Lead halide perovskite materials have attracted significant attention in the context of photovoltaics and other optoelectronic applications, and recently, research efforts have been directed to nanostructured lead halide perovskites. Collodial nanocrystals (NCs) of cesium lead halides (CsPbX₃, X = Cl, Br, I) exhibit bright photoluminescence, with emission tunable over the entire visible spectral region. However, previous studies on CsPbX₃ NCs did not address key aspects of their chemistry and photophysics such as surface chemistry and quantitative light absorption. Here, we elaborate on the synthesis of CsPbBr₃ NCs and their surface chemistry. In addition, the intrinsic absorption coefficient was determined experimentally by combining elemental analysis with accurate optical absorption measurements. ¹H solution nuclear magnetic resonance spectroscopy was used to characterize sample purity, elucidate the surface chemistry, and evaluate the influence of purification methods on the surface composition. We find that ligand binding to the NC surface is highly dynamic, and therefore, ligands are easily lost during the isolation and purification procedures. However, when a small amount of both oleic acid and oleylamine is added, the NCs can be purified, maintaining optical, colloidal, and material integrity. In addition, we find that a high amine content in the ligand shell increases the quantum yield due to the improved binding of the carboxylic acid.

Aminophosphines have recently emerged as economical, easy-to-implement precursors for making InP nanocrystals, which stand out as alternative, Cd-free quantum dots for opto-electronic applications. Here, we present a complete investigation of the chemical reactions leading to InP formation starting from InCl3 and tris(dialkylamino)phosphines. Using nuclear magnetic resonance (NMR) spectroscopy and single crystal X-ray diffraction, we demonstrate that injection of the aminophosphine in the reaction mixture is followed by a rapid transamination with oleylamine, the solvent of the reaction. In addition, mass spectrometry and NMR indicate that the formation of InP concurs with that of tetra(oleylamino)phosphoniumchloride. The chemical yield of the InP formation agrees with this 4 P(+III) → P(-III) + 3 P(+V) disproportionation reaction occuring, since full conversion of the In precursor was only attained for a 4:1 P:In ratio. Hence the double role of the aminophosphine as both precursor and reducing agent. These new insights will guide further optimization of high quality InP quantum dots and might lead to the extension of synthetic protocols towards other pnictide nanocrystals.

While the surface termination of quasi-spherical metal chalcogenide nanocrystals or quantum dots has been widely investigated, it remains unclear whether the ensuing surface chemistry models apply to similar nanocrystals with anisotropic shapes. In this work, we report on the surface-chemistry of 2D CdSe nanoplatelets, where we make use of an improved synthesis strategy that yields stable and aggregation free nanoplatelet suspensions with a photoluminescence quantum yield (PLQY) as high as 55%. We confirm that such nanoplatelets are enriched in Cd and, by means of 1H nuclear magnetic resonance spectroscopy (NMR), we show that the Cd rich surface is terminated by X-type carboxylate ligands. Not unlike CdSe quantum dots (QDs), entire cadmium carboxylate entities can be displaced by the addition of amines, and the desorption isotherm points towards a considerable binding site heterogeneity. Moreover, we find that even the slightest displacement of cadmium carboxylate ligands quenches the nanoplatelet photoluminescence. These experimental findings are further confirmed by density functional theory (DFT) calculations on a 5 mon-olayer model CdSe nanoplatelet. These simulations show that the most labile ligands are located in the vicinity of facet edges, and that the displacement of ligands from such edge sites creates midgap states that can account for the observed photoluminescence quenching. Next to extending surface chemistry insights from colloidal QDs to nanoplatelets, this work indicates that CdSe nano-platelets constitute a unique nanocrystal model system to establish a comprehensive description of mid-gap trap states, which includes their structural, chemical and electronic properties.