J. Phys. Chem. 1995,99, 12375-12378
12375
Langmuir-Blodgett Films Prepared from Ferroelectric Lead Zirconium Titanate Particles Nicholas A. Kotov, Genaro Zavala, and Janos H. Fendler* Department of Chemistry, Syracuse University, Syracuse, New York 13244-4100 Received: December 2, 1994; In Final Form: May 22, I99.P
The preparation of monoparticle-thick Langmuir films from 70-nm, lauric acid-coated (dispersion I) and alcoholcoated (dispersion 11) ferroelectric lead zirconium titanate (PZT) particles is described. Surface pressure vs surface area (A) isotherms, in situ on the water surfaces, atomic force microscopic ( A M ) , and electron microscopic ("EM) images, ex situ after their transfer to substrates, established the presence of monoparticles with appreciable and variable interparticle distances (dispersion I) and relatively densely packed monoparticles (dispersion 11) in the Langmuir films. Langmuir-Blodgett (LB) films were formed by the monoparticulate layer by monoparticulate layer transfer of the PZT film to substrates (glass slides coated by 500-A gold or indium tin oxide). Good transfer ratios (up to eight layers) were obtained if each transferred PZT film was fixed by a methanol:ethanol:2-propanolsolution of lead(I1) acetate, zirconium(1V) butoxide, and titanium(IV)isopropoxide taken in stoichiometric amounts for Pb(Zro.5Tio.5)O and cured at 80 OC for 1.5 min. Ferroelectric properties of the LB films, prepared from dispersion 11, have been demonstrated by polarization measurements using a Sawyer-Tower circuit.
(n)
Introduction
Experimental Section
The two remanent polarization states render ferroelectric thin films to be potentially useful as nonvolatile memory storage devices.'-3 Ferroelectric thin films have been prepared from such perovskite-type compounds as lead zirconium titanate (PbTi03aPbZr03, E T ) , barium titanate (BaTiOs), or lead titanate (PbTiO 3) by physical (plasma or ion-beam sputtering, for example)'.2 or chemical (sol-gel pro~essing)~ methods. Regardless of the method of preparation, the sample has to crystallize into the required ferroelectric phase.'-3 This is accomplished by high-temperature (700- 1000 "C)annealing, which is the final step in the fabrication of ferroelectric thin films. We report here a radical departure from the currently available methods of ferroelectric film fabrication. This technique may potentially lead to a new generation of ferroelectric films with improved properties and longevities. Our starting material, hydrophobic PZT particles, has been rendered ferroelectric prior to film formation. There are several advantages to ferroelectric films that are prepared from individual particles. Potentially, each particle is individually addressable, and the hydrophobic coating on the nanoparticles is likely to alleviate interparticle diffusion of ions, defects, and charge carriers and thereby increase the lifetime of the memory device. We have spread hydrophobic PZT nano- and microparticles on aqueous solutions in a Langmuir film balance and transferred them, monoparticulate layer by monoparticulate layer, to solid substrates (gold-coated glass slides) by using techniques that are analogous to those used in the formation of LangmuirBlodgett (LB)-type metallic and semiconducting particulate films5s6 The monoparticulate PZT Langmuir films have been characterized in situ on the water surface [by surface pressure (n)vs surface area (A) isotherms] and ex situ on substrates [by atomic force microscopy (AFM)and transmission electron microscopy (TEM)]. The PZT nanoparticulate LB films have been shown to be ferroelectric by the ferroelectric hysteresis loops, generated by means of a Sawyer-Tower circuit.',*
