Optical absorption and dispersion in molybdenum disulphide

Abstract

The optical absorption spectrum of cleaved single crystals of molybdenum disulphide has been measured over the temperature range 290 to 77 °K. The absorption coefficients throughout the absorption region have been determined from transmission measurements on seventy crystals ranging in thickness from 0.03 to 50 μm. At wavelengths greater than 0.55 μm thin crystals do not show the normal exponential relation between transmission and thickness. At 125 °K the long wavelength absorption edge consists of two series of exciton absorption bands A_n and B_n (5 lines and 4 lines respectively) represented by the equations V^A_n ═ 16280 – 430/n² cm^-1 and v_n^B ═ 18579 – 1099/n² cm^-1. To the short wavelength side of B_n two absorption bands C and D occur at ca. 0.5 and 0.4 μm. Absorption bands C' and A' which occur to the long wavelength side of C and A respectively are associated with the impurity content of the natural molybdenite crystal. The single crystal reflexion spectrum has been measured at 290, 173 and 77 °K for a cleaved surface and a surface containing the optic axis. The spectra are similar, the freshly cleaved surface showing reflexion peaks associated with absorption bands A₁ B₁C',C and D. The ordinary refractive index has been measured in the region of high absorption (5x10⁵ cm^-1) by using the interference fringes in thin crystals. The values obtained agree with those calculated from the reflexion spectrum. The dispersion of the ordinary refractive index is given by a Drude equation containing terms for absorption bands A₁ B₁ C' C and D. The oscillator strengths of each of these bands a t 77 °K has been determined. The birefringence has been measured and compared with a calculated value. The electrical conductivity, along and perpendicular to the optic axis, has been measured over the temperature range 230 to 500 °K and the activation energies of conduction found to be 0.874 eV (intrinsic) and 0-082 eV (impurity). The spectral distribution of photo-current has maxima at the wavelengths of absorption bands A₁ B₁, C and D. An energy level diagram has been proposed to account for the optical and electrical properties of molybdenum disulphide.