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Graphene based two dimensional hybrid nanogenerator for concurrently harvesting energy from sunlight and water flow
Carbon 105 (2016) 199e204
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Carbon journal homepage: www.elsevier.com/locate/carbon
Graphene based two dimensional hybrid nanogenerator for concurrently harvesting energy from sunlight and water flow Huikai Zhong a, b, Zhiqian Wu a, Xiaoqiang Li a, Wenli Xu a, Sen Xu a, Shengjiao Zhang a, Zhijuan Xu a, Hongsheng Chen a, b, Shisheng Lin a, b, * a b
College of Information Science and Electronic Engineering, Zhejiang University, Hangzhou, 310027, China State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, Hangzhou, 310027, China
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Article history: Received 6 February 2016 Received in revised form 5 April 2016 Accepted 13 April 2016 Available online 14 April 2016
Harvesting energy from multiple sources in our living environment is highly demanded, not only to cope with the increasing energy crises, but also to realize the self-powered electronics. Here we propose a graphene based two dimensional (2D) hybrid nanogenerator which can generate electricity through capturing sunlight as well as water flow. This 2D nanogenerator is mainly based on a graphene/silicon van der Waals Schottky diode. Two different metal electrodes are employed to build an asymmetric internal potential profile in the graphene channel, allowing for harvesting energy from sunlight in the 2D graphene plane with a maximum output power of 49.3 mW. When water flows over the graphene surface under illumination, an additional voltage can be generated simultaneously. This flow-induced voltage arises from an additional charge transfer in the graphene channel induced by a continuous doping and dedoping of the graphene, owing to a reversible wetting and dewetting effect of water during the water flowing process. The results show a physical picture of dynamic adjusting the charge transfer characteristic of graphene/silicon Schottky diode through water flow when under illumination, which may open prospects for practical applications for graphene nanogenerator. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction In order to meet the increasing global energy demands, harvesting energy form renewable and green sources has attracted worldwide attention [1]. Currently, energy harvesting devices which can harvest energy directly from the ambient environment using different transduction mechanisms have been intensively investigated and this feasible way to generate electricity shows great prospects for self-powered electronics [2e5]. Among various kinds of energy sources in our living environment, solar energy and the mechanical energy of water flow are relatively stable and widely distributed. More recently, solar cells which can convert solar energy to electricity have already become one of the most promising candidates for clean and renewable power [6], and a lot of attempts have been made to harness the mechanical energy of water flow which is inexhaustible and can be a good alternative to solar energy [7,8].
* Corresponding author. College of Information Science and Electronic Engineering, Zhejiang University, Hangzhou, 310027, China. E-mail address: [email protected] (S. Lin). http://dx.doi.org/10.1016/j.carbon.2016.04.030 0008-6223/© 2016 Elsevier Ltd. All rights reserved.
The extensively studied two dimensional (2D) material graphene, which is a single layer of carbon atoms assembled in a honeycomb lattice [9], possesses a few outstanding electrical, optical and mechanical properties, such as extremely high electron mobility, highly tunable conductivity, 2.3% constant absorption of visible light and high mechanical strength [10e13]. All these properties make graphene not only holds great potential in electronic and optoelectronic applications [14,15] but also has more promise in the pursuit of a higher energy conversion efficiency [16,17]. More recently, some solar cells based on graphene/semiconductor van der Waals Schottky diode are proposed [18e20], which is distinct from traditional PN junction or Schottky junction, as the barrier height of this type diode is highly tunable thanks to the Fermi level of graphene can be adjusted independently owing to the van der Waals contact [21]. Through introducing a graphenedielectric-graphene structure to independently tune the Fermi level of the graphene, we have achieved power conversion efficiency (PCE) as high as 18.5% with graphene/GaAs heterostructure solar cell [22]. Moreover, photocurrent detection experiments in graphene reveal a strong photoresponse near metal/graphene interface and semiconductor/graphene hybrid interface, which
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holds great promise for high speed optical communications and facilitates the capability of graphene-based optoelectronic devices [23e25]. In parallel with the extensive works on graphene based optoelectronic applications, owing to its exceptional sensitivity to external stimulus [26], a variety of approaches have been proposed recently to generate electricity from water flow in the 2D graphene plane which shows great compatibility to the photovoltaic behaviors of lateral graphene p-n junctions [27,28], but no obvious voltage responses to the flow of deionized (DI) water can be observed in previous works [29e31]. However, graphene based devices which can concurrently harvesting energy from sunlight and water flow have rarely been investigated previously, which should be important and probably push graphene based energy harvesting devices to practical application as solar energy and the mechanical energy of water flow are always available at the same time in our living environment. In this letter, we propose a graphene/Si Schottky diode based 2D hybrid nanogenerator, which can concurrently harvest energy from sunlight and water flow. Two different metal electrodes are adopted to build an asymmetric internal potential profile in the graphene channel, allowing for harvesting energy from sunlight in the 2D graphene plane. Besides, when water flows over the graphene surface under illumination, an additional voltage can be generated simultaneously. This flow-induced voltage arises from an additional charge transfer in the graphene channel induced by a continuous doping and dedoping of the graphene, owing to a reversible wetting and dewetting effect of water during the water flowing process. This water flowing process reveals a physical picture of dynamic adjusting the charge transfer characteristic of graphene/Si Schottky diode through water flow when under illumination, which is very interesting and rarely been studied. Our results demonstrate a type of 2D graphene based hybrid nanogenerator with excellent compatibility to be integrated with planar Si, which probably open prospects for practical application for graphene nanogenerator. 2. Experimental details The monolayer graphene samples used in this work were grown on 25 mm Cu foil (Alfa Aesar, item number 13382) by low pressure chemical vapor deposition with a reaction source flux ratio of CH4:H2 equals to 5:1 [32]. The n type Si wafers with a resistivity of 2e5 U cm were commercially available (Zhejiang Lijing silicon material Co., Ltd.). Native oxide on Si was removed by dipping the wafers into diluted HF solution for 3 min. 80 nm thick Al2O3 dielectric layer with an open width of 2 cm were grown by electron beam evaporation, which is used as an insulating layer to prevent electrodes/Si contact. Two asymmetric electrodes Cr/Au (5/60 nm) and Cr/Ag (5/60 nm) defined by lithography-processed mask were electron beam evaporated on the top of insulating layer respectively. After the open area of the silicon substrate was cleaned by dipping the substrate into acetone for 5 min followed with DI water rinse, monolayer graphene was transferred onto the substrate by a polymethyl methacrylate (PMMA) mediated method [33]. Then, the PMMA was removed using acetone. The Raman spectra of the graphene transferred onto SiO2/Si substrate were measured by Raman spectroscopy (Renishaw inVia Reflex) with an excitation wavelength of 532 nm. Graphene transferred onto SiO2/Si substrate was also employed to form a FET structure where the low-resistivity Si was taken as the back gate electrode, the currentevoltage characteristics of the FET structure were measured with an Agilent B1500A system. The photovoltaic behaviors of this 2D hybrid nanogenerator were tested with a solar simulator under AM1.5G condition, which was calibrated with a standard Si solar cell. The currentevoltage data were recorded
