Abstract
In recent years, silica films have emerged asa novel class of two-dimensional (2D) materials. Several groups succeeded in epitaxial growth of ultrathin SiO2 layers using different growth methods and various substrates. The structures consist of tetrahedral [SiO4] building blocks in two mirror symmetrical planes, connected via oxygen bridges. This arrangement is called a silica bilayer as it is the thinnest 2D arrangement with the stoichiometry SiO2 known today. With all bonds saturated within the nano-sheet, the interaction with the substrate is based on van der Waals forces. Complex ring networks are observed, including hexagonal honeycomb lattices, point defects and domain boundaries, as well as amorphous domains. The network structures are highly tuneable through variation of the substrate, deposition parameters, cooling procedure, introducing dopants or intercalating small species.The amorphous networks and structural defects were resolved with atomic resolution microscopy and modeled with density functional theory and molecular dynamics. Such data contribute to our understanding of the formation and characteristic motifs of glassy systems. Growth studies and doping with other chemical elements reveal ways to tune ring sizes and defects as well as chemical reactivities. The pristine films have been utilized as molecular sieves and for confining molecules in nanocatalysis. Post growth hydroxylation can be used to tweak the reactivity as well.The electronic properties of silica bilayers are favourable for using silica as insulators in 2D material stacks. Due to the fully saturated atomic structure, the bilayer interacts weakly with the substrate and can be described as quasi-freestanding. Recently, amm-scale film transfer under structure retention has been demonstrated. The chemical and mechanical stability of silica bilayers is very promising for technological applications in 2D heterostacks.Due to the impact of this bilayer system for glass science, catalysis and the field of 2D materials, a large number of theoretical and experimental studies on silica bilayers have been reported in the last years. This review aims to provide an overview on the insights gained on this material and to point out opportunities for further discovery in various fields.
Highlights
In recent years, silica films have emerged as a novel class of two-dimensional (2D) materials
The amorphous networks and structural defects were resolved with atomic resolution microscopy and modeled with density functional theory and molecular dynamics
We focus on self-saturated bilayer sheets built from [SiO4] tetrahedra and summarize the insights gained from these complex networks
Summary
We describe the reported preparations of silica bilayers, providing context by including the typical preparation of silica monolayers. Following the film growth in-situ, preferential nucleation at the metal grain boundaries was observed, indicating that Si (and possibly O) from the support migrates through the metal nanoplatelets, forming thin silica films on the surface [28] In these experiments, no additional oxygen was provided during the preparation. The solid-state growth studies highlight the need to clean the metal surface with an intense ‘‘beam shower” of electrons, only after which well-ordered films nucleated [15] In accordance with these observations, one might assume that the copper foil used as substrate in [16] was covered by graphene, blocking adsorbates from above and facilitating the growth of a silica film from below. If existing hexacelsians allow exfoliation of single layers, they might offer a simple route towards doped 2D silica films
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