497 research outputs found
Mechanical properties of polycrystalline graphene based on a realistic atomistic model
Graphene can at present be grown at large quantities only by the chemical
vapor deposition method, which produces polycrystalline samples. Here, we
describe a method for constructing realistic polycrystalline graphene samples
for atomistic simulations, and apply it for studying their mechanical
properties. We show that cracks initiate at points where grain boundaries meet
and then propagate through grains predominantly in zigzag or armchair
directions, in agreement with recent experimental work. Contrary to earlier
theoretical predictions, we observe normally distributed intrinsic strength (~
50% of that of the mono-crystalline graphene) and failure strain which do not
depend on the misorientation angles between the grains. Extrapolating for grain
sizes above 15 nm results in a failure strain of ~ 0.09 and a Young's modulus
of ~ 600 GPa. The decreased strength can be adequately explained with a
conventional continuum model when the grain boundary meeting points are
identified as Griffith cracks.Comment: Accepted for Physical Review B; 5 pages, 4 figure
From Point Defects in Graphene to Two-Dimensional Amorphous Carbon
While crystalline two-dimensional materials have become an experimental
reality during the past few years, an amorphous 2-D material has not been
reported before. Here, using electron irradiation we create an sp2-hybridized
one-atom-thick flat carbon membrane with a random arrangement of polygons,
including four-membered carbon rings. We show how the transformation occurs
step-by-step by nucleation and growth of low-energy multi-vacancy structures
constructed of rotated hexagons and other polygons. Our observations, along
with first-principles calculations, provide new insights to the bonding
behavior of carbon and dynamics of defects in graphene. The created domains
possess a band gap, which may open new possibilities for engineering
graphene-based electronic devices.Comment: 10 pages, 10 figures including supplementary informatio
Substitutional Si impurities in monolayer hexagonal boron nitride
We report the first observation of substitutional silicon atoms in
single-layer hexagonal boron nitride (h-BN) using aberration corrected scanning
transmission electron microscopy (STEM). The medium angle annular dark field
(MAADF) images reveal silicon atoms exclusively filling boron vacancies. This
structure is stable enough under electron beam for repeated imaging. Density
functional theory (DFT) is used to study the energetics, structure and
properties of the experimentally observed structure. The formation energies of
all possible charge states of the different silicon substitutions
(Si, Si and Si) are calculated. The
results reveal Si as the most stable substitutional
configuration. In this case, silicon atom elevates by 0.66{\AA} out of the
lattice with unoccupied defect levels in the electronic band gap above the
Fermi level. The formation energy shows a slightly exothermic process. Our
results unequivocally show that heteroatoms can be incorporated into the h-BN
lattice opening way for applications ranging from single-atom catalysis to
atomically precise magnetic structures
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