Layered gallium sulfide optical properties from monolayer to CVD crystalline thin films.

Interest in layered van der Waals semiconductor gallium monosulfide (GaS) is growing rapidly because of its wide band gap value between those of two-dimensional transition metal dichalcogenides and of insulating layered materials such as hexagonal boron nitride. For the design of envisaged optoelectronic, photocatalytic and photonic applications of GaS, the knowledge of its dielectric function is fundamental. Here we present a combined theoretical and experimental investigation of the dielectric function of crystalline 2H-GaS from monolayer to bulk. Spectroscopic imaging ellipsometry with micron resolution measurements are corroborated by first principle calculations of the electronic structure and dielectric function. We further demonstrate and validate the applicability of the established dielectric function to the analysis of the optical response of c-axis oriented GaS layers grown by chemical vapor deposition (CVD). These optical results can guide the design of novel, to our knowledge, optoelectronic and photonic devices based on low-dimensional GaS.

Real-time imaging of Na+ reversible intercalation in "Janus" graphene stacks for battery applications.

Sodium, in contrast to other metals, cannot intercalate in graphite, hindering the use of this cheap, abundant element in rechargeable batteries. Here, we report a nanometric graphite-like anode for Na+ storage, formed by stacked graphene sheets functionalized only on one side, termed Janus graphene. The asymmetric functionalization allows reversible intercalation of Na+, as monitored by operando Raman spectroelectrochemistry and visualized by imaging ellipsometry. Our Janus graphene has uniform pore size, controllable functionalization density, and few edges; it can store Na+ differently from graphite and stacked graphene. Density functional theory calculations demonstrate that Na+ preferably rests close to -NH2 group forming synergic ionic bonds to graphene, making the interaction process energetically favorable. The estimated sodium storage up to C6.9Na is comparable to graphite for standard lithium ion batteries. Given such encouraging Na+ reversible intercalation behavior, our approach provides a way to design carbon-based materials for sodium ion batteries.

Determining the Dielectric Tensor of Microtextured Organic Thin Films by Imaging Mueller Matrix Ellipsometry.

Polycrystalline textured thin films with distinct pleochroism and birefringence comprising oriented rotational domains of the orthorhombic polymorph of an anilino squaraine with isobutyl side chains (SQIB) are analyzed by imaging Mueller matrix ellipsometry to obtain the biaxial dielectric tensor. Simultaneous fitting of transmission and oblique incidence reflection Mueller matrix scans combined with the spatial resolution of an optical microscope allows to accurately determine the full biaxial dielectric tensor from a single crystallographic sample orientation. Oscillator dispersion relations model well the dielectric tensor components. Strong intermolecular interactions cause the real permittivity for all three directions to become strongly negative near the excitonic resonances, which is appealing for nanophotonic applications.

Fast, Noncontact, Wafer-Scale, Atomic Layer Resolved Imaging of Two-Dimensional Materials by Ellipsometric Contrast Micrography.

Adequate characterization and quality control of atomically thin layered materials (2DM) has become a serious challenge particularly given the rapid advancements in their large area manufacturing and numerous emerging industrial applications with different substrate requirements. Here, we focus on ellipsometric contrast micrography (ECM), a fast intensity mode within spectroscopic imaging ellipsometry, and show that it can be effectively used for noncontact, large area characterization of 2DM to map coverage, layer number, defects and contamination. We demonstrate atomic layer resolved, quantitative mapping of chemical vapor deposited graphene layers on Si/SiO2-wafers, but also on rough Cu catalyst foils, highlighting that ECM is applicable to all application relevant substrates. We discuss the optimization of ECM parameters for high throughput characterization. While the lateral resolution can be less than 1 µm, we particularly explore fast scanning and demonstrate imaging of a 4″ graphene wafer in 47 min at 10 µm lateral resolution, i.e., an imaging speed of 1.7 cm2/min. Furthermore, we show ECM of monolayer hexagonal BN (h-BN) and of h-BN/graphene bilayers, highlighting that ECM is applicable to a wide range of 2D layered structures that have previously been very challenging to characterize and thereby fills an important gap in 2DM metrology.

Polarisation stabilisation of vertical cavity surface emitting lasers by minimally invasive focused electron beam triggered chemistry.

Local electron triggered reactions of functional surface adsorbates were used as a maskless, dry, and minimally invasive nanolithography concept to stabilize the polarisation of individual vertical cavity surface emitting lasers (VCSELs) on a wafer in a post-processing step. Using a 30 keV focused electron beam of a scanning electron microscope and injecting volatile organo-metallic (CH(3))(2)Au(tfa) molecules, polarisation gratings were directly written on VCSELs by dissociating the surface adsorbed molecules. The electron triggered adsorbate dissociation resulted in electrically conductive Au-C nano-composite material, with gold nanocrystals embedded in a carbonaceous matrix. A resistivity of 2500 µΩcm was measured at a typical composition of 30 at.% Au. This material proved successful in suppressing polarisation switching when deposited as line gratings with a width of 200 nm, a thickness of 50 nm, and a pitch of 500 nm and 1 µm. Refractive index measurements suggest that the optical attenuation by the deposited Au-C material is much lower than by pure Au thus giving a low emission power penalty while keeping the polarisation stable.

