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Molecular Dynamics Simulations of Hydrocarbon Film Growth from Acetylene Monomers and Radicals: Effect of Substrate Temperature Mohammad Zarshenas, Bartlomiej Czerwinski, Tom Leyssens, Konstantin Moshkunov, and Arnaud Denis Delcorte J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01334 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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The Journal of Physical Chemistry

Title page

Molecular Dynamics Simulations of Hydrocarbon Film Growth from Acetylene Monomers and Radicals: Effect of Substrate Temperature Mohammad Zarshenasa, Bartlomiej Czerwinskib, Tom Leyssensc, Konstantin Moshkunova and Arnaud Delcorte*a a

Institute of Condensed Matter and Nanosciences - Bio & Soft Matter (IMCN/BSMA), Université Catholique de Louvain, Place Louis Pasteur 1 bte L4.01.10, B-1348 Louvain-la-Neuve, Belgium b

c

Division of Materials Science, Department of Engineering Sciences and Mathematics, Lulea University of Technology SE-971 87 Lulea, Sweden

Institute of Condensed Matter and Nanosciences - Molecules, Solids and Reactivity (IMCN/MOST)

Université Catholique de Louvain, Place Louis Pasteur 1, bte L4.01.03,B-1348 Louvain-La-Neuve, Belgium

Corresponding author:

Corresponding author: Prof. Arnaud Delcorte* Mailing address: Institute of Condensed Matter and Nanosciences, Bio & Soft Matter division (IMCN/BSMA), Université Catholique de Louvain, Place Louis Pasteur 1 bte L4.01.10, B-1348 Louvain-laNeuve, Belgium Tel : +3210473596 Fax : +3210472005 E-mail: [email protected]

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Abstract In an attempt to rationalize the mechanisms occurring during plasma polymerization of acetylene, classical molecular dynamics (MD) computer simulations investigating the deposition and reaction of a mixed gas of acetylene molecules and radicals on the Ag(111) substrate were performed for a wide range of substrate temperatures. Prior to that, this article establishes a methodology for film deposition and identifies the appropriate potentials for hydrocarbons by comparison with electronic calculations using the density functional theory (DFT). Based on this preliminary study, simulations of films growth are carried out at different temperatures using the REBO potential. Our results show that the rates of creation of new C-C and C-H bonds are higher at the beginning of the film growth when the substrate is still exposed, than when it is covered with polymeric chains, and these initial reaction rates are proportional to temperature. The analysis of the hybridization of carbon atoms in the films shows that the substrate temperature increase leads to the formation of coatings containing more carbon atoms in the sp2 and sp3 configurations and less in the sp configuration with sp2 becoming dominant at high temperatures. We establish a polymerization-connectivity formalism that describes the structural transformation of the film during the deposition on the basis of each atom hybridization and bonding. Within this formalism the evolution of the polymerization and the connection character of the polymers is observed and discussed.


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1. Introduction Hydrocarbon coatings formed from the gas or plasma phase are used in numerous industries and fields of applications. Their production is extensively studied either to achieve certain coating parameters or to investigate the resulting coatings, because hydrocarbon films can be both beneficial and detrimental. While the former (advantages) of hydrocarbon films are widely known, the latter happens when the films grow in areas of vacuum vessels, where their presence is not expected and/or disrupts the operation. An example of such is the growth of tritium-containing films in thermonuclear reactors.1–3 Hydrocarbon films can be grown directly by exposing the sample surface to the plasma source or indirectly, when the plasma-produced radicals travel outside of the plasma-exposed volume and hit the exposed surfaces. C:H films grow from ions, radicals and neutral monomers striking the surface and reacting with each other, former random-like interconnected structures. The mechanisms of this formation and dependencies of film parameters on various exposition conditions are of particular interest. One usually categorizes the aforementioned films as either amorphous hydrocarbons (a-C:H) or plasma polymers. In the case of a-C:H films, either soft (hydrogen-rich) or hard, the random nature of reactions and resulting connections and groups are important. on the other hand, in the case of plasma polymers, the emphasis is on the oligomerization of the monomers, which eventually lead to long connected chains, considered as quasi-polymers. In this respect, plasma polymerization is usually referred to as the growth of a thin polymeric film on a substrate exposed to the plasma produced stream of radicals, monomers and ions. The contribution to the growth process from ions versus the impact of radicals is an important discussion in the scientific society. Ions accelerated in a sheath layer have significant kinetic energies and can induce reactions and bond scissions. Therefore, they determine whether the films grow as soft (polymeric) coatings, at low ion energies, or as hard coatings with extensive cross-linking, under more energetic ion bombardment,4 and finally, as diamond-like carbon (DLC) coatings.5 Ion chemistry is another important process as the protonation or deprotonation of plasma species has proven to influence plasma polymerization.6 On the other hand, many experimental works show the importance of the radical-based growth of a-C:H films,7–10 and the work of Bauer et.al.11 clearly demonstrates the influence of different hydrocarbon simple radicals. Another study reveals the composition of radicals emitted from hydrocarbon plasmas and the component contribution to the growth of a-C:H films.12 Acetylene is one of the monomers that can be easily deposited from the gas or plasma phase to form C:H coatings and there is a lot of experimental data on plasma polymerized acetylene (PPA), regarding the empirical conditions that influence the polymerization process as well as the investigation of the electrical, optical, mechanical and biomedical properties of the PPA films.13–22 Several parameters can be controlled during the plasma polymerization which affect the structural, physical and chemical properties of the plasma polymer film for a given monomer: the monomer flow rate, discharge power, monomer pressure and substrate temperature. While the common deposition method of a-C:H films is a vacuum plasma discharge, atmospheric pressure deposition has its own benefits.23–25 Both hard25 and soft23 films can be grown in dielectric barrier discharge (DBD) using acetylene, and this kind of discharge can produce a flux of thermalized radicals greatly exceeding the ion flux due to high collision rate in the atmospheric pressure. (see ref 26 and references within).



