Fabrication of high-performance heterostructure devices requires fundamental understanding of the diffusion dynamics of metal species on 2D materials. Here, we investigate the room-temperature diffusion of Ag, Au, Cu, Pd, Pt, and Ru adatoms on graphene using ab initio and classical molecular dynamics simulations. We find that Ag, Au, Cu, and Pd follow Lévy walks, in which adatoms move continuously within ∼1–4 nm2 domains during ∼0.04 ns timeframes, and they occasionally perform ∼2–4 nm flights across multiple surface adsorption sites. This anomalous diffusion pattern is associated with a flat (<50 meV) potential energy landscape (PEL), which renders surface vibrations important for adatom migration. The latter is not the case for Pt and Ru, which encounter a significantly rougher PEL (>100 meV) and, hence, migrate via conventional random walks. Thus, adatom anomalous diffusion is a potentially important aspect for modeling growth of metal films and nanostructures on 2D materials.

We use kinetic Monte Carlo simulations and analytical modelling to study coalescence of three-dimensional (3D) nanoscale faceted silver island pairs on weakly-interacting fcc(111) substrates, with and without concurrent supply of mobile adatoms from the vapor phase. Our simulations show that for vapor flux arrival rates F < 1 monolayer/second (ML/s) coalescence manifests itself by one of the islands absorbing the other via sidewall facet migration. This process is mediated by nucleation and growth of two-dimensional (2D) layers on the island facets, while the supply of mobile atoms increases the nucleation probability and shortens the time required for coalescence completion. When F is increased above 1 ML/s, coalescence is predominantly governed by deposition from the vapor phase and the island pair reaches a compact shape via agglomeration. The crucial role of facets for the coalescence dynamics is further supported by a mean-field thermodynamic description of the nucleation energetics and kinetics. Our findings explain experimental results which show that two-dimensional film growth morphology on weakly-interacting substrates is promoted when the rate of island coalescence is suppressed. The present study also highlights that deviations of experimentally reported film morphological evolutions in weakly-interacting film/substrate systems from predictions based on the sintering and particle growth theories may be understood in light of the effect of deposition flux atoms on the energetics and kinetics of facet-layer nucleation during coalescence.

Thin films are nanoscale layers of material used to functionalize surfaces or to serve as building blocks in more complex devices. In recent years, thin metal films have become vital for modern devices within, e.g., biosensing, catalysis, and nanoelectronics, whereby synthesis of metal layers with specific morphological features on two-dimensional (2D) crystals and oxides is required. However, this entails a great scientific challenge: in most of the afore-mentioned film/substrate combinations substrate and metal atoms interact weakly, causing the latter to self-assemble without control into three-dimensional (3D) clusters.

Nowadays, a significant fraction of thin films is synthesized via condensation from the vapor phase, a far-from-equilibrium process in which film morphology is governed by the kinetic rates of atomic-scale structure-forming processes. It is, therefore, evident that knowledge-based synthesis of metal layers in high-performance devices necessitates a comprehensive understanding of the dynamic competition among these processes at the nano- and mesoscale. Such understanding is today incomplete, since experimental materials science tools are often not capable of providing nanometer and sub-nanometer insights at time scales that are relevant for thin-film synthesis. Computational approaches offer the possibility to fill the afore-mentioned gap in knowledge by allowing to explore atomistic behaviors with picosecond resolution. Hence, in the present thesis, a combination of modern computer simulation techniques is used to investigate thin metal film growth on weakly-interacting substrates from a purely atomistic point of view and to elucidate the ways by which atomic diffusion mechanisms give rise to the final film morphologies.

