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 ({C₆H₁₃NH₃) }₂PbI₄. The ns (n2) 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.