Investigation of the layer-dependent optical properties of ultrathin BiOI by spectroscopic ellipsometry

Layered semiconductor BiOI has gained considerable interests in the field of optoelectronics, but the basic knowledge of its optical and dielectric properties is still lack of a full investigation. Here, by using spectroscopic ellipsometry (SE), the complex refractive indexes and dielectric constants of ultrathin BiOI films over a wide spectral range are measured for the first time. The ellipsometric angle Δ is found to be linear with the layer number of BiOI, suggesting that SE could be a facile way to estimate the thickness of 2D BiOI. The optical properties of BiOI show a distinct layer-dependent feature, and the bandgap values of 2D BiOI increase from 1.99 to 2.33 eV with decreasing thickness. Further, the results from absorption spectrometer also show a blue shift of bandgap energy with decreasing thickness. Our work provides valuable data about the optical properties of ultrathin BiOI, and can shed a light on the exploration of original applications based on 2D BiOI.