Electric-field-induced second harmonic generation in silicon dioxide.

Electric-field-induced second harmonic generation (EFISH) as a third order nonlinear process is of high practical interest for the realization of functional nonlinear structures. EFISH in materials with vanishing χ(2) and non-zero χ(3) offers huge potential, e.g., for background-free nonlinear electro-optical sampling. In this work, we have investigated SiO2 as a potential EFISH material for such applications using DC-electric fields. We were able to observe significant second harmonic generation (SHG) in comparison to the background SHG signal. The fundamental excitation at 800 nm results in a SHG signal at 400 nm for high applied DC electric fields, which is a clear indication for EFISH. Additionally, we were are able to precisely model the EFISH signal using time-domain simulations. This numerical approach will be of great importance for efficiency enhancement and prove as a valuable tool for future device design.

Mapping metabolic activity at single cell resolution in intact volcanic fumarole sediment.

Interactions among microorganisms and their mineralogical substrates govern the structure, function and emergent properties of microbial communities. These interactions are predicated on spatial relationships, which dictate metabolite exchange and access to key substrates. To quantitatively assess links between spatial relationships and metabolic activity, this study presents a novel approach to map all organisms, the metabolically active subset and associated mineral grains, all while maintaining spatial integrity of an environmental microbiome. We applied this method at an outgassing fumarole of Vanuatu's Marum Crater, one of the largest point sources of several environmentally relevant gaseous compounds, including H2O, CO2 and SO2. With increasing distance from the sediment-air surface and from mineral grain outer boundaries, organism abundance decreased but the proportion of metabolically active organisms often increased. These protected niches may provide more stable conditions that promote consistent metabolic activity of a streamlined community. Conversely, exterior surfaces accumulate more organisms that may cover a wider range of preferred conditions, implying that only a subset of the community will be active under any particular environmental regime. More broadly, the approach presented here allows investigators to see microbial communities 'as they really are' and explore determinants of metabolic activity across a range of microbiomes.

Side-by-side comparability of batch and continuous downstream for the production of monoclonal antibodies.

Continuous processing is the future production method for monoclonal antibodies (mAbs). A fully continuous, fully automated downstream process based on disposable equipment was developed and implemented inside the MoBiDiK pilot plant. However, a study evaluating the comparability between batch and continuous processing based on product quality attributes was not conducted before. The work presented fills this gap comparing both process modes experimentally by purifying the same harvest material (side-by-side comparability). Samples were drawn at different time points and positions in the process for batch and continuous mode. Product quality attributes, product-related impurities, as well as process-related impurities were determined. The resulting polished material was processed to drug substance and further evaluated regarding storage stability and degradation behavior. The in-process control data from the continuous process showed the high degree of accuracy in providing relevant process parameters such as pH, conductivity, and protein concentration during the entire process duration. Minor differences between batch and continuous samples are expected as different processing conditions are unavoidable due to the different nature of batch and continuous processing. All tests revealed no significant differences in the intermediates and comparability in the drug substance between the samples of both process modes. The stability study of the final product also showed no differences in the stability profile during storage and forced degradation. Finally, online data analysis is presented as a powerful tool for online-monitoring of chromatography columns during continuous processing.

Bottom-Up Synthesis of Graphene Monolayers with Tunable Crystallinity and Porosity.

We present a method for a bottom-up synthesis of atomically thin graphene sheets with tunable crystallinity and porosity using aromatic self-assembled monolayers (SAMs) as molecular precursors. To this end, we employ SAMs with pyridine and pyrrole constituents on polycrystalline copper foils and convert them initially into molecular nanosheets-carbon nanomembranes (CNMs)- via low-energy electron irradiation induced cross-linking and then into graphene monolayers via pyrolysis. As the nitrogen atoms are leaving the nanosheets during pyrolysis, nanopores are generated in the formed single-layer graphene. We elucidate the structural changes upon the cross-linking and pyrolysis down to the atomic scale by complementary spectroscopy and microscopy techniques including X-ray photoelectron and Raman spectroscopy, low energy electron diffraction, atomic force, helium ion, and high-resolution transmission electron microscopy, and electrical transport measurements. We demonstrate that the crystallinity and porosity of the formed graphene can be adjusted via the choice of molecular precursors and pyrolysis temperature, and we present a kinetic growth model quantitatively describing the conversion of molecular CNMs into graphene. The synthesized nanoporous graphene monolayers resemble a percolated network of graphene nanoribbons with a high charge carrier mobility (∼600 cm2/(V s)), making them attractive for implementations in electronic field-effect devices.

Flexible automation with compact NMR spectroscopy for continuous production of pharmaceuticals.