Lead zirconium titanate [Pb(Zro.5,Tio.5)03 (PZT)] was prepared7 by calcinating stoichiometric amounts of titanium oxide (0.05 mol; Aldrich) and zirconium oxide (0.05 mol; Aldrich) in air at 1400 "C for 4 h. After pulverization, lead oxide (0.1 mol, Aldrich) was added and the well-stirred mixture was calcinated, again in air, at 800 "C twice for 2-h periods each. Exhaustive grinding of this sample lead to the stock powder of PZT that was used in the present experiments. X-ray diffraction measurements [on a Philips PW1729 automated powder diffractometer using Cu Ka radiation; the powder was mounted with a binder (methanol) on a 1 in. x 2 in. glass slide to form a thick film] established the tetragonal ferroelectric structure for the PZT powder and the absence of any residual components (Figure 1). The radiation used was Cu Ka (A =1.5418 8,). Calculating 28 (8 = the diffraction angle) from the Bragg's diffraction law (A = 2d sin 8) and taking the cell parameters for this composition as a = 4.016 8, and c = 4.118 AI6 ( a = b since the phase is tetragonal), we ascertained that the planes plotted on the figure matched with the peaks obtained on the profile, Le., 28 = 31.07 8, for the (110) plane and 28 = 55.77 8, for the (211) plane. Moreover, we can observe from the profile that peaks such as (100) and (OOI), as well as (211) and (112) are separated and that peaks (110) and (211) are higher than (001) and (112), respectively. This is required on the tetragonal phase by the multiplicity factor of the planes." No signs of a residual component were found, although a small amount of amorphous PZT might have been present which could not be revealed by X-ray diffraction. The stock PZT powder was subjected to a crude particle size separation by sedimentation from 180 proof ethanol dispersions. Only particles that remained suspended after 10 min of sedimentation were used in further experiments. The sizes of particles in these suspensions did not exceed 1 pm, as was determined by TEM and
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Abstract published in Advance ACS Abstracts, July 1, 1995.
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Adjusting the delicate balance between hydrophilic and hydrophobic properties of particles was found to be crucial for the successful formation of monoparticulate films on water surfaces. If the species being spread was too hydrophilic, it quickly sank. Conversely, considerable agglomeration occurred
0022-365419512099- 12375$09.0010 0 1995 American Chemical Society
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upon using particles that were too hydrophobic, leading to the formation of irregular, highly porous structures. Two different dispersions were employed. In dispersion I, lauric acid was added to the PZT particles, in 1:0 w/w, and dispersed in a ch1oroform:hexane = 1:4 v/v mixture. Dispersion I was sonicated (in a bath-type sonicator for 2 min) just prior to spreading. In dispersion 11, the PZT particles were suspended in 1-propano1:chloroform:isooctane= 3:5:4, v/v, or 1-butanol: ch1oroform:isooctane = 2:5:4, v/v. Surprisingly, absolutely stable Langmuir films, withstanding surface pressures of up to 80 mN/m, could be obtained from dispersion I1 without any silylation or surfactant addition. The presence of 1-propanol or 1-butanol was found to be essential for Langmuir film formation. Alcohol adsorption on the surface of PZT particles is likely to render the film to be hydrophobic. IT vs A isotherms of the spread dispersion I and dispersion I1 were determined in a MGW Lauda film balance, equipped with a barrier pressure sensor, at compression speeds of 3 cm/ min. After compression, the spread PZT particles produced a dense, macroscopically homogeneous, uniformly colored film which could be easily transferred onto various substrates by the standard Langmuir-Blodgett technique with a transfer ratio of 1.0 f 0.05. The transfer of subsequent layers required fixing the deposited layer by a solution of lead acetate, titanium isopropoxide, and zirconium ethoxide, in ratios corresponding to Pb(Zr0 5Tio 5)0, in a mixture of methanol:ethanol:2-propanol = 1:1:2, v/v. The fixing solution was deposited drop by drop near the edge of the dried (at 80 "C for 1 min) PZT film. The solution spread along the film by capillary forces uniformly damping the particulate film. Extreme care was taken to avoid the presence of excess fixing solvent. After to drying at 80 "C for 1 min, the particulate film was found to be quite firm and robust. Repetition of the fixing-monoparticulate film transferfixing cycles allowed the deposition