Impact of Shell Crystal Structure on the Thermal Stability and Photophysical Behavior of Blue Quantum dots

Epitaxial shell is vital for enhancing the optoelectronic performance of semiconductor quantum dots (QDs), especially for the wide bandgap blue QDs, yet the shell crystal structure's impact on thermal stability and photophysical behavior remains unexplored. It is reported that the ligand-directed synthesis of ZnS shells with zinc blende (ZB) or wurtzite (WZ) structure on blue WZ CdZnS cores, where sulfur precursor (Trioctylphosphine-S/octanethiol) dictates crystal phase through their thermal stability and coordination effects. WZ ZnS exhibits a lower Cd²⁺ diffusion barrier than ZB ZnS due to reduced cation density in octahedral vacancies, lowering electrostatic repulsion. Consequently, ZB-shell QDs maintain sharp core-shell boundary and energy band alignment, improving anti-thermal quenching and structural stability: fluorescence decreased by only 8% versus 17% for WZ-shell QDs with increasing temperature from 200 to 300 K, and retained 81% fluorescence intensity (vs. 36%) after 15 days at 100 °C. The rigid ZB lattice also weakens exciton-phonon coupling (simulated γLO = 39 meV vs. 67 meV for WZ). Conversely, WZ-shell QDs form alloyed interfaces enabling smooth energy band alignment, enhancing carrier delocalization. This suppresses Auger recombination, yielding a biexciton lifetime of 126 ps—nearly double that of ZB-shell QDs (69 ps)—and slower decay kinetics (τ_av = 16.7 ns vs. 55 ns).