Enhanced Long‐Term Memory Properties of ZnO Nanoparticle‐Based Optical Synaptic Devices for Neuromorphic Computing

The enhancement of long‐term memory properties in optical synaptic devices based on ZnO nanoparticles (NPs) is investigated. High‐temperature annealing improves crystal quality and carrier mobility, leading to efficient carrier generation and transport. The annealed ZnO NPs exhibit increased band edge luminescence and reduced deep‐level emission. Their larger surface grain size decreases oxygen adsorption, resulting in enhanced desorption by photoexcited carriers during UV exposure. The annealed devices show higher excitatory postsynaptic currents (EPSCs) and slower decay rates after UV termination, indicating better long‐term memory. They also demonstrate accelerated learning processes with fewer pulse cycles required to reach 100% EPSC. Overall, this research highlights the significance of high‐temperature annealing for improving long‐term memory in ZnO NP‐based optical synaptic devices, offering insights for advanced memory devices.


Introduction
[3][4][5][6] These systems, inspired by the human brain, offer potential solution to the challenges faced by traditional von Neumann architecture when processing large volumes of data.[12] Optoelectronic synapses have been developed, utilizing light stimulation instead of electrical signals to reduce energy consumption and enhance robustness and signal transmission. [13,14][23] However, to realize their full potential in memory devices, it is crucial to improve their long-term memory (LTM) properties, as their amorphous nature currently limits their practical applications in neuromorphic systems. [24,25]Consequently, researchers are actively developing various approaches to address the amorphous nature of ZnO NP-based thin films.[28][29][30] These efforts aim to optimize ZnO NP-based thin films for improved LTM characteristics, paving the way for their broader utilization in advanced neuromorphic system.[32] High-temperature annealing processes have been found to improve the crystal quality, reduce crystal defects, and enhance carrier mobility, thereby leading to improved device performance.[32][33] In this study, we focused on the analysis of the optical and electrical properties of ZnO NP-based thin films after undergoing a thermal annealing process.Building upon these findings, we fabricated optical synaptic devices with metal-ZnO NPs-metal structures, enabling the implementation of learning functions through precise control of light pulse time, frequency, and intensity.The results obtained highlight the remarkable enhancement of LTM properties in ZnO NP-based optical synaptic devices through the utilization of high-temperature annealing process, which effectively improves the crystal quality, optical properties, and EPSCs.These findings contribute to the understanding and advancement of ZnO NP-based memory devices for optoelectronic applications.

Effect of Thermal Annealing on the Surface Morphology of ZnO NP Films
The surface morphology of a ZnO thin film deposited on a (0001) sapphire substrate using the spin coating method is illustrated in Figure 1a.It indicates that the ZnO NPs were uniformly and well deposited on the substrate, exhibiting an average diameter of 34 nm and an rms surface roughness of 2.88 nm. Figure 1b,c presents the surface morphology of the ZnO NPs thin films after undergoing thermal annealing at different temperatures, specially 400 and 800 °C, respectively.Upon annealing at 400 °C, the size of surface grains increased from 34 to 65 nm, and their shape transformed from slightly elongated to circular.Consequently, the rms surface roughness decreased to 2.81 nm.When the annealing temperature was increased to 800 °C, the size of the surface grains further enlarged to 80 nm, and the rms roughness decreased to 2.68 nm.However, at temperatures above 800 °C, significant modification in the surface morphology was observed, with the size of surface grains continuing to increase, as depicted in Figure 1d.However, along with this grain growth, there was a noticeable increase in surface roughness due to surface degradation, including thermal desorption.As a result, the optimal annealing temperature for ZnO NPs films, based on surface roughness considerations, is estimated to be around 800 °C.These changes in surface morphology, including coalescence, recrystallization, and formation of larger grains with reduced surface roughness, can be attributed to the Ostwald ripening phenomenon. [30,34]This phenomenon occurs due to the variation in surface energy between small and large ZnO NPs.At high temperatures, the small ZnO NPs dissolve and migrate toward larger ZnO NPs with lower surface energy, eventually merging to form larger grains with a smoother surface.The recrystallization of ZnO NPs at elevated temperatures also leads to the formation of larger grains with improved structural quality and enhanced electrical properties.Therefore, thermal treatment proves to be an effective method for enhancing the surface morphology and quality of ZnO NPs thin films, which is crucial for their application in various optoelectronic devices. [30]

