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Continuous graphene and carbon nanotube based high flexible and transparent pressure sensor arrays
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 115501 (http://iopscience.iop.org/0957-4484/26/11/115501) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 26 (2015) 115501 (10pp)
doi:10.1088/0957-4484/26/11/115501
Continuous graphene and carbon nanotube based high flexible and transparent pressure sensor arrays Xiaoxiang Zhang1, Songtao Hu1, Meng Wang1, Jia Yu1, Qasim Khan2, Jintang Shang3 and Long Ba1 1
State Key Laboratory of Bioelectronics, School of Biology and Medical Engineering and Department of Physics, Southeast University, Nanjing 210096, People’s Republic of China 2 Research Center of Display Technology, School of Electronic Engingeering, Southeast University, Nanjing 210096, People’s Republic of China 3 Key laboratory of MEMS of Ministry of Education, School of Electronic Engineering, Southeast University, Nanjing 210096, People’s Republic of China E-mail: [email protected] Received 27 October 2014, revised 24 December 2014 Accepted for publication 7 January 2015 Published 25 February 2015 Abstract
The transparent pressure sensing arrays durable to severe deformation are fabricated by covering the continuous graphene sheets on the tip of thermal plastic polyurethane (TPU) pyramids, while most of the TPU surface is covered by a layer of densely entangled single wall carbon nanotubes. The transparency of the conducting layer exceeds 91%. The capacitance variations between TPU surface and flat electrode under compressive deformation show high sensitivity and a broad dynamic range from hundreds Pa to MPa. The measured capacitance variations show high load sensitivity and stability under repeated deformation cycles. Finite element numerical simulations present that the contact area change under deformation increases the capacitance variation. The high stability of the capacitance response to fluctuated loads demonstrates that graphene layer on the surface of TPU pyramids maintains the continuity of electric contact under a large deformation ratio and high repeating cycles. 16 × 16 arrays are connected to a circuit and a typical load distribution is regenerated by mapping the local capacitance variations on the arrays with sub-minimeter spatial resolution. Keywords: sensors/biosensors, polymeric materials, flexible, transparent, graphene (Some figures may appear in colour only in the online journal) 1. Introduction
nanorod collective piezoelectricity [17], high sensitive pressure sensor on microstructured elastomer matrix [6], strain sensors on soft material using organic electronics [18–21]. These approaches demonstrate that the high pressure sensitivity, high repeatability or high deformation ratio can be realized individually based on different principles (capacitance [6, 22, 23], contact resistance [2, 8, 10, 11], piezoelectricity [15–17], percolative piezoresistance [3, 12, 13], field effect [18, 20, 24]). Moreover, it exists huge demands for the flexible pressure sensors or sensor assays which have the combined properties of high pressure sensitivity, broad sensing range, high repeatability and large compressive ratio
There are continuous efforts toward the fabrication of flexible pressure sensors, which are essential in the fields like sensing methology of the mechanical properties of soft materials, consumer electronics, ergonomic and bionic technology [1– 10]. These technologies are integrated for the rapid innovations such as load sensitive touch screen, electronic skin and wearable electronics for health monitoring. Recent years, several technique strategies have been proposed to fabricate flexible elastomer strain sensor using deposited carbon nanotubes [8, 11], tensile and compressive stress sensor using nanoparticle fillers [3, 12–14], isolate nanowires [15, 16] or 0957-4484/15/115501+10$33.00
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© 2015 IOP Publishing Ltd Printed in the UK
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2. Experimental
without apparent loss of light transparency in the visible range. For the purpose of sensing pressure from kPa to MPa (which is the elasticity range of most elastomers), we use thermal plastic polyurethane (TPU) elastomer as matrix material because its wide stiffness range enables tunable sensing load ranges. The soft pressure sensor or sensor arrays made of polydimethylsilicone (PDMS) (PDMS were demonstrated using capacitance and field effect [6] or contact resistance [10], presenting pressure sensitivity limit can be as low as several Pa through measurements of capacitance variation or field effect or contact resistance over large area between deposited conducting layer (organic molecules or nanotubes) on structured surface. The TPUs have intrinsic advantages over PDMS as the matrix material of elastomer stress/strain sensor on several issues. Its broken elongation ratio is much higher and residual deformation is much lower than PDMS. The stiffness of most PUs can be tuned by changing the combination of their contents, while stiffness of PDMS is generally tuned by filling various volume ratio inorganic particles or by change the content of vinyl group, both methods will alter the collective properties of stiffness, processability, and light transparency of commercial material. Here we adopt the load sensing principle based on the capacitance variation between the deformed conducting pyramid arrays and flat plate. By covering the pyramids with thin conducting layer, the capacitance variation sensitivity between compressed tip and flat surface shall be much higher than planar structures. Because that the high repeating cycles of compressive deformation will cause the peeling off and cracking of the conducting layer from the elastomer surface, the method to maintain its electric connection after severe deformation is essential for this strategy. The recent developments on the fabrication and transfer of large size graphene sheets from catalystic substrate to the target flat substrate [25, 26] enable the wide applications of this high transparent and high electric conducting material. The carbon nanotubes, especially single wall carbon nanotubes (SWCNTs), have been used as flexible and transparent conducting thin film for multiple electronic devices [27–29]. We deposited large size CVD graphene on the structured TPU surface surface as continuous electrode to create the largest electric charge. To ensure the electric connection of less continuous graphene layer under deformation, we deposited SWCNTs on the surface before depositing graphene. Our tests demonstrated that the high sensitivity of the capacitance change under compressive deformation relies on the perfectness of conducting layer during deformation and the high repeatable capacitance change versus load promises application possibilities based on this less complicate and low cost design, which enables precise stress–strain measurement of soft materials and load distribution mapping with size of sensing elements less than hundreds micrometers for wide range applications from engineering sensors of soft materials to consumer electronics of tactile sensor arrays.
2.1. Fabrication of elastomer membrane with pyramidal arrays
The silicon mold with pyramidal hole arrays was fabricated using standard wet microfabrication routine. After coating photoresin on (100) silicon wafer which has 500 nm oxide layer and undertaking UV light lithograph, the exposured photoresin on silicon wafer was developed and the oxide layer on the surface of exposed squares was first etched using dilute HF aqueous solution, then etched within the mixture solution of tetramethyl ammonium hydroxide (TMAH, 25% water solution, Aldrich) and isopropanol with volume ratio of 1:1 at temperature of 70 °C for 6 h. The pyramid void arrays were generated by this heterogeneous etching. The BASF TPU (60A, Elastollan) with Shore’s hardness of 60 was used in this study. The pre-fabricated sheet of 0.5 mm thick was pressed onto the silicon mold under pressure of 0.3 MPa. Then the kit was heated to 170 ± 1 °C for 10 min in a vacuum chamber with air pressure less than 10 Pa. The surface of the TPU membrane takes the cavity geometry of the negative silicon mold after peeling off the deformed TPU. 2.2. Measurements of the mechanical, optical and electric properties
The stress–strain plots were measured on a material test device. All tests were performed under constant deformation velocity mode with the displacement rate of 0.02 mm min−1. The samples for mechanical property measurement are ∅8 mm disks. The thickness of the sample is 0.235 mm. A 0.5 mm thick copper slice with smooth surface was used as counter plate on which the tip of the pyramids was pressed for the measurements of the strain–stress plots. The optical transparency of the TPU plates covered with different SWNTs densities with and without graphene layer was measured using a SHIMADZU RF-5301PC spectrometer, while electric resistance of samples of different SWNTs densities and their contact resistance with graphene layer were measured using four probe method. The contact resistance between SWNTs and graphene was measured by template deposition of the continuous graphene on the half covered TPU plate with SWNTs. The silver paste was dotted on SWNTs region and on the graphene region on the uncovered part of TPU substrate. 2.3. Fabrication of conducting layers and piezocapacitance measurement
The SWCNTs were dispersed is sodium dodecyl sulfate aqueous solution (5% wt) by ultrasonic treatment. The solution with SWNTs density of about 0.2 mg ml−1 was dropped on the structured TPU surface to achieve transparent, flexible and electric conducting layer. The samples for electric properties were prepared by spin coating monodispersive SWNTs solution on TPU plate. The density of SWNTs was tuned by altering the coating layers. The dried sample was immersed in 10 M sulfuric acid for 10 s and then washed by DI water. The TPU surface covered by continuous indium tin oxide (ITO) 2
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Figure 1. (a) Graphene deposition on the structured TPU by softening PMMA supported graphene using hexane vapor, (b) fabrication
process of the electric conducting, flexible, transparent TPU covered by SWCNTs and graphene on the protruded pyramids.
