Flexible Organic Tribotronic Transistor for Pressure and Magnetic

Nov 3, 2017 - Here, a flexible organic tribotronic transistor (FOTT) without a top gate electrode has been demonstrated. The FOTT is fabricated on a f...
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Flexible Organic Tribotronic Transistor for Pressure and Magnetic Sensing Junqing Zhao,†,§,⊥,¶ Hang Guo,‡,¶ Yao Kun Pang,†,§,⊥ Fengben Xi,†,§,⊥ Zhi Wei Yang,†,§,⊥ Guoxu Liu,†,§,⊥ Tong Guo,†,§,⊥ Guifang Dong,*,‡ Chi Zhang,*,†,§,⊥ and Zhong Lin Wang*,†,§,⊥,∥ †

Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, People’s Republic of China Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Chemistry Department, Tsinghua University, Beijing 100084, People’s Republic of China § CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology (NCNST), Beijing 100190, People’s Republic of China ⊥ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ∥ School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States

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‡

S Supporting Information *

ABSTRACT: Flexible electronics has attracted enormous interest in wearable electronics and human−machine interfacing. Here, a flexible organic tribotronic transistor (FOTT) without a top gate electrode has been demonstrated. The FOTT is fabricated on a flexible polyethylene terephthalate film using the p-type pentacene and poly(methyl methacrylate)/Cytop composites as the conductive channel and dielectric layer, respectively. The charge carriers can be modulated by the contact electrification between the dielectric layer and a mobile triboelectric layer. Based on the fabricated FOTT, pressure and magnetic sensors have been developed, respectively, that exhibit great sensitivity, fast response time, and excellent stability. The FOTT in this simple structure shows bright potentials of tribotronics in human−machine interaction, electronic skins, wearable electronics, intelligent sensing, and so on. KEYWORDS: organic tribotronic transistor, triboelectric nanogenerator, tribotronics, pressure sensor, magnetic sensor ver the past decades, flexible electronics have attracted enormous interest in wearable devices, smart skins, and human−machine interfacing.1−4 As one of the key technologies for flexible electronics, the organic thin film transistor (OTFT), which possesses combined advantages of low cost, low temperature, and large area, has been widely studied.5−8 However, most OTFT-based sensors are usually modulated by electrical signals, which lack active and direct interactions with the environment. Therefore, an active sensing mechanism that can bridge the gap between the ambient and sensors and develop direct human−machine interaction is highly desirable. Recently, triboelectric nanogenerators (TENGs), which are based on the coupling of triboelectrification and electrostatic induction, have three major application fields: micro/nanoenergy,9−13 self-powered systems,14−21 and blue energy.22−24 Furthermore, tribotronics has been proposed by coupling with triboelectricity and a semiconductor, which can be used for information sensing,25,26 and active control.27 So far, many tribotronic devices have been demonstrated including electromechanical coupled logic circuits,28 contact-gated light-emitting LEDs,29 organic touch memory,30 adjustable phototransistors,31,32 smart tactile switches,33 electronic skins,34 tactile

sensing arrays,35 flexible transparent transistors,36 tribotronic tuning diodes,37 and force pads.38,39 All of these applications demonstrate the huge potential of tribotronics in flexible and active interaction electronics, offering a prospective strategy to design smart sensors with advantages of low cost, simple mechanism, and excellent flexibility. Here, we developed a flexible organic tribotronic transistor (FOTT) without the top gate electrode. With a pentacene conductive channel and a poly(methyl methacrylate)/cyclic transparent optical polymer (PMMA/Cytop) composite dielectric layer fabricated on a flexible polyethylene terephthalate (PET) substrate, the FOTT can be greatly modulated by contact electrification between the dielectric layer and a mobile triboelectric layer. Furthermore, the FOTT displays great sensing ability in pressure and magnetic detection with great sensitivity, fast response time, and excellent stability. The FOTT without a top gate electrode as a simple tribotronic device in this work shows great potential in human−machine