In the first part of the thesis, an in-house kinetic Monte Carlo (KMC) simulation code and analytical modelling are used to investigate the early growth stages of Ag films supported on a generic weakly-interacting substrate. The results show that the weak interaction strength between film atoms and substrates leads to the formation of strongly-faceted 3D Ag islands, whose vertical growth is mediated by the temperature-dependent upward adatom diffusion across the facets. Eventually, the 3D islands impinge on each other and coalesce via surface migration of facet layers. Migration can be promoted by an increase of the deposition flux, but it can also be hindered by material agglomeration if the flux exceeds a critical threshold. These findings provide the foundation for explaining several effects observed during thin film growth on weakly-interacting substrates, including the increase of film roughness with temperature, the transition from 3D to 2D film morphology upon suppression of coalescence, and the origin of changes in thin film roughness and grain boundary number densities when varying the magnitude of vapor flux arrival rate.

In the second part, ab initio and classical Molecular Dynamics simulations are used to investigate the diffusion dynamics of several transition metal adatoms (Ag, Au, Cu, Pd, Pt and Ru) and multi-atomic clusters (Ag, Au, Cu and Pd) on single layer graphene at room temperature (300K). The simulated diffusion trajectories reveal that diffusing adspecies experiencing a deep (hundreds of meV) potential energy landscape (PEL) on the substrate surface follow random walks; whilst those with a weak interaction with the substrate (PEL depth of a few meV) follow a superdiffusive motion pattern known as Lévy walk. This type of anomalous movement— also observed in other phenomena in physical, biological, and social systems—manifests itself as a continuous atomic motion with occasional flights over distances covering multiple adsorption sites. The fact that adspecies follow a distinctly different type of motion than what is observed in classical homoepitaxial growth theory implies that energy barriers readily available from static (0K) calculations may not be able to provide a physical accurate description of surface diffusion of metal adspecies on 2D crystals. As such, anomalous diffusion is a potentially important aspect to be considered when modelling growth of metal films and nanostructures on 2D materials.

The results and insights generated in the present thesis provide key knowledge for controlled synthesis of films and nanostructures with tailored properties. This, in turn, is relevant for developing high-performance energy-saving windows, improving the turnover frequency of catalytic reactions, and integrating 2D materials into novel nanoelectronic devices. Moreover, the techniques developed and employed herein contribute toward bringing modern computational tools closer to the field of thin film growth.

Thin films are nanoscale layers of material, with exotic properties useful in diverse areas, ranging from biomedicine to nanoelectronics and surface protection. Film properties are not only determined by their chemical composition, but also by their microstructure and roughness, features that depend crucially on the growth process due to the inherent out-of equilibrium nature of the film deposition techniques. This fact suggest that it is possible to control film growth, and in turn film properties, in a knowledge-based manner by tuning the deposition conditions. This requires a good understanding of the elementary film-forming processes, and the way by which they are affected by atomic-scale kinetics. The kinetic Monte Carlo (kMC) method is a simulation tool that can model film evolution over extended time scales, of the order of microseconds, and beyond, and thus constitutes a powerful complement to experimental research aiming to obtain an universal understanding of thin film formation and morphological evolution.

In this work, kMC simulations, coupled with analytical modelling, are used to investigate the early stages of formation of metal films and nanostructures supported on weakly-interacting substrates. This starts with the formation and growth of faceted 3D islands, that relies first on facile adatom ascent at single-layer island steps and subsequently on facile adatom upward diffusion from the base to the top of the island across its facets. Interlayer mass transport is limited by the rate at which adatoms cross from the sidewall facets to the island top, a process that determines the final height of the islands and leads non-trivial growth dynamics, as increasing temperatures favour 3D growth as a result of the upward transport. These findings explain the high roughness observed experimentally in metallic films grown on weakly-interacting substrates at high temperatures.

The second part of the study focus on the next logical step of film formation, when 3D islands come into contact and fuse into a single one, or coalesce. The research reveals that the faceted island structure governs the macroscopic process of coalescence as well as its dynamics, and that morphological changes depend on 2D nucleation on the II facets. In addition, deposition during coalescence is found to accelerate the process and modify its dynamics, by contributing to the nucleation of new facets.

This study provides useful knowledge concerning metal growth on weakly-interacting substrates, and, in particular, identifies the key atomistic processes controlling the early stages of formation of thin films, which can be used to tailor deposition conditions in order to achieve films with unique properties and applications.