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

[Display omitted] • We have calculated the electronic structure, thermal conductivity, and thermoelectric properties of a novel 2D NbOX 2 (X=Cl, Br, I) materials. • The increase in atomic number leads to a curved flat-band structure, accompanied by an enhancement in the power factor and a reduction in thermal conductivity. • The unique structure of monolayer NbOX 2 induces an anisotropy thermoelectric properties, characterized by superior electrical transport properties along the x direction and lower thermal conductivity along the y direction. • The n -type and p -type ZT values of NbOI 2 in the x direction at 300 K(500 K) are 2.91(3.96) and 2.12(3.20), respectively, indicating the promising application potential of NbOI 2 in the field of thermoelectric. 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 NbOX 2 (X=Cl, Br, I). We identified a broad and flat band below the Fermi level, mainly contributed by the d z 2 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 NbOX 2 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 NbOI 2 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 NbOI 2 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 NbOI 2 has good application potential in the realm of thermoelectrics. [ABSTRACT FROM AUTHOR]