Two-dimensional molybdenum disulfide artificial synapse with high sensitivity

This paper reports the fabrication of an artificial synapse (AS) based on two-dimensional molybdenum disulfide (MoS2) film. The AS emulates important synaptic functions such as paired-pulse facilitation, spike-rate dependent plasticity, spike-duration dependent plasticity and spike-number dependent plasticity. The spike voltage can mediate ion migration in the ion gel to regulate the conductance of MoS2 channel, thereby realizing the emulation of synaptic plasticity. More importantly, the AS stably exhibits high sensitivity in response to spike stimuli (100 mV) and low-energy consumption (∼33.5 fJ per spike). In addition, the device emulates some synaptic functions and realizes the synaptic expression of Morse code. The development of this device represents an important step toward constructing high-performance and multifunctional neuromorphic system.


Introduction
Due to the highly parallel, event-driven and energy-efficient architecture, the brain can efficiently realize information processing and storage with ultra-low energy consumption [1,2].A human brain is composed of about 10 12 neurons and 10 15 synapses that form a cross-connected neural network to achieve complex and efficient parallel operation [3,4].Synapses are the basic functional units in the nervous system to realize perception, recognition, learning, and memory [5,6].The plasticity of the synapse in response to stimuli is exploited in the brain to simultaneously realize storage and processing.These synaptic characteristics differ from the von Neumann computing architecture involving separate storage and computing.
Atomic-thin molybdenum disulfide (MoS 2 ) is a 2D transition metal dichalcogenide that has unique electronic properties including direct bandgap [20][21][22]24].A number of studies have adopted MoS 2 for fabricating ASs, and have achieved many intriguing functions and applications [25][26][27][28][29][30][31].Xie et al developed a coplanar multi-gate MoS 2 synaptic transistor, which mimicked the spatiotemporal processing functions of visual neurons [26].Yan et al used the nanomaterials constructed by MoS 2 to prepare biomimicking memristor, which successfully emulated 'learning-forgetting-relearning' processes of biological synapses [31].However, compared to the mechanical exfoliation method commonly used in these works to prepare MoS 2 , the chemical vapor deposition (CVD) method is more advantageous in terms of controllable and scalable preparation [32,33].Meanwhile, due to the intrinsic band gap and reduced shielding effect, MoS 2 is suitable for low-voltage operation and low-energy consumption, which are also the key requirements of the artificial nervous system.
In this work, we used 2D MoS 2 films grown by CVD to fabricate ASs.The ASs emulated excitatory postsynaptic current (EPSC), paired-pulse facilitation (PPF), spike-duration dependent plasticity (SDDP), spike-rate dependent plasticity (SRDP), and spike-number dependent plasticity (SNDP).In addition, the energy consumption can be reduced to ∼33.5 fJ per spike at a low gate voltage of 100 mV.Meanwhile, some important synaptic functions and synaptic expression of Morse code are realized.This work could potentially benefit the development of high-efficiency neuromorphic devices based on 2D materials.

Device fabrication
Monolayer MoS 2 continuous film was synthesized directly on sapphire substrates by CVD.MoO 3 (99.999%purity) was used as the molybdenum source, and solid sulfur (99.999% purity) was used as the sulfur source, and argon was used as the growth carrier gas.The deposition process was performed in a zone tube furnace with an 80 mm pipe diameter, in which MoO 3 was heated to 650 • C and sulfur was heated to 180 • C, and the growth pressure and time were 4000 Pa and 10 min respectively.This process yielded continuous monolayer MoS 2 film on the desired substrate.
Thermally-evaporated gold source/drain electrodes with a thickness of ∼60 nm were deposited on MoS 2 films through shadow masks.Ion gel liquid was prepared from poly (vinylidene fluoride-cohexafluoropropylene) (PVDF-HFP) and 1-ethyl-3-methylimidazolium bis (trifluoromethyl sulfonyl) imide ([EMIM][TFSI]) mixed in acetone.Then the liquid was poured onto a clean glass slide, and heated in a vacuum oven.After the acetone was completely evaporated, the prepared ion gel was peeled off from the glass and pasted on the MoS 2 channel.