This study is aimed at understanding the formation process of thin hydrocarbon films from acetylene precursors by means of molecular dynamics (MD) computer simulations, supported by ab initio calculations. Previously, a few MD studies have reported the growth of (polymer-like) thin films through deposition of various organic species and rapid chemical reactions on solid substrates. Hu and coworkers theoretically studied the deposition of organic clusters and creation of thin films on diamond utilizing MD simulations.27 MD simulations were also carried out to investigate the influence of ion energies on the growth of polythiophene thin films on silicon by Hsu et.al.28 Qi and coworkers investigated the formation of 3D hydrocarbon thin films through sequential impacts of energetic ethylene and acetylene molecular clusters with diamond using MD.29,30 These polymer-like thin films can be developed under the deposition of organic clusters with energies of 0.1-1 eV/atom.31 While MD offers unrivaled level or reaction precision and film structure, the direct simulation involving the interaction of all the species, i.e. ions, radicals, gas monomers and electrons is beyond the simulation capabilities, because of the following aspects. First, the widely used hydrocarbon potentials (REBO, AIREBO) do not consider the ionic state of the interacting atoms. Therefore, the current study focuses on the film formation by thermalized plasma-produced radicals and gas monomers (postdischarge), like in an atmospheric discharge, rather than the growth by direct exposure to energetic plasma particles. The formation of plasma polymer and a-C:H films, out of the influence of more energetic ions, occurs at deposition energies significantly lower than 0.1 eV (thermal energies ≈ 0.025 eV).32 To our knowledge, no simulation results on developing thin films at such low precursor energy have been reported yet. Second, the real flux of particles (for example, 1015 projectiles/(cm2s) or 10-8 projectiles/(nm2ns)) is beyond the scope of the MD simulation space-time (nanometers and nanoseconds), therefore the simulated flux has to be greatly increased. An efficient thermostat, however, can be applied to remove the excess energy deposited by impinging particles or released in reactions, thereby rendering the experimentally unrealistic fluxes relevant for the study. As the heat evacuation via the thermostat happens in the substrate but not in the film we do not extend our simulations to the formation of the steady state bulk films with thickness worth of many monolayers. The significant emphasis of the study is on how a flat (2D) film becomes volumetric (3D) and reaches the onset of bulk properties. As a variable parameter, we chose to explore the effect of the substrate temperature, because it is the most impactful and straightforward condition to apply in the MD system, and it is easy to adjust in an experiment as well. The general effects of the substrate temperature on the formation and structure of plasma polymerized films have been experimentally studied. The films formed at low substrate temperatures usually consist of similar atomic compositions and functional groups as the precursors.33 With increasing substrate temperature, the fragmentation of the precursor increases, and so does the cross-linking,34 possibly leading to an increased thermal stability of the resulting film.35 Our results follow these general trends. They will be described in more detail using a new formalism that helps us to visualize the statistic of carbon atom node types (termination, linear or branching) and to follow the thermal and volumetric evolution of the simulated films.