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© 2017 American Chemical Society

Received: September 12, 2017 Accepted: November 3, 2017 Published: November 3, 2017 11566

DOI: 10.1021/acsnano.7b06480 ACS Nano 2017, 11, 11566−11573

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of a mobile triboelectric layer and an OTFT without a top gate electrode. First, a 300 nm Ta2O5 film is sputtered on the PET surface. Then, the conductive channel, a 45 nm thick pentacene film, is deposited by thermal evaporation. Subsequently, two parallel Au drain/source electrodes are deposited on the pentacene film through shadow masks, and the thickness is controlled to about 45 nm. Thereafter, PMMA and Cytop are spin-coated as the dielectric layer, respectively. Figure 1(b) shows the optical graph of the as-fabricated FOTT in top view. As shown in the graph, the conductive channel of the FOTT is 60 μm in length and 1000 μm in width, respectively. Figure 1(c) presents the equivalent circuit of the FOTT. The electrical potential difference, acting as the gate voltage, is generated from the movement of the TENG by the external pressure, which can modulate the IDS. Working Mechanism and Performance of the FOTT. The FOTT, in a simple structure without a top gate electrode, presents a totally different working principle from the conventional OTFT configuration. The enhancement mode of the device is shown in Figure 2. As demonstrated in Figure 2(a), the copper (Cu) film is purposely chosen as the mobile triboelectric layer. The drain electrode of the FOTT is connected with a voltage source, while the source electrode is grounded. When the Cu film fully contacts the PMMA/Cytop dielectric, positive charges are induced on the surface of the Cu film, while negative charges are formed on the PMMA/Cytop surface according to their triboelectric series. At this moment, there is no electrical potential difference and no charge carrier concentration changes in the pentacene film because the

interaction, electronic skins, wearable electronics, intelligent sensing, and so on.

RESULTS AND DISCUSSION Structure of the FOTT. As shown in Figure 1(a), the FOTT is fabricated on a flexible PET film substrate, consisting

Figure 1. Schematic illustration of the flexible organic tribotronic transistor (FOTT). (a) Structure of the FOTT without a top gate electrode. (b) Optical graph of the as-fabricated FOTT. (c) Equivalent circuit of the FOTT.

Figure 2. Characteristics of the FOTT in enhancement mode. (a) Schematic working principle of the FOTT in the enhancement mode using a Cu film as the mobile triboelectric layer. (b) Output characteristics of the FOTT with different separation distances. (c) IDS output characteristics at a constant drain voltage (VDS) of −10 V with different separation distances (the separation distance is defined as d). The inset is the IDS−d transfer characteristics of the FOTT in enhancement mode. 11567

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Figure 3. Characteristics of the FOTT in the depletion mode. (a) Schematic working principle of the FOTT in the depletion mode using an FEP film as the mobile triboelectric layer. (b) Output characteristics with varying separation distances. (c) IDS output characteristics at a constant VDS of −10 V with different separation distances. The inset is the IDS−d transfer characteristics of the FOTT in depletion mode.

mode. As shown in the graph, the IDS rises as the separation distance increases within a VDS of 0 to −10 V. The IDS change as a function of separation distance at a constant VDS of −10 V is also studied, as shown in Figure 2(c). The inset image presents the transfer characteristics of the FOTT device. When the separation distance of the Cu film varies from 0 to 700 μm, the IDS increases from −2.84 μA to −3.25 μA, which is in good accordance with the working principle elucidated in Figure 2(a). The on/off value, defined as the ratio of the IDS generated at the two separation distances (0 and 700 μm, respectively), can be reached at 1.14. In addition, in the enhancement, the depletion mode of the FOTT device is also analyzed in Figure 3. Different from the structure in the enhancement mode, the fluorinated ethylene propylene (FEP) film acts as the mobile triboelectric layer. Figure 3(a) demonstrates the working mechanism of the FOTT in the depletion mode. Negative charges are induced on the surface of the FEP film, while positive charges are generated on the PMMA/Cytop surface when the two surfaces are in contact. Likewise, the channel carrier concentration is still not influenced at this moment. As the FEP film slowly moves away from the PMMA/Cytop surface, an inner electric field is generated, causing the holes to be repelled away from the interface of the pentacene and the PMMA/Cytop, thus creating a depletion zone in the pentacene conductive channel. Similarly, the depletion zone will be enlarged and the IDS will continually drop until a maximum separation distance of the FEP film is reached. When the FEP film is approaching the surface of PMMA/Cytop to the full contact state, the hole carriers move back to the interface of the pentacene and the PMMA/Cytop layers. In this stage, the inner charge polar-

