Defect-engineering-based titanium dioxide nanowires for artificial optoelectronic synapses

The LaAlO<sub>3</sub>/SrTiO<sub>3</sub> interface has been one of the topics studied most during the past few years due to its many intriguing properties such as the two-dimensional electron gas, transient photoconductivity (PC), persistent photoconductivity (PPC), and the coexistence of the PC and PPC. Of them, the PPC effect is the most interesting because of its potential application in exploring the photoelectric memory devices. Until now, tuning of the PPC of the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> interface under the external stimuli, such as electric or magnetic field is less addressed, while the relevant knowledge is of great value for exploring the memory devices with multifunctionality. In this paper, we report on an electric field control of the persistent PPC at the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> interface. Our LaAlO<sub>3</sub>/SrTiO<sub>3</sub> heterojunction is fabricated by growing the LaAlO<sub>3</sub> film on the SrTiO<sub>3</sub> substrates through using pulsed laser deposition. The substrate temperature is kept at 750 ℃ and the partial pressure of oxygen is maintained at 3.3 × 10<sup>–5</sup> Torr (1 Torr = 1.33322 × 10<sup>2</sup> Pa) during the deposition. The thickness of LaAlO<sub>3</sub> film is controlled to be about 2 nm by setting an appropriate deposition time. The X-ray diffraction experiment confirms that the LAO film is well epitaxial and of single phase. To guarantee the good electric contacts, Al electrodes are soldered at the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> interface and the back side of the SrTiO<sub>3</sub> respectively by ultrasonic welding. We find that the PPC at the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> interface can be significantly reinforced and modulated by the light-enhanced gating effects: that is, after a negative back gate voltage processing combined with a simultaneous light illumination, the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> interface can exhibit a notable PPC effect. And the PPC effect increases as the negative gate voltage increases, and then attains a maximum at a back gate voltage of about –70 V. Further increase of the negative gate voltage can cause the PPC to decrease. Additionally, the PPC is also found to increase monotonically with increasing the gating time. The present result can be understood in terms of the migration of the oxygen vacancies under the influence of photoelectric synergetic effect. This field enhanced PPC effects at the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> interface may find their applications in designing the photoelectric memory devices with electric tunability.

A new type artificial synapse based on the organic copolymer memcapacitor

The slow relaxation of the persistent photoconductivity (PPC) effect in sulfur-doped hydrogenated amorphous silicon (a-Si:H) has been measured as a function of temperature and illumination time. The relaxation is found to be thermally activated, with an activation energy which varies with sulfur concentration, while illuminating the film for a longer time leads to a longer relaxation time. A correlation is observed between changes of the photoconductivity during illumination and the magnitude of the PPC effect following illumination. These effects are also observed in compensated a-Si:H, suggesting that the mechanism for the PPC effect is the same in both sulfur-doped a-Si:H and compensated a-Si:H. The presence of donor and compensating acceptor states in sulfur-doped a-Si:H could arise from valence alternation pair sulfur atom defects.

The Persistent Photoconductivity (PPC) effect was studied in individual tin oxide (SnO2) nanobelts as a function of temperature, in air, helium, and vacuum atmospheres, and low temperature Photoluminescence measurements were carried out to study the optical transitions and to determine of the acceptor/donors levels and their best representation inside the band gap. Under ultraviolet (UV) illumination and at temperatures in the range of 200 to 400K we observed a fast and strong enhancement of the photoconductivity, and the maximum value of the photocurrent induced increases as the temperature or the oxygen concentration decreases. By turning off the UV illumination the induced photocurrent decays with lifetimes up to several hours. The photoconductivity and the PPC results were explained by adsorption and desorption of molecular oxygen at the surface of the SnO2 nanobelts. Based on the temperature dependence of the PPC decay an activation energy of 230 meV was found, which corresponds to the energy necessary for thermal ionization of free holes from acceptor levels to the valence band, in agreement with the photoluminescence results presented. The molecular-oxygen recombination with holes is the origin of the PPC effect in metal oxide semiconductors, so that, the PPC effect is not related to the oxygen vacancies, as commonly presented in the literature.