using a Keithley 2400 source-meter controlled by a LabView-based system. The voltage responses to the flow of the droplet of DI water were recorded in real time by a keithley 2010 multimeter which was controlled by a LabView-based data acquisition system with a sampling rate of 25 s1. 3. Results and discussion The schematic structure of this 2D hybrid nanogenerator is illustrated in Fig. 1a, which is composed of Si substrate, monolayer graphene and two asymmetric electrodes. Al2O3 dielectric insulating layer is sandwiched between the electrodes and Si substrate. The area of the graphene channel is L W (2.0 1.0) cm2. Fig. 1b shows the Raman spectrum of the graphene sample transferred onto SiO2/Si substrate, which shows a strong symmetric 2D peak, a nearly undetected defect-related D peak around 1350 cm1 and the intensity ratio of the 2D peak to G peak is 5.2, indicating the high quality monolayer of the as-grown samples. As seen from the inset of Fig. 1b, the source-drain current (Isd) of the FET based on graphene transferred onto SiO2/Si substrate achieves a minimum value at a gate voltage (Vg) of þ50 V when keeping the source-drain voltage (Vsd) constant as 1 V. The Dirac point for the graphene sample is located in the positive gate voltage region, indicating the as-grown graphene is p-type doped after the wet transferring process. Fig. 2a shows the current (I) versus voltage (V) curves of this device in the dark and under AM1.5G illumination condition. The dark I-V curve passes through zero in a straight line which simply exhibits a resistance characteristic, indicating the ohmic contact nature between the electrodes and graphene. No obvious opencircuit voltage (VOC) or short-circuit current (ISC) can be detected in the dark. Under AM1.5G illumination condition, the I-V curve is totally different where the symmetry around the origin has been broken. As we can see from the light I-V curve, the ISC of this this 2D hybrid nanogenerator with as-grown graphene becomes 0.7 mA, and the VOC achieves 0.33 V which demonstrates the harvesting energy behavior from sunlight in the 2D graphene plane. The output power P ¼ V I of this device can achieve a maximum value of 49.3 mW as we can get from the light I-V curve. When the middle region of the graphene channel is shaded, the Voc and Isc of this hybrid nanogenerator will decrease simultaneously, as shown in Fig. 2b. When the shading width increases from 0 to 1.5 cm, the Voc decreases from 0.33 to 0.256 V and the Isc decreases from 0.7 to 0.53 mA, indicating the bulk graphene channel can only make a small contribution to the photovoltage and photocurrent, whereas the regions of the graphene channel adjacent to the electrode/ graphene interfaces make the main contribution. Besides, we have also measured the I-V response to illumination of the device with the graphene channel being replaced by a 2 kohm resistance connected between the two electrodes using copper wires (Fig. S1). The straight I-V curve which passes through origin certainly reveals that the light exposed electrodes cannot generate photocurrent by themselves, demonstrating the Si contacted graphene channel is not just act as a load between the two electrodes. When the p-type doped graphene channel is contacted with the n-type Si substrate by van der Waals forces, a graphene/Si Schottky diode will be formed. Owing to the work function difference between the graphene and Si, a depletion region in Si will be formed in the Schottky junction by a build-in potential barrier (Fbarrier), which can separate the photo-generated electrons and holes into Si and graphene layer respectively, as shown in Fig. 2c. Fig. 2d presents the band profile in the lateral direction of this hybrid nanogenerator, which shows a work function tuning mechanism. By adopting the Au-Ag electrodes configuration, an asymmetric potential profile will be built in the graphene channel, where a strong
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Fig. 1. (a) Schematic structure of the 2D hybrid nanogenerator. (b) Raman spectrum of the graphene sample transferred onto SiO2/Si substrate and the inset shows the Isd of FET structure as a function of Vg when keeping Vsd constant as 1 V. (A colour version of this figure can be viewed online.)
Fig. 2. (a) IeV curves of this 2D hybrid nanogenerator in the dark and under AM1.5G illumination, the mark on the light IeV curve represents the point of maximum power output. (b) Light IeV curves of this 2D hybrid nanogenerator with the middle regions of the graphene channel with different widths are shaded from illumination. (c) Schematic electronic band profile of graphene/Si Schottky junction, which corresponding to the band profile in the vertical direction of this nanogenerator. (d) The asymmetric potential profile in the lateral direction of this nanogenerator, which leads to a directional move of the photo-generated holes in the 2D graphene plane, allowing harvest energy from sunlight. (A colour version of this figure can be viewed online.)
internal electric field exists in the narrow regions of the graphene channel adjacent to the electrode/graphene interfaces [23,34]. The photo-generated excess holes pulled to these narrow regions of the graphene channel can be efficiently drifted toward the Ag electrode owing to this strong electric field. As the excess holes in the other regions of the graphene channel can sustain a relatively long time, these excess holes can diffuse towards the Ag electrode in some extent, which leads to a small contribution to the photocurrent, as our experiment results show. In addition, the stability of the light induced voltage and current output of this structure has also been check by measuring the variation of the Voc and Isc values in 36 h under one sun illumination, where only a very small loss in voltage and current output can be observed after illumination for 36 h (Fig. S2). In order to further demonstrate the role of the electrodes and the Si contact of graphene channel in creating this photocurrent output in the 2D graphene plane, we have measured the photovoltaic response of devices with different configurations. Fig. 3a shows the photovoltaic behavior of the device consisting of two
symmetric electrodes, where both electrodes consist of Cr/Au (5/ 60 nm). The photovoltaic responses between the two electrodes are symmetric when applying a positive bias or a negative bias, indicating the mirror symmetry of the internal potential profile of the graphene channel in this case. Both the dark I-V curve and light I-V curve pass through zero, the difference is that the light I-V curve shows a larger current at the same bias which thanks to the photogenerated excess holes, no obvious VOC or ISC can be harvested between the two symmetric electrodes when under illumination. Fig. 3b shows the I-V curves of the device where the graphene/Si contact is fully prevented by the insulating layer. Even though the mirror symmetry of the internal potential profile in the graphene channel has been broken by the two different metal electrodes in this case, owing to the very short lifetime of the electronehole pairs generated in graphene (in the ps range) [35], the photo-generated carriers in the bulk graphene channel will disappear rapidly through recombination and cannot diffuse towards the electrodes. Beside, as graphene can only absorb ~2.3% of visible light [12], although the strong internal electric field which exists in the
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Fig. 3. (a) IeV curves of the device with two symmetric electrodes in the dark and under AM1.5G illumination. (b) IeV curves of the device where the graphene/Si contact is fully prevented by the insulating layer in the dark and under AM1.5G illumination. (A colour version of this figure can be viewed online.)
