  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.
