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The low resistance and high sensitivity in stretchable electrode assembled by liquid-phase exfoliated graphene
Polymer 192 (2020) 122301
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The low resistance and high sensitivity in stretchable electrode assembled by liquid-phase exfoliated graphene Yunfei Zhu a, c, d, Hongyun Chen b, c, Lixiang Jiang a, c, d, ***, Lixin Xu b, c, **, Huijian Ye, Conceptualization, Writing - review & editing b, c, * a
Beijing Institute of Spacecraft Environment Engineering, China Academy of Space Technology, Beijing, 100094, China College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou, 310014, China Joint Laboratory of Smart Materials and Compliant Mechanisms, Beijing, 100029, China d National Key Laboratory of Science and Technology on Reliability and Environmental Engineering, Beijing, 100094, China b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Flexible electrode Graphene Hyperbranched polymer Stretchable
Flexible electrodes have been extensively investigated to fulfill the development of highly advanced human interaction electronics. It’s still a challenge to develop the conductive film for the scalable device with low resistance under large deformation. In this work, we reported a stretchable conductive layer on elastomer substrates assembled by few-layer graphene, which was exfoliated in the low-boiling organic solvent with assistance of hyperbranched copolymer as stabilizer that was adsorbed on the nanosheets via CH-π non-covalent connections. The relative resistance change of graphene film is 117% as the mechanical strain reaches 35%, which retains high conductivity under tensile operation. The resistance of the graphene electrode is dependent on the overlapping of the nanosheets during the deformation, in which the slipping of nanosheets is due to the lubricant effect of the hyperbranched segments acting as dynamic CH-π interactions. This work highlights a general strategy of the stretchable conductive film for the flexible electronics, and sheds a light on the conduction mechanism for the graphene film during large deformation.
1. Introduction The integrating electronics with biological and stretchable systems have attracted huge attentions due to the tremendous demanding for body-attached electronic devices and biomedical applications in past decades. Flexible electrodes have been extensively investigated for the development of highly advanced human interaction devices, and the scalable electronic devices are also rapidly evolved [1]. Conventional electrodes for biocompatible sensors are mainly assembled by glass-carbon, gold, silver, platinum and indium-tin oxides [2–4]. It’s proposed that the structure and modification of electrode for integrating electronics is determined by its inherent rigidity and planar structure that are normally incompatible with curved biological systems [5]. The polymer transistors on flexible plastic sheets [6,7] are suggested as po tential candidates in integrated circuit [8], wearable fabric [9], motion sensor [10,11], and e-skin electronics [12], which requires deform ability of physical shape under external force. Various types of materials,
such as carbon nanotubes [11,13], graphene [10,14–17], carbon black [18–20], and metal nanowires/nanoparticles [21,22] have been employed to develop flexible electrodes. Graphene is considered to be the promising candidate for applica tions of future advanced electronics because of its high carrier mobility and large saturation velocity [23–27], which is also applied in a variety of fields for biosensors [28–30] with stable electrical property under deformation on polydimethylsiloxane (PDMS) substrate [31]. However, these graphene-based strain sensors are only suitable for compression and bending deformations. Cracking and faulting occurs in graphe ne/PDMS composite as the stress is performed, which results in the dramatic decrease of electrical conductivity that the change in resistance (ΔR) is insufficient to deliver high sensitivity of the sensor [32]. It was reported that the few-layer graphene was exfoliated from natural graphite with assistance of hyperbranched polyethylene (HBPE), which was adsorbed on the surface of graphene as polymer stabilizer via non-covalent CH-π and π-π forces [33]. The hyperbranched polymer
* Corresponding author. College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou, 310014, China. ** Corresponding author. College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou, 310014, China. *** Corresponding author. Beijing Institute of Spacecraft Environment Engineering, China Academy of Space Technology, Beijing, 100094, China. E-mail addresses: [email protected] (L. Jiang), [email protected] (L. Xu), [email protected] (H. Ye). https://doi.org/10.1016/j.polymer.2020.122301 Received 10 January 2020; Received in revised form 13 February 2020; Accepted 16 February 2020 Available online 19 February 2020 0032-3861/© 2020 Elsevier Ltd. All rights reserved.
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illustrates an approximately spherical chain topology with abundant branching ends [34,35], which is adhered to form a sandwich-like gra phene against re-stacking of nanosheets. The sliding phenomenon in this pair of adjacent nanosheets during deformation plays an important role in the conductive connection of flexible graphene electrode [36]. Although the friction and lubrication of graphene has been reported [37], the systematic properties of interlayer sliding between layers of graphene are rarely investigated. A flexible conductive film could be obtained through an appreciation of the physical phenomena e.g. the sliding of nanosheets, the lubrication effect of hyperbranched polymer and the conductive contact of gra phene. Here, a stretchable electrode was assembled by few-layer gra phene that was exfoliated in low-boiling-point solvent with assistance of hyperbranched polymer as stabilizer adsorbed on the surface of nano sheets via CH-π interactions. The graphene electrode exhibits relatively low resistance variation as the strain is lower than 35%. The sliding of few-layer graphene with the streaming effect of hyperbranched polymer during external deformation contributes to the conductive tunnel inside the film, which ensures the sensitivity of sensor for the bending and pressing motions.
2.3. The characterizations of graphene nanosheets The nuclear magnetic resonance spectroscopy (NMR) was employed to evaluate the structure and grafting ratio of monomer in HBPE-gPTFEMA copolymer with Bruker ANANCE III instrument. The copol ymer sample was dissolved in CDCl3 at room temperature with 64 scans. The Nicolet 6700 Fourier infrared spectrometer (FT-IR, TA) was used to characterize the polymer structure with testing range of 4000 to 650 cm 1 and the resolution of 4 cm 1. Transmission electron microscopy (TEM) images for the graphene were performed on a 300 kV JEM-100CX II electron microscope from FEI. The TEM samples were prepared by depositing several drops of graphene dispersion on a copper grid covered by 230-mesh carbon membrane, which was dried under an infrared lamp before observation. Raman spectroscopy was examined on a Lab RAM HRUV800 instrument with the wavenumber range of 250 cm 1 to 3200 cm 1 to clarify the defects of the nanosheets. Atomic force microscopy (AFM) images were captured on a Bruker scanning probe microscope with tapping mode. The sample for AFM characterization was deposited onto a mica surface and then dried under vacuum at 40 � C for 12 h. X-ray diffraction (XRD) measurements were performed on a PANAlytical X’Pert Pro multipurpose diffractometer with Cu Kα radia tion (λ ¼ 1.54 Å) at a scan rate of 5� min 1 from 10 to 80� . The ther mogravimetric analysis (TGA) of the graphene dispersion was accomplished using an SDT Q600 analyzer (TA) with a heating rate of � 10 C/min under nitrogen gas.
2. Experimental 2.1. Materials The bromine grafted HBPE (HBPE-Br) was synthesized by chain walking polymerization of ethylene (polymerization grade) from Hangzhou Metal Processing Special Gas Co. at 25 � C for 24 h under a typical pressure of 0.1 MPa using Pd-diimine catalyst. The monomer of trifluoroethyl methacrylate (C6H7F3O2, TFEMA, 96%) was purchased from Harbin Xuejia Fluorine Silicon Chemical Co., which was washed with 5 wt% NaOH solution and deionized water before polymerization [27,38]. The prepolymer and curing agent of polydimethylsiloxane (PDMS) (Sylgard 184) was bought from Dow Corning. The ethylene-vinyl acetate copolymer (vinyl acetate content 40 wt%) and natural graphite (99.5%) powders were bought from Sigma-Aldrich. The VHB4910 was bought from 3M China. N,N-dimethylformamide (DMF, AR) and chloroform (CHCl3, AR) were supplied by Shanghai Lingfeng Chemical Reagent Co., Ltd. The toluene (AR) from Hangzhou Shuanglin Chemical Reagent Co. was distilled under vacuum for 24 h with CaH2 prior to polymerization.
