EFFECTS OF PRESSURE AND TEMPERATURE ON EXCITON ABSORPTION AND BAND STRUCTURE OF LAYER CRYSTALS - MOLYBDENUM DISULPHIDE

The effect of pressures of up to 60 kbars on the exciton series in the spectral region of 2 eV in molybdenum disulphide has been investigated at room and liquid nitrogen temperatures, using a diamond anvil device. An electronic band model, suggested by the crystallography of the layered material and the optical spectra of the various polytypes, is made the basis for the discussion of the experimental results. An indirect energy gap of 0.25 eV determines the electrical properties of the intrinsic material, but transitions from a lower, spin-orbit split valence band produce the exciton series and fundamental absorption edge. It is shown that under these circumstances the pressure coefficient of an exciton peak can be separated into two parts; the pressure coefficient of the direct gap and the pressure dependence of the exciton binding energy which is a result of changes in the free carrier density that are necessarily related to the pressure variation of the indirect gap. © 1969.