Abstract
Poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE (70/30)) films were synthesized on a gold/glass substrate via spin coating. The films were annealed at a temperature between and . Nanoscale characterisation of the morphology, polarization switching, and local piezoresponse hysteresis loops of PVDF-TrFE film was studied using a scanning probe microscope (SPM). Ferroelectric switchable domains were identified by piezoresponse force microscopy (PFM) for all films. Small grains, with weak piezoresponse character, were observed for films annealed just above the Curie temperature. Acicular grains were obtained when the annealing temperature approached the melting point and the piezoresponse increased. Annealing above the melting point decreased the piezoresponse and the morphology changed dramatically into plate-like structures.
1. Introduction
Poly(vinylidene fluoride) (PVDF) and its copolymers, especially with trifluoroethylene (TrFE), are organic ferroelectric polymers that have been extensively studied due to their application in diverse fields such as high performance actuators [1], nonvolatile memory devices [2], energy harvesters [3], and artificial organs [4]. This broad spectrum of applications is due to the large remnant polarization, short switching time, low processing temperature, chemical stability, and excellent electromechanical properties [5]. These polymers can crystallise into four different phases: β, α, γ, and δ [5]. Only the β phase is ferroelectric. It consists of an all trans configuration. The dominant phase can be controlled by manipulating the deposition method, thermal treatment, or mechanical treatment of the polymer [5]. Alternatively, the addition of trifluoroethylene within the range of 10–50% increases and stabilizes the β phase [6].
One disadvantage in the applications of PVDF-TrFE is the high coercive field of approximately 50 MV m−1 [7]. Thus, in order to facilitate the development of devices with low operating voltages, PVDF-TrFE films with thickness less than 100 nm are necessary. Such films are commonly prepared by spin coating [8] or Langmuir-Blodgett technique [9].
The ferroelectric properties of PVDF-TrFE are determined by its crystallinity. Crystallinity is usually increased by annealing PVDF-TrFE copolymers between the Curie temperature (110°C) and melting temperature (150°C) [10]. Macroscopic polarization voltage hysteresis loops for films annealed between 120 and 155°C demonstrate optimal remnant polarization for films annealed at 140°C [11]. However, the recrystallization behaviour of PVDF-TrFE copolymers annealed above the melting point is not as well understood. Li et al. [12] have shown that the morphology of PVDF-TrFE films changes dramatically if it is annealed above the melting temperature while the structure is invariant. However, this morphology depends on the thickness of the sample. For 5 μm thick films, acicular grains are produced while for 30 nm thick samples, nanomesa morphology is obtained. Zeng et al. [11] demonstrated that films above the melting point retain ferroelectric character. Recently, piezoresponse force microscopy is used for nanoscale characterisation of ferroelectric domains and polarization related processes in PVDF-TrFE films annealed below the melting point [13, 14]. In this work, nanoscale characterisation of the morphology, polarization switching, and local piezoresponse hysteresis loops for PVDF-TrFE films annealed within the range 125 to 180°C is presented. This temperature range spans from above the Curie temperature to above the melting point.
2. Experimental
PVDF-TrFE (70/30 mol%) powder (Piezotech) was dissolved in butan-2-one without further purification to obtain a solution with 1 wt% concentration. The solution was then spin-coated onto an Au sputtered glass slide. As-deposited PVDF-TrFE was removed from the substrate and pressed into a pellet for differential scanning calorimetry (DSC) characterisation. The heating rate for the DSC measurement was 5°C/minute while the cooling rate was 3°C/min. The film was annealed at 125°C, 150°C, or 180°C for 4 hours in an oven and then allowed to cool to room temperature. Surface topography, piezoresponse image, and switching spectroscopy were performed with a commercial atomic force microscope (Cypher, Asylum Research) under ambient conditions with platinum coated silicon probes (spring constant ~2 N/m, tip radius ~28 nm). Surface roughness was determined by Igor Pro. To verify the presence of polarization switchable domains within the film, after the background poling of the film by applying +20 V bias to the AFM tip, a smaller square was poled in the opposite direction at a bias of –15 V. The thickness of the as-deposited films was about 70 nm as determined by AFM. Piezoresponse loops were obtained by superimposing a 0.5 Hz triangular square step wave on a 300 kHz ac signal with bias window up to ±30 V.
