Artificial Synapses Based on in-Plane Gate Organic Electrochemical

Sep 8, 2016 - Realization of biological synapses using electronic devices is regarded as the basic building blocks for neuromorphic engineering and ar...
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Artificial Synapses Based on in-Plane Gate Organic Electrochemical Transistors Chuan Qian,† Jia Sun,*,† Ling-an Kong,† Guangyang Gou,† Junliang Yang,† Jun He,† Yongli Gao,*,†,‡ and Qing Wan*,§ †

Hunan Key Laboratory for Super Microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha, Hunan 410083, P. R. China ‡ Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, United States § School of Electronic Science & Engineering, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, P. R. China S Supporting Information *

ABSTRACT: Realization of biological synapses using electronic devices is regarded as the basic building blocks for neuromorphic engineering and artificial neural network. With the advantages of biocompatibility, low cost, flexibility, and compatible with printing and roll-to-roll processes, the artificial synapse based on organic transistor is of great interest. In this paper, the artificial synapse simulation by ion-gel gated organic field-effect transistors (FETs) with poly(3-hexylthiophene) (P3HT) active channel is demonstrated. Key features of the synaptic behaviors, such as paired-pulse facilitation (PPF), short-term plasticity (STP), self-tuning, the spike logic operation, spatiotemporal dentritic integration, and modulation are successfully mimicked. Furthermore, the interface doping processes of electrolyte ions between the active P3HT layer and ion gels is comprehensively studied for confirming the operating processes underlying the conductivity and excitatory postsynaptic current (EPSC) variations in the organic synaptic devices. This study represents an important step toward building future artificial neuromorphic systems with newly emerged ion gel gated organic synaptic devices. KEYWORDS: organic semiconductors, ion gel, electrochemical transistors, artificial synapses, neuromorphic systems

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INTRODUCTION Recently, artificial intelligence has attracted considerable interest and inspiration from the society and scientific community because of the outstanding performance of program AlphaGo in the game of Go.1 These programs excelled in logic and accurate scientific calculations were designed and fabricated to emulate the brain behaviors. However, these conventional computers are just based on complementary metal-oxidesemiconductor (CMOS) technology and von Neumann architecture.2−4 Information processing in human’s brain is a more delicate process. The mammalian brain is the most complex system in the known universe, which contains about 1 × 1010 neurons each forming up to 1 × 105 synaptic connections.5 With CMOS technology, at least about ten transistors are needed to implement a synapse, which is too large to design massively parallel systems toward the size of brain.6 Moreover, our brain is undergoing modifications to store information and adapting to changes in the surroundings. So an important improvement in the hardware for artificial intelligence and neuromorphic computing is to fabricate an excitatory or inhibitory synapse in a single device to emulate the basic properties of synaptic computation and memory, such as synaptic plasticity, spike-timing-dependent plasticity (STDP), self-learning ability, etc. © 2016 American Chemical Society

In the past few years, several neuromorphic functions of biological synapses have been implemented in various electrolyte gated field-effect transistors (FETs).7−13 With the advantages of biocompatibility, low cost, flexibility, and compatibility with printing and roll-to-roll processes,14−18 organic semiconductors as the active layers should be one of the most promising candidates for synaptic electronics. Very recently, Malliaras and his group have demonstrated the neuromorphic functions in simple PEDOT:PSS based organic electrochemical transistor with liquid electrolyte for the first time.12 Unfortunately, liquid electrolyte is inconvenient to handle and hard to integrate into advanced functional neuromorphic devices and circuits. On the other hand, for the electrolyte gated organic field-effect transistors (OFETs), electrochemical doping is the common mode and the ions from gate dielectric manage to penetrate into the organic semiconductor under the electric field.19 For example, the open structure of semiconducting polymer, poly(3-hexylthiophene) (P3HT), results in deep penetration of the dopant ions.20 Therefore, interfacial information on the electrolyte gated Received: July 19, 2016 Accepted: September 8, 2016 Published: September 8, 2016 26169

DOI: 10.1021/acsami.6b08866 ACS Appl. Mater. Interfaces 2016, 8, 26169−26175

Research Article

ACS Applied Materials & Interfaces organic devices is critical for the OFETs based artificial synapse.21 In this work, artificial synapses based on ion-gel laterally gated OFETs were fabricated. Some important synaptic behaviors, such as excitatory postsynaptic current (EPSC), paired-pulse facilitation (PPF) and self-tuning were mimicked. Furthermore, with two presynaptic inputs, spike logic, spatiotemporal dentritic integration, and EPSC regulation were also realized. Moreover, we found that the change of the current was directly connected to the [TFSA]− transferred at the interface. In addition, the intensity of π−π* absorption was shown to be a function of the quantity of [TFSA]− penetrated into P3HT film. The present studies provide a useful guideline for designing organic-based artificial synapses for neuromorphic systems.

