120 research outputs found
Real-space study of the growth of magnesium on ruthenium
The growth of magnesium on ruthenium has been studied by low-energy electron
microscopy (LEEM) and scanning tunneling microscopy (STM). In LEEM, a
layer-by-layer growth is observed except in the first monolayer, where the
completion of the first layer in inferred by a clear peak in electron
reflectivity. Desorption from the films is readily observable at 400 K.
Real-space STM and low-energy electron diffraction confirm that sub-monolayer
coverage presents a moir\'e pattern with a 1.2 nm periodicity, which evolves
with further Mg deposition by compressing the Mg layer to a 2.2 nm periodicity.
Layer-by-layer growth is followed in LEEM up to 10 ML. On films several ML
thick a substantial density of stacking faults are observed by dark-field
imaging on large terraces of the substrate, while screw dislocations appear in
the stepped areas. The latter are suggested to result from the mismatch in
heights of the Mg and Ru steps. Quantum size effect oscillations in the
reflected LEEM intensity are observed as a function of thickness, indicating an
abrupt Mg/Ru interface.Comment: 21 pages, 10 figure
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Dynamics of metal/ceramic interface formation.
We summarize the work of the Laboratory Directed Research and Development (LDRD) project 'Dynamics of Metal/Ceramic Interface Formation.' Low-energy electron microscopy (LEEM) was used to monitor in real time how the metal/ceramic interface between the alloy NiAl and its oxide formed. The interfaces were synthesized by exposing the clean alloy to oxygen at either low or high temperature. During low-temperature exposure, an initially amorphous oxide formed. With annealing, this oxide crystallizes into one type of alumina that has two orientational domains. While the oxide is relatively uniform, it contained pinholes, which coarsened with annealing. In marked contrast, high-temperature exposure directly produced rod-shaped islands of crystalline oxide. These rods were all aligned along the substrate's [001] direction and could be many microns in length. Real-time observations showed that the rods can both grow and shrink by addition and subtraction, respectively, at their ends
Origin of the Mosaicity in Graphene Grown on Cu(111)
We use low-energy electron microscopy to investigate how graphene grows on
Cu(111). Graphene islands first nucleate at substrate defects such as step
bunches and impurities. A considerable fraction of these islands can be
rotationally misaligned with the substrate, generating grain boundaries upon
interisland impingement. New rotational boundaries are also generated as
graphene grows across substrate step bunches. Thus, rougher substrates lead to
higher degrees of mosaicity than do flatter substrates. Increasing the growth
temperature improves crystallographic alignment. We demonstrate that graphene
growth on Cu(111) is surface diffusion limited by comparing simulations of the
time evolution of island shapes with experiments. Islands are dendritic with
distinct lobes, but unlike the polycrystalline, four-lobed islands observed on
(100)-textured Cu foils, each island can be a single crystal. Thus, epitaxial
graphene on smooth, clean Cu(111) has fewer structural defects than it does on
Cu(100).Comment: Article revised following reviewer comment
In-plane orientation effects on the electronic structure, stability and Raman scattering of monolayer graphene on Ir(111)
We employ angle-resolved photoemission spectroscopy (ARPES) to investigate
the electronic structures of two rotational variants of epitaxial, single-layer
graphene on Ir(111). As grown, the more-abundant R0 variant is nearly
charge-neutral, with strong hybridization between graphene and Ir bands near
the Fermi level. The graphene Fermi surface and its replicas exactly coincide
with Van Hove singularities in the Ir Fermi surface. Sublattice symmetry
breaking introduces a small gap-inducing potential at the Dirac crossing, which
is revealed by n-doping the graphene using K atoms. The energy gaps between
main and replica bands (originating from the moir\'e interference pattern
between graphene and Ir lattices) is shown to be non-uniform along the mini-
zone boundary due to hybridization with Ir bands. An electronically mediated
interaction is proposed to account for the stability of the R0 variant. The
variant rotated 30{\deg} in-plane, R30, is p-doped as grown and K doping
reveals no band gap at the Dirac crossing. No replica bands are found in ARPES
measurements. Raman spectra from the R30 variant exhibit the characteristic
phonon modes of graphene, while R0 spectra are featureless. These results show
that the film/substrate interaction changes from chemisorption (R0) to
physisorption (R30) with in-plane orientation. Finally, graphene-covered Ir has
a work function lower than the clean substrate but higher than graphite.Comment: Manuscript plus 7 figure
Three-Fold Diffraction Symmetry in Epitaxial Graphene and the SiC Substrate
The crystallographic symmetries and spatial distribution of stacking domains
in graphene films on SiC have been studied by low energy electron diffraction
(LEED) and dark field imaging in a low energy electron microscope (LEEM). We
find that the graphene diffraction spots from 2 and 3 atomic layers of graphene
have 3-fold symmetry consistent with AB (Bernal) stacking of the layers. On the
contrary, graphene diffraction spots from the buffer layer and monolayer
graphene have apparent 6-fold symmetry, although the 3-fold nature of the
satellite spots indicates a more complex periodicity in the graphene sheets.Comment: An addendum has been added for the arXiv version only, including one
figure with five panels. Published paper can be found at
http://link.aps.org/doi/10.1103/PhysRevB.80.24140
Real-time observation of epitaxial graphene domain reorientation.
Graphene films grown by vapour deposition tend to be polycrystalline due to the nucleation and growth of islands with different in-plane orientations. Here, using low-energy electron microscopy, we find that micron-sized graphene islands on Ir(111) rotate to a preferred orientation during thermal annealing. We observe three alignment mechanisms: the simultaneous growth of aligned domains and dissolution of rotated domains, that is, 'ripening'; domain boundary motion within islands; and continuous lattice rotation of entire domains. By measuring the relative growth velocity of domains during ripening, we estimate that the driving force for alignment is on the order of 0.1 meV per C atom and increases with rotation angle. A simple model of the orientation-dependent energy associated with the moiré corrugation of the graphene sheet due to local variations in the graphene-substrate interaction reproduces the results. This work suggests new strategies for improving the van der Waals epitaxy of 2D materials
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