The optical properties of single crystals of WSe 2 and MoTe 2

Abstract

Single crystals of tungsten diselenide and molybdenum ditelluride about 200–1000 Å thick have been prepared by cleavage. With such crystals, it is possible to make optical transmission measurements in wavelength regions where the absorption coefficient reaches 10 5–10 6 cm −1. For tungsten diselenide, a strong absorption edge is observed at 7800 Å at room temperature, where the absorption coefficient rises to about 10 5 cm −1, and a second edge is observed at 6000 A. Strong absorption bands are observed on both edges. With molybdenum ditelluride, the first absorption edge is observed at 6600 Å at 77°K, and a second edge occurs at shorter wavelengths. Strong absorption bands have been observed on the first edge. The area of the crystal that gives rise to the absorption bands has been examined in the electron microscope, and dense dislocation networks are observed. For both compounds, the first absorption edge is attributed to direct transitions from a split valence band to the conduction band, and the second absorption edge to transitions from a lower valence band or at another value of the wave vector. The valence band splitting probably results from a spin—orbit interaction. The strong absorption bands are attributed to the formation of exciton states.