Continuum studies of microstructure formation in metallic and organic thin films

Abstract

In this dissertation, microstructure formation processes in binary metallic ultra-thin films and organic polycrystalline thin films are studied through a combination of theoretical model development, analysis, and numerical simulations. In binary metallic films, to investigate compositional patterning and misfit dislocation formation, a quantitative approach based on the so called phase-field crystal method is developed. Both through analysis and simulations of the model, a number of generic and limiting cases of surface alloy epitaxial systems are investigated to examine the effects of lattice mismatch, adlayer-substrate interaction potential, and line tension on equilibrium compositional domain size. A procedure is developed to quantitatively relate the parameters of the model to a specific system [CoAg/Ru(0001)], and it is demonstrated that simulations capture experimentally observed morphologies. Then, the model is employed to investigate the effects of misfit strain fields in the substrate on both heterogeneous nucleation behavior and anisotropic growth of islands at submonolayer coverages and compositional patterning at complete monolayer coverage via simulations. In particular, in the case of binary systems at complete monolayer coverage, strain-stabilized compositional domains emerge at low line tension values for both substrates. Interestingly, the compositional domains on the QC substrate inherit their symmetries at sufficiently low line tension values, while at larger line tension values, the domain structure begins to resemble the classical spinodal microstructure. These studies will enable physically-based design of nanoscale features for a broad range of applications, such as catalysis.

In organic polycrystalline films, our focus is on determining the effects of additives and substrate templating on nucleation and grain growth behavior of solution processed triethylsilylethynyl anthradithiophene films. Through a mean-field approach, it is demonstrated that with increasing additive concentrations, a universal shift in nucleation behavior from an effectively instantaneous to a constant rate process takes place. Then, to explicitly capture the microstructures of polycrystalline films, a vector phase-field model is employed to incorporate the kinetics of the grain impingement processes observed in experiments. Via numerical simulations of the model, it is demonstrated that orientational disorder in amorphous phase, substrate templating, and capillary effects have considerable impact on the microstructure formation processes of organic polycrystalline films.