Next Generation of Artificial Heterointerfaces as Building Blocks for Energy Materials

Project Information

NEXUS

Grant agreement ID: 101054572

DOI

10.3030/101054572

EC signature date 19 May 2022

Start date 1 January 2023

End date 31 December 2027

Funded under

• European Research Council (ERC)

Total cost

€ 2 491 730,50

EU contribution

€ 2 491 730,50

Coordinated by

DANMARKS TEKNISKE UNIVERSITET
Denmark

Periodic Reporting for period 1 - NEXUS (Next Generation of Artificial Heterointerfaces as Building Blocks for Energy Materials)

Reporting period: 2023-01-01 to 2025-06-30

Summary of the context and overall objectives of the project

In an era of rapid green transition changes, interfaces lie at the heart of the advances in most energy conversion and storage technologies, including batteries, Power-to-X and electrolysis. Depending on the type of device, these technologies rely upon the fast transport of atomic and electronic species across the solid-solid, solid-liquid and solid-gas interfaces. Developing viable solid-state devices requires a fundamental understanding of how ions move at the interface between two solid materials stacked together. Despite half a century of sustained research into interfaces, we still cannot answer the most critical questions about the role of interface symmetries and finding pathways for engineering fast ionic transport at room temperature. The underlying motivation to find the answers is clear: fast transport of ions provides an opportunity to accelerate energy technology. However, the fundamental
science required is extremely challenging: (1) the interfaces are buried in bulk structures and (2) possible combinations of materials are limited by the rules of epitaxy. Imagine a future where the precise tuning of materials can take place according to our aspirations by assembling ultrathin layers into new artificial heterostructures. NEXUS is the epitome of this future. In NEXUS I seek to take a leap from our present knowledge by creating artificial oxide heterostructures and hybridizing their physical properties by directly stacking freestanding membranes with different crystal structures and orientations (Figure 1). In this way I will realize novel structures with fast ionic paths potentially breaking fundamental limitations of existing energy devices. During the last decade I pioneered and matured new sets of oxide-based interfaces, exhibiting an exceptionally colourful palette of properties. The approach of NEXUS is radically different from the past work and will provide fundamental breakthroughs in the study of fast ionic transport across interfaces.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The scientific work in the project is divided into four overall work-packages. Below is the description of work performed within each workpackage.

WP1: Synthesis and Stacking of Membranes
Using Pulsed Laser Deposition (PLD), we produced millimeter-sized free-standing oxide membranes by optimizing strain through Ca doping in Sr₃Al2O₆ (SAO). We developed a robust protocol for crack-free membranes and investigated the growth modes of SrTiO₃ and LSMO on SAO layers. The membranes are then released, transferred, and stacked into new heterostructures with a patented alignment process, enabling tunable electronic, magnetic, and optical properties.

WP2: Structural and Electrical Characterization
We created bistable membranes from strain relaxation, which exhibit out-of-plane buckling influenced by cavity geometry, impacting piezoelectric responses. Magnetic tests on LSMO membranes showed stable ferromagnetism with a Curie temperature above 320 K. Elastic softening was measured via Brillouin scattering, revealing fundamental 2D effects critical for mechanical applications.

WP3: Strain-Mediated Property Enhancement
1. Step-edge-induced structural modulations in BaTiO3 (BTO) membranes: In this study, we investigated the structural modulations in freestanding BTO membranes caused by strain from one-unit-cell step-edges appearing when these membranes adhere to substrates. Using X-ray nanobeam in a MAX IV synchrotron in Lund, we examined the (020) Bragg peak with a spatial resolution of 50 nm across varying temperatures. 2. We developed a mechanical on-chip test platform capable of in-situ transmission electron microscopy (TEM) investigations to study the deformation mechanisms in freestanding thin films under various conditions. This technology could lead to insights into size-dependent effects on mechanical properties. 3. An automatic strain control system was implemented to study the relationship between strain and electronic conductivity in oxide heterointerfaces. This platform enables real-time characterization of not only conductivity but also other properties like piezoelectricity and magnetization.

WP4: Computational Interface Design
1. We employed an automated computational workflow to examine interfacial structures and Moiré patterns. Our state-of-the-art graph neural network achieved precision comparable to density functional theory, effectively studying the impact of varying orientations and surface terminations on material properties. 2. In-Operando Strain Variation Tool: The second methodology involves a new strainer designed for in-operando studies that allows continuous variation of strain across a wide temperature range. This unique tool operates down to millikelvin temperatures without requiring substrate changes. The strain is monitored with high accuracy using the capacitance of a strain gauge located beneath the sample.

The most significant achievement has been the development of a novel method for accurately twisting oxide membranes while ensuring that the interfaces between the membranes remain clean. The successful fabrication, stacking, and twisting of these membranes is a critical challenge for the NEXUS project. Our current methodology enables us to produce twisted membranes with high accuracy and reproducibility. This progress is encouraging, and we are now focusing on applying this methodology to study a wide variety of materials. This approach is currently under patent application.
Overall, new methods have been developed to realize freestanding oxide membranes [Annalen der Physik, (2022)]. Inspired by the developments in freestanding oxide membranes, we create a new platform for reassembling these ultrathin freestanding oxide membranes and twisting them into artificial heterointerfaces [Adv. Mat. (2022)]. Twisting provides a fundamentally new platform radically different from the current single freestanding approach [APL Mat., (2024)], which also allows strain engineering [Small (2024)].

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

I view the development of our methodology for fabricating accurately twisted oxide membranes as a breakthrough, as it opens new possibilities for measuring and optimizing oxide membranes, which is highly relevant in this research area. Another significant advancement is our novel strategy for dynamically tuning the phase diagram of complex oxide membranes through twisting.
NEXUS represents the first attempt to directly link the electronic structure of heterostructures with their twist angles in oxide membranes by utilizing a newly developed Quantum Twisting Microscopy (QTM) at the Weismann Institute of Science. This method enables the investigation of coherent tunneling across dynamically twisted oxide interfaces.
The QTM will lead to two independent research directions: First, it allows the creation of controllable new interfaces between two twisted freestanding oxide membranes by enabling continuous control of their twist angles. Second, it acts as a scanning microscope with direct access to the energy-momentum dispersion of electronic systems, facilitating measurements under external magnetic stimuli.
This groundbreaking approach goes beyond traditional equilibrium phase diagrams by enabling control over electronic phases through continuous twisting, resulting in more energy-efficient electrical systems and faster electronic devices.