Abstract: The possibility of application of nanomaterials is determined by our ability to control the properties of the materials, which are ultimately determined by their structure and hence their growth processes. We employ hybrid molecular dynamics / Monte Carlo (MD/MC) simulations to explore the growth of SWNTs and graphene on nickel as a catalyst, with the specific goal of unraveling the growth mechanisms. While the general observations are in agreement with the literature, we find a number of interesting phenomena to be operative which are crucial for the growth, and which are not accessible by MD simulations alone due to the associated time scale. Specifically, we observe metal mediated healing and restructuring processes to take place, reorganizing the carbon network during the initial nucleation step. In the case of carbon nanotube growth, this leads to the growth of tubes with a determinable chirality. In the case of graphene formation, we find that graphene is only formed at temperatures above 700 K. These results are of importance for understanding the growth mechanisms of these carbon nanomaterials on the fundamental level.

Abstract: Plasma technology is gaining increasing interest for gas conversion applications, such as CO2 conversion into value-added chemicals or renewable fuels, and N2 fixation from the air, to be used for the production of small building blocks for, e.g., mineral fertilizers. Plasma is generated by electric power and can easily be switched on/off, making it, in principle, suitable for using intermittent renewable electricity. In this Perspective article, we explain why plasma might be promising for this application. We briefly present the most common types of plasma reactors with their characteristic features, illustrating why some plasma types exhibit better energy efficiency than others. We also highlight current research in the fields of CO2 conversion (including the combined conversion of CO2 with CH4, H2O, or H2) as well as N2 fixation (for NH3 or NOx synthesis). Finally, we discuss the major limitations and steps to be taken for further improvement.

Abstract: 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 H₂plasma to clean the surface of monolayer WS₂grown 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 WS₂in 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 WS₂devices can be maintained by the combination of H₂plasma cleaning and OCS restoration. The low-damage plasma cleaning based on H₂and 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.

Abstract: Using density functional theory in combination with the Green’s functional formalism, we study the effect of surface functionalization on the electronic transport properties of 1D carbon allotrope—carbyne. We found that both hydrogenation and fluorination result in structural changes and semiconducting to metallic transition. Consequently, the current in the functionalization systems increases significantly due to strong delocalization of electronic states along the carbon chain. We also study the electronic transport in partially hydrogenated carbyne and interface structures consisting of pristine and functionalized carbyne. In the latter case, current rectification is obtained in the system with rectification ratio up to 50%. These findings can be useful for developing carbyne-based structures with tunable electronic transport properties.

Abstract: There is currently a growing interest in the realisation and optimization of hybrid plasma/catalyst systems for a multitude of applications, ranging from nanotechnology to environmental chemistry. In spite of this interest, there is, however, a lack in fundamental understanding of the underlying processes in such systems. While a lot of experimental research is already being carried out to gain this understanding, only recently the first simulations have appeared in the literature. In this contribution, an overview is presented on atomic scale simulations of plasma catalytic processes as carried out in our group. In particular, this contribution focusses on plasma-assisted catalyzed carbon nanostructure growth, and plasma catalysis for greenhouse gas conversion. Attention is paid to what can routinely be done, and where challenges persist.

Abstract: Despite significant progress in single walled carbon nanotube (SWCNT) production by plasma enhanced chemical vapor deposition (PECVD), the growth mechanism in this method is not clearly understood. We employ reactive molecular dynamics simulations to investigate how plasma-based deposition allows growth at low temperature. We first investigate the SWCNT growth mechanism at low and high temperatures under conditions similar to thermal CVD and PECVD. We then show how ion bombardment during the nucleation stage increases the carbon solubility in the catalyst at low temperature. Finally, we demonstrate how moderate energy ions sputter amorphous carbon allowing for SWCNT growth at 500 K. (C) 2013 Elsevier B. V. All rights reserved.

Abstract: A seriesof [TiN/TiAlN]nmultilayer coatingswith different bilayer numbers n=5, 10, 25, 50, and 100 were deposited on stainless steel substrate AISI 304 by a lateral rotating cathode arc technique in a flowing nitrogen atmosphere. The composition and microstructure of the coatings have been analyzed by using energy dispersive X-ray spectroscopy, X-ray diffraction (XRD), and conventional and high-resolution transmission electron microscopy (HRTEM). XRD analysis shows that the preferential orientation growth along the (111) direction is reduced in the multilayer coatings. TEM analysis reveals that the grain size of the coatings decreases with increasing bilayer number. HRTEMimaging of the multilayer coatings shows a high density misfit dislocation between the TiN and TiAlN layers. The cross-plane thermal conductivity of the coatings was measured by a pulsed photothermal reflectance technique. With increasing bilayer number, the multilayer coatings' thermal conductivity decreases gradually. This reduction of thermal conductivity can be ascribed to increased phonon scattering due to the disruption of columnar structure, reduced preferential orientation, decreased grain size of the coatings and present misfit dislocations at the interfaces.

Abstract: In this study, the healing of N-vacancy boron carbonitride nanosheet (NV-BC2NNS) and nanotube (NV-BC2NNT) by NO molecule is studied by means of density functional theory calculations. Two different N-vacancies are considered in each of these structures in which the vacancy site is surrounded by either three B-atoms (NB) or by two B- and one C-atom (NBC). By means of the healed BC2NNS and BC2NNT as a support, the removal of two toxic gas molecules (NO and CO) are applicable. It should be noted that the obtained energy barriers of both healing and oxidizing processes are significantly lower than those of graphene, carbon nanotubes or boron nitride nanostructures. Also, at the end of the oxidation process, the pure BC2NNS or BC2NNT is obtained without any additional defects. Therefore, by using this method, we can considerably purify the defective BC2NNS/BC2NNT. Moreover, according to the thermochemistry calculations we can further confirm that the healing process of the NV-BC2NNS and NV-BC2NNT by NO are feasible at room temperature. So, we can claim that this study could be very helpful in both purifying the defective BC2NNS/BC2NNT while in the same effort removing toxic NO and CO gases.

