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Recent advanced applications of ion-gel in ionic-gated transistor

Recent advanced applications of ion-gel in ionic-gated transistor www.nature.com/npjflexelectron REVIEW ARTICLE OPEN Recent advanced applications of ion-gel in ionic-gated transistor 1 1 1 1 1✉ 1✉ Depeng Wang , Shufang Zhao , Ruiyang Yin , Linlin Li , Zheng Lou and Guozhen Shen Diversified regulation of electrons have received much attention to realize a multi-functional transistor, and it is crucial to have a considerable control over the charge carriers in transistors. Ionic gel, as the dielectric material in transistors, facilitates a large capacitance, and high induced-carrier concentrations. This review presents the recent progress in ionic-gated transistors (IGTs) that have good mechanical stability as well as high physical and chemical stability. We first briefly introduce the various applications of IGTs in sensors, neuromorphic transistors, organic transistor circuits, and health detection. Finally, the future perspectives of IGTs are discussed and some possible solutions to the challenges are also proposed. npj Flexible Electronics (2021) 5:13 ; https://doi.org/10.1038/s41528-021-00110-2 INTRODUCTION As a kind of electrolyte, ionic gel not only has good physical/ chemical stability, but also has the characteristics of flexibility, In the past decade, with the rapid development of the Internet of lightweight, and transparency. Therefore, in the field of wearable Things and consumer electronics, human demand for high electronic devices, IGT has demonstrated unquestionable adapt- performance, portability, and wear-ability pushed the upgrade of ability, portability, and functionality. These characteristics have transistor integration density. However, due to the tunneling aroused the research enthusiasm for the application of ionic-gel to effect and other problems, this trend of continuing to use Moore’s IGT. In addition, it is well known that the nervous system controls Law will inevitably slow down. Therefore, in order to develop more the activities of the human body through electrical and chemical promising integrated circuit applications in the post-Moore era, signals. IGT has the potential advantage of being compatible with tremendous efforts are being invested to develop materials and biological signals due to its similar electrical characteristics, structures aspects to improve the performance of field-effect 1–3 structure flexible, and low toxic. IGT has also been studied in the transistors(FETs) . Many inorganic materials with high dielectric 4 5,6 context of neuromorphology, memory, and synaptic devices, and is constant have been extensively studied, like HfO and Al O . 2 2 3 24–27 expected to be applied in the field of artificial intelligence . Unfortunately, the high dielectric constant causes the carrier In this review, we will briefly list material choices, and introduce mobility decreased in the transistor because of Fröhlich polarons . the conventional and emerging applications of IGTs in flexible Meanwhile, a kind of transistor with ionic gate, ionic-gated electronics. Recent articles on the sensing function, neuromorphic, transistors (IGTs) has attracted widespread attention owing to its and IC function of IGTs will be discussed in detail. The schematic large capacitance, high carrier-inducing ability, and low operating device structure of IGT applications is summarized in Fig. 1. Finally, voltage . The function of IGT is similar to that of traditional the future development and problems of IGT sensing and MOSFET, and the channel current of IGT is also controlled by the neuromorphic systems are summarized in Section 6. gate voltage . The difference is that IGT replaces the dielectric material with the electrolyte material. When a negative voltage is applied to the gate electrode, cations in the electrolyte MATERIALS ASPECT OF IONIC-GEL-GATED TRANSISTOR accumulate at the gate/electrolyte interface under the electro- Tremendous efforts have been invested in IGTs, and the dielectric static mechanism, forming a capacitor layer C that is opposite to ge materials consist of polymer electrolytes, polyelectrolytes, ILs, ionic the charge of the gate electrode. At the same time, a capacitor gels, and inorganic nanogranular materials . Polymer electrolytes layer C is also formed at the channel layer/electrolyte interface. es and polyelectrolytes have shown an excellent capacitance Since the layer spacing of the two significant parallel plate performance, whereas, they also exhibit a slow ion migration, capacitors is ~1 nm, the capacitance effect is significantly greater which limits the application of IGTs in high-speed electronic 8,10 20,29 than that of general insulating layer materials . This type of IGT devices . ILs have good gating functions and faster responses; based on an electric double layer is also named electric double however, their intrinsic liquid state hinders the use of ILs in layer transistor (EDLT). In addition, the channel layer material may practical devices . Therefore, as an alternative to ILs, ionic gels also allow electrolyte ion implantation, thereby causing electro- solve the problem by combining polymer mechanical compat- chemical doping . Due to the typical EDL effect, the large surface ibility and swift switching response. An ionic gel is a semi-solid capacitance allows the IGT to operate well at a relatively low compound that retains charge owing to the insertion of an IL into 12 13 operating voltage and maintain good switching performance . the polymer during the drying stage. The colloidal particles in the Nowadays, the research of ion-gated transistors is booming in mixture prevent it from curing completely, but are in the form of a various disciplines, and the ion materials include electrolyte gel. Due to their excellent chemical stability and transmittance, 14–16 7,17–19 solutions and ionic liquids with liquid fluidity; as well as these materials are also widely used as battery electrolyte 20–22 23 ions/proton-conducting polymers , and inorganic materials . materials. As a result, they are also named as electrolyte gated State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences & Center of Materials Science and Optoelectronic Engineering, University of Chinese Academy of Sciences, Beijing, China. email: zlou@semi.ac.cn; gzshen@semi.ac.cn Published in partnership with Nanjing Tech University 1234567890():,; D. Wang et al. Fig. 1 Prospective applications of ionic gel-based electronics. a An artificial afferent nerve made of pressure sensors, an organic ring oscillator, and a synaptic transistor. Reproduced with permission . Copyright 2018, Elsevier. b Optical photo of Ionic-gel transistor. Reproduced with permission . Copyright 2020, Wiley-VCH. c Schematic device structure and demonstration of multiple presynaptic pulses induced synaptic transistor responses . Copyright 2019, Elsevier. d A real optical image of the OECT iontronic pressure sensor. Reproduced 84 104 with permission . Copyright 2021, IEEE. e The physical schematic diagram of the transistor-breathalyzer. Reproduced with permission . Copyright 2016, Springer Nature. f Schematics of the ionic-gel-integrated optical-transistor modulator. Reproduced with permission . Copyright 2017, American Chemical Society. g A diagram of an IGT device using a nanowire as a stress-sensing material. Reproduced with permission . Copyright 2019, Elsevier. h Schematic and physical optical photo of a single device used to prepare the inverter. Reproduced 121 129 with permission . Copyright 2010, Wiley-VCH. i A picture of completed IGZO-EGTFT device arrays. Reproduced with permission . Copyright 2015, Springer Nature. j Optical picture of a single inverter and the electrode of the substrate before preparation. Reproduced with permission . Copyright 2013, American Chemical Society. transistors (EGT). In the following section, we describe ILs and the Figure 2 shows the structures of cations and anions conventionally polymer grid used in IGTs. used in IL, and the various properties of IL are determined by the ILs are generally known as room-temperature molten salt. combination of different anions and oxygen ions. Generally, most Owing to the low vapor pressure, high ionic conductivity, and of the studies on the potential applications of ionic gel have good temperature (up to 350 °C), and chemical stabilities, ILs are advanced toward electrical and electrochemical devices, including 42 43 32,39,44–46 47,48 more suitable for polymer-in-salt systems. In 1992, since Wilkes sensors , super capacitors , batteries , actuators , 41,49 50,51 52 et al. prepared 1-ethyl-3-methylimidazole tetrafluoroborate electrochromic devices , solar batteries , FET , and optimi- ([EMIM][BF ]) with good stability in the air, the research on ionic zation of thermoelectric materials using IGTs . The manufacturing liquids has flourished . Ionic gels have a wide selection of method, materials, and performance of the reported nanoscale 31–34 anions , so ionic gels are usually classified according to the IGTs are compared in Table 1. 35,36 53,54 55,56 types of cations, including quaternary ammonium , phospho- ILs, such as [EMIM][TFSI] , [EMIM][TFSA] , and [BMIM] 37 38 39 57 nium , Imidazole , and pyridine , etc. Imidazole-based ionic [PF ] , have been reported to be widely used and have good liquids are widely used in emerging electronic devices due to their performance. Cho et al. used a chemically stable ionic liquid, 40,41 generally low melting point and many optional substituents . [EMIM] [TFSI], and photocurable mixture material to prepare npj Flexible Electronics (2021) 13 Published in partnership with Nanjing Tech University 1234567890():,; D. Wang et al. Fig. 2 The type of ionic-gel structure. Three different types of ionic-gel composition structure and main ionic-gel anion and cation types. patterned graphene transistors under ultraviolet (UV) radiation . (SOS), as the gel network and dissolved it with 1-butyl-3- The presence of ionic liquid not only maintains stability but also methylimidazolium hexafluorophosphate ([BMIM][PF ]) into exhibits excellent surface capacitance and polarization response methylene chloride (Shown in Fig. 3a). Tri-block polymers such speed. Besides being extremely sensitive to external bending and as SOS have special structural and chemical properties; the entire deformation (GF~389), the system can also implement a large- structure is divided into two parts: can be dissolved (the middle) area, highly integrated array on a flexible substrate for motion and cannot be dissolved (on two sides) in IL. Thus, they are detection. collectively referred to as ABA tri-block polymers. This character- However, for in vivo applications, these ILs have not been istic makes ABA tri-block co-polymers suitable for the preparation proven to be safe. Considering these conditions, Tang et al. of ionic gels. This easily leads to the design and preparation of an reported a biocompatible IGT composed of poly(ε-decalactone)-b- ionic gel that can be utilized to construct various transistors with poly(DLlactide)-b-poly(ε-decalactone) (DLD) and [P ][TFSI] . The different requirements. In addition to SOS, other ABA tri-block co- minimum capacitance of the 20% ionic gel could reach polymers, such as SMS, are also widely used in the preparation of −2 61 approximately 2 μFcm , ensuring that the system could be ionic gels . operated under a low voltage. The prepared IGT using P3HT as the According to application scenarios and preparation processes, channel layer showed good characteristics for printed electronic many chain polymers are also used to prepare IGTs. A transistor 2 −1 −1 5 applications: 1 cm V s mobility, an I /I ratio of 10 , and low that uses P (VDF-HFP) and a gel gate dielectric layer has the on off hysteresis. Besides, it showed promising hydrolytic degradation. characteristics of easy transfer. There is a transistor using P (VDF- More recently, Kim et al. reported an ionic gel material made of HFP) that can easily transfer the gate insulating layer through a choline and malic acid as raw materials, and the obtained dielectric “cut and paste” operation . Except good solvent compatibility, layer has good biocompatibility and biodegradability . the gel material not only has high tension and structural stability, The flexibility of material selection makes it possible to choose but also exhibits similar properties to triblock polymers in some different polymers to prepare ionic-gel gate transistors. For most specific aspects. For example, PVA, (P(VDF-HFP)) can also be used IGTs, polymer monomers are embedded into ionic solutions to form gels with [EMIM][TFSA] ILs. Compared with PS-PMMA-PS composed of polymer molecular segments, and the commonly gel, their capacitance values decrease with increasing frequency. used materials are mainly self-assembled tri-block and di-block co- The results (Fig. 3b) show that the capacitance value of P(VDF- polymers, such as poly(styrene-block-ethylene oxideblock-styrene) HFP) gel is approximately 1 μF lower than that of SMS gel at a low 53,60 (SOS) , poly(ε-decalactone)-b-poly(DL-lactide)-b-poly(ε-deca- frequency (10 Hz), however it is slightly higher than that of the 37 56 lactone) (DLD) , and poly(styrene-b-methylmethacrylate-b-styr- SMS gel at a high frequency (100 kHz) . This proves that the 61,62 ene) (SMS) . dielectric functions of the two EDL dielectric materials are The first ionic-liquid-based gel used in an organic film transistor comparable. To achieve good implantability of flexible electronic was produced by Lee et al. They chose a tri-block co-polymer, devices, biocompatibility must also be ensured for the materials Published in partnership with Nanjing Tech University npj Flexible Electronics (2021) 13 D. Wang et al. Table 1. Parameters comparison of Ionic-gel Gate Transistor. 