Multifunctional Optoelectronic Synapses Based on Arrayed MoS2 Monolayers Emulating Human Association Memory

Abstract

Optoelectronic synaptic devices integrating light-perception and signal-storage functions hold great potential in neuromorphic computing for visual information processing, as well as complex brain-like learning, memorizing, and reasoning. Herein, the successful growth of MoS₂ monolayer arrays assisted by gold nanorods guided precursor nucleation is demonstrated. Optical, spectral, and morphology characterizations of MoS₂ prove that arrayed flakes are homogeneous monolayers, and they are further fabricated as optoelectronic devices showing featured photocurrent loops and stable optical responses. Typical synaptic behaviors of photo-induced short-term potentiation, long-term potentiation, and paired pulse facilitation are recorded under different light stimulations of 450, 532, and 633 nm lasers at various excitation powers. A visual sensing system consisting of 5 × 6 pixels is constructed to simulate the light-sensing image mapped by forgetting curves in real time. Moreover, the system presents the ability of utilizing associated images to restore vague and incomplete memories, which successfully mimics human intelligent behaviors of association memory and logical reasoning. The work emulates the brain-like artificial intelligence using arrayed 2D semiconductors, which paves an avenue to achieve smart retina and complex brain-like system.

Keywords: MoS2; artificial synapse; association memory; optoelectronic device.

Figure 1

Schematics of growth, morphology, and spectral characteristics. a) Schematic view of CVD growth of arrayed MoS₂ monolayers guided by Au nanorods. The control of sulfur‐rich component in precursors and low gas velocity help to realize the monolayer growth of MoS₂. b) Optical image of 5×6 array of MoS₂ monolayers grown at the location of Au nanorods. c) SEM image of MoS₂ monolayer arrays showing clear structure morphology from the contrast. d) Mapping image of integrated PL intensity of MoS₂ monolayer arrays at 670–690 nm. Scale bars are 30 µm. e) PL spectra detected from thirty monolayers exhibiting stable luminescence peaks and uniform intensities. f) Raman spectra detected from thirty monolayers showing that the difference of Raman shifts is in the range of 19–20 cm⁻¹. g) TEM image of MoS₂ monolayer presenting clear hexagonal lattice with a spacing distance of 0.27 nm. The scale bar is 2 nm. The inset shows the corresponding FFT pattern.

Figure 2

Fabrication of arrayed devices and I–V measurements. a) Schematic of a MoS₂ synaptic transistor with a gold‐nanorod working under the light illumination. A large amount of plasmonic doping electrons can be injected into MoS₂ monolayers for the contribution of efficient photocurrent. b) The enlarged view of electrons trapped or released at the interface of MoS₂/SiO₂ revealing the working mechanism of synaptic behaviors. c) Energy diagram of heterostructures and their electron migrations at both forward and reverse electric fields. d) Optical image of arrayed electrodes fabricated on MoS₂ monolayers. Scale bar is 30 µm. e) Transfer characteristic curves of 30 devices at V _ds = 1 V in dark and under light illumination (wavelength of 532 nm, power of 0.12 mW cm⁻²). Ideal working voltage ranges from −3 to 3 V (gray region). f) I _ds–V _g curves at various drain biases of 1, 2, and 3 V.

Figure 3

Synaptic plasticity of MoS₂ optoelectronic device. a) Schematic illustration of a biological synapse with pre‐synapse stimuli, signal transduction, and postsynapse response. b) An EPSC of a synaptic transistor triggered by an optical spike. c) EPSC of a synaptic transistor triggered by a pair of optical spikes with the duration time of 0.4 s. d) The PPF index plotted as a function of inter‐spike interval (Δt) and fitted by exponential curves. STM‐to‐LTM transition induced by increasing the duration time e) and the number of pulsed light stimuli f). g) Measured “learning‐forgetting” behaviors emulated by pulsed light stimuli. All measurements were performed at V _ds = 1 V and V _g = −3 V. The wavelength, duration time, and intensity of all optical spikes are 532 nm, 50 ms, and 0.21 mW cm⁻², respectively.

Figure 4

Photoresponsive characteristics of MoS₂ devices under multiwavelength irradiations. a) Difference of reflection spectra of 30 MoS₂ monolayers (left y‐axis) in experiments. Absorption spectra of Au‐nanorod on MoS₂/SiO₂/Si substrate from simulation (left y‐axis). Intensity variation of EPSC under 450, 532,  and 633 nm irradiations (right y‐axis). Inset is the electromagnetic field distribution of a gold‐nanorod on MoS₂/SiO₂/Si substrate. b) EPSC curves stimulated with three different wavelengths at the same power density of 1.50 mW cm⁻². c) Pulse‐number‐dependent gain of A _n/A ₁ under multiwavelength stimulations. Pump‐fluence dependent forgetting curves plotted at 450 nm d), 532 nm e), and 633 nm f). All intensities of light stimuli are detected at V _ds = 1 V and V _g = −1 V with an interval of 50 ms. g) Mimicry of human visual memory in an array of 5×6 synapses. Schematics of illuminated regions with various wavelengths and light intensities (left). Color maps of photocurrent intensity at the initial state, after 5 s and after 50 s (right). h) Current decay is well fitted by the Wickelgren's power law. i) Changes in forgetting rate ψ and learning degree λ at different pulse wavelengths. j) Changes in forgetting rate ψ and learning degree λ with various light intensities of 532 nm laser.

Figure 5

Potential ability of association memory in arrayed optoelectronic synapses. a) The schematics of the overall process of human association memory. b) Association memory of optoelectronic synapse restored with three processes. c) The restored decay currents (stimulated at 633 nm) fitted by Wickelgren's power law. The inset is the statistics of accuracy of memory fitting, which reaches 97.6%, 96.8%, and 77.8% in 20, 5, and 0.4 s, respectively. d) The image of known association memory 1 (stimulated at 532 nm) decays in 50 s. e) The image of known association memory 2 (stimulated at 450 nm) decays in 50 s. f) The schematic of ineffective memory recovery with few known fragmental memory (at 40, 45, 50 s). g) The successful memory recovery (stimulated at 633 nm) with possible results using scientific association memory.