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Search Results: 1 - 10 of 444 matches for " Quentin Ramasse "
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Chemistry of the Fe2O3/BiFeO3 Interface in BiFeO3 Thin Film Heterostructures
Miryam Arredondo,Quentin M. Ramasse,Kashinath Bogle,Valanoor Nagarajan
Materials , 2010, DOI: 10.3390/ma3125274
Abstract: We investigate the interfacial chemistry of secondary Fe 2O 3 phases formed in a BiFeO 3 (BFO) layer in BFO/ La 0.67Sr 0.33MnO 3 (LSMO)/SrTiO 3 (STO) heterostructures. A combination of high-resolution spherical aberration corrected scanning TEM and spectroscopy results, reveals that specific chemical and crystallographic similarities between Fe 2O 3 and BFO, enable the BFO layer to form a facile host for Fe 2O 3.
Graphene re-knits its holes
Recep Zan,Quentin M. Ramasse,Ursel Bangert,Konstantin S. Novoselov
Physics , 2012, DOI: 10.1021/nl300985q
Abstract: Nano-holes, etched under an electron beam at room temperature in singlelayer graphene sheets as a result of their interaction with metalimpurities, are shown to heal spontaneously by filling up with either non-hexagon, graphene-like, or perfect hexagon 2D structures. Scanning transmission electron microscopy was employed to capture the healing process and study atom-by-atom the re-grown structure. A combination of these nano-scale etching and re-knitting processes could lead to new graphene tailoring approaches.
Control of Radiation Damage in MoS2 by Graphene Encapsulation
Recep Zan,Quentin M Ramasse,Rashid Jalil,Thanasis Georgiou,Ursel Bangert,Kostya S. Novoselov
Physics , 2013, DOI: 10.1021/nn4044035
Abstract: Recent dramatic progress in studying various two-dimensional (2D) atomic crystals and their heterostructures calls for better and more detailed understanding of their crystallography, reconstruction, stacking order, etc. For this, direct imaging and identification of each and every atom is essential. Transmission Electron Microscopy (TEM) and Scanning Transmission Electron Microscopy (STEM) are ideal, and perhaps the only tools for such studies. However, the electron beam can in some cases induce dramatic structure changes and radiation damage becomes an obstacle in obtaining the desired information in imaging and chemical analysis in the (S)TEM. This is the case of 2D materials such as molybdenum disulfide MoS2, but also of many biological specimens, molecules and proteins. Thus, minimizing damage to the specimen is essential for optimum microscopic analysis. In this letter we demonstrate, on the example of MoS2, that encapsulation of such crystals between two layers of graphene allows for a dramatic improvement in stability of the studied 2D crystal, and permits careful control over the defect nature and formation in it. We present STEM data collected from single layer MoS2 samples prepared for observation in the microscope through three distinct procedures. The fabricated single layer MoS2 samples were either left bare (pristine), placed atop a single-layer of graphene or finally encapsulated between single graphene layers. Their behaviour under the electron beam is carefully compared and we show that the MoS2 sample 'sandwiched' between the graphene layers has the highest durability and lowest defect formation rate compared to the other two samples, for very similar experimental conditions.
Direct Experimental Evidence of Metal-Mediated Etching of Suspended Graphene
Quentin M. Ramasse,Recep Zan,Ursel Bangert,Danil W. Boukhvalov,Young-Woo Son,Konstantin S. Novoselov
Physics , 2012, DOI: 10.1021/nn300452y
Abstract: Atomic resolution high angle annular dark field imaging of suspended, single-layer graphene, onto which the metals Cr, Ti, Pd, Ni, Al and Au atoms had been deposited was carried out in an aberration corrected scanning transmission electron microscope. In combination with electron energy loss spectroscopy, employed to identify individual impurity atoms, it was shown that nano-scale holes were etched into graphene, initiated at sites where single atoms of all the metal species except for gold come into close contact with the graphene. The e-beam scanning process is instrumental in promoting metal atoms from clusters formed during the original metal deposition process onto the clean graphene surface, where they initiate the hole-forming process. Our observations are discussed in the light of calculations in the literature, predicting a much lowered vacancy formation in graphene when metal ad-atoms are present. The requirement and importance of oxygen atoms in this process, although not predicted by such previous calculations, is also discussed, following our observations of hole formation in pristine graphene in the presence of Si-impurity atoms, supported by new calculations which predict a dramatic decrease of the vacancy formation energy, when SiOx molecules are present.
Atomically Resolved Imaging of Highly Ordered Alternating Fluorinated Graphene
Reza J. Kashtiban,M. Adam Dyson,Rahul R. Nair,Recep Zan,Swee L. Wong,Quentin Ramasse,Andre K. Geim,Ursel Bangert,Jeremy Sloan
Physics , 2014, DOI: 10.1038/ncomms5902
Abstract: One of the most desirable goals of graphene research is to produce ordered 2D chemical derivatives of suitable quality for monolayer device fabrication. Here we reveal, by focal series exit wave reconstruction, that C2F chair is a stable graphene derivative and demonstrates pristine long-range order limited only by the size of a functionalized domain. Focal series of images of graphene and C2F chair formed by reaction with XeF2 were obtained at 80 kV in an aberration-corrected transmission electron microscope. EWR images reveal that single carbon atoms and carbon-fluorine pairs in C2F chair alternate strictly over domain sizes of at least 150 nm^2 with electron diffraction indicating ordered domains >/= 0.16 square micrometer. Our results also indicate that, within an ordered domain, functionalization occurs on one side only as theory predicts. Additionally we show that electron diffraction provides a quick and easy method for distinguishing between graphene, C2F chair and fully fluorinated stoichiometric CF 2D phases.
