Optoelectronic Properties of Printed Photogating Carbon Nanotube

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Surfaces, Interfaces, and Applications

Optoelectronic Properties of Printed Photogating Carbon Nanotube Thin Film Transistors and their Application for Light-stimulated Neuromorphic Devices Lin Shao, Hailu Wang, Yi Yang, Yongli He, Yicheng Tang, Hehai Fang, Jianwen Zhao, Hongshan Xiao, Kun Liang, Miaomiao Wei, Wenya Xu, Manman Luo, Qing Wan, Weida Hu, Tianqi Gao, and Zheng Cui ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02086 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on February 28, 2019

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ACS Applied Materials & Interfaces

Optoelectronic Properties of Printed Photogating Carbon Nanotube Thin Film Transistors and their Application for Light-stimulated Neuromorphic Devices Lin Shao1,2, Hailu Wang3, Yi Yang4, Yongli He4, Yicheng Tang3, Hehai Fang3, Jianwen Zhao*1, 2, HongshanXiao2, Kun Liang2, Miaomiao Wei1,2, Wenya Xu2, Manman Luo1,2, Qing Wan*4, Weida Hu*3, Tianqi Gao1, 2, Zheng Cui1, 2 1College

of Nano Technology and Nano Bionics, University of Science and

Technology of China, 96 Jinzhai Road, Hefei, 230026, P.R. China. 2Printable

Electronics Research Centre, Suzhou Institute of Nano-Tech and

Nano-Bionics, Chinese Academy of Sciences, 398 Ruoshui Road, Suzhou 215123, P.R. China. 3State

Key Laboratory of Infrared Physics Shanghai Institute of Technical Physics,

Chinese Academy of Sciences, 500 Yutian Road, Shanghai 200083, P.R. China. 4School

of Electronic Science and Engineering, and Collaborative Innovation Center

of Advanced Microstructures, Nanjing University, 163 Xianlin Road, Nanjing 210093, P.R. China. KEYWARDS:

Printed

thin

film

transistors;

photoneuromorphic device; artificial synapses;

ACS Paragon Plus Environment

photogating;

sc-SWCNTs;

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Artificial synapse/neuron based on electronic/ionic hybrid devices have attracted wide attentions for brain-inspired neuromorphic systems since it is possible to overcome the von neumann bottleneck of the neuromorphic computing paradigm. Here, we report a novel photoneuromorphic device based on printed photogating single-walled carbon nanotube (SWCNT) thin film transistors (TFTs) using lightly n-doped Si as the gate electrode.The drain currents of the printed SWCNT TFTs can gradually increase to over 3000 times of their starting value after being pulsed with light stimulation, and the electrical signals can maintain over ten minutes. These characteristics are similar to the learning and memory functions of brain-inspired neuromorphic systems. The working mechanism of the light-stimulated neuromorphic devices is investigated and described here in detail. Important synaptic characteristics, such as low-pass filtering characteristics and the non-volatile memory ability, are successfully emulated in the printed light-stimulated artificial synapses. It demonstrates that the printed SWCNT TFT photoneuromorphic devices can act as the non-volatile memory units and perform photoneuromorphic computing, which exhibits the potentials for future neuromorphic system applications.


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1.INTRODUCTION The human brain can simultaneously handle a variety of complicated tasks including learning, memory, recognition, prediction, and control, with extremely low power consumption1-3, which are difficult for modern super-fast computers to do with the same efficiency. Imitating the human brain and developing a brain-like intelligence have been a major topic of scientific community in recent years4-6. Mimicking synaptic behaviors using novel neuromorphic devices is regarded as a valid and simple method for neuromorphic computation. Photoelectric neuromorphic devices, which have learning, recognition, memory and other information processing functions under the pulse light stimulation, are key components in the field of neuromorphic devices. These photoelectric neuromorphic devices can imitate the retinal neurons in human eyes. Recently, three-terminal photoelectric neuromorphic devices based on indium-gallium-zinc-oxide (IGZO)7, organic semiconductors8,9, and graphene10 and other 2D materials11,12 have been reported. They are transistors and phototransistors operating with electrolyte-gated electric-double-layer (EDL)13-15. Another method is photogating in which the conductance is modulated via photon induced gate voltage16. Photogated transistors have been widely used to construct high-performance photodetectors17. In particular, lightly doped silicon (Si) gate has been widely used to enhance the photosensitivity of transistors18. So far, the photogating concept has not been used in photoelectric synaptic devices. As for the transistors, fabrication technologies including vacuum evaporation, magnetron sputtering, chemical vapor deposition (CVD) and printing have been used.



