N. S. was financially supported by the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research (KAKENHI) (Grant No. 26810126). Y. Y. is grateful to the Deanship of Scientific Research, King Saud University for founding through Vice Deanship of Scientific Research Chairs.
Wide-angle XRD measurements were conducted at the Nano-Processing Facility, supported by the Innovation-Boosting Equipment Common (IBEC) Innovation Platform, National Institute for Advanced Industrial Science and Technology (AIST), Japan. Raman spectra measurement and TEM observation of the porous thin film was conducted by HORIBA TECHNO SERVICE Co., Ltd. and Foundation for Promotion of Material Science and Technology of Japan (MST), respectively. MST also conducted visualization of the strain from TEM images. We thank Andrew Jackson, PhD, from Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript.

1. Preparation of Precursor Solution
<ol>
	<li>Dissolve 50 mg of diblock copolymer PS(18000)-b-PEO(7500) in 1.5 mL of tetrahydrofuran at 40 &#176;C. Cool the polymer solution to room temperature (RT).</li>
	<li>Dissolve 127.7 mg of barium acetate in 830 &#181;L of acetic acid by stirring at 40 &#176;C for 5 min. Cool the barium acetate solution to RT. Add 170 mg of titanium butoxide to the barium acetate solution and stir the reaction mixture for 1 min.</li>
	<li>Add the polymer solution to the barium acetate solution dropwise.</li>
</ol>
2. Synthesis of Mesoporous Barium Titanate Thin Film
<ol>
	<li>Set the Si/SiOx/Ti/Pt substrate (2 cm &#215; 2 cm) on the stage of a spin-coater and drop the prepared precursor solution to entirely cover the substrate. 
	NOTE: The film thickness of SiOx, Ti, and Pt layers were approximately 1.6, 40, and 150 nm, respectively.</li>
	<li>Spin the Si/SiOx/Ti/Pt substrate at 500 rpm for 5 s (1st step), and then 3,000 rpm for 30 s (2nd step), successively.</li>
	<li>Place the as-prepared film on a hot plate and heat it to 120 &#176;C for 5 min for aging, then allow it to cool to room temperature (RT) naturally.</li>
	<li>Place the annealed film in a muffle furnace and calcine in air at 800 &#176;C for 10 min with a ramp rate of 1 &#176;C/min (for both heating and cooling).</li>
</ol>
3. Characterization
<ol>
	<li>Morphological and crystallographic characterization
	<ol>
		<li>Scanning electron microscopy (SEM) measurement
		<ol>
			<li>Put the sample on the sample stage and cover the corners with carbon tapes to fix the sample. Adjust the height of the sample holder.</li>
			<li>Set the sample holder in a loading rod and insert it to the SEM. Position the sample holder in the home position at a working distance of 8 mm.</li>
			<li>Set accelerating voltage and emission current to 5 KV and 10 mA and generate an electron beam. Display the entire image of the sample at low magnification. Move the stage to show a region of interest (ROI) and focus on the image.</li>
			<li>Increase the magnification to 50,000X. Focus on the pores and observe the porous morphology. When a suitable image is observed, save a picture.</li>
		</ol>
		</li>
		<li>Transmission electron microscopy (TEM) measurement
		<ol>
			<li>Prepare a cross-sectional specimen prior to the TEM measurement as described in step 3.3.1.</li>
			<li>Set the prepared specimen in the sample holder. Set accelerating voltage to 300 KV and generate an electron beam. Insert the holder to the TEM.</li>
			<li>Display the entire image of the sample at low magnification. Move the stage to show ROI and focus on the image.</li>
			<li>Increase the magnification to 250,000X. Focus on the sample and observe the porous morphology of the synthesized thin film (~200 nm in thickness). When a suitable image is observed, save a picture.</li>
		</ol>
		</li>
		<li>Wide-angle X-ray diffraction (XRD) measurement
		<ol>
			<li>Set up the X-ray diffractometer equipped with a Cu K&#945; source.</li>
			<li>Put the sample on the center of the sample stage.</li>
			<li>Adjust the Z-axis (i.e., height position) of the stage in which the sample blocks the half of incident X-ray. Then, adjust the &#969;-axis (i.e., grazing angle) to make the sample surface parallel to the X-ray.</li>
			<li>Repeat step 3.1.3.3 until the sample stage position becomes suitable (i.e., the sample surface is located at the center of X-ray beam and parallel to the X-ray).</li>
			<li>Fix &#969; to a small angle (e.g., 0.5&#176;), and then scan 2&#952; (i.e., detector angle) from 20&#176; to 70&#176; with a rate of 1&#176;/min. Save the data after the measurement. 
			NOTE: To suppress the background signal from the substrate, incoming X-rays were irradiated at a very small grazing angle to the synthesized thin film&#39;s surface.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Test for thermal stability of ferroelectric phase
	<ol>
		<li>Turn on the Raman microscope and the operating computer, and then open the operating software LabSpec. Click Auto Calibration to calibrate the machine.</li>
		<li>Set a glass slide on a heating stage, and then put the synthesized film on the glass. Close the cover of the stage.</li>
		<li>Heat the sample with a heating stage using a ramp ratio of 15 &#176;C/min. When the temperature reaches the target temperature, set the heating stage to keep this temperature and wait a few minutes. 
		NOTE: In this study, we selected RT and 50, 75, 100, 110, 120, 125, 130, 135, 140, and 150 &#176;C for the bulk BT, which has a Curie temperature of approximately 130 &#176;C. We selected temperatures of RT and 75, 125, 175, 225, 275, 325, 375, 425, 475, and 525 &#176;C for the mesoporous BT thin film, for which the Curie temperature was evaluated to be approximately 470 &#176;C from a previous study8.</li>
		<li>After the temperature stabilizes, measure Raman spectra at various temperatures with a confocal Raman microscope using a 300-&#181;m confocal hole and a 532-nm laser (10 mW at sample) for excitation. Click the Video icon to display the observed image, and then focus on the image. Set acquisition time and accumulations to 100 s and 3, respectively, and then click Measure icon to start the measurement. Save the data after the measurement.</li>
		<li>Continue the measurements until the spectrum changes (i.e., the peaks assignable to the ferroelectric phase disappears). 
		NOTE: Measurements were conducted at three points and the obtained spectra were averaged.</li>
	</ol>
	</li>
	<li>Visualization of the strain
	<ol>
		<li>Prepare a cross-sectional specimen from the synthesized thin film by a micro sampling method using a focused ion beam for peripheral milling, bottom and micro-bridge cutting, and thinning of the specimen. The specimen&#39;s size should be approximately 20 &#181;m in length and 4 &#181;m in thickness.</li>
		<li>Measure high-resolution transmission electron microscopy (HR-TEM) images (2,000,000X) at convex and concave surfaces induced by the porosity.</li>
		<li>Select an area (512 &#215; 512 pixels) for fast Fourier transform (FFT), and calculate the FFT pattern. Estimate the lattice spacing from the FFT pattern, and divide it by the strain-free lattice spacing to calculate the &#34;deformation ratio&#34;.</li>
		<li>Shift the FFT analysis region by 32 pixels, and repeat step 3.3.3. Continue this process until the whole area of the HR-TEM image is covered (in this study, 5,664 &#215; 5,664 pixel (162 &#215; 162 points)). 
		NOTE: The strain-free lattice spacing was estimated from the least distorted FFT image.</li>
		<li>Set the color of regions based on the calculated deformation ratio to visualize the strain. Make the histograms by counting the deformation ratio. 
		NOTE: Analysis of the deformation in HR-TEM images was conducted with the software CryStMapp.</li>
		<li>Create a detailed histogram using the software Igor Pro 
		NOTE: Because the histogram of the distortion obtained in step 3.3.5 is rough, a detailed histogram using the software Igor Pro is created.
		<ol>
			<li>Load the numerical data of the deformation ratio by selecting Data | Load Waves | Load General Text and save it as a wave with proper name. 
			NOTE: Igor Pro defines the object including a numerical array as a wave.</li>
			<li>Change the saved wave as a matrix (162 &#215; 162) by selecting Data | Change Wave Scaling.</li>
			<li>Display a wave as 2D image by selecting Image | New | Image Prot.</li>
			<li>Select Analysis | Packages | Image Processing to show Image menu.</li>
			<li>Select Image | ROI to show ROI panel. Select Start ROI Draw to select ROI. Draw the region on top of an image, and then select Finish ROI.</li>
			<li>Select Save ROI Copy to create the mask to select the area for analysis as ROI_M_Mask wave.</li>
			<li>Input &#34;ImageHistogram/R=M_ROIMASK/ S waveneame&#34; in the command line of the command window to make a histogram. Use the name set in step 3.3.6.1 in wavename.</li>
			<li>Input &#34;display W_ImageHist&#34; in the command line of the command window to show a histogram. Modify the graph, if needed.</li>
		</ol>
		</li>
	</ol>
	</li>
</ol>