negative and positive charges on the two surfaces are fully balanced. As a result, no obvious change can be observed in the IDS. Once the Cu film gradually separates from the PMMA/ Cytop surface, the negative charges on the PMMA/Cytop film surface will induce an inner charge polarization, leading to establishment of an inner electric field across the pentacene conductive channel and the PMMA/Cytop surface. This process results in holes accumulating at the interface of the pentacene and the PMMA/Cytop layers to achieve an enhancement zone in the pentacene conductive channel, and an increase in the IDS is observed. When the Cu film continues the separation movement, the inner electric field gradually increases and IDS is enhanced. After that, the Cu film begins to approach the PMMA/Cytop surface, and the enhancement zone will be depressed owing to the accumulative hole carriers being diffused away from the interface of the pentacene and the PMMA/Cytop layers. Accordingly, the IDS will decrease. When the Cu film returns to its originally full contact state, the IDS recovers to the original value. Illustrated in Figure S1 are the theoretically calculated results (performed by the software Comsol Multiphysics) of the potential difference generated from the contact and separation cycle between the mobile triboelectric layer and the PMMA/Cytop surface, indicating that this triboelectric potential varies with the separation distance of the mobile triboelectric layer. Apart from the working principle mentioned above, the relationship between the IDS and the separation distance of the Cu film is systematically studied. The separation distance of the Cu film can be accurately controlled and measured using a linear motor. Figure 2(b) plots the output characteristic of the FOTT device without external gate voltage in the enhancement 11568

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Figure 4. Structures and performances of the FOTT for pressure sensing. (a) Schematic illustration of the device structure and the sensing process. (b) IDS changes of the sensor during periodical press−release processes. (c) IDS changes as a function of the FEP film thickness (the normal pressure remains 20 Pa during the whole process). (d) IDS output characteristics at a FEP film thickness of 100 μm with different input pressures. The inset is the fitted curves.

ization is depressed, which decreases the inner electric field and the depletion zone and yet restores the hole carrier concentration in the interface and the IDS. Figure 3(b) shows the output characteristic of the FOTT at different separation distances of the FEP film without external gate voltage. Clearly shown in the graph is the increasing trend of the IDS as the separation distance. The relationship between the IDS (VDS maintains −10 V) and the separation distance of the FEP film is provided in Figure 3(c), and the inset image is the corresponding transfer characteristics of the FOTT device. A separation distance increase from 0 to 600 μm results in the IDS changing from −2.91 μA to −1.69 μA, where an on/off ratio as high as 1.72 can be realized in the depletion mode. Application of the FOTT for Pressure Sensing. On the basis of the above experimental results, the IDS can be modulated by contact electrification between a mobile triboelectric layer and the PMMA/Cytop dielectric layer. Here, if the mobile triboelectric layer moves and contacts the dielectric layer by applying an external pressure, the IDS can be used for sensing the external pressure. With this principle, we have developed a pressure sensor based on the FOTT. The structure and sensing process of the pressure sensor are displayed in Figure 4(a). As shown in the figure, the FEP film, working as a mobile triboelectric layer, is fixed on the FOTT through two polyimide (PI) tapes. The FEP film is separated from the PMMA/Cytop surface initially and can contact it by the external pressure. Due to the contact electrification, negative charges are generated on the surface of the FEP film, while positive charges are generated on the PMMA/Cytop surface. At the initial state, because of the inner charge polarization by the positive charges on the PMMA/Cytop surface, the hole carriers move away from the interface of the

