ABSTRACT The interplay between in‐plane crystal anisotropy and polytypism in ReS 2 endows it with diverse range of optical properties, including polarized excitonic emissions and layer dependent second harmonic generation. However, for this material, despite numerous studies, uncertainty remains about the number of polymorphs defined by various stacking orders. Further, there are ambiguities in correlating the spectroscopic signatures to the crystal symmetry of ReS 2 . In this work, by carrying out optical and scanning experiments and subsequently through first principles studies on ReS 2 bilayer as a prototype, we address these longstanding ambiguities. Towards this, ground state energies are calculated for different possible stacking orders, which yielded two energetically favored configurations namely AA and AB ′ XY . While the AA configuration is the natural stacking order, preserving the inversion symmetry across all thicknesses, the AB ′ XY stacking is particularly intriguing, as one layer undergoes both translational (1/2 unit cell along crystal ‘ a ’ and ‘ b ’ axes) and rotational operations relative to the other layer. This AB ′ XY stacking exhibits distinct optical signatures compared to that of AA stacking, namely, odd/even layer dependent second harmonic generation, appearance of a new Raman peak, reduced shear to breathing Raman mode intensity ratio, inverse phonon chirality, and stronger interlayer coupling. Theoretical validation of the experimental observations is provided by calculating the exciton distribution profiles. These insights, in addition to confirming the stacking and anisotropy in ReS 2 , provide a framework for investigating the structural and optical properties of other anisotropic layered materials.
Barman et al. (2026) studied this question.