Improved Thermal Stability of Ferroelectric Phase in Epitaxially Grown

May 6, 2016 - In recent years ferroelectric polymers have attracted much attention due to their potentials in flexible electronics. To satisfy the req...
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Improved Thermal Stability of Ferroelectric Phase in Epitaxially Grown P(VDF-TrFE) Thin Films Zongyuan Fu,† Wei Xia,† Weibo Chen,‡ Junhui Weng,† Jian Zhang,‡ Jianchi Zhang,‡ Yulong Jiang,*,‡ and Guodong Zhu*,† †

Department of Materials Science and ‡School of Microelectronics, Fudan University, Shanghai, China S Supporting Information *

ABSTRACT: In recent years ferroelectric polymers have attracted much attention due to their potentials in flexible electronics. To satisfy the requirements of low operation voltage and low power consumption, it is required to reduce the ferroelectric film thickness down to, for example, 100 nm. However, decreased film thickness results in low crystallinity and thus worse electrical performance. One possible solution is to realize the epitaxial growth of ferroelectric thin films via effective control of structure and orientation of ferroelectric crystals. Here we report our work on poly(tetrafluoroethylene)-template-induced ordered growth of ferroelectric thin films. We focus on the study of thermal stability of ferroelectric phase in these ferroelectric films. Our work indicates that epitaxial growth effectively increases the crystallinity and the melting and ferroelectric phase transition temperatures and implies the extended application of ferroelectric devices at higher temperature.

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crystallinity of epitaxially grown P(VDF-TrFE) films was only 70%, almost similar to that of a film directly deposited on rigid substrates, and got much small remanent polarization of 0.017 C/m2,11 while Krüger et al. emphasized that their epitaxially grown ferroelectric films were almost 100% crystallized and got remanent polarization of 0.11 C/m2,10 consistent with the expected value from PVDF crystals.12 Further work is required to well understand the influence of epitaxial growth on structural and electrical performances in ferroelectric films. Here we reported our work on the study of thermal stability in epitaxially grown P(VDF-TrFE) thin films aiming to illuminate the influence of epitaxy on phase transition behaviors of such ferroelectric films.

INTRODUCTION Organic electronics has exhibited its great potential for largearea electronic applications such as flexible displays and smart labels, in which organic nonvolatile memories are one of key elements.1 Among all reported organic memories, organic ferroelectric memories have been regarded as a promising technology for low-cost flexible memories. Ferroelectric polymers, such as poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)), and organic semiconductors, such as pentacene and P3HT, are integrated to form ferroelectric field-effect transistors, in which the polarization states of ferroelectric layer modulate the electrical conductivity of semiconducting channels realizing nonvolatile memory function.2 Because of the requirement of low voltage operation and also the large coercive field as high as 50 MV/m for P(VDFTrFE), less than 100 nm thick ferroelectric films are required. However, with the decrease of film thickness, both the degree of crystallinity and polarization decrease while leakage current increases, resulting in degraded electrical performance of ferroelectric devices.3,4 One possible solution is to realize the epitaxial growth of ferroelectric thin films via the effective control of structure and orientation of ferroelectric crystals. Epitaxial growth of polymer films has been realized on both inorganic, such as alkali halides,5 and organic substrates, such as polymethylene.6 Wittmann and Smith developed a friction transfer method to fabricate highly ordered poly(tetrafluoroethylene) (PTFE) templates7 on which highly oriented block copolymer8 and organic semiconducting9 films were well epitaxially grown. Recently, such PTFE templates were successfully extended for the epitaxial crystallization of ferroelectric P(VDF-TrFE) copolymer thin films.10,11 However, different, or even contrary, results were also reported by different groups. For example, Park et al. argued that the © XXXX American Chemical Society

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RESULTS AND DISCUSSION PTFE templates were fabricated according to the friction transfer method. The thickness of these PTFE films highly depends on all three parameters of pressure, temperature, and sliding speed during friction transfer and can vary from several to tens of nanometers. The morphology of one typical friction transferred PTFE template is shown in Figure 1a, which shows highly oriented structure. The maximum vertical distance between the highest and lowest data points is about 7.76 nm, and the root-mean-square roughness is only about 2.41 nm. A typical line profile of such PTFE template is shown in Figure S1a, in which are shown the thorn-like ridges with height undulation of several nanometers. XRD analysis in Figure 2a indicates that such PTFE films are single crystal like with only Received: March 16, 2016 Revised: April 26, 2016

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Figure 1. AFM morphologies and DSC analysis of P(VDF-TrFE) films and material. (a) AFM morphology of PTFE template. (b, c) AFM morphologies of P(VDF-TrFE)/PTFE thin films which were annealed at 135 and 180 °C, respectively. (d, e) AFM morphologies of P(VDF-TrFE)/ Si films which were annealed at 135 and 180 °C, respectively. (f) DSC analysis of P(VDF-TrFE) material. Arrows in (a−c) indicated the friction directions during the fabrication of PTFE templates. Typical line profiles for (a)−(e) are shown in Figure S1.

