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Single-Crystal-like Materials by the Self-Assembly of Cube-Shaped Lead Zirconate Titanate (PZT) Microcrystals Xiangyuan Liu,† Elizabeth F. McCandlish,‡ Larry E. McCandlish,‡ Kate Mikulka-Bolen,† Ramamoorthy Ramesh,§ Frederic Cosandey,† George A. Rossetti, Jr.,† and Richard E. Riman*,† Department of Ceramic and Materials Engineering, Rutgers, the State University of New Jersey, 607 Taylor Road, Piscataway, New Jersey 08854, Ceramare´ Corporation, 12-D Jules Lane, New Brunswick, New Jersey 08901, and Department of Materials Science and Engineering, University of California, Berkeley, California 94720 Received September 20, 2004. In Final Form: February 2, 2005 We demonstrated the formation of single-crystal-like materials that contain preferentially oriented arrays of lead zirconate titanate (PZT) cube-shaped particles by self-assembly. Hydrothermally synthesized PZT particles with a bulk composition of Zr/Ti ) 70/30 were used in making microcrystal arrays. Spreading a suspension containing PZT cube-shaped particles, 2-propanol, and mineral oil at the air-water interface produced a one-dimensional planar array of PZT particles on the water surface. The array so formed was subsequently transferred onto a flat or curved substrate. X-ray diffraction and electron backscattered diffraction analyses revealed that most of the cube-shaped particles in the array were oriented with their pseudocubic 〈001〉 direction aligned parallel to the normal direction of the substrate surface. Filling the arrays with matrixes produced monolayer or multilayer textured composites. The piezoelectric properties of oriented cube-shaped micron-sized particles in the self-assembled arrays were measured using a modified atomic force microscope to reveal the ferroelectric nature of the PZT arrays.
Single-crystal materials have many industrial and scientific applications; however, growing large single crystals is often difficult, time-consuming, and expensive. Some single crystals are especially hard to grow. For example, lead zirconate titanate (PZT) is a widely used piezoelectric and ferroelectric material that is available as an isotropic, polycrystalline ceramic. PZT has the perovskite structure with the formula PbZrxTi1-xO3. At room temperature, PZT solid solutions have tetragonal symmetry (P4mm) for x between 0 and ∼0.52 that changes abruptly to rhombohedral symmetry (R3m or R3c) for x between 0.52 and ∼0.92. For many years, researchers have attempted to grow single crystals of PZT. Conventional melt crystal growth techniques, such as the Czochralski method, are not suitable because the complete PZT solid solution (by definition) melts incongruently.1 The flux method has been suitable only for growing material with limited amounts of zirconium.2 Hydrothermal growth of PZT from aqueous KF occurs at ∼600 °C, far above the critical point of water.3 Recently, PZT crystals of sizes of up to 7 mm with compositions near the morphotropic phase boundary (x ) 0.45-0.52) were synthesized by an improved melt-based solution growth technique.4 PZT single crystals grown by the above methods are only several millimeters in size, too small for many practical applications. Efforts were also devoted to growing epitaxial PZT films under hydrothermal conditions.5 Smooth and optically transparent * To whom correspondence should be addressed. E-mail:
[email protected]. † Rutgers, the State University of New Jersey. ‡ Ceramare ´ Corporation. § University of California, Berkeley. (1) Fushimi, S.; Ikeda, T. J. Am. Ceram. Soc. 1967, 50, 129-130. (2) Clarke, R.; Whatmore, R. W. J. Cryst. Growth 1976, 33, 29-38. (3) Yangisawa, K.; Kanai, H. Jpn. J. Appl. Phys. 1997, 36, 60316034. (4) Chen, W.; Ye, Z.-G. 2003 US Navy Workshop on Acoustic Transduction Materials and Devices, The Pennsylvania State University, Pennsylvania, 2003.
