Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session P17: Self-Assembly II |
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Sponsoring Units: GSOFT Chair: Linda Hirst, University of California, Merced Room: 276 |
Wednesday, March 15, 2017 2:30PM - 2:42PM |
P17.00001: Nanoparticle microstructures templated by liquid crystal phase-transition dynamics Sheida T Riahinasab, Ahmed Elbaradei, Amir Keshavarz, Benjamin J. Stokes, Linda S. Hirst We report on the use of the isotropic to nematic phase transition of liquid crystals as a new mechanism for sorting and assembling nanoparticles into micro-scale capsules, networks, and foams. Microstructure length scales and morphologies are influenced by the dynamics of the phase transition, and therefore can be controlled by the relevant thermodynamic parameters including transition cooling rates and particle concentration in the host phase. To achieve stable microstructures a mesogenic ligand on the nanoparticle surface provides attractive short-range interactions between adjacent nanoparticles. [Preview Abstract] |
Wednesday, March 15, 2017 2:42PM - 2:54PM |
P17.00002: Formation and Characterization of Quantum Dot Clusters formed via Phase Front Propagation in Liquid Crystal Droplets Charles Melton, Linda Hirst Liquid crystal nano-composites are materials in which suspended nanoparticle organization can be tuned via an anisotropic fluid matrix. The use of a liquid crystal as a host phase allows for unique self-assembly methods that use topological defects and elastic deformation of the nematic director to sort and organize nano-scale particles. A current challenge, however, is controlling formation location in a continuous medium. Liquid crystal droplets pose a solution to this challenge, as topological defects are formed at defined location in spherical geometries. We explore the competition between cluster location at ground state defect locations and cluster location via phase change dynamics. We show that these two location control parameters form quantum dot cluster and quantum dot hollow shells at repeatable locations in the droplet. We also use the droplet geometry to study how the quantum dot clusters are packing in 3D space. [Preview Abstract] |
Wednesday, March 15, 2017 2:54PM - 3:06PM |
P17.00003: Structure and Stability of Self-Assembling Nanoparticle Dispersions Vijay D. Shah, Alan R. Denton, Samuel L. Brown, Erik K. Hobbie Nanoparticles have attracted much attention because of their unusual electronic and optical properties, e.g., photoluminescence, which are intermediate between those of bulk and molecular materials. Applications to thin-film photovoltaic devices are facilitated by self-assembly of nanoparticles into a crystalline array (superlattice). By coating nanoparticles with ligand brushes, bulk dispersions can be sterically stabilized against aggregation induced by van der Waals forces. Creating equilibrium superlattices in the laboratory, however, remains a practical challenge. To investigate the dependence of superlattice stability on interparticle forces, we perform molecular dynamics and Monte Carlo simulations to model bulk dispersions of silicon and silver nanoparticles, governed by effective pair potentials that combine short-range steric and long-range van der Waals forces. From particle configurations, we compute radial distribution functions and static structure factors to explore equilibrium structure and phase stability. By varying ligand length and bulk density, we characterize the tendency of nanoparticles to self-assemble into ordered arrays or amorphous clusters. Our results may guide the design of future experiments and fabrication of superlattices. [Preview Abstract] |
Wednesday, March 15, 2017 3:06PM - 3:18PM |
P17.00004: Macroscopic and tunable PEG-gold nanoparticle superlattices at aqueous interfaces Wenjie Wang, Honghu Zhang, Surya Mallapragada, Alex Travesset, David Vaknin To address the challenge of assembling nanoparticles (NPs) into superlattices in controlled manner, we have developed a simple, robust protocol to assemble AuNPs functionalized with synthetic polymers (i.e., polyethylene glycol, PEG) into two-dimensional (2D), macroscopic superlattices with tunable lattice parameters. Here, we report on the formation of 2D superlattices of PEG-AuNPs at aqueous surfaces characterized with surface-sensitive, grazing-incidence small-angle x-ray scattering (GISAXS) and x-ray reflectivity (XR) methods. We show that the presence of salts (i.e., potassium carbonate) in the aqueous solutions drives PEG-AuNPs to the air-water interface and form a hexagonal superlattice beyond a threshold concentration.~ Varying the nanoparticle concentration and PEG length can further regulate the lattice parameters.~ We have built a model based on polymer-brush theory that can account for the experimental observations and provide more insight on the driving force for the interfacial self-assembly and crystallization of the PEG-AuNPs. The approach we developed paves the way for 2D macroscopic and tunable supercrystal formation and can be applied to other nanoscale building blocks with various functionalities and to the formation of 3D structures. [Preview Abstract] |
