Energy &Environmental Science

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aSKKU Advanced Institute of Nanotechnolo

Nanotechnology (HINT), SKKU-Samsung Gr

(SKKU), Suwon 440-746, Republic of Korea.

290-7381; Tel: +82-31-290-7352bSchool of Advanced Materials Science and

(SKKU), Suwon 440-746, Republic of Korea

† Electronic supplementary informationexperimental setup. See DOI: 10.1039/c2e

Cite this: Energy Environ. Sci., 2013, 6,169

Received 18th September 2012Accepted 1st November 2012

DOI: 10.1039/c2ee23530g

www.rsc.org/ees

This journal is ª The Royal Society of

Highly sensitive stretchable transparent piezoelectricnanogenerators†

Ju-Hyuck Lee,a Keun Young Lee,b Brijesh Kumar,b Nguyen Thanh Tien,b

Nae-Eung Leeab and Sang-Woo Kim*ab

Here we report a new type of stretchable transparent piezoelectric nanogenerator (NG) using an organic

piezoelectric material consisting of poly(vinylidene fluoride trifluoroethylene) [P(VDF-TrFE)] sandwiched

with mobility-modified chemical vapor deposition-grown graphene electrodes by ferroelectric

polarization into P(VDF-TrFE). This new type of NG has a very high sensitivity and mechanical durability

with fully flexible, rollable, stretchable, foldable, and twistable properties. We also investigated the

mobility-modified graphene electrodes with ferroelectric P(VDF-TrFE) remnant polarization, and a

mechanism is proposed for switching the mobility of the carriers by the ferroelectric remnant

polarization. Upon exposure to the same input sound pressure, the measured output performance of

the stretchable NG with a thin polydimethylsiloxane stretchable rubber template is up to 30 times that

of a normal NG with a plastic substrate. Upon exposure to an air flow at the same speed, the measured

output voltage from the stretchable NG is about 8 times larger than that of the normal NG.

Broader context

A stretchable transparent nanogenerator (NG) with a high sensitivity having a thin PDMS rubber template which acts as a stretchable mechanical spring isinvestigated based on the modication of the mobility of graphene electrodes by ferroelectric P(VDF-TrFE) remnant polarization. This new type of piezoelectricNG has stretchable, multi-shape transformable, and mechanically durable properties with a very high sensitivity. We demonstrate that upon their exposure tothe same input sound pressure, the measured output performance of the stretchable NG is up to 30 times that of the normal NG and that upon their exposure toan air ow at the same speed, the measured output voltage from the stretchable NG shows a clear enhancement of up to about 8 times compared with that of thenormal NG.

1 Introduction

The power generation performance and commercialization ofpiezoelectric power generators with a thick and rigid templateare limited because they are usually operated in a low magni-tude and frequency situation with very minute and irregularmechanical energy sources from the living environment, suchas body movements, air ow, hydraulic pressure and acousticvibrations, which are mostly of low magnitude and lowfrequency.1–10 In addition, they do not offer multifunctionality,such as a high power output performance, high sensitivity, fullexibility, stretchability, and multi-shape transformability.Therefore, developing piezoelectric energy harvesters with highperformance, sensitivity and stretchability is extremely

gy (SAINT), Center for Human Interface

aphene Center, Sungkyunkwan University

E-mail: kimsw1@skku.edu; Fax: +82-31-

Engineering, Sungkyunkwan University

(ESI) available: An illustration of thee23530g

Chemistry 2013

important, in order to convert the various kinds of mechanicalenergies available in nature into electricity for the realization ofthe independent, sustainable, and wireless operation of lowpower-consuming devices and systems in self-poweredmode.11–20 Moreover, stretchable power generators can be easilytransferred onto various kinds of fabrics including ags,clothes, gloves, etc.

The atomically layered structure of two-dimensional gra-phene sheets with high mechanical elasticity (elastic modulusof about 1 TPa) and high transparency can be used to preparefully exible and rollable transparent piezoelectric powergenerators. However, although large-scale graphene grown bythe chemical vapor deposition (CVD) method showsoutstanding uniformity and good electrical properties, theenhancement of the electrical mobility of CVD-based grapheneis required for it to be used as an electrode material for highlysensitive stretchable transparent piezoelectric NGs.21 In thiswork, we report a new type of stretchable transparent piezo-electric NG using an organic piezoelectric material consisting ofpoly(vinylidene uoride triuoroethylene) [P(VDF-TrFE)] sand-wiched with mobility-modied CVD-grown graphene electrodesby ferroelectric polarization into P(VDF-TrFE). This new type of

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NG has a very high sensitivity and mechanical durability withfully exible, rollable, stretchable, foldable, and twistableproperties.

