Experimental Determination of Excitonic Dispersion Relation from Center-of-Mass Quantization Effect on Excitons in PbI2 Thin Films

We have investigated the center-of-mass quantization effect on excitons in layered-compound PbI 2 ultra-thin films by precisely changing the layer thickness from 7 to 20 monolayers. Crystalline PbI 2 thin films that are fully oriented along the c -axis were prepared by a vacuum deposition method. We confirmed that the surface roughness of the prepared thin films corresponds to one monolayer owing to the van der Waals interaction along the c -axis peculiar to the layered compound, which is advantageous for the realization of the center-of-mass quantization effect. The optical spectra, such as absorption, transmittance, and reflectance clearly exhibit oscillatory structures originating from the quantized excitonic states with a quantum number of up to 15: the quantization energy amounts to ∼130 meV. From the analysis of the systematic results of the center-of-mass quantization, we experimentally determined the excitonic dispersion relation of PbI 2 , which is well explained by a tight-binding exciton model instead of a usually used effective-mass model.