1992 Libro PFM

The local piezoelectric activity of thin polymer films observed by scanning tunnelingmicroscopyH. Birk, J. GlatzReichenbach, Li Jie, E. Schreck, and K. Dransfeld

Citation: Journal of Vacuum Science & Technology B 9, 1162 (1991); doi: 10.1116/1.585238 View online: http://dx.doi.org/10.1116/1.585238 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/9/2?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing

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Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 132.248.12.54 On: Tue, 19 Jan 2016 01:07:05

The local piezoelectric activity of thin polymer films observed by scanning tunneling microscopy

H. Birk, J. Glatz-Reichenbach, Li-Jie, E. Schreck, and K. Dransfeld Fakultiitfiir Physik, Universitat Konstanz, D-7750 Konstanz, Federal Republic of Germany (Received 24 July 1990; accepted 26 October 1990) Here we present first measurements of the local piezoelectric activity of thin vinylidene fluoride-trifluoroethylene (VDF-TrFE) copolymer films with a spatial resolution of about 10 nm. The average value of the piezoelectric constant d33 is - 0.03 nm/V varying typically by about 20% across a distance of 10 nm depending on the poling state of the film.

I. INTRODUCTION The copolymers ofvinyIidene fluoride and trifluoroethylene (VDF-TrFE) are scmicrystalline materials. After poling, these piezoelectric and ferroelectric copolymers are used for various technical applications as thin films or thicker foils. I Their structure was investigated, e.g., by x-ray difi'raction,2 synchrotron radiation,3 scanning and transmission electron microscopy,46 and nuclear magnetic resonance studies.7 The microstructural analysis for these semicrystal1ine mate-rials has been interpreted in terms of lamellar crystallitesH embedded in an amorphous matrix. The size of the crystal-lites seems to depend on the thickness of the film,4 decreas-ing for thinner films also in the lateral dimension. The degree of crystallinity of the copolymer films depends for a given VDF-TrFE composition on the special annealing treatment varying between 30% and 90%.6,9

In the past the piezoelectric properties of these copolymer films have mostly been explained by assuming that the fer-roelectric crystallites are responsible for the piezoelectricity of the films. 10, II But up to now the piezoelectrical effect has been studied only on macroscopic samples over regions of at least 1 mm. Here we present a first measurement of the local piezoelectric activity of thin (VDF-TrFE) copolymer films on a scale of the crystalline domains, by using the scanning tunneling microscope (STM). Employing this method in a simple experimental setup we were able to measure the local piezoactivity of the films with a resolution of about 10--20 nm.

By the STM we measured quantitatively the vibrational amplitude of the surface under the influence of an applied ac voltage. Thus we can monitor the local piezoelectric activity depending both on the lateral position and on the application of a high dc electrical field during the previous poling pro-cess.

II. SAMPLE PREPARATION On well cleaned fiat glass plates structured aluminum

electrodes were evaporated, having a thickness of about 300 nm and an area of about 2 mm2 Thereafter a small amount of the (VDF-TrFE) copolymer solution containing 60 mol % VDF, dissolved in ethyl-methyl-ketone, was applied to the substrate and-by using the spin coating technique-we produced copolymer films having a thickness of about 1 11m, The film thickness as measured by an interference mi-croscope turned out to be surprisingly uniform across the

entire electrode area. The optically observed variation of the thickness was less than 30 nm over a lateral distance of 5 mm. Subsequently the samples were annealed up to 145C for 2 h in order to enhance their crystallinity. As the last step, after cooling down again, a 20 nm gold electrode was depos-ited on the top of the copolymer film forming a condenser together with the bottom aluminum electrode. The gold film was evaporated at liquid-nitrogen temperature in order to produce a more homogeneous gold film surface.

The geometry of the polymer film, with the two electrodes and the tip of the STM is shown in Fig, 1. Between the grounded top electrode and the bottom electrode both a dc voltage could be applied for the poling process or an ac vol-tage for driving the film piezoelectrically.

III. THE LOCAL MEASUREMENT OF THE PIEZOELECTRIC ACTIVITY

The STM is a powerful instrument to measure the topo-graphy of a surface on an atomic scale by using the tunneling current as a sensor for the distance between tip and surface. Even small motions of the top electrode perpendicular to the surface-amounting only to a fraction of an angstrom-can

Piezo-Positioner''''.......

TunneliC\g-TiP~"

Ld L ---20nm . -,.

FIG, 1. Exp

1163 Blrk et al.: local piezoelectric activity of thin polymer films

thus be detected. In our case we apply a periodic driving voltage Ud (of about 10 V and a frequency of20 Hz) via the top and bottom electrodes to the poled copolymer film. In this way surface vibrations having an amplitude of about 0.3 urn are induced jfthe copolymer film is locally piezoelectric.

As is well known I2 the tunneling current It depends ex-ponentially on the tunneling gap d,

I,-exp[ -- (2Ifl)/irn!'d], with m being the electronic mass and the effective work function of gold, which for our experimental condition is in the range of 100 meV. I )

IV. THE CONSTANT-CURRENT MODE OF THE STM For a quantitative measurement of the local piezoclectri-

cal effect we used the STM in the "constant current" mode: To stimulate the piezoelectric motion ofthe copolymer film we applied a slowly varying (20 :Hz) triangular driving vol-tage Ud to the film electrodes as shown in Fig. 2.

