Page 5 - Outer Divertor Damage Characterization from Deuterium Plasma Bombardment in Graphene-Coated Tungsten

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NAVARRO et al. · OUTER DIVERTOR DAMAGE CHARACTERIZATION FROM DEUTERIUM PLASMA BOMBARDMENT
for
There are three distinct features of the Raman spectrum
graphene: the D-band, G-band, and 2D-band. The
a deuterium plasma and single-layer graphene on a tungsten surface, highlighting the main effects of these
Fig. 4. Single Raman spectra of the graphene film on the coated sample W6 before and after deuterium exposure in C-2W. As single-vacancy defects are created in the film, the defect band intensity ID at ~1350 cm−1 increases. There is no spectrum for W2 because the features shown here are only present in graphene films.
D-band is known as the disorder band; it involves the
scattering of an in-plane transverse optical-mode phonon
by a crystal defect. As defects are introduced into the
honeycomb graphene lattice, the intensity of the band
increases. The G-band is the only band coming from
a normal first-order Raman scattering process in graphene;
it is one of the two distinct features in defect-free graphene.
The ratio of the intensities in the D-band and the G-band is
used to determine crystal disorder and failure in these
membranes. Last, the 2D-band is a second-order Raman
scattering process but is permissible and always present in
graphene. The ratio of the 2D-band to the G-band
intensities is commonly used as a way to estimate the
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Fig. 5. Qualitative description of a deuterium plasma exposure of graphene-coated PCW. The green particles correspond to deuterium, magenta is helium, blue is tungsten, and black is carbon.
number of graphene layers present in a sample.
illustrates our interpretation of the interaction between
Figure 5
exposures. Low-energy ionized particles have a
Coulomb cross section for scattering with tungsten
and slow down in the lattice due to nuclear stopping.
ions can also easily interact with the carbon atoms
graphene membrane, transferring enough energy to
single-vacancy defects. In addition to creating
large atoms These in the create
these
defects, deuterium (green) also builds up in the subsurface
layer and can coalesce into bubbles, move to interstitial
sites, and become trapped in energy sinks. The deuterium
has the added effect of adsorbing to the surface layer,
introducing amorphization of the membrane and
transforming the hybridization states of the graphene
24,28,29
and as we start introducing defects (ID is increasing), the
electrons.
honeycomb bonds break, and graphene starts to create new
Graphene has a honeycomb structure,
FUSION SCIENCE AND TECHNOLOGY · VOLUME 75 · AUGUST 2019