Статьи на перевод PVDF_P(VDF-TrFE) / Enhanced Ferroelectric Switching Characteristics of P(VDF-TrFE) for

Introduction

Integration of ferroelectrics for nonvolatile memory application, i.e., ferroelectric random access memory (FeRAM), is gaining rising attention. In many aspects, organic ferroelectrics are promising candidates for ferroelectric memories, owing to their processing practicality and cost-effectiveness. Poly(vinylidene fluoride-ran-trifluoroethylene), P(VDF-TrFE) is among the most well-studied organic ferroelectrics. It is a semicrystalline polymer with at least two main crystalline phases, the nonpolar R-phase and the ferroelectric -phase, which coexist with the amorphous part. Several excellent reviews exploring its characteristics and properties are available in the literature.1-4 Several reports have demonstrated its excellent potential to be incorporated in various types of nonvolatile memory device based on capacitance and resistance switching.5-8 One notable issue in P(VDF-TrFE) is that the polarization switching of this material is relatively slow, 100 ms at 50 MV/m and room temperature,9 which is not favorable to the operation (writing- reading-erasing) of memory devices. Nanosecond switching in P(VDF-TrFE) could be achieved by applying very high electric field above 200 MV/m.10 Such approach, however, is not practical as it requires high operating voltage or ultrathin film when translated into devices.

The switching dynamics in ferroelectric polymers is generally governed by polarization of the crystalline phase and the amorphous phase as well as the interface between them. According to this, the presence of free charges is necessary to compensate and stabilize the polarization of the crystallites.11 The polarization and the switching time are improved significantly when adequate amount of free charges is present in the system, as implied from the ferroelectric measurement on samples with conducting polymer PEDOT:PSS12 and PPy:PSS13 as interfacial layer between the P(VDF-TrFE) and aluminum electrodes, whereby the presence of conducting polymer facilitates charge injection into the P(VDF-TrFE) films.14 The role of these free charges in stabilizing the polarization domain has

* To whom correspondence should be addressed. Fax: (65)-6790 9081. Tel: (65)-6790 6661. E-mail: pslee@ntu.edu.sg.

been recognized in both organic and inorganic ferroelectrics alike.15,16 Studies of ferroelectric switching transient of P(VDFTrFE) have been intensively performed by Furukawa et al. by applying step function of high electric field across the P(VDFTrFE) thin film. The amount of reversible polarization, which equals twice the remnant polarization (16 µC/cm2), is independent of electric field. On the other hand, the switching time follows an exponential law with the activation energy being temperature dependent.17

Polarization reversal processes in P(VDF-TrFE) are often expressed on the basis of nucleation and growth model.18 Nucleation process initiates when a sufficient number of molecular chains with reversed polarity are present. In this state, the polarization reversal progresses very slowly and is strongly dependent on the applied electric field. In the case of P(VDFTrFE) when the polarization reaches 4% of total reversed polarization, domain growth begins to take place by the movement of domain wall and thereupon the polarization reversal progresses rapidly.19 By using sequence of unipolar on-off short electric field pulses, Nakajima et al. were able to observe a delayed, but eventually a completed, polarization reversal. They observed that the polarization switching progresses in two steps corresponding to nucleation and growth.19 This is consistent with the intrinsic polarization switching theory. The activation energy of the domain nucleation is typically in the range 0.8-1.8 GV/m,9,17,18,20 whereas the domain growth proceeds at a much lower activation field of 87 MV/m.21 This indicates that the domain nucleation dominates the entire switching behaviors and in most of the cases, polarization reversal of ferroelectric polymers occurs at the nucleation-limited regime. However, in some cases, the presence of defects such as head-to-head and tail-to-tail defects, local strain, cross-linking, and impurities has been known to induce heterogeneous domain nucleation.22 When the heterogeneous domain nucleation occurs at much lower activation field, polarization reversal will be limited by the domain growth process (growth-limited regime).

In this paper, we demonstrate a facile method to enhance the switching dynamics of P(VDF-TrFE) with the incorporation of gold nanoparticles, with the intention of promoting heteroge-

neous nucleation of the polarization domain. In the presence of gold nanoparticles, the structural properties and electrical behavior of the blend were evaluated accordingly, to provide an insight on the improved polarization switching behavior.

