Probing optical anisotropy and stability of NbOCl2 and NbOI2 using Mueller matrix spectroscopic ellipsometry

The niobium oxide dihalide family NbOX2 (X = Cl, Br, and I) exhibits strong in-plane anisotropy and hence polarization-dependent properties, making them great candidates for electronic, optoelectronic, and quantum photonic applications. In this work, the dielectric functions of NbOCl2 and NbOI2 nanoflakes were investigated using Mueller matrix spectroscopic ellipsometry. Despite the monoclinic symmetry of the bulk materials, we propose a biaxial orthorhombic optical model and observe an excellent match to the ellipsometry data collected at different azimuth orientations. Density functional theory (DFT) calculations reveal that the principal axes of the dielectric tensor are indistinguishably close to the c and a* axes (the layer normal). Good agreement is also observed in the principal dielectric tensor obtained from DFT calculation and ellipsometry analysis. Onset of absorptions along the principal polar b-axis occurs at higher energies compared to the non-polar c-axis, which defines the material bandgap of 1.75 eV for NbOI2 and 2.1 eV for NbOCl2 nanoflakes. The real part of the dielectric function tends to be higher for NbOI2 than for NbOCl2, in every principal direction. Remarkable linear dichroism was observed from UV to the visible spectral range. NbOCl2 remained optically stable in ambient storage, while the optical properties of NbOI2 changed dramatically over time, indicating material degradation under ambient conditions.