Hierarchical PbTiO3 Nanostructures Grown on SrTiO3 Substrates

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CRYSTAL GROWTH & DESIGN

Hierarchical PbTiO3 Nanostructures Grown on SrTiO3 Substrates

2009 VOL. 9, NO. 4 1979–1984

Per Martin Rørvik, Tor Grande, and Mari-Ann Einarsrud* Department of Materials Science and Engineering, Norwegian UniVersity of Science and Technology, 7491 Trondheim, Norway ReceiVed NoVember 27, 2008; ReVised Manuscript ReceiVed January 16, 2009

ABSTRACT: A synthesis route, based on a hydrothermal treatment of an amorphous PbTiO3 precursor at 180 °C, has been developed to grow hierarchical PbTiO3 nanostructures on single-crystal SrTiO3 substrates. Initially, highly oriented PbTiO3 platelets grew parallel to the 〈100〉 substrate orientations. PbTiO3 nanorods with squared cross-section were shown to grow perpendicular out of the platelets. The length of the rods could be controlled by the vertical position of the substrate in the autoclave. Furthermore, changing the crystallographic orientation of the substrate resulted in a systematic change in the orientation of the nanorods. Finally, growth of PbTiO3 nanorods perpendicular to the substrate surface was demonstrated by hindering the initial growth of PbTiO3 platelets. Introduction Chemical methods to produce micro/nanostructures are widely sought to assist or replace physical methods such as lithographic techniques. Chemical methods are more cost effective and can more easily be scaled up to large-area fabrication. Future applications and nanomanufacturing will strongly rely on largescale patterned and designed growth of nanostructures.1 An essential step in integrating nanostructures with existing technologies is to control the location, size, and orientation of the nanostructures. Nanostructures obtained by chemical bottomup processes are therefore highly desired. PbTiO3 is a prototype ferroelectric material with the perovskite structure. Above 490 °C, PbTiO3 transforms from the ferroelectric tetragonal phase to a paraelectric cubic phase. Hydrothermal synthesis enables growth of PbTiO3 thin films with the perovskite structure far below the transition temperature,2,3 in contrast to other methods such as sol-gel processing, sputtering, or pulsed laser deposition.4 The hydrothermal growth of PbTiO3 on SrTiO3 substrates typically progresses from initial formation of nanosized islands, to coalescence of these islands into a thin film covering the substrate.2,5 Chien et al.2 have shown that the morphology of the PbTiO3 film is highly influenced by the crystallographic orientation of the substrate, with (110)- and (111)-oriented SrTiO3 substrates resulting in thin films of crystals with {100}-facetted surfaces. Morita et al.6 have shown that the vertical position of the substrate relative to the height of the liquid level in the autoclave influences the morphology of PbZr1-xTixO3 thin films grown on SrTiO3 substrates. In a previous study, we reported growth of PbTiO3 nanorod arrays on PbTiO3 and SrTiO3 substrates by a hydrothermal method, using amorphous PbTiO3 as precursor and a surfactant as structure-directing agent.7 An advantage of using the hydrothermal method to produce arrays of nanorods is that singlecrystalline nanorods are typically formed,7-12 in contrast to other methods of producing PbTiO3 nanorod or nanotube arrays, such as template-assisted methods.13-19 Here we show that we can control the morphology of PbTiO3 nanostructures grown on SrTiO3 substrates by controlling synthesis parameters such as substrate position, synthesis time, and crystallographic orientation of the substrate. This has enabled growth of hierarchical * To whom correspondence should be addressed. Phone: +47 73594002. E-mail: [email protected].

Figure 1. Experimental setup with the polished substrate surface facing downward in the autoclave.

