Abstract: Benefiting from the fast progress of artificial intelligence, a number of evolutions in the areas of human-machine interface, bio-inspired sensing systems, robots and prosthetics have been achieved. However,because of data explosion and the requirement for intelligent human-machine interaction, novel technology should be developed to overcome current bottlenecks. Different from existing extensive-energy-consumption neural networks implemented at the software level based on the conventional von Neumann architecture, the human brain only has a power consumption of about 20 W. Therefore, it is of a great importance to design a new neuromorphic computing system that is executed by parallel operation with high speed and low power consumption resemble to the human brain. Artificial synapses, either based on transistor or memristor structure, can be used as basic building blocks to achieve large-scale neural network parallelism. In addition, the spike based information processing in biological systems can also be mimicked with the employment of thue artificial synapses, which is beneficial to the construction of intelligent human-machine interface. Therefore, much efforts have been made to optimize the performance of the synaptic devices in terms of materials, fabrication process and structures. Consequently, a series of biorealistic synaptic behaviors, such as visual information reprocessing, movement control and learning-forgetting process, have been emulated using flexible artificial synapses. Despite the great achievement in the study of artificial synapses, several underlying mechanisms have not been uncovered. First, the modulation of the post-synaptic signals varies from each individual, which requires specific analysis in order to make it to be compatible with the neural signal. In addition, dendrites in biological systems can collect, integrate, and modulate thousands of pre-synaptic input signals, and transmit these signals to post-synaptic neurons. That is to say, spatiotemporal information can be modulated. Therefore, exploration of the underlying mechanism and optimization of the device structure mimicking the biological dendrite can contribute to the simulation of dynamic logic induced by spatiotemporal synaptic stimulation. Besides, most of the reported researches were performed on the rigid substrates, which are not compatible with the biological systems. Therefore, fabrication of the devices on flexible substrates and investigation of the relationship between the electrical properties and interface quality are critical. Herein, an overview of the recent progress of the artificial synapses is presented in terms of device structure, material selection, and working mechanism. Future challenges, research directions, and possible applications are also discussed. This review is hoped to provide a guidance for the design and fabrication of the flexible artificial synapses towards neuromorphic computing and intelligent human-machine interface.
