Piezoelectricity and ferroelectricity of cellular

Piezoelectricity and ferroelectricity of cellular polypropylene electrets films characterized by piezoresponse force microscopy Hongchen Miao, Yao Sun, Xilong Zhou, Yingwei Li, and Faxin Li Citation: Journal of Applied Physics 116, 066820 (2014); doi: 10.1063/1.4891395 View online: http://dx.doi.org/10.1063/1.4891395 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/116/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Cellular-foam polypropylene ferroelectrets with increased film thickness and reduced resonance frequency Appl. Phys. Lett. 103, 252901 (2013); 10.1063/1.4850523 Nanoscale ferroelectric switching behavior at charged domain boundaries studied by angle-resolved piezoresponse force microscopy Appl. Phys. Lett. 99, 142909 (2011); 10.1063/1.3646761 Thermal variation of piezoresponse in microscopically poled poly(vinylidene fluoride-trifluoroethylene) ferroelectric copolymers approaching Curie temperature J. Appl. Phys. 110, 052008 (2011); 10.1063/1.3623774 Time dependence of piezoelectric d 33 coefficient of cellular ferroelectret polypropylene film Appl. Phys. Lett. 98, 122902 (2011); 10.1063/1.3569950 Micromechanical prediction of the effective electromechanical properties of cellular ferroelectrets J. Appl. Phys. 108, 054101 (2010); 10.1063/1.3481435

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 162.105.25.163 On: Tue, 19 Aug 2014 12:02:38

JOURNAL OF APPLIED PHYSICS 116, 066820 (2014)

Piezoelectricity and ferroelectricity of cellular polypropylene electrets films characterized by piezoresponse force microscopy Hongchen Miao,1 Yao Sun,1 Xilong Zhou,1 Yingwei Li,1 and Faxin Li1,2,a) 1

LTCS and Department of Mechanics and Engineering Science, College of Engineering, Peking University, Beijing 100871, China 2 HEDPS and Center for Applied Physics and Technology, Peking University, Beijing, China

(Received 19 December 2013; accepted 2 May 2014; published online 11 August 2014) Cellular electrets polymer is a new ferroelectret material exhibiting large piezoelectricity and has attracted considerable attentions in researches and industries. Property characterization is very important for this material and current investigations are mostly on macroscopic properties. In this work, we conduct nanoscale piezoelectric and ferroelectric characterizations of cellular polypropylene (PP) films using piezoresponse force microscopy (PFM). First, both the singlefrequency PFM and dual-frequency resonance-tracking PFM testings were conducted on the cellular PP film. The localized piezoelectric constant d33 is estimated to be 7–11pC/N by correcting the resonance magnification with quality factor and it is about one order lower than the macroscopic value. Next, using the switching spectroscopy PFM (SS-PFM), we studied polarization switching behavior of the cellular PP films. Results show that it exhibits the typical ferroelectric-like phase hysteresis loops and butterfly-shaped amplitude loops, which is similar to that of a poly(vinylidene fluoride) (PVDF) ferroelectric polymer film. However, both the phase and amplitude loops of the PP film are intensively asymmetric, which is thought to be caused by the nonzero remnant polarization after poling. Then, the D-E hysteresis loops of both the cellular PP film and PVDF film were measured by using the same wave form as that used in the SS-PFM, and the results show significant differences. Finally, we suggest that the ferroelectric-like behavior of cellular electrets films should be distinguished from that of typical ferroelectrics, both macroscopically and microscopically. C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4891395] V

I. INTRODUCTION

Piezoelectrics and ferroelectrics have been widely used and intensively studied in the past century due to their peculiar electromechanical coupling properties, quick response, and compact size.1,2 All the ferroelectric materials are piezoelectric and most of the widely used piezoelectric materials are ferroelectric although there are some non-ferroelectric piezoelectrics, such as quartz, ZnO, AlN, etc. Typically, the piezoelectrics/ferroelectrics (P/F) are ionic crystal. Their brittleness, however, unfortunately limited their applications in high reliable devices. P/F can also be molecular type and flexible, such as the semicrystalline poly (vinylidene fluoride) (PVDF) and its copolymers with trifluoroethylene P(VDF-TrFE). The recently reported molecular ferroelectrics of Diisopropylammonium Bromide can even have a spontaneous polarization of 23 lC/cm2,3 comparable to that of BaTiO3. In comparison to the ionic and molecular P/F in which the piezoelectricity/ferroelectricity arises inherently from the crystal structure, electrets polymers may also exhibit apparent piezoelectricity and/or ferroelectricity caused by space charges, namely so called piezo-/ferro-electrets. However, the microscopic mechanisms of piezoelectricity in cellular polypropylene electrets and traditional piezoelectric materials are essentially different: piezoelectricity in a)

Author to whom all correspondence should be addressed. Electronic mail: [email protected]. Telephone: 86 10 62757454.

