Thick lead-free ferroelectric films with high Curie temperatures through nanocomposite-induced strain

Abstract

Ferroelectric materials are used in applications ranging from energy harvesting to high-power electronic transducers. However, industry-standard ferroelectric materials contain lead, which is toxic and environmentally unfriendly. The preferred alternative, BaTiO(3), is non-toxic and has excellent ferroelectric properties, but its Curie temperature of ∼130 °C is too low to be practical. Strain has been used to enhance the Curie temperature of BaTiO(3) (ref. 4) and SrTiO(3) (ref. 5) films, but only for thicknesses of tens of nanometres, which is not thick enough for many device applications. Here, we increase the Curie temperature of micrometre-thick films of BaTiO(3) to at least 330 °C, and the tetragonal-to-cubic structural transition temperature to beyond 800 °C, by interspersing stiff, self-assembled vertical columns of Sm(2)O(3) throughout the film thickness. The columns, which are 10 nm in diameter, strain the BaTiO(3) matrix by 2.35%, forcing it to maintain its tetragonal structure and resulting in the highest BaTiO(3) transition temperatures so far.

Origin of thermally stable ferroelectricity in a porous barium titanate thin film synthesized through block copolymer templating

“…In the Raman spectrum of the thin film, a peak attributed to the (ferroelectric) tetragonal phase at ∼710 cm 1 was observed at room temperature and remained stable until a much higher temperature than T C of bulk BT (∼130 • C). Analyzing high-resolution TEM images of the thin film by the FFTM method revealed compressive stress along the [1][2][3][4][5][6][7][8][9][10] direction, which enhanced ferroelectricity because of the increased c/a ratio (i.e., deformation). These results clarify that the introduction of porosity thermally stabilizes and enhances the ferroelectricity of BT.…”

“…To increase the BT film thickness while preventing stress relaxation, a superlattice (periodic structure of very thin layers) and a threedimensional (3D) heteronanostructure have been developed. Harrington et al 5 synthesized vertical mesostructures of BT and Sm 2 O 3 to obtain micrometer-order thick films with T C above 800 • C. Inspired by this study, we chemically synthesized a 3D nanocomposite by introducing a precursor solution of BT onto a mesoporous strontium titanate (SrTiO 3 , hereafter denoted as ST) thin film. 6 T C of the resulting ST/BT nanocomposite reached 230 • C.…”

“…In FFTM images, we used the "deformation ratio," which was defined as the ratio of the distances between adjacent atoms deduced by tracking the positions of FFT peaks to the strain-free distance between those atoms. The FFTM image for the [1][2][3][4][5][6][7][8][9][10] direction in a convex area revealed that the outermost convex surface was slightly expanded, which caused lattice relaxation and weakened ferroelectricity. Conversely, the areas below the surface were compressed.…”

“…This indicates that there was little stress in the direction. In contrast, the histograms for the [1][2][3][4][5][6][7][8][9][10] direction contained marked peaks at a deformation ratio of around 0.99, indicating areas where compressive stress existed in the BT thin film. These results reveal that anisotropic compressive stress in the [1][2][3][4][5][6][7][8][9][10] direction existed in the porous BT thin films.…”

“…In contrast, the histograms for the [1][2][3][4][5][6][7][8][9][10] direction contained marked peaks at a deformation ratio of around 0.99, indicating areas where compressive stress existed in the BT thin film. These results reveal that anisotropic compressive stress in the [1][2][3][4][5][6][7][8][9][10] direction existed in the porous BT thin films. Compressive stress along the [1][2][3][4][5][6][7][8][9][10] direction increased the c/a ratio, causing further displacement of Ti 4+ cations from the center of the lattice.…”