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In this work, we report a colloidal synthesis of atomically flat PbSe nanoplatelets (NPLs) of different thicknesses from 3 to 6 monolayers via cation exchange from the corresponding CdSe NPLs. In order to preserve the delicate structure of the NPLs, we performed the reaction at mild conditions using a temperature of 80 °C and lead bromide dissolved in oleylamine and octadecene as a Pb precursor. Cation exchange affected the size and the shape of the resulting NPLs depending on their thickness, i.e., the thinnest platelets (3 monolayers) underwent a partial degradation, although retaining their 2D structure, while the thicker ones were etched on their edges. We observed thickness-dependent near-infrared (NIR) photoluminescence (PL) with maxima ranging from 1330 to 1550 nm and quantum yields (PL QY) reaching 15%. The PbSe NPLs exhibited an excellent stability of the optical properties upon their storage in ambient atmosphere for several weeks, which we attribute to an efficient surface passivation with bromide ions or PbBr 2 as X-and Z-type ligands, respectively. Moreover, we achieved extremely narrow full width at half-maximum values of their PL spectra down to 80 meV, which in combination with significantly high PL QYs make these nanomaterials excellent NIR emitters for exploitation in NIR light-emitting diodes, lasing, and telecommunications.

2638www.MaterialsViews.com wileyonlinelibrary.com approaches, [ 15,16 ] effi ciency, [ 17,18 ] extending their spectral range [19][20][21] and environmental friendliness using less toxic materials [22][23][24] sparked industrial applications. Those started with the demonstration of a 40 in. display prototype presented by Samsung [ 25 ] and followed by the foundation of QD Vision, Inc., [ 26 ] whose QDs are now used in the latest series of Sony products. All of the mentioned applications require long-term stability of the QDs under various conditions including high temperatures as well as high intensity illumination. Packaging of the QDs within polymer or inorganic matrices is one way to address these issues, while improving the processability of the QDs at the same time. Commonly used polymers, such as polystyrene and poly(methylmethacrylate) [ 27 ] are relatively less stable and tight in comparison to their inorganic counterparts. Inorganic matrices on the other hand can incorporate the QDs directly from their melt, e.g., using the Czochralski approach, [ 28 ] coated as a thin fi lm directly on their surface [ 29,30 ] or, as recently developed by our group, grown as mixed crystals at ambient temperatures from a saturated salt solution. [ 31 ] Using this method, different types of QDs can be incorporated due to the low thermal stress and, by choosing the proper matrix-QD system, the photoluminescence quantum yields (PL-QYs) are enhanced upon incorporation. [ 32,33 ] Here, a new, fast, and versatile method for the incorporation of colloidal quantum dots (QDs) into ionic matrices enabled by liquid-liquid diffusion is demonstrated. QDs bear a huge potential for numerous applications thanks to their unique chemical and physical properties. However, stability and processability are essential for their successful use in these applications. Incorporating QDs into a tight and chemically robust ionic matrix is one possible approach to increase both their stability and processability. With the proposed liquid-liquid diffusion-assisted crystallization (LLDC), substantially accelerated ionic crystallization of the QDs is shown, reducing the crystallization time needed by one order of magnitude. This fast process allows to incorporate even the less stable colloids including initially oil-based ligandexchanged QDs into salt matrices. Furthermore, in a modifi ed two-step approach, the seed-mediated LLDC provides the ability to incorporate oilbased QDs directly into ionic matrices without a prior phase transfer. Finally, making use of their processability, a proof-of-concept white light emitting diode with LLDC-based mixed QD-salt fi lms as an excellent color-conversion layer is demonstrated. These fi ndings suggest that the LLDC offers a robust, adaptable, and rapid technique for obtaining high quality QD-salts.

Atomically flat colloidal semiconductor CdSe nanoplatelets (NPLs) with precisely controlled thickness possess a range of unique optoelectronic properties. Here, we study the growth of CdSe, CdTe, and CdS NPLs with the aim of synthesizing thicker NPLs in order to extend their optical activity further into the lower energy/larger wavelength range. We employ cadmium halides, which lead to faster reaction kinetics as confirmed by control experiments with cadmium hydroxide as a Cd-precursor. Addition of halides in all cases led to the formation of thicker NPL species, as compared with the corresponding syntheses without these additives. Analysis of a recent theoretical model of the platelet growth mechanism confirms an earlier suggestion that reducing both the reaction enthalpy and the surface energy of CdSe, by replacing acetate ligands with chloride ions, should indeed lead to thicker NPLs as observed. We noticed a formation of Cd0-metal nanoparticles in the first stage of the synthesis by preparing the Cd-precursor, which is another key finding of our work. We assume that these particles can serve as an active cadmium source facilitating the growth of the NPLs. The resulting 6 ML CdSe NPLs exhibited bright photoluminescence with quantum yield of up to 50%, exceptionally narrow spectrum centered at 582 nm with full width at half-maximum of approx. 11 nm, and small Stokes shift of 2 nm. Moreover, we demonstrated the synthesis of heterostructured core/shell CdSe/CdS NPLs based on 6 ML CdSe platelets, which also exhibited bright fluorescence. This work shows the possibility to overcome energetic barrier limiting the size (thickness) control by using appropriate promoters of the growth of CdSe, CdTe, and CdS 2D structures.

A straightforward, rapid method to create colloidally stable and brightly luminescent core/shell CdSe‐based nanoplatelets (NPLs) with fluorescence quantum yields (QYs) up to 50% is demonstrated. A layer‐by‐layer deposition technique is used which is based on a two‐phase mixture—consisting of a nonpolar phase, which includes the NPLs, and a saturated ionic polar phase—to separate the reagents and hinder the nucleation of the shell material. The deposition of the first sulfur layer leads to a significant redshift (by more than 100 nm) of the optical absorption and emission of the NPLs. Hence, by varying either the sulfur precursor content or the reaction time, one can precisely and continuously tune the absorption and emission maxima from 520 to 630 nm. This evolution of the absorption onset during the shell growth is explained quantitatively using density‐functional theory and atomistic statistical simulations. The emission can be further enhanced by exposure of the NPL solution to ambient sunlight. Finally, it is demonstrated that the core/shell NPLs can be transferred from the organic solution to aqueous media with no reduction of their QY, which opens the door to a broad range of practical applications.