Phase‐Pure and Size‐Tunable Tin Halide Perovskite Quantum Dots

ABSTRACT Tin halide perovskites represent an emerging alternative to lead halide perovskites, offering comparably advantageous electronic structures, lower toxicity, and bandgaps that extend into the near‐infrared region. However, the synthesis of tin halide perovskite quantum dots (QDs) remains challenging due to the tendency toward lower‐dimensional phase formation, limited control over QD size and composition, and high sensitivity to intrinsic defects. Here, we introduce a room‐temperature synthesis protocol employing cesium oleate and tin halide adducts with trioctylphosphine oxide in the absence of oleylamine. This non‐templating route effectively suppresses 2D impurities and yields monodisperse CsSnI 3 QDs, finely tunable between 4 and 22 nm, providing access to size‐dependent optical bandgap energies across the strong‐to‐weak confinement regimes. Increasing the availability of Sn(II) through an additional tin carboxylate source reduces defect densities and leads to the emergence of pronounced and spectrally well‐resolved excitonic absorption in strongly confined QDs. Variation of the tin halide and A‐cation yields formamidinium, methylammonium, and cesium tin bromide and iodide, all with narrow size distributions. This synthetic route sets the stage for further defect engineering to yield bright tin halide perovskite emitters.