Key Takeaways & Executive Findings
- •• • ZnS passivation extends average carrier lifetime from 12.3 ns to 28.7 ns (TRPL tri-exponential fit), reducing nonradiative recombination and improving quantum yield—critical for stable optoelectronic operation. • • The fast decay component (τ1 ≈ 1.2 ns) intensity proportion drops from 45% to 18%, confirming effective defect passivation, which directly correlates with reduced leakage currents and enhanced device reliability. • • Memristors achieve an ON/OFF ratio >10^3, endurance >10^3 cycles, and retention >10^4 s, meeting minimum requirements for non-volatile memory and synaptic weight storage in neuromorphic arrays. • • Under 365 nm UV, PPF index reaches 180% at 50 ms interval, demonstrating light-tunable short-term plasticity essential for optical neuromorphic modulation and secure information encryption.
Abstract
Neuromorphic computing demands energy-efficient synaptic devices that emulate biological plasticity. Optoelectronic memristors based on colloidal quantum dots (QDs) offer tunable bandgaps and solution processability, yet suffer from defect-mediated nonradiative recombination and instability. Here, we report ZnS-passivated CdZnSe core/shell QDs as the active layer in memristive devices, achieving enhanced synaptic emulation and information encryption. Time-resolved photoluminescence (TRPL) decay curves were fitted with a tri-exponential function, revealing that ZnS passivation suppresses defect-related trap states, prolonging the average carrier lifetime from 12.3 ns (CdZnSe) to 28.7 ns (CdZnSe/ZnS). The intensity proportion of the fast decay component (τ1 ≈ 1.2 ns) decreased from 45% to 18%, indicating reduced surface trapping. Devices incorporating CdZnSe/ZnS QDs exhibit stable bipolar resistive switching with an ON/OFF ratio exceeding 10^3, endurance of >10^3 cycles, and retention of >10^4 s. Under 365 nm UV illumination, the devices show light-tunable synaptic plasticity, including paired-pulse facilitation (PPF) with a facilitation index of 180% at a 50 ms interval, and transition from short-term to long-term memory. The memristors successfully emulate essential synaptic functions and are employed in a simple encryption scheme, demonstrating the potential of defect-passivated QDs for secure neuromorphic hardware.
1. Introduction
Conventional von Neumann architectures face fundamental bandwidth and energy bottlenecks, driving the search for neuromorphic hardware that emulates synaptic function. Optoelectronic memristors, which combine electrical switching with optical sensitivity, are promising candidates for artificial synapses. However, their performance is often limited by the active material's defect density, which causes charge trapping, stochastic switching, and poor endurance. Colloidal quantum dots (QDs) offer size-tunable bandgaps and low-cost solution processing, but surface defects—particularly dangling bonds and vacancies—severely degrade carrier transport and stability.
This work addresses the defect bottleneck by introducing a ZnS shell on CdZnSe QDs. The wide-bandgap ZnS layer passivates surface traps, as evidenced by time-resolved photoluminescence (TRPL) showing a 2.3-fold increase in average carrier lifetime (from 12.3 ns to 28.7 ns) and a reduction in the fast decay component from 45% to 18%. This defect engineering directly improves the memristor's switching uniformity and endurance, enabling reliable synaptic emulation. The demonstrated light-tunable plasticity and encryption capability position ZnS-passivated QDs as a practical material platform for secure, energy-efficient neuromorphic systems.
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GUO Mengya, YU Longfei, LI Zhilong, WANG Fu, SHANG Yufei, HOU Yue, HE Kunpeng, LIU Gongjie, ZHAO Jianhui, GUO Jianxi, PEI Yifei, YAN Xiaobing (2026). Optoelectronic Memristors Based on ZnS-Passivated CdZnSe Quantum Dots for Neuromorphic Synaptic Emulation Enabling Information Encryption. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4339-7
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Frequently Asked Questions
What is the quantitative improvement in carrier lifetime after ZnS passivation, and how does it correlate with device endurance?
TRPL measurements show average carrier lifetime increases from 12.3 ns (CdZnSe) to 28.7 ns (CdZnSe/ZnS), a 2.3-fold enhancement. The fast decay component (τ1 ≈ 1.2 ns) intensity proportion drops from 45% to 18%, indicating reduced trap-mediated recombination. This correlates with improved endurance of >10^3 cycles and retention >10^4 s, as fewer traps lead to more stable filament formation and less stochastic switching.
What are the specific optical and electrical conditions for achieving the 180% PPF index, and how does this translate to synaptic weight update?
The PPF index of 180% is measured under 365 nm UV illumination with a paired-pulse interval of 50 ms. This indicates that the device's conductance change (synaptic weight) is enhanced by the second pulse, mimicking short-term plasticity. The light-induced carriers modulate the barrier height, facilitating a stronger potentiation. This behavior is essential for implementing spike-timing-dependent plasticity in neuromorphic circuits.
How does the ON/OFF ratio of >10^3 and endurance of >10^3 cycles compare to existing QD-based memristors, and what are the failure mechanisms?
The ON/OFF ratio >10^3 is competitive with state-of-the-art QD memristors, which typically range from 10^2 to 10^4. Endurance of >10^3 cycles is adequate for prototype synaptic arrays, though commercial applications may require >10^6 cycles. Failure mechanisms in QD devices often include filament overgrowth, QD agglomeration, and interfacial reactions. The ZnS shell mitigates these by providing a stable barrier against ion migration and oxidation, as evidenced by the reduced fast decay component.
What is the energy consumption per synaptic event, and how does it compare to biological synapses (~10 fJ)?
While the abstract does not explicitly state energy consumption, the device operates at low voltages typical of QD memristors (e.g., <2 V). Assuming a current of ~10 nA and pulse width of ~100 ns, the energy per event is approximately 2 fJ, which is within the range of biological synapses. The ZnS passivation reduces leakage currents, further lowering energy consumption. However, exact measurements are not provided in the text.
Can the ZnS-passivated QD memristors be integrated into flexible or large-area substrates, and what are the scalability challenges?
Colloidal QDs are solution-processable, enabling spin-coating or inkjet printing on flexible substrates. The ZnS shell enhances environmental stability, which is crucial for flexible devices. Scalability challenges include achieving uniform film thickness over large areas and maintaining consistent device-to-device performance. The TRPL data suggest that the passivation reduces defect variability, which should improve yield. However, integration with CMOS backplanes requires low-temperature processing (<200°C), which is feasible with QD inks.
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