Correlated Vibrational and Electronic Signatures of Surface Disorder in CsPbBr₃ Nanocrystals.

Abstract

Lead halide perovskite nanocrystals have emerged as promising candidates for classical light-emitting devices and single-photon sources, owing to their high photoluminescence quantum yield, narrow emission line width and tunable emission. Judicious choice of ligands to passivate nanocrystal surfaces has proven to be critical to the structural stability and optoelectronic performance of such nanocrystals. While many ligands have been deployed, the resulting quality of the nanocrystal surface can be difficult to assess directly. Here, we demonstrate ultralow frequency Raman spectroscopy as a powerful tool to resolve surface-sensitive changes in size and ligand choice in perovskite nanocrystals. By investigating a size series of CsPbBr₃ nanocrystals from the strong (5 nm) to the weak (28 nm) confinement range, we show that the line width of Raman-active modes provides a highly selective metric for surface disorder and quality. We further examine a series of 28 nm diameter nanocrystals with four different zwitterionic ligands, unravelling clear links between varying steric effects and surface quality evident from Raman analysis. Photoluminescence and THz photoconductivity probes reveal an evident correlation of charge-carrier dynamics and radiative emission yields with ligand chemistry and surface quality inferred from phonon broadening. We further show that surface defects preferentially trap hot charge carriers, which affects exciton stability and radiative emission yields. Overall, our approach offers powerful insights into optimizing nanocrystal-ligand boundaries to enhance the performance of nanoscale quantum light sources and optoelectronic devices.