Fig. 1 Fabrication process of a stretchable NG (a) PDMS stretchable rubbertemplate on the Cu foil, (b) transparent graphene sheet transferred to PDMS/Cu,(c) P(VDF-TrFE) and graphene sheet transferred to graphene/PDMS/Cu, and (d)stretchable NG with sandwiched graphene electrodes.

2 Experimental section

To grow the graphene sheets, a Cu foil was placed in a rapidthermal CVD chamber and the temperature was increased fromroom temperature to 1000 �C with a 10 standard cubic centi-meters per minute (sccm) ow of H2 (1 Torr). When thetemperature reached 1000 �C, the Cu foil was annealed for 30min to clean its surface by the reduction of copper(II) oxide andto grow the grain size. The synthesis of the graphene sheets wasachieved with a mixture of CH4 (20 sccm) and H2 (10 sccm) for agrowth time of 30 min (1 Torr). Aer the growth was completed,the gas supply was terminated and the chamber was cooled tobelow 100 �C at a cooling rate of 160 �C min�1. A solution ofP(VDF-TrFE) (10 wt%) dissolved in N,N-dimethylformamide(DMF) solvent was spun on the graphene electrode to a layerthickness of 2 mm, followed by drying at 60 �C to remove theDMF solvent. Next, this layer was maintained at 140 �C for 2 hand then naturally cooled down to room temperature in anitrogen atmosphere, in order to enhance the crystallinity of theb phase.

The electrical properties of graphene were measured usingHall Measurement systems (ECOPIA, HMS-3000). The soundwave was generated by a speaker using a function generator(Tektronix, AFG3021B). The magnitude of the generated soundwave was measured by a dB meter. The air ow consisted ofnitrogen gas from a blowing gun. The speed of the air ow wasmeasured by an anemometer. The device was placed on a rigidsquare hollow stand in front of the speaker and the blowing gunwhich were 1 cm apart. The performance of the power genera-tors was measured using an oscilloscope (Tektronix, DPO5054)(see ESI, Fig. S1 for the experimental setup†).

Fig. 2 (a) Raman spectra of the graphene monolayer on SiO2/Si. (b) SynchrotronXRD result of the b-phase (110/200) of P(VDF-TrFE).

3 Results and discussion

Fig. 1 shows a schematic illustration of the experimentalprocedures employed to integrate the stretchable NG. First, toprepare one electrode for the stretchable NG, a polydimethyl-siloxane (PDMS) thin lm was spin deposited on a copper (Cu)foil using spin coating at 3000 rpm for 60 s (Fig. 1a). Fig. 1areveals a cross-sectional eld-emission scanning electronmicroscopy (FE-SEM) image of the 20 mm thick PDMS thin lmon the Cu foil. This PDMS thin lm acts as a stretchablemechanical spring and a stretchable rubber template,enhancing the mechanical durability of the device. Then, thelarge-scale monolayer graphene grown on a Cu foil using theCVD method was transferred onto a PDMS/Cu foil using thewell-known wet transfer method (Fig. 1b).22,23 The transferprocess was iterated three times to make a more conductivegraphene electrode.

In a further step, a piezoelectric polymer thin lm of P(VDF-TrFE) was spin-coated on a three-monolayer graphene sheet at800 rpm for 60 s and subsequently annealed at 140 �C for 2 h.The FE-SEM image in Fig. 1c shows a typical P(VDF-TrFE) thin

170 | Energy Environ. Sci., 2013, 6, 169–175

lm morphology. To construct the stretchable NG, in the nextstep, the P(VDF-TrFE)/graphene sheet multilayer was trans-ferred onto the graphene/PDMS/Cu foil template using a drytransfer method.24 Finally, the stretchable transparent NG wasobtained by removing the Cu foil using a Cu etchant (Fig. 1d).The electrical poling process was carried out by applying anelectric eld between the top and bottom electrodes of gra-phene by applying an electric eld of 100 MV m�1 for 30 min.

The Raman spectrum obtained from the graphene mono-layer transferred onto SiO2 reveals no D band peak, indicating alow defect density in the graphene layer (Fig. 2a). The b-phase(110/200) formation in the P(VDF-TrFE) thin lm was conrmedusing synchrotron X-ray diffraction (XRD) measurements(Fig. 2b).25

This journal is ª The Royal Society of Chemistry 2013

Fig. 3 (a) Carrier mobility of graphene sheets vs. the electric field applied toP(VDF-TrFE) in the poling process; the inset is a schematic of a sample prepared forHall measurements. (b) Proposed mechanism for the carrier mobility switching ofgraphene when subjected to ferroelectric remnant polarization with negativelyand positively poled P(VDF-TrFE). (c) Ferroelectric remnant polarization andpiezoelectric effect with strain of P(VDF-TrFE).

