Carbon Nanosheets: Mechanically Stacked 1‐nm‐Thick Carbon Nanosheets: Ultrathin Layered Materials with Tunable Optical,Chemical, and Electrical Properties (Small 7/2011)

Carbon nanosheets are mechanically stable, free‐standing two‐dimensional materials with a thickness of ≈1 nm and well defined physical and chemical properties. They are made by radiation‐induced cross‐linking of aromatic self‐assembled monolayers. Herein, a route is presented to the scalable fabrication of multilayer nanosheets with tunable electrical, optical, and chemical properties on insulating substrates. Stacks of up to five nanosheets with sizes of ≈1 cm² on oxidized silicon are studied. Their optical characteristics are investigated by visual inspection, optical microscopy, UV–vis reflection spectroscopy, and model calculations. Their chemical composition is studied by X‐ray photoelectron spectroscopy. The multilayer samples are then annealed in an ultrahigh vacuum at various temperatures up to 1100 K. A subsequent investigation by Raman, X‐ray photoelectron, and UV–vis reflection spectroscopy, as well as by electrical four‐point probe measurements, demonstrates that the layered nanosheets transform into nanocrystalline graphene. This structural and chemical transformation is accompanied by changes in the optical properties and electrical conductivity and opens up a new path for the fabrication of ultrathin functional conductive coatings.     

Mechanically stacked 1-nm-thick carbon nanosheets: ultrathin layered materials with tunable optical, chemical, and electrical properties

Carbon nanosheets are mechanically stable, free-standing two-dimensional materials with a thickness of ≈1 nm and well defined physical and chemical properties. They are made by radiation-induced cross-linking of aromatic self-assembled monolayers. Herein, a route is presented to the scalable fabrication of multilayer nanosheets with tunable electrical, optical, and chemical properties on insulating substrates. Stacks of up to five nanosheets with sizes of ≈1 cm(2) on oxidized silicon are studied. Their optical characteristics are investigated by visual inspection, optical microscopy, UV-vis reflection spectroscopy, and model calculations. Their chemical composition is studied by X-ray photoelectron spectroscopy. The multilayer samples are then annealed in an ultrahigh vacuum at various temperatures up to 1100 K. A subsequent investigation by Raman, X-ray photoelectron, and UV-vis reflection spectroscopy, as well as by electrical four-point probe measurements, demonstrates that the layered nanosheets transform into nanocrystalline graphene. This structural and chemical transformation is accompanied by changes in the optical properties and electrical conductivity and opens up a new path for the fabrication of ultrathin functional conductive coatings.     

A nonlinearity error calibration technique for pipelined ADCs

This paper presents a digital background calibration technique that measures and cancels offset, linear and nonlinear errors in each stage of a pipelined analog to digital converter (ADC) using a single algorithm. A simple two-step subranging ADC architecture is used as an extra ADC in order to extract the data points of the stage-under-calibration and perform correction process without imposing any changes on the main ADC architecture which is the main trend of the current work. Contrary to the conventional calibration methods that use high resolution reference ADCs, averaging and chopping concepts are used in this work to allow the resolution of the extra ADC to be lower than that of the main ADC.     

Stereoisomeric flavour compounds LXXIV: 2-phenylpropanol, 2-phenylpropanal and 2-phenylpropanal dimethyl acetal: structure elucidation and structure-function relationship

The direct enantioseparation of 2-phenylpropanol, 2-phenylpropanal and 2-phenylpropanal dimethyl acetal was achieved, using enantioselective gas chromatography and heptakis-[2,3-di-0-acetyl-6-0-tert.butyldimethylsilyl]--cyclodextrin (DIAC--CD) or octakis-[2,3-di-0-acetyl-6-0-tert.butyldimethylsilyl]--cyclodex-trin (DIAC--CD) as the chiral stationary phase. Their odour characteristics and threshold values were investigated by enantioselective gas chromatography/olfactometry. Starting from commercially available (R)-2-phenylpropionic acid, enantiopure (R)-configured reference compounds were synthesized.     

Stereoisomere Aromastoffe

Zusammenfassung Es werden Methoden zur racemisie-rungsfreien Stereodifferenzierung der 2-Methylbuttersäure (direkt bzw. nach Esterhydrolyse) aus Äpfeln und apfelhaltigen Lebensmitteln beschrieben. Die Enantiomerenverteilung der 2-Methylbuttersäure kann direkt an permethylierter-Cyclodextrinphase gaschromatogra-phisch bestimmt werden. Es wird stets hoher Enantiomerenüberschuß zugunsten des (S)-Isomeren der Säure nachgewiesen. Proben von naturidentischem und natürlichem (biotechnologisch hergestelltem) 2-Methyl-buttersäureethylester sowie Äpfel, Apfelsaft, Apfellikör, Calvados, Apfelwein, Apfelmarmelade und Joghurts mit Apfelzusatz werden untersucht. Ein Zusatz von naturidentischen, racemischen 2-Methylbuttersäureestern ist einfach nachweisbar.

