A Flexible and Transparent Graphene-Based Triboelectric

IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 15, NO. 3, MAY 2016

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A Flexible and Transparent Graphene-Based Triboelectric Nanogenerator Smitha Ankanahalli Shankaregowda, Chandrashekar Banankere Nanjegowda, Xiao-Liang Cheng, Ma-Yue Shi, Zhong-Fan Liu, and Hai-Xia Zhang

Abstract—This paper presents a new high-output, flexible and transparent nanogenerator using chemical vapor deposition grown graphene as one of the friction layer. Graphene on copper is transferred onto polyethylene terephthalate by wet transfer method makes graphene-based triboelectric nanogenerator (TENG) have electrical conductivity and high optical transmittance. We have fabricated plasma treated thin layer of polydimethylsiloxane structure as another layer to improve the output performance of nanogenerator. Using this graphene-based TENG, maximum output voltage 650 V and current 12 μA is achieved at 4.3 Hz frequency. As a power source, LCD and 50 commercial blue light-emitting diodes are lighted up. It is low cost, simple, and robust approach for harvesting ambient vibration energy. Index Terms—Chemical vapor deposition (CVD) grown graphene, flexibility, optical transmittance, triboelectric nanogenerator (TENG).

I. INTRODUCTION HE serious energy crisis can be a major factor that limits the quality of life [1]–[3]. Vast amount of mechanical energy from human activities, traffic, airflows, and so forth are ubiquitous but regretfully being ignored. Nowadays, harvesting vibration energy have attracted extensive interest and are considered to be effective and promising approaches for solving the energy crisis due to the great abundance of mechanical energy existing in our living environment and industrial production [4]. Harvesting vibration energy mainly focuses on electrostatic, piezoelectric, magnetostrictive effect, aiming at building up self-powdered systems to drive small electronics [5]–[9]. Recently, triboelectric nanogenerator (TENG), a conjunction use of the universally known contact electrification effect and

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Manuscript received September 29, 2015; revised February 11, 2016; accepted February 28, 2016. Date of publication March 10, 2016; date of current version May 6, 2016. This work was supported by the National Natural Science Foundation of China under Grants 61176103, 91023045, and 91323304, the National Hi-Tech Research and Development Program of China (“863” Project) under Grant 013AA041102, the Beijing Science & Technology Project under Grant Z141100003814003, and the Beijing Natural Science Foundation of China under Grant 4141002. The review of this paper was arranged by Associate Editor SIROMA Guest Editors. S. A. Shankaregowda, X.-L. Cheng, M. Shi, and H.-X. Zhang are with the Institute of Microelectronics, Peking University, Beijing 100871, China (e-mail: [email protected]; [email protected]; [email protected]. cn; [email protected]). C. B. Nanjegowda is with the Center for Nanochemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China (e-mail: [email protected]). Z. Liu is with the Centre for Nano Chemistry, Peking University, Beijing 100871, China (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNANO.2016.2540958

electrostatic induction has gained more attention for harvesting mechanical energy [10]–[16]. Triboelectrification is a process of charge separation and transfer between two materials through mechanical contact and friction. When two materials with oppositely polarized triboelectric charges are subject to periodic contact and separation either by press and release motion or planar sliding motion, the induced potential difference between the two electrodes can be changed cyclically, thus driving the alternating current flowing through the external load [17]. The demand of new materials of increased conductivity, flexibility and transparency are required for variety of potential applications [18]–[21]. The electronic devices with the integration of flexible and transparent characteristics are most expected to meet emerging technological demands for the next generation of flexible electronic, foldable electronic and optoelectronic devices [22]. Graphene, a monolayer honeycomb lattice structure of sp2 bonded carbon atoms, has become a subject of great interest due to its fascinating advantages of high electrical conductivity, optical transparency, gas barrier property, robustness, flexibility and environmental stability [23]–[25]. The combination of these remarkable electrical and mechanical properties leads us to expect that graphene can be used as stable conductor under any kinds of substrate deformation [26]–[31]. A number of theoretical studies on the electrostatic behavior of graphene have been reported, and it has been concluded that graphene can store an electric charge for a period of time, which adds to its suitability for TENGs [32]–[36]. Chemical vapor deposition (CVD) grown graphene have wrinkled and ripples which make the graphene structure and more suitable for high output voltage applications because of significant amount of friction and surface charge created by triboelectric effect [37]–[38]. The need of mechanical flexibility and optical transparency on demand of various electronic applications, here we attempted to fabricate the TENGs to obtained high performance by harvesting low frequency ambient vibration energy. Recently, a new type electrical energy harvesting from graphene by mechanical stressing was intensively studied [39], [40]. Furthermore, through rational design, this new mode of power generation can be developed to build a high-output, flexible, and transparent TENG. A challenge to TENG is its relative low output current and high output voltage. Hence graphene based TENG is designed to overcome this problem. Here, an arch shaped graphene based TENG for harvesting ambient vibration energy is demonstrated. By utilizing the coupling between the triboelectric effect and the electrostatic effect, the periodic contact/separation between the graphene film and polydimethylsiloxane (PDMS) film