2. Methodology and Computational details This study was conducted using ab-initio calculations and molecular dynamics (MD) computer simulations. The ab-initio calculations were carried out using density functional theory (DFT) in the 4 ACS Paragon Plus Environment

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framework of the Gaussian 09 computational package. All the investigated structures were optimized by employing the hybrid density functional DFT/B3LYP36–38 method and the 6-31G (d,p) basis set. The LAMMPS39 simulation package was utilized to perform the MD simulations. In MD, integrating Hamilton's equation of motion determines the position and velocity of each atom in the system of interest. The forces among the atoms were described by a combination of many-body and pairwise additive potential energy functions. The Ag-C and Ag-H interactions are described by using the LennardJones potential and the embedded-atom method (eam)40,41 was used for the Ag-Ag interactions. The reactive empirical bond order (REBO),42 the adaptive intermolecular potential (AIREBO),43 the ReaxFF potential with different parametrizations44–49 and the charge-optimized many-body potential (COMB)50 were used and compared for H-H, H-C and C-C interactions.

2.1.

Simulation approach

Our approach in order to model the plasma polymerization process was to create a substrate and deposit the acetylene precursors from an arranged cell, by giving them initial kinetic energy toward the substrate. As shown in Fig. 1, the side borders (x-z direction) of the created system are governed by periodic boundary conditions and the films were grown in the y-direction. The silver substrate contains 2016 atoms and has a surface area of (40Å×40Å) and the thickness of ~19Å corresponds to 9 layers of atoms. These atomic layers were divided into three zones. The rigid zone contained the first two layers from the bottom (marked with pink color in Fig. 1) and they were fixed to keep up the substrate structure. The next five layers belonged to the stochastic zone (marked with green color in Fig. 1) where a Langevin heat bath was implemented in order to regulate the temperature. Finally, the reaction zone (marked with light gray color in Fig. 1) contains the last two atomic layers. One of the layers in the latter zone is connected to the mentioned heat bath and the upper one is the region where the reactions happen. To mimic the cyclic or continuous flow of the gas toward the surface, successive depositions of the precursors cells were performed. A reflective wall was located at the top of the cell in order to backscatter precursors toward the substrate and increase the amount of deposited mass at each deposition stage. The size of the precursors cell was 40Å×810Å×40Å. The initial cell contained 1200 acetylene molecules. 90% of the total weight of the cell was attributed to acetylene molecules and 10% to the reactive species of the acetylene plasma.



FIG. 1. Schematic representation of the initial system arrangement for deposition of the acetylene precursors on the silver substrate. The rigid zone, stochastic zone and reaction zone of the substrate are colored in pink, green and light gray, respectively. Red is for C atoms and white for H atoms. The yellow frame represents the boundary of the periodic cell. The reflective wall is denoted by light blue color at the top of the cell.

In order to better understand the distribution of reactive species in our simulations, it is useful to concisely address the different reactive species that can possibly be formed in the plasma. As was mentioned before, collisions between energetic electrons and the monomer gas result in bond scissions and ionizations forming the reactive species of the plasma. The energy of electrons in the plasma used for polymerization is in the range of 2-5 eV.51 This energy is adequate to dissociate chemical bonds with the energy range of 3-4 eV,52 while the minimum energy for ionization is around 10 eV,53 which limits the generation of ions in the plasma. Therefore, radical bond scissions of organic monomers are the most probable events during the electron-monomer gas collision. In the acetylene plasma, C-containing radicals with a lower number of hydrogen atoms (C2H, CH and C2) are found to be more reactive than those with a higher number of hydrogen atoms.54 In our simulations, these three radicals were chosen as the reactive species in the acetylene precursors cell. In practice, 10% of the acetylene molecules in the 6 ACS Paragon Plus Environment

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cell were divided equally between these radicals. The numbers of each radical types (weight percentage of each radical in the cell) are as follows: 40 C2H (3.2%), 40 C2 (3.1%), 80 CH (3.3%) and 120 H (0.4%). The scheme of the collision between C2H2 molecules and electrons leading to the considered radicals through bond scission is as follows:

 40C 2 H  40C 2 H 2 + e  40C 2 H+40H     40C 2 120C 2 H 2 →  40C 2 H 2 + e →  40C 2 + 40H+40H →   40C H + e  40CH+40CH 80CH 2 2   120H

In order to accumulate statistics and validate our results, the process of film growth was repeated with three randomly arranged cells. The size of the different cells and proportions of molecules and radicals were kept identical. Initially, the distance between the atom with the lowest vertical position in the precursors cell and the uppermost atom of the substrate was kept to 5 Å at each deposition step. The initial kinetic energy per precursor was set to 0.025 eV,32 as estimated from experiments with velocity vector directed towards the substrate. Total simulation time for growing the films at each temperature was 18 ns which corresponds to 60 steps of sequential acetylene precursors cell deposition. A constant time step of 0.25 fs was chosen.

2.2.

