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Keys AS and Glotzer SC
How Do Quasicrystals Grow?
Phys. Rev. Lett. 99, 235503
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View Abstract
Using molecular simulations, we show that the aperiodic growth of quasicrystals is controlled by the
ability of the growing quasicrystal nucleus to incorporate kinetically trapped atoms into the solid phase with minimal rearrangement. In the system under investigation, which forms a dodecagonal quasicrystal, we show that this process occurs through the assimilation of stable icosahedral clusters by the growing quasicrystal. Our results demonstrate how local atomic interactions give rise to the long-range aperiodicity of quasicrystals.
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Singh C, Ghorai PK, Horsch MA, Jackson AM, Larson RG, Stellacci F, Glotzer SC
Entropy-Mediated Patterning of Surfactant-Coated Nanoparticles and Surfaces
Physical Review Letters 99, 226106 (2007)
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View Abstract
We perform atomistic and mesoscale simulations to explain the origin of experimentally observed
stripelike patterns formed by immiscible ligands coadsorbed on the surfaces of gold and silver nano-
particles. We show that when the conformational entropy gained via this morphology is sufficient,
microphase-separated stripelike patterns form. When the entropic gain is not sufficient, we instead predict bulk phase-separated Janus particles. We also show corroborating experimental results that confirm our simulational predictions that stripes form on flat surfaces as well as on curved nanoparticle surfaces.
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Ghorai, PK and Glotzer, SC
Molecular Dynamics Simulation Study of Self-Assembled Monolayers of Alkanethiol Surfactants on Spherical Gold Nanoparticles
J. Phys. Chem. C, 111, 15857-15862, 2007
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View Abstract
Atomistic molecular dynamics (MD) simulations of self-assembled alkanethiol monolayers are performed to
investigate the ligand shell organization of homoligand surfactants on spherical gold nanoparticle surfaces as
a function of temperature, nanoparticle size, and ligand tail length. At high temperature, we show that the
ligands orient randomly with respect to the surface normal with a small tilt angle. As the temperature decreases,
the molecules order and adopt a larger tilt angle. The effects of alkanethiol tail length and nanoparticle size
on the tilt structure are also significant. At low temperature, we find the equilibrium conformation of alkanethiols
obeys the crystallographic model, whereas at high temperature the continuous model is valid. The dependence
of tilt angle on different parameters and comparison with self-assembled monolayers on flat surfaces are also
discussed.
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Striolo, A, McCabe, CM, Cummings, PT, Chan, ER and Glotzer, SC
Aggregation of POSS Monomers in Liquid Hexane: A Molecular Simulation Study
J. Phys. Chem. B, 111(42); 12248-12256 (2007)
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Polyhedral oligomeric silsesquioxanes (POSS) are multifunctional molecules that can be employed as building
blocks to develop nanocomposite materials whose mechanical properties often improve upon those of traditional
polymeric materials. We report here molecular simulation results for the effective potential of mean force
between octamethyl POSS monomers and between POSS monomers in which one methyl group has been
substituted by a linear alkane chain of nine carbon atoms in liquid normal hexane at 300 and 400 K. The
results are discussed and compared to available data for the effective interactions between octamethyl POSS
monomers in normal hexadecane. Our results show that the effective short-ranged POSS-POSS attraction is
significantly weaker in hexane than it is in hexadecane, perhaps explaining why normal hexane is often the
solvent of choice for the preparation of POSS-containing materials. Additionally, we provide results for the
radial distribution functions between selected sites in the POSS monomers that can be used both to understand
the association between POSS monomers in solution and to parametrize coarse-grained simulation models.
Such models will be used to study the formation of POSS-containing supramolecular structures such as lamellae
or micelles that are currently not accessible by atomistic simulation and can be compared to experimental
observations.
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Glotzer SC and Solomon MJ
Anisotropy of building blocks and their assembly into complex structures
Nature Materials 6 (8): 557-562 AUG 2007
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A revolution in novel nanoparticles and colloidal building blocks has been enabled by recent breakthroughs in particle synthesis. These new particles are poised to become the ‘atoms’ and ‘molecules’ of tomorrow’s materials if they can be successfully assembled into useful structures. Here, we discuss the recent progress made in the synthesis of nanocrystals and colloidal particles and draw analogies between these new particulate building blocks and better-studied molecules and supramolecular objects. We argue for a conceptual framework for these new building blocks based on anisotropy attributes and discuss the prognosis for future progress in exploiting anisotropy for materials design and assembly.
