104 research outputs found
Confined step-flow growth of Cu intercalated between graphene and a Ru(0001) surface
By comparing the growth of Cu thin films on bare and graphene-covered
Ru(0001) surfaces, we demonstrate the role of graphene as a surfactant allowing
the formation of flat Cu films. Low-energy electron microscopy, X-ray
photoemission electron microscopy and X-ray absorption spectroscopy reveal that
depositing Cu at 580 K leads to distinct behaviors on both types of surfaces.
On bare Ru, a Stranski-Krastanov growth is observed, with first the formation
of an atomically flat and monolayer-thick wetting layer, followed by the
nucleation of three-dimensional islands. In sharp contrast, when Cu is
deposited on a graphene-covered Ru surface under the very same conditions, Cu
intercalates below graphene and grows in a step-flow manner: atomically-high
growth fronts of intercalated Cu form at the graphene edges, and extend towards
the center of the flakes. Our findings suggest potential routes in metal
heteroepitaxy for the control of thin film morphology.Comment: 9 pages, 4 figure
Twins and their boundaries during homoepitaxy on Ir(111)
The growth and annealing behavior of strongly twinned homoepitaxial films on
Ir(111) has been investigated by scanning tunneling microscopy, low energy
electron diffraction and surface X-ray diffraction. In situ surface X-ray
diffraction during and after film growth turned out to be an efficient tool for
the determination of twin fractions in multilayer films and to uncover the
nature of side twin boundaries. The annealing of the twin structures is shown
to take place in a two step process, reducing first the length of the
boundaries between differently stacked areas and only then the twins
themselves. A model for the structure of the side twin boundaries is proposed
which is consistent with both the scanning tunneling microscopy and surface
X-ray diffraction data.Comment: 13 pages, 11 figure
Strains Induced by Point Defects in Graphene on a Metal
Strains strongly affect the properties of low-dimensional materials, such as
graphene. By combining in situ, in operando, reflection high energy electron
diffraction experiments with first-principles calculations, we show that large
strains, above 2%, are present in graphene during its growth by chemical vapor
deposition on Ir(111) and when it is subjected to oxygen etching and ion
bombardment. Our results unravel the microscopic relationship between point
defects and strains in epitaxial graphene and suggest new avenues for graphene
nanostructuring and engineering its properties through introduction of defects
and intercalation of atoms and molecules between graphene and its metal
substrate
Intercalating cobalt between graphene and iridium (111): a spatially-dependent kinetics from the edges
Using low-energy electron microscopy, we image in real time the intercalation
of a cobalt monolayer between graphene and the (111) surface of iridium. Our
measurements reveal that the edges of a graphene flake represent an energy
barrier to intercalation. Based on a simple description of the growth kinetics,
we estimate this energy barrier and find small, but substantial, local
variations. These local variations suggest a possible influence of the graphene
orientation with respect to its substrate and of the graphene edge termination
on the energy value of the barrier height. Besides, our measurements show that
intercalated cobalt is energetically more favorable than cobalt on bare
iridium, indicating a surfactant role of graphene
Modulating charge density and inelastic optical response in graphene by atmospheric pressure localized intercalation through wrinkles
The intercalation of an oxide barrier between graphene and its metallic
substrate for chem- ical vapor deposition is a contamination-free alternative
to the transfer of graphene to dielectric supports, usually needed for the
realization of electronic devices. Low-cost pro- cesses, especially at
atmospheric pressure, are desirable but whether they are achievable remains an
open question. Combining complementary microscopic analysis, providing
structural, electronic, vibrational, and chemical information, we demonstrate
the spontaneous reactive intercalation of 1.5 nm-thick oxide ribbons between
graphene and an iridium substrate, at atmospheric pressure and room
temperature. We discover that oxygen-containing molecules needed for forming
the ribbons are supplied through the graphene wrinkles, which act as tunnels
for the efficient diffusion of molecules entering their free end. The
intercalated oxide ribbons are found to modify the graphene-support
interaction, leading to the formation of quasi-free-standing high quality
graphene whose charge density is modulated in few 10-100 nm-wide ribbons by a
few 10^12 cm-2, where the inelastic optical response is changed, due to a
softening of vibrational modes - red-shifts of Raman G and 2D bands by 6 and 10
cm-1, respectively.Comment: Carbon (2013) available onlin
Local deformations and incommensurability of high quality epitaxial graphene on a weakly interacting transition metal
We investigate the fine structure of graphene on iridium, which is a model
for graphene weakly interacting with a transition metal substrate. Even the
highest quality epitaxial graphene displays tiny imperfections, i.e. small
biaxial strains, ca. 0.3%, rotations, ca. 0.5^{\circ}, and shears over
distances of ca. 100 nm, and is found incommensurate, as revealed by X-ray
diffraction and scanning tunneling microscopy. These structural variations are
mostly induced by the increase of the lattice parameter mismatch when cooling
down the sample from the graphene preparation temperature to the measurement
temperature. Although graphene weakly interacts with iridium, its thermal
expansion is found positive, contrary to free-standing graphene. The structure
of graphene and its variations are very sensitive to the preparation
conditions. All these effects are consistent with initial growth and subsequent
pining of graphene at steps
Foreword: Recent advances in 2D materials Physics
International audienc
Universal classification of twisted, strained and sheared graphene moir\'e superlattices
Moir\'e superlattices in graphene supported on various substrates have opened
a new avenue to engineer graphene's electronic properties. Yet, the exact
crystallographic structure on which their band structure depends remains highly
debated. In this scanning tunneling microscopy and density functional theory
study, we have analysed graphene samples grown on multilayer graphene prepared
onto SiC and on the close-packed surfaces of Re and Ir with ultra-high
precision. We resolve small-angle twists and shears in graphene, and identify
large unit cells comprising more than 1,000 carbon atoms and exhibiting
non-trivial nanopatterns for moir\'e superlattices, which are commensurate to
the graphene lattice. Finally, a general formalism applicable to any hexagonal
moir\'e is presented to classify all reported structures.Comment: 14 pages, 6 figure
Magnetism of cobalt nanoclusters on graphene on iridium
The structure and magnetic properties of Co clusters, comprising from 26 to
2700 atoms, self-organized or not on the graphene/Ir(111) moir\'e, were studied
in situ with the help of scanning tunneling microscopy and X-ray magnetic
circular dichroism. Surprisingly the small clusters have almost no magnetic
anisotropy. We find indication for a magnetic coupling between the clusters.
Experiments have to be performed carefully so as to avoid cluster damage by the
soft X-rays
Epitaxial graphene prepared by chemical vapor deposition on single crystal thin iridium films on sapphire
Uniform single layer graphene was grown on single-crystal Ir films a few
nanometers thick which were prepared by pulsed laser deposition on sapphire
wafers. These graphene layers have a single crystallographic orientation and a
very low density of defects, as shown by diffraction, scanning tunnelling
microscopy, and Raman spectroscopy. Their structural quality is as high as that
of graphene produced on Ir bulk single crystals, i.e. much higher than on metal
thin films used so far.Comment: To appear in Appl. Phys. Let
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