Choice of the potential

We performed initial simulations in order to select the best potential to describe the interactions among acetylene molecules and radicals and the formation of polymeric chains. In order to investigate the interactions, the potential energy was calculated as a function of the reacting atom interdistances and the results were compared to electronic structure calculations. To establish the curves, we took one radical and one molecule and fixed a specific distance between two atoms in the radical and the molecule which were supposed to react with each other (signified as x and y in Figs 2-3). Then, the system was left to relax around these two fixed atoms. The C2H radical was chosen for this purpose since it was found to be one of the most reactive radicals in the acetylene plasma.54 The last step was to calculate the potential energy of the final structure for the specific distance. The same procedure was repeated for a range of interdistances, in order to fully describe the reaction energetics along that coordinate. The protocol was firstly carried out with DFT and then repeated with four different MD potentials for hydrocarbons. To check the accuracy of each MD potential, the results were compared with DFT. In addition, to make sure that all the MD potentials results are compatible with the DFT, the exact DFT coordinates for the atoms were used to examine the interaction. For the considered reaction, DFT doesn’t show any barrier and predicts the first step of the polymerization (Fig. 2a). Concerning the MD potentials, it can be seen that, except for REBO42 (0 eV) and the Aryanpour parametrization of ReaxFF47 (0.088 eV), all the potentials indicate the existence of a considerable reaction barrier, which means that the reaction would probably not occur at low energy and within the nanoseconds time scale of the 7 ACS Paragon Plus Environment


simulations. In order to further test the potentials, the same procedure was repeated for the second step of the polymerization reaction (Fig. 2b).

FIG. 2. Potential energies as a function of the reacting atom interdistances for the first (a) and second (b) step of the polymerization reaction.

Again, DFT predicts reaction without any activation energy where an acetylene molecule links to the growing radical, but all the MD potentials predict a significant energy barrier. By observing the potential barriers in terms of molecule-radical reactions, specifically in the second step of interaction, one may worry that the utilized method is improper to grow acetylene plasma polymers. However, the type of structures forming on the substrate during a full-scale simulation were monitored, and it was detected that acetylene molecules still take part in the different types of interactions. This was verified by growing a PPA film on Ag(111) for 6 ns with a substrate temperature fixed to 300 K. In additional simulations of precursors cell deposition with REBO and the Aryanpour parametrization of ReaxFF, we observed that these two potentials provided equivalent reactions and structures. All these preliminary results indicate that the kinetic energy of the molecules and radicals, partly transformed in internal energy upon impact, is sufficient to overcome the potential barrier for these two potentials. As an example, the energetics of one more reaction occurring during the film growth is illustrated in Fig. 3a, using the same procedure as for the first and second steps of reactions in Fig. 2. The 0 eV of barrier found with REBO and 0.046 eV found for the Aryanpour parametrization of ReaxFF are reasonable and explain the occurrence of these reactions. Such reactions justify the existence of both the acetylene molecules and the radicals in the PPA chains. In addition, the formation of linear polymeric 8 ACS Paragon Plus Environment

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chains which are the result of the continual interaction of acetylene molecule (C2H2) with the reaction sites was observed as well (Fig. 3b). Mass spectrometry confirms such a growth model,55 which validates our MD approach to develop and study acetylene plasma polymer films. The development of these chains at the substrate temperature of 300 K means that the initial kinetic energy of the precursors, the thermal energy of the substrate and the REBO (or the Aryanpour parametrization of ReaxFF) potential constitute a suitable combination to model the formation of acetylene polymer films. Considering the widespread use of the REBO potential for growing films based on carbon and hydrogen,56–59 as well as its faster computation, REBO was therefore selected for the remainder of this study.

FIG. 3. (a) REBO potential and Aryanpour parametrization of ReaxFF potential for the interaction between acetylene molecule and the possible reaction site. Here x and y indicate the carbon atom which is a reaction site and carbon atom in the acetylene molecule, respectively. (b) Formation of CnHm PPA chains (yellow circled) after 6 ns of precursors deposition at the substrate temperature of 300 K.

Our choice of potential, favoring the most realistic representation of the precursor reactivity, neglects the description of the long-range interactions in the formed hydrocarbon films. For this reason, the description of the sticking and adsorption of non-bonded acetylene molecules or radicals in the 3D polymeric film will not be described adequately. Given the triple point of acetylene (192.4 K; 1.28 bar), the effect should be negligible at room temperature and beyond, for pressures up to the ambient. Nevertheless this point will be reminded in the discussion.

