Electric control of straight stripe conductive mixed-phase nanostructures in La-doped BiFeO3

This study examines the atomic force microscope (AFM) tip-based electrical formation of tens of microns long stripe (and ∼100 nm wide) inorganic one-dimensional nanostructures based on the morphotropic phase boundary of La-doped BiFeO3 epitaxial thin films. The substitution of Lanthanum into bismuth ferrite not only produces the formation of straight stripe mixed-phase patterns but also improves the spatial continuity drastically by two orders of magnitude. We create, switch and erase stripe nanostructures in a reversible and deterministic way. We demonstrate that electrically formed areas with a nearly single variant alignment can be overwritten with different alignments repeatedly, which reflects the reversible and nonvolatile nature of the switching process. In addition, we explore the functionality of the created nanostructures by clarifying ferroelectric polarizations and observing the improvement of the electronic conduction at the phase boundary. Our findings provide new pathways to one-dimensional rewritable nanostructures and inspire researchers to conceive various multifunctional devices by combining the superb electromechanical property with their unique interfacial electronic conduction properties at nanoscale phase boundaries. Scanning a La-doped BiFeO3 epitaxial thin film with a biased AFM tip, we produce straight stripe one-dimensional nanostructures in which two competing polymorphic ferroelectric phases appear alternately at the interval of ∼100 nm showing the enhancement of electronic conduction at the phase boundaries. We can create, switch and erase the eight-variant stripe nanostructures in a reversible and deterministic way by controlling the tip scanning direction and tip biased voltage. The findings provide a new pathway into one-dimensional nanostructures and versatile patterns of ferroelectric domains and interfaces. Some materials exhibit ‘morphotropic phase boundaries’ at which their crystal structure abruptly changes from a tetrahedral to a rhombohedral lattice. Such phase boundaries are created under high pressure or strain and are accompanied by a change in the material's characteristics. For example, there is great interest in controlling mixed-phase domains composed of ‘T’ and ‘R” phases in thin films of bismuth ferrite (BiFeO3, also known as BFO) for electromechanical applications. Now, a South Korea-Australia-based team led by Chan-Ho Yang has used an atomic force microscope tip to create - and further manipulate or erase - well-aligned, alternating straight stripes of the T and R' domains on a BFO thin film doped with lanthanum. In the absence of lanthanum, the stripes are curved rather than straight. The T-R' phase boundary shows enhanced electronic conduction compared to the bulk phases. These precise and rewritable nanostructures might serve to develop functional materials.