0021-8979/2014/116(6)/066820/8/$30.00

electrets results from the deformation of the charged voids, while traditional piezoelectric materials rely on ion displacement in a lattice.4 In the past one and half decades, a considerable number of cellular (or voided) electret polymers have been developed typically with the piezoelectric d33 of several hundred pC/N,5,6 which is one order higher than that in conventional ferroelectric polymers and comparable to that in ferroelectric ceramics. Currently, most researches on cellular electrets polymers are focused on developing new materials (mostly in the form of thin films with the thickness ranging from tens of lm to hundreds of lm) or new fabrication methods to realize higher piezoelectricity, better thermal stability, etc.7–11 Device applications have also been suggested or implemented.12 The apparent piezoelectric properties of this type of materials are quite different from conventional piezoelectric ceramics or crystals with a high piezoelectricity along the thickness direction (d33) but almost negligible piezoelectric responses in the transverse directions (d31 or d32). Also, the piezoelectricity of cellular polymers is strongly dependent on the Young’s modulus and the charge densities of the voids.13,14 Characterization of the d33 is thus of great importance to materials design and applications. So far, there are typically five characterization methods for d33 measurement of the cellular electrets polymers films, i.e., the quasi-static method, dynamic method, interferometric method, resonance method, and acoustic method. The quasi-static method is the simplest and direct method, while it typically gives the

116, 066820-1

C 2014 AIP Publishing LLC V


066820-2

Miao et al.

overlarge d33 values of up to several thousand pC/N as it cannot separate the charge contributions from the direct piezoelectricity from other effects such as the space charge.15,16 The dynamic method and interferometric method are also direct methods to measure the direct d33 or the converse d33 with the typical frequency range of 0 1000 Hz for the former and within several hundred kHz for the latter.13,16,17 The resonance method and acoustic method are both indirect methods and usually have a larger data scatter than the dynamic method and interferometric method. It should be noted that the above-mentioned methods are all macroscopic methods which can only get the averaged piezoelectric property of the sample. Actually, there exists a microscopic piezoelectricity characterization method based on the atomic force microscopy, i.e., piezoresponse force microscopy (PFM).18,19 In principle, PFM is very similar to the interferometric method which excites the sample’s thickness vibration at a high frequency of tens to hundreds of kHz and measures the vibration amplitude. Generally, there are two main differences between PFM and the interferometric method: one is that the sample is excited by a localized field in PFM, whereas it is a macroscopic field in the interferometric method. The other is that PFM is a contact method which measures the vibration via a conductive cantilever, while the interferometric method is non-contact and detects the vibration by the laser spot. On the other hand, it is well known that cellular ferroelectrets polymers may exhibit ferroelectric-like D-E hysteresis loop by means of dielectric barrier discharges. In fact, the switching in ferroelectrets is the change of charged state, which can be regarded as an analog of the polarization switching in a ferroelectric material. Thus, the threshold voltage for dielectric breakdown in cellular ferroelectrets polymers can be viewed as the analog of the coercive voltage of ferroelectric materials. We may get a D-E hysteresis loop in ferroelectrets when the applied voltage is well above the threshold voltage. However, there is a distinct difference between ferroelectrics and cellular ferroelectrets. The polarization in ferroelectrics is always thermodynamically stable, while the trapped-charge polarization in ferroelectrets is metastable.20,21 Since the switching behavior of ferroelectrics can also be characterized by the switching spectroscopy function of PFM (SS-PFM), in forms of the phase hysteresis loops and amplitude butterfly loops,22 the cellular ferroelectrets polymer may also be microscopically ferroelectret if it also exhibits these two types of loops. In this work, we first mapped the microscopic piezoelectric properties of a cellular polypropylene (PP) film using both single-frequency PFM and dual-frequency resonancetracking PFM, and compared with the measurement results of a PVDF film. It is found that the d33 measured by PFM is about one order smaller than that measured macroscopically. Next, the PP film is characterized by SS-PFM and shows ferroelectric-like phase hysteresis loops and amplitude butterfly loops, similar to that of the PVDF film but with intensive asymmetries. Then, using the same waveform driving field as that adopted in the SS-PFM, we measured the macroscopic D-E hysteresis loops of the PP films and found they would never be saturated with a switchable polarization of

