The role of flexoelectric coupling and chemical strains in the emergence of polar chiral nano-structures

This theoretical review examines the conditions that give rise to the formation of flexo-sensitive chiral polar structures in thin films and core–shell ferroelectric nanoparticles. It also analyzes possible mechanisms by which the flexoelectric effect impacts the polarization structure in core–shell ferroelectric nanoparticles. Special attention is given to the role of the anisotropic flexoelectric effect in forming a unique type of polarization states with distinct chiral properties, referred to as “flexons.” In the first half of the review, we study the influence of the flexoelectric coupling on the polarity, chirality, and branching of metastable labyrinthine domain structures in uniaxial ferroelectric core–shell nanoparticles. We reveal that the transition from sinuous branched domain stripes to spiral-like domains occurs gradually as the flexoelectric coupling strength is increased. Our findings indicate that the joint action of the flexoelectric effect and chemical strains, termed as “flexo-chemical” coupling, can significantly influence the effective Curie temperature, polarization distribution, domain morphology, and chirality in multiaxial ferroelectric core–shell nanoparticles. Furthermore, we demonstrate that the combination of flexo-chemical coupling and screening effects leads to the appearance and stabilization of a chiral polarization morphology in nanoflakes of van der Waals ferrielectrics covered by a shell of ionic-electronic screening charge. In the second half of the review, we discuss several advanced applications of flexo-sensitive chiral polar structures in core–shell ferroelectric nanoparticles for nanoelectronics elements and cryptography. We underline the possibilities of the flexoelectric control of multiple-degenerated labyrinthine states, which may correspond to a differential negative capacitance (NC) state stabilized in the uniaxial ferroelectric core by the presence of a screening shell. We show that the paraelectric-like state of van der Waals ferrielectric nanoflakes covered by a shell of ionic-electronic screening charge exhibits a pronounced NC effect over a relatively wide range of nanoflake thicknesses, flexo-chemical strains, and surface charge densities.