Subnanometer-Scale Probing of Excitonic Behavior in 2D Semiconductor CrSBr

CrSBr is a promising material for optoelectronics due to its robust excitonic resonance and the intricate interplay among its magnetic, optical, and electronic properties. Here, electron energy loss spectroscopy performed in a scanning transmission electron microscope at room temperature reveals an unconventional blueshift in CrSBr exciton energy with increasing thickness, contrary to the typical redshift observed in other two-dimensional semiconductors. This blueshift is attributed to the decrease in bandgap size with increasing thickness being less pronounced than the reduction in exciton binding energy. This phenomenon may stem from the charge-transfer bandgap and strong frequency-dependent dielectric screening in CrSBr. Furthermore, analysis of excitonic behavior across interfaces with varying thicknesses at subnanometer resolution reveals a relatively localized exciton nature in CrSBr's paramagnetic state. This study enhances comprehension of intrinsic excitonic properties in CrSBr at room temperature and provides valuable insights into its band structure, facilitating exciton control for future optoelectronic applications.