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EXPERIMENTAL SECTION

Film Growth and Device Fabrication. Before dried by N2 flow and treated by ozone, the 200 nm-SiO2/Si (Si-Mat, Silicon Materials) and glass substrate were ultrasonically cleaned in acetone, alcohol and deionized water, respectively. Semiconducting P3HT is purchased from Sigma-Aldrich, which was spin-coated from dichlorobenzene (15 mg/mL) and then cured at 50 °C for 2 h. Thermally deposited Au electrodes were used as the source, drain, and in-plane gate, which define a channel with a length of 80 μm and a width of 1600 μm, and a larger device dimension for interface characterization with a length of 2 mm and a width of 8 mm. To prepare the ion-gel films, we codissolved the poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) (purchased from Sigma-Aldrich) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMI][TFSA]) (purchased from TCI Chemicals) in acetone in a 1:4:7 weight ratio. To remove the residual solvent, we placed spin-coated ion-gel films in oven at 70 °C for 1 day. The ion gels were then laminated onto the in-plane gate as the gate dielectric of OFETs, according to a previously reported method.22 Characterization. Electrical properties and neuromorphic functions of the devices were measured by a Keithley 4200 semiconductor parameter analyzer in air and at room temperature. The spectroscopies of films were measured by X-ray photoelectron spectroscopy (XPS) system (ESCALAB 250Xi, ThermoFisher-VG Scientific) in an ultrahigh vacuum (UHV) system and the based pressure of analysis chamber is superior to 5 × 10−10 mbar. The surface morphology of the P3HT films was characterized by atomic force microscopy (AFM, Agilent Technologies) in air at ambient temperature. The absorption spectra were in situ performed for operating P3HT OFETs on glass substrate with an ultraviolet−visible spectrophotometer (UV−vis, Puxi, T9, China). Two Keithley 2400 sourcemeters were used to apply the voltage.

Figure 1. (a) Schematic image of the P3HT-based OFET with inplane gate structure. (b) Simple schematic picture of an artificial synapse based on a P3HT-based OFET. (c) EPSC triggered by the presynaptic spikes (−1.0 V) are shown versus time with duration of 20 ms. (d, e) Schematically show the ion-gel gated P3HT transistor structure in the different states.

postsynaptic neuron. To represent the synaptic strength, a potential spike signal is applied on the presynaptic neuron, which can trigger an ionic EPSC. In our organic artificial synapse, mobile [TFSA]− ions in the ion gel between the gate electrode and P3HT channel play an essential role to trigger EPSC. The conductance of channel is considered as the synaptic weight. Figure 1c shows the typical temporal response of the P3HT synapse to a −1.0 V presynaptic spike with 20 ms duration. During the test of neuromorphic functions, the voltage applied on the source is always a constant potential Vds = −0.2 V. The initial channel current is about 0.1 μA at Vg = 2.0 V. The schematic diagram of this state is shown in Figure 1d. Most of negative ions are close to gate electrode and the current of P3HT channel is small. As shown schematically in Figure 1e, when applying a presynaptic spike at the gate electrode (−1.0 V), ions move from ion gel to semiconductor, then a double-layer is formed at the interface between the P3HT film and the ion gel and a lot of holes are accumulated near the surface of the P3HT. By applying a voltage between source and drain electrodes, a high current of ∼1.85 μA in the channel has just produced. When the spike is finished, the EPSC gradually decays back to the initial current in about 190 ms and this mimicked behavior is similar to EPSC in a biological excitatory synapse.23 As shown in Figure 1d, the negative ions are returned to the interface of gate electrode/ion gel after removing the pulse and gate voltage back to 2.0 V. The P3HT layer is reversibly dedoped to its initial state with the decrease of the carrier concentrations and the decay of the postsynaptic current. The EPSC with 15 ms duration versus time is shown in Figure S2. When a presynaptic spike is applied, a positive current of 2.6 μA is observed first, which is different with the phenomenon of synapse applied spike with 20 ms duration. There are a part of ions drifted between the gate and source electrode before sufficient ions reached the interface. So the probed positive current at this time is mainly from the