2 −1 −1 μ (cm V s )I ratio SS (mV) Vth (V) Ionic liquid Polymer Channel Method Ref F On/Off 5 55 20 10 158 −0.13 V [EMI] [TFSA] P(VDF-HFP) In O Cut and stick 2 3 4 53 20 10 110 −0.2 V [EMIM] [TFSI] PS-PMMA-PS PQT-12 Cut and stick 5 53 20 10 270 −0.5 V [EMIM] [TFSI] PS-PEO-PS PQT-12 Cut and stick 5 67 20 10 nr 0.12 V Proton Wheat flour IZO Drop 5 25 210 670 0 V [EMIM][TFSI] P(VDF-HFP) IWO Cut and stick 5 57 nr 10 nr 12 V [BMIM][PF ] PS-PEO-PS P3HT Cut and stick 5 53 20 10 280 −0.7 V [BMIM][PF ] PS-PMMA-PS P3HT Cut and stick 5 54 2.06 10 nr −1 V [EMI] [TFSI] PS-PMMA-PS ZnO Spin-coating 5 60 nr 10 73 −0.8 V [EMIM][TFSI] P(VDF-HFP) P3HT EHD 6 132 nr 10 nr −1V [P ] [TFSI] SOS-N P3HT Photo-pattern 14 3 5 133 7.8 10 135 1 V Proton Chitosan ZnO Spin-coating 5 134 11 10 90–110 0.65 V [EMIM][TFSI] PS−PMMA SWCNT Printing 2 135 1.8 10 nr 1.2 V [EMIM][TFSI] PS-PMMA-PS WO Cut and stick 5 122 nr 10 nr 0.5 V [EMI] [TFSI] SEAS-N ZnO Screen-printed 2 136 nr 10 nr −0.2 V [EMIM][TFSI] P(VDF-HFP) CuSCN Cut and stick 5 56 2.3 10 nr 0 V [EMI][TFSA] P(VDF-HFP) P3HT Cut and stick used in co-polymers. Therefore, some researchers use biocompa- stretchability, have attracted much attention in the field of 59 63,64 tible materials, such as polysaccharides , chitosan , and wearable electronics. Not only can it be used as a mechanical 65 66 proteins instead of cross-linked polymers . As mentioned sensor to sense pressure, but it can also be used as a proximity before, Kim et al. proposed an ionic gel made of Lewandan sensor or smart sensor in sensing, thereby improving signal polysaccharide and choline-based water-soluble IL cast by a processing efficiency and reducing system power consumption. In the field of optoelectronics, phototransistors and optical simple solution method (shown in Fig. 3c). This dielectric has the characteristics of good biocompatibility, biodegradability, high modulators take full advantage of IGTs . Phototransistors that transparency, free stranding, and solid state . Besides, Yang et al. receive optical signals and convert them into electrical signals can dissolved wheat flour and acetic acid in water and prepared a detect light in various bands, therefore, enabling applications of wheat flour electrolyte membrane by a drop-casting method. The night vision, three-dimensional object recognition, health mon- 71,72 itoring, and optical communications . Owing to their three- resulting IZO-based thin-film transistor exhibited a switching ratio 5 2 −1 −1 terminal configurations, phototransistors can effectively reduce of 10 , and have excellent saturation mobility (15 cm V s ). Accordingly, they realized the simulation of synaptic functions, noise and amplify electrical signals. Meanwhile, ionic gel such as excitatory postsynaptic current (EPSC) and paired-pulse electrolytes are introduced due to its high capacitance and 67 73 facilitation (PPF) . transparency to solve the problem of high operating voltages . Furthermore, some mixed copolymer has more application Thus, the ionic gel has been used to achieve low operation voltage and high carrier concentration, while the low mobility of compatibility and enables realize photo-crosslinking, specific recognition, acid-base resistance, hydrophilic, and hydrophobic the ions and formation of the layered effect can help realize control, etc. by controlling the internal components. A UV-curable optical modulation. ionic gel can easily realize the preparation of ultra-tiny IGTs .In In order to obtain a strong electric field at the interface between addition, Liu et al. also prepared a fully stretchable polarized the gate and the channel, EDLT is a promising choice, which can transistor by controlling the PFPE content in the PFPE-DMA gate generate high carrier density at low pressure. Many studies have used IGTs to achieve electrical and optical multi-modal control of dielectric layer. They demonstrated a skin-like display that is the channel layer, and further applied to neuromorphological formed by integrating stretchable TFT array and stretchable 69 55 organic light-emitting electrochemical cells (OLECs) (Fig. 3d) . research . Much more interesting, Ionic-gel-based photonic Through the preparation method compatible with traditional devices with simple processing and high gate-capacitance, MEMS technology, the application of IGTs in fields, like health dynamically regulate the graphene chemical potential (Fermi level) (Fig. 4) . The charge density is electrically tuned at the ionic monitoring and logic operation, will be further expanded. gel–graphene interface, and the chemical potential of graphene is changed to control its intensity of light absorption. At 1.55 μm SENSING APPLICATIONS OF IONIC-GATE TRANSISTORS (IGTS) wavelength, the transverse-magnetic (TM) mode is increased from In 1970, Bergveld et al. discovered that after removing the gate of −10.8 dB to −8.8 dB, and the transverse electric (TE) mode is the FET and immersing it in a liquid, the ions in the insulating layer reduced from −9.0 dB to −12.1 dB. Depending on these data, the would affect the conductivity of the channel layer. Therefore, the waveguide sample with an ionic-gel gate dielectric can be used as original IGT was used as a sensitive ion sensor and used in a broadband TM transmission polarizer. In addition, the optical physiological detection. Nowadays, IGT has become a hot research behavior of the ion-gated modulator clearly shows hysteresis due topic among multidisciplinary areas including electronics, neuro- to the slow polarization response time. morphology, biology, and engineering. Various exciting applica- Further, a research study has recently demonstrated that tions of IGT have been developed continuously. For example, due photoluminescence (PL) can be reversibly tuned by EDLTs up to to the excellent light transmittance of ionic gels, IGT also has 1–2 orders of magnitude at a low gate voltage (Fig. 4c, d) . The broad application prospects in the field of optics . The properties density of electron traps via the interaction of trap with oxygen of IGT, such as stable structure, stable chemical properties, and ion at V < 0 is reduced, owing to the transverse gate-electric-field npj Flexible Electronics (2021) 13 Published in partnership with Nanjing Tech University D. Wang et al. Fig. 3 Four different types of polymer materials used for an ionic-gel transistor. a SOS, a schematic diagram of the structure of a triblock (ABA) polymer. Reproduced with permission . Copyright 2014, American Chemical Society. Capacitance-frequency dependence of four different ionic-gel. Reproduced with permission . Copyright 2009, American Chemical Society. b Comparison of the frequency dependence curves of the maximum capacitance of SMS and P (VDF-HFP) materials. The inset is a schematic diagram of the molecular structure and device structure of P(VDF-HFP). Reproduced with permission . Copyright 2012, Wiley-VCH. c The molecular structure of Lewandan, the synthetic ionic liquid based on choline and malate ions, and the chemical structure and schematic diagram of the expected synthesized LSE membrane. On the below side is the Nyquist chart of LSE. Reproduced with permission . Copyright 2020, Wiley-VCH. d The schematic diagram shows the material design used to pattern stretchable transistors on elastomer dielectrics by inkjet printing. The figure on the right shows the dielectric constant of the PFPE-DMA film subjected to uniaxial strain. Reproduced with permission . Copyright 2020, Springer Nature. generated by ionic gels, leading to lower probabilities of non- coefficient of up to 10 . Resulting from merging single atomic layer radiative recombination and photogenerated carriers with nanoscience with ultrafast optoelectronics, this heterogeneous increased mobility, and accordingly, the PL density is enhanced. graphene microcavity can further enhance our understanding of The function of reversible control of perovskite luminescence can dynamic frequency combs and ultrafast optoelectronics. be advantageous to the potential applications of photons in The FET-type pressure device is one of the basic component of perovskite materials. electronic skin devices that achieve advanced sensing perfor- Moreover, optical frequency combs are the basis of modern mance, including multi-parameter tracking, high sensitivity, high 79–82 frequency measurement, astronomical observation, precision spec- resolution, and low crosstalk signaling .Inanearlier study, 76,77 troscopy, ultrafast optics, and quantum information . Yao et al. ionic-gel-based FETs have been observed to function by pioneered the application of IGTs in graphene-based tunable optical inducing mobile charges in the semi-conductor channel and frequency combs (Fig. 4e) . By coupling the ionic-gel gate-tunable are utilized to fabricate pressure sensors with low power 58,83 light guide to the silicon nitride photonic microresonator, the consumption and high transparency . For example, Khadem- second- and higher-order dispersion can be modulated by tuning hosseini et al. demonstrate an IGT-based pressure sensor .The the Fermi level. They have successfully demonstrated a double-layer dielectric layer is fabricated by microstructured ionic hydrogel, IGT that can adjust the Fermi level of graphene to 0.45–0.65 eV, showninFig. 5a–d. As external pressure is applied to the top of besides, the double-layer IGT is able to retain a cavity mass the transistor, the microstructure hydrogel deforms and changes Published in partnership with Nanjing Tech University npj Flexible Electronics (2021) 13 D. Wang et al. Fig. 4 Applications of phototransistor formed by ionic-gel. a Schematics of ionic-gel-integrated optical-transistor modulator. Reproduced with permission . Copyright 2017, American Chemical Society. b Thepolar imagemeasuredunder thepresenceof the ionic-geland theoptical transmission of phototransistor according to the top gate voltage at a wavelength of 1.55 μm. c Scheme of the IGT photoluminescence devices with a lead-halide perovskite single-crystal integrated with an ionic gel. Reproduced with permission . Copyright 2019, Elsevier. d The relationship between V -induced modulation of PL intensity and the device geometry (the crystal thickness and photoexcitation area) of perovskite EDLTs. e Schematic architecture of the graphene-based microresonator. The gray object showed is the silicon nitride. Reproduced with permission . Copyright 2018, Springer Nature. f Electric field distribution of the graphene–nitride heterogeneous waveguide, with a Si N cross-section of 1.2 × 0.8 μm .The 3 4 distance between the Si N waveguide and the graphene layer is 100 nm. The graphene and the top-gate probe are separated by 1 μmwiththe 3 4 interlayer ionic-gel capacitor. In this structure, transverse electric (TE) mode is applied. g Optical micrographs show the bus waveguide (red arrows), ring resonator, and Au/Ti metallized patterns. An etched window is designed to ensure both graphene–light interaction and reduced propagation loss. Here the graphene-covered area is marked by the gray dashed box; the etched window label refers to the whole horizontal area between the two central lines. CW continuous wave, MI modulated intensity. Scale bar, 100 μm. h Soliton state with crystal-like defects modulated by heterostructure transistor. The middle and right graphs are the corresponding spectral measurements and the frequency-resolved second harmonic autocorrelation graphs of soliton pulses. The gray curve here shows the real-time autocorrelation intensity curve. the capacitance of the hydrogel/gate interface, as a result, ionic-gel gate capacitance under the double capacitance effect is −2 affecting the number of ions in the channel. Due to the large (7.9 μFcm ), which ensures that the graphene FET works at outstanding amplify capability of the organic electrochemistry a low gate voltage of less than 2 V. Between the source and drain transistor (OECT), the pressure sensor can detect pressure as low electrodes, the interdigitated electrode at the bottom is in as 20 Pa in this manner. Furthermore, the IGT operates at a lower contact with the graphene backplate under external pressure, voltage (0–1 V) due to the high capacitance of the EDL effect, resulting in a change in resistance (shown in Fig. 5e). It exhibited −1 and the entire power consume of sensing device used is only high-pressure sensitivity (0.12 kPa ) and outstanding mechanical 10–1000 μW. Based on these two advantages, ionic gel-based durability (more than 2500 cycles). With the help of this structure, pressure sensors are also competitive in wearable and implan- the fabrication of common-gate array devices can be easily table sensor applications. realized. The pressures on the 4 × 4 GFET matrices can be mapped Cho et al. proposed a structure for the fabrication of pressure spatially (Fig. 5f). sensor matrices with a simple process and excellent perfor- However, the use of transistors as direct pressure sensors is mance . The ionic-gel connects the drain and the gate, and the limited by structural design difficulties and poor stability, npj Flexible Electronics (2021) 13 Published in partnership with Nanjing Tech University D. Wang et al. Fig. 5 Piezo electronics sensor made of ionic-gel transistor. a