Hindawi Publishing Corporation Advances in Condensed Matter Physics Volume 2013, Article ID 435938, 5 pages http://dx.doi.org/10.1155/2013/435938 Research Article Effect of Annealing Temperature on the Morphology and Piezoresponse Characterisation of Poly(vinylidene fluoride-trifluoroethylene) Films via Scanning Probe Microscopy K. Lau, 1 Y. Liu, 1 H. Chen, 2 and R. L. Withers 1 1 Research School of Chemistry, e Australian National University, Canberra, ACT 0200, Australia 2 Centre for Advanced Microscopy, e Australian National University, Canberra, ACT 0200, Australia Correspondence should be addressed to K. Lau; [email protected] and Y. Liu; [email protected] Received 6 June 2013; Accepted 30 September 2013 Academic Editor: Jianhua Hao Copyright © 2013 K. Lau et al. is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE (70/30)) films were synthesized on a gold/glass substrate via spin coating. e films were annealed at a temperature between 125 ∘ C and 180 ∘ C. 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, espe- cially with trifluoroethylene (TrFE), are organic ferroelectric polymers that have been extensively studied due to their application in diverse fields such as high performance actu- ators [1], nonvolatile memory devices [2], energy harvesters [3], and artificial organs [4]. is broad spectrum of applica- tions is due to the large remnant polarization, short switching time, low processing temperature, chemical stability, and excellent electromechanical properties [5]. ese polymers can crystallise into four different phases: , , , and [5]. Only the phase is ferroelectric. It consists of an all trans configuration. e 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 phase [6]. One disadvantage in the applications of PVDF-TrFE is the high coercive field of approximately 50 MV m −1 [7]. us, 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]. e ferroelectric properties of PVDF-TrFE are deter- mined 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 rem- nant 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 m 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
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Hindawi Publishing CorporationAdvances in Condensed Matter PhysicsVolume 2013, Article ID 435938, 5 pageshttp://dx.doi.org/10.1155/2013/435938
Research ArticleEffect of Annealing Temperature on the Morphologyand Piezoresponse Characterisation of Poly(vinylidenefluoride-trifluoroethylene) Films via Scanning Probe Microscopy
K. Lau,1 Y. Liu,1 H. Chen,2 and R. L. Withers1
1 Research School of Chemistry, The Australian National University, Canberra, ACT 0200, Australia2 Centre for Advanced Microscopy, The Australian National University, Canberra, ACT 0200, Australia
Correspondence should be addressed to K. Lau; [email protected] and Y. Liu; [email protected]
Received 6 June 2013; Accepted 30 September 2013
Academic Editor: Jianhua Hao
Copyright © 2013 K. Lau et al. This is an open access article distributed under the Creative Commons Attribution License, whichpermits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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 125∘C and 180∘C. Nanoscale characterisation of the morphology, polarizationswitching, 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 weakpiezoresponse character, were observed for films annealed just above the Curie temperature. Acicular grains were obtained whenthe annealing temperature approached the melting point and the piezoresponse increased. Annealing above the melting pointdecreased the piezoresponse and the morphology changed dramatically into plate-like structures.
1. Introduction
Poly(vinylidene fluoride) (PVDF) and its copolymers, espe-cially with trifluoroethylene (TrFE), are organic ferroelectricpolymers that have been extensively studied due to theirapplication in diverse fields such as high performance actu-ators [1], nonvolatile memory devices [2], energy harvesters[3], and artificial organs [4]. This broad spectrum of applica-tions is due to the large remnant polarization, short switchingtime, low processing temperature, chemical stability, andexcellent electromechanical properties [5]. These polymerscan crystallise into four different phases: 𝛽, 𝛼, 𝛾, and 𝛿[5]. Only the 𝛽 phase is ferroelectric. It consists of an alltrans configuration. The dominant phase can be controlledby 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 isthe high coercive field of approximately 50MVm−1 [7].Thus,
in order to facilitate the development of devices with lowoperating voltages, PVDF-TrFE films with thickness less than100 nm are necessary. Such films are commonly prepared byspin coating [8] or Langmuir-Blodgett technique [9].