PbTiO3 nanostructures consisting of platelets with nanorods growing perpendicular to the platelet surfaces. Experimental Section A Pb-Ti sol was made as follows: titanium(IV) isopropoxide (10 mmol, Acros Organics, >98%) was dissolved in aqueous citric acid (20 mL, 1.5 mol/L, Merck, >99%) at 60 °C. The pH value was raised to 5 by adding ammonia solution (25 wt % NH3, Merck, p.a.). Lead(II) acetate trihydrate (10 mmol, Merck, >99.5%) and ethylene glycol (67.7 mmol, Merck, >99%) were added and dissolved. The Pb/Ti ratio was 1.00. Potassium hydroxide (10 g, Merck, >85%) was then added to the sol (pH g 14), with instant formation of a white precipitate of amorphous PbTiO3-0.5x(OH)x. Sodium dodecylbenzenesulfonate (SDBS, 10 mmol, Sigma, >80%) was added, and the dispersion was stirred for 30 min. The dispersion (total volume ≈ 50 mL) was then poured into a Teflon-lined autoclave (125 mL, Parr Instrument Co.). A SrTiO3 substrate with (100), (110), or (111) orientation (10 × 10 mm2, Crystal GmbH) was mounted on a Teflon holder in the autoclave. To avoid sedimentation of particles on the substrate, a modified setup compared to the previous study7 was used, with the polished substrate surface normally facing downward in the autoclave (see Figure 1). The height of the substrate above the autoclave bottom (h) was varied from 9 to 29 mm. The autoclave was heated to 180 °C for 0-48 h. A summary of the experimental conditions for the various syntheses is given in Table 1. The substrate was taken out of the autoclave after cooling, washed with distilled water and ethanol, and finally dried at ∼100 °C for 1 h. The phase composition of the products was studied by X-ray diffraction (XRD) using a Siemens D5000 diffractometer with Cu KR radiation. The morphology of the products was studied by scanning electron microscopy (SEM, Hitachi S-3400N) and field emission scanning electron microscopy (FESEM, Zeiss Ultra 55).

Results The optimal vertical position of the substrate in the dispersion was determined in the first four syntheses. By varying the

10.1021/cg8012969 CCC: $40.75  2009 American Chemical Society Published on Web 03/02/2009

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Table 1. Synthesis Conditions and Product Morphology for the Different Syntheses synthesis

substrate heighta [mm]

synthesis time [h]

substrate orientation

product morphology

1 2 3 4 5 6 7 8 9 10 11 12 13

29 22 15 9 15 15 15 15 15 15 15 15 19

48 48 48 48 0b 1 6 24 1 48 1 48 6 + 42c

(100) (100) (100) (100) (100) (100) (100) (100) (110) (110) (111) (111) (100)

platelets with very short nanorods platelets with short nanorods platelets with nanorods platelets with disordered nanorods small platelets platelets platelets with short nanorods platelets with nanorods platelets platelets with nanorods platelets platelets with nanorods carbonaceous layer, nanorods growing from the substrate surface in small areas

a The distance between the polished side of the substrate and the autoclave bottom (h in Figure 1). b The autoclave was heated to 180 °C and thereafter directly cooled. The programmed heating and cooling rates were 200 °C/h. c The autoclave was heated at 180 °C for 6 h without any substrate present and thereafter cooled to room temperature. The upper layer of organic material was removed, a substrate was introduced with the polished substrate surface facing upward, and the autoclave was reheated to 180 °C for 42 h.

Figure 2. SEM images of PbTiO3 grown hydrothermally (48 h) on (100)-oriented SrTiO3 substrates located at different vertical positions (syntheses 1-4): h ) (a) 29, (b) 22, (c) 15, and (d) 9 mm. The scale bar in the insets indicates 1 µm. The crystallographic directions shown in a are similar for all images.

vertical position, the morphology of the PbTiO3 products gradually changed as shown in Figure 2. Generally, regular arrays of platelets were formed, with nanorods growing perpendicular from the platelet surfaces. At h ) 29 mm (see Figure 1), the growth was dominated by platelets oriented along the 〈100〉 directions of the SrTiO3 substrate (Figure 2a). Nanorods with lengths of ∼200 nm grew out from the platelets, perpendicular to the platelet surfaces. Platelets were also observed at lower vertical positions of the substrate (h ) 22

