Enhanced High-Resolution Triboelectrification-Induced

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Enhanced High-Resolution Triboelectrification-Induced Electroluminescence for Self-Powered Visualized Interactive Sensing Ying Wang, Hai Lu Wang, Hua Yang Li, Xiao Yan Wei, Zhong Lin Wang, and Guang Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02313 • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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ACS Applied Materials & Interfaces

Enhanced High-Resolution TriboelectrificationInduced Electroluminescence for Self-Powered Visualized Interactive Sensing Ying Wang†,‡, Hai Lu Wang†,‡, Hua Yang Li †,‡, Xiao Yan Wei †,‡ , Zhong Lin Wang†,‡,‖, Guang Zhu*,†,‡,§

†CAS

Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-nano

Energy and Sensor, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China. ‡School

of Nanoscience and Technology, University of Chinese Academy of Sciences,

Beijing 100049, China. §New

Materials Institute, Department of Mechanical, Materials and Manufacturing

Engineering, University of Nottingham Ningbo China, Ningbo 315100, China. ‖ School

of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA

30332, USA

ACS Paragon Plus Environment

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*Email: [email protected] KEYWORDS: triboelectrification, electroluminescence, self-powered, tactile sensing, slipping sensor ABSTRACT: Transforming dynamic mechanical interactions into visualized luminescence represents a research frontier in the detection of tactile stimuli. Here, we report a self-powered high-resolution triboelectrification-induced electroluminescence (HR-TIEL) sensor for visualizing the contact profile and the dynamic trajectory of a contact object. As dynamic interactions occurs, triboelectric charges at the contact interface generates a transient electric field that excites the phosphor. From numerical simulation, a conductive layer based on transparent silver nanowires (AgNWs) guides the direction of the electric field and confines it within the profile boundary of the connect object. As a result, a sharp change of the electric field at the profile boundary greatly promotes the luminescence intensity as well as the lateral spatial resolution. Compared to a TIEL sensor without the conductive layer, the luminescence intensity is enhanced by 90%, and the lateral spatial resolution of ~ 500 μm is achieved. The HR-TIEL is then demonstrated to reveal the surface texture on a nitrile glove. It relies on neither additional power supplies


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nor complex wiring/circuit design. This work paves the way for a feasible detection of tactile stimuli such as touch and slipping, which will be potentially used in robotics, humanmachine interface, flexible and wearable electronics, etc.

INTRODUCTION

Interactive sensing is a stimuli-response process with a human/machine readable response, and it has widespread applications in microfluidic networks1-3, organic electrochromics4,5, piezophotonic6-8 and electroluminescence.9,10 A stimuli-response process, primarily physical contact between the sensor and an object surface11-14, can be realized by collecting readable signals, such as electrical15-18, acoustic19,20 and optical signals.21,22 For the optical signal, it needs to have an accurate visibility and high responsivity at a real-time working environment.23-25 Kim et al. developed an organic lightemitting board that detected external stimuli and response with an accurate display.26 Chou et al. reported an e-skin whose color could be easily controlled by using different applied pressure.27 However, these optical signals relied on either an externally applied


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voltage or large pressure, which might cause problems of power consumption or materials failure, respectively. In this context, a recently reported motion-driven luminescence called triboelectrification-induced electroluminescence (TIEL) turned out be promising for the interactive sensing.28,29 The TIEL relying on the combination of triboelectrification and electroluminescence proved to be self-powered and nondestructive to materials. A sensor based on the TIEL did not require complex electric circuit and could be fabricated in a straightforward way. However, in the previously reported TIEL, the electrostatic field that generated the electroluminescence tended to diverge in the lateral direction without effective confinement. As a result, the true profile of the contact object could not be accurately revealed because of the degraded lateral spatial resolution. Therefore, although the TIEL has an advantage of low threshold stress28, its luminescence intensity, resolution and responsivity still need to be promoted.

