Alexander Hexemer

Soft x-ray scattering facility at the Advanced Light Source with real-time data processing and analysis

We present the development and characterization of a dedicated resonant soft x-ray scattering facility. Capable of operation over a wide energy range, the beamline and endstation are primarily used for scattering from soft matter systems around the carbon K-edge (∼285 eV). We describe the specialized design of the instrument and characteristics of the beamline. Operational characteristics of immediate interest to users such as polarization control, degree of higher harmonic spectral contamination, and detector noise are delineated. Of special interest is the development of a higher harmonic rejection system that improves the spectral purity of the x-ray beam. Special software and a user-friendly interface have been implemented to allow real-time data processing and preliminary data analysis simultaneous with data acquisition.

Biomimetic self-templating supramolecular structures

In nature, helical macromolecules such as collagen, chitin and cellulose are critical to the morphogenesis and functionality of various hierarchically structured materials. During tissue formation, these chiral macromolecules are secreted and undergo self-templating assembly, a process whereby multiple kinetic factors influence the assembly of the incoming building blocks to produce non-equilibrium structures. A single macromolecule can form diverse functional structures when self-templated under different conditions. Collagen type I, for instance, forms transparent corneal tissues from orthogonally aligned nematic fibres, distinctively coloured skin tissues from cholesteric phase fibre bundles, and mineralized tissues from hierarchically organized fibres. Nature’s self-templated materials surpass the functional and structural complexity achievable by current top-down and bottom-up fabrication methods. However, self-templating has not been thoroughly explored for engineering synthetic materials. Here we demonstrate the biomimetic, self-templating assembly of chiral colloidal particles (M13 phage) into functional materials. A single-step process produces long-range-ordered, supramolecular films showing multiple levels of hierarchical organization and helical twist. Three distinct supramolecular structures are created by this approach: nematic orthogonal twists, cholesteric helical ribbons and smectic helicolidal nanofilaments. Both chiral liquid crystalline phase transitions and competing interfacial forces at the interface are found to be critical factors in determining the morphology of the templated structures during assembly. The resulting materials show distinctive optical and photonic properties, functioning as chiral reflector/filters and structural colour matrices. In addition, M13 phages with genetically incorporated bioactive peptide ligands direct both soft and hard tissue growth in a hierarchically organized manner. Our assembly approach provides insight into the complexities of hierarchical assembly in nature and could be expanded to other chiral molecules to engineer sophisticated functional helical-twisted structures.

A SAXS/WAXS/GISAXS Beamline with Multilayer Monochromator

We discuss the construction of a new SAXS/WAXS beamline at the Advanced Light Source at Lawrence Berkeley Laboratory. The beamline is equipped with a multilayer monochromator in order to obtain a high X-ray flux. The detrimental effects that the increased bandwidth transmitted by this monochromator could have on the data quality of the SAXS and WAXS patterns is shown to be negligible for the experimental program intended to be operated on this beamline.

Conjugated Block Copolymer Photovoltaics with near 3% Efficiency through Microphase Separation

Organic electronic materials have the potential to impact almost every aspect of modern life including how we access information, light our homes, and power personal electronics. Nevertheless, weak intermolecular interactions and disorder at junctions of different organic materials limit the performance and stability of organic interfaces and hence the applicability of organic semiconductors to electronic devices. Here, we demonstrate control of donor-acceptor heterojunctions through microphase-separated conjugated block copolymers. When utilized as the active layer of photovoltaic cells, block copolymer-based devices demonstrate efficient photoconversion well beyond devices composed of homopolymer blends. The 3% block copolymer device efficiencies are achieved without the use of a fullerene acceptor. X-ray scattering results reveal that the remarkable performance of block copolymer solar cells is due to self-assembly into mesoscale lamellar morphologies with primarily face-on crystallite orientations. Conjugated block copolymers thus provide a pathway to enhance performance in excitonic solar cells through control of donor-acceptor interfaces.

Spin-cast bulk heterojunction solar cells: a dynamical investigation.

Spin-coating is extensively used in the lab-based manufacture of organic solar cells, including most of the record-setting solution-processed cells. We report the first direct observation of photoactive layer formation as it occurs during spin-coating. The study provides new insight into mechanisms and kinetics of bulk heterojunction formation, which may be crucial for its successful transfer to scalable printing processes.

