169,833 research outputs found
Recommended from our members
Molecular-level Design of Heterogeneous Chiral Catalysts
No full textIt has been shown previously that the adsorption of a chiral 2-butanol template on Pd(111) leads to enantioselective adsorption of chiral propylene oxide probe molecules. Enantioselectivity is expressed over a narrow coverage range where the maximum value of enantioselectivity ratio (ER defined as Θ(R-propylene oxide)/Θ(S-propylene oxide), where Θ is the coverage) reaches ~2. Probe coverages in this case were measured using either reflection-absorption infrared spectroscopy (RAIRS) or temperature-programmed desorption (TPD) [1,2]. The enantioselectivity disappears when the 2-butanol-covered surface was heated to ~200 K since the adsorbed butoxy species decomposes by a β-hydride elimination reaction to yield a non-chiral ketone. Monte Carlo calculations of the effect of chiral modifiers have yielded results that are consistent with these experimental observations [3,4]. Similar experiments using 2-methyl butanoic acid as a template, where the chiral center is identical to that in 2-butanol but is now anchored by a carboxylate group rather than by an alkoxide, shows no enantioselectivity. In this case, propylene oxide coverages were measured using the King and Wells method. RAIRS experiments and density functional calculations suggest that the 2-butyl group of the 2-butoxy species is oriented parallel to the surface. A possible origin for the lack of enantioselectivity of a 2-methyl butanoic acid-covered surface may be that the 2-butyl group is farther from the surface, allowing it to rotate more freely, averaging out any asymmetry, resulting in a loss of chirality. In order to test this idea, the alkyl group on the carboxylic acid was functionalized with an amine to anchor the chiral center to the surface. Using the amino-acids alanine and 2-amino butanoic acid as templates restored the enantioselectivity and yielded ER values of 2.0 ± 0.2 and 1.75 ± 0.15 respectively. These results suggest that a two-point attachment of the chiral template is required, one for surface adsorption and the other to allow the enantioselectivity to be expressed. Low-energy electron diffraction (LEED) intensity versus energy (I/E) measurements are used to measure the structure of templates and probes on the Pd(111) surface, where these results will be compared with calculations carried out by the Sholl group. Since the aminoacids are relatively large, initial experiments were carried out to determine the structure of carboxylates on the surface to determine the carboxylate group anchoring site. Since carboxylates do not form ordered structures on Pd(111), we have exploited a method recently developed in collaboration with Professor Saldin to measure structures of disordered overlayers [5]. Results show that the formate OCO plane is oriented perpendicular to the surface with the oxygen atoms located across a short bridge on the (111) surface. The effect of the size of the functional group on the amino acid template (RCH(NH2)COOH) was also investigated where the maximum ER values obtained using propylene oxide were 2.0 ± 0.2 (R=CH3), 1.75 ± 0.15 (R=C2H5), 1.65 ± 0.15 (R=C3H6) and 1.30 ± 0.15 (R=CH2CH(CH3)2) thus showing a decreasing trend with increasing size of the side chain. The enantioselectivity of S-(1-naphthyl) ethylamine-covered surfaces have been explored using propylene oxide as a probe, but these systems showed no enantioselectivity. However, using 2-butanol as a probe lead to enantioselective chemisorption implying that one-to-one modification requires a direct hydrogen-bonding interaction between the probe and modifier. 1. Enantioselective Chemisorption on a Chirally Patterned Surface in Ultrahigh Vacuum: Adsorption of Propylene Oxide on 2-butoxy-Covered Pd(111), D. Stacchiola, L. Burkholder and W.T. Tysoe, J. Am. Chem. Soc., 124, 8984 (2002) 2. Enantioselective Chemisorption on a Chirally Modified Surface in Ultrahigh Vacuum: Adsorption of Propylene Oxide on 2-butoxide-Covered Pd(111), Darío Stacchiola, Luke Burkholder and Wilfred T. Tysoe, J. Mol. Catal A: Chemical, 216, 215 (2004) 3. Theoretical Analysis of the Coverage Dependence of Enantioselective Chemisorption on a Chirally Patterned Surface, F. Roma, D. Stacchiola, G. Zgrablich and W. T. Tysoe, Journal of Chemical Physics, 118, 6030 (2003) 4. Lattice-gas Modeling of Enantioselective Adsorption by Template Chiral Substrates, F. Romá, D. Stacchiola, W.T. Tysoe and G. Zgrablich, Physica A., 338, 493 (2004) 5. Structure Determination of Disordered Organic Molecules on Surfaces from the Bragg Spots of Low Energy Electron Diffraction and Total Energy Calculations, H. C. Poon, M. Weinert, D. K. Saldin, D. Stacchiola, T. Zheng and W. T. Tysoe, Phys. Rev. B., 69, 35401 (2004