J. Appl. Phys. 116, 066820 (2014)

only 0.3 mC/m2 after poling by a 175 kV/mm field, quite different from the typical ferroelectric D-E hysteresis loop of a PVDF film with the saturated and switchable polarization of 1.8 lC/cm2. In addition, both the superiority and limitation of PFM in the piezoelectricity and ferroelectricity characterization of cellular electrets films (or flexible ferroelectrics) have been discussed. II. EXPERIMENTS A. Method

Piezoresponse force microscopy (PFM) is a powerful tool to study electromechanical coupling in piezoelectric and ferroelectric materials at nanoscale. For the basic principle of PFM, readers can refer to the recent reviews.18,19 Here, we only briefly present the principle of switching spectroscopy PFM (SS-PFM), which was proposed by Kalinin et al.23 to characterize the local switching behavior of ferroelectrics. In SS-PFM, the excitation voltage applied onto the cantilever is a high frequency AC voltage superimposed on a time variant dc bias voltage, as shown in Fig. 1(a). At the “ON” state or “OFF” state, the piezoelectric response is measured by applying the high-frequency AC voltage and getting the amplitude and phase responses. By plotting the amplitude/phase versus the dc bias voltage, an amplitude butterfly curve and a phase hysteresis loop (see Fig. 1(b)) will be obtained, if the sample is ferroelectric and the applied dc bias voltage is large enough to induce local polarization switching. Generally, the curves are measured at the “OFF” state to characterize the switching behavior, which can minimize the effects of electrostatic interactions. B. Materials

The material to be tested is a cellular polypropylene electrets film of 50 lm thick provided by Tongji University, Shanghai, China. The film is poled by a corona poling method and the macroscopic d33 value is measured to be about 280 pC/N at room temperature with a quasi-static method, which is close to the value measured using other methods in the literatures.24 In comparison to the ferroelectrets PP films, we also conduct testing on a poly (vinylidene fluoride) (PVDF) film which is both ferroelectrics and ferroelectrets. The PVDF sample is provided by Jinzhou Kexin Electronics Ltd, China, with the thickness of 45 50 lm. The as-received film is poled with the d33 value of 21 pC/N. C. Testing procedure

First, we conduct both conventional (or single-frequency) PFM testing and DFRT PFM testing on the cellular PP film, and estimate the d33 value by correcting the quality factor Q.25 The DFRT PFM testing is also conducted on the PDVF film for comparison. Then, we conduct SS-PFM testing on both the cellular PP film and the PVDF film to characterize the microscopic ferroelectricity. Finally, we conduct the macroscopic D-E hysteresis loops measurements on both types of films using a similar waveform field with that in SSPFM. In all the PFM testing, we use the Olympus AC240 cantilever with the nominal stiffness of 2 N/m and the first


066820-3

Miao et al.

J. Appl. Phys. 116, 066820 (2014)

FIG. 1. (a) The waveform of excitation voltage in SS-PFM; (b) the typical amplitude butterfly loop and (c) phase hysteresis loop obtained by SS-PFM testing on a ferroelectric ceramics.

free resonance frequency of 70 kHz. All the PFM experiments were conducted using a commercial scanning probe microscope (SPM) (Asylum Research MFP-3D). III. RESULTS AND DISCUSSIONS A. Piezoelectricity characterization using single-frequency PFM

