Molecular Dynamics & Adaptive Kinetic Monte Carlo Growth Study of Hydrocarbon Flakes

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1 Molecular Dynamics & Adaptive Kinetic Monte Carlo Growth Study of Hydrocarbon Flakes Amit R. Sharma Department of Physics Wright State University Dayton, Ohio, USA

2 Hydrocarbon co-deposits in tokamak C. Martin et al. / Journal of Nuclear Materials (2007) a:ch film deposition in arc discharge. Modeling: Monte Carlo method. W. Bohmeyer et al., Journal of Nuclear Materials (2005)

3 ToreSupra experiments volumetric measurements through adsorption isotherms and of structural measurements through scanning electron microscopy (SEM) and transmission electron microscopy (TEM). pores with a typical size lower than 2 nm, ( 11%), mesoporosity ( 5%) and macroporosity, (typical size more than 50 nm.) C. Martin et al. / Journal of Nuclear Materials (2007)

4 Plasma generator PSI-2 a stationary high current arc discharge; used for deposition experiments. Well defined amounts of CH 4 or C 2 H 4 are injected into the plasma through a nozzle at the plasma edge. injected gas molecules (CH 4 ) has been analyzed using the 3-D Monte Carlo code ERO which has been adapted to the geometry of the plasma generator PSI-2. W. Bohmeyer et al. / Journal of Nuclear Materials (2005) W. Bohmeyer et al. / Journal of Nuclear Materials (2007)

5 Plasma-wall interaction in Fusion reactors D-T retention? Material choice is a critical issue. 5

6 Multiscale model development Max-Planck IPP Greifswald. Ralf Schneider, Manoj Warrier, Abha Rai, Bas Braams. 6

7 Classical MD simulation to compute sticking/reflection 986 atoms with a H:C-ratio of 0.66 with periodic boundary conditions in three-dimensions. cycles of heating up to 3000 K and subsequent cooling down to 200 K Modified Brenner potential. A.R. Sharma et al. / Journal of Nuclear Materials (2007)

8 Orientation dependence Local chemistry dominates the events and atomic scale surface roughness (dangling bonds) is important factor. Numerous surface sites were sampled (error bars) 8

9 Atomic & dimer carbon reflection 9

10 C 2 H x Hydrocarbon reflection 10

11 C 1 H x reflection 11

12 Breakup pattern 12

13 Amorphous hydrocarbon growth, Mechanism? Dynamics? 13

14 Also of interest - CVD CNT growth 14

15 Kinetic Monte Carlo In solid state systems, atoms reside near their equilibrium positions and reactions are rare events. Reactions occur on a much longer timescale than the typical vibrational period of the system (~100 fs), it is not possible to use molecular dynamics (MD) simulations to model solid-state reactions. Adaptive kinetic Monte Carlo (AKMC) is a powerful tool for simulating the dynamics of solid-state systems that works by identifying the saddle points between the stable minima and then modeling the reactions between them using statistical mechanics (computationally expensive) 15

16 Adaptive KMC 16

17 Kinetic Monte Carlo the saddle points are located by running high temperature MD trajectories. trajectories should be hot enough that a few picoseconds of dynamics will be sufficiently long for a reaction to occur. The saddle points are located by periodically minimizing the MD trajectory to detect when the trajectories minimizes to a new geometry. Once the new minimum is located, the saddle point is found by running a nudged elastic band (NEB) calculation. 17

18 Kinetic Monte Carlo saddle points are identified, the reaction rates can be estimated using harmonic transition state theory (HTST). In KMC, the system is represented using a Markov chain. Each state of the Markov chain represents a stable state of the system (minimum) and the transition probabilities between the states are related to the reaction rates (determined from the saddle points). 18

19 akmc simulation of surface atom diffusion initial configuration will be a 4 layer p(4x4)-pt(100) slab (FCC) with a single adatom occupying a four-fold hollow site. MD temperature 2000K; KMC temperature 300K Confidence 0.95 Barrier 0.72, k=2.77x10 1 (1/s) 19

20 akcm migration via surface hopping Adatom diffusion via surface hopping has a higher barrier and slower rate Barrier 0.93, rate=1.424x10-3 (1/s) 20

21 Surface atom diffusion in Tungsten Initial geometry optimization with max force tolerance of 0.01 ev/ang, max step length of 0.2 Angstrom, MD trajectory at 2000K don t yield saddle point. After cranking up the MD temperature to 3000K some saddle points and minima is located. No noticeable surface deformation at 2000K 21

22 Growth of amorphous hydrocarbon codeposits Initial geometry optimization. MD trajectory at elevated temperature does not generate Markov chain samples for saddle point optimization. No noticeable surface deformation at 2000K EAM potential Juslin, N and Erhart, P and Traskelin, P and Nord, J and Henriksson, Krister OE and Nordlund, K and Salonen, E and Albe, K, Analytical interatomic potential for modeling nonequilibriumprocesses in the W--C--H system, Journal of applied physics, 98, pp ,

23 Dynamics/Kinetics of hydrocarbon MD trajectory at elevated temperature generates local Markov chain samples. 23

24 Dynamics/Kinetics of hydrocarbon MD trajectory at elevated temperature is used to generate Markov chain samples for saddle point optimization. Transition rate is calculated. 24

25 Summary Adaptive KMC simulation of surface adatom diffusion (self/foreign) is shows new pathways. akmc is being used to understand PES landscape for hydrocarbon to amorphous C:H flake formation on metal surface. The reflection coefficients of atomic carbon, C 2, CH, CH 2, CH 3, CH 4, C 2 H, C 2 H 2, C 2 H 3 and C 2 H 4 show a decreasing trend with increasing energy. The trend in the reflection coefficient also holds for surfaces with H to C different than 0.66 The energy dependence of the reflection coefficient is used in the ERO modeling of PSI-2 improved modeling results! 25