Postsynthesis ligand exchange has been employed extensively on lead halide perovskite (LHP) nanocrystals (NCs), but the complex ligand shell composition of the starting NCs prevented a clear understanding of the exchange process, and the surface chemistry of the final NCs remained poorly characterized. Here, we describe a ligand exchange strategy involving the displacement of both cationic and anionic ligands on native model systems of CsPbBr 3 NCs, which are exclusively coated with Cs-oleate. These ligands are exchanged with various quaternary ammonium bromides (R 4 NBr), and complete exchange is confirmed by nuclear magnetic resonance (NMR) spectroscopy analysis. The displacement of the native Cs-oleate ligands with proton-free R 4 NBr delivers NCs with excellent colloidal stability and near-unity PLQY, which is preserved after washing with polar solvents, over 3 weeks of storage in air, and after heating a solution of NCs to 80 °C, as confirmed by NMR analysis. The results, together with density functional theory calculations, suggest that the higher stability of quaternary ammonium capped NCs is not due to a stronger binding interaction to the surface but rather to weaker solvent−ligand interactions of R 4 NBr compared to Cs-oleate, driving the former to the surface of the NCs.
Fully inorganic cesium
lead halide perovskite (CsPbX3) nanocrystals (NCs) have
been extensively studied due to their excellent
optical properties, especially their high photoluminescence quantum
yield (PLQY) and the ease with which the PL can be tuned across the
visible spectrum. So far, most strategies for synthesizing CsPbX3 NCs are highly sensitive to the processing conditions and
ligand combinations. For example, in the synthesis of nanocubes of
different sizes, it is not uncommon to have samples that contain various
other shapes, such as nanoplatelets and nanosheets. Here, we report
a new colloidal synthesis method for preparing shape-pure and nearly
monodispersed CsPbBr3 nanocubes using secondary amines.
Regardless of the length of the alkyl chains, the oleic acid concentration,
and the reaction temperature, only cube-shaped NCs were obtained.
The shape purity and narrow size distribution of the nanocubes are
evident from their sharp excitonic features and their ease of self-assembly
in superlattices, reaching lateral dimensions of up to 50 μm.
We attribute this excellent shape and phase purity to the inability
of secondary amines to find the right steric conditions at the surface
of the NCs, which consequently limits the formation of low-dimensional
structures. Furthermore, no contamination from other phases was observed,
not even from Cs4PbBr6, presumably due to the
poor ability of secondary aliphatic amines to coordinate to PbBr2 and, hence, to provide a reaction environment that is depleted
in Pb.
The
photoluminescence (PL), color purity, and stability of lead
halide perovskite nanocrystals depend critically on surface passivation.
We present a study on the temperature-dependent PL and PL decay dynamics
of lead bromide perovskite nanocrystals characterized by different
types of A cations, surface ligands, and nanocrystal sizes. Throughout,
we observe a single emission peak from cryogenic to ambient temperature.
The PL decay dynamics are dominated by surface passivation, and a
postsynthesis ligand exchange with a quaternary ammonium bromide (QAB)
results in more stable passivation over a larger temperature range.
The PL intensity is highest from 50 to 250 K, which indicates that
ligand binding competes with the thermal energy at ambient temperature.
Despite the favorable PL dynamics of nanocrystals passivated with
QAB ligands (monoexponential PL decay over a large temperature range,
increased PL intensity and stability), surface passivation still needs
to be improved to achieve maximum emission intensity in nanocrystal
films.
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