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	<li>Choi, K. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Enhancement+of+Ferroelectricity+in+Strained+BaTiO3+Thin+Films.">Enhancement of Ferroelectricity in Strained BaTiO3 Thin Films.</a> Science. 306 (5698), 1005-1009 (2004).</li><li>Nagarajan, V., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Misfit+dislocation+in+nanoscale+ferroelectric+heterostructures.">Misfit dislocation in nanoscale ferroelectric heterostructures.</a> Appl. Phys. Lett. 86 (19), 192910 (2005).</li><li>Wimbush, S. C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Interfacial+Strain-Induced+Oxygen+Disorder+as+the+Cause+of+Enhanced+Critical+Current+Density+in+Superconducting+Thin+Films.">Interfacial Strain-Induced Oxygen Disorder as the Cause of Enhanced Critical Current Density in Superconducting Thin Films.</a> Adv. Funct. Mater. 19 (6), 835-841 (2009).</li><li>Harrington, S. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thick+lead-free+ferroelectric+films+with+high+Curie+temperatures+through+nanocomposite-induced+strain.">Thick lead-free ferroelectric films with high Curie temperatures through nanocomposite-induced strain.</a> Nat. Nanotechnol. 6 (8), 491-495 (2011).</li><li>Suzuki, N., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Synthesis+of+Highly+Strained+Mesostructured+SrTiO3/BaTiO3+Composite+Films+with+Robust+Ferroelectricity.">Synthesis of Highly Strained Mesostructured SrTiO3/BaTiO3 Composite Films with Robust Ferroelectricity.</a> Chem. -Eur. J. 19 (14), 4446-4450 (2014).</li><li>Kresge, C. T., Leonowicz, M. E., Roth, W. J., Vartuli, J. C., Beck, J. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ordered+mesoporous+molecular+sieves+synthesized+by+a+liquid-crystal+template+mechanism.">Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism.</a> Nature. 359 (6397), 710-712 (1992).</li><li>Yang, P., Zhao, D., Margolese, D. I., Chmelka, B. F., Stucky, G. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Generalized+syntheses+of+large-pore+mesoporous+metal+oxides+with+semicrystalline+frameworks.">Generalized syntheses of large-pore mesoporous metal oxides with semicrystalline frameworks.</a> Nature. 396 (6707), 152-155 (1998).</li><li>Suzuki, N., Jiang, X., Salunkhe, R. R., Osada, M., Yamauchi, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chemical+Preparation+of+Ferroelectric+Mesoporous+Barium+Titanate+Thin+Films:+Drastic+Enhancement+of+Curie+Temperature+Induced+by+Mesopore-Drived+Strain.">Chemical Preparation of Ferroelectric Mesoporous Barium Titanate Thin Films: Drastic Enhancement of Curie Temperature Induced by Mesopore-Drived Strain.</a> Chem. -Eur. J. 20 (36), 11283-11286 (2014).</li><li>Tenne, D. A., Xi, X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Raman+Spectroscopy+of+Ferroelectric+Thin+Films+and+Superlattices.">Raman Spectroscopy of Ferroelectric Thin Films and Superlattices.</a> J. Am. Ceram. Soc. 91 (6), 1820-1834 (2008).</li><li>Ide, T., Sakai, A., Shimizu, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nanometer-Scale+Imaging+of+Lattice+Deformation+with+Transmission+Electron+Micrograph.">Nanometer-Scale Imaging of Lattice Deformation with Transmission Electron Micrograph.</a> Jpn. J. Appl. Phys., Part 2. 37 (12B), L1546-L1548 (1998).</li><li>Hoshina, T., Wada, S., Kuroiwa, Y., Tsurumi, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Composite+structure+and+size+effect+of+barium+titanate+nanoparticles.">Composite structure and size effect of barium titanate nanoparticles.</a> Appl. Phys. Lett. 93 (19), 192914 (2008).</li><li>Feng, C., Zhou, D. X., Gong, S. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Core-shell+structure+and+size+effect+in+barium+titanate+nanoparticle.">Core-shell structure and size effect in barium titanate nanoparticle.</a> Phys. B. 406 (6-7), 1317-1322 (2011).</li><li>Suzuki, N., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Origin+of+thermally+stable+ferroelectricity+in+a+porous+barium+titanium+thin+film+synthesized+through+block+copolymer+templateing.">Origin of thermally stable ferroelectricity in a porous barium titanium thin film synthesized through block copolymer templateing.</a> APL Mater. 5 (7), 076111 (2017).</li></ol>