pentacene and the PMMA/Cytop layers, and a depletion zone is created in the pentacene conductive channel. When the pressure is applied, the FEP film is deformed to approach the PMMA/Cytop surface, which depresses the inner charge polarization and the depletion zone and yet restores the hole carrier concentration in the interface to increase the IDS. When the pressure gradually increases until the FEP film and the PMMA/Cytop completely contact each other, the IDS value increases to the maximum level. Therefore, the IDS change can be determined by the applied pressure. Figure 4(b) reveals the IDS change of the FOTT during the press−release processes. Consistent with the analysis above is the IDS increase/decrease when the FEP film is pressed/released. In order to visualize the IDS change under the modulation of the external force, the sensor is utilized as the switch through an amplifier circuit to an LED light for illumination. The LED is turned on when the sensor is in a pressed state and turned off in a released state. Dynamic demonstrations can be found in Video S1. An interesting phenomenon is that the FEP film thickness will affect the performance of the sensor. The IDS shown in Figure 4(c) decreases obviously with the increase in the FEP film thickness under the same external pressure. This may be ascribed to the reduced distortion of the FEP film at large thickness, which diminishes the modulation effect on the IDS. In this work, a 100 μm thick FEP film has been selected to fabricate a pressure sensor, and the pressure sensing properties are illustrated in Figure 4(d). The sensor exhibits a good sensing performance with a high linear correlation coefficient of about 0.980 within the pressure range of 20−1000 Pa. The sensitivity of this pressure sensor is defined as S = (ΔI/I0)/ΔP, where ΔI represents the relative change in current, I0 is the current of the sensor without pressure loading, and ΔP is the 11569

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Figure 5. Structures and performances of the FOTT for magnetic sensing. (a) Schematic illustration of the device structure and the sensing process. (b) Atomic force microscope (AFM) image of the PDMS/Fe3O4 composites. (c) Photograph of a flexible sensing device demonstrating its excellent flexibility. (d) IDS changes of the sensor with a magnetic field on and off. (e) IDS changes corresponding to a magnetic field of 200 mT with different PDMS/Fe3O4 composite thicknesses. (f) IDS output characteristics with different magnetic fields. A fitted curve is displayed in the inset. (g) Stability test of the sensor. VDS remains at −10 V during the whole experiment. A small hysteresis and good repeatbility are attained even after about 10 000-cycle tests.

a magnetic sensor is developed. Figure 5(a) presents the structure and sensing process of the magnetic sensor. To begin with, the Fe3O4/PDMS magnetic composites, spin-coated on the top surface of the FEP film, are fixed on the FOTT. Figure 5(b) illustrates the atomic force microscope (AFM) image of the PDMS/Fe3O4 composites. Figure S4 reveals the detailed fabrication process. The Fe3O4/PDMS magnetic composites will deform and get close to the PMMA/Cytop dielectric layer by magnetic force when a magnet touches the bottom surface, which is equal to narrowing the depletion zone in the pentacene conductive channel. This will increase the IDS. However, when the magnet is taken away, the Fe3O4/PDMS magnetic composites will recover to the initial state and the depletion zone will increase, leading to the decrement of the

relative change in the applied pressure. By taking advantage of the distinct response of the device as a function of the applied pressure, a maximum sensitivity of 21% Pa−1 is calculated. The temporal response of the pressure sensor during a quick press− release process shows a fast response time of 110 ms and a recovery time of 120 ms, as depicted in Figure S2. Moreover, the persistence and repeatability experiment of the pressure sensor is depicted in Figure S3. As a consequence, the FOTT as a pressure sensor has shown promising prospects in wearable electronics and intelligent sensing. Application of the FOTT for Magnetic Sensing. In addition to being a contact pressure sensor, another intriguing application of the FOTT is noncontact magnetic sensing. By further adding magnetic composites into the triboelectric layer, 11570