1 Liao X, Xiao L, Yang C, et al. Frontiers of Computer Science, 2014, 8, 345. 2 Markram H. Nature Reviews Neuroscience, 2006, 7, 153. 3 Nawrocki R A, Voyles R M, Shaheen S E. IEEE Transactions on Electron Devices, 2016, 63, 3819. 4 Ho V M, Lee J A, Martin K C. Science, 2011, 334, 623. 5 Zucker R S, Regehr W G. Annual Review of Physiology, 2002, 64, 355. 6 Wang X, Li G, Liu R, et al. Journal of Materials Chemistry, 2012, 22, 21824. 7 Wang X, Xiong Z, Liu Z, et al. Advanced Materials, 2015, 27, 1370. 8 Wang X, Gu Y, Xiong Z, et al. Advanced Materials, 2014, 26, 1309. 9 Gu Y, Wang X, Gu W, et al. Nano Research, 2017, 10, 2683. 10 Yao H B, Ge J, Wang C F, et al. Advanced Materials, 2013, 25, 6692. 11 Kim Y, Chortos A, Xu W, et al. Science, 2018, 360, 998. 12 Zidan M A, Strachan J P, Lu W D. Nature Electronics, 2018, 1, 22. 13 Tian H, Zhao L, Wang X, et al. ACS Nano, 2017, 11, 12247. 14 Zhang C, Ye W B, Zhou K, et al. Advanced Functional Materials, 2019, 29, 1808783. 15 Hu L, Fu S, Chen Y, et al. Advanced Materials, 2017, 29, 1606927. 16 Yu H, Gong J, Wei H, et al. Materials Chemistry Frontiers, 2019, 3, 941. 17 Tuma T, Pantazi A, Le Gallo M, et al. Nature Nanotechnology, 2016, 11, 693. 18 Prezioso M, Merrikh-Bayat F, Hoskins B, et al. Nature, 2015, 521, 61. 19 Xu W, Lee Y, Min S Y, et al. Advanced Materials, 2016, 28, 527. 20 Jiang J, Hu W, Xie D, et al. Nanoscale, 2019, 11, 1360. 21 John R A, Ko J, Kulkarni M R, et al. Small, 2017, 13, 1701193. 22 Arnold A J, Razavieh A, Nasr J R, et al. ACS Nano, 2017, 11, 3110. 23 Wan C, Chen G, Fu Y, et al. Advanced Materials, 2018, 30, 1801291. 24 He Y, Yang Y, Nie S, et al. Journal of Materials Chemistry C, 2018, 6, 5336. 25 Tian H, Mi W, Wang X F, et al. Nano Letters, 2015, 15, 8013. 26 Gou G, Sun J, Qian C, et al. Journal of Materials Chemistry C, 2016, 4, 11110. 27 Sanchez Esqueda I, Yan X, Rutherglen C, et al. ACS Nano, 2018, 12, 7352. 28 Gao W T, Zhu L Q, Tao J, et al. ACS Applied Materials & Interfaces, 2018, 10, 40008. 29 Jo S H, Chang T, Ebong I, et al. Nano Letters, 2010, 10, 1297. 30 Yang J J, Strukov D B, Stewart D R. Nature Nanotechnology, 2013, 8, 13. 31 Chae B G, Seol J B, Song J H, et al. Advanced Materials, 2017, 29, 1701752. 32 Vishwanath S K, Kim J. Journal of Materials Chemistry C, 2016, 4, 10967. 33 Wang M, Bi C, Li L, et al. Nature Communications, 2014, 5, 4598. 34 Zhang Q, Shi Z, Yin K, et al. Nano Letters, 2018, 18, 5070. 35 Shmitz F, Kirsch M, Wagner H. European Journal of Cell Biology, 1989, 49, 207. 36 Kandel E R, Schwartz J H, Jessell T M, et al. Principles of neural science, McGraw-hill, New York, 2000. 37 Abbott L, Regehr W G. Nature, 2004, 431, 796. 38 Rotman Z, Deng P Y, Klyachko V A. Journal of Neuroscience, 2011, 31, 14800. 39 Reyes A D. Hearing Research, 2011, 279, 60. 40 Hebb D. The Organization of Behavior, John Wiley & Sons, New York, 1949. 41 Caporale N, Dan Y. Annual Review of Neuroscience, 2008, 31, 25. 42 Wang Z, Joshi S, Savel’ev S E, et al. Nature Materials, 2017, 16, 101. 