The authors have nothing to disclose.

The splitting of the A1(TO) mode in the Raman spectrum of a porous BT thin film (Figure 3b) originates from compressive strain. This feature was clearly observed by the FFTM method (Figure 4) and its anisotropy in the [1-10] direction was determined from the histogram of distortion (Figure 5). Compressive strain along the [1-10] direction has a similar effect of inducing biaxial compressive strain in the (001) surface, which enhances the ferroelectricity in BT1. Porous driven anisotropic strain elongates the crystal lattice toward the c-axis, causing further dislocation of Ti4+ from the center of the lattice. This dislocation is expected to increase the electric dipole moment, which in turn enhances its ferro(piezo)electricity. Indeed, the piezoelectricity of a mesoporous BT film is superior to that of a non-porous film8.
The strain induced in the BT crystal lattice stabilizes the distorted tetragonal phase. Thus, the thermal stability of the lattice is expected to be enhanced. The Raman spectrum of the porous BT thin film showed the tetragonal phase origin peak (at 710 cm&#8722;1) remained visible until 375 &#176;C, although the peak gradually became weaker and broader (Figure 6b). This trend was similar to that found in a previous study, in which the Tc was estimated to be 470 &#176;C8. Thus, we confirmed the assumption that the pore-driven strain in the BT thin film effectively thermally stabilized the tetragonal phase.
Through this study, we clarified that pore-induced strain formed by a simple and inexpensive chemical procedure has a similar effect to that of strain at a hetero-interface originated from lattice mismatch. These findings provide novel insights into strain engineering.