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ACS Nano IDS. A sensing device with excellent flexibility is shown in Figure 5(c). Moreover, the IDS changes of the sensor with a 200 mT magnetic field on and off are shown in Figure 5(d) (the device operates under a constant VDS of −10 V). When no magnetic field is loaded, the IDS remains unchanged and no obvious current modulation is observed. Once a magnetic field is introduced, the IDS increases obviously, which indicates a good potential in magnetic sensing. A demonstration of IDS change by switching an LED light through an amplifier circuit is shown in Video S2. Notably, the I D S is related to the Fe 3 O 4 /poly(dimethylsiloxane) (PDMS) magnetic composite thickness, with corresponding experimental results shown in Figure 5(e). A rising trend of the IDS before the magnetic film thickness reaches about 250 μm is observed, and after that, it begins to decrease. This can be explained as the multieffects of magnetic particle content and the thickness. Commonly, the magnetic particle content increases with increasing thickness, as well as the magnetic force. However, the increasing thickness also hinders the distortion process. Accordingly, 250 μm is selected as the optimal thickness for PDMS/Fe 3 O 4 magnetic composites. With these magnetic composites, a flexible FOTT magnetic sensor has been developed with characteristics as shown in Figure 5(f). From the figure, the current dramatically rises from −2.44 μA to −2.83 μA as the magnetic field increases within the range of 1−150 mT. Furthermore, the linear correlation coefficient between the magnetic field and the IDS is 0.983. The sensitivity of our magnetic sensor is defined as S = (ΔI/I0)/ΔB, where ΔI denotes the relative change in current, I0 is the current of the sensor without magnetic loading, and ΔB is the relative change in the applied magnetic field. By calculation, we obtained a maximum sensitivity of 16% mT−1. In order to further assess the sensing performance of the FOTT-based magnetic sensor, we investigate the magneticfield-dependent current change response, which exhibits a fast response and recovery time (seen in Figure S5). Compared with previous magnetic sensors,40,41 which exhibit a response time of 130 ms to 2.85 s, the FOTT-based magnetic sensor exhibits a faster response time of 120 ms. Furthermore, previously reported magnetic sensors exhibit sensitivities of 0.093% to 2.5% mT−1 in a magnetic field of several hundreds of mT,42−44 which demonstrated that the sensitivity of our device is obviously higher than that of previous reports. Moreover, the measurements for about ten thousand cycles are carried out to validate the stability and repeatability of the device, as shown in Figure 5(g). Even after ten thousands test cycles, the changes in the IDS amount to less than 5%, showing its small hysteresis and excellent reproducibility. One important cause of the excellent stability is that there is no traditional top gate electrode in the FOTT, which can prevent the device performance degradation caused by electrode damage in the device bending process as well as simplify the preparation process. All these results may promise a bright future of this magnetic sensor in intelligent sensing and control.

mode. This FOTT can be further used as a pressure sensor that exhibits a great sensitivity as high as 21% Pa−1 and fast response time of 110 ms in the pressure range of 20−1000 Pa. Moreover, this FOTT can also be used for magnetic sensing with a sensitivity of 16% mT−1, fast response time of 120 ms, and excellent stability for more than 10 000 cycles when the sensor operates at 1−150 mT. The FOTT in the simple structure, with pressure-magnetic-sensing capability in this work, shows bright potentials of tribotronics in human−machine interactions, smart skins, wearable devices, intelligent sensing, and so on.