We first conduct single-frequency (SF) PFM testing on the cellular PP films with the 2 N/m cantilever. During testing, the probe is pressed tightly onto the testing sample with a static pressing force of about 40 nN. The contact resonance frequency (CRF) of the cantilever tip-sample system is about 285 kHz, and the driving frequency for SF-PFM testing is 100 kHz, far from the CRF. Therefore, we applied a large driving voltage of 10 V during testing to get large amplitude responses. The scan rate is 1 Hz so that it takes about 4mins to get a group of 256 256 images. Larger scan rate is also possible but not recommended because it may destroy the sample surface and/or the cantilever tip in that case. Figure 2 shows the obtained SF-PFM images of the cellular PP film within a 5 lm 5 lm scan area in which all the images of topography, amplitude, and phase are presented. It can be seen in Fig. 2(a) that the surface of the as-received cellular PP film is fairly flat with the roughness of tens of nanometers. Thanks to the large applied voltage of 10 V, the PFM amplitude is over 100 pm, large enough to be detected using the lock-in amplifier. It can be seen from Fig. 2(b) that

the distribution of the piezoelectricity is inhomogeneous in the scan area. The piezoelectric response we detected in PFM is localized piezoelectricity, since the sample is excited by a localized electric field. Thus if the local areas are easily compressible and highly charged, the obtained piezoelectric response will be large.13 The phase image in Fig. 2(c) shows little contrast, which may indicate that the poling of the cellular PP film is complete that the remnant polarization has the same directions within the scanning area. B. d33 measurement using DFRT PFM

Then we conduct dual-frequency resonance-tracking (DFRT) PFM testing on the cellular PP film using the same cantilever of 2 N/m. During testing, the static pressing force applied on the sample is about 40 nN. Because the amplitude in DFRT PFM is enhanced by the resonance effect, the amplitude of the AC driving voltage is 3 V. As it takes additional time using the DFRT method to track the resonance frequency of the sample-tip system, the scan rate is set to be 0.5 Hz and a faster scan rate will make the DFRT process ineffective. Figure 3 shows the DFRT PFM images of the cellular PP film in which besides the topography, amplitude and phase images, the images of quality factor, contact resonance frequency (CRF) and the calculated d33 by correcting the quality factor Q.25 It can be seen that in DFRT PFM images, the amplitude (Fig. 3(c)) is fairly enhanced by the resonance effect, with the maximum value of about 700 pm under a

FIG. 2. Single-frequency PFM images of a cellular PP electrets film within a 5lm 5lm scan. (a) topography; (b) amplitude; (c) phase.


066820-4

Miao et al.

FIG. 3. DFRT PFM images of a cellular PP electrets film within a 1 lm 1 lm scan. (a) topography; (b)quality factor; (c) amplitude; (d) phase; (e)contact resonance frequency; (f) d33 image.

driving field of 3 V. Different from that obtained in the single-frequency PFM testing, the DFRT PFM phase image (Fig. 3(d)) shows considerable contrast with the phase difference typically less than 70 . Furthermore, the contrast pattern in the phase image is in good accordance with that in the amplitude image, and they are both in good accordance with that in the topography image in Fig. 3(a). As we know, the surface roughness can have great influence on PFM testing. The amplitude and phase shift may result from the fact that sometimes the tip may lose firm contact with the sample such as in a valley location. The CRF image in Fig. 3(e) may represent the relative contact stiffness within the scanning area, i.e., a larger CRF corresponds to large contact stiffness thus larger local modulus.26 It can be found that the localized piezoelectricity is inversely proportional to the local Young’ modulus by comparing the d33 image Fig. 3(f) with the contact resonance frequency image, which is consistent with previous studies.13 It can also be seen from Fig. 3 that the d33 measured in this way varies from 7 to 11 pC/N, which is about one order lower than that of 200 pC/N measured using the interferometric method.16 In fact, it is not quite reasonable to compare the d33 coefficients obtained from PFM with the macroscopic d33 value, since they are measured under very different configurations. Currently, there still exist great challenges for quantitative determination of piezoelectric coefficients using PFM because the position effect of the laser spot and the background effect of the SPM setup are typically difficult to remove. For d33 measurement in cellular PP films using PFM, there are another two challenges: one is that it is almost impossible to estimate the electric field distribution because the sample is intensively inhomogeneous. The other is that a fairly large pre-stress will be applied onto the sample during