2.4. The morphology and conductivity of stretchable graphene film Field emission scanning electron microscope (SEM) images of gra phene films for surface and cross-section morphologies were obtained using a Nano SEM 450 microscope of FEI. The sample was nitrogen frozen-fractured, and the surface was sprayed with a thin layer of Au prior to characterization. The capacitance was measured with the fre quency range of 102–106 Hz using an Agilent 4294A LCR precision impedance analyzer (Agilent), and the testing area was coated with silver electrode before characterization. The surface resistivity was examined with an RTS-8 four-probe system embedding a ZC-90 high resistivity meter (Taiou Electronics). Electrochemical workstation (CHI 700E, Shanghai Chenhua Instrument Co., Ltd.) with 3-electrode method was employed to record the volt-ampere characteristic curve. A constant voltage of 5 V was applied to detect the variation of the current during the deformation of sensor.
2.2. Preparation of flexible graphene film
3. Results and discussion
The graphene dispersion was prepared by the exfoliation of natural graphite ultrasonically in chloroform with HBPE-g-PTFEMA as polymer stabilizer [39]. Briefly, 320.0 mg of natural graphite and 320.0 mg of HBPE-g-PTFEMA were added into a glass vessel with 80 mL of CHCl3. The mixture was then sonicated for 48 h with a continuous stream of water to retain the room temperature, and the dispersion was then centrifuged at 4000 rpm for 30 min to remove residual large-sized particles. The supernatant was carefully collected and further vacuum filtered using 100-nm polyester membrane to eliminate excess HBPE-g-PTFEMA copolymer. Finally, the black filtration cake was re-dispersed in DMF under sonication to reach a stable dispersion with concentration CB ¼ 1.0 mg mL 1. The mixture with ratio of PDMS prepolymer to curing agent as 10:1 was used to prepare elastomer substrate. Normally, 5 g of prepolymer and 0.5 g of curing agent were stirred for 30min, and was poured into glass dish after eliminating the bubbles, which was cured at 80 � C for 2 h. Then 3.3 mL of graphene dispersion was dropped on the surface of the PDMS, which was transferred into an oven at 120 � C for 10 h. Finally, a stretchable conductive film on the PDMS elastomer was received.
3.1. Morphology and structure of graphene The few-layer graphene and boron nitride nanosheets were exfoli ated in low-boiling-point solvent with assistance of hyperbranched polymer, which was attached on the surface of flakes against re-stacking [27]. In order to improve the compatibility of few-layer graphene and PDMS substrate, the PTFEMA segments were introduced into hyper branched polymer. The schematic diagram for the exfoliation of gra phene and the preparation of conductive film with PDMS substrate is illustrated in Fig. 1. The HBPE-Br macroinitiator was synthesized with palladium-diimine catalyst in one step, and the TFEMA monomer was grafted on the hyperbranched ends through atom transfer radical poly merization (ATRP). The 1H NMR curve of the HBPE-g-PTFEMA copol ymer is shown in the Supporting Information Fig. S1, which demonstrates that the copolymer has been successfully synthesized with hyperbranched structure via ATRP technique. The few-layer graphene was exfoliated from natural graphite in chloroform under sonication. The FT-IR technique is applied to verify the fluoro copolymer attached on the surface of nanosheets, and the spectra are shown in Fig. S2a. In order to thoroughly remove the free HBPE-g-PTFEMA, the dispersion was vacuum filtered and the filter cake was washed by fresh chloroform 2
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Fig. 1. Schematic illustration for the exfoliation of graphene with assistance of HBPE-g-PTFEMA and the preparation of conductive film with PDMS substrate.
under sonication and further filtration. This washing procedure was performed totally for 3 cycles and the final filtrate was collected. Considering that the copolymer has very low concentration in the final filtrate, it is reasonable to deduce that the fluoro copolymer detected here is irreversibly adsorbed on graphene surface. The hyperbranched copolymer illustrates an approximately spherical topology and abun dant branch ends, which was attached on the surface of the nanosheets via non-covalent CH-π interactions [33]. Graphene is suggested as promising candidate for conductive coatings due to its morphological advantages such as high aspect ratio and surface functionalization. The TGA characterization of exfoliated dispersion without free stabilizer to evaluate the ratio of graphene to copolymer in the received dispersion. The TGA curves are shown in Supporting Information Fig. S2b, and the mass proportion of the copolymer is 24%. Finally, the graphene film is prepared by a simple deposition method as conductive layer in a densely stacked manner. As shown in Supporting Information Fig. S3, the elec trode assembled by few-layer graphene can effectively illuminate the light bulb, which verifies that the conductive film is well-formed in dense structure. In the process of tensile deformation, the conductive path depending on the tunneling effect is conversed, thereby yields a variation in electrical resistance, which is employed as a highly sensitive sensor. In addition, since a small amount of HBPE-g-PTFEMA adheres to the graphene surface serving as an intermediate layer during stretching, the buffer layer favors the sliding between layers due to lubricant effect of hyperbranched polymer. Therefore, in the deformation process the electrical conductivity of electrode is improved. The morphological and structural characterizations of few-layer graphene exfoliated in DMF with assistance of hyperbranched copol ymer are displayed in Fig. 2. The TEM image for few-layer graphene in Fig. 2a illustrates translucent and flat nanosheets, indicating that the thicknesses of exfoliated flakes are thin. The inset image is the largeproduction graphene dispersion in DMF. The high-resolution TEM image of large nanosheet with the folded edge is shown in Fig. 2b and c, from which the folded structure is observed that could be applied to distinguish the number of layer by direct visualization. It’s suggested that the nanosheets are partially parallel under folded or lifted edge that are respectively dominated by dark lines. The number of layer is also obtained by diffraction analysis of nanosheet with the angle of inci dence. The layer spacing of graphene is found to be 0.34 nm, which is
consistent with the thickness of mono-layer graphene reported previ ously [40]. Inset the Fig. 2b is the selected area electron diffraction of few-layer graphene with a clear six-membered ring array, which strengthens that the crystal structure of exfoliated graphene is still intact. The linear segments of the inner ring and outer diffraction are drawn in Fig. S4. The intensity of the inner ring is much stronger than that of the outer one, indicating that the few-layer nanosheets is ob tained during exfoliation with the polymer stabilizer [41]. The trans verse dimension of nanosheets has been evaluated with the sample set of 100 pieces in Fig. 2g, in which large portion of the resultant flakes ranges from 150 nm to 300 nm. AFM characterization was employed to examine the surface morphology. The graphene from the AFM image is horizontally located about 200–300 nm, which is consistent with the TEM results. The height profiles of the nanosheets in AFM image are shown in Fig. 2e, and the thicknesses of the isolated flakes in Fig. 2d are 1.58 nm and 1.85 nm, respectively. The three-dimensional trans formation of nanosheet is displayed in Fig. S5. Also, the statistical thickness of graphene with sample set of 100 flakes was estimated, and the distribution is displayed in Fig. 2f. The thickness for exfoliated graphene is between 1.4 and 1.8 nm, which verifies the received gra phene within 3–8 layers. The Raman spectra of graphite and graphene with the corresponding enlarged 2D peak is presented in Fig. 2h. The intensity ratio of ID/IG for graphene is estimated to be 0.82, higher than that of natural graphite as 0.10, which proves an increase of defects because the patterned bulk was exfoliated into disordered few-layer nanosheets with relatively small size [42]. The overall quality of the graphene is still intact with low defects. In addition, the position of the peak for graphene is blue-shifted, and the curve shape becomes more symmetrical compared with natural graphite, which evidences that graphene was exfoliated into an oligo layer. The XRD patterns of natural graphite, HBPE-g-PTFEMA, graphene and electrode film are displayed in Fig. 2i. There is a strong diffraction at 2θ ¼ 26.4� assigning as (002) crystal plane for graphite. The intensity of this peak for few-layer graphene is significantly reduced, indicating that the natural ordered bulk has been converted into a disorderly nanosheet with low crystallization, which strengthens that the few-layer graphene is obtained by liquid exfoliation with assistance of hyperbranched polymer.