43 Wang Z, Midya R, Joshi S, et al. In: the 2018 IEEE International Symposium on Circuits and Systems. Florence, 2018, pp.18228325. 44 Upadhyay N K, Jiang H, Wang Z, et al. Advanced Materials Technologies, 2019, 4, 1800589. 45 Chen L, He Z Y, Wang T Y, et al. Electronics, 2018, 7, 80. 46 van de Burgt Y, Lubberman E, Fuller E J, et al. Nature Materials, 2017, 16, 414. 47 Tian H, Mi W, Zhao H, et al. Nanoscale, 2017, 9, 9275. 48 Wu C, Kim T W, Choi H Y, et al. Nature Communications, 2017, 8, 752. 49 van De Burgt Y, Melianas A, Keene S T, et al. Nature Electronics, 2018, 1, 386. 50 Zhao Y, Jiang J. Journal of Nanoscience and Nanotechnology, 2018, 18, 8003. 51 Lee Y, Park J, Choe A, et al. Advanced Functional Materials, 2019, 1904523. 52 Wan C, Cai P, Wang M, et al. Advanced Materials, 2019, 1902434. 53 Zhou F, Zhou Z, Chen J, et al. Nature Nanotechnology, 2019, 14, 776. 54 Van Tho L, Baeg K J, Noh Y Y. Nano Convergence, 2016, 3, 10. 55 Ren Y, Yang J Q, Zhou L, et al. Advanced Functional Materials, 2018, 28, 1805599. 56 Zhang H, Zhang Y, Yu Y, et al. ACS Photonics, 2017, 4, 2220. 57 Diorio C, Hasler P, Minch A, et al. IEEE transactions on Electron Devices, 1996, 43, 1972. 58 Kim S, Choi B, Lim M, et al. ACS Nano, 2017, 11, 2814. 59 Ren Y, Yang J Q, Zhou L, et al. Advanced Functional Materials, 2018, 28, 1805599. 60 Sakai S, Ilangovan R, Takahashi M. Japanese Journal of Applied Physics Part 1-Regular Papers Brief Communications & Review Papers, 2004, 43, 7876. 61 Kato Y, Kaneko Y, Tanaka H, et al. Japanese Journal of Applied Phy-sics, 2014, 47, 2719. 62 Hatanaka T, Takahashi M, Sakai S, et al. IEICE Transactions on Electronics, 2011, 94, 539. 63 Miyasako T, Trinh B N Q, Onoue M, et al. Japanese Journal of Applied Physics, 2011, 50, 04DD09. 64 Kaneko Y, Yu N, Ueda M, et al. Applied Physics Letters, 2011, 99, 3311. 65 Hoffman J, Xiao P, Reiner J W, et al. Advanced Materials, 2010, 22, 2957. 66 Kim E J, Kim K A, Yoon S M. Journal of Physics D Applied Physics, 2016, 49, 075105. 67 Kim S T, Kim D J, Kim T J, et al. Nano Letters, 2010, 10, 2877. 68 Yoon S M, Tokumitsu E, Ishiwara H. IEEE Electron Device Letters, 1999, 20, 229. 69 Yoon, S M Y, Tokumitsu E, Ishiwara H. Japanese Journal of Applied Physics, 2000, 39, 2119. 70 Boscke T S, Muller J, Brauhaus D, et al. Applied Physics Letters, 2011, 99, 5397. 71 Müller J, Böscke T S, Bräuhaus D, et al. Applied Physics Letters, 2011, 99, 947. 72 Starschich S, Griesche D, Schneller T, et al. Applied Physics Letters, 2014, 104, 051604. 73 Mueller S, Mueller J, Singh A, et al. Advanced Functional Materials, 2012, 22, 2412. 74 Jerry M, Chen P Y, Zhang J, et al. In: the 2017 IEEE International Electron Devices Meeting (IEDM). San Francisco, 2017, pp.17524699. 75 Jang S, Jang S, Lee E H, et al. ACS Applied Materials & Interfaces, 2018, 11, 1071. 76 Yu S. Proceedings of the IEEE, 2018, 106, 260. 77 Kim S H, Hong K, Xie W, et al. Advanced Materials, 2013, 25, 1822. 78 Lee J, Kaake L G, Cho J H, et al. The Journal of Physical Chemistry C, 2009, 113, 8972. 