3. Results and Discussion Using this approach, six series of three PPA films were grown at the substrate temperatures of 0, 100, 200, 300, 400 and 500 Kelvin, respectively. The films were characterized at different substrate temperatures by calculating a selection of parameters for each film. Films were investigated at two different timeframes. First, after 2.4 ns, when the substrate is still exposed and the films grow horizontally (2D films) and second after 18 ns, when the substrate has been covered by 2D acetylene polymer chains and the films grow vertically (3D films). Fig. 4 illustrates the growth of 2D and 3D films 9 ACS Paragon Plus Environment


for a substrate temperature of 300 K. Amorphous and pinhole-free structures are morphological features of the films formed after 18 ns of acetylene precursors deposition, particularly at high substrate temperatures. At 0 K, the substrate atoms are frozen and the incoming precursors do not receive any thermal energy from the substrate. This makes it possible to investigate the effect of other parameters such as the initial kinetic energy of the precursors on the creation of new bonds.

FIG. 4. Top view of the 2-dimensional film growth after 2.4 ns of acetylene precursors deposition (a) and side view of the 3-dimensional film growth after 18 ns of acetylene precursors deposition (b) at a substrate temperature of 300 K.

3.1.

Creation of new bonds

One of the most noticeable characteristics of hydrocarbons deposited from plasmas is the high rate of formation of new C-C bonds which leads to a high degree of branching and cross-linking. Beside the creation of new bonds, one may expect C-C or C-H bond scissions during the precursor cell deposition, leading to some etching of the film in competition with the growth. However, no bond breaking due to collision of new precursors with each other or with the formed film on the substrate were observed in this study. This is probably due to the thermal deposition energy of the precursors (0.025 eV), which is below the activation barrier of 0.43 eV reported in the literature.60,61 In our simulations, triple bond opening and successive polymerization by addition of acetylene molecules and C-containing radicals (C2H) is the main mechanism of film formation. Hydrogen radicals also participate in the bond creation, via opening of triple or double C-C bonds. Therefore, here we 10 ACS Paragon Plus Environment

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evaluate the influence of substrate temperature on the creation of both new C-C and C-H bonds in PPA films. To do so, we investigated a parameter defined as the bonding ratio (BR). This parameter is calculated as the ratio of the number of new C-C (C-H) bonds to the total number of bonds in the film:

100%

(1)

The substrate temperature dependence of BR for new C-C (BRC-C) and new C-H (BRC-H) bonds at 2.4 and 18 ns are shown in Fig. 5a and Fig. 5b, respectively. The bonding ratios increase monotonically with temperature. This behavior is due to the increase of the reaction rate at high substrate temperatures. Creation of new bonds at low temperatures, especially at 0 K, is caused by two factors. First, the large number of deposited molecules and radicals increases the chance of encounter of the reaction partners and second, the initial kinetic energy of the precursors in some cases provides sufficient energy for the atoms to overcome the potential barriers and create new bonds.



FIG. 5. Bonding Ratio as a function of the substrate temperature for both 2D (a) and 3D (b) PPA films. New C-C bonds as a function of deposition time at indicated temperatures (c) and new C-H bonds as a function of deposition time at indicated temperatures (d).

The increase of BRC-C as a function of temperature is more pronounced at 2.4 ns when the substrate is still exposed. From 100 K to 500K, the slope of increasing BR tends to saturate at 18 ns. At that time the substrate is covered with either polymeric chains or non-reacted acetylene molecules. The rate of creation of new bonds rises steeply with the substrate temperature during the growth of 2D chains, when precursors can directly perceive the thermal excitation of the substrate atoms. During the growth of 3D films, new precursors encounter polymeric chains (not the substrate directly). However, the mild increase of BRC-C with temperature during the growth of 3D films shows that the rate of creating new C-C bonds is still incremental, but less than the rate of growth of 2D chains. The time distributions of BRC-C and BRC-H are calculated for all substrate temperatures and reported in Fig. 5c and Fig. 5d. The evolution of BRC-C over the first 2.4 ns indicates a considerable increase in the magnitude of BRC-C as a function of temperature during the formation of 2D films (Fig. 5c). After 2.4 ns when the PPA films start growing vertically, BRC-C still shows an increasing behavior but with the much lower rate. At low substrate temperatures BRC-C shows a relatively high rate of increase until 12 ns and afterwards tends to saturate to a constant value. At 0 K this value is about half of the saturation values reached for 300 K and beyond (~18% vs. ~35%). This large difference in saturation level at 0 K probably reveals the difficulty to overcome the energy barrier of certain reactions (as observed in Section 1.2) without additional thermal excitation. The behavior of BRC-C as a function of time shows that a faster formation of 2D and consequently 3D films on the substrate leads to a milder increase in the rate of BRC-C after 2.4 ns. This is likely due to the fact that, by covering the substrate with cross-linked 3D PPA film, the carbon atoms of new precursors (acetylene molecules and radicals) mostly react with the carbon atoms in the surface layers of the PPA film and no longer with the reaction sites in deeper layers. The saturation almost reached at every temperature suggests that one has entered a regime of 3D growth, where the substrate effect is negligible. During the formation of PPA films, new C-H bonds form just due to the reaction of free H radicals and under-coordinated C atoms or C atoms in sp or sp2 configurations. Highly reactive free H atoms can easily approach carbon atoms and make a new C-H bond. In contrast with BRC-C, BRC-H continues to increase beyond 2.4 ns for all temperatures, with no sign of saturation, before and during the formation of 3D films (Fig. 5d). Unlike large C-containing molecules and radicals, single H atoms, smaller and more mobile, are able to readily penetrate and make new bonds in the depth of the 3D film.

