Electrical and Optical Properties and Electronic Structures of LnCuOS (Ln = La∼Nd)

A series of layered oxysulfides, LnCuOS (Ln = La∼Nd), was studied to examine the influence of these lanthanide ions on their optical and electrical transport properties. PrCuOS and NdCuOS showed wide-gap p-type semiconducting properties similar to LaCuOS, which is a p-type semiconductor with an energy gap of 3.1 eV. A sharp photoluminescence peak near the absorption edge, which originates from excitons, was observed in LnCuOS (Ln = La, Pr, and Nd), indicating that these lanthanide ions do not influence the fundamental optical properties and excitons are confined in (Cu 2 S 2 ) 2 - sulfide layers sandwiched between (Ln 2 O 2 ) 2+ oxide layers. On the other hand, CeCuOS showed p-type degenerate semiconducting behavior and its color was black. The electronic structures of LnCuOS (Ln = La∼Nd) were investigated by ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy to understand these differences derived from the lanthanide ions. Although no remarkable difference in the electronic structures was found among LnCuOS (Ln = La∼Nd), only an inverse photoemission spectrum of CeCuOS clearly showed an unoccupied band near the Fermi level in the conduction band. It is considered that the formation of this unoccupied band at the conduction band minimum is responsible for the unique electrical and optical features of CeCuOS.