was fabricated using conventional rf. sputtering technique with a layer 10–20 nm thick SiO2 and a layer of 60–80 nm thick ITO. The graphene layer was deposited by using template transfer method. First, the CVD graphene on copper foil (obtained from Xiamen G-CVD Material Technology) was covered by a layer of poly(methylmethacrylate) (PMMA) using spin coating with thickness of about 200 nm on one side and the graphene layer on the other side was scratched off, because the graphene was grown on both side of copper foil and capper layer of the side without polymer template shall be exposed to be dissolved in solution. The PMMA is obtained from Sigma Aldrich with Mw of about 996 K and dissolved within ethylacetate with concentration of 5% wt. After baked at 180 °C for 10 min, the foil was immersed within the aqueous solution of iron nitride for 24 h. First the clean PMMA/ graphene bilayers was transferred on the top of TPU structure from DI water with the graphene layer contact to the TPU. Second, the bilayers covered sample with a thin layer of water between bilayers and sample surface was put into the vapor of hexane. The covered beaker with output leakage enables the simultaneous evaporation of hexane and drying of water on the sample (figure 1(a)). After 12 h exposure to the hexane vapor at room temperature, the sample was taken out of hexane vapor and fully dried at 50 °C. The fabrication process is described in figure 1(b). The capacitance between conducting layer on TPU and counter electrode was measured by applying an ac voltage onto the conducting layer on TPU and the grounded copper counter electrode, which was covered by a layer of 5 μm polystyrene. The capacitance was calculated from the current which was measured by using a Stanford 830 lock-in amplifier, while its internal signal voltage source was used as ac voltage with Vpp of 1 V and frequency of 38.7 kHz. The current was recorded simultaneously when the sample was vertically compressed on the material test device. The capacitance variations and stress versus strain relation of ten
sequential tests were acquired. The capacitance variations under fluctuated load were recorded by keeping a constant load on an electromagnetic actuator and the sample. A modulated current was send to the actuator to change the load on the sample. The capacitance was recorded in accordance with fluctuated load. 2.4. Numerical simulation of the deformation of elastomer pyramid under large strain
The numerical simulation of individual pyramid was conducted using the finite element simulation software ABAQUS 6.11.1. The calculation was established based on the model that the truncated tip pyramid with base plate pressed by rigid plate. 3D models were established to simulate the load versus deformation relations. The Poisson’s ratios of the materials were assumed as 0.45 and the strain–stress response of polyurethane was taken from experimental data pressed on a 1 mm thick ∅8 mm plate from 0 to 0.87 MPa. Mooney– Rivilin model was used for the recovery of material data of biaxial tensile from experimental results of uniaxial compression. The hybrid standard quadratic elements C3D10H were applied to 3D models. All simulations use standard static and constant displacement boundary condition. 2.5. Load distribution measurement
The 16 × 16 matrix was designed for capacitance measurement, which allows each element contains one pyramid by slicing the arrays into parallel strips using laser. The capacitance of each element was measured by applying an ac voltage between one pyramid strip and the pressed electrodes of perpendicularly aligned ITO strips on polyester (PE) film. The current through each element was read through a switch circuit and a Stanford 830 lock-in amplifier. The 38.7 kHz ac voltage with Vpp of 1 V was alternately applied to one of the parallel lines connecting the pyramids while all other 3
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friction constant between elastomer and top hard die plates was set to 0.15, while the bottom face was pinned to constrain normal displacement. A quarter of 3D asymmetric models were meshed and loaded with constant displacement (figure 3(a)). Considering the sharp tapped geometry, we assigned the Tet 3D meshes to the whole pyramid, the numerical calculations converge smoothly with compression ratio of 30% of the tapered region. The S Mises over all the meshes (figure 3(b)) indicate that the highest stress locates at the center region of the pyramid, nearly 1/4 height to the tip of the pyramid region. The outer surface of the pyramid contact or uncontact to the die plate mainly suffers bending deformation. Simulation shows that the largest positive strains happen at the oblique edges contact with the die. This means that any non-elastic conducting layer attached to the out surface could be broken by extensive strain under continuous multiple deformations. The application like transparent tactile sensor needs repeatability at least more than 104 deformations. Though there is strong adhesion of TPU with most metal and oxide conducting materials, the large deformation will cause the cracking of the conducting layer and loss of conductivity. The static apparent stress versus strain (σ/ε) response of the TPU slices with surface pyramids structure was measured under constant compressive deformation velocity (figure 3(c)). To calculate the deformation process and stress, strain distribution, the hyperelastic model was used, and the uniaxial strain versus stress test data were inputted. The Mooney–Rivilin model was selected to fit the material property data. The apparent stiffness of structured samples is obviously lower than bulk sample, which means the excessive compressive deformation of this viscoelastic material at the tip of pyramid has no obvious hardening effect. The analytical models established for largely compressed rubber can give out calculations of the apparent elastic modulus, with the apparent modulus and contact stress expressed as the function of Young’s modulus of the soft material and the shape factor [30]. The recent analytical and numerical study on the compressive rubber block shows that the apparent modulus and contact stress derived based on the assumptions of linear elastic incompressible rubber and plane strain are validated by finite element simulation and experiments [31].
Figure 2. Illustration of the measuring circuit of load distribution.
electrode lines were connected to ground. By picking the voltage drop on the sampling resistors which were connected to the counter electrodes biased to ac voltage, the capacitance between one pyramid and counter electrode was recorded (figure 2). Scanning the bias to all pyramid lines and recording the all the voltage drops on the sampling resistors, the capacitance variation on each pyramid was recorded. A 0.3 mm thick star was pressed on the surface of the films. Total load of 18 N was uniformly pressed on the star. The static capacitance distribution of each element was recorded for total 16 × 16 elements across the arrays before and after loading. The capacitance distribution was off-line processed after normalizing the capacitance of each element without loading. The load distribution of the star applied on the sample was visualized by converting the normalized capacitance change into bar diagram using MATLAB. 2.6. Scanning electron microscopy observation of pyramids and conducting layer under multiple deformations
The surface structure of the sample was observed using a Zeiss Ultraplus field-emission scanning electron microscope (Zeiss Ultraplus FESEM, Carl Zeiss SMT) under conditions of 10 kV, 10−7 Pa.
3. Results and discussions 3.1. Deformation of pyramid under vertical compression
3.2. Microstructure of the SWNTs and graphene covered pyramid arrays, optical transparency of SWNTs covered by graphene and contact resistance between graphene and SWNTs layers
Due to the geometric structure of the pyramid array, the effect coverage of the pyramid surface is essential for capacitance sensing of this device. The sensitivity and stability of capacitance variation under load or deformation is also affected by the effective electric connection of the conducting layer on the pyramids with the conducting layer on the flat region. We first analyzed the deformation of TPU pyramid arrays using experimental and numerical methods. The simulation results using the same geometric parameters of the measured sample (height H: 135 μm and tapered angle θ: 54.7° of the tip, thickness h: 100 μm of the base layer, size of the root square L1: 200 μm and size of base L2: 300 μm) give the deformation process of the soft tip pressed by flat plane. The
From the SEM images, the SWCNTs were observed being uniformly deposited on both the flat region and the surface of the pyramids. Due to surface tension, the density on the tip region is much lower than the root and flat regions (figures 4(b) and (c)). The most of the deposited graphene sheets covered the top end of the tip, which can be indicated by the brighter contrast covered region of the surface of pyramid (figure 4(d)). The original continuous ITO layer (figure 4(e)) deposited on the TPU cracks on both the pyramid and the flat regions, and loss conductivity after repeating 4
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Figure 3. (a) Geometry and mesh model of finite element simulation, (b) S Mises distribution of the compressed pyramid model. (c)
Measured and simulated stress versus strain relations of the TPU pyramid arrays with size of L: 200 μm, H: 135 μm, gap: 50 μm.
Figure 4. SEM images of SWCNTs, SWCNTs plus graphene, ITO, and ITO plus graphene coated TPU pyramid arrays, (a) SWCNTs
deposited on a pyramid, (b) tip region of a pyramid covered by SWCNTs, (c) root region of a pyramid covered by SWCNTs, (d) above sample covered by graphene layer through template transfer, (e) as-fabricated ITO covered pyramid arrays, (f) ITO covered sample after times compression, (g), (h) SWNTs deposited sample covered by graphene. Scale of image A–H is 30, 5, 2, 30, 100, 90, 60 and 150 μm, respectively.
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Table 1 shows the electric conductivity and optical transparency of the SWCNTs layer of various repeating coating layers. Comparing the transparency loss of SWCNTs coated TPU with the graphene on TPU of similar resistance, the transparency of graphene is higher than SWCNTs. Thus the graphene on entangled nanotube structure on the TPU pyramid arrays only increases the electric conductivity at the tip region and does not decreases the total optical transparency. The much high contact resistance than the resistance of both nanotube layer and graphene layer means there probably is heterojunction between graphene and SWCNTs. The details of the graphene covered on the tip of the pyramid (figure 5(a)) and flat SWCNTs layer (figure 5(b)) clear indicate that graphene layer keeps continuous at the scale of tens micrometers on both tip and the flat SWCNTs. Though the contact resistance is high, the continuity of the graphene sheet guarantees the stability of capacitance variation during prolong deformation cycles.
