Single atomic layer detection of Ca and defect characterization of Bi-2212 with EELS in HA-ADF STEM

By forming a small electron probe in a scanning transmission electron microscope equipped with a high-angle annular dark-field (HA-ADF) detector, the Bi–O atomic planes in Bi2Sr2CaCu2O8+δ (Bi-2212) can be directly observed with the incoherent Z-contrast imaging technique. Using a combination of electron energy loss spectroscopy (EELS) and HA-ADF imaging, we were able to detect the Ca signals from individual Ca atomic planes in this structure, so that the Ca distribution could be probed within individual unit cells. This high-spatial-resolution EELS technique has been successfully applied to characterize planar defects such as the half-unit cell intergrowth of Bi-2201 on the nanometer scale. Present results show that EELS, in conjunction with high-resolution HA-ADF imaging, provides a powerful tool to study chemical and structural nature of this material on the atomic scale.