Strong Bidentate Coordination for Surface Passivation and Ligand-Shell Engineering of Lead Halide Perovskite Nanocrystals in the Strongly Quantum-Confined Regime
Aaron Malinoski, Guoxiang Hu, Chen Wang
Abstract
The surface of lead halide perovskite nanocrystals (PNCs) is unique compared to that of conventional metal chalcogenide or pnictogenide semiconductor nanoparticles for their ionic character and dynamic ligand layers, which makes them unstable in stock solutions and hinders the development of surface engineering strategies. This work employs a chelating strategy to form stable coordination on the PNC surface. Through screening a series of heterocyclic aromatic carboxylates, we found the best ligand, picolinate (PIC), with an exceptional passivation effect on the surface traps of CsPbBr3 PNCs in the strongly quantum-confined regime, resulting in >0.8 photoluminescence quantum yields (PLQYs). The exciton lifetime in the passivated PNC approaches the radiative decay limit in various solvents. From a nuclear magnetic resonance (NMR) titration experiment, the binding affinity of PIC is estimated to be at least 15- to 30-fold stronger than the original ligand from synthesis. The NMR and Fourier transform infrared (FT-IR) spectroscopic data and first-principles calculations elucidate the bidentate nature of the PIC coordination at the surface Pb site and the co-adsorption of the ammonium-PIC ion pair. In apolar solvents, such as cyclohexane, the binding of PIC is stoichiometric to the available surface sites, suggesting the structure as a potent candidate for anchoring functional molecular structures to the PNC surface. In polar solvents, the strong affinity of PIC on the PNC surface provides protection for carrying out the precipitation-redissolution purification procedure that removes synthetic residual from the as-synthetic PNC samples. By modifying the purification procedure, we also develop a cation-exchange procedure to replace the original oleylammonium cation with desired structures that consist of an ammonium-anchoring group. Our results provide a direction for constructing strong interactions to protect the vulnerable surface of PNCs and pave the way for developing surface engineering strategies to functionalize these nanoparticles.