The tunneling current is kept constant by a feedback loop and thus the tunneling tip accurately follows the motion of the top electrode. By measuring the magnitude of the feed-back signal with a lock-in amplifier, we could observe direct-ly the vibrational amplitude perpendicular to the sample sur-face (z direction in Fig. 2) and thus determine the local value ofthe piezoelectric constant d3,. In our experiments, we ob-tained an average value of about 0.03 nm/V on a poled co-polymer film in good agreement with macroscopic observa-tions on thicker films of nearly the same copolymer material. 10

In these experiments we took special care to electrostati-cally screen the tip from the ac driving field Ud If the shield-ing by the thin top electrode would not be complete a dis-placement current to the tip-being proportional to (d U d I dt) -could in principle be added to the tunnel current and thus disturb the measurement. In our experiments we used such a low frequency (20 Hz) that no "pick-up" signal was observed.

101 Ibl

FIG. 2. The piezoelectric surface motion of a (60-40) VDF-TrFE copoly-mer film as measured by the STM in the constant current moue. The slowly varying triangular driving voltage U" (lower trace) and the feedback vol-tage VI (upper trace, typically 40 m V) afe plotted as a function of time for (a) positive and for (b) negative poling. The absolute value of the surface amplitude tu is derived from the calibration of the z piczopositioner. The resulting observed local piezoelectric constant d'1 ~,- 0.03 nm/V is near-ly equal for both directions of the polarization. The driving frequency was 20Hz.

J. 'lac. Sci. Techno\. B, Vol. 9, No, 2, Marl Apr 1991

1163

v. THE INFLUENCE OF HIGH ELECTRICAL FIELDS ON THE SURFACE MOTION DURING PIEZOELECTRIC STIMULATION OF THE COPOLYMER FILM

If we superimpose (in addition to the driving voltage Ud at 20 Hz) a poling voltage Up of a lower frequency (typically 10 mHz) to the copolymer film, we can follow the time-dependent variation of its local piezoactivity. The value of the piezoelectric constant d u is a function of the poling vol-tage and its saturation is reached for an applied field of 90 MV 1m (see Fig. 3). The coercive field turned out to be 50 MV 1m for our 0.85 /-lm thin copolymer films, which is about 10 MV 1m higher than reported for films which are several /-lm thick. 14 For a sudden application of small poling vol-tages (of approximately 35 MV 1m), the corresponding change of the piezoelectric activity needs several seconds to build up which we can clearly see as a time dependence of the hysteresis loop (such as shown by the inner curve in Fig. 3) where the poling voltage varies with a frequency of 10 mHz. If, however, the poling field amplitude is high (Up> 100 MV 1m) the local switching time is faster than 1 ms.9

Additionally, we saw a shift of the whole hysteresis loop parallel to the positive vertical axis particularly for a film thickness below 1 pm and for an applied poling field well above the coercive field of 50 MV 1m. At present, this phe-nomenon is not yet understood.

VI. PIEZOELECTRICAL IMAGING Subsequently we moved the tip at constant tunneling cur-

rent in a one-dimensional scan across the sample over a later-al distance of 180 nm at a speed of 0.2 nm/s while the driving voltage Uel was applied. The vibrational amplitude was mea-sured locally with the lock-in technique. Simultaneously we monitored the topology of the gold covered copolymer film (see Fig. 4). We found no clear correlation between the am-

1.50, ~

:' :::1 ~ 4;~ I t 0,"1 iZ~_/ I ~ -0.50 t .,/17 11 I ~ r ~-~-,I..~-'B I 1E -1.00 1_ I

1.50-~--------~--- -----.~ 120 80 40 0 40 80 120

poling field [MV/m]

FIG. 3. The 11ystcresis of the piezoelectric activity for a 0.85 pm thick (60-40) VDF-TrFE copolymer film: The inner curve (A) shows a hysteresis for a poling voltage well below the saturation field strength. For higher poling voltages outer curve (D) clearly shows the saturation ofthe surface motion. For both curves a small driving voltage Ud (4 V amplitUde at a frequency of 20 Hz) and a variable poling voltage (up to 100 V amplitude at a frequency of 10 mHz) arc simultaneously applied. The ae piezoelectric surface motion (caused hy the driving voltage U,,) is plotted as a function of the poling field. Saturation of the surface motion is reached for eleCtrical fields above 90 MV 1m. The coercive field tumed out to be of about 50 MV 1m.


1164 Birk et af.: Local piezoelectric activity of thin polymer films

~120 0

5> 90 o

1165 Bir!< at al.: local piezoelectric activity of thin polymer films

7 J. F. Legrand, P. J. Schuele, V. H. Schmidt, and M. Menier, Polymer 26, 1683 (1985).

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"'T. Furukawa. IEEE Trans. Electr. Insul. Ei-24, 375 (1989). 11 M. G. Broadhurst and G. T. Davis, in Topics in Applied Physics, edited

J. Vac. Sci. Techno!' S, Vol. 9, No.2, Marl Apr 1991

by G. M. Sessler (Springer, Berlin, 1980), Vol. 33, p. 347. 12p. K. Hansma and J. Tersoff', J. App!. Phys. 61, Rl (1987). "G. Binnig and H. Rohrer, IBM J. Res. Develop. 30, 355 (1986).

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'''T. Vagi, Y. Higashihata. K. Fukuyama, and J. Sako, Ferroelectrics 57, 327 (1984).

15 P. Giithner, J. Glatz-Reichenbach, and K. Dransfeld, J. AppL Phys. (to be published).


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