Making thin film transistors (TFTs) by printing is of great advantages19 because of its simple process, low cost20, suitable for flexible substrates and easy to scale up21,22. There have been researched on printing TFTs using organic23,24 and metal oxide semiconductor inks25. Semiconducting single-walled carbon nanotubes (sc-SWCNTs) are considered to be a better ink material for printing TFTs due to their high electron and hole mobility, high solubility for making inks, good physical/chemical stability and low processing temperature26-28. In the last few years, advances in sc-SWCNT purification techniques29, printing technologies30 and post-treatment technologies have significantly improved the performance of printed SWCNT TFTs. In the past, printed SWCNT TFTs have been mainly used in traditional applications including display, logic gates31 and circuits32, gas sensors33, biosensors34 and wearable electronics35. Recently, printed SWCNT TFTs have been extended to construct neuromorphic devices using EDL dielectric layers and reasonably good performance has been demonstrated36. In this work, photoneuromorphic devices based on printed TFTs have been developed using lightly n-doped Si as photogate and polymer-sorted sc-SWCNTs as printable semiconductor ink materials. The as-prepared photoelectrical neuromorphic devices can rapidly respond to the pulsed lights with wavelengths from 520 to 1310 nm. The output currents of neuromorphic devices based on printed photogating SWCNT TFTs are gradually increased under the pulsed light illumination and this effect can be retained after light stimulation is removed. Key synaptic functions including learning, memory and signal filtering characteristics, were successfully


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emulated in the printed photogating neuromorphic devices. The results present a promising approach to fabricate SWCNT-based synaptic devices for neuromorphic system applications.

2 EXPERIMENTAL SECTION 2.1 Materials and Instruments Single side polished lightly phosphorus-doped n-type Si wafers were purchased from Zhejiang Lijing Silicon Material Co., Ltd (the electrical resistivity of 14-16 Ωcm).Arc discharge SWCNTs (P2) were purchased from Carbon Solutions (USA). Poly-diketopyrrolopyrrole 5-Thiophene (P-DPPb5T) was synthesized in our lab. All electrical measurements were measured in ambient using an Agilent Semiconductor Analyzer B1500. Optical absorption spectra were performed in a Perkin Elmer Lambda 750 UV-Vis-NIR spectrometer. Sorted sc-SWCNT solutions were printed by an aerosol Jet 300P system (Optomec Inc., USA). 50 nm HfO2 thin films were deposited on lightly n-doped Si by atomic layer deposition (ALD, Cambridge Nano-tech Inc.).SEM images of carbon nanotube thin films in device channels were obtained using a Hitachi S-4800 instrument (Hitachi Co. Japan). 2.2 Fabrication of printed SWCNT TFT devices The fabrication process of printed SWCNT TFT devices is illustrated in Scheme1. Firstly, a 50 nm thickness HfO2 thin film was deposited on the lightly n-doped Si substrates at 250 °C by ALD. Subsequently, pre-patterned interdigitated Au/Ti (50 nm Au/5 nm Ti) electrode arrays were fabricated on the 50 nm HfO2/Si



substrates by photolithography, e-beam deposition and lift-off process (the channel width, length and interdigital electrode space are 1000, 20 and 20 μm, respectively.). After that, the substrates were treated by oxygen plasma (100 W) for 3 min, and then sc-SWCNT inks were printed into the device channels by Aerosol Jet printer, followed by washing with toluene. Finally, the device are annealed on the hot plate at 120 °C for 30 min at which point the SWCNT TFTs are ready.

