11 research outputs found
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Potential economic impact of COVID-19-related school closures
Severe disruptions in school education during the coronavirus disease (COVIDā19) pandemic has impacted children through their formative years which will affect their employment opportunities and earning potential for many years after school ages. This paper examines the medium-to-long-term economic scarring effects, using data available through the Global Trade Analysis Project, a computable general equilibrium model, with empirical study focusing on the impact of school closures on economic growth and employment. The estimated results show significant declines in global gross domestic product (GDP) and employment. Moreover, the losses in global GDP and employment increase over time. Declines in global GDP amount to 0.19% in 2024, 0.64% in 2028, and 1.11% in 2030. In absolute terms, the cost to the global economy in 2030 alone is $943 billion. The scarring effects are greater in economies with significant student populations from rural areas, those in the poorest and second wealth quintile. Learning and earning losses are also significant in economies where the share of unskilled labor employment in the overall labor force is high.</p
Heterogeneous Pyrolysis: A Route for Epitaxial Growth of hBN Atomic Layers on Copper Using Separate Boron and Nitrogen Precursors
Growth
of hBN on metal substrates is often performed via chemical vapor deposition
from a single precursor (e.g., borazine) and results in hBN monolayers
limited by the substrates catalyzing effect. Departing from this paradigm,
we demonstrate close control over the growth of mono-, bi-, and trilayers
of hBN on copper using triethylborane and ammonia as independent sources
of boron and nitrogen. Using density functional theory (DFT) calculations
and reactive force field molecular dynamics, we show that the key
factor enabling the growth beyond the first layer is the activation
of ammonia through heterogeneous pyrolysis with boron-based radicals
at the surface. The hBN layers grown are in registry with each other
and assume a perfect or near perfect epitaxial relation with the substrate.
From atomic force microscopy (AFM) characterization, we observe a
moireĢ superstructure in the first hBN layer with an apparent
height modulation and lateral periodicity of ā¼10 nm. While
this is unexpected given that the moireĢ pattern of hBN/Cu(111)
does not have a significant morphological corrugation, our DFT calculations
reveal a spatially modulated interface dipole layer which determines
the unusual AFM response. These findings have improved our understanding
of the mechanisms involved in growth of hBN and may help generate
new growth methods for applications in which control over the number
of layers and their alignment is crucial (such as tunneling barriers,
ultrathin capacitors, and graphene-based devices)
Configurational-Bias Monte Carlo Back-Mapping Algorithm for Efficient and Rapid Conversion of Coarse-Grained Water Structures into Atomistic Models
Coarse-grained
molecular dynamics (MD) simulations represent a
powerful approach to simulate longer time scale and larger length
scale phenomena than those accessible to all-atom models. The gain
in efficiency, however, comes at the cost of atomistic details. The
reverse transformation, also known as back mapping, of coarse-grained
beads into their atomistic constituents represents a major challenge.
Most existing approaches are limited to specific molecules or specific
force fields and often rely on running a long-time atomistic MD of
the back-mapped configuration to arrive at an optimal solution. Such
approaches are problematic when dealing with systems with high diffusion
barriers. Here, we introduce a new extension of the configurational-bias
Monte Carlo (CBMC) algorithm, which we term the crystalline-configurational-bias
Monte Carlo (C-CBMC) algorithm, which allows rapid and efficient conversion
of a coarse-grained model back into its atomistic representation.
Although the method is generic, we use a coarse-grained water model
as a representative example and demonstrate the back mapping or reverse
transformation for model systems ranging from the iceāliquid
water interface to amorphous and crystalline ice configurations. A
series of simulations using the TIP4P/Ice model are performed to compare
the new CBMC method to several other standard Monte Carlo and molecular
dynamics-based back-mapping techniques. In all of the cases, the C-CBMC
algorithm is able to find optimal hydrogen-bonded configuration many
thousand evaluations/steps sooner than the other methods compared
within this paper. For crystalline ice structures, such as a hexagonal,
cubic, and cubic-hexagonal stacking disorder structures, the C-CBMC
was able to find structures that were between 0.05 and 0.1 eV/water
molecule lower in energy than the ground-state energies predicted
by the other methods. Detailed analysis of the atomistic structures
shows a significantly better global hydrogen positioning when contrasted
with the existing simpler back-mapping methods. The errors in the
radial distribution functions (RDFs) of back-mapped configuration
relative to reference configuration for the C-CBMC, MD, and MC were
found to be 6.9, 8.7, and 12.9, respectively, for the hexagonal system.