Wednesday, March 15, 2017 3:18PM - 3:30PM |
P17.00005: Self-assembling Gold Nanoparticle Monolayers in a Three-phase System -- Overcoming Ligand Size Limitations Guang Yang, Jagjit Nanda, Boya Wang, Gang Chen, Daniel T. Hallinan Jr. An effective self-assembly technique was developed to prepare centimeter-scale monolayer gold nanoparticle (Au NP) films of long-range order with hydrophobic ligands. Aqueous Au NPs were entrapped in the organic/aqueous interface where the Au NP surface was in situ modified with different types of amine ligands, including amine-terminated polystyrene. The Au NPs then spontaneously relocated to the air/water interface to form an NP monolayer. The spontaneous formation of an Au NP film at the organic/water interface was due to the minimization of the system Helmholtz free energy. Self-assembled Au NP films has a hexagonal close packed structure. The interparticle spacing was dictated by the amine ligand length. Thus-assembled Au NP monolayers exhibit tunable surface plasma resonance and excellent spacial homogeneity of surface-enhanced Raman-scattering. The ``air/water/oil'' self-assembly method developed in this study not only benefits the fundamental understanding of NP ligand conformations, but is also promising to scale up the manufacture of plasmonic nanoparticle devices with precisely designed optical properties. [Preview Abstract] |
Wednesday, March 15, 2017 3:30PM - 3:42PM |
P17.00006: Fabrication of non-hexagonal close packed colloidal array on a substrate by transfer Meneka Banik, Rabibrata Mukherjee Self-organized colloidal arrays find application in fabrication of solar cells with advanced light management strategies. We report a simple spincoating based approach for fabricating two dimensional colloidal crystals with hexagonal and non-hexagonal close packed assembly on flat and nanopatterned substrates. The non-HCP arrays were fabricated by spin coating the particles onto soft lithographically fabricated substrates. The substrate patterns impose directionality to the particles by confining them within the grooves. We have developed a technique by which the HCP and non-HCP arrays can be transferred to any surface. For this purpose the colloidal arrays were fabricated on a UV degradable PMMA layer, resulting in transfer of the particles on UV exposure. This allows the colloidal structures to be transported across substrates irrespective of their surface energy, wettability or morphology. Since the particles are transferred without exposing it to any kind of chemical or thermal environment, it can be utilized for placing particles on top of thin film solar cells for improving their absorption efficiency. [Preview Abstract] |
Wednesday, March 15, 2017 3:42PM - 3:54PM |
P17.00007: Self Assembly of Poly(ethylene oxide-b-lysine-b-leucine) in Dilute Aqueous Solution Craig Machado, Brooke Barnes, Kyle Bentz, Daniel Savin In general, the self-assembly of amphiphilic block polymers is dictated by the balance of three thermodynamic parameters. When one or more of the blocks exhibits a specific interaction, this balance can be shifted. In this study, the self-assembly of block polypeptides in dilute aqueous solution is examined via light scattering and transmission electron microscopy (TEM). Triblock polymers of poly(ethylene oxide-b-lysine-b-leucine) (PEO-Lys-Leu) with varying lengths of the poly(leucine) block were synthesized in order to study the effect of poly(leucine) block length on assembly behavior. It was observed that the presence of the leucine block facilitates formation of elongated structures such as nanotubes, hydrogels and hierarchical fractal assemblies. In all cases, radius of gyration (Rg) was greater than the hydrodynamic radius (Rh). A clear increase in size of the aggregates can be seen with increasing degree of polymerization of the poly(leucine) block. [Preview Abstract] |
Wednesday, March 15, 2017 3:54PM - 4:06PM |
P17.00008: Morphological Control of Melting Gel Materials by Electrospray Lin Lei, Ji Hyun Ryu, Kutaiba Al-Marzoki, Daniel Sullivan, Lisa Klein, Assimina Pelegri, Jonathan Singer, Gabriela Rodriguez, Andrei Jitianu Melting gel materials are fluorosilane polymers that possess glass transition temperatures near room temperature and a consolidation temperature \textgreater 150C above which they irreversibly dehydrate into silica glass. Because of this, they can be processed as a thermoplastic liquid into a desired form and then converted into a permanent structure. In this study, we deposit melting gel materials by electrospray in order to investigate the kinetics principles of different experimental conditions affect the final morphologies of melting gel films. Electrospray is an ideal method for the deposition of structured thin films. Due to the electrostatic breakup mechanism, electrospray results in uniform droplets down to hundreds of nanometers. By using dilute loadings, these microdroplets can deliver extremely small quantities of material at a continuous rate. By controlling spray composition, substrate temperature, flow rate, and collection distance, kinetics of solvent evaporation and melting gel consolidation. The results reveal that these can be used to controllably tune surface structure from smooth, to rough, to cellular hard coatings. Further, as the solutions are dilute, sparingly soluble nanoparticles may also be incorporated into the final structures. For example, sacrificial porogenic nanoparticles can add porosity to make highly fractal structures or low density coatings. [Preview Abstract] |
