Session P03: Self-assembly of Nanomaterials: Supramolecular Self-assembly II

Intentional self-assembly of nonequilibrium structures: when can kinetic trapping be useful?

The aim of molecular self-assembly in the laboratory is usually to make an equilibrium structure rather than a nonequilibrium one, in part because many nonequilibrium structures are disordered and not useful. In this talk I will use theory and simulation to argue that it is possible to make nonequilibrium colloidal structures that have a high degree of order and are potentially useful. The structures in question are two-component crystals whose component types are intermingled in way that resembles the spins of a ferromagnet at a critical point. I will describe possible experimental realizations of these materials.   

Block Copolymer Self Assembly Directed Nanomaterials

Global problems including energy conversion and storage, clean water and human health require increasingly complex, multi-component hybrid materials with unprecedented control over composition, structure, and order down to the nanoscale. This talk will give examples for the rational design of novel functional nanomaterials directed by block copolymer self assembly. Discussion will include polymer-nanoparticle self-assembly derived synthetic porous materials with amorphous, polycrystalline, and epitaxially grown single-crystal structures as well as asymmetric membranes structures. Experiments will be compared to theoretical predictions to provide physical insights into formation principles and specific function. The aim of the described work is to understand the underlying fundamental chemical, thermodynamic and kinetic formation principles as well as nanostructure-property correlations enabling generalization of results over a wide class of materials systems. Examples will cover the formation of hierarchical structures at equilibrium as well as via processes far away from equilibrium. Targeted functions of the prepared systems will include nanostructured hybrids for energy conversion and storage devices, metamaterials with specific optical and phononic properties, as well as the formation of first self-assembled superconductors.

  

Synthetic Pathways to ABA and ABC Triblock Copolymer Derived Ordered Mesoporous Niobium Nitrides

Ordered mesoporous transition metal nitrides are of substantial interest in areas as diverse as catalysis, electrochemical energy conversion and storage, and superconductivity. We developed a route to synthesize such nitrides via a highly tunable route based on high-temperature ammonia treatment of mesostructured oxides derived from ABC poly(isoprene-b-styrene-b-ethylene oxide) triblock terpolymers. Such ABC triblocks are not widely available, however, greatly limiting potential work with mesoporous nitrides. We report pathways to mesoporous niobium nitride based on the commercial Pluronics family of ABA triblock copolymers, a synthetic route previously thought to be impossible due to the small pore wall thicknesses in Pluronics-derived materials and rapid crystallization of the nitride. We expect that the availability of synthetic routes to nitrides using these polymers will allow wider exploration of the impacts of mesoscale order on transition metal nitrides.

Ref.: Discovering Synthesis Routes to Hexagonally Ordered Mesoporous Niobium Nitrides Using Poloxamer/Pluronics Block Copolymers, Chem. Mater. (2017) 10.1021/acs.chemmater.7b03834   

Twists and Turns in Bicontinuous Cubic Phases

We recently reported ^[1] that all compounds, when in the triple-network cubic liquid crystal phase with Im-3m symmetry ^[2] (now considered possibly I432) display strong optical activity, even though the compounds themselves are intrinsically not chiral. In contrast, the double gyroid cubic phase Ia-3d is never optically active. We have explained these intriguing observations by a twist around the axis of network segment and a long-range propagation of chiral sense through chirality matching at network junctions. ^[1] The absence of net chirality in the Ia-3d phase was attributed to cancelation between the two antichiral networks. Now we report on another bicontinuous phase that contains two isochiral networks which therefore has high net chirality, whether or not it contains chiral molecules. We will also report on the remarkable effects of chiral environment on the nucleation and the domain shape of these phases due to the high barrier to helix reversal.

1. C. Dressel, F. Liu, M. Prehm, X.B. Zeng, G. Ungar, C. Tschierske “Dynamic mirror-symmetry breaking in bicontinuous cubic phases”, Angew. Chem. Int. Ed., 2014, 53, 13115 –13120.
2. XB Zeng, G Ungar, M Impéror-Clerc, “A Triple-Network Tricontinuous Cubic Liquid Crystal”, Nat. Mater., 2005 4 562-567.   

Characterizing the Conformer Dependence on the Self-Assembly of Diphenylalanine

Diphenylalanine (FF) self-assembles into highly ordered and remarkably stable nanotubes which have a wide variety of potential biomaterial applications. Explicating the FF self-assembly mechanism would enable the rational design of a FF-based biomaterial. Utilizing all-atom molecular dynamics, we demonstrate that dispersed FF monomers initially aggregate into a disordered aggregate by means of backbone–backbone interactions. Results from quantum mechanical calculations suggest the individual monomers within this FF aggregate then undergo a large structural reorientation into conformers with higher internal energies. Although in a less stable conformation, the dihedral space of this conformer allows for further hydrophobic packing. Steric sidechain size and phenyl–backbone interactions are the source of the energetic barrier between conformers, suggesting modification resulting in increased rigidity of the dihedral state would alter the ordering in FF self-assembly.   

