Ultraviolet-visible absorption spectra of nanoscaled EuS thin films reveal a blue shift of the energy between the top-valence and bottom-conduction bands. This band-gap tuning changes smoothly with decreasing film thickness and becomes significant below the exciton Bohr diameter ~3.5nm indicating strong quantum confinement effects. The results are reproduced in the framework of the potential morphing method in Hartree Fock approximation. The large values of the effective mass of the holes, due to localization of the EuS ƒ-states, limit the blue shift to about 0.35eV. This controllable band-gap tuning of magnetic semiconductor EuS renders it useful for merging spintronics and optoelectronics.
