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.

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In this work, we demonstrate that a preferential Ga-for-Zn cation exchange is responsible for the increase in photoluminescence that is observed when gallium oleate is added to InZnP alloy QDs. By exposing InZnP QDs with varying Zn/In ratios to gallium oleate and monitoring their optical properties, composition, and size, we conclude that Ga3+ preferentially replaces Zn2+, leading to the formation of InZnP/InGaP core/graded-shell QDs. This cation exchange reaction results in a large increase of the QD photoluminescence, but only for InZnP QDs with Zn/In ≥ 0.5. For InP QDs that do not contain zinc, Ga is most likely incorporated only on the quantum dot surface, and a PL enhancement is not observed. After further growth of a GaP shell and a lattice-matched ZnSeS outer shell, the cation-exchanged InZnP/InGaP QDs continue to exhibit superior PL QY (over 70%) and stability under long-term illumination (840 h, 5 weeks) compared to InZnP cores with the same shells. These results provide important mechanistic insights into recent improvements in InP-based QDs for luminescent applications.@en

Colloidal quantum dots (QDs) show great promise as LED phosphors due to their tunable narrow-band emission and ability to produce high-quality white light. Currently, the most suitable QDs for lighting applications are based on cadmium, which presents a toxicity problem for consumer applications. The most promising cadmium-free candidate QDs are based on InP, but their quality lags much behind that of cadmium based QDs. This is not only because the synthesis of InP QDs is more challenging than that of Cd-based QDs, but also because the large lattice parameter of InP makes it difficult to grow an epitaxial, defect-free shell on top of such material. Here, we propose a viable approach to overcome this problem by alloying InP nanocrystals with Zn²⁺ ions, which enables the synthesis of In_xZn_yP alloy QDs having lattice constant that can be tuned from 5.93 Å (pure InP QDs) down to 5.39 Å by simply varying the concentration of the Zn precursor. This lattice engineering allows for subsequent strain-free, epitaxial growth of a ZnSe_zS_1-z shell with lattice parameters matching that of the core. We demonstrate, for a wide range of core and shell compositions (i.e.; varying x, y, and z), that the photoluminescence quantum yield is maximal (up to 60%) when lattice mismatch is minimal.

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