Schematic diagram of the device structure of an OECT. b quivalent circuits in an OECT and schematic diagram of the proposed microstructured hydrogel-gated OECT iontronic pressure sensor. The deformation of the hydrogel determines the number of ions delivered into the channel. c)V and V change after application of external pressure on the gate g sol electrode. d SEM image of gelatin methacryloyl (GelMA) hydrogel with pyramidical microstructures on the surface; Lower picture is a real optical image of the OECT iontronic pressure sensor. Reproduced with permission . Copyright 2021, IEEE. e Schematic diagram of a GFET pressure sensor fabricated with interdigitated source electrodes. f Spatial pressure graph of GFET pressure sensor matrix. g Pressure sensing characteristics of GFET pressure sensor mounted on PDMS rubber substrate. The illustration shows the GFET matrix making angular contact with human hands or table tennis. Reproduced with permission . Copyright 2014, Wiley-VCH. h Mechanism of GT strain sensing based on piezoelectric potential gating. i Sensitivity and stability characteristics of piezoelectric gated GT strain sensor. Reproduced with permission . Copyright 2015, Wiley-VCH. g Process for manufacturing multi-level non-volatile memory array for piezoelectric potential programming. k Real-time programming and erasing steps applied to piezoelectric potential programming memory. Reproduced with permission . Copyright 2016, American Chemical Society. prompting researchers to turn to other methods. The piezoelectric Ionic-gel electrolyte materials are extensively applied in piezo- effect is defined as the electric polarization of the piezoelectric tronic transistors owing to their large capacitance that enables 91,92 material that originates from materials that lack inversion low-voltage transistor operation . The long-range polarization symmetry, caused by the action of external mechanical forces; characteristics of ions in the ionic gel allow the placement of the piezoelectric materials convert random mechanical energy into gate electrode and channel to be coplanar. Sun et al. described a 85,86 electric energy (piezopotential power) . The piezotronic piezopotential-powered active strain sensor matrix array combin- transistor, utilizing inner-crystal piezopotential to harvests bio- ing piezoelectric nanogenerators (PENGs) and coplanar-gate 87,88 chemical and biomechanical energy , while achieving self- graphene transistors based on P(VDF-TrFE), as shown in Fig. 5h. powered active digital data processing capacities, such as pressure The output pressure sensor signal is maintained at the corre- 89,90 arrays with high resolution and human-machine interaction . sponding value based on the applied strain. The ionic-gel Published in partnership with Nanjing Tech University npj Flexible Electronics (2021) 13 D. Wang et al. dielectrics can effectively couple the piezoelectric potential to the monitoring. Unlike previous studies that have fabricated ultra-thin graphene channel, thereby maintaining the output sensor signal inorganic semi-conductors or have designed “wavy” structures to 109,110 at the corresponding stress value. As a result, excellent sensitivity achieve flexibility , intrinsically stretchable conductive poly- (GF = 389), ultra-high detection limit (0.008%), and high mechan- mers have become a growing trend. Kim et al. synthesized ical durability (> 3000 cycles) are obtained (Fig. 5i). They further biocompatible, biodegradable, highly transparent, free-standing, developed a piezopotential-programmed non-volatile memory and solid-state electrolytes consisting of levan polysaccharide and array by integrating ionic-gel-gated FETs and PENGs . The choline-based water-soluble IL (as shown in Fig. 6e). Levan storage device is programmed by applying external pressure to polysaccharides function as free-standing and solid-state poly- the PENG, and by grounding the top electrode of the PENG. In meric matrices that are translucent, versatile, and water-soluble, contrast, it is easy to flash the stored data by grounding the upper while choline-type water-soluble IL reacts with malic acid, utilizing electrode of the PENG (Fig. 5g). Under various external bending choline as the cation and malic acid as the anion . Biodegrad- strains, multi-level data storage of more than 3 orders of able electrolyte-based organic transistors exhibit satisfactory magnitude is achieved with a good memory efficiency, including performance, such as low operating voltage (V = −1.0 V and ds 3 3 a high programming/erasing current ratio (> 10 ), 2-bit multi-level V = −2.0 V), good stability, and high on/off ratio (over 10 ). gs data storage (over 4 levels), efficiency reliability over 100 cycles, Organic transistors were incorporated with bio-integrated instru- and reliable data retention over 3000 s. ments, such as electrocardiogram (ECG) recordings on human skin With the advent of the Internet of Things (IoTs), non-invasive and rat heart with a better signal-to-noise ratio (shown in Fig. 6f). measurements and real-time health monitoring have become A thread-based transistor (TBT) realized by electrolyte gating of increasingly important in modern medicine, particularly to address semiconducting carbon nanotube networks on linen threads was the urgent needs of the aging population . In the process of proposed by Sameer et al. The thread has an interchangeable medical monitoring and diagnosis, equipment with high-quality module to fit the biological tissue required, can be minimally signals and long-term stability is essential . In addition, soft invasive (for example, sutures), and enables the provision of three- devices with good physiological comfort and biological compat- dimensional tissues or organs with a natural interface . The ibility can effectively minimize the harm to the human body and electrolyte is composed of colloidally scattered nanoparticles of 2,95–99 discomfort of wearing . IGTs have the key advantages of low silica and IL gel (a form of an ionic gel) that enables CNT 100,101 threshold voltages, high amplification, and biocompatibility transistors to be electrostatically gated. To achieve a multiplexed that ensure an accurate diagnosis of physical illness. Therefore, diagnostic system based on complete threads, they linked TBTs IGTs that are intrinsically stretchable, adequately meet the needs with thread-based electrochemical sensors (TBEs). As seen in of physiological monitoring. Fig. 6g, completely threaded platforms are thin, extremely In conventional methods, medical monitoring and diagnosis durable, and compatible, allowing them to be worn directly on mainly rely on physiological signals (blood pressure, heart rate, the skin without any silicone substrate or without using a skin and respiratory rate) and the real-time concentration of biochem- needle. As mentioned above, most ionic gels consisting of + + ical molecules (Na ,K , glucose). Transistors can achieve an polymers and ILs are not proven to be biocompatible and effective amplification of these data and thereby effectively biodegradable, and some of them are even toxic to humans. improve the sensitivity of medical sensors. For example, Someya Therefore, it is necessary to apply natural biomaterials such as et al. fabricated a biocompatible electrode by integrating a cellulose or hydrogels with non-toxic synthesis procedures. gelatinous composite into transistor amplifiers to realize a direct detection of pericardial electrocardiography (Fig. 6a). One-pot NEUROMORPHOLOGY APPLICATIONS OF IONIC-GATE fabrication of a conductive gel that is responsive to glucose TRANSISTORS (IGTS) without enzyme, is integrated into OECTs to detect the patient’s glucose level clinically . Alcohol dehydrogenase (ADH) and its Another emerging application of IGT is neuromorphic devices, nicotinamide adenine dinucleotide (NAD ) cofactor are readily particularly synaptic transistors. In order to simulate the human immobilized on the OECT to detect ethanol in the range of 0.01 % brain, much efforts have been invested in the ability of parallel to 0.2 % blood alcohol content (Fig. 6b) . processing of data and weight modulation through algorithms 112,113 Although gel-based transistors have realized various applications and software on the existing computer hardware . However, in the medical field, there is still a lot of scope for improvement. The the human brain performs better than traditional computers while first problem that needs to be solved in this field is the requirement processing real-time sensory data such as images, videos, sounds, of the high-level accuracy of physiological signals to provide an and navigation. In addition, compared with the high energy accurate diagnosis, and this is dependent on the SNR (signal-to- consumption of supercomputers, low energy consumption is also 105,106 114 noise ratio) . To obtain a high SNR, a preamplifier may be a major advantage of the biological brain . Synapse transistor is integrated in close proximity to the sensing electrode to reduce the imitated as a synapse, where two neurons are connected influence of the wiring noise, thereby realizing active elements with functionally, and they are also the key parts of signal transmission, 102,107 local signal amplifications . The structure of OECT without the as shown in Fig. 7a. The human brain is a complex network 11 15 gate dielectric layer makes direct electrolytic operation possible, composed of nearly 10 neurons and 10 synapses that are allowing the conversion amplifier to be directly loaded at the highly interconnected, massively parallel, and structurally vari- location of the target, thereby largely reducing the ratio of noise. able . Synaptic plasticity, as a functional feature of nerve cell Someya et al. designed an OECT-based electrophysiological interconnection, gives biology the ability to learn and remember. transducer with a high SNR (24 dB). They successfully solved the The appearance of hysteresis of EDLT devices limits its switching issue of the inability of transistors to operate for a long time due to speed, and is not suitable as a conventional transistor, but is more electrolyte drying, by integrating an ultra-thin OECT with a suitable as a functional device that mimics synaptic transmission. nonvolatile gel electrolyte, as shown in Fig. 6c. OECT made of thin The bionic simulation of brain nerve structure provides many gels can adhere well to the skin and effectively monitor the heart feasible research scopes. Based on the phenomena of dendrites signal. It demonstrated stable performance during long-term receiving multi-synaptic signals, Wan et al. controlled the continuous monitoring (for example, within 3 h) and multiple reuses information transmission of neuromorphic transistors through in tests, lasting more than a week (Fig. 6d). multiple in-plane gates (Fig. 7b). Owing to the different Another major achievement is the technological advances in distances between the source and drain electrodes and the flexible and biocompatible materials that not only allow patients multiple gates, the spatial position of each gate on the plane is to have a better wearing experience, but also facilitates real-time defined by different coordinates. Obviously, the spatial coordinate npj Flexible Electronics (2021) 13 Published in partnership with Nanjing Tech University D. Wang et al. Fig. 6 Bio-sensors made of ionic-gel transistor. a Circuit diagram of a unit of an organic amplifier with a conductive gel for an in vivo electrocardiograph and Pictures of an ultra-flexible circuit on a rat’s heart. Reproduced with permission . Copyright 2016, Springer Nature. b Enzymatic reaction of ethanol and ADH in an electrolyte. Reproduced with permission . Copyright 2016, Springer Nature. c Schematic diagram of OECT in a nonvolatile gel electrolyte containing glycerol and ions. d OECT-based sensors measured ECG data at different electrolytes (0.7 m NaCl, hydrogels, and glycerin gels) Reproduced with permission . Copyright 2019, Wiley-VCH. e Schematic of a biodegradable organic transistor. f Standard equipment and biodegradable organic transistors record the ECG signals from the human skin. Reproduced with permission . Copyright 2020, Wiley-VCH. g Schematic diagram of a thread-based integrated transistor system for advanced sensing of bio-related ions. Reproduced with permission . Copyright 2019, American Chemical Society. of the gate closer to the channel layer contributes more as the dielectric materials to prepare neuromorphic memtransistor significantly to the weight of the transistor. The mapping of elements .Bydefining different gate voltages as different distance and EPSC contribution is shown in Fig. 7c. They present harmful inputs, they successfully used IGTs to simulate the EPSC current diagrams for different stimulation directions and the modulation of the salient weight of pain receptors. Similar to I/O relationship between the rate coding scheme and neurons in bio-nociceptors, the satellite threshold adjusting receptors (STARs) different directions. They further verified the influence of stimuli can adjust the transfer characteristic curve of the device according directivity on multi-gate neuromorphic transistors (Fig. 7d). to the intensity of the harmful stimulus (V ) to make it more gs Real-time control can be achieved through a programmable sensitive and avoid repeated damage. When V is higher than the gs circuit design. John et al. also utilized P(VDF-HFP) and [EMIM][TFSI] threshold voltage and the duration increases, significant transistor Published in partnership with Nanjing Tech University npj Flexible Electronics (2021) 13 D. Wang et al. Fig. 7 The scheme and functions of multi-synapse. a Schematic illustration of a Biological network. Reproduced with permission . Copyright 2020, Springer Nature. b Schematic picture of the visual system and 2D MoS neuromorphic transistor with a grid of 3 × 3 coplanar- gate arrays. Reproduced with permission . Copyright 2018, American Chemical Society. c, d 2D EPSCs surface summarized as a function of spatial coordinate, and each gate has a different coordinate. EPSC responses driven by dual spike inputs and its different orientation relationships. 