Quantitative study of the effects of surface ligand concentration on CdSe nanocrystal photoluminescence

Abstract

We study the impact of surface chemistry on the photoluminescence (PL) quantum yield of CdSe core, CdSe/CdS core/shell, and CdSe/CdZnS/ZnS core/shell/shell nanocrystals prepared by multiple synthetic routes. We expose as-synthesized particles to varying concentrations of n-alkylamine and n-alkanethiol ligands and verify that the addition of n-alkanethiols to CdSe and CdSe/CdS nanocrystal solutions quenches their PL. We also show that the addition of n-alkylamines to nanocrystal solutions can increase or decrease nanocrystal PL, an effect that depends on the concentration of both nanocrystals and ligands. We demonstrate the importance of considering the nanocrystal concentration when fitting ligand binding curves, and show that common solvent impurities can affect the PL and ligand binding data. While alkanethiols quench CdSe nanocrystals prepared using multiple synthetic procedures, we find the exact shape of the quenching curve depends on the synthetic route chosen. We emphasize that the ligand binding data extracted from PL quenching curves are contingent on the assumptions made during fitting. By fitting our PL quenching curves to a Langmuir isotherm and accounting for the particle surface sites, we estimate a lower limit for the equilibrium CdSe-alkanethiol binding constant on the order of 10(9) M-1 with different numbers of thiol binding sites depending on the method of nanocrystal synthesis.