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Imaging of ferroelectric vinylidene fluoride and trifluoroethylene copolymer films by scanning tunneling microscopy
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Applied Surface Science 254 (2008) 2487–2492 www.elsevier.com/locate/apsusc
Imaging of ferroelectric vinylidene fluoride and trifluoroethylene copolymer films by scanning tunneling microscopy GuoDong Zhu, ZhiGang Zeng, Li Zhang, XueJian Yan * Department of Materials Science, Fudan University, 220 Handan Road, Shanghai 200433, China Received 30 June 2007; received in revised form 28 August 2007; accepted 25 September 2007 Available online 29 September 2007
Abstract In this paper, we reported the possibility to image non-conducting P(VDF-TrFE) copolymer films by STM. The films had the thickness of 25.0 nm and were spin-coated onto Au or graphite substrates. For films deposited on Au substrates, STM images showed grain structures of 100 nm, much larger than the grains of bare Au substrate. With increased scan rate, the film surface was damaged by STM tip and extreme protrusions and holes were observed. For films deposited on graphite substrates, we only obtained an image of very flat plane and could not observe the topography of the film surface. It seemed that the tip had pierced through the uppermost P(VDF-TrFE) layers and only imaged the layers nearest to the substrate. Asymmetrical current–voltage curves were observed from copolymer films deposited on HOPG. Experimental results were discussed. # 2007 Elsevier B.V. All rights reserved. PACS : 68.37.Ef; 68.55.a; 73.40.Ei Keywords: Scanning tunneling microscopy (STM); Atomic force microscopy (AFM); P(VDF-TrFE); Surface morphology; Current–voltage characteristic
1. Introduction Ferroelectric polymers such as poly(vinylidene fluoride) (PVDF) and its copolymer P(VDF-TrFE) have been well studied because of their applications in transducers, sensors, actuators, and potential ultra-high-density data storage [1,2]. Especially in recent years, scanning probe microscopy has been widely utilized to investigate the local structures and electrical properties of P(VDF-TrFE) thin films. Gu¨thner demonstrated the possibility to control and image local polarized domains on P(VDF-TrFE) thin films by using atomic force microscope (AFM) [3]; Matsushige et al. studied local piezoelectric properties of PVDF and P(VDF-TrFE) thin films prepared by spin-coating and vacuum-deposition methods whose thickness were on the order of several nanometers to several tens of nanometers [4,5]; Bune, et al. succeeded in preparing twomonolayer copolymer film by Langmuir–Brodgett (LB)
* Corresponding author. Tel.: +86 021 6564 2872; fax: +86 021 6564 2872. E-mail address: [email protected] (X. Yan). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.09.072
method and discovered two-dimensional ferroelectricity by means of scanning tunneling microscope (STM) [6]. Sheats [7] had reported the possibility to image nonconducting polymer films with the thickness up to 110 nm by STM. P(VDF-TrFE) was also non-conducting, but STM imaging of such films was only obtained from its LB films, of which the thickness was only about 1.0 nm [8]. Studies on its thicker films (such as tens of nanometers) by STM have not been reported yet. In this paper, we attempted to image spin-coated P(VDF-TrFE) films by STM and to compare the differences in STM topographies of films deposited on different substrates. 2. Experiment Copolymer films were spin-coated from a 1.0% by weight solution of 60/40 P (VDF-TrFE) in butanone onto different conducting substrates (highly oriented pyrolytic graphite, HOPG, or Au substrates), the thickness of these as-cast films was about 25.0 10.0 nm, which was measured by AFM’s line profile. HOPG substrates were freshly cleaved and Au substrates were prepared by vacuum depositing 40 nm Au films onto mica.