10,117 drift occurs, as shown in Fig. 8a–f. After a period of time or of different wavelengths . Further, inspired by biological visual application of the recovery voltage, the threshold characteristic of and nervous systems, Sun et al. demonstrated a flexible, dual- IGT can be restored to its original level, as shown in Fig. 8e. The modulation, tunable synaptic FET based on ZnO nanowires as the robot can change its sensitivity after receiving harmful stimuli and channel, and sodium alginate as the gate . IGT is combined with can realize self-healing and functional recovery after being ZnO that has an excellent UV light response, it can also output a damaged. The decentralized intelligence they proposed is higher current than the normal state after multiple UV light different from traditional centralized intelligence and is more stimulations. The optical image, transfer characteristics, and suitable for the requirements of future robot intelligence in terms optical responses of the synaptic field-effect transistor (SFET) on of power consumption and application prospects. a PI substrate are shown in Fig. 9b. The threshold voltage V of th Owing to the inherent flexibility of ionic gels, some flexible the SFET moves to the left under the various forces of UV neuromorphic transistors have also been reported. Shim et al. radiation, demonstrating the implementation potential of the fabricated rubber-like synaptic transistors using full rubbery system in terms of artificial optical synapses. The light spike elastomeric materials . Based on the all-rubber materials, this stimulates the transistor as the action of a positive gate voltage. A rubber synaptic transistor has realized synaptic behaviors such as matrix of SFETs has been manufactured and verified to investigate pair-pulse facilitation (PPF), synaptic filtering, and current the uses of SFETs in artificial visual systems (As shown in Fig. 9d). enhancement after excitation. They studied the excitatory postsynaptic current drops from 82.21 µA to 77.15, 44.54, and IC APPLICATIONS OF IONIC-GATE TRANSISTORS (IGTS) 41.8 µA without damage, when the rubber synaptic transistor was stretched by 10% and 30% to 50%, as shown in Fig. 8h. With an Much effort has been made in applying IGT to IC functions, like increase in the pulse width at the frequency of 5 Hz, the synaptic inverter, which is the basic function of advanced integrated device exhibits the functions of sensory memory, short-term circuits . The most commonly used structures of inverters in memory, and long-term memory (Fig. 8i). Figure 8j shows that the recent years are two forms: single-component FET and comple- flexible synaptic transistor is excited at 20 Hz, and the excitatory mentary FET . The single-component FET inverter is consists of postsynaptic current response of 0% and 50% strain is significantly one FET and a resistor. Frisbie et al. reported an EDL-FET using different. They defined the gain strength of A /A and used it to SMS ionic-gel as gate dielectric on a poly (ethylenenaphthalate) 20 1 measure the effect of strain. Obviously, the strength of the gain is (PEN) substrate (Fig. 10a). They adopted whole organic material to also related to the pulse frequency, pulse width, and pulse peak fabricate this low-cost OFET, instead of Au. The switching V is th value. They further used triboelectric nanogenerator as power zero, and completed within 0.5 V, shows a high gain at 7 . supplies to build an integrated soft adaptive neural robot (Fig. 8k). Furthermore, they used IGT to prepare a CMOS inverter with high This soft neurorobotic system receives external pressure stimuli response speed and low operating voltage. Frisbie et al used and produces bending movements, which is suitable for the stenciling technology and traditional photolithography to prepare application of building humanoid robots in the future. SEAS-N ionic-gel gate dielectric layer transistors (Fig. 10c). The Many inorganic nanomaterials or organic semi-conductor fabricated transistor is powered by a power supply sub-3 V and materials have excellent response characteristics to light waves the inverter stage delay is close to 50 µs. More importantly, they npj Flexible Electronics (2021) 13 Published in partnership with Nanjing Tech University D. Wang et al. Fig. 8 The function of artificial synapse realized by IGT. a The working principle of biological nociceptors and their central nervous control. Reproduced with permission . Copyright 2020, Springer Nature. b The weight change in SWARM follows the anti-Hebbian rule. c–e The low- amplitude voltage V = 1 V indicates the normal state, and the high-amplitude voltage indicates damage and can cause sensitive reactions. gs f Sensorimotor platform capable of detecting and associating noxious stimuli. g Scheme diagram of the all flexible ionic-gel gated transistor. Reproduced with permission . Copyright 2019, Elsevier. h Scheme of the SM, STM, and LTM. I Schematic diagram of pulse width adjustment at 5 Hz. j Measured EPSCs of IGTs in response to presynaptic pulses at 20 Hz, under the strain with 0% and 50%. k Schematic illustration of the soft neurorobotics and its programmed operation based on robotic memory decoded signals. claim that through further optimization, especially in terms of the IGZO, and the organic amine ligand molecules therein serve as the ion conductivity of gel, the stage delay can be shortened to the nano-scale tunneling dielectric of the gold nanoparticle core and the order of 1 µs. There is no doubt that this will greatly expand the IGZO channel, which are not present in the evaporated gold potential applications of ionic-gel transistor circuits. nanoparticles. The tunneling dielectric delays the charge transfer In addition to the functional devices of traditional integrated between the Au nanoparticles and IGZO channel, and has a better circuits such as inverters, ionic-gel transistors can also be used as retention effect on the programming/erasing signal. The memory 123–125 organic memory devices due to their reverse ion characteristics . window obtained when the program/erase voltage is 9 V, which is Koo et al. reported nonvolatile transistor memories (NvTMs) devices summarized in Fig. 10g as a function of the colloidal dispersion that depend on the formation of EDL (Shown in Fig. 10e). The IGZO concentration to form the gold nanoparticle layer. It is verified that an film pattern was prepared by spin coating and UV curing. The transfer increase in the coverage area of gold nanoparticles on IGZO is characteristics of Au nanoparticles device are shown in Fig. 10f. They beneficial to storing more charges. More importantly, the preservation separately analyzed the induction of the gate on the IGZO surface characteristics of the memory are improved relative to the through the EDL effect under the write voltage V and the erase nanoparticles prepared by thermal evaporation due to the position prog voltage V . Colloidal gold nanoparticles are used at the interface of of the insulating organic ligand inserted between the IGZO channel era Published in partnership with Nanjing Tech University npj Flexible Electronics (2021) 13 D. Wang et al. Fig. 9 The scheme and functions of a visual synapse. a Schematic diagram of device structure and bionic function. Reproduced with permission . Copyright 2020, Wiley-VCH. b Optical photo of ZnO/SA-based SFET. c Electrical characteristics of the bioinspired flexible ZnO/ SA-based SFET. d Artificial visual memory systems based on SFETs. and the gold nanoparticle sheet. Figure 10h shows the retention have low processing accuracy, and the task load is indeed heavy characteristics of the programming and erasing signals of the IGZO when dealing with high-precision systems. Merging various nonvolatile memory prepared with colloidal gold nanoparticles technologies (R2R combination transfer), maybe more suitable −1 (5 mg ml ). Ion-gel gated IGZO NvTM with multiple programming/ for rapid and low-cost preparation and application of high- erase functions showed stable transfer characteristics in a series of precision IGTs, however further research is needed in terms of cycling tests (Fig. 10i). Obviously, the drive storage device with the material adhesion treatment and precision control. participation of electrolyte has a different working mode and control The good compatibility of ionic-gel materials with organic FETs ability from the previous device. further improves the shortcomings of flexible electronic technol- ogy in terms of dielectric materials. The integration of IGTs with portable power can serve a wide range of IoTs applications such as CONCLUSION health monitoring and industrial sensing. In addition, integrated We have reviewed the advances in the realization of the emerging neuromorphic transistors can also provide technical support for IGTs technology. As mentioned before, the carriers induced by the artificial intelligence and soft robotics. Although the development large EDL-capacitance enable IGTs to be driven at much lower of neuromorphic devices has attracted much attention, current voltages than traditional electronic transistors, which has shown progress is still limited to a few nerve or synaptic devices, and the the potential contribution of IGTs in addressing energy security functions they realize are all around the basic biological nerve challenges. However, there are un-matched challenges for the functions. In addition, the energy consumption is much lower than processing and preparation of the ionic-gel layers for mass-scale traditional transistors, but still higher than the biological level. industrialization. Spin coating, printing, and other methods Finally, the multi-input neuromorphic transistor provides an have the problem of contaminating the semi-conductor layer alternative model for simulating the multiple connections of material of the transistor, while the transfer methods generally biological nerves, the weight modulation between multiple gates npj Flexible Electronics (2021) 13 Published in partnership with Nanjing Tech University D. Wang et al. Fig. 10 Integrated circuit functions formed by ion-gel-gated transistor. a The circuit diagram of a Single-component inverter. Reproduced with permission . Copyright 2010, Wiley-VCH. The inset is a scheme of an all-printed ion-gel-gated P3HT transistor with PEDOT: PSS electrodes. b Characteristics of a single-component inverter. c The circuit scheme of the complementary inverter. The upper inset shows the optical image of this complementary inverter. d The dynamic performance of the complementary inverter. e Schematic cross-section of the memory device with colloidal Au NPs with ligands. The chemical structure of the ligand molecule, oleylamine. f Transfer characteristics for an ionic-gel-gated IGZO NvTM containing Au NPs that were applied with different V and V . g Summary of the voltage drift in the transfer prog era characteristics of devices prepared from colloidal gold dispersions of different concentrations at ± 9 V program/erase voltages. h Retention characteristics of the programmed and erased signals for an ionic-gel-gated IGZO NvTM. i A series of transfer characteristic curves of ionic-gel gated IGZO NvTM with multiple application/erase operations. 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C. 6, adaptation, distribution and reproduction in any medium or format, as long as you give 1980–1987 (2018). appropriate credit to the original author(s) and the source, provide a link to the Creative 136. Lee, H. J. et al. Ultrahigh-mobility and solution-processed inorganic P-channel Commons license, and indicate if changes were made. The images or other third party thin-film transistors based on a transition-metal halide semiconductor. ACS Appl. material in this article are included in the article’s Creative Commons license, unless Mater. Interfaces 11, 40243–40251 (2019). indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly ACKNOWLEDGEMENTS from the copyright holder. To view a copy of this license, visit http://creativecommons. This work was supported by the National Science Foundation of China (NSFC, Grant org/licenses/by/4.0/. No. 61625404, 61874111, 61888102, 62022079), Young Elite Scientists Sponsorship Program by CAST (2018QNRC001), and the Youth Innovation Promotion Association of Chinese Academy of Sciences under Grant No. 2020115. © The Author(s) 2021 npj Flexible Electronics (2021) 13 Published in partnership with Nanjing Tech University http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png npj Flexible Electronics Springer Journals

Recent advanced applications of ion-gel in ionic-gated transistor