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Fig. 2. The morphological and structural characterizations of few-layer graphene exfoliated in DMF with assistance of hyperbranched copolymer: (a) TEM image, and inset is the digit image of graphene dispersion, (b) TEM image with the folded edges, and inset is selected area electron diffraction, (c) enlarged image for the folded edge, (d) AFM image, (e) the corresponding heights of flakes, (f) the statistical thickness distribution with the sample set of 100 flakes, (g) the statistical lateral size distribution with the sample set of 100 flakes, (h) Raman spectra, and (i) XRD patterns of natural graphite, HBPE-g-PTFEMA, graphene electrode and few-layer graphene. The excess copolymer was eliminated prior to characterizations.
3.2. Surface resistance of flexible graphene electrode
stretching-release cycles, the surface resistance of graphene film retains low value, indicating the stability of electrode, which is ascribed to the good compatibility of PDMS and nanosheets. The relative resistance change (ΔR/R) in Fig. 3b augments dramatically with the increasing strain, indicating that the slip of nanosheet is insufficient to compensate for the large deformation. The conductive path is gradually dis integrated, and the long-range conductive network is destroyed under large deformation. The photo images for stretching progress of flexible graphene electrode on the PDMS substrate are shown in Supporting Information Fig. S8. The nanosheets are stacked as an interface layer that is attached on the substrate, thereby retains the conductive per formance of the electrode under stretching. From Fig. S7 obvious gullies and cracks are not observed from graphene film when the strain is less than 30%. In order to examine the lubricate role of hyperbranched polymer, the graphene film without HBPE-g-PTFEMA was also deposited on the elastomer, and the results are displayed in Supporting Informa tion Fig. S9. The resistance of pure graphene film increases hugely with the increasing strain, while the curve of graphene film exfoliated with
In order to evaluate the strain sensing capability of graphene film, the conductive layer was prepared by the simple droplet coating of graphene dispersion on the PDMS substrate and the resistivity-strain behavior was investigated. The dense structure of conductive film is critical for the motion of charge carrier and thus the conductivity of film. The cross-section and surface morphologies under different de formations were characterized by SEM, and the images are shown in Figs. S6 and S7. The thickness of graphene film is around 6 μm. The digital images of deformation progress for flexible electrode on the PDMS substrate are presented in Fig. S6. The Four-probes method was applied to examine the electrical conductivity of flexible film [41]. As shown in Fig. 3a, the surface resistance gradually becomes large with the increase of the stretching ratio. The curve exhibits the linear tendency as the elongation is lower than 35%, which is ascribed to the relative viscous slip of nanosheets in favor of the lubricate effect of hyper branched polymer. In addition, as shown in Fig. 3a, after 10 4
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Fig. 3. The electric performance of flexible graphene film on PDMS substrate: (a) surface resistance, (b) the relative resistance variation, (c) voltammetric curve, (d) partial enlarged view of voltammetric curve, (e) capacitance, and (d) dielectric loss.
assistance of HBPE-g-PTFEMA retains relatively stable at strain ε < 35%. The presence of HBPE-g-PTFEMA between layers is presumed to improve the inter-laminar mobility of nanosheets due to the hyper branched topology, which is usually applied to enhance the process ability of polymer [35]. In order to examine the conductivity of flexible film, an electro chemical workstation was employed to characterize the volt-ampere characteristics with �0.3 V scan voltage. The number of scans per ten sion test is 20, and the red/black arrows are the stretching/release processes, respectively. In a typical electromechanical characterization, the circuit current gradually decreases with stretching and increases with relaxation under applied voltage, which is consistent with the proposed operating principle. The volt-ampere characteristic curves at different elongations are shown in Fig. 3c, and the partial linear view of the diagram is displayed in Fig. 3d. The contact resistance increases with increasing strain, which is ascribed to the decrease of overlap region
inside film, and the current of the circuit decreases accordingly. Theo retically, slipping phenomenon occurs between the adjacent nanosheets due to the enlarged inter-space during deformation. The long-distance conductive network is destroyed due to the macroscopic fracture as the stretching ratio turns large, which results in a sharp increase in resistance. Conversely, during the relaxation of the strain sensor, since the graphene sheets are recovered to the original position, the circuit current gradually increases to the original level. When a constant voltage of ~5 V is applied to the device, the strain is detected in the form of current or resistance due to the variety of contact resistance between the nanosheets during deformation, which provides the foundation for excellent sensor stability and repeatability in a large strain range. In order to further examine the performance of the prepared flexible electrode, the capacitance of PDMS with the graphene film under deformation was characterized. The capacitance and dielectric loss for PDMS elastomer are illustrated in Fig. 3e and f. The capacitance at 40% 5
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strain is 14.2 pF, which is 83% of the original PDMS with capacitance of 17.1 pF. This demonstrates good conductivity of graphene electrode under large deformation. As the deformation progresses, the peak of dielectric loss shifts to low frequency, which is related to the intrinsic relaxation properties of graphene induced by the orientation and gra phene slip pairs [43].
electrode. The gauge factor GF ¼ 19.2 at ε ¼ 35%, and the GF value rises monotonously with the increase of the strain, e.g. GF ¼ 34.8 at 50% strain [45]. The molecular dynamics was applied to simulate the interlayer mo tion of two-dimensional materials during deformation [36]. The sliding of the few-layer graphene with 3–8 layers is usually regarded as sticky slip, leading to the variation of resistance [46]. In order to appreciate the conductive mechanism of graphene film under deformation, a simple framework is suggested to describe the sliding behavior based on tunneling conduction. The diagram for the slipping progress of graphene with lubricate effect of hyperbranched polymer is shown in Fig. 5. The electrical model of graphene with the sliding consists of four stages: overlapping conductive network, conductive path, tunneling effect and non-conducting situation [46,47]. The connection resistance between the overlap region of graphene during the sliding process is primary for the effective transport of charge carriers. The resistance of the graphene layer is mainly based on the tunneling effect from two parts: the intrinsic resistance of the graphene (RG) and the connection resistance between the nanosheets (Rl), while the contribution of the latter graph to the overall resistance is huge, especially for the large surface resistance. Here the graphene itself resistance (RG) is neglected, which is simplified to evaluate the resistance model. The relationship between R and the tunneling distance d can be approximated as [48–50]: � � 8π hl Rl ¼ (3) eXd 2 3A XdN
3.3. Sensor performance of stretchable graphene electrode The stretchable electrodes are generally embedded in the sensors and portable devices, here we set-up a simple device with flexible graphene film to detect the signal in practical application. For the biological sensor device, we cut the prepared graphene film onto a VHB4910 substrate with a soft copper as extension of testing electrode that was connected with an electrochemical workstation to record the variation of current during stretching and relaxation [44]. A constant 5 V voltage was applied on the device with the initial current of 0.18 mA. As shown in Fig. 4a and b, the amount of current is observed as the periodicity of the finger motion. Fig. 4c and d record the signals for different weights, from which the 100-mg weight can be easily perceived. The detection performance for the bending of the human body exhibits superior pre cision that sensitivity and short response time are preferred for the ap plications in biological device. A common parameter of the sensitivity of electrical displacement to mechanical deformation is the gauge factor (GF) [48–50], which is related to the change of electrical resistance with applied strain: GF ¼
ΔR 1 � R0 ε
(2)
X¼
pffiffiffiffiffiffiffiffiffi 4π 2mϕ h
(4)
where X is the tunneling parameter, h the Planck constant, l the number of particles forming a single conductive path, N the number of
where R0 is the resistance at original state, R the instantaneous resis tance with strain, and ε reflects the shape variable of the flexible
Fig. 4. Sensing performance of flexible graphene electrode: (a) pressing sensor, (b) bending sensor with the finger joint, (c) the loading weight of 100 g, and (d) the loading weight of 100 mg. 6
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Fig. 5. The diagram for the slipping progress of graphene with lubricate effect of hyperbranched polymer: (a) the interlocking arrangement of graphene nanosheets, and (b) different types of nanosheets under deformation.
