Crossover from strong to weak exciton confinement in thickness-controlled epitaxial PbI2 thin films

Spatially confined excitons undergo two distinct quantization effects depending on the confinement length, the enhanced binding energy under strong confinement, and the center-of-mass quantization under weak one. However, the transition between them has not been experimentally identified in two-dimensional (2D) materials due to the lack of thin films satisfying large-scale uniformity and atomic-level flatness in a wide thickness range. Here, we reveal the crossover in high-quality epitaxial thin films of a 2D semiconductor PbI2 grown by molecular beam epitaxy. The absorption spectra exhibit oscillatory structures manifesting the exciton center-of-mass quantization, and the quantization energies show an additional blue shift associated with the strong confinement effect below five-layer thickness (35 Å). The precise control of exciton quantum states will lead to the further development of optoelectronic functionalities of 2D materials.