Modular plants using intensified continuous processes represent an appealing concept for the production of pharmaceuticals. It can improve quality, safety, sustainability, and profitability compared to batch processes; besides, it enables plug-and-produce reconfiguration for fast product changes. To facilitate this flexibility by real-time quality control, we developed a solution that can be adapted quickly to new processes and is based on a compact nuclear magnetic resonance (NMR) spectrometer. The NMR sensor is a benchtop device enhanced to the requirements of automated chemical production including robust evaluation of sensor data. Beyond monitoring the product quality, online NMR data was used in a new iterative optimization approach to maximize the plant profit and served as a reliable reference for the calibration of a near-infrared (NIR) spectrometer. The overall approach was demonstrated on a commercial-scale pilot plant using a metal-organic reaction with pharmaceutical relevance. Graphical abstract.

Transferable Organic Semiconductor Nanosheets for Application in Electronic Devices.

A method has been developed to stabilize and transfer nanofilms of functional organic semiconductors. The method is based on crosslinking of their topmost layers by low energy electron irradiation. The films can then be detached from their original substrates and subsequently deposited onto new solid or holey substrates retaining their structural integrity. Grazing incidence X-ray diffraction, X-ray specular reflectivity, and UV-Vis spectroscopy measurements reveal that the electron irradiation of ≈50 nm thick pentacene films results in crosslinking of their only topmost ≈5 nm (3-4 monolayers), whereas the deeper pentacene layers preserve their pristine crystallinity. The electronic performance of the transferred pentacene nanosheets in bottom contact field-effect devices is studied and it is found that they are fully functional and demonstrate superior charge injection properties in comparison to the pentacene films directly grown on the contact structures by vapor deposition. The new approach paves the way to integration of the organic semiconductor nanofilms on substrates unfavorable for their direct growth as well as to their implementation in hybrid devices with unusual geometries, e.g., in devices incorporating free-standing sheets.

One-step electrochemical synthesis of nitrogen and sulfur co-doped, high-quality graphene oxide.

High-quality graphene oxide (GO) with high crystallinity and electrical conductivity as well as in situ doped with nitrogen and sulfur is obtained via the electrochemical exfoliation of graphite. Furthermore, iron incorporated GO sheets show promising catalytic activity and stable methanol tolerance durability when used as electrocatalysts for the oxygen reduction reaction.

Direct Growth of Patterned Graphene.

The direct growth of single-layer graphene patterns via electron irradiation of aromatic self-assembled monolayers and subsequent annealing is demonstrated. In this way, a reduction in the number of necessary manufacturing steps is achieved. The formed micro- and nanostructures can be arbitrarily shaped and eventually implemented in a manifold of applications.

Chemical vapor deposition of high quality graphene films from carbon dioxide atmospheres.

The realization of graphene-based, next-generation electronic applications essentially depends on a reproducible, large-scale production of graphene films via chemical vapor deposition (CVD). We demonstrate how key challenges such as uniformity and homogeneity of the copper metal substrate as well as the growth chemistry can be improved by the use of carbon dioxide and carbon dioxide enriched gas atmospheres. Our approach enables graphene film production protocols free of elemental hydrogen and provides graphene layers of superior quality compared to samples produced by conventional hydrogen/methane based CVD processes. The substrates and resulting graphene films were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and Raman microscopy, sheet resistance and transport measurements. The superior quality of the as-grown graphene films on copper is indicated by Raman maps revealing average G band widths as low as 18 ± 8 cm(-1) at 514.5 nm excitation. In addition, high charge carrier mobilities of up to 1975 cm(2)/(V s) were observed for electrons in transferred films obtained from a carbon dioxide based growth protocol. The enhanced graphene film quality can be explained by the mild oxidation properties of carbon dioxide, which at high temperatures enables an uniform conditioning of the substrates by an efficient removal of pre-existing and emerging carbon impurities and a continuous suppression and in situ etching of carbon of lesser quality being co-deposited during the CVD growth.

A universal scheme to convert aromatic molecular monolayers into functional carbon nanomembranes.

Free-standing nanomembranes with molecular or atomic thickness are currently explored for separation technologies, electronics, and sensing. Their engineering with well-defined structural and functional properties is a challenge for materials research. Here we present a broadly applicable scheme to create mechanically stable carbon nanomembranes (CNMs) with a thickness of ~0.5 to ~3 nm. Monolayers of polyaromatic molecules (oligophenyls, hexaphenylbenzene, and polycyclic aromatic hydrocarbons) were assembled and exposed to electrons that cross-link them into CNMs; subsequent pyrolysis converts the CNMs into graphene sheets. In this transformation the thickness, porosity, and surface functionality of the nanomembranes are determined by the monolayers, and structural and functional features are passed on from the molecules through their monolayers to the CNMs and finally on to the graphene. Our procedure is scalable to large areas and allows the engineering of ultrathin nanomembranes by controlling the composition and structure of precursor molecules and their monolayers.