films with a thickness ranging between 32 and 395 nm were uniformly distributed on the substrates. However, significant technological advances will be necessary to achieve film thicknesses on the order of millimeters. There exists a need for materials with performance properties comparable to those of single crystals, which can be synthesized economically in large size and quantity. An alternative to single-crystal materials are “singlecrystal-like” materials, which have been demonstrated to possess better properties than ceramics (which contain randomly oriented grains) and exhibit properties approaching those of true single crystals.6 We define singlecrystal-like materials as aligned arrays of single-crystal particles whose positions and orientations are fixed by binding to a substrate and/or embedding in a matrix. A highly textured ceramic is an example of such a material.7 PZT compositions with rhombohedral symmetry are particularly interesting for use in creating microcrystal arrays. Rhombohedral PZT compositions have interaxial angles of R ) β ) γ ∼ 89.7° and lattice parameters of a ) b ) c ∼ 0.4 nm, and therefore, these compositions are highly pseudosymmetric with the parent cubic (Pm3m) prototype perovskite structure. Thus, when rhombohedral PZT crystallizes as particles bounded by {001} planes, we can refer to these particles as pseudocubes. Recent calculations show that the electromechanical properties of PZT single crystals are strong functions of crystal structure and crystal orientation.8,9 PZT compositions having rhombohedral symmetry are predicted to (5) Suchanek, W. L.; Lencka, M. M.; McCandlish, L. E.; Pfeffer, R. L.; Oledzka, M.; Mikulka-Bolen, K.; Rossetti, G. A., Jr.; Riman, R. E. Cryst. Growth Des., submitted for publication, 2004. (6) Riman, R. E.; McCandlish, L. E. Single-Crystal-Like Materials, Pub. No. US2003/0211369, Pub. Date: Nov. 13, 2003 (patent application). Riman, R. E.; Liu, X.; McCandlish, L. E. Single-Crystal-Like Materials, PCT Pub. No. 04/106,599, Pub. Date: December 9, 2004 (patent application). (7) Cullity, B. D. Elements of X-ray diffraction; Addison-Wesley Publishing Company, Inc.: Reading, MA, 1956; p 272. (8) Du, X.; Belegundu, U.; Uchino, K. Jpn. J. Appl. Phys. 1997, 36, 5580-5587.
10.1021/la047655o CCC: $30.25 © 2005 American Chemical Society Published on Web 03/16/2005
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Figure 1. Influence of an applied electric field on an array of single-crystal rhombohedral PZT cubes: (a) In the absence of an electric field, the crystals have polarization vectors along the eight directions related to 〈111〉 in a cube, where the net polarization of the array is zero. (b) When the array is subjected to an electric field, the polarization vectors align so that most of the polarization vectors are pointing upward. The net polarization is nonzero and perpendicular to the plane of the substrate.
have higher piezoelectric constants, d33, than PZT compositions having tetragonal symmetry. A maximum d33 is calculated to occur very close to the pseudocubic 〈001〉 direction for a rhombohedral crystal electrically poled along this direction. (If an isotropic ferroelectric ceramic is subjected to an electric field sufficient to reorient the randomly oriented dipoles and give the ceramic a net polarization, the ceramic is said to be poled). Similarly, the electromechanical coupling constant, k33, is a maximum in approximately the same direction. The variation in d33 and k33 due to slight misorientation from the 〈001〉 direction is small. It can be seen from Figure 1a that if PZT pseudocubes were to be assembled into a planar array with their {001} faces resting on a flat substrate, the polarization vectors would cancel, to give a zero net polarization. If the microcube array were then subjected to an electric field (poled), as shown in Figure 1b, the polarization vectors would realign to give a net polarization perpendicular to the plane of the substrate. In this paper, we report the fabrication of partially ordered composites, by making particles self-assemble on a water surface. Self-assembly occurs even when the particles are much denser than water; for example, PZT has a density of ∼8 g/cm3. We apply this technique to cube-shaped PZT particles synthesized under mild hydrothermal conditions and arrange them with their {001} faces resting on a flat substrate. An important generic advantage of making single-crystal-like materials from pseudocubic single-crystal particles can be realized when utilizing assembly on surfaces of variable topology to provide densely packed structures that are oriented with respect to the substrate surface. For instance, PZT cubes assembled on a cylindrical surface will present the 〈001〉 direction for all surface normals. Such structures are not possible with shaped conventional single crystals, since their orientation changes with the direction of the surface topology. For example, a cylindrical crystal with the c-axis oriented in the axial direction presents all possible 〈hk0〉 directions on the convex surface with the ends presenting only the 〈001〉 direction. For pseudocubic particles of PZT, it does not matter which of the {001} faces are parallel (9) Du, X.; Zheng, J.; Belegundu, U.; Uchino, K. Appl. Phys. Lett. 1998, 72, 2421-2423.