In this work, we report a self-powered high-resolution triboelectrification-induced electroluminescence (HR-TIEL) sensor for visualizing the contact profile and the dynamic trajectory of a contact object. Triboelectric charges produced by contact electrification


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produces a transient electric field that excites the electroluminescence of phosphors along the trajectory of the contact object. Attributed to a transparent conductive layer made of silver nanowires (AgNWs) that guides the direction of the electric field, the electric field is confined within the profile boundary of the contact object. Then the change rate of the electric field at the profile boundary becomes significant. Compared to a TIEL sensor without the conductive layer, the luminescence intensity is enhanced by 88%, and the responsivity is increased by 17%. The lateral spatial resolution of ~ 500 μm is achieved. As a demonstration, the surface texture on a nitrile glove can be clearly revealed by the HR-TIEL. Besides, it does not require additional power supplies nor complex wiring as well as circuit design. This work paves the way for developing a novel method for visualized detection of tactile stimuli such as touch and slipping, which will be potentially used in smart robotics30, human-machine interface26, flexible and wearable electronics31, etc.

RESULTS AND DISCUSSION The configuration of the HR-TIEL sensor, as illustrated in Figure 1a, consists of four layers.


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Briefly, it comprises a 50 μm thick electrification layer made of fluorinated ethylene propylene (FEP), a luminescent layer based on polydimethylsiloxane (PDMS) matrix and ZnS:Cu phosphor, a transparent conductive layer made of silver nanowires (AgNWs) and a 100 μm thick polyethylene terephthalate (PET) substrate. Compared with the TIEL sensor reported previously, the HR-TIEL sensor has an additional transparent conductive AgNWs layer. Figure 1b shows an SEM image of the AgNWs layer. The AgNWs have a diameter and a length of 25~30 nm and 20~30 μm, respectively. Figure 1c shows a cross-sectional view of the ZnS:Cu phosphor and the PDMS matrix in an SEM image. The thickness of the luminescent layer is 50 μm after structural optimization. The average diameter of the phosphor particles is 20 μm. A photograph of the HRTIEL sensor is displayed in Figure 1d. To quantify the luminescence intensity and responsivity, a test platform was constructed as shown in Figure 1e. The contact object slid on the electrification layer in a lateral plane as driven by a linear stepper. At the same time, an optic-fiber probe and a force sensor were placed underneath the substrate. The optic-fiber collected and directed the light emission to a spectrometer, and the force sensor monitored the vertical force at the contact interface. With the dynamic interactions between contact interfaces, triboelectric charges were produced. They generated a transient varying electric field that excited the electroluminescence (EL) of the phosphor along the sliding trajectory. The obtained luminescence spectrum peak value was at 519 nm, which corresponds to Commission Internationale de l’Éclairage (CIE) coordinates of (0.25, 0.56). Compared to the previously reported TIEL sensor, the HR-TIEL sensor proves to be superior in two aspects. First, the luminescence intensity of the HR-TIEL sensor is substantially improved by 88%, as shown in Figure 1f. Second, the lateral spatial resolution of the HR-TIEL sensor also turns out to be much higher. To assess the resolution of the HR-TIEL sensor, a capital letter “A”


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made of thermoplastic polyurethane (TPU) was used as a contact object on the HR-TIEL sensor, as shown in Figure 2a. The contact object “A” was pressed onto electrification layer at a contact force of 0.8 N, which corresponded to a pressure of 24 kPa. Meanwhile, it slid against the electrification layer back and forth. The EL image was captured by a camera as shown in Figures 2b and 2d. After image processing via MatLab, a quantitative mapping of the luminescence intensity could be obtained, which is presented in Figures 2c and 2e. As shown in Figures 2b, 2c and 2f, a blurred image of the contact object was obtained from the TIEL sensor, which could not reveal the actual profile of the contact interface. Along the dashed line defined in Figure 2c, the luminescence intensity at the positions of “1”, “2”,“3” and “4” are clearly indicated in Figure 2f. Compared with the TIEL sensor, the HR-TIEL sensor could clearly present the outline of the letter “A” as shown in Figures 2d and 2e. The details of the test processing are shown in Movie S1 (Supporting Information). Along the dashed line in Figure 2e, four sharp peaks can be identified in Figure 2g. The slope of the peaks in Figure 2g is enhanced by 679 % compared with the case in Figure 2f, which indicates a much more clear-cut profile of the obtained image. As for the repeatability of the HR-TIEL, a cyclic test was conducted. The cyclic characteristic was measured when the contact object repeatedly slid against the electrification layer in a reciprocating way (frequency: 1 Hz, sliding velocity: 6 cm/s, contact pressure: 20 kPa). The peak luminescence intensity of each cycle was recorded and as shown in Figure S1 (Supporting Information). After 10 000 cycles, the luminescence intensity only slightly dropped 2.9 %, demonstrating a good durability of the HR-TIEL sensor. Therefore, the HR-TIEL sensor possesses not only an enhanced luminescence intensity but also a promoted spatial resolution and a good repeatability. The reason lies in the AgNWs-based conductive layer, which is elaborated below. Here a numerical simulation via COMSOL modeling was conducted. Three-