79 Xu W, Min S Y, Hwang H, et al. Science Advances, 2016, 2, e1501326. 80 Sharbati M T, Du Y, Torres J, et al. Advanced Materials, 2018, 30, 1802353. 81 Yu J, Liang L, Hu L, et al. Nano Energy, 2019, 62, 772. 82 Lai Q, Zhang L, Li Z, et al. Advanced Materials, 2010, 22, 2448. 83 Shi J, Ha S D, Zhou Y, et al. Nature Communications, 2013, 4, 2676. 84 Yang C S, Shang D S, Liu N, et al. Advanced Materials, 2017, 29, 1700906. 85 Ha S D, Shi J, Meroz Y, et al. Physical Review Applied, 2014, 2, 064003. 86 Wan C J, Liu Y H, Zhu L Q, et al. ACS Applied Materials & Interfaces, 2016, 8, 9762. 87 Wan C, Zhou J, Shi Y, et al. IEEE Electron Device Letters, 2014, 35, 414. 88 Zhu L Q, Wan C J, Guo L Q, et al. Nature Communications, 2014, 5, 3158. 89 Liu Y H, Zhu L Q, Feng P, et al. Advanced Materials, 2015, 27, 5599. 90 Jin W C, Qiang Z L, Mei Z J, et al. Nanoscale, 2014, 6, 4491. 91 Li H K, Chen T P, Liu P, et al. Journal of Applied Physics, 2016, 119, 243. 92 Lee Y, Oh J Y, Xu W, et al. Science Advances, 2018, 4, eaat7387. 93 Wang Y, Lv Z, Chen J, et al. Advanced Materials, 2018, 30, 1802883. 94 Sun Y, Qian L, Xie D, et al. Advanced Functional Materials, 2019, 1902538. 95 Gao S, Liu G, Yang H, et al. ACS Nano, 2019, 13, 2634. 96 Yang Y, He Y, Nie S, et al. IEEE Electron Device Letters, 2018, 39, 897. 97 He Y, Nie S, Liu R, et al. IEEE Electron Device Letters, 2019, 40, 818. 98 John R A, Ko J, Kulkarni M R, et al. Small, 2017, 13,1701193. 99 Bichler O, Zhao W, Alibart F, et al. IEEE Transactions on Electron Devices, 2010, 57, 3115. 100 Alibart F, Pleutin S, Guérin D, et al. Advanced Functional Materials, 2010, 20, 330. 101 Qian C, Kong L, Yang J, et al. Applied Physics Letters, 2017, 110, 083302. 102 Qian C, Sun J, Kong L, et al. ACS Applied Materials & Interfaces, 2016, 8, 26169. 103 Gkoupidenis P, Koutsouras D A, Lonjaret T, et al. Scientific Reports, 2016, 6, 27007. 104 Kim C H, Sung S, Yoon M H. Scientific Reports, 2016, 6, 33355. 105 Gkoupidenis P, Schaefer N, Garlan B, et al. Advanced Materials, 2015, 27, 7176. 106 Shen A M, Chen C L, Kim K, et al. ACS Nano, 2013, 7, 6117. 107 Gacem K, Retrouvey J M, Chabi D, et al. Nanotechnology, 2013, 24, 384013. 108 Wan C J, Liu Y H, Feng P, et al. Advanced Materials, 2016, 28, 5878. 109 Sharbati M T, Du Y, Torres J, et al. Advanced Materials, 2018, 30, 1870273. 110 Tian H, Guo Q, Xie Y, et al. Advanced Materials, 2016, 28, 4991. 111 John R A, Liu F, Chien N A, et al. Advanced Materials, 2018, 30, 1800220. 112 Zhu J, Yang Y, Jia R, et al. Advanced Materials, 2018, 30, 1800195. 113 Bagdzevicius S, Maas K, Boudard M, et al. Journal of Electroceramics, 2017, 39, 157. 114 Yang J J, Miao F, Pickett M D, et al. Nanotechnology, 2009, 20, 215201. 115 Medeiros-Ribeiro G, Perner F, Carter R, et al. Nanotechnology, 2011, 22, 095702. 116 Hirose Y, Hirose H. Journal of Applied Physics, 1976, 47, 2767. 117 Yang Y, Gao P, Gaba S, et al. Nature Communications, 2012, 3, 732. 118 Yang Y, Peng G, Li L, et al. Nature Communications, 2014, 5, 4232. 