3.2.

Hybridization of carbon atoms

Hybridization of carbon atoms is another parameter which was investigated to characterize the PPA films. To do so, we calculated a hybridization ratio (HR) for each film, as follows:

100%, (2)


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where Nx is a number of carbon atoms with spx (x=1,2 or 3) hybridization and NC is a total number of carbon atoms. Fig. 6 illustrates the substrate temperature-dependent distribution of carbon atoms with sp, sp2 and sp3 hybridizations during the formation of 2D and 3D PPA films developed after 2.4 ns and 18 ns of acetylene precursors deposition, respectively. Carbon atoms in the acetylene molecule have a sp hybridization. The high HRsp calculated at low temperatures is mostly due to the carbon atoms of acetylene molecules sticking to the substrate.

FIG. 6. Distribution of carbon atoms with sp, sp2 and sp3 hybridization as a function of the substrate temperature for both 2D and 3D PPA films.

The grafting of radicals and molecules on the growing sides of the chains increases with substrate temperature. This results in PPA films containing more carbon atoms in sp2 and sp3 configurations, and less in sp configuration. sp2 is the dominant type of hybridization for carbon atoms at high temperatures. The evolution of the HRsp2 curve is in good correlation with BRC-C, which increases with increasing substrate temperature during the formation of both 2D and 3D films. The possible configurations of the carbon atoms with sp, sp2 and sp3 hybridization are investigated as well, in order to refine our structural description of the films. The results are presented in Fig. 7. There are two possible configurations for the carbon atoms with sp hybridizations in the PPA films (Fig. 7a). In the first one, the C atom is bonded to one C and one H (sp(C,H)), like C atoms in the C2H2 molecule and, in the second one, the C atom is bonded to two other C atoms (sp(C,C)). The percentage of sp(C,H) is higher than the one of sp(C,C) in the 2D films at low temperatures (0 K to 200 K) and it is due to the high number of acetylene molecules which are deposited on the substrate without reacting. With increasing temperature and rate of reaction, sp(C,C) carbon atoms become dominant. The existence of sp(C,H) carbon atoms at high temperatures (300 K to 500 K), where the films are fully linked, is due to C and H atoms that are located at the end of polymeric chains. For the 3D films, the percentage of sp(C,H) remains higher than sp(C,C) at 0 K (Fig. 7a). But overall, the relative numbers of sp(C,C) are higher than for the 2D films, which is consistent with the formation of longer chains. 13 ACS Paragon Plus Environment


FIG. 7. Substrate temperature distribution of possible configurations for the carbon atoms with sp (a), sp2 (b) and sp3 (c) hybridizations in both 2D and 3D PPA films. Carbon atoms with their possible configurations are colorcoded at the bottom of each graph.

The three possible configurations for sp2 carbon atoms in the PPA films are shown in Fig. 7b, where the carbon is bonded to: 1) two C atoms and one H atom (sp2(2C,H)), 2) two H atoms and one C atom (sp2(C,2H)) and 3) three C atoms (sp2(3C)). In both 2D and 3D films at all temperatures, the vast majority is sp2(2C,H) carbon atoms, which corresponds to the sequential reactions of the C2H2 molecule and the growth of PPA chains. The reaction of single H atoms and C atoms of acetylene molecule in PPA chain results in the formation of sp2(C,2H) carbon atoms. These C atoms have the smallest contribution among sp2 carbons and they are mostly observed at the end of polymeric chains. The rising fraction of sp2(3C) carbons is due to the increase of reaction between carbon atoms in the polymeric chains with increasing substrate temperature. In order to validate this hypothesis, a new series of simulations was set up. To do so, we took the PPA film which was grown at 0 K and increased the substrate temperature to 500 K and let the substrate heat up for 0.5 ns.


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FIG. 8. Representation of polymeric chains before and after reaction of carbons atoms and creation of (a) sp2(3C) and (b) sp3(3C,H) carbons. Reacting carbon atoms are colored in yellow and green, yellow being for the new sp2(3C) and sp3(3C,H) carbons. Here we cut and illustrated the desired region of the thin film.