of a dense PZT film in any desired thickness. Firing the PZT films (typically containing four to five monoparticulate layers) at c a 250 "C for 2 h bumed off the organic ingredients and cemented the separate ferroelectric PZT particles together by nonferroelectric PZT (derived from the fixing solution). A thin layer of gold (500-A thick) was deposited on top of the prepared films by using an aluminum foil mask. Electrical polarization loops were taken by a Sawyer-Tower circuit, equipped with an adjustable resistor to compensate for the linear dielectric loss of the LB films. A graphite lead and a tungsten STM tip were used as electrodes in contact with the gold deposited on top of the PZT film. The second electrode was fastened to the conducting substrate onto which the ferroelectric LB films had been deposited. Upon the application of external voltage, the resistor was adjusted to compensate for the linear dielectric loss and the signals were collected by an HP-5410A digitizing oscilloscope. The realtime monitoring of the polarization loops was realized by a Kikisui 5021 two-channel oscilloscope. AFh4 images of monoparticle-thick PZT films, transferred from the water surface onto freshly cleaved mica or gold-coated
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Figure 2. (A) Consecutive surface pressure isotherms of a PZT-lauric acid monolayer (dispersion I); the first, second, and third compression cycles appear from right to left. 0.2 g of PZT-0.04 g of lauric acid dispersed in a 10 mL of chloroform/ hexane mixture 4:l v/v. Compression speed = 30 cm2/min. (B) Two consecutive surface pressure isotherms of a PZT monolayer (dispersion 11); the first and second compression cycles appear from right to left. PZT (0.3 g) dispersed in a 10 mL 1-propano1:chloroform:isooctanemixture 3 5 4 vlv. Compression speed = 30 cm2/min.
glass substrates (flat within 1 nm), were taken by a Topometrix Explorer scanning probe microscope in both the contact and noncontact modes using standard silicon nitride tips (force constant of 0.12 N/m) and silicon tips (force constant of 3664 N/m), respectively. Reproducible images were obtained both in air and under water. Transmission electron micrographs were taken by a JEOL JEM-2000 EX 80-keV instrument. The monoparticulate PZT film was transferred (from the water surface) onto a 400-mesh copper grid by horizontal lifting and was air dried prior to the electron microscopic measurements.
Results and Discussion Spreading of dispersion I on the water surface resulted in the formation of an extremely thin, hardly visible film whose behavior resembled that of a usual monolayer, prepared from simple surfactants or lipids? The IT vs A isotherms of dispersion I were not reversible, even below a surface pressure of 30 mN/m (Figure 2). A wellpronounced phase transition region is evident in the first compression curve. It corresponds to the collapse and extrusion of the lauric acid moiety from the interface region. Upon compression, the film attained a faint yellowish coloration due to PZT. Multiple compression-expansion cycles (0 < ll < 40 mN/m) resulted in a 3-fold decrease of the critical surface area (see Figure 2A) which signaled the enrichment of the surface in PZT particles and the gradual dissolution of the acid in the aqueous subphase. Thus, an increasing number of compression-expansion cycles progressively decreased the PZT interparticle distances. This behavior is documented by the AFM images of PZT films picked up from the water surfaces subsequent to one to three compression and expansion cycles (Figure 3).
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J. Phys. Chem., Vol. 99, No. 33, 1995 12377 2 5 (rm
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Figure 3. Atomic force microscopic images of a PZT-lauric acid monolayer deposited on a glass substrate coated with a 500-81 layer of gold; 0.2 g of PZT-0.04 g of lauric acid dispersed in a 10 mL chlorofodhexane mixture 4:1 v/v at various stages of compression: ll = 10 mN/m, first compression cycle (A, top); ll = 25 mN/m, first compression cycle (B, middle); and ll = 55 mN/m, third compression cycle (C, bottom).
