Pressure dependence of direct optical transitions in ReS2 and ReSe2

Robert Oliva,Magdalena Laurien,Oleg Rubel,Sefaattin Tongay,Ying Qin,Filip Dybala,Jan Kopaczek,Robert Kudrawiec

doi:10.1038/s41699-019-0102-x

Abstract

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.

Highlights

The ReX2 crystals (X = S, Se) are semiconductors from the family of two-dimensional layered transition metal dichalcogenides (TMDCs) that exhibit special properties
These properties result from their particular band structure and reduced crystal symmetry, as well as a strong 2D character that has been attributed to weak van der Waals interlayer bonding even in their bulk form.[1,2]
Spite of the fundamental properties of this crystal system being Two main features can be observed for all samples, which relatively well-known at ambient pressure, HP optical measure- correspond to the direct excitonic transitions A and B

Summary

INTRODUCTION

Rhenium-based TMDCs have received increasing interest during the last few years owing to their large in-plane anisotropic properties These properties result from their particular band structure and reduced crystal symmetry, as well as a strong 2D character that has been attributed to weak van der Waals interlayer bonding even in their bulk form.[1,2]. Different modulation spectroscopies have shown to be very useful for studying the optical transitions of ReX2: piezoreflectance,[19] electrolyte electroreflectance,[20] thermoreflectance,[21] and polarization-dependent measurements[22–24] revealed two and three excitonic transitions for ReS2 and ReSe2, respectively. These works provided evidence that these excitons, which exhibit a strongly polarized dipole character, were confined within single layers. HP X-ray diffraction measurements show that the bulk modulus of ReX2 (23–31 GPa)[30,31] is significantly lower than group 6 TMDCs (57–72 GPa).[32–35] HP Raman measurements on ReS2

RESULTS

DISCUSSION

METHODS