at low temperatures (≤250 °C), the ε r values are as low as approximately 30 and are probably caused by the presence of the amorphous phase.The ε r value increases to 206 with increasing coating temperature for the film deposited at 400 °C owing to the improved crystallinity.However, the NaNbO 3 films deposited at higher temperatures (≥350 °C) exhibits a high dielectric loss, possibly owing to the defects originating from the volatilization of Na  300 °C, the d 33 value decreases to 89 pm V À1 for the film deposited at 400 °C owing to the presence of defects originating from the evaporation of Na 2 O.The d 33 value of the NaNbO 3 thin film, formed at room temperature via chemical deposition and subsequently annealed at 650 °C, was reported to be approximately 35 pm V À1 . [53]Moreover, the NaNbO 3 piezoceramic exhibited a low d 33 value in the range of approximately 25-30 pm V À1 . [53]Therefore, the [001]-oriented NaNbO 3 film deposited at 300 °C displayed a high d 33 value.In addition, the NaNbO 3 film exhibits excellent ferroelectric and insulating properties (Figure S8a-e, Supporting Information).Figure 6c shows the changes in the d 33 Â g 33 and g 33 values of the NaNbO 3 films deposited at different temperatures.The films deposited at low temperatures (≤250 °C) exhibit low d 33 Â g 33 values (<2.54 Â 10 À12 m 2 N À1 ) because of their small d 33 value.However, the film deposited at 300 °C exhibits a high d 33 Â g 33 (15.3Â 10 À12 m 2 N À1 ) owing to its large d 33 value.The d 33 Â g 33 value decreases for the films deposited at high temperatures (>300 °C), possibly because of the reduced d 33 and increased ε r values.The g 33 value shows a similar trend, and the NaNbO 3 film deposited at 300 °C exhibits the highest g 33 value of 0.12 Vm N À1 .Therefore, the nanogenerator synthesized using the NaNbO 3 film, which was deposited at 300 °C, should exhibit the largest voltage and power.
A PNG was fabricated using a NaNbO 3 film deposited on the SNi substrate at 300 °C with a single Sr 2 Nb 3 O 10 monolayer.A schematic of the nanogenerator is shown in Figure S10, Supporting Information.The NaNbO 3 film was adequately deposited on the SNi substrate along the [001] direction with good crystallinity (Figure 6d).The ε r and dielectric loss of this film are approximately 153 and 2.87% at 100 kHz, respectively (Figure 6e).The d 33 value of this film is approximately 119 pC N À1 (Figure 6f ).Moreover, the d 33 Â g 33 and g 33 values of this film are approximately 10.5 Â 10 À12 m 2 N À1 and 0.09 Vm N À1 , respectively.Therefore, the dielectric and piezoelectric characteristics of this film are similar to those of the film deposited on the STSS substrate.
The output voltage (V out ) of the NaNbO 3 nanogenerator was measured at various strains and strain rates, and it increases with increasing strain and strain rate (Figure S9a,b, Supporting Information).The largest strain and strain rate obtained from the bending machine were 2.87% and 4.24% s À1 , respectively, and were used to measure the V out of the nanogenerator.Figure 7a shows the V out of the NaNbO 3 nanogenerator measured at different load resistances (R L ).The V out increases with an increase in R L , and the highest V out (0.9 V) is obtained above 3.0 MΩ.The output current (I out ) and power (P out ) of the nanogenerator were obtained using the root mean square value of the output voltage (V RMS ); the results are shown in Figure 7b.The highest P out (0.32 μW) is observed at 1.0 MΩ.The V out of the nanogenerator operated by the bending machine (<1.0 V) was too low to operate the NaNbO 3 memristor.Therefore, the nanogenerator was manually bent, resulting in an average strain and strain rate of 3.31% and 4.69% s À1 , respectively.The nanogenerator exhibits an increased V out of 1.8 V at 1.0 MΩ with a maximum P out of 3.2 μW at 1.0 MΩ (Figure 7c,d).Therefore, the V out generated by the NaNbO 3 nanogenerator via hand bending can supply sufficient voltage to operate the NaNbO 3 memristor.
Figure 8a displays a schematic of the NaNbO 3 memristor connected to the NaNbO 3 nanogenerator to measure the endurance.Negative and positive pulses were generated when the NaNbO 3 nanogenerator was bent and released, respectively, (Figure 8a), and were used to measure the currents of the memristor in the LRS and HRS at 0.1 V using a pulse generator.The endurance of the NaNbO 3 memristor powered by the NaNbO 3 nanogenerator is shown in Figure 8b.The NaNbO 3 memristor exhibits stable R HRS and R LRS ratios of approximately 10, implying that the NaNbO 3 memristor can be operated using the NaNbO 3 nanogenerator.The artificial synaptic properties of the NaNbO 3 memristor powered by the NaNbO 3 nanogenerator were also investigated.Figure 8c shows a schematic of the NaNbO 3 artificial synapse linked to the NaNbO 3 nanogenerator used to measure the nonlinear transmission properties of the biological synapse.A bridge rectifier was used to produce the P-and D-spikes via the input voltage generated by the NaNbO 3 nanogenerator (Figure 8c).The P-and D-spikes applied to the NaNbO 3 artificial synapse were À1.4 and 1.45 V/4 ms, respectively (Figure 8d).When a P-spike was applied to the NaNbO 3 artificial synapse, a current of approximately 30 μA was produced, which increased to 45 μA after applying 100 Pspikes (Figure 8d).The current of the NaNbO 3 artificial synapse decreased when the D-spike number increased to 30 μA after a supply of 100 D-spikes, indicating that the nonlinear transmission characteristic of the biological synapse is realized in the NaNbO 3 artificial synapse powered by the nanogenerator.Therefore, it may be assumed that the NaNbO 3 thin films coated at 300 °C can be applied to self-powered ReRAM devices and artificial synapses.
In addition, a self-powered and self-rectifying artificial synapse was fabricated for the first time using a piezoelectric NaNbO 3 crystalline film and Sr 2 Nb 3 O 10 monolayers.Piezoelectric NaNbO 3 crystalline films were used to fabricate PNG for self-powered devices, and Sr 2 Nb 3 O 10 monolayers were utilized as insulating layers to fabricate self-rectifying memristors.Moreover, a simple device structure was used to determine the feasibility of the self-powered and self-rectifying artificial synapse.In the future, a more complex structure can be used to improve the memristor properties of the NaNbO 3 /Sr 2 Nb 3 O 10 artificial synapses.