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Four groups of samples were used in experiments: the first group was as-cast P(VDF-TrFE) films deposited on Au substrate, indexed as S1; the second was films deposited on Au substrate and annealed at 132 8C for 6 h to increase their crystallinity, indexed as S2; the third was as-cast films deposited on HOPG, indexed as S3; and the last was films deposited on HOPG and annealed at 132 8C for 6 h, indexed as S4. Topographies of these films were characterized by AFM (UltraObjective, Surface Imaging Systems) and STM (AJ-I, Aijian Inc.), respectively. The tungsten tips used in STM measurements were dc-etched in 1.0 mol/L KOH solution, the bias voltage (Vbias) was applied to the substrate. Both AFM and STM images were taken in air and at room temperature. 3. Results 3.1. P(VDF-TrFE) films deposited on Au substrate AFM morphologies of P(VDF-TrFE) films deposited on Au substrates were shown in Fig. 1. The surface of as-cast sample S1 (Fig. 1a) was covered by fine grains with the diameter of tens of nanometers, and the boundaries of these grains were blurry. The root mean square (RMS) roughness of such films was about 1.48 nm. Morphology of the annealed film S2 (Fig. 1b) showed even larger grains, but no distinct differences in morphology between S1 and S2 samples could be observed. The RMS roughness of annealed film S2 was about 2.55 nm. Topographies of S1 and S2 were also obtained by STM, which could be seen in Fig. 2. During the scan of all these four images, STM was operated in the same conditions: Vbias = 100 mV, setpoint current Iset = 0.5 nA. As a comparison, Fig. 2a indicated the STM morphology of bare Au substrate. Au film was covered with elliptical grains with diameter of 10–20 nm. Its RMS roughness was only about 0.78 nm. Fig. 2b showed the STM image of as-cast sample S1. The film surface consisted of large grains with diameter of 50– 100 nm, much larger than those in bare Au substrate, and some of adjacent grains united into clusters and the boundaries of such grains were blurred. The RMS roughness of S1 in STM measurements was about 1.90 nm. The STM morphology of S2
was shown in Fig. 2c. The film surface was still covered with grain structures as large as 100 nm, however, compared with the STM morphology of S1 (Fig. 2b), the grains shown in annealed films (Fig. 2c) were isolated, and more subtle details were observed in these grains: it seemed that the annealed grains consisted of some lamellar or strip-like structures. The RMS roughness of S2 was about 2.69 nm. The roughness of copolymer films revealed by STM was consistent with that revealed by AFM. The scan rate for Fig. 2b and c was 1500 nm/s, and at such a slow scan rate, we could obtain repeatable images. Varying the bias voltage Vbias from 50 to 600 mV or changing its polarity or changing the setpoint current Iset from 0.1 to 5.0 nA did not alter the images significantly. However, when the scan rate was increased to 4000 nm/s, the image soon deteriorated grossly, and extreme topography (several hundred nanometers, much greater than the film thickness) appeared. Fig. 2d showed one of such results, which was obtained on S2 surface with high scan rate (4000 nm/s). Extreme protrusions and holes were observed. It was believed that the tip had scratched out or piled up part of the P(VDF-TrFE) film. This damage did not occur on the surface of bare Au substrate, even when the scan rate was increased to 10,000 nm/s. From all these experimental measurements, it could be concluded that the images observed by STM on the surface of non-conducting film (Fig. 2b,c) should be attributed, at least partly, to the topography of copolymer film itself. Perhaps the Au substrate also had some contributions to the imaging of nonconducting film, but in this paper we could not judge to what extent the STM images in Fig. 2b and c could be attributable to the Au substrate. 3.2. P(VDF-TrFE) films deposited on HOPG AFM topography of S3 spin-coated on HOPG resembled that observed on S1 sample spin-coated on Au substrate, which was shown in Fig. 1a. After the film was annealed at 132 8C for 6 h, the surface of the sample S4 was covered with rod-like structures (Fig. 3a). The width of these rods was about 100 nm and their length could be as long as 500 nm. Such rod-like
Fig. 1. AFM morphologies of as-cast (a) and annealed (b) P(VDF-TrFE) films spin-coated on Au substrates.
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Fig. 2. STM morphologies of Au substrate (a), as-cast (b), annealed (c) and deteriorated (d) P(VDF-TrFE) films spin-coated on Au substrates.
structures were similar to those observed by Kobayashi [9]. The RMS roughness of S4 was about 2.72 nm. However, whether on the surface of as-cast sample S3 or on the surface of annealed one S4, images obtained by STM were extremely different from those obtained by AFM, although we varied Vbias from 800 to +800 mV and Iset from 0.1 to 8.0 nA. What we observed by STM was only a very flat plane, much similar to the plane of bare HOPG. Fig. 3b indicated one typical STM image of such a flat plane. The image was obtained on the surface of annealed film S4, Vbias was 100 mV and Iset was 1.0 nA. Fig. 3b displayed the steps of HOPG substrate buried under the copolymer film rather than the rod-like structures shown in AFM image (Fig. 3a).
High resolution images could also be performed by STM on the surface of annealed P(VDF-TrFE) film (Fig. 4a, Vbias = 200 mV, Iset = 1.0 nA). Though large noises blurred the image, an ordered strip-like structure could still be clearly observed in the center of Fig. 4a. The spacing of these parallel ˚ . As a comparison, atomic resolution strips was about 3.5 A image of bare HOPG substrate was also shown in Fig. 4b (Vbias = 300 mV, Iset = 2.0 nA). The spacing of adjacent carbon ˚ , which was much smaller than the spacing atoms was 2.5 A ˚ was consistent with the shown in Fig. 4a. The spacing of 3.5 A interchain spacing (3.3 0.1 nm) observed on the surface of P(VDF-TrFE) LB films by STM [10], so it seemed to suggest that the white strips in Fig. 4a should correspond to the
Fig. 3. Morphologies of annealed P(VDF-TrFE) film spin-coated on HOPG, imaged by AFM (a) and STM (b).
