Selective radial thickness growth of compositionally graded shells on colloidal quantum rods for more efficient light-emitting diodes

Colloidal quantum nanorods (NRs) exhibit linearly polarized emission and fast radiative recombination owing to their two-dimensional confinement, enabling high-performance light-emitting diodes (LEDs). However, due to facet-dependent growth kinetics, anisotropic shell growth often leads to preferential overgrowth along the long axis and insufficient radial (short axis) coverage. This structural imbalance weakens exciton confinement within the core and compromises the suppression of Förster resonance energy transfer. Here, we report a dual-ligand (organic phosphorus/carboxylic acids) slow-injection strategy to synthesize wurtzite CdSe/CdZnSe/ZnSeS core/shell NRs featuring compositionally graded thick shell (≈ 4.5 nm) with ZnSe as the dominant component. The organic phosphorus ligands create an environment of high monomer concentration (cadmium monomer concentration from 1.4% to 2% of cadmium by mass) to drive anisotropic growth, while the carboxylic acids promote isotropic growth due to their nearly equal binding energy across different crystal facets. Simultaneously epitaxial growth of compositionally graded alloy shells alleviates lattice mismatch at core/shell interfaces while reducing defect density and boosting carrier recombination efficiency. The resulting NR-LEDs achieve an external quantum efficiency of 32% for anisotropic nanocrystals. This work establishes graded thick-shell NRs as a generalizable platform for efficient, stable and polarized optoelectronics.