903 research outputs found
Structure and stability of graphene nanoribbons in oxygen, carbon dioxide, water, and ammonia
We determine, by means of density functional theory, the stability and the
structure of graphene nanoribbon (GNR) edges in presence of molecules such as
oxygen, water, ammonia, and carbon dioxide. As in the case of
hydrogen-terminated nanoribbons, we find that the most stable armchair and
zigzag configurations are characterized by a non-metallic/non-magnetic nature,
and are compatible with Clar's sextet rules, well known in organic chemistry.
In particular, we predict that, at thermodynamic equilibrium, neutral GNRs in
oxygen-rich atmosphere should preferentially be along the armchair direction,
while water-saturated GNRs should present zigzag edges. Our results promise to
be particularly useful to GNRs synthesis, since the most recent and advanced
experimental routes are most effective in water and/or ammonia-containing
solutions.Comment: accepted for publication in PR
Structure, Stability, Edge States and Aromaticity of Graphene Ribbons
We determine the stability, the geometry, the electronic and magnetic
structure of hydrogen-terminated graphene-nanoribbons edges as a function of
the hydrogen content of the environment by means of density functional theory.
Antiferromagnetic zigzag ribbons are stable only at extremely-low ultra-vacuum
pressures. Under more standard conditions, the most stable structures are the
mono- and di-hydrogenated armchair edges and a zigzag edge reconstruction with
one di- and two mono-hydrogenated sites. At high hydrogen-concentration
``bulk'' graphene is not stable and spontaneously breaks to form ribbons, in
analogy to the spontaneous breaking of graphene into small-width nanoribbons
observed experimentally in solution. The stability and the existence of exotic
edge electronic-states and/or magnetism is rationalized in terms of simple
concepts from organic chemistry (Clar's rule)Comment: 4 pages, 3 figures, accepted for publication by Physical Review
Letter
Insights into Tikhonov regularization: application to trace gas column retrieval and the efficient calculation of total column averaging kernels
Insights are given into Tikhonov regularization and its application
to the retrieval of vertical column densities of atmospheric trace
gases from remote sensing measurements. The study builds upon the
equivalence of the least-squares profile-scaling approach and
Tikhonov regularization method of the first kind with an infinite
regularization strength. Here, the vertical profile is expressed
relative to a reference profile. On the basis of this, we propose a
new algorithm as an extension of the least-squares profile scaling
which permits the calculation of total column averaging kernels on
arbitrary vertical grids using an analytic expression. Moreover, we
discuss the effective null space of the retrieval, which comprises
those parts of a vertical trace gas distribution which cannot be
inferred from the measurements.
Numerically the algorithm
can be implemented in a robust and efficient manner. In particular
for operational data processing with challenging demands on
processing time, the proposed inversion method in combination with
highly efficient forward models is an asset. For demonstration
purposes, we apply the algorithm to CO column retrieval from
simulated measurements in the 2.3 ÎĽm spectral region and
to O<sub>3</sub> column retrieval from the UV. These represent ideal
measurements of a series of spaceborne spectrometers such as
SCIAMACHY, TROPOMI, GOME, and GOME-2. For both spectral ranges, we
consider clear-sky and cloudy scenes where clouds are modelled as an
elevated Lambertian surface. Here, the smoothing error for the
clear-sky and cloudy atmosphere is significant and reaches several
percent, depending on the reference profile which is used for
scaling. This underlines the importance of the column averaging
kernel for a proper interpretation of retrieved column densities.
