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2005
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Glotzer SC, Horsch MA, Iacovella CR, Zhang ZL, Chan ER, Zhang X
Self-assembly of anisotropic tethered nanoparticle shape amphiphiles
CURRENT OPINION IN COLLOID = INTERFACE SCIENCE 10 (5-6): 287-295 DEC 2005
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The varied and exotic shapes of new nanoscale organic and inorganic building blocks provide new opportunities to engineer materials possessing specific functionality and physical properties dictated by the unique packings of these particles. We briefly review some of the current strategies for inducing the self-assembly of these building blocks focusing on one strategy in particular-the attachment of tethers to the building blocks at precise locations to create tethered nanoparticle "shape amphiphiles". We use Computer simulation to demonstrate that the resulting anisotropy imparted to nanocrystals or nanocolloids by the tethers can be used to encode simple design rules into the building blocks that ultimately result in a unique self-assembled structure. We present a general classification scheme for tethered nanoparticles wherein the anisotropy of a shape amphiphile is described by a vector comprised of one or more axes each describing a measure of anisotropy. (c) 2005 Elsevier Ltd. All rights reserved.
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Zhang ZL, Keys AS, Chen T, Glotzer SC
Self-assembly of patchy particles into diamond structures through molecular mimicry
LANGMUIR 21 (25): 11547-11551 DEC 6 2005
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Fabrication of diamond structures by self-assembly is a fundamental challenge in making three-dimensional photonic crystals. We simulate a system of model hard particles with attractive patches and show that they can self-assemble into a diamond structure from an initially disordered state. We quantify the extent to which the formation of the diamond structure can be facilitated by "seeding" the system with small diamond crystallites or by introducing a rotation interaction to mimic a carbon-carbon antibonding interaction. Cur results suggest patchy particles may serve as colloidal "atoms" and "molecules" for the bottom-up self-assembly of three-dimensional crystals.
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Zhang X, Chan ER, Glotzer SC
Self-assembled morphologies of monotethered polyhedral oligomeric silsesquioxane nanocubes from computer simulation
JOURNAL OF CHEMICAL PHYSICS 123 (18): Art. No. 184718 NOV 8 2005
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Self-assembly of functionalized nanoscale building blocks is a promising strategy for "bottom-up" materials design. Recent experiments have demonstrated that the self-assembly of polyhedral oligomeric silsesquioxane (POSS) "nanocubes" functionalized with organic tethers can be utilized to synthesize novel materials with highly ordered, complex nanostructures. We have performed molecular simulations for a simplified model of monotethered POSS nanocubes to investigate systematically how the parameters that control the assembly process and the resulting equilibrium structures, including concentration, temperature, tether lengths, and solvent conditions, can be manipulated to achieve useful structures via self-assembly. We report conventional lamellar and cylindrical structures that are typically found in block copolymer and surfactant systems, including a thermotropic order-order transition, but with interesting stabilization of the lamellar phase caused by the bulkiness and cubic geometry of the POSS nanocubes.
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Iacovella CR, Horsch MA, Zhang Z, Glotzer SC
Phase diagrams of self-assembled mono-tethered nanospheres from molecular simulation and comparison to surfactants
LANGMUIR 21 (21): 9488-9494 OCT 11 2005
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We perform Brownian dynamics simulations on model 3-D systems of mono-tethered nanospheres (TNS) to study the equilibrium morphologies formed by their self-assembly in a selective solvent. We predict that in contrast to flexible amphiphiles the nanospheres are locally ordered and there is an increase in the local order with an increase in concentration or relative nanoparticle diameter. We present the temperature vs concentration phase diagram for a system of TNS and propose a dimensionless scaling factor F-v (headgroup volume/tether volume) that allows a comparison between the morphologies formed from TNS and traditional surfactants.
