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See detailConnectivity percolation and thin film growth containing rods
Dixit, Mohit UL

Doctoral thesis (2018)

The evolution of structure in thin films is usually discussed using a kinetic description. This description, mainly developed for metallic films at low temperature, may not be applicable at higher temper ... [more ▼]

The evolution of structure in thin films is usually discussed using a kinetic description. This description, mainly developed for metallic films at low temperature, may not be applicable at higher temper- atures as the film grown is close to the equilibrium where the bulk and surface thermodynamics play an essential role, e.g. in case of deposition of organic molecules. Weaker intermolecular interactions and molecular anisotropy add to the complexity of understanding the structure of a growing film. In this thesis, we analyze simplified, coarse–grained models which have potential for addressing the interplay of equilibrium phases and structure formation using Monte Carlo (MC) simulations. Anisotropic particles are modeled by rods with steric exclusion (hard–rods), mutual attractions and interactions with a substrate. First, investigations are shown for formation of the films that con- tain rods, which have mutual attractions and substrate interaction, using MC simulations and kinetic growth model. The model de- scribed by kinetic rate equations requires microscopic quantities as input parameters such as the deposition rate, diffusion constant, and attachment and detachment rates. These parameters are extracted from MC simulations, and we compare simulated growth dynamics to predictions by rate equations. It is observed that the widely ac- cepted strategy of adopting simple kinetic rate equations to specific film growth problems fails in the case of systems with orientational degrees of freedom, such as most organic semiconductor materials. Next, by analyzing the hard–rod monolayer case, the equilibrium properties are obtained which serve as a template for growth stud- ies. Additionally, the MC simulations are compared with classical density functional theory and lattice-based simulations which allow a good methodological control in order to study more complicated and detailed models. In equilibrium, a continuous “standing–up" transition is observed with or without interactive substrate, while the equilibrium properties of the monolayer dominate growth dy- namics. Lastly, and relevant to the charge transport properties of a film, we investigated connectivity percolation in suspensions of attractive rods using the same MC simulation methods mentioned above. The simulation results are then compared with connectivity percolation theory. [less ▲]

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See detailMonolayers of hard rods on planar substrates. II. Growth
Klopotek, Miriam; Hansen-Goos, Hendrik; Dixit, Mohit UL et al

in Journal of Chemical Physics (2017), 146

Growth of hard-rod monolayers via deposition is studied in a lattice model using rods with discrete orientations and in a continuum model with hard spherocylinders. The lattice model is treated with ... [more ▼]

Growth of hard-rod monolayers via deposition is studied in a lattice model using rods with discrete orientations and in a continuum model with hard spherocylinders. The lattice model is treated with kinetic Monte Carlo simulations and dynamic density functional theory while the continuum model is studied by dynamic Monte Carlo simulations equivalent to diffusive dynamics. The evolution of nematic order (excess of upright particles, “standing-up” transition) is an entropic effect and is mainly governed by the equilibrium solution, rendering a continuous transition [Paper I, M. Oettel et al., J. Chem. Phys. 145, 074902 (2016)]. Strong non-equilibrium effects (e.g., a noticeable dependence on the ratio of rates for translational and rotational moves) are found for attractive substrate potentials favoring lying rods. Results from the lattice and the continuum models agree qualitatively if the relevant characteristic times for diffusion, relaxation of nematic order, and deposition are matched properly. Applicability of these monolayer results to multilayer growth is discussed for a continuum-model realization in three dimensions where spherocylinders are deposited continuously onto a substrate via diffusion. [less ▲]

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See detailMonolayers of hard rods on planar substrates: I. Equilibrium
Oettel, Martin; Klopotek, Miriam; Dixit, Mohit UL et al

in Journal of Chemical Physics (2016), 145

The equilibrium properties of hard rod monolayers are investigated in a lattice model (where position and orientation of a rod are restricted to discrete values) as well as in an off-lattice model ... [more ▼]

The equilibrium properties of hard rod monolayers are investigated in a lattice model (where position and orientation of a rod are restricted to discrete values) as well as in an off-lattice model featuring spherocylinders with continuous positional and orientational degrees of freedom. Both models are treated using density functional theory and Monte Carlo simulations. Upon increasing the density of rods in the monolayer, there is a continuous ordering of the rods along the monolayer normal (“standing up” transition). The continuous transition also persists in the case of an external potential which favors flat-lying rods in the monolayer. This behavior is found in both the lattice and the continuum models. For the lattice model, we find very good agreement between the results from the specific DFT used (lattice fundamental measure theory) and simulations. The properties of lattice fundamental measure theory are further illustrated by the phase diagrams of bulk hard rods in two and three dimensions. [less ▲]

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See detailConnectivity percolation in suspensions of attractive square-well spherocylinders
Dixit, Mohit UL; Meyer, Hugues UL; Schilling, Tanja UL

in Physical Review. E : Statistical, Nonlinear, and Soft Matter Physics (2016), (93), 012116

We have studied the connectivity percolation transition in suspensions of attractive square-well spherocylinders by means ofMonte Carlo simulation and connectedness percolation theory. In the 1980s the ... [more ▼]

We have studied the connectivity percolation transition in suspensions of attractive square-well spherocylinders by means ofMonte Carlo simulation and connectedness percolation theory. In the 1980s the percolation threshold of slender fibers has been predicted to scale as the fibers’ inverse aspect ratio [Phys. Rev. B 30, 3933 (1984)]. The main finding of our study is that the attractive spherocylinder system reaches this inverse scaling regime at much lower aspect ratios than found in suspensions of hard spherocylinders. We explain this difference by showing that third virial corrections of the pair connectedness functions, which are responsible for the deviation from the scaling regime, are less important for attractive potentials than for hard particles. [less ▲]

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