Reversible Temperature-Induced Structural Transformations in PbS Nanocrystal Superlattices

摘要

The precise structure of nanocrystal (NC) solids is a delicate balance between energetic and entropic interactions. Studying the temperature-dependent structure of NC solids can inform understanding of these interactions as well as the stability of different self-assembled phases. Using synchrotron-based grazing-incidence small- and wide-angle X-ray scattering (GISAXS/GIWAXS), we measured the distortion of colloidal PbS NC superlattices (SLs) between room temperature and cryogenic temperatures. Experimental data show that regardless of the NC size, dispersity, or bound/free ligand populations, the SL reversibly distorts by contracting along one axis and expanding along two axes. Several thermal markers of the surface-bound ligands delineate this distortion pathway through the typical cubic-family SL phase diagram: freezing point, glass transition, and shift in gauche ligand-backbone conformer defects. Hysteresis in the pathway is observed and attributed to a ligand order/disorder transition, presenting a thermodynamic barrier upon cooling which is absent during subsequent reheating. Unexpected, nonmonotonic trends of the unit cell volume upon cooling are observed. This is attributed to a mutual ligand-induced increase of the ligand-shell thickness for large core diameters (low curvature) with high ligand coverages (low spacing between binding groups) as confirmed by molecular dynamics (MD) simulations of single NCs. These results highlight the central importance of the ligands in dictating SL structure and demonstrate the necessity of full characterization for understanding temperature-dependent structural transitions. This fundamental understanding also serves as a potential route for engineering chosen lattice polymorphs at a specific temperature.