Figure 2. XRD analyses of PTFE template and P(VDF-TrFE) films. (a) XRD spectra of the rubbed PTFE templates and P(VDF-TrFE)/PTFE films. (b) XRD spectra of P(VDF-TrFE) films directly deposited on bare Si substrates. Before XRD measurements, ferroelectric films were annealed at various temperatures. Inset in (b) is the comparison of XRD spectra of both P(VDF-TrFE)/PTFE and P(VDF-TrFE)/Si films annealed at 135 °C.

one sharp characteristic peak at 2θ ≈ 18.1°, corresponding to (100) reflection of PTFE crystals. P(VDF-TrFE) thin films were spin-coated onto these PTFE templates. Annealing treatment was required to realize the epitaxial growth of P(VDF-TrFE) films on rubbed PTFE templates. For simplification, such epitaxially grown ferroelectric films are marked as P(VDF-TrFE)/PTFE films. Without special declaration, the thickness of all P(VDFTrFE) films was about 380 nm. In Figures 1b and 1c are shown the AFM topographies of such P(VDF-TrFE)/PTFE thin films which are annealed at temperatures of 135 and 180 °C, respectively. The corresponding line profiles are shown in Figures S1b and S1c. The surfaces of both annealed films show highly ordered and parallel stripe-like crystallites, the direction of which is perpendicular to the friction direction of PTFE templates beneath. The dimensions of these crystallites are about 0.5−1.2 μm of length and 80−100 nm of width. Further XRD analyses in Figure 2a indicate that for all P(VDF-TrFE)/

PTFE films annealed at 135, 150, and 180 °C, characteristic peaks occur at 2θ ≈ 19.8°, corresponding to the superimposition of (200) and (110) reflections of ferroelectric βpolar phase.13 Note that due to the preferred orientation of P(VDF-TrFE) crystallites on PTFE templates, X-ray spectra should be dependent on the relationship between incident XRD beam and friction direction of PTFE templates. Therefore, during XRD measurements of P(VDF-TrFE)/ PTFE films, all samples were carefully positioned so that the incident X-ray beam was parallel to the friction direction of PTFE templates. All XRD spectra also present a peak at 2θ ≈ 18.1°. This peak is attributed to the (100) reflection of PTFE templates beneath P(VDF-TrFE) films. Furthermore, annealing treatment at higher temperature, for example 180 °C, results in the decrease of peak intensity, indicating the slight decrease of crystallinity. In fact, based on such highly ordered PTFE templates, even thinner P(VDF-TrFE) films could be expected. In Figure S2 is shown the AFM morphology of P(VDF-TrFE) B

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Figure 3. Local thermal analyses of P(VDF-TrFE) thin films. (a, b) AFM morphologies of P(VDF-TrFE)/PTFE and P(VDF-TrFE)/Si thin films, respectively. (c) The corresponding nanoTA deflection−temperature curves of both films. The solid circles in (a, b) indicate where the local thermal analyses were performed. Both films were annealed at 135 °C before these measurements.