conductive paths, d the tunneling distance, A2 the effective connection section, ϕ the height of the barrier between adjacent nanosheets, e and m are the charge and mass of electron, respectively. The strain is expressed as:
ε¼
d
d0 d0
¼
Δd d0
The experimental data and the fitting curves based on the expression (7) are plotted in Fig. 6. The agreement between the experimental data and the theoretical curve is achieved at Xd0 ¼ 4.96 as the strain ϵ < 35%. It’s suggested that graphene nanosheets are in contact that the layer becomes conductive by yielding threshold with an empirical value X � 14.59 nm [46]. The maximum value coincides with the Van Der Waals distance d0 ¼ 0.34 nm [52]. This tunnel conduction model fits well with the experimental results under 35% strain, which illustrates that the tunnel theory plays a main role in the transport of carriers in the gra phene film. The composite materials usually exhibit desirable conduc tivity with a strain less than 20% [53,54]. Obviously, the conductive network constructed here retains stable at a large strain. The small amount of hyperbranched polymer is adsorbed on the graphene surface, which enhances the sliding behavior of nanosheets during deformation. The flexible graphene film as electrodes has the potential to improve current strain sensor technology for promising applications. In addition, the resistance of the electrode is abruptly increased as the strain ϵ > 35%, which is ascribed to the reconstruction of the conductive path. The dense pattern of graphene layer is destructed, and the conductive network becomes disconnected. The dependence of conducting path N under applied strain is estimated as following [54–56]:
(5)
According to equations (3) and (5), the relationship between ln R/R0 and ε is obtained, where R0 and R reflect the resistance before and after the applied strain: ln
R ¼ Xd0 ε R0
lnðε þ 1Þ
(6)
From the equation (6), a small shape variable can be found that d0 is the power exponent of the surface resistance for the stretchable elec trode. The distance between them is exponential positively. As the variable deformation becomes large, the value of d increases, and the connection resistance Rl rises exponentially, which leads to the rapid increase of surface resistance for the conductive film. The sensitivity of the sensor improves exponentially with the expansion between gra phene as the film being elongation. Similarly, the surface resistance is inversely proportional to overlapping area, which gradually increases with the deformation of the film. For uniaxial shear strain, the relative slip distance of graphene is also described as: s/s0 ¼ (1 þ ϵ) [51]. Ac cording to the equation (6), the ΔR=R is directly proportional to the deformation ϵ, which obeys the tightly stacked (Fig. 5 Type one) model: ΔR eXd0 ε ¼ R ðε þ 1Þ2
1
N¼
N0 e½AεþBε2 �
(8)
where A and B are constants. Therefore, the change of the normalized resistance is also described as: � ðXd0 εþAεþBε2 ΔR e ¼ 1 (9) R ðε þ 1Þ2
(7)
In Fig. 6 the agreement between the experimental data and the theoretical curve is fitted well at the strain 35% < ε < 55% (Xd0þA ¼ 7.1 and B approaching to 0). The overlapping of graphene is decreased as the strain is larger than 35%, which leads to the high resistance. The flexible graphene film is assembled by the few-layer twodimension lamellae by the deposition technology. Considering that the hyperbranched polymer is attached on the surface of nanosheets that is inadequate motion for electron among adjacent layers, it’s deduced that the charge transfer accounts for the low resistance of graphene film in this work. The distinguishable interlock-tile re-allocation of nanosheets in a near-parallel construction is responsible for the compliant nano sheets on the horizontal direction [57]. The external force results in the uniform arrangement of stress, which is transferred through the shear motion of interlamellar, in which the hyperbranched polymer with spherical topology improves the sliding of adjacent nanosheets. Based on the combination of graphene sliding and tunneling effect, the inter laminar shearing effect of few-layer nanosheets retains low resistance of
Fig. 6. Experimental data (dots) and theoretical curves (solid lines) of ΔR/R as a function of strain for graphene film. 7
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the graphene film during large deformation, which provides a new insight of flexible electrode for advanced electronics.
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4. Conclusions In summary, we reported a general preparation scheme for flexible film electrode, which could be applied in a sensing device. The few-layer graphene was liquid-phase exfoliated with HBPE-PTFEMA as stabilizer attached on the surface of nanosheets via CH- π non-covalent in teractions. The graphene film was deposited on the elastomer as a stacked conductive layer to monitor the resistance variation under deformation. The relative resistance change is 117% at strain of 35%, which is ascribed to the reversible motion of interlaminar nanosheets with the lubricant effect of hyperbranched polymer. The resistance of the graphene two-dimensional electrode material in the deformation process is dependent on the overlapping area of graphene and the dis tance between the nanosheets. The sliding behavior of few-layer nano sheets involves multiple interactions and interlaminar shear motion, which promotes the appreciation of the patterns for nanosheets under large deformation. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Yunfei Zhu: Formal analysis, Investigation, Writing - original draft. Hongyun Chen: Investigation, Data curation, Writing - original draft. Lixiang Jiang: Writing - review & editing. Lixin Xu: Writing - review & editing. Huijian Ye: Conceptualization, Writing - review & editing. Acknowledgements The financial support from the National Natural Science Foundation of China (51707175, 11502025) and Natural Science Foundation of Zhejiang Province of China (LTZ20E070001, LY18B040005) is greatly appreciated. This work is also supported by China Postdoctoral Science Foundation (2018M640572). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.polymer.2020.122301. References [1] J.A. Rogers, T. Someya, Y. Huang, Science 327 (5973) (2010) 1603–1607. [2] Q. Zhai, X. Zhang, Y. Han, J. Zhai, J. Li, E. Wang, Anal. Chem. 88 (1) (2015) 945–951. [3] X. Zhu, Q. Zhai, W. Gu, J. Li, E. Wang, Anal. Chem. 89 (22) (2017) 12108–12114. [4] H. Xing, X. Zhang, Q. Zhai, J. Li, E. Wang, Anal. Chem. 89 (7) (2017) 3867–3872. [5] Q. Zhai, X. Zhang, J. Li, E. Wang, Nanoscale 8 (33) (2016) 15303–15308. [6] F. Garnier, R. Hajlaoui, A. Yassar, P. Srivastava, Science 265 (16) (1994) 1684–1686. [7] Z.N. Bao, Y. Feng, A. Dodabalapur, V.R. Raju, Andrew J. Lovinger, Chem. Mater. 9 (6) (1997) 1299–1301. [8] D.H. Kim, J.H. Ahn, W.M. Choi, H.S. Kim, T.H. Kim, J. Song, Y.Y. Huang, Z. Liu, C. Lu, J.A. Rogers, Science 320 (5875) (2008) 507–511. [9] X. Li, P. Sun, L. Fan, M. Zhu, K. Wang, M. Zhong, J. Wei, D. Wu, Y. Cheng, H. Zhu, Sci. Rep. 2 (395) (2012) 1–8. [10] C.S. Boland, U. Khan, C. Backes, A. O’Neill, J. McCauley, S. Duane, R. Shanker, Y. Liu, I. Jurewicz, A.B. Dalton, J.N. Coleman, ACS Nano 8 (9) (2014) 8819–8830. [11] D.J. Lipomi, M. Vosgueritchian, B.C.K. Tee, S.L. Hellstrom, J.A. Lee, C.H. Fox, Z. N. Bao, Nat. Nanotechnol. 6 (2011) 788–792.
