We have studied the spreading of phospholipid vesicles on photochemically patterned n-octadecylsiloxane monolayers using epifluorescence and imaging ellipsometry measurements. Self-assembled monolayers of n-octadecylsiloxanes were patterned using short-wavelength ultraviolet radiation and a photomask to produce periodic arrays of patterned hydrophilic domains separated from hydrophobic surroundings. Exposing these patterned surfaces to a solution of small unilamellar vesicles of phospholipids and their mixtures resulted in a complex lipid layer morphology epitaxially reflecting the underlying pattern of hydrophilicity. The hydrophilic square regions of the photopatterned OTS monolayer reflected lipid bilayer formation, and the hydrophobic OTS residues supported lipid monolayers. We further observed the existence of a boundary region composed of a nonfluid lipid phase and a lipid-free moat at the interface between the lipid monolayer and bilayer morphologies spontaneously corralling the fluid bilayers. The outer-edge of the boundary region was found to be accessible for subsequent adsorption by proteins (e.g., streptavidin and BSA), but the inner-edge closer to the bilayer remained resistant to adsorption by protein or vesicles. Mechanistic implications of our results in terms of the effects of substrate topochemical character are discussed. Furthermore, our results provide a basis for the construction of complex biomembrane models, which exhibit fluidity barriers and differentiate membrane properties based on correspondence between lipid leaflets. We also envisage the use of this construct where two-dimensionally fluid, low-defect lipid layers serve as sacrificial resists for the deposition of protein and other material patterns.

Graphene oxide (GO) contains several chemical functional groups that are attached to the graphite basal plane and can be manipulated to tailor GO for specific applications. It is now revealed that the reaction of GO with ozone results in a high level of oxidation, which leads to significantly improved ionic (protonic) conductivity of the GO. Freestanding ozonated GO films were synthesized and used as efficient polymer electrolyte fuel cell membranes. The increase in protonic conductivity of the ozonated GO originates from enhanced proton hopping, which is due to the higher content of oxygenated functional groups in the basal planes and edges of ozonated GO as well as the morphology changes in GO that are caused by ozonation. The results of this study demonstrate that the modification of dispersed GO presents a powerful opportunity for optimizing a nanoscale material for proton-exchange membranes.

We report a general procedure to prepare functional organic thin films for biological assays on oxide surfaces. Silica surfaces were functionalized by self-assembly of an amine-terminated silane film using both vapor- and solution-phase deposition of 3'-aminopropylmethyldiethoxysilane (APMDES). We found that vapor-phase deposition of APMDES under reduced pressure produced the highest quality monolayer films with uniform surface coverage, as determined by atomic force microscopy (AFM), ellipsometry, and contact angle measurements. The amine-terminated films were chemically modified with a mixture of carboxylic acid-terminated poly(ethylene glycol) (PEG) chains of varying functionality. A fraction of the PEG chains (0.1-10 mol %) terminated in biotin, which produced a surface with an affinity toward streptavidin. When used in pseudo-sandwich assays on waveguide platforms for the detection of Bacillus anthracis protective antigen (PA), these functional PEG surfaces significantly reduced nonspecific binding to the waveguide surface while allowing for highly specific binding. Detection of PA was used to validate these films for sensing applications in both buffer and complex media. Ultimately, these results represent a step toward the realization of a robust, reusable, and autonomous biosensor.

Platinum group metal-free (PGM-free) electrocatalysts for the oxygen reduction reaction (ORR) often exhibit a complex functionalized graphitic structure. Because of this complex structure, limited understanding exists about the design factors for the synthesis of high-performing materials. Graphene, a two-dimensional hexagonal structure of carbon, is amenable to structural and functional group modifications, making it an ideal analogue to study crucial properties of more complex graphitic materials utilized as electrocatalysts. In this paper, we report the synthesis of active nitrogen-doped graphene oxide catalysts for the ORR in which their activity and four-electron selectivity are enhanced using simple solvent and electrochemical treatments. The solvents, chosen based on Hansen’s solubility parameters, drive a substantial change in the morphology of the functionalized graphene materials by (i) forming microporous holes in the graphitic sheets that lead to edge defects and (ii) inducing 3D structure in the graphitic sheets that promotes ORR. Additionally, the cycling of these catalysts has highlighted the multiplicity of the active sites, with different durability, leading to a highly selective catalyst over time, with a minimal loss in performance. High ORR activity was demonstrated in an alkaline electrolyte with an onset potential of ∼1.1 V and half-wave potential of 0.84 V vs RHE. Furthermore, long-term stability potential cycling showed minimal loss in half-wave potential (<3%) in both N2- and O2-saturated solutions with improved selectivity toward the four-electron reduction after 10000 cycles. The results described in this work provide additional understanding about graphitic electrocatalysts in alkaline media that may be utilized to further enhance the performance of PGM-free ORR electrocatalysts.

