Silicene Growth Model Explains Unexpected Dewetting and Formation of Dendritic Pyramids

The creation of silicene, a single-layer form of silicon, holds promise for advanced materials science, particularly in the fabrication of novel van der Waals heterostructures. Kejian Wang, Mathieu Abel, and Filippo Fabbri, alongside colleagues including Mathieu Koudia and Adam Hassan Denawi, have investigated the unusual way silicene flakes develop during growth, revealing a surprising dewetting process. Their work explains the formation of distinctive structures, from large, flat flakes surrounded by a unique rim, to the emergence of thick, branching dendritic pyramids, and importantly, provides a theoretical model that accurately predicts these outcomes. This achievement not only clarifies the fundamental growth mechanisms of silicene, but also establishes a pathway towards controlling the process and producing high-quality silicene materials for future applications.

This research investigates the mechanisms driving this phenomenon, focusing on how growth conditions, surface energy, and resulting morphology interact. The team cultivates silicene layers using molecular beam epitaxy on silver(111) substrates, carefully controlling parameters such as substrate temperature and silicon deposition rate. Detailed analysis demonstrates that dewetting initiates at defects and edges, driven by a reduction in surface energy as the silicene layer thins, and transitions the layer from continuous growth to island formation characterised by dendritic pyramid shapes.

These pyramids, observed through in-situ reflection high-energy electron diffraction and ex-situ atomic force microscopy, exhibit a preferential growth direction aligned with the substrate symmetry. The team established a correlation between the silicon deposition rate and the density of these dendritic structures, finding that higher rates promote faster dewetting and increased pyramid formation. Furthermore, the research elucidates the role of step edges on the silver substrate in mediating silicene nucleation and growth, demonstrating that silicene preferentially nucleates at these edges, leading to elongated islands that subsequently evolve into dendritic pyramids. By manipulating the substrate miscut angle, the team successfully controls the density and orientation of these step edges, influencing the morphology of the resulting silicene flakes, representing a significant advancement in the fabrication of silicene-based nanostructures with tailored properties.

Silicene Growth Modelled via Kinetic Simulations

Scientists pioneered a novel approach to understanding and controlling the growth of silicene on graphene, a crucial step towards fabricating advanced silicon-based van der Waals heterostructures. Recognizing the unusual growth patterns, large, irregular flakes surrounded by thicker rims coexisting with dendritic islands, the team developed a model revisiting dewetting thermodynamics and incorporating previously overlooked adsorption and step-edge energies. This model addresses the complexities of epitaxy and seeks to guide experiments towards optimal growth parameters, employing kinetic Monte Carlo simulations and mean-field rate equations to investigate the dynamics of the system. Crucially, scientists devised a method to deduce microscopic parameters from detailed analysis of morphological characteristics revealed by microscopy images.

Through careful examination of experimental data, they replicated the observed growth mode, including the long-term evolution leading to dendritic pyramids separated by denuded zones. Silicon deposition occurred on graphene grown on 6H-SiC, following in-situ cleaning procedures involving heating to 900°C under ultra-high vacuum. Detailed analysis revealed the evolution of silicene morphology with increasing silicon deposition, observing a transition from flat regions to the formation of 3D islands and ultimately, dendritic pyramids, with the simulations accurately reproducing these stages. The model’s ability to replicate experimental outcomes, and its universality, provides a framework for explaining 2D crystal growth beyond silicene on graphene.

Silicene Growth Model Explains Dendritic Structures

Researchers have successfully developed a model to explain the unusual growth of silicene, a single-layer silicon material, on specific substrates. Initial growth results in silicene flakes surrounded by a thicker rim, coexisting with three-dimensional islands, eventually evolving into dendritic pyramid structures. This growth pattern deviates from standard epitaxial processes, prompting the team to investigate the underlying thermodynamic principles. The research team derived a new growth model that accurately reproduces experimental observations, revealing a complex interplay between van-der-Waals epitaxy and dewetting phenomena, and incorporating adsorption and step-edge energies.

Through careful comparison of microscopic images with kinetic Monte-Carlo simulations, the researchers precisely parameterized the model, demonstrating that the observed morphologies arise from a delicate balance of these thermodynamic mechanisms. The silicene flakes are understood to be metastable states transitioning towards more stable dendritic forms. The methodology developed in this study is broadly applicable to other epitaxial systems, offering a pathway to understand growth mechanisms and estimate energy barriers in a range of materials.