Ethene adsorption and decomposition on the Cu(410) surface
No full textThe influence of open steps on the surface properties is shown by investigating the interaction of molecular ethene with Cu(410). We find a surprisingly low-temperature, site-selective chemistry at the strongly under-coordinated step sites. Ethene bonds either in a pi-bonded or in a di-sigma-bonded state or undergoes complete dehydrogenation. All pathways involve the low-coordination sites at the step, since the first species is partially stabilized with respect to low-Miller-index Surfaces, while the other two are observed only on Cu(410). When annealing the surface, dehydrogenation and transformation into the di-sigma-bonded moiety proceed, both processes being favored by faster heating rates. The so-generated carbon (presumably C(2) admolecules) decorates the step edges, thereby blocking the active sites for subsequent dissociation and permitting only pi-bonding of ethene. The dip de loss of carbon disappears ill high-resolution electron energy loss spectroscopy when annealing to room temperature, indicating that carbon moves to more coplanar or even to subsurface sites where it still influences the sin-face chemistry. The surface reactivity is recovered when heating the crystal to 900 K since C dissolves then deep enough into the bulk
Probing the outermost layer of thin gold films by XPS and density functional theory
No full textThis work examines gold thin films of various thicknesses varying from 5 to over 100 nm along a 7 cm distance to investigate the effect on the surface sensitivity of the XPS Au4f signal. Increasing the relative intensity of the surface XPS signal can be useful for studying properties of the surface atoms in case of functionalized gold, and for shedding light on reactions occurring at the interface with the metal surface as well as adsorption geometry. The surface-to-bulk signal ratio increased with decreasing thickness for thicknesses greater than 15 nm. Below this value, the intensity of the surface signal decreased. The binding-energy shift of the surface component with respect to the bulk component was found to be in agreement with that calculated by density functional theory (DFT). The calculated shift was found to be essentially unaffected, whether or not final-state effects were included. KPFM was also performed to investigate correlations between the measured work function and the surface/bulk BE shift, revealing that the contact potential difference changes with thickness because of the influence of the substrate. Gradients were prepared by a novel evaporation method, by template stripping, and by photolithography, yielding stripes of increasing thickness in the same sample
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Mitomycin C in highly myopic eyes - Author reply
No full textOphthalmology. 2005 Feb;112(2):208-18; discussion 219.
Mitomycin C modulation of corneal wound healing after photorefractive keratectomy in highly myopic eyes.
Gambato C, Ghirlando A, Moretto E, Busato F, Midena E.
SourceRefractive Surgery Service and Antimetabolite Therapy Research Unit, Department of Ophthalmology, University of Padova, Padova, Italy.
Abstract
PURPOSE: To evaluate the role of topical mitomycin C in corneal wound healing (CWH) after photorefractive keratectomy (PRK) in highly myopic eyes.
DESIGN: Prospective, double-masked, randomized clinical trial.
PARTICIPANTS: Seventy-two eyes of 36 patients affected by high (>7 diopters) myopia.
METHODS: In each patient, one eye was randomly assigned to PRK with intraoperative topical 0.02% mitomycin C application, and the fellow eye was treated with a placebo. Postoperatively, mitomycin C-treated eyes received artificial tears (3 times daily, tapered in 3 months), whereas the fellow eye was treated with fluorometholone sodium 2% and artificial tears (3 times daily, tapered in 3 months).
MAIN OUTCOME MEASURES: Uncorrected visual acuity (UCVA) and best-corrected visual acuity (BCVA), contrast sensitivity, manifest refraction, and biomicroscopy. Contrast sensitivity was determined using the Pelli-Robson chart. Corneal confocal microscopy documented CWH.
