Coassembled molecular structures are known to exhibit a large variety of geometries and morphologies. A grand challenge of self-assembly design is to find techniques to control the crystal symmetries and overall morphologies of multicomponent systems. By mixing +3 and -1 ionic amphiphiles, we assemble crystalline ionic bilayers in a large variety of geometries that resemble polyhedral cellular crystalline shells and archaea wall envelopes. We combine TEM with SAXS and WAXS to characterize the coassembled structures from the mesoscopic to nanometer scale. The degree of ionization of the amphiphiles and their intermolecular electrostatic interactions are controlled by varying pH. At low and high pH values, we observe closed, faceted vesicles with two-dimensional hexagonal molecular arrangements, and at intermediate pH, we observe ribbons with rectangular-C packing. Furthermore, as pH increases, we observe interdigitation of the bilayer leaflets. Accurate atomistic molecular dynamics simulations explain the pH-dependent bilayer thickness changes and also reveal bilayers of hexagonally packed tails at low pH, where only a small fraction of anionic headgroups is charged. Coarse-grained simulations show that the mesoscale geometries at low pH are faceted vesicles where liquid-like edges separate flat crystalline domains. Our simulations indicate that the curved-to-polyhedral shape transition can be controlled by tuning the tail density in regions where sharp bends can form the polyhedral edges. In particular, the pH acts to control the overall morphology of the ionic bilayers by changing the local crystalline order of the amphiphile tails.

Organic thin film transistor (OTFT) performance is highly materials interface-dependent, and dramatic performance enhancements can be achieved by properly modifying the semiconductor/gate dielectric interface. However, the origin of these effects is not well understood, as this is a classic "buried interface" problem that has traditionally been difficult to address. Here we address the question of how n-octadecylsilane (OTS)-derived self-assembled monolayers (SAMs) on Si/SiO(2) gate dielectrics affect the OTFT performance of the archetypical small-molecule p-type semiconductors P-BTDT (phenylbenzo[d,d]thieno[3,2-b;4,5-b]dithiophene) and pentacene using combined in situ sum frequency generation spectroscopy, atomic force microscopy, and grazing incidence and reflectance X-ray scattering. The molecular order and orientation of the OTFT components at the dielectric/semiconductor interface is probed as a function of SAM growth mode in order to understand how this impacts the overlying semiconductor growth mode, packing, crystallinity, and carrier mobility, and hence, transistor performance. This understanding, using a new, humidity-specific growth procedure, leads to a reproducible, scalable process for highly ordered OTS SAMs, which in turn nucleates highly ordered p-type semiconductor film growth, and optimizes OTFT performance. Surprisingly, the combined data reveal that while SAM molecular order dramatically impacts semiconductor crystalline domain size and carrier mobility, it does not significantly influence the local orientation of the overlying organic semiconductor molecules.

Bimetallic hollow, porous noble metal nanoparticles are of broad interest for biomedical, optical and catalytic applications. The most straightforward method for preparing such structures involves the reaction between HAuCl and well-formed Ag particles, typically spheres, cubes, or triangular prisms, yet the mechanism underlying their formation is poorly understood at the atomic scale. By combining in situ nanoscopic and atomic-scale characterization techniques (XAFS, SAXS, XRF, and electron microscopy) to follow the process, we elucidate a plausible reaction pathway for the conversion of citrate-capped Ag nanospheres to AgAu nanocages; importantly, the hollowing event cannot be explained by the nanoscale Kirkendall effect, nor by Galvanic exchange alone, two processes that have been previously proposed. We propose a modification of the bulk Galvanic exchange process that takes into account considerations that can only occur with nanoscale particles. This nanoscale Galvanic exchange process explains the novel morphological and chemical changes associated with the typically observed hollowing process.

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The behavior of poly(n-butyl acrylate) (PnBA) spread at the air-water interface has been studied for a full range of surface coverages and several molecular weights. At low and intermediate surface coverages, the surface pressure-area isotherm behavior of the polymer is found to follow the expected scaling laws. In the dilute regime the pressure is an increasing function of surface coverage and a decreasing function of molecular weight. In the semidilute regime the surface pressure becomes independent of molecular weight, and a Flory exponent for the twodimensional radius of gyration is found to be ν = 0.57 ( 0.02. Beginning in the high coverage concentrated regime, at a surface pressure of around 15 mN/m, and through the full coverage regime (where the water in the subphase is fully covered and not exposed to air), X-ray reflectivity (XR) measurements show the formation of a continuous waterfree monolayer (i.e., one monomer thick) film of the polymer. At surface concentrations above the transition point to the full coverage regime (alternatively called the "collapsed" regime hereafter for the reason that will become apparent below), Brewster angle microscopy (BAM) shows that the excess polymer material does not distribute uniformly in the polymer film layer but instead leads to formation of micrometer-scale isolated globular domains of roughly uniform size. Further, it was observed that the number of such domains increases as the surface polymer concentration is increased, whereas the size of the globular domains is largely unaffected by the concentration variation. X-ray grazing incidence diffraction (GID) indicates that these domains are regions of bulklike (amorphous) polymer. These and other observations, including the invariant nature of the monolayer throughout the compression (confirmed by XR), the plateau nature of surface pressure-area isotherm throughout the collapsed regime, and the reversible nature of the domain formation (evidenced by BAM), suggest that the globular domains formed at high surface concentrations of PnBA are in a type of coexistence with the uniform monolayer. A simple thermodynamic model considering the entropic penalty of confining the polymer chains to monolayer, the translational entropy of the domains, and the surface energy of the interface is made in order to understand the behavior of the polymer as it becomes excluded from the monolayer. This argument suggests that the excess polymer should form a single large domain in order to minimize the large surface energy at the water-polymer interface. The presence of many small domains suggests the domains are kinetically trapped in a local, rather than global, equilibrium.