Nanocrystal-ligand interactions and ligand exchange processes are usually described by a uniform distribution of equal binding sites. Here, we analyze this assumption by a quantitative study of the displacement of Z-type cadmium oleate ligands from CdSe nanocrystals by addition of L-type ligands. First, we determined the stoichiometry of the displacement reaction by analyzing the equilibrium upon dilution using solution nuclear magnetic resonance spectroscopy. We found that 1 equivalent of tetramethylethylene-1,2-diamine (TMEDA) or two equivalents of n-butylamine or benzylamine bind the displaced cadmium oleate. We only reached a comprehensive description of the displacement isotherm by including two types of

Inductively coupled plasma mass spectrometry (ICP-MS) was combined with UV-vis absorption spectroscopy and transmission electron microscopy to determine the size, composition, and intrinsic absorption coefficient μ of 4 to 11 nm sized colloidal CsPbBr nanocrystals (NCs). The ICP-MS measurements demonstrate the nonstoichiometric nature of the NCs, with a systematic excess of lead for all samples studied. Rutherford backscattering measurements indicate that this enrichment in lead concurs with a relative increase in the bromide content. At high photon energies, μ is independent of the nanocrystal size. This allows the nanocrystal concentration in CsPbBr nanocolloids to be readily obtained by a combination of absorption spectroscopy and the CsPbBr sizing curve.

Although solvent-ligand interactions play a major role in nanocrystal synthesis, dispersion formulation and assembly, there is currently no direct method to study this. Here we examine the broadening of 1 H NMR resonances associated with bound ligands, and turn this poorly understood descriptor into a tool to assess solvent-ligand interactions. We show that the line broadening has both a homogeneous and a heterogeneous component. The former is nanocrystal-size dependent and the latter results from solvent-ligand interactions. Our model is supported by experimental and theoretical evidence that correlates broad NMR lines with poor ligand solvation. This correlation is found across a wide range of solvents, extending from water to hexane, for both hydrophobic and hydrophilic ligand types, and for a multitude of oxide, sulfide and selenide nanocrystals. Our findings thus put forward NMR line shape analysis as an indispensable tool to form, investigate and manipulate nanocolloids.

In this work, we present a combined experimental and theoretical analysis of two-component ligand shells passivating CdSe quantum dots. Using nuclear magnetic resonance spectroscopy, we first show that exposing oleate-capped quantum dots to primary carboxylic acids results in a one-for-one exchange that preserves the overall ligand surface concentration. Exposure to straight-chain acids leads to a binary ligand shell that behaves as an ideal mixture and that has a composition matching the overall acid composition of the dispersion. In the case of branched-chain acids, the exchange is restricted to about 25% of the original ligands. Based on molecular dynamics simulations, we argue that this behavior reflects the more favorable packing of oleates compared to branched carboxylates on the (100) facets of CdSe quantum dots.

While reports on stimulated emission and lasing by perovskites show great promise, a comprehensive quantification of their optical gain characteristics is lacking. Here, bulk-like colloidal perovskite nanocrystals (NC) are used as a test bed to quantify optical gain by lead halide perovskites. By analyzing colloidal dispersions of such NC, we can deploy a unique combination of broadband transient absorption and ultrafast fluorescence spectroscopy to measure gain coefficients, clarify the gain mechanism, and explore the gain dynamics. Opposite from current literature, we show that optical gain in perovskite NCs is supported by stimulated emission from free carriers and not from excitons or bi-excitons. Importantly, we demonstrate that the concomitant gain coefficients agree with literature results reported for thin films. Finally, we show that, even in the case of fully inorganic lead halide perovskites, a cooling bottleneck hampers the development of net stimulated emission at high excitation density. Weakly Confined Perovskites Gain Spectroscopy with Varying Pump Cooling Bottleneck At high density, carrier cooling slows down from a sub-ps to close to 10 ps average phonon emission time. The bottleneck arises from overheated phonon modes and gives rise to the observed reduction in maximum material gain for off-resonant pumping.