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www.nature.com/npjflexelectron REVIEW ARTICLE OPEN Recent advanced applications of ion-gel in ionic-gated transistor 1 1 1 1 1✉ 1✉ Depeng Wang , Shufang Zhao , Ruiyang Yin , Linlin Li , Zheng Lou and Guozhen Shen Diversified regulation of electrons have received much attention to realize a multi-functional transistor, and it is crucial to have a considerable control over the charge carriers in transistors. Ionic gel, as the dielectric material in transistors, facilitates a large capacitance, and high induced-carrier concentrations. This review presents the recent progress in ionic-gated transistors (IGTs) that have good mechanical stability as well as high physical and chemical stability. We first briefly introduce the various applications of IGTs in sensors, neuromorphic transistors, organic transistor circuits, and health detection. Finally, the future perspectives of IGTs are discussed and some possible solutions to the challenges are also proposed. npj Flexible Electronics (2021) 5:13 ; https://doi.org/10.1038/s41528-021-00110-2 INTRODUCTION As a kind of electrolyte, ionic gel not only has good physical/ chemical stability, but also has the characteristics of flexibility, In the past decade, with the rapid development of the Internet of lightweight, and transparency. Therefore, in the field of wearable Things and consumer electronics, human demand for high electronic devices, IGT has demonstrated unquestionable adapt- performance, portability, and wear-ability pushed the upgrade of ability, portability, and functionality. These characteristics have transistor integration density. However, due to the tunneling aroused the research enthusiasm for the application of ionic-gel to effect and other problems, this trend of continuing to use Moore’s IGT. In addition, it is well known that the nervous system controls Law will inevitably slow down. Therefore, in order to develop more the activities of the human body through electrical and chemical promising integrated circuit applications in the post-Moore era, signals. IGT has the potential advantage of being compatible with tremendous efforts are being invested to develop materials and biological signals due to its similar electrical characteristics, structures aspects to improve the performance of field-effect 1–3 structure flexible, and low toxic. IGT has also been studied in the transistors(FETs) . Many inorganic materials with high dielectric 4 5,6 context of neuromorphology, memory, and synaptic devices, and is constant have been extensively studied, like HfO and Al O . 2 2 3 24–27 expected to be applied in the field of artificial intelligence . Unfortunately, the high dielectric constant causes the carrier In this review, we will briefly list material choices, and introduce mobility decreased in the transistor because of Fröhlich polarons . the conventional and emerging applications of IGTs in flexible Meanwhile, a kind of transistor with ionic gate, ionic-gated electronics. Recent articles on the sensing function, neuromorphic, transistors (IGTs) has attracted widespread attention owing to its and IC function of IGTs will be discussed in detail. The schematic large capacitance, high carrier-inducing ability, and low operating device structure of IGT applications is summarized in Fig. 1. Finally, voltage . The function of IGT is similar to that of traditional the future development and problems of IGT sensing and MOSFET, and the channel current of IGT is also controlled by the neuromorphic systems are summarized in Section 6. gate voltage . The difference is that IGT replaces the dielectric material with the electrolyte material. When a negative voltage is applied to the gate electrode, cations in the electrolyte MATERIALS ASPECT OF IONIC-GEL-GATED TRANSISTOR accumulate at the gate/electrolyte interface under the electro- Tremendous efforts have been invested in IGTs, and the dielectric static mechanism, forming a capacitor layer C that is opposite to ge materials consist of polymer electrolytes, polyelectrolytes, ILs, ionic the charge of the gate electrode. At the same time, a capacitor gels, and inorganic nanogranular materials . Polymer electrolytes layer C is also formed at the channel layer/electrolyte interface. es and polyelectrolytes have shown an excellent capacitance Since the layer spacing of the two significant parallel plate performance, whereas, they also exhibit a slow ion migration, capacitors is ~1 nm, the capacitance effect is significantly greater which limits the application of IGTs in high-speed electronic 8,10 20,29 than that of general insulating layer materials . This type of IGT devices . ILs have good gating functions and faster responses; based on an electric double layer is also named electric double however, their intrinsic liquid state hinders the use of ILs in layer transistor (EDLT). In addition, the channel layer material may practical devices . Therefore, as an alternative to ILs, ionic gels also allow electrolyte ion implantation, thereby causing electro- solve the problem by combining polymer mechanical compat- chemical doping . Due to the typical EDL effect, the large surface ibility and swift switching response. An ionic gel is a semi-solid capacitance allows the IGT to operate well at a relatively low compound that retains charge owing to the insertion of an IL into 12 13 operating voltage and maintain good switching performance . the polymer during the drying stage. The colloidal particles in the Nowadays, the research of ion-gated transistors is booming in mixture prevent it from curing completely, but are in the form of a various disciplines, and the ion materials include electrolyte gel. Due to their excellent chemical stability and transmittance, 14–16 7,17–19 solutions and ionic liquids with liquid fluidity; as well as these materials are also widely used as battery electrolyte 20–22 23 ions/proton-conducting polymers , and inorganic materials . materials. As a result, they are also named as electrolyte gated State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences & Center of Materials Science and Optoelectronic Engineering, University of Chinese Academy of Sciences, Beijing, China. email: zlou@semi.ac.cn; gzshen@semi.ac.cn Published in partnership with Nanjing Tech University 1234567890():,; D. Wang et al. Fig. 1 Prospective applications of ionic gel-based electronics. a An artificial afferent nerve made of pressure sensors, an organic ring oscillator, and a synaptic transistor. Reproduced with permission . Copyright 2018, Elsevier. b Optical photo of Ionic-gel transistor. Reproduced with permission . Copyright 2020, Wiley-VCH. c Schematic device structure and demonstration of multiple presynaptic pulses induced synaptic transistor responses . Copyright 2019, Elsevier. d A real optical image of the OECT iontronic pressure sensor. Reproduced 84 104 with permission . Copyright 2021, IEEE. e The physical schematic diagram of the transistor-breathalyzer. Reproduced with permission . Copyright 2016, Springer Nature. f Schematics of the ionic-gel-integrated optical-transistor modulator. Reproduced with permission . Copyright 2017, American Chemical Society. g A diagram of an IGT device using a nanowire as a stress-sensing material. Reproduced with permission . Copyright 2019, Elsevier. h Schematic and physical optical photo of a single device used to prepare the inverter. Reproduced 121 129 with permission . Copyright 2010, Wiley-VCH. i A picture of completed IGZO-EGTFT device arrays. Reproduced with permission . Copyright 2015, Springer Nature. j Optical picture of a single inverter and the electrode of the substrate before preparation. Reproduced with permission . Copyright 2013, American Chemical Society. transistors (EGT). In the following section, we describe ILs and the Figure 2 shows the structures of cations and anions conventionally polymer grid used in IGTs. used in IL, and the various properties of IL are determined by the ILs are generally known as room-temperature molten salt. combination of different anions and oxygen ions. Generally, most Owing to the low vapor pressure, high ionic conductivity, and of the studies on the potential applications of ionic gel have good temperature (up to 350 °C), and chemical stabilities, ILs are advanced toward electrical and electrochemical devices, including 42 43 32,39,44–46 47,48 more suitable for polymer-in-salt systems. In 1992, since Wilkes sensors , super capacitors , batteries , actuators , 41,49 50,51 52 et al. prepared 1-ethyl-3-methylimidazole tetrafluoroborate electrochromic devices , solar batteries , FET , and optimi- ([EMIM][BF ]) with good stability in the air, the research on ionic zation of thermoelectric materials using IGTs . The manufacturing liquids has flourished . Ionic gels have a wide selection of method, materials, and performance of the reported nanoscale 31–34 anions , so ionic gels are usually classified according to the IGTs are compared in Table 1. 35,36 53,54 55,56 types of cations, including quaternary ammonium , phospho- ILs, such as [EMIM][TFSI] , [EMIM][TFSA] , and [BMIM] 37 38 39 57 nium , Imidazole , and pyridine , etc. Imidazole-based ionic [PF ] , have been reported to be widely used and have good liquids are widely used in emerging electronic devices due to their performance. Cho et al. used a chemically stable ionic liquid, 40,41 generally low melting point and many optional substituents . [EMIM] [TFSI], and photocurable mixture material to prepare npj Flexible Electronics (2021) 13 Published in partnership with Nanjing Tech University 1234567890():,; D. Wang et al. Fig. 2 The type of ionic-gel structure. Three different types of ionic-gel composition structure and main ionic-gel anion and cation types. patterned graphene transistors under ultraviolet (UV) radiation . (SOS), as the gel network and dissolved it with 1-butyl-3- The presence of ionic liquid not only maintains stability but also methylimidazolium hexafluorophosphate ([BMIM][PF ]) into exhibits excellent surface capacitance and polarization response methylene chloride (Shown in Fig. 3a). Tri-block polymers such speed. Besides being extremely sensitive to external bending and as SOS have special structural and chemical properties; the entire deformation (GF~389), the system can also implement a large- structure is divided into two parts: can be dissolved (the middle) area, highly integrated array on a flexible substrate for motion and cannot be dissolved (on two sides) in IL. Thus, they are detection. collectively referred to as ABA tri-block polymers. This character- However, for in vivo applications, these ILs have not been istic makes ABA tri-block co-polymers suitable for the preparation proven to be safe. Considering these conditions, Tang et al. of ionic gels. This easily leads to the design and preparation of an reported a biocompatible IGT composed of poly(ε-decalactone)-b- ionic gel that can be utilized to construct various transistors with poly(DLlactide)-b-poly(ε-decalactone) (DLD) and [P ][TFSI] . The different requirements. In addition to SOS, other ABA tri-block co- minimum capacitance of the 20% ionic gel could reach polymers, such as SMS, are also widely used in the preparation of −2 61 approximately 2 μFcm , ensuring that the system could be ionic gels . operated under a low voltage. The prepared IGT using P3HT as the According to application scenarios and preparation processes, channel layer showed good characteristics for printed electronic many chain polymers are also used to prepare IGTs. A transistor 2 −1 −1 5 applications: 1 cm V s mobility, an I /I ratio of 10 , and low that uses P (VDF-HFP) and a gel gate dielectric layer has the on off hysteresis. Besides, it showed promising hydrolytic degradation. characteristics of easy transfer. There is a transistor using P (VDF- More recently, Kim et al. reported an ionic gel material made of HFP) that can easily transfer the gate insulating layer through a choline and malic acid as raw materials, and the obtained dielectric “cut and paste” operation . Except good solvent compatibility, layer has good biocompatibility and biodegradability . the gel material not only has high tension and structural stability, The flexibility of material selection makes it possible to choose but also exhibits similar properties to triblock polymers in some different polymers to prepare ionic-gel gate transistors. For most specific aspects. For example, PVA, (P(VDF-HFP)) can also be used IGTs, polymer monomers are embedded into ionic solutions to form gels with [EMIM][TFSA] ILs. Compared with PS-PMMA-PS composed of polymer molecular segments, and the commonly gel, their capacitance values decrease with increasing frequency. used materials are mainly self-assembled tri-block and di-block co- The results (Fig. 3b) show that the capacitance value of P(VDF- polymers, such as poly(styrene-block-ethylene oxideblock-styrene) HFP) gel is approximately 1 μF lower than that of SMS gel at a low 53,60 (SOS) , poly(ε-decalactone)-b-poly(DL-lactide)-b-poly(ε-deca- frequency (10 Hz), however it is slightly higher than that of the 37 56 lactone) (DLD) , and poly(styrene-b-methylmethacrylate-b-styr- SMS gel at a high frequency (100 kHz) . This proves that the 61,62 ene) (SMS) . dielectric functions of the two EDL dielectric materials are The first ionic-liquid-based gel used in an organic film transistor comparable. To achieve good implantability of flexible electronic was produced by Lee et al. They chose a tri-block co-polymer, devices, biocompatibility must also be ensured for the materials Published in partnership with Nanjing Tech University npj Flexible Electronics (2021) 13 D. Wang et al. Table 1. Parameters comparison of Ionic-gel Gate Transistor. 