A binary ligand system composed of aliphatic carboxylic acids and primary amines of various chain lengths is commonly employed in diverse synthesis methods for CsPbBr3 nanocrystals (NCs). In this work, we have carried out a systematic study examining how the concentration of ligands (oleylamine and oleic acid) and the resulting acidity (or basicity) affects the hot-injection synthesis of CsPbBr3 NCs. We devise a general synthesis scheme for cesium lead bromide NCs which allows control over size, size distribution, shape, and phase (CsPbBr3 or Cs4PbBr6) by combining key insights on the acid–base interactions that rule this ligand system. Furthermore, our findings shed light upon the solubility of PbBr2 in this binary ligand system, and plausible mechanisms are suggested in order to understand the ligand-mediated phase control and structural stability of CsPbBr3 NCs.

We propose here a new colloidal approach for the synthesis of both all-inorganic and hybrid organic–inorganic lead halide perovskite nanocrystals (NCs). The main limitation of the protocols that are currently in use, such as the hot injection and the ligand-assisted reprecipitation routes, is that they employ PbX2 (X = Cl, Br, or I) salts as both lead and halide precursors. This imposes restrictions on being able to precisely tune the amount of reaction species and, consequently, on being able to regulate the composition of the final NCs. In order to overcome this issue, we show here that benzoyl halides can be efficiently used as halide sources to be injected in a solution of metal cations (mainly in the form of metal carboxylates) for the synthesis of APbX3 NCs (in which A = Cs+, CH3NH3+, or CH(NH2)2+). In this way, it is possible to independently tune the amount of both cations and halide precursors in the synthesis. The APbX3 NCs that were prepared with our protocol show excellent optical properties, such as high photoluminescence quantum yields, low amplified spontaneous emission thresholds, and enhanced stability in air. It is noteworthy that CsPbI3 NCs, which crystallize in the cubic α phase, are stable in air for weeks without any postsynthesis treatment. The improved properties of our CsPbX3 perovskite NCs can be ascribed to the formation of lead halide terminated surfaces, in which Cs cations are replaced by alkylammonium ions.

Postsynthesis ligand exchange has been employed extensively on lead halide perovskite (LHP) nanocrystals (NCs), but the complex ligand shell composition of the starting NCs prevented a clear understanding of the exchange process, and the surface chemistry of the final NCs remained poorly characterized. Here, we describe a ligand exchange strategy involving the displacement of both cationic and anionic ligands on native model systems of CsPbBr 3 NCs, which are exclusively coated with Cs-oleate. These ligands are exchanged with various quaternary ammonium bromides (R 4 NBr), and complete exchange is confirmed by nuclear magnetic resonance (NMR) spectroscopy analysis. The displacement of the native Cs-oleate ligands with proton-free R 4 NBr delivers NCs with excellent colloidal stability and near-unity PLQY, which is preserved after washing with polar solvents, over 3 weeks of storage in air, and after heating a solution of NCs to 80 °C, as confirmed by NMR analysis. The results, together with density functional theory calculations, suggest that the higher stability of quaternary ammonium capped NCs is not due to a stronger binding interaction to the surface but rather to weaker solvent−ligand interactions of R 4 NBr compared to Cs-oleate, driving the former to the surface of the NCs.

Fully inorganic cesium lead halide perovskite (CsPbX3) nanocrystals (NCs) have been extensively studied due to their excellent optical properties, especially their high photoluminescence quantum yield (PLQY) and the ease with which the PL can be tuned across the visible spectrum. So far, most strategies for synthesizing CsPbX3 NCs are highly sensitive to the processing conditions and ligand combinations. For example, in the synthesis of nanocubes of different sizes, it is not uncommon to have samples that contain various other shapes, such as nanoplatelets and nanosheets. Here, we report a new colloidal synthesis method for preparing shape-pure and nearly monodispersed CsPbBr3 nanocubes using secondary amines. Regardless of the length of the alkyl chains, the oleic acid concentration, and the reaction temperature, only cube-shaped NCs were obtained. The shape purity and narrow size distribution of the nanocubes are evident from their sharp excitonic features and their ease of self-assembly in superlattices, reaching lateral dimensions of up to 50 μm. We attribute this excellent shape and phase purity to the inability of secondary amines to find the right steric conditions at the surface of the NCs, which consequently limits the formation of low-dimensional structures. Furthermore, no contamination from other phases was observed, not even from Cs4PbBr6, presumably due to the poor ability of secondary aliphatic amines to coordinate to PbBr2 and, hence, to provide a reaction environment that is depleted in Pb.

We devised a colloidal approach for the synthesis of CsPbBr 3 nanocrystals (NCs) in which the only ligands employed are alkyl phosphonic acids. Compared to more traditional syntheses of CsPbBr 3 NCs, the present scheme delivers NCs with the following distinctive features: (i) The NCs do not have cubic but truncated octahedron shape enclosed by Pb-terminated facets. This is a consequence of the strong binding affinity of the phosphonate groups toward Pb 2+ ions. (ii) The NCs have near unity photoluminescence quantum yields (PLQYs), with no need of postsynthesis treatments, indicating that alkyl phosphonic acids are effectively preventing the formation of surface traps. (iii) Unlike NCs coated with alkylammonium or carboxylate ligands, the PLQY of phosphonate coated NCs remains constant upon dilution, suggesting that the ligands are tightly bound to the surface.