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Fig. 4. High resolution imaging of annealed P(VDF-TrFE) film spin-coated on HOPG (a) and bare HOPG substrate (b).
molecular chains of P(VDF-TrFE). The intrachain spacing could not be distinguished in Fig. 4a because of too large noises. It seemed that the STM tip pierced through the uppermost P(VDFTrFE) layers and only imaged the layers nearest to HOPG substrate. In fact, this phenomenon had been reported on the study of n-alkylcyanobiphenyl liquid crystal monolayers [11]. ˚ was much smaller than the bulk The interchain spacing of 3.5 A ˚ [12]. Cai, et al. close packing spacing between chains of 4.25 A [10] believed that such a reduced interchain spacing was caused by the interaction between the HOPG substrate and the copolymer layers nearest to the substrate. The scan rate for Fig. 3b was 2000 nm/s. Different from the STM imaging of P(VDF-TrFE) deposited on Au substrate, even after the scan rate had been increased to 8000 nm/s, we still obtained the flat images, just as shown in Fig. 3b, and no holes or protrusions were observed. 3.3. Current–voltage characterization of P(VDF-TrFE) films By STM, the current–voltage (I–V) curves were also measured on bared HOPG substrate and on ferroelectric films
Fig. 5. I–V characteristics obtained by STM and on the surfaces of bared HOPG and P(VDF-TrFE) films deposited on HOPG and Au substrates, respectively.
deposited on HOPG (S4) and Au (S2) substrates, respectively. The results were shown in Fig. 5. I–V curves from HOPG or from copolymer film deposited on Au substrate showed a symmetric structure, that was to say, whether Vbias was in negative or in positive polarity, the same magnitude of Vbias would correspond to the same magnitude of the tunneling current. However, A rectification effect was observed on the surface of P(VDF-TrFE) films deposited on HOPG. At the range of 500.0 mV < jVbiasj < 1500.0 mV, with the increase of jVbiasj, the slope of I–V curve increased. For the same magnitude of Vbias, the magnitude of the tunneling current induced by the negative Vbias was far larger than that induced by the positive Vbias (for example, when Vbias = +1000.0 mV, the resultant tunneling current I = 34.0 nA; while, when Vbias = 1000.0 mV, I = 94.0 nA). Vbias in negative polarity resulted in sharper I–V characteristic. 4. Discussions 4.1. STM imaging of nanoscale non-conducting films STM is expected to be severely limited in its application to organic materials, since it requires electrical conductivity. Though ultrathin layers (monolayers) of a variety of molecules have been imaged successfully in molecular details [11,13], only few reports exist on STM imaging of thicker nonconducting films. Tang, et al. [14] describes the imaging of a polyhydroxycellulose film (several hundred angstroms thick); Sheats [7] has imaged several kinds of non-conducting polymer films with the thickness up to 110 nm. Imaging of these surfaces is not possible within the context of conventional STM theory [15], the thickness of such films is too great for electrons to tunnel through directly. Some models or mechanisms for imaging non-conducting thicker films have been put forward (absorbed water layer model [16], pressureinduced resonance mechanism [17], dielectric breakdown model [14] and so on). But to date it is still an open question on how STM works in the imaging of thicker non-conducting polymer films.
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In our experiments, compared with HOPG substrate, the high conductivity of Au substrates seem to have increased the possibility of dielectric breakdown between the tip and the film [14], which causes a large current that can be detected by STM over the non-conducting film. However, this breakdown does not seem to occur between the tip and the film deposited on HOPG because of the lower conductivity of HOPG. To keep the tunneling current constant, preset by Iset, the tip has to pierce the uppermost P(VDF-TrFE) layers until the detected tunneling current reaches the preset value Iset. 4.2. Asymmetrical I–V characteristics An obvious rectification characteristic is observed on the surfaces of copolymer films deposited on HOPG. However, a symmetric I–V characteristic is obtained on the surfaces of bared HOPG, which suggested that such asymmetric I–V characteristics should not originate from the instrumental errors or from the asymmetric structures of the tip-air gap-sample systems. In fact, several papers have referred to the effect of substrate materials (especially HOPG substrate) on the structures of P(VDF-TrFE) copolymer films and the orientations of molecular dipoles. In the study of local poling of ferroelectric P(VDF-TrFE) ultrathin films (about 23 nm thick, spin-coated on HOPG), Chen, et al. [4] observed that the negatively local poled areas had lower piezoelectric amplitudes than the positively poled areas, and explained this by the local orientational changes of dipoles induced by the substrate. Qu et al. [18] imaged two-monolayer P(VDF-TrFE) ultrathin films prepared by LB deposition technique, observed the same asymmetric I–V characteristic by STM, and believed that the substrate had oriented the nearest dipoles upward. Based on all these studies mentioned above, we put forward the model of substrate-induced orientations of dipoles (shown in Fig. 6) to understand the results in our STM measurements. HOPG substrate has induced the orientation of those molecular dipoles which are adjacent to the substrate. These oriented dipoles are indicated as grey and directed upward in Fig. 6. A
Fig. 6. A model for the asymmetrical I–V characteristic in P(VDF-TrFE) films deposited on HOPG.