Scheme 1 Schematic diagram of the fabrication process of printed SWCNT TFTs. (a) The lightly n-doped Si substrate, (b) 50nm HfO2 thin films deposited on the Si substrate by ALD, (c) gold electrodes fabricated on the HfO2/Si substrates, (d) HfO2/Si treated by oxygen plasma, (e) printing sc-SWCNT ink, (f) obtaining printed SWCNT TFT after washing with toluene and annealing. 2.3 Electrical properties of printed TFTs and light-stimulated neuromorphic devices The photocurrents and electrical properties of printed SWCNT TFTs were measured by an Agilent Semiconductor Analyzer B1500 under pulsed light illumination at gate voltage of 2 V with a drain-source voltage of -0.5 V. The linear


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field μ

effect

mobility

dI DS L 1 × × dVGS W COX VDS

(µ)

was

calculated

using

the

following

formula

at VDS of -0.25 V. Here, Cox is the capacitance density of dielectric

layers, measured by an HIOKI IM3533 LCR meter, and L and W are the channel length and width, which are 20 μm and 1000 μm, respectively. The devices were exposed to the wavelengths of 520, 940 and 1310 nm by using lasers as light sources.

3. RESULTS AND DISCUSSION 3.1 Characterization of sc-SWCNT ink and electrical properties base on SWCNT TFT devices The inset of Figure 1a shows the chemical structure of P-DPPb5T with large conjugated units, which is used to sort sc-SWCNTs from commercial arc-discharge SWCNTs. Figure 1a is the absorption spectrum of a sorted sc-SWCNT ink with the weight ratio of SWCNTs to P-DPPb5T of 1:4. Semiconducting peaks (S22) located at 1000-1300 nm are very sharp, demonstrating that SWCNTs disperse well in toluene with the aid of P-DPPb5T. The sc-SWCNT ink with the S22 peak height of 0.27 was directly used to deposit in device channels by Aerosol Jet printer to construct SWCNT TFTs. Figure 1b shows the optical image of a device. Morphologies of the carbon nanotube thin films in device channels were observed by a Hitachi S-4800 instrument. It can be seen from Figure 1b, the sc-SWCNT thin film is very homogeneous with density as high as 42 tubes/µm2. Typical transfer and output characteristics were shown in Figure 1c and 1d. Printed SWCNT TFTs showed high on/off ratios (106), high mobility (27.46 cm2V-1s-1) and low leakage currents (0.027 nA) at the gate



voltages in the range of -2 to 2 V (shown in Figure 1c), which suggests that metallic SWCNTs are eliminated after being sorted by P-DPPb5T. Figure 2d shows the output curves of printed SWCNT TFTs. The output current is linear at low VDS that indicating there is little contact resistance between the sc-SWCNT channel and the gold electrode. The drain current saturation can be clearly seen at relatively low VDS(< 2 V), and the saturation current can be up to 432 μA at VDS=-2.5V and VGate=-2V. The capacitance of HfO2 thin film was measured at a frequency from 1 to 105 Hz. It is evident that the capacitance value mostly keeps at a constant value of 260 nF/cm2 shown in Figure S1. We use this capacitance value to calculate the mobility of printed sc-SWCNT TFTs in this work.

Figure 1. (a) Absorption spectrum of a sc-SWCNT ink( mSWCNTs : mP-DPPb5T = 3mg : 12mg, the inset is the chemical structure of P-DPPb5T), (b) typical optical image of a device and SEM image of sc-SWCNT thin film in device channel, (c) transfer curve and (d) output curves of a printed SWCNT TFT.


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3.2 Photoelectric properties of the printed photogating SWCNT TFTs.

