• Manna, Liberato
• Toso, Stefano
• Baranov, Dmitry

Abstract

<p class="articleBody_abstractText">Following the impressive developmentof bulk lead-based perovskite photovoltaics, the “perovskite fever” didnot spare nanochemistry. In just a few years, colloidal cesium leadhalide perovskite nanocrystals have conquered researchers worldwide withtheir easy synthesis and color-pure photoluminescence. Thesenanomaterials promise cheap solution-processed lasers, scintillators,and light-emitting diodes of record brightness and efficiency. However,that promise is threatened by poor stability and unwanted reactivityissues, throwing down the gauntlet to chemists.</p><p class="articleBody_abstractText">Moregenerally, Cs–Pb–X nanocrystals have opened an exciting chapter in thechemistry of colloidal nanocrystals, because their ionic nature andbroad diversity have challenged many paradigms established bynanocrystals of long-studied metal chalcogenides, pnictides, and oxides.The chemistry of colloidal Cs–Pb–X nanocrystals is synonymous withchange: these materials demonstrate an intricate pattern of shapes andcompositions and readily transform under physical stimuli or the actionof chemical agents. In this Account, we walk through four types ofCs–Pb–X nanocrystal metamorphoses: change of structure, color, shape,and surface. These transformations are often interconnected; forexample, a change in shape may also entail a change of color.</p><p class="articleBody_abstractText">Theionic bonding, high anion mobility due to vacancies, and preservationof cationic substructure in the Cs–Pb–X compounds enable fast anionexchange reactions, allowing the precise control of the halidecomposition of nanocrystals of perovskites and related compounds (e.g.,CsPbCl₃ ⇄ CsPbBr₃ ⇄ CsPbI₃ and Cs₄PbCl₆ ⇄ Cs₄PbBr₆ ⇄ Cs₄PbI₆)and tuning of their absorption edge and bright photoluminescence acrossthe visible spectrum. Ion exchanges, however, are just one aspect of aricher chemistry.</p><p class="articleBody_abstractText">Cs–Pb–Xnanocrystals are able to capture or release (in short, trade) ions oreven neutral species from or to the surrounding environment, causingmajor changes to their structure and properties. The trade of neutralPbX₂ units allows Cs–Pb–X nanocrystals to cross the boundaries among four different types of compounds: 4CsX + PbX₂ ⇄ Cs₄PbBr₆ + 3PbX₂ ⇄ 4CsPbBr₃ + PbX₂ ⇄ 4CsPb₂X₅. These reactions do not occur at random, because the reactant and product nanocrystals are connected by the Cs⁺cation substructure preservation principle, stating that ion tradereactions can transform one compound into another by means ofdistorting, expanding, or contracting their shared Cs⁺ cation substructure.</p><p class="articleBody_abstractText">Thenanocrystal surface is a boundary between the core and the surroundingenvironment of Cs–Pb–X nanocrystals. The surface influences nanocrystalstability, optical properties, and shape. For these reasons, the dynamicsurface of Cs–Pb–X nanocrystals has been studied in detail, especiallyin CsPbX₃ perovskites. Two takeaways have emerged from thesestudies. First, the competition between primary alkylammonium and cesiumcations for the surface sites during the CsPbX₃ nanocrystalnucleation and growth governs the cube/plate shape equilibrium.Short-chain acids and branched amines influence that equilibrium andenable shape-shifting synthesis of pure CsPbX₃ cubes,nanoplatelets, nanosheets, or nanowires. Second, quaternary ammoniumhalides are emerging as superior ligands that extend the shelf life ofCs–Pb–X colloidal nanomaterials, boost their photoluminescence quantumyield, and prevent foreign ions from escaping the nanocrystals. That isaccomplished by combining reduced ligand solubility, due to the branchedorganic ammonium cation, with the surface-healing capabilities of thehalide counterions, which are small Lewis bases.</p>

Topics

• perovskite
• impedance spectroscopy
• surface
• compound
• photoluminescence
• mobility
• random
• amine