Temperature-Dependent ²⁰⁷Pb Nuclear Magnetic Resonance Spectroscopy: A Spectroscopic Probe for the Local Electronic Structure of Lead Halide Perovskites

Lead halide perovskites (LHPs) have garnered considerable interest, owing to their advantageous optoelectronic properties and ease of synthesis. However, understanding their intricate structure-property relationships remains challenging, for both bulk and nanoscale forms, such as colloidal quantum dots (QDs). In this study, in addition to conventional characterization by X-ray diffraction and optical absorption, we show that variable temperature solid-state nuclear magnetic resonance spectroscopy, complemented by computational modeling, provides unique insight into the local coordination geometry and electronic structure of LHPs in relation to the moderate change in composition or materials morphology. For CsPbBr₃ and FAPbBr₃ in the form of QDs and bulk, we uncover nuanced disparities between their orthorhombic and on-average cubic structures, respectively, reflected in their temperature-dependent ²⁰⁷Pb chemical shifts and optical band gaps. Specifically, the mode of thermal expansion, be it the increase of the Pb-Br-Pb angles in the orthorhombic structure or the elongation of the Pb-Br bonds in a cubic lattice, gives rise to an increase of the chemical shift by 0.63 or 1.53 ppm/K and optical band gap by 0.18 or 0.66 meV/K, respectively. Identifying the chemical shift as a spectroscopic descriptor, in particular as a lattice ruler, is highly instrumental also for LHP QDs, capturing the difference between CsPbBr₃ and FAPbBr₃. In a broader perspective, establishing relations across spectroscopic and structural descriptors for diverse LHP compositions and morphologies paves the way for informed design strategies in next-generation optoelectronic devices.