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Polymer journal homepage: http://www.elsevier.com/locate/polymer
The low resistance and high sensitivity in stretchable electrode assembled by liquid-phase exfoliated graphene Yunfei Zhu a, c, d, Hongyun Chen b, c, Lixiang Jiang a, c, d, ***, Lixin Xu b, c, **, Huijian Ye, Conceptualization, Writing - review & editing b, c, * a
Beijing Institute of Spacecraft Environment Engineering, China Academy of Space Technology, Beijing, 100094, China College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou, 310014, China Joint Laboratory of Smart Materials and Compliant Mechanisms, Beijing, 100029, China d National Key Laboratory of Science and Technology on Reliability and Environmental Engineering, Beijing, 100094, China b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Flexible electrode Graphene Hyperbranched polymer Stretchable
Flexible electrodes have been extensively investigated to fulfill the development of highly advanced human interaction electronics. It’s still a challenge to develop the conductive film for the scalable device with low resistance under large deformation. In this work, we reported a stretchable conductive layer on elastomer substrates assembled by few-layer graphene, which was exfoliated in the low-boiling organic solvent with assistance of hyperbranched copolymer as stabilizer that was adsorbed on the nanosheets via CH-π non-covalent connections. The relative resistance change of graphene film is 117% as the mechanical strain reaches 35%, which retains high conductivity under tensile operation. The resistance of the graphene electrode is dependent on the overlapping of the nanosheets during the deformation, in which the slipping of nanosheets is due to the lubricant effect of the hyperbranched segments acting as dynamic CH-π interactions. This work highlights a general strategy of the stretchable conductive film for the flexible electronics, and sheds a light on the conduction mechanism for the graphene film during large deformation.
1. Introduction The integrating electronics with biological and stretchable systems have attracted huge attentions due to the tremendous demanding for body-attached electronic devices and biomedical applications in past decades. Flexible electrodes have been extensively investigated for the development of highly advanced human interaction devices, and the scalable electronic devices are also rapidly evolved [1]. Conventional electrodes for biocompatible sensors are mainly assembled by glass-carbon, gold, silver, platinum and indium-tin oxides [2–4]. It’s proposed that the structure and modification of electrode for integrating electronics is determined by its inherent rigidity and planar structure that are normally incompatible with curved biological systems [5]. The polymer transistors on flexible plastic sheets [6,7] are suggested as po tential candidates in integrated circuit [8], wearable fabric [9], motion sensor [10,11], and e-skin electronics [12], which requires deform ability of physical shape under external force. Various types of materials,
such as carbon nanotubes [11,13], graphene [10,14–17], carbon black [18–20], and metal nanowires/nanoparticles [21,22] have been employed to develop flexible electrodes. Graphene is considered to be the promising candidate for applica tions of future advanced electronics because of its high carrier mobility and large saturation velocity [23–27], which is also applied in a variety of fields for biosensors [28–30] with stable electrical property under deformation on polydimethylsiloxane (PDMS) substrate [31]. However, these graphene-based strain sensors are only suitable for compression and bending deformations. Cracking and faulting occurs in graphe ne/PDMS composite as the stress is performed, which results in the dramatic decrease of electrical conductivity that the change in resistance (ΔR) is insufficient to deliver high sensitivity of the sensor [32]. It was reported that the few-layer graphene was exfoliated from natural graphite with assistance of hyperbranched polyethylene (HBPE), which was adsorbed on the surface of graphene as polymer stabilizer via non-covalent CH-π and π-π forces [33]. The hyperbranched polymer
* Corresponding author. College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou, 310014, China. ** Corresponding author. College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou, 310014, China. *** Corresponding author. Beijing Institute of Spacecraft Environment Engineering, China Academy of Space Technology, Beijing, 100094, China. E-mail addresses: [email protected] (L. Jiang), [email protected] (L. Xu), [email protected] (H. Ye). https://doi.org/10.1016/j.polymer.2020.122301 Received 10 January 2020; Received in revised form 13 February 2020; Accepted 16 February 2020 Available online 19 February 2020 0032-3861/© 2020 Elsevier Ltd. All rights reserved.
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illustrates an approximately spherical chain topology with abundant branching ends [34,35], which is adhered to form a sandwich-like gra phene against re-stacking of nanosheets. The sliding phenomenon in this pair of adjacent nanosheets during deformation plays an important role in the conductive connection of flexible graphene electrode [36]. Although the friction and lubrication of graphene has been reported [37], the systematic properties of interlayer sliding between layers of graphene are rarely investigated. A flexible conductive film could be obtained through an appreciation of the physical phenomena e.g. the sliding of nanosheets, the lubrication effect of hyperbranched polymer and the conductive contact of gra phene. Here, a stretchable electrode was assembled by few-layer gra phene that was exfoliated in low-boiling-point solvent with assistance of hyperbranched polymer as stabilizer adsorbed on the surface of nano sheets via CH-π interactions. The graphene electrode exhibits relatively low resistance variation as the strain is lower than 35%. The sliding of few-layer graphene with the streaming effect of hyperbranched polymer during external deformation contributes to the conductive tunnel inside the film, which ensures the sensitivity of sensor for the bending and pressing motions.
2.3. The characterizations of graphene nanosheets The nuclear magnetic resonance spectroscopy (NMR) was employed to evaluate the structure and grafting ratio of monomer in HBPE-gPTFEMA copolymer with Bruker ANANCE III instrument. The copol ymer sample was dissolved in CDCl3 at room temperature with 64 scans. The Nicolet 6700 Fourier infrared spectrometer (FT-IR, TA) was used to characterize the polymer structure with testing range of 4000 to 650 cm 1 and the resolution of 4 cm 1. Transmission electron microscopy (TEM) images for the graphene were performed on a 300 kV JEM-100CX II electron microscope from FEI. The TEM samples were prepared by depositing several drops of graphene dispersion on a copper grid covered by 230-mesh carbon membrane, which was dried under an infrared lamp before observation. Raman spectroscopy was examined on a Lab RAM HRUV800 instrument with the wavenumber range of 250 cm 1 to 3200 cm 1 to clarify the defects of the nanosheets. Atomic force microscopy (AFM) images were captured on a Bruker scanning probe microscope with tapping mode. The sample for AFM characterization was deposited onto a mica surface and then dried under vacuum at 40 � C for 12 h. X-ray diffraction (XRD) measurements were performed on a PANAlytical X’Pert Pro multipurpose diffractometer with Cu Kα radia tion (λ ¼ 1.54 Å) at a scan rate of 5� min 1 from 10 to 80� . The ther mogravimetric analysis (TGA) of the graphene dispersion was accomplished using an SDT Q600 analyzer (TA) with a heating rate of � 10 C/min under nitrogen gas.
2. Experimental 2.1. Materials The bromine grafted HBPE (HBPE-Br) was synthesized by chain walking polymerization of ethylene (polymerization grade) from Hangzhou Metal Processing Special Gas Co. at 25 � C for 24 h under a typical pressure of 0.1 MPa using Pd-diimine catalyst. The monomer of trifluoroethyl methacrylate (C6H7F3O2, TFEMA, 96%) was purchased from Harbin Xuejia Fluorine Silicon Chemical Co., which was washed with 5 wt% NaOH solution and deionized water before polymerization [27,38]. The prepolymer and curing agent of polydimethylsiloxane (PDMS) (Sylgard 184) was bought from Dow Corning. The ethylene-vinyl acetate copolymer (vinyl acetate content 40 wt%) and natural graphite (99.5%) powders were bought from Sigma-Aldrich. The VHB4910 was bought from 3M China. N,N-dimethylformamide (DMF, AR) and chloroform (CHCl3, AR) were supplied by Shanghai Lingfeng Chemical Reagent Co., Ltd. The toluene (AR) from Hangzhou Shuanglin Chemical Reagent Co. was distilled under vacuum for 24 h with CaH2 prior to polymerization.
