Insight into long-period pattern by depth sectioning using aberration-corrected scanning transmission electron microscope

Long-period patterns (LPPs) are widely observed by transmission electron microscopy (TEM) in the study of nanoscale materials. Identifying the origin of LPPs is of significant importance when interpreting TEM images, and for an in-depth understanding of material characteristics. However, the two most common LPP categories, modulated structure and moiré patterns, are not easily differentiated by conventional TEM (CTEM). In this work, an LPP was observed in Cu_2-xSe nanoplates by CTEM. And then the depth sectioning with an aberration-corrected scanning transmission electron microscope (AC STEM) has been performed to determine the LPP type. Two misorientated layers were recognized from the depth-series of atomic resolution images of an LPP region, confirming the LPP is a moiré pattern caused by two twisted stacked crystal flakes which commonly exists in nanosized materials. This depth sectioning method is generally applicable for structural characterization of layered systems, and is a powerful approach for the in-situ structural probe of nanomaterials. It is promising to be extended to fast three-dimensional (3D) reconstruction.