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Creators/Authors contains: "Lee, Brian"

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  1. Embedding percolating networks of nanoparticles (NPs) within polymers is a promising approach for mechanically reinforcing polymers and for introducing novel electronic, transport, and catalytic properties into otherwise inert polymers. While such networks may be obtained through kinetic assembly of unary system of NPs, the ensuing structures exhibit limited morphologies. Here, we investigate the possibility of increasing the diversity of NP networks through kinetic assembly of multiple species of NPs. Using lattice Monte Carlo simulations we show that networks obtained from co-assembly of two NP species of different sizes exhibit significantly more diverse morphology than those assembled from a single species.more »In particular, we achieved considerable variations in the particle spatial distribution, proportions of intra- and interspecies contacts, fractal dimension, and pore sizes of the networks by simply modulating the stoichiometry of the two species and their intra and inter-species affinities. We classified these distinct morphologies into “integrated”, “coated”, “leaved”, and “blocked” phases, and provide relevant phase diagrams for achieving them. Our findings are relevant to controlled and predictable assembly of particle networks for creating multifunctional composites with improved properties.« less
  2. Surface functionalization of nanoparticles with polymer grafts was recently shown to be a viable strategy for controlling the relative orientation of shaped nanoparticles in their higher-order assemblies. In this study, we investigated in silico the orientational phase behavior of coplanar polymer-grafted nanocubes confined in a thin film. We first used Monte Carlo simulations to compute the two-particle interaction free-energy landscape of the nanocubes and identify their globally stable configurations. The nanocubes were found to exhibit four stable phases: those with edge–edge and face–face orientations, and those exhibiting partially overlapped slanted and parallel faces previously assumed to be metastable. Moreover, themore »edge–edge configuration originally thought to involve kissing edges instead displayed partly overlapping edges, where the extent of the overlap depends on the attachment positions of the grafts. We next formulated analytical scaling expressions for the free energies of the identified configurations, which were used for constructing a comprehensive phase diagram of nanocube orientation in a multidimensional parameter space comprising of the size and interaction strength of the nanocubes and the Kuhn length and surface density of the grafts. The morphology of the phase diagram was shown to arise from an interplay between polymer- and surface-mediated interactions, especially differences in their scalings with respect to nanocube size and grafting density across the four phases. The phase diagram provided insights into tuning these interactions through the various parameters of the system for achieving target configurations. Overall, this work provides a framework for predicting and engineering interparticle configurations, with possible applications in plasmonic nanocomposites where control over particle orientation is critical.« less
  3. Rats exhibit ‘empathy’ making them a model to understand the neural underpinnings of such behavior. We show data consistent with these findings, but also that behavior and dopamine (DA) release reflects subjective rather than objective evaluation of appetitive and aversive events that occur to another. We recorded DA release in two paradigms: one that involved cues predictive of unavoidable shock to the conspecific and another that allowed the rat to refrain from reward when there were harmful consequences to the conspecific. Behavior and DA reflected pro-social interactions in that DA suppression was reduced during cues that predicted shock in themore »presence of the conspecific and that DA release observed on self-avoidance trials was present when the conspecific was spared. However, DA also increased when the conspecific was shocked instead of the recording rat and DA release during conspecific avoidance trials was lower than when the rat avoided shock for itself.« less