  The photoluminescence (PL), color purity, and stability of lead halide
perovskite nanocrystals depend critically on the 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 the surface passivation, and a post-synthesis ligand exchange with
a quaternary ammonium bromide (QAB) results in a more stable passivation over a
larger temperature range. The PL intensity is highest from 50K-250K, which
indicates that the 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), the surface passivation still needs improvement
toward increased emission intensity in nanocrystal films.