Furthermore, we show that the smoothing due to regularization can be
underestimated by calculating the column averaging kernel on a too
coarse vertical grid. For both retrievals, this effect becomes
negligible for a vertical grid with 20–40 equally thick layers
between 0 and 50 km
Editorial: Towards a Unifying Pan-Arctic Perspective of the Contemporary and Future Arctic Ocean
An international symposium addressing pan-Arctic perspectives of the marine ecosystems of
the Arctic Ocean took place in October 2017 and this editorial introduces the publications that
derived from the conference. The symposium focused in particular upon physical forcing and
biogeochemical cycling in surface waters of the Arctic Ocean, connectivity between surface and
deep waters in the central basins and adjacent slopes and the ecology of the lesser-known shelf
ecosystems. The symposium was the fourth in a sequence that has pan-Arctic integrations of Arctic
Ocean ecosystems at its core. The series started in 2002 and its first volume was published under
the title Structure and function of contemporary food webs on Arctic shelves (Wassmann, 2006). At
the 2002-meeting, a suite of marine Arctic researchers from the main nations that work in the
Arctic Ocean started applying the now-ubiquitous term pan-Arctic. The term underlined that the
applied research goals and directions were more than a circumarctic perspective, but distinctly
considered the entire expanse of the Arctic Ocean. Based upon this exercise, increased interest
in the Arctic and some of the scientific endeavors of the 4th International Polar Year central
projects and key oceanographers operating in the pan-Arctic region convened at the 2nd pan-Arctic
integration symposium, entitled Arctic Marine Ecosystems in an Era of Rapid Climate Change in
2009 (Wassmann, 2011). After a decade of pan-Arctic research and building upon the foundation
presented in Wassmann (2006, 2011) a 3rd conference was initiated in 2012, entitled Overarching
perspectives of contemporary and future ecosystems in the Arctic Ocean (Wassmann, 2015).
This Research Topic brings together 13 publications from the 4th pan-arctic integration
symposium held in 2017, entitled Toward a Unifying Pan-Arctic Perspective of the Contemporary
and Future Arctic Ocean. We, the editors of the Research Topic, are delighted with the breadth,
quality and diversity of the papers. We introduce the essence of the publications under three,
summarizing headlines
• Physical connectivity, yet regionality
• What shapes pan-Arctic primary production
• The fate of production.
Toward the end we incorporate the knowledge presented in this volume into the overall progress.
and status of pan-Arctic marine ecosystem integration that has been achieved, so far, through the
four pan-Arctic integration symposia
Potential of alternate wetting and drying irrigation practices for the mitigation of ghg emissions from rice fields: Two cases in central luzon (philippines)
Reducing methane (CH) emission from paddy rice production is an important target for many Asian countries in order to comply with their climate policy commitments. National greenhouse gas (GHG) inventory approaches like the Tier-2 approach of the Intergovernmental Panel on Climate Change (IPCC) are useful to assess country-scale emissions from the agricultural sector. In paddy rice, alternate wetting and drying (AWD) is a promising and well-studied water management technique which, as shown in experimental studies, can effectively reduce CH) emissions. However, so far little is known about GHG emission rates under AWD when the technique is fully controlled by farmers. This study assesses CH) and nitrous oxide (N)O) fluxes under continuous flooded (CF) and AWD treatments for seven subsequent seasons on farmers’ fields in a pumped irrigation system in Central Luzon, Philippines. Under AWD management, CH) emissions were substantially reduced (73% in dry season (DS), 21% in wet season (WS)). In all treatments, CH) is the major contributor to the total GHG emission and is, thus, identified as the driving force to the global warming potential (GWP). The contribution of N)O emissions to the GWP was higher in CF than in AWD, however, these only offset 15% of the decrease in CH) emission and, therefore, did not jeopardize the strong reduction in the GWP. The study proves the feasibility of AWD under farmers’ management as well as the intended mitigation effect. Resulting from this study, it is recommended to incentivize dissemination strategies in order to improve the effectiveness of mitigation initiatives. A comparison of single CH) emissions to calculated emissions with the IPCC Tier-2 inventory approach identified that, although averaged values showed a sufficient degree of accuracy, fluctuations for single measurement points have high variation which limit the use of the method for field-level assessments