J. Appl. Phys. 116, 066820 (2014)

testing. The quasti-static pressing force applied in our DFRT PFM testing is 40 nN, and the dynamic pressing force is estimated to be 2 nN using the amplitude and the cantilever stiffness of 2 N/m. As the cellular PP film is very soft (with the Young’s modulus of only several MPa), the contact stress will dramatically reduce the measured piezoelectric constant d33.17,27 Since in PFM testing, a pressing force must be applied onto the sample to keep the cantilever tip in good contact with the sample, we think that PFM is not suitable to measure the piezoelectric properties in very soft materials such as PP films in which the d33 constant is very sensitive to the applied pre-stress. To make a comparison, we also conducted DFRT PFM testing on a 50 lm thick PVDF film using an AC driving field of 4 V and a static pressing force of 40 nN, and the results are shown in Fig. 4 in which only the images of amplitude, phase, CRF and calculated d33 are presented. It can be seen that amplitude image shows distinct contrast, while the phase image shows poor contrast, which may also indicate that the PVDF is well poled with the polarization directions uniform within the scanning area. The CRF image of Fig. 4(c) shows that the averaged CRF of the cantileverPVDF sample system is 352 kHz, considerably higher than that of about 285 kHz for the cantilever-PP sample system, which is in good accordance with the fact the PVDF film (with the Young’s modulus of about 2.5 GPa) is much stiffer than the PP film (with the Young’s modulus of only several MPa).26 The calculated d33 image of Fig. 4(d) shows that the d33 value of the PVDF film measured using DFRT PFM ranges in 5 8 pC/N, although still smaller than but is close to that of 17 pC/N measured using the Berlincourt Meter. This implies that the d33 of PVDF film is less sensitive to the applied pre-stress (this is the case in d33 measurement using macroscopic methods) and DFRT PFM is thus suitable to estimate the piezoelectric properties of PVDF. C. Polarization switching characterization using SS-PFM

We further conduct the SS-PFM testing on the cellular PP film to characterize the local polarization switching behavior. Limited by the high-voltage module of the testing

FIG. 4. DFRT PFM images of a PVDF thin film within a 1 lm 1 lm scan. (a) amplitude; (b) phase; (c) contact resonance frequency; (d) d33 image.


066820-5

Miao et al.

J. Appl. Phys. 116, 066820 (2014)

FIG. 5. SS-PFM testing results on a cellular PP thin film. (a) DFRT PFM d33 image superposed on topography in which the three SS-PFM testing points are indicated; (b) amplitude butterfly curve and phase hysteresis loop of Point 2; (c) amplitude butterfly curves and (d) phase hysteresis loops of the three testing points.

setup, the maximum applied DC voltage in our SS-PFM testing is set between 216 V and 216 V. The SS-PFM testing results of the PP film are shown in Fig. 5 in which the results of three typical points are presented. It can be seen the cellular PP film can also exhibit amplitude butterfly curve and phase hysteresis loop, as shown in Fig. 5(b). However, both these two curves of the PP film are intensively asymmetric, with the amplitude at the maximum positive DC field much larger than that at the maximum negative DC field, quite different from that of typical ferroelectrics. Furthermore, from Figs. 5(c) and 5(d), it can be seen that the appearance of asymmetric amplitude butterfly curves and phase hysteresis loops are repeatable, although the shapes are somewhat different for different testing points. If we regard the switching of the charge stage in PP as an analog of the polarization switching in a ferroelectric material, Fig. 5(b) means that the applied voltage of about 200 V in PFM can make the polarity in the 50 lm-thick cellular PP film switch. This is impossible at the macroscopic scale since 200 V/50 lm ¼ 4 MV/m is far below the threshold field of 10–30 MV/m.28,29 However, it is possible in PFM testing because the electric field is highly concentrated near the cantilever tip, which may reach above 109 V/m under a tip with the radius of 50 nm and 5 V on.30 In PFM testing of cellular PP thin films, the field concentration effect will be dramatically reduced because of the existence of the cover layer which typically has a thickness of 0.5 7 lm.31 Bearing in mind that the field around the tip drops off approximately as 1/r2 (where r is the distance from the tip), then the local field applied on the voids can be within a large range of 4 MV/m 400 MV/m under applied voltage of 200 V, which makes the switching of polarity possible in cellular PP films by means of internal Paschen breakdown during SS-PFM testing. It should also be noted that the large distribution of the microvoid size in PP will result in a broad distribution of the breakdown voltages. Thus in some areas, when the maximum local field is still below the