2 −1 −1 μ (cm V s )I ratio SS (mV) Vth (V) Ionic liquid Polymer Channel Method Ref F On/Off 5 55 20 10 158 −0.13 V [EMI] [TFSA] P(VDF-HFP) In O Cut and stick 2 3 4 53 20 10 110 −0.2 V [EMIM] [TFSI] PS-PMMA-PS PQT-12 Cut and stick 5 53 20 10 270 −0.5 V [EMIM] [TFSI] PS-PEO-PS PQT-12 Cut and stick 5 67 20 10 nr 0.12 V Proton Wheat flour IZO Drop 5 25 210 670 0 V [EMIM][TFSI] P(VDF-HFP) IWO Cut and stick 5 57 nr 10 nr 12 V [BMIM][PF ] PS-PEO-PS P3HT Cut and stick 5 53 20 10 280 −0.7 V [BMIM][PF ] PS-PMMA-PS P3HT Cut and stick 5 54 2.06 10 nr −1 V [EMI] [TFSI] PS-PMMA-PS ZnO Spin-coating 5 60 nr 10 73 −0.8 V [EMIM][TFSI] P(VDF-HFP) P3HT EHD 6 132 nr 10 nr −1V [P ] [TFSI] SOS-N P3HT Photo-pattern 14 3 5 133 7.8 10 135 1 V Proton Chitosan ZnO Spin-coating 5 134 11 10 90–110 0.65 V [EMIM][TFSI] PS−PMMA SWCNT Printing 2 135 1.8 10 nr 1.2 V [EMIM][TFSI] PS-PMMA-PS WO Cut and stick 5 122 nr 10 nr 0.5 V [EMI] [TFSI] SEAS-N ZnO Screen-printed 2 136 nr 10 nr −0.2 V [EMIM][TFSI] P(VDF-HFP) CuSCN Cut and stick 5 56 2.3 10 nr 0 V [EMI][TFSA] P(VDF-HFP) P3HT Cut and stick used in co-polymers. Therefore, some researchers use biocompa- stretchability, have attracted much attention in the field of 59 63,64 tible materials, such as polysaccharides , chitosan , and wearable electronics. Not only can it be used as a mechanical 65 66 proteins instead of cross-linked polymers . As mentioned sensor to sense pressure, but it can also be used as a proximity before, Kim et al. proposed an ionic gel made of Lewandan sensor or smart sensor in sensing, thereby improving signal polysaccharide and choline-based water-soluble IL cast by a processing efficiency and reducing system power consumption. In the field of optoelectronics, phototransistors and optical simple solution method (shown in Fig. 3c). This dielectric has the characteristics of good biocompatibility, biodegradability, high modulators take full advantage of IGTs . Phototransistors that transparency, free stranding, and solid state . Besides, Yang et al. receive optical signals and convert them into electrical signals can dissolved wheat flour and acetic acid in water and prepared a detect light in various bands, therefore, enabling applications of wheat flour electrolyte membrane by a drop-casting method. The night vision, three-dimensional object recognition, health mon- 71,72 itoring, and optical communications . Owing to their three- resulting IZO-based thin-film transistor exhibited a switching ratio 5 2 −1 −1 terminal configurations, phototransistors can effectively reduce of 10 , and have excellent saturation mobility (15 cm V s ). Accordingly, they realized the simulation of synaptic functions, noise and amplify electrical signals. Meanwhile, ionic gel such as excitatory postsynaptic current (EPSC) and paired-pulse electrolytes are introduced due to its high capacitance and 67 73 facilitation (PPF) . transparency to solve the problem of high operating voltages . Furthermore, some mixed copolymer has more application Thus, the ionic gel has been used to achieve low operation voltage and high carrier concentration, while the low mobility of compatibility and enables realize photo-crosslinking, specific recognition, acid-base resistance, hydrophilic, and hydrophobic the ions and formation of the layered effect can help realize control, etc. by controlling the internal components. A UV-curable optical modulation. ionic gel can easily realize the preparation of ultra-tiny IGTs .In In order to obtain a strong electric field at the interface between addition, Liu et al. also prepared a fully stretchable polarized the gate and the channel, EDLT is a promising choice, which can transistor by controlling the PFPE content in the PFPE-DMA gate generate high carrier density at low pressure. Many studies have used IGTs to achieve electrical and optical multi-modal control of dielectric layer. They demonstrated a skin-like display that is the channel layer, and further applied to neuromorphological formed by integrating stretchable TFT array and stretchable 69 55 organic light-emitting electrochemical cells (OLECs) (Fig. 3d) . research . Much more interesting, Ionic-gel-based photonic Through the preparation method compatible with traditional devices with simple processing and high gate-capacitance, MEMS technology, the application of IGTs in fields, like health dynamically regulate the graphene chemical potential (Fermi level) (Fig. 4) . The charge density is electrically tuned at the ionic monitoring and logic operation, will be further expanded. gel–graphene interface, and the chemical potential of graphene is changed to control its intensity of light absorption. At 1.55 μm SENSING APPLICATIONS OF IONIC-GATE TRANSISTORS (IGTS) wavelength, the transverse-magnetic (TM) mode is increased from In 1970, Bergveld et al. discovered that after removing the gate of −10.8 dB to −8.8 dB, and the transverse electric (TE) mode is the FET and immersing it in a liquid, the ions in the insulating layer reduced from −9.0 dB to −12.1 dB. Depending on these data, the would affect the conductivity of the channel layer. Therefore, the waveguide sample with an ionic-gel gate dielectric can be used as original IGT was used as a sensitive ion sensor and used in a broadband TM transmission polarizer. In addition, the optical physiological detection. Nowadays, IGT has become a hot research behavior of the ion-gated modulator clearly shows hysteresis due topic among multidisciplinary areas including electronics, neuro- to the slow polarization response time. morphology, biology, and engineering. Various exciting applica- Further, a research study has recently demonstrated that tions of IGT have been developed continuously. For example, due photoluminescence (PL) can be reversibly tuned by EDLTs up to to the excellent light transmittance of ionic gels, IGT also has 1–2 orders of magnitude at a low gate voltage (Fig. 4c, d) . The broad application prospects in the field of optics . The properties density of electron traps via the interaction of trap with oxygen of IGT, such as stable structure, stable chemical properties, and ion at V < 0 is reduced, owing to the transverse gate-electric-field npj Flexible Electronics (2021) 13 Published in partnership with Nanjing Tech University D. Wang et al. Fig. 3 Four different types of polymer materials used for an ionic-gel transistor. a SOS, a schematic diagram of the structure of a triblock (ABA) polymer. Reproduced with permission . Copyright 2014, American Chemical Society. Capacitance-frequency dependence of four different ionic-gel. Reproduced with permission . Copyright 2009, American Chemical Society. b Comparison of the frequency dependence curves of the maximum capacitance of SMS and P (VDF-HFP) materials. The inset is a schematic diagram of the molecular structure and device structure of P(VDF-HFP). Reproduced with permission . Copyright 2012, Wiley-VCH. c The molecular structure of Lewandan, the synthetic ionic liquid based on choline and malate ions, and the chemical structure and schematic diagram of the expected synthesized LSE membrane. On the below side is the Nyquist chart of LSE. Reproduced with permission . Copyright 2020, Wiley-VCH. d The schematic diagram shows the material design used to pattern stretchable transistors on elastomer dielectrics by inkjet printing. The figure on the right shows the dielectric constant of the PFPE-DMA film subjected to uniaxial strain. Reproduced with permission . Copyright 2020, Springer Nature. generated by ionic gels, leading to lower probabilities of non- coefficient of up to 10 . Resulting from merging single atomic layer radiative recombination and photogenerated carriers with nanoscience with ultrafast optoelectronics, this heterogeneous increased mobility, and accordingly, the PL density is enhanced. graphene microcavity can further enhance our understanding of The function of reversible control of perovskite luminescence can dynamic frequency combs and ultrafast optoelectronics. be advantageous to the potential applications of photons in The FET-type pressure device is one of the basic component of perovskite materials. electronic skin devices that achieve advanced sensing perfor- Moreover, optical frequency combs are the basis of modern mance, including multi-parameter tracking, high sensitivity, high 79–82 frequency measurement, astronomical observation, precision spec- resolution, and low crosstalk signaling .Inanearlier study, 76,77 troscopy, ultrafast optics, and quantum information . Yao et al. ionic-gel-based FETs have been observed to function by pioneered the application of IGTs in graphene-based tunable optical inducing mobile charges in the semi-conductor channel and frequency combs (Fig. 4e) . By coupling the ionic-gel gate-tunable are utilized to fabricate pressure sensors with low power 58,83 light guide to the silicon nitride photonic microresonator, the consumption and high transparency . For example, Khadem- second- and higher-order dispersion can be modulated by tuning hosseini et al. demonstrate an IGT-based pressure sensor .The the Fermi level. They have successfully demonstrated a double-layer dielectric layer is fabricated by microstructured ionic hydrogel, IGT that can adjust the Fermi level of graphene to 0.45–0.65 eV, showninFig. 5a–d. As external pressure is applied to the top of besides, the double-layer IGT is able to retain a cavity mass the transistor, the microstructure hydrogel deforms and changes Published in partnership with Nanjing Tech University npj Flexible Electronics (2021) 13 D. Wang et al. Fig. 4 Applications of phototransistor formed by ionic-gel. a Schematics of ionic-gel-integrated optical-transistor modulator. Reproduced with permission . Copyright 2017, American Chemical Society. b Thepolar imagemeasuredunder thepresenceof the ionic-geland theoptical transmission of phototransistor according to the top gate voltage at a wavelength of 1.55 μm. c Scheme of the IGT photoluminescence devices with a lead-halide perovskite single-crystal integrated with an ionic gel. Reproduced with permission . Copyright 2019, Elsevier. d The relationship between V -induced modulation of PL intensity and the device geometry (the crystal thickness and photoexcitation area) of perovskite EDLTs. e Schematic architecture of the graphene-based microresonator. The gray object showed is the silicon nitride. Reproduced with permission . Copyright 2018, Springer Nature. f Electric field distribution of the graphene–nitride heterogeneous waveguide, with a Si N cross-section of 1.2 × 0.8 μm .The 3 4 distance between the Si N waveguide and the graphene layer is 100 nm. The graphene and the top-gate probe are separated by 1 μmwiththe 3 4 interlayer ionic-gel capacitor. In this structure, transverse electric (TE) mode is applied. g Optical micrographs show the bus waveguide (red arrows), ring resonator, and Au/Ti metallized patterns. An etched window is designed to ensure both graphene–light interaction and reduced propagation loss. Here the graphene-covered area is marked by the gray dashed box; the etched window label refers to the whole horizontal area between the two central lines. CW continuous wave, MI modulated intensity. Scale bar, 100 μm. h Soliton state with crystal-like defects modulated by heterostructure transistor. The middle and right graphs are the corresponding spectral measurements and the frequency-resolved second harmonic autocorrelation graphs of soliton pulses. The gray curve here shows the real-time autocorrelation intensity curve. the capacitance of the hydrogel/gate interface, as a result, ionic-gel gate capacitance under the double capacitance effect is −2 affecting the number of ions in the channel. Due to the large (7.9 μFcm ), which ensures that the graphene FET works at outstanding amplify capability of the organic electrochemistry a low gate voltage of less than 2 V. Between the source and drain transistor (OECT), the pressure sensor can detect pressure as low electrodes, the interdigitated electrode at the bottom is in as 20 Pa in this manner. Furthermore, the IGT operates at a lower contact with the graphene backplate under external pressure, voltage (0–1 V) due to the high capacitance of the EDL effect, resulting in a change in resistance (shown in Fig. 5e). It exhibited −1 and the entire power consume of sensing device used is only high-pressure sensitivity (0.12 kPa ) and outstanding mechanical 10–1000 μW. Based on these two advantages, ionic gel-based durability (more than 2500 cycles). With the help of this structure, pressure sensors are also competitive in wearable and implan- the fabrication of common-gate array devices can be easily table sensor applications. realized. The pressures on the 4 × 4 GFET matrices can be mapped Cho et al. proposed a structure for the fabrication of pressure spatially (Fig. 5f). sensor matrices with a simple process and excellent perfor- However, the use of transistors as direct pressure sensors is mance . The ionic-gel connects the drain and the gate, and the limited by structural design difficulties and poor stability, npj Flexible Electronics (2021) 13 Published in partnership with Nanjing Tech University D. Wang et al. Fig. 5 Piezo electronics sensor made of ionic-gel transistor. a Schematic diagram of the device structure of an OECT. b quivalent circuits in an OECT and schematic diagram of the proposed microstructured hydrogel-gated OECT iontronic pressure sensor. The deformation of the hydrogel determines the number of ions delivered into the channel. c)V and V change after application of external pressure on the gate g sol electrode. d SEM image of gelatin methacryloyl (GelMA) hydrogel with pyramidical