59 research outputs found
Quantifying the impact of vibrational nonequilibrium in plasma catalysis: Insights from a molecular dynamics model of dissociative chemisorption
The rate, selectivity and efficiency of plasma-based conversion processes is
strongly affected by nonequilibrium phenomena. High concentrations of
vibrationally excited molecules are such a plasma-induced effect. It is
frequently assumed that vibrationally excited molecules are important in plasma
catalysis because their presence lowers the apparent activation energy of
dissociative chemisorption reactions and thus increases the conversion rate. A
detailed atomic-level understanding of vibrationally stimulated catalytic
reactions in the context of plasma catalysis is however lacking. Here, we
couple a recently developed statistical model of a plasma-induced vibrational
nonequilibrium to molecular dynamics simulations, enhanced sampling methods,
and machine learning techniques. We quantify the impact of a vibrational
nonequilibrium on the dissociative chemisorption barrier of H2 and CH4 on
nickel catalysts over a wide range of vibrational temperatures. We investigate
the effect of surface structure and compare the role of different vibrational
modes of methane in the dissociation process. For low vibrational temperatures,
very high vibrational efficacies are found, and energy in bend vibrations
appears to dominate the dissociation of methane. The relative impact of
vibrational nonequilibrium is much higher on terrace sites than on surface
steps. We then show how our simulations can help to interpret recent
experimental results, and suggest new paths to a better understanding of plasma
catalysis
A route towards the fabrication of 2D heterostructures using atomic layer etching combined with selective conversion
Heterostructures of low-dimensional semiconducting materials, such as transition metal dichalcogenides (MX2), are promising building blocks for future electronic and optoelectronic devices. The patterning of one MX2 material on top of another one is challenging due to their structural similarity. This prevents an intrinsic etch stop when conventional anisotropic dry etching processes are used. An alternative approach consist in a two-step process, where a sacrificial silicon layer is pre-patterned with a low damage plasma process, stopping on the underlying MoS2 film. The pre-patterned layer is used as sacrificial template for the formation of the top WS2 film. This study describes the optimization of a cyclic Ar/Cl2 atomic layer etch process applied to etch silicon on top of MoS2, with minimal damage, followed by a selective conversion of the patterned Si into WS2. The impact of the Si atomic layer etch towards the MoS2 is evaluated: in the ion energy range used for this study, MoS2 removal occurs in the over-etch step over 1–2 layers, leading to the appearance of MoOx but without significant lattice distortions to the remaining layers. The combination of Si atomic layer etch, on top of MoS2, and subsequent Si-to-WS2 selective conversion, allows to create a WS2/MoS2 heterostructure, with clear Raman signals and horizontal lattice alignment. These results demonstrate a scalable, transfer free method to achieve horizontally individually patterned heterostacks and open the route towards wafer-level processing of 2D materials
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Reactive plasma cleaning and restoration of transition metal dichalcogenide monolayers
The cleaning of two-dimensional (2D) materials is an essential step in the fabrication of future devices, leveraging their unique physical, optical, and chemical properties. Part of these emerging 2D materials are transition metal dichalcogenides (TMDs). So far there is limited understanding of the cleaning of “monolayer” TMD materials. In this study, we report on the use of downstream H2 plasma to clean the surface of monolayer WS2 grown by MOCVD. We demonstrate that high-temperature processing is essential, allowing to maximize the removal rate of polymers and to mitigate damage caused to the WS2 in the form of sulfur vacancies. We show that low temperature in situ carbonyl sulfide (OCS) soak is an efficient way to resulfurize the material, besides high-temperature H2S annealing. The cleaning processes and mechanisms elucidated in this work are tested on back-gated field-effect transistors, confirming that transport properties of WS2 devices can be maintained by the combination of H2 plasma cleaning and OCS restoration. The low-damage plasma cleaning based on H2 and OCS is very reproducible, fast (completed in a few minutes) and uses a 300 mm industrial plasma etch system qualified for standard semiconductor pilot production. This process is, therefore, expected to enable the industrial scale-up of 2D-based devices, co-integrated with silicon technology
A robust human norovirus replication model in zebrafish larvae.
