Active Matrix Electronic Skin Strain Sensor Based on

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Active Matrix Electronic Skin Strain Sensor Based on Piezopotential-Powered Graphene Transistors

Qijun Sun , Wanchul Seung , Beom Joon Kim , Soonmin Seo , Sang-Woo Kim ,* and Jeong Ho Cho*

Dr. Q. Sun, B. J. Kim, Prof. S.-W. Kim, Prof. J. H. Cho SKKU Advanced Institute of Nanotechnology (SAINT) Sungkyunkwan University Suwon 440-746 , South Korea E-mail: [email protected]; [email protected] W. Seung, Prof. S.-W. Kim School of Advanced Materials Science and Engineering Sungkyunkwan University Suwon 440-746 , South Korea Prof. J. H. Cho School of Chemical Engineering Sungkyunkwan University Suwon 440-746 , South Korea Prof. S. Seo College of BioNano Technology Gachon University Seongnam 461-701 , South Korea

DOI: 10.1002/adma.201500582

processes. New transistor architectures are, therefore, required, such as the coplanar-gate structures of a transistor with an ion gel dielectric adopted in this study. [ 27,28 ] The long-range polari-zation of ions in an ion gel allows the gate electrode position to be coplanar with the channel position. Piezoelectric poly-mers may then be simply patterned on top of the extended gate region to realize piezopotential gating. Furthermore, the high capacitance of the ion gel enables low-voltage transistor opera-tion, [ 29–32 ] which mitigates the requirement for high piezoelec-tric potential in self-powered systems.

This paper describes the demonstration of piezopotential-powered active matrix strain sensor arrays using a combina-tion of piezoelectric nanogenerators (NGs) and coplanar-gate graphene transistors (GTs). A coplanar-gate geometry in a GT simplifi es device fabrication by requiring only a two-step photolithography process that takes advantage of the semime-tallic properties of graphene. Piezoelectric polymers are easily patterned onto laterally positioned gate regions in an active matrix by utilizing a conformal polymeric mask. The piezopo-tentials induced by external strains were effectively coupled to the channels of a GT through ion gel gate dielectrics, which led to charge accumulation in the channel. Unlike piezoelec-tric self-powered strain sensors that exhibit instantaneous pulse output signals under external strains, the output drain cur-rents measured from the integrated devices maintained their output values under the continuous applied strains, which is important for real-time strain sensing applications. The resulting strain sensor exhibited excellent performance prop-erties, including a high sensitivity (gauge factor = 389) and a minimum detectable strain as low as 0.008%. Excellent device durability was observed after 3000 bending-releasing cycles. A transparent and conformal strain sensor fabricated on a rubber substrate was mounted onto the hand of a human test sub-ject and was observed to detect continuous hand movements. Finally, an active-matrix strain sensor matrix was fabricated for use in quantitative strain mapping. Notably, only a 0.1 V drain voltage was applied to operate the piezopotential-gated GT matrix strain sensors, whereas the piezopotential induced by an external strain was suffi cient to gate the GT active matrix. This work constitutes a signifi cant advance toward the development of e-skins that operate at extremely low voltages.

The procedure used to fabricate an active-matrix strain-sensor array based on the piezopotential-gated GTs (4 × 4 GT array on a 2.5 × 2.5 cm 2 polyethylene terephthalate (PET) substrate) is illustrated in Figure 1 a. The Au contact wiring electrodes used to transmit the measurements were patterned onto the PET substrate. Large-area high-quality monolayer graphene (see the Raman spectrum in Figure S1, Supporting Information) was then transferred onto the substrate. [ 33,34 ] The graphene patterns,