The reaction between carbon atoms of two different polymeric chains and creation of sp2(3C) carbons are illustrated in Fig. 8a. This additional simulation indicates that temperature promotes the reaction in the film even without deposition of new monomers and radicals. In fact, at high substrate temperature, the rotational and vibrational motion of the dangling chains and their interaction in the coated films are sufficient to induce new reactions leading to the sp2(3C) configuration. Returning to Fig. 7c, the four possibilities of configurations of sp3 carbon atoms in the PPA films are: 1) The C atom is bonded to one C atom and three H atoms (sp3(C,3H)), 2) C atom is bonded to two C atoms and two H atoms (sp3(2C,2H)), 3) C atom is bonded to three C atoms and one H atom (sp3(3C,H)) and 4) C atom is bonded to four C atoms (sp3(4C)). sp3(C,3H) carbon atoms are located at the end of CnHm chains. The contribution of this configuration decreases with increasing substrate temperature, which indicates that the C-H interaction at low substrate temperature mostly results in termination of polymeric chains rather than the creation of reaction sites. sp3(2C,2H) carbon atoms have the largest contribution among the sp3 ones and they correspond to the reaction of either a H atom and sp2(2C,H) carbon atoms or a C atom and sp2(C,2H) carbon atoms. Increased occurrence of C-C reactions with increasing substrate temperature leads to high percentages of sp3(3C,H) carbon atoms in the PPA films. The reaction of carbon atoms in the polymeric chains and the creation of sp3(3C,H) carbon atoms are indicated in Fig. 8b showing that, as for sp2(3C), temperature alone is sufficient to induce new reactions in the films. sp3(4C) carbon atoms pertain to the reaction of C2 and C2H radicals with another three carbon atoms and according to our results, such a reaction seldom happens during the formation of PPA chains.



3.3.

Characterization of the polymerization and cross-linking

In this section, the film formation is analyzed from the perspectives of polymerization, branching and cross-linking. First, the average number of carbon chains in each film is investigated (Table 1). Here our data concern 3D films which are grown after 18 ns of acetylene precursors deposition. According to our results, the number of non-connected chain types decreases with increasing substrate temperature, going towards larger and larger carbon networks.

Type of species CeHf Total

0K 151 5 1 2 159

Average number of species at each substrate temperature 100 K 200 K 300 K 400 K 500 K 31 4 1 2 1 1 1 1 34 5 1 1 1

TAB. 1. The average number of hydrocarbon chains for 3D films grown at different substrate temperature. According to their structure a, b, c and d are less than or equal to 11, 50, 100 and 500, respectively, and 500< (e and f) 1). C=1 corresponds to a tree-like structure with short (low P) or long branches (high P). As C becomes larger, the network becomes more interlinked. In the limit of C→∞ the film is fully interconnected, a lattice for example, which is usually a carbonized film with graphite and/or diamondlike inclusions. The P parameter shows how much the film can be represented as a polymer rather than a17 ACS Paragon Plus Environment


C:H or diamond-like carbon. In the diagram it scales the type of structure defined by C by growing the polymeric chains between related B/T units.

FIG. 9. Polymerization-connectivity diagram for 2D and 3D films grown at different substrate temperature.

The P and C values for both 2D and 3D films are plotted in Fig. 9, so that the structural transition due to the vertical growth of the films can be seen. The extreme 0 K point starts with non-linked short chains (P=0.4) on the surface. They grow longer with the film expansion in the third dimension, along with the introduction of a small degree of linking. At 0 K, the precursors and particularly acetylene molecules are initially deposited on the substrate with little to no interaction. As the temperature increases, both linking and polymerization of the film increase for 2D films (see Fig. 4a). Up to 200 K, both parameters also increase significantly when the film goes tridimensional, essentially indicating more reacted precursors and longer chains. However, above 300 K, active cross-linking overcomes the polymerization tendency and thus reduces the average length of the polymer chains, as compared to the 2D structure. This coincides with the general trend of C:H films becoming harder with temperature and it is known that 18 ACS Paragon Plus Environment

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cross-linking leads to polymer structures with higher rigidity.65–67 Still, for the planar films the P parameter increases even up to 500 K, which means that the 2D structure is leaning to be more polymeric than interconnected, probably because the 2D structure is limited in the number of points where a joint can happen (Fig. 4a). As a caveat, it should be reminded here that the REBO potential, chosen for its better accuracy in describing reaction barriers, does not integrate long-range interactions. Therefore, the amount of nonreacted molecules or radicals trapped in the 3D film should be underestimated, at least at low temperature (0-200 K), where the van der Waals interactions might suffice to retain them in the film. A more accurate description of this effect might result in a lateral displacement of the corresponding data points towards the left of the graph in Fig. 9. This, however, would not change the conclusions of our analysis. Finally, the H/C ratio for the grown 3D films is in the range of 0.7-0.9, which characterizes them as soft a-C:H, and it does not exhibit a clear dependency on the temperature. As it was found experimentally, the H/C ratio for the soft plasma-deposited films does not change significantly below 500 K.68 This is in agreement with our results.