The ferroelectric properties of PVDF-TrFE are deter-mined by its crystallinity. Crystallinity is usually increasedby annealing PVDF-TrFE copolymers between the Curietemperature (110∘C) and melting temperature (150∘C) [10].Macroscopic polarization voltage hysteresis loops for filmsannealed between 120 and 155∘C demonstrate optimal rem-nant polarization for films annealed at 140∘C [11]. However,the recrystallization behaviour of PVDF-TrFE copolymersannealed above the melting point is not as well understood.Li et al. [12] have shown that the morphology of PVDF-TrFEfilms changes dramatically if it is annealed above the meltingtemperature while the structure is invariant. However, thismorphology depends on the thickness of the sample. For5 𝜇m thick films, acicular grains are produced while for30 nm thick samples, nanomesa morphology is obtained.Zeng et al. [11] demonstrated that films above the melting
2 Advances in Condensed Matter Physics
point retain ferroelectric character. Recently, piezoresponseforce microscopy is used for nanoscale characterisation offerroelectric domains and polarization related processes inPVDF-TrFE films annealed below the melting point [13, 14].In this work, nanoscale characterisation of the morphology,polarization switching, and local piezoresponse hysteresisloops for PVDF-TrFE films annealed within the range 125 to180∘C is presented. This temperature range spans from abovethe Curie temperature to above the melting point.
2. Experimental
PVDF-TrFE (70/30mol%) powder (Piezotech) was dissolvedin butan-2-one without further purification to obtain a solu-tion with 1 wt% concentration. The solution was then spin-coated onto an Au sputtered glass slide. As-deposited PVDF-TrFEwas removed from the substrate and pressed into a pelletfor differential scanning calorimetry (DSC) characterisation.The heating rate for the DSC measurement was 5∘C/minutewhile the cooling rate was 3∘C/min.The film was annealed at125∘C, 150∘C, or 180∘C for 4 hours in an oven and then allowedto cool to room temperature. Surface topography, piezore-sponse image, and switching spectroscopy were performedwith a commercial atomic forcemicroscope (Cypher, AsylumResearch) under ambient conditions with platinum coatedsilicon probes (spring constant ∼2N/m, tip radius ∼28 nm).Surface roughness was determined by Igor Pro. To verify thepresence of polarization switchable domains within the film,after the background poling of the filmby applying +20Vbiasto the AFM tip, a smaller square was poled in the oppositedirection at a bias of –15V. The thickness of the as-depositedfilms was about 70 nm as determined by AFM. Piezoresponseloops were obtained by superimposing a 0.5Hz triangularsquare step wave on a 300 kHz ac signal with bias windowup to ±30V.
3. Results and Discussion
Consistent with previous reports [16, 17], from the DSC ofas-deposited PVDF-TrFE, the Curie temperature is observedat 111∘C upon heating (𝑇C1) and around 68∘C upon cooling(𝑇C2). The melting and crystallization temperature are at152∘Cand 142∘C, respectively (Figure 1). PVDF-TrFE films aregenerally annealed between the Curie and melting tempera-ture because the chain mobility is higher in the paraelectricphase as compared to the ferroelectric phase [16]. Moreover,chain mobility increases as a function of temperature [18]. Ahigher chain mobility favours the lowest energy conforma-tion (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 lowestchain mobility so the degree of crystallization is expectedto be low. The degree of crystallinity of the film annealedat 150∘C is higher due to the higher annealing temperature.Chain mobility in the film annealed at 180∘C is the high-est, however, it will be recrystallized as it cools to roomtemperature. The degree of crystallinity, chain mobility, andwhether recrystallization takes place are likely to affect the
Endo
ther
mic
Temperature (∘C)
Cooling
Heating
60 80 100 120 140 160 180 200
TC2: 68∘C
TC1: 111∘C
Tm: 152∘C
Tcrystallization: 142∘C
Figure 1: DSC curve for as-deposited PVDF-TrFE (70/30mol%).
morphology, polarization switching, and local piezoresponsehysteresis loops.
The AFM images of the resulting film after annealingat different temperatures are shown in Figures 2(a)–2(c).After annealing at 125∘C, the grains are relatively smallwith an average length of 130 nm and 70 nm diameter.As the annealing temperature is increased to 150∘C, thegrains coalesce and acicular grains are obtained with anaverage length of 1.5 𝜇m and diameter of 160 nm. There isa concomitant increase in surface roughness. The surfaceroughness is 3.3 nm and 12.7 nm, respectively, for the filmannealed at 125∘C and 150∘C.This is in agreement with worksby other authors [12, 19, 20]. In the needle like grains, themolecular chains are preferentially oriented parallel to thesubstrate in lamellar crystals which are perpendicular to thesubstrate (Figure 3(a)) [7, 15]. These edge-on lamellae formacicular grains. Above the melting point, the morphologychanges drastically due to the melting and recrystallizationof the film. In this situation, the chain axis is dominantlynormal to the surface, and the crystalline lamellae stack ontop of one another resulting in a relatively smooth topography(Figure 3(b)) [15]. The surface roughness is 6.3 nm.