Electronic skin (e-skin) has attracted signifi cant research invest-ment toward the development of novel stimuli sensing and monitoring applications. [ 1–8 ] Strain sensors are fundamental components of e-skin devices, and the preparation of highly sensitive large-area strain sensors is critical to the develop-ment of e-skins. [ 9 ] Piezopotential-powered sensor devices [ 10 ] for use in e-skin and wearable applications are particularly desirable or even required for certain biomedical implants. [ 11 ] Piezopotential-powered sensor matrices are applicable to a wider variety of devices and they tend to reduce the size and weight of a system. Recently, several capacitor-type piezoelectric sensor arrays have been developed for use in large-area strain sensing. [ 12,13 ] A parasitic capacitance that results from a cou-pling interaction with adjacent pixels or interconnections can dissipate an induced output voltage signal and disrupt applied strain sensing. An active matrix network based on transistors integrated with piezoelectric materials could provide advanced sensing performances, including a low degree of signal cross-talk, multi-parameter monitoring, a high sensitivity, and a high spatial resolution. [ 14 ]

Piezoelectric materials, [ 15–17 ] including lead zirconate titanate (PZT), barium titanate, and zinc oxide (ZnO), have been extensively studied in piezoelectric nanogenerator applica-tions, which offer one approach to developing piezopotential-powered systems. [ 18–23 ] Solution-processable polymeric piezo-electric materials, which provide an excellent fl exibility and low-temperature processability, perform better than inorganic piezoelectric materials in fl exible or conformable e-skin appli-cations; [ 24–26 ] however, connecting the materials with traditional transistor necessitates complicated layered-structures that are easily damaged during subsequent solution-based deposition

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including the channel, gate-source-drain electrodes, and matrix layout lines were formed using one-step photolithography and RIE etching. [ 28 ] Poly(vinylidenefl uoride- co -trifl uoroethylene) (P(VDF-TrFE)) was utilized to generate piezopotentials that gated the GTs because P(VDF-TrFE) assumes a stable piezo-electric crystalline β -phase at room temperature. The addition of a third fl uoride into the TrFE monomer unit increased steric hindrance and favored an all-trans conformation, which favored the formation of the piezoelectric β -phase [ 26,35 ] (confi rmed by X-ray diffraction measurements, as shown in Figure S2, Sup-porting Information). As shown in Figure 1 a, P(VDF-TrFE) was patterned on top of the coplanar gate electrodes of the GTs using a conformal polyurethane acrylate (PUA) mask fabricated by soft lithography. [ 36 ] The polymeric mask fabrication process is shown in detail in Figure S3, Supporting Information. The surface morphology of the patterned P(VDF-TrFE) was char-acterized by scanning electron microscopy (SEM) (Figure S4, Supporting Information). Precise alignment of the P(VDF-TrFE) pattern onto the graphene gate electrode was critical for enhancing the gate coupling performance and minimizing undesired leakage currents. The Au top electrodes for the NGs were deposited onto the P(VDF-TrFE) patterns by thermal evaporation. Finally, the ion gel gate dielectrics, consisting of bis(trifl uoromethyl sulfonyl)imide ([EMIM][TFSI]) ion liquid, poly(ethylene glycol) diacrylate (PEGDA) monomer, and 2-hydroxy-2-methylpropiophenone (HOMPP) photo-initiator (with a weight ratio of 90:7:3), was photopatterned across the graphene channel and a portion of the graphene gate electrode. Under UV exposure, the HOMPP photoinitiator generates radi-cals that can react with the acrylates in the PEGDA monomers and initiate the polymerization. Thereby, the exposed region was cross-linked and patterned, while the unexposed region was washed away with DI water. The coplanar geometry of the GTs signifi cantly simplifi ed the matrix fabrication steps, relative

to the steps used to fabricate conventional transistors. This geometry was made possible by the long-range polarization of the ion gel dielectrics. In the matrix, the graphene channel width and length were designed to be 200 and 3500 µm, respec-tively, with a 200 µm distance between the gate and channel. Figure 1 b,c shows, respectively, photographic images and the circuit designs used to create the fl exible strain-sensor matrices based on piezopotential-gated GTs. Four GTs were connected to form one group through a single grounded electrode, and the other three groups were positioned parallel to one another.