and 15 mm) (Figure 2b and 2c). The length of the nanorods growing from the platelets increased to 600-800 (h ) 22 mm) and 800-1000 nm (h ) 15 mm). At the lowest position (h ) 9 mm) (Figure 2d), platelets were also formed, but in this case they were completely covered by nanorods. The growth of nanorods was less organized on substrates located in the lowest position, and the nanorods were generally longer, up to 2 µm. All the substrates had a homogeneous surface coverage with only small variations in the morphology across the surface. The variations of the morphology that were observed in our previous study7 were therefore eliminated by positioning the substrate horizontally with the polished surface facing downward in the dispersion. The X-ray diffractograms of the products from syntheses 1-4, given in Figure 3a-d, confirmed the formation of tetragonal PbTiO3. The diffractograms clearly demonstrate a preferential orientational relationship of PbTiO3 to the substrate, by comparing the relative intensities of the diffraction lines. For instance, the (001) lines in Figure 3a-d have a significantly higher intensity than the (101) and (110) lines, while for bulk PbTiO3 the (101) and (110) lines are more intense than the (001) line (Figure 3e). Moving the substrate from the top to the bottom of the autoclave, an increased intensity of especially the (00l) PbTiO3 diffraction lines was observed. The intensity of the diffraction lines from traces of metallic Pb did also decrease from the top to the bottom. The substrate was located at h ) 15 mm for all the following experiments. At this position, nanorods were formed (Figure 2c), while a certain distance from the nanocrystalline PbTiO3 powder formed at the bottom of the autoclave was preserved. Figure 4 shows SEM images of the products synthesized for 0, 1, 6, and 24 h. Initially, square-shaped platelets grew parallel to the substrate surface, and triangular-shaped platelets grew perpendicular to the substrate surface (Figure 4a and 4b). The platelets were oriented along the 〈100〉 directions of the substrate with the edges of the platelets perpendicular to the 〈110〉 directions of the substrate. The triangular-shaped platelets grew from size ∼800 × 150 × 400 nm3 (length × width × heigth) after heating/cooling (0 h) (Figure 4a and 4b) to ∼2000 × 300 × 800 nm3 size after 1 h (Figure 4c and 4d). A pronounced change in the shape is also evident. After 6 h nanorods grew perpendicular out of the platelet side surfaces (Figure 4e and 4f) and upward from the platelet top surfaces (inset in Figure 4e). The nanorods grew longer with prolonged time (24 h, Figure 4g and 4h). Especially the nanorods growing from the top of

Hierarchical PbTiO3 Nanostructures

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Figure 4. SEM and FESEM images of PbTiO3 grown on (100)-oriented SrTiO3 substrates located at constant vertical position (h ) 15 mm) for different hydrothermal treatment times (syntheses 5-8): (a and b) 0, (c and d) 1, (e and f) 6, and (g and h) 24 h. In a, c, e, and g, the substrates were tilted 43° when the images were recorded. The scale bar in the insets indicates 500 nm. The crystallographic directions shown in b are similar for all images.

Figure 3. X-ray diffractograms of PbTiO3 grown hydrothermally (48 h) on (100)-oriented SrTiO3 substrates located at different vertical positions (syntheses 1-4): h ) (a) 29, (b) 22, (c) 15, and (d) 9 mm. (e) Standard PDF pattern for tetragonal PbTiO3.

the platelets had grown noticeably longer after 24 h, as the growth in that direction was not sterically hindered (inset in Figure 4g). The X-ray diffractograms of the products from syntheses 6-8, given in Figure 5, clearly show the increasing growth and crystallinity of PbTiO3 with time. After 1 h, broad (00l) PbTiO3 diffraction lines appeared (Figure 5b), and after 6 and 24 h the increased intensity of these lines shows the gradual growth and increased crystallinity of PbTiO3 (Figure 5c and 5d, respectively). No PbTiO3 lines were observed in the diffractogram of the product from synthesis 5 (0 h, not shown). The orientation of the PbTiO3 platelets and nanorods could be changed by changing the crystallographic orientations of the SrTiO3 substrates. PbTiO3 grown on (110)-oriented substrates is shown in Figure 6, while PbTiO3 grown on (111)-oriented substrates is shown in Figure 7. On both types of substrates, the PbTiO3 platelets were oriented parallel to the 〈100〉 orientations of the substrate (Figures 6a and 7a). The nanorods grew perpendicular from the platelet surfaces, which resulted in nanorods arranged in two directions on the (110)-oriented

