Thanks to a novel encapsulation technique, researchers at Fudan University in Shanghai, China, have succeeded in imaging the surface structure of the atomically thin topological quantum material MnBi2Te4 (MBT) for the first time. The work could help advance our understanding of the intrinsic properties of this technologically important class of materials and enable their use in novel devices.
The electronic, magnetic and topological properties of materials thinned down to atomic-scale thicknesses are very different to those of their bulk counterparts. They can, for example, be employed to study exotic quantum states that could prove useful for a host of advanced applications, such as next-generation spintronics and dissipationless electronics.
Before such devices see the light of day, however, we need to understand these emergent properties, which means accurately determining the surface atomic structure of these materials. This is no easy task because these surface structures are extremely sensitive to their environment. What is more, the most common technique used to study them, transmission electron microscopy (TEM), requires samples to be prepared as lamellae, which degrades them, changing their structure. The high-energy electron irradiation beam in the microscope does them no good either.
In the new work, a team led by Yuanbo Zhang of the State Key Laboratory of Surface Physics and department of physics at Fudan University, studied the topological quantum material MBT. This is the only intrinsic magnetic topological insulator made in the laboratory so far and can be used to study novel topological states – such as the quantum anomalous Hall effect and axion-insulator physics – many of which are fundamentally governed by surface states.
MBT’s surface is exceptionally vulnerable
Like other sensitive 2D materials, explains study co-author Jingjing Gao, MBT’s surface is exceptionally vulnerable to external perturbations, such as ambient air exposure, heat or standard electron microscopy preparation. These degrade the surface layer and irreversibly alter its structure. Indeed, a previous TEM study also revealed that the septuple-layer structure of an MBT flake transforms into a Bi2Te3-like quintuple layer configuration under ambient conditions.
To overcome this problem, Gao and colleagues developed a protective encapsulation technique that successfully shields the delicate surface structure of MBT from its environment and protects it during TEM sample preparation.
The researchers examined two types of encapsulating materials. The first was hexagonal boron nitride (hBN), which is chemically inert and already routinely employed to protect 2D materials. The second was a thin flake of MBT itself, which provides a “homomaterial” capping layer. They encapsulated the MBT in one of these two materials immediately after they had exfoliated (or shaved off) flakes of MBT from a bulk sample. They did this inside an inert argon-filled glovebox.
An atomic-scale barrier
The protective layer acts like an atomic-scale barrier that isolates the MBT surface from oxygen, moisture, ion-beam damage and other environmental disturbances produced during the focused-ion beam processing used to prepare the samples for TEM. It also protects the MBT from the high-energy electron irradiation in the imaging microscope.
The researchers found that the homomaterial encapsulation is especially protective because the atomically similar layers can intimately contact with a minimal interfacial gap.
The technique strongly reduces defect formation, so the original lattice structure remains stable throughout the entire TEM workflow, explains Gao. “We therefore succeeded in safeguarding the ‘true’ surface throughout, allowing us to visualize the intrinsic septuple-layer structure at atomic resolution for the first time.”
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The work will be important for both understanding intrinsic magnetic topological insulators and for developing quantum devices from these materials in the future, she tells Physics World. “Making such devices will demand exceptionally stringent requirements for sample surface and interface quality. This is because the MBT’s unique topological and magnetic properties are hosted by its surface states and even minor atomic reconstruction can distort its intrinsic quantum behaviour.”
Looking ahead, the researchers, who detail their work in Chinese Physics Letters, say they will now be looking to construct heterostructure devices with high-quality intrinsic surfaces and interfaces based on MBT. They will also make use of their encapsulation strategy to fabricate and study magnetic topological heterostructures.
“We will also explore topological superconductivity by interfacing MBT with superconductors, thereby advancing the development of topological quantum computing platforms,” reveals Gao.
