  We investigate the variation of the band gap across the Ruddlesden-Popper
(RP) series (An+1BnX3n+1) in model chalcogenide, oxide, and halide materials to
understand the factors influencing band gap evolution. In contrast to the
oxides and halides, we find the band gap of the chalcogenides evolve
differently with the thickness of the perovskite blocks in these natural
superlattices. We show that octahedral rotations (i.e. deviation of the B-X-B
bond angles from 180) and quantum confinement effects compete to decide the
band gap evolution of RP phases. The insights gained here will allow us to
rationally design layered perovskite phases for electronics and
optoelectronics.
