Scattering Studies of Ligand Effects on The Structure and Dynamics of Atomically Precise InP Clusters

Colloidal semiconductor nanocrystals demonstrate great promise as solution-processable materials for a multitude of applications in solar photovoltaics, biological imaging, light emitting devices and quantum photonics. Synthetic advances increasingly mitigate nanocrystal polydispersity and the inhomogeneous line broadening of colloidal ensembles. As a result, PL emission line widths of select nanocrystal ensembles are now reduced to that of a single particle, where homogeneous line broadening is the dominant contribution. These advances warrant detailed study of homogeneous broadening phenomena that arise from the local structural order of the nanocrystal, vibrational modes, surface chemistry and the steric considerations imposed by the ligand shell. High surface to volume ratios grant terminating ligands a nontrivial role in altering atomic structure, electronic structure, and transition dipole moments of the nanoparticle, thus influencing stability, reactivity and optoelectronic performance.¹ There lacks a robust understanding, however, of ligand influence on nanoparticle surface and core, thermal motions, and how these in turn impact desired properties.<br/>Atomically precise or magic-sized clusters (MSCs) are model systems to understand the effects of ligand-induced strain, particle rigidity and thermal motions on the optoelectronic properties of semiconductor nanoparticles. MSCs offer the advantages of guaranteed ensemble monodispersity, structures of often less than 100 atoms, and detailed understanding of the binding modes of terminating ligands.² These advantages facilitate atomistic modeling of experimental results and high levels of theory applied at reasonable computational cost.<br/>Here we present structural characterization of In37P20(O2CR)51 MSCs using a combination of X-ray and neutron scattering techniques. We employ temperature-dependent Pair Distribution Function (PDF) analysis from X-ray total scattering, a technique necessary to accurately describe the structure of this small, non-crystalline particle. We will discuss current atomistic modeling strategies of discrete nanoparticles, by which our PDF analysis reveals structural modulation of the In37P20 core by steric effects of the ligand shell. Further, we present results from quasielastic neutron scattering (QENS) describing rotation and precession motions of carboxylate ligands about the inorganic core. This combined characterization approach offers an encompassing view of ligand effects on the structure and dynamics of In37P20 MSCs and provides compelling insights alongside collaborative results from optical spectroscopy and computational modeling.<br/><br/>1. Nguyen, H. A. et al., Design Rules for Obtaining Narrow Luminescence from Semiconductors Made in Solution. <i>Chem. Rev.</i> <b>2023</b>, 123, 12, 7890–7952<br/>2. Gary, D. C. et al., Single-Crystal and Electronic Structure of a 1.3 nm Indium Phosphide Nanocluster. <i>J. Am. Chem. Soc.</i> <b>2016</b>,<b> </b>138, 1510–1513