The strain-sensing mechanism underlying the piezopotential-gated GT was investigated by fabricating a single GT coupled with a piezoelectric P(VDF-TrFE), as shown in Figure S5, Sup-porting Information. The graphene channel width and length were 500 and 700 µm, respectively, with a 100 µm distance between gate and channel. Figure S6, Supporting Information, shows the typical transfer characteristics of the coplanar-gate GT. The device exhibited a hole mobility of 498 cm 2 V −1 s −1 and an electron mobility of 178 cm 2 V −1 s −1 with an on/off ratio of around 10. Upon application of a gate voltage, ions in the ion gel gate dielectric migrated to the graphene gate–ion gel and the ion gel–graphene channel interfaces, and then formed electrical double layers (EDLs) at each interface. [ 37 ] The formation of highly capacitive EDLs led to carrier accumulation at the electrolyte/graphene channel interface. For the piezopotential-gated GT, the channel conductance of GT could be effi ciently modulated by the piezopotentials of the NG under external applied strains, which were coupled to the graphene channel through the ion gel dielec-tric. Figure 2 a shows the integration of NG and GT (or coupled NG–GT). The piezopotential induced by external strains could supply the power for gating the graphene transistor. Meanwhile, the measured output currents in GT indicated different external strain levels (applied to the NG) because the charge density in the GT channel was modulated by the piezopotentials coupled

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Figure 1. a) Procedure used to fabricate fl exible active-matrix strain sensors based on piezopotential-gated coplanar-gate GTs. b) Photographic image of the active-matrix strain sensor array. Inset shows an optical microscopy image of the patterned graphene on a PET substrate. c) Circuit diagram of the active-matrix strain sensor array.

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to ion gel gate dielectric. The operating mechanism of the cou-pled NG–GT could be explained as follows. The piezopotential induced by an external strain repelled the electrons to the gra-phene gate electrode, while compensating cations in the ion gel would be attracted to the interface between graphene gate and ion gel. Meanwhile, the anions in the ion gel would migrate to the other interface (leaving a charge-neutral electrolyte between the two EDLs), and thereby leading to the holes accumulation in graphene channel (lower panel in Figure 2 a). Notably, most coupled piezopotential was dropped across the EDLs, resulting in high electric fi eld at the interfaces and negligible inside bulk electrolyte (Figure S7, Supporting Information). Figure 2 b shows the output characteristics measured under an applied gate voltage and the piezopotential gating induced by different strains. The drain current ( I D ) increased gradually as the applied strain increased, similar to the performance of the GT under increased gate voltages. These results demonstrated that the strain-induced piezopotential could effectively gate the GT. The piezopotential generated at a strain of 0.20% was equivalent with a gate voltage of 0.3 V, representing an I D of 150 µA.

The aligned dipoles present in a P(VDF-TrFE) fi lm subject to an external strain (0.20%) are strengthened at both interfaces

between P(VDF-TrFE) layer and top (or bottom) electrode, where they further attract the opposite charges and repel the same charges. The repelled charges are neutralized through the external circuit, yielding a positive pulse output signal (upper panel in Figure 2 c). The excellent insulating properties of P(VDF-TrFE) enable the attracted charges to accumulate at the interface regions. As the strain is released, the aligned dipoles recover to their initial states. The attracted charges then fl ow back, yielding a pulse with the opposite voltage. These two oppo-site voltage pulses represent typical piezoelectric NG signals. [ 15 ]

The NG integrated with a GT, however, displayed a markedly different output signal (lower panel in Figure 2 c). During the initial bending motion, the output I D increased immediately as a result of the piezopotential-induced local charges distribution at the interface between graphene gate electrode and ion gel. As the strain remained fi xed, I D decreased and reached a saturated level above the initial value after the piezopotential-induced charges redistributed and reached an equilibrium state. Similar behaviors have also been reported in ZnO-based photoelectro-chemical anode [ 38,39 ] and hybrid piezoelectric-solar cells. [ 40 ] This behavior differed from the operation of a single NG, in which the charges that were repelled under strain were neutralized

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Figure 2. a) The mechanism underlying strain sensing by the piezopotential-gated GT. b) Output characteristics of the GT under applied gate voltages (left), and the piezopotential-gated GT under applied strains (right). c) NG device performance (upper panel) and piezopotential-gated GT device per-formance (lower panel). d) Output I D signals of the piezopotential-gated GT at a bending speed of 0.25 mm s −1 . e) Output I D signal of a piezopotential-gated GT under tensile or compression strains.


