One or Many: Using single particle microscopy to investigate perovskite quantum dot surface chemistry

Abstract

Since their initial discovery, colloidal quantum dots (QDs) have emerged as promising materials for a wide range of optoelectronic applications including Quantum Dot Light Emitting Diodes for display applications, solution-processible lasers, and photodetectors. Perovskite quantum dots are a class of materials that are uniquely promising, due to their high photoluminescence quantum yield (PLQY), narrow ensemble linewidths and tunable bandgap. In particular, cesium lead halide nanocrystals have shown efficiency comparable to commercial Organic LEDs, and have even shown promise as quantum emitters, due to their high degree of coherent single photon emission relative to other colloidal quantum dots. Remarkably, perovskite quantum dots achieve these desirable optoelectronic properties without an inorganic shell, which is required in II-VI or III-V colloidal quantum dots. Instead, perovskite nanocrystals have organic ligand shells as their surface termination. As a result, the properties of perovskite nanocrystals are highly dependent on the surface ligand identity. Despite this, efforts to systematically investigate the effect of ligand composition on single particle properties are in a nascent stage. To optimize the surface chemistry of perovskite quantum dots for single particle applications we need to gain a more systematic understanding of the role that surface ligand plays in photoluminescence blinking. In the first project we investigate the effect of ligand binding equilibrium on photoluminescence blinking in cesium lead bromide quantum dots. Here we compared QDs capped with lecithin, a multidentate tightly binding ligand, to QDs capped with a binary ligand system of Oleic Acid and Oleylamine (OA/OAm). We find that lecithin acts to suppress blinking in CsPbBr3 QDs. Under common experimental conditions lecithin-capped QDs are 7.5 times more likely to be non-blinking and spend 2.5 times longer in their most emissive state than OA/OAm QDs, despite having comparable ensemble PLQY post synthesis. We measure photoluminescence as a function of dilution and show via ligand re-addition that the differences between lecithin and OA/OAm capping emerge at low concentrations, because of differences in the ligand binding affinity. This work shows the importance of ligand binding affinity to single particle properties otherwise obfuscated by ensemble dynamics. In the second project, we are investigating the effect of varying the ligand head group by comparing diquaternary amine (DC) and cationic ligand surface treatments, such as didodecylammonmum bromide (DDAB) capped QDs to those capped by zwitterionic ligands, such as lecithin and recently reported primary amine zwitterionic (PEA-C8C12). These cation and zwitterion-capped QDs show comparable ensemble properties, including near unity PLQY and narrow (<20nm fwhm) ensemble photoluminescence spectra. We further investigate the role of these ligand exchanges by investigating photoluminescence blinking and linewidth. We find that zwitterionic ligands reduce blinking and show narrower 4K linewidth compared to their cationic counterparts. We can rationalize these results by considering the geometry and ligand surface density. The cationic ligands have a lower number of ligands per QD and a more amorphous crystal habit compared to their zwitterionic counterparts. This suggest that zwitterionic ligands are a more favorable choice for ligand exchange reactions to improve single particle properties.