No abstract

Recently, rapidly increased demands of integration and miniaturization continuously challenge energy densities of dielectric capacitors. New materials with high recoverable energy storage densities become highly desirable. Here, by structure evolution between fluorite HfO2 and perovskite hafnate, we create an amorphous hafnium-based oxide that exhibits the energy density of ~155 J/cm3 with an efficiency of 87%, which is state-of-the-art in emergingly capacitive energy-storage materials. The amorphous structure is owing to oxygen instability in between the two energetically-favorable crystalline forms, in which not only the long-range periodicities of fluorite and perovskite are collapsed but also more than one symmetry, i.e., the monoclinic and orthorhombic, coexist in short range, giving rise to a strong structure disordering. As a result, the carrier avalanche is impeded and an ultrahigh breakdown strength up to 12 MV/cm is achieved, which, accompanying with a large permittivity, remarkably enhances the energy storage density. Our study provides a new and widely applicable platform for designing high-performance dielectric energy storage with the strategy exploring the boundary among different categories of materials.

Recently, flexoelectric effect has attracted considerable attention owing to ubiquitous existence in all dielectrics, regardless of the symmetry. It promises intriguingly physical phenomena, such as strain gradient-induced electric polarizations, photocurrents, and interfacial transports, as well as their electromechanical coupling with external force loading, in diverse materials for multifunctional applications in electronics. In this work, we report the flexoelectric-modulation on surface potential of LaFeO3 (LFO) thin-film heterostructures. The LFO thin film with or without the flexoelectric effect has been achieved by controlling epitaxial misfit against a substrate. Lattice structures and strain behaviors are observed by atomic-resolution high-angle annular dark-field imaging. Grown on a LaAlO3 substrate, a giant strain gradient of ∼3 × 106 m−1 is generated in the LFO thin film due to the gradual relaxation of large misfit strain with increasing thickness, yielding a robust flexoelectric polarization pointing to the heterostructure surface. In contrast, the LFO is almost fully strained on a SrTiO3 substrate due to the small lattice mismatch. The flexoelectric polarization results in an increase in surface potential in the LFO heterostructure due to the incomplete screening of positive polarization bound charges, as observed by scanning kelvin probe microscopy. Furthermore, x-ray photoelectron spectroscopy reveals that the flexoelectric polarization can downward bend the band alignment of the LFO layer and modulate the interfacial potential barriers. These results provide the way for experimental observations of the flexoelectric effect and deliver physical insight into deep understanding of interfacial electronic structures of flexoelectric-based devices.

Dielectric capacitors are fundamental for electric power systems due to the fast charging/discharging rate and high-power density.[1,2] Recently, rapidly increased demands of miniaturization and integration continuously challenge energy storage density of dielectric capacitors, especially for that could be compatible with the complementary metal-oxide-semiconductor (CMOS) technology, for developing energy-autonomous systems and implantable/wearable electronics, where high-κ capacitors become increasingly desirable in the next-generation applications.[3-5] However, their recoverable energy storage densities (Urec) are low in emerging capacitive energy storage materials. Here, by structure evolution between fluorite HfO2 and perovskite hafnate who have similar metal sublattices, we create an amorphous hafnium-based oxide that exhibits a giant Urec of ~155 J/cm3 with an efficiency (η) of 87%, which is record-high in high-κ materials and state-of-the-art in dielectric energy storage. The improved energy density is owing to the strong structure disordering in both short and long ranges induced by oxygen instability in between the two energetically-favorable crystalline forms. As a result, the carrier avalanche is impeded and an ultrahigh breakdown strength (Eb) up to 12 MV/cm is achieved, which, accompanying with a large permittivity (εr), remarkably enhances the dielectric energy storage. Our study provides a new and widely applicable playground for designing high-performance dielectric energy storage with the strategy exploring the boundary among different categories of materials.