Vacancy-Induced Z-Contrast Anomaly in Self-Assembled (Ti,V)O₂ Heterostructure

Annular dark-field scanning transmission electron microscopy (ADF-STEM) imaging is often called as the Z-contrast, where the element with a higher atomic number gives rise to the brighter contrast. ADF-STEM imaging on the self-assembled (Ti,V)O₂ heterostructure, exhibiting the alternating V-rich and Ti-rich layers, unexpectedly reveals the higher contrast in V-rich layers in spite of the nearly identical atomic numbers of Ti (Z = 22) and V (Z = 23). Our analyses of local strain mapping and electron energy loss spectroscopy (EELS) confirm that oxygen vacancies are formed dominantly in the V-rich and thus the higher contrast in the V-rich layers is attributable to the accumulation of oxygen vacancies facilitated by the inherent [001] channel pathways of the rutile structure. Our results highlight the critical role of crystallographic pathways in guiding oxygen vacancy diffusion within rutile-based heterostructures.