Abstract:
Lead halide perovskites such as CsPbBr3 in the form of colloidal nanocrystals possess amenable electronic and optical properties that find a wide variety of applications in optoelectronics and catalysis. However, the susceptibility of such nanocrystals towards surface defect formation during purification adversely affects their photochemical properties. A common strategy to overcome this problem is the use of long-chain charged organic ligands such as amines, carboxylates, and phosphonates for surface passivation. In spite of intense ongoing research focused towards understanding ligand binding on nanocrystal surfaces using techniques such as photoluminescence spectroscopy, infrared spectroscopy, and solution state nuclear magnetic resonance (NMR) spectroscopy, an experimentally determined atomic-level description of the ligand-binding mechanism on the perovskite surface sites still does not exist. In this project, we have aimed to characterize ligand-surface interactions on CsPbBr3 nanocrystals at the atomic level using dynamic nuclear polarization (DNP) enhanced solid- state NMR complemented by density functional theory (DFT) calculations. From 1H-13C CP- HETCOR and 13-C15N TEDOR experiments, we have obtained the first reported experimental evidence of one-on-one cooperative interactions between the ammonium and carboxylate functional groups of the ligands, characterized by interaction distances of 3 – 6 Å obtained by spin dynamics simulations. 3D 13C-15N TEDOR experiments exhibit at least three distinct carboxylate environments and two distinct ammonium environments, signifying multiple conformations and binding modes. We have analyzed the 15N and 13C NMR spectra of the ligand functional groups by ab-initio chemical shift calculations of adsorption and substitution modes of ligand interaction with the different available nanocrystal facets. The computed binding energies for the different binding modes show that Cs substitution by the ammonium functional group of the protonated amine ligand is the most favored. DFT-calculated chemical shifts for such conformations correlate with the NMR spectra at low temperature. Co- substitution studies of the two types of ligands also support the TEDOR analysis. The results of this project provided much needed clarity regarding ligand-surface interactions in perovskite nanocrystals that will be very helpful in ligand engineering aimed towards improving surface passivation of more environment-friendly perovskite nanomaterials, as well as enhancing nanocrystal properties for applications such as LEDs and photocatalysis.
