Revealing the Quantum-Confined Free Exciton A Anisotropic Emission in a CdS/CdS:SnS₂ Superlattice Nanocone via Angle-Resolved Photoluminescence Spectroscopy

The nanostructures with exciton anisotropic emission are important in many applications due to the directionality of the flow of photons and the exciton confinement. Moreover, it is still a challenge to observe exciton anisotropic emission directly due to the limitation of measurement techniques and materials. Here, we observed the anisotropy of free exciton A (FXA) in a CdS/CdS:SnS₂ superlattice nanocone by angle-resolved photoluminescence (ARPL) spectroscopy. Raman spectra and in situ spectral mapping indicate quasi-periodic SnS₂ layers perpendicular to the nanocone within the CdS/CdS:SnS₂ superlattice nanocone. Polarization-dependent ARPL spectra discriminated two exciton series, namely, FXA and free exciton B (FXB). By comparing different CdS nanostructures, FXA is found to be omniscient in different samples, but it is even significant in the CdS/CdS:SnS₂ superlattice nanocone due to the local dielectric confinement of SnS₂ layers in the nanocone and the enhanced anisotropic exciton–phonon coupling. Meanwhile, ARPL spectra indicated that the FXA emission blue shifts with reducing cross-sectional radius, while the FXB emission does not. The comprehensive understanding of the FXA anisotropic emission and quantum confinement effect in the CdS/CdS:SnS₂ superlattice nanocone may trigger the potential applications in future anisotropy-polarized devices and wavelength-tuning lasers.