Graphene, a recently discovered two-dimensional form of carbon, is a strong candidate for many future electronic devices. There is, however, still much debate over how the electronic properties of graphene behave on ultrashort time scales. Here by employing the technique of time-resolved photoemission, we obtain the evolving quantum distributions of the electrons and holes: on an ultrashort 500 fs time scale, the electron and hole populations can be described by two separate Fermi–Dirac distributions, whereas on longer time scales the populations coalesce to form a single Fermi–Dirac distribution at an elevated temperature. These studies represent the first direct measure of carrier distribution dynamics in monolayer graphene after ultrafast photoexcitation.

We report infrared studies of the insulator-to-metal transition (IMT) in GaAs doped with either magnetic (Mn) or non-magnetic acceptors (Be). We observe a resonance with a natural assignment to impurity states in the insulating regime of Ga1−xMnxAs, which persists across the IMT to the highest doping (16%). Beyond the IMT boundary, behavior combining insulating and metallic trends also persists to the highest Mn doping. Be doped samples however, display conventional metallicity just above the critical IMT concentration, with features indicative of transport within the host valence band.The insulator-to-metal transition (IMT) becomes exceptionally complex when magnetism is involved, as proven in materials such as mixed-valence manganites, rare-earth chalcogenides, and Mn-doped III-V compounds [1,2]. In all these systems, the electronic and magnetic properties are typically interconnected, creating an entising challenge to understand how magnetism affects the IMT physics. A promising route to isolate differences attributable to the presence of magnetism on the IMT physics is to investigate either magnetic or non-magnetic dopants in the same host. p-doped GaAs is well suited for the task since metallicity in this material can be initiated by non-magnetic (Zn, Be, C) and magnetic (Mn) acceptors. Infrared (IR) experiments reported here for Ga 1−x Be x As and Ga 1−x Mn x As monitor the charge dynamics and electronic structure in the course of the IMT. Our results establish that the onset arXiv:1109.0310v1 [cond-mat.mtrl-sci] 1 Sep 2011

In this contribution, six conjugated polymers consisting of benzo[1,2-b:4,5-b′]dithiophene–bithiophene (BDT-BT) and benzo[1,2-b:4,5-b′]dithiophene–benzothiadiazle (BDT-BTD) as building blocks in the main chain were synthesized by coupling polymerization and utilized for photovoltaic applications. By directly attaching three kinds of alkylthienyl side chains to the conjugated main chain, the resulted two-dimensional configuration revealed a broader absorption range due to the ground state electron transition of their corresponding alkylthienyl units and polymer backbone. Temperature-dependent absorbance, emission spectra, and thermal annealing further verify that the shoulder band(s) were originated from the aggregated (crystalline) species of polymers. The photovoltaic properties of the donor–acceptor polymers revealed well-defined side chain geometries, physical, and electronic structures and showed the highest power conversion efficiency of 4.25% among polymer solar cells based on two-dimensional (2-D) bithienyl- or terthienyl-substituted benzodithiophene.

Single bilayer membranes of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) were formed on ordered nanocomposite and nanoporous silica thin films by fusion of small unilamellar vesicles. The structure of these membranes was investigated using neutron reflectivity. The underlying thin films were formed by evaporation induced self-assembly to obtain periodic arrangements of silica and surfactant molecules in the nanocomposite thin films, followed by photocalcination to oxidatively remove the organics and render the films nanoporous. We show that this platform affords homogeneous and continuous bilayer membranes that have promising applications as model membranes and sensors.