- Email: [email protected]
Structures and electrical properties of ferroelectric copolymer ultrathin films
European Polymer Journal 40 (2004) 987–992 www.elsevier.com/locate/europolj
Structures and electrical properties of ferroelectric copolymer ultrathin films Kei Kobayashi a
a,b,*
, Hiroyuki Masuda b, Hirofumi Yamada b, Kazumi Matsushige a,b
International Innovation Center, Kyoto University, Yoshida-Honmachi, Sakyo, Kyoto 606-8501, Japan b Department of Electronic Science and Engineering, Kyoto University, Kyoto 615-8510, Japan Received 5 September 2003; received in revised form 20 January 2004; accepted 21 January 2004
Abstract Ultrathin films of ferroelectric copolymer vinylidenefluoride and trifluoroethylene, P(VDF–TrFE), were successfully obtained by spin-coating and their nanoscale structures and electrical properties were studied utilizing atomic force microscopy (AFM). We succeeded in obtaining ultrathin copolymer films on graphite whose thickness ranged from 1 nm to several tens of nanometers by controlling concentration of copolymer solutions in methylethylketone. We found that ultrathin films thinner than 4 nm showed layered structures whose layer thickness was about 0.5 nm. On the other hand, films thicker than 4 nm formed typical edge-on lamellar crystal structures. Furthermore, we investigated surface potential distribution and piezoelectric property by AFM-based techniques and discussed interaction between electrical dipoles in the molecular chains and graphite substrate. 2004 Elsevier Ltd. All rights reserved. Keywords: Ferroelectric polymer; Ultrathin film; Atomic force microscopy; Piezoresponse force microscopy
1. Introduction Copolymer of vinylidenefluoride and trifluoroethylene, P(VDF–TrFE), has been a well-known ferroelectric polymer material. Its ferroelectric properties are due to electrical dipoles, formed by positive hydrogen atoms and negative fluorine atoms, which are perpendicular to its main chain. Structures and electrical properties of P(VDF–TrFE) copolymer has been intensively studied for its potential applications to piezoelectric and pyroelectric devices [1,2]. Recently, scanning probe microscopy techniques have been utilized to investigate local structures and electrical properties of P(VDF–TrFE) copolymer thin films. Since
* Corresponding author. Address: International Innovation Center, Kyoto University, Yoshida-Honmachi, Sakyo, Kyoto 606-8501, Japan. Tel.: +81-75-753-9153; fax: +81-75-753-9145. E-mail address: [email protected] (K. Kobayashi).
G€ uthner and Dransfeld demonstrated that it is possible to control and image local polarized domains on P(VDF– TrFE) copolymer thin films by using atomic force microscope (AFM) [3], we have been studying local piezoelectric properties of P(VDF–TrFE) copolymer thin films on graphite prepared by spin-coating whose thickness were on the order of several tens of nanometers [4–6]. On the other hand, Bune et al. succeeded in preparing two-monolayer (2-ML) copolymer film by Langmuir– Brodgett (LB) method [7]. Blinov et al. investigated ferroelectric properties of such P(VDF–TrFE) LB film utilizing electric force microscopy (EFM) [8]. Furthermore, Qu and coworkers have recently reported scanning tunneling microscopy (STM) experiments which suggested switching of electrical dipoles in individual molecules in ultrathin P(VDF–TrFE) LB film on graphite by applying electric field with an STM tip [9]. In spite of such impressive experimental results on P(VDF–TrFE) copolymer ultrathin films reported so far, their ferroelectric switching mechanisms are not elucidated yet.
0014-3057/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.01.029
988
K. Kobayashi et al. / European Polymer Journal 40 (2004) 987–992
In this study, we prepared ultrathin copolymer films by spin-coating technique and investigated their local structures depending on their thickness by dynamicmode AFM. We also studied their electrical properties by AFM-based techniques in terms of interaction between electrical dipoles in copolymer chains and graphite substrate.
nance frequency were 0.82 N/m and 66 kHz, respectively. While a cantilever tip was in contact with the sample surface, an AC modulation voltage (6 kHz, 2 V peak-to-peak) was applied and an induced cantilever deflection was detected by LIA. An in-phase signal component (A cos /) from LIA was recorded for PFM image.
