Polar Opposites: Ligand-Mediated Polarity Inversion for Perovskite Quantum Dots with Sub-Nanometer Ligand Shells

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Perovskite nanocrystals (pNCs) and quantum dots (pQDs) are exciting bright emitters with tremendous potential for efficient and affordable classical and quantum light. (1,2) Although promising, their ionic lattice makes it challenging to process them in polar solvents, leading to rapid dissolution and degradation. To solve this problem, several strategies have been developed (see Supporting Information (SI), Table SI1). For instance, encapsulating pNCs in polymeric ligand shells (like Solutol, polystyrene or polystyrene-block-poly(acrylic acid)) (3−7) results in large shells in the range of 10–100 nm, allowing the pNCs to be stable in polar solvents. However, these thick polymeric ligand shells often sacrifice colloidal stability and render the pNCs nonconductive, thus hindering the formation of dense and conductive QD films. An alternative strategy is encapsulating pNCs in inorganic shells like silica or aluminum oxide. (8−11) In most cases, these shells are often several nanometers thick, but recent encapsulation of alumina-gel-coated NCs demonstrated pQDs with thin shells of <0.5 nm, dispersible in polar solvents like butanol. (11) However, such inorganic shells require anhydrous and inert synthesis conditions, as metallo-organic precursors (like aluminum ethoxide) are moisture sensitive. This limits the practical use of such inorganic shells and opposes the promise of pQDs being cheap and scalable light emitters. However, strategies developed for conventional QDs, like molecular inorganic ligands, (12) will not work, as the ionic pQDs still require protection from the polar solvents due to their high ionic solubility. Thus, for pQDs to be dispersible in polar solvents using only ligands, we must develop new ligand chemistry tailored for pQDs.

Here, we demonstrate that pQDs can be dispersed in polar solvents like ethanol (EtOH) when capped with dicationic Gemini ligands terminated with hydroxyl groups. These ligands act as a polar membrane, with the dicationic ammonium groups binding to the pQDs, the alkyl chains providing an apolar barrier, and the hydroxyl heads providing a polar outer layer (see TOC). This strategy works for pQDs, as the hydroxyl groups have no affinity to the pQD surface, ensuring a unilateral binding of the ligands. Such ligands result in thin ligand shells of 0.7 nm and colloidally and chemically stable pQDs. Together with the pQDs’ size and composition tunability, and the range of green polar solvents to stabilize them in, it creates new processing, photocatalysis and electrochemistry opportunities for pQDs.

We first synthesized CsPbBr₃ QDs with apolar ligands, (13) using Gemini bromide ligands with dodecyl tails, as these are short enough for efficient perovskite LEDs, (14) and a hexyl linker (referred to as 12–6–12 Gemini, Figure 1a and SI1), which has been proven to stabilize pNCs. (15) These ligands are ideal for CsPbBr₃ QDs, as the dicationic quaternary ammonium groups strongly bind to the perovskite surface while the Br^– aids in trap passivation. (15) As shown in Figure 1a, this results in pQDs that are dispersible in apolar solvents (toluene). The pQDs’ monodispersity is supported by High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM, Figures 1b and SI2), giving a size of 7.1 nm. These apolar pQDs exhibit a high photoluminescence quantum yield (PLQY) of 73% . To switch the pQDs’ polarity, we synthesized 12–6–12 Gemini ligands with hydroxyl terminated tails (12–6–12-OH Gemini, Figures 1c and SI3). The 12–6–12-OH Gemini-coated pQDs, now exhibiting a polar surface, can be dispersed in polar solvents like EtOH. These polar pQDs exhibit the same absorption spectrum, PLQY (74%) and size as their apolar counterparts (Figures1c,d and SI4). The high PLQY’s are also supported by a fast, single exponential 6.0 ns PL lifetime (Figure SI5). Nuclear Magnetic Resonance spectroscopy (NMR) on dissolved pQDs showed no residual apolar ligands after synthesis (Figure SI6) and a rough ligand coverage of 1 ligand/nm², matching with half of the reported 1.2–1.7 ligands/nm² for monodentate ligands, see SI and Figure SI7. (16) The EtOH-dispersed pQDs exhibit remarkable long-term chemical and colloidal stability in solution, with no degradation after two months (Figure SI8). Polar QD films fully preserve the absorption and single exponential PL lifetime of 6.1 ns, and exhibit good PL stability under continuous excitation (Figure SI9). The change in the pQDs’ polarity is supported by a biphasic mixture of mineral oil and EtOH (see TOC and Figure SI10); the apolar QDs reside exclusively in the mineral oil, and the polar pQDs in the EtOH layer.

Figure 1

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To demonstrate that the polar ligand shells are small (<1 nm), we performed dynamic light scattering (DLS, Figure 1e). We found that the hydrodynamic size of the polar pQDs in EtOH was about 8.5 nm, which, compared to the 7.1 nm from HAADF-STEM, suggests a ligand shell of about 0.7 nm. This matches the interparticle distance of 1.4 nm obtained via HAADF-STEM (Figure SI11). Although the ligand shell is small, the pQDs exhibit excellent colloidal stability, with solutions up to 10 mg/mL (Figure 1f). We also dispersed the polar pQDs in various alcohols, including isopropanol cyclopropanol and n-butanol (Figures 1g and SI12, see Supporting Note). These solvents are also nontoxic and green solvents for sustainable chemistry. (17) As QD dispersions are generally a few wt %, switching to green solvents will significantly lower their chemical footprint.

As shown in Figure 2a and SI13, the size of the polar pQDs is tunable (see SI), with the first excitonic peak tunable from 478 to 507 nm, corresponding to pQDs of roughly 5 to 10 nm. (13) The hydrodynamic sizes (Figure 2b) of these pQDs further confirm an average 1.9 nm difference in their respective core sizes, resulting in an average ligand shell thinner than 1 nm (Figure SI14). The composition of the polar pQDs can be tuned via anion exchange using ZnCl₂ and ZnI₂ (Figures 2c and SI15). (13) Further engineer of the ligand shell thickness and polarity can be achieved by changing the ligand tail length as well as using alkoxy and carboxylic acid tail groups (Figure SI16). Finally, to confirm the thin ligand shell, we performed quenching experiments with a nonbinding electron scavenger (p-benzoquinone), demonstrating a nearly order of magnitude fast quenching for 12–6–12-OH capped QDs compared to methyl-16-bromohexadecanoate Gemini and lecithin capped QDs (Figure SI17).

Figure 2

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To summarize, we demonstrate the synthesis of size- and composition-tunable pQDs dispersed in polar solvents by capping them with polar membrane like Gemini ligands. Although these ligand shells are thin (<1 nm), the pQDs exhibit a remarkable chemical and colloidal stability toward polar solvents. This opens opportunities for polar solvent processing pQDs optoelectronic devices using green polar solvents, orthogonal solvent processing, and apolar QD transport layers. Likewise, the polar pQDs will result in exciting new studies on photocatalysis and electrochemistry, which require pQDs to be stable in polar solvents.