microstructures on the surface; Lower picture is a real optical image of the OECT iontronic pressure sensor. Reproduced with permission . Copyright 2021, IEEE. e Schematic diagram of a GFET pressure sensor fabricated with interdigitated source electrodes. f Spatial pressure graph of GFET pressure sensor matrix. g Pressure sensing characteristics of GFET pressure sensor mounted on PDMS rubber substrate. The illustration shows the GFET matrix making angular contact with human hands or table tennis. Reproduced with permission . Copyright 2014, Wiley-VCH. h Mechanism of GT strain sensing based on piezoelectric potential gating. i Sensitivity and stability characteristics of piezoelectric gated GT strain sensor. Reproduced with permission . Copyright 2015, Wiley-VCH. g Process for manufacturing multi-level non-volatile memory array for piezoelectric potential programming. k Real-time programming and erasing steps applied to piezoelectric potential programming memory. Reproduced with permission . Copyright 2016, American Chemical Society. prompting researchers to turn to other methods. The piezoelectric Ionic-gel electrolyte materials are extensively applied in piezo- effect is defined as the electric polarization of the piezoelectric tronic transistors owing to their large capacitance that enables 91,92 material that originates from materials that lack inversion low-voltage transistor operation . The long-range polarization symmetry, caused by the action of external mechanical forces; characteristics of ions in the ionic gel allow the placement of the piezoelectric materials convert random mechanical energy into gate electrode and channel to be coplanar. Sun et al. described a 85,86 electric energy (piezopotential power) . The piezotronic piezopotential-powered active strain sensor matrix array combin- transistor, utilizing inner-crystal piezopotential to harvests bio- ing piezoelectric nanogenerators (PENGs) and coplanar-gate 87,88 chemical and biomechanical energy , while achieving self- graphene transistors based on P(VDF-TrFE), as shown in Fig. 5h. powered active digital data processing capacities, such as pressure The output pressure sensor signal is maintained at the corre- 89,90 arrays with high resolution and human-machine interaction . sponding value based on the applied strain. The ionic-gel Published in partnership with Nanjing Tech University npj Flexible Electronics (2021) 13 D. Wang et al. dielectrics can effectively couple the piezoelectric potential to the monitoring. Unlike previous studies that have fabricated ultra-thin graphene channel, thereby maintaining the output sensor signal inorganic semi-conductors or have designed “wavy” structures to 109,110 at the corresponding stress value. As a result, excellent sensitivity achieve flexibility , intrinsically stretchable conductive poly- (GF = 389), ultra-high detection limit (0.008%), and high mechan- mers have become a growing trend. Kim et al. synthesized ical durability (> 3000 cycles) are obtained (Fig. 5i). They further biocompatible, biodegradable, highly transparent, free-standing, developed a piezopotential-programmed non-volatile memory and solid-state electrolytes consisting of levan polysaccharide and array by integrating ionic-gel-gated FETs and PENGs . The choline-based water-soluble IL (as shown in Fig. 6e). Levan storage device is programmed by applying external pressure to polysaccharides function as free-standing and solid-state poly- the PENG, and by grounding the top electrode of the PENG. In meric matrices that are translucent, versatile, and water-soluble, contrast, it is easy to flash the stored data by grounding the upper while choline-type water-soluble IL reacts with malic acid, utilizing electrode of the PENG (Fig. 5g). Under various external bending choline as the cation and malic acid as the anion . Biodegrad- strains, multi-level data storage of more than 3 orders of able electrolyte-based organic transistors exhibit satisfactory magnitude is achieved with a good memory efficiency, including performance, such as low operating voltage (V = −1.0 V and ds 3 3 a high programming/erasing current ratio (> 10 ), 2-bit multi-level V = −2.0 V), good stability, and high on/off ratio (over 10 ). gs data storage (over 4 levels), efficiency reliability over 100 cycles, Organic transistors were incorporated with bio-integrated instru- and reliable data retention over 3000 s. ments, such as electrocardiogram (ECG) recordings on human skin With the advent of the Internet of Things (IoTs), non-invasive and rat heart with a better signal-to-noise ratio (shown in Fig. 6f). measurements and real-time health monitoring have become A thread-based transistor (TBT) realized by electrolyte gating of increasingly important in modern medicine, particularly to address semiconducting carbon nanotube networks on linen threads was the urgent needs of the aging population . In the process of proposed by Sameer et al. The thread has an interchangeable medical monitoring and diagnosis, equipment with high-quality module to fit the biological tissue required, can be minimally signals and long-term stability is essential . In addition, soft invasive (for example, sutures), and enables the provision of three- devices with good physiological comfort and biological compat- dimensional tissues or organs with a natural interface . The ibility can effectively minimize the harm to the human body and electrolyte is composed of colloidally scattered nanoparticles of 2,95–99 discomfort of wearing . IGTs have the key advantages of low silica and IL gel (a form of an ionic gel) that enables CNT 100,101 threshold voltages, high amplification, and biocompatibility transistors to be electrostatically gated. To achieve a multiplexed that ensure an accurate diagnosis of physical illness. Therefore, diagnostic system based on complete threads, they linked TBTs IGTs that are intrinsically stretchable, adequately meet the needs with thread-based electrochemical sensors (TBEs). As seen in of physiological monitoring. Fig. 6g, completely threaded platforms are thin, extremely In conventional methods, medical monitoring and diagnosis durable, and compatible, allowing them to be worn directly on mainly rely on physiological signals (blood pressure, heart rate, the skin without any silicone substrate or without using a skin and respiratory rate) and the real-time concentration of biochem- needle. As mentioned above, most ionic gels consisting of + + ical molecules (Na ,K , glucose). Transistors can achieve an polymers and ILs are not proven to be biocompatible and effective amplification of these data and thereby effectively biodegradable, and some of them are even toxic to humans. improve the sensitivity of medical sensors. For example, Someya Therefore, it is necessary to apply natural biomaterials such as et al. fabricated a biocompatible electrode by integrating a cellulose or hydrogels with non-toxic synthesis procedures. gelatinous composite into transistor amplifiers to realize a direct detection of pericardial electrocardiography (Fig. 6a). One-pot NEUROMORPHOLOGY APPLICATIONS OF IONIC-GATE fabrication of a conductive gel that is responsive to glucose TRANSISTORS (IGTS) without enzyme, is integrated into OECTs to detect the patient’s glucose level clinically . Alcohol dehydrogenase (ADH) and its Another emerging application of IGT is neuromorphic devices, nicotinamide adenine dinucleotide (NAD ) cofactor are readily particularly synaptic transistors. In order to simulate the human immobilized on the OECT to detect ethanol in the range of 0.01 % brain, much efforts have been invested in the ability of parallel to 0.2 % blood alcohol content (Fig. 6b) . processing of data and weight modulation through algorithms 112,113 Although gel-based transistors have realized various applications and software on the existing computer hardware . However, in the medical field, there is still a lot of scope for improvement. The the human brain performs better than traditional computers while first problem that needs to be solved in this field is the requirement processing real-time sensory data such as images, videos, sounds, of the high-level accuracy of physiological signals to provide an and navigation. In addition, compared with the high energy accurate diagnosis, and this is dependent on the SNR (signal-to- consumption of supercomputers, low energy consumption is also 105,106 114 noise ratio) . To obtain a high SNR, a preamplifier may be a major advantage of the biological brain . Synapse transistor is integrated in close proximity to the sensing electrode to reduce the imitated as a synapse, where two neurons are connected influence of the wiring noise, thereby realizing active elements with functionally, and they are also the key parts of signal transmission, 102,107 local signal amplifications . The structure of OECT without the as shown in Fig. 7a. The human brain is a complex network 11 15 gate dielectric layer makes direct electrolytic operation possible, composed of nearly 10 neurons and 10 synapses that are allowing the conversion amplifier to be directly loaded at the highly interconnected, massively parallel, and structurally vari- location of the target, thereby largely reducing the ratio of noise. able . Synaptic plasticity, as a functional feature of nerve cell Someya et al. designed an OECT-based electrophysiological interconnection, gives biology the ability to learn and remember. transducer with a high SNR (24 dB). They successfully solved the The appearance of hysteresis of EDLT devices limits its switching issue of the inability of transistors to operate for a long time due to speed, and is not suitable as a conventional transistor, but is more electrolyte drying, by integrating an ultra-thin OECT with a suitable as a functional device that mimics synaptic transmission. nonvolatile gel electrolyte, as shown in Fig. 6c. OECT made of thin The bionic simulation of brain nerve structure provides many gels can adhere well to the skin and effectively monitor the heart feasible research scopes. Based on the phenomena of dendrites signal. It demonstrated stable performance during long-term receiving multi-synaptic signals, Wan et al. controlled the continuous monitoring (for example, within 3 h) and multiple reuses information transmission of neuromorphic transistors through in tests, lasting more than a week (Fig. 6d). multiple in-plane gates (Fig. 7b). Owing to the different Another major achievement is the technological advances in distances between the source and drain electrodes and the flexible and biocompatible materials that not only allow patients multiple gates, the spatial position of each gate on the plane is to have a better wearing experience, but also facilitates real-time defined by different coordinates. Obviously, the spatial coordinate npj Flexible Electronics (2021) 13 Published in partnership with Nanjing Tech University D. Wang et al. Fig. 6 Bio-sensors made of ionic-gel transistor. a Circuit diagram of a unit of an organic amplifier with a conductive gel for an in vivo electrocardiograph and Pictures of an ultra-flexible circuit on a rat’s heart. Reproduced with permission . Copyright 2016, Springer Nature. b Enzymatic reaction of ethanol and ADH in an electrolyte. Reproduced with permission . Copyright 2016, Springer Nature. c Schematic diagram of OECT in a nonvolatile gel electrolyte containing glycerol and ions. d OECT-based sensors measured ECG data at different electrolytes (0.7 m NaCl, hydrogels, and glycerin gels) Reproduced with permission . Copyright 2019, Wiley-VCH. e Schematic of a biodegradable organic transistor. f Standard equipment and biodegradable organic transistors record the ECG signals from the human skin. Reproduced with permission . Copyright 2020, Wiley-VCH. g Schematic diagram of a thread-based integrated transistor system for advanced sensing of bio-related ions. Reproduced with permission . Copyright 2019, American Chemical Society. of the gate closer to the channel layer contributes more as the dielectric materials to prepare neuromorphic memtransistor significantly to the weight of the transistor. The mapping of elements .Bydefining different gate voltages as different distance and EPSC contribution is shown in Fig. 7c. They present harmful inputs, they successfully used IGTs to simulate the EPSC current diagrams for different stimulation directions and the modulation of the salient weight of pain receptors. Similar to I/O relationship between the rate coding scheme and neurons in bio-nociceptors, the satellite threshold adjusting receptors (STARs) different directions. They further verified the influence of stimuli can adjust the transfer characteristic curve of the device according directivity on multi-gate neuromorphic transistors (Fig. 7d). to the intensity of the harmful stimulus (V ) to make it more gs Real-time control can be achieved through a programmable sensitive and avoid repeated damage. When V is higher than the gs circuit design. John et al. also utilized P(VDF-HFP) and [EMIM][TFSI] threshold voltage and the duration increases, significant transistor Published in partnership with Nanjing Tech University npj Flexible Electronics (2021) 13 D. Wang et al. Fig. 7 The scheme and functions of multi-synapse. a Schematic illustration of a Biological network. Reproduced with permission . Copyright 2020, Springer Nature. b Schematic picture of the visual system and 2D MoS neuromorphic transistor with a grid of 3 × 3 coplanar- gate arrays. Reproduced with permission . Copyright 2018, American Chemical Society. c, d 2D EPSCs surface summarized as a function of spatial coordinate, and each gate has a different coordinate. EPSC responses driven by dual spike inputs and its different orientation relationships. 