The dedicated high-resolution grazing-incidence X-ray scattering beamline 8-ID-E at the Advanced Photon Source.

As an increasingly important structural-characterization technique, grazing-incidence X-ray scattering (GIXS) has found wide applications for in situ and real-time studies of nanostructures and nanocomposites at surfaces and interfaces. A dedicated beamline has been designed, constructed and optimized at beamline 8-ID-E at the Advanced Photon Source for high-resolution and coherent GIXS experiments. The effectiveness and applicability of the beamline and the scattering techniques have been demonstrated by a host of experiments including reflectivity, grazing-incidence static and kinetic scattering, and coherent surface X-ray photon correlation spectroscopy. The applicable systems that can be studied at 8-ID-E include liquid surfaces and nanostructured thin films.

Dynamic investigation of gold nanocrystal assembly using in situ grazing-incidence small-angle X-ray scattering.

Here we investigate the dynamic self-assembly pathway of ordered gold nanocrystal arrays during the self-assembly of gold nanocrystal micelles, with and without the presence of colloidal silica precursors, using grazing-incidence X-ray scattering performed at a synchrotron source. With silica precursors present, a lattice with rhombohedral symmetry is formed from the partial collapse of a face-centered cubic structure. In the absence of silica, a transient body-centered orthorhombic phase appears, which rapidly collapses into a glassy nanocrystal film. The appearance of face-centered and body-centered structures is consistent with a phase diagram for charged colloidal particles with assembly modulated via Coulomb screening.

Effect of interfacial interaction on the cross-sectional morphology of tobacco mosaic virus using GISAXS.

We have investigated the effect of the interfacial interaction on the cross-sectional morphology of the tobacco mosaic virus (TMV) in solution and on two types of solid substrates, SiOx (polar) on Si(100) and polystyrene film (nonpolar) on Si(100), using small-angle X-ray scattering (SAXS) and grazing incidence small-angle X-ray scattering (GISAXS), respectively. Results reveal that the flexible chains at the outer surface of TMV either expand or contract depending on the nature of the substrate. Although the unfavorable interaction between the TMV and the PS causes a minimal effect, the stronger attractive interaction between the outer protein surface of TMV and the SiOx substrate induces pronounced deformation of its cross-sectional morphology.

Particle dynamics in polymer-metal nanocomposite thin films on nanometer-length scales.

X-ray photon correlation spectroscopy was used in conjunction with resonance-enhanced grazing-incidence small-angle x-ray scattering to probe slow particle dynamics and kinetics in gold/polystyrene nanocomposite thin films. Such enhanced coherent scattering enables, for the first time, measurement of the particle dynamics at wave vectors up to approximately 1 nm(-1) (or a few nanometers spatially) in a disordered system, well in the regime where entanglement, confinement, and particle interaction dominate the dynamics and kinetics. Measurements of the intermediate structure factor f(q,t) indicate that the particle dynamics differ from Stokes-Einstein Brownian motion and are explained in terms of viscoelastic effects and interparticle interactions.

Surface organization, light-driven surface changes, and stability of semifluorinated azobenzene polymers.

A series of polymers with 4-perfluoroalkyl-modified azobenzene side groups was investigated for its light-induced changes in surface properties. The ultraviolet (UV) light activated trans to cis isomerization of the azobenzene group, and the influence of molecular order and orientation on this process were studied using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and water contact angle measurements. Light-induced molecular reorganization in the near-surface region was studied by NEXAFS using in situ UV irradiation of polymer thin films. Differential scanning calorimetry and wide-angle X-ray scattering studies showed that sufficiently long fluoroalkyl groups formed well-ordered smectic mesophases in the bulk, as well as on the surface, which was evidenced by NEXAFS. The disruption of mesogen packing by photoisomerization was found to be influenced by the fluoroalkyl segment length. Surfaces with perfluorohexyl and perfluorooctyl groups that showed high orientational order were also highly resistant to light-induced changes. In such cases, the trans-cis isomerization resulted in greater lowering of the azobenzene phenyl ring order parameters than the perfluoroalkyl order parameters. UV exposure caused reorientation of the phenyl rings of the azobenzene group, but the terminal perfluoroalkyl segments remained more or less ordered.

Generalized coating route to silica and titania films with orthogonally tilted cylindrical nanopore arrays.

We describe a simple, inexpensive coating method to produce thin silica and titania films with surfactant templated, orthogonally tilted cylindrical nanopore arrays. These films can be deposited onto any substrate because orientation of the 2D hexagonally close packed (HCP) mesophases out of the plane of the film is directed by a chemically neutral sacrificial copolymer layer. Orientation of the HCP mesophases through the entire thickness of films cured in open air is achieved by limiting the coating thickness. This generalizes the coating method by making it possible to deposit oriented films on substrates of any curvature and size. We find a critical thickness between 70 and 100 nm, below which the triblock copolymer surfactant-templated HCP phase aligns completely out of the plane of glass and silicon wafer substrates. Above this thickness, the effect of the chemically neutral bottom layer does not propagate across the entire film, and alignment of the HCP mesophases parallel to the (nonpolar) air interface produces a mixed orientation.

Self-directed self-assembly of nanoparticle/copolymer mixtures.

The organization of inorganic nanostructures within self-assembled organic or biological templates is receiving the attention of scientists interested in developing functional hybrid materials. Previous efforts have concentrated on using such scaffolds to spatially arrange nanoscopic elements as a strategy for tailoring the electrical, magnetic or photonic properties of the material. Recent theoretical arguments have suggested that synergistic interactions between self-organizing particles and a self-assembling matrix material can lead to hierarchically ordered structures. Here we show that mixtures of diblock copolymers and either cadmium selenide- or ferritin-based nanoparticles exhibit cooperative, coupled self-assembly on the nanoscale. In thin films, the copolymers assemble into cylindrical domains, which dictate the spatial distribution of the nanoparticles; segregation of the particles to the interfaces mediates interfacial interactions and orients the copolymer domains normal to the surface, even when one of the blocks is strongly attracted to the substrate. Organization of both the polymeric and particulate entities is thus achieved without the use of external fields, opening a simple and general route for fabrication of nanostructured materials with hierarchical order.

Control of surface properties using fluorinated polymer brushes produced by surface-initiated controlled radical polymerization.

Surface-grafted styrene-based homopolymer and diblock copolymer brushes bearing semifluorinated alkyl side groups were synthesized by nitroxide-mediated controlled radical polymerization on planar silicon oxide surfaces. The polymer brushes were characterized by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and time-dependent water contact angle measurements. Angle-resolved XPS studies and water contact angle measurements showed that, in the case of the diblock copolymer brushes, the second block to be added was always exposed at the polymer-air interface regardless of its surface energy. Values of z*/Rg were estimated based on the radius of gyration, Rg, of the grafted homopolymer or block copolymer chains for the grafted brushes and thickness of the brush, z*. The fact that z*/Rg > 1 suggests that all these brushes are stretched. These results support the idea that after grafting the first block onto the surface the nitroxide-end capped polymer chains were able to polymerize the second block in a "living" fashion and the stretched brush so formed was dense enough that the outermost block in all cases completely covers the surface. NEXAFS analysis showed a relationship between the surface orientation of the fluorinated side chains and brush thickness with thicker brushes having more oriented side chains. Time-dependent water contact angle measurements revealed that the orientation of the side chains of the brush improved the surface stability toward reconstruction upon prolonged exposure to water.