In the last decade, all-inorganic halide perovskite CsPbX₃ (X = Cl, Br, I) colloidal quantum dots (QDs)/nanocrystals (NCs) have shown dramatic application prospect in optoelectronic devices such as solar cells and light-emitting diodes (LEDs). In particular, this class of all-inorganic semiconductor nanomaterials is featured with excellent photophysical properties (e.g high luminous efficiency, narrow emission bandwidth, and tunable optical bandgap in the visible region) and low-cost solution preparation. Additionally, their structural stability is considerably enhanced compared to organometallic hybrid halide perovskite counterparts. To date, there has been a proliferation of methods for the synthesis of CsPbX₃ QDs, which is one of the important components of this emerging field for all-inorganic halide perovskite QDs. The diversity of synthesis methods also lays a favorable material foundation for further practical applications of such functional nanomaterials. Among numerous synthesis methods, the most commonly methods are the conventional hot-injection (HI) and room temperature (RT) ligand-assisted reprecipitation (LARP) technique. However, compared to the conventional HI method, RT synthesis of photoactive pure-iodine all-inorganic perovskite QDs is still a challenge due to the thermal unequilibrium-induced metastable (perovskite)-to-stable (non-perovskite) phase transition. Besides, RT synthesis without inert gas protection has attracted more attention due to its easy manipulation and mild reaction conditions. Thus, it is very necessary to develop a simple and feasible RT synthesis approach for synthetizing pure-iodine all-inorganic perovskite QDs.

Although it is notoriously difficult to stabilize small-sized metal halide perovskite QDs due to the inherent ionic nature and significantly increased surface defects, the RT stability of perovskite-phase iodine-based QDs can be highly improved by the reduced QD size and increased surface Gibbs energy. Besides, the doping/alloying of B-site divalent ions in CsPbI₃ QDs through HI method is an effective strategy for defect passivation and stability improvement of iodine-based perovskite QDs. However, to our knowledge, the studies on direct and rapid RT alloying of CsPbI₃ QDs with CsSnI₃ in open air have never been reported before. Therefore, the RT preparation of small-sized and stable pure-iodine all-inorganic Sn-Pb alloyed perovskite QDs is worth further exploration, and the underlying mechanisms behind the superior properties upon RT alloying also need to be clearly elaborated.

In our recent work published in Nature Communications, a direct and rapid RT strongly confined spontaneous crystallization strategy in open air is proposed for preparing pure-iodine all-inorganic CsSn_xPb_1-xI₃ perovskite colloidal QDs with bright yellow luminescence. The experimental and theoretical studies both confirm that the yellow emission is ascribed to the increased bandgap caused by strong size confinement. This study showcases that the yellow emission with such a wider bandgap is presented for iodine-based all-inorganic Sn-Pb alloyed perovskite QDs. Most notably, the size confinement and photoluminescence (PL) emission of the resultant perovskite QDs can be regulated, which benefits from the management in Cs/Pb feed molar ratio. The optical bandgaps of bromine- and chlorine-based all-inorganic Sn-Pb perovskite QDs also show a remarkable increase compared with those in the reported works. This RT synthesis strategy is also applicable to pure-iodine organic-inorganic hybrid perovskite QDs. In addition, compared with the pristine CsPbI₃ QDs synthesized using this RT method, the pure-iodine all-inorganic CsSn_0.09Pb_0.91I₃ perovskite QDs show the similar PL characteristics with superior emission properties and colloidal stability, resulting from size-stabilized crystallization and direct RT alloying of stannous ions.

The key points of this work include the following aspects: (1) A completely new RT reaction system with simple preparation of precursor solutions is proposed for pure-iodine all-inorganic alloyed perovskite colloidal QDs. The entire synthesis processes are carried out in a RT atmospheric environment. (2) Because the crystallization rate of halide perovskite QDs is extremely fast, it is very easy for iodine-based reaction system to directly form a stable nonphotoactive yellow phase with a larger crystal size through a RT process. However, the RT stability of perovskite-phase iodine-based QDs can be highly improved by the reduced QD size, strong quantum confinement effect and increased surface Gibbs energy. In this work, the yellow emissive pure-iodine all-inorganic perovskite QDs are successfully synthesized through RT strongly confined spontaneous crystallization in open air. The successful preparation of photoactive perovskite-phase iodine-based all-inorganic QDs benefited from strong size confinement and enhanced phase stability. (3) Based on this size-stabilized synthesis process, the pure-iodine all-inorganic Sn-Pb alloyed perovskite QDs are directly prepared for the first time at RT. The resultant all-inorganic CsSn_0.09Pb_0.91I₃ QDs exhibit the enhanced PL intensity, prolonged fluorescence lifetime and improved colloidal stability as compared with CsPbI₃ QDs. This work has pioneered the direct RT preparation of pure-iodine all-inorganic Sn-Pb alloyed perovskite QDs, enriched the synthesis pathways of pure-iodine CsSn_xPb_1-xI₃ QDs, and filled the gap in related research.

This work titled“Spontaneous crystallization of strongly confined CsSn_xPb_1-xI₃ perovskite colloidal quantum dots at room temperature”was published in the Nature Communications journal (https://www.nature.com/articles/s41467-024-45945-1).