Surface-Mediated Interactions and Self-Assembly Pathways of Colloidal Nanocrystals

Colloidal nanocrystals (NCs) stabilized by compact, inorganic ligands offer a promising route toward strongly coupled, electronically functional superlattices. However, despite their potential, the colloidal behavior and interparticle interactions of electrostatically stabilized sub‑10 nm NCs remain poorly understood, limiting the rational design of their assemblies. In this dissertation, I investigate the dispersion thermodynamics, interparticle forces, and self‑assembly pathways of PbS NCs functionalized with thiostannate metal chalcogenide complex (MCC) ligands (PbS-Sn₂S₆^4-). Using small‑angle X‑ray scattering (SAXS), I show that these NCs exhibit pronounced deviations from ideal solution behavior, governed by long‑range electrostatic repulsion that contrasts sharply with the short‑range steric interactions of oleate‑capped PbS NCs. Screening these interactions with multivalent electrolytes reduces the repulsive length scale, enabling controlled transitions from amorphous films to crystalline domains during drying and spin‑coating. Second virial coefficients extracted from SAXS measurements quantify how ligand chemistry, solvent environment, and NC size shape the pair potentials that ultimately dictate colloidal stability and assembly.

Building on this understanding of interparticle interactions, I demonstrate that PbS NCs capped with strongly charged MCC ligands (Sn₂S₆^4-, AsS₄^3- etc.) self‑assemble into all‑inorganic superlattices with both long‑range translational order and atomic‑lattice orientational alignment. Structural characterization reveals an unexpected edge‑to‑edge configuration of NCs, and numerical simulations show that this orientational order is thermodynamically stabilized by many‑body ion correlations within the dense electrolyte environment. Remarkably, these superlattices exhibit reversible oriented attachment, where unwashed PbS-Sn₂S₆^4- NC superlattices containing incorporated K₃AsS₄ can be fully disassembled back into their original colloidal NCs, preserving their size and shape distributions. Partial removal of the incorporated electrolyte during washing induces permanent lattice contraction and epitaxial neck formation, rendering the assemblies irreversible. This behavior highlights a delicate interplay between ligand chemistry, electrolyte incorporation, and lattice mechanics that governs the transition between reversible and permanent oriented attachment.

Together, these findings advance the fundamental understanding of electrostatically stabilized NCs and establish a framework for designing nanocrystal solids with tunable order, coupling, and reversibility. The discovery of atomically aligned yet reconfigurable superlattices opens new opportunities for dynamic, stimuli‑responsive, and defect‑correcting materials with potential applications in adaptive optoelectronics, self‑healing architectures, and next‑generation nanostructured devices.