Atomic Coherence Achieved in Twisted NaNbO3 Membranes Via Controlled Oxygen Treatment

Scientists are tackling the challenge of achieving truly coherent interfaces in twisted oxide membranes, a key hurdle in harnessing their potential for novel electronic devices. Young-Hoon Kim (ORNL), Reza Ghanbari (North Carolina State University), and Min-Hyoung Jung (Sungkyunkwan University) et al. report a breakthrough in establishing atomically bonded interfaces within twisted NaNbO3 heterostructures, overcoming the limitations imposed by amorphous layers that typically disrupt coherent coupling. Their research, detailed through controlled oxygen treatment, reveals a chemically reconstructed interface , evidenced by ordered perovskite registry, lattice contraction and modified electronic structure , rather than simple physical adhesion. This reconstruction facilitates asymmetric strain propagation and long-range electromechanical coupling, paving the way for the creation of robust, strain-tunable oxide moiré superlattices and ultimately, the engineering of advanced ferroic and electronic functionalities.

This breakthrough addresses a significant challenge in the field of Twistronics, the presence of amorphous interfacial layers that hinder coherent coupling and suppress moiré-induced interactions in conventional oxide heterostructures. Researchers achieved this by employing controlled oxygen annealing, a method that avoids the elemental loss often associated with high-temperature thermal treatments, particularly crucial for materials containing volatile elements like sodium. The study reveals the realization of ordered perovskite registry at the twisted interface, accompanied by systematic lattice contraction and modified electronic structure, clear indicators of chemical reconstruction rather than simple physical adhesion.

Atomic-resolution imaging and spectroscopy confirmed these findings, providing direct evidence of the newly formed bonds. This reconstructed interface mediates a highly asymmetric strain propagation, where the bottom membrane remains largely relaxed while the top membrane accommodates substantial shear strain, effectively establishing a strain gradient throughout the twisted oxide membranes. Experiments show that this unique strain distribution enables long-range electromechanical coupling, opening exciting possibilities for manipulating and controlling the material’s properties. By resolving the nature of the reconstructed interface, the team establishes a robust pathway for achieving coherent and strain-tunable oxide moiré superlattices.
This work is particularly significant because it overcomes limitations associated with volatile elements, which often degrade during high-temperature processing. The research establishes a fundamental understanding of interfacial bonding in twisted oxide bilayers, paving the way for engineering emergent ferroic and electronic functionalities. Specifically, the team fabricated epitaxial heterostructures of 6.7nm-thick NaNbO3 thin films on 20nm-thick La0.7Sr0.3MnO3 buffer layers grown on (001)-oriented SrTiO3 substrates via pulsed laser deposition. Following growth, selective etching released freestanding NaNbO3 membranes, which were then transferred and processed to achieve the atomically coherent interface, demonstrating a viable route towards advanced oxide-based devices.

Oxidative Annealing Creates Coherent Oxide Interfaces

Scientists engineered atomically coherent interfaces in twisted NaNbO3 heterostructures via controlled oxygen treatment, overcoming limitations imposed by amorphous interfacial layers common in conventional oxide heterostructures. The research team addressed the issue of volatile element loss during high-temperature processing by implementing a precise oxidative annealing process, successfully eliminating carbon-containing amorphous phases formed during aqueous processing. This innovative technique yielded atomically sharp, chemically bonded interfaces, directly confirmed through atomic-resolution imaging and spectroscopy. To assess strain distribution within the twisted membranes, researchers acquired atomic-resolution annular dark-field (ADF) images along the zone axes of both the top and bottom membranes.

The peak pairs analysis (PPA) method was then employed to extract and quantify local strain states, specifically focusing on shear strain (Exy). Shear strain mapping revealed that strain propagation wasn’t limited to the interface but extended throughout the top membrane, demonstrating effective modulation across the entire film. Quantitative analysis indicated a shear strain of 0.9 ±0.7% within the top membrane, as determined from Exy histograms. In stark contrast, the bottom membrane exhibited minimal strain, registering 0 ±0.4%, signifying a nearly relaxed state. Further verification involved analysing in-plane (Exx) and out-of-plane (Eyy) strain components, confirming pronounced variations in the top membrane while the bottom membrane remained largely strain-free.

Additional Exy mapping consistently corroborated this asymmetric strain distribution, demonstrating the reproducibility of strain modulation within the top membrane. This asymmetry arises because the bottom membrane is mechanically constrained by the Si substrate and interfacial reconstruction, while the top membrane’s mechanical freedom allows for lattice adjustments and strain redistribution. This approach enables the creation of twisted bilayer structures independent of epitaxial registry, relaxing substrate-induced mechanical constraints and facilitating long-range electromechanical coupling throughout the membranes. The study pioneers a robust pathway for achieving coherent and strain-tunable oxide moiré superlattices, potentially leading to engineered ferroic and electronic functionalities.

