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Mechanosensation-Active Matrix Based on Direct-Contact Tribotronic Planar Graphene Transistor Array ACS Nano Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 09/08/18. For personal use only.
Yanfang Meng,†,‡,§,△ Junqing Zhao,†,‡,§,△ XiXi Yang,†,‡,§ Chunlin Zhao,†,‡,§ Shanshan Qin,†,‡,§ Jeong Ho Cho,∥ Chi Zhang,*,†,‡,# Qijun Sun,*,†,‡,# and Zhong Lin Wang*,†,‡,#,⊥ †
Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, China § University of Chinese Academy of Sciences, Beijing 100049, China ∥ SKKU Advanced Institute of Nanotechnology (SAINT), School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, South Korea # Center on Nanoenergy Research, School of Physical Science and Technology, Guangxi University, Nanning 530004, China ⊥ School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ‡
S Supporting Information *
ABSTRACT: Mechanosensitive electronics aims at replicating the multifunctions of human skin to realize quantitative conversion of external stimuli into electronic signals and provide corresponding feedback instructions. Here, we report a mechanosensation-active matrix based on a direct-contact tribotronic planar graphene transistor array. Ion gel is utilized as both the dielectric in the graphene transistor and the friction layer for triboelectric potential coupling to achieve highly efficient gating and sensation properties. Different contact distances between the ion gel and other friction materials produce different triboelectric potentials, which are directly coupled to the graphene channel and lead to different output signals through modulating the Fermi level of graphene. Based on this mechanism, the tribotronic graphene transistor is capable of sensing approaching distances, recognizing the category of different materials, and even distinguishing voices. It possesses excellent sensing properties, including high sensitivity (0.16 mm−1), fast response time (∼15 ms), and excellent durability (over 1000 cycles). Furthermore, the fabricated mechanosensation-active matrix is demonstrated to sense spatial contact distances and visualize a 2D color mapping of the target object. The tribotronic active matrix with ion gel as dielectric/ friction layer provides a route for efficient and low-power-consuming mechanosensation in a noninvasive fashion. It is of great significance in multifunction sensory systems, wearable human−machine interactive interfaces, artificial electronic skin, and future telemedicine for patient surveillance. KEYWORDS: electronic skin, graphene transistor, direct-contact tribotronic devices, mechanosensation, triboelectric nanogenerator
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trixes mainly concentrate on replication of the human somatosensory system to sense various mechanical stimuli.15−18 To develop an artificial skin sensation system for practical applications, it is still required (1) to realize noninvasive mechosensation for long-term durability; (2) to simplify the fabrication process of a complicated sensory system; and (3) to decrease the power consumption during the operation of a large number of sensors. Therefore, researchers have been endeavoring to seek out sophisticated materials and technologies to solve the above-mentioned problems.
uman skin is regarded as a sophisticated and highly integrated sensory system composed of various sensation receptors. The development of electronic artificial skins, epidermal electronics, and a mechanosensationactive matrix intensively aims to mimic the complex sensing functions of human skin, convert external gentle stimuli into quantified electronic signals, and further provide feedback instructions.1−6 Until now, skin-inspired electronic devices have covered scalable sensory arrays with excellent stretchability and self-healing capability based on material design and structure optimization.7−9 Epidermal electronics (or transient electronics) with biocompatibility or biodegradability have driven significant advances in biomedical devices for physiological signal monitoring.10−14 Mechanosensation ma© XXXX American Chemical Society
Received: June 13, 2018 Accepted: September 5, 2018 Published: September 5, 2018 A
DOI: 10.1021/acsnano.8b04490 ACS Nano XXXX, XXX, XXX−XXX
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Figure 1. (a) Schematic illustration of the mechanosensation-active matrix based on a tribotronic planar graphene transistor array. Inset is an enlarged illustration of a single sensing pixel. (b) Circuit diagram of the tribotronic GFET array. (c) UV−vis transmittance spectrum of PET, graphene on PET, and GFET on PET, respectively. Inset is a photoimage of the transparent GFET array.
tribotronic gating. Different contact distances between the ion gel and other friction materials produced different triboelectric potentials, which were directly coupled to the graphene channel and affected its Fermi level. The output currents were related with both the contact distances and category of materials. The planar design of the tribotronic graphene transistor also promised a facile fabrication process for a planar complementary inverter. A single tribotronic planar graphene transistor was successfully demonstrated for wearable sensation tests including approach sensing and voice distinction. It exhibited excellent sensing properties, including high sensitivity (0.16 mm−1), fast response time (∼15 ms), and excellent durability (over 1000 cycles). The tribotronic sensor could also recognize different categories of materials, such as different metals and organic materials. Finally, we demonstrated that the active matrix could sense contact distances and realize a 2D color mapping of an object. The proposed tribotronic active matrix with ion gel as dielectric/friction layer opens a way for efficient and low-energy-consuming mechanosensation in a noninvasive fashion. It is believed to have great promise in human−robot interfaces, electronic artificial skin, multifunctional sensors, and smart wearable devices.
