77 research outputs found
Water formation on bare grains: When the chemistry on dust impacts interstellar gas
Context. Water together with O2 are important gas phase ingredients to cool
dense gas in order to form stars. On dust grains, H2 O is an important
constituent of the icy mantle in which a complex chemistry is taking place, as
revealed by hot core observations. The formation of water can occur on dust
grain surfaces, and can impact gas phase composition. Aims. The formation of
molecules such as OH, H2 O, HO2, H2 O2, as well as their deuterated forms and
O2 and O3 is studied in order to assess how the chemistry varies in different
astrophysical environments, and how the gas phase is affected by grain surface
chemistry. Methods. We use Monte Carlo simulations to follow the formation of
molecules on bare grains as well as the fraction of molecules released into the
gas phase. We consider a surface reaction network, based on gas phase
reactions, as well as UV photo-dissociation of the chemical species. Results.
We show that grain surface chemistry has a strong impact on gas phase
chemistry, and that this chemistry is very different for different dust grain
temperatures. Low temperatures favor hydrogenation, while higher temperatures
favor oxygenation. Also, UV photons dissociate the molecules on the surface,
that can reform subsequently. The formation-destruction cycle increases the
amount of species released into the gas phase. We also determine the time
scales to form ices in diffuse and dense clouds, and show that ices are formed
only in shielded environments, as supported by observations.Comment: Accepted in A&
Imaging and Dynamics of Light Atoms and Molecules on Graphene
Observing the individual building blocks of matter is one of the primary
goals of microscopy. The invention of the scanning tunneling microscope [1]
revolutionized experimental surface science in that atomic-scale features on a
solid-state surface could finally be readily imaged. However, scanning
tunneling microscopy has limited applicability due to restrictions, for
example, in sample conductivity, cleanliness, and data aquisition rate. An
older microscopy technique, that of transmission electron microscopy (TEM) [2,
3] has benefited tremendously in recent years from subtle instrumentation
advances, and individual heavy (high atomic number) atoms can now be detected
by TEM [4 - 7] even when embedded within a semiconductor material [8, 9].
However, detecting an individual low atomic number atom, for example carbon or
even hydrogen, is still extremely challenging, if not impossible, via
conventional TEM due to the very low contrast of light elements [2, 3, 10 -
12]. Here we demonstrate a means to observe, by conventional transmision
electron microscopy, even the smallest atoms and molecules: On a clean
single-layer graphene membrane, adsorbates such as atomic hydrogen and carbon
can be seen as if they were suspended in free space. We directly image such
individual adatoms, along with carbon chains and vacancies, and investigate
their dynamics in real time. These techniques open a way to reveal dynamics of
more complex chemical reactions or identify the atomic-scale structure of
unknown adsorbates. In addition, the study of atomic scale defects in graphene
may provide insights for nanoelectronic applications of this interesting
material.Comment: 9 pages manuscript and figures, 9 pages supplementary informatio
HD and H2 formation in low-metallicity dusty gas clouds at high redshift
Context: The HD and H2 molecules play important roles in the cooling of
primordial and very metal-poor gas at high redshift. Aims: Grain surface and
gas phase formation of HD and H2 is investigated to assess the importance of
trace amounts of dust, 10^{-5}-10^{-3} Zo, in the production of HD and H2.