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RESULTS AND DISCUSSION Shown in Figure 1a is the device structure of an in-plane-gate P3HT OFET with ion-gel coupling on a SiO2/Si substrate. As illustrated in Figure 1b, the simple picture of an artificial synapse based on a P3HT-based OFET is presented. The fabricated devices cannot replace biologic synapse, but is used to mimic neuromorphic functions. The transfer and output characteristic curves of the P3HT-based OFET are shown in Figure S1. The device can be switched between ON and OFF at low gate voltages and low source-drain voltages. The turn-on voltage (Von) of the devices is about 2.1 V, when the applied gate voltage smaller than Von, the Ids is increasing from 3 × 10−6 A to 7.4 × 10−4 A. The electrical-double-layer (EDL) transistor is regarded as an artificial synapse, which consist of presynaptic terminal (gate electrode) and postsynaptic terminal (P3HT channel). For this synapses, transmission of information occurs via the motion of ions from the presynaptic neuron to the 26170

DOI: 10.1021/acsami.6b08866 ACS Appl. Mater. Interfaces 2016, 8, 26169−26175

Research Article

ACS Applied Materials & Interfaces

indicates that the quantity of ions moving between P3HT and ion gel is constant in the repetitive process. The following depression behavior is not result to ions exhausted, but the persistent drift of ions affected on the structure of the P3HT film, which reduces the conductivity of the neutral P3HT to balance the change. This is a kind of self-tuning, which is discussed in biological excitatory synapses.5 Neural electrochemical activity is subject to homeostasis in physiological systems. Network activity need keeping stable in the face of changes in synaptic strength by decreasing neuronal excitation. In neural network, a single neuron receives spatiotemporally correlated inputs from different neurons via dendrites and these synaptic inputs are integrated in the postsynaptic neuron.731−33 To emulate the spatiotemporal logic, a P3HT-based synaptic transistor with two input gates is designed, as shown in Figure 3a. Figure 3b shows the “OR” logic function. The amplitudes of

contribution of the electrolyte-ion-polarization-induced leakage current in ion gel between gate and source electrodes. The mimicked typical EPSCs triggered by presynaptic spikes are shown as a function of the presynaptic spike duration of 20 to 100 ms in Figure 2a. The process is reversible and the EPSC

Figure 2. (a) Pulse duration dependent EPSCs of the P3HT synaptic transistor. (b) EPSCs triggered by paired pulses with pulse interval (Δt) of 100 ms. A1 and A2 are the peak EPSCs of the first and second spikes, respectively. (c) Paired-pulse facilitation ratio as a function of Δt. (d) EPSCs recorded in response to the train of pulses applied at the gate electrode.

amplitude is linearly increased from 1.8 to 7.9 μA with the increasing duration time. More time the spike spends, more ions move onto the interface of ion gel/semiconductor. The channel current as a function of duration of applied voltage is further discussed in the last XPS part. Furthermore, we emulate the PPF in our artificial synapses. The behavior of PPF is a kind of short-term plasticity (STP), which is very important for decoding temporal information.24−27 The amplitude and the duration of a presynaptic spike for PPF measurement were fixed at −1.0 V and 50 ms with pulse interval time (Δt) ranging from 50 to 600 ms. As shown in Figure 2b, a pair of presynaptic pulses with Δt of 100 ms is applied at the gate electrode and the postsynaptic channel current is measured as a function of time. The EPSC triggered by the first presynaptic spike is 0.7 times smaller than the EPSC by the second spike, which is similar to the phenomenon of PPF in the biological system.28 The ions triggered by the first spike do not have sufficient time diffuse back to the ion gel before the second pulse arrives. So the second EPSC is facilitated by the residual ions and increased. The ratio of two amplitudes (A2/A1) is plotted versus Δt in Figure 2c. The ratio reaches the maximum value (198%) when Δt = 50 ms, and gradually decreases to 100% with increasing Δt. However, with larger pulse interval Δt ≥ 600 ms, ions triggered by the prior spike will drift back to the ion gel and facilitated EPSC can not be observed. To reproduce STP, we applied a train of pulses on the gate electrode.26,27 Figure 2d shows the EPSCs recorded in response to the train of pulses. The EPSC is initially sensitive and subsequently adapts to the train of pulses. The first two spikes have the similar behavior of PPF. Then the channel current decreases with the increasing of the number of spikes. Such STP behavior is quite similar to the biological synaptic depression.29,30 After each spikes removed, the minimum current has a small change, keeping around 1.0 μA. This result

Figure 3. (a) Schematic image of the ion-gel gated organic synaptic transistors with two in-plane presynaptic inputs. (b) Input−output characteristics of the OR pulse logic triggered by presynaptic spikes (−1 V, 50 ms) applied on presynapse 1 (G1) and presynapse 2 (G2). The measured sum plotted as a function of expected sum of two EPSCs from two presynapses triggered by (c) presynaptic spikes of −1 V with different pulse duration from 40 to 90 ms and (d) different presynaptic spikes (from −1 V to −2 V) with the same pulse duration of 50 ms.