Figure 2. The photoresponse performance of printed SWCNT TFTs using lightly n-doped Si as bottom gates. (a) Schematic 3D view of a printed SWCNT phototransistor device under illumination of different light wavelengths, (b) the photoresponse characteristics of a printed SWCNT TFT device under 520 nm pulse light illumination with the frequency of 0.5 Hz (optical resource power is 0.34mW), and photoresponse characteristics under pulsed illumination with (c) 520 and 940 nm, and (d) 1310 and 1550 nm laser at frequency of 0.2 Hz (light resource power is 0.025 mW). The photoresponse characteristics of printed SWCNT TFTs using lightly n-doped Si as the photogate were characterized using Agilent semiconductor analyzer B1500 under the pulse light illumination at VDS=-0.5 V and VGate = 2 V. Figure 2a represents the schematic 3D view of a printed SWCNT phototransistor under pulsed



light illumination with a 520, 940, 1310 or 1550 nm laser. Generally, drain currents of printed SWCNT TFTs increase gradually under continuous visible light illumination, and they are over 1000 times greater than those in dark after illumination for 100 s in at VGate= -2 V and VDS= -0.5 V (the light power is 0.34 mW). It should be pointed out again that the devices are in their off states at VDS=-0.5 V and VGate = 2 V. Furthermore, this phenomenon is observed not only on the devices using lightly n-doped Si as the bottom gate, but also on those using other electrodes as bottom gates on the glass substrate, suggesting that some carriers are generated in the device channel under the light illumination37. Figure 2b shows the typical photoresponse characteristics of printed SWCNT TFTs using lightly n-doped Si as the photogate under the pulsed light (520nm laser and frequency of 0.5Hz) illumination with a power of 0.34 mW. As shown in Figure 2b, printed SWCNT TFTs exhibit negative photoresponse characteristics. i.e., drain currents of printed SWCNT TFTs rapidly decrease and then keep the constant (called as off current) under light illumination, while the current will rapidly increase and then reach the stable value (called as on current) when the light is turned off. In addition, on current increases gradually after every stimulation by pulsed light shown in Figure 2b, and reaches to 85 nA after 53 cycles, which is 250 times greater than the background current. At the same time, the photoelectrical currents can be retained for several minutes after turning off the laser light. However, this phenomenon can not be observed when lightly n-doped Si is replaced by heavily n-doped Si or Mo electrode or the printed silver electrode as shown in Figure S2, demonstrating that the photogating effect only comes from


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lightly n-doped Si under light illumination. The photoelectrical properties of printed SWCNT TFT devices were also investigated under 520, 940, 1310 and 1550 nm light pulse illumination with the frequency of 0.2 Hz. We can see from Figure 2c and 2d that the photocurrents can be detected under 520, 940, 1310 and 1550 nm (the power of 0.025 mW) pulsed light illumination. The photocurrents can reach 3 and 1.5 nA under 520 and 940 nm laser stimulations with the frequency of 0.2 Hz after 8 pulse cycles. Although similar photoresponse characteristics can also be observed under pulsed illumination by 1310 nm light, the photocurrents is very weak (only 5 pA) as shown in Figure 2d. However, no obvious changes can be detected when the wavelength is changed to 1550 nm (Figure 2d). We speculate the photocurrents are dependent on the energy of the illumination light as well as on the wavelengths of the illumination source. 3.3 The photoresponse mechanism of printed SWCNT TFTs To study the photoresponse mechanism of printed SWCNT TFTs, the photogate voltages and the changes of electrical properties of printed SWCNT TFTs were investigated. Figure 3a represents the typical relationship between the photogate voltage and time under the pulsed visible light ( 520 nm ) illumination at frequency of 0.5 Hz at VDS = -0.25 V. It is evident that the gate voltage shows the sudden increase under pulsed light illumination. The photovoltages of printed SWCNT TFTs increase ~5 mV (defined as ΔVPV) under each light pulse (light power ~0.025 mW) with the response time being only 40 ms shown in Figure 3a38. Additionally, the background lines of photovoltages39 gradually increased with the increase of the detection time



due to give rise the induced electrical field (VIE) in lightly n-doped Si as shown in Figure S3a and S3b40. At the same time, the transfer curves of printed SWCNT TFTs were measured before and after illumination for 3 min by strong visible light (the light power is 80 mW). It can be seen from Figure 3b that off current is increased from 10-10 to 10-8 after illumination for 3 min, together with threshold voltage shifted towards the positive direction, implying that some carriers were generated and stored in the device channel after the light illumination41-44. It should be pointed out that the devices were in off state when measuring the photoresponse characteristics since the gate voltage and the drain-source voltage were set to 2 and -0.5 V, respectively.

