Bulk and monolayer thermoelectric and optical properties of anisotropic NbS2Cl2

Recently, the chalcogenide-based materials are attracting the scientific community due to their promising thermoelectric properties. Monoclinic compounds of the chalcogenide class of materials are the least explored for thermoelectric and optoelectronic device applications. In our present study, we have taken NbS2Cl2 to analyse its electronic properties of bulk and monolayer to understand its electronic and thermal transport properties along with its optical properties, using the density functional theory framework. The investigated compound has direct bandgap values of 1.41 eV and 1.67 eV in its bulk and monolayer phases, respectively, making it a semiconductor. The lattice thermal conductivity(K-1), being a crucial parameter for a thermoelectric material, is observed to be ultra-low along 'c' direction for the bulk (0.44 Wm/K) and the same along 'b' direction for monolayer (0.36 Wm/K). We also predict a great potential for nanostructuring with 72% reduction in the thermal conductivity for the crystal grain size of 10 nm. Further, carrier lifetimes for various concentrations and temperatures are estimated by incorporating different scattering mechanisms in the calculations, such as acoustic deformation scattering (ADP), ionized impurity scattering (IMP) and polar optical scattering (POP). Based on thermoelectric performance, we predict n-type doping would be more favourable. At 700 K, n-type bulk has a maximum thermoelectric performance (ZT) of 0.4, higher than the well-known monoclinic GaTe (0.24) TE material. In addition, the calculated absorption coefficient reveals the optical anisotropy. The predicted absorption coefficients for bulk and monolayer in the visible range are 7.5 x 10(5)/cm and 3x10(5)/cm, respectively. The current work highlights its extremely low thermal conductivity and significant absorption coefficients, which could lead to future applications in thermoelectric and optical devices.