Barium titanate (BaTiO3, hereafter BT) is a typical perovskite-type ferroelectric material. Although its ferroelectric properties were discovered in the 1940s, it is still widely used today because of its well-balanced ferroelectric and piezoelectric responses and favorable dielectric constant. Furthermore, because BT is a lead-free, eco-friendly material, it has attracted great interest as a replacement for lead zirconate titanate (PZT). At room temperature, the crystal phase of BT is tetragonal, where the ratio of c and a lattice parameters (c/a) is not equal to 1. In the tetragonal phase, the BT lattice is slightly elongated toward the c-axis and cations (Ba2+, Ti4+) and anions (O2&#8722;) are displaced in opposite directions. This displacement results in the spontaneous polarization of BT. When the temperature increases to the Curie temperature (Tc), a phase transition to the cubic phase occurs. In the cubic phase of BT, which has c/a = 1, the lattice distortion is relaxed, and its ferroelectricity is lost owing to the electrical neutrality originating from the inversion symmetry of the lattice. Recently, the use of high temperature ferroelectric materials has expanded. However, the Tc of BT is relatively low (~130 &#176;C) and bulk BT does not meet these demands.
To increase the Tc of BT, the ferroelectric (tetragonal) phase has been stabilized by the application of strain at the hetero-interface. For example, Choi et al. enhanced the ferroelectricity of BT films epitaxially grown on GdScO3 (110) and DyScO3 (110) substrates through the use of biaxial compressive strain caused by lattice mismatch1. However, the increase of Tc is limited to very thin films (tens of nanometers thick)2,3, which is impractical for device applications.
To increase the BT film thickness while preventing strain relaxation, superlattice (periodic structure of very thin layers) and three-dimensional (3D) hetero-nanostructures have been developed. Harrington et al. synthesized a vertical mesostructure of BT and Sm2O3 and obtained a micrometer-scale-ordered thick film without strain relaxation. In this sample, spontaneous polarization is oriented perpendicular to the substrate owing to uniaxial expansion of the BT unit cell; thus, a large remnant polarization was maintained at high temperature (i.e., the Tc was greater than 800 &#176;C)4. The obtained properties were satisfactory; however, a complicated and expensive physical process (pulsed-laser deposition) was required for the fabrication, which is a drawback for practical applications.
As an alternative facile and inexpensive fabrication process, we have proposed the chemical synthesis of 3D nanocomposites by the introduction of a precursor solution of BT into pores of a porous strontium titanate (SrTiO3, hereafter ST) thin film5. In the study, the porous ST thin film was synthesized by a surfactant-assisted sol-gel method, in which the self-assembly of amphipathic surfactant micelles was used as an organic template6,7. The method is illustrated schematically in Figure 1. Because the obtained ST thin film has a complex 3D porous structure with a large surface area, strain at the BT/ST hetero-interface is introduced into the nanocomposite, leading to the stabilization of the ferroelectric phase of BT (the Tc of ST/BT nanocomposite reached 230 &#176;C).
We hypothesized that porosity could directly introduce strain in BT and enhance the thermal stability of ferroelectric properties. In this study, we used a surfactant-assisted sol-gel method to fabricate porous BT and scrutinize the pore-induced strain. In addition, we compared the thermal stability between porous BT and nonporous bulk BT. We found that the introduced pores induced an anisotropic strain, which elongated the BT crystal lattice. This effect might be favorable for stabilizing the ferroelectric phase. Because the synthesis process used here is very simple, it has advantages over conventional physical processes for 3D hetero-nanostructures.

<table><tbody><tr><td>Diblock Copolymer PS(18000)-&lt;em&gt;b&lt;/em&gt;-PEO(7500)</td><td>Polymer Source, Inc.</td><td>#8399-SEO</td><td></td></tr><tr><td>Acetic acid (37 wt.%)</td><td>Wako</td><td>017-00256</td><td></td></tr><tr><td>Tetrahydrofuran</td><td>Wako</td><td>204-08745</td><td></td></tr><tr><td>Barium acetate</td><td>Sigma-Aldrich</td><td>243671-100G</td><td></td></tr><tr><td>Titanium(IV) butoxide</td><td>Sigma-Aldrich</td><td>244112-100G</td><td></td></tr><tr><td>Reference bulk BT single crystal</td><td>Crystal Base Co., Ltd.</td><td></td><td></td></tr><tr><td>Balance</td><td>Sartorius</td><td></td><td></td></tr><tr><td>Hot stirrer</td><td>IKA</td><td>RCT basic</td><td></td></tr><tr><td>Spin coater</td><td>Active</td><td>ACT-300DII</td><td></td></tr><tr><td>Hot plate</td><td>As one</td><td>ND-1</td><td></td></tr><tr><td>Muffle Furnace</td><td>Yamato Scientific Co., Ltd.</td><td>FO series</td><td></td></tr><tr><td>Scanning electron microscopy</td><td>Hitachi</td><td>SU-8000</td><td></td></tr><tr><td>Transmission electron microscopy</td><td>Hitachi</td><td>H-9000NAR</td><td></td></tr><tr><td>Wide-angle X-ray diffraction</td><td>Rigaku</td><td>RINT-Ultima III</td><td></td></tr><tr><td>Raman microscope</td><td>Horiba</td><td>XploRA Plus</td><td></td></tr></tbody></table>

chemical synthesis of porous barium titanate thin film and thermal stabilization of ferroelectric phase by porosity-induced strain

Barium titanate (BaTiO3, hereafter BT) is an established ferroelectric material first discovered in the 1940s and still widely used because of its well-balanced ferroelectricity, piezoelectricity, and dielectric constant. In addition, BT does not contain any toxic elements. Therefore, it is considered to be an eco-friendly material, which has attracted considerable interest as a replacement for lead zirconate titanate (PZT). However, bulk BT loses its ferroelectricity at approximately 130 &#176;C, thus, it cannot be used at high temperatures. Because of the growing demand for high-temperature ferroelectric materials, it is important to enhance the thermal stability of ferroelectricity in BT. In previous studies, strain originating from the lattice mismatch at hetero-interfaces has been used. However, the sample preparation in this approach requires complicated and expensive physical processes, which are undesirable for practical applications.
In this study, we propose a chemical synthesis of a porous material as an alternative means of introducing strain. We synthesized a porous BT thin film using a surfactant-assisted sol-gel method, in which self-assembled amphipathic surfactant micelles were used as an organic template. Through a series of studies, we clarified that the introduction of pores had a similar effect on distorting the BT crystal lattice, to that of a hetero-interface, leading to the enhancement and stabilization of ferroelectricity. Owing to its simplicity and cost effectiveness, this fabrication process has considerable advantages over conventional methods.