Optical and Structural Characterization of ZnO NPs Films with Different Thermal Annealing Temperatures
The optical and structural properties of ZnO NPs films were investigated by analyzing their absorption spectra and photoluminescence (PL) spectra, focusing on the effect of different thermal annealing temperatures.Figure 2a displays the absorption spectra, which reveal that regardless of the annealing temperatures, the absorption of higher energy photons increases rapidly from around 3.27 eV, which indicates that the bandgap of ZnO NPs thin films was formed in the 3.27 eV region.The optical bandgap of the as-grown ZnO NPs film was estimated to be 3.272 eV using the extension line method, where a tangent line is drawn to the steepest part of the absorption curve and extended to intersect the x-axis. [35]With an increasing in the thermal annealing temperature to 900 °C, the optical bandgap energy of the ZnO NPs film increased to 3.284 eV.[32][33] Thermal annealing can lead to the removal of impurities or the reordering of the crystal structure, resulting in a more perfect crystal lattice with fewer defects. [34]he decrease in defects contributes to an increase in bandgap energy, as defects introduce additional energy states within the bandgap, trapping charge carriers and reducing the mobility of the material.Therefore, the observed increase in bandgap energy in the ZnO NPs thin films annealed above 800 °C can be attributed to a combination of reduced grain boundaries and decreased defects and impurities within the crystal structure.Figure 2b is the room temperature PL spectra of ZnO NPs films with different thermal annealing process.The spectra show a strong emission at the bandedge (%3.271 eV) and a deep-level emission in the green emission region (480-600 nm), associated with oxygen-related defects such as oxygen vacancies (V o ), oxygen interstitials (O i ), and O antisite defects (O Zn ) in the ZnO NPs.These defects act as trap states for electron and holes, contributing to nonradiative recombination process. [36,37]The PL intensity of the bandedge emission in the ZnO NPs film annealed at 400 °C slightly increased compared to the as-deposited ZnO NPs film.However, in the ZnO NPs film annealed above 800 °C, a significant increase in the bandedge emission intensity and a decrease in the deep level emission associated with V o , O i , and O Zn were observed.These improvements can be attributed to the enhanced crystallinity of the ZnO NPs film due to recrystallization during the annealing process.Recrystallization leads to the formation of larger, more uniform grains with fewer defects, resulting in a more ordered and continuous crystal structure. [32,38]This, in turn, reduces nonradiative recombination processes and enhances the efficiency of bandedge emission.Moreover, the slight redshift in the bandedge emission was observed for the ZnO NPs film annealed above 800 °C, with a value of 3.264 eV, while the bandedge emission of the ZnO film annealed at 400 °C remained at 3.271 eV, which is consistent with the as-deposited ZnO NPs.The increase in bandedge emission intensity and the observed redshift in the ZnO NPs film annealed above 800 °C are believed to be caused the reduction of crystal defects, specifically V o , O i , and O Zn , resulting from grain growth during the thermal annealing process.
The X-ray ω/2θ scan spectra in Figure 2c  temperatures.The as-deposited ZnO NPs thin films exhibit no other crystalline peaks associated with ZnO, except for a very weak and broad peak near 34°, which is associated with the (0002) ZnO plane.This is because the ZnO NPs were applied using the spin coating method, which results in the random orientation of crystallites.However, as the annealing temperature increases to 400 °C, a weak and broad peak related to (0002) ZnO plane starts to emerge.With further increase in annealing temperature to 900 °C, the (0002) ZnO peak becomes more prominent and well defined.This indicates that the crystallinity of the ZnO NPs is greatly enhanced by the thermal annealing process, with the most significant improvement observed at 900 °C. [31,34,39]In addition, a peak corresponding to (101) ZnO was observed at 36.4°, near the right shoulder of the (002) ZnO peak.This indicates that in the 900 °C annealing process, (002) ZnO is significantly dominant, but there is a slight presence of (101) ZnO , suggesting the occurrence of crystallographic defects, such as dislocations, caused by the two crystal planes during thermal treatment above 800 °C.The preferential orientation of the amorphous ZnO NPs thin film toward the (002) crystal plane of ZnO is believed to occur as the film crystallizes during the thermal annealing process.Consequently, the recrystallization of the ZnO NPs at 900 °C, as depicted in Figure 1, leads to an increase in crystal size and the corresponding intensification of diffraction peaks satisfying Bragg's law, particularly those associated with the (002) ZnO .It indicates that the thermal treatment process plays a crucial role in transforming the amorphous structure of the ZnO NPs film into a more crystalline one.