narrow regions (~0.2 mm) adjacent to the electrode/graphene interfaces can separate the electronehole pairs generated in these narrow regions of graphene [23,34], the contribution of these narrow regions to the photocurrent between the two electrodes can be neglected. As a result, the light I-V curve of this configuration has no obvious difference compared to the dark I-V curve as shown in Fig. 3b, both pass through zero in a straight line, indicating we cannot generate electricity effectively from sunlight in the 2D graphene plane without its contact with Si substrate. In addition to solar energy, the mechanical energy of water flow is always available around the clock in our living environment, so concurrently harvest solar energy and the mechanical energy of water flow is highly attractive. More excitingly, the mechanical energy of water flow can be simultaneously harvested with the solar energy by this 2D graphene/Si Schottky junction based nanogenerator we proposed. Fig. 4a shows the experimental setup, where the device is fixed on a slope with an inclination q of 30 with ground level. The area of the graphene channel in this case is increased to 6.0 2.0 cm2 in order to study the generating
electricity behavior from water flow and poly(dimethysiloxane) (PDMS) polymer is used to seal the electrodes in order to prevent the electrodes from exposing to water. A burette with a capacity of 10 mL is used to control the volume of the water droplet in our experiment, which is fixed in a few millimeters distance up away from the top end of graphene channel. A keithley 2010 multimeter is used to record the voltage responses to the flow of the droplet of DI water from the top end of the graphene channel to the bottom end driven by its own gravity. Fig. 4b shows the Raman spectra of graphene at different conditions. As we can see, the G peak position of graphene is blue-shifted from 1592.5 cm1 to 1594.1 cm1 when the graphene is wetted by DI water, and it is shifted back to 1593.1 cm1 after 10 min, which demonstrates graphene can be slightly p-type doped when contacted with DI water and this doping effect is reversible [26,36]. Fig. 4c shows the voltage responses to the flow of the droplet of DI water towards the direction of Ag electrode (0.1 mL in volume, unless indicated otherwise) in the dark and under room light (measured as 1.08 mW/cm2) illumination condition. In the dark, there is no noticeable voltage
Fig. 4. (a) Schematic diagram of experimental set-up to study the voltage response to the flow of DI water. (b) Raman G peak positions of the graphene at three conditions. (c) Voltage responses to the flow of DI water over graphene/Si Schottky diode in the dark and under room light illumination. (d) Voltage responses to the flow of DI water towards different directions. (A colour version of this figure can be viewed online.)
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response when the droplet of DI water flows over the graphene surface, indicating no interaction exists between DI water and graphene/Si Schottky diode in the water flowing process. When the device is under illumination, the voltage response is totally different. An initial voltage value around 25.6 mV is generated. During the water flowing process, the generated voltage increases with time to a peak voltage of ~28.14 mV when the water droplet flows to the bottom end of the graphene channel, indicating that an additional peak voltage of around 2.54 mV can be generated through the flow of a droplet of DI water. Once the droplet of DI water flows away from the graphene surface through the electrode, the voltage decreases sharply to an intermediate value around 26.5 mV. Then the voltage recovers to the initial value with a rate of 0.625 mV/s, which can be attributed to the effect of residual water on the graphene surface. When we reverse the water flowing direction, an additional voltage response with reversed sign can be generated. As shown in Fig. 4d which are measured under the same room light illumination using another device with the same configuration, when the droplet of DI water flows over the graphene surface towards Ag electrode, an additional voltage increment can be generated corresponding to an initial value of around 14 mV. However, an additional voltage reduction will be induced if the droplet of DI water flows towards Au electrode. This asymmetric voltage response is well in agreement with the asymmetry potential profile of the graphene channel. The slight fluctuation of the induced voltages can be attributed to fluctuations in flowing process of the droplet of DI water. This distinguishable voltage response generated in the flowing process of the droplet of DI water when under room light illumination should reflect the existing interaction between DI water and this 2D hybrid nanogenerator. In order to clarify the origin of this behavior, we have measured the voltage responses to the flow of DI water droplet with the device where the graphene/Si contact is
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fully prevented by Al2O3 insulating layer when under the same illumination (Fig. S3). We observe, no obvious voltage response can be generated, considering the electrodes are sealed to avoid exposing to water [28], we suggest that the distinguish voltage response should arises from the interaction between the DI water and graphene/Si Schottky diode instead of the wateregraphene interaction or the watereelectrodes interaction. In addition, owing to the flow-induced voltage in our experiments has the same direction with the flowing direction, the influence of the Hþ and OH ions which might be generated by photocatalysis effect can be excluded as a voltage with reversed sign will be generated with enough photocatalytic Hþ and OH ions [27e29]. Fig. 5a shows the charge distribution of the graphene/Si Schottky diode when under illumination, where holes are pulled to the graphene layer and electrons are pulled to the bulk Si substrate by the built-in electric field. When the graphene is contacted with a droplet of DI water, the big dipole moments of the water molecules which absorbed onto the graphene surface can act as a local electric field, which will lead to an electron transfer from the graphene to the water droplet, as a result, the graphene will be slightly p-type doped [26,37,38], which is also demonstrated by the shift of G peak of graphene as shown in Fig. 4b. As the Fermi level of graphene shifts downward owing to the p-type doping, the Fbarrier of the graphene/Si Schottky diode will become higher, the width of depletion region in Si will be increased and more holes can be pulled to graphene layer, as shown in Fig. 5b. When the droplet of DI water is flowing along the graphene surface, the graphene channel at the front side of the droplet is going to be wetted and p-type doped by the water, which indicates more holes will be pulled to this region from the depletion region in Si substrate. Meanwhile, the graphene channel at the rear side of the droplet is going to be dewetted and dedoped, which means excess holes will flow back to the depletion region in Si substrate as the Fbarrier of the graphene/Si Schottky diode becomes
Fig. 5. (a) Charge distribution of the graphene/Si Schottky diode when under illumination. (b) Charge distribution of the graphene/Si Schottky diode at the stable state when the water droplet keeps still. (c) Dynamic charge transfer process in the graphene/Si Schottky diode when the water droplet is flowing along the graphene surface, more photogenerated carriers will be separated at the front side of the droplet whereas the excessive photo-generated carriers will flow back to the depletion region in Si substrate at the rear side of the droplet. (d) Overall charge transfer process in the graphene channel during the water flowing process, the excess holes in graphene can be driven to move from the rear side to the front side of the droplet. (A colour version of this figure can be viewed online.)
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lower, as shown in Fig. 5c. As long as the water droplet keeps flowing, the graphene channel will be continuously doped and dedoped, this dynamic process will drive the additional portion of photo-generated holes which induced by the doping effect moving from the rear side to the front side of the water droplet, across the graphene channel under the droplet, which will lead to an additional potential difference between the two sides of the droplet, as shown in Fig. 5d. We suggest the physical picture of dynamic adjusting the charge transfer characteristic of graphene/Si Schottky diode through water flow when under illumination as the plausible mechanism for this flow-induced voltage. 4. Conclusions
[10]
[11]
[12]
[13] [14] [15] [16]
In conclusion, we propose a graphene/Si Schottky diode based 2D hybrid nanogenerator, which can concurrently harvest energy from sunlight and water flow. Two different metal electrodes are adopted in order to harvest energy from sunlight in the 2D graphene plane. When water flows over the graphene surface at the same time, an additional voltage can be generated simultaneously. This flow-induced additional voltage arises from a continuous doping and dedoping of the graphene during the water flowing process. This process reveals a physical picture of dynamic adjusting the charge transfer characteristic of graphene/Si Schottky diode through water flow when under illumination. Our results demonstrate a type of 2D graphene based hybrid nanogenerator with excellent compatibility to be integrated with planar Si, which probably open prospects for practical application for graphene nanogenerator.
[17]
[18] [19] [20] [21]
[22]
[23]
[24]
Acknowledgment
[25]
The authors thank the support from the National Natural Science Foundation of China (No.51202216, 51502264 and 51551203) and Special Foundation of Young Professor of Zhejiang University (No. 2013QNA5007).
[26]
[27] [28]
Appendix A. Supplementary data
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Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.carbon.2016.04.030.