Characterization
The surface morphologies of the MoS 2 film were investigated using an atomic force microscope (AFM) operated in tapping mode (Bruker dimension icon microscope).High-resolution tunneling electron microscopy images were obtained using an electron microscope (FEI Talos F200X).Raman spectra were obtained on a laser Raman Spectrometer (EnSpectr R532) operated with 532 nm excitation.Photoluminescence (PL) was characterized using a fluorescence spectrometer (EnSpectr R532).Scanning electronic microscopy (SEM) was performed using a field emission microscope (FEI-Apreo-S).Optical microscopy (OM) images were measured using a Leica DM2700M.
All electrical characterizations were performed using a probe station in a nitrogen-filled glove box, and the data were collected using a Keithley 4200A semiconductor parameter analyzer.The electrical measurements were performed in dark.

Results and discussion
A typical cortical neuron has up to ten thousand synaptic connections, and a biological synapse is generally composed of three parts, including the presynaptic membrane, the synaptic cleft, and the postsynaptic membrane (figure 1(a)) [6].When the presynaptic membrane receives the action potential triggered by the neuron, it releases neurotransmitters that diffuse across the synaptic cleft.These neurotransmitters then interact with specific receptors on the postsynaptic membrane, manifested by generating excitatory or inhibitory postsynaptic potentials [5,6].
To emulate biological synapses, we fabricated ASs that exploited the characteristics of MoS 2 film (figure 1(b)).The synaptic transistors consist of a metal probe, ion gel, MoS 2 film, and source/drain electrodes.The metal probe that represents the presynaptic membrane was in contact with the ion gel to provide electrical spikes.Those spikes are analogous to the presynaptic spikes.Electrical spikes induced ion migration in the ion gel (PVDF-HFP, [EMIM][TFSI]), which mimicked the transmission of neurotransmitters in a synaptic cleft.The conductance change in the MoS 2 semiconducting channel emulated the receipt of transient signals by a postsynaptic membrane.
The highly crystalline structures of the MoS 2 film was confirmed by the periodic hexagonal lattices in the TEM and the radial symmetric spots in the diffraction pattern (figure 2(a)) [34,35].SEM and OM images (figures S1 and S2 (https://stacks.iop.org/NCE/2/014004/mmedia))show that the monolayer MoS 2 film grown by CVD was continuous and flat.Further, the atomic force microscopy (AFM) reveal that the surface of MoS 2 film deposited on the sapphire substrate (figure 2(b)) is highly uniform and flat with a root mean squared  roughness less than 0.74 nm.The atomically-flat surface of the 2D material reduces its screening effect, so the physical properties can be easily adjusted by application of various stimuli [36,37].
Raman spectroscopy was used to analyze the quality and number of layers of MoS 2 crystals [38].The in-plane vibration and out-of-plane vibration of MoS 2 can be represented by Raman peaks E 1 2g and A 1g respectively (figure 2(c)).The characteristic peaks of the prepared MoS 2 film were at 385.5 cm −1 (E 1 2g ) and 405.3 cm −1 (A 1g ), while the difference is 19.8 cm −1 , which is characteristic of monolayer MoS 2 [38,39].
The PL signal of monolayer MoS 2 is very strong due to the existence of a direct bandgap [40].The PL spectrum (figure 2(d)) shows a strong peak A at ∼672.9 nm, which corresponds to the excitonic emission of MoS 2 .The band gap E g (eV) was calculated from the emission wavelength λ (nm) as E g = 1240/λ = 1.84 eV, which is consistent with the previous studies [41].The observation of this single strong exciton emission peak A confirms that the monolayer MoS 2 is of good quality [41][42][43].The presence of the weak excitation peak B at a wavelength of 626 nm indicates energy splitting caused by the spin-orbit coupling in MoS 2 [40,44].By sweeping the gate voltage from −3 V to 3 V, figure S3 shows the current-voltage transfer characteristics with a fixed bias of V DS = 0.1 V.The obvious anti-clockwise hysteresis in the transfer curve may be attributed to the presence of mobile ions in the ion gel [26,29].In addition, the transfer characteristics showed good stability and reproducibility after 10 consecutive cycles (figure S4).We also evaluated the device-to-device variations (figure S5).According to the statistical distribution of the on-state current (I on ) in the transfer curves of four devices, the I on was roughly maintained in the range of 29-34 μA.
During device testing, we emulated nerve signals by applying voltage spikes to the metal probe.The transport of ions in the ion gel emulates the diffusion of neurotransmitters in the synaptic cleft.Before the voltage was applied, anions and cations in the ion gel are randomly distributed (figure 3(a) left).When a positive gate voltage spike is applied, the cations in the ion gel migrate and accumulate at the ion gel/MoS 2 interface to attract extra electrons in the conductive channel (figure 3(a) center).Such process generates a channel current in the MoS 2 semiconductor layer.When the voltage spike stops, the cations gradually drift back (figure 3(a), right).Consequently, the channel conductivity decreases, and the drain current gradually decreases back to the quiescent current level after a characteristic relaxation time.These processes can be exploited to emulate the short-term plasticity in the MoS 2 -film ASs.
EPSC was obtained by applying a constant drain voltage (0.1 V) as the read voltage, and applying a single spike stimulus (3 V, 51 ms) to the top gate to evoke postsynaptic current (figure 3(b)).When a presynaptic potential was applied, ions gathered at the ion gel/MoS 2 interface and attracted additional electrons in the conductive channel, manifested as the EPSC.After the spike stimulation was ceased, the accumulated ions gradually returned to equilibrium state within relaxation time, reflected by the gradual decay of the EPSC.