Figure 3 (a) the changes of gate voltage of a printed SWCNT TFT with pulsed 520 nm light at frequency of 0.5 Hz (VDS= -0.25 V, light resource power is 0.025 mW), (b) the transfer curves of a printed SWCNT TFT before and after illumination with strong visible light (light resource power is 80 mW).


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Figure 4. Schematic illustrations of carrier distributions and photogating effect under pulsed light illumination. The first light pulse cycle (a) without and (b) with light illumination, and the second light pulse cycle (c) without and (d) with light illumination. On the basis of above data, the photoresponse process of printed SWCNT TFTs under pulse light illumination is schematically illustrated in Figure 4. Initially, there are no obvious drain currents observed without light illumination due to the low carrier concentrations in the device channel (Figure 4a, also shown in Figure 2b). Upon light pulse illumination, a few carriers are generated and part of them are trapped in the device channels (Figure 4b). In this case, the gate voltage of devices rapidly shifts to the positive direction (2 V+VPV +VIE) due to photogating effects (VPV) and the induced effects (VIE) shown in Figure S3c, and the device is switched off when illuminated by light. In addition, the photocurrents (which can be calculated



by the equation of (dIDS/dVGate)·ΔVPV) are less pronounced. Therefore, the drain current rapidly decreases under light illumination due to the photogating effect shown in Figure 4b and Figure 2c. Photogate effects disappear immediately along with increasing the induced electrical field (VIE) in lightly n-doped Si45,46, resulting in the gate voltage shifts to 2 V+VIE shown in Figure 4c and Figure S3d when the light is turned off. It can be seen from Figure 3a that both ΔVIE and ΔVPV are very small, and ΔVIE less than ΔVPV (5 mV). Furthermore, the background of gate voltage increases with the detecting time shown in Figure 3a. In other words, the gate voltage of the device in dark (2 V+VIE) is always less than that (2 V+VPV +VIE) under light illumination in the same pulse period. At the same time, the concentrations of active carriers in the device channel in dark are more than those under light illumination. As a result, these carriers in the device channel will be rapidly depleted under the pulsed light illumination, resulting in rapidly increasing the drain currents with a relatively small amplitude. When the light is turned on again, the active carrier generation, accumulation and photogating effect simultaneously take place again (Figure 4d). As a result, the drain currents rapidly decrease again. Drain currents in dark continuously increase with the increase of stimulation times due to the photoinduction of carriers released in the device channel (Figure 2c). At the same time, current signals can be retained for several minutes due to the relatively slow discharge of accumulated carriers in the device channel47,48. This phenomenon is very similar to the synapse behaviors (learning and memory).


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Based on the above analysis, the photoresponse characteristics of printed SWCNT TFTs under pulsed light illumination can be explained as following: photo generated holes are trapped in device channel because of the photogating effect of lightly n-doped Si and released gradually from trapping sites under the pulsed light illumination. More and more holes are accumulated in device channel with the increased number of light pulses, which exceeds the released holes, resulting in the gradual increase of drain currents. 3.4 The effect of light power, VDS and oxygen concentration on the photoresponse characteristics of printed SWCNT TFTs.

Figure 5. EPSC characteristics with different VDS and laser power and vacuum level at V

Gate=2V

and VDS = -0.5 V (laser wavelength is 520 nm, and optical pulse

frequency is 0.5 Hz, the power is 0.34 mW unless otherwise specified). (a) Schematic illustration of the biological synapse, (b) the EPSC is defined the change of IDS at the first stimulation in light and every stimulation in dark, (c) the amplitude of the EPSC