119 Liu Q, Sun J, Lv H, et al. Advanced Materials, 2012, 24, 1844. 120 Sun Y, Tai M, Song C, et al. Journal of Physical Chemistry C, 2018, 122, 6431. 121 Tian X, Yang S, Zeng M, et al. Advanced Materials, 2014, 26, 3649. 122 Chae B, Seol J, Song J, et al. Advanced Materials, 2017, 29, 1701752. 123 Pan F, Gao S, Chen C, et al. Materials Science & Engineering R-reports, 2014, 83, 1. 124 Kwon D, Kim K, Jang J H, et al. Nature Nanotechnology, 2010, 5, 148. 125 Pickett M D, Julien B, Joshua J Y, et al. Advanced Materials, 2011, 23, 1730. 126 Lee M J, Han S, Jeon S H, et al. Nano Letters, 2009, 9, 1476. 127 Nili H, Walia S, Balendhran S, et al. Advanced Functional Materials, 2014, 24, 6741. 128 Domaradzki J. Surface & Coatings Technology, 2016, 290, 28. 129 Muenstermann R, Menke T, Dittmann R, et al. Advanced Materials, 2010, 22, 4819. 130 Gu C, Lee J. ACS Nano, 2016, 10, 5413. 131 Waser R, Dittmann R, Staikov G, et al. Advanced Materials, 2009, 21, 2632. 132 Hansen M, Ziegler M, Kolberg L, et al. Scientific Reports, 2015, 5, 13753. 133 Hu Z, Li Q, Li M, et al. Applied Physics Letters, 2013, 102, 102901. 134 Han J S, Van Le Q, Choi J, et al. ACS Applied Materials & Interfaces, 2019, 11, 8155. 135 Chanthbouala A, Garcia V, Cherifi R O, et al. Nature Materials, 2012, 11, 860. 136 Kim D, Lu H, Ryu S, et al. Nano Letters, 2012, 12, 5697. 137 Hamdioui S, Aziza H, Sirakoulis G C, In: 2014 9th IEEE International Conference on Design & Technology of Integrated Systems in Nanoscale Era. Santorini, 2014, pp.14447109. 138 Li D, Wu B, Zhu X, et al. ACS Nano, 2018, 12, 9240. 139 Strukov D B, Snider G S, Stewart D R, et al. Nature, 2008, 453, 80. 140 Padovani A, Woo J, Hwang H, et al. IEEE Electron Device Letters, 2018, 39, 672. 141 Zhu X, Du C, Jeong Y, et al. Nanoscale, 2017, 9, 45. 142 Tan Z, Yang R, Terabe K, et al. Advanced Materials, 2016, 28, 377. 143 Pillai P B, De Souza M M. ACS Applied Materials & Interfaces, 2017, 9, 1609. 144 Hubbard W A, Kerelsky A, Jasmin G, et al. Nano Letters, 2015, 15, 3983. 145 Cao X, Li X, Gao X, et al. Journal of Applied Physics, 2009, 106, 073723. 146 Tan Z H, Yang R, Terabe K, et al. Advanced Materials, 2016, 28, 377. 147 Babu S, Thanneeru R, Inerbaev T, et al. Nanotechnology, 2009, 20, 085713. 148 Guo Z, Zhu L, Zhou J, et al. Journal of Materials Chemistry C, 2015, 3, 4081. 149 Jiang H, Stewart D A. ACS Applied Materials & Interfaces, 2017, 9, 16296. 150 Long S, Liu Q, Lv H, et al. Applied Physics A, 2011, 102, 915. 151 Zhu X J, Shang J, Li R W. Frontiers of Materials Science, 2012, 6, 183. 152 Yuan Y, Huang J. Accounts of Chemical Research, 2016, 49, 286. 153 Choi J, Van Le Q, Hong K T, et al. ACS Applied Materials & Interfaces, 2017, 9, 30764. 154 Walsh A. The Journal of Physical Chemistry C, 2015, 119, 5755. 155 Noel N K, Stranks S D, Abate A, et al. Energy & Environmental Science, 2014, 7, 3061. 156 Shi J, Ha S D, Zhou Y, et al. Nature Communications, 2013, 4, 2676. 157 Du N, Kiani M, Mayr C G, et al. Frontiers in Neuroscience, 2015, 9, 227. 