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build-in electrical field Ein has been set up for the orderly alignment of these oriented dipoles. However, the dipoles, far away from the substrate still oriented disorderly because of the negligible effect of the substrate. When the Vbias is in negative polarity, relative to the substrate, the STM tip lies in a higher voltage level, and the applied electrical filed Eout is downward. The direction of Ein is parallel to the direction of Eout, so the existence of Ein should accelerate the tunneling electrons through the oriented layer in copolymer film, as induces a relatively large tunneling current. While, when Vbias is in positive polarity, the STM tip is in a lower level, and the direction of Ein is antiparallel to the direction of Eout, so Ein should slower the tunneling electrons through the oriented layer and should correspond to a reduced current. The model discussed above is different from that given by Qu et al. [18]. The thickness of P(VDF-TrFE) LB ultrathin films studied in reference 18 is two monolayers (1.0 nm), and 0.4 V < jVbiasj < 1.0 V is the range of Vbias, in which a asymmetric I–V characteristic is observed. In their experiments, the strength of the electrical field is about 0.4 1.0 GV/m, which is near or even much larger than the coercive filed Ec of the ultrathin P(VDF-TrFE) films (about 0.5 GV/m [8]), so in their model, there are two contributions to the asymmetry of I– V characteristic: one is substrate-induced orientation of dipoles, and the other is the ferroelectric switching, caused by the high enough electrical field. However, in our STM measurements, the thickness of the spin-coated copolymer films is about 25.0 10.0 nm, the range of Vbias is 0.5 V < jVbiasj < 1.5 V, and then the resultant electrical field is about 25–75 MV/m, which is near or just a bit larger than Ec (50 MV/m [19]) of the bulk materials or thicker films (such as 800 nm [20]); but several reports have proved that Ec of P(VDF-TrFE) films increases with the decreased film thickness, and the Ec for 25 nm thick film should be about 360 MV/m [21], far larger than the applied electrical field in our measurements, so Vbias cannot cause the ferroelectric switching, and the only contribution to the asymmetric I–V characteristic is the orientation of dipoles induced by the substrate. Our model seems to imply that, with the increased film thickness, the effect of Ein on tunneling current become negligible and we should observe symmetric I–V curves. However, in our experiments it is hard to validate this implication since the STM tip pierces through the uppermost layers and only images the layers nearest to HOPG substrate. The I–V curves from films on Au substrates displayed symmetrical characteristic, which seems to be a proof that thick films will weaken the effect of the orientated dipole layers and show a symmetrical I–V characteristic. However, such a symmetrical I–V characteristic can be attributed to the existence of Au substrates, which perhaps cannot induce the orientation of dipoles, or to the weak effect of orientated layers because of the increased thickness, or both. Though we have put forward the model of substrate-induced orientation of dipoles to explain the asymmetry of I–V characteristic, we still do not know why and how the substrates oriented the adjacent dipoles, and perhaps it is the electrically positive free dangling bonds on the surface of substrates that
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attract electrically negative fluorine atoms to form such an orderly alignment of dipoles. 5. Conclusion In this paper, we successfully imaged nano-scale nonconducting ferroelectric copolymer films of 25.0 nm by STM. STM images of P(VDF-TrFE) films spin-coated on different substrates (Au or HOPG) displayed different ‘‘topographies’’. STM images of films deposited on Au substrates showed grains of 100 nm, much larger than the grains of Au substrate and consistent with the AFM images of such films. When the scan rate was increased, extreme protrusions and holes were observed which indicated the film surface had been damaged by STM tip. On the surface of films deposited on HOPG, STM images did not show the rod-like structures, which appeared in AFM images, but displayed very flat planes. It was believed the tip had pierced through the uppermost P(VDF-TrFE) layers and only imaged the layers nearest to the substrate. References [1] T.T. Wang, J.M. Herbert, A.M. Glass, The Application of Ferroelectric Polymers, Blackie & Son, Glasgow, 1988. [2] K. EL-Hami, M. Hara, H. Yamada, K. Matsushige, Ann. Chim. Sci. Mat. 26 (2001) 217.