on different VDS by applying 31 spikes, (d) the relation between the EPSC change and light powers (VDS=-0.5V). In a neural system, excitatory post-synaptic currents (EPSCs) or inhibitory postsynaptic currents in a postsynaptic neuron can be triggered by voltage pulses from presynaptic neurons49. Similarly, trapped carriers in the printed SWCNT TFT interfaces as well as the pulsed photovoltage generated by the pulsed light stimulation can modulate the drain currents of the printed SWCNT TFT channel.Therefore, our devices exhibit the characteristics of neuromorphic devices, which can be exploited to emulate some functions of synapse in a neural system.The pulsed light applied on the lightly n-doped Si gate electrode is regarded as presynaptic inputs and the current across the SWCNT channel is the equivalent of EPSC, as shown in Figure 5a. We first explored the EPSC characteristics with different VDS values, laser powers and ambient pressures (VGate= 2 V, 520 nm pulsed laser at frequency of 0.5 Hz). Figure 5b is the typical photoresponse curve of the printed SWCNT TFT under the pulsed light illumination with a frequency of 0.5 Hz, which displays the relationship between n EPSC and VDS. Ion is are defined as the drain current in dark and Ioff is the drain n current under light spikes (31in total), where “n” in the Ion is the stimulation times

n (cycles) and EPSCn=| Ion -Ioff |. As shown in Figure 5c, EPSC increased rapidly with

the increase of VDS from-0.1 to -0. 5 V and spike times, which was only 3 nA at VDS of -0.1 V, but reached 22 and 43 nA at VDS of -0.3 and -0.5 V after 31 presynaptic light spikes. Figure 5d exhibits EPSC curves at the laser powers of 0.025, 0.34, 1.39 and 2.17 mW, respectively. It is clearly seen that higher EPSC values are obtained


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when stimulated by higher light power with the same stimulation length and frequency. For example, EPSCs are 1, 86, 392 and 563 nA after stimulation 50 times with light power of 0.025, 0.34, 1.39 and 2.17 mW at frequency of 0.5 Hz. The relationship between EPSC and the ambient pressure is shown in Figure S4. EPSC values at 105 Pa are significantly higher than those at 10 and 10-2 Pa of ambient pressure, suggesting that ambient oxygen plays a key role on EPSC responses. Oxygen molecules adsorbed on SWCNTs and the HfO2 surface can act as carrier trapping sites. There are more carrier trapping sites in the device channel in air due to the higher oxygen concentration50, leading to more photoinduced hole carriers to be trapped

in

the

device

channel

under

light

illumination

(Figure

S4).

As previously described, a great part of these trapped holes can rapidly be released and give rise to the sudden increasing of drain currents when the light is turned off (at relatively low pulse frequency). 3.5 Low-pass filtering characteristics of the printed neuromorphic devices In biological synapses, it has been found that synapses having a low initial probability of vesicle release can act as low-pass filters. To evaluate the filtering characteristics for the printed SWCNT TFTs, EPSCs were recorded when a series of presynaptic light spike trains with different frequencies are applied on the input. Each stimulation train contains 30 optical spikes. The amplitudes of drain currents raises gradually with applying successive spike stimulation at a frequency of 0.5 Hz, however, no obvious changes of drain currents are observed at frequency of 10 Hz. Figure 6a represents the relationship between EPSC and light stimulation times with



frequency from 0.5 to 10 Hz. It is clearly seen that EPSC increases with the increase of stimulation times and there are higher EPSC values with lower stimulation frequency in the range of 0.5 to 10 Hz. The EPSC are 40 and 0.5 nA after stimulating 30 times at frequency of 0.5 and 10 Hz, respectively. The EPSC amplitude has no obvious changes during the course of every 30 spikes when the frequency is more than 5 Hz. It is likely that carriers including photogenerated and trapped carriers cannot be released under pulse frequency of more than 5 Hz. The amplitude gain, which is defined as |(A30-A1)/A1|where A30 and A1 are EPSC amplitude of the last and first spikes, corresponds as a function of the spike frequency shown in Figure 6b. It can be seen that the amplitude gain decreases rapidly with the increase of the stimulation frequency. For a frequency of 0.5, 1, 2, 3, 5 and 10 Hz, the amplitude gains are 40.7, 20.1, 6.9, 4.1, 1.2 and 1.1 respectively, suggesting that our neuromorphic devices show stronger coupling between light spikes at a lower stimulation frequency.