Table 1. Optical transparency, resistance of the coated SWCNTs layer on TPU, graphene sheet on TPU and contact resistance graphene on SWNTs layer (Rc□).
Layers 2 4 10 20 Graphene
I/I0 (450 nm)
I/I0 (650 nm)
R□(Ω)
Rc□(Ω)
98.7% 92.8% 93.1% 93.3% 94.6%
98.3% 92.6% 93.0% 93.2% 95.1%
4.3 M 33.4 k 3.1 k 1.5 k 4.2 k
259 k 136 k 84 k
I/I0 (450 nm) and (650 nm) means the measured optical transparency at wavelength of 450 nm and 650 nm.
compressive deformation (figure 4(f)) (square resistance increase from 200 Ω at as fabricated state to 3 MΩ after several compressive deformation). After transferring the CVD graphene grown on copper foil using PMMA [25] onto the SWCNTs covered TPU surface, the graphene/PMMA bilayers were on the top of the pyramid arrays, enabling the broken graphene attaches to the surface of TPU (figures 4(g) and (h)). The bright region on each pyramid tip means that the electric conductivity at this region is lower than root region. This could be introduced by low conductivity of PMMA template layer, and show the pattern of the deposited graphene (figure 4(g)). Considering the TPU can be swollen by most organic solvent, which can give additional expansion of the substrate, the deposited graphene will be generated more wrinkles than unexpanded substrates. These wrinkles tolerate the extensive deformation and the graphene sheets can maintain electric connection under severe compressive deformation. The square resistance of the structured surface covered by SWCNTs with density of about 0.05 mg cm−2 (estimated from the density of the SWCNTs solution dropped and dried on structured TPU) is ∼5 kΩ. After depositing of graphene and multiple bending or compressing deformations (100 times), the electric conductivity keeps no obvious change. For this asymmetry structure, the largest deformation is the macroscopic buckle, which will not happen for the symmetry structure of tapped cone. The peeling and cracking of the conducting surface layer is caused by the strain mismatch between conducting layer and matrix material and the low elasticity of the metallic or ceramic conductor. The graphene sheet composes large amount of wrinkles and has very low bending stiffness, it can withstand moderate tensile or large compressive deformation. In our case, the coverage of the large continuous graphene sheets on the pyramids arrays is the region of largest deformation. The PMMA layer acts as both the support during template transfer process and the anchoring effect after graphene was deposited on TPU. The much enhanced electric conductivity stability under severe deformation demonstrates that the peeling and cracking of SWCNTs layer and graphene sheets are less significant after repeating deformation. This opens wide opportunities for the application of this soft and transparent conducting elastomer structure.
3.3. Sensitivity and stability of capacitance variation under constant and fluctuated loads
The microscopy observation proves that large graphene sheets deposited on the TPU structured surface cover the region suffered severe deformation. The protruded geometry causes the planar graphene can only cover the part of the pyramid surface, while the flat region of the TPU surface remains uncovered. The coverage of large continuous graphene sheets on this bump surface mainly covers the top end and will provide higher charge density than nanotubes, thus the higher capacitance variation. Because the direct measurement of effect electric conductivity between graphene and nanotube layer is difficult, the comparison of capacitance change between the samples with and without graphene layer gives the evidence of the effect of graphene layer. The measured ΔC/C0 plots under compression deformation exhibit much higher sensitivity for the sample with graphene layer than that without graphene layer (figure 6). The sensitivity of the sample covered with graphene at 0.1 MPa is 4.79 MPa−1 (average from first ten tests), which that of the sample without graphene coverage is 0.54 MPa−1, almost nine times lower than that with graphene layer. It can be noted from FE simulation that the contact area increases from 8 × 10−2 to 2% of the sample area when pressure varies from zero to 0.57 MPa. This means that there is plenty space to increase the sensitivity for this approach by decrease the stray capacitance of the measurement circuit. The ΔC/C0 plots of the sample covered by ITO layer were also measured. The sensitivity of fresh made sample is two folds higher than graphene covered sample. The repeatability of the capacitance variation for the graphene covered sample also shows high consistent. The decreased ΔC/C0 of first several compressive tests could be caused by the stress relaxation of TPU and the trivial electric contact loss of the graphene layer under repeating load. This decrease is not introduced by the deterioration of the conducting layer or the pyramid structure, because this inchworm can also be reflected in load-displacement plots. The ΔC/C0 versus calibrated strain shall have 6
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Figure 5. SEM images show the details of graphene deposited on SWCNTs coated TPU pyramid (a) and on the SWCNTs coated flat TPU
surface, scale 20 μm (a) and 5 μm (b).
Figure 6. Measured collective capacitance variation normalized over background under compressive deformation of initial four tests and the tenth of the sample covered by graphene and one test of the sample without graphene. Data were acquired during compressing process.
less inchworm drift during prolong loading cycles. The measurement on more loading cycles will be performed to clarify the stability of ΔC/C0 versus strain. The capacitance variations under multiple deformation cycles were recorded at different load scale. The fluctuated load was applied to the sample by an electromagnetic actuator between the die pair on the material test device, with constant load supplied and controlled by the device. It can be seen that the capacitance fluctuation takes the details of the pattern of the fluctuated applied load at any constant load scales (figure 7). Notice the tiny features of the measured fluctuated normal capacitance and compare the load fluctuation plots (figure 7(b)), the accuracy of the capacitance corresponds to the pressure can reach 0.1 kPa (three times S/N ratio), exceeding the load sensing limit of the material test device we used here. This means that the load sensing dynamic range reaches the order of 105. Comparing to thesensitivities and dynamic ranges of capacitance type load sensors based on
Figure 7. Dynamic measurements of capacitance under fluctuated
load at different load platures, (a) at near 0.4 MPa and (b) at near 0.1 MPa.
PDMS [6, 32] and polyolefin foam [22], our sensitivity is higher or similar than these devices, but our approach shows much broader sensing range. Comparing to the resistance type load sensor [10], which shows much higher load sensitivity 7
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and sensing low limit, our moderate sensitivity and broad sensing range enable even wider applications. The graphene sheets on the pyramid surface can tolerate very high repeating deformation with both full load to the scale of MPa and fluctuated load of kPa p–p scale. Our repeatability tests prove that the contact resistance between most graphene sheets and SWCNTs layer remain near constant at least after tens full scale compression to the load of 0.4 MPa and hundreds fluctuation of p–p 0.05 MPa. The very quick decay of the sensitivity of the ITO covered sample presents a negative example that the loss of perfectness of the conducting layer on the tip limits the sensing stability of brittle conducting layer covered sample. The measured ΔC/C0 sensitivity limit corresponding to the load or deformation of our approach depends on the sensitivity of capacitance measurements, the size of the sample and stray capacitance of the electric circuit. The noise level from lock-in amplifier is 10−2 mV. Higher detection sensitivity can be realized by suitably depressing stray capacitance with more precious deformation steps.
C Δ Cij /Cij
0.15 0.1 0.05 0 -0.05 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
3.4. Mapping of load distribution with sub-minimeter spatial resolution
The transparency of the pyramid arrays deposited by SWCNTs and graphene sheets can be seen in the photographic image (figures 8(a) and (d)). Comparing the samples before and after deposition of SWCNTs and graphene, the visible light transparency is 91%. By slicing the conducting layer of both the pyramid arrays and counter electrode into parallel lines, each pyramid was connected to the sampling resistor (figure 2). Stacking the pyramid arrays onto the counter electrode by perpendicularly crossing the sliced lines of electric connected pyramids and ITO lines of the same width on the counter electrodes, the pyramids are right on top of flat counter ITO strips. Scanning the ac bias voltage applied to one pyramid strip while keeping the voltage on the rest at ground, the capacitance variation between individual pyramid and counter electrode was measured after alternately scanning all the pyramid strips and measuring the voltage drops on each sampling resistor. A 0.3 mm thick star on the flat glass substrate (figure 8(b)) was pressed on the top of counter electrode to simulate a load distribution. The measured load distribution on 16 × 16 pixels were plotted in (figure 8(c)), with the normalized ΔCij/Cij as signal heights. The size of individual pyramid is the lateral resolution of the load distribution mapping. Though the higher resolution could be possible by fabricating smaller pyramids, the capacitance variation on smaller pyramid will also be smaller. The realization of higher later load resolution depends mainly on the effective eliminating of stray capacitance. The stray capacitance of single element of our set-up is about 35 fF. The uneven electric conductivity change of each pyramid before and after deformation will also add measurement errors. We applied PE as counter electrode substrate. The high stiffness of PE causes the spreading of load when local uneven distributed load was applied on the top of PE layer. This causes the distortion in the reconstructed image (figure 8(c)). The
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Figure 8. (a) Transparent TPU pyramid arrays covered with graphene and nanotubes, (b) the plastic star on glass substrate as loading embossment, (c) reconstructed 3D image of load distribution where coloured bars indicate the location and height indicates the intensity, and (d) tortured TPU sample with transparent conducting layer.
spatial and load resolution of the sensor arrays can be improved by more effectively depressing the stray capacitance in the measurement circuit and using more flexible counter electrode.