PPF was generated by stimulating the prepared synaptic device with a pair of 3 V spikes separated by an interval Δt S = 102 ms (figure 3(c)).Two consecutive EPSCs have amplitudes A1 and A2, respectively.After two consecutive spikes, EPSC can temporarily increase, and such change is quantified by PPF index = 100%•A2/A1.In a biological nerve, the amplification of synaptic connections is a result of increase in presynaptic Ca 2+ concentration, and a consequent increase in the release of neurotransmitters in synapses.In our synaptic device, when the spike interval is shorter than the relaxation time of the mobile cations, some of the migrated cations cannot return to their equilibrium position before the arrival of the second spike.These residual cations induce additional charges in the channel, so the channel current increases.We also quantified the effect of Δt S on PPF index (figure 4(a)) by stimulating the gate electrode with paired spikes (3 V) separated by different Δt S .The larger Δt S allows more migrating cations to drift back to their equilibrium position before the second spike.Therefore, the increase in Δt S weakens channel-current modulation and decreases the PPF index [45].
SRDP is a persistent change of synapse reactivity in response to frequent synaptic stimuli [46].Application of presynaptic spikes at a high rate can lead to a significant potentiation of the synaptic strength [47].A series of 10 spike stimuli (3 V) with different rates (0.563 s/spike R S 0.102 s/spike) were applied to the AS.Increase in R S caused increase in the SRDP index that represents the degree of EPSC gain.When the spike interval time is less than 0.204 s, the increasing trend of EPSC gain caused by R S increase tends to be saturated, possibly as a result of limitations to the number of mobile ions that can cause EPSC gain.
In neural networks, continuous stimulation can increase the amount of chemical neurotransmitters released, and thereby amplify synaptic strength.Here, SDDP was investigated as a function of increasing duration D S of presynaptic spikes (3 V) (figure 4(c)).As D S increased from 0.05 s to 0.46 s, the EPSC gain (SDDP index) continued to increase.The increase in D S may cause additional ions to migrate from the equilibrium position of the ion gel, and thereby increase the number of cations at the ion gel/MoS 2 film interface, and consequently increase the carrier movement in the MoS 2 channel.
SNDP is an important learning rule by which repeated stimuli contribute to an increase in the number of neurotransmitters in the synaptic cleft, and to an increase in ion flux through the postsynaptic membrane [48].In synaptic devices, a continuous increase in the number N S of spikes can imitate the repeated rehearsal process, and thereby achieve the purpose of regulating synaptic plasticity [49,50].When N S (3 V) was increased from 1 to 9, the SNDP index increased (figure 4(d)).Such trend shows that the EPSC gain gradually increased.The number of ions that migrate and accumulate on the ion gel/MoS 2 interface continued to increase as N S , so EPSC amplitude increased.
In neuromorphic electronics, high sensitivity to spike stimuli and low-energy consumption are important requirements for constructing efficient artificial biological systems.Here, sensitivity refers to the minimum amplitude of the external spike stimuli required for the device to produce an effective EPSC response.Due to the atomically-flat surface and the reduced screening effect, the MoS 2 channel layer is likely to be adjusted at a low stimulation voltage, which is beneficial for synaptic applications [36,37].To test the sensitivity and energy consumption of the prepared device, the amplitude of the presynaptic spike was further reduced to 100 mV.Energy consumption was estimated as E = AIW/2, where A (0.2 mV) was the driving voltage, I (5.85 nA) was the current flowing across the device, and W (∼57 ms) was the width of the presynaptic spike.The energy consumption of a single synaptic event was calculated as ∼33.5 fJ per spike (figure 5(a)).The energy consumption of this AS is close to those of biological synapses.
We further tested the synaptic plasticity of the AS under 100 mV spike stimulation.As the applied spike rate increased, the postsynaptic current increased (figure 5(b)).Such trend is attributed to the limitation of the time available for the reverse diffusion of cations from the interface.Additionally, under a fixed amplitude of 100 mV, as the duration and number of spikes continued to increase, synaptic weight was also adjusted and the EPSC gain continues to increase (figures 5(c) and (d)).These results show that the device can still emulate important synaptic functions with high sensitivity to spike stimuli and low energy consumption.Finally, the AS successfully realized the synaptic expression of Morse code (figure S6), which were achieved by the combination of dot (EPSC triggered by a single 100 mV spike) and dash (EPSC triggered by three consecutive 100 mV spikes).Through the different arrangements of two kinds of spikes, the device successfully expresses different English letters and numbers.The high sensitivity to spike amplitude and low-energy consumption of the device make it possible to expand synaptic functions under low-voltage operation [36].In addition, the device can be used as an important signal processing unit for building a variety of afferent and efferent nerves through connections with front-end sensors and end-effector actuators [36,37].Finally, 2D materials are expected to be integrated with CMOS circuit/device to achieve advanced logic/computation functions [32,33,51].