Experimental Methods

P(VDF-TrFE) (70-30 mol %) was obtained from Solvay and used without further purification. Polymer pellets were dissolved in methyl ethyl ketone (MEK) at a concentration of 30 mg/mL. The solution was then passed through 0.4 µm pore size PTFE filter and labeled as reference solution. Gold nanoparticles were synthesized from HAuCl4 · 2H2O salt and sodium citric in a boiling aqueous solution, according to the method described elsewhere.23 The resulting citric-stabilized gold nanoparticles were 10-12 nm in size. A polymer-nanoparticles blend was prepared with gold nanoparticle concentration ranging from 1 × 10-2 to 1 × 10-6 wt %. The reference solution and the blends were then spin-coated onto ITO-coated glass substrates. An average film thickness of 320 nm was revealed from the surface profiler measurements. Subsequently, the films were annealed at 140 °C for 2 h in N2 atmosphere, followed by slow cooling to room temperature. The film crystallinity was determined by Bruker D8 discover GADDS X-ray diffractometer (XRD) (40 kV, 40 mA). Piezoresponse force microscopy (PFM) was carried out on an atomic force microscope (Asylum Research MFP3D) using Pt/Ir coated tip to study the ferroelectric domain structure.

A two-electrode capacitor device was fabricated for the electrical characterization. Counter electrodes were formed by evaporation of gold (99.9% purity) through a shadow mask (electrode surface area 0.64 mm2) onto the annealed film sample. The leakage current was measured using Keithley 4200 analyzer, and capacitance data was obtained by using Agilent E4980A precision LCR meter (ac 50 mV, 1 kHz). Both polarization hysteresis and switching time measurements were performed using Radiant ferroelectric tester HVS6000. For the switching time measurement, a series of electric field pulses were applied across the film. The polarization response from the sample was recorded and plotted as a function of time. The pulse sequence consists of three consecutive voltage steps with pulse width ranging from 5 ms to 0.5 s (Figure 2). The first pulse of magnitude -100 MV/m imparts full polarization in one direction. The second pulse, with magnitude varying from 50 to 100 MV/m, switches the polarization into the opposite direction. The identical final pulse switches the already-reversed dipole once again, accounting for the contribution of the nonswitching current. The net polarization switching response was obtained from the difference in polarization induced by the second and the third pulses.

Results and Discussion

The electrical behavior of P(VDF-TrFE)-gold nanoparticles blend was studied by the means of capacitance-voltage (C-V) and current-voltage (I-V) measurements as presented in Figure 1. The C-V and I-V curves for Au nanoparticles of 1 × 10-5 and 1 × 10-6 wt % are shown in the Supporting Information, Figure S1. The hysteretic C-V “butterfly” curve is the characteristic of the ferroelectric capacitor, indicating the dielectric nonlinearity. The dielectric constant extracted from the C-V curve at zero bias does not change significantly in the presence of nanoparticles up to 1 × 10-4 wt % Au NP. Further addition of Au nanoparticles lowered the dielectric constant of P(VDF-TrFE). Similar behavior was also observed from the I-V characteristic. The leakage current saturates around 1 µA/cm2 for pristine P(VDF-TrFE) and increases, within the tolerable limit, with increasing Au nanoparticles content up to 1 × 10-4 wt %. The leakage current becomes excessively high when the Au nanoparticles content reaches 1 × 10-2 wt %. These behaviors are expected since ferroelectricity and electrical conductivity are mutually exclusive in a single system. Enhancement of ferroelectricity, e.g., by blending with conductors, strongly reduces the ferroelectric polarization due to additional free charge carriers that neutralizes the polarization dipoles.6 As the excessive leakage current deters the measurement of switching time, we have limited our study to the 1 × 10-6 wt % AuNP-blended sample.