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to or perpendicular to the substrate. We anticipate that these microcrystal arrays will have ferroelectric properties intermediate between those of poled PZT ceramics and PZT single crystals. In addition, the ability to create microcrystal arrays provides a means to fabricate devices with discrete addressable elements at the micron length scale. The assembly of individual micron-scale or nanoscale elements into a desired structure is a challenging problem for materials scientists interested in innovative materials fabrication. The self-assembly of colloidal spheres into ordered two-dimensional arrays at the air-liquid interface10-13 has been reported by several research groups. Other methods have also been used, including stirring, electromagnetic fields, and convective flow.14-17 Here, we report the assembly of cube-shaped PZT particles into microcube arrays on assorted surfaces. The arrays are made by first treating cubes with 2-propanol and mineral oil in the correct proportions. When the slurry is placed dropwise on a water surface, the balance among spreading tension, surface tension, interfacial tension, wetting forces, and capillary forces18 causes the cubes to self-assemble into a monolayer that is densely packed. The assembled array is roughly analogous to a LangmuirBlodgett monolayer, but the particles are microcrystals, instead of molecules. The monolayer array can be deposited on a microscope slide using a method similar to that which produces a Langmuir-Blodgett film. Early reports of texturing via particle morphology have been reported.19-21 In contrast to our work, neither self-assembly nor monolayer formation was the focus of these studies. Cube-shaped PZT particles are obtained by hydrothermal synthesis of PZT below the critical point of water, using H4EDTA (ethylenediamine tetraacetic acid) as a chelating agent (synthesis details can be found in the Methods section). The cubes are uniform in shape and size, with slightly truncated corners, and 2-3 µm on a side (Figure 2). They have a bulk Zr/Ti ratio of 70/30 and have rhombohedral (R3c) symmetry at room temperature. Before assembly, no ordered arrangement can be observed (Figure 2a). After assembly and transfer to a microscope slide, the array consists primarily of a monolayer of cubes, which covers ∼80% of the surface. Most of the cubes have their {001} faces in contact with the slide. There are some areas of multiple layers, misaligned cubes, and other defects and some highly ordered areas, as shown in Figure 2c. The process for making the microcube arrays is illustrated in Figure 3. The PZT cubes are suspended in (10) Onoda, G. Y. Phys. Rev. Lett. 1985, 55 (2), 226-229. (11) Lenzmann, F.; Li, K.; Kitai, A. H.; Sto¨ver, H. D. Chem. Mater. 1994, 6, 156-159. (12) Kondo, M.; Shinozaki, K.; Bergstro¨m, L.; Mizutani, N. Langmuir 1995, 11, 394-397. (13) Hurd, A. J.; Schaefer, D. W. Phys. Rev. Lett. 1985, 54, 10431046. (14) Bowden, N.; Terfort, A.; Carbeck, J.; Whitesides, G. M. Science 1997, 276, 233-235. (15) Grzybowski, B. A.; Stone, H. A.; Whitesides, G. M. Nature 2000, 405, 1033-1036. (16) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Magayama, K. Langmuir 1992, 8, 3183-3190. (17) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Magayama, K. Nature 1993, 361, 26. (18) Jaycock, M. J.; Parfitt, G. D. Chemistry of Interfaces; Ellis Horwood Series in Physical Chemistry; Ellis Horwood Limited: Chichester, U.K., 1981. (19) Boatner, L. A.; O., J. L. B.; Abraham, M. M. J. Am. Ceram. Soc. 1990, 73 (8), 2333-2344. (20) O., J. L. B.; Boatner, L. A.; Abraham, M. M. J. Am. Ceram. Soc. 1990, 73 (8), 2345-2359. (21) Steadman, G. W.; Brewster, J. R.; Budai, J. D.; Boatner, L. A. Mater. Res. Soc. Symp. Proc. 1993, 286, 33-38.
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Figure 2. Scanning electron microscopy (SEM) pictures of cube-shaped PZT particles (a) as synthesized (2000×) and (b) after self-assembly on a microscope cover slide (500×). Most of the cubes have their 〈001〉 direction normal to the plane of the substrate, with domains of higher order. Part c shows a domain containing three-dimensional oriented PZT cube-shaped particles (5000×).
2-propanol containing a small amount of mineral oil (processing details can be found in the Methods section and ref 22). The suspension is added continuously to a water surface in a small container. As long as the slurry continues to be added, the cubes are pushed (presumably by the spreading tension at the oil-water-2-propanol interface) to the far end of the container. The cubes are entrained in the mineral oil, which floats on the surface of the water. Some of the cubes will sink, but enough will float so that a clearly visible “monolayer” forms. This layer is much more stable and densely packed than a layer formed in the absence of mineral oil. In the absence of mineral oil, most of the PZT cubes sink to the bottom of the container. Other volatile solvents and hydrophobic oily phases can also be used.22 The cube array can be lifted off the surface of the water by continuously confining the array, in a densely packed form, as a microscope slide moves through the surface of the water. An array can be transferred to a surface that is flat compared to the cubes. Glass, metal, plastic, and some fluorinated hydrocarbons such as polyvinylidene fluoride can be used. In addition, (22) Riman, R. E.; Liu, X.; McCandlish, L. E. Fluid forming of single phase and composite materials. U.S. Patent pending, P-25,937, 2003.