Human noroviruses (HuNoVs) are the most common cause of foodborne illness, with a societal cost of $60 billion and 219,000 deaths/year. The lack of robust small animal models has significantly hindered the understanding of norovirus biology and the development of effective therapeutics. Here we report that HuNoV GI and GII replicate to high titers in zebrafish (Danio rerio) larvae; replication peaks at day 2 post infection and is detectable for at least 6 days. The virus (HuNoV GII.4) could be passaged from larva to larva two consecutive times. HuNoV is detected in cells of the hematopoietic lineage and the intestine, supporting the notion of a dual tropism. Antiviral treatment reduces HuNoV replication by >2 log10, showing that this model is suited for antiviral studies. Zebrafish larvae constitute a simple and robust replication model that will largely facilitate studies of HuNoV biology and the development of antiviral strategies
Plasma-liquid interactions: a review and roadmap
Plasma-liquid interactions represent a growing interdisciplinary area of research involving plasma science, fluid dynamics, heat and mass transfer, photolysis, multiphase chemistry and aerosol science. This review provides an assessment of the state-of-the-art of this multidisciplinary area and identifies the key research challenges. The developments in diagnostics, modeling and further extensions of cross section and reaction rate databases that are necessary to address these challenges are discussed. The review focusses on non-equilibrium plasmas
Stabilities of Bimetallic Nanoparticles for Chirality-Selective Carbon Nanotube Growth and the Effect of Carbon Interstitials
Bimetallic
nanoparticles play a crucial role in various applications.
A better understanding of their properties would facilitate these
applications and possibly even enable chirality-specific growth of
carbon nanotubes (CNTs). We here examine the stabilities of NiFe,
NiGa, and FeGa nanoparticles and the effect of carbon dissolved in
NiFe nanoparticles through density functional theory (DFT) calculations
and Born–Oppenheimer molecular dynamics (BOMD) simulations.
We establish that nanoparticles with more Fe in the core and more
Ga on the surface are more stable and compare these results with well-known
properties such as surface energy and atom size. Furthermore, we find
that the nanoparticles become more stable with increasing carbon content,
both at 0 K and at 700 K. These results provide a basis for further
research into the chirality-specific growth of CNTs
Influence of the Material Dielectric Constant on Plasma Generation inside Catalyst Pores
Abstract: Plasma catalysis is gaining increasing interest for various environmental applications, but the crucial question is whether plasma can be created inside catalyst pores and under which conditions. In practice, various catalytic support materials are used, with various dielectric constants. We investigate here the influence of the dielectric constant on the plasma properties inside catalyst pores and in the sheath in front of the pores, for various pore sizes. The calculations are performed by a two-dimensional fluid model for an atmospheric pressure dielectric barrier discharge in helium. The electron impact ionization rate, electron temperature, electron and ion density, as well as the potential distribution and surface charge density, are analyzed for a better understanding of the discharge behavior inside catalyst pores. The results indicate that, in a 100 \u3bcm pore, the electron impact ionization in the pore, which is characteristic for the plasma generation inside the pore, is greatly enhanced for dielectric constants below 300. Smaller pore sizes only yield enhanced ionization for smaller dielectric constants, i.e., up to \u3b5r = 200, 150, and 50 for pore sizes of 50, 30, and 10 \u3bcm. Thus, the most common catalyst supports, i.e., Al2O3 and SiO2, which have dielectric constants around \u3b5r = 811 and 4.2, respectively, should allow more easily that microdischarges can be formed inside catalyst pores, even for smaller pore sizes. On the other hand, ferroelectric materials with dielectric constants above 300 never seem to yield plasma enhancement inside catalyst pores, not even for 100 \u3bcm pore sizes. Furthermore, it is clear that the dielectric constant of the material has a large effect on the extent of plasma enhancement inside the catalyst pores, especially in the range between \u3b5r = 4 and \u3b5r = 200. The obtained results are explained in detail based on the surface charge density at the pore walls, and the potential distribution and electron temperature inside and above the pores. The results obtained with this model are important for plasma catalysis, as the production of plasma species in catalyst pores might affect the catalyst properties, and thus improve the applications of plasma catalysis
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