Ultra-low thermal conductivity induced by significant anharmonic phonon scattering in two-dimensional graphitic metal (Sb and Bi) carbides

Two-dimensional (2D) thermoelectric (TE) materials have attracted much attention in recent years. One major factor that limits the TE performance of 2D materials is their relatively high thermal conductivity. Searching for 2D materials with low thermal conductivity is therefore one of the central goals in theoretical and computational studies of 2D TE materials. Here, we report a comprehensive first-principles study of the TE properties of 2D graphitic metal carbides (g-MCs), recently proposed highly stable 2D crystals made of carbon and metal. After theoretically examining the entire family of 2D g-MCs, we found that 2D g-MCs (M = Sb, Bi, Al, Ga, Rh, Ir, and W) possess intrinsically low thermal conductivity and the two members among them, 2D g-SbC and g-BiC, exhibit ultra-low lattice thermal conductivities of 0.78 and 0.90 W m^-1 K^-1, respectively, at room temperature. Detailed analysis shows that the underlying ultra-low thermal conductivities of 2D g-SbC and g-BiC are the significant anharmonic effects in acoustic phonon modes originating from the extended metal-carbon bonds in their unique structure, which lead to strong anharmonic phonon scattering and highly reduced phonon group velocities. With a combination of low thermal conductivity and favorable electron conductivity, p-type Sb₂C₁₂ shows high TE figures-of-merit of around 0.98 at 300 K and 2.55 at 900 K. These findings offer new opportunities for future applications of 2D TE materials.