Aliphatic carboxylates are the most common class of surface ligands to stabilize colloidal nanocrystals. The widely used approach to identify the coordination modes between surface cationic sites and carboxylate ligands is based on the empirical infrared (IR) spectroscopic assignment, which is often ambiguous and thus hampers the practical control of surface structures. In this report, multiple techniques based on nuclear magnetic resonance (NMR) and IR spectra are applied to distinguish the different coordination structures in a series of zinc-blende CdSe nanocrystals with unique facet structures, including nanoplatelets dominated with {100} basal planes, hexahedrons with only three types of low-index facets (i.e., {100}, {110}, and {111}), and spheroidal dots without well-defined facets. Interpretation and assignment of NMR and IR signals were assisted by density functional theory (DFT) calculations. In addition to the identification of facet-sensitive bonding modes, the present methods also allow a nondestructive quantification of mixed ligands.

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Formation of InP quantum dots (QDs) in a noncoordinating solvent is divided into four stages for studying the chemical reactions. By introducing tertiary phosphines, such as trioctylphosphine (TOP), in the first stage, the four stages are all altered significantly, which enables the formation of InP QDs with high optical quality, that is, with a well-defined first-exciton absorption peak and a high-energy absorption shoulder in their ultraviolet−visible spectra. The first stage is the formation of a less sterically hindered complex with three monodentate carboxylates and one TOP ligand [In(TOP)(St) 3 ] by reacting indium stearate [In(St) 3 ] with TOP, which is soluble and reactive at room temperature. The second stage is the formation of InP clusters with near-unity yield and very small size by reacting In(TOP)(St) 3 with tris(trimethylsilyl)phosphine [(TMS) 3 P] at ambient temperatures (20−50 °C). During the third stage, tiny InP clusters formed with the In(TOP)(St) 3 precursor enable the formation of nearly monodisperse InP QDs by a hot-injection approach. In the following fourth stage, the InP QDs formed in the third stage grow further in the same pot by the secondary injections of the InP clustersthe ones formed at ambient temperatures with the In(TOP)(St) 3 precursor for efficient "self-focusing of size distribution"to finally obtain high-quality InP QDs with their absorption peak covering most part of the visible window (between ∼480 and 660 nm).

Zinc-blende CdSe quantum dots (QDs) encased in six equal (100) facets are synthesized in a noncoordinating solvent. Their monodispersed size, unique facet structure, and single crystallinity render the narrowest ensemble photoluminescence for CdSe QDs (full width at half-maximum being 52 meV). The nucleation stage can selectively form small-size CdSe QDs (≤3 nm) as seeds suited for the growth of cube-shaped QDs by reducing the concentration of cadmium carboxylates (Cd­(RCOO)2) as the sole source of ligands. While resulting in poorly controlled nucleation, chloride-ion ligands introduced in the form of soluble CdCl x (RCOO)1–x (x = 0.1∼0.2) would thermodynamically stabilize the cadmium-terminated (100) facets yet kinetically accelerate the deposition of selenium ions onto the (100) facets. Results suggest that it is fully feasible to synthesize QDs simultaneously with monodisperse size and surface structure through ligand-controlled nucleation and growth.

Zinc-blende CdSe, CdS, and CdSe/CdS core/shell nanocrystals with a structure-matched shape (cube-shaped, edge length ≤30 nm) are synthesized via a universal scheme. With the edge length up to five times larger than exciton diameter of the bulk semiconductors, the nanocrystals exhibit novel properties in the weakly confined size regime, such as near-unity single exciton and biexciton photoluminescence (PL) quantum yields, singlenanocrystal PL nonblinking, mixed PL decay dynamics of exciton and free carriers with sub-microsecond monoexponential decay lifetime, and stable yet extremely narrow PL full width at half maximum (FWHM < 0.1 meV) at 1.8 K. Their monodisperse edge length, shape, and facet structure enable demonstration of unexpected yet size-dependent PL properties at room temperature, including unusually broad and abnormally size-dependent PL FWHM (∼100 meV), nonmonotonic size dependence of PL peak energy, and dual-peak single-exciton PL. Calculations suggest that these unusual properties should be originated from the band-edge electron/hole states of the dynamic-exciton, whose exciton binding energy is too small to hold the photogenerated electron−hole pair as a bonded Wannier exciton in a weakly confined nanocrystal.