[3] P. Gu¨uthner, K. Dransfeld, Appl. Phys. Lett. 61 (1992) 1137. [4] X.Q. Chen, H. Yamada, Y. Terai, T. Horiuchi, K. Matsushige, P.S. Weiss, Thin Solid Films 353 (1999) 259. [5] K. Noda, K. Ishida, A. Kubono, T. Horiuchi, H. Yamada, K. Matsushige, J. Appl. Phys. 93 (2003) 2866. [6] A. Bune, V.M. Fridkin, S. Durcharme, L.M. Blinov, S.P. Palto, A.V. Sorokin, S.G. Yudin, A. Zlatkin, Nature 391 (1998) 874. [7] J.R. Sheats, Langmuir 10 (1994) 2044. [8] L.M. Blinov, V.M. Fridkin, S.P. Palto, A.V. Bune, P.A. Dowben, S. Ducharme, Phys. Usp. 43 (2000) 243. [9] K. Kobayashi, H. Masuda, H. Yamada, K. Matsushige, Eur. Polym. J. 40 (2004) 987. [10] L. Cai, H. Qu, C. Lu, S. Ducharme, P.A. Dowben, J. Zhang, Phys. Rev. B 70 (2004) 155411. [11] D.P. Smith, H. Horber, Ch. Gerber, G. Binnig, Science 245 (1989) 43. [12] S. Hayashi, A. Imamura, J. Polym. Sci. B 30 (1992) 769. [13] S.N. Magonov, M.H. Whangbo, Surface Analysis with STM and AFM, VCH, New York, 1996. [14] S.L. Tang, A.J. McGhie, A. Suna, Phys. Rev. B 47 (1993) 3850. [15] S.N. Magonov, Appl. Spectrosc. Rev. 28 (1993) 1. [16] J.Y. Yuan, Z. Shao, C. Gao, Phys. Rev. Lett. 67 (1991) 863. [17] S.M. Lindsay, O.F. Sankey, Y. Li, C. Herbst, A. Rupprecht, J. Phys. Chem. 94 (1990) 4655. [18] H. Qu, W. Yao, T. Garcia, J. Zhang, A.V. Sorokin, S. Ducharme, P.A. Dowben, V.M. Fridkin, Appl. Phys. Lett. 82 (2003) 4322. [19] T. Furukawa, Phase Transitions 18 (1989) 143. [20] J. Li, C. Baur, B. Koslowski, K. Dransfeld, Physica B 204 (1995) 318. [21] L.M. Blinov, V.M. Fridkin, S.P. Palto, A.V. Sorokin, S.G. Yudin, Thin Solid Films 284–285 (1996) 474.
Applied Surface Science 254 (2008) 2487–2492 www.elsevier.com/locate/apsusc
Imaging of ferroelectric vinylidene fluoride and trifluoroethylene copolymer films by scanning tunneling microscopy GuoDong Zhu, ZhiGang Zeng, Li Zhang, XueJian Yan * Department of Materials Science, Fudan University, 220 Handan Road, Shanghai 200433, China Received 30 June 2007; received in revised form 28 August 2007; accepted 25 September 2007 Available online 29 September 2007
Abstract In this paper, we reported the possibility to image non-conducting P(VDF-TrFE) copolymer films by STM. The films had the thickness of 25.0 nm and were spin-coated onto Au or graphite substrates. For films deposited on Au substrates, STM images showed grain structures of 100 nm, much larger than the grains of bare Au substrate. With increased scan rate, the film surface was damaged by STM tip and extreme protrusions and holes were observed. For films deposited on graphite substrates, we only obtained an image of very flat plane and could not observe the topography of the film surface. It seemed that the tip had pierced through the uppermost P(VDF-TrFE) layers and only imaged the layers nearest to the substrate. Asymmetrical current–voltage curves were observed from copolymer films deposited on HOPG. Experimental results were discussed. # 2007 Elsevier B.V. All rights reserved. PACS : 68.37.Ef; 68.55.a; 73.40.Ei Keywords: Scanning tunneling microscopy (STM); Atomic force microscopy (AFM); P(VDF-TrFE); Surface morphology; Current–voltage characteristic
1. Introduction Ferroelectric polymers such as poly(vinylidene fluoride) (PVDF) and its copolymer P(VDF-TrFE) have been well studied because of their applications in transducers, sensors, actuators, and potential ultra-high-density data storage [1,2]. Especially in recent years, scanning probe microscopy has been widely utilized to investigate the local structures and electrical properties of P(VDF-TrFE) thin films. Gu¨thner demonstrated the possibility to control and image local polarized domains on P(VDF-TrFE) thin films by using atomic force microscope (AFM) [3]; Matsushige et al. studied local piezoelectric properties of PVDF and P(VDF-TrFE) thin films prepared by spin-coating and vacuum-deposition methods whose thickness were on the order of several nanometers to several tens of nanometers [4,5]; Bune, et al. succeeded in preparing twomonolayer copolymer film by Langmuir–Brodgett (LB)
* Corresponding author. Tel.: +86 021 6564 2872; fax: +86 021 6564 2872. E-mail address: [email protected] (X. Yan). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.09.072
method and discovered two-dimensional ferroelectricity by means of scanning tunneling microscope (STM) [6]. Sheats [7] had reported the possibility to image nonconducting polymer films with the thickness up to 110 nm by STM. P(VDF-TrFE) was also non-conducting, but STM imaging of such films was only obtained from its LB films, of which the thickness was only about 1.0 nm [8]. Studies on its thicker films (such as tens of nanometers) by STM have not been reported yet. In this paper, we attempted to image spin-coated P(VDF-TrFE) films by STM and to compare the differences in STM topographies of films deposited on different substrates. 2. Experiment Copolymer films were spin-coated from a 1.0% by weight solution of 60/40 P (VDF-TrFE) in butanone onto different conducting substrates (highly oriented pyrolytic graphite, HOPG, or Au substrates), the thickness of these as-cast films was about 25.0 10.0 nm, which was measured by AFM’s line profile. HOPG substrates were freshly cleaved and Au substrates were prepared by vacuum depositing 40 nm Au films onto mica.