Photoreactive Properties of ZnO NPs Films with Different Thermal Treatments
Figure 3a,b demonstrates the dark current and photocurrent characteristics of the Al-ZnO-Al two-terminal synaptic device using a ZnO NPs thin film, respectively, as a function of applied voltage.The as-deposited ZnO NPs device without thermal annealing process shows a very low dark current of 53 pA at 10 V, as shown in Figure 3a.However, for the device subjected to thermal treatment from 400 to 900 °C, the dark current values increase significantly, measuring 37 nA at 400 °C and 56.1 μA at 900 °C when an applied voltage of 10 V is used.The increase in dark current values with higher thermal annealing temperatures can be attributed to the generation of surface point defects with shallow energy levels during the high temperature annealing process, leading to an increase in intrinsic carriers.[32] In addition, to evaluate the photoresponse properties of ZnO NP-based devices, the devices were illuminated with 365 nm UV excitation light, and the photocurrent was measured by subtracting the dark current from UV current.Figure 3b illustrates the photocurrent as a function of applied voltage for ZnO NPs devices.For the as-deposited ZnO NPs devices without thermal treatment, the photocurrent at 10 V was found to be 21.9 nA.In contrast, the photocurrents for the ZnO NPs two-terminal optical devices annealed at 400 and 800 °C were significantly higher, measuring 26.2 and 943 μA, respectively.However, it was observed that the photocurrent of ZnO NPs devices decreased at an annealing temperature of 900 °C.These results demonstrate a significant increase in photocurrent with increasing thermal annealing temperature up to 800 °C.However, above 800 °C, the photoexcitation characteristics begin to degrade, which is believed to be due to the surface thermal degradation of the ZnO NPs film by the high-temperature annealing treatment, resulting in a decrease in UV current, although the dark current still increases.In addition, in Figure 3d, the calculated quantum efficiency and photoresponsivity are plotted as a function of thermal annealing temperature for the Al-ZnO NPs-Al two-terminal optical device.The results show that both quantum efficiency and photoresponsivity increase with increasing thermal treatment temperature.As shown in Figure 2a-c, the improvement in optical and crystal properties of ZnO NPs with increasing thermal annealing temperature leads to an increase in the number of photoexcited carriers upon UV irradiation.This is believed to be the reason for the significant increase in photocurrent observed in the annealed ZnO NPs devices.Figure 3e presents the time-dependent photocurrents of the ZnO NPs devices annealed at different temperatures (ranging from 25 to 900 °C) under four cycles of 20 s UV exposure followed by 20 s of UV-off states.The as-deposited ZnO NPs device exhibits a photocurrent at the nA level, which rapidly decreases upon turning off the UV light.However, higher photocurrents were observed at higher annealing temperatures, except for 900 °C, where surface thermal degradation occurs.Moreover, the rate of decrease in photocurrent upon UV termination became slower as the annealing temperature increased up to 900 °C.In particular, the ZnO NPs device annealed at 800 °C exhibits a significantly higher photocurrent at the sub-mA level, and even after turning off UV light, the decay rate is less than 10%.It indicates that the high-temperature annealed ZnO NPs devices are effective in photogenerating carriers in ZnO NPs films through UV light exposure.Upon UV illumination, the electron-hole pairs are generated in the ZnO NP film, leading to an increase in carrier density and subsequent increase in conductivity.The holes migrate toward the surface and recombine with the electrons trapped in the adsorbed oxygen atoms resulting in the release of oxygen atoms from the surface.This process creates an excess of unpaired electrons, which become the dominant carriers contributing to the photocurrent.However, after the termination of UV illumination, the unpaired electrons that remain in the ZnO NP film are susceptible to readsorption by oxygen molecules on the surface.When these unpaired electrons are trapped by readsorbed oxygen, the photocurrent gradually decreases over time due to the reduced number of available carriers.This decrease in the photocurrent also leads to a corresponding decline in the EPSC. [33,37]The hightemperature (>800 °C) annealing process improves the PL bandedge emission properties of ZnO NP film, primarily attributed to the (002) ZnO crystallization, as demonstrated in Figure 2c.This implies an increasing in the radiative recombination rate of electron-hole pairs, leading to a larger population of photoexcited carriers available for conduction.The enhanced bandedge emission properties indicate a reduction of nonradiative recombination pathways and improved carrier transport in the ZnO NPs film, ultimately resulting in an improved photoresponse.Indeed, the as-deposited ZnO NPs device exhibits fast response and recovery times, which make it suitable for applications that require quick photodetection.On the other hand, the hightemperature annealed ZnO NPs devices exhibit the ability to maintain photocurrent for a longer duration, indicating their potential application in long-term optical memory devices.The improved stability and sustained photocurrent of the hightemperature annealed ZnO NPs devices make them well suited for optical synaptic devices that require LTM capabilities.

