Photoreflectance study of the excitonic transitions of rhenium disulphide layer compounds

Excitonic transitions of the ReS₂ triclinic layer semiconductor are studied using polarized photoreflectance (PPR) spectroscopy with optical polarizations along and perpendicular to the b axis in the temperature range of 25--300 K. The low-temperature PPR spectra reveal a prominent and enlarged excitonic series positioned at the higher-energy side with respect to previously identified band-edge excitons E₁ᵉˣ and E₂ᵉˣ [Phys. Rev. B 55, 15 608 (1997)]. A detailed line-shape analysis and the photosensitive characteristic of the excitonic sequence provide conclusive evidence that the series and those of band-edge excitons E₁ᵉˣ and E₂ᵉˣ are interband excitonic transitions originated from different origins. The possible origin of the excitonic sequence originates from nonbonding Re $5d$ t2g (dxy,d_x²-y²) states to antibonding S $3p$ σ* states. The sequence is shown to correspond to the Rydberg series starting with a principal quantum number $n=2.$ The temperature dependence of transition energies of the exciton series is analyzed. The parameters that describe the temperature variations of the excitonic transitions in ReS₂ layers are evaluated and discussed.