Probing in-plane anisotropy and interlayer interactions in ReS$_2$ and ReSe$_2$ by Raman spectroscopy

We address the intriguing Raman response of rhenium disulfide (ReS$_2$) and rhenium diselenide (ReSe$_2$). These layered semiconductors belong to the family of transition metal dichalcogenides and exhibit significant in-plane anisotropy and can be represented as a distorted $1T$-phase (octahedral), with considerably lower symmetry than the more extensively studied $2H$-phase (trigonal prismatic) compounds based on molybdenum or tungsten. Nevertheless, we will demonstrate that the low-frequency rigid layer vibrational modes of $N$-layer ReS$_2$ and ReSe$_2$ can, on the one hand, be described using a linear chain model but, on the other hand, make it possible to directly probe the in-plane anisotropy and to determine the crystal orientation. Since in-plane anisotropy also has a direct impact on the optical and electron transport properties, our work opens avenues for engineering novel optoelectronic devices relying on ReS$_2$ and ReSe$_2$.