[30] [31]
References [1] Z.L. Wang, Towards self-powered nanosystems: from nanogenerators to nanopiezotronics, Adv. Funct. Mater. 18 (22) (2008) 3553e3567. [2] S. Ghosh, A.K. Sood, N. Kumar, Carbon nanotube flow sensors, Science 299 (5609) (2003) 1042e1044. [3] B. Tian, X. Zheng, T.J. Kempa, Y. Fang, N. Yu, G. Yu, et al., Coaxial silicon nanowires as solar cells and nanoelectronic power sources, Nature 449 (7164) (2007) 885e889. [4] G.J. Snyder, E.S. Toberer, Complex thermoelectric materials, Nat. Mater. 7 (2) (2008) 105e114. [5] S. Xu, Y. Qin, C. Xu, Y. Wei, R. Yang, Z.L. Wang, Self-powered nanowire devices, Nat. Nanotechnol. 5 (5) (2010) 366e373. [6] N.S. Lewis, Toward cost-effective solar energy use, Science 315 (5813) (2007) 798e801. [7] Z.H. Lin, G. Cheng, S. Lee, K.C. Pradel, Z.L. Wang, Harvesting water drop energy by a sequential contact-electrification and electrostatic-induction process, Adv. Mater. 26 (27) (2014) 4690e4696. [8] M. Ma, L. Guo, D.G. Anderson, R. Langer, Bio-inspired polymer composite actuator and generator driven by water gradients, Science 339 (6116) (2013) 186e189. [9] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, et al.,
[32]
[33]
[34] [35]
[36]
[37]
[38]
Electric field effect in atomically thin carbon films, Science 306 (5696) (2004) 666e669. K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, M.I. Katsnelson, I.V. Grigorieva, et al., Two-dimensional gas of massless Dirac fermions in graphene, Nature 438 (7065) (2005) 197e200. C. Lee, X. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science 321 (5887) (2008) 385e388. R. Nair, P. Blake, A. Grigorenko, K. Novoselov, T. Booth, T. Stauber, et al., Fine structure constant defines visual transparency of graphene, Science 320 (5881) (2008), 1308-. A.K. Geim, Graphene: status and prospects, Science 324 (5934) (2009) 1530e1534. F. Bonaccorso, Z. Sun, T. Hasan, A.C. Ferrari, Graphene photonics and optoelectronics, Nat. Photonics 4 (9) (2010) 611e622. K.S. Novoselov, V.I. Fal’ko, L. Colombo, P.R. Gellert, M.G. Schwab, K. Kim, A roadmap for graphene, Nature 490 (7419) (2012) 192e200. J. Yin, J. Zhou, X. Li, Y. Chen, G. Tai, W. Guo, Enhanced gas-flow-induced voltage in graphene, Appl. Phys. Lett. 99 (7) (2011) 073103. X. Li, J. Yin, J. Zhou, Q. Wang, W. Guo, Exceptional high Seebeck coefficient and gas-flow-induced voltage in multilayer graphene, Appl. Phys. Lett. 100 (18) (2012) 183108. C.C. Chen, M. Aykol, C.C. Chang, A.F. Levi, S.B. Cronin, Graphene-silicon Schottky diodes, Nano Lett. 11 (5) (2011) 1863e1867. E. Shi, H. Li, L. Yang, L. Zhang, Z. Li, P. Li, et al., Colloidal antireflection coating improves graphene-silicon solar cells, Nano Lett. 13 (4) (2013) 1776e1781. P. Wang, X. Li, Z. Xu, Z. Wu, S. Zhang, W. Xu, et al., Tunable graphene/indium phosphide heterostructure solar cells, Nano Energy 13 (2015) 509e517. X. Miao, S. Tongay, M.K. Petterson, K. Berke, A.G. Rinzler, B.R. Appleton, et al., High efficiency graphene solar cells by chemical doping, Nano Lett. 12 (6) (2012) 2745e2750. X. Li, W. Chen, S. Zhang, Z. Wu, P. Wang, Z. Xu, et al., 18.5% efficient graphene/ GaAs van der Waals heterostructure solar cell, Nano Energy 16 (2015) 310e319. F. Xia, T. Mueller, R. Golizadeh-Mojarad, M. Freitag, Y-m Lin, J. Tsang, et al., Photocurrent imaging and efficient photon detection in a graphene transistor, Nano Lett. 9 (3) (2009) 1039e1044. J. Park, Y. Ahn, C. Ruiz-Vargas, Imaging of photocurrent generation and collection in single-layer graphene, Nano Lett. 9 (5) (2009) 1742e1746. J. Meng, H.D. Song, C.Z. Li, Y. Jin, L. Tang, D. Liu, et al., Lateral graphene p-n junctions formed by the graphene/MoS(2) hybrid interface, Nanoscale 7 (27) (2015) 11611e11619. F. Schedin, A.K. Geim, S.V. Morozov, E.W. Hill, P. Blake, M.I. Katsnelson, et al., Detection of individual gas molecules adsorbed on graphene, Nat. Mater. 6 (9) (2007) 652e655. J. Yin, Z. Zhang, X. Li, J. Yu, J. Zhou, Y. Chen, et al., Waving potential in graphene, Nat. Commun. 5 (2014) 3582. J. Yin, Z. Zhang, X. Li, J. Zhou, W. Guo, Harvesting energy from water flow over graphene? Nano Lett. 12 (3) (2012) 1736e1741. J. Yin, X. Li, J. Yu, Z. Zhang, J. Zhou, W. Guo, Generating electricity by moving a droplet of ionic liquid along graphene, Nat. Nanotechnol. 9 (5) (2014) 378e383. P. Dhiman, F. Yavari, X. Mi, H. Gullapalli, Y. Shi, P.M. Ajayan, et al., Harvesting energy from water flow over graphene, Nano Lett. 11 (8) (2011) 3123e3127. H. Zhong, X. Li, Z. Wu, S. Zhang, Z. Xu, H. Chen, et al., Two dimensional graphene nanogenerator by coulomb dragging: moving van der Waals heterostructure, Appl. Phys. Lett. 106 (24) (2015) 243903. X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, et al., Large-area synthesis of highquality and uniform graphene films on copper foils, Science 324 (5932) (2009) 1312e1314. X. Li, Y. Zhu, W. Cai, M. Borysiak, B. Han, D. Chen, et al., Transfer of large-area graphene films for high-performance transparent conductive electrodes, Nano Lett. 9 (12) (2009) 4359e4363. F. Xia, T. Mueller, Y.M. Lin, A. Valdes-Garcia, P. Avouris, Ultrafast graphene photodetector, Nat. Nanotechnol. 4 (12) (2009) 839e843. F. Wang, G. Dukovic, L.E. Brus, T.F. Heinz, Time-resolved fluorescence of carbon nanotubes and its implication for radiative lifetimes, Phys. Rev. Lett. 92 (17) (2004) 177401. A. Das, S. Pisana, B. Chakraborty, S. Piscanec, S.K. Saha, U.V. Waghmare, et al., Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor, Nat. Nanotechnol. 3 (4) (2008) 210e215. O. Leenaerts, B. Partoens, F.M. Peeters, Water on graphene: Hydrophobicity and dipole moment using density functional theory, Phys. Rev. B 79 (23) (2009). F. Yavari, C. Kritzinger, C. Gaire, L. Song, H. Gulapalli, T. Borca-Tasciuc, et al., Tunable bandgap in graphene by the controlled adsorption of water molecules, Small 6 (22) (2010) 2535e2538.
