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In general, it is very limited to control charge transfer through the triboelectricity in the existing materials of the triboelec-tric generators because the work functions of the materials should be modulated for controlling charge transfer. However, fer-roelectrics are advantageous over other materials. The charge transfer can be readily controlled in the ferroelectric materials since the ferroelectric materials possess polarization charges that may influence the charge transfer. In other words, the output performances of the ferroelectric material-based triboelectric energy harvesters can be easily enhanced or reduced through controlling of polari-zation. Further, the sign of the output per-formances can be controlled as well. This could allow improving performance of energy harvesters because the manipula-

tion and/or enhancement of polarization have been extensively studied so far.

In fact, polarization states in ferroelectric materials can be switched by applying an external electric field.[11,12] Further-more, multi-level stable polarization states have been reported to be achieved by controlling the magnitude of an external elec-tric field.[13,14] The variation in the polarization states in ferro-electric materials can be achieved due to smaller depolarization energy for multi-level polarization states than for single-level states. This indicates that the surface polarization states can be different, even in the same samples. If we fully understand the triboelectric phenomena in ferroelectric materials, we can manipulate the amount of charge transfer, and furthermore, the direction of the charge transfer during contact electrification.

In this work, using ferroelectric polarization, we explore controlling both the amount and direction of charge transfer in a ferroelectric film using atomic force microscopy (AFM). We have chosen a poly(vinylidenefluoride-co-trifluoroethylene) [P(VDF-TrFE)] film as a model system to explore the triboelec-tricity in the ferroelectric materials because it is one of the most promising ferroelectric copolymers and can potentially be used in a number of applications, including in nonvolatile, low-cost memory devices and nanogenerators.[15,16] Furthermore, whereas oxide ferroelectrics exhibit other phenomena, such as a piezochemical effect during contact electrification,[17,18] the P(VDF-TrFE) can exclude such potential issues during contact electrification. Hence, we observed the significant contribution of the polarization direction in the P(VDF-TrFE) film to the tri-boelectricity of the ferroelectric surfaces through a combination

Controllable Charge Transfer by Ferroelectric Polarization Mediated Triboelectricity

Keun Young Lee, Sung Kyun Kim, Ju-Hyuck Lee, Daehee Seol, Manoj Kumar Gupta, Yunseok Kim,* and Sang-Woo Kim*

Next-generation memory and energy harvesting devices require a higher output performance for charging lectric devices. Generally, it is very limited to control charge transfer through triboelectricity in triboelectric materials. Here, using ferroelectric polarization, it is found that both the amount and direction of charge transfer in triboelectric materials can be controlled. The ferroelec-tric-dependent triboelectricity in a ferroelectric co-polymer film is explored using atomic force microscopy (AFM). Ferroelectric surfaces are rubbed with the AFM tip after poling with positive and negative bias voltages to achieve a triboelectric effect. The surface potential of the positively (negatively) poled area becomes smaller (larger) after rubbing the film surface with the AFM tip. Furthermore, the power output from the triboelectric nanogenerator is dependent on the ferroelectric polarization state. The results indicate that the amount and direction of the charge transfer in triboelectricity can be con-trolled by the ferroelectric polarization state.

DOI: 10.1002/adfm.201505088

Dr. K. Y. Lee, S. K. Kim, D. Seol, Dr. M. K. Gupta, Prof. Dr. Y. Kim, Prof. Dr. S.-W. KimSchool of Advanced Materials Science and EngineeringSungkyunkwan University (SKKU)Suwon 440-746, Republic of KoreaE-mail: [email protected]; [email protected]. Lee, Prof. Dr. S.-W. KimSKKU Advanced Institute of Nanotechnology (SAINT)Center for Human Interface Nanotechnology (HINT)Sungkyunkwan University (SKKU)Suwon 440-746, Republic of Korea

1. Introduction

The triboelectric phenomenon consists of contact-induced elec-trification with which a material becomes electrically charged after it has made contact with a different material through fric-tion.[1,2] Recently, this phenomenon has been studied for use in multiple applications, and it has been successfully applied in several useful technologies, including photocopying,[3] laser printing,[4] electrostatic separations,[5] and nanogenerators.[6–9] Nevertheless, remarkably little is known of the mechanism underlying this phenomenon, especially in ferroelectric sys-tems. Charge transfer is commonly accepted to be derived from the work function difference between the contacting mate-rials assisted by friction heat,[10] which acts as a driving force. Accordingly, the friction conditions can significantly affect the charge transfer during triboelectricity.