4. Conclusions By depositing SWCNTs on the TPU surface with pyramid surface structure, then depositing graphene sheet on the 8
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surface of pyramids, the flexible and visible light transparent pressure sensing arrays are fabricated. The surface electric resistance of this elastomer device remains at level of 5 kΩ after severe deformation. Numerical simulation using infinite element method presents the stress and local deformation distribution. The capacitance variation of under compressive deformation on the pyramid arrays shows high stability, high sensitivity and dynamic range from sub-kPa to MPa. Microscopic observation shows that the graphene layer covers the tip end of the pyramids, while nanotubes cover the most region of the TPU surface. The comparison of the capacitance variation between flat electrode and surface structured TPU covered by SWCNTs plus graphene and only SWCNTs proves that the graphene layer on the pyramids has enough electric connection with the SWCNTs and much enhanced capacitance sensitivity. The measurements of capacitance variation under steady loads present that the load detect sensitivity is 4.79 MPa−1, nine times of that obtained from the sample without graphene, while the measurements under fluctuated loads present that load sensing limit can reach 0.1 kPa, and sensing stability keeps stable at least after hundreds. A 16 × 16 arrays are connected to a circuit and a typical load distribution is regenerated by mapping the capacitance variation of the arrays.
[8] Yamada T, Hayamizu Y, Yamamoto Y, Yomogida Y, Izadi-Najafabadi A, Futaba D N and Hata K 2011 A stretchable carbon nanotube strain sensor for human-motion detection Nat. Nanotechnology 6 296–301 [9] Schwartz G, Tee B C K, Mei J G, Appleton A L, Kim D H, Wang H L and Bao Z N 2013 Flexible polymer transistors with high pressure sensitivity for application in electronic skin and health monitoring Nat. Commun. 4 1859 [10] Wang X W, Gu Y, Xiong Z P, Cui Z and Zhang T 2014 Silkmolded flexible ultrasensitive and highly stable electronic skin for monitoring human physiological signals Adv. Mater. 26 1336–42 [11] Lipomi D J, Vosgueritchian M, Tee B C K, Hellstrom S L, Lee J A, Fox C H and Bao Z N 2011 Skin-like pressure and strain sensors based on transparent elastic films of carbon nanotubes Nat. Nanotechnology 6 788–92 [12] Chun K Y, Oh Y, Rho J, Ahn J H, Kim Y J, Choi H R and Baik S 2010 Highly conductive printable and stretchable composite films of carbon nanotubes and silver Nat. Nanotechnology 5 853–7 [13] Cochrane C, Koncar V, Lewandowski M and Dufour C 2007 Design and development of a flexible strain sensor for textile structures based on a conductive polymer composite Sensors 7 473–92 [14] Gullapalli H, Vemuru V S M, Kumar A, Botello-Mendez A, Vajtai R, Terrones M, Nagarajaiah S and Ajayan P M 2010 Flexible piezoelectric ZnO–paper nanocomposite strain sensor Small 6 1641–6 [15] He R R and Yang P D 2006 Giant piezoresistance effect in silicon nanowires Nat. Nanotechnology 1 42–6 [16] Wu W Z, Wen X N and Wang Z L 2013 Taxel addressable matrix of vertical-nanowire piezotronic transistors for active and adaptive tactile imaging Science 340 952–7 [17] Liu N S, Fang G J, Wan J W, Zhou H, Long H and Zhao X Z 2011 Electrospun PEDOT PSS–PVA nanofiber based ultrahigh-strain sensors with controllable electrical conductivity J. Mater. Chem. 21 18962–6 [18] Sekitani T, Yokota T, Zschieschang U, Klauk H, Bauer S, Takeuchi K, Takamiya M, Sakurai T and Someya T 2009 Organic nonvolatile memory transistors for flexible sensor arrays Science 326 1516–9 [19] Sekitani T, Zschieschang U, Klauk H and Someya T 2010 Flexible organic transistors and circuits with extreme bending stability Nat. Mater. 9 1015–22 [20] Cosseddu P, Tiddia G, Milita S and Bonfiglio A 2013 Continuous tuning of the mechanical sensitivity of pentacene OTFTs on flexible substrates: from strain sensors to deformable transistors Org. Electron. 14 206–11 [21] Nam S H, Jeon P J, Min S W, Lee Y T, Park E Y and Im S 2014 Highly sensitive non-classical strain gauge using organic heptazole thin-film transistor circuit on a flexible substrate Adv. Funct. Mater. 24 4413–9 [22] Metzger C, Fleisch E, Meyer J, Dansachmuller M, Graz I, Kaltenbrunner M, Keplinger C, Schwodiauer R and Bauer S 2008 Flexible-foam-based capacitive sensor arrays for object detection at low cost Appl. Phys. Lett. 92 013506 [23] Hu W L, Niu X F, Zhao R and Pei Q B 2013 Elastomeric transparent capacitive sensors based on an interpenetrating composite of silver nanowires and polyurethane Appl. Phys. Lett. 102 083303 [24] Trung T Q, Tien N T, Kim D, Jang M, Yoon O J and Lee N E 2014 A flexible reduced graphene oxide field-effect transistor for ultrasensitive strain sensing Adv. Funct. Mater. 24 117–24 [25] Li X S et al 2009 Large area synthesis of high quality and uniform graphene films on copper foils Science 324 1312–4 [26] Li X S, Zhu Y W, Cai W W, Borysiak M, Han B Y, Chen D, Piner R D, Colombo L and Ruoff R S 2009 Transfer of large area graphene films for high performance transparent conductive electrodes Nano Lett. 9 4359–63
Acknowledgments The authors acknowledge the financial supports from the Natural Science Foundation of China (No. 10774022) and Jiangsu Provincial Social Development Foundation (No. BE2009665).