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Four groups of samples were used in experiments: the first group was as-cast P(VDF-TrFE) films deposited on Au substrate, indexed as S1; the second was films deposited on Au substrate and annealed at 132 8C for 6 h to increase their crystallinity, indexed as S2; the third was as-cast films deposited on HOPG, indexed as S3; and the last was films deposited on HOPG and annealed at 132 8C for 6 h, indexed as S4. Topographies of these films were characterized by AFM (UltraObjective, Surface Imaging Systems) and STM (AJ-I, Aijian Inc.), respectively. The tungsten tips used in STM measurements were dc-etched in 1.0 mol/L KOH solution, the bias voltage (Vbias) was applied to the substrate. Both AFM and STM images were taken in air and at room temperature. 3. Results 3.1. P(VDF-TrFE) films deposited on Au substrate AFM morphologies of P(VDF-TrFE) films deposited on Au substrates were shown in Fig. 1. The surface of as-cast sample S1 (Fig. 1a) was covered by fine grains with the diameter of tens of nanometers, and the boundaries of these grains were blurry. The root mean square (RMS) roughness of such films was about 1.48 nm. Morphology of the annealed film S2 (Fig. 1b) showed even larger grains, but no distinct differences in morphology between S1 and S2 samples could be observed. The RMS roughness of annealed film S2 was about 2.55 nm. Topographies of S1 and S2 were also obtained by STM, which could be seen in Fig. 2. During the scan of all these four images, STM was operated in the same conditions: Vbias = 100 mV, setpoint current Iset = 0.5 nA. As a comparison, Fig. 2a indicated the STM morphology of bare Au substrate. Au film was covered with elliptical grains with diameter of 10–20 nm. Its RMS roughness was only about 0.78 nm. Fig. 2b showed the STM image of as-cast sample S1. The film surface consisted of large grains with diameter of 50– 100 nm, much larger than those in bare Au substrate, and some of adjacent grains united into clusters and the boundaries of such grains were blurred. The RMS roughness of S1 in STM measurements was about 1.90 nm. The STM morphology of S2
was shown in Fig. 2c. The film surface was still covered with grain structures as large as 100 nm, however, compared with the STM morphology of S1 (Fig. 2b), the grains shown in annealed films (Fig. 2c) were isolated, and more subtle details were observed in these grains: it seemed that the annealed grains consisted of some lamellar or strip-like structures. The RMS roughness of S2 was about 2.69 nm. The roughness of copolymer films revealed by STM was consistent with that revealed by AFM. The scan rate for Fig. 2b and c was 1500 nm/s, and at such a slow scan rate, we could obtain repeatable images. Varying the bias voltage Vbias from 50 to 600 mV or changing its polarity or changing the setpoint current Iset from 0.1 to 5.0 nA did not alter the images significantly. However, when the scan rate was increased to 4000 nm/s, the image soon deteriorated grossly, and extreme topography (several hundred nanometers, much greater than the film thickness) appeared. Fig. 2d showed one of such results, which was obtained on S2 surface with high scan rate (4000 nm/s). Extreme protrusions and holes were observed. It was believed that the tip had scratched out or piled up part of the P(VDF-TrFE) film. This damage did not occur on the surface of bare Au substrate, even when the scan rate was increased to 10,000 nm/s. From all these experimental measurements, it could be concluded that the images observed by STM on the surface of non-conducting film (Fig. 2b,c) should be attributed, at least partly, to the topography of copolymer film itself. Perhaps the Au substrate also had some contributions to the imaging of nonconducting film, but in this paper we could not judge to what extent the STM images in Fig. 2b and c could be attributable to the Au substrate. 3.2. P(VDF-TrFE) films deposited on HOPG AFM topography of S3 spin-coated on HOPG resembled that observed on S1 sample spin-coated on Au substrate, which was shown in Fig. 1a. After the film was annealed at 132 8C for 6 h, the surface of the sample S4 was covered with rod-like structures (Fig. 3a). The width of these rods was about 100 nm and their length could be as long as 500 nm. Such rod-like
Fig. 1. AFM morphologies of as-cast (a) and annealed (b) P(VDF-TrFE) films spin-coated on Au substrates.
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Fig. 2. STM morphologies of Au substrate (a), as-cast (b), annealed (c) and deteriorated (d) P(VDF-TrFE) films spin-coated on Au substrates.
structures were similar to those observed by Kobayashi [9]. The RMS roughness of S4 was about 2.72 nm. However, whether on the surface of as-cast sample S3 or on the surface of annealed one S4, images obtained by STM were extremely different from those obtained by AFM, although we varied Vbias from 800 to +800 mV and Iset from 0.1 to 8.0 nA. What we observed by STM was only a very flat plane, much similar to the plane of bare HOPG. Fig. 3b indicated one typical STM image of such a flat plane. The image was obtained on the surface of annealed film S4, Vbias was 100 mV and Iset was 1.0 nA. Fig. 3b displayed the steps of HOPG substrate buried under the copolymer film rather than the rod-like structures shown in AFM image (Fig. 3a).