In the PFM phase image (Figures 2(d)–2(f)), purpleregions correspond to a c− domain (polarization antiparallelto surface normal), and yellow regions correspond to a c+domain (polarization parallel to surface normal). Polariza-tion switchable domains are present in the PVDF-TrFE films,regardless of the annealing temperature, as a smaller square ofopposite polarization can be poled at −15 V after backgroundpoling at +20V. The quality of the pattern appears to beoptimum for the film annealed at 150∘C. This is probablydue to its high degree of crystallinity and low number ofdefects [12]. The irregularities in the film annealed at 125∘Care 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 fora single point obtained from PFM for PVDF-TrFE filmsannealed at different temperatures is presented in Figure 4.The loops are slightly shifted to a negative voltage. This is
Advances in Condensed Matter Physics 3
(a)
Topo
grap
hy
200nm
(a)
(b) 1𝜇m
(b)
(c) 1𝜇m
(c)
(d) 200nm
125 ∘C
PFM
pha
se
(d)
(e)
150 ∘C
1𝜇m
(e)
(f) 1𝜇m
180 ∘C
(f)
Figure 2: Surface topography obtained by AFM for films annealed at (a) 125∘C, (b) 150∘C, and (c) 180∘C.White regions are higher than blackregions. The scan size is (a) 1 × 1 𝜇m and (b), (c) 5 × 5𝜇m.The black to white contrast is (a) 12 nm, (b) 50 nm, and (c) 30 nm. (d), (e), and (f)PFM phase image of the corresponding area after background poling at +20V tip bias followed by a poling of a smaller square at −15 V tipbias. c− domains are purple, and c+ domains are yellow.
(a) (b)
Figure 3: Schematic structure of (a) edge-on (b) face-on lamellar structure for films annealed (a) below the melting temperature, (b) abovethe melting temperature. The red lines are representative of a single PVDF-TrFE molecular chain folding to form crystalline lamellae [15].
commonly attributed to the presence of an internal fieldor to the asymmetry of the tip/polymer/electrode sample[14]. All films demonstrate ferroelectricity with comparablecoercive voltages. As expected, the piezoresponse is highestfor the film annealed at 150∘C, lowest for the film annealedat 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 loopsof 550 nm PVDF-TrFE (72/28mol%) demonstrate a similartrend [12]. The vertical shift in the loops is likely to be causedby nonswitchable dipoles in the film [21]. Contrary to thequality of the switching pattern, the piezoresponse for the film
4 Advances in Condensed Matter PhysicsPi
ezor
espo
nse (
a.u.)
8
4
0
−4
−8
−30 −20 −10 0 10 20 30
125∘C150∘C180∘C
Applied voltage (V)
Figure 4: Typical piezoresponse hysteresis loops for PVDF-TrFEfilms annealed at different temperatures.
annealed at 150∘C is greater than that for the film annealedat 125∘C. This could be caused by a large local variationin piezoresponse for the film annealed at 150∘C. Thus, thelocal hysteresis loop shows good piezoresponse, however, thequality of the switching pattern is not as desirable.
4. Conclusion
The effect of annealing PVDF-TrFE spin-coated films acrossa broad range of annealing temperatures was studied. Forfilms annealed below the melting point, PVDF-TrFE edge-oncrystalline lamellae grains are observed. As the temperature isincreased the crystalline grain enlarges as the grains coalesce.Annealing above the melting point drastically changes themorphology since face-on lamellae are formed. Irrespectiveof the annealing temperature, all the films produced demon-strate polarization switchable domains as evidenced by polinga small square of opposite polarization after background pol-ing. The film annealed at 150∘C exhibits the best ferroelectriccharacter due to the quality of the switching pattern and thehigh local piezoresponse.
Acknowledgment
K. Lau, Y. Liu, and R. L. Withers acknowledge financialsupport from the Australian Research Council (ARC) in theform of an ARC Discovery Project. Y. Liu thanks the ARCFuture Fellowship for funding.
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