Optical Features of Nanosize Iron and Molybdenum Sulfide Clusters

Abstract

In the bulk state FeS₂ and MoS₂ are optically opaque, narrow bandgap semiconductors with no optical applications. We demonstrate that nanosize FeS₂ and MoS₂ have bandgaps that can be adjusted to the visible and even UV region of the spectrum by control of the cluster size. This opens up a host of applications of these materials as inexpensive solar photocatalysts. We demonstrate that the band-gap of both materials shifts to the blue with decreasing size but ceases shifting when a size of ∼ 3 nm (in the case of MoS₂) is attained. We interpret this observation as a change from bulk quantum confinement of the hole-electron pair of a tiny semiconductor to a set of discrete molecular-like transitions more characteristic of a large molecule. Room temperature photoemission studies of these clusters demonstrate that, while photoemission shifts to the blue with increasing bandgap for large clusters, small clusters have photoemission exclusively from trapped sub-bandgap surface states. Chemical modification of the surface to introduce hole or electron traps can result in either an enhancement or a decrease in the photoluminescence. In addition, we report our results concerning chemical purification and preliminary surface characterization of MoS₂ clusters by chromatography.