Using the X-ray standing wave method, scanning tunneling microscopy, low energy electron diffraction, and density functional theory, we precisely determine the lateral and vertical structure of hexagonal boron nitride on Ir(111). The moiré superstructure leads to a periodic arrangement of strongly chemisorbed valleys in an otherwise rather flat, weakly physisorbed plane. The best commensurate approximation of the moiré unit cell is (12 × 12) boron nitride cells resting on (11 × 11) substrate cells, which is at variance with several earlier studies. We uncover the existence of two fundamentally different mechanisms of layer formation for hexagonal boron nitride, namely, nucleation and growth as opposed to network formation without nucleation. The different pathways are linked to different distributions of rotational domains, and the latter enables selection of a single orientation only.
The interaction of Fe deposited on graphene grown on Ir(111) was studied in detail to better understand the growth, intercalation, and oxidation of Fe ultrathin films on and under graphene. The study has combined a multiple technique approach that allows extracting at once the chemical, topographic, and precise atomic structure of the system submitted to different conditions of growth and atmospheric environment. For instance, scanning tunneling microscopy (STM) measurements allowed us to follow the formation of Fe nanostructures during Fe deposition and intercalation. Synchrotron-based high-resolution X-ray photoelectron spectroscopy (HR-XPS) untangled the different chemical environments for C–Fe bonds. We have also used photoelectron diffraction experiments to site-specifically unravel the atomic structure of the intercalated Fe under graphene. Oxidation experiments were also performed for samples prepared under different conditions which revealed that indeed one can set the sample temperature to selectively protect or oxidize the intercalated/supported materials, which open interesting possibilities to engineer complex metal-oxide graphene-based devices.
We studied the effect of alkali metal intercalation (Cs and Li) on the geometry of graphene on Ir(111) using the X-ray standing waves technique. For both alkali metals, the increase in the mean height of the carbon layer does not depend on the lateral structure or the density of the intercalated layer. For Li, full delamination of graphene from the metal substrate is found already for a small amount of intercalant. Even though lithium lifts graphene to a smaller height, it is much more efficient in ironing out the corrugation of pristine graphene on Ir(111).
The structure of graphene on Ru(0001) has been extensively studied over the last decade, yet with no general agreement. Here, we analyze graphene's valleys and hills using a combination of X-ray standing wave (XSW) and density functional theory (DFT). The chemical specificity of XSW allows an independent analysis of valleys and hills, which, together with the DFT model, results in the precise determination of the distance between the flat, strongly bounded valleys of graphene and the substrate as well as the corrugation presented in the weakly bounded hills. From the theoretical viewpoint, the good agreement with the experiment validates the choices regarding the unit cell size and the nonlocal correlation functional.
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