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

The overall goal of this procedure is the facile enhancement of the ferroelectricity of barium titanate by chemically introduced pore-induced strain. This method can help answer key questions in the strain and gelling field about simple and inexpensive ways to introduce strain. The main advantage of this technique is that it directly introduces strain without using a lattice mismatch at the hetero-interface.

To begin the procedure, dissolve 50 milligrams of polystyrene-block-poly(ethylene oxide)in 1.5 milliliters of tetrahydrofuran at 40 degrees Celsius. Allow the polymer solution to cool to room temperature. Then, combine 127.7 milligrams of barium acetate and 830 microliters of acetic acid.

Heat the mixture to 40 degrees Celsius, and stir for five minutes to dissolve the barium acetate. Allow the barium acetate solution to cool to room temperature. Then, add to the barium acetate solution 170 milligrams of titanium butoxide, and stir the mixture for one minute.

Then, add the polymer solution dropwise to the stirring barium acetate solution to form the precursor solution. Next, fix a two-centimeter-by-two-centimeter silicon/silica/titanium/platinum substrate on the sample stage of a spin coater. Cover the surface of the substrate with the precursor solution.

Spin coat the substrate with a sequence of 500 rpm for five seconds, followed by 3, 000 rpm for 30 seconds. Bake the thin film sample on a hotplate at 120 degrees Celsius for five minutes. Then, remove the sample from the hotplate, and allow it to cool to room temperature.

Finally, transfer the annealed sample to a muffle furnace. Calcine the sample at 800 degrees Celsius in air for 10 minutes to obtain the mesoporous barium titanate thin film. Fix the thin film sample on a scanning electron microscope sample holder with carbon tape.

Adjust the sample holder height as needed. Use a loading rod to insert the sample holder into the SEM. Position the sample holder to achieve a working distance of eight millimeters.

Set the accelerating voltage and emission current to five kilovolts and 10 milliamps, respectively, and start the electron beam. Inspect the sample at 30 to 100X magnification. Select a region of interest on the sample, and focus the image.

Gradually increase the magnification to 500 to 1, 000X, and focus the image again on the region of interest. Then, increase the magnification to 50, 000X. Focus the image on the pores to allow observation of the porous morphology.

Save one or more images of the sample. Prepare an X-ray diffractor equipped with a copper K-alpha source. Place the sample at the center of the sample stage.

Adjust the sample stage height so that the sample blocks half of incident X-ray beam. Adjust the grazing angle so that the sample surface is parallel to the beam. Continue alternately adjusting the height and grazing angle until the sample surface is at the center of and parallel to the X-ray beam.

Set a small grazing angle, such as 0.5 degrees, and scan two theta from 20 degrees to 70 degrees at one degree per minute. Save and process the X-ray diffraction data. Following visualization and initial analysis of the strain and the thin film sample, import the deformation ratio values into analysis software as a wave.

Use the Change Wave Scaling tool to change the wave to a 162-by-162 matrix. Then, display the wave as a 2D image plot using automatic X and Y scaling. Open the image processing unit, and draw a region of interest on the image.

Save the ROI as a mask, indicating the area for analysis, using the Save ROI Copy tool. Use the Histogram Generation tool to make and display the histogram of the deformation ratio. Adjust the color, font, and size of the histogram as desired.

Save the histogram when finished. The mesoporous barium titanate film was composed of vertically stacked crystallites spaced apart to form pores. Wide-angle X-ray diffraction showed strong peaks from the barium titanate crystalline framework.

Raman spectroscopy indicated a predominately tetragonal crystal phase. Fast Fourier transform mapping was used to determine the spatial distribution of the strain in convex and concave areas of the barium titanate framework. The outermost convex surface in the one negative one zero direction was expanded, suggesting a paraelectric cubic phase.

The inner area within the framework was compressed, suggesting a ferroelectric tetragonal phase. Some expanded areas also occurred within the framework, primarily at kinks and grain boundaries. Compression within the framework was observed in concave areas in the one negative one zero direction, but expansion was not clearly observed at the surface.

Minimal deformation was observed in the one one negative one direction for both convex and concave areas. The degree of deformation was quantified by the ratio of the distances between adjacent lattices in target and reference areas. The histogram for the one one negative one direction was nearly symmetric about 1.00 for both convex and concave areas.

The histogram for the one negative one zero direction had a peak centered at approximately 0.99 in both convex and concave areas, indicating increased comprehensive strain. After watching this video, you should have a good understanding of how to prepare and analyze a mesoporous thin film.