growth of P(VDF-TrFE) films on PTFE templates and argued that their degree of crystallinity was probably almost 100%.10 Though from the XRD and DSC analyses and also from AFM morphology in Figure 1e we understand that annealing at 180 °C induces the melting and recrystallization of P(VDFTrFE) films directly deposited on Si substrates, we cannot arbitrarily judge that the same process occurs for those epitaxially grown P(VDF-TrFE) films. Both crystallinity and substrates can influence the thermodynamic behavior of thin films. It is much difficult to determine the Tm of such thin films via the DSC method, so here we conduct AFM-based local thermal analysis, i.e., nanoTA mode, to characterize the thermodynamic behaviors of both P(VDF-TrFE)/PTFE and P(VDF-TrFE)/Si films at nanoscale. Local thermal analysis is based on the thermomechanical response and used to determine the local glass transition T g or melting T m temperatures of various polymer materials and films, the mechanism of which can refer to those reported work.16,17 In brief, the AFM tip is placed on the surface of the sample, and then the temperature of the heated tip is ramped with time, while the cantilever bending due to thermal expansion of the film is monitored. At the point of phase transition, the material beneath the tip softens and the tip penetrates into the sample. Thus, we can measure the characteristic temperatures of the sample such as Tg or Tm. typical nanoTA deflection− temperature curves are shown in Figure 3, which are obtained on the crystallites of both P(VDF-TrFE)/PTFE and P(VDFTrFE)/Si films. As for P(VDF-TrFE)/Si films, an obvious turning point occurs at 141 °C in the deflection−temperature curve, which should be attributed to the melting point. This value is a little smaller than that of 149.6 °C from DSC ayalysis shown in Figure 1f, which should be due to the difference of operation mechanisms of macroscopic DSC and microscopic nanoTA analyses. As for P(VDF-TrFE)/PTFE films, the turning point occurs at 158 °C. From both results, we confirm that the epitaxial growth of P(VDF-TrFE) thin films induces the increase of Tm up to 158 °C, 17 °C higher than that of annealed P(VDF-TrFE) film deposited directly on bare substrates. The increase may be attributed to the increase of crystallite size, which can be proved by the AFM observation in Figure 1 and also by the XRD results in Figure 2. Though the width (80−100 nm) of stripe-like crystallites in P(VDF-TrFE)/ PTFE films (Figure 1b) is a little smaller than that (100−140 nm) of P(VDF-TrFE)/Si films (Figure 1d), the length (0.5− 1.2 μm) of crystallites in P(VDF-TrFE)/PTFE films is much

thin film with thickness of about 70 nm. Such thinner P(VDFTrFE)/PTFE films still showed obviously ordered and parallel stripe-like crystallites just like those shown in Figure 1b. However, with the further decrease of film thickness, for example, to less than 50 nm, due to dewetting between PTFE and P(VDF-TrFE), large holes usually occur inside these thin films, resulting in bad film quality. We also deposited P(VDF-TrFE) films directly on bare Si substrates, marked as P(VDF-TrFE)/Si films, in order to emphasize the influence of PTFE templates. DSC analysis shown in Figure 1f indicates that the phase transition (Tc) and melting (Tm) temperatures of the P(VDF-TrFE) material are about 105 and 149.6 °C, respectively. Ferroelectric films deposited on bare Si substrates were annealed at 135 and 180 °C, respectively. One temperature was a little below Tm to increase film crystallinity, and the other was above Tm to melt and recrystallize the film. Interestingly, the surface of P(VDFTrFE) films shows disordered and rod-like crystallites with much smaller length of 250−400 nm and larger width of 100− 140 nm after annealing treatment at 135 °C (Figure 1d), while, after annealing treatment at 180 °C (Figure 1e), these P(VDFTrFE) thin films melt and recrystallize into large and straight ridges, resulting in nearly complete loss of characteristic peak of ferroelectric phase as seen in Figure 2b. From the XRD spectra in Figure 2b, the as-cast film shows neglectable crystallinity. After the annealing treatment at 135 °C, the crystallinity of ferroelectric phase greatly enhances. With further increase of annealing temperature to near Tm, such as 150 °C, part of film recrystallizes, resulting in the decrease of the crystallinity of ferroelectric phase. Even higher annealing temperature, such as 180 °C, results in the nearly complete loss of ferroelectric phase. In fact, melting-induced loss of ferroelectric phase has been reported by several groups.14,15 Park et al. observed the irreversible extinction of ferroelectric polarization in P(VDFTrFE) thin films upon melting and recrystallization, which was attributed to the rotation of b-axis to the direction perpendicular to the electric field.14 The inset in Figure 2b is shown the XRD spectra obtained from both P(VDF-TrFE)/PTFE and P(VDF-TrFE)/Si thin films, both of which were annealed at 135 °C. Obviously P(VDF-TrFE)/PTFE film shows much stronger diffraction at 2θ ≈ 19.8°, which may imply that epitaxially grown ferroelectric films obtain well-improved crystallinity. This result is supported by a previous observation where the authors utilized the modified Bridgman crystal growth method for the epitaxial C