EPSC of Al-ZnO NPs-Al Optical Synaptic Devices under UV Illumination
To investigate the effect of the annealing temperature on EPSCs, we comparatively characterized the EPSCs of as-deposited ZnO NPs optical synaptic devices with devices annealed at 400 and at 800 °C, where the photocurrent is maximized.Figure 4a-l illustrates the ESPCs over time for as-deposited, 400 °C-annealed and 800 °C-annealed ZnO NPs optical synaptic devices under different photostimulation conditions.The ESPCs values were as a function of varying photosimulation durations, light powers, frequencies, and exposure numbers.The observed linear increase in ESPC for all ZnO NPs optical synaptic devices indicates their effective response to changes in input parameters such as time (0.5-3 s), light source power (10.6-94.3μW cm À2 ), exposure frequency (20-200 mHz), and exposure number (1-20).This characteristic makes the device well suited for integration into neuromorphic computing systems. [10,11]Furthermore, when the UV light was switched off, the rate of decrease in ESPC was slower with longer photostimulation durations, higher light powers, frequencies, and exposure numbers.This finding suggests the potential of ZnO NPs optical synaptic devices to exhibit LTM properties through UV light stimulation and light output. [12,24,25]In particular, noteworthy is the substantial increase in EPSC observed as the thermal annealing temperature increased from 400 to 800 °C in ZnO NPs optical synaptic devices.The EPSC values escalated from nA level to several hundred μA, depending on factors such as UV exposure time, light intensity, frequency, and number of exposures.Notably, even after terminating UV exposure, the EPSC values remained higher at higher annealing temperatures.These results demonstrate that a higher annealing temperature not only enhances the EPSC during photostimulation but also mitigates the decline in the remaining EPSC value after UV exposure termination.Consequently, this improvement signifies the enhanced LTM properties of the optical synaptic devices.In particular, the ZnO NPs optical synaptic device annealed at 800 °C exhibits a remarkable responsiveness to UV light, generating a significantly higher ESPC compared to the as-deposited ZnO NPs optical synaptic device.It is attributed to the improved crystalline structure and optical properties of the ZnO NPs optical synaptic device annealed at high temperatures, leading to increased generation of photoexcited carriers and enhanced photoconductivity.Furthermore, it was observed that the as-deposited ZnO NPs optical synaptic device without any thermal annealing process exhibited a rapid decay in photocurrent upon tuning off the UV light.This slow decay rate indicates a prolong maintenance of ESPC even after UV light termination, as shown in Figure 4i-l.The high EPSC and slower decay observed in the 800 °C-annealed ZnO NPs optical synaptic devices can be attributed to several factors resulting from the annealing process and the enhanced optical/crystal properties.The increase in bandedge emission, as shown in Figure 2b, suggests an increase in the generation of photoexcited carrier within the high temperature annealed ZnO NPs film.This higher concentration of photoexcited carriers facilitates the enhanced desorption of oxygen adsorbed on the surface, leading to a high EPSC during UV exposure.Furthermore, the abundance of photoexcited carriers inhibits the adsorption of oxygen after UV termination, resulting in a slow decrease in photocurrent and a correspondingly low decrease in EPSC.This phenomenon occurs when the number of photoexcited carriers in the ZnO NP film is significantly higher than the number of oxygen atoms adsorbed on the surface at the end of UV illumination.The carriers reduced by the adsorption of oxygen on the surface are relatively small compared to the total number of photoexcited carriers, which contributes to the slow decrease in EPSC.Another contributing factor is the reduction in surface Zn atoms, as indicated by the decreased in green PL emission associated with antisite defects (O Zn ), which hinders oxygen adsorption