3. Results and Discussion
Consistent with previous reports [16, 17], from the DSC of as-deposited PVDF-TrFE, the Curie temperature is observed at 111°C upon heating () and around 68°C upon cooling (). The melting and crystallization temperature are at 152°C and 142°C, respectively (Figure 1). PVDF-TrFE films are generally annealed between the Curie and melting temperature because the chain mobility is higher in the paraelectric phase as compared to the ferroelectric phase [16]. Moreover, chain mobility increases as a function of temperature [18]. A higher chain mobility favours the lowest energy conformation (all trans) thereby increasing its ferroelectric character [5]. In this work, the annealing temperatures are at 125°C, 150°C, and 180°C. The film annealed at 125°C has the lowest chain mobility so the degree of crystallization is expected to be low. The degree of crystallinity of the film annealed at 150°C is higher due to the higher annealing temperature. Chain mobility in the film annealed at 180°C is the highest, however, it will be recrystallized as it cools to room temperature. The degree of crystallinity, chain mobility, and whether recrystallization takes place are likely to affect the morphology, polarization switching, and local piezoresponse hysteresis loops.
The AFM images of the resulting film after annealing at different temperatures are shown in Figures 2(a)–2(c). After annealing at 125°C, the grains are relatively small with an average length of 130 nm and 70 nm diameter. As the annealing temperature is increased to 150°C, the grains coalesce and acicular grains are obtained with an average length of 1.5 μm and diameter of 160 nm. There is a concomitant increase in surface roughness. The surface roughness is 3.3 nm and 12.7 nm, respectively, for the film annealed at 125°C and 150°C. This is in agreement with works by other authors [12, 19, 20]. In the needle like grains, the molecular chains are preferentially oriented parallel to the substrate in lamellar crystals which are perpendicular to the substrate (Figure 3(a)) [7, 15]. These edge-on lamellae form acicular grains. Above the melting point, the morphology changes drastically due to the melting and recrystallization of the film. In this situation, the chain axis is dominantly normal to the surface, and the crystalline lamellae stack on top of one another resulting in a relatively smooth topography (Figure 3(b)) [15]. The surface roughness is 6.3 nm.
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In the PFM phase image (Figures 2(d)–2(f)), purple regions correspond to a c− domain (polarization antiparallel to surface normal), and yellow regions correspond to a c+ domain (polarization parallel to surface normal). Polarization switchable domains are present in the PVDF-TrFE films, regardless of the annealing temperature, as a smaller square of opposite polarization can be poled at −15 V after background poling at +20 V. The quality of the pattern appears to be optimum for the film annealed at 150°C. This is probably due to its high degree of crystallinity and low number of defects [12]. The irregularities in the film annealed at 125°C are presumably due to its lower degree of crystallinity [11]. Similarly, the lower crystallinity of films annealed at 180°C [11] affects the quality of the switching.
A comparison of typical local piezoresponse loops for a single point obtained from PFM for PVDF-TrFE films annealed at different temperatures is presented in Figure 4. The loops are slightly shifted to a negative voltage. This is commonly attributed to the presence of an internal field or to the asymmetry of the tip/polymer/electrode sample [14]. All films demonstrate ferroelectricity with comparable coercive voltages. As expected, the piezoresponse is highest for the film annealed at 150°C, lowest for the film annealed at 125°C, and intermediate for the film annealed at 150°C. This trend is consistent with the degree of crystallinity. Moreover, macroscopic polarization voltage hysteresis loops of 550 nm PVDF-TrFE (72/28 mol%) demonstrate a similar trend [12]. The vertical shift in the loops is likely to be caused by nonswitchable dipoles in the film [21]. Contrary to the quality of the switching pattern, the piezoresponse for the film annealed at 150°C is greater than that for the film annealed at 125°C. This could be caused by a large local variation in piezoresponse for the film annealed at 150°C. Thus, the local hysteresis loop shows good piezoresponse, however, the quality of the switching pattern is not as desirable.
4. Conclusion
The effect of annealing PVDF-TrFE spin-coated films across a broad range of annealing temperatures was studied. For films annealed below the melting point, PVDF-TrFE edge-on crystalline lamellae grains are observed. As the temperature is increased the crystalline grain enlarges as the grains coalesce. Annealing above the melting point drastically changes the morphology since face-on lamellae are formed. Irrespective of the annealing temperature, all the films produced demonstrate polarization switchable domains as evidenced by poling a small square of opposite polarization after background poling. The film annealed at 150°C exhibits the best ferroelectric character due to the quality of the switching pattern and the high local piezoresponse.
Acknowledgment
K. Lau, Y. Liu, and R. L. Withers acknowledge financial support from the Australian Research Council (ARC) in the form of an ARC Discovery Project. Y. Liu thanks the ARC Future Fellowship for funding.