The morphology of the obtained mesoporous BT thin film was examined by electron microscopy. A top-view SEM image confirmed the porous features of the synthesized BT thin film (Figure 2a). The morphological features in the depth direction were investigated with a cross-sectional TEM image (Figure 2b). Large crystallites with diameters of several tens of nanometers were vertically stacked, and the gaps between these crystallites were pores. The estimated thickness of the BT thin film was approximately 200 nm.
The crystallinity of the BT framework was examined by wide-angle XRD measurements. Very weak peaks of BaCO3 and TiO2 were detectable and prominent peaks assignable to BT crystals were clearly observed (Figure 3a). However, it was difficult to distinguish between the (ferroelectric) tetragonal and (paraelectric) cubic phases. This is because the XRD patterns of both phases are quite similar. The main difference is that the peak at 2&#952; = 45&#176; for the cubic phase is split for the tetragonal phase. In this study, the detection of such splitting was difficult because the polycrystalline nature of the film broadened the peak width. Thus, to clarify the crystal phase of the thin film, its Raman spectrum was measured (Figure 3b).The Raman spectrum of a bulk single BT crystal at room temperature showed peaks centered at 275, 305, 515, and 720 cm&#8722;1, which were assigned to A1(TO), B1+E(TO+LO), E(TO)+A1(TO), and E(LO)+A1(LO) modes of the tetragonal phase9. In the spectrum of the porous BT thin film, although splitting of the A1(TO) mode occurred, the main features of the spectrum were retained. Thus, the synthesized porous BT thin film was tetragonal.
The spatial distribution of the strain in the BT framework of the thin film was examined by the fast Fourier transform mapping (FFTM) method10. This method analyzes and visualizes tiny distortions in FFT patterns of high-resolution (HR)-TEM images. Figure 4 depicts HR-TEM images of areas of the thin film with convex and concave surfaces and the corresponding FFTM images. The FFTM image of the [1-10] direction in a convex area revealed that the outermost convex surface was slightly expanded, an arrangement which should cause lattice relaxation and weakens ferroelectricity. Conversely, the areas just below the surface were compressed, and the compressed areas were observed entirely inside the framework. This result is consistent with previous reports that indicate the surface of BT nanoparticles feature a paraelectric cubic phase, while the inner core is a ferroelectric tetragonal phase11,12. Within the BT framework, some expanded areas were also found, mainly at kinks and/or grain boundaries (Figure 4c). For concave areas, although the deformation of the outermost surface was not clearly observed, likely because the surface was polygonal rather than curved, compression within the framework was detected (Figure 4d). Conversely, the FFTM images of the [11-1] direction in both the convex and concave areas were unclear (Figure 4e, f), suggesting that there was little deformation of the BT unit cell in this direction.
To examine the deformation of the BT lattice more quantitatively, the degree of deformation was summarized in histograms (Figure 5). From these histograms, we determined the &#34;deformation ratio&#34;, which is defined as the ratio of the distance between adjacent lattice spacing in the target and reference areas, as a measure of deformation. In the [11-1] direction, the histograms were centered at a deformation ratio of 1.00 and were nearly symmetrical for both convex and concave areas. This result indicates that there was little strain in the [11-1] direction, consistent with the results of FFTM mentioned above. Conversely, the histograms for the [1-10] direction contained marked peaks at a deformation ratio of approximately 0.99, showing the area where compressive strain increased in the BT thin film.
The thermal stability of the ferroelectric tetragonal phase was examined from the temperature dependence of its Raman spectrum (Figure 6). In a bulk BT single crystal, the sharp peaks at 305 and 720 cm&#8722;1 disappeared at 140 &#176;C, which is consistent with the Tc of bulk BT (~130 &#176;C). Conversely, the peak at 710 cm&#8722;1 from the tetragonal phase remained at much higher temperatures, detectable up to 375 &#176;C for the synthesized porous thin film.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/57441/57441fig1.jpg" /> 
Figure 1: Schematic image of surfactant-assisted sol-gel method. Self-assembly of amphipathic surfactant micelles was used as a template. By combining the organic-template and inorganic sol, an organic/inorganic hybrid is created. Finally, calcination was conducted to create pores by removing the organic template and crystallizing the inorganic framework. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57441/57441fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/57441/57441fig2v3.jpg" /> 
Figure 2: Electron microscope image of mesoporous BT thin film. (a) Top-view SEM and (b) cross-sectional TEM image. This figure has been modified from Suzuki, N. et al.13 in accordance with the Creative Commons Attribution (CC BY) license. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57441/57441fig2v3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/57441/57441fig3.jpg" /> 
Figure 3: Spectroscopic data for the crystal phase of mesoporous BT thin film. (a) Wide-angle XRD pattern and (b) Raman spectrum of the synthesized porous BT thin film at room temperature. The spectrum of a bulk BT single crystal is also included for reference. This figure has been modified from Suzuki, N. et al.13 in accordance with the Creative Commons Attribution (CC BY) license. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57441/57441fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/57441/57441fig4v2.jpg" /> 
Figure 4: Visualization of porosity-induced strain. (a,b) High-resolution TEM images and (c-f) fast Fourier transform mapping (FFTM) images of (a, c, e) convex and (b, d, f) concave areas of the BT thin films. The orientations of the FFTM images are (c, d) [1-10] and (e, f) [11-1]. In the FFTM images, green and red areas represent regions where compressive and tensile strain are applied, respectively, while yellow indicates the reference area. The orientation used for the analysis is also included (left). This figure has been modified from Suzuki, N. et al.13 in accordance with the Creative Commons Attribution (CC BY) license. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57441/57441fig4v2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/57441/57441fig5.jpg" /> 
Figure 5: Analysis of porosity-induced strain. Histogram of distortion analyzed in (a) convex (Figure 4a, Figure 4c, and Figure 4e) and (b) concave (Figure 4b, Figure 4d, Figure 4f) areas of the porous BT thin film. This figure has been modified from Suzuki, N. et al.13&#160;in accordance with the Creative Commons Attribution (CC BY) license. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57441/57441fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/57441/57441fig6.jpg" /> 
Figure 6: Thermal stability of ferroelectric phase. Temperature dependence of the Raman spectrum of (a) a bulk BT single crystal and (b) a synthesized porous BT thin film. This figure has been modified from Suzuki, N. et al.13&#160;in accordance with the Creative Commons Attribution (CC BY) license. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57441/57441fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Chemical Synthesis of Porous Barium Titanate Thin Film and Thermal Stabilization of Ferroelectric Phase by Porosity-Induced Strain at JoVE.com