Conclusion
A Sr 2 Nb 3 O 10 nanosheet was deposited on a TiN/SiO 2 /Si substrate via the Langmuir-Blodgett (LB) technique and used as a seed layer for the formation of a NaNbO 3 crystalline film at low temperatures.The [001]-oriented crystalline NaNbO 3 thin film was deposited on the STSS substrate at 300 °C.The NaNbO 3 film deposited on the Sr 2 Nb 3 O 10 monolayer exhibited bipolar switching behavior, attributed to the construction and destruction of oxygen vacancy filaments.Moreover, the memristor imitates the synaptic properties of biological synapses.However, self-rectifying characteristics were not observed in the NaNbO 3 film deposited on a single Sr 2 Nb 3 O 10 monolayer because the Sr 2 Nb 3 O 10 monolayer did not behave as an insulating layer.The NaNbO 3 film deposited on the STSS with two Sr 2 Nb 3 O 10 monolayers exhibited self-rectifying properties because the two Sr 2 Nb 3 O 10 monolayers behaved as a tunnel barrier in the NaNbO 3 memristor.This memristor showed good rectification and on/off ratios of 48 and 15.7, respectively, at À1.4 V. Schottky emission and tunneling were the conduction mechanisms in the HRS at low and high voltages, respectively, and the current in the LRS was subject to the tunneling mechanism.The NaNbO 3 memristor with two Sr 2 Nb 3 O 10 monolayers exhibited artificial synaptic characteristics and could thus be applied to ANNs.The [001]-oriented crystalline NaNbO 3 film deposited at 300 °C, which showed high d 33 Â g 33 (15.3Â 10 À12 m 2 N À1 ) and g 33 (0.12 Vm N À1 ) values, was used to produce the PNG, generating a large V out (1.8 V) and P out (3.2 μW).Furthermore, the endurance of the ReRAM device and nonlinear transmission characteristics of the biological synapse were emulated using a NaNbO 3 memristor powered by a NaNbO 3 nanogenerator.Therefore, the [001]-oriented crystalline NaNbO 3 film deposited at 300 °C could be applied to self-powered artificial synapses.