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Fig. 1. Schematic diagram showing the structural design of the arch- shaped Graphene based TENG.

can induce charge transfer between the graphene electrode and indium tin oxide (ITO), resulting in the flow of the electrons across an external load resistor. The fabricated TENG can deliver open-circuit voltage about 650 V and a short circuit current about 12 μA. The output power of a single device with a size of 3.2 cm by 2.8 cm is high enough to simultaneously light up more than 50 commercial light-emitting diodes (LEDs) and LCD, unambiguously demonstrating its feasibility of powering portable electronics, and sensors for environmental and infrastructure monitoring and security. II. EXPERIMENTAL SECTION By applying cyclic compressive force on arch-shaped structure TENG, it is easy to achieve complete contact and separation between the surfaces [46]. The ability of an arch-shaped structure and rough surface to enhance the TENG output performance has been demonstrated by previous work [38], [39] and [46]. Hence, this graphene based TENG was also appropriately designed to be arch-shape as is shown in Fig. 1. The top layer is graphene/ethylene vinyl acetate (EVA)/polyethylene terephthalate (PET) and the bottom layer is PDMS/ITO/PET.

Fig. 2. Structure and fabrication process of CVD grown Graphene on PET/EVA. (a)–(e) Fabrication flow chart of top layer of the graphene based TENG.

A. Large Area CVD Graphene Growth and Transfer Procedure The schematic view of large area CVD graphene grow and transfer procedure is as shown in Fig. 2. Low pressure CVD was used for the growth of large area graphene on copper foil. First, dry polycrystalline copper foil (99.9% purity, 18-μm thickness, 1-μm roughness, ∼100 RMB/kg) was placed inside the furnace tube. The furnace was then heated to 980 °C, under the gas flow of 50 sccm H2 . After reaching 980 °C, the sample was annealed for 30 min without changing the gas flow. Growth was carried out under a gas mixture of 20 sccm of CH4 and 50 sccm of H2 for 30 min. Finally the CVD system was cooled to room temperature under H2 . After the CVD growth, graphene/copper stack was pressed against the target substrate EVA/PET shown in Fig. 2(a). The Cu/G/EVA/PET stack was passed between the two rollers for the hot lamination process which offers uniform adhesion of graphene from copper onto EVA PET shown in Fig. 2(b), (c). In the subsequent step, the copper substrate was then etched away by an aqueous solution of 0.1 M ammonium persulphate

as shown in Fig. 2(d). The Graphene/EVA/PET samples were rinsed in deionized water and blow-dried with a nitrogen gun finally as shown in Fig. 2(e). B. PDMS Preparation The base solution and curing agent of PDMS (Sylgard 184, Dow Corning Corporation) were mixed thoroughly in the quantity ratio of 10: 1 and degassed for 30 min. Then the mixture was spin-coated (at 1000 r/min) onto a transparent ITO-coated PET and cured at 75 °C for 60 min in an oven. Finally the Surface chemical modification of PDMS on PET/ITO was done. An inductively coupled plasma (ICP) etcher (Surfacing Technology Systems plc, Multiplex ICP 48443) was used to conduct fluorocarbon plasma treatment. The PDMS film was treated by the fluorocarbon plasma with octafluorocyclobutane (C4 F8 ) for the reactive gas. In fluorocarbon Plasma treatment modification process, the RF power, platen power, gas flow rate, pressure,

SHANKAREGOWDA et al.: FLEXIBLE AND TRANSPARENT GRAPHENE-BASED TRIBOELECTRIC NANOGENERATOR

Fig. 4.

Fig. 3. Schematic and fabrication process illustration of bottom layer graphene based TENG. (a)–(d) shows the flow diagram of PDMS preparation and C4 F8 treatment.

and time were set as 400 W, 0 W, 80 sccm, 5 Pa and 180 s respectively. C. Fabrication of Graphene Based TENG To make the nanogenerator transparent and flexible, accordingly materials are chosen and employed to fabricate the device. The graphene acts as electrode as well as one of the friction layers of the TENG device. A thin layer C4 F8 plasma treated PDMS was coated on ITO/PET. ITO film on PET acts as an induced charge collector and electrode for the connecting to the external circuit. PET/Graphene and PDMS/ITO/PET were faced opposite to each other and both of their edges were connected using a plastic (polyimide) spacer to obtain the device in arc shape. The triboelectric phenomenon depends not only on the electronic properties of the materials but also on their elastic and surface characteristics. It is found that the fluorocarbon plasma treatment [40], [41] can significantly modify the surface and strengthen the TENG output performance. The use of spacer in the device significantly improves the capacitance of the system in the deformation process because of the presence of air void between graphene and PDMS, which increase the dipole moments formed during mechanical deformation [38]. III. PRINCIPLE DESIGN The Fig. 4 shows the schematic diagram for the working mechanism of Graphene based TENG; it can be included into four steps, (a) contact (b) releasing (c) electrical equilibrium

