Stacking and Layer Parity Dependent Photoluminescence in ReS2

ReS₂ crystallizes in low crystal symmetry distorted triclinic (1T’) structure, and has anisotropic optical properties. Recently, two different stacking orders named as AA and AB phases, have been identified in ReS₂ with distinct characteristics, such as stacking order dependent Raman shift, carrier dynamics, and second harmonic generation (SHG). However, the low crystal symmetry structure combined with different stacking sequences complicate the understanding of optical responses of this material and thus instigating the debate on interlayer coupling, number of excitonic emissions, and quantum confinement. In this work, the open questions on the optical responses of ReS₂ are addressed by combining angle resolved photoluminescence (PL) at cryogenic temperatures and SHG spectroscopy on different numbers of layers of two polymorphs. Careful identification of stacking sequences allows to conclusively establish that few layer AA phase ReS₂ has two excitons, while AB phase demonstrates five emission peaks. SHG and PL imaging confirm layer parity dependent optical responses for AB phase ReS₂. Furthermore, bulk and few layer thick samples are found to have distinct PL, suggesting that bulk ReS₂ cannot be assumed as decoupled monolayers. The findings of this study hold significance toward understanding the optical phenomena occurring in polymorphs of anisotropic 2D layered materials.