Experimental Section
KSr 2 Nb 3 O 10 ceramics with a layered perovskite structure were produced using a traditional sintering technique and used to prepare Sr 2 Nb 3 O 10 nanosheets via proton exchange and exfoliation methods. [54,55]he Sr 2 Nb 3 O 10 nanosheets were coated on TiN/SiO 2 /Si and Ni substrates using the LB method at room temperature; the deposition processes have been described in previous studies. [31,32,44,56]The STSS and SNi substrates were heated at 350 °C under N 2 atmosphere for 30 min to remove tetrabutylammonium-related organic defects in the Sr 2 Nb 3 O 10 nanosheet that were likely induced during the exfoliation procedure. [31,32]Thereafter, the STSS and SNi substrates were heated at 300 °C under 50 Torr O 2 pressure to remove the oxygen vacancies formed during heating under N 2 atmosphere. [31,32]The Sr 2 Nb 3 O 10 nanosheets were used as the seed layers for the growth of NaNbO 3 crystalline films at low temperatures.Radiofrequency magnetron sputtering was used to induce the growth of the crystalline NaNbO 3 films on the STSS and SNi substrates at various temperatures.A NaNbO 3 ceramic with a radius of 1.0 inch was used as the sputtering target, and the NaNbO 3 films were coated using a sputtering power of 100 W in a vacuum chamber under mixed atmosphere of Ar and O 2 (Ar:O 2 = 4:1) with a total pressure of 10 mTorr.
The NaNbO 3 crystalline films were coated onto the STSS substrates, which composed of Sr 2 Nb 3 O 10 nanosheets with different thicknesses.Circular Pt with a radius of 140 μm was deposited on the NaNbO 3 film using DC sputtering to form the top electrode.The crystalline NaNbO 3 films deposited on the SNi substrates were used as PNGs.A conductive fabric was then formed on the deposited NaNbO 3 film to form the top electrode of the nanogenerator.Finally, the conductive fabric/NaNbO 3 / SNi device was affixed to PET to produce a NaNbO 3 nanogenerator, as shown in Figure S10, Supporting Information.The detailed measurement processes for the various physical properties of the NaNbO 3 memristors and the output properties of the NaNbO 3 nanogenerators are described in Section S10, Supporting Information.

Figure
Figure 1a shows an atomic force microscopy (AFM) image of the surface of the Sr 2 Nb 3 O 10 monolayer deposited on the TiN/SiO 2 / Si substrate.The Sr 2 Nb 3 O 10 monolayer formed a homogeneous cover (96%) on the substrate.The average roughness of the Sr 2 Nb 3 O 10 monolayer was approximately 0.8 nm, implying that it has a smooth surface.The thickness of the Sr 2 Nb 3 O 10 monolayer was approximately 1.50 nm, as detected using the AFM probe tip; scanning was conducted from the Sr 2 Nb 3 O 10 monolayer to the TiN/SiO 2 /Si surface (Figure 1b).The measured thickness of the Sr 2 Nb 3 O 10 monolayer is similar to the ideal thickness at 1.47 nm.Hence, the Sr 2 Nb 3 O 10 monolayer was successfully deposited on the TiN/SiO 2 /Si substrate.The X-ray diffraction (XRD) patterns of the NaNbO 3 films deposited on the STSS substrate with a single Sr 2 Nb 3 O 10 monolayer at different temperatures are shown in Figure1c.Peaks for the crystalline NaNbO 3 thin film are not detected in the films deposited at low temperatures (≤250 °C), implying that these films have an amorphous NaNbO 3 phase.However, a highintensity (001) peak is detected for the film deposited at 300 °C, and its intensity increases with increasing coating