breakdown fields, there will be no polarization switching and neither the phase hysteresis loop nor the amplitude butterfly loop can be obtained. To make a comparison, we also conduct SS-PFM testing on the PVDF thin film and the results are shown in Fig. 6. It can be seen that for the ferroelectric PVDF thin film, the amplitude butterfly curve is only slightly asymmetric and the phase hysteresis loop is almost a rectangular, which indicate that ferroelectric polarization switching occurs during the SS-PFM testing. The SS-PFM curves of other typical ferroelectrics, such as PZT ceramics, have the similar shape as that in Fig. 6 and will not be discussed here. The mechanism of ferroelectric-like amplitude butterfly curves and phase hysteresis loops of the PP thin film in SSPFM testing can be explained in a simple way as illustrated in Fig. 7. As we know, the polarization in cellular PP electrets film is not intrinsic or stable polarization but space charges trapped at the opposite sides of the pores. During SS-PFM

FIG. 6. Amplitude butterfly curve and phase hysteresis loop of a PVDF thin film measured using SS-PFM.


066820-6

Miao et al.

J. Appl. Phys. 116, 066820 (2014)

some hard ferroelectrics with large internal bias field can also be intensively asymmetric. D. Polarization switching measurement using SawyerTower circuit method

FIG. 7. Mechanism of amplitude butterfly curve and phase hysteresis loop of cellular PP thin film during SS-PFM testing.

testing, when the DC bias field is zero, i.e., Point A in Fig. 7, the remnant polarization is positive and detectable amplitude can be obtained. When a positve DC field is applied, say at Point B in Fig. 7, the positive polarization is enhanced, since cellular PP electrets experience a barrier discharge. Thus, the amplitude is greatly increased and the phase remains the same as that at Point A. When the voltage decreases, the amplitude will also decrease, which is attributed to the inverse barrier discharge in cellular PP films.21 When the DC field changes negative, as denoted by Point C in Fig. 7, the polarization reverses to be negative but its magnitude is considerably smaller than the value of Point B because of the large positive internal bias field induced during corona poling. Therefore, the amplitude at Point C is considerably smaller than that at Point B, leading to intensively asymmetric amplitude butterfly curve shown in Fig. 7(a). Meanwhile, as the polarization direction at Point C is opposite to that at Point B, the phase difference is 180 , which is also close to the experimental observations in Fig. 5. From above, it can be seen that like ferroelectrics, cellular PP electrets films can also show amplitude butterfly curves and phase hysteresis loops during SS-PFM testing. However, as we know, the cellular PP electrets film is not ferroelectrics because the polarization inside is not intrinsic dipole but oriented space charges by poling. Therefore, one cannot identify ferroelectrics just by measuring the amplitude butterfly curves and phase hysteresis loops using SSPFM. On the other hand, the SS-PFM curves of cellular PP electrets films are strongly asymmetric, which is quite different from that of typical ferroelectrics. Thus, it seems one may distinguish cellular PP electrets films from ferroelectrics based on the shapes of the SS-PFM curves. However, this may not be very effective because the SS-PFM curves of

To further elucidate the mechanism of SS-PFM and its function in characterization of ferroelectrics, we conduct the D-E measurement on both PP electrets films and PVDF ferroelectric films using the traditional Sawyer-Tower circuit. In the testing, we use two different waveform driving field: one is the conventional triangle waveform; the other is the same as what we used in the SS-PFM testing. Figure 8 shows the measured D-E hysteresis loops of a cellular PP electrets film using the two different waveform driving fields. It can be seen that using the conventional triangle waveform field, the cellular PP electrets film shows no hysteresis at all under a field less than 25 kV/mm. When the field increases to 175 kV/mm, very significant D-E hysteresis loop occurs as shown in Fig. 8(a). While the hysteresis loop shows no saturation, we cannot tell whether there exist polarization switching in PP thin films because some lossy dielectric materials can also show hysteresis loops as what is shown in Fig. 8(a). In comparison, under the SS-PFM waveform driving field, the PP film shows a remnant polarization of about 0.3 mC/m2 at the OFF state, indicating that there really exists polarization switching in the material under a large driving field. Furthermore, from Fig. 8, we can conclude that the SS-PFM waveform driving field is superior to the conventional waveform field in identifying polarization switching, since it can reduce the contributions from the conductance and capacitance of sample, which is similar to the modified Sawyer-Tower circuit.32 We also conduct D-E hysteresis measurement on the PVDF thin film using these two waveform driving field and the results are presented in Fig. 9. As expected, under the conventional triangle waveform field, PVDF shows no D-E hysteresis loops when the field is 40 kV/mm (well below the coercive field of about 75 kV/mm) because no polarization switching occurs. When the field gradually increases to 140 kV/mm, significant polarization switching occurs at 75 kV/mm and it saturates when the field is above 100 kV/mm, which is typical responses of ferroelectrics. The total switchable polarization is about 1.8 lC/cm2 and it can be deduced that the remnant polarization of as-received PVDF thin film is 1.5 lC/cm2. When the driving field changed to be SS-PFM waveform type, the D-E