Adv. Funct. Mater. 2016, DOI: 10.1002/adfm.201505088

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of piezoresponse force microscopy (PFM) and Kelvin probe force microscopy (KPFM). We were also able to control the triboelectric charge transfer in triboelectric nanogenera-tors (TNGs) by manipulating the polarization states.

2. Results and Discussion

In order to observe the ferroelectric polar-ization-dependent triboelectricity at the nanoscale, it is necessary to explore charge dynamics on the ferroelectric surfaces during the friction process. PFM is well known to allow detecting polarization states based on the detection of local piezoelectric defor-mation of a ferroelectric sample[19,20] while KPFM allows the detection of the surface potential, which can be used to interpret surface charges.[21,22] Hence, a combination of PFM and KPFM makes it possible to dis-tinguish the origin of the surface charges and to eventually understand the surface charge dynamics during the triboelectric phenomena.[21,23,24]

For obtaining ferroelectric materials, we synthesized the P(VDF-TrFE) film and per-formed subsequent annealing to transform the α-phase to a β-phase (Figure S1, Sup-porting Information). Two different poling states are achieved by applying positive and negative bias voltages to the conductive probe in the P(VDF-TrFE) film (Figure 1). As shown in Figure 1a, the positively and nega-tively poled areas of the P(VDF-TrFE) film exhibit a clear contrast between dark and bright, respectively. The dark (bright) con-trast in the PFM phase image corresponds to the downward (upward) polarizations, which indicates that the positively and negatively poled areas are fully switched in the desired directions. This can be deduced even from the hyster-esis loops of Figure S2 in the Supporting Information. We note that the as-grown state shows rather noise-like phase signal due to the nonpenetrated vertical polarization along the thickness direction.[25]

In the KPFM images of Figure 1b, the positively (negatively) poled area exhibits a positive (negative) surface potential, and as-grown region shows a negative surface potential. On the fer-roelectric surfaces, the polarization charges are compensated by the screen charges, which come from the intrinsic/extrinsic surface state or the free charges, for achieving charge balance. An in-complete screening was reported to be generally observed in ferroelectric surfaces as a stable surface charge state.[22,26,27] In such a case, the surface potential dominantly originates from the polarization charge. Indeed, as shown in Figure 1b, we were able to observe partial screening, of which surface potential is negative, which is dominantly originated from the negative polarization charge. However, when an electric field

is applied to the sample by the conductive probe, the charge injection occurs onto the sample surface. Even though partially or completely screened surface are the usual cases of the ferro-electric surfaces, the charge injection during the poling proce-dure frequently leads to an over-screened surface due to a large amount of charge injected during the poling process through the AFM tip.[22,24,28] Similarly to a previous report, when a positive (negative) voltage is applied to the left (right) yellow dotted area in Figure 1b, the positive (negative) charges that are injected dominantly exist on that area. Accordingly, the sign of the surface potential is measured as a positive (negative) value.

However, this state is not thermodynamically stable because the amount of injected charges far exceeds that of the polariza-tion charges.[22,28] When a larger amount of the injected charges is relaxed, the surface charge distribution could achieve a stable state. After a 12 h period, the surface potential with an oppo-site contrast compared to the surface potential that was initially obtained could be observed (see Figure 1c). Furthermore, this state was still observed even after 24 h. This indicates that the P(VDF-TrFE) surface has a stable charge state after 12 h. We



Figure 1. Polarization switching and surface potential distribution of the P(VDF-TrFE) thin film: a) PFM phase image after the poling process and b) KPFM images after the poling process, c) after 12 h of the poling process, and d) after friction over an area indicated with white dotted line of the P(VDF-TrFE) thin film. The left (right) yellow dotted square indicates the positively (negatively) poled area by applying +20 V (−20 V) to the conductive probe during the poling process.

