The surface composition of colloidal quantum dots (CQDs) influences their performance across various applications. To date, most highly emissive CQDs are cation-terminated and coated with anionic ligands, while reports on highly emissive anion-terminated CQDs are rare. This study presents a strategy to achieve highly emissive (photoluminescence quantum yield > 80%) ammonium sulfide-coated CdSe/CdS core/shell CQDs within 30 min at room temperature. A facile prepared ammonium polysulfide was used to react surface metal salts in non-polar solvent. The polysulfide was then reduced to sulfide by trioctylphosphine and passivated by ammonium. Although alkyl ammoniums with varying numbers of alkyl groups provide similar colloidal stability, primary ammonium offers superior surface passivation due to its minimal steric hindrance. This strategy is applicable to CQDs with different ligands and compositions. These unique ammonium sulfide-coated, highly emissive CQDs serve as novel platforms for exploring new properties of CQDs and provide valuable experimental evidence for theoretical studies.

The impact of alkali metal carboxylates on the synthesis of colloidal quantum dots (CQDs) was investigated. Through a ligand removal experiment, we demonstrated that due to its high hydrophilic nature, sodium oleate dispersed in n-octadecene (ODE) with the formation of micelles with the help of other polar molecules, which resulted in reduced concentration of oleic acid and cadmium oleate both in the solution and on the surface of CQDs. These effects allow for control the size of CdSe CQDs in a wide range when synthesizing them by solely changing the amount of sodium oleate, under either cation-rich or anion-rich conditions. Additionally, enhanced ligand dynamics promote morphology transformation and suppress size deviation caused by different morphologies' existence in CQDs synthesis. Alkali metal oleate not only stabilized anion-rich CdSe CQDs but also results in highly crystallized wurtzite structure of CdSe CQDs when synthesizing them with excess anions. Furthermore, under anion-rich synthetic condition, anisotropic growth can be realized, leading to nanorods and nanoplatelets based on the alkali metal ions used. Given their outstanding effects and widely applicable synthetic conditions, alkali metal carboxylates offer new possibilities for designing efficient methods for synthesizing CQDs.

The diffusion-controlled growth mode is widely used to narrow the size distribution of colloidal quantum dots. However, this growth mode always suffers from size broadening at the later growth stage. By monitoring the growth process of CdS colloidal quantum dots, we show the size broadening is a result of different growth rates of CdS colloidal quantum dots (CQDs) with different morphologies. Monomer concentration-dependent growth experiments demonstrate the different growth rates are caused by the different ligand permeabilities of CdS CQDs. The cubic ones have lower ligand permeability but higher saturated surface reaction rate than the noncubic ones, leading to unexpected narrower size distribution under higher monomer concentration. More efficient narrowing can be obtained by the addition of chloride ions, which can increase the ligand permeability of all CdS CQDs, as well as the opposite discrepancies in ligand permeability and surface reaction between cubic and noncubic CdS CQDs. The photoluminescence (PL) full width at half maximum (FWHM) of CdS CQDs can be narrowed down to below 80 meV for PL peaks from 430 to 500 nm. Given the inevitable usage of the ligands in the solution synthesis of colloidal nanocrystals, the influence of morphology difference on growth rate should be common. Our results can provide an alternative solution to realize size focusing for the synthesis of colloidal nanocrystals.

Alkanoate-coated CdSe/CdS core/shell quantum dots (QDs) with near-unity photoluminescence (PL) quantum yield and mono-exponential PL decay dynamics are applied for studying quasi-stationary charge transfer from photo-excited QDs to quinone derivatives physically-adsorbed within the ligand monolayer of a QD. Though PL quenching efficiency due to electron transfer can be up to > 80%, transient PL and transient absorption spectra reveal that the charge transfer rate ranges from single-digit nanoseconds to sub-nanoseconds, which is ~ 3 orders of magnitude slower than that of static charge transfer and ~ 2 orders of magnitude faster than that of collisional charge transfer. The physically-adsorbed acceptors can slowly (500–1, 000 min dependent on the size of the quinone derivatives) desorb from the ligand monolayer after removal of the free acceptors. Contrary to collisional charge transfer, the efficiency of quasi-stationary charge transfer increases as the ligand length increases by providing additional adsorption compartments in the elongated hydrocarbon chain region. Because ligand monolayer commonly exists for a typical colloidal nanocrystal, the quasi-stationary charge transfer uncovered here would likely play an important role when colloidal nanocrystals are involved in photocatalysis, photovoltaic devices, and other applications related to photo-excitation.