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Figure 4. Varied-temperature XRD analyses of P(VDF-TrFE) thin films. (a−c) The temperature dependences of the X-ray (200, 110) reflection. (d−f) The lattice spacing as a function of temperature. P(VDF-TrFE)/PTFE films were annealed at (a, d) 135 °C and (b, e) 180 °C (c, f). As a control sample, P(VDF-TrFE) film was directly deposited on Si substrate and then annealed at 135 °C.

is expected that such P(VDF-TrFE)/PTFE films can endure even higher temperature as long as PTFE templates are preserved. Note that here the melting point is obtained from PTFE rod and should be different to some extent from that of friction transferred PTFE templates. It is difficult to determine the melting point of such PTFE templates by the local thermal analysis method above because PTFE films are too thin for nanoTA measurement to get good signal-to-noise ratio. To further validate the expectation that such epitaxially grown ferroelectric P(VDF-TrFE) films can endure higher temperature, P(VDF-TrFE)/PTFE films were annealed at 220, 290, and 340 °C, and then XRD and AFM measurements were performed to determine phase and structural characteristics of these high-temperature annealed films. Results are shown in Figure S3. Films annealed at 220 and 290 °C still display obvious characteristic peaks of ferroelectric phase at 2θ ≈ 19.8° (Figure S3b) and ordered surfaces with parallel stripe-like crystallites (Figure S3c). Annealing at even higher temperature of 340 °C results in the melting of PTFE templates, which damages the structural regularity of PTFE templates and thus results in the failure of epitaxial growth of P(VDF-TrFE) films. The XRD spectrum in Figure S3b indicates great degradation of the degree of crystallinity for both PTFE and P(VDF-TrFE) films, while the AFM result in Figure S3d also shows the nearly complete loss of ordered stripe-like crystallites. All these results

larger than that (250−400 nm) of P(VDF-TrFE)/Si films. Furthermore, from the XRD spectra of both P(VDF-TrFE)/ PTFE and P(VDF-TrFE)/Si films in the inset of Figure 2, the crystallite size can be roughly estimated according to the Scherrer relation D = 0.9λ/(cos(θ)β),18 where D is the averaged crystallite size, β is the full width at half-maximum (fwhm) of the crystalline peak, and λ is the wavelength of the X-ray (λ = 1.5418 Å). The calculated crystallite sizes are 13.80 nm for P(VDF-TrFE)/PTFE films and 13.08 nm for P(VDFTrFE)/Si films. So both AFM and XRD results validate the increase of crystallite size, which contributes to the increase of melting point. Now we can further prove that 180 °C is high enough to cause the melting and recrystallization of P(VDF-TrFE)/PTFE thin films. According to the observations in Figures 1 and 2, it is convenient to conclude that due to the existence of PTFE templates, even if P(VDF-TrFE) films suffer high temperature above their melting point, they can still recrystallize into ferroelectric phase with high degree of crystallinity, much different from those ferroelectric films deposited directly on substrates. This observation was also reported by a Korean group, though that work lacked a direct proof on whether such epitaxially grown ferroelectric films really melted.11 Because of the high melting point (∼329 °C) of PTFE material which was measured by DSC method (Figure S3a), it D