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note that the sign of the surface potential on the negatively poled area is still negative even after achieving the stable charge state. Since an entire as-grown surface shows nega-tive surface potential, it might affect surface potential level because the KPFM is based on the long-range electrostatic interactions.[29]

After fully relaxing the injected charges on the surface, we performed an experiment to examine the triboelectric effect on the stable surface charge states. Figure 1d shows sur-face potential after rubbing the poled areas with a Pt-coated AFM tip for 20 times. After friction, the absolute surface potential has changed, and its difference between two poled areas becomes more significant. This can be a result of the triboelectric charge transfer from the triboelectric effect between the AFM tip and the ferroelectric surface. However, there can be several contribu-tions from the rubbing of the film surface: triboelectricity, flexoelectricity, piezochem-istry, and band gap modulation. Since the strain is expected to be immediately relaxed after releasing the strain, the strain-induced band gap modulation may not significantly affect the surface electrical properties. Fur-ther, since the piezochemical effect can be observed in oxides by modulating stoichi-ometry,[17] it can be excluded as one of major effects for the ferroelectric copolymers. In addition, the flexoelectric effect can affect the surface electrical properties through the modulation of the ferroelectric charges. However, there was no significant change on the polarization states even after scan-ning the surface as shown in Figure 2. In fact, since the applied force, here 30 nN, is significantly lower than those in previous reports,[30,31] it is not expected to show significant influence from the flexoelectricity. We note that electrostatic interaction can affect the measured PFM response. However, while at least 5 dc voltage was applied to the samples in the previous reports, maximum change in the surface potential induced by the friction is only about 0.1 V, which is significantly smaller[32,33] than those in the previous reports. Thus, the reasonable major effect in this study could be triboelectricity.

To further examine the triboelectric phenomenon as a func-tion of the numbers of scans, i.e., the number of times for fric-tion or rub, the surface potential, and the corresponding charge density were analyzed (Figure 3). As shown in Figure 3a, the sur-face potential line profiles of the positively and negatively poled areas before and after friction exhibit a more visible change in the surface potential. It is evident that the surface potential of the positively (negatively) poled area becomes smaller (larger) after rubbing the film surface with the AFM tip. As shown in Figure 3b, after the first scan, the charge density in the posi-tively poled area decreases from −17.60 to −20.68 nC cm–2 while the charge density in the negatively poled area increases from

−10.34 to −10.19 nC cm–2 (the calculation of the charge den-sity can be found in the Supporting Information). The decrease (increase) in the surface potential indicates that hole (electron) transfer occurs from the surface to the AFM tip. The amount of change in the corresponding electron density for the first scan is 1.92 × 1010 cm−2 and 8.96 × 108 cm−2 for the positively and negatively poled areas, respectively.

The observed phenomenon is used to schematically illustrate the electron/hole transfer between the P(VDF-TrFE) thin films and the Pt-coated AFM tip (Figure 3c,d). If we consider the gen-eral mechanism of triboelectricity, the charge transfer between the P(VDF-TrFE) and the AFM tip can be strongly dependent on the their relative work function values.[34] However, the ferroelectric surface can exhibit very distinct charge transfer behavior due to the existence of polarization. As mentioned above, even though the surface potential exhibits a relatively smaller (larger) value in the positively (negatively) poled area, the positively (negatively) poled surface is still covered by holes (electrons) due to screening of the polarization charges. In such a situation, scanning the surface can supply sufficient energy to overcome the activation energy to induce the charge transfer of



Figure 2. PFM a,b) phase and c,d) amplitude images before and after rubbing the surface for 20 times. The left (right) yellow dotted square indicates the positively (negatively) poled area by applying +20 V (−20 V) to the conductive probe. To avoid potential influence of the PFM measurements on the surface potential, the PFM measurements were separately performed with KPFM measurements.

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these surface screen charges. Since the positively (negatively) poled surface is covered by holes (electrons), it is favorable to induce hole (electron) transfer from the P(VDF-TrFE) surface to the AFM tip (bottom figures in Figure 3c,d). Thus, the charge transfer can be concluded to be significantly affected by the polarization state of the ferroelectric surface.

If the observed phenomenon is unique only in the ferroelec-tric materials, then it should not be observed in nonferroelec-tric materials. Thus, the unique phenomena observed in the ferroelectric materials are confirmed by performing the same experiments in the nonferroelectric poly(methyl methacrylate) (PMMA) film (Figure 4). Since PMMA is an insulator as well as a nonferroelectric polymer, it has been chosen as the model system. To examine the triboelectric effect in the nonferroelec-tric materials, we also prepared PMMA thin films on an ITO/PEN substrate. After applying bias voltages of +5 and −5 V (see Figure 4a), we did not observe any distinct contrast in the PFM phase image, indicating that the material is not ferroelectric. However, a clear contrast in the two differently biased areas can

be observed on the PMMA surface, as shown in Figure 4b. In the case of the PMMA film, the surface potential (Figure 4c) is not reversed after 12 h and, furthermore, is nearly the same with that just after the biasing process (Figure 4b). This means that most of excess charges still remained after 12 h, unlike for the ferroelectric P(VDF-TrFE) film (Figure 1). This difference might originate from different levels of surface states.