2. Experimental 3. Results and discussion P(VDF–TrFE) ultrathin films were prepared as follows. A P(VDF–TrFE) powder with VDF 75% and TrFE 25% molar contents was dissolved in methylethylketone. A droplet of solution with a typical volume of about 5 ll was placed on a freshly-cleaved graphite substrate using a syringe. Immediately after placing a droplet, we span the sample at 1200 rpm for 3 s and 1600 rpm for 100 s. By precisely controlling concentration in a droplet while maintaining other parameters, we succeeded in preparing various thickness of films while maintaining other parameters. Obtained films were annealed in a vacuum oven in order to improve their crystallinity. We annealed the films at 140 C for 2 h except for the film used for piezoelectric response measurement, which was annealed at 120 C for 2 h in order to obtain a film having a large continuous area without a pin-hole. All experiments were carried out in an ambient condition using a commercially-available apparatus (Seiko Instruments Inc. SPA300). For dynamic-mode AFM and Kelvin-probe force microscopy (KFM), we used a gold-coated silicon cantilever (Nanosensors ZEILR) whose nominal spring constant and resonance frequency were 1.6 N/m and 27 kHz, respectively. The measured Q-factor was about 200. Cantilever was vibrated at a fixed frequency which was higher than the resonance frequency by about 60 Hz using a piezoelectric actuator. Distance between the tip and the sample was regulated by maintaining a damped vibration amplitude constant in an intermittent-contact regime. For KFM imaging, we applied an AC modulation voltage (15 kHz, 3.5 V peak-to-peak) between a mechanically-vibrated AFM tip and graphite and then a lock-in amplifier (LIA) (NF Corp. 5610B) was utilized to detect an induced electrostatic force. A signal component (A cos /) from LIA, indicating magnitude of the induced vibration which is in-phase to the modulation voltage, was fed to feedback electronics to control an additional offset bias voltage for compensating contact potential difference (CPD) between the tip and the sample. The offset bias voltage required to null CPD was recorded for KFM image. For piezoresponse force microscopy (PFM), we used a gold-coated silicon nitride cantilever (Olympus RC800PB), whose nominal spring constant and reso-
Fig. 1(a) shows a dynamic-mode AFM image of an ultrathin copolymer film prepared from 0.25 mg/ml solution. Scanned area is 500 nm · 500 nm. We can see needle-like aggregates whose thickness is ranging from 2 to 3 nm. We can also recognize a graphite step in the center of the image. The long axes of these rod-like aggregates are preferentially oriented with a sixfold symmetry. This is probably due to a strong interaction between P(VDF–TrFE) copolymer molecular chains and graphite. One of the possible explanations for these structures is that these aggregates are bundles of extended molecular chains. On the other hand, Fig. 1(b) shows a dynamic-mode AFM image of the same film after annealing at 140 C for 2 h. Scanned area is again 500 nm · 500 nm, but not the same area as Fig. 1(a). We can see that a flat ultrathin film was obtained whose typical thickness was about 1.0 nm. The film is as thin as those prepared by LB method thus we consider the obtained film is a 2-ML-film. However, graphite substrate was not fully covered by the copolymer ultrathin film in this case. It should be noted that there is some overlayers, whose thickness was 0.5 nm (1 ML) or 1.0 nm (2 MLs), on the 2-ML-film. By increasing polymer concentration in solutions, we could easily obtain ultrathin films with larger thickness. Fig. 2(a) and (b) are dynamic-mode AFM images of the ultrathin copolymer films prepared from 0.5 and 0.75 mg/ml solutions after annealing, respectively. Scanned area for each image is 800 nm · 800 nm. From these two images, we can see that ultrathin copolymer films grow layer by layer. The average thickness of films in Fig. 2(a) and (b) are about 2 nm and about 3 nm, respectively. Fig. 3(a) shows a dynamic-mode AFM image of an annealed ultrathin copolymer film prepared from 1 mg/ ml solution. Scanned area is 400 nm · 400 nm. In this image, we can see many grooves on the continuous film surface. We should note that the films studied in this study was not in a thermodynamic equilibrium. For example, when we annealed this film for more than 8 h, we observed edge-on lamellar crystals piled up as shown in Fig. 3(b). We believe that molecular packing structures in the film shown in Fig. 3(a) is quite close to that in lamellar crystals even though the average thickness of the film is as thin as 4 nm.
