Stacking Faults and Intercalants in (Ta1–x Ti x )Se2 Revealed by Cross-Sectional Transmission Electron Microscopy

Structural defects in the layered compounds (Ta1–x Ti x )­Se2 (0 ≤ x ≤ 1), characterized by anisotropy in electrical conductivity, have been studied by cross-sectional observation using chemically sensitive high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Intercalants were detected in the widest space between Se atoms across the van der Waals (vdW) gap, regardless of the Ti content (except TiSe2) or thermal history after the crystal growth. Binary TaSe2 with both 3R and 2H structures includes a high density of stacking faults, while Ti addition stabilizes the 1T structure with no stacking faults. We show that the chemical composition dominates the structural defects in the (Ta1–x Ti x )­Se2 compounds. Substitution of Ta with Ti acts as a hole addition, which reduces the number of conduction electrons in the layer, while an intercalated atom in the vdW gap may not act as a donor of conduction electrons.