Strain-driven autonomous control of cation distribution for artificial ferroelectrics

Theoretical material design and experimental synthesis have advanced in the past few decades with a key role in the development of functional materials, useful for next generation technologies. Ultimately, however, the goal of synthesis science remains to be achieved in order to locate atoms in a specific position of matter. In a new report now published on Science Advances, Changhee Sohn and researchers in materials science and nanostructure physics in the U.S. and the Republic of Korea developed a unique method to inject elements in a specific crystallographic position in a composite material via strain engineering. The team showed a powerful way to use strain to artificially manipulate the atomic position for the synthesis of new materials and structures. The results are applicable to a wide range of systems to provide a new route to functional materials.

Using strain to develop new materials.

Epitaxial strain originates from the lattice mismatch between a film and a substrate to manipulate important physical properties of materials. They have also revolutionized industries to develop fast central computing processors. Ferroelectricity and its potential for ultrahigh density memory shows the importance of strain engineering in future technologies. In a recent theoretical prediction, researchers proposed an unreported role of strain to develop new materials by inserting and repositioning individual atoms in a site-specific manner within a unit cell of materials. Using this strain-driven method, Sohn et al. combined layered perovskite materials such as Bi₄Ti₃O₁₂ (abbreviated as BiT) and simple perovskites with the general formula ABO₃. The BiT is a unique ferroelectric material with three oxygen octahedral sublayers sandwiched between two BiO₂^- layers. In a separate synthetic approach, Sohn et al. formed a composite Bi₅Ti₃FeO₁₂ (BiTF) at the subunit level by strain and controlled the inserted iron (Fe) ions at the subunit level. During the experiments, they used pulsed laser deposition with two targets Bi₄Ti₃O₁₂ (abbreviated as BiT) and bismuth ferrite (BiFeO₃), abbreviated BFO, to demonstrate the growth control of composite materials by alloying BFO with layered BiT. During the experiments, they ablated the material at subunit cell level on strontium titanate (SrTiO₃) substrates to precisely control their composition. Using scanning transmission electron microscopy (STEM), the team visualized the complete insertion of the additional octahedral layers between adjunct BiO₂^- layers. They obtained high-angle annular dark-field (HAADF) images of BiT and BiTF films grown on strontium titanate substrates, where the bright and intense signals arrived from heavy bismuth (Bi) ions and weaker signals arrived from the lighter titanium and iron ions. Using the two-target method, Sohn et al. also synthesized epitaxial BiTF thin films on various substrates with different directions and magnitudes of strain.

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Experimental observations

To understand the strain-dependent distribution of Fe ions in the materials at the atomic scale, Sohn et al. conducted energy-dispersive X-ray spectroscopy mapping combined with STEM (scanning transmission electron microscopy) on BiTF films. Using atomically resolved energy dispersive X-ray (EDX) mapping, the team revealed the distinct evolution of the material. The excellent agreement of the role of strain with the theoretical prediction supported its role in controlling the Fe ion distribution in the film. The scientists were also keen to understand how inserting and positioning Fe ions into BiT affected the macroscopic properties of the film. To accomplish this, they fist focused on the optical properties important to understand electronic structures at the fundamental level and for technical applications. After inserting BFO blocks the scientists observed bandgap reduction. Sohn et al. also observed the relationship between ferroelectricity of BiTF films and the cationic distribution of iron ions. Thereafter, using Kelvin probe force microscopy (cKPFM), they examined the piezoelectric properties of the films to note strong substrate dependence on lateral and vertical ferroelectricity.

Outlook

In this way, Changhee Sohn and colleagues demonstrated the unique strain-driven synthetic paradigm that allowed researchers to insert atoms and autonomously direct them to a specific crystallographic position of matter. The method is distinct from well-known methods of synthesis such as conventional heterostructure engineering or simple alloying of two different materials. The strain-driven artificial control of atomic positions can boost research in materials science and condensed matter physics to develop multifunctional composite systems. Based on this method, Sohn et al. expect to synthesize multiferroic materials and control their magnetic ground state through cationic distribution.

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