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Macromolecules imply that P(VDF-TrFE)/PTFE films can retain its ferroelectric phase with high crystallinity as long as the annealing temperature they suffer is lower than the melting point of PTFE templates. This fact greatly improves the thermal stability of P(VDF-TrFE) related ferroelectric devices. Next we consider ferroelectric−paraelectric phase transition of epitaxially grown P(VDF-TrFE) thin films. Phase transition temperature Tc is a crucial factor for ferroelectric materials which determines the temperature at which ferroelectric devices can work. Undoubtedly, we hope for a high Tc to extend the operation temperature range. The ferroelectric phase transition was first discovered in 1980 for P(VDF-TrFE) with 55% VDF content,19 which displayed a characteristic of second-order transition.20 Extensive studies indicated that the transitional behavior of P(VDF-TrFE) was dependent on VDF content. With an increment of VDF content the transition point shifted to the higher temperature side and the thermal hysteresis becomes larger, characteristic of first-order transition, for example, for the copolymer with VDF content 70−80%.21 We conducted varied-temperature XRD analyses to study the influence of PTFE-based epitaxy on ferroelectric phase transition. Typical results are shown in Figure 4, where three kinds of P(VDF-TrFE) films were studied: 135 and 180 °C annealed P(VDF-TrFE)/PTFE films and 135 °C annealed P(VDF-TrFE)/Si film. From XRD spectra in Figure 4a−c, we find that all these three samples show the same changes with the temperature. During heating process, at relatively low temperature, only the ferroelectric low-temperature (LT) phase exists with characteristic peak at 2θ ≈ 19.8°, corresponding to the X-ray (200, 110) reflections. The peak shifts slightly toward lower 2θ side due to thermal expansion of lattice spacing (Figure 4d−f). Note that in Figure 4a,b we observe again the (100) reflection of PTFE templates at 2θ ≈ 18.1°. As the temperature rises close to the transition region, the X-ray reflections of the LT phase decrease in intensity and new reflections at 2θ ≈ 18.0° appear at the positions different from those of the original LT phase. These new peaks correspond to the paraelectric high-temperature (HT) phase and also shift to lower 2θ side with increased temperature due to thermal expansion. At high enough temperature, the LT phase disappears and only strong reflections of HT phase exist in XRD spectra. During the cooling process, the reverse situation occurs. With the decrease of temperature, the reflections of HT phase decrease and then disappear, while the reflections of LT phase appear at a certain temperature and then get stronger with decreased temperature. Varied-temperature XRD analyses indicate that for all three samples the XRD spectra change clearly and reversibly between the LT and HT phases. No evidence indicates the existence of cooled (CL) phase. This observation is a little different from the results from thick P(VDF-TrFE) films with VDF content of 72%,21 where the authors observed the reversible phase transition between LT and HT phases for oriented P(VDF-TrFE) thick films while for unoriented P(VDF-TrFE) films, they found the existence of CL phase though its XRD reflections were much weak compared to the LT and HT phases. Though the XRD analyses indicate that all three samples show the same changes with the temperature, the temperature ranges of ferroelectric phase transition are a little different from each other. Here we especially care about four temperature points of phase transition: Tc0H, the temperature at which the HT phase occurs during heating process; Tc1H, the temperature at which the LT phase completely disappears during heating

process; Tc0C, the temperature at which the LT phase occurs during cooling process; and Tc1C, the temperature at which the HT phase completely disappears during cooling process. Tc0H and Tc1H are especially important which determine at how high temperature the ferroelectric devices can work. What makes the quantitative analyses difficult is the superposition of the XRD reflections both from HT phase of P(VDF-TrFE) and from the (100) reflection of PTFE templates. Fortunately, compared with the reflections of HT phase, the (100) reflections of PTFE templates are much weaker and less dependent on temperature. So we can determine the four phase transition temperatures according to the sharp changes in the reflections of both LT and HT phases with temperature. The four temperatures are listed in Table 1 for all three ferroelectric samples. The data Table 1. Comparison of Tc0H, Tc1H, Tc0C, and Tc0C for All Three Ferroelectric Samplesa samples 135 °C-P(VDF-TrFE)/Si 135 °C-annealed P(VDF-TrFE)/ PTFE 180 °C-annealed P(VDF-TrFE)/ PTFE a

Tc0H (°C)

Tc1H (°C)

Tc0C (°C)

Tc1C (°C)

80 95

105 115

65 65

50 50

75

100

60

50

Data are extracted from the xrd analyses shown in Figure 4.

show obvious thermal hysteresis between heating and cooling processes, supporting the first-order phase transition type. Furthermore, all three P(VDF-TrFE) samples show obviously different temperature points especially on the heating process. For annealed P(VDF-TrFE)/Si film, on the heating process, phase transition begins at 80 °C (Tc0H) and finishes at 105 °C (Tc1H, at which ferroelectricity is completely lost), while for 135 °C-annealed P(VDF-TrFE)/PTFE film, Tc0H shifts up to 95 °C and the complete loss of ferroelectricity occurs at 115 °C (Tc1H). Tc0H gets an effective increment by 15 °C, indicating the extended operation temperature for P(VDF-TrFE)/PTFEbased devices. However, as for 180 °C-annealed P(VDFTrFE)/PTFE film, data show no obvious improvement for both Tc0H and Tc1H. The transition behavior in P(VDF-TrFE) films can be influenced by several factors, such as crystallite and domain sizes,22−24 the number of defects,23,24 and the VDF content.21 As for our epitaxially grown P(VDF-TrFE) thin films, crystallite and domain sizes and defects in crystallites may do the major contribution to the Curie temperature. In fact, the influence of annealing treatment on Tc has been comprehensively studied. Odajima studied the dependence of Tc of P(VDF-TrFE) copolymer with VDF 73 molar % on annealing temperature and found that Tc increased for larger crystallite size.22 Ohigashi further interpreted the change in Tc on the basis ferroelectric domains in crystallites and believed that the formation of large domains resulted in the increase of Tc.23 Tashiro further indicated that the defects in crystallites, i.e., trans−gauche conformational disorder, resulted in the decrease of the transition temperature.24 As for the 135 °C-annealed P(VDFTrFE)/PTFE film, XRD and AFM results have proved the increase of crystallite size. Though there is no evidence of simultaneous increase of ferroelectric domain size, it is expected so according to Tashiro’s illumination that annealing at a temperature higher than Tc resulted in the growth of ferroelectric domains accompanied by the increase of trans− gauche conformational disordering. With no measurements of E

