Synthesis and Single Particle Spectroscopic Measurements of Strongly Confined Perovskite Quantum Dots

Colloidal lead halide perovskite quantum dots (QDs) have attracted much attention owing to their facile synthesis and high photoluminescence (PL) quantum yields. To understand their exotic optical properties, synthetic control over the size and shape of perovskite QDs is necessary. However, perovskite QDs grow very fast due to their low crystal formation energy, making their kinetic growth control difficult. Using CsPbBr₃ as an example, traditional hot-injection synthesis often produces large (&gt; 7 nm) QDs with inadequate size uniformity. Instead of kinetically controlled growth, a thermodynamic equilibrium-controlled method was developed to produce strongly confined perovskite QDs with narrow QD size distributions from 3.5 nm to 7 nm. During this synthesis, QD growth is mediated by metastable nanoclusters with sizes around 2.4 nm. These nanoclusters can grow into strongly confined QDs if sufficient Cs-precursors are present. Otherwise, nanoclusters will self-assemble and fuse into nanoplatelets. By controlling the precursor ratio and concentrations, strongly confined perovskite QDs can be synthesized at the gram scale and are thermodynamically stable in solution at elevated temperatures for hours. The high structural stability enabled their surface morphology control and impurity doping synthetic control.<br/><br/>Despite the improved synthetic control of perovskite QDs, accurately determining their optical properties requires single-particle spectroscopic studies. Efforts on spectroscopic measurements of individual perovskite QDs were mainly focused on large, weakly-confined nanocrystals. The poor photostability of small (&lt; 7 nm) CsPbBr₃ QDs makes their single-particle studies extremely challenging. In particular, they exhibit severe PL blinking and photodarkening since strong quantum confinement makes them more sensitive to surface defects. We found that the repulsive intermolecular interaction in QDs covered by conventional entropic ligands with bulky carbon chains will reduce the surface ligand coverage when QDs are isolated and solidified for single QD studies. We present that phenethylammonium (PEA) ligands with low steric effect and attractive inter-ligand interaction promote near-epitaxial surface passivation of single CsPbBr₃ QDs. Our QDs are nearly non-blinking and exhibit non-observable spectral shifting. Moreover, these QDs remain non-blinking after 12 hours of continuous measurements with no photodarkening. Size-dependent exciton properties of perovskite QDs are hence accurately determined using single-particle spectroscopies.