Remote epitaxy of Ag(Nb,Ta)O3 thin films through graphene - A synergistic approach for improving energy storage performance

One of the key components for the realization of clean and smart energy technologies are high-performance energy storage (ES) systems. Electrostatic ES based on dielectric capacitors is an optimal enabler of fast charging and discharging speed, and ultrahigh power density. Nonlinear dielectrics, particularly antiferroelectrics (AFE) and relaxor ferroelectrics (RFE), are at the forefront of research on electrostatic ES. Despite the immense progress achieved in the field, new breakthroughs are sought after. Ag(Nb,Ta)O3 (ANT) has been widely studied in the past as an important material for the development of tunable microwave devices. Recently, ANT has received a great deal of attention in ES materials research, owing to its tunability between AFE and RFE behavior. The energy density values obtained for ANT ceramics are comparable those of state-of-the-art Pb-based systems. While research on ANT thin films is still in its infancy, recent reports indicate the films hold promise for high recoverable energy density (Wreco) and efficiency. Thin films offer additional degrees of freedom that can be utilized to engineer the properties of the films. Among the thin-film deposition techniques, pulsed-laser deposition (PLD) is recognized as a technique with many adjustable parameters that enable the tuning and stacking of layers with atomic precision. While substrate-induced strain can unlock new phenomena not present in bulk, strain and especially substrate clamping can be detrimental to the functional response of the films. In the proposed project, a novel approach, based on the introduction of a 2D material at the substrate-film interface will be developed. The idea is for the 2D material to change the strain relaxation mechanism at the interface. A dislocation-free film will have a higher dielectric breakdown strength (DBS). Enhanced DBS improves ES performance and the unlocks the possibility to study the material at high electric fields, which is especially important for studying the phase transitions in materials with complex phase diagrams. The DBS will be further enhanced by compositional engineering, mainly focused on the prevention of Ag-loss, where knowledge obtained in our previous work on Pb-based systems will be exploited. The overall aim of the project is to study the synergistic effects of strain and dislocation-free relaxation mechanism to stabilize the desired phases and achieve coherent growth of films with high DBS. In the first stage, ANT thin films will be grown on oxide substrates to deepen the understanding of the effects of epitaxial strain on the structural and electrical properties of the films. In the second stage, the films will be doped with rare-earths, which have been shown to increase structural heterogeneity and induce relaxor-like behavior, thereby improving the ES efficiency. In the third stage, a 2D material (graphene) will be introduced to the interface to realize remote-epitaxy. Films with a complete absence of epitaxial strain will be prepared via van der Waals growth on amorphous substrates (covered with graphene). The samples will undergo multiscale analysis, by combining methods sensitive to the lattice strain, defects, chemical arrangements and compositions, with local electrical and strain-mapping techniques. The obtained results will be related to the macroscopic dielectric and ferroelectric properties, and Wreco and ES efficiency values will be calculated. Beyond state-of-the-art performance is expected to result from the proposed approach. In addition to serving as a platform to study alternative relaxation mechanisms and declamping the response of the films, graphene can be used as an enabler for the integration of functional oxides with Si. Therefore, the results of the proposed study will contribute to paving the way for on-chip energy storage, which would propel the development of the internet of things, flexible and wearable electronics, where Pb-free materials are highly desirable.