METHODS Fabrication Process of the FOTT. First, a 300 nm Ta2O5 film is sputtered on the surface of a flexible PET substrate that is cleaned by ultrasound in deionized water four times. Then, a 45 nm pentacene (Sigma-Aldrich, >99%, used as received) film is thermally evaporated at a deposition rate of 0.01−0.02 nm s−1 under a 1 × 10−4 Pa vacuum. After that, 45 nm Au drain/source electrodes are deposited on two sides of the pentacene film through shadow masks. Finally, PMMA (Sigma-Aldrich) and Cytop are spin-coated (3000 and 1500 rpm s−1) as the dielectric layer, respectively. The final step is annealing the device in a glovebox at 100 °C for 1 h. Fabrication of the Pressure Sensor. The pressure sensor is proposed by attaching two PI tapes of 300 μm on the FOTT to fix the FEP as the mobile triboelectric layer. Fabrication of the Magnetic Sensor. The Fe3O4 nanoparticles (Aladdin, particle size: 300−500 nm) are added into PDMS (Dow Corning 184) and then stirred. Then, Fe3O4/PDMS composites are spin-coated (2000 rpm s−1) onto the FEP film and dried at 60 °C. The sample is peeled off after 1 h. Finally, the Fe3O4/PDMS composites are fixed on the FOTT by two PI tapes. Thereby, the magnetic sensor is well-prepared. Electrical Measurements of the FOTT. The I−V curves in Figure 2 and Figure 3 are measured using a Keysight B1500A semiconductor parameter analyzer. The other experimental data are measured by a DS 345 synthesized function generator in conjunction with an SR570 low-noise current amplifier (Stanford Research System). Different magnetic intensities are measured by a magnetometer through changing the distance between the magnet and the bottom surface of the PET substrate.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b06480. Figures S1−S5 (PDF) Tribotronic transistor for pressure sensing (AVI) Tribotronic transistor for magnetic sensing (AVI)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] (G. Dong). *E-mail: [email protected] (C. Zhang). *E-mail: [email protected] (Z. L. Wang). ORCID

Hang Guo: 0000-0001-7004-1527 Guifang Dong: 0000-0003-2299-0212 Chi Zhang: 0000-0002-7511-805X Zhong Lin Wang: 0000-0002-5530-0380

CONCLUSIONS In summary, a flexible organic tribotronic transistor without a top gate electrode has been developed by using the p-type pentacene and PMMA/Cytop composites as the conductive channel and the dielectric layer, respectively. By making the mobile triboelectric layer contact with and separate from the PMMA/Cytop dielectric surface, the IDS will increase in the enhancement mode and decrease dramatically in the depletion