Figure 6. (a) EPSCs recorded for presynaptic spike trains with different frequency, (b) the amplitude raise, defined as |(A30 –A1 )/A1|, is the corresponding the frequency. A30


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and A1 are EPSC amplitude of the last and first spikes, respectively. The inset in Figure 6b represents the schematic of low-pass filtering of a synapse.

3.6 Synaptic plasticity characteristics of the printed SWCNT TFT The long-term plasticity characteristics of light-stimulated neuromorphic devices are also investigated. When a presynaptic spikes by the pulsed light (520 nm or 940 nm at the power of 0.025 mW), the EPSC shows a sudden increase, and then gradually decays, as shown in Figure 7a. The amplitudes of EPSC decrease from 6.57 to 5.12 nA and from 5.21 to 4.58 nA within 100 s after stimulation by 520 and 940 nm laser, representing the current signals decay of 22% and 12% after 100 s. This means that the printed SWCNT TFTs have the long-term plasticity characteristics. The slow decay characteristic of EPSC is attributed to the fact that photogenerated carriers can be quickly trapped in SWCNT and HfO2 surfaces, then released slowly. When using a high intensity white light (80 mW) to stimulate a printed photogating neuromorphic device in a single pulse, the EPSC increases quickly from 0.34 to 1180 nA (greater 3470 times) in a matter of seconds, and then gradually decays to 1070 (90.7%), 440 (37.3%) and 43 nA (3.64%) after 100, 660 and 2040 s, respectively (shown in Figure 7b). It suggests that the learning and memory ability of the printed neuromorphic device increase with the increase of the stimulation intensity. When the printed neuromorphic device is stimulated again, no obvious changes of the response curves are observed shown in Figure 7b, suggesting the printed neuromorphic devices are of excellent repeatability.



Figure 7 Synaptic plasticity characteristics of the printed SWCNT TFT. (a) the attenuation characteristics of neuromorphic devices after stimulated by a presynaptic light spike (520 and 940 nm laser, the power of 0.025 mW), (b) the attenuation and repeatability characteristics of neuromorphic devices after stimulated by a presynaptic white light spike (the power is 80 mW), (c) EPSCs recorded for presynaptic spike trains with a frequency of 40 Hz. EPSC amplitude increases sharply with last spike stimulation. E5 and E5' are EPSC amplitude of the first and last spikes at 5th measurement, respectively, (d) the relationship of the amplitude ratio and the measurement times (the amplitude ratio is defined as E'/E, and E' and E are EPSC


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amplitude of the last and first spikes, respectively), (e) A1 is EPSC at first spike stimulation and A2 is an EPSC obtained after a series of laser pulses at frequency of 40 Hz and then waiting for 1, 5 and 10 min under the power-off conditions, (f) the multiple of amplitude change, defined as A2/A1, is plotted to the pulsed times and the waiting time. Next, the long-term plasticity51,52 characteristics of neuromorphic devices were studied at higher frequency (40Hz) with or without the supply power (the laser power of 0.025 mW). Figure 7c shows that the EPSC amplitude increases sharply after 50 spikes with frequency of 40 Hz. E5 and E5' are the EPSC amplitudes after the first spike and after 50 spikes in 5th measurement, respectively. However, the EPSC amplitude increase gradually and keep a constant value after the first 50 pulsed light stimulation. The amplitude ratio of EPSC (E'/E) is plotted to the measurement times. The result shows that the E'/E increases almost linearly with the increase of the measurement time shown in Figure 7d, suggesting that the neuromorphic devices have a linearly cumulative effect of EPSC. To further study the learning and memory ability (or the decay of EPSC) of the neuromorphic devices, we design a new test program to control the supply power of laser shown on top of Figure 7e. i.e., giving a pulsed light, along with recording one EPSC (A1), then giving 200, 400, 800, 1500 and 1800 pulsed stimulations with frequency of 40 Hz, finally, recording the EPSC (A2) again after waiting for 1, 5 and 10 min. Here, we define as A2/A1 is multiple of amplitude change. The relationship between A2/A1 with the waiting time and the stimulation times is shown in Figure 7f. The A2/A1increases with the increase the



