Optical-absorption spectra of inorganic fullerenelikeMS2(M=Mo,W)

Abstract

Optical-absorption spectroscopy of inorganic fullerenelike ${\mathrm{MoS}}_2$ and ${\mathrm{WS}}_2$ (${\mathrm{I}\mathrm{F}-\mathrm{M}\mathrm{o}\mathrm{S}}_2$ and ${\mathrm{I}\mathrm{F}-\mathrm{W}\mathrm{S}}_2$) is reported in the range 400–800 nm, at temperatures between 4–300 K, and compared to the corresponding bulk (2H) material. A systematic study of the effect of IF size and number of atomic layers on the optical properties shows that the semiconductivity of the layered material is preserved in the IF structures. Nevertheless, all IF with number of layers $\mathrm{}\left(n\right)>6\mathrm{}$ exhibit a decrease in the $A$ and $B$ exciton energies. This redshift becomes larger as additional inner layers are formed, until a saturation value is reached $\mathrm{}(n>\mathrm{}10\mathrm{}).$ We assign this redshift to the deformations, curvature, and discommensuration between 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 $\mathrm{}(n<\mathrm{}5\mathrm{}).$ This blueshift is attributed to a quantum confinement in the $z$ direction. Band-structure calculations show that an expansion of $>7\%$ along the $c$ axis leads to a convergence of the levels $K_1$ and $K_4,$ which is displayed in the absorption spectra of IF with 1 or 2 layers.