The triboelectric nanogenerator (TENG) has been demonstrated to be a cost-efficient and highly productive way to convert mechanical energy into electricity by coupling of triboelectrification and electrostatic induction.19−23 Selfpowered sensory systems conjugated with a TENG have been demonstrated to monitor displacement, velocity, temperature, humidity, and other physical parameters.24−26 Furthermore, the induced triboelectric potential is able to couple with field effect transistors (FETs) for controlling the transport properties of semiconductor channels and helping to achieve functional devices.27−29 The tribotronic devices show great prospects for applications in logic gates, memory, phototransistors, tactile switches, etc.30−33 FET-based sensors are considered to be significant components in electronic skin and exhibit advanced sensing properties, including high sensitivity and resolution, multiparameter monitoring, and alleviation of signal crosstalk. To achieve high-performance mechanosensation devices, tribotronic graphene transistors gating through an ion gel dielectric may offer a more efficient way of modulating the Fermi level of graphene channels and acquiring higher sensitivity.34 Long-range polarization of the ion gel also enables a transistor gate patterned in coplanarity with source−drain electrodes. Direct contact-electrification gating between target objects and the ion gel dielectric will have the following advantages: (1) simplified fabrication process with compact geometry design; (2) ion gel functionalization as a neutral material to recognize other friction materials with different polarities (or electronegativities). In this work, we first reported a mechanosensation-active matrix based on a direct-contact tribotronic planar graphene transistor array for distance mapping and material recognition. In this active matrix, all the graphene transistors were efficiently gated by a triboelectric potential instead of applying gate voltages. An ion gel as the electrolyte gate dielectric of the graphene planar transistor was adopted as the electrification layer for compact mechanosensation design and efficient
RESULTS AND DISCUSSION Figure 1a shows a schematic illustration of the mechanosensation-active matrix (4 × 4 graphene field effect transistors (GFETs) with planar geometry) on a flexible polyethylene terephthalate (PET) substrate. Large-area and high-quality graphene was grown by chemical vapor deposition (CVD) and transferred to a PET substrate at first. Raman spectra of graphene exhibited two characteristic peaks of a G band at ∼1597 cm−1 and a 2D band at ∼2694 cm−1. The monolayer formation was confirmed by both the full width at halfmaximum (∼29 cm−1) of the symmetric 2D band and the 2D/ G intensity ratio (∼2.5) (Figure S1). The blue shift of the G band of graphene from 1578 cm−1 to 1597 cm−1 indicated B
DOI: 10.1021/acsnano.8b04490 ACS Nano XXXX, XXX, XXX−XXX
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Figure 2. Electrical performance of the tribotronic GFET and inverter. (a) Schematic illustration of the triboelectric GFET with ion gel as friction layer. (b) Circuit diagram of tribotronic GFET contact with PTFE. (c) Output performance and corresponding transfer curve of tribotronic GFET contact with PTFE. (d) Output performance and corresponding transfer curve of tribotronic GFET contact with Cu. (e) Energy band diagram of the tribotronic GFET device contact with PTFE. (f) Energy band diagram of the tribotronic GFET device contact with Cu. (g) Schematic illustration of the tribotronic complementary inverter. (h) Voltage transfer curve of the inverter under applied input voltage. Inset is the circuit diagram of the inverter. (i) Time-dependent output voltage of a tribotronic inverter. Inset is the circuit diagram.