Understanding the self-assembly of the inkjet-printed CNT inks: Towards developing printable CNT inks

The printability of carbon nanotubes (CNTs) has attracted massive attention in the past few years due to its high thermal and electronic conductivities as compared to traditional metallic materials. 3D printed CNT has emerged as an important material that is extensively used in applications that involve the use of conductive wires with fine dimension and sophisticated configurations. However, the solubility of CNT in water is too low to enable any large-scale manufacturing, so graphene oxide and cellulose nanocrystal are included to help the CNT dispersion. The ink is printed through an inkjet 3D printer under an optimized pressure on a polymer surface; subsequently, the printed ink particles assemble together as the water evaporates out. The morphology and self-assembly dynamics vary dramatically with the variation in the CNT: GO/CNC ratio. Most importantly, we carry out a detailed electron microscopy analysis to relate the microscopic structural features of the ink of various compositions that result from the corresponding self-assembly with the overall printability of the ink. We anticipate that our research will be a precursor of a green technology to enable the large-scale printing of CNT.   

Probing Early-Stage Gelation Behavior of Low Molecular Mass Organogelators

Molecular gels are formed by the supramolecular aggregation of low molecular weight gelators (LMWGs) in organic solvents and/or water. Hierarchical self-assembly of small gelator molecules lead to three-dimensional complex fibrillar networks that immobilize solvent and results in viscous solid like materials or gels. These gels have drawn significant attentions for their potential applications for drug delivery, tissue engineering, sensors, etc. Self-assembly of gelator molecules into one-dimensional fibers is not well understood, although that is very important to design new gelators for desired applications. Here, we present molecular dynamics study that provides molecular level insight into early stage aggregation of selected gelator, di-Fmoc-L-lysine in binary mixture of organic solvent and water. We will present the role of different functional groups of gelator molecule such as aromatic ring, amide, and carboxylic group on aggregation. We will also present the effect of concentrations of gelator and solvent on self-assembly of gelators. This study has captured helical fiber growth and branching of fiber, which is in good agreement with experimental observations.   

Confined Phase Separation of Aqueous-Organic Nanodroplets

Nano-confined supercooled water often occurs in aqueous-organic aerosol nanodroplets. We used classical molecular dynamics (MD) simulations to study the structures of binary SPC/E water-TraPPE butanol nanodroplets, [PCCP 19, 26839 (2017)]. Water-butanol cross-interaction LJ parameters were adjusted to reproduce the experimentally observed mutual solubilities of water-butanol at 295 K. Simulations at 250 K show three different nanodroplet structures depending on the butanol concentration. At low concentrations, core-shell (CS) structures occur in which a butanol shell completely wets a water-rich core. At high concentrations, well-mixed (WM) structures occur as the water and butanol become fully miscible. At intermediate concentrations, phase-separated Russian Doll-Shell (RDS) structures occur. The RDS structure consists of a water-rich droplet partially wetted by a well-mixed water/butanol lens (a RD), and this lens-droplet structure is coated by a thin shell of butanol. At 295 K the RDS structure transforms into a WM droplet due to the increased mutual solubility of water and butanol. A classical DFT model produces structures similar to those found using MD.   

Challenges in barrier thinning of home-grown anodic aluminum oxide template on aluminum foil

Home-grown AAO templates allow for tuning the pore diameter and length and also ensure that the pores are uniform in size resulting in nanowires that have smooth surfaces. The biggest challenge with home-grown templates is the dissolution of the barrier layer. Depending on the anodizing potential, a thin barrier exists at the metal-dielectric interface. There are several strategies to thin and remove this barrier layer including: (i) a controlled voltage reduction process, (ii) chemical etching of the barrier after the AAO template is released from the aluminum foil and (iii) reverse anodization to thin the barrier layer. The removal or thinning of this barrier layer is critical for the use of the AAO template in applications, such as template growth of uniform arrays of nanowires and nanotubes on the aluminum host substrate. Results will be presented to show the growth of Co-InSb multi-layers in home-made anodic alumina oxide (AAO) templates that were prepared by critically controlling the barrier thinning and subsequent removal. The as-grown nanowires were characterized to study their electrical and structural properties.   

Insight on the Silver Catalyst Distribution During Silicon Nanowire Array Formation: an X-ray Reflectivity Study

Although metal-catalysts are commonly used to form nanoscale surface structures, little is quantitatively known about the vertical distribution of these metal nanoparticles. Using X-ray specular reflectivity, we report the first quantitative investigation, with nanoscale resolution, of the Ag catalyst depth distribution profile during Si nanowire (SiNW) array formation by metal-assisted chemical etching (MACE). Despite challenges from the extraordinarily rough interfaces, it is revealed that Ag nanoparticles unexpectedly distribute along the length of the nanowires, which are not tapered or pitted. Coarsening over the course of etching indicates that the Ag ions are highly mobile – interestingly the Ag remain in the SiNW region rather than migrating into the etch bath. The Ag mobility and vertical distribution suggests a strong chemical force that attracts Ag towards the etch front. Results of this study provide important new insight into the poorly-understood metal-assisted chemical etching process.   

Roughening Temperature of Facets in Au Electrodeposition

A theory has been established to describe the formation of facets on metal crystals grown from their melt. There exists a temperature for each crystallographic orientation below which it will form a facet, and above which the metal surface will be rounded and atomically rough. Here we have extended this theory to electrodeposition. We used potentiostatic electrochemical impedance spectroscopy to measure the approximate surface energy of Au at different applied potentials to determine the roughening temperature for each potential.