Contents lists available at ScienceDirect
Carbon journal homepage: www.elsevier.com/locate/carbon
Graphene based two dimensional hybrid nanogenerator for concurrently harvesting energy from sunlight and water flow Huikai Zhong a, b, Zhiqian Wu a, Xiaoqiang Li a, Wenli Xu a, Sen Xu a, Shengjiao Zhang a, Zhijuan Xu a, Hongsheng Chen a, b, Shisheng Lin a, b, * a b
College of Information Science and Electronic Engineering, Zhejiang University, Hangzhou, 310027, China State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, Hangzhou, 310027, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 February 2016 Received in revised form 5 April 2016 Accepted 13 April 2016 Available online 14 April 2016
Harvesting energy from multiple sources in our living environment is highly demanded, not only to cope with the increasing energy crises, but also to realize the self-powered electronics. Here we propose a graphene based two dimensional (2D) hybrid nanogenerator which can generate electricity through capturing sunlight as well as water flow. This 2D nanogenerator is mainly based on a graphene/silicon van der Waals Schottky diode. Two different metal electrodes are employed to build an asymmetric internal potential profile in the graphene channel, allowing for harvesting energy from sunlight in the 2D graphene plane with a maximum output power of 49.3 mW. When water flows over the graphene surface under illumination, an additional voltage can be generated simultaneously. This flow-induced voltage arises from an additional charge transfer in the graphene channel induced by a continuous doping and dedoping of the graphene, owing to a reversible wetting and dewetting effect of water during the water flowing process. The results show a physical picture of dynamic adjusting the charge transfer characteristic of graphene/silicon Schottky diode through water flow when under illumination, which may open prospects for practical applications for graphene nanogenerator. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction In order to meet the increasing global energy demands, harvesting energy form renewable and green sources has attracted worldwide attention [1]. Currently, energy harvesting devices which can harvest energy directly from the ambient environment using different transduction mechanisms have been intensively investigated and this feasible way to generate electricity shows great prospects for self-powered electronics [2e5]. Among various kinds of energy sources in our living environment, solar energy and the mechanical energy of water flow are relatively stable and widely distributed. More recently, solar cells which can convert solar energy to electricity have already become one of the most promising candidates for clean and renewable power [6], and a lot of attempts have been made to harness the mechanical energy of water flow which is inexhaustible and can be a good alternative to solar energy [7,8].
* Corresponding author. College of Information Science and Electronic Engineering, Zhejiang University, Hangzhou, 310027, China. E-mail address: [email protected] (S. Lin). http://dx.doi.org/10.1016/j.carbon.2016.04.030 0008-6223/© 2016 Elsevier Ltd. All rights reserved.
The extensively studied two dimensional (2D) material graphene, which is a single layer of carbon atoms assembled in a honeycomb lattice [9], possesses a few outstanding electrical, optical and mechanical properties, such as extremely high electron mobility, highly tunable conductivity, 2.3% constant absorption of visible light and high mechanical strength [10e13]. All these properties make graphene not only holds great potential in electronic and optoelectronic applications [14,15] but also has more promise in the pursuit of a higher energy conversion efficiency [16,17]. More recently, some solar cells based on graphene/semiconductor van der Waals Schottky diode are proposed [18e20], which is distinct from traditional PN junction or Schottky junction, as the barrier height of this type diode is highly tunable thanks to the Fermi level of graphene can be adjusted independently owing to the van der Waals contact [21]. Through introducing a graphenedielectric-graphene structure to independently tune the Fermi level of the graphene, we have achieved power conversion efficiency (PCE) as high as 18.5% with graphene/GaAs heterostructure solar cell [22]. Moreover, photocurrent detection experiments in graphene reveal a strong photoresponse near metal/graphene interface and semiconductor/graphene hybrid interface, which
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holds great promise for high speed optical communications and facilitates the capability of graphene-based optoelectronic devices [23e25]. In parallel with the extensive works on graphene based optoelectronic applications, owing to its exceptional sensitivity to external stimulus [26], a variety of approaches have been proposed recently to generate electricity from water flow in the 2D graphene plane which shows great compatibility to the photovoltaic behaviors of lateral graphene p-n junctions [27,28], but no obvious voltage responses to the flow of deionized (DI) water can be observed in previous works [29e31]. However, graphene based devices which can concurrently harvesting energy from sunlight and water flow have rarely been investigated previously, which should be important and probably push graphene based energy harvesting devices to practical application as solar energy and the mechanical energy of water flow are always available at the same time in our living environment. In this letter, we propose a graphene/Si Schottky diode based 2D hybrid nanogenerator, which can concurrently harvest energy from sunlight and water flow. Two different metal electrodes are adopted to build an asymmetric internal potential profile in the graphene channel, allowing for harvesting energy from sunlight in the 2D graphene plane. Besides, when water flows over the graphene surface under illumination, an additional voltage can be generated simultaneously. This flow-induced voltage arises from an additional charge transfer in the graphene channel induced by a continuous doping and dedoping of the graphene, owing to a reversible wetting and dewetting effect of water during the water flowing process. This water flowing process reveals a physical picture of dynamic adjusting the charge transfer characteristic of graphene/Si Schottky diode through water flow when under illumination, which is very interesting and rarely been studied. Our results demonstrate a type of 2D graphene based hybrid nanogenerator with excellent compatibility to be integrated with planar Si, which probably open prospects for practical application for graphene nanogenerator. 2. Experimental details The monolayer graphene samples used in this work were grown on 25 mm Cu foil (Alfa Aesar, item number 13382) by low pressure chemical vapor deposition with a reaction source flux ratio of CH4:H2 equals to 5:1 [32]. The n type Si wafers with a resistivity of 2e5 U cm were commercially available (Zhejiang Lijing silicon material Co., Ltd.). Native oxide on Si was removed by dipping the wafers into diluted HF solution for 3 min. 80 nm thick Al2O3 dielectric layer with an open width of 2 cm were grown by electron beam evaporation, which is used as an insulating layer to prevent electrodes/Si contact. Two asymmetric electrodes Cr/Au (5/60 nm) and Cr/Ag (5/60 nm) defined by lithography-processed mask were electron beam evaporated on the top of insulating layer respectively. After the open area of the silicon substrate was cleaned by dipping the substrate into acetone for 5 min followed with DI water rinse, monolayer graphene was transferred onto the substrate by a polymethyl methacrylate (PMMA) mediated method [33]. Then, the PMMA was removed using acetone. The Raman spectra of the graphene transferred onto SiO2/Si substrate were measured by Raman spectroscopy (Renishaw inVia Reflex) with an excitation wavelength of 532 nm. Graphene transferred onto SiO2/Si substrate was also employed to form a FET structure where the low-resistivity Si was taken as the back gate electrode, the currentevoltage characteristics of the FET structure were measured with an Agilent B1500A system. The photovoltaic behaviors of this 2D hybrid nanogenerator were tested with a solar simulator under AM1.5G condition, which was calibrated with a standard Si solar cell. The currentevoltage data were recorded
