Researchers Demonstrate the Feasibility of High-Quality Twisted Bilayer Oxide Nanomembrane Constructions

By Pritam RoyReviewed by Danielle Ellis, B.Sc.Nov 3 2022

In an article published in ACS Applied Materials & Interfaces, researchers demonstrated the creation of higher-order artificial oxide heterostructure based on various materials and symmetries. They established the material basis for examining moiré related electronic effects in a broader range of twisted bilayer oxide thin films.

Study: Observation of Moiré Patterns in Twisted Stacks of Bilayer Perovskite Oxide Nanomembranes with Various Lattice Symmetries. Image Credit: 3DStach/Shutterstock.com

Background

A new race to conceptualize and produce artificial lattice structures has been spurred by the design and synthesis of modern quantum instruments in which exotic phenomena result from moiré physics. This interest is expanded to thin-film transition metal oxide research to create a twisted bilayer of perovskite oxides that simultaneously reveal moiré landscapes.

Here, the authors demonstrated that high-quality twisted bilayer oxide nanomembrane constructions were feasible using a sacrificial-layer-based strategy. Discrete moiré patterns were observed at the atomic scale that was directly created by various twist angles. Also, the symmetry-inequivalent nanomembrane could be piled together to develop new complicated moiré configurations.

The Advent of the Twisted Bilayer Nanomembrane

The development of magic-angle twisted bilayer graphene systems has opened important areas for research. This introduction of artificial periodic lattices offers unprecedented control for adjusting band configurations and electronic correlations. Since then, a wide range of unique quantum phenomena has been identified. Also, a new class of device architectures known as twistronics has demonstrated tremendous promise for the development of quantum devices in the future.

Other twisted bilayered two-dimensional van der Waals (vdW) nano instruments based on black phosphorus, transition-metal dichalcogenides, and hexagonal boron nitrides and their heterojunctions have been inspired by utilizing van der Waals epitaxy. These instruments have seen enormous opportunities provided by tuning material structure, twist angle, symmetry, and interlayer coupling strength.

Due to translational symmetry breaking, the emergent quantum states primarily result from superlattice structures creating moiré patterns. Such patterns have been observed using several visualization methods highlighting a hexagon-like innovative paradigm materials platform with configurable lattice parameters.

However, most of these stacked vdW materials have been the focus of this twist-control stacking strategy until now. Due to their unexfoliated nature, complex transition-metal oxides (TMOs) are frequently not bonded by vdW forces. They display numerous physical features deriving from the d electrons, usually unsuitable for moiré engineering. 

In this work, the authors aimed to create twisted bilayer thin films of the standard perovskite oxide material, strontium titanate, SrTiO₃ (STO), for the first time. It was more challenging to generate coherent superlattice structures with good crystallinity than the atomically thin vdW twisted stacks because of more complex surface chemistry, crystal structure, and strain relaxation during the release procedure.

Direct observation of moiré patterns was established in rotating STO twisted bilayer nanomembrane in real space using high-resolution transmission electron microscopy (HRTEM) by refining the growth and transfer technique. In addition, a unique method was presented for creating atypical homogenous "epitaxy" of any perovskite oxides by stacking STO nanomembrane films with different orientations onto silicon substrates to generate two other forms of twisted bilayer stacks.

These findings demonstrated the appearance of moiré physics at the twisted bilayer of oxide nanomembrane, opening a potentially new and practical path to understanding strong electronic interactions that were previously inaccessible.

The Experimental Set-Up

Utilizing a Bruker D8 Discover X-ray diffractometer having Cu Ka1 radiation, the crystallographic characterization of the freestanding single-layer and the twisted bilayer nanomembrane was carried out. Plan-view transmission electron microscopy (TEM) specimens of the single-layer and twisted bilayer nanomembrane stacks were generated for HRTEM investigation by periodically direct transferring the single-layer nanomembrane on a holey carbon TEM grid.

2q-w X-ray diffraction (XRD) images of several single-layer and twisted bilayer STO samples on Si substrates with diverse configurations were observed. The significant silicon peak was visible with distinct reflections in the single-layer freestanding STO nanomembrane.

Also, the absence of secondary phases or phase segregations demonstrated the high crystallinity of the single-layer nanomembrane samples on silicon wafers. Moreover, depending on whether the two layers had the same crystallographic orientation, two groups of samples were prepared for the twisted bilayer samples. These samples were labeled as the HoO-twisted bilayer STO sample and the HeO-twisted bilayer sample.

The zoom-in HRTEM image demonstrated a periodic moiré pattern resembling a Chinese knot in the twisted bilayer STO region. In general, the lattice mismatch between the interlayers and twist could both cause moiré patterns. Since there was no lattice mismatch between the two layers in the HoO-twisted bilayer samples, the moiré pattern shown here could only be influenced by the twist angle.

From the HRTEM images, it was observed that the moiré morphology varied as the twist angle increased. This distinct modulation upon altering the twist angle created a pathway for microscopic moiré engineering in these nonhexagonal lattices, offering fascinating chances for novel orbital engineering for the associated electrons. 

Twisted Bilayer Oxide Stacks and the Future of Quantum Device Fabrication

In conclusion, the authors effectively created twisted bilayer oxide stacks made of SrTiO₃ nanomembrane with various crystallographic orientations by tuning the thin-film development on different single-crystalline substrates. The emergence of several moiré patterns was observed by adjusting the twist angle, demonstrating the potential offered by the unique van der Waals heterostructures made of nonlayered materials.

A complex oxide version of a moiré lattice provided an additional degree of freedom, or crystallographic direction, that was particularly highlighted by the intricate pattern originating from the twisted bilayer perovskite oxide nanomembrane. This degree of freedom was referred to as twistronics. The study also presented a simple technique for measuring the twist angles in these systems using XRD in-plane azimuthal scans. 

The findings provided new opportunities for fabricating quantum devices and manipulating quantum states in oxide nanoelectronics, a relatively uncharted territory in the study of oxide twisted stacks.

Reference

Shen, J et al. (2022). Observation of Moiré Patterns in Twisted Stacks of Bilayer Perovskite Oxide Nanomembranes with Various Lattice Symmetries. ACS Applied Materials & Interfaces. https://pubs.acs.org/doi/10.1021/acsami.2c14746

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