substrate (Figure 6b and 6c), and in three directions on the (111)oriented substrate (Figure 7b), as a result of the 〈100〉 orientations of the substrates (Figures 6d and 7c). The X-ray diffractograms of the products from syntheses 9-12, presented in Figure 8, confirmed the preferential orientations of the growth of PbTiO3 on the (110)- and (111)-oriented substrates. The (101) and (202) PbTiO3 diffraction lines dominated for the (110)-oriented substrates (Figure 8a and 8b), while the (111) PbTiO3 diffraction line dominated for the (111)oriented substrate (Figure 8c and 8d). The origin of the diffraction lines marked with arrows in Figure 8a is uncertain. Experiments were also performed to facilitate growth of nanorods directly on the substrate by avoiding the initial platelet growth. The dispersion was first heat treated for 6 h, before the substrate was inserted, and the autoclave was thereafter heat treated for 42 h. This procedure led to the formation of a layer of cracked carbonaceous material on the substrate. The amount of carbonaceous material was reduced by orienting the substrate vertically or facing upward instead of facing downward. Nevertheless, in small areas on the substrate with the polished side facing upward, growth of nanorods aligned perpendicular to the substrate surface was observed directly on the substrate (Figure 9). Figure 9b shows that the cross-section of the nanorods is square shaped. The side faces of the nanorods were oriented in the crystallographic 〈110〉 directions of the underlying substrate. The areas with growth of nanorods instead of

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Figure 5. X-ray diffractograms of a (100)-oriented SrTiO3 substrate and of PbTiO3 grown on (100)-oriented SrTiO3 substrate located at constant vertical position (h ) 15 mm) for different hydrothermal treatment times (syntheses 6-8): (a) Untreated (100)-oriented SrTiO3 substrate, (b) 1 h, (c) 6 h, and (d) 24 h. The line identities refer to cubic SrTiO3 (PDF 35-734) in a and tetragonal PbTiO3 (PDF 6-452) in d.

carbonaceous material constituted approximately 3% of the exposed substrate surface. Discussion The results show that novel PbTiO3 nanostructures can be grown on SrTiO3 substrates by a hydrothermal synthesis method. The structures were hierarchical, consisting of platelets with nanorods growing perpendicular out of the platelet surfaces. The shape and orientation of the structures can be controlled in detail by varying the synthesis parameters, such as the vertical position of the substrate in the autoclave, the synthesis time, and the crystallographic orientation of the substrate. The possibility of controlling the morphology of such complex structures is considered to be important for applications. With the present procedure, initially platelets were formed parallel to the 〈100〉 orientations of the substrate. This was followed by growth of nanorods out from the surfaces of the platelets after 1-6 h. The nanorods steadily grew longer with prolonged synthesis time. The hydrothermal growth can therefore be divided into two regimes: first, a platelet growth regime, and after 1-6 h a nanorod growth regime. By avoiding the initial growth of platelets, it was shown that PbTiO3 nanorods can be grown directly on a SrTiO3 substrate (Figure 9). Compared to our previous study,7 the growth of PbTiO3 presented here was much more homogeneous across the substrate surface. The main reason is that the exposed substrate

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Figure 6. SEM and FESEM images of PbTiO3 grown on (110)-oriented SrTiO3 substrates located at constant vertical position (h ) 15 mm) for different hydrothermal treatment times (syntheses 9 and 10): (a) 1 and (b and c) 48 h. In c, the substrate was tilted 45° when the image was recorded. The crystallographic directions shown in a are similar for images a-c. (d) Crystallographic directions for the (110) substrate (black, parallel to the substrate surface; red, perpendicular to the substrate surface; blue, 45° angle to the substrate surface).

