Research Seminar - Christian Tanner

Self-assembly of colloidal nanocrystals (NCs) into superlattices (SLs), long-range ordered arrays, represents an appealing strategy toward the bottom-up design of hierarchically organized functional and multifunctional materials. SLs are typically formed using NCs with organic surface ligands whose insulating nature make it difficult to couple the electronic states of individual NCs within a SL, precluding the formation of collective states that resemble bands of conventional solids and limiting the related emergent technologies. My collaborators have recently developed a novel method to self-assemble strongly coupled metallic NC SLs from electrostatically stabilized colloidal NCs in electrolytic environments. While we can characterize the initial and final states of the assembly, little is known about the kinetic and dynamic processes that govern the transition from the colloid to SL states for this system and in general. In this talk, I will share how we probe the self-assembly of strongly coupled NC SLs in electrolytic environments in situ using small angle X-ray scattering (SAXS). The kinetics of SL coarsening, strain relaxation, and lattice contraction as a function of solvent ionic strength indicate quench-dependent self-assembly mechanisms. Deeper quenches lead to larger, more contracted, and more highly strained SLs with grain-merging limited coarsening due to deeper interaction potentials and the constraints of growing SLs form a finite bath of colloidal NCs. Dynamics of individual SL annealing events are obtained via X-ray photon correlation spectroscopy (XPCS) and reveal timescales of collective NC motion within SL grains. These results help us map out a generalized thermodynamic phase diagram for NCs with short-ranged interactions, which in addition to the kinetic and dynamic information helps inform on the most efficient routes toward the self-assembly of large, defect free SLs as well as new classes of strongly coupled semiconducting NC SLs.

Christian’s work in the Ginsberg group focuses on investigating the self-assembly of nanocrystals (NCs) into long-range, ordered arrays or superlattices (SLs). SLs are usually formed from NCs with insulating, organic surface ligands, which prevent the development of extended electronic states (bands) and limit energy carrier transport. Christian uses a variety of in situ X-ray scattering techniques to study the self-assembly of strongly coupled NC SLs that form in complex, electrolytic environments. Currently he is working with the Limmer group to map the thermodynamic, kinetic, and dynamic processes that govern self-assembly of metallic NC SLs, with the goal to inform new protocols for forming novel classes of strongly coupled semiconducting NC SLs.