Graphene nanostructures, where quantum confinement opens an energy gap in the band structure, hold promise for future electronic devices. To realize the full potential of these materials, ...atomic-scale control over the contacts to graphene and the graphene nanostructure forming the active part of the device is required. The contacts should have a high transmission and yet not modify the electronic properties of the active region significantly to maintain the potentially exciting physics offered by the nanoscale honeycomb lattice. Here we show how contacting an atomically well-defined graphene nanoribbon to a metallic lead by a chemical bond via only one atom significantly influences the charge transport through the graphene nanoribbon but does not affect its electronic structure. Specifically, we find that creating well-defined contacts can suppress inelastic transport channels.
The electronic properties of graphene edges have been predicted to depend on their crystallographic orientation. The so-called zigzag (ZZ) edges haven been extensively explored theoretically and ...proposed for various electronic applications. However, their experimental study remains challenging due to the difficulty in realizing clean ZZ edges without disorder, reconstructions, or the presence of chemical functional groups. Here, we propose the ZZ-terminated, atomically sharp interfaces between graphene and hexagonal boron nitride (BN) as experimentally realizable, chemically stable model systems for graphene ZZ edges. Combining scanning tunneling microscopy and numerical methods, we explore the structure of graphene–BN interfaces and show them to host localized electronic states similar to those on the pristine graphene ZZ edge.
Abstract
African desert dust is emitted and long-range transported with multiple effects on climate, air quality, cryosphere, and ecosystems. On 21–23 February 2021, dust from a sand and dust storm ...in northern Africa was transported to Finland, north of 60°N. The episode was predicted 5 days in advance by the global operational SILAM forecast, and its key features were confirmed and detailed by a retrospective analysis. The scavenging of dust by snowfall and freezing rain in Finland resulted in a rare case of substantial mineral dust contamination of snow surfaces over a large area in the southern part of the country. A citizen science campaign was set up to collect contaminated snow samples prepared according to the scientists’ instructions. The campaign gained wide national interest in television, radio, newspapers and social media, and dust samples were received from 525 locations in Finland, up to 64.3°N. The samples were utilised in investigating the ability of an atmospheric dispersion model to simulate the dust episode. The analysis confirmed that dust came from a wide Sahara and Sahel area from 5000 km away. Our results reveal the features of this rare event and demonstrate how deposition samples can be used to evaluate the skills and limitations of current atmospheric models in simulating transport of African dust towards northern Europe.
The Baltic Sea is a highly frequented shipping area with busy shipping lanes close to
densely populated regions. Exhaust emissions from ship traffic into the atmosphere
do not only enhance air ...pollution, they also affect the Baltic Sea environment
through acidification and eutrophication of marine waters and surrounding terrestrial
ecosystems. As part of the European BONUS project SHEBA (Sustainable Shipping and
Environment of the Baltic Sea region), the transport, chemical transformation and fate
of atmospheric pollutants in the Baltic Sea region were simulated with three regional
chemistry transport model (CTM) systems, CMAQ, EMEP/MSC-W and SILAM, with grid
resolutions between 4 and 11 km. The main goal was to quantify
the effect that shipping emissions have on the regional air quality in the Baltic Sea
region when the same shipping emission dataset but different CTMs are used in their typical
set-ups. The performance of these models and the shipping contribution to
the results of the individual models were evaluated for sulfur dioxide (SO2),
nitrogen dioxide (NO2), ozone (O3) and particulate matter
(PM2.5). Model results from the three CTMs for total air pollutant concentrations
were compared to observations
from rural and urban background stations of the AirBase monitoring network in the
coastal areas of the Baltic Sea region. Observed PM2.5
in summer was underestimated strongly by CMAQ and to some extent by EMEP/MSC-W.
Observed PM2.5 in winter was underestimated by SILAM.
In autumn all models were in better agreement with observed PM2.5.
The spatial average of the annual mean O3 in the EMEP/MSC-W simulation
was ca. 20 %
higher compared to the other two simulations, which is mainly the
consequence of using a different set of boundary conditions for the European model
domain. There are significant differences in the calculated ship contributions to the
levels of air pollutants among the three models.
EMEP/MSC-W, with the coarsest grid, predicted weaker ozone depletion through NO
emissions in the proximity of the main shipping routes than the other two models.
The average contribution of ships to PM2.5 levels in coastal land areas is
in the range of 3.1 %–5.7 % for the three CTMs.
Differences in ship-related PM2.5 between the models are mainly attributed
to differences in the schemes for inorganic aerosol formation.
Differences in the ship-related elemental carbon (EC) among the CTMs can be
explained by differences in the meteorological conditions, atmospheric transport
processes and the applied wet-scavenging parameterizations.
Overall, results from the
present study show the sensitivity of the ship contribution to combined uncertainties
in boundary conditions, meteorological data and aerosol formation and deposition schemes.
This is an important step towards a more reliable evaluation of policy options regarding emission
regulations for ship traffic and the planned introduction of a nitrogen emission control
area (NECA) in the Baltic Sea and the North Sea in 2021.
We implement, optimize, and validate the linear-scaling Kubo–Greenwood quantum transport simulation on graphics processing units by examining resonant scattering in graphene. We consider two ...practical representations of the Kubo–Greenwood formula: a Green–Kubo formula based on the velocity auto-correlation and an Einstein formula based on the mean square displacement. The code is fully implemented on graphics processing units with a speedup factor of up to 16 (using double-precision) relative to our CPU implementation. We compare the kernel polynomial method and the Fourier transform method for the approximation of the Dirac delta function and conclude that the former is more efficient. In the ballistic regime, the Einstein formula can produce the correct quantized conductance of one-dimensional graphene nanoribbons except for an overshoot near the band edges. In the diffusive regime, the Green–Kubo and the Einstein formalisms are demonstrated to be equivalent. A comparison of the length-dependence of the conductance in the localization regime obtained by the Einstein formula with that obtained by the non-equilibrium Green’s function method reveals the challenges in defining the length in the Kubo–Greenwood formalism at the strongly localized regime.
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