K. Kobayashi et al. / European Polymer Journal 40 (2004) 987–992
989
Fig. 1. (a) Dynamic-mode AFM image of P(VDF–TrFE) film prepared by spin-coating of 0.25 mg/ml solution on graphite. Scanned area is 500 nm · 500 nm. Molecules are aligned preferentially with sixfold symmetry reflecting strong interaction between molecular chains and graphite. (b) Dynamic-mode AFM image of the same film after annealing. Scanned area is 500 nm · 500 nm. Average film thickness is about 1.0 nm.
Fig. 2. Dynamic-mode AFM images of annealed P(VDF–TrFE) films which were prepared from 0.5 and 0.75 mg/ml solutions. Scanned area for each image is 800 nm · 800 nm.
In addition, from a line-profile shown in Fig. 3(c), we can recognize that there is thinner regions within the holes whose thickness is about 1.0 nm thus we consider that these regions are again 2-ML regions. Fig. 4 shows dynamic-mode AFM images of annealed ultrathin copolymer films prepared from 2.5 and 10 mg/ml, respectively. Scanned area for each image is 1.6 lm · 1.6 lm. Now these films consist of edge-on lamellar crystals continuously covering over graphite. Comparing Fig. 3(a) and these two images, we can see that gaps between lamellar crystals are wider for thicker film when prepared in the same condition. This is probably because that interaction between copolymer molecular chains and substrate is rapidly decreased as
the thickness of the film increases. It should be noted that the width of rod-like crystals (lamellar thickness) is apparently larger for the thick film. We studied structures of ultrathin copolymer films with a various thickness using dynamic-mode AFM. In order to investigate local electrical properties of these films, we performed surface potential (SP) mapping by KFM and imaged polarized domains by PFM. Fig. 5(a) and (b) are a topographic image and a simultaneously-obtained KFM image of annealed copolymer film prepared from 1 mg/ml solution. Corresponding line-profiles were shown below as Fig. 5(c) and (d). We can see from line-profiles that SP on the film is lower than graphite and SP is larger for thicker regions.
990
K. Kobayashi et al. / European Polymer Journal 40 (2004) 987–992
Fig. 3. (a) Dynamic-mode AFM image of annealed P(VDF–TrFE) film which was prepared from 1 mg/ml solution. Scanned area is 400 nm · 400 nm. (b) Dynamic-mode AFM image of P(VDF–TrFE) film after annealing for 8 h which was prepared from 1 mg/ml solution. Scanned area is 2 lm · 2 lm. (c) Line-profile measured on the black dot line in (a).
Fig. 4. Dynamic-mode AFM images of annealed P(VDF–TrFE) films which were prepared from 2.5 and 10 mg/ml solutions. Scanned area for each image is 1.6 lm · 1.6 lm.
This result supports the frozen dipole model suggested in previous studies where the very first copolymer layers on substrate is oriented with their electrical dipoles downward [6,8]. Furthermore, we measured piezoelectric response by PFM. We prepared a copolymer ultrathin film from 3.5 mg/ml solution and annealed the film at 120 C for 2 h in this case. Although lamellar crystals were not obtained due to lower annealing temperature, we could obtain very flat and wide surface without a pin-hole which was suitable for PFM experiment. The thickness of the film measured from AFM line-profile across a hole made by scratching AFM tip after PFM experiment (not shown
here) was about 15 nm. We made 6 differently poled regions by applying bias voltage to the sample at +6, )6, +6, 0, )6, +6 V, respectively, while scanning 1.2 lm · 1.2 lm with a cantilever tip in contact with the film surface. Each region has an area of 1.2 lm · 200 nm. Fig. 6(a) and (b) are a contact-mode AFM image and a simultaneously-obtained PFM image obtained immediately after polarization as described above. These images were obtained with a scan angle of 90 (scan with fast scan axes perpendicular to the cantilever long axis). Scanned area is 2.0 lm · 2.0 lm. As we expected, in PFM image we obtained bright (dipole upward), dark (downward), bright, zero (random), dark, bright con-
K. Kobayashi et al. / European Polymer Journal 40 (2004) 987–992
991
Fig. 5. (a) Dynamic-mode AFM image of annealed P(VDF–TrFE) film which was prepared from 1 mg/ml solution. Scanned area is 1.6 lm · 1.6 lm. (b) Simultaneously-obtained Kelvin-probe force microscopy (KFM) image showing surface potential distribution. (c) Line-profile measured on the white dot line in (a). (d) Line-profile measured on the white dot line in (b).