Chemical Synthesis of Porous Barium Titanate Thin Film and Thermal Stabilization of Ferroelectric Phase by Porosity-Induced Strain

Here, we present a protocol for the synthesis of porous barium titanate (BaTiO3) thin film by a surfactant-assisted sol-gel method, in which self-assembled amphipathic surfactant micelles are used as an organic template.

Barium titanate (BaTiO3, hereafter BT) is an established ferroelectric material first discovered in the 1940s and still widely used because of its ...

chemical-synthesis-porous-barium-titanate-thin-film-thermal

Nanyang Technological University

Research

JoVE Journal

Chemistry

11.0K Views.  Tokyo University of Science (TUS). Hier presenteren we een protocol voor de synthese van poreuze barium titanate (BaTiO3) dunne film door een oppervlakteactieve stof-bijgewoonde sol-gel-methode, waarbij zelf geassembleerde amphipathic oppervlakteactieve stof micellen worden gebruikt als een biologische sjabloon.

Video: Chemical Synthesis of Porous Barium Titanate Thin Film and Thermal Stabilization of Ferroelectric Phase by Porosity-Induced Strain

Fabrication of Spatially Confined Complex Oxides

Complex materials such as high Tc superconductors, multiferroics, and colossal magnetoresistors have electronic and magnetic properties that arise from the inherent strong electron correlations that reside within them. These materials can also possess electronic phase separation in which regions of vastly different resistive and magnetic behavior can coexist within a single crystal alloy material. By reducing the scale of these materials to length scales at and below the inherent size of the electronic domains, novel behaviors can be exposed. Because of this and the fact that spin-charge-lattice-orbital order parameters each involve correlation lengths, spatially reducing these materials for transport measurements is a critical step in understanding the fundamental physics that drives complex behaviors. These materials also offer great potential to become the next generation of electronic devices 1-3. Thus, the fabrication of low dimensional nano- or micro-structures is extremely important to achieve new functionality. This involves multiple controllable processes from high quality thin film growth to accurate electronic property characterization. Here, we present fabrication protocols of high quality microstructures for complex oxide manganite devices. Detailed descriptions and required equipment of thin film growth, photo-lithography, and wire-bonding are presented.

We describe the use of pulsed laser deposition (PLD), photolithography and wire-bonding techniques to create micrometer scale complex oxides devices. The PLD is utilized to grow epitaxial thin films. Photolithography and wire-bonding techniques are introduced to create practical devices for measurement purposes.

Complex materials such as high Tc superconductors, multiferroics, and colossal magnetoresistors have electronic and magnetic properties that arise from ...

Engineering

Synthesis of Non-uniformly Pr-doped SrTiO3 Ceramics and Their Thermoelectric Properties

We demonstrate a novel synthesis strategy for the preparation of Pr-doped SrTiO3 ceramics via a combination of solid state reaction and spark plasma sintering techniques. Polycrystalline ceramics possessing a unique morphology can be achieved by optimizing the process parameters, particularly spark plasma sintering heating rate. The phase and morphology of the synthesized ceramics were investigated in detail using X-ray diffraction, scanning electron microcopy and energy-dispersive X-ray spectroscopy. It was observed that the grains of these bulk Pr-doped SrTiO3 ceramics were enhanced with Pr-rich grain boundaries. Electronic and thermal transport properties were also investigated as a function of temperature and doping concentration. Such a microstructure was found to give rise to improved thermoelectric properties. Specifically, it resulted in a significant improvement in carrier mobility and the thermoelectric power factor. Simultaneously, it also led to a marked reduction in the thermal conductivity. As a result, a significant improvement (> 30%) in the thermoelectric figure of merit was achieved for the whole temperature range over all previously reported maximum values for SrTiO3-based ceramics. This synthesis demonstrates the steps for the preparation of bulk polycrystalline ceramics of non-uniformly Pr-doped SrTiO3.

Synthesis of Non-uniformly Pr-doped SrTiO3 Ceramics and Their Thermoelectric Properties

A protocol for the synthesis and processing of polycrystalline SrTiO3 ceramics doped non-uniformly with Pr is presented along with the investigation of their thermoelectric properties.

We demonstrate a novel synthesis strategy for the preparation of Pr-doped SrTiO3 ceramics via a combination of solid state reaction and spark plasma ...