Subsequently, the device was subjected to switching time measurement. A series of electrical pulses were employed to induce the polarization switching as shown in Figure 2. Application of the first negative pulse sets the electrical dipole P(VDF-TrFE) away from the substrate. When the second positive pulse is applied, the dipole switches its polarization into reverse direction toward the substrate. However, only part of the polarization can be retained as soon as the second pulse is turned off. The observed changes in polarization are contributed by the ferroelectric as well as the dielectric polarization. While the ferroelectric polarization is preserved, the dielectric contribution depolarized as soon as the applied field is removed. The final positive pulse is therefore necessary to account for the dielectric polarization alone. By subtracting the nonswitching from the switching response, net ferroelectric switching current is obtained. The magnitude of this switching current (δD) is consistent with the one obtained from the D-E polarization hysteresis, which is approximately twice the remnant polarization (16 µC/cm2). The polarization response was plotted against log time. The time required from the start of the pulse to the peak maximum of δD/δ log(t) plot was defined as the switching time (Figure 3a).

Ferroelectric Switching Time of P(VDF-TrFE) Film

Figure 2. Sequence of electric field pulses used to induce polarization switching on the device. The field strength is plotted against time and ∆t is kept constant throughout the experiment to maintain consistency. The first pulse was fixed at -100 MV/m and 0.5 ms to induce homogeneous initial polarization. The second and third pulses were varied in magnitude, and the pulse wide was set accordingly to allow complete polarization switching. The responses from polarization, both the switching and the nonswitching contributions, were recorded accordingly. The switching time data is extracted from the net polarization against time plot.

Figure 3 shows the switching time of the device as a function of electric field for both reference and nanoparticle-blended samples. The switching time for reference P(VDF-TrFE) samples strongly depends on the applied field. As the electric field increases, the switching time decreases from around 20 ms at 50 MV/m down to 20 µs at 80 MV/s. The dependence of switching time τs on field strength E can be modeled by the exponential law, as confirmed by the linear fitting between log switching times against 1/E plot

In our experiment, the gradient of log τs against 1/E corresponds to the activation field Ea of 0.5 GV/m as obtained from linear fitting of the switching time data for the pristine P(VDF-TrFE). These results follow closely with the observations made by Furukawa et al. and Nakajima et al. on similar devices.17,19 In contrast, the switching time for P(VDF- TrFE)-gold nanoparticles blend samples shows pronounced enhancement, especially at low and moderate field strength. Significant enhancement of switching time is achieved even at low nanoparticles content of 1 × 10-6 wt %. The P(VDF- TrFE)-nanoparticles blend sample switches 10 times faster at 70 MV/m compared to the reference sample. The enhancement ratio is as high as 1000 times at 50 MV/m. The difference becomes negligible at higher electric fields above 80 MV/m. The activation field for the Au NP-blended P(VDF-TrFE) is found to be 10 times lower at 30 MV/m.

From the findings, we consider the following possible causes responsible for the observed enhancement in switching time, including generation of additional free charges into the system, alteration of the crystallinity, and phase structure of P(VDFTrFE) as well as promotion of heterogeneous domain nucleation at lowered activation field in the presence of nanoparticles. The presence of the nanoparticles may introduce additional free charges required to compensate and stabilize the polarization domain. However, in our case, the amount of nanoparticles is at the minute concentration of 1 × 10-6 wt % (approximately one nanoparticle in every 125 µm3). Considering only a few free charges can be supplied from such nanoparticles,24 it is unlikely that the presence of additional free charges is the main cause for the observed switching time enhancement.

To investigate the crystallinity and the phase structure of P(VDF-TrFE) in the presence of nanoparticles, a 2D XRD analysis was performed and the integrated intensity diffraction results are presented in Figure 4a. From the XRD diffraction results, coexistence of paraelectric R and ferroelectric phases at much higher amount is observed for the samples with gold nanoparticles. The relative amount of both phases does not

change much with the addition of gold nanoparticles, except for a very high concentration at 1 × 10-2 wt %. It is noteworthy from the 2D XRD patterns that the (110) and (200) planes of the phase in the nanoparticle-blended P(VDF-TrFE) samples show preferential orientation (Figure 4b). In the presence of the nanoparticles, these (200) and (110) planes oriented at γ angle of about 90°. It means that most of the planes oriented nearly parallel to the substrate surface. On the other hand, the (200) and (110) planes from the pristine P(VDF-TrFE) show random orientation. Such preferential orientation is advantageous as the effective electric field acting on the planes is at its optimum, whereas for the randomly oriented plane, only portion of electric field is actually acting on the planes. This leads to the difference in the effective electric field acting on the ferroelectric (200) and (110) planes, whereby it would be perceived stronger in the case of nanoparticle-blended P(VDFTrFE) samples.