10,117 drift occurs, as shown in Fig. 8a–f. After a period of time or of different wavelengths . Further, inspired by biological visual application of the recovery voltage, the threshold characteristic of and nervous systems, Sun et al. demonstrated a flexible, dual- IGT can be restored to its original level, as shown in Fig. 8e. The modulation, tunable synaptic FET based on ZnO nanowires as the robot can change its sensitivity after receiving harmful stimuli and channel, and sodium alginate as the gate . IGT is combined with can realize self-healing and functional recovery after being ZnO that has an excellent UV light response, it can also output a damaged. The decentralized intelligence they proposed is higher current than the normal state after multiple UV light different from traditional centralized intelligence and is more stimulations. The optical image, transfer characteristics, and suitable for the requirements of future robot intelligence in terms optical responses of the synaptic field-effect transistor (SFET) on of power consumption and application prospects. a PI substrate are shown in Fig. 9b. The threshold voltage V of th Owing to the inherent flexibility of ionic gels, some flexible the SFET moves to the left under the various forces of UV neuromorphic transistors have also been reported. Shim et al. radiation, demonstrating the implementation potential of the fabricated rubber-like synaptic transistors using full rubbery system in terms of artificial optical synapses. The light spike elastomeric materials . Based on the all-rubber materials, this stimulates the transistor as the action of a positive gate voltage. A rubber synaptic transistor has realized synaptic behaviors such as matrix of SFETs has been manufactured and verified to investigate pair-pulse facilitation (PPF), synaptic filtering, and current the uses of SFETs in artificial visual systems (As shown in Fig. 9d). enhancement after excitation. They studied the excitatory postsynaptic current drops from 82.21 µA to 77.15, 44.54, and IC APPLICATIONS OF IONIC-GATE TRANSISTORS (IGTS) 41.8 µA without damage, when the rubber synaptic transistor was stretched by 10% and 30% to 50%, as shown in Fig. 8h. With an Much effort has been made in applying IGT to IC functions, like increase in the pulse width at the frequency of 5 Hz, the synaptic inverter, which is the basic function of advanced integrated device exhibits the functions of sensory memory, short-term circuits . The most commonly used structures of inverters in memory, and long-term memory (Fig. 8i). Figure 8j shows that the recent years are two forms: single-component FET and comple- flexible synaptic transistor is excited at 20 Hz, and the excitatory mentary FET . The single-component FET inverter is consists of postsynaptic current response of 0% and 50% strain is significantly one FET and a resistor. Frisbie et al. reported an EDL-FET using different. They defined the gain strength of A /A and used it to SMS ionic-gel as gate dielectric on a poly (ethylenenaphthalate) 20 1 measure the effect of strain. Obviously, the strength of the gain is (PEN) substrate (Fig. 10a). They adopted whole organic material to also related to the pulse frequency, pulse width, and pulse peak fabricate this low-cost OFET, instead of Au. The switching V is th value. They further used triboelectric nanogenerator as power zero, and completed within 0.5 V, shows a high gain at 7 . supplies to build an integrated soft adaptive neural robot (Fig. 8k). Furthermore, they used IGT to prepare a CMOS inverter with high This soft neurorobotic system receives external pressure stimuli response speed and low operating voltage. Frisbie et al used and produces bending movements, which is suitable for the stenciling technology and traditional photolithography to prepare application of building humanoid robots in the future. SEAS-N ionic-gel gate dielectric layer transistors (Fig. 10c). The Many inorganic nanomaterials or organic semi-conductor fabricated transistor is powered by a power supply sub-3 V and materials have excellent response characteristics to light waves the inverter stage delay is close to 50 µs. More importantly, they npj Flexible Electronics (2021) 13 Published in partnership with Nanjing Tech University D. Wang et al. Fig. 8 The function of artificial synapse realized by IGT. a The working principle of biological nociceptors and their central nervous control. Reproduced with permission . Copyright 2020, Springer Nature. b The weight change in SWARM follows the anti-Hebbian rule. c–e The low- amplitude voltage V = 1 V indicates the normal state, and the high-amplitude voltage indicates damage and can cause sensitive reactions. gs f Sensorimotor platform capable of detecting and associating noxious stimuli. g Scheme diagram of the all flexible ionic-gel gated transistor. Reproduced with permission . Copyright 2019, Elsevier. h Scheme of the SM, STM, and LTM. I Schematic diagram of pulse width adjustment at 5 Hz. j Measured EPSCs of IGTs in response to presynaptic pulses at 20 Hz, under the strain with 0% and 50%. k Schematic illustration of the soft neurorobotics and its programmed operation based on robotic memory decoded signals. claim that through further optimization, especially in terms of the IGZO, and the organic amine ligand molecules therein serve as the ion conductivity of gel, the stage delay can be shortened to the nano-scale tunneling dielectric of the gold nanoparticle core and the order of 1 µs. There is no doubt that this will greatly expand the IGZO channel, which are not present in the evaporated gold potential applications of ionic-gel transistor circuits. nanoparticles. The tunneling dielectric delays the charge transfer In addition to the functional devices of traditional integrated between the Au nanoparticles and IGZO channel, and has a better circuits such as inverters, ionic-gel transistors can also be used as retention effect on the programming/erasing signal. The memory 123–125 organic memory devices due to their reverse ion characteristics . window obtained when the program/erase voltage is 9 V, which is Koo et al. reported nonvolatile transistor memories (NvTMs) devices summarized in Fig. 10g as a function of the colloidal dispersion that depend on the formation of EDL (Shown in Fig. 10e). The IGZO concentration to form the gold nanoparticle layer. It is verified that an film pattern was prepared by spin coating and UV curing. The transfer increase in the coverage area of gold nanoparticles on IGZO is characteristics of Au nanoparticles device are shown in Fig. 10f. They beneficial to storing more charges. More importantly, the preservation separately analyzed the induction of the gate on the IGZO surface characteristics of the memory are improved relative to the through the EDL effect under the write voltage V and the erase nanoparticles prepared by thermal evaporation due to the position prog voltage V . Colloidal gold nanoparticles are used at the interface of of the insulating organic ligand inserted between the IGZO channel era Published in partnership with Nanjing Tech University npj Flexible Electronics (2021) 13 D. Wang et al. Fig. 9 The scheme and functions of a visual synapse. a Schematic diagram of device structure and bionic function. Reproduced with permission . Copyright 2020, Wiley-VCH. b Optical photo of ZnO/SA-based SFET. c Electrical characteristics of the bioinspired flexible ZnO/ SA-based SFET. d Artificial visual memory systems based on SFETs. and the gold nanoparticle sheet. Figure 10h shows the retention have low processing accuracy, and the task load is indeed heavy characteristics of the programming and erasing signals of the IGZO when dealing with high-precision systems. Merging various nonvolatile memory prepared with colloidal gold nanoparticles technologies (R2R combination transfer), maybe more suitable −1 (5 mg ml ). Ion-gel gated IGZO NvTM with multiple programming/ for rapid and low-cost preparation and application of high- erase functions showed stable transfer characteristics in a series of precision IGTs, however further research is needed in terms of cycling tests (Fig. 10i). Obviously, the drive storage device with the material adhesion treatment and precision control. participation of electrolyte has a different working mode and control The good compatibility of ionic-gel materials with organic FETs ability from the previous device. further improves the shortcomings of flexible electronic technol- ogy in terms of dielectric materials. The integration of IGTs with portable power can serve a wide range of IoTs applications such as CONCLUSION health monitoring and industrial sensing. In addition, integrated We have reviewed the advances in the realization of the emerging neuromorphic transistors can also provide technical support for IGTs technology. As mentioned before, the carriers induced by the artificial intelligence and soft robotics. Although the development large EDL-capacitance enable IGTs to be driven at much lower of neuromorphic devices has attracted much attention, current voltages than traditional electronic transistors, which has shown progress is still limited to a few nerve or synaptic devices, and the the potential contribution of IGTs in addressing energy security functions they realize are all around the basic biological nerve challenges. However, there are un-matched challenges for the functions. In addition, the energy consumption is much lower than processing and preparation of the ionic-gel layers for mass-scale traditional transistors, but still higher than the biological level. industrialization. Spin coating, printing, and other methods Finally, the multi-input neuromorphic transistor provides an have the problem of contaminating the semi-conductor layer alternative model for simulating the multiple connections of material of the transistor, while the transfer methods generally biological nerves, the weight modulation between multiple gates npj Flexible Electronics (2021) 13 Published in partnership with Nanjing Tech University D. Wang et al. Fig. 10 Integrated circuit functions formed by ion-gel-gated transistor. a The circuit diagram of a Single-component inverter. Reproduced with permission . Copyright 2010, Wiley-VCH. The inset is a scheme of an all-printed ion-gel-gated P3HT transistor with PEDOT: PSS electrodes. b Characteristics of a single-component inverter. c The circuit scheme of the complementary inverter. The upper inset shows the optical image of this complementary inverter. d The dynamic performance of the complementary inverter. e Schematic cross-section of the memory device with colloidal Au NPs with ligands. The chemical structure of the ligand molecule, oleylamine. f Transfer characteristics for an ionic-gel-gated IGZO NvTM containing Au NPs that were applied with different V and V . g Summary of the voltage drift in the transfer prog era characteristics of devices prepared from colloidal gold dispersions of different concentrations at ± 9 V program/erase voltages. h Retention characteristics of the programmed and erased signals for an ionic-gel-gated IGZO NvTM. i A series of transfer characteristic curves of ionic-gel gated IGZO NvTM with multiple application/erase operations. 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Nano Lett. 19, reprints 1712–1718 (2019). 133. Lu, G. et al. Realization of artificial synapse and inverter based on oxide electric- Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims double-layer transistor gated by a chitosan biopolymer electrolyte. Semicond. in published maps and institutional affiliations. Sci. Technol. 35, 075014 (2020). 134. Robin, M. et al. Overcoming electrochemical Instabilities of printed silver elec- trodes in all-printed ion gel gated carbon nanotube thin-film transistors. ACS Appl. Mater. Interfaces 11, 41531–41543 (2019). Open Access This article is licensed under a Creative Commons 135. Barbosa, M. S. et al. Tungsten oxide ion gel-gated transistors: how structural and Attribution 4.0 International License, which permits use, sharing, electrochemical properties affect the doping mechanism. J. Mater. Chem. C. 6, adaptation, distribution and reproduction in any medium or format, as long as you give 1980–1987 (2018). appropriate credit to the original author(s) and the source, provide a link to the Creative 136. Lee, H. J. et al. Ultrahigh-mobility and solution-processed inorganic P-channel Commons license, and indicate if changes were made. The images or other third party thin-film transistors based on a transition-metal halide semiconductor. ACS Appl. material in this article are included in the article’s Creative Commons license, unless Mater. Interfaces 11, 40243–40251 (2019). indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly ACKNOWLEDGEMENTS from the copyright holder. To view a copy of this license, visit http://creativecommons. This work was supported by the National Science Foundation of China (NSFC, Grant org/licenses/by/4.0/. No. 61625404, 61874111, 61888102, 62022079), Young Elite Scientists Sponsorship Program by CAST (2018QNRC001), and the Youth Innovation Promotion Association of Chinese Academy of Sciences under Grant No. 2020115. © The Author(s) 2021 npj Flexible Electronics (2021) 13 Published in partnership with Nanjing Tech University

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