High quality, size-selected, nano-size clusters of FeS2 have been succ
essfully grown inside inverse micellar cages and their optical absorpt
ion studied. Remarkable structure in the absorption spectrum is observ
ed and large blue shifts in the absorption edge and other spectral fea
tures are observed upon crossing from the weak to strong quantum confi
nement regimes with decreasing cluster size. The bandgap appears to re
main indirect down to the smallest cluster size studied (2 nm).