Another plausible mechanism responsible for the observed enhancement in switching time is that the nanoparticles act as heterogeneous nucleation centers for ferroelectric domains during the polarization switching. Comparing the switching time plot for both samples (Figure 3), in the presence of the nanoparticles the activation field for polarization reversal reduces significantly from 0.5 GV/m to 30 MV/m. The obtained polarization reversal activation field is closer to those of growthlimited (87 MV/m) rather than the nucleation-limited ( 1 GV/ m) regimes.20,21 This is an indication that the polarization reversal behavior has been shifted from the domain nucleationlimited into the domain growth-limited regime. To investigate the ferroelectric domain structure of P(VDF-TrFE), PFM technique was employed. Figure 5 presents the ferroelectric domain structure for pristine (left) and nanoparticle-blended (right) P(VDF-TrFE). The color contrasts indicate domains with opposite polarization direction. Higher contrast found in the nanoparticle-blended P(VDF-TrFE) samples indicates strong disparity between opposite polarization direction. This is consistent with the XRD results whereby the ferroelectric (200) and (110) planes are preferentially aligned parallel to the substrate surface. In addition, the region between the two domains (i.e., the phase boundary) for nanoparticles-blended samples shows sharper transition. As for the pristine P(VDFTrFE), a diffuse phase boundary is observed instead. The movement of domain walls for sample with lower activation field will proceed at faster rate, resulting in a sharp-edged phase

Figure 5. Lateral PFM phase image for pristine (left) and nanoparticleblended P(VDF-TrFE) (right). The contrast indicates ferroelectric domains with opposite polarization direction.

boundary being detected in the phase image. The sharp contrast indicates the polarization domain growth process whereas diffuse contrast implies higher activation energy is required for the switching, an indication of ferroelectric domain nucleationlimited regime.21,25 This enhanced heterogeneous domain nucleation has been considered to play an important part in the enhancement of switching time in addition to the apparent enhanced alignment of the ferroelectric (200) and (110) planes which are deemed responsible for the observed enhanced switching. These insights provide the impetus for further development and realization of fast responsive ferroelectric polymers that are valuable in various applications ranging from fast writing-reading organic ferroelectric memories and rapid sensing capability in sensor and actuator to the rapid charge- discharge cycle in high energy storage applications.

Conclusion

We have demonstrated a facile method to improve the ferroelectric switching behavior of P(VDF-TrFE). Addition of gold nanoparticles has been shown to significantly enhance the switching time of P(VDF-TrFE) by more than 3 orders of magnitude, especially at moderate and low field strength. Comparable dielectric constant and leakage current behavior with the pristine samples can be achieved for gold nanoparticle concentration below 10-4 wt %, above which higher leakage starts to take place. We have successfully established a significant enhancement in switching time with an optimum concentration of Au nanoparticles in the P(VDF-TrFE) blends. This has been attributed to the preferred crystal orientation and heterogeneous ferroelectric domain nucleation.

Ferroelectric Switching Time of P(VDF-TrFE) Film

Acknowledgment. The work is financially supported by the Ministry of Education (MOE2009-T2-1-045). We gratefully thank Prof. T. Furukawa for insightful discussion.

Supporting Information Available: Current-voltage (I-V) and capacitance-voltage (C-V) graphs depicting the leakage current and dielectric behavior of Au nanoparticle-blended samples from pristine to 1 × 10-4 wt %, including the 1 × 10-5 and 1 × 10-6 wt % concentration. This material is available free of charge via the Internet at http://pubs.acs.org.

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