References [1] Zhou J, Gu Y D, Fei P, Mai W J, Gao Y F, Yang R S, Bao G and Wang Z L 2008 Flexible piezotronic strain sensor Nano Lett. 8 3035–40 [2] Pang C, Lee G Y, Kim T I, Kim S M, Kim H N, Ahn S H and Suh K Y 2012 A flexible and highly sensitive strain-gauge sensor using reversible interlocking of nanofibres Nat. Mater. 11 795–801 [3] Knite M, Teteris V, Kiploka A and Kaupuzs J 2004 Polyisoprene–carbon black nanocomposites as tensile strain and pressure sensor materials Sensors Actuators A 110 142–9 [4] Someya T, Sekitani T, Iba S, Kato Y, Kawaguchi H and Sakurai T 2004 A large-area flexible pressure sensor matrix with organic field-effect transistors for artificial skin applications Proc. Natl. Acad. Sci. USA 101 9966–70 [5] Kim D H et al 2011 Epidermal electronics Science 333 838–43 [6] Mannsfeld S C B, Tee B C K, Stoltenberg R M, Chen C, Barman S, Muir B V O, Sokolov A N, Reese C and Bao Z N 2010 Highly sensitive flexible pressure sensors with microstructured rubber dielectric layers Nat. Mater. 9 859–64 [7] Engel J, Chen J and Liu C 2003 Development of polyimide flexible tactile sensor skin J. Micromech. Microeng. 13 359–66 9
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flexible transparent conducting films J. Am. Chem. Soc. 129 7758 [30] Gent A N and Lindley P B 1959 Internal rupture of bonded rubber cylinders in tension Proc. R. Soc. A 249 195 [31] Suh J B and Kelly S G 2012 Stress response of a rubber block under vertical loading J. Eng. Mech. ASCE. 138 770–83 [32] Lee H K, Chang S I and Yoon E 2006 A flexible polymer tactile sensor: fabrication and modular expandability for large area deployment J. Microelectromech. Syst. 15 1681–6
[27] Zhang M, Fang S L, Zakhidov A A, Lee S B, Aliev A E, Williams C D, Atkinson K R and Baughman R H 2005 Strong, transparent, multifunctional carbon nanotube sheets Science 309 1215–9 [28] Zhang D H, Ryu K, Liu X L, Polikarpov E, Ly J, Tompson M E and Zhou C W 2006 Transparent conductive, and flexible carbon nanotube films and their application in organic light-emitting diodes Nano Lett. 6 1880–6 [29] Geng H Z, Kim K K, So K P, Lee Y S, Chang Y and Lee Y H 2007 Effect of acid treatment on carbon nanotube-based
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Continuous graphene and carbon nanotube based high flexible and transparent pressure sensor arrays
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Nanotechnology Nanotechnology 26 (2015) 115501 (10pp)
doi:10.1088/0957-4484/26/11/115501
Continuous graphene and carbon nanotube based high flexible and transparent pressure sensor arrays Xiaoxiang Zhang1, Songtao Hu1, Meng Wang1, Jia Yu1, Qasim Khan2, Jintang Shang3 and Long Ba1 1
State Key Laboratory of Bioelectronics, School of Biology and Medical Engineering and Department of Physics, Southeast University, Nanjing 210096, People’s Republic of China 2 Research Center of Display Technology, School of Electronic Engingeering, Southeast University, Nanjing 210096, People’s Republic of China 3 Key laboratory of MEMS of Ministry of Education, School of Electronic Engineering, Southeast University, Nanjing 210096, People’s Republic of China E-mail: [email protected] Received 27 October 2014, revised 24 December 2014 Accepted for publication 7 January 2015 Published 25 February 2015 Abstract
The transparent pressure sensing arrays durable to severe deformation are fabricated by covering the continuous graphene sheets on the tip of thermal plastic polyurethane (TPU) pyramids, while most of the TPU surface is covered by a layer of densely entangled single wall carbon nanotubes. The transparency of the conducting layer exceeds 91%. The capacitance variations between TPU surface and flat electrode under compressive deformation show high sensitivity and a broad dynamic range from hundreds Pa to MPa. The measured capacitance variations show high load sensitivity and stability under repeated deformation cycles. Finite element numerical simulations present that the contact area change under deformation increases the capacitance variation. The high stability of the capacitance response to fluctuated loads demonstrates that graphene layer on the surface of TPU pyramids maintains the continuity of electric contact under a large deformation ratio and high repeating cycles. 16 × 16 arrays are connected to a circuit and a typical load distribution is regenerated by mapping the local capacitance variations on the arrays with sub-minimeter spatial resolution. Keywords: sensors/biosensors, polymeric materials, flexible, transparent, graphene (Some figures may appear in colour only in the online journal) 1. Introduction
nanorod collective piezoelectricity [17], high sensitive pressure sensor on microstructured elastomer matrix [6], strain sensors on soft material using organic electronics [18–21]. These approaches demonstrate that the high pressure sensitivity, high repeatability or high deformation ratio can be realized individually based on different principles (capacitance [6, 22, 23], contact resistance [2, 8, 10, 11], piezoelectricity [15–17], percolative piezoresistance [3, 12, 13], field effect [18, 20, 24]). Moreover, it exists huge demands for the flexible pressure sensors or sensor assays which have the combined properties of high pressure sensitivity, broad sensing range, high repeatability and large compressive ratio
There are continuous efforts toward the fabrication of flexible pressure sensors, which are essential in the fields like sensing methology of the mechanical properties of soft materials, consumer electronics, ergonomic and bionic technology [1– 10]. These technologies are integrated for the rapid innovations such as load sensitive touch screen, electronic skin and wearable electronics for health monitoring. Recent years, several technique strategies have been proposed to fabricate flexible elastomer strain sensor using deposited carbon nanotubes [8, 11], tensile and compressive stress sensor using nanoparticle fillers [3, 12–14], isolate nanowires [15, 16] or 0957-4484/15/115501+10$33.00
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© 2015 IOP Publishing Ltd Printed in the UK
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2. Experimental
without apparent loss of light transparency in the visible range. For the purpose of sensing pressure from kPa to MPa (which is the elasticity range of most elastomers), we use thermal plastic polyurethane (TPU) elastomer as matrix material because its wide stiffness range enables tunable sensing load ranges. The soft pressure sensor or sensor arrays made of polydimethylsilicone (PDMS) (PDMS were demonstrated using capacitance and field effect [6] or contact resistance [10], presenting pressure sensitivity limit can be as low as several Pa through measurements of capacitance variation or field effect or contact resistance over large area between deposited conducting layer (organic molecules or nanotubes) on structured surface. The TPUs have intrinsic advantages over PDMS as the matrix material of elastomer stress/strain sensor on several issues. Its broken elongation ratio is much higher and residual deformation is much lower than PDMS. The stiffness of most PUs can be tuned by changing the combination of their contents, while stiffness of PDMS is generally tuned by filling various volume ratio inorganic particles or by change the content of vinyl group, both methods will alter the collective properties of stiffness, processability, and light transparency of commercial material. Here we adopt the load sensing principle based on the capacitance variation between the deformed conducting pyramid arrays and flat plate. By covering the pyramids with thin conducting layer, the capacitance variation sensitivity between compressed tip and flat surface shall be much higher than planar structures. Because that the high repeating cycles of compressive deformation will cause the peeling off and cracking of the conducting layer from the elastomer surface, the method to maintain its electric connection after severe deformation is essential for this strategy. The recent developments on the fabrication and transfer of large size graphene sheets from catalystic substrate to the target flat substrate [25, 26] enable the wide applications of this high transparent and high electric conducting material. The carbon nanotubes, especially single wall carbon nanotubes (SWCNTs), have been used as flexible and transparent conducting thin film for multiple electronic devices [27–29]. We deposited large size CVD graphene on the structured TPU surface surface as continuous electrode to create the largest electric charge. To ensure the electric connection of less continuous graphene layer under deformation, we deposited SWCNTs on the surface before depositing graphene. Our tests demonstrated that the high sensitivity of the capacitance change under compressive deformation relies on the perfectness of conducting layer during deformation and the high repeatable capacitance change versus load promises application possibilities based on this less complicate and low cost design, which enables precise stress–strain measurement of soft materials and load distribution mapping with size of sensing elements less than hundreds micrometers for wide range applications from engineering sensors of soft materials to consumer electronics of tactile sensor arrays.
2.1. Fabrication of elastomer membrane with pyramidal arrays
The silicon mold with pyramidal hole arrays was fabricated using standard wet microfabrication routine. After coating photoresin on (100) silicon wafer which has 500 nm oxide layer and undertaking UV light lithograph, the exposured photoresin on silicon wafer was developed and the oxide layer on the surface of exposed squares was first etched using dilute HF aqueous solution, then etched within the mixture solution of tetramethyl ammonium hydroxide (TMAH, 25% water solution, Aldrich) and isopropanol with volume ratio of 1:1 at temperature of 70 °C for 6 h. The pyramid void arrays were generated by this heterogeneous etching. The BASF TPU (60A, Elastollan) with Shore’s hardness of 60 was used in this study. The pre-fabricated sheet of 0.5 mm thick was pressed onto the silicon mold under pressure of 0.3 MPa. Then the kit was heated to 170 ± 1 °C for 10 min in a vacuum chamber with air pressure less than 10 Pa. The surface of the TPU membrane takes the cavity geometry of the negative silicon mold after peeling off the deformed TPU. 2.2. Measurements of the mechanical, optical and electric properties
The stress–strain plots were measured on a material test device. All tests were performed under constant deformation velocity mode with the displacement rate of 0.02 mm min−1. The samples for mechanical property measurement are ∅8 mm disks. The thickness of the sample is 0.235 mm. A 0.5 mm thick copper slice with smooth surface was used as counter plate on which the tip of the pyramids was pressed for the measurements of the strain–stress plots. The optical transparency of the TPU plates covered with different SWNTs densities with and without graphene layer was measured using a SHIMADZU RF-5301PC spectrometer, while electric resistance of samples of different SWNTs densities and their contact resistance with graphene layer were measured using four probe method. The contact resistance between SWNTs and graphene was measured by template deposition of the continuous graphene on the half covered TPU plate with SWNTs. The silver paste was dotted on SWNTs region and on the graphene region on the uncovered part of TPU substrate. 2.3. Fabrication of conducting layers and piezocapacitance measurement
The SWCNTs were dispersed is sodium dodecyl sulfate aqueous solution (5% wt) by ultrasonic treatment. The solution with SWNTs density of about 0.2 mg ml−1 was dropped on the structured TPU surface to achieve transparent, flexible and electric conducting layer. The samples for electric properties were prepared by spin coating monodispersive SWNTs solution on TPU plate. The density of SWNTs was tuned by altering the coating layers. The dried sample was immersed in 10 M sulfuric acid for 10 s and then washed by DI water. The TPU surface covered by continuous indium tin oxide (ITO) 2
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Figure 1. (a) Graphene deposition on the structured TPU by softening PMMA supported graphene using hexane vapor, (b) fabrication
process of the electric conducting, flexible, transparent TPU covered by SWCNTs and graphene on the protruded pyramids.