Fig. 4 Proposed mechanism for voltage and current output from the piezo-electric NG based on the graphene-sandwiched P(VDF-TrFE) structure. (a) and (b)are before and after applied strain, respectively. (c) and (d) are output voltage andoutput current density generated from the NG by bending (the strain of 0.081%),respectively.

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The transparent, mechanically durable, and high carriermobility properties of graphene made it a promising materialfor use as an electrode material for the stretchable NG in thiswork. Interestingly, it was found that the ferroelectric remnantpolarization of P(VDF-TrFE) affects the electrical properties ofthe graphene sheet. The carrier type, carrier concentration, andmobility of graphene are investigated before and aer poling atroom temperature by Hall-effect measurements. The electricalproperties of the three monolayer graphene sheet with non-poled P(VDF-TrFE) show p-type conduction with a carrier

This journal is ª The Royal Society of Chemistry 2013

mobility of 748 cm2 V�1 s�1 and a sheet carrier density of 3.84�1013 cm�2. When the ferroelectric P(VDF-TrFE) is poled in bothdirections, it dopes the graphene with either electrons or holes.We observed the change of the charge carrier mobility in thegraphene sheets depending on the direction of the appliedelectric eld, as shown in Fig. 3a. On the application of anelectric eld of �100 MV m�1, the graphene sheets exhibited acarrier mobility of 822 cm2 V�1 s�1, which is approximately 10%higher than that of graphene with non-poled P(VDF-TrFE). Onthe other hand, on the application of an electric eld of +100MV m�1, the graphene sheets exhibited a carrier mobility of665 cm2 V�1 s�1, which is approximately 10% lower than that ofgraphene with non-poled P(VDF-TrFE).

Fig. 3b and c show the proposed mechanism for the carriermobility switching of the graphene sheets according to thepoling direction of P(VDF-TrFE). In general, non-poled ferro-electric materials, such as P(VDF-TrFE) in the pristine state, inwhich the molecular dipoles are randomly oriented, do notpossess a net eld due to their lack of polarization in anyspecic direction. Ferroelectric P(VDF-TrFE) is characterized bytwo symmetrical remnant polarizations, i.e. negative poling andpositive poling for the upward and downward dipole congu-rations, respectively.26

As we discussed previously in Fig. 3a, the hole mobility of thegraphene sheets increases and decreases upon negative andpositive poling, respectively. In the polarized regime, theferroelectric dipole charges are aligned along the direction ofpoling. Hence, the positive dipole charge (V+) and the negativedipole charge (V�) in P(VDF-TrFE) repel and attract holes in thep-type conducting graphene sheets, respectively. Therefore, thehole mobility increases, owing to the extra repulsive forceexerted on the holes and decrease in the hole concentrations asa result of electrostatic doping of electrons with V+ uponnegative poling, while V� reduces the hole mobility uponpositive poling.27,28 On the other hand, the mobility of theelectrons in graphene increases and decreases upon positive

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and negative poling, respectively, as shown in Fig. 3b. Further,as illustrated in Fig. 3c, it is expected that the remnant polari-zation will increase more when the P(VDF-TrFE) is subjected totensile strain. Thus, the hole mobility upon negative polingfurther increases as compared to that upon positive poling inFig. 3b, because of the stronger positive dipole charge (V++)originating from the tensile strain generated in the case ofnegative poling, while the stronger negative dipole charge (V��)originating from the tensile strain increases the electronmobility in the case of positive poling.

As illustrated in Fig. 4a, in the graphene-sandwiched P(VDF-TrFE) structure, V+ repels holes and traps electrons at the topgraphene sheet, while V� repels electrons and traps holes at thebottom graphene sheet. When the sandwiched structure is sub-jected to tensile strain, V�� drives the electron ow from thebottom graphene sheet to the top graphene sheet through anexternal load resistor, while V++ generates aowof holes from thetop graphene sheet to the bottom graphene sheet (Fig. 4b). Thus,the mobility of holes and electrons at both graphene sides isincreased.Due to the large dielectric property of P(VDF-TrFE), theelectrons (holes) accumulate at the interface region between thetop (bottom) graphene sheet andP(VDF-TrFE).When the strain isreleased, the piezoelectric potential immediately vanishes andthe accumulated electrons (holes) ow back through the externalcircuit, giving rise to opposite voltage and current pulses.