EPSCs triggered by presynaptic spikes (−1.0 V, 50 ms) are applied on presynapse 1 (G1) and presynapse 2 (G2). When the input spikes are first triggered individually and then triggered simultaneously on two presynapse, the three postsynaptic spike currents can be observed. The amplitudes of EPSCs triggered by the two individual spikes are −1.2 and −1.26 μA, respectively. With the lateral coupling effect, the EPSC amplitude (−1.53 μA) triggered simultaneously on two presynapse is larger than the EPSC amplitudes triggered individually. The sum of two individual responses, −2.46 μA, is larger than the response to two inputs, indicating the sublinear synaptic integration. The computing and memory-related functions in the neural network are based on synaptic integrations.34 The measured sum plotted as a function of expected sum of two EPSCs from two presynapses are shown in Figure 3c, d. When the pulse duration is less than 70 ms or 26171

DOI: 10.1021/acsami.6b08866 ACS Appl. Mater. Interfaces 2016, 8, 26169−26175

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ACS Applied Materials & Interfaces

in the P3HT film. We also apply the constant modulation voltage (V2) of 2.0 V and −2.0 V on the in-plane G2 electrode, as showed in Figure 4c. Under a constant V2 on the modulatory terminal G2, an obvious modulation of EPSC could be observed. The average of EPSC amplitude measured with modulation voltage V2 of 2.0 V and −2.0 V are calculated to be 1.15 and 13.48 μA, respectively. Because of large interval (about 5 s) between spikes, there not exists PPF behavior. The amplitude of EPSC is relatively stable and these operations are repeatable, derived from the reliability of P3HT-based synaptic transistors. We conclude that there is the correlation between the modulation voltages V2, ranging from 2.0 V to −2.0 V, and the corresponding peak EPSC in Figure 4d. Under this modulating, the value of EPSC amplitude has an above 1 order of magnitude increasing. When the V2 is smaller than −1.0 V, the change is more significant. Applied more negative voltage on the modulatory terminal, a more effective electric field will be formed and more ions will move onto the interface of ion gel/semiconductor to modulate the EPSC amplitude. As discussed above, the EPSC is related to the motion of ions in the ion-gel gate dielectrics. To understand the doping behaviors of ion-gel gated organic synaptic transistors, we further studied the interface doping processes in ion-gel gated organic transistors. Because of the limitation of the instrument, we have to use a larger channel to characterize the interface between the ion gel and the semiconductor. Transfer characteristic curves (Ids vs Vg) of an ion-gel gated P3HT transistor with a large device dimension are shown in Figure S4. The Von of the devices is also 2.1 V, when the applied gate voltage smaller than Von, the Ids is increasing from 7.7 × 10−6 A to 1 × 10−3 A. For the measurements of XPS and AFM, the ion-gel dielectrics were peeled off from the operated FETs using tweezers. Then, the doped P3HT films were ultrasonic clean by deionized water and annealing. Because of the solubility of ionic liquids in water, the device was washed by deionized water to remove the physical adsorbed ionic liquids after removing the ion gel. We applied XPS to confirm the compositional changes in the films.36,37 The molecular structure of the P3HT and [EMI][TFSA] used in this study is given in the inset of Figure 5a, b. As can be seen, each [EMI][TFSA] unit has six F atoms while neutral P3HT is without F atom. Though there are S atoms in both P3HT and [EMI][TFSA]. Because of the different chemical bonds, the peaks of S 2s are widely separated (see Figure S5). The atomic ratio in such OFETs can be sensitively detected by the XPS method. The atomic ratio of F and S (nF/nS) can be calculated by the Avantage program. In a P3HT monomer unit, the S atom is very stable and these four kinds of films have similar atomic concentration of S, as shown in Figure 5a. By calculating the atomic ratio of F and S, we can observe the compositional changes of the P3HT film with different doping levels. As seen in Figure 5b, there is no F 1s in the neutral P3HT film without F atom. The atomic concentrations of F in processed P3HT thin film steadily increase with enlarging scanning range of the gate bias. The nF/nS as a function of ΔVg is summed up in Figure 5c. The atomic ratio increases with ΔVg, in good agreement with the increasing of Ids. For the device operated with ΔVg of 1.0 V, the nF/nS is 0.033. After the scanning scale enlarged to 4.0 V, the value increased to 0.238, 7 folds higher than that of the device with ΔVg of 1.0 V. The XPS results demonstrate that there are a lot of ions penetrating into the semiconductor layer as gate voltage changing from positive to negative voltages. Figure 5d shows the Ids as a function of time