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dimensional models of the TIEL (Figure 3a) sensor and the HR-TIEL (Figure 3e) sensor were built to illustrate the origin of the enhancement. The detailed modeling parameters are discussed in the Methods. In particular, the surface charge density on the electrification layer and on the contact object were set to be -3.5 μC/m2 and +100 μC/m2, respectively. The principles of the TIEL and the HR-TIEL are elaborated through the cross-sectional views of the numerically calculated electric potential distribution in Figure 3b and Figure 3f, respectively. When the contact object (letter “A” in this case) slid on the electrification layer, triboelectrification created charge transfer at the contact interface, with positive charges on the TPU and equivalent negative ones on the FEP. The electric field lines (white arrows in Figure 3b) within the TIEL sensor diverge away from the positively charged contact object, as shown in the zoom-in view of Figure 3c. The white arrows indicate the direction of the electric field lines. The slow variation of the electric potential within the luminescent layer means a low electric field. To illustrate the distribution of the electric field lines, the charge distribution in a cross-sectional view is shown in Figure 3d. For the HR-TIEL sensor, the electric field lines tend to be confined as a result of the AgNWs-based conductive layer in Figure 3f. Then, the changing rate of


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the electric potential within the luminescent layer becomes much more significant, which results in a higher electric field as shown in Figure 3g. Due to electrostatic induction, a layer of positive induced charges and a layer of negative induced charges are produced within the AgNWs conductive layer, as shown in Figure 3h. The positive induced charges on the AgNWs layer can draw the electric field lines perpendicularly towards the AgNWs conductive layer, reducing their divergence in the lateral direction. This effect actually confines the distribution of electric field within the profile boundary of the contact object. As a result, a sudden and sharp variation of the electric field at the profile boundary causes not only the enhanced luminescence intensity but also promoted the spatial resolution. To quantify the effect discussed above, the distribution of the electric potential and the electric field within the luminescent layer are shown in Figure 4. The area underneath the contact object “A” had a significantly higher electric potential than the surrounding area as shown in Figures 4a and 4b. Following the dashed line defined in Figures 4a and 4b, the lateral distribution of the electric potential within the luminescent layer is depicted in Figure 4c. At the profile boundary, the electric potential distribution within the HR-TIEL


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sensor has swiftly changing curves that can follow the actual profile of the contact object. In contrast, the slowly changing electric potential within the TIEL sensor merges into a smooth curve, which could not outline the profile of the contact object. By obtaining the derivative of the electric potential, the electric field distribution in the perpendicular direction was calculated, as is demonstrated in Figures 4d and 4e. Again, along the dashed lines in Figures 4d and 4e, the amplitude of the electric field can be obtained in Figure 4f. According to the numerical simulation results, the electric field intensity experiences an enhancement by 60%. The four peaks for the HR-TIEL sensor has not only much higher amplitude but also steeper slope than that for the TIEL sensor. They represent the four local regions in which the swiftly changing electric field is applied onto the luminescent layer. As a result, bright luminescence was locally observed with a clearcut boundary, which is consistent to the captured photograph in Figure 2d. In comparison, the electric field distribution for the TIEL is much broader with less amplitude. Calculated from the width of the electric field distribution, the lateral spatial resolution of the HR-TIEL sensor is at a scale of hundreds micrometers, which is dozens of times higher than the TIEL sensor.