Preparation of Macroporous Epitaxial Quartz Films on Silicon by Chemical Solution Deposition

This work describes the detailed protocol for preparing piezoelectric macroporous epitaxial quartz films on silicon(100) substrates. This is a three-step process based on the preparation of a sol in a one-pot synthesis which is followed by the deposition of a gel film on Si(100) substrates by evaporation induced self-assembly using the dip-coating technique and ends with a thermal treatment of the material to induce the gel crystallization and the growth of the quartz film. The formation of a silica gel is based on the reaction of a tetraethyl orthosilicate and water, catalyzed by HCl, in ethanol. However, the solution contains two additional components that are essential for preparing mesoporous epitaxial quartz films from these silica gels dip-coated on Si. Alkaline earth ions, like Sr2+ act as glass melting agents that facilitate the crystallization of silica and in combination with cetyl trimethylammonium bromide (CTAB) amphiphilic template form a phase separation responsible of the macroporosity of the films. The good matching between the quartz and silicon cell parameters is also essential in the stabilization of quartz over other SiO2 polymorphs and is at the origin of the epitaxial growth.

A protocol is presented for the preparation of piezoelectric macroporous epitaxial films of quartz on silicon by solution chemistry using dip-coating and thermal treatments in air.

This work describes the detailed protocol for preparing piezoelectric macroporous epitaxial quartz films on silicon(100) substrates. This is a three-step ...

Epitaxial Growth of Perovskite Strontium Titanate on Germanium via Atomic Layer Deposition

Atomic layer deposition (ALD) is a commercially utilized deposition method for electronic materials. ALD growth of thin films offers thickness control and conformality by taking advantage of self-limiting reactions between vapor-phase precursors and the growing film. Perovskite oxides present potential for next-generation electronic materials, but to-date have mostly been deposited by physical methods. This work outlines a method for depositing SrTiO3 (STO) on germanium using ALD. Germanium has higher carrier mobilities than silicon and therefore offers an alternative semiconductor material with faster device operation. This method takes advantage of the instability of germanium's native oxide by using thermal deoxidation to clean and reconstruct the Ge (001) surface to the 2&#215;1 structure. 2-nm thick, amorphous STO is then deposited by ALD. The STO film is annealed under ultra-high vacuum and crystallizes on the reconstructed Ge surface. Reflection high-energy electron diffraction (RHEED) is used during this annealing step to monitor the STO crystallization. The thin, crystalline layer of STO acts as a template for subsequent growth of STO that is crystalline as-grown, as confirmed by RHEED. In situ X-ray photoelectron spectroscopy is used to verify film stoichiometry before and after the annealing step, as well as after subsequent STO growth. This procedure provides framework for additional perovskite oxides to be deposited on semiconductors via chemical methods in addition to the integration of more sophisticated heterostructures already achievable by physical methods.

This work details the procedures for the growth and characterization of crystalline SrTiO3 directly on germanium substrates by atomic layer deposition. The procedure illustrates the ability of an all-chemical growth method to integrate oxides monolithically onto semiconductors for metal-oxide semiconductor devices.

Atomic layer deposition (ALD) is a commercially utilized deposition method for electronic materials. ALD growth of thin films offers thickness control and ...

Spark Plasma Sintering Apparatus Used for the Formation of Strontium Titanate Bicrystals

A spark plasma sintering apparatus was used as a novel method for diffusion bonding of two single crystals of strontium titanate to form bicrystals with one twist grain boundary. This apparatus utilizes high uniaxial pressure and a pulsed direct current for rapid consolidation of material. Diffusion bonding of strontium titanate bicrystals without fracture, in a spark plasma sintering apparatus, is possible at high pressures due to the unusual temperature dependent plasticity behavior of strontium titanate. We demonstrate a method for the successful formation of bicrystals at accelerated time scales and lower temperatures in a spark plasma sintering apparatus compared to bicrystals formed by conventional diffusion bonding parameters. Bond quality was verified by scanning electron microscopy. A clean and atomically abrupt interface containing no secondary phases was observed using transmission electron microscopy techniques. Local changes in bonding across the boundary was characterized by simultaneous scanning transmission electron microscopy and spatially resolved electron energy-loss spectroscopy.

A viable technique for the formation of strontium titanate bicrystals at high pressure and fast heating rate via the spark plasma sintering apparatus is developed.

A spark plasma sintering apparatus was used as a novel method for diffusion bonding of two single crystals of strontium titanate to form bicrystals with ...

A Fabrication and Measurement Method for a Flexible Ferroelectric Element Based on Van Der Waals Heteroepitaxy

Flexible non-volatile memories have received much attention as they are applicable for portable smart electronic device in the future, relying on high-density data storage and low-power consumption capabilities. However, the high-quality oxide based nonvolatile memory on flexible substrates is often constrained by the material characteristics and the inevitable high-temperature fabrication process. In this paper, a protocol is proposed to directly grow an epitaxial yet flexible lead zirconium titanate memory element on muscovite mica. The versatile deposition technique and measurement method enable the fabrication of flexible yet single-crystalline non-volatile memory elements necessary for the next generation of smart devices.

In this paper, we present a protocol to directly grow an epitaxial yet flexible lead zirconium titanate memory element on muscovite mica.

Flexible non-volatile memories have received much attention as they are applicable for portable smart electronic device in the future, relying on ...