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Figure 3. Process for making arrays of cube-shaped particles. Initially, the particles are dispersed in 2-propanol that contains a small amount of mineral oil. (a) The suspension is transferred onto the water surface. Particles self-assemble into a monolayer on the water surface. (b) The array is picked up on a microscope slide. (c) The array may also be picked up on a curved surface.
macroscopically flat and curved surfaces have been coated. When the arrays are first lifted off the water surface, they can be put back down with no apparent changes. However, after a few seconds, when the excess liquid has drained from the array, the cubes preferentially adhere almost completely to the microscope slide. Multiple layers of cubes can be assembled in this way. If desired, the arrays can be heat treated in a furnace at 550 °C to remove the mineral oil. Heat treating the sample at such temperatures does not modify the crystal assembly, even if the temperature exceeds 550 °C. From the physical perspective, on the basis of FESEM (field emission scanning electron microscopy) measurements, heat treatment at a temperatures of 700 °C did not generate any changes of morphology, surface roughness, or aggregation for the cubes. From the chemical perspective, a comparison of the XRD patterns for as-synthesized PZT cubes and the cubes annealed at 900 °C in air for 2 h did not reveal any differences in the number of peaks or peak positions, thereby indicating that the phase assemblage and phase composition was unaffected by the heat treatment.
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Figure 4. X-ray diffraction patterns of hydrothermally synthesized PZT cubes, Cu KR radiation, Ni filter: (a) randomly oriented (ground) PZT; (b) diffraction pattern of 〈001〉-oriented PZT cubes (a small (110) peak appears because there are misoriented particles (defects) in the array); (c) a 〈001〉 X-ray pole figure of a planar array; (d) a 〈011〉 X-ray pole figure of a PZT array; (e) a 〈001〉 electron backscattered diffraction pole figure of a PZT planar array. An area of about 20 × 20 µm2 was sampled during the backscattering experiment.
We believe that cube surface modification, surface tension, spreading tension, and capillary force are key factors for forming monolayer cube particle arrays on the water surface. Hydrothermally synthesized PZT cubes are hydrophilic. However, by dispersing PZT cubes in a mixture of volatile 2-propanol and mineral oil, the cube surfaces are modified to be hydrophobic. This hydrophobicity enables the cubes to float on the water surface by virtue of the surface tension of water and a high contact angle between water and the surface modified PZT particle. When a low boiling point alcohol is added onto the water surface, spreading tension creates a thin alcohol layer on the water surface.18 The spreading of 2-propanol at the water surface propels large clusters of PZT cubes into close proximity of one another. As this process proceeds, 2-propanol evaporates as the mineral oil concentrates around the PZT clusters. Once adjacent cubes are close to one another, capillary tensional forces exerted by the mineral oil densify the cube matrix by rearranging clusters of particles and creating a maximum of cube face contacts (Figure 2c). This rearrangment process produces domains of oriented particles (Figure 2b). X-ray diffraction was used to examine the crystallographic orientation of the microcube arrays. The powder pattern obtained for a random polycrystalline sample of a 70/30 PZT composition is shown in Figure 4a. In contrast, a 2-theta scan of a microcube array (Figure 4b) shows a preferred 〈001〉 direction. The small peak assigned to {110} shows that a fraction of the cubes are misaligned. The pole figures in Figure 4c,d show that most of the cubes are oriented with two of their six {001} faces parallel to the substrate surface. The symmetrical ring of diffracted intensity in Figure 4d reveals that the cubes are randomly rotated in the plane of the array over the area sampled by the X-ray beam. The small central peak in Figure 4d indicates some misaligned cubes. The X-ray results are consistent with electron microscope observations (Figure 2b,c). The high degree of texture is further supported by pole figures obtained using electron backscattered dif-
Figure 5. FESEM picture of a PZT planar array infiltrated with PMMA matrix. The cross section of the formed film was made by removing a monolayer coating from a gold-coated glass microscope slide. Note that the structure of the assembled PZT psuedocube array was retained after matrix infiltration.
fraction (EBSD) (Figure 4e), which look similar to the diffraction patterns obtained with X-rays. The misoriented particles (defects) seen in the diffraction patterns and images come from a variety of sources. The cubes are not perfectly uniform in size, and they are occasionally twinned or malformed. Small PZT particles (