4. Summary and conclusions Our simulations indicate that the main mechanism of hydrocarbon film formation, upon acetylene molecule and radical deposition from the gas phase, is through the opening of the triple bond and polymerization by successive addition of acetylene molecules and C-containing radicals (C2H). Hydrogen radicals also participate in the bond creation, via the opening of triple or double C-C bonds. C-C and C-H bonds scissions (film etching) have not been observed in this study, probably because of the thermal energy of the precursors (0.025 eV). With increasing substrate temperature, in addition to the interaction of the precursors with the growing chains, dangling chains also happen to react with each other in the coated films, increasing cross-linking. The structures of the deposited hydrocarbon films, from planar to tridimensional, exhibit several important features. First, the films become fully cross-linked for a substrate temperature of 300 K and by that we mean that all C chains are connected into a single network. Second, the proposed polymerization-connectivity formalism allowed us to track the nature of crosslinking and to show that planar films tend to become more polymerized at higher temperatures, while 3D films change their connection structure from branched to cross-linked with increasing temperature, with their polymerization degree remaining almost unchanged.

5. Acknowledgments This work was supported by the Belgian Science Policy Office through the Interuniversity Attraction Pole (IAP phase VII, P7/34: Physical chemistry of Plasma-Surface Interaction). We are grateful to the Consortium des Équipements de Calcul Intensif (CÉCI) for providing computational resources, funded by the Fonds de la Recherche Scientifique de Belgique (F.R.S.-FNRS) under Grant No. 2.5020.11.



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Table of contents graphic



FIG. 1. Schematic representation of the initial system arrangement for deposition of the acetylene precursors on the silver substrate. The rigid zone, stochastic zone and reaction zone of the substrate are colored in pink, green and light gray, respectively. Red is for C atoms and white for H atoms. The yellow frame represents the boundary of the periodic cell. The reflective wall is denoted by light blue color at the top of the cell. 167x279mm (300 x 300 DPI)

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FIG. 2. Potential energies as a function of the reacting atom interdistances for the first (a) and second (b) step of the polymerization reaction. 286x172mm (300 x 300 DPI)



FIG. 3. (a) REBO potential and Aryanpour parametrization of ReaxFF potential for the interaction between acetylene molecule and the possible reaction site, Here x and y indicate the carbon atom which is a reaction site and carbon atom in the acetylene molecule, respectively. (b) Formation of CnHm PPA chains (yellow circled) after 6 ns of plasma deposition at the substrate temperature of 300 K. 254x92mm (300 x 300 DPI)


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FIG. 4. Top view of the 2-dimensional film growth after 2.4 ns of acetylene precursors deposition (a) and side view of the 3-dimensional film growth after 18 ns of acetylene precursors deposition (b) at a substrate temperature of 300 K. 355x187mm (300 x 300 DPI)



FIG. 5. Bonding Ratio as a function of the substrate temperature for both 2D (a) and 3D (b) PPA films. New C-C bonds as a function of deposition time at indicated temperatures (c) and new C-H bonds as a function of deposition time at indicated temperatures (d). 237x189mm (300 x 300 DPI)


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FIG. 6. Distribution of carbon atoms with sp, sp2 and sp3 hybridization as a function of the substrate temperature for both 2D and 3D PPA films. 307x120mm (300 x 300 DPI)



FIG. 7. Substrate temperature distribution of possible configurations for the carbon atoms with sp (a), sp2 (b) and sp3 (c) hybridizations in both 2D and 3D PPA films. Carbon atoms with their possible configurations are color-coded at the bottom of each graph. 264x160mm (300 x 300 DPI)


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FIG. 8. Representation of polymeric chains before and after reaction of carbons atoms and creation of (a) sp2(3C) and (b) sp3(3C,H) carbons. Reacting carbon atoms are colored in yellow and green, yellow being for the new sp2(3C) and sp3(3C,H) carbons. Here we cut and illustrated the desired region of the thin film. 313x152mm (300 x 300 DPI)



FIG. 9. Polymerization-connectivity diagram for 2D and 3D films grown at different substrate temperature. 127x156mm (300 x 300 DPI)


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Molecular Dynamics Simulations of Hydrocarbon Film Growth from

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