<p><b>Two-dimensional lead
halide perovskite nanoplatelets (2D LHP NPLs) have been emerging as one of the most
promising semiconductor nanomaterials due to their narrow absorption and
emission line widths, tunable bandgaps, high exciton binding energies, high
defect tolerance as well as highly localized energy states. Colloidal synthesis
of 2D LHP NPLs is generally performed using hot-injection or ligand assisted
precipitation techniques (LARP). In the LARP method, perovskites are
synthesized in polar solvents, which decrease the stability of the 2D LHP NPLs
due to their weakly bonded nature. In fact, the presence of residual polar
solvent in the LHP NPL colloid can cause deterioration of thickness uniformity,
degradation of NPLs to parent precursors, and undesired phase transformations. Herein,
for the first time, we report facile seed-mediated synthesis route of
monolayer, 2-monolayers, and thicker lead halide perovskite nanoplatelets
without using A site cation halide salt (AX</b><b>;</b><b>
A = Cesium, methylammonium, formamidinium and, X = Cl, Br, I) and long chain
alkylammonium halide salts (LX; L = oleylammonium, octylammonium, butylammonium
and, X = Cl, Br, I). The seed solution has been synthesized by reacting lead
(II) halide salt and coordinating ligands (oleylamine or octylamine and oleic
acid) in nonpolar high boiling solvent (1-octadecene). The seed mediated
synthesis has been carried out in hexane by reacting seed solution with A-site
cation precursors (Cs-oleate, FA-oleate, or diluted MA solution in hexane) under
ambient conditions. More importantly, the seed mediated growth of NPLs has been
tracked for the first time by performing in-situ optical measurements.
Furthermore, the optical properties and morphologies of the seeds have been extensively
studied. We find that our facile synthesis route provides highly stable,
monodisperse NPLs with narrow absorption, and photoluminescence line widths (68-201
meV), and high PLQY (37.6-1.66% for 2ML NPLs). Furthermore, anion exchange
reactions have been performed by mixing pre-synthesized LHP NPLs with counter
halide seeds. The optical properties of NPLs have been affectively tuned by
postsynthetic chemical reactions without changing the thickness of the NPLs. We
anticipate that our new synthetic route provides
further understanding of growth dynamics of LHP NPLs.</b></p>
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