surface faced downward, while it faced upward in the previous study. Sedimentation of particles onto the substrate was therefore prevented, which made the growth process much more reproducible. In the previous study, high resolution transmission electron microscopy studies showed that the interface between the SrTiO3 substrate and hydrothermally grown PbTiO3 was atomically sharp.7 Therefore, when PbTiO3 nucleates at the substrate, the crystallographic orientation of the substrate will influence the orientation of the product, resulting in epitaxial structures, as long as the growth process is well controlled. Here, the growth orientation of the PbTiO3 platelets and nanorods corresponded to the three different orientations of the (100), (110), and (111) SrTiO3 substrates used, which shows that the orientation of the underlying substrate was governing the orientation of the PbTiO3 nanostructures. During the platelet growth regime, small platelets were formed on the substrate surface, with the platelets oriented parallel to the 〈100〉 directions of the SrTiO3 substrate (Figure 4b). The platelet growth is then limited in the direction perpendicular to the platelet facet, indicating growth initiation in the 〈100〉 orientation of PbTiO3, exposing the {001} surfaces, as observed by Peterson and Slamovich.20 In addition, the edges of the platelets were terminated in the 〈110〉 directions of the SrTiO3 substrate (Figure 4b). The anisometric shape of the platelets is probably caused by anisotropic surface energy20 and possibly surfactant adsorption. Upon further growth, some

Hierarchical PbTiO3 Nanostructures

Figure 7. SEM images of PbTiO3 grown on (111)-oriented SrTiO3 substrates located at constant vertical position (h ) 15 mm) for different hydrothermal treatment times (syntheses 11 and 12): (a) 1 and (b) 48 h. (c) Crystallographic directions for the (111) substrate seen from the side (left) and top (right) (black, parallel to the substrate surface; red, perpendicular to the substrate surface; blue, 35.3° angle to the substrate surface). The crystallographic directions shown to the right in c are valid for both images.

platelets grew at the sacrifice of other platelets (Figure 4d). The gradual transition from platelet growth to nanorod growth occurred between 1 and 6 h synthesis time. Contrasting the platelet growth, the nanorods grew only in one direction, with the width of the nanorods apparently equal after 6, 24, and 48 h (Figure 4f and 4h and inset in Figure 2c). In addition to the variations of the synthesis time, variations in the vertical position of the substrate in the autoclave also gave a systematic variation in the morphology. An increasing rod length and an increasing disorder in the growth direction was observed when going from the top (h ) 29 mm) to the bottom (h ) 9 mm) of the autoclave (Figure 2). This implies a concentration gradient of the species resulting in rod growth in the autoclave, at least after a certain time, leading to increased rod growth in the lower part of the autoclave. The concentration of ions dissolved in the solution is likely to be essentially equal independent of the vertical position, because of diffusion, while the number of particles per volume (both the amorphous precursor particles and the precipitated nanocrystals of PbTiO3) will likely be higher toward the bottom of the autoclave because of sedimentation. To evaluate the effect of the particle gradient, experiments using an autoclave with stirring could be performed, as the gradient should then disappear. The results from an initial experiment using stirring indicate that the gradient effect is then eliminated.

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Figure 8. X-ray diffractograms of PbTiO3 grown on (110)- and (111)oriented SrTiO3 substrates located at constant vertical position (h ) 15 mm) for different hydrothermal treatment times (syntheses 11-14): (a) (110) for 1 h, (b) (110) for 48 h, (c) (111) for 1 h, and (d) (111) for 48 h. The lines marked with an asterisk originate from the substrates. The origin of the lines marked with an arrow is uncertain. The line identities refer to cubic SrTiO3 (PDF 35-734) and tetragonal PbTiO3 (PDF 6-452).

Figure 9. FESEM images of PbTiO3 nanorods from a small area with nanorod growth on a (100)-oriented SrTiO3 substrate that was inserted after 6 h (synthesis 13): (a) 45° tilted view and (b) top view.