Methods: We consider carbonaceous and silicate grains and include both
physisorption and chemisorption, tunneling, and realistic grain surface
barriers. We find, for a collapsing gas cloud environment with coupled chemical
and thermal balance, that dust abundances as small as 10^{-5} solar lead to a
strong boost in the H2 formation rate due to surface reactions. As a result of
this enhancement in H2, HD is formed more efficiently in the gas phase through
the D+ + H2 reaction. Direct formation of HD on dust grains cannot compete well
with this gas phase process for dust temperatures below 150 K. We also derive
up-to-date analytic fitting formulae for the grain surface formation of H2 and
HD, including the different binding energies of H and D. Results: Grain surface
reactions are crucial to the availability of H2 and HD in very metal-poor
environments. Above metallicities of 10^{-5} solar, the grain surface route
dominates the formation of H2, which in turn, drives the formation of HD in the
gas phase. At dust temperatures above 150 K, laboratory experiments and
theoretical modelling suggest that H2 formation on grains is suppressed while
HD formation on grains is not.Comment: typos corrected, accepted for publication in Astronomy and
Astrophysic
Modeling of hydrogen and hydroxyl group migration on graphene
Density functional calculations of optimized geometries for the migration of
single hydrogen and hydroxyl groups on graphene are performed. It is shown that
the migration energy barrier for the hydroxyl group is three times larger than
for hydrogen. Crucial role of supercell size for the values of the migration
barriers are discussed. The paired migration of hydrogen and hydroxyl groups
has also been examined. It could be concluded that hydroxyl groups based
magnetism is rather stable in contrast with unstable hydrogen based magnetism
of functionalized graphene. The role of water in the migration of hydroxyl
groups is also discussed, with the results of the calculations predicting that
the presence of water weakens the covalent bonds and makes these groups more
fluid. Increasing of number of water molecules associated with hydroxyl group
provides grown of the migration energy.Comment: 17 pages, 5 figures, to appear in PCC
Prospects for hydrogen storage in graphene
Hydrogen-based fuel cells are promising solutions for the efficient and clean
delivery of electricity. Since hydrogen is an energy carrier, a key step for
the development of a reliable hydrogen-based technology requires solving the
issue of storage and transport of hydrogen. Several proposals based on the
design of advanced materials such as metal hydrides and carbon structures have
been made to overcome the limitations of the conventional solution of
compressing or liquefying hydrogen in tanks. Nevertheless none of these systems
are currently offering the required performances in terms of hydrogen storage
capacity and control of adsorption/desorption processes. Therefore the problem
of hydrogen storage remains so far unsolved and it continues to represent a
significant bottleneck to the advancement and proliferation of fuel cell and
hydrogen technologies. Recently, however, several studies on graphene, the
one-atom-thick membrane of carbon atoms packed in a honeycomb lattice, have
highlighted the potentialities of this material for hydrogen storage and raise
new hopes for the development of an efficient solid-state hydrogen storage
device. Here we review on-going efforts and studies on functionalized and
nanostructured graphene for hydrogen storage and suggest possible developments
for efficient storage/release of hydrogen at ambient conditions
Interaction of Hydrogen with Graphitic Surfaces, Clean and Doped with Metal Clusters
ProducciĂłn CientĂficaHydrogen is viewed as a possible alternative to the fossil fuels in transportation.
The technology of fuel-cell engines is fully developed, and the outstanding
remaining problem is the storage of hydrogen in the vehicle. Porous materials,
in which hydrogen is adsorbed on the pore walls, and in particular nanoporous
carbons, have been investigated as potential onboard containers. Furthermore,
metallic nanoparticles embedded in porous carbons catalyze the dissociation of
hydrogen in the anode of the fuel cells. For these reasons the interaction of
hydrogen with the surfaces of carbon materials is a topic of high technological
interest. Computational modeling and the density functional formalism (DFT)
are helping in the task of discovering the basic mechanisms of the interaction
of hydrogen with clean and doped carbon surfaces. Planar and curved graphene
provide good models for the walls of porous carbons. We first review work on
the interaction of molecular and atomic hydrogen with graphene and graphene nanoribbons, and next we address the effects due to the presence of metal clusters
on the surface because of the evidence of their role in enhancing hydrogen
storage.Ministerio de EconomĂa, Industria y Competitividad (Grant MAT2014-54378-R
Etude Ab initio des mécanismes réactionnels dans la phase initiale du dépôt par couches atomiques des oxydes à moyenne et forte permittivité sur silicium