Silicon-carbon bond inversions driven by 60 keV electrons in graphene
Toma Susi,Jani Kotakoski,Demie Kepaptsoglou,Clemens Mangler,Tracy C. Lovejoy,Ondrej L. Krivanek,Recep Zan,Ursel Bangert,Paola Ayala,Jannik C. Meyer,Quentin Ramasse
Physics , 2014, DOI: 10.1103/PhysRevLett.113.115501
Abstract: We demonstrate that 60 keV electron irradiation drives the diffusion of threefold coordinated Si dopants in graphene by one lattice site at a time. First principles simulations reveal that each step is caused by an electron impact on a C atom next to the dopant. Although the atomic motion happens below our experimental time resolution, stochastic analysis of 38 such lattice jumps reveals a probability for their occurrence in a good agreement with the simulations. Conversions from three- to fourfold coordinated dopant structures and the subsequent reverse process are significantly less likely than the direct bond inversion. Our results thus provide a model of non-destructive and atomically precise structural modification and detection for two-dimensional materials.
Liquid exfoliation of solvent-stabilised black phosphorus: applications beyond electronics
Damien Hanlon,Claudia Backes,Evie Doherty,Clotilde S. Cucinotta,Nina C. Berner,Conor Boland,Kangho Lee,Peter Lynch,Zahra Gholamvand,Andrew Harvey,Saifeng Zhang,Kangpeng Wang,Glenn Moynihan,Anuj Pokle,Quentin M. Ramasse,Niall McEvoy,Werner J. Blau,Jun Wang,Stefano Sanvito,David D. ORegan,Georg S. Duesberg,Valeria Nicolosi,Jonathan N. Coleman
Physics , 2015, DOI: 10.1038/ncomms9563
Abstract: Few layer black phosphorus is a new two-dimensional material which is of great interest for applications, mainly in electronics. However, its lack of stability severely limits our ability to synthesise and process this material. Here we demonstrate that high-quality, few-layer black phosphorus nanosheets can be produced in large quantities by liquid phase exfoliation in the solvent N-cyclohexyl-2-pyrrolidone (CHP). We can control nanosheet dimensions and have developed metrics to estimate both nanosheet size and thickness spectroscopically. When exfoliated in CHP, the nanosheets are remarkably stable unless water is intentionally introduced. Computational studies show the degradation to occur by reaction with water molecules only at the nanosheet edge, leading to the removal of phosphorus atoms and the formation of phosphine and phosphorous acid. We demonstrate that liquid exfoliated black phosphorus nanosheets are potentially useful in a range of applications from optical switches to gas sensors to fillers for composite reinforcement.
Estrogen-induced DNA synthesis in vascular endothelial cells is mediated by ROS signaling
Quentin Felty
BMC Cardiovascular Disorders , 2006, DOI: 10.1186/1471-2261-6-16
Abstract: The effect of estrogen on the production of intracellular oxidants and the role of estrogen-induced ROS on cell growth was studied in human umbilical vein endothelial cells. ROS were measured by monitoring the oxidation of 2'7'-dichlorofluorescin by spectrofluorometry. Endothelial cell growth was measured by a colorimetric immunoassay based on BrdU incorporation into DNA.Physiological concentrations of estrogen (367 fmol and 3.67 pmol) triggered a rapid 2-fold increase in intracellular oxidants in endothelial cells. E2-induced ROS formation was inhibited to basal levels by cotreatment with the mitochondrial inhibitor rotenone (2 μM) and xanthine oxidase inhibitor allopurinol (50 μM). Inhibitors of NAD(P)H oxidase, apocynin and DPI, did not block E2-induced ROS formation. Furthermore, the NOS inhibitor, L-NAME, did not prevent the increase in E2-induced ROS. These findings indicate both mitochondria and xanthine oxidase are the source of ROS in estrogen treated vascular endothelial cells. E2 treated cells showed a 2-fold induction of BrdU incorporation at 18 h which was not observed in cells exposed to vehicle alone. Cotreatment with ebselen (20 μM) and NAC (1 mM) inhibited E2-induced BrdU incorporation without affecting the basal levels of DNA synthesis. The observed inhibitory effect of NAC and ebselen on E2-induced DNA synthesis was also shown to be dose dependent.We have shown that estrogen exposure stimulates the rapid production of intracellular ROS and they are involved in growth signaling of endothelial cells. It appears that the early estrogen signaling does not require estrogen receptor genomic signaling because we can inhibit estrogen-induced DNA synthesis by antioxidants. Findings of this study may further expand research defining the underlying mechanism of how estrogen may promote vascular lesions. It also provides important information for the design of new antioxidant-based drugs or new antioxidant gene therapy to protect the cardiovascular health of
The Metaphysical Necessity of Natural Laws.
Quentin Smith
Philosophica , 2001,
La libertad de las repúblicas: un tercer concepto de libertad?
Skinner, Quentin
Isegoría , 2005,
Abstract: In this article the author tries to show that there is a third concept of liberty beyond the two coined by Isaiah Berlin. In order to do that he embarks himself in a historical reconstruction of both hobbesian liberty and liberty before liberalism. He concludes that although liberty as non-interference was a feasible ideal in post-war western societies there are other concepts of liberty equally valuable depending on what account we believe should be given of the normative character of human nature. En este artículo se quiere mostrar que hay un tercer concepto de libertad aparte de los dos descritos por Isaiah Berlin. Para llevar a cabo su propósito el autor realiza una reconstrucción histórica del concepto hobbesiano de libertad y del concepto de libertad al que éste se opuso. Se concluye se alando que, aunque el concepto de libertad como no interferencia pudo se un ideal valioso en el mundo occidental de la posguerra, hay otros conceptos de libertad que son igualmente valiosos en relación a la concepción que tengamos del carácter normativo de la naturaleza humana.
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