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Stereoisomeric flavour compounds. XLII. Enantiomer distribution of 2-methylbutanoic acid in apples and apple-containing food

Summary The direct Stereodifferentiation of 2-methyl-butanoic acid and its esters (after hydrolysis) from apples and apple-containing food is described, using permethyl-ated-cyclodextrin as the chiral CGC phase. In all cases, high enantiomeric excess in favour of the (S)-enantiomer was ascertained. The method is applicable to apples, apple juice, apple liquor, calvados, apple wine, apple jam and apple-containing yoghurts. The addition of race-mic (nature-identical) esters may be easily detected.

     

A new method to characterize bifacial solar cells

We present a new method to characterize bifacial solar cells under standard test conditions (STC). The method considers the bifacial operation of the cell and provides the characteristics for simultaneous front and rear side illumination rather than providing the front and the rear side characteristics separately. The method involves measurements of front side electrical parameters (efficiency, open‐circuit voltage, short‐circuit current and fill factor) and rear side short‐circuit current under STC. Two new parameters are introduced, namely bifacial 1.x efficiency (effective efficiency) and gain‐efficiency product, which are calculated from the measured STC parameters. The former provides information related to the cell design considering the bifacial operation, whereas the latter provides the end‐use benefits from the modules with bifacial cells for a particular installation. To calculate the bifacial 1.x efficiency and the gain‐efficiency product, a one‐diode solar cell equivalent circuit is used. Characteristic plots are shown for the newly introduced parameters as a function of rear‐side illumination for various example solar cells. A sensitivity analysis is performed to understand the influence of each single‐sided STC solar cell parameter on the newly introduced parameters. This sensitivity analysis shows that the fill factor and the rear‐to‐front current ratio are the most critical parameters for bifacial solar cells. Copyright © 2012 John Wiley & Sons, Ltd.     

Raman microspectroscopic analysis of changes in the chemical structure and reactivity of soot in a diesel exhaust aftertreatment model system

Raman microspectroscopy has been applied to follow structural changes in spark discharge (GfG) soot and light-duty diesel vehicle (LDV) soot upon oxidation and gasification by nitrogen oxides and oxygen in a diesel exhaust aftertreatment model system at 523 and 573 K. Raman spectra have been recorded before and during the oxidation process, and spectral parameters have been determined by curve fitting with five bands (G, D1-D4). For GfG soot, a steep initial decrease of the relative intensity of the D3 band suggested rapid preferential oxidation of a highly reactive amorphous carbon fraction, while a less steep but also substantial decrease of band widths (in particular, the D1 band) indicated a slower overall increase of chemical homogeneity and structural order in the partially oxidized material. The spectroscopic changes are in agreementwith a strong decrease of chemical reactivity at increasing mass conversion of GfG soot. In contrast, the spectral parameters and reactivity of partially oxidized LDV soot remained largely unchanged throughout the oxidation process. Overall, the spectroscopic and kinetic findings suggest that Raman spectroscopic parameters provide information about the relative abundance and structural order of graphitelike and amorphous carbon and can be used as proxies for the chemical reactivity of soot undergoing oxidation and gasification. Thus, Raman spectroscopy promisesto become an efficient tool forfurther investigation and optimization of diesel exhaust aftertreatment in continuously regenerating traps and particle filters.     

Enzymatic hydrolysis of polyester based coatings

The potential of two hydrolytic enzymes, namely a lipase from Thermomyces lanuginosus (TlL) and a cutinase from Humicola insolens (HiC) for hydrolysis of the phthalic acid backbone based polyester coatings was assessed. Two phthalic acid/trimethylolpropane based model substrates resembling the structure of the polyester backbone of coatings were synthesized. Out of both enzymes, only the cutinase was able to hydrolyze both model substrates while the larger substrate was hydrolyzed at a lower rate. The cutinase was also able to hydrolyze a coating (alkyd resin) both in suspension and as dried film. LC–MS analysis of the hydrolysis products released from the coating revealed the presence of oleic acid, its monoglyceride, phthalic acid and 2-((3-((2-((2,3-dihydroxypropoxy)carbonyl)benzoyl)oxy)-2-hydroxypropoxy)carbonyl)benzoic acid. These results indicate that the enzyme was able to hydrolyze the polyester backbone as well as to release fatty acid side chains. Consequently, enzymatic hydrolysis has a potential for the removal of coatings, their recycling or their functionalization.