Sharpened Dirac Cone and Elevated Carrier Mobility in Bi ₄ Te ₃ Engineered by a High [Bi ₂ ] Bilayer Content for Thermoelectrics

The [Bi₂]_m[Bi₂Te₃]_n family features a natural heterostructure of metallic [Bi₂] bilayers (BL) and topological insulator [Bi₂Te₃] quintuple layers (QL). We demonstrate that increasing the BL ratio ((m/(m + n))) enhances key thermoelectric properties. Bi₄Te₃ (m = 3; n = 3) exhibits a sharpened Dirac cone and a high carrier mobility (117 cm² V^-1 s^-1) twice that of Bi₁Te₁ (m = 1; n = 2). This stems from strengthened σ-bonding interlayer coupling, which reduces the carrier effective mass. Concurrently, the abundant [Bi₂]-BL lower Te vacancy formation energy, suppressing electron carrier concentration and synergistically boosting the Seebeck coefficient. This work provides the first direct experimental and theoretical evidence for the sharpening of the Dirac cone via a metallic [Bi₂] bilayer engineering strategy. It addresses the key limitations of Bi₁Te₁, and the established structure-property relationship, together with the metallic [Bi₂]-BL engineering strategy for sharpening the Dirac cone, further offers valuable insights for the rational design of thermoelectric materials and performance optimization across the entire Bi/Te material family.