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the repelled charges from P(VDF-TrFE) NG would reside on the graphene gate electrode, while the oppositely-charged ions in ion gel would be located at the interface between graphene gate and ion gel (i.e., EDL as shown in Figure 2 a). This EDL formation in ion gel gate dielectrics caused the accumulation of charges in graphene channel. Importantly, ion gel is an ioni-cally conducting but electronically insulating dielectric, so the repelled charges from NG could not be neutralized through external circuit. Thus, the output current ( I D ) (depending on the carrier density in graphene channel) reached a saturated level under a continuous applied strain. Once the strain was released (the piezoelectric potential in P(VDF-TrFE) disap-peared), repelled charges fl owed back to the NG and the poten-tial drop in EDLs disappeared. Therefore, the I D decreased abruptly as the coupled charges, which modulated the channel conductance, were restored to recover the initial I D level (Video I, Supporting Information). Figure 2 d shows the value of I D as a function of time in the piezoelectric-gated GT. The piezopoten-tial-gated GT mounted on a bending system was bent at a con-stant speed (0.25 mm s −1 ) until the strain had reached 0.20%. I D was then measured under a constant drain voltage ( V D ) of 0.1 V. The observed gradual increase in I D (Video II, Supporting Information) was attributed to an increase in the piezopotential with the gradually increasing strain. The charges induced by the piezopotential were progressively coupled to the graphene channel, and I D underwent a gradual increase, indicating that the piezopotential acted as an effective gate for the GT. Figure 2 e shows the output I D signals of the piezopotential-gated GT characterized under tensile and compressive strains. As the tensile (0.06%) and compressive (–0.06%) strains were applied

to the NG, output I D signals with opposite directions were observed because the Dirac voltage of the GT was positioned at around 1 V (Figure S6, Supporting Information). Note that the Dirac voltage was required to be either positive or negative in order to distinguish the tensile from the compressive strains. The tensile and compressive strains induced piezopotentials in the NG with opposing signs. Therefore, the holes that accumu-lated in the channel increased under an applied tensile strain and decreased under an applied compressive strain.

The performances of the strain sensor devices prepared using the piezopotential-gated GTs were further character-ized. Figure 3 a shows the output I D signals of the strain sensor under different tensile strains at a constant V D = 0.1 V. Dif-ferent strains (0.04, 0.06, 0.09, 0.10, 0.11, and 0.20%) were applied for 15 s, and then the strains were released and held for 15 s. The I D increased abruptly upon application of the strain, maintained a certain level, decreased upon strain release, and recovered to its initial level. Different output I D values at the corresponding strains were clearly observed. The application of a larger strain to the device increased the piezopotential coupled to the graphene channel, thereby producing a higher output current. A higher output current under the same strain conditions could be obtained using NG with a larger P(VDF-TrFE) pattern size (Figure S8, Supporting Information). The stepwise output I D increased under a stepwise strain increase.

The sensitivity of a strain sensor is generally characterized by a gauge factor (GF), [ 41 ] which is defi ned as (Δ I / I 0 )/ε, where I 0 is the initial I D , Δ I is the I D variations under a certain strain, and ε is the applied strain. The results shown in Figure 3 b were used to calculate the values of GF in region I (389) and region II (69). It should be noted that GF in region I is by far the highest

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Figure 3. a) Output I D signals as a function of time obtained from a piezopotential-gated GT strain sensor under different tensile strains. b) Sensitivity characteristics of the piezopotential-gated GT strain sensor. c) Durability test over 3000 bending cycles (0.20% tensile strains). d) Characterization of the minimum detectable strain. e) A piezopotential-gated GT strain sensor mounted on the fi nger of a test subject for sensing fi nger bending, holding, and releasing.