The surface potential images in the two differently biased areas of the PMMA with an increasing number of scans are shown in Figure 5. In the case of the positively (negatively) biased PMMA film, the surface potential initially shows more (less) positive surface potential. Up to the fifth scan, the surface potential of the positively biased area drastically decreases. How-ever, that of the negatively biased area increases. In the PMMA surface, as mentioned above, the relaxation time of the injected charges seems to be much longer than that of the P(VDF-TrFE) surface. Nevertheless, since an unstable excess charge exists on the PMMA surface, these charges can readily move to the AFM tip, which is grounded during the scanning process.[22,28]

In other words, even though a typical charge transfer induced by the friction, i.e., charge transfer originated from work function differ-ence, can occur, it is more favorable to remove the excess charges on the PMMA surface as a result of the potential difference between the PMMA surface and the AFM tip, i.e., the grounded tip effect.[22,28] However, after all excess charges on the PMMA surface have been removed, both the surface potentials of the positively and the negatively biased areas progressively increase as the number of scans increases. Since the excess charges that are injected have already been removed by sev-eral initial scans, additional scans can induce the charge transfer with different mechanism



Figure 3. Surface potential before and after friction (20 times) in the P(VDF-TrFE) thin film: a) surface potential profile and b) charge density of the positively (blue) and negatively (red) poled P(VDF-TrFE) film depend on the number of the scan. The surface potential profile was obtained from Figure 1. c,d) Schematics of the charge transfer mechanism in the c) positively and d) negatively poled P(VDF-TrFE) film.

Figure 4. PFM phase and KPFM images of the PMMA thin film: a) the PFM phase and b) cor-responding KPFM image of the PMMA thin film after the biasing process. The left (right) yellow dotted square indicates the positively (negatively) poled area by applying +5 V (−5 V) to the conductive probe during the biasing process. c) KPFM image after 12 h of the biasing process.

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after achieving stable surface states. In this case, the electrons are released from the PMMA surface to the AFM tip as a result of the conventional mechanism originating from the work function difference between the PMMA and the AFM tip. We note that since the charge transfer in this stage is solely induced by the work function difference, both positively and negatively biased areas show an increase in the surface potential (see more details in Figure S5, Supporting Information). Therefore, these results confirm that the sign of the charge transfer in the ferroelectric materials can be controlled by the polarization states.

To further confirm the polarization-mediated charge transfer in the ferroelectric materials, we examined it in the ferroelectric material-based energy harvesting device structures. A mechan-ical force stimulator was used to cyclically apply pushing forces on the device to investigate the output performance of the P(VDF-TrFE) film-based TNGs. Figure 6 shows the output voltage obtained from the devices when the force is applied in

a periodical contact-separation mode. Interestingly, the sign of the output voltages in the positively-poled P(VDF-TrFE)-based TNGs was the opposite of that of the negatively poled P(VDF-TrFE)-based TNGs. The change in the sign of the output volt-ages is expected to be a result of the influence of the polariza-tion direction, as was previously discussed. These results, thus, confirm that the polarization direction indeed determines the sign of the charge transfer during the triboelectric phenom-enon. Furthermore, while the bare P(VDF-TrFE) device exhibits an output voltage of about 15 V, positively and negatively poled P(VDF-TrFE)-based TNGs under a vertical compressive force of 2 kg show output voltages that reach up to a measured 400 and 100 V, respectively, which are much larger than that of bare P(VDF-TrFE)-based TNGs. Since the bare P(VDF-TrFE) is not fully switched in one direction, this can be well explained. The different levels of output voltages also confirm that the amount of charge transfer can be controlled by modulating the



Figure 5. KPFM images of the PMMA thin film: a) KPFM images before and after scanning the PMMA film surface and b) corresponding surface potential profiles. The rubbed regions are indicated with blue dotted lines.