RESULTS: Mean follow-up was 18 months (range, 12-36). No side effects or toxic effects were documented. At 12-month follow-up examination, UCVAs (logarithm of the minimum angle of resolution) were 0.4+/-0.48 and 0.5+/-0.53 (P = .03) in mitomycin C-treated eyes and corticosteroid-treated eyes, respectively. At 1 year, corneal haze developed in 20% of corticosteroid-treated eyes, versus 0% of mitomycin C-treated eyes. At 12, 24, and 36 months, corneal confocal microscopy showed activated keratocytes and extracellular matrix significantly more evident in untreated eyes (Ps = 0.004, 0.024, and 0.046, respectively).
CONCLUSION: Topical intraoperative application of 0.02% mitomycin C can reduce haze formation in highly myopic eyes undergoing PRK.
Comment in
Ophthalmology. 2006 Feb;113(2):357; author reply 357-8
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Ethylene Decomposition at Undercoordinated Sites on Cu(410)
No full textControlling hydrocarbon chemistry on metal surfaces is important
for both heterogeneous catalysis and nanotechnology. Formation
of C-C bonds and C-H scission is central to carbon nanotube
synthesis whereas the inhibition of carbon growth (C-C breaking)
is important for preventing the poisoning of industrial catalysts.
Although the importance of special surface active sites for catalysis has been known for many years, only a few investigations deal with the influence of undercoordinated sites on reaction selectivity.
In this communication we demonstrate the bond breaking selectivity for molecular ethylene adsorbed on Cu(410). While on terraces ethylene is reversibly π-bonded, the step sites induce not only unexpected di-σ-bonding but also complete dehydrogenation at rather low crystal temperatures, T. The resulting carbon decorates the step edges
A Multi-Language Comparison of Influences on Author Verification using Character N-Grams
No full textWe create a new multi-language corpus for author verification based on Wikipedia talkpages, and evaluate the influence that differences in topic and time have on character n-gram author profiles. Topic alignment between two texts is found to increase author verification precision, and an authors writing style is found to change over time, but not more significantly after 3 years than after 1 year.Information ArchitectureWISElectrical Engineering, Mathematics and Computer Scienc
A 0.12mm<sup>2</sup> Wien-Bridge Temperature Sensor with 0.1°C (3σ) Inaccuracy from -40°C to 180°C
No full text Resistor-based temperature sensors can achieve much higher resolution and energy efficiency than conventional BJT-based sensors [1], but they typically occupy more area (> 0.25 mm 2 ) and have lower operating temperatures (le 125 {circ} {C}) [2]-[4]. This work describes a 0.12mm 2 resistor-based sensor that uses a Wien-bridge (WB) filter to achieve 0.1 {circ} {C} (3 sigma) inaccuracy from - 40 {circ} {C} to 180 {circ} {C}. Compared to a state-of-the-art WB sensor [4], it occupies 6 × less area and achieves comparable relative accuracy over a 76% wider operating range. Session 10.3 Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Electronic InstrumentationMicroelectronic
A ±25A Versatile Shunt-Based Current Sensor with 10kHz Bandwidth and ±0.25% Gain Error from -40°C to 85°C Using 2-Current Calibration
No full textAccurate current sensing is critical in many industrial applications, such as battery management and motor control. Precise shunt-based current sensors have been reported with gain errors of less than 1% over the industrial temperature range (-40°C to 85°C) [1]–[4]. However, since they are intended for coulomb counting, their bandwidth is limited to a few tens of Hz, making them unsuitable for battery impedance or motor-current sensing. This paper presents a current sensor with a wide (10kHz) bandwidth and a tunable temperature compensation scheme (TCS), which allows it to be flexibly used with different types of shunts while maintaining high accuracy. A low-cost room-temperature calibration scheme is proposed to optimize gain flatness over temperature by exploiting the shunt's self-heating at large currents. Over the industrial temperature range and a ±25A current range, it achieves state-of-the-art gain error (±0.25%) with both low-cost PCB and stable metal-alloy shunts.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Electronic InstrumentationMicroelectronic
- …