Ultrasensitive artificial synapse based on conjugated polyelectrolyte

Electroluminescent synaptic devices with logic functions

Various hippocampal and neocortical synapses of mammalian brain show both short-term plasticity and long-term plasticity, which are considered to underlie learning and memory by the brain. According to Hebb’s postulate, synaptic plasticity encodes memory traces of past experiences into cell assemblies in cortical circuits. However, it remains unclear how the various forms of long-term and short-term synaptic plasticity cooperatively create and reorganize such cell assemblies. Here, we investigate the mechanism in which the three forms of synaptic plasticity known in cortical circuits, i.e., spike-timing-dependent plasticity (STDP), short-term depression (STD) and homeostatic plasticity, cooperatively generate, retain and reorganize cell assemblies in a recurrent neuronal network model. We show that multiple cell assemblies generated by external stimuli can survive noisy spontaneous network activity for an adequate range of the strength of STD. Furthermore, our model predicts that a symmetric temporal window of STDP, such as observed in dopaminergic modulations on hippocampal neurons, is crucial for the retention and integration of multiple cell assemblies. These results may have implications for the understanding of cortical memory processes.

Emulating synaptic functionalities in optoelectronic devices is significant in developing artificial visual-perception systems and neuromorphic photonic computing. Persistent photoconductivity (PPC) in metal oxides provides a facile way to realize the optoelectronic synaptic devices, but the PPC performance is often limited due to the oxygen vacancy defects that release excess conduction electrons without external stimuli. Herein, a high-performance optoelectronic synapse based on the stoichiometry-controlled LaAlO3/SrTiO3 (LAO/STO) heterostructure is developed. By increasing La/Al ratio up to 1.057:1, the PPC is effectively enhanced but suppressed the background conductivity at the LAO/STO interface, achieving strong synaptic behaviors. The spectral noise analyses reveal that the synaptic behaviors are attributed to the cation-related point defects and their charge compensation mechanism near the LAO/STO interface. The short-term and long-term plasticity is demonstrated, including the paired-pulse facilitation, in the La-rich LAO/STO device upon exposure to UV light pulses. As proof of concepts, two essential synaptic functionalities, the pulse-number-dependent plasticity and the self-noise cancellation, are emulated using the 5×5 array of La-rich LAO/STO synapses. Beyond the typical oxygen deficiency control, the results show how harnessing the cation stoichiometry can be used to design oxide heterostructures for advanced optoelectronic synapses and neuromorphic applications.

Detection and recognition of latent fingerprints play crucial roles in identification and security. However, the separation of sensor, memory, and processor in conventional ex-situ fingerprint recognition system seriously deteriorates the latency of decision-making and inevitably increases the overall computing power. In this work, a photoelectronic reservoir computing (RC) system, consisting of DUV photo-synapses and nonvolatile memristor array, is developed to detect and recognize the latent fingerprint with in-sensor and parallel in-memory computing. Through the Ga-rich design, we achieve amorphous GaOx (a-GaOx) photo-synapses with an enhanced persistent photoconductivity (PPC) effect. The PPC effect, which induces nonlinearly tunable conductivity, renders the a-GaOx photo-synapses an ideal deep ultraviolet (DUV) photoelectronic reservoir, thus mapping the complex input vector into a dimensionality-reduced output vector. Connecting the reservoirs and a memristor array, we further construct an in-sensor RC system for latent fingerprint identification. The system maintains over 90% recognition accuracy for latent fingerprint within 15% stochastic noise level via the proposed dual-feature strategy. This work provides a subversive prototype system of DUV in-sensor RC for highly efficient recognition of latent fingerprints.

Just as biological synapses provide basic functions for the nervous system, artificial synaptic devices serve as the fundamental building blocks of neuromorphic networks; thus, developing novel artificial synapses is essential for neuromorphic computing. By exploiting the band alignment between 2D inorganic and organic semiconductors, the first multi-functional synaptic transistor based on a molybdenum disulfide (MoS2 )/perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) hybrid heterojunction, with remarkable short-term plasticity (STP) and long-term plasticity (LTP), is reported. Owing to the elaborate design of the energy band structure, both robust electrical and optical modulation are achieved through carriers transfer at the interface of the heterostructure, which is still a challenging task to this day. In electrical modulation, synaptic inhibition and excitation can be achieved simultaneously in the same device by gate voltage tuning. Notably, a minimum inhibition of 3% and maximum facilitation of 500% can be obtained by increasing the electrical number, and the response to different frequency signals indicates a dynamic filtering characteristic. It exhibits flexible tunability of both STP and LTP and synaptic weight changes of up to 60, far superior to previous work in optical modulation. The fully 2D MoS2 /PTCDA hybrid heterojunction artificial synapse opens up a whole new path for the urgent need for neuromorphic computation devices.