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Working mechanism of graphene based TENG.

and (d) pressing. At original stage, the device is in neutral state; therefore no charge is on the surface of graphene and PDMS. When the device is compressed the positive and negative electrostatic charges are generated and distributed at each surface of the PDMS and CVD graphene film according to triboelectric tendency [42]–[45]. Since the produced opposite triboelectric charges and opposite polarities are equally balanced, there is no electron flow between the top and bottom electrodes at the pressed state. Once the deformation starts to be released, the opposite triboelectric charges separate with an air gap and form a dipole moment, which drives electron to flow from bottom ITO electrode to upper graphene electrode. The flow of the electrons can last until PDMS and graphene film reaches the maximum separation. If the device is pressed again, the dipole moment disappears, resulting in the reverse flow of electrons between the electrodes. This electrostatic induction process produces the output signals until both surfaces are completely contacted, that is, pressed state. IV. RESULT AND DISCUSSION A. Transparence of the Graphene Based TENG Fig. 5(a)–(e) provides the structural characterization of the materials used for the fabrication of graphene based TENG. Large area monolayer graphene grown on copper foil using LPCVD was transferred onto the EVA/PET by wet transfer method. Fig. 5(a) shows a scanning electron microscopy image of the resulting graphene on EVA/PET. Features such as bilayer graphene spots (solid arrow), wrinkles (hollow arrow) and continuous monolayer graphene (grey color) with copper polycrystalline morphology are clearly seen on flexible and transparent EVA/PET. The presence of bilayer graphene spots between the wrinkles helps to enhance the electrical conductance in the CVD monolayer graphene. Fig. 5(b) shows the transmittance of the graphene film up to 97.5% at the wavelength of 550 nm,

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Fig. 6. Performance characterization of a typical transparent TENG. (a) Output voltages and (b) the relation between voltage and current (c) Short circuit current of the TENG in continuous operation. (d) Power versus Load.

Fig. 5. Characterization of graphene based TENG (a) Scanning electron microscope image of graphene on EVA/PET. (b) and (c) UV/Visible transmittance of PET/EVA/Graphene (Inset is the photograph of PET/Graphene) and whole device (Inset is the photograph of graphene based TENG) respectively. (d) Photograph of an as fabricated device showing high transparency (e) Flexibility of device.

corresponding to the single layer graphene. The transmittance of the whole TENG device shows 78% at the wavelength of 550 nm as shown in Fig. 5(c). The commercial ITO-coated PET film before and after the PDMS layer exhibits a transmittance of 82% [46]. TENG device is composed of sandwiched structure with graphene/EVA/PET and ITO coated PET covered by PDMS thin film. Fig. 5(d) shows the photograph of the TENG


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wrinkles and ripples, hence significant amount of surface charge and friction is created which makes graphene more suitable for high output voltage application. To investigate the output performance of the Graphene based TENG as a power source voltage and current were measured with different external loads. Fig. 6(c) shows the resistance dependence of both output current and voltage from 10 to 100 MΩ. The output currents decrease with the increasing load resistance while the output voltages show the reverse trend. The output currents decrease with the increasing load resistance while the output voltages show the reverse trend. Therefore, the effective electrical power of the TENG is closely related to the external load and reaches a maximum value of 3.28 mW at a load resistance ∼60 MΩ. To prove the capability of the Graphene based TENG as a sustainable power source, when the device is tapped a 20 pin Liquid Crystal Display is displayed and 50 commercial blue LEDs connected in series with a rectifier circuit were lighted up as shown in Fig. 7(a) and (b). V. CONCLUSION Fig. 7. Graphene based TENG applied as power source. (a) Photograph of 50 blue LED lamps in a series simultaneously lighted. (b) LCD was powered (Letter “A” is displayed).

