Role of Dimensionality on Excitonic Properties of BiSeI using Many-body Perturbative Approaches

The mechanical exfoliation of two-dimensional materials has sparked significant interest in the study of low-dimensional structures. In this work, we investigate the bulk and low-dimensional derivatives of BiSeI, a quasi-one-dimensional anisotropic crystal known for its remarkable stability and novel electronic properties. Using the density functional theory and many-body perturbation theory, we examine the influence of dimensionality on their electronic, optical, and excitonic properties. Quasi-particle $\mathit{G_0W_0}$ calculations reveal a significant increase in the band gap with a decrease in dimensionality, driven by quantum confinement effects and reduced dielectric screening. By solving the Bethe-Salpeter equation, we identify a transition from weakly bound Wannier-Mott excitons in bulk BiSeI to strongly bound excitons its low-dimensional forms. These structures feature band gaps spanning the infrared to the visible spectrum and exhibit large exciton binding energies, making them promising for next-generation optoelectronics and excitonic applications. Our findings provide a theoretical foundation for future experimental studies on BiSeI and its low-dimensional counterparts.