Conclusions
We used 2D MoS 2 film as the semiconducting channel in artificial synaptic devices.This device emulated important synaptic functions, such as EPSC, PPF, SRDP, SDDP, SNDP, and it can also stably achieve lowenergy consumption (∼33.5 fJ per spike) under 100 mV spike stimulation, and the synaptic expression of Morse code by using the device output is demonstrated.This AS show great potential in high-sensitivity and low consumption applications for neuromorphic electronics.

Figure 1 .
Figure 1.(a) Schematic diagram of biological nerve synapse.(b) Schematic diagram of MoS 2 based AS.

Figure 2 .
Figure 2. (a) TEM image and the corresponding FFT patterns (insets) of monolayer M O S 2 film.(b) AFM image of monolayer M O S 2 film.(c) Raman spectra of monolayer MOS 2 film containing in-plane E 1 2g mode and out-of-plane A 1g mode.(d) PL spectra of monolayer M O S 2 film.

Figure 3 .
Figure 3. (a) Schematic diagram of ion migration in ion gel.(b) EPSC triggered by an applied external spike (gate voltage amplitude: 3 V) recorded at a constant drain voltage of 0.1 V. (c) EPSCs are triggered by two successive presynaptic spikes (gate voltage amplitude: 3 V) separated by an interval of 102 ms, and emulate the biological process of PPF.