and allows EPSC to persist even after UV termination.In general, the persistent photoconductivity (PCC) phenomenon is often associated with the presence of defects that exhibit metastability between shallow and deep energy states. [40]One such defect is the deep unknown (DX) center, which is formed when shallow donors undergo significant lattice relaxation and convert into deep donors. [40,41]he increase in dark current values observed with higher thermal annealing temperatures, as shown in Figure 3a, can be attributed to the generation of point defect defects with shallow levels during the annealing process.These defects result in an increased population of intrinsic n-type carriers, which can subsequently convert into DX centers after a large relaxation.In addition, the PL analysis reveals a slight decrease in green emission related to deep-level defects associated with oxygen after hightemperature annealing.However, it is primarily the increased population of shallow-level defects that contributes to the formation of DX centers.Consequently, the high-temperature annealed ZnO NP film is believed to exhibit an enhanced presence of n-type shallow levels and DX centers, which play a crucial role in the occurrence of PCC.This suggests that high-temperature annealed ZnO NP film is capable of retaining the optical stimulation and demonstrate long-term memory effects.Moreover, the increase in surface grain size results in a decrease in the surface area, which limits the number of oxygen atoms available for adsorption, thereby reducing the adsorption area for atmospheric oxygen.This reduction in surface Zn atoms further hinders oxygen adsorption, contributing to the persistence of EPSCs even after UV termination. [31,33]Therefore, it is believed that the combination of improved optical/crystal qualities, the presence of trapping centers, and the effects of reduced oxygen adsorption enable the device to sustain a higher EPSC and exhibit longer LTM properties even when UV excitation is turned off.
In general, PPF is a measure of the synaptic connection strength in response to two closely spaced stimuli.Specially, it quantifies the ratio of the ESPC response to the second stimulus (A 2 ) compared to that of the first stimulus (A 1 ) for different exposure interval times. [10,11]A high PPF value indicates that the synaptic connection has good short-term plasticity, which is important for information processing and memory formation in the brain.In Figure 5a, the ESPC is plotted as a function time for as-deposited, 400 and 800 °C-annealed ZnO NPs optical synaptic devices under two cycles of 0.5 s UV excitation followed by 0.5 s off states.The as-deposited ZnO NPs optical synaptic device demonstrates a relatively small increase in EPSC at the nA level upon photoexcitation, accompanied by a relatively rapid decay once the photoexcitation ends.However, as the thermal annealing temperature increases, the EPSC upon photoexcitation shows a significant enhancement, reaching the μA level, and maintains a high EPSC even after the photoexcitation is terminated.Furthermore, it is observed that the EPSC during secondary photoexcitation also exhibits higher values with increasing annealing temperature.It means that the ZnO NPs optical synaptic devices annealed at higher temperatures have higher PPF values, indicating better short-term plasticity.In particular, the EPSC of the as-deposited ZnO NPs optical synaptic device shows a rapid decay within 3.5 s after photoexcitation, whereas the ZnO NPs synaptic device annealed at 800 °C exhibits a slower decay rate and maintains a high EPSC after photoexcitation is terminated.These results indicate that higher temperature annealing (%800 °C) of ZnO NPs optical synaptic devices can improve not only their photoexcitation-induced EPSCs but also their shortand long-term memory properties due to the enhanced optical and structural properties.When the interval time (Δt) is as short as 0.5 s, the PPF of the as-deposited ZnO NPs optical synaptic device is 160.1%, which is lower than the 180% observed for the ZnO NPs optical synaptic device annealed at 800 °C.