device placed on the color logo of PKU showing visible which indicates that the complete TENG device is transparent. As fabricated TENG device is deformed by human fingers exhibits the good flexibility [seeFig. 5(e)]. B. Electrical Output of the Graphene Based TENG The electrical measurements of arc-shaped TENG were characterized via a mechanical vibration system. The vibration system incorporates a power amplifier, oscilloscope (RIGOLDS1102E) and a vibrator. The output voltage and current were measured using an oscilloscope (RIGOL DS1102E) with an input impedance of 100 MΩ and low-noise current preamplifier (Stanford Research SR570). In order to characterize the electrical output performance of the graphene based TENG, a vibration system was used to apply a periodic contact and separation process. During the periodical vibration at a certain frequency (at 4.3 Hz), the maximum output voltage and current of a typical TENG device with a size of 3.5 cm × 4.5 cm reach up to 650 V and 12 μA, respectively [see Fig. 6(a) and (b)]. The experimental results indicate that the output performance of this TENG is much greater than that of other types of transparent generators [43]–[46]. A digital oscilloscope with 100 MΩ probe was used to measure the voltage directly, while the current was measure by using low noise preamplifier with 1 Ω probe. The improvement in the result is due to surface modification and CVD graphene. The fluorocarbon plasma treatment modifies the PDMS surface and enhances the output performance of the TENG by chemical modification of the friction surface. The CVD -grown graphene samples have

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Smitha Ankanahalli Shankaregowda received the B.E. degree from the Department of Electronics and Communication Engineering, SJBIT College, Bangalore, India, and the M.Tech. degree in digital electronics and communication, NMAMIT College, Nitte, Visvesvaraya Technological University, India, in 2009 and 2011, respectively. She was an Assistant Professor for two years in the Coorg Institute of Technology, Karnataka, India. She is currently a Visiting Scholar (2014) at the National Key Laboratory of Nano/Micro Fabrication Technology, Peking University, Beijing, China. Her research includes nanogenerators for biomedical applications.

Chandrashekar Banankere Nanjegowda received the B.Sc. degree from the Department of Chemistry, the M.Sc. and Ph.D. degrees in industrial chemistry from Kuvempu University, Shimoga, India, in 2006, 2008, and 2013, respectively. He is currently a Postdoctoral Researcher in the Center for Nano Chemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing, China. His research interests include electrode design for the biosensors, 2-D material synthesis, roll-to-roll CVD graphene synthesis, and fabrication of functional materials for the applications of energy conversion.

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Xiao-Liang Cheng received the B.S. degree from the University of Electronic Science and Technology of China, Chengdu, in 2014. He is currently working toward the Ph.D. degree at the National Key Laboratory of Nano/Micro Fabrication Technology, Peking University, Beijing, China. His research interests mainly include design and fabrication of nanogenerator and mechanical energy harvester.

Ma-Yue Shi (M’14) was born in Shaanxi, China, in 1989. He received the B.S. degree in electronic science and technology from the Xi’an Jiaotong University, Xi’an, China, in 2012. Since 2013, he is a Master Student in microelectronics, Peking University, Beijing, China. His research interests include self-powered sensors and systems, micro energy harvesters, micro electromechanical systems, and artificial intelligence.

Zhong-Fan Liu received the Ph.D. degree from the University of Tokyo, Tokyo, Japan, in 1990. After a Postdoctoral fellowship at the Institute for Molecular Science, Okazaki, Japan, he became an Associate Professor in 1993, a Full Professor in 1993, and a Cheung Kong Chair Professor in 1999 of Peking University. He was elected as the Member of Chinese Academy of Sciences in 2011 and as one of the six outstanding scientists in Ten-Thousand-Talents Program in 2013. He is currently the Director of the Center for Nanoscale Science and Technology, the Center for Nanochemistry of Peking University, and the Beijing Science and Engineering Center for Low Dimensional Carbon Materials. He is the Chairman of Nanochemistry Committee in Chinese Chemical Society and the past President of Chinese Electrochemical Society. His research interests include low-dimensional carbon materials and novel 2-D atomic crystals targeting nanoelectronic and energy conversion devices together with the exploration of fundamental phenomena in nanoscale systems. He has received many academic awards and honors, including Chinese Chemical Society-Akzo Nobel Chemical Sciences Award (2012), Baogang Outstanding Teacher Award (2012), National Award for Natural Science (second class, 2008), etc. He is currently the Fellow of Royal Society of Chemistry and the Fellow of Institute of Physics, U.K.

Hai-Xia Zhang (SM’10) received the Ph.D. degree in mechanical engineering from the Huazhong University of Science and Technology, Wuhan, China, 1998. She is currently a Professor with the Institute of Microelectronics, Peking University, Beijing, China. She joined the Faculty of the Institute of Microelectronics in 2001 after her postdoctoral position with Tsinghua University. Her research interests include MEMS design and fabrication technology and micro energy technology. She has served on the General Chair of the IEEE NEMS 2013 Conference and the Organizing Chair of Transducers 2011. As the Founder of the International Contest of Innovation, she has been organizing this world-wide event since 2007. In 2006, she received the National Invention Award of Science and Technology.