Wednesday, March 15, 2017 4:06PM - 4:18PM |
P17.00009: Robust peptide bundles designed computationally Michael Haider, Huixi Violet Zhang, Kristi Kiick, Jeffery Saven, Darrin Pochan Peptides are ideal candidates for the design and controlled assembly of nanoscale materials due to their potential to assemble with atomistic precision as in biological systems. Unlike other work utilizing natural proteins and structural motifs, this effort is completely de novo in order to build arbitrary structures with desired size for the specific placement and separation of functional groups. We have successfully computationally designed soluble, coiled coil, peptide, tetramer bundles which are robust and stable. Using circular dichroism we demonstrated the thermal stability of these bundles as well as confirmed their alpha helical and coiled coil nature. The stability of these bundles arises from the computational design of the coiled coil interior core residues. The coiled coil tetramer was confirmed to be the dominant species by analytical ultra-centrifugation sedimentation studies. We also established how these bundles behave in solution using small angle neutron scattering. The form factor of the bundles is well represented by a cylinder model and their behavior at high concentrations is modeled using a structure factor for aggregates of the cylinders. All of these experiments support our claim that the designed coiled coil bundles were achieved in solution. [Preview Abstract] |
Wednesday, March 15, 2017 4:18PM - 4:30PM |
P17.00010: Noise-induced creation and annihilation of dissipative solitons (DS) in a passively mode-locked laser Tesfay Teamir, Parviz Elahi, Ghaith Makey, Ilday Fatih Passive mode-locking, resulting in self-organized formation of femtoseconds-long laser pulses, constitutes a far-from-equilibrium steady state. Mode-locking is not only important for laser technology, but also of fundamental interest for broad class of systems. Despite numerous studies on their nonlinear dynamics, there is little understanding of the transitions that intrinsic noise can induce. We show that transitions between single-DS and multi-DS states can be triggered. Near critical points, DS states are observed to repeatedly exchange energy among themselves, form DS clusters with varying or vibrating temporal separation, often followed by random transformations among different states. This critical behavior appears to be caused by soliton-soliton or soliton-generated dispersive wave interactions. Irrespective of the specifics of the state, the measured noise level of the laser starts at a moderate value, is then reduced, as the DS's energy is increased. Further increases in power (nonlinearity) drives it towards a noisy critical state, where creation or annihilation of pulses occurs just before a new steady state is formed. These noise-induced transitions between steady states can shed light on the thermodynamics of far-from-equilibrium systems. [Preview Abstract] |
Wednesday, March 15, 2017 4:30PM - 4:42PM |
P17.00011: Similarity of ensembles of trajectories of reversible and irreversible growth processes Katherine Klymko Models of bacterial growth tend to be `irreversible', allowing for the number of bacteria in a colony to increase but not to decrease. By contrast, models of molecular self-assembly are usually `reversible', allowing for addition and removal of particles to a structure. Such processes differ in a fundamental way because only reversible processes possess an equilibrium. Here we show at mean- field level that dynamic trajectories of reversible and irreversible growth processes are similar in that both feel the influence of attractors, at which growth proceeds without limit but the intensive properties of the system are invariant. Attractors of both processes undergo nonequilibrium phase transitions as model parameters are varied, suggesting a unified way of describing reversible and irreversible growth. We also demonstrate an efficient method for sampling the rare events in these growth models. [Preview Abstract] |
Wednesday, March 15, 2017 4:42PM - 4:54PM |