On the basis of the present and previous observations,7-10 we propose that the platelets are formed by nucleation on the substrate and subsequent growth, while the nanorods are formed by self-

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assembly of cube-shaped or facetted PbTiO3 nanocrystals. The basis of this growth model is a dissolution-reprecipitation mechanism: dissolution of the amorphous precursor into ions, followed by nucleation of crystalline nanoparticles which ripen into cube-shaped or facetted nanocrystals.7 We cannot exclude the possibility that also the nanorods grow by a classic nucleation and growth mechanism based on Ostwald ripening; however, the transition of growth from platelets to nanorods after 1-6 h indicates two different growth mechanisms for the platelets and the nanorods. The transition from the platelet growth regime to the nanorod growth regime will take place after the nanocrystals have nucleated and ripened into cube shape or facetted shape. In the meantime, platelets will continue to grow by supply of ions from the solution. Equal concentration of the ions in the solution at all heights in the dispersion will result in platelet growth on the substrate regardless of the vertical position of the substrate in the autoclave (Figure 2). Because of sedimentation of the amorphous precursor, there will be a gradient in the number of PbTiO3 nanocrystals per volume formed in the dispersion. The nanorod growth by self-assembly of nanocrystals will therefore occur faster on substrates closer to the bottom of the autoclave because of a higher number of nanocrystals per volume. This will result in both increased rod length and increased growth perpendicular to the substrate surface in the bottom of the autoclave (h ) 9 and 15 mm) (Figure 2). We have previously shown that the elongated direction of the nanorods is in the [001] direction (polar direction) of the tetragonal perovskite structure,7-10 while the side faces of the nanorods have {110} orientation.8-10 In the growth of nanorods out of platelets, it was not possible to determine the orientation of the rod facets. However, the nanorods that grew directly on the substrate in synthesis 13 had a clear side face orientation with relation to the substrate; the side faces were oriented in the 〈110〉 directions of the underlying SrTiO3 substrate (Figure 9b). The self-assembly growth into nanorods in the [001] direction is probably highly influenced by the dipole interaction between individual nanocrystals because of the ferroelectric polarization. In addition, the surfactant (SDBS) has been shown to be necessary for the nanorod formation.9 The surfactant is more likely to adsorb on the charged {110} surfaces than on the neutral {100} surfaces. The stability of the {110} side faces of the nanocrystals and the nanorods is therefore probably caused by surfactant adsorption, which hinders further growth in the radial direction and promotes growth in the [001] direction. The initial 〈110〉-oriented edge termination of the platelets (Figure 4b) may also be caused by preferential SDBS adsorption. Figure 9 shows that nanorod growth could be initiated directly from the substrate. Although the area with such growth was small, the principle of PbTiO3 nanorods grown directly on substrates is very interesting and useful for future applications. For instance, by growing nanorods on a conducting substrate (such as SrRuO3 thin film or Nb-doped SrTiO3), these nanorods could be interesting for use in energy-harvesting devices21 or ferroelectric random access memory.22 The formation of the carbonaceous layer will then have to be avoided, as it prevents the nucleation of PbTiO3 on the substrate surface. Optimization of both the synthesis procedure and the synthesis parameters may result in homogeneous growth of PbTiO3 nanorods over the entire substrate. Conclusions Hierarchical structures of PbTiO3, consisting of platelets with nanorods growing out from the platelet surfaces, have been grown

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on SrTiO3 substrates by hydrothermal synthesis at 180 °C. Initially, platelets grew oriented in the crystallographic 〈100〉 orientations of the underlying SrTiO3 substrate. Nanorods with a square-shaped cross-section then grew from the platelet surfaces. The morphology of the PbTiO3 nanostructures was varied by controlling the vertical position of the substrate in the autoclave, the synthesis time, and the crystallographic orientation of the SrTiO3 substrates. A growth model, based on initial growth of platelets from solution, followed by self-assembly of cube-shaped or facetted nanocrystals into nanorods was proposed. The ability to tailor the morphology of PbTiO3 nanostructures on substrates by chemical synthesis, as demonstrated here, is important for the nanomanufacturing of devices for future applications. Acknowledgment. This work was financially supported by the Strategic Area of Materials at the Norwegian University of Science and Technology and the Research Council of Norway (NANOMAT, grant no. 158518/431).

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