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The luminescence intensity and the responsivity of the HR-TIEL sensor were influenced by two parameters of the AgNWs conductive layer, including the optical transmittance and the conductivity. To investigate these two factors, five groups of HRTIEL sensors were prepared with different concentrations of the AgNWs solution (See Method for details). Figure 5a shows the optical transmittances spectra of the five different AgNWs conductive layers. The optical transmittances of the five samples are 91.48%, 87.57%, 82.96%, 75.85% and 67.78% at 519 nm. All of the photographs and the SEM images of these samples can be seen in Figure S2 (Supporting Information). With the increasing concentration of the AgNWs solution, the optical transmittance decreases gradually, and the sheet resistance drops dramatically as depicted in Figure 5a. To quantify the effect of the AgNWs conductive layer on the HR-TIEL sensor, six sensors were prepared by using different AgNWs conductive layers plus an ITO layer. They are labeled from HR-TIEL 1 to HR-TIEL 5 as well as TIEL (ITO), respectively. From Figure 5b, the HR-TIEL 3 sensor has the strongest luminescence intensity at a contact pressure of 20 kPa. It is slightly lower than the TIEL (ITO) sensor, in which the ITO layer has the optical transmittances of 84.69% and the sheet resistance of 39.5 Ω/sq. The increase of


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the conductivity had a positive correlation with luminescence intensity to a certain extent. Experiment results showed that either too high or too low sheet resistance of the AgNWs reduced the luminescence intensity. On one hand, high sheet resistance means low conductivity that weakens the confinement effect of the conductive layer on the electric field. On the other hand, low sheet resistance is obtained at the expense of the optical transmittance of the conductive layer, which also reduces the emitted luminescence. As a result, an optimal value of the sheet resistance for the HR-TIEL was observed. Two characters that represent the HR-TIEL sensor were investigated in two categories, i.e., luminescence intensity and responsivity. Here, normalized luminescence intensities of HR-TIEL 1 to HR-TIEL 5 and TIEL (ITO) sensors under different stresses are plotted in Figure 5c. All of the HR-TIEL sensors had similar luminescence intensity trend, which increased rapidly in a low-stress region (＜ 20 kPa). Compared with the TIEL sensor, the HR-TIEL 3 sensor experienced a 90% enhancement at 20 kPa, which is in agreement with the theoretical electric field enhancement in Figure 4f. With the saturation of tribocharge density, the luminescence intensity increased at a sluggish pace at a largestress region and experienced a 1.63-fold enhancement at 80 kPa. At a stress of 20 kPa,


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the HR-TIEL 3 sensor and the TIEL (ITO) sensor could reach 69.8% and 72.3% of luminescence saturation intensity while the TIEL sensor barely reached 60.5% as shown in Figure S3. As mentioned in literatures23, the responsivity was defined as ΔI/ Δσ, where ΔI is variation of the luminescence intensity and Δσ is the stress increment. The responsivities of the TIEL sensor, the HR-TIEL 3 sensor and the TIEL (ITO) sensor were found to be 0.030 kPa-1, 0.035 kPa-1 and 0.036 kPa-1, representing a responsivity increase of 17% in a low-stress region as depicted in Figure 5d. The HR-TIEL sensor in this work can be potentially used for self-powered visualized interactive sensing. Here, it was demonstrated to visualize the texture of the contact interface. A textured nitrile gloved finger was used as a contact object as shown in Figures 6a and 6b. From Figures 6c to 6d, the texture of nitrile glove could be clearly captured by the HR-TIEL sensor instead of the TIEL sensor. The confined electric field with swiftly changing rate reduced the lateral diffusion of the emitted luminescence so that the HRTIEL sensor could achieve a lateral spatial resolution of ~ 500 μm (Figures 6d and 6h). The maximal luminescence intensity experienced a 2.2-fold enhancement with the HRTIEL sensor as shown in Figures 6g and 6h. In comparison, the luminescence from the


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TIEL sensor laterally diffused without a distinct texture pattern (Figure 6c). This characteristic distribution of the luminescence intensity can reflect the clear-cut morphology of a contact object. It needs to be pointed out that the HR-TIEL does not require sophisticated fabrication of high-density sensor units to achieve high resolution. As a result, the demonstration of a self-powered interactive sensing indicates feasible applications of the HR-TIEL, such as flexible displays and even robotic hands for detecting slipping during grabbing.