using a Keithley 2400 source-meter controlled by a LabView-based system. The voltage responses to the flow of the droplet of DI water were recorded in real time by a keithley 2010 multimeter which was controlled by a LabView-based data acquisition system with a sampling rate of 25 s1. 3. Results and discussion The schematic structure of this 2D hybrid nanogenerator is illustrated in Fig. 1a, which is composed of Si substrate, monolayer graphene and two asymmetric electrodes. Al2O3 dielectric insulating layer is sandwiched between the electrodes and Si substrate. The area of the graphene channel is L W (2.0 1.0) cm2. Fig. 1b shows the Raman spectrum of the graphene sample transferred onto SiO2/Si substrate, which shows a strong symmetric 2D peak, a nearly undetected defect-related D peak around 1350 cm1 and the intensity ratio of the 2D peak to G peak is 5.2, indicating the high quality monolayer of the as-grown samples. As seen from the inset of Fig. 1b, the source-drain current (Isd) of the FET based on graphene transferred onto SiO2/Si substrate achieves a minimum value at a gate voltage (Vg) of þ50 V when keeping the source-drain voltage (Vsd) constant as 1 V. The Dirac point for the graphene sample is located in the positive gate voltage region, indicating the as-grown graphene is p-type doped after the wet transferring process. Fig. 2a shows the current (I) versus voltage (V) curves of this device in the dark and under AM1.5G illumination condition. The dark I-V curve passes through zero in a straight line which simply exhibits a resistance characteristic, indicating the ohmic contact nature between the electrodes and graphene. No obvious opencircuit voltage (VOC) or short-circuit current (ISC) can be detected in the dark. Under AM1.5G illumination condition, the I-V curve is totally different where the symmetry around the origin has been broken. As we can see from the light I-V curve, the ISC of this this 2D hybrid nanogenerator with as-grown graphene becomes 0.7 mA, and the VOC achieves 0.33 V which demonstrates the harvesting energy behavior from sunlight in the 2D graphene plane. The output power P ¼ V I of this device can achieve a maximum value of 49.3 mW as we can get from the light I-V curve. When the middle region of the graphene channel is shaded, the Voc and Isc of this hybrid nanogenerator will decrease simultaneously, as shown in Fig. 2b. When the shading width increases from 0 to 1.5 cm, the Voc decreases from 0.33 to 0.256 V and the Isc decreases from 0.7 to 0.53 mA, indicating the bulk graphene channel can only make a small contribution to the photovoltage and photocurrent, whereas the regions of the graphene channel adjacent to the electrode/ graphene interfaces make the main contribution. Besides, we have also measured the I-V response to illumination of the device with the graphene channel being replaced by a 2 kohm resistance connected between the two electrodes using copper wires (Fig. S1). The straight I-V curve which passes through origin certainly reveals that the light exposed electrodes cannot generate photocurrent by themselves, demonstrating the Si contacted graphene channel is not just act as a load between the two electrodes. When the p-type doped graphene channel is contacted with the n-type Si substrate by van der Waals forces, a graphene/Si Schottky diode will be formed. Owing to the work function difference between the graphene and Si, a depletion region in Si will be formed in the Schottky junction by a build-in potential barrier (Fbarrier), which can separate the photo-generated electrons and holes into Si and graphene layer respectively, as shown in Fig. 2c. Fig. 2d presents the band profile in the lateral direction of this hybrid nanogenerator, which shows a work function tuning mechanism. By adopting the Au-Ag electrodes configuration, an asymmetric potential profile will be built in the graphene channel, where a strong
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Fig. 1. (a) Schematic structure of the 2D hybrid nanogenerator. (b) Raman spectrum of the graphene sample transferred onto SiO2/Si substrate and the inset shows the Isd of FET structure as a function of Vg when keeping Vsd constant as 1 V. (A colour version of this figure can be viewed online.)
Fig. 2. (a) IeV curves of this 2D hybrid nanogenerator in the dark and under AM1.5G illumination, the mark on the light IeV curve represents the point of maximum power output. (b) Light IeV curves of this 2D hybrid nanogenerator with the middle regions of the graphene channel with different widths are shaded from illumination. (c) Schematic electronic band profile of graphene/Si Schottky junction, which corresponding to the band profile in the vertical direction of this nanogenerator. (d) The asymmetric potential profile in the lateral direction of this nanogenerator, which leads to a directional move of the photo-generated holes in the 2D graphene plane, allowing harvest energy from sunlight. (A colour version of this figure can be viewed online.)
internal electric field exists in the narrow regions of the graphene channel adjacent to the electrode/graphene interfaces [23,34]. The photo-generated excess holes pulled to these narrow regions of the graphene channel can be efficiently drifted toward the Ag electrode owing to this strong electric field. As the excess holes in the other regions of the graphene channel can sustain a relatively long time, these excess holes can diffuse towards the Ag electrode in some extent, which leads to a small contribution to the photocurrent, as our experiment results show. In addition, the stability of the light induced voltage and current output of this structure has also been check by measuring the variation of the Voc and Isc values in 36 h under one sun illumination, where only a very small loss in voltage and current output can be observed after illumination for 36 h (Fig. S2). In order to further demonstrate the role of the electrodes and the Si contact of graphene channel in creating this photocurrent output in the 2D graphene plane, we have measured the photovoltaic response of devices with different configurations. Fig. 3a shows the photovoltaic behavior of the device consisting of two
symmetric electrodes, where both electrodes consist of Cr/Au (5/ 60 nm). The photovoltaic responses between the two electrodes are symmetric when applying a positive bias or a negative bias, indicating the mirror symmetry of the internal potential profile of the graphene channel in this case. Both the dark I-V curve and light I-V curve pass through zero, the difference is that the light I-V curve shows a larger current at the same bias which thanks to the photogenerated excess holes, no obvious VOC or ISC can be harvested between the two symmetric electrodes when under illumination. Fig. 3b shows the I-V curves of the device where the graphene/Si contact is fully prevented by the insulating layer. Even though the mirror symmetry of the internal potential profile in the graphene channel has been broken by the two different metal electrodes in this case, owing to the very short lifetime of the electronehole pairs generated in graphene (in the ps range) [35], the photo-generated carriers in the bulk graphene channel will disappear rapidly through recombination and cannot diffuse towards the electrodes. Beside, as graphene can only absorb ~2.3% of visible light [12], although the strong internal electric field which exists in the
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Fig. 3. (a) IeV curves of the device with two symmetric electrodes in the dark and under AM1.5G illumination. (b) IeV curves of the device where the graphene/Si contact is fully prevented by the insulating layer in the dark and under AM1.5G illumination. (A colour version of this figure can be viewed online.)