trasts from left to right. However, the first bright region has almost zero contrast. The same phenomenon was already recognized in the previous experiments (Fig. 1 in Ref. [8]). We assume that this is because the most of the upward dipoles were turned back to random dipoles by positive surface charges which were deposited when the next regions were scanned with an opposite bias voltage. On the other hand, the second dark region was not so much affected by the subsequent upward polarization. These results suggest that upward dipoles were less sta-
ble than downward dipoles against counter charges deposited when the adjacent area was poled. This idea is also consistent with the previous experiment showing that the diffused area and density for surface charge was larger and lower, respectively, for the positive charges [5]. It is interesting to see that the last bright contrast was expanded to the right. Electrical dipoles in this region were probably poled upward by the deposited negative charges on the surface. On the other hand, regions with downward dipoles were not apparently
Fig. 6. (a) Contact-mode AFM image of annealed P(VDF–TrFE) film which was prepared from 3.5 mg/ml solution. Scanned area is 2.0 lm · 2.0 lm. (b) Simultaneously-obtained piezoresponse force microscopy (PFM) image revealing polarized domains.
992
K. Kobayashi et al. / European Polymer Journal 40 (2004) 987–992
expanded. This is probably due to lower density of the diffused positive charges which was not enough to create an electric field higher than its coercive field. From these two electrical measurements by KFM and PFM, we suggest that copolymer molecules in the very first layers are oriented with their electrical dipoles downward. Further studies including D–E hysteresis measurements by AFM have to be carried out. Such experiments in a vacuum condition would be interesting for further understanding of switching mechanisms of P(VDF–TrFE) copolymer thin films.
4. Conclusions P(VDF–TrFE) ultrathin copolymer films obtained by spin-coating technique were characterized by AFM. Results of surface potential measurement and piezoresponse measurement suggest that copolymer molecules in the very first layers are oriented with their electrical dipoles downward.
Acknowledgements The authors would like to thank Daikin Industries Ltd. for providing P(VDF–TrFE). This work was supported by an Innovative Cluster Creation Project and a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sport, Science and Technology of Japan.
References [1] Kimura K, Ohigashi H. Ferroelectric properties of poly(vinylidenefluoride–trifluoroethylene) thin films. Appl Phys Lett 1983;43(9):834–6. [2] Koga K, Ohigashi H. Piezoelectricity and related properties of vinylidenefluoride and trifluoroethylene copolymers. J Appl Phys 1986;59(6):2142–50. [3] G€ uthner P, Dransfeld K. Local poling of ferroelectric polymers by scanning force microscopy. Appl Phys Lett 1992;61(9):1137–9. [4] Chen XQ, Yamada H, Horiuchi T, Matsushige K. Structures and local polarized domains of ferroelectric organic films studied by atomic force microscopy. Jpn J Appl Phys 1998;37(6B):3834–7. [5] Chen XQ, Yamada H, Horiuchi T, Matsushige K. Investigation of surface potential of ferroelectric organic molecules by scanning probe microscopy. Jpn J Appl Phys 1999; 38(6B):3932–5. [6] Chen XQ, Yamada H, Terai Y, Horiuchi T, Matsushige K, Weiss PS. Strong substrate effect in local poling of ultrathin ferroelectric polymer films. Thin Solid Films 1999;353: 259–63. [7] Bune A, Fridkin VM, Durcharme S, Blinov LM, Palto SP, Sorokin AV, et al. Two-dimensional ferroelectric films. Nature 1998;391:874–7. [8] Blinov LM, Barben R, Palto SP, De Santo MP, Yudin SG. Switching of a ferroelectric polymer Langmuir–Brodgett film studied by electrostatic force microscopy. J Appl Phys 2001;89(7):3960–6. [9] Qu H, Yao W, Garcia T, Zhang J, Sorokin AV, Ducharme S, et al. Nanoscale polarization manipulation and conductance switching in ultrathin films of a ferroelectric copolymer. Appl Phys Lett 2003;82(24):4322–4.