We observed an output voltage of 8 V (peak-to-peak) with acurrent density of 0.8 mA cm�2 from a NG fabricated usinggraphene-sandwiched P(VDF-TrFE) with a 120 mm thick poly-ethylene naphthalate (PEN) substrate (Fig. 4c and d). Bybending the device into an arc shape with a dimension asillustrated in Fig. 5, the strain in the length direction (3y) of thedevice is,

R$2q ¼ L0

sin q ¼ sin L0/2R ¼ L/2R

3n ¼ h/2R

where h is thickness of the device (120 mm, including thepolymer substrate because the strain is approximately equal to

Fig. 5 Illustration of device bending to set up the strain.

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the strain of the outer surface of the polymer substrate), R is thebending radius that varies between 71.95 and 22.1 mm. 3y is0.083–0.271%. With relation of Poisson’s ratio (n, n ¼ �3x/3yPoisson’s ratio of P(VDF-TrFE) is 0.3), the strain in the thicknessdirection of the device is 3x ¼ 0.025–0.081%. This resultsuggests that the graphene-sandwiched P(VDF-TrFE) structureis a promising architecture for highly sensitive piezoelectricNGs.

In order to demonstrate the high sensitivity of the stretch-able NG in a low magnitude and frequency situation with a veryminute and irregular mechanical energy source, we comparedthis device with a normal NG with a plastic PEN substrate.Fig. 6a is an illustration of the stretchable NG and normal NGstructures. When they are exposed to sound waves, the devicesexperience the sound pressure and, subsequently, a

Fig. 6 (a) An illustration of the stretchable NG and normal NG structures. (b)Piezoelectric peak-to-peak output voltage generated from the stretchable NGand the normal NG under the same sound wave pressure. (c) Switching polaritytest of voltage generated signals with an applied input sound wave.

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Fig. 7 Output voltages as a function of applied sound wave pressure (dB): (a) at85 dB, (b) at 90 dB, (c) at 95 dB, (d) at 100 dB, (e) at 105 dB, and (f) at 110 dB onthe stretchable NG; (g) at 85 dB, (h) at 90 dB, (i) at 95 dB, (j) at 100 dB, (k) at 105dB, and (l) at 110 dB on the normal NG.

Fig. 8 (a) Output voltage vs. input power of the applied sound wave at a fixedfrequency of 100 Hz. (b) Output voltage vs. frequency of the applied sound waveat a fixed power at 100 dB.

Fig. 9 Output voltage as a function of the frequency of the applied sound wave:(a) at 100 Hz, (b) at 200 Hz, (c) at 400 Hz, and (d) at 800 Hz on the stretchable NG.

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piezoelectric potential is generated across the thickness ofP(VDF-TrFE). This generated piezoelectric potential drives thecarriers from the electrodes to the external circuit; the outputvoltage and output current are measured from the stretchableNG and normal NG devices upon the application of varioussound pressures. With the exposure of the devices to sound byvarying the input powers from 82 to 110 decibels (dB) at 100 Hz,the measured peak to peak output voltage of the stretchable NGis from 50 mV to 600 mV, which shows a clear enhancement ofabout 30 times compared with that of the normal NG which isfrom 10 mV to 22 mV (Fig. 6b). We clearly observed a voltageoutput even upon the exposure of the stretchable NG device tosound waves with a low input power of 85 dB. On the otherhand, we only observed noise signals upon the exposure of thenormal NG to sound waves with an input power of 85–95 dB(Fig. 7). Hence, the clear observation of voltage output upon theexposure of the stretchable NG device to sound waves with amuch lower input power conrms its high sensitivity in a lowmagnitude and frequency situation.

To verify that the measured signals were from the stretchableNG rather than the measurement system, a switching polaritytest was performed, in which the electrodes were connected inreverse and the signals were also reversed. Moreover, a phasedifference between the sound wave and the measured piezo-electric voltage signal due to the impedance of the intrinsiccapacitance and the reactance within the piezoelectric circuit isobserved (Fig. 6c). Also, the functional relationship of the

This journal is ª The Royal Society of Chemistry 2013

parameters used in the sound-driven piezoelectric NGs can bedescribed as

V2 f P/f

where P, V, and f are the power (which is dened as the inputintensity of the sound wave multiplied by the surface area), the

Energy Environ. Sci., 2013, 6, 169–175 | 173

Fig. 10 (a) Output voltage generated from stretchable and normal NGs as afunction of air flow speed. (b) Measured output voltage from stretchable andnormal NGs at 3.0 m s�1. (c) and (d) show output voltage signals from stretchableand normal NGs, respectively, for one cycle between the turning on and turningoff of the air flow (3.0 m s�1).