Abstract: In this paper we employ first-principles calculations to investigate the effect of substitutional Si doping in the amorphous calcium-phosphate (a-HAP) structure on the work of adhesion, integral charge transfer, charge density difference and theoretical tensile strengths between an a-HAP coating and amorphous titanium dioxide (a-TiO2) substrate systemically. Our calculations demonstrate that substitution of a P atom by a Si atom in a-HAP (a-Si-HAP) with the creation of OH-vacancies as charge compensation results in a significant increase of the bonding strength of the coating to the substrate. The work of adhesion of the optimized Si-doped interfaces reaches a value of up to -2.52 J m(-2), which is significantly higher than for the stoichiometric a-HAP/a-TiO2. Charge density difference analysis indicates that the dominant interactions at the interface have significant covalent character, and in particular two Ti-O and three Ca-O bonds are formed for a-Si-HAP/a-TiO2 and one Ti-O and three Ca-O bonds for a-HAP/a-TiO2. From the stress-strain curve, the Young's modulus of a-Si-HAP/a-TiO2 is calculated to be about 25% higher than that of the a-HAP/a-TiO2, and the yielding stress is about 2 times greater than that of the undoped model. Our calculations therefore demonstrate that the presence of Si in the a-HAP structure strongly alters not only the bioactivity and resorption rates, but also the mechanical properties of the a-HAP/a-TiO2 interface. The results presented here provide an important theoretical insight into the nature of the chemical bonding at the a-HAP/a-TiO2 interface, and are particularly significant for the practical medical applications of HAP-based biomaterials.

Abstract: Molecular dynamics simulations are used to study the effects of different cations on the permeation of charged polymers through flat capillaries with heights below 2 nm. Interestingly, we found that, despite being monovalent, Li+ , Na+ , and K+ cations have different effects on polymer permeation, which consequently affects their transmission speed throughout those capillaries. We attribute this phenomenon to the interplay of the cations' hydration free energies and the hydrodynamic drag in front of the polymer when it enters the capillary. Different alkali cations exhibit different surface versus bulk preferences in small clusters of water under the influence of an external electric field. This paper presents a tool to control the speed of charged polymers in confined spaces using cations.

Abstract: Plasma medicine has been claimed to provide a novel route to heal wounds and regenerate skin, although very little is currently known about the elementary processes taking place. We carried out a series of ReaxFF-based reactive molecular dynamics simulations to investigate the interaction of O and OH radicals with lipids, more specifically with α-linolenic acid as a model for the free fatty acids present in the upper skin layer. Our calculations predict that the O and OH radicals most typically abstract a H atom from the fatty acids, which can lead to the formation of a conjugated double bond, but also to the incorporation of alcohol or aldehyde groups, thereby increasing the hydrophilic character of the fatty acids and changing the general lipid composition of the skin. Within the limitations of the investigated model, no formation of possibly toxic products was observed.

Abstract: Plasma catalysis holds great promise for environmental applications, provided that the process viability can be maximized in terms of energy efficiency and product selectivity. This requires a fundamental understanding of the various processes taking place and especially the mutual interactions between plasma and catalyst. In this review, we therefore first examine the various effects of the plasma on the catalyst and of the catalyst on the plasma that have been described in the literature. Most of these studies are purely experimental. The urgently needed fundamental understanding of the mechanisms underpinning plasma catalysis, however, may also be obtained through modelling and simulation. Therefore, we also provide here an overview of the modelling efforts that have been developed already, on both the atomistic and the macroscale, and we identify the data that can be obtained with these models to illustrate how modelling and simulation may contribute to this field. Last but not least, we also identify future modelling opportunities to obtain a more complete understanding of the various underlying plasma catalytic effects, which is needed to provide a comprehensive picture of plasma catalysis.

Abstract: Modelling at the molecular or atomic scale can be very useful for obtaining a better insight in plasma medicine. This paper gives an overview of different atomic/molecular scale modelling approaches that can be used to study the direct interaction of plasma species with biomolecules or the consequences of these interactions for the biomolecules on a somewhat longer time-scale. These approaches include density functional theory (DFT), density functional based tight binding (DFTB), classical reactive and non-reactive molecular dynamics (MD) and united-atom or coarse-grained MD, as well as hybrid quantum mechanics/molecular mechanics (QM/MM) methods. Specific examples will be given for three important types of biomolecules, present in human cells, i.e. proteins, DNA and phospholipids found in the cell membrane. The results show that each of these modelling approaches has its specific strengths and limitations, and is particularly useful for certain applications. A multi-level approach is therefore most suitable for obtaining a global picture of the plasma–biomolecule interactions.

Abstract: Cold atmospheric pressure plasmas in and in contact with liquids represent a growing field of research for various applications. Understanding the interactions between the plasma generated species and the liquid is crucial. In this work we perform molecular dynamics (MD) simulations based on a quantum mechanical method, i.e. density-functional based tight-binding (DFTB), to examine the interactions of OH radicals and O atoms in bulk water. Our calculations reveal that the transport of OH radicals through water is not only governed by diffusion, but also by an equilibrium reaction of H-abstraction with water molecules. Furthermore, when two OH radicals encounter each other, they either form a stable cluster, or react, resulting in the formation of a new water molecule and an O atom. In addition,
the O atoms form either oxywater (when in singlet configuration) or they remain stable in solution (when in triplet configuration), stressing the important role that O atoms can play in aqueous solution, and in contact with biomolecules. Our observations are in line with both experimental and ab initio results from the literature.