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and 180 °C-annealed P(VDF-TrFE)/PTFE films can only work up to 91.8 °C. Finally, we reported the ferroelectric and piezoelectric properties of such epitaxially grown P(VDF-TrFE) thin films. The P(VDF-TrFE)/PTFE thin film was annealed at 135 °C, and its ferroelectric switching process was recorded by homemade Sawyer−Tower circuit. A typical result is shown in Figure 6a. Switching current peaks occurs at −22.4 and 20.7

Raman and IR spectra, here we cannot estimate the number of trans−gauche conformational disorders. However, our experimental observations imply that the positive influence of increased crystallite and domain sizes on Tc is dominant over the negative contribution of increased trans−gauche conformational disordering when films are annealed between Tc and Tm of ferroelectric films. For 180 °C-annealed P(VDF-TrFE)/ PTFE film, the melting and recrystallization process may induce more defects, such as trans−gauche conformational disorder, inside crystallites which is expected to result in the decrease of transition temperature.23,24 In Figure 4d−f is shown the temperature dependence of LT and HT lattice spacing for all three samples. LT and HT phases have obviously different lattice spacing, which have been proved by IR and Raman spectra.25,26 The low-temperature ferroelectric phase consists of polar packing of all-trans chains, while the high-temperature paraelectric phase is structured by an arrangement of gauche-type chains with a sequence of TG, TG̅ , TTTG, and TTTG̅ isomers. Because of this conformational change, the lattice spacing expands largely in the hightemperature phase. Lattice spacing of both LT and HT phases increases linearly with temperature due to thermal expansion. From Figure 4d−f, it is convenient to extract those data shown in Table 1. It is supposed that the areas of ferroelectric characteristic peaks at 2θ ≈ 19.8° reflect the content of ferroelectric phase; thus, it is possible to quantitatively determine the change of ferroelectric phase with temperature. In Figure 5 is shown such

Figure 5. Temperature dependence of the normalized areas of characteristic peaks of the LT ferroelectric phase.

results from all three ferroelectric films. Obviously, at relatively lower temperature, the area slightly decreases with temperature, indicating the slight degradation of ferroelectricity. When temperature reaches Tc0H, the area begins to sharply decrease down to zero at Tc1H, indicating the quick degradation of ferroelectricity. On the cooling, the reverse situation occurs, but the heating and cooling curves do not follow the same trace but show large thermal hysteresis, the characteristic of first-order phase transition. The widths of thermal hysteresis for both annealed P(VDF-TrFE)/Si and 180 °C-annealed P(VDFTrFE)/PTFE films are about 30 °C, while for 135 °C-annealed P(VDF-TrFE)/PTFE films, this value expands to 40 °C. Furthermore, if we suppose the temperature corresponding to 30% attenuation as the lower limit for the operation of ferroelectric devices, the 135 °C-annealed P(VDF-TrFE)/ PTFE film can work up to 104.5 °C while P(VDF-TrFE)/Si

Figure 6. Ferroelectric and piezoelectric properties of P(VDF-TrFE) thin films. (a) Ferroelectric switching current and P−V hysteresis loops of P(VDF-TrFE)/PTFE film. (b) The local piezoelectric properties of both P(VDF-TrFE)/Ni and P(VDF-TrFE)/PTFE/Ni films. The bottom panel shows the schematic diagram of triangular voltage used to drive ferroelectric film to vibrate.