was fabricated using conventional rf. sputtering technique with a layer 10–20 nm thick SiO2 and a layer of 60–80 nm thick ITO. The graphene layer was deposited by using template transfer method. First, the CVD graphene on copper foil (obtained from Xiamen G-CVD Material Technology) was covered by a layer of poly(methylmethacrylate) (PMMA) using spin coating with thickness of about 200 nm on one side and the graphene layer on the other side was scratched off, because the graphene was grown on both side of copper foil and capper layer of the side without polymer template shall be exposed to be dissolved in solution. The PMMA is obtained from Sigma Aldrich with Mw of about 996 K and dissolved within ethylacetate with concentration of 5% wt. After baked at 180 °C for 10 min, the foil was immersed within the aqueous solution of iron nitride for 24 h. First the clean PMMA/ graphene bilayers was transferred on the top of TPU structure from DI water with the graphene layer contact to the TPU. Second, the bilayers covered sample with a thin layer of water between bilayers and sample surface was put into the vapor of hexane. The covered beaker with output leakage enables the simultaneous evaporation of hexane and drying of water on the sample (figure 1(a)). After 12 h exposure to the hexane vapor at room temperature, the sample was taken out of hexane vapor and fully dried at 50 °C. The fabrication process is described in figure 1(b). The capacitance between conducting layer on TPU and counter electrode was measured by applying an ac voltage onto the conducting layer on TPU and the grounded copper counter electrode, which was covered by a layer of 5 μm polystyrene. The capacitance was calculated from the current which was measured by using a Stanford 830 lock-in amplifier, while its internal signal voltage source was used as ac voltage with Vpp of 1 V and frequency of 38.7 kHz. The current was recorded simultaneously when the sample was vertically compressed on the material test device. The capacitance variations and stress versus strain relation of ten
sequential tests were acquired. The capacitance variations under fluctuated load were recorded by keeping a constant load on an electromagnetic actuator and the sample. A modulated current was send to the actuator to change the load on the sample. The capacitance was recorded in accordance with fluctuated load. 2.4. Numerical simulation of the deformation of elastomer pyramid under large strain
The numerical simulation of individual pyramid was conducted using the finite element simulation software ABAQUS 6.11.1. The calculation was established based on the model that the truncated tip pyramid with base plate pressed by rigid plate. 3D models were established to simulate the load versus deformation relations. The Poisson’s ratios of the materials were assumed as 0.45 and the strain–stress response of polyurethane was taken from experimental data pressed on a 1 mm thick ∅8 mm plate from 0 to 0.87 MPa. Mooney– Rivilin model was used for the recovery of material data of biaxial tensile from experimental results of uniaxial compression. The hybrid standard quadratic elements C3D10H were applied to 3D models. All simulations use standard static and constant displacement boundary condition. 2.5. Load distribution measurement
The 16 × 16 matrix was designed for capacitance measurement, which allows each element contains one pyramid by slicing the arrays into parallel strips using laser. The capacitance of each element was measured by applying an ac voltage between one pyramid strip and the pressed electrodes of perpendicularly aligned ITO strips on polyester (PE) film. The current through each element was read through a switch circuit and a Stanford 830 lock-in amplifier. The 38.7 kHz ac voltage with Vpp of 1 V was alternately applied to one of the parallel lines connecting the pyramids while all other 3
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friction constant between elastomer and top hard die plates was set to 0.15, while the bottom face was pinned to constrain normal displacement. A quarter of 3D asymmetric models were meshed and loaded with constant displacement (figure 3(a)). Considering the sharp tapped geometry, we assigned the Tet 3D meshes to the whole pyramid, the numerical calculations converge smoothly with compression ratio of 30% of the tapered region. The S Mises over all the meshes (figure 3(b)) indicate that the highest stress locates at the center region of the pyramid, nearly 1/4 height to the tip of the pyramid region. The outer surface of the pyramid contact or uncontact to the die plate mainly suffers bending deformation. Simulation shows that the largest positive strains happen at the oblique edges contact with the die. This means that any non-elastic conducting layer attached to the out surface could be broken by extensive strain under continuous multiple deformations. The application like transparent tactile sensor needs repeatability at least more than 104 deformations. Though there is strong adhesion of TPU with most metal and oxide conducting materials, the large deformation will cause the cracking of the conducting layer and loss of conductivity. The static apparent stress versus strain (σ/ε) response of the TPU slices with surface pyramids structure was measured under constant compressive deformation velocity (figure 3(c)). To calculate the deformation process and stress, strain distribution, the hyperelastic model was used, and the uniaxial strain versus stress test data were inputted. The Mooney–Rivilin model was selected to fit the material property data. The apparent stiffness of structured samples is obviously lower than bulk sample, which means the excessive compressive deformation of this viscoelastic material at the tip of pyramid has no obvious hardening effect. The analytical models established for largely compressed rubber can give out calculations of the apparent elastic modulus, with the apparent modulus and contact stress expressed as the function of Young’s modulus of the soft material and the shape factor [30]. The recent analytical and numerical study on the compressive rubber block shows that the apparent modulus and contact stress derived based on the assumptions of linear elastic incompressible rubber and plane strain are validated by finite element simulation and experiments [31].
Figure 2. Illustration of the measuring circuit of load distribution.
electrode lines were connected to ground. By picking the voltage drop on the sampling resistors which were connected to the counter electrodes biased to ac voltage, the capacitance between one pyramid and counter electrode was recorded (figure 2). Scanning the bias to all pyramid lines and recording the all the voltage drops on the sampling resistors, the capacitance variation on each pyramid was recorded. A 0.3 mm thick star was pressed on the surface of the films. Total load of 18 N was uniformly pressed on the star. The static capacitance distribution of each element was recorded for total 16 × 16 elements across the arrays before and after loading. The capacitance distribution was off-line processed after normalizing the capacitance of each element without loading. The load distribution of the star applied on the sample was visualized by converting the normalized capacitance change into bar diagram using MATLAB. 2.6. Scanning electron microscopy observation of pyramids and conducting layer under multiple deformations
The surface structure of the sample was observed using a Zeiss Ultraplus field-emission scanning electron microscope (Zeiss Ultraplus FESEM, Carl Zeiss SMT) under conditions of 10 kV, 10−7 Pa.
3. Results and discussions 3.1. Deformation of pyramid under vertical compression
3.2. Microstructure of the SWNTs and graphene covered pyramid arrays, optical transparency of SWNTs covered by graphene and contact resistance between graphene and SWNTs layers
Due to the geometric structure of the pyramid array, the effect coverage of the pyramid surface is essential for capacitance sensing of this device. The sensitivity and stability of capacitance variation under load or deformation is also affected by the effective electric connection of the conducting layer on the pyramids with the conducting layer on the flat region. We first analyzed the deformation of TPU pyramid arrays using experimental and numerical methods. The simulation results using the same geometric parameters of the measured sample (height H: 135 μm and tapered angle θ: 54.7° of the tip, thickness h: 100 μm of the base layer, size of the root square L1: 200 μm and size of base L2: 300 μm) give the deformation process of the soft tip pressed by flat plane. The
From the SEM images, the SWCNTs were observed being uniformly deposited on both the flat region and the surface of the pyramids. Due to surface tension, the density on the tip region is much lower than the root and flat regions (figures 4(b) and (c)). The most of the deposited graphene sheets covered the top end of the tip, which can be indicated by the brighter contrast covered region of the surface of pyramid (figure 4(d)). The original continuous ITO layer (figure 4(e)) deposited on the TPU cracks on both the pyramid and the flat regions, and loss conductivity after repeating 4
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Figure 3. (a) Geometry and mesh model of finite element simulation, (b) S Mises distribution of the compressed pyramid model. (c)
Measured and simulated stress versus strain relations of the TPU pyramid arrays with size of L: 200 μm, H: 135 μm, gap: 50 μm.
Figure 4. SEM images of SWCNTs, SWCNTs plus graphene, ITO, and ITO plus graphene coated TPU pyramid arrays, (a) SWCNTs
deposited on a pyramid, (b) tip region of a pyramid covered by SWCNTs, (c) root region of a pyramid covered by SWCNTs, (d) above sample covered by graphene layer through template transfer, (e) as-fabricated ITO covered pyramid arrays, (f) ITO covered sample after times compression, (g), (h) SWNTs deposited sample covered by graphene. Scale of image A–H is 30, 5, 2, 30, 100, 90, 60 and 150 μm, respectively.
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Table 1 shows the electric conductivity and optical transparency of the SWCNTs layer of various repeating coating layers. Comparing the transparency loss of SWCNTs coated TPU with the graphene on TPU of similar resistance, the transparency of graphene is higher than SWCNTs. Thus the graphene on entangled nanotube structure on the TPU pyramid arrays only increases the electric conductivity at the tip region and does not decreases the total optical transparency. The much high contact resistance than the resistance of both nanotube layer and graphene layer means there probably is heterojunction between graphene and SWCNTs. The details of the graphene covered on the tip of the pyramid (figure 5(a)) and flat SWCNTs layer (figure 5(b)) clear indicate that graphene layer keeps continuous at the scale of tens micrometers on both tip and the flat SWCNTs. Though the contact resistance is high, the continuity of the graphene sheet guarantees the stability of capacitance variation during prolong deformation cycles.