2.4. The morphology and conductivity of stretchable graphene film Field emission scanning electron microscope (SEM) images of gra phene films for surface and cross-section morphologies were obtained using a Nano SEM 450 microscope of FEI. The sample was nitrogen frozen-fractured, and the surface was sprayed with a thin layer of Au prior to characterization. The capacitance was measured with the fre quency range of 102–106 Hz using an Agilent 4294A LCR precision impedance analyzer (Agilent), and the testing area was coated with silver electrode before characterization. The surface resistivity was examined with an RTS-8 four-probe system embedding a ZC-90 high resistivity meter (Taiou Electronics). Electrochemical workstation (CHI 700E, Shanghai Chenhua Instrument Co., Ltd.) with 3-electrode method was employed to record the volt-ampere characteristic curve. A constant voltage of 5 V was applied to detect the variation of the current during the deformation of sensor.
2.2. Preparation of flexible graphene film
3. Results and discussion
The graphene dispersion was prepared by the exfoliation of natural graphite ultrasonically in chloroform with HBPE-g-PTFEMA as polymer stabilizer [39]. Briefly, 320.0 mg of natural graphite and 320.0 mg of HBPE-g-PTFEMA were added into a glass vessel with 80 mL of CHCl3. The mixture was then sonicated for 48 h with a continuous stream of water to retain the room temperature, and the dispersion was then centrifuged at 4000 rpm for 30 min to remove residual large-sized particles. The supernatant was carefully collected and further vacuum filtered using 100-nm polyester membrane to eliminate excess HBPE-g-PTFEMA copolymer. Finally, the black filtration cake was re-dispersed in DMF under sonication to reach a stable dispersion with concentration CB ¼ 1.0 mg mL 1. The mixture with ratio of PDMS prepolymer to curing agent as 10:1 was used to prepare elastomer substrate. Normally, 5 g of prepolymer and 0.5 g of curing agent were stirred for 30min, and was poured into glass dish after eliminating the bubbles, which was cured at 80 � C for 2 h. Then 3.3 mL of graphene dispersion was dropped on the surface of the PDMS, which was transferred into an oven at 120 � C for 10 h. Finally, a stretchable conductive film on the PDMS elastomer was received.
3.1. Morphology and structure of graphene The few-layer graphene and boron nitride nanosheets were exfoli ated in low-boiling-point solvent with assistance of hyperbranched polymer, which was attached on the surface of flakes against re-stacking [27]. In order to improve the compatibility of few-layer graphene and PDMS substrate, the PTFEMA segments were introduced into hyper branched polymer. The schematic diagram for the exfoliation of gra phene and the preparation of conductive film with PDMS substrate is illustrated in Fig. 1. The HBPE-Br macroinitiator was synthesized with palladium-diimine catalyst in one step, and the TFEMA monomer was grafted on the hyperbranched ends through atom transfer radical poly merization (ATRP). The 1H NMR curve of the HBPE-g-PTFEMA copol ymer is shown in the Supporting Information Fig. S1, which demonstrates that the copolymer has been successfully synthesized with hyperbranched structure via ATRP technique. The few-layer graphene was exfoliated from natural graphite in chloroform under sonication. The FT-IR technique is applied to verify the fluoro copolymer attached on the surface of nanosheets, and the spectra are shown in Fig. S2a. In order to thoroughly remove the free HBPE-g-PTFEMA, the dispersion was vacuum filtered and the filter cake was washed by fresh chloroform 2
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Fig. 1. Schematic illustration for the exfoliation of graphene with assistance of HBPE-g-PTFEMA and the preparation of conductive film with PDMS substrate.
under sonication and further filtration. This washing procedure was performed totally for 3 cycles and the final filtrate was collected. Considering that the copolymer has very low concentration in the final filtrate, it is reasonable to deduce that the fluoro copolymer detected here is irreversibly adsorbed on graphene surface. The hyperbranched copolymer illustrates an approximately spherical topology and abun dant branch ends, which was attached on the surface of the nanosheets via non-covalent CH-π interactions [33]. Graphene is suggested as promising candidate for conductive coatings due to its morphological advantages such as high aspect ratio and surface functionalization. The TGA characterization of exfoliated dispersion without free stabilizer to evaluate the ratio of graphene to copolymer in the received dispersion. The TGA curves are shown in Supporting Information Fig. S2b, and the mass proportion of the copolymer is 24%. Finally, the graphene film is prepared by a simple deposition method as conductive layer in a densely stacked manner. As shown in Supporting Information Fig. S3, the elec trode assembled by few-layer graphene can effectively illuminate the light bulb, which verifies that the conductive film is well-formed in dense structure. In the process of tensile deformation, the conductive path depending on the tunneling effect is conversed, thereby yields a variation in electrical resistance, which is employed as a highly sensitive sensor. In addition, since a small amount of HBPE-g-PTFEMA adheres to the graphene surface serving as an intermediate layer during stretching, the buffer layer favors the sliding between layers due to lubricant effect of hyperbranched polymer. Therefore, in the deformation process the electrical conductivity of electrode is improved. The morphological and structural characterizations of few-layer graphene exfoliated in DMF with assistance of hyperbranched copol ymer are displayed in Fig. 2. The TEM image for few-layer graphene in Fig. 2a illustrates translucent and flat nanosheets, indicating that the thicknesses of exfoliated flakes are thin. The inset image is the largeproduction graphene dispersion in DMF. The high-resolution TEM image of large nanosheet with the folded edge is shown in Fig. 2b and c, from which the folded structure is observed that could be applied to distinguish the number of layer by direct visualization. It’s suggested that the nanosheets are partially parallel under folded or lifted edge that are respectively dominated by dark lines. The number of layer is also obtained by diffraction analysis of nanosheet with the angle of inci dence. The layer spacing of graphene is found to be 0.34 nm, which is
consistent with the thickness of mono-layer graphene reported previ ously [40]. Inset the Fig. 2b is the selected area electron diffraction of few-layer graphene with a clear six-membered ring array, which strengthens that the crystal structure of exfoliated graphene is still intact. The linear segments of the inner ring and outer diffraction are drawn in Fig. S4. The intensity of the inner ring is much stronger than that of the outer one, indicating that the few-layer nanosheets is ob tained during exfoliation with the polymer stabilizer [41]. The trans verse dimension of nanosheets has been evaluated with the sample set of 100 pieces in Fig. 2g, in which large portion of the resultant flakes ranges from 150 nm to 300 nm. AFM characterization was employed to examine the surface morphology. The graphene from the AFM image is horizontally located about 200–300 nm, which is consistent with the TEM results. The height profiles of the nanosheets in AFM image are shown in Fig. 2e, and the thicknesses of the isolated flakes in Fig. 2d are 1.58 nm and 1.85 nm, respectively. The three-dimensional trans formation of nanosheet is displayed in Fig. S5. Also, the statistical thickness of graphene with sample set of 100 flakes was estimated, and the distribution is displayed in Fig. 2f. The thickness for exfoliated graphene is between 1.4 and 1.8 nm, which verifies the received gra phene within 3–8 layers. The Raman spectra of graphite and graphene with the corresponding enlarged 2D peak is presented in Fig. 2h. The intensity ratio of ID/IG for graphene is estimated to be 0.82, higher than that of natural graphite as 0.10, which proves an increase of defects because the patterned bulk was exfoliated into disordered few-layer nanosheets with relatively small size [42]. The overall quality of the graphene is still intact with low defects. In addition, the position of the peak for graphene is blue-shifted, and the curve shape becomes more symmetrical compared with natural graphite, which evidences that graphene was exfoliated into an oligo layer. The XRD patterns of natural graphite, HBPE-g-PTFEMA, graphene and electrode film are displayed in Fig. 2i. There is a strong diffraction at 2θ ¼ 26.4� assigning as (002) crystal plane for graphite. The intensity of this peak for few-layer graphene is significantly reduced, indicating that the natural ordered bulk has been converted into a disorderly nanosheet with low crystallization, which strengthens that the few-layer graphene is obtained by liquid exfoliation with assistance of hyperbranched polymer.