V, corresponding to an apparent coercive field of 57 MV/m. Here we use “apparent” because not all external voltage is applied to the P(VDF-TrFE) layer due to the voltage drop on PTFE layer. The polarization−voltage hysteresis loop is obtained by integrating the switching current with time. Here we get a remanent polarization Pr of 0.064 C/m2, a little lower than its expected value of 0.11 C/m2 for PVDF crystals.12 We note that an even lower Pr value of 0.017 C/m2 was reported on P(VDF-TrFE)/PTFE films with both layer thicknesses of 40 F

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directly on rigid substrates, epitaxially grown P(VDF-TrFE) films showed obviously increased melting and ferroelectric− paraelectric phase transition temperatures by about 17 and 15 °C, respectively. Furthermore, due to the existence of ordered PTFE templates, even if ferroelectric films melted, these films could still recrystallize into ferroelectric phase with high crystallinity. Such improved thermal stability of ferroelectric phase implied that the operation temperature could be extended to even higher temperature for ferroelectric polymer based devices.

and 30 nm, respectively and the authors attributed this low value both to the voltage drop on PTFE layer and also to the different molecular orientation of epitaxially grown P(VDFTrFE) lamellae.11 They suggested a structural model in which the polar axis was perpendicular to the electric field applied parallel to the surface normal of the film, resulting in a 13% decrease of polarization compared to the expected polarization of P(VDF-TrFE) crystals whose b-axis was completely parallel to the electric field.12 However, we have to further notice another work from one German group;10 on PTFE templates the researchers obtained 4 μm thick and ordered P(VDF-TrFE) films and observed Pr value as high as 0.11 C/m2, well consistent with the expected value. These authors argued that their films were nearly 100% crystallized. These different experimental results from both groups may be due to different film thickness and/or different preferred orientation of P(VDFTrFE) crystallites. Further work is required to well understand the complicated correlation between structural and electrical properties. In Figure 6b are shown the observations of local piezoelectricity recorded by the conductive AFM probe. The voltages used to polarize ferroelectric films were supplied by the AFM system and were always applied to the bottom Ni electrode while conductive AFM probe was electrically grounded. Because of maximum output voltage limit, thinner ferroelectric films were prepared from 1.0 wt % solution, resulting in film thickness of about 70 nm. Before local piezoelectric measurements, the local area of 4.0 × 4.0 μm2 of ferroelectric films was scanned, and simultaneously a voltage of +10 V was applied between Ni electrode and AFM probe to polarize the local area. Then probe was positioned on one polarized crystallite and positive triangular voltage with peak-topeak amplitude of 10 V, and frequency of 0.5 kHz was applied to the bottom electrode. A schematic diagram of this triangular voltage is shown in the bottom panel of Figure 6b. Note that here the polarity of triangular voltage was the same as that of poling voltage to avoid polarization reversal caused by this driving voltage. Here we neglected the depolarization after the removal of poling voltage since triangular voltage was immediately applied for piezoelectric measurement. The voltage-driven changes in film thickness (i.e., vibration in Figure 6b) were recorded as a function of time. From both films of P(VDF-TrFE)/Ni and P(VDF-TrFE)/PTFE/Ni, AFM probe detects obvious local vibrations with averaged peak-topeak value of 0.42 and 0.41 nm, corresponding to reverse piezoelectric constants d33 of −0.42 and −0.41 Å/V, respectively. Note that both values are a little lower than the expected value of −0.46 Å/V, which was calculated from molecular simulation of PVDF crystals.27 The difference can be attributed to imperfect crystallization and also the introduction of TrFE into VDF polymer. Anyway, from local piezoelectric measurements, we obtain the similar d33 values from epitaxially grown film and also the control sample. However, we have to mention that the voltage applied to the epitaxially grown film is less than 10 V due to the voltage drop on PTFE layer. It is expected the real d33 value of such epitaxially grown films may be even closer to the expected one.

