Observation of anisotropic interlayer Raman modes in few‐layer ReS₂

Abstract

ReS$_2$ has recently emerged as a new member in the rapidly growing family of two-dimensional materials. Unlike MoS$_2$ or WSe$_2$, the optical and electrical properties of ReS$_2$ are not isotropic due to the reduced symmetry of the crystal. Here, we present layer-dependent Raman measurements of ReS$_2$ samples ranging from monolayers to ten layers in the ultralow frequency regime. We observe layer breathing and shear modes which allow for easy assignment of the number of layers. Polarization-dependent measurements give further insight into the crystal structure and reveal an energetic shift of the shear mode which stems from the in-plane anisotropy of the shear modulus in this material

Splitting of Interlayer Shear Modes and Photon Energy Dependent Anisotropic Raman Response in N-Layer ReSe2 and ReS2

“…these anisotropic systems depends subtly on the relative orientation of the crystal axis and of the exciting laser field and also on the angle between the polarizations of the incoming and Raman scattered photons [14,18,19,24,25]. Nevertheless, most studies on ReX 2 have focused on the complex manifold of intralayer Raman modes, which exhibits minor variations as a function of the number of layers [14,18,24].…”

“…Nevertheless, most studies on ReX 2 have focused on the complex manifold of intralayer Raman modes, which exhibits minor variations as a function of the number of layers [14,18,24]. In contrast, the low-frequency interlayer shear modes (LSM) and breathing modes (LBM) in TMD make it possible to unambiguously determine the number of layers and evaluate the strength of van der Waals interlayer coupling [19,[25][26][27][28][29][30][31]. In the case of N -layer ReX 2 , a splitting of the (in-plane) LSM is expected [19] and should provide an invaluable tool to investigate in-plane anisotropy.…”

“…3 the fan diagrams associated with the two non-degenerate LSM and with the LBM in N -layer ReX 2 . As previously introduced for other layered materials [25,[28][29][30][31]33], a linear chain model can be used to fit the frequencies of the observed modes and extract the interlayer force constants. The LSM and LBM frequencies write…”

Basic Concepts and Recent Advances of Crystallographic Orientation Determination of Graphene by Raman Spectroscopy

“…There are armchair edges or zigzag edges in most of graphene [37,132]. Therefore, the crystallographic orientation of graphene can be estimated from the crystallographic properties of graphene under optical microscopes, similar to ReS 2 nanosheets [133,134]. Figure 6a shows the crystallographic directions that form armchair edges and zigzag edges.…”

Anisotropic Saturable and Excited‐State Absorption in Bulk ReS2

“…The additional d valence electron from Re atoms contributes to the formation of zig‐zag Re chains parallel to the b ‐axis of the crystal, which drastically reduces its symmetry and affects its physical properties. The highly anisotropic optical, vibrational, thermal, and electronic transport properties demonstrated in ReS 2 make it promising for applications in sensors and electronic devices, such as polarization‐sensitive detectors based on the anisotropic in‐plane optical absorption . Comparing with black phosphorous, whose in‐plane anisotropic properties have also been studied, the excitonic and band‐edge transitions of ReS 2 have decoupled polarization dependence between the regularly observed Exciton I (1.51 eV) and Exciton II (1.57 eV), allowing for tuning of individual transitions .…”