Structures and electrical properties of ferroelectric copolymer ultrathin films Kei Kobayashi a
a,b,*
, Hiroyuki Masuda b, Hirofumi Yamada b, Kazumi Matsushige a,b
International Innovation Center, Kyoto University, Yoshida-Honmachi, Sakyo, Kyoto 606-8501, Japan b Department of Electronic Science and Engineering, Kyoto University, Kyoto 615-8510, Japan Received 5 September 2003; received in revised form 20 January 2004; accepted 21 January 2004
Abstract Ultrathin films of ferroelectric copolymer vinylidenefluoride and trifluoroethylene, P(VDF–TrFE), were successfully obtained by spin-coating and their nanoscale structures and electrical properties were studied utilizing atomic force microscopy (AFM). We succeeded in obtaining ultrathin copolymer films on graphite whose thickness ranged from 1 nm to several tens of nanometers by controlling concentration of copolymer solutions in methylethylketone. We found that ultrathin films thinner than 4 nm showed layered structures whose layer thickness was about 0.5 nm. On the other hand, films thicker than 4 nm formed typical edge-on lamellar crystal structures. Furthermore, we investigated surface potential distribution and piezoelectric property by AFM-based techniques and discussed interaction between electrical dipoles in the molecular chains and graphite substrate. 2004 Elsevier Ltd. All rights reserved. Keywords: Ferroelectric polymer; Ultrathin film; Atomic force microscopy; Piezoresponse force microscopy
1. Introduction Copolymer of vinylidenefluoride and trifluoroethylene, P(VDF–TrFE), has been a well-known ferroelectric polymer material. Its ferroelectric properties are due to electrical dipoles, formed by positive hydrogen atoms and negative fluorine atoms, which are perpendicular to its main chain. Structures and electrical properties of P(VDF–TrFE) copolymer has been intensively studied for its potential applications to piezoelectric and pyroelectric devices [1,2]. Recently, scanning probe microscopy techniques have been utilized to investigate local structures and electrical properties of P(VDF–TrFE) copolymer thin films. Since
* Corresponding author. Address: International Innovation Center, Kyoto University, Yoshida-Honmachi, Sakyo, Kyoto 606-8501, Japan. Tel.: +81-75-753-9153; fax: +81-75-753-9145. E-mail address: [email protected] (K. Kobayashi).
G€ uthner and Dransfeld demonstrated that it is possible to control and image local polarized domains on P(VDF– TrFE) copolymer thin films by using atomic force microscope (AFM) [3], we have been studying local piezoelectric properties of P(VDF–TrFE) copolymer thin films on graphite prepared by spin-coating whose thickness were on the order of several tens of nanometers [4–6]. On the other hand, Bune et al. succeeded in preparing two-monolayer (2-ML) copolymer film by Langmuir– Brodgett (LB) method [7]. Blinov et al. investigated ferroelectric properties of such P(VDF–TrFE) LB film utilizing electric force microscopy (EFM) [8]. Furthermore, Qu and coworkers have recently reported scanning tunneling microscopy (STM) experiments which suggested switching of electrical dipoles in individual molecules in ultrathin P(VDF–TrFE) LB film on graphite by applying electric field with an STM tip [9]. In spite of such impressive experimental results on P(VDF–TrFE) copolymer ultrathin films reported so far, their ferroelectric switching mechanisms are not elucidated yet.
0014-3057/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.01.029
988
K. Kobayashi et al. / European Polymer Journal 40 (2004) 987–992
In this study, we prepared ultrathin copolymer films by spin-coating technique and investigated their local structures depending on their thickness by dynamicmode AFM. We also studied their electrical properties by AFM-based techniques in terms of interaction between electrical dipoles in copolymer chains and graphite substrate.
nance frequency were 0.82 N/m and 66 kHz, respectively. While a cantilever tip was in contact with the sample surface, an AC modulation voltage (6 kHz, 2 V peak-to-peak) was applied and an induced cantilever deflection was detected by LIA. An in-phase signal component (A cos /) from LIA was recorded for PFM image.
