Halide-assisted Al-doped graded shells for emission tunability and photostability in CdSe NPLs

We report a facile one-pot synthetic strategy for producing CdSe/Cd_1-xZn_xS core/shell nanoplatelets (NPLs) with a chemical composition gradient, enabled by the introduction of halide ions, which promote the formation of a uniform-thickness shell. The use of a compositionally graded shell eliminates abrupt interfaces between the core and shell, leading to both high photoluminescence quantum yield and enhanced photostability. This graded shell architecture also induces significant Stokes shift, particularly due to heterogeneous growth of the NPL vertices, thereby effectively suppressing self-absorption. By varying the halide precursor concentration, both the emission wavelength and shell uniformity can be finely tuned, while the Auger recombination rate is significantly reduced with increasing shell thickness. Furthermore, we introduce an Al-doped ZnS outer shell that functions as a robust inorganic diffusion barrier, markedly enhancing resistance to photo-oxidation by effectively blocking oxygen and moisture penetration. Together, the halide-assisted graded shell and Al-doped outer passivation enable the direct synthesis of thick NPLs with optimized electronic structure and long-term stability, making them promising candidates for optoelectronic applications such as light-emitting diodes and laser devices.