Author Contributions ¶

J. Zhao and H. Guo contributed equally to this work.

Notes

The authors declare no competing financial interest. 11571

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ACKNOWLEDGMENTS The authors thank the support of National Key Research and Development Program of China (2016YFA0202704), National Natural Science Foundation of China (No. 51475099), Beijing Nova Program (No. Z171100001117054), the Youth Innovation Promotion Association, CAS (No. 2014033), and the “Thousands Talents” program for the pioneer researcher and his innovation team, China. The authors also thank Xianpeng Fu and Tianzhao Bu for their contributions in the whole experiment. REFERENCES (1) Rogers, J. A.; Someya, T.; Huang, Y. Materials and Mechanics for Stretchable Electronics. Science 2010, 327, 1603−1607. (2) Kim, D.-H.; Lu, N.; Ma, R.; Kim, Y.-S.; Kim, R.-H.; Wang, S.; Wu, J.; Won, S. M.; Tao, H.; Islam, A.; Yu, K. J.; Kim, T.-I.; Chowdhury, R.; Ying, M.; Xu, L.; Li, M.; Chung, H.-J.; Keum, H.; McCormick, M.; Liu, P.; Zhang, Y.-W.; Omenetto, F. G.; Huang, Y.; Coleman, T.; Rogers, J. A. Epidermal Electronics. Science 2011, 333, 838−843. (3) Chortos, A.; Bao, Z. Skin-Inspired Electronic Devices. Mater. Today 2014, 17, 321−331. (4) Hammock, M. L.; Chortos, A.; Tee, B. C. K.; Tok, J. B. H.; Bao, Z. 25th Anniversary Article: The Evolution of Electronic Skin (ESkin): A Brief History, Design Considerations, and Recent Progress. Adv. Mater. 2013, 25, 5997−6038. (5) Sun, Q.; Kim, D. H.; Park, S. S.; Lee, N. Y.; Zhang, Y.; Lee, J. H.; Cho, K.; Cho, J. H. Transparent, Low-Power Pressure Sensor Matrix Based on Coplanar-Gate Graphene Transistors. Adv. Mater. 2014, 26, 4735−4740. (6) Sun, Q.; Seung, W.; Kim, B. J.; Seo, S.; Kim, S. W.; Cho, J. H. Active Matrix Electronic Skin Strain Sensor Based on PiezopotentialPowered Graphene Transistors. Adv. Mater. 2015, 27, 3411−3417. (7) Knopfmacher, O.; Hammock, M. L.; Appleton, A. L.; Schwartz, G.; Mei, J.; Lei, T.; Pei, J.; Bao, Z. Highly Stable Organic Polymer Field-Effect Transistor Sensor for Selective Detection in the Marine Environment. Nat. Commun. 2014, 5, 2954. (8) Chou, H.-H.; Nguyen, A.; Chortos, A.; To, J. W. F.; Lu, C.; Mei, J.; Kurosawa, T.; Bae, W.-G.; Tok, J. B.-H.; Bao, Z. A ChameleonInspired Stretchable Electronic Skin with Interactive Colour Changing Controlled by Tactile Sensing. Nat. Commun. 2015, 6, 8011. (9) Zhang, C.; Zhou, T.; Tang, W.; Han, C.; Zhang, L.; Wang, Z. L. Rotating-Disk-Based Direct-Current Triboelectric Nanogenerator. Adv. Energy Mater. 2014, 4, 1301798. (10) Zhang, C.; Tang, W.; Han, C.; Fan, F.; Wang, Z. L. Theoretical Comparison, Equivalent Transformation, and Conjunction Operations of Electromagnetic Induction Generator and Triboelectric Nanogenerator for Harvesting Mechanical Energy. Adv. Mater. 2014, 26, 3580−3591. (11) Zhou, T.; Zhang, C.; Bao Han, C.; Ru Fan, F.; Tang, W.; Lin Wang, Z. Woven Structured Triboelectric Nanogenerator for Wearable Devices. ACS Appl. Mater. Interfaces 2014, 6, 14695−14701. (12) Zhou, T.; Zhang, L.; Xue, F.; Tang, W.; Zhang, C.; Wang, Z. L. Multilayered Electret Films Based Triboelectric Nanogenerator. Nano Res. 2016, 9, 1442−1451. (13) Huang, L. B.; Bai, G.; Wong, M. C.; Yang, Z.; Xu, W.; Hao, J. Magnetic-Assisted Noncontact Triboelectric Nanogenerator Converting Mechanical Energy into Electricity and Light Emissions. Adv. Mater. 2016, 28, 2744−2751. (14) Bao Han, C.; Zhang, C.; Li, X. H.; Zhang, L.; Zhou, T.; Hu, W.; Lin Wang, Z. Self-Powered Velocity and Trajectory Tracking Sensor Array Made of Planar Triboelectric Nanogenerator Pixels. Nano Energy 2014, 9, 325−333. (15) Tang, W.; Zhou, T.; Zhang, C.; Ru Fan, F.; Bao Han, C.; Lin Wang, Z. A Power-Transformed-and-Managed Triboelectric Nanogenerator and Its Applications in a Self-Powered Wireless Sensing Node. Nanotechnology 2014, 25, 225402. 11572

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