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Gebremichael Y, Vogel M, Bergroth MNJ, Starr FW, Glotzer SC
Spatially heterogeneous dynamics and the Adam-Gibbs relation in the Dzugutov liquid
JOURNAL OF PHYSICAL CHEMISTRY B 109 (31): 15068-15079 AUG 11 2005
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We perform molecular dynamics simulations of a one-component glass-forming liquid and use the inherent structure formalism to test the predictions of the Adam-Gibbs (AG) theory and to explore the possible connection between these predictions and spatially heterogeneous dynamics. We calculate the temperature dependence of the average potential energy of the equilibrium liquid and show that it obeys the Rosenfeld-Tarazona T-3/5 law for low temperature T, while the average inherent structure energy is found to be inversely proportional to temperature at low T, consistent with a Gaussian distribution of potential energy minima. We investigate the shape of the basins around the local minima in configuration space via the average basin vibrational frequency and show that the basins become slightly broader upon cooling. We evaluate the configurational entropy S-conf, a measure of the multiplicity of potential energy minima sampled by the system, and test the validity of the AG relation between S-conf and the bulk dynamics. We quantify the dynamically heterogeneous motion by analyzing the motion of particles that are mobile on short and intermediate time scales relative to the characteristic bulk relaxation time. These mobile particles move in one-dimensional "strings", and these strings form clusters with a well-defined average cluster size. The AG approach predicts that the minimum size of cooperatively rearranging regions (CRR) of molecules is inversely proportional to Sconf, and recently (Phys. Rev. Lett. 2003, 90, 085506) it has been shown that the mobile-particle clusters are consistent with the CRR envisaged by Adam and Gibbs. We test the possibility that the mobile-particle strings, rather than clusters, may describe the CRR of the Adam-Gibbs approach. We find that the strings also follow a nearly inverse relation with S-conf. We further show that the logarithm of the time when the strings and clusters are maximum, which occurs in the late-beta-relaxation regime of the intermediate scattering function, follows a linear relationship with 1/TSconf, in agreement with the AG prediction for the relationship between the configurational entropy and the characteristic time for the liquid to undergo a transition to a new configuration. Since strings are the basic elements of the clusters, we propose that strings are a more appropriate measure of the minimum size of a CRR that leads to configurational transitions. That the cluster size also has an inverse relationship with S-conf may be a consequence of the fact that the clusters are composed of strings.
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Horsch MA, Zhang ZL, Glotzer SC
Self-assembly of polymer-tethered nanorods
PHYSICAL REVIEW LETTERS 95 (5): Art. No. 056105 JUL 29 2005
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We present results of molecular simulations that predict the phases formed by self-assembly of nanorods functionalized by a polymer "tether." Microphase separation of the immiscible tethers and rods coupled with the liquid crystal ordering of the rods induces the formation of a cubic phase, a smectic C phase, a tetragonally perforated lamellar phase, and a honeycomb phase; the latter two have been observed experimentally but have not been predicted. We also predict a new phase-a racemic mixture of hexagonally ordered chiral cylinders that self-assemble from these achiral building blocks.
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Chan ER, Zhang X, Lee CY, Neurock M, Glotzer SC
Simulations of tetra-tethered organic/inorganic nanocube-polymer assemblies
MACROMOLECULES 38 (14): 6168-6180 JUL 12 2005
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We perform molecular simulations to study the self-assembly of tetratethered nanoparticles with a cubic geometry. We develop a minimal model of the tethered nanoscale building block (NBB) to represent a polyhedral oligomeric silsesquioxane (POSS) molecule with polymeric functionalities based on information about the molecular structure and interactions obtained from ab initio density functional theory calculations. Using this model, we explore the rich nanostructures formed from self-assembly of the NBBs and make analogies with the morphologies observed in block copolymer, surfactant, and liquid crystalline systems. On the basis of the assembled structures produced and determination of the location and nature of the order-disorder transitions in the system, we propose phase diagrams to describe the behavior of these molecules. We find that qualitative similarities exist between the phase diagrams for the tetratethered NBBs and those for block copolymer and surfactant systems.
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Bergroth MNJ, Vogel M, Glotzer SC
Examination of dynamic facilitation in molecular dynamics simulations of glass-forming liquids
JOURNAL OF PHYSICAL CHEMISTRY B 109 (14): 6748-6753 APR 14 2005
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Using data from molecular dynamics computer simulations of the one-component Dzugutov liquid and of BKS silica in metastable equilibrium supercooled states, we examine ideas introduced by Garrahan and Chandler (GC) in their dynamic facilitation (DF) model of the glass transition. Utilizing a recently introduced measure of DF, we find that DF is important for particle motion in both the supercooled Dzugutov liquid and in the BKS silica melt, that mobility propagates continuously, and that this effect becomes increasingly pronounced with decreasing T. We show that, in both systems, dynamic facilitation is strongest on the time scale of the late-beta relaxation, where clusters of highly mobile neighboring particles escaping from their cages are largest and, except for the silicon atoms in BKS silica, stringlike motion is most prominent. By comparing the two systems, we show that the temperature dependence of one measure of DF as the mode-coupling temperature is approached from high temperature is similar, once the temperature dependence of the structural relaxation time in each system is scaled out.