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Fig. 2. The morphological and structural characterizations of few-layer graphene exfoliated in DMF with assistance of hyperbranched copolymer: (a) TEM image, and inset is the digit image of graphene dispersion, (b) TEM image with the folded edges, and inset is selected area electron diffraction, (c) enlarged image for the folded edge, (d) AFM image, (e) the corresponding heights of flakes, (f) the statistical thickness distribution with the sample set of 100 flakes, (g) the statistical lateral size distribution with the sample set of 100 flakes, (h) Raman spectra, and (i) XRD patterns of natural graphite, HBPE-g-PTFEMA, graphene electrode and few-layer graphene. The excess copolymer was eliminated prior to characterizations.
3.2. Surface resistance of flexible graphene electrode
stretching-release cycles, the surface resistance of graphene film retains low value, indicating the stability of electrode, which is ascribed to the good compatibility of PDMS and nanosheets. The relative resistance change (ΔR/R) in Fig. 3b augments dramatically with the increasing strain, indicating that the slip of nanosheet is insufficient to compensate for the large deformation. The conductive path is gradually dis integrated, and the long-range conductive network is destroyed under large deformation. The photo images for stretching progress of flexible graphene electrode on the PDMS substrate are shown in Supporting Information Fig. S8. The nanosheets are stacked as an interface layer that is attached on the substrate, thereby retains the conductive per formance of the electrode under stretching. From Fig. S7 obvious gullies and cracks are not observed from graphene film when the strain is less than 30%. In order to examine the lubricate role of hyperbranched polymer, the graphene film without HBPE-g-PTFEMA was also deposited on the elastomer, and the results are displayed in Supporting Informa tion Fig. S9. The resistance of pure graphene film increases hugely with the increasing strain, while the curve of graphene film exfoliated with
In order to evaluate the strain sensing capability of graphene film, the conductive layer was prepared by the simple droplet coating of graphene dispersion on the PDMS substrate and the resistivity-strain behavior was investigated. The dense structure of conductive film is critical for the motion of charge carrier and thus the conductivity of film. The cross-section and surface morphologies under different de formations were characterized by SEM, and the images are shown in Figs. S6 and S7. The thickness of graphene film is around 6 μm. The digital images of deformation progress for flexible electrode on the PDMS substrate are presented in Fig. S6. The Four-probes method was applied to examine the electrical conductivity of flexible film [41]. As shown in Fig. 3a, the surface resistance gradually becomes large with the increase of the stretching ratio. The curve exhibits the linear tendency as the elongation is lower than 35%, which is ascribed to the relative viscous slip of nanosheets in favor of the lubricate effect of hyper branched polymer. In addition, as shown in Fig. 3a, after 10 4
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Fig. 3. The electric performance of flexible graphene film on PDMS substrate: (a) surface resistance, (b) the relative resistance variation, (c) voltammetric curve, (d) partial enlarged view of voltammetric curve, (e) capacitance, and (d) dielectric loss.
assistance of HBPE-g-PTFEMA retains relatively stable at strain ε < 35%. The presence of HBPE-g-PTFEMA between layers is presumed to improve the inter-laminar mobility of nanosheets due to the hyper branched topology, which is usually applied to enhance the process ability of polymer [35]. In order to examine the conductivity of flexible film, an electro chemical workstation was employed to characterize the volt-ampere characteristics with �0.3 V scan voltage. The number of scans per ten sion test is 20, and the red/black arrows are the stretching/release processes, respectively. In a typical electromechanical characterization, the circuit current gradually decreases with stretching and increases with relaxation under applied voltage, which is consistent with the proposed operating principle. The volt-ampere characteristic curves at different elongations are shown in Fig. 3c, and the partial linear view of the diagram is displayed in Fig. 3d. The contact resistance increases with increasing strain, which is ascribed to the decrease of overlap region
inside film, and the current of the circuit decreases accordingly. Theo retically, slipping phenomenon occurs between the adjacent nanosheets due to the enlarged inter-space during deformation. The long-distance conductive network is destroyed due to the macroscopic fracture as the stretching ratio turns large, which results in a sharp increase in resistance. Conversely, during the relaxation of the strain sensor, since the graphene sheets are recovered to the original position, the circuit current gradually increases to the original level. When a constant voltage of ~5 V is applied to the device, the strain is detected in the form of current or resistance due to the variety of contact resistance between the nanosheets during deformation, which provides the foundation for excellent sensor stability and repeatability in a large strain range. In order to further examine the performance of the prepared flexible electrode, the capacitance of PDMS with the graphene film under deformation was characterized. The capacitance and dielectric loss for PDMS elastomer are illustrated in Fig. 3e and f. The capacitance at 40% 5
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strain is 14.2 pF, which is 83% of the original PDMS with capacitance of 17.1 pF. This demonstrates good conductivity of graphene electrode under large deformation. As the deformation progresses, the peak of dielectric loss shifts to low frequency, which is related to the intrinsic relaxation properties of graphene induced by the orientation and gra phene slip pairs [43].
electrode. The gauge factor GF ¼ 19.2 at ε ¼ 35%, and the GF value rises monotonously with the increase of the strain, e.g. GF ¼ 34.8 at 50% strain [45]. The molecular dynamics was applied to simulate the interlayer mo tion of two-dimensional materials during deformation [36]. The sliding of the few-layer graphene with 3–8 layers is usually regarded as sticky slip, leading to the variation of resistance [46]. In order to appreciate the conductive mechanism of graphene film under deformation, a simple framework is suggested to describe the sliding behavior based on tunneling conduction. The diagram for the slipping progress of graphene with lubricate effect of hyperbranched polymer is shown in Fig. 5. The electrical model of graphene with the sliding consists of four stages: overlapping conductive network, conductive path, tunneling effect and non-conducting situation [46,47]. The connection resistance between the overlap region of graphene during the sliding process is primary for the effective transport of charge carriers. The resistance of the graphene layer is mainly based on the tunneling effect from two parts: the intrinsic resistance of the graphene (RG) and the connection resistance between the nanosheets (Rl), while the contribution of the latter graph to the overall resistance is huge, especially for the large surface resistance. Here the graphene itself resistance (RG) is neglected, which is simplified to evaluate the resistance model. The relationship between R and the tunneling distance d can be approximated as [48–50]: � � 8π hl Rl ¼ (3) eXd 2 3A XdN
3.3. Sensor performance of stretchable graphene electrode The stretchable electrodes are generally embedded in the sensors and portable devices, here we set-up a simple device with flexible graphene film to detect the signal in practical application. For the biological sensor device, we cut the prepared graphene film onto a VHB4910 substrate with a soft copper as extension of testing electrode that was connected with an electrochemical workstation to record the variation of current during stretching and relaxation [44]. A constant 5 V voltage was applied on the device with the initial current of 0.18 mA. As shown in Fig. 4a and b, the amount of current is observed as the periodicity of the finger motion. Fig. 4c and d record the signals for different weights, from which the 100-mg weight can be easily perceived. The detection performance for the bending of the human body exhibits superior pre cision that sensitivity and short response time are preferred for the ap plications in biological device. A common parameter of the sensitivity of electrical displacement to mechanical deformation is the gauge factor (GF) [48–50], which is related to the change of electrical resistance with applied strain: GF ¼
ΔR 1 � R0 ε
(2)
X¼
pffiffiffiffiffiffiffiffiffi 4π 2mϕ h
(4)
where X is the tunneling parameter, h the Planck constant, l the number of particles forming a single conductive path, N the number of
where R0 is the resistance at original state, R the instantaneous resis tance with strain, and ε reflects the shape variable of the flexible
Fig. 4. Sensing performance of flexible graphene electrode: (a) pressing sensor, (b) bending sensor with the finger joint, (c) the loading weight of 100 g, and (d) the loading weight of 100 mg. 6
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Fig. 5. The diagram for the slipping progress of graphene with lubricate effect of hyperbranched polymer: (a) the interlocking arrangement of graphene nanosheets, and (b) different types of nanosheets under deformation.
