7,010 research outputs found
First-principles Study of the Luminescence of Eu2+-doped Phosphors
The luminescence of fifteen representative Eu-doped phosphors used for
white-LED and scintillation applications is studied through a Constrained
Density Functional Theory. Transition energies and Stokes shift are deduced
from differences of total energies between the ground and excited states of the
systems, in the absorption and emission geometries. The general applicability
of such methodology is first assessed: for this representative set, the
calculated absolute error with respect to experiment on absorption and emission
energies is within 0.3 eV. This set of compounds covers a wide range of
transition energies that extents from 1.7 to 3.5 eV. The information gained
from the relaxed geometries and total energies is further used to evaluate the
thermal barrier for the crossover, the full width at half-maximum of
the emission spectrum and the temperature shift of the emission peak, using a
one-dimensional configuration-coordinate model. The former results indicate
that the crossover cannot be the dominant mechanism for the thermal
quenching behavior of Eu-doped phosphors and the latter results are
compared to available experimental data and yield a 30 mean absolute
relative error. Finally, a semi-empirical model used previously for
Ce-doped hosts is adapted to Eu-doped hosts and gives the
absorption and emission energies within 0.9 eV of experiment, underperforming
compared to the first-principles calculation.Comment: 17 pages, 13 figures, (Phys. Rev. B 2017 Accept
First-principles study of Ce doped lanthanum silicate nitride phosphors: Neutral excitation, Stokes shift, and luminescent center identification
We study from first principles two lanthanum silicate nitride compounds,
LaSiN and LaSiN, pristine as well as doped with
Ce ion, in view of explaining their different emission color, and
characterising the luminescent center. The electronic structures of the two
undoped hosts are similar, and do not give a hint to quantitatively describe
such difference. The neutral excitation of the Ce
ions is simulated through a constrained density-functional theory method
coupled with a SCF analysis of total energies, yielding absorption
energies. Afterwards, atomic positions in the excited state are relaxed,
yielding the emission energies and Stokes shifts. Based on these results, the
luminescent centers in LaSiN:Ce and LaSiN:Ce are
identified. The agreement with the experimental data for the computed
quantities is quite reasonable and explains the different color of the emitted
light. Also, the Stokes shifts are obtained within 20\% difference relative to
experimental data.Comment: 12 pages, 10 figure
Assessment of First-Principles and Semiempirical Methodologies for Absorption and Emission Energies of Ce-Doped Luminescent Materials
In search of a reliable methodology for the prediction of light absorption
and emission of Ce-doped luminescent materials, 13 representative
materials are studied with first-principles and semiempirical approaches. In
the first-principles approach, that combines constrained density-functional
theory and SCF, the atomic positions are obtained for both ground and
excited states of the Ce ion. The structural information is fed into
Dorenbos' semiempirical model. Absorption and emission energies are calculated
with both methods and compared with experiment. The first-principles approach
matches experiment within 0.3 eV, with two exceptions at 0.5 eV. In contrast,
the semiempirical approach does not perform as well (usually more than 0.5 eV
error). The general applicability of the present first-principles scheme, with
an encouraging predictive power, opens a novel avenue for crystal site
engineering and high-throughput search for new phosphors and scintillators.Comment: 12 pages, 3 figure
The war for the future
The following essay was born out of the authorsâ participation in the Hamburg (Insecurity) Sessions 2019: Un-Cancelling the Future, and the discussions that took place in the workshop on Future Weapons and Emerging Technologies. The workshop participants critically addressed such problems as the future of defence technologies and their sociotechnical environments, cybersecurity and surveillance proliferation and the improvised instruments of insurgency. Our task was to weave the ideas and insights of the workshopâs participants into a vision of the world in 2040 and use it to motivate an analysis of the technoscientific imaginaries emerging in the present. From the workshop presentations and discussions, we identified three key areas of that allowed us to imagine the outlines and interactions of global security and technoscientific practice in 2040: ecopolitics: the exploitation of ecological systems for strategic ends; technonationalism: the use of advanced technologies to pursue racialised and nationalistic geopolitical agendas; and the security continuum: the extension of conflict modes to all aspects of social life and the open-source proliferation of security tools and techniques
Simultaneous quantification of depolymerization and mineralization rates by a novel 15N tracing model
The depolymerization of soil organic matter, such as proteins and (oligo-)peptides, into monomers (e.g. amino acids) is currently considered to be the rate-limiting step for nitrogen (N) availability in terrestrial ecosystems. The mineralization of free amino acids (FAAs), liberated by the depolymerization of peptides, is an important fraction of the total mineralization of organic N. Hence, the accurate assessment of peptide depolymerization and FAA mineralization rates is important in order to gain a better process-based understanding of the soil N cycle. In this paper, we present an extended numerical 15N tracing model Ntrace, which incorporates the FAA pool and related N processes in order to provide a more robust and simultaneous quantification of depolymerization and gross mineralization rates of FAAs and soil organic N. We discuss analytical and numerical approaches for two forest soils, suggest improvements of the experimental work for future studies, and conclude that (i) when about half of all depolymerized peptide N is directly mineralized, FAA mineralization can be as important a rate-limiting step for total gross N mineralization as peptide depolymerization rate; (ii) gross FAA mineralization and FAA immobilization rates can be used to develop FAA use efficiency (NUEFAA), which can reveal microbial N or carbon (C) limitation
Preventing Ventilator Associated Pneumonia
Ventilator Associated Pneumonia is a serious infection in the lungs that affects individuals who are being mechanically ventilated. These patients, who are already critically ill, have decreased defense mechanisms that allow pathogens such as bacteria to invade the sterile respiratory tract (Cooper, 2021). Many of the risk factors for the development of VAP may be preventable, and it is up to the ICU nurses, who care for these mechanically ventilated patients, to play a role in the implementation of certain guidelines and/or bundles that may help with the prevention of ventilator associated pneumonia ( Aysegul et al., 2020). Although ventilator associated pneumonia may never be 100% prevented, the existing prevention guidelines are the best available resources to improve outcomes for ventilated patients. In our presentation, we elaborated on the most common prevention guidelines used in practice today
When group dispersal and Allee effect shape metapopulation dynamics
The dispersal ability of a species will be critical for how population dynamics are realized in spatially structured systems. To date, the effect of group dispersal on metapopulation dynamics is poorly understood. Here, we investigate how group dispersal and Allee effects shape metapopulation dynamics identifying conditions in which group dispersal can be an advantage over independent dispersal. We approach this question by building and analysing a Markovian random walk for metapopulation dynamics including group dispersal and Allee effect. This Markovian random walk is analogous to the discrete-time Stochastic Patch Occupancy Model (SPOM). We find that intermediate group sizes may lead to larger and more sustainable metapopulations in the presence of an Allee effect. Hence, understanding how group size variation and realized (meta) population dynamics are linked offers an exciting future venue for research that is expected to yield key insights into the ecology and evolution of populations occupying spatially structured environments.Peer reviewe
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