Before demonstrating the VDS-tunable neuromorphic device performance, we first characterized the basic synaptic transistor performance of the 2D MOF-based device (Figure \ref{369822}, S7). Except the erase process of the device (-15 V), the gate voltage applied on the device was kept at the value of 0 V to investigate the light effect of the device. In our device, the source-drain channel is regarded as the post-synaptic neuron, the IDS is regarded as the post-synaptic signal and the channel conductance is treated as the synaptic weight. The photosensitive 2D Zn2(ZnTCPP) MOFs-PMMA charge trapping layer can be regarded as a light-stimulated pre-synaptic neuron and the silicon gate can be regarded as an electrical-stimulated pre-synaptic neuron. In bio-synapse, the pre-synaptic neurons contain neurotransmitters and the post-synaptic neurons contain neurotransmitter receptors. The signal reached the pre-synaptic neuron would open the Ca2+ channel on the pre-synaptic membrane and therefore the membrane would release the neurotransmitters into the synaptic cleft\cite{marin2006,holcman2018,lossi2011}. The neurotransmitters in the synaptic cleft eventually interact with the neurotransmitter receptors to induce pre-synaptic signals transmission to post-synaptic neurons, triggering the EPSC and the IPSC (Inhibitory post-synaptic current)\cite{forsythe2000}. The investigation of the EPSC behaviors with the light spike at various wavelengths ranging from 365 to 530 nm exhibits that the 430 nm light spike can induce the largest EPSC amplitude (Figure \ref{369822}b), which is consistent with the UV-Vis spectrum of 2D Zn2(ZnTCPP) MOFs-PMMA film (Figure \ref{369822}b and Figure S6a). To assess the response of our neuromorphic device to light stimulation (430 nm), we further investigated EPSCs with different light spike durations (Figure \ref{369822}c) and intensities (Figure \ref{369822}d), respectively. The EPSCs could be enhanced with the increment of light spike duration or intensity.
We fabricated a device based on pentacene/PMMA structure without 2D MOFs. The photo-reponse performance of the device was characterized and compared with that of pentacene/2D MOFs-PMMA device. As shown in Figure S8, when the illumination was removed, the current of pentacene/PMMA device immediately dropped to the initial state, while the current of the pentacene/2D MOFs-PMMA device decreased slowly. The results indicates that the generation, transportation, and trapping processes of the photogenerated changes on the MOFs polymer and organic semiconductor heterojunction attributed to the transistors with different synaptic behaviors.
Paired-pulse facilitation (PPF) is an essential behavior in the bio-synaptic system for temporary information processing, where the post-synaptic conductance (synaptic weight) can be enhanced via two consecutive pre-synaptic stimulations, resulting in the device showing higher conductance after the second spike than that after the first spike\cite{regehr1996}. To show that our device can simulate PPF behavior, a pair of optic-signal spikes with a certain spike interval was utilized as a pre-synaptic signal spike. The typical PPF behaviors of the device under two consecutive light spikes was characterized. As shown in Figure S9, two consecutive 430 nm 100 μW/cm2 light spikes (1 s) with 1 s interval were applied to our synaptic device. The larger value of the EPSC triggered by the 2nd light spike than that value triggered by the 1st light spike was observed. The PPF ratio can be defined by the following equation:
\(PPF\ ratio=\frac{A_{2}}{A_{1}}\times100\%\)
where the values A1 and A2 are the EPSC amplitudes of the first and the second light spike, respectively. Tuning the interval time can change the ratio of two-spike synaptic current. The PPF ratio would decrease when we increase the interval time (tinter) (Figure \ref{369822}e). The maximum value of the PPF is ~149%, which was obtained at the minimum tinter of 100 ms. The electrons trapping in the 2D Zn2(ZnTCPP) MOFs-PMMA layer induced by the first light spike have insufficient time to recombine with holes before the 2nd light spike was applied, which results in an enhanced EPSC amplitude after the second light spike. The PPF decay can be described as the combination of rapid decay and slow decay, defined by the following equation:
\(W=\frac{EPSC}{V_{DS}}\)
By changing the light spike parameters, we achieved the simulation of different types of synaptic plasticity in our 2D Zn2(ZnTCPP) MOFs-based neuromorphic devices, suggesting the potential of our device for future neuromorphic computing applications.
Temperature stability is one of the key parameters when the device is practically operated. Therefore, we characterized the temperature effect on the device optoelectronic performance. As shown in Figure S10, with the increase of temperature, although the device baseline current was gradually enhanced (Figure S10a), our device can maintain decent photo-synaptic behaviors (Figure S10b), which suggests a certain degree of temperature stability of our pentacene/2D MOF-PMMA device.
Demonstration of Emotion-Tunable Simulation
Humans have abundant emotions (such as happy, sad, plain, mild), which are essential in human learning and memory. Positive emotion would improve vitality and enhance learning efficiency. By contrast, negative emotion would depress vitality, resulting in a low learning rate (Figure \ref{234487}a)\cite{tunariu2019,logan2012,zhou2020a}. The VDS-tunable photoresponse of our 2D Zn2(ZnTCPP) MOFs-based neuromorphic device can be utilized to mimic emotion-tunable memory and learning behaviors. Figure \ref{234487}b displays the energy alignment controlled by VDS of the pentacene/2D Zn2(ZnTCPP) MOFs-PMMA structure, which can be used to describe the mechanism of the VDS tunning capability. The kelvin probe force microscope (KPFM) potential variation with the light illumination confirmed the hole accumulation in pentacene under illumination (Figure S11). The level of the VDS determines the energy band structure. At high VDS, the large energy difference between the drain electrode and gate electrode enhances the bending of the energy band in the pentacene and MOF layers. Light-generated carriers are easily transferred to the OSC (organic semiconductor) channel, which results in a large ΔEPSC value. The recombination rate of the light-induced accumulated charge carriers in pentacene would be also depressed, delivering a long retention time. On the contrary, the recombination rate of light-generated carriers would be increased at a low VDS level because of the weak bending of the energy band. Furthermore, at the ultra-low VDS or even zero-level state, the transfer process of light-induced carriers would be in chaos.