Estrogens and genomic instability in human cancer cells-involvement of Src/Raf/Erk signaling in micronucleus formation by estrogenic chemicals
This article is available open access through the publisher’s website. Copyright @ 2008 The Authors.Reports of the ability of estrogenic agents such as 17β-estradiol (E2), estriol (E3) and bisphenol A (BPA) to induce micronuclei (MN) in MCF-7 breast cancer cells have prompted us to investigate whether these effects are linked to activation of the estrogen receptor (ER) α. Coadministration of tamoxifen and the pure ER antagonist ICI 182 780 to cells treated with E2 and E3 did not lead to significant reductions in micronucleus frequencies. Since these antiestrogens interfere with the transcriptional activity of the ER and block promotion of ER-dependent gene expression, it appears that this process is not involved in micronucleus formation. However, ER activation also triggers rapid signaling via the Src/Raf/extracellular signal-regulated kinase (Erk) pathway. When MCF-7 cells were exposed to E2 and BPA in combination with the specific kinase inhibitors pyrazolopyrimidine and 2′-amino-3′-methoxyflavone, reductions in micronucleus frequencies occurred. These findings suggest that the Src/Raf/Erk pathway plays a role in micronucleus formation by estrogenic agents. Enhanced activation of the Src/Raf/Erk cascade disturbs the localization of Aurora B kinase to kinetochores, leading to a defective spindle checkpoint with chromosome malsegregation. Using antikinetochore CREST antibody staining, a high proportion of micronucleus containing kinetochores was observed, indicating that such processes are relevant to the induction of MN by estrogens. Our results suggest that estrogens induce MN by causing improper chromosome segregation, possibly by interfering with kinase signaling that controls the spindle checkpoint, or by inducing centrosome amplification. Our findings may have some relevance in explaining the effects of estrogens in the later stages of breast carcinogenesis.European Commissio
Biomechanical analysis of temporomandibular joint dynamics based on real-time magnetic resonance imaging
Aim: The traditional hinge axis theory of temporomandibular joint (TMJ) dynamics is increasingly being replaced by the theory of instantaneous centers of rotation (ICR). Typically, ICR determinations are based on theoretical calculations or three-dimensional approximations of finite element models. Materials and methods: With the advent of real-time magnetic resonance imaging (MRI), natural physiologic movements of the TMJ may be visualized with 15 frames per second. The present study employs real-time MRI to analyze the TMJ biomechanics of healthy volunteers during mandibular movements, with a special emphasis on horizontal condylar inclination (HCI) and ICR pathways. The Wilcoxon rank sum test was used to comparatively analyze ICR pathways of mandibular opening and closure. Results: Mean HCI was 34.8 degrees (± 11.3 degrees) and mean mandibular rotation was 26.6 degrees (± 7.2 degrees). Within a mandibular motion of 10 to 30 degrees, the resulting x- and y-translation during opening and closure of the mandible differed significantly (10 to 20 degrees, x: P = 0.02 and y: P 30 degrees showed no significant differences in x- and y-translation. Near occlusion movements differed only for y-translation (P < 0.01). Conclusion: Real-time MRI facilitates the direct recording of TMJ structures during physiologic mandibular movements. The present findings support the theory of ICR. Statistics confirmed that opening and closure of the mandible follow different ICR pathways, which might be due to muscular activity discrepancies during different movement directions. ICR pathways were similar within maximum interincisal distance (MID) and near occlusion (NO), which might be explained by limited extensibility of tissue fibers (MID) and tooth contact (NO), respectively
Experimentally Engineering the Edge Termination of Graphene Nanoribbons
The edges of graphene nanoribbons (GNRs) have attracted much interest due to
their potentially strong influence on GNR electronic and magnetic properties.
Here we report the ability to engineer the microscopic edge termination of high
quality GNRs via hydrogen plasma etching. Using a combination of
high-resolution scanning tunneling microscopy and first-principles
calculations, we have determined the exact atomic structure of plasma-etched
GNR edges and established the chemical nature of terminating functional groups
for zigzag, armchair and chiral edge orientations. We find that the edges of
hydrogen-plasma-etched GNRs are generally flat, free of structural
reconstructions and are terminated by hydrogen atoms with no rehybridization of
the outermost carbon edge atoms. Both zigzag and chiral edges show the presence
of edge states.Comment: 16+9 pages, 3+4 figure
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