High resolution images could also be performed by STM on the surface of annealed P(VDF-TrFE) film (Fig. 4a, Vbias = 200 mV, Iset = 1.0 nA). Though large noises blurred the image, an ordered strip-like structure could still be clearly observed in the center of Fig. 4a. The spacing of these parallel ˚ . As a comparison, atomic resolution strips was about 3.5 A image of bare HOPG substrate was also shown in Fig. 4b (Vbias = 300 mV, Iset = 2.0 nA). The spacing of adjacent carbon ˚ , which was much smaller than the spacing atoms was 2.5 A ˚ was consistent with the shown in Fig. 4a. The spacing of 3.5 A interchain spacing (3.3 0.1 nm) observed on the surface of P(VDF-TrFE) LB films by STM [10], so it seemed to suggest that the white strips in Fig. 4a should correspond to the
Fig. 3. Morphologies of annealed P(VDF-TrFE) film spin-coated on HOPG, imaged by AFM (a) and STM (b).
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Fig. 4. High resolution imaging of annealed P(VDF-TrFE) film spin-coated on HOPG (a) and bare HOPG substrate (b).
molecular chains of P(VDF-TrFE). The intrachain spacing could not be distinguished in Fig. 4a because of too large noises. It seemed that the STM tip pierced through the uppermost P(VDFTrFE) layers and only imaged the layers nearest to HOPG substrate. In fact, this phenomenon had been reported on the study of n-alkylcyanobiphenyl liquid crystal monolayers [11]. ˚ was much smaller than the bulk The interchain spacing of 3.5 A ˚ [12]. Cai, et al. close packing spacing between chains of 4.25 A [10] believed that such a reduced interchain spacing was caused by the interaction between the HOPG substrate and the copolymer layers nearest to the substrate. The scan rate for Fig. 3b was 2000 nm/s. Different from the STM imaging of P(VDF-TrFE) deposited on Au substrate, even after the scan rate had been increased to 8000 nm/s, we still obtained the flat images, just as shown in Fig. 3b, and no holes or protrusions were observed. 3.3. Current–voltage characterization of P(VDF-TrFE) films By STM, the current–voltage (I–V) curves were also measured on bared HOPG substrate and on ferroelectric films
Fig. 5. I–V characteristics obtained by STM and on the surfaces of bared HOPG and P(VDF-TrFE) films deposited on HOPG and Au substrates, respectively.
deposited on HOPG (S4) and Au (S2) substrates, respectively. The results were shown in Fig. 5. I–V curves from HOPG or from copolymer film deposited on Au substrate showed a symmetric structure, that was to say, whether Vbias was in negative or in positive polarity, the same magnitude of Vbias would correspond to the same magnitude of the tunneling current. However, A rectification effect was observed on the surface of P(VDF-TrFE) films deposited on HOPG. At the range of 500.0 mV < jVbiasj < 1500.0 mV, with the increase of jVbiasj, the slope of I–V curve increased. For the same magnitude of Vbias, the magnitude of the tunneling current induced by the negative Vbias was far larger than that induced by the positive Vbias (for example, when Vbias = +1000.0 mV, the resultant tunneling current I = 34.0 nA; while, when Vbias = 1000.0 mV, I = 94.0 nA). Vbias in negative polarity resulted in sharper I–V characteristic. 4. Discussions 4.1. STM imaging of nanoscale non-conducting films STM is expected to be severely limited in its application to organic materials, since it requires electrical conductivity. Though ultrathin layers (monolayers) of a variety of molecules have been imaged successfully in molecular details [11,13], only few reports exist on STM imaging of thicker nonconducting films. Tang, et al. [14] describes the imaging of a polyhydroxycellulose film (several hundred angstroms thick); Sheats [7] has imaged several kinds of non-conducting polymer films with the thickness up to 110 nm. Imaging of these surfaces is not possible within the context of conventional STM theory [15], the thickness of such films is too great for electrons to tunnel through directly. Some models or mechanisms for imaging non-conducting thicker films have been put forward (absorbed water layer model [16], pressureinduced resonance mechanism [17], dielectric breakdown model [14] and so on). But to date it is still an open question on how STM works in the imaging of thicker non-conducting polymer films.
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In our experiments, compared with HOPG substrate, the high conductivity of Au substrates seem to have increased the possibility of dielectric breakdown between the tip and the film [14], which causes a large current that can be detected by STM over the non-conducting film. However, this breakdown does not seem to occur between the tip and the film deposited on HOPG because of the lower conductivity of HOPG. To keep the tunneling current constant, preset by Iset, the tip has to pierce the uppermost P(VDF-TrFE) layers until the detected tunneling current reaches the preset value Iset. 4.2. Asymmetrical I–V characteristics An obvious rectification characteristic is observed on the surfaces of copolymer films deposited on HOPG. However, a symmetric I–V characteristic is obtained on the surfaces of bared HOPG, which suggested that such asymmetric I–V characteristics should not originate from the instrumental errors or from the asymmetric structures of the tip-air gap-sample systems. In fact, several papers have referred to the effect of substrate materials (especially HOPG substrate) on the structures of P(VDF-TrFE) copolymer films and the orientations of molecular dipoles. In the study of local poling of ferroelectric P(VDF-TrFE) ultrathin films (about 23 nm thick, spin-coated on HOPG), Chen, et al. [4] observed that the negatively local poled areas had lower piezoelectric amplitudes than the positively poled areas, and explained this by the local orientational changes of dipoles induced by the substrate. Qu et al. [18] imaged two-monolayer P(VDF-TrFE) ultrathin films prepared by LB deposition technique, observed the same asymmetric I–V characteristic by STM, and believed that the substrate had oriented the nearest dipoles upward. Based on all these studies mentioned above, we put forward the model of substrate-induced orientations of dipoles (shown in Fig. 6) to understand the results in our STM measurements. HOPG substrate has induced the orientation of those molecular dipoles which are adjacent to the substrate. These oriented dipoles are indicated as grey and directed upward in Fig. 6. A
Fig. 6. A model for the asymmetrical I–V characteristic in P(VDF-TrFE) films deposited on HOPG.