For the cubic system, the relative errors of the RDFs for the C-CBMC,
MD, and MC were found to be 18.2, 34.6, and 39.0, respectively. Our
results demonstrate the efficiency and efficacy of our new back-mapping
approach, especially for crystalline systems where simple force-field-based
relaxations have a tendency to get trapped in local minima
<i>Ab Initio</i>-Based Bond Order Potential to Investigate Low Thermal Conductivity of Stanene Nanostructures
We
introduce a bond order potential (BOP) for stanene based on
an <i>ab initio</i> derived training data set. The potential
is optimized to accurately describe the energetics, as well as thermal
and mechanical properties of a free-standing sheet, and used to study
diverse nanostructures of stanene, including tubes and ribbons. As
a representative case study, using the potential, we perform molecular
dynamics simulations to study staneneās structure and temperature-dependent
thermal conductivity. We find that the structure of stanene is highly
rippled, far in excess of other 2-D materials (e.g., graphene), owing
to its low in-plane stiffness (stanene: ā¼ 25 N/m; graphene:
ā¼ 480 N/m). The extent of staneneās rippling also shows
stronger temperature dependence compared to that in graphene. Furthermore,
we find that stanene based nanostructures have significantly lower
thermal conductivity compared to graphene based structures owing to
their softness (i.e., low phonon group velocities) and high anharmonic
response. Our newly developed BOP will facilitate the exploration
of stanene based low dimensional heterostructures for thermoelectric
and thermal management applications
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Theme 1 - reseaux et systemes - Projet ResedasSIGLEAvailable from INIST (FR), Document Supply Service, under shelf-number : 14802 E, issue : a.2000 n.3869 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc
Defect Dynamics in 2āD MoS<sub>2</sub> Probed by Using Machine Learning, Atomistic Simulations, and High-Resolution Microscopy
Structural
defects govern various physical, chemical, and optoelectronic
properties of two-dimensional transition-metal dichalcogenides (TMDs).
A fundamental understanding of the spatial distribution and dynamics
of defects in these low-dimensional systems is critical for advances
in nanotechnology. However, such understanding has remained elusive
primarily due to the inaccessibility of (a) necessary time scales <i>via</i> standard atomistic simulations and (b) required spatiotemporal
resolution in experiments. Here, we take advantage of supervised machine
learning, <i>in situ</i> high-resolution transmission electron
microscopy (HRTEM) and molecular dynamics (MD) simulations to overcome
these limitations. We combine genetic algorithms (GA) with MD to investigate
the extended structure of point defects, their dynamical evolution,
and their role in inducing the phase transition between the semiconducting
(2H) and metallic (1T) phase in monolayer MoS<sub>2</sub>. GA-based
structural optimization is used to identify the long-range structure
of randomly distributed point defects (sulfur vacancies) for various
defect densities. Regardless of the density, we find that organization
of sulfur vacancies into extended lines is the most energetically
favorable. HRTEM validates these findings and suggests a phase transformation
from the 2H-to-1T phase that is localized near these extended defects
when exposed to high electron beam doses. MD simulations elucidate
the molecular mechanism driving the onset of the 2H to 1T transformation
and indicate that finite amounts of 1T phase can be retained by increasing
the defect concentration and temperature. This work significantly
advances the current understanding of defect structure/evolution and
structural transitions in 2D TMDs, which is crucial for designing
nanoscale devices with desired functionality
Development of a Modified Embedded Atom Force Field for Zirconium Nitride Using Multi-Objective Evolutionary Optimization
Zirconium nitride (ZrN) exhibits
exceptional mechanical, chemical,
and electrical properties, which make it attractive for a wide range
of technological applications, including wear-resistant coatings,
protection from corrosion, cutting/shaping tools, and nuclear breeder
reactors. Despite its broad usability, an atomic scale understanding
of the superior performance of ZrN, and its response to external stimuli,
for example, temperature, applied strain, and so on, is not well understood.
This is mainly due to the lack of interatomic potential models that
accurately describe the interactions between Zr and N atoms. To address
this challenge, we develop a modified embedded atom method (MEAM)
interatomic potential for the ZrāN binary system by training
against formation enthalpies, lattice parameters, elastic properties,
and surface energies of ZrN (and, in some cases, also Zr<sub>3</sub>N<sub>4</sub>) obtained from density functional theory (DFT) calculations.