slight p-type doping of graphene by oxygen and moisture.35 A UV-cross-linkable ion gel34 (composed of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) ion liquid, poly(ethylene glycol) diacrylate (PEGDA) monomers, and 2-hydroxy-2-methylpropiophenone (HOMPP) photoinitiator in a weight ratio of 90:7:3) was patterned across the graphene channels. The ion gel was utilized as both the GFET gate dielectric and triboelectrification layer. Notably, only two steps of photolithography were used to pattern the sensation matrix, which was greatly simplified compared with previously reported eskin arrays. The detailed fabrication process is shown in Figure S2. The inset of Figure 1a shows an enlarged illustration of a single sensing pixel. In each pixel, graphene plays two roles: the part in contact with the ion gel acts as the transistor channel, while the remainder forms the layout lines and source−drain electrodes. The width and length of the graphene channel are 500 and 2000 μm, respectively. Planar GFET with ion gel dielectric is functionalized as the sensing component, while the target objects are conformably located on top of the ion gel as the friction layer. The friction layer and ion gel construct a contact−separation mode TENG. The produced triboelectric potential is directly coupled to the graphene channel and efficiently gates the GFET (the detailed working mechanism will be discussed below). The corresponding circuit diagram of
the mechanosensation-active matrix is shown in Figure 1b. Four GFETs are aligned in one column, sharing the same source electrode (grounding line). Then four columns of GFET are located side by side. Top friction materials are necessary to be grounded. The mechanosensation matrix exhibited good optical transparency, confirmed by UV−vis spectroscopy. As shown in Figure 1c, the commercial PET substrate exhibited a transmittance of 91.5% in the visible and near-infrared region (from 400 to 900 nm). After patterning graphene and the ion gel, the overall transmittance was ∼83%. The inset of Figure 1c shows a photoimage of the flexible and transparent mechanosensation matrix. Prior to investigating the mechanosensation properties of the matrix, the electrical performance of a single tribotronic GFET was first explored. A schematic illustration of the tribotronic planar GFET is shown in Figure 2a. The graphene channel width and length were 50 and 300 μm, respectively. Typical output characteristics of a GFET under an applied gate voltage are shown in the right panel of Figure 2b. The drain current (ID) was increased from 44.04 μA to 236.37 μA with a gate voltage (VG) increasing from 0 to 2 V at a drain voltage (VD) of 0.5 V. The transfer curve of a GFET under an applied gate is plotted in Figure S3. It was observed that the ID increased with an increment in VG in both the positive and negative directions, indicating the ambipolar charge transport C
DOI: 10.1021/acsnano.8b04490 ACS Nano XXXX, XXX, XXX−XXX
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negative voltage to the GFET, which shifted the Fermi level of the graphene channel downward, decreased the charge injection barrier, and led to an increased drain current. Different from previously reported tribotronic devices, the tribotronic GFET worked in a direct-contact-gating mode; that is, the adopted ion gel worked as both the dielectric layer of the GFET and friction layer of the TENG (Figure S4a). Therefore, the triboelectric potential was directly coupled to the GFET. In contrast, the separated coupling of the triboelectric potential is a series connection between the FET and the NG (Figure S4b). We supposed the direct coupling of the triboelectric potential would show a higher electric performance compared with separated coupling of the TENG and GFET. We compared the output performance of two TENGs composed of ion-gel/Cu and PTFE/Cu to verify our assumption at first. The equivalent output voltage of iongel/Cu TENG (0.47 V) was higher than that of PTFE/Cu TENG (0.25 V), which can be explained by the increased capacitance effect in the ion gel friction layer due to the formation of EDLs during the electrification process (Figure S5).36−40 The electrical performance of the tribotronic GFET upon external TENG gating was then investigated in detail in Figure S6 to compare with the gating situation through direct triboelectrification with the ion gel. A direct-contact triboelectrification gated GFET produced an output current variation of 6 μA at a contact−separation distance of 1 mm, twice higher than the counterpart of the external-TENG-gated GFET with a value of 3 μA (Figure S7). To get insight into the advantage of ion gel as a friction layer and verify the enhanced capacitance effect, capacitance analysis and impedance spectroscopy were conducted on the ion gel dielectric layer upon friction with PTFE and copper, respectively. A planar Au/ion-gel/Au structure was utilized, and the external friction layer (PTFE or Cu) was controlled to approach the ion gel layer through a linear motor (Figure S8a and e). Figure S8b depicts the capacitance versus frequency under various distances between the ion gel and PTFE. The varied distances did not alter the shape of the capacitance− frequency curve, indicating that the triboelectric potential gating did not change the frequency-dependent capacitance properties of the ion gel. However, the varied distances altered the values of the effective capacitance during the EDL formation process. Detailed explanations are provided in the Supporting Information. The highly efficient tribotronic gating and feasible fabrication procedure of the GFET in planar geometry promised its potential applications in tribotronic logic devices. The planar tribotronic inverter comprising two tribotronic GFETs connected together is shown in Figure 2g. One graphene transistor was connected to the supplied voltage (VDD), and the other one was connected to a ground. The two transistors shared one output terminal and the same ion gel dielectric as the triboelectrification layer (tribotronic gate input). Unlike complementary inverters composed of both ntype and p-type transistors,42,43 the tribotronic inverter operated based on two identical ambipolar transistors. Under tribotronic input voltage (VIN) sweeping (i.e., external friction material approaching the ion gel process), the effective potential drop imposed to each transistor varied respectively according to the drain voltage superposition effect.44 Consequently, an inverted signal between the input and the output voltage (VOUT) was produced following the relationship between transistor channel resistances (R1 and R2) and VDD:
property of graphene. The GFET could operate at a low gate voltage (