Spreading dispersion I1 on the water surface resulted in the formation of highly visible domains which coalesced into a uniform film upon compression to 40-50 mN/m (Figure 2B). The surface pressure rose immediately to ca. 30 mN/m and then stabilized at 10 mN/m. The initial equilibrium surface pressure was found to be quite reproducible from experiment to experiment and was found to depend very little (9-12 mN/m) on the
Figure 4. (A, top) Atomic force microscopic image of a PZT monolayer deposited on a glass substrate coated with a 500-81layer of gold at ll = 55 mN/m (second compression cycle); 0.3 g of PZT dispersed in a 10 mL 1-propano1:chloroform:isooctanemixture, 3:5:4 v/v. On a water surface, the monolayer appeared as a dense, uniform, opaque film. (B, bottom) Transmission electron microscopic image of a PZT monolayer deposited at ll = 55 mN/m (second compression cycle); 0.3 g of PZT dispersed in a 10 mL 1-propano1:chloroform: isooctane mixture, 3:5:4v/v. The pinholes between the particles will be filled upon the deposition of succeeding layers and treatment with a lead acetate, titanium isopropoxide, and zirconium ethoxide solution in a methanol:chloroform:2-propanolmixture, 1: 1:2 v/v. Scale bar = 1 pm.
amount of particles placed on the water surface. Upon the visible formation of a continuous particulate film, the surface pressure started rising steeply. If dispersion II exceeded 60 mN/m, the formation of wrinkles along the moving barrier was observed, which completely disappeared during the expansion process. When the surface pressure dropped back to ca. 20 mN/ m, the uniform film broke into 1-5 mm pieces resembling glass splinters. Upon expansion to the original area, these domains dissipated into microscopic dendritic strings of PZT particles. The compression-expansion process for dispersion I1 was highly reversible, provided that II .c 60 mN/m (Figure 2). Multiple compression-expansion cycles at II -= 50 mN/m resulted in a slight reduction (ca. 10%) of the surface covered with the film. This is likely to be due to the sinking of a small amount of PZT particles to the bottom of the trough. Films formed from dispersion I1 could be deposited by lifting a substrate through the interface. The first layer displayed excellent deposition properties (the deposition ratio was always around l.O), whereas the second layer never deposited well unless the previous one was fixed by a mixture of organometallic
12378 J. Phys. Chem., Vol. 99, No. 33, 1995
Letters an STM tip as electrodes, are shown in Figure 5. The remanent polarizations determined were 2.2 pC/cm2 (graphite electrode, film 1) and 2.55 pC/cm2 (STM electrode). These values are lower than those obtained for PZT films prepared by other techniques (12- 17 p C / ~ m * ) ; ' ~ -however, '~ the determined values for the coercive field, 101.5 kV/cm (graphite electrode) and 148.1 kV/cm (STM electrode), are comparable to those obtained previously (45- 100 kV/cm).I3-I5 Elimination of the annealing step in the formation of particulate ferroelectric films is the major accomplishment of the present work. The transition to the ferroelectric phase requires heating to 650-850 OC, which can result in unrecoverable damage to the contacts of the integral circuits, owing to increased diffusion, different thermal expansion, and other factors. Construction of films from particles with controllable sizes, size distributions, and interparticle distances which are already in their ferroelectric phase opens the door to important fundamental investigations and to novel device fabrication.
Ferroelectric Loops
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Figure 5. Ferroelectric hysteresis loops of two PZT films, taken with a Sawyer-Tower circuit with a sine waveform of 4 kHz. The top electrode is connected to the Sawyer-Tower circuit by a graphite lead for film 1 and by a tungsten-STM tip for film 2. Both films were 2 p m cm2 and film 2 of 3.8 x thick. Film 1 has an area of 2.8 x cm2. The remanent polarization (Pr) for film 1 is 2.2 pC/cm2 and 2.55 pClcm2 for film 2 (compared to 12-17 pClcm2 obtained in sol-gel PZT thin film^'^-'^). The coercive field (Ec) for film 1 is 101.5 and 148.1 kVlcm for film 2 (compared to 45-100 kV/cm obtained in solgel fiImsl3-I5).