This work attempts to bring a new light on the understanding of some critical aspects of the physicochemical processes that control Alumina, Zirconia and Hafnia ALD growth, yet not sufficiently understood. These materials are addressed as potentially best candidates to replace gate dielectric SiO2 in the near future electronic applications. Most accurate ab initio correlated methods, like couple-cluster CCSD(T) and CISD(T), with different basis sets functions, as well as the available experimental data have been used for testing by a systematic study the accuracy and the reliability of DFT B3LYP functional. Our results have claimed this hybrid-DFT method to be chosen in predicting of high accurate static and dynamic properties throughout the family of organometallic-like (AlxCyHzOt) and transition metal-based (Zr/HfxClyOzHt) molecular systems. First systematic study of torsional potential surfaces of TMA has been performed and the related features of the hindered rotors of the methyl groups revealed with high accuracy. Laying on these accurate results we have also proposed least-squared fit methods to determine frequency scaling factors subject to different thermodynamic properties and/or thermal conditions. Many-step reaction mechanisms of ALD gas phase precursors of each of the three oxides with residual water, or regime of low pressure H2OÓALD pulses, have been studied in detail. Strong anharmonic internal movements of molecular species throughout the hydrolysis reactions have been observed and qualitatively discussed in relation with their possible effects on the reactions' kinetics. TMA/H2O reactions have been validated as strongly exothermic, while Hafnium and Zirconium tetrachlorides have founded to react endothermically with single H2O molecule. We have also studied in detail reaction mechanisms of the related on-surface ALD-complexes with water vapors. Our theoretical investigations address to the initial stage of ALD growth, more s pecifically on SiO2/Si(001)-2x1 like surfaces. The proposed many-step mechanisms, similar to those discussed for the gas phase, confirmed again the strong reactivity of H2O molecule with on-surface Aluminum hydroxymethylides, and responds strong endothemically as for the hydroxylation of Zirconium and Hafnium on-surface hydroxychlorides. The last two proved a very similar surface chemistry. Finally the cooperative effects of H2O molecules have been considered in our models of reactions, and have revealed dramatic influences on the reactivity Zirconium- and Hafnium hydroxychlorides surfaces. Our results proved the importance of both cooperative interactions of on-surface complexes and H2O molecules in the case of the Zirconia and HafniaÓALD growth, while for Aluminum oxide, presently considered ideal for ALD growth, these effects seem of secondary importance.L'objectif de ce travail est d'apporter un éclairage nouveau à la compréhension des mécanismes physico-chimiques qui contrôlent la croissance des trois oxydes d'Aluminium, de Zirconium, de Hafnium. Ces matériaux sont considérés comme les meilleurs candidats pour remplacer la silice en tant qu'oxyde de grille dans le futur composant MOS. La précision et la fiabilité de la méthode DFT associé à la fonctionnelle B3LYP, ont été testées à l'aide des résultats expérimentaux et des méthodes ab initio les plus précis telles que CCSD(T) et CISD(T), en utilisant différents ensembles des fonctions de bases. Nos résultats montrent que et la méthode hybride DFT peut prédire de façon précise les propriétés statistiques et dynamiques de la famille d'organométalliques (AlxCyHzOt) et des systèmes moléculaires à base de métaux de transition (Zr/HfxClyOzHt). Les premières études systématiques des surfaces d'énergie potentielle de TMA ont été présentes et les caractéristiques des rotors constitués des groups méthyles ont été rapportées avec une grande précision. Les mécanismes réactionnels, à plusieurs étapes, entre les molécules précurseurs de trois oxydes et les molécules d'eau résiduelle phase gazeuse ont été étudies en détail. Les mouvements internes fortement anharmoniques, des espèces moléculaires présentes touts au long du processus d'hydrolyse ont été déterminés. Les effets qualitatifs sur les cinétiques des réactions ont été discutés. La forte exothermicité de la réaction TMA/H2O a été démontrée, alors que la réaction avec les tétrachlorures de Zirconium et Hafnium ont montré un caractère plutôt endothermique. Nous avons aussi étudié les mécanismes réactionnels de la vapeur d'eau avec d'espèces moléculaires chimisorbés en surface. Les réactions interviennent dans les cycles initiales d'ALD sur en substrat de Si(001)-2x1 légèrement oxydé. Les mécanismes que nous proposons sont qualitativement proches des mécanismes d'hydrolyse discutés dans la phase gaz euse : confirmation de la forte réactivité exothermique avec les hydroxyméthyliques d'Aluminium, endothermicité des réactions avec hydroxychlorures de Zirconium et Hafnium. Les composés avec le Zirconium et le Hafnium ont des comportements similaires. Enfin, les effets de coopérativité, à la fois au niveau des molécules de vapeur d'eau, qu'au niveau des complexes en surface, sur les réactions ont été mis en évidence et discutés. Ils montrent des comportements tout à fait intéressants dans le cas des hydroxychlorures des Zirconium et Hafnium. Par contre, ces effets sont de moindre importance dans les cas de l'oxyde d'aluminium, qui est actuellement considéré comme le matériau le plus compatible avec la croissance par AL
Etude ab initio des mécanismes réactionnels dans la phase initiale du dépôt par par couches atomiques des oxydes à moyenne et forte permittivité sur silicium
TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF
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