158 Nili H, Walia S, Kandjani A E, et al. Advanced Functional Materials, 2015, 25, 3172. 159 Xu W, Cho H, Kim Y H, et al. Advanced Materials, 2016, 28, 5916. 160 Hwang B, Lee J S. Nanoscale, 2018, 10, 8578. 161 Nili H, Ahmed T, Walia S, et al. Nanotechnology, 2016, 27, 505210. 162 Liu G, Wang C, Zhang W, et al. Advanced Electronic Materials, 2016, 2, 1500298. 163 Erokhin V, Schüz A, Fontana M P. International Journal of Unconventional Computing, 2010, 6,15. 164 Erokhin V, Berzina T, Smerieri A, et al. Nano Communication Networks, 2010, 1, 108. 165 Yang X, Wang C, Shang J, et al. RSC Advances, 2016, 6, 25179. 166 Bandyopadhyay A, Sahu S, Higuchi M. Journal of the American Chemical Society, 2011, 133, 1168. 167 McFarlane T M, Zdyrko B, Bandera Y, et al. Journal of Materials Chemistry C, 2018, 6, 2533. 168 Xie W, Willa K, Wu Y, et al. Advanced Materials, 2013, 25, 3478. 169 Zschieschang U, Ante F, Kälblein D, et al. Organic Electronics, 2011, 12, 1370. 170 Zschieschang U, Kang M J, Takimiya K, et al. Journal of Materials Chemistry, 2012, 22, 4273. 171 Kim C H. IEEE Electron Device Letters, 2018, 39, 1736. 172 Qin S, Dong R, Yan X, et al. Organic Electronics, 2015, 22, 147. 173 Ge J, Zhang S, Liu Z, et al. Nanoscale, 2019, 11, 6591. 174 Park Y, Lee J S. ACS Nano, 2017, 11, 8962. 175 Yan X, Li X, Zhou Z, et al. ACS Applied Materials & Interfaces 2019, 11, 18654. 176 Novoselov K S, Geim A K, Morozov S V, et al. Science, 2004, 306, 666. 177 Wan C J, Zhu L Q, Liu Y H, et al. Advanced Materials, 2016, 28, 3557. 178 He Q, Wu S, Yin Z, et al. Chemical Science, 2012, 3, 1764. 179 Tian H, Yang Y, Xie D, et al. Scientific Reports, 2014, 4, 3598. 180 Wang Y, Shi Z, Huang Y, et al. The Journal of Physical Chemistry C, 2009, 113, 13103. 181 Cheng P, Sun K, Hu Y H. Nano Letters, 2015, 16, 572. 182 Ge R, Wu X, Kim M, et al. Nano Letters, 2017, 18, 434. 183 Wang W, Panin G N, Fu X, et al. Scientific Reports, 2016, 6, 31224. 184 Zhou J, Lin J, Huang X, et al. Nature, 2018, 556, 355. 185 Jeong H Y, Kim J Y, Kim J W, et al. Nano Letters, 2010, 10, 4381. 186 Zhuge F, Hu B, He C, et al. Carbon, 2011, 49, 3796. 187 Xu R, Jang H, Lee M H, et al. Nano Letters, 2019, 19, 2411. 188 Khan A K, Lee B H. AIP Advances, 2016, 6, 095022. 189 He H K, Yang R, Zhou W, et al. Small, 2018, 14, 1800079. 190 Di J, Zhang X, Yong Z, et al. Advanced Materials, 2016, 28, 10529. 191 Liang K D, Huang C H, Lai C C, et al. ACS Applied Materials & Interfaces, 2014, 6, 16537. 192 Nagashima K, Yanagida T, Kanai M, et al. Japanese Journal of Applied Physics, 2012, 51, 11PE09. 193 Sun Y, Yan X, Zheng X, et al. Nano Research, 2016, 9, 1116. 194 Nagashima K, Yanagida T, Oka K, et al. Nano letters, 2010, 10, 1359. 195 Bae S H, Lee S, Koo H, et al. Advanced Materials, 2013, 25, 5098. 196 Yao P, Wu H, Gao B, et al. Nature Communications, 2017, 8, 15199. 197 Cai F, Correll J M, Lee S H, et al. Nature Electronics, 2019, 2, 290. 198 Hu M, Graves C E, Li C, et al. Advanced Materials, 2018, 30, 1705914.