FIG. 8. D-E hysteresis loops of a cellular PP electrets film measured using two different waveform driving field: (a) triangle waveform; (b) SS-PFM waveform.


066820-7

Miao et al.

J. Appl. Phys. 116, 066820 (2014)

FIG. 9. D-E hysteresis loops of the PVDF thin film measured using two different waveform driving field: (a) triangle waveform; (b) SS-PFM waveform.

hysteresis loop for the ON state is almost the same as what shows under the triangle waveform field. As expected, at the OFF state, the polarization saturated without dielectric responses at large field and the remnant polarization is the same as that at the ON state. Therefore, for typical ferroelectrics, the D-E hysteresis loop is not so sensitive to the waveform of the driving field. In another word, for typical ferroelectrics, one can characterize polarization switching via the D-E hysteresis loops measurement using conventional waveform driving field. From all the above measurements, it can also be seen that the SS-PFM waveform field is generally more suitable to be used to identify polarization switching, both microscopically and macroscopically. However, even if we can identify polarization switching using the SS-PFM waveform filed, typically we still cannot tell whether it is ferroelectrics or not because non-ferroelectric electrets can also show ferroelectric-like polarization switching. While if the OFF state curve is like what is shown in Fig. 8(b), we can tell that it is not ferroelectrics when the driving field is large enough. That is to say, the macroscopic D-E hysteresis measurement is more reliable than the SS-PFM measurement when identify ferroelectrics, although it is still not the gold criterion as pointed out by Scott.33

IV. CONCLUSIONS

In summary, we characterized both piezoelectric and ferroelectric-like behavior of cellular PP electrets thin films using PFM, SS-PFM and macroscopic D-E hysteresis measurement using the SS-PFM type waveform field. For comparison, we also conduct these testing on another electrets and ferroelectric PVDF thin film. Results show that the d33 of cellular PP thin film measured using PFM is about one order lower than the value measured using macroscopic methods, which indicates that PFM is not suitable to measure piezoelectric properties of PP thin film because the large pressing stress (typically several MPa) applied during measurement, since the d33 value of cellular PP film is very sensitive to the pre-stress. SS-PFM testing shows that PP thin film can also show ferroelectric-like amplitude butterfly curves and phase hysteresis loops, but these two curves are intensively asymmetric, different from that of typical ferroelectrics. We further measure the D-E hysteresis loops of PP thin film using the similar waveform field as that used in

SS-PFM and finalized that there exist polarization switching in cellular PP thin films under large electric field but the switching cannot be saturated even under a high field of 175 kV/mm. As we know in advance that cellular PP electrets film is not ferroelectrics, when identifying ferroelectrics we suggest that caution must be taken using SS-PFM testing and in general, the macroscopic D-E hysteresis loop measurement method using SS-PFM type waveform field is more reliable than the microscopic SS-PFM testing. ACKNOWLEDGMENTS

F.L. greatly thanks Professor Yongping Wan (Tongji University, China) for providing the cellular PP thin films for testing. Financial support from the Natural Science Foundation of China (under Grant Nos. 11090331 and 11002002) is also acknowledged.