CONCLUSION In summary, we developed a HR-TIEL sensor for visualizing tactile stimuli especially slipping by transforming dynamic mechanical interaction into optical luminescence. A highly conductive, transparent and flexible AgNWs layer plays a vital role in promoting the luminescence intensity, the responsivity, and the spatial resolution of the HR-TIEL. The conductive layer can effectively confine the triboelectrification-induced electric field within the profile boundary of the contact object. As a result, the profile and surface morphology of the contact object can be explicitly revealed. It needs to be noticed that


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the HR-TIEL is self-powered without relying on additional power supplies. Besides, it does not require complex wiring and circuit design. These merits make the HR-TIEL a viable, feasible and cost-effective technique for tactile sensing, which may have applications in industrial automation, human-machine interface and wearable electronics, etc.

METHODS Preparation of silver nanowires (AgNWs) conductive layer. The AgNWs conductive layer was prepared by a thermal removal method.32 AgNWs (Beijing Hi-nano Technology Co. Ltd) were centrifuged with 5000 r/min and washed three times with ethanol as reported previously. The AgNWs dispersed in a mixed solvent consisting of 1:9 v/v water/ethanol and formed different concentrations of AgNWs solutions. Polyethylene oxide (PEO) (average Mv ≈ 600,000, powder, Aladdin Reagent Co. Ltd. Shanghai, China) was first dissolved in a mixed solvent of 10 vol % water and 90 vol % ethanol and then mixed with different AgNWs solutions under magnetic stirring to obtain a AgNWs /PEO composites solution. Five different AgNWs conductive layers labeled from Sample 1 to Sample 5 were prepared with AgNWs (1 mg/mL)/PEO (1 mg/mL), AgNWs (2 mg/mL)/PEO (1 mg/mL), AgNWs (5 mg/mL)/PEO (1 mg/mL), AgNWs (8 mg/mL)/PEO (1 mg/mL) and AgNWs (10 mg/mL)/PEO (1 mg/mL). The composite films with different concentrations of AgNWs solutions were prepared on polyethylene terephthalate (PET) substrate by Meyer Rod (Bar No.23) and then immediately put in an oven at 50 °C to dry for 5 min before being transferred to a 165 °C oven for 10 min to remove PEO.


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Fabrication of the HR-TIEL sensor. A layer of 100 μm thick PET transparent substrate was cut into 4 cm × 4 cm by laser cutting. And then a AgNWs conductive layer was prepared on PET substrate as described in the previous step. The ZnS:Cu phosphor particles were mixed with PDMS matrix at a mass ratio of 2:1 to form a phosphor paste. Such a ratio was optimized to achieve high luminescence intensity. The phosphor paste was deposited by spinning coating at 300 rpm for 10 s and 1500 rpm for 20s. Then the phosphor paste was cured in an oven at 80 °C for 2 h. A layer of FEP tape of 50 μmin thickness was adhered onto the luminescence layer as an electrification layer. A capital letter “A” made of thermoplastic polyurethane (TPU) was used as a contact object on the HR-TIEL sensor. COMSOL model settings. 3D models of the HR-TIEL sensor and the TIEL sensor were used to reveal the electric potential and electric field distribution. The dimensions of the PET substrate were 30 mm × 30 mm × 1 mm. The thicknesses of the FEP and AgNWs conductive layer were set to be 0.05 mm. The thickness of contact objectletter “A” was set to be 1 mm. The surface charge density on the FEP and contact object is set to -3.5 μC/m2 and +100 μC/m2, respectively. The entire setup was surrounded by a grounded air sphere of 60 mm in diameter. The dielectric permittivity used in the simulation were ε = 1.0 for air and AgNWs conductive layer, ε = 3.6 for PET, ε = 2.4 for FEP and ε = 4.0 for TPU contact object. Characterization of the HR-TIEL sensor. The morphology of the AgNWs conductive layer and luminescent layer were characterized by a field emission scanning electron microscope (SEM, Nova Nano 450, FEI, Japan) at an accelerating voltage of 10 kV. The sheet resistances of AgNWs conductive layers were detected by a standard four-probe method (RTS-9, PROBES TECH, China); the optical transmittances were measured with a uv-vis-nir spectrometer (UV-3600,


