Complete determination of crystallographic orientation of ReX2 (X=S, Se) by polarized Raman spectroscopy

TL;DR

This study develops a reliable polarized Raman spectroscopy method to accurately determine the crystallographic orientations of monolayer ReX2 (X=S, Se), crucial for understanding their anisotropic physical properties.

Contribution

The paper introduces a new procedure combining polarized Raman spectroscopy and electron microscopy to precisely identify all three principal directions of ReX2 monolayers, including face differentiation.

Findings

Established laser excitation energies for ReS2 and ReSe2 to optimize orientation determination.

Demonstrated the ability to distinguish face types and Re-chain directions unambiguously.

Provided a practical method for correlating Raman polarization dependence with crystal axes.

Abstract

Polarized Raman spectroscopy on few-layer ReS2 and ReSe2 was carried out to determine the crystallographic orientations. Since monolayer ReX2 (X=S or Se) has a distorted trigonal structure with only an inversion center, there is in-plane anisotropy and the two faces of a monolayer crystal are not equivalent. Since many physical properties vary sensitively depending on the crystallographic orientation, it is important to develop a reliable method to determine the crystal axes of ReX2. By comparing the relative polarization dependences of some representative Raman modes measured with three different excitation laser energies with high-resolution scanning transmission electron microscopy, we established a reliable procedure to determine the all three principal directions of few-layer ReX2 including a way to distinguish the two types of faces: a 2.41-eV laser for ReS2 or a 1.96-eV laser for ReSe2 should be chosen as the excitation source of polarized Raman measurements; then the relative directions of the maximum intensity polarization of the Raman modes at 151 and 212 cm-1 (124 and 161 cm-1) of ReS2 (ReSe2) can be used to determine the face types and the Re-chain direction unambiguously.