Optical property of the near band-edge transitions in rhenium disulfide and diselenide

An optical study of the band-edge property of ReX2 (X = S, Se) has been carried out using the techniques of transmittance, photoreflectance (PR), and piezoreflectance (PzR). The polarized transmittance measurements indicate that the absorption edge of Eparallel tob polarization presents a significant red-shift behavior with respect to that of the E perpendicular to b polarization. Band-edge excitons E-1(ex) and E-2(ex) for ReX2 are characterized using polarized PzR measurements. The polarization dependence of E' and E' provides conclusive evidence that the band-edge excitons are interband excitonic transitions originated from different origins. In addition higher energy excitonic series in ReX2 layered compounds is also studied using low-temperature PR measurements at 25 K. Prominent and enlarged excitonic features positioned at higher energy side with respect to the E-1(ex) and E-2(ex) are observed in the PR spectra. The observed excitonic sequence has been shown to be corresponding to the Rydberg series starting with principal quantum number n = 2. From the experimental results together with the theoretical density-of-states calculations, the near band-edge property of rhenium disulfide (ReS2) and diselenide (ReSe2) are characterized and discussed. (C) 2004 Elsevier B.V. All rights reserved.