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Shimadzu, Japan) in the 400–800 nm wavelength range at a resolution of 0.5 nm. XRD patterns were recorded on an X-ray diffractometer (X’pert 3, PANalytical, Netherlands). The XRD patterns shows that PEO could be well removed after heat treatment for its relatively low thermally decomposed temperature. The AgNWs/PEO composite film has an additional peak at around 23°, which disappears after heat treatment as shown in Figure S4. The XRD patterns of the AgNWs conductive layer before and after the heat treatment indicate that PEO had been mostly removed.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Video of the TIEL sensor and the HR-TIEL sensor (AVI) Cycling test of HR-TIEL sensor; SEM images and photographs of different AgNWs conductive layers; Luminescence intensities of different sensors under different stresses; XRD patterns of AgNWs layer before and after heat treatment (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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ORCID Guang Zhu: 0000-0003-2350-0369 Notes The authors declare no competing financial interest. ACKNOWLEDGMETS This research was supported by the National Key R&D Project from the Ministry of Science and Technology, China (Grant Nos. 2016YFA0202701 and 2016YFA0202703), Zhejiang Provincial Natural Science Foundation of China (Grant No. LR19F010001), National Science Foundation of China (Grant No. 51572030), Natural Science Foundation of Beijing Municipality (Grant No. 2162047). REFERENCES

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Figure 1. Structure of HR-TIEL sensor. (a) Schematic of the layer-by-layer HR-TIEL sensor. (b) SEM image of the silver nanowires (AgNWs). (c) SEM image of the luminescent layer showing ZnS:Cu particles embedded in PDMS matrix. (d) Photograph of HR-TIEL sensor. (e) Schematic diagram of the test platform for quantitative measurement of the HR-TIEL sensor. (f) Wavelength spectrums of HR-TIEL and TIEL sensors and their corresponding CIE coordinate.


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Figure 2. Comparison of TIEL and HR-TIEL sensors. (a) Photograph of TPU contact object. (b) Luminescence image of TIEL sensor. (c) Corresponding mapping of luminescence intensity with TIEL sensor. (d) Luminescence image of HR-TIEL sensor. (e) Corresponding mapping of luminescence intensity with HR-TIEL sensor. (f) Luminescence intensity distribution along the dashed line in (c). (g) Luminescence intensity distribution along the dashed line in (e).


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Figure 3. Theoretical analysis and schematic illustrations of TIEL sensor and HR-TIEL sensor. (a) Structure illustration of TIEL sensor. (b) Electric potential distribution of the intersection of TIEL sensor calculated by COMSOL. (c) Enlarged view of dashed area in (b). (d) Diagram of charge distribution of TIEL sensor. (e) Structure illustrations of HRTIEL sensor. (f) Electric potential distribution of the intersection of HR-TIEL sensor calculated by COMSOL. (g) Enlarged view of dashed area in (f). (h) Diagram of charge distribution of HR-TIEL sensor.


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Figure 4. Theoretical analysis of TIEL sensor and HR-TIEL sensor. Electric potential distribution of (a) TIEL and (b) HRTIEL sensors. (c) 1D electric potential distribution of TIEL and HR-TIEL sensors along white dashed line. Electric field distribution of (d) TIEL and (e) HR-TIEL sensors. (f) 1D electric field distribution of TIEL and HR-TIEL sensors along black dashed line.


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Figure 5. Characters of AgNWs conductive layers and luminescent spectra measurement. (a) Optical transmittances and sheet resistances of AgNWs layers with different concentration AgNWs solution. (b) Wavelength spectrums of HR-TIEL sensors under 20 kPa. (c) Luminescent intensities of HR-TIEL sensors under different stresses. (d) Responsivities of TIEL sensor, HR-TIEL 3 sensor and TIEL (ITO) sensor under different stresses.


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Figure 6. Demonstrations of visualizing texture by touch with TIEL and HR-TIEL sensors. (a) Photograph of textured nitrile gloved palm. (b) Surface morphology of textured nitrile glove. (c) Luminescence image of textured nitrile gloved finger with TIEL sensor. (d) Luminescence image of textured nitrile gloved finger with HR-TIEL sensor. (e) Corresponding mapping of luminescence intensity with TIEL sensor. (f) Corresponding mapping of luminescence intensity with HR-TIEL sensor. (g) Luminescence intensity distribution along the dashed line in (e). (h) Luminescence intensity distribution along the dashed line in (f).

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