Functional single-layer graphene sheets from aromatic monolayers.

Self-assembled monolayers of aromatic molecules on copper substrates can be converted into high-quality single-layer graphene using low-energy electron irradiation and subsequent annealing. This two-dimensional solid state transformation is characterized on the atomic scale and the physical and chemical properties of the formed graphene sheets are studied by complementary microscopic and spectroscopic techniques and by electrical transport measurements. As substrates, Cu(111) single crystals and the technologically relevant polycrystalline copper foils are successfully used.

Energy-filtered transmission electron microscopy of biological samples on highly transparent carbon nanomembranes.

Ultrathin carbon nanomembranes (CNM) comprising crosslinked biphenyl precursors have been tested as support films for energy-filtered transmission electron microscopy (EFTEM) of biological specimens. Due to their high transparency CNM are ideal substrates for electron energy loss spectroscopy (EELS) and electron spectroscopic imaging (ESI) of stained and unstained biological samples. Virtually background-free elemental maps of tobacco mosaic virus (TMV) and ferritin have been obtained from samples supported by ∼1nm thin CNM. Furthermore, we have tested conductive carbon nanomembranes (cCNM) comprising nanocrystalline graphene, obtained by thermal treatment of CNM, as supports for cryoEM of ice-embedded biological samples. We imaged ice-embedded TMV on cCNM and compared the results with images of ice-embedded TMV on conventional carbon film (CC), thus analyzing the gain in contrast for TMV on cCNM in a quantitative manner. In addition we have developed a method for the preparation of vitrified specimens, suspended over the holes of a conventional holey carbon film, while backed by ultrathin cCNM.

Time of flight-photoemission electron microscope for ultrahigh spatiotemporal probing of nanoplasmonic optical fields.

Nanoplasmonic excitations as generated by few-cycle laser pulses on metal nanostructures undergo ultrafast dynamics with timescales as short as a few hundred attoseconds (1 as = 10(-18) s). So far, the spatiotemporal dynamics of optical fields localized on the nanoscale (nanoplasmonic field) have been hidden from direct access in the real space and time domain. An approach which combines photoelectron emission microscopy and attosecond streaking spectroscopy and which provides direct and non-invasive access to the nanoplasmonic field with nanometer-scale spatial resolution and temporal resolution of the order of 100 as has been proposed (Stockman et al 2007 Nat. Photon. 1 539). To implement this approach, a time of flight-photoemission electron microscope (TOF-PEEM) with ∼25 nm spatial and ∼50 meV energy resolution, which has the potential to detect a nanoplasmonic field with nanometer spatial and attosecond temporal resolution, has been developed and characterized using a 400 nm/60 ps pulsed diode laser. The first experimental results obtained using this newly developed TOF-PEEM in a two-photon photoemission mode with a polycrystalline Cu sample and an Ag microstructure film show that the yield and the kinetic energy of the emitted photoelectrons are strongly affected by the nanolocalized plasmonic field.

Three-dimensional characterization of extreme ultraviolet mask blank defects by interference contrast photoemission electron microscopy.

A photoemission electron microscope based on a new contrast mechanism "interference contrast" is applied to characterize extreme ultraviolet lithography mask blank defects. Inspection results show that positioning of interference destructive condition (node of standing wave field) on surface of multilayer in the local region of a phase defect is necessary to obtain best visibility of the defect on mask blank. A comparative experiment reveals superiority of the interference contrast photoemission electron microscope (Extreme UV illumination) over a topographic contrast one (UV illumination with Hg discharge lamp) in detecting extreme ultraviolet mask blank phase defects. A depth-resolved detection of a mask blank defect, either by measuring anti-node peak shift in the EUV-PEEM image under varying inspection wavelength condition or by counting interference fringes with a fixed illumination wavelength, is discussed.

At-wavelength inspection of sub-40 nm defects in extreme ultraviolet lithography mask blank by photoemission electron microscopy.

A new at-wavelength inspection technology to probe nanoscale defects buried underneath Mo/Si multilayers on an extreme ultraviolet (EUV) lithography mask blank has been implemented using EUV photoemission electron microscopy (EUV-PEEM). EUV-PEEM images of programmed defect structures of various lateral and vertical sizes recorded at an ~13.5 nm wavelength show that 35 nm wide and 4 nm high buried line defects are clearly detectable. The imaging technique proves to be sensitive to small phase jumps, enhancing the edge visibility of the phase defects, which is explained in terms of a standing wave enhanced image contrast at resonant EUV illumination.