Non–close-packed nanoparticle arrays: Films of colloidal nanoparticles usually form dense, close-packed lattices. If binary lattices could be made and one component removed, then a more open array could form, as long as the remaining nanoparticles could be stabilized. Udayabhaskararao et al. formed binary superlattices of gold and magnetite nanoparticles at an air-liquid interface that could then be transferred to carbon-coated surfaces (see the Perspective by Kotov). Selective etching of either of the nanoparticles created non–close-packed arrays with vacancies stabilized by the carbon surface. Science , this issue p. 514 ; see also p. 448

Among perovskites, the research on cesium lead iodides (CsPbI₃) has attracted a large research community, owing to their all-inorganic nature and promising solar cell performance. Typically, the CsPbI₃ solar cell devices are prepared at various heterojunctions, and working at fluctuating temperatures raises questions on the material stability-related performance of such devices. The fundamental studies reveal that their poor stability is due to a lower side deviation from Goldschmidt’s tolerance factor, causing weak chemical interactions within the crystal lattice. In the case of organic–inorganic hybrid perovskites, where their stability is related to the inherent chemical nature of the organic cations, which cannot be manipulated to improve the stability drastically whereas the stability of CsPbI₃ is related to surface and lattice engineering. Thus, the challenges posed by CsPbI₃ could be overcome by engineering the surface and inside the CsPbI₃ crystal lattice. A few solutions have been proposed, including controlled crystal sizes, surface modifications, and lattice engineering. Various research groups have been working on these aspects and had accumulated a rich understanding of these materials. In this review, at first, we survey the fundamental aspects of CsPbI₃ polymorphs structure, highlighting the superiority of CsPbI₃ over other halide systems, stability, the factors (temperature, polarity, and size influence) leading to their phase transformations, and electronic band structure along with the important property of the defect tolerance nature. Fortunately, the factors stabilizing the most effective phases are achieved through a size reduction and the efficient surface passivation on the delicate CsPbI₃ nanocrystal surfaces. In the following section, we have provided the up-to-date surface passivating methods to suppress the non-radiative process for near-unity photoluminescence quantum yield, while maintaining their optically active phases, especially through molecular links (ligands, polymers, zwitterions, polymers) and inorganic halides. We have also provided recent advances to the efficient synthetic protocols for optically active CsPbI₃ NC phases to use readily for solar cell applications. The nanocrystal purification techniques are challenging and had a significant effect on the device performances. In part, we summarized the CsPbI₃-related solar cell device performances with respect to the device fabrication methods. At the end, we provide a brief outlook on the view of surface and lattice engineering in CsPbI₃ NCs for advancing the enhanced stability which is crucial for superior optical and light applications.

Precise control of quantum structures in hybrid nanocrystals requires advancements in scientific methodologies. Here, on the design of tunable CsPbBr₃/Cs₄PbBr₆ quantum dots are reported by developing a unique discrete phase transformation approach in Cs₄PbBr₆ nanocrystals. Unlike conventional hybrid systems that emit solely in the green region, this current strategy produces adjustable luminescence in the blue (450 nm), cyan (480 nm), and green (510 nm) regions with high photoluminescence quantum yields up to 45%, 60%, and 85%, respectively. Concentration‐dependent studies reveal that phase transformation mechanisms and the factors that drive CsBr removal occur at lower dilutions while the dissolution–recrystallization process dominates at higher dilutions. When the polymer‐CsPbBr₃/Cs₄PbBr₆ integrated into a field‐effected transistor the resulting phototransistors featured enhanced photosensitivity exceeding 10⁵, being the highest reported for an n‐type phototransistor, while maintaining good transistor performances as compared to devices consisting of polymer‐CsPbBr₃ NCs.

Lithium's interaction with CsPbBr₃ nanocrystals (NCs), can enhancing its intrinsic electrical conductivity (σ) for high‐performance device applications. Herein, two distinctly different modes of Li⁺ interaction with CsPbBr₃ NCs: minor lattice insertion (0.07% relative to Cs) and predominant surface passivation is reported through Li_mPb_n alloy formation. In contrast, Li⁺ exhibits significantly reduced interaction with Cs₄PbBr₆ NCs, which could be due to the persence of lower amount of Pb²⁺ on the surface of these structures. The σ of CsPbBr₃:xLi⁺ NCs through bottom‐contact devices exhibited a gradual increase from 2.1 × 10⁻⁷ to as high as 2.5 × 10⁻⁶ S m⁻¹, which is a 50‐fold improvement compared to CsPbBr₃ NCs. The enhanced σ is attributed to the presence of Li⁺ doping and surface passivation of CsPbBr₃ by the Li_mPb_n ligated complexes. DFT calculations revealed electron movement from the valence and to conduction band and a reduced bandgap further supporting the inferences from experimental studies. The unique feature of the increased luminescence and σ of CsPbBr₃:Li⁺ NCs is explored for fabricating white light emitting diodes. The luminescence efficacy of the device is in the range of 88.5 to 112.5 lm W⁻¹ which is higher compared to pure CsPbBr₃ NCs (96.5 lm W⁻¹), offering a pathway for advanced optoelectronic applications.