narrow regions (~0.2 mm) adjacent to the electrode/graphene interfaces can separate the electronehole pairs generated in these narrow regions of graphene [23,34], the contribution of these narrow regions to the photocurrent between the two electrodes can be neglected. As a result, the light I-V curve of this configuration has no obvious difference compared to the dark I-V curve as shown in Fig. 3b, both pass through zero in a straight line, indicating we cannot generate electricity effectively from sunlight in the 2D graphene plane without its contact with Si substrate. In addition to solar energy, the mechanical energy of water flow is always available around the clock in our living environment, so concurrently harvest solar energy and the mechanical energy of water flow is highly attractive. More excitingly, the mechanical energy of water flow can be simultaneously harvested with the solar energy by this 2D graphene/Si Schottky junction based nanogenerator we proposed. Fig. 4a shows the experimental setup, where the device is fixed on a slope with an inclination q of 30 with ground level. The area of the graphene channel in this case is increased to 6.0 2.0 cm2 in order to study the generating
electricity behavior from water flow and poly(dimethysiloxane) (PDMS) polymer is used to seal the electrodes in order to prevent the electrodes from exposing to water. A burette with a capacity of 10 mL is used to control the volume of the water droplet in our experiment, which is fixed in a few millimeters distance up away from the top end of graphene channel. A keithley 2010 multimeter is used to record the voltage responses to the flow of the droplet of DI water from the top end of the graphene channel to the bottom end driven by its own gravity. Fig. 4b shows the Raman spectra of graphene at different conditions. As we can see, the G peak position of graphene is blue-shifted from 1592.5 cm1 to 1594.1 cm1 when the graphene is wetted by DI water, and it is shifted back to 1593.1 cm1 after 10 min, which demonstrates graphene can be slightly p-type doped when contacted with DI water and this doping effect is reversible [26,36]. Fig. 4c shows the voltage responses to the flow of the droplet of DI water towards the direction of Ag electrode (0.1 mL in volume, unless indicated otherwise) in the dark and under room light (measured as 1.08 mW/cm2) illumination condition. In the dark, there is no noticeable voltage
Fig. 4. (a) Schematic diagram of experimental set-up to study the voltage response to the flow of DI water. (b) Raman G peak positions of the graphene at three conditions. (c) Voltage responses to the flow of DI water over graphene/Si Schottky diode in the dark and under room light illumination. (d) Voltage responses to the flow of DI water towards different directions. (A colour version of this figure can be viewed online.)
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response when the droplet of DI water flows over the graphene surface, indicating no interaction exists between DI water and graphene/Si Schottky diode in the water flowing process. When the device is under illumination, the voltage response is totally different. An initial voltage value around 25.6 mV is generated. During the water flowing process, the generated voltage increases with time to a peak voltage of ~28.14 mV when the water droplet flows to the bottom end of the graphene channel, indicating that an additional peak voltage of around 2.54 mV can be generated through the flow of a droplet of DI water. Once the droplet of DI water flows away from the graphene surface through the electrode, the voltage decreases sharply to an intermediate value around 26.5 mV. Then the voltage recovers to the initial value with a rate of 0.625 mV/s, which can be attributed to the effect of residual water on the graphene surface. When we reverse the water flowing direction, an additional voltage response with reversed sign can be generated. As shown in Fig. 4d which are measured under the same room light illumination using another device with the same configuration, when the droplet of DI water flows over the graphene surface towards Ag electrode, an additional voltage increment can be generated corresponding to an initial value of around 14 mV. However, an additional voltage reduction will be induced if the droplet of DI water flows towards Au electrode. This asymmetric voltage response is well in agreement with the asymmetry potential profile of the graphene channel. The slight fluctuation of the induced voltages can be attributed to fluctuations in flowing process of the droplet of DI water. This distinguishable voltage response generated in the flowing process of the droplet of DI water when under room light illumination should reflect the existing interaction between DI water and this 2D hybrid nanogenerator. In order to clarify the origin of this behavior, we have measured the voltage responses to the flow of DI water droplet with the device where the graphene/Si contact is
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fully prevented by Al2O3 insulating layer when under the same illumination (Fig. S3). We observe, no obvious voltage response can be generated, considering the electrodes are sealed to avoid exposing to water [28], we suggest that the distinguish voltage response should arises from the interaction between the DI water and graphene/Si Schottky diode instead of the wateregraphene interaction or the watereelectrodes interaction. In addition, owing to the flow-induced voltage in our experiments has the same direction with the flowing direction, the influence of the Hþ and OH ions which might be generated by photocatalysis effect can be excluded as a voltage with reversed sign will be generated with enough photocatalytic Hþ and OH ions [27e29]. Fig. 5a shows the charge distribution of the graphene/Si Schottky diode when under illumination, where holes are pulled to the graphene layer and electrons are pulled to the bulk Si substrate by the built-in electric field. When the graphene is contacted with a droplet of DI water, the big dipole moments of the water molecules which absorbed onto the graphene surface can act as a local electric field, which will lead to an electron transfer from the graphene to the water droplet, as a result, the graphene will be slightly p-type doped [26,37,38], which is also demonstrated by the shift of G peak of graphene as shown in Fig. 4b. As the Fermi level of graphene shifts downward owing to the p-type doping, the Fbarrier of the graphene/Si Schottky diode will become higher, the width of depletion region in Si will be increased and more holes can be pulled to graphene layer, as shown in Fig. 5b. When the droplet of DI water is flowing along the graphene surface, the graphene channel at the front side of the droplet is going to be wetted and p-type doped by the water, which indicates more holes will be pulled to this region from the depletion region in Si substrate. Meanwhile, the graphene channel at the rear side of the droplet is going to be dewetted and dedoped, which means excess holes will flow back to the depletion region in Si substrate as the Fbarrier of the graphene/Si Schottky diode becomes
Fig. 5. (a) Charge distribution of the graphene/Si Schottky diode when under illumination. (b) Charge distribution of the graphene/Si Schottky diode at the stable state when the water droplet keeps still. (c) Dynamic charge transfer process in the graphene/Si Schottky diode when the water droplet is flowing along the graphene surface, more photogenerated carriers will be separated at the front side of the droplet whereas the excessive photo-generated carriers will flow back to the depletion region in Si substrate at the rear side of the droplet. (d) Overall charge transfer process in the graphene channel during the water flowing process, the excess holes in graphene can be driven to move from the rear side to the front side of the droplet. (A colour version of this figure can be viewed online.)
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lower, as shown in Fig. 5c. As long as the water droplet keeps flowing, the graphene channel will be continuously doped and dedoped, this dynamic process will drive the additional portion of photo-generated holes which induced by the doping effect moving from the rear side to the front side of the water droplet, across the graphene channel under the droplet, which will lead to an additional potential difference between the two sides of the droplet, as shown in Fig. 5d. We suggest the physical picture of dynamic adjusting the charge transfer characteristic of graphene/Si Schottky diode through water flow when under illumination as the plausible mechanism for this flow-induced voltage. 4. Conclusions
[10]
[11]
[12]
[13] [14] [15] [16]
In conclusion, we propose a graphene/Si Schottky diode based 2D hybrid nanogenerator, which can concurrently harvest energy from sunlight and water flow. Two different metal electrodes are adopted in order to harvest energy from sunlight in the 2D graphene plane. When water flows over the graphene surface at the same time, an additional voltage can be generated simultaneously. This flow-induced additional voltage arises from a continuous doping and dedoping of the graphene during the water flowing process. This process reveals a physical picture of dynamic adjusting the charge transfer characteristic of graphene/Si Schottky diode through water flow when under illumination. Our results demonstrate a type of 2D graphene based hybrid nanogenerator with excellent compatibility to be integrated with planar Si, which probably open prospects for practical application for graphene nanogenerator.
[17]
[18] [19] [20] [21]
[22]
[23]
[24]
Acknowledgment
[25]
The authors thank the support from the National Natural Science Foundation of China (No.51202216, 51502264 and 51551203) and Special Foundation of Young Professor of Zhejiang University (No. 2013QNA5007).
[26]
[27] [28]
Appendix A. Supplementary data
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Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.carbon.2016.04.030.
