Optical Features of Nanosize Iron and Molybdenum Sulfide Clusters
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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.
Introduction FeS2 and MoS2 materials in bulk form have significant technological applications. For example, cubic iron sulfide is considered to be the most important mineral present in coal formations and likely plays a critical catalytic role in the hydrogenolysis of coal[l]. It has also been suggested as an inexpensive material for photovoltaic applications. MoS2 is the most widely used catalyst for the removal of sulfur from crude oil and coal and, due to its graphitelike two-dimensional structure, is widely used in lubrication applications (e.g. axle grease). However, because both materials are optically opaque and have bandgaps in the near IR, photochemical applications of. these materials are non-existent. As with other nanosize semiconductor materials, however, we have shown that in nanosize form the bandgaps of both materials can be shifted into the visible and even UV region of the optical spectrum and thus matched to the solar spectrum[2]. The concomitant shifts in the valence and conduction band potentials theoretically allow chemical oxidation and reduction of a wide range of reactants. This opens up a host of possibilities for these materials as photooxidation catalysts for destruction of chlorinated hydrocarbons in water and H2 fuel production via splitting of water or H2S. It should be noted that compared to the only other extant material available for these applications, TiO2, covalent materials such as MoS2 and WS2 can be much better matched to the solar spectrum and, unlike II-VI semiconductor materials such as CdS, are chemically robust and stable in the presence of molecular oxygen. FeS2, while oxygen sensitive, can be coated with a thin layer of inert metal such as gold as we demonstrate and so is also potentially useful as a
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