How Exciton-Phonon Coupling Impacts Photoluminescence in Halide Perovskite Nanoplatelets

Semiconductor nanocrystals are receiving increased interest as narrow-band emitters for display applications. Here, we investigate the underlying photoluminescence (PL) linewidth broadening mechanisms in thickness-tunable 2D halide perovskite (Cs_n-1Pb_nBr_3n+1) nanoplatelets (NPLs). Temperature-dependent PL spectroscopy on NPL thin films reveals a blue-shift of the PL maximum for thicker NPLs, no shift for three monolayer (ML) thick NPLs, and a red-shift for the thinnest (2 ML) NPLs with increasing temperature. Emission linewidths also strongly depend on NPL thickness, with the thinnest NPLs showing the smallest temperature-induced broadening. We determine the combined interaction of exciton-phonon coupling and thermal lattice expansion to be responsible for both effects. Additionally, the 2 ML NPLs exhibit a significantly larger Fröhlich coupling constant and optical phonon energy, possibly due to an inversion in the exciton fine structure. These results illustrate that ultrathin halide perovskite NPLs could illuminate the next generation of displays, provided a slightly greater sample homogeneity and improved stability.