Abstract
Nanocrystal quantum dots are generally coated with an organic ligand layer. These layers are a necessary consequence of their chemical synthesis, and in addition they play a key role in controlling the optical and electronic properties of the system. Here we describe a method for quantitative measurement of the ligand layer in 3 nm diameter lead sulfide–oleic acid quantum dots. Complementary small-angle X-ray and neutron scattering (SAXS and SANS) studies give a complete and quantitative picture of the nanoparticle structure. We find greater-than-monolayer coverage of oleic acid and a significant proportion of ligand remaining in solution, and we demonstrate reversible thermal cycling of the oleic acid coverage. We outline the effectiveness of simple purification procedures with applications in preparing dots for efficient ligand exchange. Our method is transferrable to a wide range of colloidal nanocrystals and ligand chemistries, providing the quantitative means to enable the rational design of ligand-exchange procedures.
Highlights
3 nm diameter lead sulfide−oleic acid quantum dots
The distribution and conformation of the bound ligand layer is of importance both for understanding the quantum dot (QD) properties before and after exchange and for influencing the interaction between the native bound ligand and the incoming ligand species intended for exchange
We demonstrate a rigorous structural approach, utilizing complementary techniques, that is applicable to a wide range of QD and ligand chemistries to resolve, in detail, the coverage of ligands, the ligand density distribution away from the QD surface, and the residual ligand concentration in solution
Summary
3 nm diameter lead sulfide−oleic acid quantum dots. Complementary small-angle Xray and neutron scattering (SAXS and SANS) studies give a complete and quantitative picture of the nanoparticle structure. Cores (X-ray scattering length density, xSLD, of PbS for a Ga source = 50.7 × 10−6 Å−2) translates into an excellent X-ray contrast with most organic materials and solvents
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