Figure 1 .
Figure 1.a) AFM image of the Sr 2 Nb 3 O 10 monolayer coated on the TiN/SiO 2 /Si substrate.b) Line scan of the AFM probe tip from the Sr 2 Nb 3 O 10 monolayer to the TiN/SiO 2 /Si surface.c) XRD patterns of the NaNbO 3 film coated on the STSS at various temperatures.d) SEM image of the crystalline NaNbO 3 film coated on the STSS at 300 °C.
, Supporting Information).Hence, we concluded that the Sr 2 Nb 3 O 10 nanosheets acted as templates for the deposition of the [001]-oriented crystalline NaNbO 3 thin films.A scanning electron microscopy (SEM) image of the NaNbO 3 film deposited on the STSS at 300 °C is shown in Figure 1d.The figure shows that the 75 nm thick NaNbO 3 thin film adequately coats the STSS substrate, and a continuous and sharp interface is formed between the [001]-oriented NaNbO 3 thin film and STSS substrate.

Figure 2 .
Figure 2. SEM images of the surface of the NaNbO 3 film formed on the STSS substrate with a single Sr 2 Nb 3 O 10 monolayer at various temperatures: a) 250, b) 300, and c) 400 °C.d) EDX spectrum of the NaNbO 3 film formed at 300 °C.Inset shows the semiquantitative results of the composition analysis of this film.

Figure 3 .
Figure 3. a) I-V curves of the crystalline NaNbO 3 film deposited on the STSS substrate with a Sr 2 Nb 3 O 10 monolayer at 300 °C.XPS O1s spectra of the TiN electrode of the NaNbO 3 memristor in b) HRS and c) LRS.d) Resistance of the NaNbO 3 film in the LRS and HRS measured at various temperatures.

Figure 4 .
Figure 4. a) I-V curves, b) rectification and on/off ratios, and c) retention characteristics at different reading voltages for the NaNbO 3 memristor with two Sr 2 Nb 3 O 10 monolayers.d) ln( J/T 2 ) versus E 1/2 curve of the NaNbO 3 memristor with two Sr 2 Nb 3 O 10 monolayers in the HRS at low voltage (>À0.85V). ln(I/V 2 ) versus (1/V ) plot of the NaNbO 3 memristor with two SNO monolayers in the e) HRS at high voltage (≤À0.85) and f ) LRS.

Figure 5 .
Figure 5. a) I-V plots of the NaNbO 3 memristor with the application of the five negative and positive voltage swings.b) Change in conductance with the application of 100 P-spikes and 100 D-spikes.c) Variation in synaptic weight according to the P-spike number with diverse pulse intervals.d) Variation in Δw with regard to Δt.
2 O during coating.The NaNbO 3 film grown at 300 °C shows a ε r value of 116 with a low dielectric loss of 1.15% at 100 kHz.The d 33 values of the NaNbO 3 films deposited on the Sr 2 Nb 3 O 10 monolayer are shown in Figure 6b.The d 33 value of the film deposited at low temperatures (≤250 °C) is small (<30 pm V À1 at 10 V) owing to the presence of the amorphous phase.However, the NaNbO 3 film deposited at 300 °C demonstrates a large d 33 of 125 pm V À1 at 7.4 V.When the growth temperature exceeded

Figure 6 .
Figure 6.a) ε r and dielectric loss, b) d 33 , and c) changes in the d 33 Â g 33 and g 33 values of the NaNbO 3 film deposited on a single Sr 2 Nb 3 O 10 monolayer at different temperatures.d) XRD pattern, e) ε r and the dielectric loss, and f ) d 33 of the NaNbO 3 deposited on the SNi substrate with one Sr 2 Nb 3 O 10 monolayer.Inset of (d) displays the SEM image of the NaNbO 3 film.

Figure 7 .
Figure 7. a) V out and b) changes in V RMS , I out , and P out with regard to the R L for the NaNbO 3 nanogenerator operated by the bending machine.c) V out and d) changes in V RMS , I out , and P out with regard to the R L for the NaNbO 3 nanogenerator operated by hand.

Figure 8 .
Figure 8. a) Schematic of the NaNbO 3 memristor connected to the NaNbO 3 nanogenerator.b) Endurance of the NaNbO 3 memristor powered by the NaNbO 3 nanogenerator.c) Schematic of the NaNbO 3 artificial synapse connected to the NaNbO 3 nanogenerator.d) Change in conductance with the application of 100 P-spikes and 100 D-spikes using the nanogenerator.