Connectivity percolation and thin film growth containing rods

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 ▲]

Monolayers of hard rods on planar substrates: I. Equilibrium

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 ▲]