Metamorphoses of Cesium Lead Halide Nanocrystals

Summary, in English

Following the impressive development
of bulk lead-based perovskite photovoltaics, the “perovskite fever” did
not spare nanochemistry. In just a few years, colloidal cesium lead
halide perovskite nanocrystals have conquered researchers worldwide with
their easy synthesis and color-pure photoluminescence. These
nanomaterials 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 reactivity
issues, throwing down the gauntlet to chemists.

More
generally, Cs–Pb–X nanocrystals have opened an exciting chapter in the
chemistry of colloidal nanocrystals, because their ionic nature and
broad diversity have challenged many paradigms established by
nanocrystals of long-studied metal chalcogenides, pnictides, and oxides.
The chemistry of colloidal Cs–Pb–X nanocrystals is synonymous with
change: these materials demonstrate an intricate pattern of shapes and
compositions and readily transform under physical stimuli or the action
of chemical agents. In this Account, we walk through four types of
Cs–Pb–X nanocrystal metamorphoses: change of structure, color, shape,
and surface. These transformations are often interconnected; for
example, a change in shape may also entail a change of color.

The
ionic bonding, high anion mobility due to vacancies, and preservation
of cationic substructure in the Cs–Pb–X compounds enable fast anion
exchange reactions, allowing the precise control of the halide
composition 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 across
the visible spectrum. Ion exchanges, however, are just one aspect of a
richer chemistry.

Cs–Pb–X
nanocrystals are able to capture or release (in short, trade) ions or
even neutral species from or to the surrounding environment, causing
major changes to their structure and properties. The trade of neutral
PbX₂ 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 trade
reactions can transform one compound into another by means of
distorting, expanding, or contracting their shared Cs⁺ cation substructure.

The
nanocrystal surface is a boundary between the core and the surrounding
environment of Cs–Pb–X nanocrystals. The surface influences nanocrystal
stability, optical properties, and shape. For these reasons, the dynamic
surface of Cs–Pb–X nanocrystals has been studied in detail, especially
in CsPbX₃ perovskites. Two takeaways have emerged from these
studies. First, the competition between primary alkylammonium and cesium
cations for the surface sites during the CsPbX₃ nanocrystal
nucleation and growth governs the cube/plate shape equilibrium.
Short-chain acids and branched amines influence that equilibrium and
enable shape-shifting synthesis of pure CsPbX₃ cubes,
nanoplatelets, nanosheets, or nanowires. Second, quaternary ammonium
halides are emerging as superior ligands that extend the shelf life of
Cs–Pb–X colloidal nanomaterials, boost their photoluminescence quantum
yield, and prevent foreign ions from escaping the nanocrystals. That is
accomplished by combining reduced ligand solubility, due to the branched
organic ammonium cation, with the surface-healing capabilities of the
halide counterions, which are small Lewis bases.

• Author: Stefano Toso; Dmitry Baranov; Liberato Manna
• Publishing year: 2021-02-02
• Language: English
• Pages: 498-508
• Publication/Series: Accounts of Chemical Research
• Volume: 54
• Issue: 3
• Links: http://dx.doi.org/10.1021/acs.accounts.0c00710
• Document type: Journal article review
• Publisher: The American Chemical Society (ACS)
• Status: Published
• ISBN/ISSN/Other: ISSN: 0001-4842