We use solution NMR techniques to analyze the organic/inorganic interface of CdSe quantum dots (Q-CdSe) synthesized using oleic acid as a surfactant. It is shown that the resulting Q-CdSe are stabilized by tightly bound oleic acid species that only exchange upon addition of free oleic acid. The NMR analysis points toward a two-step exchange mechanism where free ligands are initially physisorbed within the ligand shell to end up as bound, chemisorbed ligands in a second step. Importantly, we find that every ligand is involved in this exchange process. By addition of oleic acid with a deuterated carboxyl headgroup, we demonstrate that the bound ligands are oleate ions and not oleic acid molecules. This explains why a dynamic adsorption/desorption equilibrium only occurs in the presence of excess free oleic acid, which donates the required proton. Comparing the number of oleate ligands to the excess cadmium per CdSe quantum dot, we find a ratio of 2:1. This completes the picture of Q-CdSe as organic/inorganic entities where the surface excess of Cd(2+) is balanced by a double amount of oleate ligands, yielding overall neutral nanoparticles.

We show that adjusting the reaction rate in a hot injection synthesis is a viable strategy to tune the diameter of colloidal nanocrystals at the end of the size distribution focusing, i.e., the postfocused diameter. The approach is introduced by synthesis simulations, which describe nucleation and growth of colloidal nanocrystals from a solute or monomer that is formed in situ out of the injected precursors. These simulations indicate that the postfocused diameter is reached at almost full yield and that it can be adjusted by the rate of monomer formation. We implement this size-tuning strategy using a particular CdSe quantum dot synthesis that shows excellent agreement with the model synthesis. After demonstrating that the reaction rate depends in first order on the Cd and Se precursor concentration, the proposed strategy of size control is explored by varying the precursor concentration. This enables the synthesis of colloidal nanocrystals with a predefined size at almost full yield and sharp size distributions. In addition, we demonstrate that the same tuning strategy applies to the synthesis of CdS quantum dots. This result is highly relevant especially in the context of reaction upscaling and automation. Moreover, the results obtained challenge the traditional interpretation of the hot injection synthesis, in particular the link between hot injection, burst nucleation, and sharp size distributions.

Various literature studies show that increasing the concentration of free acid in the hot injection synthesis of colloidal nanocrystals raises the diameter of the resulting nanocrystals. We analyze this reaction chemistry/nanocrystal property relation by combining reaction simulations with an experimental study on a particular CdSe nanocrystal synthesis. We find that increasing the free acid concentration has the same effect on a real synthesis as raising the solute solubility in the simulations. Both lead to larger sizes and a deterioration of the size dispersion at constant reaction rate. Since free acids are used to coordinate the cation precursors in these syntheses, this leads to a meaningful link between a parameter in reaction simulations and the composition of an experimental reaction mixture. We thus explain the increase of the nanocrystal size with the acid concentration as resulting from an enhanced consumption of the solute by nanocrystal growth, which reduces the number of nanocrystals formed. This link between a simulation parameter and the composition of the reaction mixture provides a rational basis to further explore and understand reaction chemistry/nanocrystal property relations in the hot injection synthesis.

We analyze the optical properties of CdTe quantum dots, including the sizing curve, the absorption coefficient, and the oscillator strength of the band gap transition, by combining absorption spectroscopy, elemental analysis, and electron microscopy imaging. At short wavelengths, the absorption coefficient spectrum is still affected by quantum confinement, yet a largely constant value, close to that of bulk CdTe, is found at around 410 nm. At shorter wavelengths, remaining quantum confinement effects on the CdTe E 1 transition are present even for the largest quantum dots studied (11 nm). For the band gap transition, we find an integrated absorption coefficient μ gap that scales almost proportionally to the inverse of the quantum dot volume. Especially for the smaller diameters, deviations up to a factor of 3 are found as compared to widely used literature values. The corresponding oscillator strength f gap is almost size-independent in the diameter range 3−7 nm. The correspondence between radiative lifetimes predicted based on f gap and literature values is discussed.

We report an alternative procedure to incorporate gold nanoparticles into 3D ordered colloidal crystal film. The size of gold nanoparticles within the films can be controlled from about 10 nm to about 60 nm by simply varying the gold plating time. The application of the as-prepared films in surface-enhanced Raman spectroscopy (SERS) is investigated by using Rhodamine 6G (R6G) as probe molecules. It is found that the resultant gold-coated 3D ordered colloidal crystal films can be used as SERS substrates, exhibiting excellent enhancement ability.