Heterostructures for Piezoelectric Applications and Energy storage

The perovskite relaxor ferroelectrics (RFs) are complex materials with intriguing dielectric and piezoelectric properties. They are characterized by the coexistence of polar nanoregions (PNRs) with normal ferroelectric domains which results in a diffuse phase transition ¹. The RFs in bulk form have been intensively studied by both experimental and theoretical methods due to the great number of potential applications, such as in energy harvesting, medical devices, information storage, and so on.^2–4 Pb(Mg_1/3Nb_2/3)O_3-xPbTiO₃ (PMN-PT) solid solution, composed of a relaxor PMN and a classical ferroelectric PT is one of the most popular RFs which have received considerable interest from both scientific and industry community due to its extraordinary electromechanical performance at morphotrophic phase boundaries (MPBs)^5–8. In thin films form, PMN-xPT has received little attention compared to its bulk counterparts, particularly the study of strain effect and the correlations between the local atomic domain structures and the functional properties. Thus, the understanding of the nature of domain structures in RF thin films and their evolution under external stimuli is crucial for practical applications. In this spirit, we aim to elucidate and understand the contribution of the misfit strain and domain structures on piezoelectric and ferroelectric performance of prototypical RF PMN-PT thin films in order to develop nano-materials with high performance for microelectromechanical systems (MEMS) and energy storage (ES) devices. Noting that, the most of the reports were devoted to the growth of oriented PMN-PT thick films. So far, there are only few reports concerning the study of the effect of strain in the epitaxial fully strained PMN-PT films⁹. However, more experimental and theoretical investigation on strain engineering is needed to address RF thin films in order to enhance their functional properties.

The main aim of this proposal is to obtain a thorough understanding of the effect of compressive and tensile strain, as well as the interfaces on piezoelectric response and ES performance of epitaxial PMN-PT thin film heterostructures/superlattices. The control of the epitaxial strain at the atomic level and the profound understanding of its effect on the structural characteristics require samples with very high quality which are known to be challenging due to the compositional complexity of the PMN-PT material and the easy formation of pyrochlore phases. During the last years we realized a significant improvement in the understanding of the growth of the epitaxial PMN-PT thick films on STO substrates using PLD and their relation to the functional properties^10,11. To overcome the Pb and Mg volatility, in the present project we aim to use an optimized PbO and MgO excess in the in house prepared PMN-PT targets. In addition, the further optimization of PLD process parameters permit growing stoichiometric high quality PMN-PT thin films which is of a great importance for the strain control and the study of local domain structures. In this project, PMN-PT thin films will be grown on an atomically single terminated STO and REScO₃ (RE= Dy, Tb, Gd, Sm, Nd and Pr) substrates (figure 1(b)). These substrates permit to apply different strain state (both compressive and extensive) on different PMN-xPT film compositions (figure 1(a)). Our approach is to adopt thin film processing using PLD to growth differently strained PMN-xPT with compositions across the MPB on single terminated substrates, resulting in layers of high crystalline quality (figure 1 (c,d)). PLD method allows us to engineer strain state, interface quality, and domain arrangement within the material by modifying the growth parameters, substrate, film thickness, and electrode material. To the best of our knowledge, no such complete study has yet been conducted and represents a milestone in the understanding and engineering of nano-materials for the production of energy storage and MEMS devices. In addition, establishing the structure-property and a dip understanding of physical mechanisms and relevant keys parameters in the prototypical lead based PMN-PT thin films can provide new guidelines for the design of lead free environmentally friendly next-generation nano-materials.

 

In this project we will perform comprehensive in-situ and ex-situ analyses of PMN-xPT thin film with various compositions around MPB. Materials will be deposited using PLD on substrates with different unit-cell sizes. The growth modes and interfaces/surfaces quality of the thin film heterostructures will be monitored in situ by RHEED. In situ Laser-Ablation Inductively Coupled-Plasma Mass Spectrometry will be applied to determine quantitatively the spatial element composition of the targets and the as prepared films. The composition, as well as films` structural and chemical properties will be analysed ex situ using advanced methodology (RBS, aberration-corrected STEM, high-resolution XRD, Raman, and XPS). Ferroelectric domain structure and local piezoelectric coefficients d33 will be determined by PFM. The macroscopic electrical measurements will be performed to characterize films` electrical properties and determine the ES performance. Specific compositions of PMN-xPT thin films with the best functional properties will be processed and integrated into prototype devices and validated.