the amplitude is larger than −2.0 V, the sum of two individual responses (expected sum) is larger than the response to two inputs (measured sum) and sublinear synaptic integration is obtained. On the contrary, when the pulse duration is greater than or equal to 70 ms or the amplitude is equal to −2.0 V, the expected sum is smaller than the measured sum and a superlinear synaptic integration is obtained. These results illustrate that spatial summation is sublinear for weak stimuli and superlinear for strong stimuli, which are very similar to some biological experiments.35 The transfer curve of the device with different V2 is shown in Figure S3a and current integration versus time is shown in Figure S3b. In the previous reported synaptic transistors based on inorganic materials, the response to two inputs was saturated at weak stimuli and back to sublinear synaptic integration at high spike voltages.10 However, the synaptic integration transformation of our organic synaptic transistors is different with the previous results. As shown in the Figure S3b, the integrated current of two inputs gradually increases. In a short period, the current is hard to saturate. Maybe because the speed of ions penetrated from ion gels into the P3HT layer is very slow. Synaptic modulation is a significant part of learning mechanisms in physiological systems. By modulating synapses, the self-healing can been achieved and the neural network will been enriched. As showed in Figure 4a, to simulate the behavior

Figure 4. EPSC triggered by a train of pulses applied on the G1 electrode with different constant modulation voltage (a) VB (10 V and −40 V) and (c) V2 (2 V and −2 V). Scattergram for the correlation between the average peak EPSC and the modulation voltages applied on the (b) bottom gate of Si or (d) the G2 electrode. Each value of EPSC is the average of the ten peaks triggered by a train of pulses applied on the G1.

of modulation, a train of presynaptic spikes with duration time of 50 ms are applied on G1 electrode and the constant modulation voltage (VB) of 10 V or −40 V is applied on the bottom gate of Si, simultaneously. When the VB is 10 V, the average of EPSC amplitude is about 0.84 μA; when the VB is −40 V, the value is up to 1.28 μA. Figure 4b shows the correlation between the average peak EPSC and VB (from 10 V to −40 V). Each value of EPSC is the average of the ten peaks triggered by a train of pulses applied on the G1. Under the electrostatic modulation of VB, there are limited holes increased 26172

DOI: 10.1021/acsami.6b08866 ACS Appl. Mater. Interfaces 2016, 8, 26169−26175

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Figure 5. (a) XPS S 2s and (b) F 1s spectra of processed P3HT thin film after operated with different scanning scale of Vg (ΔVg). The molecular structure of the P3HT and [EMI][TFSA] used in this study is given in the inset of a and b. (c) Atomic ratio of S and F as a function of ΔVg. (d) Current between source and drain electrodes with P3HT channel as a function of time obtained under the Vg of 2 V and Vds of −2 V, and corresponding atomic ratio of F and S extracted from XPS results.

Figure 6. (a) XPS S 2s and (b) F 1s spectra of processed P3HT thin film after being operated via a single presynaptic spike. (c) EPSC triggered by the presynaptic spikes (−1.0 V) are shown versus time with duration of 1.5 s. The inserted values correspond to the atomic ratio of S and F of different states. (d) Absorption spectra as a function of the wavelength of the operating P3HT OFETs biased at various gate voltages.