However, as Δt increases to 500 s, the PPF of the as-deposited ZnO NPs optical synaptic device decreases significantly to 109%, while the PPF for the ZnO NPs optical synaptic device annealed at higher temperatures shows no significant decrease with increasing Δt.Specifically, for the ZnO NPs optical synaptic devices annealed at 400 and 800 °C, the PPFs remain high at 143% and 161%, respectively, even when Δt is increased to 500 s.The gradual decrease in PPF values with an increase in the interval between photoexcitation exposures is consistent with the characteristics of the optical synaptic device and time dependence of the PPF index in biological synapses, where higher PPF indexes are observed at shorter intervals between stimuli. [42]owever, as the interval time between UV exposure periods increases, the PPF index of the as-deposited ZnO NPs optical synaptic device decreases rapidly, indicating that its short-term memory properties are fading quickly.In contrast, the EPSC of the ZnO NPs optical synaptic devices annealed at 800 °C exhibit a smaller decrease in the PPF value, indicating that better maintenance of their short-and long-term memory properties over time.This improved performance can be attributed to the enhanced optical and crystal properties of the ZnO NPs film at higher annealing temperatures, as depicted in Figure 1 and 2. [30,31]

Dynamic Learning and Forgetting Simulation Process of ZnO NPs Optical Synaptic Devices
The learning process is demonstrated by the gradual increase in EPSC with each UV light pulse, indicating an increase in synaptic weight.Conversely, the forgetting process is characterized by the gradual decrease in EPSC after turning off the UV light, indicating a decrease in synaptic weight. [11]Figure 6a,b shows the learning and forgetting processes of as-deposited and 800 °C-annealed ZnO NPs optical synaptic devices, respectively.These processes are observed by monitoring the changes in EPSC during 100-cycle pulses of UV light.To determine the pulse cycles required for learning and the time for forgetting, the experiments were repeated twice for validation purpose. [12]he learning process is considered successful when the EPSC reached at least 80% of the maximum value, while the forgetting process occurs when the EPSC drops below 80%.In the first learning process, both as-deposited and 800 °C-annealed ZnO NPs optical synaptic devices demonstrate successful learning, with EPSC increasing from 0 to 17 nA and from 0 to 1.87 mA, respectively, using 100 pulses.The 800 °C-annealed ZnO NPs devices show faster learning ability, requiring only 56 pulses to reach the maximum EPSC, compared to 63 pulses in the as-deposited ZnO NPs device.During the first forgetting process, the EPSC of the 800 °C-annealed ZnO NPs device remains above 80% even after 350 s of UV light termination, which is approximately 26 times longer than the retention time (13 s) of the as-deposited ZnO NPs device.This demonstrates significant enhancement in LTM characteristics of the 800 °C-annealed ZnO NPs optical synaptic devices.In the second learning process, the EPSCs required to induce learning in subsequent repetitions decrease from 63 to 33 pulses in the as-deposited ZnO NPs device and from 56 to 30 pulses in the 800 °C annealed ZnO NPs device.This suggests that the second learning process is faster than the first learning process, and the 800 °C-annealed ZnO NPs device exhibits a faster learning process compared to the as-deposited ZnO NPs optical synaptic device.Furthermore, after 350 s of UV light termination, the EPSCs of both devices remain above 80%, indicating improved retention capabilities.The 800 °C-annealed ZnO NPs device results in higher EPSC formation, leading to better EPSC retention after 350 s, demonstrating the positive influence of hightemperature annealing on the optical properties of ZnO NPs.
In addition, the simulated experiments were performed to observe the behavior of EPSCs for a 3 Â 3 array.Nine adjacent ZnO NPs optical synaptic devices from a single wafer were selected to represent the 3 Â 3 pixel layout.Each individual device underwent stimulation for a fixed period of 100 s, comprising 100 pulses with a pulse width of 0.5 μs and a duty cycle of 50%.The EPSCs values for these pulse inputs were utilized to encode the predefined pixel colors and intensity into the devices.A 3 Â 3 array was used to observe the decay of EPSCs over time after achieving maximum EPSCs with 100 cycles in the first and second learning-forgetting processes.The as-deposited ZnO NPs optical synaptic device exhibits rapid memory decay, nearly decaying completely within 10 s during the first learning process.However, during the second learning process, the device shows slower decay of ESPC over the same time period.Similarly, the 800 °C-annealed ZnO NPs optical synaptic device demonstrates superior retention of LTM compared to the as-deposited ZnO NPs optical synaptic device.It maintains over 80% of its memory for up to 300 s during the first learning process, and it exhibits higher EPSC during the second learning process.These findings suggest that the high temperature annealing of ZnO NPs improves the LTM properties of the optical synaptic device, allowing for longer-lasting memory without easy fading.

