Image charge effect on two-dimensional excitons in an inorganic-organic quantum-well crystal

We report a clear experimental evidence of the image charge effect on excitons in the optical and electroabsorption spectra of an inorganic-organic quantum-well crystal ${({\mathrm{C}}_{6}{\mathrm{H}}_{13}\mathrm{N}{\mathrm{H}}_{3})}_{2}\mathrm{Pb}{\mathrm{I}}_{4}$. The ns $(n\ensuremath{\geqslant}2)$ Wannier series are ideal two-dimensional excitons whose binding energies are enhanced by both the spatial confinement and image charge effects. Exceptionally large binding energy of the $1s$ exciton (361 meV) is also due to the prominent image charge effect. We also observed unique blueshifts of the $1s$ excitons under the electric field perpendicular to the quantum-well layers, for which we demonstrated with calculations that the image charge effect plays a significant role.