Physical properties of layer structures : optical properties and photoconductivity of thin crystals of molybdenum disulphide

Abstract

Very thin crystals of molybdenum disulphide, less than 100 Å thick, have been prepared by cleavage. The optical absorption spectra in the thickness range several micrometres to less than 100 Å are similar. Absorption coefficients have been measured to values close to 106 cm-1. The absorption bands observed with thin crystals are associated with bulk rather than surface properties. Deviations from an exponential relation between transmission and thickness are observed as the crystal thickness is decreased. The transmission begins to oscillate and thick crystals may transm it more light than thinner ones. Three absorption edges have been observed in the absorption spectrum at about 7000, 5000 and 3000 Å, and structure is observed on the first edge at 4 °K. It is suggested that the levels at the top of the valence band are split by spin-orbit interaction. The edges at 5000 and 3000 Å may be due to transitions at other values of the wave vector or from deeper valence bands. The absorption peaks correspond to exciton lines close to the conduction band. Photoconductivity has been observed at 77 °K. A response at both absorption edges is detected, and photoconduction peaks are observed at the wavelengths of the exciton bands. Values for the refractive index have been obtained in the region of the first absorption edge, and out to 2 μm. Comparison of the optical and low-frequency dielectric constants indicate that the bonding is covalent. The absorption edges and the exciton bands of single crystals and polycrystalline films are compared. The impurity content of the natural crystals has no effect on the properties observed.