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Chan ER, Striolo A, McCabe C, Cummings PT, Glotzer SC
Coarse-grained force field for simulating polymer-tethered silsesquioxane self-assembly in solution
J. Chem. Phys. 127, 114102 (2007)
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View Abstract
A coarse-grained model has been developed for simulating the self-assembly of nonyl-tethered polyhedral oligomeric silsesquioxane (POSS) nanoparticles in solution. A mapping scheme for groups of atoms in the atomistic molecule onto beads in the coarse-grained model was established. The coarse-grained force field consists of solvent-mediated effective interaction potentials that were derived via a structural-based coarse-graining numerical iteration scheme. The force field was obtained from initial guesses that were refined through two different iteration algorithms. The coarse-graining scheme was validated by comparing the aggregation of POSS molecules observed in simulations of the coarse-grained model to that observed in all-atom simulations containing explicit solvent. At 300 K the effective coarse-grained potentials obtained from different initial guesses are comparable to each other. At 400 K the differences between the force fields obtained from different initial guesses, although small, are noticeable. The use of a different iteration algorithm employing identical initial guesses resulted in the same overall effective potentials for bare cube corner bead sites. In both the coarse-grained and all-atom simulations, small aggregates of POSS molecules were observed with similar local packings of the silsesquioxane cages and tether conformations. The coarse-grained model afforded a savings in computing time of roughly two orders of magnitude. Further comparisons were made between the coarse-grained monotethered POSS model developed here and a minimal model developed in earlier work. The results suggest that the interactions between POSS cages are long ranged and are captured by the coarse-grained model developed here. The minimal model is suitable for capturing the local intermolecular packing of POSS cubes at short separation distances.
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Zhang ZL, Tang ZY, Kotov NA, Glotzer SC
Simulations and analysis of self-assembly of CdTe nanoparticles into wires and sheets
NANO LETTERS 7 (6): 1670-1675 JUN 2007
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Recent experiments have reported the self-assembly of TGA- and DMAET-stabilized CdTe nanoparticles (NPs) into wires and sheets, respectively, depending upon the stabilizer used. We develop a mesoscale model based on quantum mechanical calculations and perform Monte Carlo simulations of these NPs to elucidate the conditions under which these two structures will form. We show that consideration of NP shape, directional attraction, and electrostatic interactions is key to determining the anisotropy of the NP-NP interaction and final self-assembled structures.
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Chen T, Glotzer SC
Simulation studies of a phenomenological model for elongated virus capsid formation
PHYSICAL REVIEW E 75 (5): Art. No. 051504 Part 1, MAY 2007
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We study a phenomenological model in which the simulated packing of hard, attractive spheres on a prolate spheroid surface with convexity constraints produces structures identical to those of prolate virus capsid structures. Our simulation approach combines the traditional Monte Carlo method with a modified method of random sampling on an ellipsoidal surface and a convex hull searching algorithm. Using this approach we identify the minimum physical requirements for nonicosahedral, elongated virus capsids, such as two aberrant flock house virus particles and the prolate prohead of bacteriophage phi 29, and discuss the implication of our simulation results in the context of recent experimental findings. Our predicted structures may also be experimentally realized by the evaporation-driven assembly of colloidal spheres under appropriate conditions.
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Chen T, Zhang Z-L, Glotzer SC
Simulation studies of the self-assembly of cone-shaped particles
LANGMUIR 23 (12): 6598-6605 JUN 5 2007
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We investigate the self-assembly of anisotropic cone-shaped particles decorated by ringlike attractive "patches". In a recent paper, we demonstrated that the self-assembled clusters, which arise due to the conical particle's anisotropic shape combined with directional attractive interactions, are precise for certain cluster sizes, resulting in a precise packing sequence of clusters of increasing sizes with decreasing cone angles (Chen et al. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 717-722). Here we explore the dependence of cluster packing on the cone angle and cooling rate and discuss the "stability" and "metastability" of the resulting structures as well as polymorphism of non-"magic-number" clusters. We investigate large clusters of cones and discuss the implication of our simulation results in the context of the Israelachvili packing rule for surfactants and a recent geometrical packing analysis on hard cones in the limit of large numbers of cones.
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Iacovella CR, Keys AS, Horsch MA, Glotzer SC
Icosahedral packing of polymer-tethered nanospheres and stabilization of the gyroid phase
PHYSICAL REVIEW E 75 (4): Art. No. 040801 Part 1, APR 2007
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We present results of simulations that predict the phases formed by the self-assembly of model nanospheres functionalized with a single polymer "tether," including double gyroid, perforated lamella, and crystalline bilayer phases. We show that microphase separation of the immiscible tethers and nanospheres causes confinement of the nanoparticles, which promotes local icosahedral packing that in turn stabilizes the gyroid. We present a new metric for determining the local arrangement of particles based on spherical harmonic "fingerprints," which we use to quantify the extent of icosahedral ordering.
