Pressure dependence of direct optical transitions in ReS2 and ReSe2

The ReX2 system (X = S, Se) exhibits unique properties that differ from other transition metal dichalcogenides. Remarkably, its reduced crystal symmetry results in a complex electronic band structure that confers this material in-plane anisotropic properties. In addition, multilayered ReX2 presents a strong 2D character even in its bulk form. To fully understand the interlayer interaction in this system, it is necessary to obtain an accurate picture of the electronic band structure. Here, we present an experimental and theoretical study of the electronic band structure of ReS2 and ReSe2 at high-hydrostatic pressures. The experiments are performed by photoreflectance spectroscopy and are analyzed in terms of ab initio calculations within the density functional theory. Experimental pressure coefficients for the two most dominant excitonic transitions are obtained and compared with those predicted by the calculations. We assign the transitions to the Z k-point of the Brillouin zone and other k-points located away from high-symmetry points. The origin of the pressure coefficients of the measured direct transitions is discussed in terms of orbital analysis of the electronic structure and van der Waals interlayer interaction. The anisotropic optical properties are studied at high pressure by means of polarization-resolved photoreflectance measurements.