[30] [31]
References [1] Z.L. Wang, Towards self-powered nanosystems: from nanogenerators to nanopiezotronics, Adv. Funct. Mater. 18 (22) (2008) 3553e3567. [2] S. Ghosh, A.K. Sood, N. Kumar, Carbon nanotube flow sensors, Science 299 (5609) (2003) 1042e1044. [3] B. Tian, X. Zheng, T.J. Kempa, Y. Fang, N. Yu, G. Yu, et al., Coaxial silicon nanowires as solar cells and nanoelectronic power sources, Nature 449 (7164) (2007) 885e889. [4] G.J. Snyder, E.S. Toberer, Complex thermoelectric materials, Nat. Mater. 7 (2) (2008) 105e114. [5] S. Xu, Y. Qin, C. Xu, Y. Wei, R. Yang, Z.L. Wang, Self-powered nanowire devices, Nat. Nanotechnol. 5 (5) (2010) 366e373. [6] N.S. Lewis, Toward cost-effective solar energy use, Science 315 (5813) (2007) 798e801. [7] Z.H. Lin, G. Cheng, S. Lee, K.C. Pradel, Z.L. Wang, Harvesting water drop energy by a sequential contact-electrification and electrostatic-induction process, Adv. Mater. 26 (27) (2014) 4690e4696. [8] M. Ma, L. Guo, D.G. Anderson, R. Langer, Bio-inspired polymer composite actuator and generator driven by water gradients, Science 339 (6116) (2013) 186e189. [9] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, et al.,
[32]
[33]
[34] [35]
[36]
[37]
[38]
Electric field effect in atomically thin carbon films, Science 306 (5696) (2004) 666e669. K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, M.I. Katsnelson, I.V. Grigorieva, et al., Two-dimensional gas of massless Dirac fermions in graphene, Nature 438 (7065) (2005) 197e200. C. Lee, X. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science 321 (5887) (2008) 385e388. R. Nair, P. Blake, A. Grigorenko, K. Novoselov, T. Booth, T. Stauber, et al., Fine structure constant defines visual transparency of graphene, Science 320 (5881) (2008), 1308-. A.K. Geim, Graphene: status and prospects, Science 324 (5934) (2009) 1530e1534. F. Bonaccorso, Z. Sun, T. Hasan, A.C. Ferrari, Graphene photonics and optoelectronics, Nat. Photonics 4 (9) (2010) 611e622. K.S. Novoselov, V.I. Fal’ko, L. Colombo, P.R. Gellert, M.G. Schwab, K. Kim, A roadmap for graphene, Nature 490 (7419) (2012) 192e200. J. Yin, J. Zhou, X. Li, Y. Chen, G. Tai, W. Guo, Enhanced gas-flow-induced voltage in graphene, Appl. Phys. Lett. 99 (7) (2011) 073103. X. Li, J. Yin, J. Zhou, Q. Wang, W. Guo, Exceptional high Seebeck coefficient and gas-flow-induced voltage in multilayer graphene, Appl. Phys. Lett. 100 (18) (2012) 183108. C.C. Chen, M. Aykol, C.C. Chang, A.F. Levi, S.B. Cronin, Graphene-silicon Schottky diodes, Nano Lett. 11 (5) (2011) 1863e1867. E. Shi, H. Li, L. Yang, L. Zhang, Z. Li, P. Li, et al., Colloidal antireflection coating improves graphene-silicon solar cells, Nano Lett. 13 (4) (2013) 1776e1781. P. Wang, X. Li, Z. Xu, Z. Wu, S. Zhang, W. Xu, et al., Tunable graphene/indium phosphide heterostructure solar cells, Nano Energy 13 (2015) 509e517. X. Miao, S. Tongay, M.K. Petterson, K. Berke, A.G. Rinzler, B.R. Appleton, et al., High efficiency graphene solar cells by chemical doping, Nano Lett. 12 (6) (2012) 2745e2750. X. Li, W. Chen, S. Zhang, Z. Wu, P. Wang, Z. Xu, et al., 18.5% efficient graphene/ GaAs van der Waals heterostructure solar cell, Nano Energy 16 (2015) 310e319. F. Xia, T. Mueller, R. Golizadeh-Mojarad, M. Freitag, Y-m Lin, J. Tsang, et al., Photocurrent imaging and efficient photon detection in a graphene transistor, Nano Lett. 9 (3) (2009) 1039e1044. J. Park, Y. Ahn, C. Ruiz-Vargas, Imaging of photocurrent generation and collection in single-layer graphene, Nano Lett. 9 (5) (2009) 1742e1746. J. Meng, H.D. Song, C.Z. Li, Y. Jin, L. Tang, D. Liu, et al., Lateral graphene p-n junctions formed by the graphene/MoS(2) hybrid interface, Nanoscale 7 (27) (2015) 11611e11619. F. Schedin, A.K. Geim, S.V. Morozov, E.W. Hill, P. Blake, M.I. Katsnelson, et al., Detection of individual gas molecules adsorbed on graphene, Nat. Mater. 6 (9) (2007) 652e655. J. Yin, Z. Zhang, X. Li, J. Yu, J. Zhou, Y. Chen, et al., Waving potential in graphene, Nat. Commun. 5 (2014) 3582. J. Yin, Z. Zhang, X. Li, J. Zhou, W. Guo, Harvesting energy from water flow over graphene? Nano Lett. 12 (3) (2012) 1736e1741. J. Yin, X. Li, J. Yu, Z. Zhang, J. Zhou, W. Guo, Generating electricity by moving a droplet of ionic liquid along graphene, Nat. Nanotechnol. 9 (5) (2014) 378e383. P. Dhiman, F. Yavari, X. Mi, H. Gullapalli, Y. Shi, P.M. Ajayan, et al., Harvesting energy from water flow over graphene, Nano Lett. 11 (8) (2011) 3123e3127. H. Zhong, X. Li, Z. Wu, S. Zhang, Z. Xu, H. Chen, et al., Two dimensional graphene nanogenerator by coulomb dragging: moving van der Waals heterostructure, Appl. Phys. Lett. 106 (24) (2015) 243903. X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, et al., Large-area synthesis of highquality and uniform graphene films on copper foils, Science 324 (5932) (2009) 1312e1314. X. Li, Y. Zhu, W. Cai, M. Borysiak, B. Han, D. Chen, et al., Transfer of large-area graphene films for high-performance transparent conductive electrodes, Nano Lett. 9 (12) (2009) 4359e4363. F. Xia, T. Mueller, Y.M. Lin, A. Valdes-Garcia, P. Avouris, Ultrafast graphene photodetector, Nat. Nanotechnol. 4 (12) (2009) 839e843. F. Wang, G. Dukovic, L.E. Brus, T.F. Heinz, Time-resolved fluorescence of carbon nanotubes and its implication for radiative lifetimes, Phys. Rev. Lett. 92 (17) (2004) 177401. A. Das, S. Pisana, B. Chakraborty, S. Piscanec, S.K. Saha, U.V. Waghmare, et al., Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor, Nat. Nanotechnol. 3 (4) (2008) 210e215. O. Leenaerts, B. Partoens, F.M. Peeters, Water on graphene: Hydrophobicity and dipole moment using density functional theory, Phys. Rev. B 79 (23) (2009). F. Yavari, C. Kritzinger, C. Gaire, L. Song, H. Gulapalli, T. Borca-Tasciuc, et al., Tunable bandgap in graphene by the controlled adsorption of water molecules, Small 6 (22) (2010) 2535e2538.