Exciton Splitting in Thin Copper Indium Disulfide Nanosheets

Nanosheets of Cu2-xS having discrete thicknesses of five, six, and eight crystallographic monolayers are prepared by adaptation of literature methods. These are converted to CuInS2 nanosheets having the same discrete monolayer thicknesses by cation-exchange reactions. The six- and eight-monolayer CuInS2 nanosheets exhibit broad, nearly featureless extinction (absorption) spectra that are generally characteristic of CuInS2 nanocrystals. However, the thinnest, five-monolayer CuInS2 nanosheets exhibit structured extinction spectra having a well-resolved doublet feature with peaks at 2.74 and 2.54 eV. An exciton-splitting (valence-band-splitting) model assigns the transitions to excitons derived from inequivalent A, B, and C holes from the bulk band structure of CuInS2. This splitting is analogous to the heavy-hole, light-hole splitting observed in the absorption spectra of III-V quantum wells and pseudo-two-dimensional II-VI nanocrystals. The model semiquantitatively accounts for the magnitude of the observed splitting, which is largely due to quantum confinement and the differing A, B, and C hole effective masses. The lack of resolved splitting in CuInS2 nanocrystals having larger dimensions is also explained.