Conclusion
We investigated the performance of ZnO NPs optical synaptic devices with different annealing temperatures.The results demonstrated that higher annealing temperatures, such as 800 °C, led to significant improvements in the optical and structural properties of ZnO NPs device, resulting in enhanced synaptic functionality.The annealed devices exhibited higher EPSC values upon photoexcitation, indicating improved short-term plasticity.Moreover, their PPF values remained high even with increased interval times between stimuli, suggesting better short-term memory retention.These findings highlight the importance of thermal annealing in optimizing synaptic device performance.Dynamic learning and forgetting simulations revealed that the annealed devices had faster learning processes, requiring fewer pulse cycles to achieve maximum EPSCs, and better LTM retention.The annealed devices demonstrated a significantly longer LTM characteristic, retaining over 80% of their memory for an extended period compared to the as-deposited devices.Overall, the high-temperature annealing of ZnO NPs optical synaptic devices not only improved their photoexcitationinduced EPSCs but also enhanced their short-term and LTM properties.These findings contribute to the understanding of synaptic device behavior and provide insights for the development of advanced neuromorphic systems and memory storage applications.

Experimental Section
A (0001) sapphire was used as a substrate for the deposition of ZnO NPs using the spin coating method at a speed of 2000 RPM for 60 s.The ZnO NPs, provided by Sigma-Aldrich Corporation, were mixed with isopropyl alcohol as a solvent and the solvent was removed by annealing the ZnO NPs/sapphire on a hot plate at 100 °C for 20 min.Additional thermal annealing process was carried out at 400 and 800 °C for 5 min to improve the crystallinity of the ZnO NPs films.After depositing 50 nmthick ZnO NPs thin films on sapphire substrates, we fabricated a metal-ZnO NPs thin film-metal structure optical synaptic device with a sensing area of 50 Â 1000 μm 2 using a shadow mask.As a metal electrode, 100 nm-thick Al film as an electrode was deposited by thermal evaporation.
The ZnO NPs thin films were characterized using various analytical techniques to determine their surface and crystallographic properties, as well as their optical and electrical properties.The thickness of ZnO NPs thin film was determined by measuring its cross-sectional image using scanning electron microscopy.AFM was used to measure the surface structure of the thin films, while high-resolution XRD Ω/2θ scan was used to analyze their crystallographic properties.To evaluate the optical properties of the ZnO NPs films, room temperature PL properties using 266 nm laser excitation light and absorption using UV-vis spectroscopy were measured.The dark current and photocurrent of ZnO NPs thin film optical synaptic devices were also evaluated by measuring their response to 365 nm UV light source.To characterize the optical synaptic properties, EPSCs of as-deposited and annealed ZnO NPs optical synaptic devices were measured at a voltage of 10 V.The measurements were performed by using different exposure times, light source powers, exposure frequencies, and exposure numbers.Overall, these analytical techniques were used to gain a comprehensive understanding of the properties of the ZnO NPs thin films as a function of thermal treatment temperature, which is critical for optimizing the performance of optical synaptic devices.

Figure 1 .
Figure 1.Surface morphologies measured by atomic force microscopy of a) as-deposited ZnO NP films grown on sapphire substrate and annealed at b) 400 °C, c) 800 °C, and d) 900 °C.e) RMS roughness and f ) surface grain size of ZnO NPs films as a function of annealing temperature.

Figure 5 .
Figure 5. a) The EPSCs of ZnO NPs optical synaptic devices with different annealing temperatures measured at 10.0 V using two consecutive light pulse injection of 0.5 s exposure and 0.5 s off.b) Schematic of an Al/ZnO NPs/Al optical synaptic device and the transmission of presynaptic and postsynaptic signals at the synapse.c) PPF as a function of different time intervals under a constant 0.5 s pulse exposure time for as-deposited, 400 and 800 °Cannealed ZnO NPs synaptic devices.

Figure 6 .
Figure 6.The measured learning and forgetting-experience behaviors under pulsed light stimuli (365 nm-UV light with 0.5 s pulse width and 50% duty cycle) and mimicry of human visual memory using a) as-deposited and b) 800 °C-annealed ZnO NPs optical synaptic devices.In the mimicry of human visual memories, the forgetting processes examined over time, starting from the first learning input images encoded with 100 pulses for both the as-deposited and 800 °C-annealed ZnO NPs.