OPTICAL-ABSORPTION SPECTRA OF INORGANIC FULLERENE-LIKE MS2 (M = MO, W)

Optical-absorption spectroscopy of inorganic fullerenelike MoS2 and WS 2 (IF-MoS2 and IF-WS2) is reported in the range 400-800 nm, at tempera tures between 4-300 K, and compared to the corresponding bulk (2H) mat erial. A systematic study of the effect of IF size and number of atomi c layers on the optical properties shows that the semiconductivity of the layered material is preserved in the IF structures. Nevertheless, all TF with number of layers (n)>6 exhibit a decrease in the A and B e xciton energies. This redshift becomes larger as additional inner laye rs are formed, until a saturation value is reached (n>10). We assign t his redshift to the deformations, curvature, and discommensuration bet ween adjacent atomic layers the structure must accommodate in order to form an IF structure. An increase in the exciton energies is observed in IF consisting of a few sulfide layers (n<5). This blueshift is att ributed to a quantum confinement in the z direction. Band-structure ca lculations show that an expansion of >7% along the c axis leads to a c onvergence of the levels K-1 and K-4, which is displayed in the absorp tion spectra of IF with 1 or 2 layers.