compounds following the procedure described in the Experimental Section. The fixing process is necessary to render the PZT film on a substrate to be hydrophilic. The hydrophobic PZT film floating on water did not adhere at all to equally hydrophobic PZT film on a substrate, which can be attributed to the presence of a thin layer of water being carried away by surface particles. Fixed by the sol-gel solution, subsequent layers of films deposited also demonstrated a transfer ratio of 1.0 f 0.1. AFM images of the PZT monoparticulate film, prepared from dispersion 11, indicated the presence of well packed particles (Figure 4). However, even after multiple compressionexpansion cycles, a great number of pinholes were observed in one-layer films due to the irregular shape of the particles. The mean diameter of these PZT particles was determined, from the TEM images, to be 0.7 p m (Figure 4). The deposition of several layers and damping of the film with a sol-gel precursor of PZT are expected to fill the gaps between the particles, thus providing a good-quality film. The PZT-LB films prepared from dispersion I1 exhibited ferroelectric properties, as was evidenced by polarization measurements. Ferroelectric hysteresis loops of two 2-pm thick PZT films (with areas of 2.8 x and 3.8 x cm2 for films 1 and 2, respectively), taken with a graphite tip and with
Acknowledgment. Support of this work by a grant from the National Science Foundation is gratefully acknowledged. G.Z. appreciates the support of CONACYT (Mexico) and the Fulbright Foundation. We would also like to acknowledge Dr. Youxin Yuan (SUNY-ESF) for her generous help with the TEM measurements. We thank Dr. Fiona C. Meldrum for developing methodologies for forming monoparticulate BaTiO3 films. References and Notes (1) Xu, Y . Ferroelectric Materials and Their Applications; NorthHolland: The Netherlands, 1991. (2) Lines, M. W.; Glass, A. M. Principles and Applications of Ferroelectrics and Related Materials; Clarendon Press: Oxford, 1977. (3) Scott, J. F.; Paz de Araujo, C. A. Science 1989, 246, 1400. (4) Brinker, C. J.; Scherer, G. W. Sol-Gel Science. The Physics and Chemisty of Sol-Gel Processing; Academic Press, Inc.: San Diego, CA, 1990. ( 5 ) Fendler, J. H. Membrane-Mimetic Approach to Advanced Materials; Springer-Verlag: Berlin, 1994; Advances in Polymer Science Series, Vol. 113. (6) Fendler, J. H.; Meldrum, F. C. Adv. Mater., in press. (7) Shrout, T. R.; Papet, P.; Kim, S.; Lee, G . S. J A m . Ceram. SOC. 1990, 73, 1862. (8) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. Langmuir 1994, 10, 2035. (9) Ulman, A. An Introduction to Ultrathin Organic Films. From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (10) Pieranski, P. Phys. Rev. Lett. 1980, 45, 569. (11) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. J. Phys. Chem. 1994, 98, 4506. (12) Kamalasanan, M. N.; Chandra, S.; Joshi, P. C.; Masingh, A. Appl. Phys. Lett. 1991, 59, 3541. (13) Dey, S. K.; Zuleeg, R. Ferroelectrics 1990, 108, 37. (14) Sanchez, L. E.; Wu, S.; Naik, I. K. Appl. Phys. Lett. 1990, 56, 2399. (15) Canano, J.; Sudhama, C.; Chikarmane, V.; Lee, J.; Tasch, A.; Shepherd, W.; Abt, N. IEEE Trans. On Ultrasonics, Ferroelectrics and Frequency Control 1991, 38, 690. (16) Shirane, G.; Suzuki, K. J. Phys. SOC. Jpn. 1952, 7, 333. (17) Cullity, B. D. Elements of X-ray Di'ruction, 2nd ed.; AddisonWesley Publishing Co., Inc.: Reading, MA, 1978; p 127. JF'943207L
