6 research outputs found
Graphene Coatings: Probing the Limits of the One Atom Thick Protection Layer
The limitations of graphene as an effective corrosion-inhibiting coating on metal surfaces, here exemplified by the hex-reconstructed Pt(100) surface, are probed by scanning tunneling microscopy measurements and density functional theory calculations. While exposure of small molecules directly onto the Pt(100) surface will lift the reconstruction, a single graphene layer is observed to act as an effective coating, protecting the reactive surface from O<sub>2</sub> exposure and thus preserving the reconstruction underneath the graphene layer in O<sub>2</sub> pressures as high as 10<sup>–4</sup> mbar. A similar protective effect against CO is observed at CO pressures below 10<sup>–6</sup> mbar. However, at higher pressures CO is observed to intercalate under the graphene coating layer, thus lifting the reconstruction. The limitations of the coating effect are further tested by exposure to hot atomic hydrogen. While the coating can withstand these extreme conditions for a limited amount of time, after substantial exposure, the Pt(100) reconstruction is lifted. Annealing experiments and density functional theory calculations demonstrate that the basal plane of the graphene stays intact and point to a graphene-mediated mechanism for the H-induced lifting of the reconstruction
Self-assembly of ordered graphene nanodot arrays
<p>Raw data associated with the publication "Self-assembly of ordered graphene nanodot arrays" published in Nature Communications under the DOI: 10.1038/s41467-017-00042-4</p>
<p>Abstract: The ability to fabricate nanoscale domains of uniform size in two-dimensional (2D) materials could potentially enable new applications in nanoelectronics and the development of innovative metamaterials. However, achieving even minimal control over the growth of 2D lateral heterostructures at such extreme dimensions has proven exceptionally challenging. Here we show the spontaneous formation of ordered arrays of graphene nano-domains (dots), epitaxially embedded in a 2D boron-carbon-nitrogen alloy. These dots exhibit a strikingly uniform size of 1.6nm ± 0.2nm and strong ordering, and the array periodicity can be tuned by adjusting the growth conditions. We explain this behaviour with a model incorporating dot-boundary energy, a moiré-modulated substrate interaction, and long-range repulsion between dots. This new 2D material, which theory predicts to be an ordered composite of uniform-size semiconducting graphene quantum dots laterally integrated within a larger-bandgap matrix, holds promise for novel electronic and optoelectronic properties, with a variety of potential device applications.</p
Controlling Hydrogenation of Graphene on Ir(111)
Combined fast X-ray photoelectron spectroscopy and density functional theory calculations reveal the presence of two types of hydrogen adsorbate structures at the graphene/Ir(111) interface, namely, graphane-like islands and hydrogen dimer structures. While the former give rise to a periodic pattern, dimers tend to destroy the periodicity. Our data reveal distinctive growth rates and stability of both types of structures, thereby allowing one to obtain well-defined patterns of hydrogen clusters. The ability to control and manipulate the formation and size of hydrogen structures on graphene facilitates tailoring of its properties for a wide range of applications by means of covalent functionalization
Quantum Dots Embedded in Graphene Nanoribbons by Chemical Substitution
Bottom-up
chemical reactions of selected molecular precursors on a gold surface
can produce high quality graphene nanoribbons (GNRs). Here, we report
on the formation of quantum dots embedded in an armchair GNR by substitutional
inclusion of pairs of boron atoms into the GNR backbone. The boron
inclusion is achieved through the addition of a small amount of boron
substituted precursors during the formation of pristine GNRs. In the
pristine region between two boron pairs, the nanoribbons show a discretization
of their valence band into confined modes compatible with a Fabry–Perot
resonator. Transport simulations of the scattering properties of the
boron pairs reveal that they selectively confine the first valence
band of the pristine ribbon while allowing an efficient electron transmission
of the second one. Such band-dependent electron scattering stems from
the symmetry matching between the electronic wave functions of the
states from the pristine nanoribbons and those localized at the boron
pairs
Symmetry-Driven Band Gap Engineering in Hydrogen Functionalized Graphene
Band
gap engineering in hydrogen functionalized graphene is demonstrated
by changing the symmetry of the functionalization structures. Small
differences in hydrogen adsorbate binding energies on graphene on
Ir(111) allow tailoring of highly periodic functionalization structures
favoring one distinct region of the moiré supercell. Scanning
tunneling microscopy and X-ray photoelectron spectroscopy measurements
show that a highly periodic hydrogen functionalized graphene sheet
can thus be prepared by controlling the sample temperature (<i>T</i><sub>s</sub>) during hydrogen functionalization. At deposition
temperatures of <i>T</i><sub>s</sub> = 645 K and above,
hydrogen adsorbs exclusively on the HCP regions of the graphene/Ir(111)
moiré structure. This finding is rationalized in terms of a
slight preference for hydrogen clusters in the HCP regions over the
FCC regions, as found by density functional theory calculations. Angle-resolved
photoemission spectroscopy measurements demonstrate that the preferential
functionalization of just one region of the moiré supercell
results in a band gap opening with very limited associated band broadening.
Thus, hydrogenation at elevated sample temperatures provides a pathway
to efficient band gap engineering in graphene <i>via</i> the selective functionalization of specific regions of the moiré
structure
Exciting H<sub>2</sub> Molecules for Graphene Functionalization
Hydrogen functionalization
of graphene by exposure to vibrationally
excited H<sub>2</sub> molecules is investigated by combined scanning
tunneling microscopy, high-resolution electron energy loss spectroscopy,
X-ray photoelectron spectroscopy measurements, and density functional
theory calculations. The measurements reveal that vibrationally excited
H<sub>2</sub> molecules dissociatively adsorb on graphene on Ir(111)
resulting in nanopatterned hydrogen functionalization structures.
Calculations demonstrate that the presence of the Ir surface below
the graphene lowers the H<sub>2</sub> dissociative adsorption barrier
and allows for the adsorption reaction at energies well below the
dissociation threshold of the H–H bond. The first reacting
H<sub>2</sub> molecule must contain considerable vibrational energy
to overcome the dissociative adsorption barrier. However, this initial
adsorption further activates the surface resulting in reduced barriers
for dissociative adsorption of subsequent H<sub>2</sub> molecules.
This enables functionalization by H<sub>2</sub> molecules with lower
vibrational energy, yielding an avalanche effect for the hydrogenation
reaction. These results provide an example of a catalytically active
graphene-coated surface and additionally set the stage for a re-interpretation
of previous experimental work involving elevated H<sub>2</sub> background
gas pressures in the presence of hot filaments