Chemically bonded interfaces drive strain coupling

Scientists achieved atomically coherent, chemically bonded interfaces in twisted NaNbO3 heterostructures through controlled oxygen treatment. Atomic-resolution imaging and spectroscopy revealed ordered perovskite registry accompanied by systematic lattice contraction and modified electronic structure at the twisted interface, confirming chemical reconstruction rather than simple physical adhesion. This reconstructed interface mediates highly asymmetric strain propagation, where the bottom membrane remains nearly relaxed while the top membrane accommodates substantial shear strain, establishing a strain gradient that enables long-range electromechanical coupling throughout the twisted oxide membranes. By resolving the nature of the reconstructed interface, this work establishes a robust pathway for achieving coherent and strain-tunable oxide moire superlattices, opening avenues to engineer emergent ferroic and electronic functionalities.

Researchers synthesized epitaxial heterostructures consisting of 6.7nm-thick NaNbO3 thin films and 20nm-thick La0.7Sr0.3MnO3 buffer layers on (001)-oriented single-crystalline SrTiO3 substrates via pulsed laser deposition (PLD). Following growth, the buffer layer was selectively etched using a mixed diluted solution of hydrochloric acid and potassium iodide to release freestanding NaNbO3 membranes. X-ray diffraction analysis of the released membranes revealed high crystallinity and a single-phase perovskite structure, as shown in Figure S1. A second NaNbO3 membrane was prepared identically and stacked onto the first with a relative angular offset of approximately 47°, forming twisted NaNbO3 bilayers (t-NNO), as demonstrated by plan-view annular dark-field (ADF) imaging in Figure 1b.

Cross-sectional ADF imaging, however, revealed an interfacial region exhibiting structural characteristics distinct from the adjacent crystalline layers. Both the top and bottom membranes maintained crystalline periodicity of approximately 4nm, with a consistent pseudo-cubic lattice spacing of d(001)pc = 0.392nm, but an unintended layer with reduced crystalline order was observed at the interface. Low-magnification ADF imaging corroborated the existence of interfacial roughness and discontinuity extending laterally over 100nm, detailed in Figure S2a, while atomic-resolution analysis confirmed a reduction in ADF intensity within the interlayer region, as seen in Figures S2b-d. Further cross-sectional analyses confirmed high crystalline ordering and structural integrity, although an interfacial layer of approximately 1-3nm thickness persisted, as shown in Figure S3.

Energy-dispersive X-ray spectroscopy (EDX) elucidated the chemical characteristics of the interfacial layer. Elemental mapping revealed uniform distribution of Na, Nb, and O across the crystalline membrane regions, but these constituents exhibited approximately 50% reduction at the interface, accompanied by high carbon enrichment, as shown in Figure 2a and 2b. The spatial extent of this depletion corresponded to the amorphous layer thickness determined via cross-sectional ADF imaging. Consequently, the carbon signal was sharply localized at the interface, indicating carbonaceous contamination constituted the primary component of the disordered region.

These findings suggest the amorphous interlayer originated from residual PMMA contaminants not completely removed during acetone cleaning. To address this, researchers applied post-annealing treatment in a tube furnace under flowing oxygen. AFM analysis revealed a continuous decrease in root-mean-square (RMS) roughness with increasing annealing temperature, from 960pm in the as-transferred t-NNO to 186pm after annealing at 660°C, as shown in Figure S4. The emergence of step terraces on membranes annealed at 660°C for two hours suggests effective interface reconstruction.

Chemically Bonded Interfaces Enable Strain Gradients and Enhanced

Scientists have demonstrated the creation of atomically coherent, chemically bonded interfaces in twisted NaNbO3 heterostructures through controlled oxygen treatment. Atomic-resolution imaging and spectroscopy confirm ordered perovskite registry alongside systematic lattice contraction and modified electronic structure at the twisted interface, indicating chemical reconstruction rather than simple adhesion. This reconstructed interface facilitates highly asymmetric strain propagation, where the bottom membrane remains relaxed while the top membrane accommodates significant shear strain, establishing a strain gradient for long-range electromechanical coupling. The findings resolve a long-standing challenge in oxide twistronics by eliminating the amorphous interfacial layer commonly found in oxide membranes, achieved through a practical oxidative annealing process.

Researchers identified the origin of interfacial disorders in twisted freestanding NaNbO3 and successfully removed a carbon-containing amorphous phase formed during aqueous processing. Observation of differing strain states in the top and bottom membranes reveals asymmetric strain propagation due to the lattice softness of NaNbO3, providing insights into engineering strain distributions in twisted oxides. The authors acknowledge that their study focuses on fundamental interfacial bonding mechanisms, rather than direct device applications, and note limitations related to the specific materials and processing conditions employed. Future work could explore the application of this technique to other oxide combinations and investigate the potential for designing chemically coherent, strain-tunable architectures hosting emergent ferroic and electronic phenomena.