Surface Structure of Lecithin-Capped Cesium Lead Halide Perovskite Nanocrystals Using Solid-State and Dynamic Nuclear Polarization NMR Spectroscopy.

Abstract

Inorganic colloidal cesium lead halide perovskite nanocrystals (NCs) encapsulated by surface capping ligands exhibit tremendous potential in optoelectronic applications, with their surface structure playing a pivotal role in enhancing their photophysical properties. Soy lecithin, a tightly binding zwitterionic surface-capping ligand, has recently facilitated the high-yield synthesis of stable ultraconcentrated and ultradilute colloids of CsPbX₃ NCs, unlocking a myriad of potential device applications. However, the atomic-level understanding of the ligand-terminated surface structure remains uncertain. Herein, we use a versatile solid-state nuclear magnetic resonance (NMR) spectroscopic approach, in combination with dynamic nuclear polarization (DNP) and atomistic molecular dynamics (MD) simulations, to explore the effect of lecithin on the core-to-surface structures of CsPbX₃ (X = Cl or Br) perovskites, sized from micron to nanoscale. Surface-selective (cross-polarization, CP) solid-state and DNP NMR (¹³³Cs and ²⁰⁷Pb) methods were used to differentiate the unique surface and core chemical environments, while the head-groups {trimethylammonium [-N(CH₃)₃⁺] and phosphate (-PO₄^-)} of lecithin were assigned via ¹H, ¹³C, and ³¹P NMR spectroscopy. A direct approach to determining the surface structure by capitalizing on the unique heteronuclear dipolar couplings between the lecithin ligand (¹H and ³¹P) and the surface of the CsPbCl₃ NCs (¹³³Cs and ²⁰⁷Pb) is demonstrated. The ¹H-¹³³Cs heteronuclear correlation (HETCOR) DNP NMR indicates an abundance of Cs on the NC surface and an intimate proximity of the -N(CH₃)₃⁺ groups to the surface and subsurface ¹³³Cs atoms, supported by ¹H{¹³³Cs} rotational-echo double-resonance (REDOR) NMR spectroscopy. Moreover, the ¹H-³¹P{²⁰⁷Pb} CP REDOR dephasing curve provides average internuclear distance information that allows assessment of -PO₄^- groups binding to the subsurface Pb atoms. Atomistic MD simulations of ligand-capped CsPbCl₃ surfaces aid in the interpretation of this information and suggest that ligand -N(CH₃)₃⁺ and -PO₄^- head-groups substitute Cs⁺ and Cl^- ions, respectively, at the CsCl-terminated surface of the NCs. These detailed atomistic insights into surface structures can further guide the engineering of various relevant surface-capping zwitterionic ligands for diverse metal halide perovskite NCs.