Metal halide perovskites represent a flourishing area of research, which is driven by both their potential application in photovoltaics and optoelectronics and by the fundamental science behind their unique optoelectronic properties. The emergence of new colloidal methods for the synthesis of halide perovskite nanocrystals, as well as the interesting characteristics of this new type of material, has attracted the attention of many researchers. This review aims to provide an up-to-date survey of this fast-moving field and will mainly focus on the different colloidal synthesis approaches that have been developed. We will examine the chemistry and the capability of different colloidal synthetic routes with regard to controlling the shape, size, and optical properties of the resulting nanocrystals. We will also provide an up-to-date overview of their postsynthesis transformations, and summarize the various solution processes that are aimed at fabricating halide perovskite-based nanocomposites. Furthermore, we will review the fundamental optical properties of halide perovskite nanocrystals by focusing on their linear optical properties, on the effects of quantum confinement, and on the current knowledge of their exciton binding energies. We will also discuss the emergence of nonlinear phenomena such as multiphoton absorption, biexcitons, and carrier multiplication. Finally, we will discuss open questions and possible future directions.

Among the various postsynthesis treatments of colloidal nanocrystals that have been developed to date, transformations by cation exchange have recently emerged as an extremely versatile tool that has given access to a wide variety of materials and nanostructures. One notable example in this direction is represented by partial cation exchange, by which preformed nanocrystals can be either transformed to alloy nanocrystals or to various types of nanoheterostructures possessing core/shell, segmented, or striped architectures. In this review, we provide an up to date overview of the complex colloidal nanostructures that could be prepared so far by cation exchange. At the same time, the review gives an account of the fundamental thermodynamic and kinetic parameters governing these types of reactions, as they are currently understood, and outlines the main open issues and possible future developments in the field.

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.

We show here the first colloidal synthesis of double perovskite Cs2AgInCl6 nanocrystals (NCs) with a control over their size distribution. In our approach, metal carboxylate precursors and ligands (oleylamine and oleic acid) are dissolved in diphenyl ether and reacted at 105 °C with benzoyl chloride. The resulting Cs2AgInCl6 NCs exhibit the expected double perovskite crystal structure, are stable under air, and show a broad spectrum white photoluminescence (PL) with quantum yield of ∼1.6 ± 1%. The optical properties of these NCs were improved by synthesizing Mn-doped Cs2AgInCl6 NCs through the simple addition of Mn-acetate to the reaction mixture. The NC products were characterized by the same double perovskite crystal structure, and Mn doping levels up to 1.5%, as confirmed by elemental analyses. The effective incorporation of Mn ions inside Cs2AgInCl6 NCs was also proved by means of electron spin resonance spectroscopy. A bright orange emission characterized our Mn-doped Cs2AgInCl6 NCs with a PL quantum yield as high as ∼16 ± 4%.

We report a high-yield, low cost synthesis route to colloidal Cu 1-x InS 2 nanocrystals with a tunable amount of Cu vacancies in the crystal lattice. These are then converted into quaternary Cu−In−Zn−S (CIZS) nanocrystals by partial exchange of Cu + and In 3+ cations with Zn 2+ cations. The photoluminescence quantum yield of these CIZS nanocrystals could be tuned up to a record 80%, depending on the amount of copper vacancies.

Synthesis approaches to colloidal Cu3P nanocrystals (NCs) have been recently developed, and their optical absorption features in the near-infrared (NIR) have been interpreted as arising from a localized surface plasmon resonance (LSPR). Our pump–probe measurements on platelet-shaped Cu3-xP NCs corroborate the plasmonic character of this absorption. In accordance with studies on crystal structure analysis of Cu3P dating back to the 1970s, our density functional calculations indicate that this material is substoichiometric in copper, since the energy of formation of Cu vacancies in certain crystallographic sites is negative, that is, they are thermodynamically favored. Also, thermoelectric measurements point to a p-type behavior of the majority carriers from films of Cu3-xP NCs. It is likely that both the LSPR and the p-type character of our Cu3-xP NCs arise from the presence of a large number of Cu vacancies in such NCs. Motivated by the presence of Cu vacancies that facilitate the ion diffusion, we have additionally exploited Cu3-xP NCs as a starting material on which to probe cation exchange reactions. We demonstrate here that Cu3-xP NCs can be easily cation-exchanged to hexagonal wurtzite InP NCs, with preservation of the anion framework (the anion framework in Cu3-xP is very close to that of wurtzite InP). Intermediate steps in this reaction are represented by Cu3-xP/InP heterostructures, as a consequence of the fact that the exchange between Cu+ and In3+ ions starts from the peripheral corners of each NC and gradually evolves toward the center. The feasibility of this transformation makes Cu3-xP NCs an interesting material platform from which to access other metal phosphides by cation exchange.

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.

We report a new colloidal synthesis of niobium-doped TiO2 anatase nanocrystals (NCs) that allows for the preparation of ∼10 nm NCs with control over the amount of Nb doping up to ∼14%. The incorporation of niobium ions leads to the appearance of a tunable, broad absorption peak that ranges from the visible range to the mid-infrared. This optical behavior is attributed to the substitution of Nb5+ on Ti4+ sites generating free carriers inside the conduction band of the TiO2 NCs as supported by optical and electron paramagnetic resonance spectroscopic investigations. At the same time, the incorporation of progressively more niobium ions drives an evolution of the shape of the NCs from tetragonal platelets to “peanutlike” rods.