Complete-series excitonic dipole emissions in few layer ReS2 and ReSe2 observed by polarized photoluminescence spectroscopy

Excitonic emission is a key factor for a two-dimensional (2D) semiconductor to be fabricated in light-emitting device for optoelectronics application. Especially when the 2D material has a specific in-plane optical anisotropy, its excitons would become polarized dipole emitters. In this study, the whole series excitonic emissions of few-layer ReS2 and ReSe2 have been detailed characterized using polarized micro-photoluminescence (mu PL) measurement. There are totally five excitonic emissions denoted as E-1(ex) , E-2(ex), E-3(ex) , E-s1(ex) and E-s2(ex) can be detected by the mu PL experiment for both the few-layer ReS2 and ReSe2. The angular dependencies of the polarized excitonic emissions with respect to b axis (R-e4 diamond-chain direction) are determined. The emission feature E-1(ex) is the band-edge exciton while it shows orthogonal polarization with respect to both E-2(ex) and E-3(ex) in layered ReX2 (X = S, Se). The formation of Re-4 diamond chains (DCs) in the 1 T'-phase ReS2 and ReSe2 will also assist the development of in-plane built-in electric field and forming dipoles around the DC regions. The built-in electric field effect greatly enhances the higher E-s1(ex) and E-s2(ex) excitonic features as evident in the photo-modulated reflectance spectra of the layered ReX2 with in-plane electrical field modulation. The binding energy and transition origin of all the excitons are also analyzed and discussed.