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value obtained from the state-of-the-art strain sensors, [ 42 ] such as conventional metal strain gauges (1–5), thick resistor-type strain sensors (100), or single-crystal Si strain sensors (200). The durability properties of the piezopotential-gated GTs were tested, as shown in Figure 3 c. Bending motions under a tensile strain of 0.20% were applied at a fi xed V D of 0.1 V. Over a 3000 cycle bending and releasing test, the output I D currents were maintained at a stable value. The minimum detectable strain was found to be 0.008% (Figure 3 d), half of the value reported for reduced graphene oxide FET strain sensors. [ 43 ] These results were attributed to the intrinsic high strain sensitivity of the piezoelectric P(VDF-TrFE) layer. A piezopotential-gated GT on a PET substrate was mounted onto a human test subject's fi nger to monitor continuous fi nger movements (Figure 3 e and Video III, Supporting Information). The current was found to increase, maintain a high value, and decrease, depending on the fi nger bending, holding, and release motions. Toward applica-tion to e-skin, fast tension-release test was important. Therefore,

fast bending test (with tension-release cycle within 1s) through a bending system, and monitoring hand for grabbing and releasing a bottle (cycle below 2s) were respectively demon-strated in Video IV and V, Supporting Information. Results are shown in Figure S9, Supporting Information, respectively.

A strain sensor matrix based on piezopotential-gated GTs was successfully fabricated onto a plastic substrate using the fabri-cation procedure illustrated in Figure 1 a. Prior to mapping the spatial pattern of the strains applied to the active matrix, the elec-trical performances of the 16 GTs were characterized, as shown in Figure S10, Supporting Information. All GTs operated well, and the output currents remained stable, varying by less than 1 µA. Gauge factors of each pixel in the matrix were also extracted and their distribution ranged from 252–267 (Figure S10e, Supporting Information). This level of stability is very impor-tant for achieving accurate strain measurements at different positions in the matrix. The external strains applied to the active matrix were spatially mapped, as shown in Figure 4 a. Two edges

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Figure 4. a) Output currents distribution for each pixel under bending and 2D mapping of the strains distribution on the active matrix. b) Hand move-ment monitoring by a conformal piezopotential-gated GT strain sensor fabricated on a PDMS substrate.


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or two corners of the matrix were fi xed, and only a 0.1 V poten-tial was applied to the bit lines. The output currents of all pixels were then measured (middle panel in Figure 4 a), and the 2D color mapping of the strains distribution on the active matrix was demonstrated (right panel in Figure 4 a). Finally, a trans-parent and stretchable piezopotential-gated GT was fabricated onto a polydimethylsiloxane (PDMS) substrate to achieve con-formal contact with human skin. A fabrication process similar to that shown in Figure S5, Supporting Information, was used here. The conformable device was attached onto the hand of a human test subject, as shown in the upper panel of Figure 4 b. The current outputs depended on the test subject’s motions and were clearly captured (lower panel of Figure 4 b).

In conclusion, an active-matrix strain sensor array was suc-cessfully demonstrated using piezopotential-gated coplanar GTs. The piezoelectric NGs based on P(VDF-TrFE) were fabri-cated onto the coplanar gate electrodes of the GTs. The output sensing signals were maintained at relevant values according to the applied different strains. This was because the piezo-potential could be effectively coupled to the graphene channel through the ion gel dielectrics. The resulting strain sensor exhibited excellent sensitivity (GF = 389), a minimum detect-able strain of as low as 0.008%, and a high mechanical dura-bility (>3000 cycles bending test). The strains applied to the matrix were quantitatively visualized in a 2D color map. More-over, the transparent and stretchable strain sensors were also fabricated on a PDMS substrate to monitor human hand move-ments. This design concept described in this work represents a signifi cant advance in the fi eld of piezopotential-powered artifi -cial skin.