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build-in electrical field Ein has been set up for the orderly alignment of these oriented dipoles. However, the dipoles, far away from the substrate still oriented disorderly because of the negligible effect of the substrate. When the Vbias is in negative polarity, relative to the substrate, the STM tip lies in a higher voltage level, and the applied electrical filed Eout is downward. The direction of Ein is parallel to the direction of Eout, so the existence of Ein should accelerate the tunneling electrons through the oriented layer in copolymer film, as induces a relatively large tunneling current. While, when Vbias is in positive polarity, the STM tip is in a lower level, and the direction of Ein is antiparallel to the direction of Eout, so Ein should slower the tunneling electrons through the oriented layer and should correspond to a reduced current. The model discussed above is different from that given by Qu et al. [18]. The thickness of P(VDF-TrFE) LB ultrathin films studied in reference 18 is two monolayers (1.0 nm), and 0.4 V < jVbiasj < 1.0 V is the range of Vbias, in which a asymmetric I–V characteristic is observed. In their experiments, the strength of the electrical field is about 0.4 1.0 GV/m, which is near or even much larger than the coercive filed Ec of the ultrathin P(VDF-TrFE) films (about 0.5 GV/m [8]), so in their model, there are two contributions to the asymmetry of I– V characteristic: one is substrate-induced orientation of dipoles, and the other is the ferroelectric switching, caused by the high enough electrical field. However, in our STM measurements, the thickness of the spin-coated copolymer films is about 25.0 10.0 nm, the range of Vbias is 0.5 V < jVbiasj < 1.5 V, and then the resultant electrical field is about 25–75 MV/m, which is near or just a bit larger than Ec (50 MV/m [19]) of the bulk materials or thicker films (such as 800 nm [20]); but several reports have proved that Ec of P(VDF-TrFE) films increases with the decreased film thickness, and the Ec for 25 nm thick film should be about 360 MV/m [21], far larger than the applied electrical field in our measurements, so Vbias cannot cause the ferroelectric switching, and the only contribution to the asymmetric I–V characteristic is the orientation of dipoles induced by the substrate. Our model seems to imply that, with the increased film thickness, the effect of Ein on tunneling current become negligible and we should observe symmetric I–V curves. However, in our experiments it is hard to validate this implication since the STM tip pierces through the uppermost layers and only images the layers nearest to HOPG substrate. The I–V curves from films on Au substrates displayed symmetrical characteristic, which seems to be a proof that thick films will weaken the effect of the orientated dipole layers and show a symmetrical I–V characteristic. However, such a symmetrical I–V characteristic can be attributed to the existence of Au substrates, which perhaps cannot induce the orientation of dipoles, or to the weak effect of orientated layers because of the increased thickness, or both. Though we have put forward the model of substrate-induced orientation of dipoles to explain the asymmetry of I–V characteristic, we still do not know why and how the substrates oriented the adjacent dipoles, and perhaps it is the electrically positive free dangling bonds on the surface of substrates that
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attract electrically negative fluorine atoms to form such an orderly alignment of dipoles. 5. Conclusion In this paper, we successfully imaged nano-scale nonconducting ferroelectric copolymer films of 25.0 nm by STM. STM images of P(VDF-TrFE) films spin-coated on different substrates (Au or HOPG) displayed different ‘‘topographies’’. STM images of films deposited on Au substrates showed grains of 100 nm, much larger than the grains of Au substrate and consistent with the AFM images of such films. When the scan rate was increased, extreme protrusions and holes were observed which indicated the film surface had been damaged by STM tip. On the surface of films deposited on HOPG, STM images did not show the rod-like structures, which appeared in AFM images, but displayed very flat planes. It was believed the tip had pierced through the uppermost P(VDF-TrFE) layers and only imaged the layers nearest to the substrate. References [1] T.T. Wang, J.M. Herbert, A.M. Glass, The Application of Ferroelectric Polymers, Blackie & Son, Glasgow, 1988. [2] K. EL-Hami, M. Hara, H. Yamada, K. Matsushige, Ann. Chim. Sci. Mat. 26 (2001) 217.
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