2. Experimental 3. Results and discussion P(VDF–TrFE) ultrathin films were prepared as follows. A P(VDF–TrFE) powder with VDF 75% and TrFE 25% molar contents was dissolved in methylethylketone. A droplet of solution with a typical volume of about 5 ll was placed on a freshly-cleaved graphite substrate using a syringe. Immediately after placing a droplet, we span the sample at 1200 rpm for 3 s and 1600 rpm for 100 s. By precisely controlling concentration in a droplet while maintaining other parameters, we succeeded in preparing various thickness of films while maintaining other parameters. Obtained films were annealed in a vacuum oven in order to improve their crystallinity. We annealed the films at 140 C for 2 h except for the film used for piezoelectric response measurement, which was annealed at 120 C for 2 h in order to obtain a film having a large continuous area without a pin-hole. All experiments were carried out in an ambient condition using a commercially-available apparatus (Seiko Instruments Inc. SPA300). For dynamic-mode AFM and Kelvin-probe force microscopy (KFM), we used a gold-coated silicon cantilever (Nanosensors ZEILR) whose nominal spring constant and resonance frequency were 1.6 N/m and 27 kHz, respectively. The measured Q-factor was about 200. Cantilever was vibrated at a fixed frequency which was higher than the resonance frequency by about 60 Hz using a piezoelectric actuator. Distance between the tip and the sample was regulated by maintaining a damped vibration amplitude constant in an intermittent-contact regime. For KFM imaging, we applied an AC modulation voltage (15 kHz, 3.5 V peak-to-peak) between a mechanically-vibrated AFM tip and graphite and then a lock-in amplifier (LIA) (NF Corp. 5610B) was utilized to detect an induced electrostatic force. A signal component (A cos /) from LIA, indicating magnitude of the induced vibration which is in-phase to the modulation voltage, was fed to feedback electronics to control an additional offset bias voltage for compensating contact potential difference (CPD) between the tip and the sample. The offset bias voltage required to null CPD was recorded for KFM image. For piezoresponse force microscopy (PFM), we used a gold-coated silicon nitride cantilever (Olympus RC800PB), whose nominal spring constant and reso-
Fig. 1(a) shows a dynamic-mode AFM image of an ultrathin copolymer film prepared from 0.25 mg/ml solution. Scanned area is 500 nm · 500 nm. We can see needle-like aggregates whose thickness is ranging from 2 to 3 nm. We can also recognize a graphite step in the center of the image. The long axes of these rod-like aggregates are preferentially oriented with a sixfold symmetry. This is probably due to a strong interaction between P(VDF–TrFE) copolymer molecular chains and graphite. One of the possible explanations for these structures is that these aggregates are bundles of extended molecular chains. On the other hand, Fig. 1(b) shows a dynamic-mode AFM image of the same film after annealing at 140 C for 2 h. Scanned area is again 500 nm · 500 nm, but not the same area as Fig. 1(a). We can see that a flat ultrathin film was obtained whose typical thickness was about 1.0 nm. The film is as thin as those prepared by LB method thus we consider the obtained film is a 2-ML-film. However, graphite substrate was not fully covered by the copolymer ultrathin film in this case. It should be noted that there is some overlayers, whose thickness was 0.5 nm (1 ML) or 1.0 nm (2 MLs), on the 2-ML-film. By increasing polymer concentration in solutions, we could easily obtain ultrathin films with larger thickness. Fig. 2(a) and (b) are dynamic-mode AFM images of the ultrathin copolymer films prepared from 0.5 and 0.75 mg/ml solutions after annealing, respectively. Scanned area for each image is 800 nm · 800 nm. From these two images, we can see that ultrathin copolymer films grow layer by layer. The average thickness of films in Fig. 2(a) and (b) are about 2 nm and about 3 nm, respectively. Fig. 3(a) shows a dynamic-mode AFM image of an annealed ultrathin copolymer film prepared from 1 mg/ ml solution. Scanned area is 400 nm · 400 nm. In this image, we can see many grooves on the continuous film surface. We should note that the films studied in this study was not in a thermodynamic equilibrium. For example, when we annealed this film for more than 8 h, we observed edge-on lamellar crystals piled up as shown in Fig. 3(b). We believe that molecular packing structures in the film shown in Fig. 3(a) is quite close to that in lamellar crystals even though the average thickness of the film is as thin as 4 nm.