Experimental Section Materials Preparation : Large-area (8 × 8 cm 2 ) high-quality monolayer

graphene was grown onto a Cu catalyst surface using chemical vapor deposition (CVD) techniques, as described previously. A cleaned Cu foil (treated with piranha solution for 15 min) was folded and crimped to form an enclosure prior to inserting into a quartz tube. Once the pressure in the tube had fallen below 5 × 10 −3 Torr, 10 sccm H 2 was introduced and the tube was heated to 1000 °C for Cu annealing. CH 4 was then introduced at a low fl ow rate of 5 sccm to enable graphene growth over 4 h under continuous H 2 fl ow. After cooling to room temperature under a H 2 atmosphere, high-quality monolayer graphene was obtained at the inside face of the Cu enclosure. The P(VDF-TrFE) (ARKEMA Inc.) polymer was dissolved in N , N -dimethylformamide (DMF) at a ratio of 20 wt%. The solution was stirred over 2 d to obtain a uniform solution.

PUA Mask Fabrication : The SU-8 mold (the same pattern with fi nal PUA mask) used for the PDMS replication step was fi rstly patterned on a Si wafer using standard photolithography techniques. The reverse reliefs were then replicated onto the PDMS stamp. UV-curable PUA (Minuta Tech, MINS 301RM) was injected between the replicated PDMS stamp and another fl at PDMS stamp. A certain pressure was applied to create full conformal contact between the reliefs on the top PDMS and the bottom fl at PDMS layer. The uncontacted parts were then fi lled with the PUA solution. After UV (λ = 350 nm) curing over 3 min, the PUA molecules were cross-linked and solidifi ed. Peeling away the PDMS stamp yielded a conformal and highly fl exible free-standing PUA stencil bearing the required patterns.

Strain Sensor Matrix Fabrication : A PET substrate used for the matrix was cleaned with acetone, IPA, and DI water using ultrasonication. Au wiring electrodes and asteroid aligner markers were patterned on the PET

substrate. Monolayer graphene was transferred onto the PET substrate, following wet transfer assisted by PMMA, which was subsequently dissolved in acetone. The matrix layout lines, GT channel, and gate–source–drain electrodes composed of graphene were patterned in one step by photolithography and RIE etching. A poly[(vinylidenefl uoride-co- trifl uoroethylene] (P(VDF-TrFE)) (P(VDF-TrFE)) solution was spin-coated on top of the gate electrodes of the GTs using a PUA mask, which was conformally contacted with the PET substrate to effectively prevent the P(VDF-TrFE) solution from escaping from the designated regions on the substrate. After patterning, the P(VDF-TrFE), DMF solvent was removed at 60 °C, and the assembly was annealed at 140 °C for 3 h in a N 2 atmosphere to enhance the piezoelectric β -phase of the P(VDF-TrFE). Au electrodes for grounding were then thermally evaporated on top of the patterned P(VDF-TrFE). Finally, the ion gel was patterned onto the graphene channel and parts of the gate electrodes to enable piezopotential coupling. The GT performances were measured in air using a Keithley 2600 working group.

Conformable Strain Sensor Fabrication : As-grown graphene was patterned on a Cu foil and then transferred onto a PDMS substrate using the photoresist AZ 5214 instead of PMMA. P(VDF-TrFE) and the ion gel were patterned using the method shown in Figure S5, Supporting Information. Finally, a top graphene electrode was transferred onto the top of the P(VDF-TrFE) layer using a dry transfer method.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements Q.S. and W.S. contributed equally to this work. This work was supported by a grant from the Center for Advanced Soft Electronics (CASE) under the Global Frontier Research Program (2013M3A6A5073177) and Basic Science Research Program (2009-0083540) of the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology, Korea.Note: the units for mobility on page 3412 were corrected on June 5, 2015, after initial publication online.

Received: February 3, 2015 Revised: March 20, 2015

Published online: April 27, 2015

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