(127h) Plasmonic Coupling in Self-Assembled Nanocrystal Gels and Superlattices

Inorganic nanocrystals have remarkable plasmonic properties that can be tuned through nanocrystal size, shape, and chemical composition. The plasmonic response of closely spaced nanocrystals couple together, so their collective optoelectronic properties are also sensitive to structure. Materials self-assembled from colloidal dispersions of plasmonic nanocrystals take advantage of this structure-dependent behavior and are promising for scalable and highly tunable functional materials, particularly as photovoltaics, catalysts, and electrolytes. Computer simulations and statistical thermodynamics models have helped connect how the microscopic details of the nanocrystal building blocks and their environment affect the assembled morphologies, but probing the effective plasmonic properties remains a major bottleneck. The necessary optical calculations require expensive numerical solutions to many-bodied systems of equations and are therefore limited to configurations of only a handful of particles, which are not representative of large-scale structural features and heterogeneities. This limitation has hindered the rational design of self-assembled plasmonic materials. To address these shortcomings, we have developed computational tools to rapidly determine the optical response of nanocrystal materials, allowing for tens of thousands of particles to be probed at a time. We use these tools to investigate plasmonic coupling in two important classes of self-assembled materials: disordered nanocrystal gels and randomly mixed binary superlattices. The gels are assembled in molecular dynamics simulations using linker molecules to mediate bonding between nanocrystals. Gelation changes the optical response of the nanocrystals, but in qualitatively different ways depending on the physical characteristics of the nanocrystals and the linkers. Elucidating this dependence helps understand recent experimental efforts investigating plasmonic nanocrystal gels. For binary superlattices, we show how plasmonic coupling emerges excess of that expected from simple weighted mixing of pure species spectra. This excess coupling can be leveraged to create “doped” superlattices, analogous to atomic doping, by substituting dopant nanocrystals into a host superlattice to tune the effective optoelectronic properties.