Anisotropic transport and optical birefringence of triclinic bulk and monolayer NbX₂Y₂ (X = S, Se and Y = Cl, Br, I)

A systematic analysis of the electronic, thermoelectric and optical properties of triclinic van der Waal's solids NbX2Y2 (X = S, Se and Y = Cl, Br, I) is carried out within the framework of density functional theory for bulk and monolayer. The investigated compounds are semiconductors in bulk and monolayer, with band gap values ranging from 1.1 to 1.8 eV. We observed huge anisotropy in the electrical conductivity with the in-plane conductivity being 40 times higher than out-of-plane conductivity in NbS2I2. The observed high power factor and low thermal conductivity in NbX2Y2 render these compounds as potential thermoelectric materials. In addition, the calculated optical properties such as refractive index and absorption coefficient reveal the optical anisotropy. We have calculated birefringence for all the studied compounds and a large value of 0.313 is observed for NbSe2I2. The monolayer electronic properties indicate the presence of anomalous quantum confinement. The giant birefringence along with promosing monolayer properties are the highlights of present work which might fetch future device applications in both bulk as well as monolayer.