Christopher K. Ober

Attogram detection using nanoelectromechanical oscillators

We report on the fabrication of nanometer-scale mass sensors with subattogram sensitivity. Surface micromachined polycrystalline silicon and silicon nitride nanomechanical oscillators were used to detect the presence of well-defined mass loading. Controlled deposition of thiolate self-assembled monolayers on lithographically defined gold dots were used for calibrated mass loading. We used a dinitrophenyl poly(ethylene glycol) undecanthiol-based molecule (DNP-PEG4-C11thiol) as a model ligand for this study. Due to the fact that the gold mass is attached at the distance l0 from the end x=l of the cantilever beam, an additional moment evolves in the boundary condition of the oscillator, which was taken into consideration through the rotational inertia of the attached mass. We showed that the corresponding correction of the frequency is on the order of γ(l0/l), where γ is the attached mass normalized to the mass of the beam. The rotational inertia correction to the frequency is on the order of γ(l0/l)2. The a...

Advances in polymers for anti-biofouling surfaces

Self-assembling polymers and nanostructured polymer thin films are being actively explored as advanced coatings for marine and biomedical applications. This review highlights recent advances in the design and synthesis of polymers that can resist fouling by biomolecules, cells and organisms. Current understanding of the mechanisms of anti-biofouling activity is also discussed.

Self-Assembled Smectic Phases in Rod-Coil Block Copolymers

Rod-coil block copolymers are self-assembling polymers that combine the physics of orientational ordering of rodlike polymers and the microphase separation of coil-coil block copolymers. Several new solid-state morphologies were observed in a series of anionically synthesized model poly(hexyl isocyanate-b-styrene) rod-coil diblock copolymers examined by transmission electron microscopy and selected-area electron diffraction. The rod-coils formed smectic C-like and O-like morphologies with domain sizes ranging from tens of nanometers to almost 1 micrometer. Both structural and orientational changes were found for increasing rod volume fractions. In addition, some morphologies exhibited spontaneous long-range orientational order over many tens of micrometers.

Block copolymer patterns and templates

This review describes the chemical and physical aspects of patternable block copolymers and their use for nanostructure fabrication. The patternability of block copolymers results from their ability to self-assemble into microdomains and the manipulation of these patterns by a variety of physical and chemical means. Procedures for achieving long-range lateral order, as well as orientation order of microdomain patterns, are discussed. The level of control that these strategies afford has enabled block copolymers to be used as templates for fabricating a variety of nanostructures.

Study of the interlayer expansion mechanism and thermal-mechanical properties of surface-initiated epoxy nanocomposites

Abstract The exfoliation mechanism and thermal–mechanical properties of surface-initiated epoxy nanocomposites were studied. Time-resolved high-temperature X-ray diffraction, DSC, and isothermal rheological analyses revealed that the interlayer expansion mechanism might be separated into three stages. These stages relate to the initial interlayer expansion, the steady-state interlayer expansion, and the cessation of interlayer expansion. It was found that differences in the activation energies of interlayer expansion and of curing influence the final nanostructures of the materials. The thermal–mechanical properties of the nanocomposites were studied using dynamic mechanical thermal analysis. Variations in ultimate properties were attributed to the formation of an interphase layer, where the interphase is hypothesized to be the epoxy matrix plasticized by surfactant chains.

Liquid crystal polymers with flexible spacers in the main chain

This review is be concerned primarily with the synthesis and properties of such polymers which show thermotropic behaviour.

An overview of supercritical CO 2 applications in microelectronics processing

Supercritical CO2 technologies have only recently been proposed for microelectronic applications in response to needs for material-compatible cleaning systems, small-dimension developing solvents, and low chemical-use processes. Numerous proposed CO2 applications that are both process enabling and provide potential for chemical abatement in microelectronics engineering are discussed to present readers with the current scope of research in the field. Examples include stripping of photoresist and residues, drying, developing, and spinning of resist, chemical fluid deposition of metals, silylation, and formation and patterning of low-dielectric materials.

Liquid crystalline and rigid-rod networks

Abstract Starting about a decade ago, researchers began to explore the use of LC networks as a means of coupling mechanical forces to the mesomorphic character of networks. In so doing, the field of LC elastomers was created and the resulting ferroelectric and piezoelectric materials are beginning to revolutionize the field of polymers. More recently, industry has begun to investigate rigid-rod and LC thermosets as a means of introducing the desirable physical properties of LC thermoplastics in thermoset systems. These polymer networks offer such capabilities as tuned coefficients of thermal expansion and increased fracture toughness. Finally, efforts are being made to develop networks which while not necessarily mesomorphic, do have highly oriented structures to produce networks with NLO properties. Long-term orientational stability is one of the features offered by these new materials. This review will examine all three types of networks, with the major focus being on LC thermosets.

Dissociation behavior of weak polyelectrolyte brushes on a planar surface.

Poly(acrylic acid) (PAA) brushes and poly(methacrylic acid) (PMAA) brushes on gold substrates were synthesized by surface-initiated atom-transfer radical polymerization of sodium acrylate and sodium methacrylate in water media at room temperature. Fourier transform infrared spectroscopy (FTIR) titration and contact angle titration methods were used in combination to investigate the dissociation behavior of these two brushes. Whereas FTIR titration gives effective bulk pKa values of the polyacid brushes (pKabulk of PAA brushes is 6.5-6.6 and pKabulk of PMAA brushes is 6.9-7.0), contact angle titration provides effective surface pKa of the brushes (pKasurf of PAA brushes is 4.4+/-0.01 and pKasurf of PMAA brushes is approximately 4.6+/-0.1). The difference between pKabulk and pKasurf suggests that acid groups further from the substrate surface are easier to ionize and have smaller pKa values. Although such behavior of weak polyelectrolyte brushes has been predicted by theoretical simulation, here we provide the first experimental evidence of this behavior.

Control of Self-Assembly of Lithographically Patternable Block Copolymer Films

Poly(alpha-methylstyrene)-block-poly(4-hydroxystyrene) acts as both a lithographic deep UV photoresist and a self-assembling material, making it ideal for patterning simultaneously by both top-down and bottom-up fabrication methods. Solvent vapor annealing improves the quality of the self-assembled patterns in this material without compromising its ability to function as a photoresist. The choice of solvent used for annealing allows for control of the self-assembled pattern morphology. Annealing in a nonselective solvent (tetrahydrofuran) results in parallel orientation of cylindrical domains, while a selective solvent (acetone) leads to formation of a trapped spherical morphology. Finally, we have self-assembled both cylindrical and spherical phases within lithographically patterned features, demonstrating the ability to precisely control ordering. Observing the time evolution of switching from cylindrical to spherical morphology within these features provides clues to the mechanism of ordering by selective solvent.