ConspectusThe surface chemistry of lead
halide perovskite nanocrystals (NCs)
plays a major role in dictating their colloidal and structural stability
as well as governing their optical properties. A deep understanding
of the nature of the ligand shell, ligand–NC, and ligand–solvent
interactions is therefore of utmost importance. Our recent studies
have revealed that such knowledge can be harnessed following a multidisciplinary
approach comprising chemical, structural, and spectroscopic analyses
coupled with atomistic modeling. We show that specific surface terminations
can be produced only by employing flexible and versatile syntheses
that enable to work under desired conditions. In this Account, we
first describe our studies aimed at synthesizing CsPbBr3NCs with various surface terminations. These include CsPbBr3NCs prepared under Br- and oleylamine-rich conditions, which
feature a ligand shell composed of alkylammonium-Br species and a
photoluminescence quantum yield (PLQY) of ∼90%. On the other
hand, taking advantage of the inability of secondary amines to bind
to the perovskite NCs surface, we could prepare cuboidal CsPbBr3NCs bearing a Cs-oleate surface termination and a PLQY of
70% by employing oleic acid and secondary alkylamines. In the quest
to identify ligands that can bind more strongly than oleates or primary
alkylammonium ions to the surface of NCs already in the synthesis
step, we used phosphonic acids as the sole ligands in the CsPbBr3NCs synthesis, which yielded NCs with a truncated octahedron
shape, high PLQY (∼100%), and a PbBr2-terminated
surface passivated by hydrogen phosphonates and phosphonic acid anhydride.
The surface chemistry and the stability of perovskite NCs were investigated
via ad-hoc postsynthesis treatments. We found, for example, that reacting
oleylammonium-Br-terminated NCs with stoichiometric amounts of neutral
primary alkylamines (or their conjugated acids) led to a partial replacement
of oleylammonium ions with new alkylammonium ions (following a deprotonation/protonation
mechanism), which resulted in a boost of the PLQY (up to 100%) and
of the NCs’ colloidal stability. Similar results in terms of
optical properties were achieved by treating Cs-oleate-terminated
NCs with alkylammonium-carboxylate or quaternary ammonium-Br (namely,
didodecyldimethylammonium-Br, DDA-Br) couples. Interestingly, when
the native NCs are ligand exchanged with DDA-Br, the ligand shell
is then composed of species not bearing any proton. This, in turn,
enabled us to study the interaction of such NCs with a variety of
ligands under completely aprotic conditions wherein these DDA-Br-capped
NCs were basically inert. The only exceptions were carboxylic, phosphonic,
and sulfonic acids that were capable of stripping surface DDA-Br couples.
As a note, most studies on CsPbBr3NCs to date have focused
primarily on choosing ligands with specific anchoring groups rather
than on tuning the length and type of alkyl chains, as this is time-consuming
and requires a large number of syntheses. Our recent developments
in the computational chemistry of colloidal NCs are expected to provide
a pivotal role in this direction since they can be integrated with
machine learning models to investigate with greater details the ligand–NC,
ligand–ligand, and ligand–solvent interactions and ultimately
find optimal candidates through the prediction of surfactant properties
using high-throughput data sets.