conductive paths, d the tunneling distance, A2 the effective connection section, ϕ the height of the barrier between adjacent nanosheets, e and m are the charge and mass of electron, respectively. The strain is expressed as:
ε¼
d
d0 d0
¼
Δd d0
The experimental data and the fitting curves based on the expression (7) are plotted in Fig. 6. The agreement between the experimental data and the theoretical curve is achieved at Xd0 ¼ 4.96 as the strain ϵ < 35%. It’s suggested that graphene nanosheets are in contact that the layer becomes conductive by yielding threshold with an empirical value X � 14.59 nm [46]. The maximum value coincides with the Van Der Waals distance d0 ¼ 0.34 nm [52]. This tunnel conduction model fits well with the experimental results under 35% strain, which illustrates that the tunnel theory plays a main role in the transport of carriers in the gra phene film. The composite materials usually exhibit desirable conduc tivity with a strain less than 20% [53,54]. Obviously, the conductive network constructed here retains stable at a large strain. The small amount of hyperbranched polymer is adsorbed on the graphene surface, which enhances the sliding behavior of nanosheets during deformation. The flexible graphene film as electrodes has the potential to improve current strain sensor technology for promising applications. In addition, the resistance of the electrode is abruptly increased as the strain ϵ > 35%, which is ascribed to the reconstruction of the conductive path. The dense pattern of graphene layer is destructed, and the conductive network becomes disconnected. The dependence of conducting path N under applied strain is estimated as following [54–56]:
(5)
According to equations (3) and (5), the relationship between ln R/R0 and ε is obtained, where R0 and R reflect the resistance before and after the applied strain: ln
R ¼ Xd0 ε R0
lnðε þ 1Þ
(6)
From the equation (6), a small shape variable can be found that d0 is the power exponent of the surface resistance for the stretchable elec trode. The distance between them is exponential positively. As the variable deformation becomes large, the value of d increases, and the connection resistance Rl rises exponentially, which leads to the rapid increase of surface resistance for the conductive film. The sensitivity of the sensor improves exponentially with the expansion between gra phene as the film being elongation. Similarly, the surface resistance is inversely proportional to overlapping area, which gradually increases with the deformation of the film. For uniaxial shear strain, the relative slip distance of graphene is also described as: s/s0 ¼ (1 þ ϵ) [51]. Ac cording to the equation (6), the ΔR=R is directly proportional to the deformation ϵ, which obeys the tightly stacked (Fig. 5 Type one) model: ΔR eXd0 ε ¼ R ðε þ 1Þ2
1
N¼
N0 e½AεþBε2 �
(8)
where A and B are constants. Therefore, the change of the normalized resistance is also described as: � ðXd0 εþAεþBε2 ΔR e ¼ 1 (9) R ðε þ 1Þ2
(7)
In Fig. 6 the agreement between the experimental data and the theoretical curve is fitted well at the strain 35% < ε < 55% (Xd0þA ¼ 7.1 and B approaching to 0). The overlapping of graphene is decreased as the strain is larger than 35%, which leads to the high resistance. The flexible graphene film is assembled by the few-layer twodimension lamellae by the deposition technology. Considering that the hyperbranched polymer is attached on the surface of nanosheets that is inadequate motion for electron among adjacent layers, it’s deduced that the charge transfer accounts for the low resistance of graphene film in this work. The distinguishable interlock-tile re-allocation of nanosheets in a near-parallel construction is responsible for the compliant nano sheets on the horizontal direction [57]. The external force results in the uniform arrangement of stress, which is transferred through the shear motion of interlamellar, in which the hyperbranched polymer with spherical topology improves the sliding of adjacent nanosheets. Based on the combination of graphene sliding and tunneling effect, the inter laminar shearing effect of few-layer nanosheets retains low resistance of
Fig. 6. Experimental data (dots) and theoretical curves (solid lines) of ΔR/R as a function of strain for graphene film. 7
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the graphene film during large deformation, which provides a new insight of flexible electrode for advanced electronics.
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4. Conclusions In summary, we reported a general preparation scheme for flexible film electrode, which could be applied in a sensing device. The few-layer graphene was liquid-phase exfoliated with HBPE-PTFEMA as stabilizer attached on the surface of nanosheets via CH- π non-covalent in teractions. The graphene film was deposited on the elastomer as a stacked conductive layer to monitor the resistance variation under deformation. The relative resistance change is 117% at strain of 35%, which is ascribed to the reversible motion of interlaminar nanosheets with the lubricant effect of hyperbranched polymer. The resistance of the graphene two-dimensional electrode material in the deformation process is dependent on the overlapping area of graphene and the dis tance between the nanosheets. The sliding behavior of few-layer nano sheets involves multiple interactions and interlaminar shear motion, which promotes the appreciation of the patterns for nanosheets under large deformation. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Yunfei Zhu: Formal analysis, Investigation, Writing - original draft. Hongyun Chen: Investigation, Data curation, Writing - original draft. Lixiang Jiang: Writing - review & editing. Lixin Xu: Writing - review & editing. Huijian Ye: Conceptualization, Writing - review & editing. Acknowledgements The financial support from the National Natural Science Foundation of China (51707175, 11502025) and Natural Science Foundation of Zhejiang Province of China (LTZ20E070001, LY18B040005) is greatly appreciated. This work is also supported by China Postdoctoral Science Foundation (2018M640572). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.polymer.2020.122301. References [1] J.A. Rogers, T. Someya, Y. Huang, Science 327 (5973) (2010) 1603–1607. [2] Q. Zhai, X. Zhang, Y. Han, J. Zhai, J. Li, E. Wang, Anal. Chem. 88 (1) (2015) 945–951. [3] X. Zhu, Q. Zhai, W. Gu, J. Li, E. Wang, Anal. Chem. 89 (22) (2017) 12108–12114. [4] H. Xing, X. Zhang, Q. Zhai, J. Li, E. Wang, Anal. Chem. 89 (7) (2017) 3867–3872. [5] Q. Zhai, X. Zhang, J. Li, E. Wang, Nanoscale 8 (33) (2016) 15303–15308. [6] F. Garnier, R. Hajlaoui, A. Yassar, P. Srivastava, Science 265 (16) (1994) 1684–1686. [7] Z.N. Bao, Y. Feng, A. Dodabalapur, V.R. Raju, Andrew J. Lovinger, Chem. Mater. 9 (6) (1997) 1299–1301. [8] D.H. Kim, J.H. Ahn, W.M. Choi, H.S. Kim, T.H. Kim, J. Song, Y.Y. Huang, Z. Liu, C. Lu, J.A. Rogers, Science 320 (5875) (2008) 507–511. [9] X. Li, P. Sun, L. Fan, M. Zhu, K. Wang, M. Zhong, J. Wei, D. Wu, Y. Cheng, H. Zhu, Sci. Rep. 2 (395) (2012) 1–8. [10] C.S. Boland, U. Khan, C. Backes, A. O’Neill, J. McCauley, S. Duane, R. Shanker, Y. Liu, I. Jurewicz, A.B. Dalton, J.N. Coleman, ACS Nano 8 (9) (2014) 8819–8830. [11] D.J. Lipomi, M. Vosgueritchian, B.C.K. Tee, S.L. Hellstrom, J.A. Lee, C.H. Fox, Z. N. Bao, Nat. Nanotechnol. 6 (2011) 788–792.
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