7 research outputs found
Mesoscopic Structures and Coexisting Phases in Silica Films
Silica films represent a unique two-dimensional film system, exhibiting both crystalline and vitreous forms. While much scientific work has focused on the atomic-scale features of this film system, mesoscale structures can play an important role for understanding confined space reactions and other applications of silica films. Here, we report on mesoscale structures in silica films grown under ultrahigh vacuum and examined with scanning tunneling microscopy (STM). Silica films can exhibit coexisting phases of monolayer, zigzag, and bilayer structures. Both holes in the film structure and atomic-scale substrate steps are observed to influence these coexisting phases. In particular, film regions bordering holes in silica bilayer films exhibit vitreous character, even in regions where the majority film structure is crystalline. At high coverages mixed zigzag and bilayer phases are observed at step edges, while at lower coverages silica phases with lower silicon densities are observed more prevalently near step edges. The STM images reveal that silica films exhibit rich structural diversity at the mesoscale
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Bending Rigidity of 2D Silica
A chemically stable bilayers of SiO2 (2D silica) is a new, wide band gap 2D material. Up till now graphene has been the only 2D material where the bending rigidity has been measured. Here we present inelastic helium atom scattering data from 2D silica on Ru(0001) and extract the first bending rigidity, κ, measurements for a nonmonoatomic 2D material of definable thickness. We find a value of κ=8.8  eV±0.5  eV which is of the same order of magnitude as theoretical values in the literature for freestanding crystalline 2D silica
Two-Dimensional Iron Tungstate: A Ternary Oxide Layer With Honeycomb Geometry
The
exceptional physical properties of graphene have sparked tremendous
interests toward two-dimensional (2D) materials with honeycomb structure.
We report here the successful fabrication of 2D iron tungstate (FeWO<sub><i>x</i></sub>) layers with honeycomb geometry on a Pt(111)
surface, using the solid-state reaction of (WO<sub>3</sub>)<sub>3</sub> clusters with a FeO(111) monolayer on Pt(111). The formation process
and the atomic structure of two commensurate FeWO<sub><i>x</i></sub> phases, with (2 × 2) and (6 × 6) periodicities,
have been characterized experimentally by combination of scanning
tunneling microscopy (STM), low-energy electron diffraction (LEED),
X-ray photoelectron spectroscopy (XPS), and temperature-programmed
desorption (TPD) and understood theoretically by density functional
theory (DFT) modeling. The thermodynamically most stable (2 ×
2) phase has a formal FeWO<sub>3</sub> stoichiometry and corresponds
to a buckled Fe<sup>2+</sup>/W<sup>4+</sup> layer arranged in a honeycomb
lattice, terminated by oxygen atoms in Fe–W bridging positions.
This 2D FeWO<sub>3</sub> layer has a novel structure and stoichiometry
and has no analogues to known bulk iron tungstate phases. It is theoretically
predicted to exhibit a ferromagnetic electronic ground state with
a Curie temperature of 95 K, as opposed to the antiferromagnetic behavior
of bulk FeWO<sub>4</sub> materials
A Two Dimensional Zigzag Silica Polymorph on a Metal Support
We
present a new polymorph of the two-dimensional (2D) silica film
with a characteristic ‘zigzag’ line structure and a
rectangular unit cell which forms on a Ru(0001) metal substrate. This
new silica polymorph may allow for important insights into growth
modes and transformations of 2D silica films as a model system for
the study of glass transitions. Based on scanning tunneling microscopy,
low energy electron diffraction, infrared reflection absorption spectroscopy,
and X-ray photoelectron spectroscopy measurements on the one hand,
and density functional theory calculations on the other, a structural
model for the ‘zigzag’ polymorph is proposed. In comparison
to established monolayer and bilayer silica, this ‘zigzag’
structure system has intermediate characteristics in terms of coupling
to the substrate and stoichiometry. The silica ‘zigzag’
phase is transformed upon reoxidation at higher annealing temperature
into a SiO<sub>2</sub> silica bilayer film which is chemically decoupled
from the substrate