In situ removal and purification of biosurfactants by automated surface enrichment.

A new method is described to remove and separate biosurfactants from complex mixtures by compressing and harvesting the liquid surface layer. This method was applied to Bacillus subtilis cultures, in which the lipopeptide antibiotic fengycin as well as the polyketide antibiotic bacillaene were produced. The automated harvesting and collection in a custom-built glass body called 'flounder' was repeated several hundred times. The fengycin concentration in the fractions was found to be four times higher than in the culture centrifugate. Of the overall fengycin, 50% (w/w) were recovered after 300 cycles, 95% (w/w) after 800 harvesting cycles. A separation of fengycin from the less surface-active bacillaene could be achieved due to stronger surface activity of fengycin. The ratio of partition coefficients of fengycin and bacillaene was nine times higher compared to foam fractionation. A stepwise increase of the equilibrium surface tension in the centrifugate from 29 to 33 mN/m indicated a fractionated separation of different surface-active substances. The utilization of cell containing culture broth instead of centrifugate had only slight effects on separation efficiency. These results demonstrate the possibility to separate biosurfactants directly from cultivation without the use of extraction solvents or foam formation.

Chemical indices and methods of multivariate statistics as a tool for odor classification.

Industrial and agricultural off-gas streams are comprised of numerous volatile compounds, many of which have substantially different odorous properties. State-of-the-art waste-gas treatment includes the characterization of these molecules and is directed at, if possible, either the avoidance of such odorants during processing or the use of existing standardized air purification techniques like bioscrubbing or afterburning, which however, often show low efficiency under ecological and economical regards. Selective odor separation from the off-gas streams could ease many of these disadvantages but is not yet widely applicable. Thus, the aim of this paper is to identify possible model substances in selective odor separation research from 155 volatile molecules mainly originating from livestock facilities, fat refineries, and cocoa and coffee production by knowledge-based methods. All compounds are examined with regard to their structure and information-content using topological and information-theoretical indices. Resulting data are fitted in an observation matrix, and similarities between the substances are computed. Principal component analysis and k-means cluster analysis are conducted showing that clustering of indices data can depict odor information correlating well to molecular composition and molecular shape. Quantitative molecule describtion along with the application of such statistical means therefore provide a good classification tool of malodorant structure properties with no thermodynamic data needed. The approximate look-alike shape of odorous compounds within the clusters suggests a fair choice of possible model molecules.

Comparison of modified supports on the base of glycoprotein interaction studies and of adsorption investigations.

The features of matrices, suitable for affinity chromatography, have been extensively investigated and got subject for several reviews. But these investigations show, that there is still a lack in adsorbent characterization and a demand of comparative investigations of adsorbents, based on different materials with a range of different surface functionalities. In this work the performance of self-prepared silica and cellulose-based adsorbents were compared with commercially available polymeric supports. A model system was chosen comprising the lectins concanavalin A (ConA) and wheat germ agglutinin (WGA), which were covalently attached to the support matrices, combining the selectivity of the lectin-sugar interaction with the chemical and mechanical properties of the support that influence the efficacy of the prepared adsorbent. The verification of the different supports provides information about tayloring carbohydrate specific lectin adsorbents. The characterization outlines the main features of the different adsorbents and takes into account the properties of the pure supports. It encompasses immobilization kinetics and isotherms as well as the description of the binding capacity of the adsorbents by depicting adsorption isotherms. The separation performances were also investigated in terms of glycoprotein purification factors and recoveries. Further, detailed information about binding of GOD to immobilized ConA are obtained.

Process development in affinity separation of glycoconjugates with lectins as ligands.

Process development for affinity separation is a crucial prerequisite for a successful biospecific isolation of biological active substances like glycoconjugates or enzymes. The functionalization of polymer and silica based adsorbents and their influence on the adsorption behaviour of the modified adsorbents are presented. Improvement of the immobilization conditions for different lectins lead to a stable binding of more than 90% within 4 h of ligands applied to the immobilization solution. The prepared adsorbents are characterized according to specificity, stability and capacity. The isolation of the glycoprotein fetuin from fetal calf serum with wheat germ agglutinin adsorbents and the purification of horseradish peroxidase with concanavalin A (Con A) adsorbent are described. The Langmuir model, using glucose oxidase as glycoprotein and Con A adsorbents, expresses the sorption behaviour. The fixed bed separation is represented by the dispersion model. The process simulation supports the process development evaluating design parameters and investigating and optimizing process conditions. The influence of the flow as well as the concentration of contaminants competing with the valuable product for the ligands on the separation performance are demonstrated and discussed.