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Keys AS, Abate AR, Glotzer SC, Durian DJ
Measurement of growing dynamical length scales and prediction of the jamming transition in a granular material
NATURE PHYSICS 3 (4): 260-264 APR 2007
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Supercooled liquids and dense colloidal suspensions exhibit anomalous behaviour known as 'spatially heterogeneous dynamics' (SHD), which becomes increasingly pronounced as the system approaches the glass transition(1-3). Recently, the observation of SHD in confined granular packings under slow shear near the onset of jamming has bolstered speculation that the two transitions are related(4-6). Here, we report measurements of SHD in a system of air-driven granular beads, as a function of both density and effective temperature. On approach to jamming, the dynamics becomes progressively slower and more spatially heterogeneous. The rapid growth of timescales and dynamical length scales characterizing the heterogeneities can be described both by mode-coupling theory(7) and the Vogel - Tammann Fulcher (VTF) equation(8), such as in glass-forming liquids. The value of the control variable at the VTF transition coincides with point J (refs 9,10), the random close-packed jamming density at which all motion ceases, in analogy to a zero-temperature ideal glass transition. Our findings demonstrate further universality of the jamming concept and provide a significant step forward in the quest for a unified theory of jamming in disparate systems.
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Zhang X, Zhang Z-L, Glotzer SC
Simulation study of cyclic-tethered nanocube self-assemblies: effect of tethered nanocube architectures
NANOTECHNOLOGY 18 (11): Art. No. 115706 MAR 21 2007
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Self-assembly of functionalized nano building blocks (NBBs) is a promising avenue for 'bottom-up' nanomaterials design. Experimental studies on functionalized polyhedral oligomeric silsesquioxane (POSS) nanocubes have revealed a wide variety of nanostructures from their assemblies. Our previous simulation studies have reproduced some of these nanostructures and predicted unusual phase behaviours imparted by the unique geometry of the nanocubes and their close packing patterns. Recent experiments further inspire us to explore the effects of tether topologies on functionalized nanocube self-assemblies. We use a simplified model and perform stochastic molecular dynamics simulations to map the morphological phase diagrams of cyclic tethered nanocubes with varying tether topology, tether number, and tether placement. Our results illustrate that the steric influence of the tethers can be manipulated to confer precise control over the self-assembled nanostructures and the phase behaviour. The novel controlling factors investigated in our study suggest new opportunities in controlling functionalized NBB self-assemblies.
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Zhang X, Zhang Z-L, Glotzer SC
Simulation study of dipole-induced self-assembly of nanocubes
JOURNAL OF PHYSICAL CHEMISTRY C 111 (11): 4132-4137 MAR 22 2007
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ecent experiments have demonstrated that nanoparticles with dipoles can self-assemble into interesting one-dimensional and two-dimensional nanostructures. In particular, nanocubes with dipoles are found to form straight nanowires and nanorings with potential applications for nanodevices. In this paper, we use a minimal model to study dipole-induced self-assembly of nanocubes with varying dipole directions, dipole strengths and both with and without face-face attractions arising from dispersive or solvophobic interactions. We reproduce the structures observed in experiments and illustrate that the self-assembled morphologies are dictated by the head-to-tail alignment of the dipoles, the orientation of the dipoles within the cubes, and the face-to-face packing of the nanocubes. Our results show how the self-assembly of dipolar nanocubes differs from that of dipolar spheres in which the only anisotropy is the dipole itself and how system parameters can be manipulated to control the assembled morphologies and the phase behavior. Our simulation model, which uses the plane separating algorithm for efficient detection of nanoparticle overlaps, can be utilized to investigate the self-assembly of other smooth, convex polyhedral-shaped nanoparticles to facilitate novel nanomaterials design.
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Chen T, Zhang Z-L, Glotzer SC
A precise packing sequence for self-assembled convex structures
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA 104 (3): 717-722 JAN 16 2007
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Molecular simulations of the self-assembly of cone-shaped particles with specific, attractive interactions are performed. Upon cooling from random initial conditions, we find that the cones self-assemble into clusters and that clusters comprised of particular numbers of cones (e.g., 4-17, 20, 27, 32, and 42) have a unique and precisely packed structure that is robust over a range of cone angles. These precise clusters form a sequence of structures at specific cluster sizes (a "precise packing sequence") that for small sizes is identical to that observed in evaporation-driven assembly of colloidal spheres. We further show that this sequence is reproduced and extended in simulations of two simple models of spheres self-assembling from random initial conditions subject to convexity constraints, including an initial spherical convexity constraint for moderate- and large-sized clusters. This sequence contains six of the most common virus capsid structures obtained in vivo, including large chiral clusters and a cluster that may correspond to several nonicosahedral, spherical virus capsids obtained in vivo. Our findings suggest that this precise packing sequence results from free energy minimization subject to convexity constraints and is applicable to a broad range of assembly processes.
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Brick CM, Chan ER, Glotzer SC, Marchal JC, Martin DC
Self-lubricating nano-ball-bearings
ADVANCED MATERIALS 19 (1): 82- JAN 8 2007
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View Abstract
Friedel-Crafts alkylation of octaphenyl-octasilsesquioxane (OPS) introduces a hydrocarbon layer around the rigid OPS core (see figure). Although periodic order is maintained even in the melt, as determined from X-ray diffraction measurements, this layer provides access to low-melting solids for butyl, hexyl, octyl, and decyl substituents. Because the alkyl groups interdigitate, the butyl and hexyl melts are stable to temperatures of ca. 400 degrees C, which is ca. 100 degrees C higher than the octyl and decyl compounds.