Figure 6. Structure of the TNGs in P(VDF-TrFE) and its power generation depending on the poling condition: output voltages of a) bare P(VDF-TrFE) and b) positively and c) negatively poled P(VDF-TrFE).

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polarization states. Overall, Figure 6 shows that the sign and the amount of charge transfer can be controlled by the polariza-tion states.

3. Conclusion

In summary, we have explored the ferroelectric polarization-dependent triboelectricity in ferroelectric co-polymer surfaces using PFM and KPFM. After rubbing the surface with the AFM tip, the surface potential of the positively (negatively) poled area becomes smaller (larger). This indicates that holes (electrons) on the surface of the film surface can be removed by the AFM tip in the positively (negatively) poled area. However, an insu-lating type nonferroelectric PMMA displays the different trend, and only one directional charge transfer is allowed on both pos-itively and negatively poled areas after removing excess charges injected during the biasing process. Furthermore, the output voltages of the TNGs are dependent on the ferroelectric polari-zation state. These results, thus, indicate that the amount and direction of the charge transfer in triboelectricity can be con-trolled by the ferroelectric polarization state. Hence, our studies offer an effective protocol to understand and control the charge transfer behavior of ferroelectric materials through the tribo-electric effect for devices based on next-generation memory technologies.

4. Experimental SectionMaterials—Fabrication of High Crystalline Ferroelectric P(VDF-

TrFE) Film: A solution of P(VDF-TrFE) (20 wt%) dissolved in N,N-dimethylformamide (DMF) solvent was spun on an ITO/PEN substrate, followed by drying at 60 °C to remove the DMF solvent. Next, this layer was maintained at 140 °C for 2 h and was then naturally cooled down to room temperature in a nitrogen atmosphere to improve the crystallinity of the β-phase.[26]

Fabrication of PMMA Film: PMMA (Mw ≈ 15 000) was purchased from Sigma-Aldrich and was used as received. 150 nm thick PMMA thin films were spin-coated on the ITO/PEN substrate, followed by drying at 80 °C for 30 min on a hot plate.

Measurements: X-ray diffraction and Fourier transform infrared spectroscopy measurements were performed to conduct a structural investigation of the ferroelectric P(VDF-TrFE) film. The AFM-based investigations were carried out by using a commercial system (XE100, Park Systems). The ferroelectric and triboelectric properties of the P(VDF-TrFE) film (thickness ≈ 150 nm) and the PMMA film (thickness ≈ 150 nm) were measured via PFM and KPFM with a conductive probe (Multi75E-G, BudgetSensors). For the PFM measurements, a 2 Vac signal with frequency of 17 kHz was applied to the sample by the lock-in amplifier. The KPFM measurements were carried out in the noncontact mode with a 2 Vac signal with a frequency of 17 kHz. In order to avoid potential issues related to the contamination, we have used two separate tips of the same model for performing the experiments: one is only used for scanning the surface and the other is only used for measuring KPFM, respectively. Further, the AFM measurements were performed under the same measuring conditions (temperature = 21 °C, humidity = 17%, and contact force = 30 nN).

Triboelectric Nanogenerators: To fabricate the TNGs, two pieces of acrylic were cut as substrates with dimensions of 40 mm × 40 mm × 1 mm. The Al film was glued on the substrate as the top electrode and active contact material. The P(VDF-TrFE) film (thickness ≈ 7 μm) spin-coated on ITO/PEN was assembled on the other acrylic substrate

with a P(VDF-TrFE) film facing the Al on the top substrate. For the electrical poling process, the Al and the P(VDF-TrFE) were brought into full mechanical contact by an external force. Then, an electric field of 100 MV m−1 was applied between the top and bottom electrodes for 30 min. The electrical measurement of the TNGs was performed 24 h after the poling process to achieve a stable surface state. A pushing tester (Model No. ET-126-4, Labworks Inc.) was utilized to create strain in the TNGs. A digital phosphor oscilloscope (DPO 3052, Tektronix) and a low-noise current preamplifier (SR570, Stanford Research Systems Inc.) were used for the electrical measurements.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsK.Y.L. and S.K.K. contributed equally to this work. This research was supported by the Basic Science Research program through the National Research Foundation (NRF) of Korea, funded by the Ministry of Science, ICT & Future Planning (2014R1A4A1008474, 2015R1A2A1A05001851, and 2009–0083540).

Received: November 27, 2015Revised: December 21, 2015

Published online:

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