Table 1. Optical transparency, resistance of the coated SWCNTs layer on TPU, graphene sheet on TPU and contact resistance graphene on SWNTs layer (Rc□).
Layers 2 4 10 20 Graphene
I/I0 (450 nm)
I/I0 (650 nm)
R□(Ω)
Rc□(Ω)
98.7% 92.8% 93.1% 93.3% 94.6%
98.3% 92.6% 93.0% 93.2% 95.1%
4.3 M 33.4 k 3.1 k 1.5 k 4.2 k
259 k 136 k 84 k
I/I0 (450 nm) and (650 nm) means the measured optical transparency at wavelength of 450 nm and 650 nm.
compressive deformation (figure 4(f)) (square resistance increase from 200 Ω at as fabricated state to 3 MΩ after several compressive deformation). After transferring the CVD graphene grown on copper foil using PMMA [25] onto the SWCNTs covered TPU surface, the graphene/PMMA bilayers were on the top of the pyramid arrays, enabling the broken graphene attaches to the surface of TPU (figures 4(g) and (h)). The bright region on each pyramid tip means that the electric conductivity at this region is lower than root region. This could be introduced by low conductivity of PMMA template layer, and show the pattern of the deposited graphene (figure 4(g)). Considering the TPU can be swollen by most organic solvent, which can give additional expansion of the substrate, the deposited graphene will be generated more wrinkles than unexpanded substrates. These wrinkles tolerate the extensive deformation and the graphene sheets can maintain electric connection under severe compressive deformation. The square resistance of the structured surface covered by SWCNTs with density of about 0.05 mg cm−2 (estimated from the density of the SWCNTs solution dropped and dried on structured TPU) is ∼5 kΩ. After depositing of graphene and multiple bending or compressing deformations (100 times), the electric conductivity keeps no obvious change. For this asymmetry structure, the largest deformation is the macroscopic buckle, which will not happen for the symmetry structure of tapped cone. The peeling and cracking of the conducting surface layer is caused by the strain mismatch between conducting layer and matrix material and the low elasticity of the metallic or ceramic conductor. The graphene sheet composes large amount of wrinkles and has very low bending stiffness, it can withstand moderate tensile or large compressive deformation. In our case, the coverage of the large continuous graphene sheets on the pyramids arrays is the region of largest deformation. The PMMA layer acts as both the support during template transfer process and the anchoring effect after graphene was deposited on TPU. The much enhanced electric conductivity stability under severe deformation demonstrates that the peeling and cracking of SWCNTs layer and graphene sheets are less significant after repeating deformation. This opens wide opportunities for the application of this soft and transparent conducting elastomer structure.
3.3. Sensitivity and stability of capacitance variation under constant and fluctuated loads
The microscopy observation proves that large graphene sheets deposited on the TPU structured surface cover the region suffered severe deformation. The protruded geometry causes the planar graphene can only cover the part of the pyramid surface, while the flat region of the TPU surface remains uncovered. The coverage of large continuous graphene sheets on this bump surface mainly covers the top end and will provide higher charge density than nanotubes, thus the higher capacitance variation. Because the direct measurement of effect electric conductivity between graphene and nanotube layer is difficult, the comparison of capacitance change between the samples with and without graphene layer gives the evidence of the effect of graphene layer. The measured ΔC/C0 plots under compression deformation exhibit much higher sensitivity for the sample with graphene layer than that without graphene layer (figure 6). The sensitivity of the sample covered with graphene at 0.1 MPa is 4.79 MPa−1 (average from first ten tests), which that of the sample without graphene coverage is 0.54 MPa−1, almost nine times lower than that with graphene layer. It can be noted from FE simulation that the contact area increases from 8 × 10−2 to 2% of the sample area when pressure varies from zero to 0.57 MPa. This means that there is plenty space to increase the sensitivity for this approach by decrease the stray capacitance of the measurement circuit. The ΔC/C0 plots of the sample covered by ITO layer were also measured. The sensitivity of fresh made sample is two folds higher than graphene covered sample. The repeatability of the capacitance variation for the graphene covered sample also shows high consistent. The decreased ΔC/C0 of first several compressive tests could be caused by the stress relaxation of TPU and the trivial electric contact loss of the graphene layer under repeating load. This decrease is not introduced by the deterioration of the conducting layer or the pyramid structure, because this inchworm can also be reflected in load-displacement plots. The ΔC/C0 versus calibrated strain shall have 6
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Figure 5. SEM images show the details of graphene deposited on SWCNTs coated TPU pyramid (a) and on the SWCNTs coated flat TPU
surface, scale 20 μm (a) and 5 μm (b).
Figure 6. Measured collective capacitance variation normalized over background under compressive deformation of initial four tests and the tenth of the sample covered by graphene and one test of the sample without graphene. Data were acquired during compressing process.
less inchworm drift during prolong loading cycles. The measurement on more loading cycles will be performed to clarify the stability of ΔC/C0 versus strain. The capacitance variations under multiple deformation cycles were recorded at different load scale. The fluctuated load was applied to the sample by an electromagnetic actuator between the die pair on the material test device, with constant load supplied and controlled by the device. It can be seen that the capacitance fluctuation takes the details of the pattern of the fluctuated applied load at any constant load scales (figure 7). Notice the tiny features of the measured fluctuated normal capacitance and compare the load fluctuation plots (figure 7(b)), the accuracy of the capacitance corresponds to the pressure can reach 0.1 kPa (three times S/N ratio), exceeding the load sensing limit of the material test device we used here. This means that the load sensing dynamic range reaches the order of 105. Comparing to thesensitivities and dynamic ranges of capacitance type load sensors based on
Figure 7. Dynamic measurements of capacitance under fluctuated
load at different load platures, (a) at near 0.4 MPa and (b) at near 0.1 MPa.
PDMS [6, 32] and polyolefin foam [22], our sensitivity is higher or similar than these devices, but our approach shows much broader sensing range. Comparing to the resistance type load sensor [10], which shows much higher load sensitivity 7
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and sensing low limit, our moderate sensitivity and broad sensing range enable even wider applications. The graphene sheets on the pyramid surface can tolerate very high repeating deformation with both full load to the scale of MPa and fluctuated load of kPa p–p scale. Our repeatability tests prove that the contact resistance between most graphene sheets and SWCNTs layer remain near constant at least after tens full scale compression to the load of 0.4 MPa and hundreds fluctuation of p–p 0.05 MPa. The very quick decay of the sensitivity of the ITO covered sample presents a negative example that the loss of perfectness of the conducting layer on the tip limits the sensing stability of brittle conducting layer covered sample. The measured ΔC/C0 sensitivity limit corresponding to the load or deformation of our approach depends on the sensitivity of capacitance measurements, the size of the sample and stray capacitance of the electric circuit. The noise level from lock-in amplifier is 10−2 mV. Higher detection sensitivity can be realized by suitably depressing stray capacitance with more precious deformation steps.
C Δ Cij /Cij
0.15 0.1 0.05 0 -0.05 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
3.4. Mapping of load distribution with sub-minimeter spatial resolution
The transparency of the pyramid arrays deposited by SWCNTs and graphene sheets can be seen in the photographic image (figures 8(a) and (d)). Comparing the samples before and after deposition of SWCNTs and graphene, the visible light transparency is 91%. By slicing the conducting layer of both the pyramid arrays and counter electrode into parallel lines, each pyramid was connected to the sampling resistor (figure 2). Stacking the pyramid arrays onto the counter electrode by perpendicularly crossing the sliced lines of electric connected pyramids and ITO lines of the same width on the counter electrodes, the pyramids are right on top of flat counter ITO strips. Scanning the ac bias voltage applied to one pyramid strip while keeping the voltage on the rest at ground, the capacitance variation between individual pyramid and counter electrode was measured after alternately scanning all the pyramid strips and measuring the voltage drops on each sampling resistor. A 0.3 mm thick star on the flat glass substrate (figure 8(b)) was pressed on the top of counter electrode to simulate a load distribution. The measured load distribution on 16 × 16 pixels were plotted in (figure 8(c)), with the normalized ΔCij/Cij as signal heights. The size of individual pyramid is the lateral resolution of the load distribution mapping. Though the higher resolution could be possible by fabricating smaller pyramids, the capacitance variation on smaller pyramid will also be smaller. The realization of higher later load resolution depends mainly on the effective eliminating of stray capacitance. The stray capacitance of single element of our set-up is about 35 fF. The uneven electric conductivity change of each pyramid before and after deformation will also add measurement errors. We applied PE as counter electrode substrate. The high stiffness of PE causes the spreading of load when local uneven distributed load was applied on the top of PE layer. This causes the distortion in the reconstructed image (figure 8(c)). The
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Figure 8. (a) Transparent TPU pyramid arrays covered with graphene and nanotubes, (b) the plastic star on glass substrate as loading embossment, (c) reconstructed 3D image of load distribution where coloured bars indicate the location and height indicates the intensity, and (d) tortured TPU sample with transparent conducting layer.
spatial and load resolution of the sensor arrays can be improved by more effectively depressing the stray capacitance in the measurement circuit and using more flexible counter electrode.