Bulk and Thin Film Synthesis of Compositionally Variant Entropy-stabilized Oxides

Here, we present a procedure for the synthesis of bulk and thin film multicomponent (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (Co variant) and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O (Cu variant) entropy-stabilized oxides. Phase pure and chemically homogeneous (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (x = 0.20, 0.27, 0.33) and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O (x = 0.11, 0.27) ceramic pellets are synthesized and used in the deposition of ultra-high quality, phase pure, single crystalline thin films of the target stoichiometry. A detailed methodology for the deposition of smooth, chemically homogeneous, entropy-stabilized oxide thin films by pulsed laser deposition on (001)-oriented MgO substrates is described. The phase and crystallinity of bulk and thin film materials are confirmed using X-ray diffraction. Composition and chemical homogeneity are confirmed by X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy. The surface topography of thin films is measured with scanning probe microscopy. The synthesis of high quality, single crystalline, entropy-stabilized oxide thin films enables the study of interface, size, strain, and disorder effects on the properties in this new class of highly disordered oxide materials.

The synthesis of high quality bulk and thin film (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O entropy-stabilized oxides is presented.

Here, we present a procedure for the synthesis of bulk and thin film multicomponent (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (Co variant) and ...

Fabrication of Mechanically Tunable and Bioactive Metal Scaffolds for Biomedical Applications

Biometal systems have been widely used for biomedical applications, in particular, as load-bearing materials. However, major challenges are high stiffness and low bioactivity of metals. In this study, we have developed a new method towards fabricating a new type of bioactive and mechanically reliable porous metal scaffolds-densified porous Ti scaffolds. The method consists of two fabrication processes, 1) the fabrication of porous Ti scaffolds by dynamic freeze casting, and 2) coating and densification of the porous scaffolds. The dynamic freeze casting method to fabricate porous Ti scaffolds allowed the densification of porous scaffolds by minimizing the chemical contamination and structural defects. The densification process is distinctive for three reasons. First, the densification process is simple, because it requires a control of only one parameter (degree of densification). Second, it is effective, as it achieves mechanical enhancement and sustainable release of biomolecules from porous scaffolds. Third, it has broad applications, as it is also applicable to the fabrication of functionally graded porous scaffolds by spatially varied strain during densification.

Bioactive and mechanically reliable metal scaffolds have been fabricated through a method which consists of two processes, dynamic freeze casting for the fabrication of porous Ti, and coating and densification of the Ti scaffolds. The densification process is simple, effective and applicable to the fabrication of functionally graded scaffolds.

Biometal systems have been widely used for biomedical applications, in particular, as load-bearing materials. However, major challenges are high stiffness ...

Bioengineering

Visualizing Uniaxial-strain Manipulation of Antiferromagnetic Domains in Fe1+YTe Using a Spin-polarized Scanning Tunneling Microscope

The quest to understand correlated electronic systems has pushed the frontiers of experimental measurements toward the development of new experimental techniques and methodologies. Here we use a novel home-built uniaxial-strain device integrated into our variable temperature scanning tunneling microscope that enables us to controllably manipulate in-plane uniaxial strain in samples and probe their electronic response at the atomic scale. Using scanning tunneling microscopy (STM) with spin-polarization techniques, we visualize antiferromagnetic (AFM) domains and their atomic structure in Fe1+yTe samples, the parent compound of iron-based superconductors, and demonstrate how these domains respond to applied uniaxial strain. We observe the bidirectional AFM domains in the unstrained sample, with an average domain size of ~50-150 nm, to transition into a single unidirectional domain under applied uniaxial strain. The findings presented here open a new direction to utilize a valuable tuning parameter in STM, as well as other spectroscopic techniques, both for tuning the electronic properties as for inducing symmetry breaking in quantum material systems.

Visualizing Uniaxial-strain Manipulation of Antiferromagnetic Domains in Fe1+YTe Using a Spin-polarized Scanning Tunneling Microscope

Using uniaxial strain combined with spin-polarized scanning tunneling microscopy, we visualize and manipulate the antiferromagnetic domain structure of Fe1+yTe, the parent compound of iron-based superconductors.

The quest to understand correlated electronic systems has pushed the frontiers of experimental measurements toward the development of new experimental ...

Applying Dynamic Strain on Thin Oxide Films Immobilized on a Pseudoelastic Nickel-Titanium Alloy

Direct alteration of material structure/function through strain is a growing area of research that has allowed for novel properties of materials to emerge. Tuning material structure can be achieved by controlling an external force imposed on materials and inducing stress-strain responses (i.e., applying dynamic strain). Electroactive thin films are typically deposited on shape or volume tunable elastic substrates, where mechanical loading (i.e., compression or tension) can affect film structure and function through imposed strain. Here, we summarize methods for straining n-type doped titanium dioxide (TiO2) films prepared by a thermal treatment of a pseudo-elastic nickel-titanium alloy (Nitinol). The main purpose of the described methods is to study how strain affects electrocatalytic activities of metal oxide, specifically hydrogen evolution and oxygen evolution reactions. The same system can be adapted to study the effect of strain more broadly. Strain engineering can be applied for optimization of a material function, as well as for design of adjustable, multifunctional (photo)electrocatalytic materials under external stress control.

Dynamic, tensile strain is applied on TiO2 thin films to study the effects of strain on electrocatalysis, specifically proton reduction and water oxidation. TiO2 films are prepared by thermal treatment of the pseudo-elastic NiTi alloy (Nitinol).

Direct alteration of material structure/function through strain is a growing area of research that has allowed for novel properties of materials to ...

Chemical Synthesis of Porous Barium Titanate Thin Film and Thermal Stabilization of Ferroelectric Phase by Porosity-Induced Strain

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