Two-dimensional van der Waals heterostructures provide a versatile platform for multifunctional optoelectronic devices. WS2/PdSe2 heterostructures, characterized by n-type conductive behavior, exhibit persistent photoconductivity in high vacuum under negative gate voltages. This phenomenon has been leveraged to simulate synaptic functions of the human brain, including short- and long-term plasticity. Short-term plasticity was simulated, resulting in paired-pulse facilitation and post-tetanic potentiation indices of approximately 140% and 300%, respectively. By stimulating the WS2/PdSe2 heterostructure with consecutive 1 s light pulses, the learning, forgetting, and re-learning phases associated with knowledge acquisition in the human brain were replicated. Furthermore, the device was demonstrated as an optoelectronic memory, programmable with light pulses and erasable via electrical pulses on the gate electrode.

Optoelectronic synaptic devices outperform electrical synapses in speed, energy efficiency, and integration density. Recent progress in visual sensing and optogenetics has led to the integration of light-sensitive materials in these devices, promising unmatched speed, connectivity, and bandwidth. Here, we present a copper phthalocyanine (CuPc) based optoelectronic synaptic device boasting femto Joule power consumption stable at room temperature. The optoelectronic synapse can be operated with energy consumption as low as 430.4 fJ which is very attractive from the point of view of low-power neuromorphic devices. By modulating light pulses, the neuromorphic behavior can be emulated including excitatory post-synaptic current (EPSC), paired-pulse facilitation (PPF), transitioning from short-term plasticity (STP) to long-term plasticity (LTP), spike-rate dependent plasticity (SRDP) and spike-number dependent plasticity (SNDP), etc. Optical potentiation and electrical depression are observed with combined optical and electrical stimulation, proving the multi-functionality of the synapse. Furthermore, the device demonstrates classical associative learning behaviors like Pavlovian conditioning using optical and electrical stimuli. We have established the pain conditioning processes such as hyperalgesic response and pain extinction effects with varying optical pulse amplitudes. These results render the CuPc-based devices as multifunctional and highly versatile artificial synaptic devices for future computing applications, offering unprecedented efficiency and functionality in neuromorphic systems.

Xanthan gum biopolymer gated low voltage-operating photo synaptic organic field-effect transistor for neuromorphic electronics

Synapse is one of the key components of brain nervous system. As the fourth electronic component, memristor can be used to act as synaptic device and construct bionic neural network, in which the synapses can be stimulated by either electrical excitation or optical irradiation. Up to now, there have been many researches on the optoelectronic synaptic devices and the main functional materials include binary oxides, perovskite complex oxides, solid electrolytes, organic and polymer materials, etc. Here, we report anoptoelectronic synaptic device based on SnO₂monolayer embedded with Ag nanocrystals and structured as ITO/SnO₂:Ag/p++-Si. It is found that the SnO2:Ag thin film can effectively absorb light illumination in the broadband range of 450 nm to 635 nm, and that the device has been successfully simulated with multiple synaptic behaviors in the broadband range, including excitatory postsynaptic current (EPSC), paired pulse facilitation (PPF) and the transition from short-term plasticity (STP) to long-term plasticity (LTP). Our newly developed optoelectronic synapse device shows great application potential in neuromorphological computing.

Li+ electrolyte-gated transistors (EGTs) have attracted significant attention as artificial synapses because of the fast response of Li+ ion, low operating voltage, and applicability to flexible electronics. Due to the inherent nature of Li+ ion, Li+ EGTs show, however, limitations, such as poor long-term synaptic plasticity and nonlinear/nonsymmetric conductance update, which hinder the practical applications of artificial synapses. Herein, Li+ EGTs integrated with poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) ferroelectric polymer as a channel–electrolyte interlayer are presented. Owing to the polarized domains of PVDF-TrFE, the transport of Li+ ions at the channel–electrolyte interface is accelerated, and Li+ ions effectively penetrate the channel. Moreover, the self-diffusion of Li+ ions from the channel to the electrolyte is suppressed by the downward polarized domains. Li+ EGTs, therefore, successfully demonstrate synaptic characteristics, including excitatory postsynaptic current, short-/long-term synaptic plasticity, and paired-pulse facilitation. Also, conductance update in Li+ EGTs shows a dynamic range (Gmax/Gmin) of 92.42, high linearity, and distinct stability over 100 cycles. Based on their synaptic characteristics, inference simulations using a convolution neural network for the CIFAR-10 dataset imply that Li+ EGTs are suitable as artificial synapses with an inference accuracy of 89.13%. The new methodological approach addressing modulation of ion dynamics at the interface is introduced for developing practical synaptic devices.