We introduce a general surface passivation mechanism for cesium lead halide perovskite materials (CsPbX 3 , X = Cl, Br, I) that is supported by a combined experimental and theoretical study of the nanocrystal surface chemistry. A variety of spectroscopic methods are employed together with ab initio calculations to identify surface halide vacancies as the predominant source of charge trapping. The number of surface traps per nanocrystal is quantified by 1 H NMR spectroscopy, and that number is consistent with a simple trapping model in which surface halide vacancies create deleterious under-coordinated lead atoms. These halide vacancies exhibit trapping behavior that differs between CsPbCl 3 , CsPbBr 3 , and CsPbI 3. Ab initio calculations suggest that introduction of anionic X-type ligands can produce trap-free bandgaps by altering the energetics of lead-based defect levels. General rules for selecting effective passivating ligand pairs are introduced by considering established principles of coordination chemistry. Introducing softer, anionic, X-type Lewis bases that target under-coordinated lead atoms results in absolute quantum yields approaching unity and monoexponential luminescence decay kinetics, thereby indicating full trap passivation. This work provides a systematic framework for preparing highly luminescent CsPbX 3 nanocrystals with variable compositions and dimensionalities, thereby improving fundamental understanding of these materials and informing future synthetic and post-synthetic efforts towards trap-free CsPbX 3 nanocrystals.

Replacing lead in halide perovskites is of great interest due to concerns about stability and toxicity. Recently, lead free double perovskites in which the unit cell is doubled and two divalent lead cations are substituted by a combination of mono- and trivalent cations have been synthesized as bulk single crystals and as thin films. Here, we study stability and optical properties of all-inorganic cesium silver(I) bismuth(III) chloride and bromide nanocrystals with the double perovskite crystal structure. The cube-shaped nanocrystals are monodisperse in size with typical side lengths of 8 to 15 nm. The absorption spectrum of the nanocrystals presents a sharp peak, which we assign to a direct bismuth s-p transition and not to a quantum confined excitonic transition. Using this spectroscopic handle combined with high-resolution transmission electron microscopy (TEM) based elemental analysis, we conduct stoichiometric studies at the single nanocrystal level as well as decomposition assays in solution and observe that Ag diffusion and coalescence is one of the pathways by which this material degrades. Drying the nanocrystals leads to self-assembly into ordered nanocrystal solids, and these exhibit less degradation than nanocrystals in solution. Our results demonstrate that CsAgBiX (X = Cl, Br) nanocrystals are a useful model system to study structure-function relationships in the search for stable nontoxic halide perovskites.

Lead toxicity has sparked interest into alternative halide nanomaterials with properties similar to CsPbX 3 perovskites. A promising alternative suggested from

The structure of Cu nanocrystals as catalysts for the electrochemical reduction of CO2 is a subject of considerable contemporary interest. Recent efforts have focused mainly on the preparation of Cu nanocrystals, but the question of their stability is equally relevant and has not been considered as extensively. Herein, we report on the reconstruction of Cu nanocrystals during CO2 reduction and discuss the factors influencing the observed changes with computerbased quantitative analysis and spectroscopic techniques. The timelines of opposing phenomena, sintering and declustering, previously reported separately, are detailed with a focus on two forces affecting the final morphology: applied potential and reaction intermediates. This intriguing system demonstrates the need for fundamental understanding of catalyst behavior preceding the ability to control its performance.