Concentration-dependent photophysics of InP/ZnS quantum dots: surface still matters despite thick shells

Abstract

Core–shell InP/ZnS quantum dots (QDs) are promising non-toxic alternatives to cadmium-based emitters, yet their photophysical stability remains underexplored. Here, we investigate the optical properties of oleic acid-capped InP/ZnS QDs with varying emission energies spanning the visible spectrum. Using steady-state absorption, absolute photoluminescence (PL) quantum yield (QY) measurements, and time-resolved PL spectroscopy, we assess the impact of particle size, concentration, and host environment on radiative performance. Despite thick ZnS shells (6–13 monolayers) that should, in principle, insulate the exciton from the environment, both PL lifetimes and QY exhibit strong, monotonic decreases upon sample dilution. Spectrally resolved lifetime measurements reveal quantum-confinement (QC)-driven trends: larger dots display longer lifetimes, consistent with QC model. However, the dilution-induced suppression of PL efficiency points to surface-related quenching mechanisms where partial desorption of oleate ligands from ZnS surfaces can create defect-mediated nonradiative channels, amplified under ambient oxygen. When plotted against integrated surface area, PL lifetime and QY collapse onto a universal trend across different QD sizes, reinforcing the surface-origin of the observed behavior. Incorporation of QDs into solid polymer matrices further highlights environmental sensitivity: poly(methyl methacrylate) (PMMA) preserves most of the colloidal PL efficiency, whereas polydimethylsiloxane (PDMS) causes severe quenching due to ligand incompatibility and increased oxidative trapping. These results reveal that even in type-I heterostructures with thick shells, excitonic wavefunctions remain susceptible to surface chemistry. The findings underscore the need for ligand engineering and optimized host matrices to achieve stable, high-efficiency InP/ZnS QD emitters for optoelectronic applications.