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stimulation times under the same waiting time. For example, A2/A1 are ~9.4, 4.1 and 2.7 with the 1800 pulses and the waiting time of 1, 5 and 10 min, respectively. In this case, EPSC signal (A2) is 2.7 times greater than that of A1 after 10 min, suggesting our photoneuromorphic device can still “memorize” the light stimulation signals. It is noted that the printed neuromorphic devices are under power-off condition after the continuous pulsed stimulations. It demonstrates that the printed photoneuromorphic devices have good non-volatile long-term memory after 1800 pulses. However, when the pulse number is changed to 200, ΔEPSCs are changed to ~2.9, 1.3 and 1 with the waiting time of 1, 5 and 10 min, respectively. EPSC signal (A2) is the same as A1 after 10 min, indicating that the “memory” is totally removed after 10 min. Overall, the neuromorphic behaviors of printed SWCNT TFTs, such as learning and memory, lower-pass filtering characteristics, are due to the photovoltage generated at the interface between Si and HfO2 thin film, together with the trapped carriers in device interfaces.

4. SUMMARY A low-voltage light-stimulated neuromorphic devices is presented which is based on photogating of printed SWCNT TFTs using lightly n-doped Si substrates as photogates and a 50 nm thick HfO2 thin film as dielectric layer. The printed photoneuromorphic

devices

exhibit

learning,

memory

low-pass

filtering

characteristics under pulsed light stimulation due to photogenerated carriers and the trapping states in the device interfaces, which can be used as the non-volatile memory


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units and photoneuromorphic computing. In addition, they have the ability to response to light with wavelengths within the visible and IR spectrum (less than 1310 nm). The novel printing approach to constructing photoneuromorphic devices presents new possibilities for neuromorphic system applications as well as for high-performance infrared photoneuromorphic devices and infrared detectors.

ASSOCIATED CONTENT Supporting Information Figures S1-S4 as described in the text, the relationship between the capacitance of HfO2 dielectric layer vs. frequency ranges of 1 to 105 Hz (Figure S1), the photoresponse characteristics of a printed SWCNT TFT device using heavily n-doped Si, printed silver electrodes and magnetron sputtering Mo electrodes as bottom gates (Figure S2.), schematic illustrations of carrier distributions in lightly n-doped Si and photogating effect at the interface between Si and HfO2 under the pulsed light illumination (Figure S3), and the photoresponse characteristics and the changes of EPSC of a printed SWCNT TFT device with different stimulation cycles under different atmospheric level (Figure S4). The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Authors *E-mail:[email protected].



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*E-mail: [email protected] * E-mail: [email protected] ORCID Jianwen Zhao:0000-0002-5548-5469 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by Natural Science Foundation of China (61874132), Key Research Program of Frontier Science of Chinese Academy of Sciences (QYZDB-SSW-SLH031), National Key Research and Development Program of China

(2016YFB0401100),

CAS

President’s

International

Fellowship

for

Postdoctoral Researchers (2019PT0020), Industry-University-Research Joint Project of Shenzhen China Star Optoelectroncis Technology Co., Ltd, and Shanghai Mifang Science and Technology Co., Ltd., the Basic Research Program of Jiangsu Province (BK20161263), Science and Technology Program of Guangdong Province, China (2016B090906002) and Basic Research Programme of Suzhou Institute of Nano-tech and Nano-bionics (Y5AAY21001), and Cooperation Project of Vacuum Interconnect Nano X Research Facility (NANO-X) of Suzhou Institute of Nano-tech and Nano-bionics, CAS (H060).

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Table of Contents Printed photogating single-walled carbon nanotube (SWCNT) thin film transistors (TFTs) using lightly n-doped Si as gate electrode are proposed for photoneuromorphic devices. Some important synaptic characteristics, such as low-pass filtering characteristics, signal learning and non-volatile memory ability, are successfully emulated in as-proposed printed light-stimulated artificial synapses under the pulsed light illumination. This work demonstrates that printed SWCNT TFTs have potentials for future neuromorphic systems.


Optoelectronic Properties of Printed Photogating Carbon Nanotube

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