Fig. 11 (a) Output voltage as a function of the applied air flow speed: (a) at 0.5m s�1, (b) at 1.0 m s�1, (c) at 1.5 m s�1, (d) at 2.0 m s�1, (e) at 2.5 m s�1, and (f) at3.0 m s�1 on the stretchable NG; (g) at 0.5 m s�1, (h) at 1.0 m s�1, (i) at 1.5 m s�1,(j) at 2.0 m s�1, (k) at 2.5 m s�1, and (l) at 3.0 m s�1 on the normal NG.

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output voltage obtained from the NG, and the frequency of theapplied sound wave, respectively.2 The output voltage obtainedfrom the NG as functions of the input power of the appliedsound wave at a xed frequency of 100 Hz and the frequency ofthe applied sound wave at a xed power at 100 dB are shown inFig. 8a and b respectively. At a constant frequency (100 Hz), theV2 generated from the stretchable NG is proportional to theapplied input sound intensity impacting on the stretchable NG.The V2 generated from the stretchable NG increases linearlywithin the intensity range of 0–100 mW m�2, which can beconverted to power in the range of 0 to 10 mW considering thedevice size of 1 cm2. When the input sound intensity was kept at�100 dB, V2 was linearly dependent on the frequency of theapplied sound wave. The measured output voltage signals of thestretchable NG depending on the frequency of the appliedsound wave (100 Hz, 200 Hz, 400 Hz, and 800 Hz) are also shownin Fig. 9. These relationships obviously provide evidence thatthe piezoelectric output is generated by the sound wave.2

We further demonstrate the wind-driven power-generatingperformance of the stretchable NG compared to the normal NG.The generated piezoelectric voltages are measured fromstretchable NG and normal NG devices by varying the appliedair ow pressure. Upon its exposure to an air ow, with speedvarying from 0.5 m s�1 to 3 m s�1, the measured output voltage(peak to peak) from the stretchable NG ranges from 0.3 V to3.9 V, while that from the normal NG ranges from 0.1 V to 0.5 V(Fig. 10a, b and 11). As clearly shown in Fig. 10b, the measuredoutput voltage from the stretchable NG at 3.0 m s�1 is approx-imately 8 times larger than that from the normal NG at the sameair ow speed of 3.0 m s�1. Interestingly, we observed differentbehavior in terms of the generated output voltages from thestretchable NG and the normal NG in response to the airow. As shown in Fig. 10c, at a constant air ow with a speed of3.0 m s�1, the stretchable NG produces a continuous output

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voltage owing to the vibration of the PDMS which acts as astretchable mechanical spring. Similar behavior of piezoelectricoutput voltage signals generated from a PVDF microbelt hasbeen demonstrated recently.5,6 On the other hand, we onlyobserved an output voltage associated with noise from thenormal NG during the application of a constant air owbetween the turning on and turning off of the air ow due to thethick substrate (Fig. 10d).

4 Conclusions

In summary, a stretchable transparent NG with a high sensi-tivity having a thin PDMS rubber template which acts as astretchable mechanical spring is investigated based on themodication of the mobility of graphene electrodes by ferro-electric P(VDF-TrFE) remnant polarization. This new type ofpiezoelectric NG has stretchable, multi-shape transformable,and mechanically durable properties with a very high sensi-tivity. We demonstrated that upon its exposure to the sameinput sound pressure, the measured output performance of thestretchable NG is up to 30 times that of the normal NG andupon exposure to an air ow at the same speed, the measuredoutput voltage from the stretchable NG shows a clearenhancement of up to about 8 times compared with that of thenormal NG. We believe that the approach described hereininvolving the fabrication of a stretchable NG with grapheneelectrodes is very promising for harvesting electrical energyfrom very minute and irregular mechanical energy sources in a

This journal is ª The Royal Society of Chemistry 2013

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living environment, such as body movements, air ow, andacoustic vibrations of low magnitude and low frequency.

Acknowledgements

This work was nancially supported by the National ResearchFoundation of Korea (NRF) grant funded by the Ministry ofEducation, Science and Technology (MEST)(2012R1A2A1A01002787 and 2010-0015035), the FusionResearch Program for Green Technologies through the NRFfunded by the MEST (2010-0019086), and the Energy Interna-tional Collaboration Research & Development Program of theKorea Institute of Energy Technology Evaluation and Planning(KETEP) funded by the Ministry of Knowledge Economy (MKE)(2011-8520010050).

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