Exciton spectra in thin crystals

The optical absorption edge of crystals of molybdenum disulphide consists of two series of exciton absorption bands, the A and B series, which are due to the formation of delocalized excitons by direct allowed transitions from a split valence band. In ‘thick’ crystals at 77 °K the wavenumber position v_n of the absorption bands is given by VnA=15952−(364/n2)cm−1andVnB=18230−(1056/n2)cm−1. Transmission and reflexion measurements on crystals of different thickness ranging from 0∙05 to 0∙8 μm have shown that the exciton bands move to higher energies in thin crystals as a result of the increased exciton binding energy in small crystal volume. This effect is most marked for excitons of large radii and for the larger excitons of the A series. The variation of absorption coefficient and refractive index over the exciton spectra have been measured. The oscillator strength of the A series bands varies as ca. n ^-3. Absorption bands A₁ and B₁ are unaffected by the crystal size effect and their observed behaviour in thin crystals is predicted by classical optics. This includes the measured oscillation in transmission with changing crystal thickness which occurs at the peaks of the n = 1 exciton bands.