K. Kobayashi et al. / European Polymer Journal 40 (2004) 987–992
989
Fig. 1. (a) Dynamic-mode AFM image of P(VDF–TrFE) film prepared by spin-coating of 0.25 mg/ml solution on graphite. Scanned area is 500 nm · 500 nm. Molecules are aligned preferentially with sixfold symmetry reflecting strong interaction between molecular chains and graphite. (b) Dynamic-mode AFM image of the same film after annealing. Scanned area is 500 nm · 500 nm. Average film thickness is about 1.0 nm.
Fig. 2. Dynamic-mode AFM images of annealed P(VDF–TrFE) films which were prepared from 0.5 and 0.75 mg/ml solutions. Scanned area for each image is 800 nm · 800 nm.
In addition, from a line-profile shown in Fig. 3(c), we can recognize that there is thinner regions within the holes whose thickness is about 1.0 nm thus we consider that these regions are again 2-ML regions. Fig. 4 shows dynamic-mode AFM images of annealed ultrathin copolymer films prepared from 2.5 and 10 mg/ml, respectively. Scanned area for each image is 1.6 lm · 1.6 lm. Now these films consist of edge-on lamellar crystals continuously covering over graphite. Comparing Fig. 3(a) and these two images, we can see that gaps between lamellar crystals are wider for thicker film when prepared in the same condition. This is probably because that interaction between copolymer molecular chains and substrate is rapidly decreased as
the thickness of the film increases. It should be noted that the width of rod-like crystals (lamellar thickness) is apparently larger for the thick film. We studied structures of ultrathin copolymer films with a various thickness using dynamic-mode AFM. In order to investigate local electrical properties of these films, we performed surface potential (SP) mapping by KFM and imaged polarized domains by PFM. Fig. 5(a) and (b) are a topographic image and a simultaneously-obtained KFM image of annealed copolymer film prepared from 1 mg/ml solution. Corresponding line-profiles were shown below as Fig. 5(c) and (d). We can see from line-profiles that SP on the film is lower than graphite and SP is larger for thicker regions.
990
K. Kobayashi et al. / European Polymer Journal 40 (2004) 987–992
Fig. 3. (a) Dynamic-mode AFM image of annealed P(VDF–TrFE) film which was prepared from 1 mg/ml solution. Scanned area is 400 nm · 400 nm. (b) Dynamic-mode AFM image of P(VDF–TrFE) film after annealing for 8 h which was prepared from 1 mg/ml solution. Scanned area is 2 lm · 2 lm. (c) Line-profile measured on the black dot line in (a).
Fig. 4. Dynamic-mode AFM images of annealed P(VDF–TrFE) films which were prepared from 2.5 and 10 mg/ml solutions. Scanned area for each image is 1.6 lm · 1.6 lm.
This result supports the frozen dipole model suggested in previous studies where the very first copolymer layers on substrate is oriented with their electrical dipoles downward [6,8]. Furthermore, we measured piezoelectric response by PFM. We prepared a copolymer ultrathin film from 3.5 mg/ml solution and annealed the film at 120 C for 2 h in this case. Although lamellar crystals were not obtained due to lower annealing temperature, we could obtain very flat and wide surface without a pin-hole which was suitable for PFM experiment. The thickness of the film measured from AFM line-profile across a hole made by scratching AFM tip after PFM experiment (not shown
here) was about 15 nm. We made 6 differently poled regions by applying bias voltage to the sample at +6, )6, +6, 0, )6, +6 V, respectively, while scanning 1.2 lm · 1.2 lm with a cantilever tip in contact with the film surface. Each region has an area of 1.2 lm · 200 nm. Fig. 6(a) and (b) are a contact-mode AFM image and a simultaneously-obtained PFM image obtained immediately after polarization as described above. These images were obtained with a scan angle of 90 (scan with fast scan axes perpendicular to the cantilever long axis). Scanned area is 2.0 lm · 2.0 lm. As we expected, in PFM image we obtained bright (dipole upward), dark (downward), bright, zero (random), dark, bright con-
K. Kobayashi et al. / European Polymer Journal 40 (2004) 987–992
991
Fig. 5. (a) Dynamic-mode AFM image of annealed P(VDF–TrFE) film which was prepared from 1 mg/ml solution. Scanned area is 1.6 lm · 1.6 lm. (b) Simultaneously-obtained Kelvin-probe force microscopy (KFM) image showing surface potential distribution. (c) Line-profile measured on the white dot line in (a). (d) Line-profile measured on the white dot line in (b).