1

B. Jaffe, W. R. Cook, and H. Jaffe, Piezoelectric Ceramic (Academic Press, London, New York, 1971). 2 Y. Xu, Ferroelectric Materials and Their Applications (Amsterdam Press, North-Holland, 1991). 3 D. W. Fu, H. L. Cai, Y. M. Liu, Q. Ye, W. Zhang, Y. Zhang, X. Y. Chen, G. Giovannetti, M. Capone, J. Y. Li, and R. G. Xiong, Science 339, 425 (2013). 4 S. Bauer, R. Gerhard-Multhaupt, and G. M. Sessler, Phys. Today 57(2), 37 (2004). 5 X. L. Qiu, J. Appl. Phys. 108, 011101 (2010). 6 M. Wegener and S. Bauer, Chemphyschem 6, 1014 (2005). 7 J. Hillenbrand and G. M. Sessler, J. Appl. Phys. 103, 074103 (2008). 8 W. Wirges, M. Wegener, O. Voronina, L. Zirkel, and R. GerhardMulthaupt, Adv. Funct. Mater. 17, 324 (2007). 9 X. Q. Zhang, G. X. Cao, Z. L. Sun, and Z. F. Xia, J. Appl. Phys. 108, 064113 (2010). 10 X. Q. Zhang, G. M. Sessler, and J. Hillenbrand, J. Electrostat. 65, 94 (2007). 11 S. Zhukov, S. Fedosov, and H. von Seggern, J. Phys. D. Appl. Phys. 44, 105501 (2011). 12 S. Bauer, IEEE Trans. Dielectr. El. In 13, 953 (2006). 13 J. Hillenbrand and G. M. Sessler, IEEE Trans. Dielectr. El. In 7, 537 (2000). 14 A. Mellinger and O. Mellinger, IEEE Trans. Dielectr. El. In 18, 43 (2011). 15 R. A. C. Altafim, H. C. Basso, R. A. P. Altafim, L. Lima, C. V. de Aquino, L. G. Neto, and R. Gerhard-Multhaupt, IEEE. Trans. Dielectr. El. In 13, 979 (2006). 16 X. Zhang, J. Hillenbrand, and G. M. Sessler, J. Appl. Phys. 101, 054114 (2007). 17 P. Fang, L. Hollander, W. Wirges, and R. Gerhard, Meas. Sci. Technol. 23, 035604 (2012). 18 N. Balke, I. Bdikin, S. V. Kalinin, and A. L. Kholkin, J. Am. Ceram. Soc. 92, 1629 (2009). 19 E. Soergel, J. Phys. D. Appl. Phys. 44, 464003 (2011).


066820-8 20

Miao et al.

M. Lindner, S. Bauer-Gogonea, S. Bauer, M. Paajanen, and J. Raukola, J. Appl. Phys. 91, 5283 (2002). 21 X. L. Qiu, A. Mellinger, M. Wegener, W. Wirges, and R. Gerhard, J. Appl. Phys. 101, 104112 (2007). 22 S. Jesse, H. N. Lee, and S. V. Kalinin, Rev. Sci. Instrum. 77, 073702 (2006). 23 S. Jesse, A. P. Baddorf, and S. V. Kalinin, Appl. Phys. Lett. 88, 062908 (2006). 24 Y. P. Wan, L. T. Xie, K. X. Lou, X. Q. Zhang, and Z. Zhong, J. Mech. Phys. Solids 60, 1310 (2012). 25 S. H. Xie, A. Gannepalli, Q. N. Chen, Y. M. Liu, Y. C. Zhou, R. Proksch, and J. Y. Li, Nanoscale 4, 408 (2012). 26 D. C. Hurley, in Applied Scanning Probe Methods XI, edited by B. Bushan and H. Fuchs (Springer, Berlin, 2009), p. 97.

J. Appl. Phys. 116, 066820 (2014) 27

R. Kressmann, J. Appl. Phys. 90, 3489 (2001). X. L. Qiu, A. Mellinger, W. Wirges, and R. Gerhard, Appl. Phys. Lett. 91, 132905 (2007). 29 H. von Seggern, S. Zhukov, and S. Fedosov, IEEE Trans Dielectr. El. In 18, 49 (2011). 30 L. L. Tian, V. R. Aravind, and V. Gopalan, in Scanning Probe Microscopy of Functional Materials, edited by S. V. Kalinin and A. Gannepalli (Springer, London, New York, 2010). 31 Y. P. Wan, L. T. Xie, X. Q. Zhang, and Z. Zhong, Appl. Phys. Lett. 98, 122902 (2011). 32 X. L. Qiu, L. Hollander, W. Wirges, R. Gerhard, and H. C. Basso, J. Appl. Phys. 113, 224106 (2013). 33 J. F. Scott, J. Phys.-Condens. Mater. 20, 021001 (2008). 28
