Encapsulation-Assisted TEM Resolves the Intrinsic Surface Structure of 2D Material MnBi₂Te₄

Thinning two-dimensional (2D) materials to atomic-scale thickness often unlocks novel quantum phenomena dominated by surface and interface effects. Understanding these emergent properties requires accurately determining the surface atomic structure. However, probing the intrinsic atomic structure of sensitive 2D materials faces significant challenges, primarily due to the inherent instability of the materials and the severe surface degradation induced during transmission electron microscopy (TEM) lamella preparation. Here, we demonstrate a protective encapsulation strategy for reliable surface characterization, using MnBi₂Te₄ (MBT), a topological quantum material of intense interest, as an example. We show that unprotected MBT surfaces are highly susceptible to degradation during standard characterization, undergoing a structural collapse from the pristine septuple-layer (SL) to a Bi₂Te₃-like quintuple-layer (QL) configuration. By employing hexagonal boron nitride (hBN) or a homomaterial (MBT) encapsulation strategy, we effectively enable direct TEM observation of the intrinsic surface atomic structure of unstable materials. Crucially, we find that the protected MBT surface retains the pristine SL structure after the complete TEM lamella fabrication process. This work resolves the current controversy regarding the surface configuration of MBT and establishes a generalizable protocol for reliable atomic-scale characterization of unstable 2D quantum materials.