trasts from left to right. However, the first bright region has almost zero contrast. The same phenomenon was already recognized in the previous experiments (Fig. 1 in Ref. [8]). We assume that this is because the most of the upward dipoles were turned back to random dipoles by positive surface charges which were deposited when the next regions were scanned with an opposite bias voltage. On the other hand, the second dark region was not so much affected by the subsequent upward polarization. These results suggest that upward dipoles were less sta-
ble than downward dipoles against counter charges deposited when the adjacent area was poled. This idea is also consistent with the previous experiment showing that the diffused area and density for surface charge was larger and lower, respectively, for the positive charges [5]. It is interesting to see that the last bright contrast was expanded to the right. Electrical dipoles in this region were probably poled upward by the deposited negative charges on the surface. On the other hand, regions with downward dipoles were not apparently
Fig. 6. (a) Contact-mode AFM image of annealed P(VDF–TrFE) film which was prepared from 3.5 mg/ml solution. Scanned area is 2.0 lm · 2.0 lm. (b) Simultaneously-obtained piezoresponse force microscopy (PFM) image revealing polarized domains.
992
K. Kobayashi et al. / European Polymer Journal 40 (2004) 987–992
expanded. This is probably due to lower density of the diffused positive charges which was not enough to create an electric field higher than its coercive field. From these two electrical measurements by KFM and PFM, we suggest that copolymer molecules in the very first layers are oriented with their electrical dipoles downward. Further studies including D–E hysteresis measurements by AFM have to be carried out. Such experiments in a vacuum condition would be interesting for further understanding of switching mechanisms of P(VDF–TrFE) copolymer thin films.
4. Conclusions P(VDF–TrFE) ultrathin copolymer films obtained by spin-coating technique were characterized by AFM. Results of surface potential measurement and piezoresponse measurement suggest that copolymer molecules in the very first layers are oriented with their electrical dipoles downward.
Acknowledgements The authors would like to thank Daikin Industries Ltd. for providing P(VDF–TrFE). This work was supported by an Innovative Cluster Creation Project and a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sport, Science and Technology of Japan.
References [1] Kimura K, Ohigashi H. Ferroelectric properties of poly(vinylidenefluoride–trifluoroethylene) thin films. Appl Phys Lett 1983;43(9):834–6. [2] Koga K, Ohigashi H. Piezoelectricity and related properties of vinylidenefluoride and trifluoroethylene copolymers. J Appl Phys 1986;59(6):2142–50. [3] G€ uthner P, Dransfeld K. Local poling of ferroelectric polymers by scanning force microscopy. Appl Phys Lett 1992;61(9):1137–9. [4] Chen XQ, Yamada H, Horiuchi T, Matsushige K. Structures and local polarized domains of ferroelectric organic films studied by atomic force microscopy. Jpn J Appl Phys 1998;37(6B):3834–7. [5] Chen XQ, Yamada H, Horiuchi T, Matsushige K. Investigation of surface potential of ferroelectric organic molecules by scanning probe microscopy. Jpn J Appl Phys 1999; 38(6B):3932–5. [6] Chen XQ, Yamada H, Terai Y, Horiuchi T, Matsushige K, Weiss PS. Strong substrate effect in local poling of ultrathin ferroelectric polymer films. Thin Solid Films 1999;353: 259–63. [7] Bune A, Fridkin VM, Durcharme S, Blinov LM, Palto SP, Sorokin AV, et al. Two-dimensional ferroelectric films. Nature 1998;391:874–7. [8] Blinov LM, Barben R, Palto SP, De Santo MP, Yudin SG. Switching of a ferroelectric polymer Langmuir–Brodgett film studied by electrostatic force microscopy. J Appl Phys 2001;89(7):3960–6. [9] Qu H, Yao W, Garcia T, Zhang J, Sorokin AV, Ducharme S, et al. Nanoscale polarization manipulation and conductance switching in ultrathin films of a ferroelectric copolymer. Appl Phys Lett 2003;82(24):4322–4.