(a) Schematic band structure of 2D TIs. Here, τ₁ and τ₂ denote the scattering times within and outside the bulk-gap region, respectively. τ₁/τ₂ ≫ 1. Red (blue) colored lines represent the edge states of up (down) spins. (b) The Fermi level E_F dependence of TE properties for the edge states. E_F is referenced to the bottom of the bulk conduction band. (c) The dependence of room-temperature zT on the geometric parameters of nanorribon structures for the 2D TI material, fluorinated stanene. Adapted from Ref. [35].