The best set of MEAM parameters are determined by employing a multiobjective
global optimization scheme driven by genetic algorithms. Our newly
developed MEAM potential accurately reproduces structure, thermodynamics,
energetic ordering of polymorphs, as well as elastic and surface properties
of ZrāN compounds, in excellent agreement with DFT calculations
and experiments. As a representative application, we employed molecular
dynamics simulations based on this MEAM potential to investigate the
atomic scale mechanisms underlying fracture of bulk and nanopillar
ZrN under applied uniaxial strains, as well as the impact of strain
rate on their mechanical behavior. These simulations indicate that
bulk ZrN undergoes brittle fracture irrespective of the strain rate,
while ZrN nanopillars show quasi-plasticity owing to amorphization
at the crack front. The MEAM potential for ZrāN developed in
this work is an invaluable tool to investigate atomic-scale mechanisms
underlying the response of ZrN to external stimuli (e.g, temperature,
pressure etc.), as well as other interesting phenomena such as precipitation
Ultrafast Three-Dimensional Xāray Imaging of Deformation Modes in ZnO Nanocrystals
Imaging
the dynamical response of materials following ultrafast excitation
can reveal energy transduction mechanisms and their dissipation pathways,
as well as material stability under conditions far from equilibrium.
Such dynamical behavior is challenging to characterize, especially <i>operando</i> at nanoscopic spatiotemporal scales. In this letter,
we use X-ray coherent diffractive imaging to show that ultrafast laser
excitation of a ZnO nanocrystal induces a rich set of deformation
dynamics including characteristic āhardā or inhomogeneous
and āsoftā or homogeneous modes at different time scales,
corresponding respectively to the propagation of acoustic phonons
and resonant oscillation of the crystal. By integrating the 3D nanocrystal
structure obtained from the ultrafast X-ray measurements with a continuum
thermo-electro-mechanical finite element model, we elucidate the deformation
mechanisms following laser excitation, in particular, a torsional
mode that generates a 50% greater electric potential gradient than
that resulting from the flexural mode. Understanding of the time-dependence
of these mechanisms on ultrafast scales has significant implications
for development of new materials for nanoscale power generation
Ultrafast Three-Dimensional Xāray Imaging of Deformation Modes in ZnO Nanocrystals
Imaging
the dynamical response of materials following ultrafast excitation
can reveal energy transduction mechanisms and their dissipation pathways,
as well as material stability under conditions far from equilibrium.
Such dynamical behavior is challenging to characterize, especially <i>operando</i> at nanoscopic spatiotemporal scales. In this letter,
we use X-ray coherent diffractive imaging to show that ultrafast laser
excitation of a ZnO nanocrystal induces a rich set of deformation
dynamics including characteristic āhardā or inhomogeneous
and āsoftā or homogeneous modes at different time scales,
corresponding respectively to the propagation of acoustic phonons
and resonant oscillation of the crystal. By integrating the 3D nanocrystal
structure obtained from the ultrafast X-ray measurements with a continuum
thermo-electro-mechanical finite element model, we elucidate the deformation
mechanisms following laser excitation, in particular, a torsional
mode that generates a 50% greater electric potential gradient than
that resulting from the flexural mode. Understanding of the time-dependence
of these mechanisms on ultrafast scales has significant implications
for development of new materials for nanoscale power generation
Effect of the Hydrofluoroether Cosolvent Structure in Acetonitrile-Based Solvate Electrolytes on the Li<sup>+</sup> Solvation Structure and LiāS Battery Performance
We evaluate hydrofluoroether
(HFE) cosolvents with varying degrees of fluorination in the acetonitrile-based
solvate electrolyte to determine the effect of the HFE structure on
the electrochemical performance of the LiāS battery. Solvates
or sparingly solvating electrolytes are an interesting electrolyte
choice for the LiāS battery due to their low polysulfide solubility.
The solvate electrolyte with a stoichiometric ratio of LiTFSI salt
in acetonitrile, (MeCN)<sub>2</sub>āLiTFSI, exhibits limited
polysulfide solubility due to the high concentration of LiTFSI. We
demonstrate that the addition of highly fluorinated HFEs to the solvate
yields better capacity retention compared to that of less fluorinated
HFE cosolvents. Raman and NMR spectroscopy coupled with ab initio
molecular dynamics simulations show that HFEs exhibiting a higher
degree of fluorination coordinate to Li<sup>+</sup> at the expense
of MeCN coordination, resulting in higher free MeCN content in solution.
However, the polysulfide solubility remains low, and no crossover
of polysulfides from the S cathode to the Li anode is observed