P17.00012: Self-assembly of thin, triangular prisms into open networks at a flat air-water interface Michael Solomon, Joseph Ferrar, Deshpreet Bedi, Shangnan Zhou, Xiaoming Mao We observe capillary-driven binding between thin, equilateral triangle microprisms at a flat air-water interface. The triangles are fabricated from epoxy resin via SU-8 photolithography. For small thickness to length (T/L) ratios, two distinct pairwise particle-particle binding events occur with roughly equal frequency, and optical and environmental scanning electron microscopy (eSEM) demonstrate that these two distinct binding events are driven by the specific manner in which the interface is pinned to the particle surface. Additionally, particle bending is observed for the lowest T/L ratios, which leads to enhanced interface curvature and thus enhanced strength of capillary-driven attractions, and may also play a pivotal role in the dichotomy in particle-particle binding. Dichotomy in particle-particle binding is not observed at thicker T/L ratios, although capillary-driven binding still occurs. Ultimately, the particles self-assemble into space-spanning open networks, and the results suggest design parameters for the fabrication of building blocks of ordered open structures, such as the Kagome lattice. [Preview Abstract] |
Wednesday, March 15, 2017 4:54PM - 5:06PM |
P17.00013: Self-assembly of triangular particles via capillary interactions Deshpreet Bedi, Shangnan Zhou, Joseph Ferrar, Michael Solomon, Xiaoming Mao Colloidal particles adsorbed to a fluid interface deform the interface around them, resulting in either attractive or repulsive forces mediated by the interface. In particular, particle shape and surface roughness can produce an undulating contact line, such that the particles will assume energetically-favorable relative orientations and inter-particle distances to minimize the excess interfacial surface area. By expediently selecting specific particle shapes and associated design parameters, capillary interactions can be utilized to promote self-assembly of these particles into extended regular open structures, such as the kagome lattice, which have novel mechanical properties. We present the results of numerical simulations of equilateral triangle microprisms at an interface, including individually and in pairs. We show how particle bowing can yield two distinct binding events and connect it to theory in terms of a capillary multipole expansion and also to experiment, as presented in an accompanying talk. We also discuss and suggest design principles that can be used to create desirable open structures. [Preview Abstract] |
Wednesday, March 15, 2017 5:06PM - 5:18PM |
P17.00014: Microsphere Wetting, Meniscus Structure, and Capillary Interactions on a Curved Liquid Interface Paul Kim, Anthony Dinsmore, David Hoagland, Thomas Russell A small spherical microparticle on a cylindrically curved liquid interface locally induces a quadrupolar interface deformation to maintain a constant contact angle about its wetted periphery. Measured by optical profilometry, this deformation was compared to a recent theoretical expression, and good agreement was noted for contact line shape, particle vertical position, and deformation vs. (distance, angle, particle size, interfacial curvature). Interface quadrupoles lead to particle capillary interactions in analogy to 2D electrostatic quadrupoles, and as one consequence, spheres on a cylindrical interface assemble tetragonally, i.e., into a square lattice. This assembly was monitored in the optical microscope, with particles interacting as predicted, into a square lattice aligned with the underlying cylindrical axis. These particles and assemblies were driven to the middle of the curved interface by capillary interaction with pinned liquid contact lines on each side of the liquid cylindrical section used in the experiments. These phenomena can inform the directed interfacial assembly of micro-sized spherical objects, with potential application in fabrication of functional devices and materials, encapsulation, and emulsification. [Preview Abstract] |
Wednesday, March 15, 2017 5:18PM - 5:30PM |
P17.00015: Site-Specific Colloidal Crystal Nucleation by Template-enhanced Particle Transport Chandan K Mishra, A K Sood, Rajesh Ganapathy The deliberate positioning of nano- and microstructures on surfaces is often a prerequisite for fabricating functional devices. While template-assisted nucleation is a promising route to self-assemble these structures, its success hinges on particles reaching target sites prior to nucleation and for nano/microscale particles, this is hampered by their small surface mobilities. We tailored surface features, which in the presence of attractive depletion interactions not only directed micrometer-sized colloids to specific sites but also subsequently guided their growth into ordered crystalline arrays of well-defined size and symmetry. By following the nucleation kinetics with single-particle resolution, we demonstrate control over nucleation density in a growth regime that has hitherto remained inaccessible. Our findings pave the way towards realizing non-trivial surface architectures composed of complex colloids/nanoparticles as well. [Preview Abstract] |
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