obtained under the Vg of 2.0 V and Vds of −2.0 V. The XPS S 2s and F 1s spectra of processed P3HT thin film after being operated via a gate bias of 2.0 V with different time are shown in Figure S6. We extracted the corresponding nF/nS from this XPS results. It clearly can be seen that the Ids shows a strong dependence on the nF/nS. The device operated via a gate bias of 2.0 V with 60 s shows the largest Ids about 10 times that of its counterpart with 5 s. The observed increase in Ids with increasing nF/nS results from an enhanced quantity of ions penetrated from ion gels into P3HT. The small decreased Ids observed at the time over 60 s is due to the saturation of ions near the interface and the process to balance. The AFM morphologies of P3HT with different electrochemical doping levels are shown in Figure S7. Similar to the pure P3HT, under the gate bias of 0 and 2.0 V for 60 s, the film morphologies showed a uniform and pinhole free surface (the RMS maintains about 2.8 nm). When the gate bias reaches to −3.0 V, the RMS of the highly doped P3HT film is increases to 4.89 nm. In high gate electric-field, a lot of the [TFSA]− penetrated into the P3HT films and the nanoaggregations of the films are interrupted. This is maybe related to the electrochemical reaction between the ion liquid and P3HT. We also detected the change of nF/nS in the process of a single presynaptic spike applied on the device by the same interfacial XPS method. The XPS S 2s and F 1s spectra of processed P3HT thin film after being operated via a single presynaptic spike are shown in Figure 6a, b. Figure 6c shows the typical temporal response of the P3HT synapse with a large channel to a −1.0 V presynaptic spike. In the device with large dimension, ions need more time to move and the response of device become slower. When the pulse is applied, the atomic concentrations of F in processed P3HT thin film greatly raise. The nF/nS is increased from 0.054 to 0.16. A lot of the [TFSA]− penetrated into the P3HT films and a large channel current is observed. After a pulsing, the channel current return to initial value and nF/nS is back to 0.071. Most of the [TFSA]− in P3HT is return to the ion gels under an bias of 2.0 V. As shown in Figure 6d, the results of an investigation of gate modulated π−π* absorption is reported and we use the absorption spectra

characterizing the operating P3HT OFETs biased at various gate voltages. The spectrum of neutral P3HT indicates the maximum absorption of P3HT is approximately at 555 nm with a weak vibronic shoulder. The intensity of π−π* absorption rapidly decreases with decreasing bias from 2.0 to 0 V, whereas the absorption peak shifts to a long wavelength region, which attribute to the polaron absorption of P3HT.38 For the spectra obtained under the bias of 1.0 to approximately −3.0 V, the changes are characteristic of electrochemical p-type doping from the [TFSA]− and these behaviors of P3HT film are consistent with the previous report.39,40 Under the application of −1.0 V and −3.0 V, the π−π* absorption increase in magnitude back a little, because of the saturation of doping ions, similar to the behavior in the Figure 5d. Then we get a spectrum without any bias. To keep the ion gels’ electroneutrality, some ions move back to the ion gels and the π−π* absorption of P3HT film increase a little again. But, without a dedoping bias, the spectrum is quite different with the spectrum of neutral P3HT. Comparing to the reduced intensity of π−π* absorption in the doping processes, the increased intensity in this case is much smaller. It indicates that the penetrated [TFSA]− in P3HT is hard to return to the ion gels. Therefore, the XPS method used to characterize the doping of the OFETs is reliable, but these results cannot be used to quantitatively analyze the quantity of injected carriers. The spectrum of π−π* absorption obtained under the bias of 3.0 V overlap with the pure P3HT spectrum, indicating that the electrochemical doping takes place reversibly under the influence of dedoping voltage.41 The obtained results are consistent with the repeatable EPSC by the spike signal applied on the presynaptic neuron.

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CONCLUSION In summary, artificial synapses based on ion-gel laterally gated organic electrochemical transistors were demonstrated, and some important synaptic behaviors, such as EPSC, PPF, and self-tuning, were mimicked. Furthermore, with two presynaptic inputs, spike logic, spatiotemporal dentritic integration, and EPSC regulation were also emulated. EDL modulation and 26173

DOI: 10.1021/acsami.6b08866 ACS Appl. Mater. Interfaces 2016, 8, 26169−26175

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ACS Applied Materials & Interfaces

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interfacial electrochemical doping processes were studied. Comparing the XPS and in situ absorption spectra results with electrical measurements, we find that the change of the EPSC current, atomic ratio and π−π* absorption is directly connected to the [TFSA]− transferred at the interface. Our results provide a potential approach to construct neuromorphic platforms based on organic synaptic transistors.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b08866. Transfer characteristic curve and output characteristic curves of P3HT-based OFET, EPSC triggered by the presynaptic spikes with duration of 15 ms, the current integration versus time, XPS spectra and AFM images (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Funding

This project was supported in part by the National Natural Science Foundation of China (61306085, 11334014, 61222406) and the Fundamental Research Funds for the Central Universities of Central South University (2016zzts018). Y.G. acknowledges support by National Science Foundation DMR-1303742 and CBET-1437656. Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acsami.6b08866 ACS Appl. Mater. Interfaces 2016, 8, 26169−26175