Coatings based on side-chain ether-linked poly(ethylene glycol) and fluorocarbon polymers for the control of marine biofouling.

The preparation of side group modified polystyrene-based surface-active block copolymers (SABC) for use as marine fouling resistance/release applications is described. Modifying moieties such as poly(ethylene glycol) (PEG) and semifluorinated segments were used. A novel bilayer methodology has been employed that provides both suitable mechanical properties through the use of an elastomeric primer layer of styrene-ethylene/butylene-styrene (SEBS) and control of surface-chemistry through use of the SABCs. This approach has potential as a cost-effective technology for environmentally benign coatings that resist and release marine biofouling. Initial testing of these materials included determination of captive bubble contact angles and protein adsorption. Testing against marine fouling organisms was performed using settlement and adhesion bioassays with zoospores of the green alga Enteromorpha . The results showed that all surfaces had markedly reduced levels of zoospore settlement compared with glass controls and that adhesion strength was strongly affected by the semifluorinated SABC. The results are discussed in terms of surface properties.

Advanced grazing-incidence techniques for modern soft-matter materials analysis

Advanced grazing-incidence techniques have developed significantly during recent years. With the ongoing progress in instrumentation, novel methods have emerged which allow for an in-depth morphology characterization of modern soft-matter materials. Examples are in situ and in operando grazing-incidence small-angle X-ray scattering (GISAXS), micro- and nanofocused GISAXS, time-of-flight (TOF) grazing-incidence small-angle neutron scattering (GISANS) and surface-sensitive resonant soft X-ray scattering techniques, including the potential to investigate polarization. Progress in software for data analysis is another important aspect.

An In Situ Grazing Incidence X-Ray Scattering Study of Block Copolymer Thin Films During Solvent Vapor Annealing

In situ grazing-incidence small-angle X-ray scattering experiments on thin films of block copolymers during annealing in neutral solvent vapors are reported. By removing the solvent in a controlled manner, the period of the microphase separated morphology is found to increase with increasing block copolymer concentration in a power law manner with an exponent ∼ 2/3. By venting the systems at different rates during the solvent removal process, kinetically arresting the system, the period of the microphase separated morphology in the dried film can be varied.

The Crystallization of PEDOT:PSS Polymeric Electrodes Probed In Situ during Printing

C. M. Palumbiny, Prof. P. Müller-Buschbaum Lehrstuhl für Funktionelle Materialien Physik-Department Technische Universität München James-Franck-Str. , 1, 85748 Garching , Germany E-mail: muellerb@ph.tum.de Dr. F. Liu, Prof. T. P. Russell Department of Polymer Science and Engineering University of Massachusetts Amherst 120 Governors Drive , Amherst , MA 01003 , USA Dr. F. Liu, Prof. T. P. Russell Materials Sciences Division Lawrence Berkeley National Laboratory 1 Cyclotron Road , Berkeley , CA 94720 , USA Dr. A. Hexemer, Dr. C. Wang Advanced Light Source Lawrence Berkeley National Laboratory 1 Cyclotron Road Berkeley , CA 94720 , USA

Defining the nanostructured morphology of triblock copolymers using resonant soft X-ray scattering.

The morphologies of a poly(1,4-isoprene)-block-polystyrene-block-poly(2-vinyl pyridine) (IS2VP) copolymer were investigated using resonant soft X-ray scattering (RSoXS) together with scanning force microscopy, small-angle X-ray scattering, and electron microscopy. Differences in the nanoscopic morphologies in the bulk and thin film samples were observed arising from the competition between segmental interactions between the blocks and the substrate and the surface energies of each block. Using soft X-rays at selected photon energies to isolate the scattering contribution from different polymer blocks, RSoXS unambiguously defined the complex morphology of the triblock copolymer. In the bulk sample, two nested, hexagonal arrays of P2VP and PI cylindrical microdomains residing in the PS matrix were observed. The cylindrical microdomains of one component were found to be located at the interstitial sites of the hexagonal array of the other component that has the larger d spacing. In solvent-annealed thin films with 40 nm in thickness, a hexagonal array of core-shell microdomains of P2VP cores with PS shells that reside in a PI matrix were observed.