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EXPERIMENTAL SECTION

PTFE rods and ferroelectric polymer P(VDF-TrFE) (VDF/TrFE molar ratio: 70/30) were purchased from NICHIAS, Japan, and Kunshan Hisense Electronic Co., respectively, and used as received. Highly doped n-type silicon wafers (n+-Si) with resistivity of 0.001 Ω cm were used as substrates, and before deposition of PTFE templates, the native oxide was removed by HF solution. Highly ordered thin PTFE films were fabricated according to the reported friction transfer method.7 In brief, PTFE thin films were deposited on the cut n+-Si substrates by pressing a PTFE rod with a pressure of about 1.0 MPa and sliding it against the n+-Si substrates with a rate of 0.5 mm/s, during which the surface temperature of the substrates was kept at 120 °C. The deposited PTFE films were about 5−15 nm thick, which were determined by atomic force microscope (AFM, Dimension Edge, Bruker Inc.). Ferroelectric P(VDF-TrFE) thin films were then deposited by the spin-coating method from a 4.0 wt % solution of 70/30 P(VDF-TrFE) in cyclohexanone onto these PTFE-coated substrates and then annealed at various temperatures to study the influence of annealing treatment on film structures and electrical properties. The thickness of such annealed ferroelectric films was about 380 nm. Note that we chose cyclohexanone as the solvent because continuous P(VDF-TrFE) thin films could be easily coated on PTFE templates from its cyclohexanone solution. As control samples, P(VDF-TrFE) thin films were also directly spin coated from a 4.0 wt % solution onto Si substrates. The revolution speed of the spin coater was elaborately controlled to make sure that the control samples had nearly the same film thickness of 380 nm as epitaxially grown films did. Even thinner ferroelectric films were also prepared on PTFE templates from a 1.0 wt % solution, resulting in film thickness of about 70 nm. Note that to make sure that all ferroelectric films were annealed under the same condition, we carefully controlled the annealing parameters. Films were annealed in an oven, which was heated to the preset temperature at a rate of 5 °C/min. Films kept at this preset temperature for 5 h and then slowly cooled down to room temperature at cooling rate of 5 °C/min. The melting temperature Tm of P(VDF-TrFE) and PTFE materials was determined by differential scanning calorimetry (DSC, PerkinElmer Inc.). Structural and morphological characteristics were determined by AFM and XRD (D8, Bruker Inc.). Note that during XRD measurements of P(VDF-TrFE)/PTFE films, all samples were carefully positioned so that the incident X-ray beam was parallel to the friction direction of PTFE templates. To determine the local melting temperature of ferroelectric thin films, local thermal analyses were conducted by nanoTA mode of AFM (AFM+, Anasys Inc.). Before thermal analysis, the probe temperature was calibrated by polystyrene and poly(methyl methacrylate) standards. For polarization−voltage (P−V) measurements of epitaxially grown thin films deposited from a 4.0 wt % solution, before friction transfer of PTFE templates, 50 nm thick nickel films were thermally deposited onto n+-Si substrates and then PTFE and P(VDF-TrFE) layers were deposited. Finally top circular Al electrodes with diameter of 0.5 mm were thermally deposited onto the annealed ferroelectric films. P−V measurements were performed by homemade Sawyer−Tower circuit. Local piezoelectric properties were determined by AFM. Voltages were supplied by the AFM system and applied between conductive probe and substrate to polarize the ferroelectric films and also to drive the films to vibrate. Because of the maximum output voltage limit

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CONCLUSION In summary, we fabricated epitaxially grown ferroelectric P(VDF-TrFE) thin films on friction transferred PTFE templates, in which improved thermal stability of ferroelectric phase was observed. Compared to those films deposited G

DOI: 10.1021/acs.macromol.6b00532 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (±10 V) of AFM system, thinner ferroelectric films were required for the poling process. PTFE templates were friction transferred onto Nicoated n+-Si substrates, and then P(VDF-TrFE) were spin-coated from a 1.0 wt % solution and finally annealed at 135 °C for 5 h. As a control sample, P(VDF-TrFE) thin films were also directly deposited from a 1.0 wt % solution on Ni-coated n+-Si substrates. Film thickness was about 70 nm. AFM worked in contact mode, and its conductive probe was used to detect the changes of film thickness which were excited by triangular voltage pulses applied between the bottom Ni electrode and the conductive AM probe. During local piezoelectric measurements, voltage was always applied to the bottom Ni electrode while conductive AFM probe was electrically grounded.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00532. Figures S1−S3 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (Y.J.). *E-mail [email protected] (G.Z.). Author Contributions

Z.F. carried out the experiments with W.X. W.C., J.W., J.Z., and J.Z. conducted the fabrication and measurements of control samples. Y.J. and G.Z. were in charge of designing and supervision of the experiments. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

This work was financially supported by STCSM (13NM1400600) and NSAF (U1430106). Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.macromol.6b00532 Macromolecules XXXX, XXX, XXX−XXX