4. Conclusions By depositing SWCNTs on the TPU surface with pyramid surface structure, then depositing graphene sheet on the 8
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surface of pyramids, the flexible and visible light transparent pressure sensing arrays are fabricated. The surface electric resistance of this elastomer device remains at level of 5 kΩ after severe deformation. Numerical simulation using infinite element method presents the stress and local deformation distribution. The capacitance variation of under compressive deformation on the pyramid arrays shows high stability, high sensitivity and dynamic range from sub-kPa to MPa. Microscopic observation shows that the graphene layer covers the tip end of the pyramids, while nanotubes cover the most region of the TPU surface. The comparison of the capacitance variation between flat electrode and surface structured TPU covered by SWCNTs plus graphene and only SWCNTs proves that the graphene layer on the pyramids has enough electric connection with the SWCNTs and much enhanced capacitance sensitivity. The measurements of capacitance variation under steady loads present that the load detect sensitivity is 4.79 MPa−1, nine times of that obtained from the sample without graphene, while the measurements under fluctuated loads present that load sensing limit can reach 0.1 kPa, and sensing stability keeps stable at least after hundreds. A 16 × 16 arrays are connected to a circuit and a typical load distribution is regenerated by mapping the capacitance variation of the arrays.
[8] Yamada T, Hayamizu Y, Yamamoto Y, Yomogida Y, Izadi-Najafabadi A, Futaba D N and Hata K 2011 A stretchable carbon nanotube strain sensor for human-motion detection Nat. Nanotechnology 6 296–301 [9] Schwartz G, Tee B C K, Mei J G, Appleton A L, Kim D H, Wang H L and Bao Z N 2013 Flexible polymer transistors with high pressure sensitivity for application in electronic skin and health monitoring Nat. Commun. 4 1859 [10] Wang X W, Gu Y, Xiong Z P, Cui Z and Zhang T 2014 Silkmolded flexible ultrasensitive and highly stable electronic skin for monitoring human physiological signals Adv. Mater. 26 1336–42 [11] Lipomi D J, Vosgueritchian M, Tee B C K, Hellstrom S L, Lee J A, Fox C H and Bao Z N 2011 Skin-like pressure and strain sensors based on transparent elastic films of carbon nanotubes Nat. Nanotechnology 6 788–92 [12] Chun K Y, Oh Y, Rho J, Ahn J H, Kim Y J, Choi H R and Baik S 2010 Highly conductive printable and stretchable composite films of carbon nanotubes and silver Nat. Nanotechnology 5 853–7 [13] Cochrane C, Koncar V, Lewandowski M and Dufour C 2007 Design and development of a flexible strain sensor for textile structures based on a conductive polymer composite Sensors 7 473–92 [14] Gullapalli H, Vemuru V S M, Kumar A, Botello-Mendez A, Vajtai R, Terrones M, Nagarajaiah S and Ajayan P M 2010 Flexible piezoelectric ZnO–paper nanocomposite strain sensor Small 6 1641–6 [15] He R R and Yang P D 2006 Giant piezoresistance effect in silicon nanowires Nat. Nanotechnology 1 42–6 [16] Wu W Z, Wen X N and Wang Z L 2013 Taxel addressable matrix of vertical-nanowire piezotronic transistors for active and adaptive tactile imaging Science 340 952–7 [17] Liu N S, Fang G J, Wan J W, Zhou H, Long H and Zhao X Z 2011 Electrospun PEDOT PSS–PVA nanofiber based ultrahigh-strain sensors with controllable electrical conductivity J. Mater. Chem. 21 18962–6 [18] Sekitani T, Yokota T, Zschieschang U, Klauk H, Bauer S, Takeuchi K, Takamiya M, Sakurai T and Someya T 2009 Organic nonvolatile memory transistors for flexible sensor arrays Science 326 1516–9 [19] Sekitani T, Zschieschang U, Klauk H and Someya T 2010 Flexible organic transistors and circuits with extreme bending stability Nat. Mater. 9 1015–22 [20] Cosseddu P, Tiddia G, Milita S and Bonfiglio A 2013 Continuous tuning of the mechanical sensitivity of pentacene OTFTs on flexible substrates: from strain sensors to deformable transistors Org. Electron. 14 206–11 [21] Nam S H, Jeon P J, Min S W, Lee Y T, Park E Y and Im S 2014 Highly sensitive non-classical strain gauge using organic heptazole thin-film transistor circuit on a flexible substrate Adv. Funct. Mater. 24 4413–9 [22] Metzger C, Fleisch E, Meyer J, Dansachmuller M, Graz I, Kaltenbrunner M, Keplinger C, Schwodiauer R and Bauer S 2008 Flexible-foam-based capacitive sensor arrays for object detection at low cost Appl. Phys. Lett. 92 013506 [23] Hu W L, Niu X F, Zhao R and Pei Q B 2013 Elastomeric transparent capacitive sensors based on an interpenetrating composite of silver nanowires and polyurethane Appl. Phys. Lett. 102 083303 [24] Trung T Q, Tien N T, Kim D, Jang M, Yoon O J and Lee N E 2014 A flexible reduced graphene oxide field-effect transistor for ultrasensitive strain sensing Adv. Funct. Mater. 24 117–24 [25] Li X S et al 2009 Large area synthesis of high quality and uniform graphene films on copper foils Science 324 1312–4 [26] Li X S, Zhu Y W, Cai W W, Borysiak M, Han B Y, Chen D, Piner R D, Colombo L and Ruoff R S 2009 Transfer of large area graphene films for high performance transparent conductive electrodes Nano Lett. 9 4359–63
Acknowledgments The authors acknowledge the financial supports from the Natural Science Foundation of China (No. 10774022) and Jiangsu Provincial Social Development Foundation (No. BE2009665).
References [1] Zhou J, Gu Y D, Fei P, Mai W J, Gao Y F, Yang R S, Bao G and Wang Z L 2008 Flexible piezotronic strain sensor Nano Lett. 8 3035–40 [2] Pang C, Lee G Y, Kim T I, Kim S M, Kim H N, Ahn S H and Suh K Y 2012 A flexible and highly sensitive strain-gauge sensor using reversible interlocking of nanofibres Nat. Mater. 11 795–801 [3] Knite M, Teteris V, Kiploka A and Kaupuzs J 2004 Polyisoprene–carbon black nanocomposites as tensile strain and pressure sensor materials Sensors Actuators A 110 142–9 [4] Someya T, Sekitani T, Iba S, Kato Y, Kawaguchi H and Sakurai T 2004 A large-area flexible pressure sensor matrix with organic field-effect transistors for artificial skin applications Proc. Natl. Acad. Sci. USA 101 9966–70 [5] Kim D H et al 2011 Epidermal electronics Science 333 838–43 [6] Mannsfeld S C B, Tee B C K, Stoltenberg R M, Chen C, Barman S, Muir B V O, Sokolov A N, Reese C and Bao Z N 2010 Highly sensitive flexible pressure sensors with microstructured rubber dielectric layers Nat. Mater. 9 859–64 [7] Engel J, Chen J and Liu C 2003 Development of polyimide flexible tactile sensor skin J. Micromech. Microeng. 13 359–66 9
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X Zhang et al
flexible transparent conducting films J. Am. Chem. Soc. 129 7758 [30] Gent A N and Lindley P B 1959 Internal rupture of bonded rubber cylinders in tension Proc. R. Soc. A 249 195 [31] Suh J B and Kelly S G 2012 Stress response of a rubber block under vertical loading J. Eng. Mech. ASCE. 138 770–83 [32] Lee H K, Chang S I and Yoon E 2006 A flexible polymer tactile sensor: fabrication and modular expandability for large area deployment J. Microelectromech. Syst. 15 1681–6
[27] Zhang M, Fang S L, Zakhidov A A, Lee S B, Aliev A E, Williams C D, Atkinson K R and Baughman R H 2005 Strong, transparent, multifunctional carbon nanotube sheets Science 309 1215–9 [28] Zhang D H, Ryu K, Liu X L, Polikarpov E, Ly J, Tompson M E and Zhou C W 2006 Transparent conductive, and flexible carbon nanotube films and their application in organic light-emitting diodes Nano Lett. 6 1880–6 [29] Geng H Z, Kim K K, So K P, Lee Y S, Chang Y and Lee Y H 2007 Effect of acid treatment on carbon nanotube-based
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