Highly anisotropic thermoelectric properties of the monolayer NbOX2 (X=Cl, Br, I) via first-principles calculations

In recent years, there has been increasing interest in two-dimensional (2D) thermoelectric materials owing to their potential to achieve enhanced thermoelectric conversion efficiency, driven by quantum size effects. Here, we employed density functional theory (DFT) to investigate the thermoelectric and mechanisms properties of a novel monolayer NbOX₂ (X=Cl, Br, I). We identified a broad and flat band below the Fermi level, mainly contributed by the d_z² orbitals of the niobium atoms. This flat band will result in a relatively large effective mass (m*) and manifests as a peak in the density of states (DOS), known as a Van Hove singularity. Notably, the monolayer NbOX₂ exhibits outstanding electronic transport properties along the x-direction and demonstrates reduced lattice thermal conductivity (κ_l) along the y-direction. The anisotropic κ_l of monolayer NbOI₂ is measured at only 0.76 (0.45) W/m/K along the x (y) direction at room temperature, potentially attributed to strong nonharmonic interaction. Moreover, as the atomic number of the halogen elements increases, it leads to an enhancement of the power factor and a reduction in κ_l. At 300 K, the maximum anisotropic ZT values with n-type and p-type for NbOI₂ in the x (y) direction are recorded at 2.91 (0.87) and 2.12 (0.97), respectively. When the temperature rises to 500 K, its p(n)-type ZT in the x direction can attain values as high as 3.96 (3.20), indicating that the NbOI₂ has good application potential in the realm of thermoelectrics.