Figure 3. High resolution transmission electron micrographs of the
graphene that solves the problems regarding insufficient exfoliation of
the graphite powder and deterioration in the electrical conductivity and
thermal conductivity.
The Raman spectra are illustrated in Figure 4 for the graphene that
solves the problems regarding insufficient exfoliation of the graphite
powder and deterioration in the electrical conductivity and thermal
conductivity. These laser-induced graphene features may be characterized
by the presence of a distinct 2D-band in the Raman spectra. This can be
fit with a single Lorentzian and is very similar to that observed for
single layer exfoliated graphene and epitaxial graphene. The 2D mode
results from a two-phonon resonant scattering process and is normally
considered an overtone of the defect mediated D-band. The 2D feature is
always observed with widths defined by the thickness and stacking order
along the c-axis. For single layer graphene, a single Lorenztian line
shape centered at 2690 per centimeter can indicate electronic structure
that is dominated by Dirac-Weyl dispersion. It is therefore accepted as
one of better optical signatures regarding the presence of graphene. It
is different than that from highly oriented pyrolytic graphite.
Specifically, the G-band is much broader and blue shifted, there is a
strong D-band at 1345 per centimeter, and the 2D-band is nearly absent.
These changes are correlated with structural changes and defects
associated with the hydroxyl and epoxy groups in the basal plane and a
variety of alkyl and oxygen-containing functional groups terminating the
edges. The 2D-band is observed and there is a narrowing of the G-band as
well as diminution of the D-band. When using nitrogen background gas,
the 2D-band is further enhanced, the G-band is much narrower and the
D-band is nearly removed. Formation of the 2D-band and G-band narrowing
also occurs. However, the D-band feature, though reduced relative to
untreated graphene, remains independent of the laser flux. This
indicates an intrinsic difference in the defect density and material
quality relative to that formed using continuous-wave excitation. When
using 355 nm photons, the 2D-band is produced at the lowest powers and
the integrated D and 2D peak intensity ratio is about 0.8-2.0. Though
sample surface non-uniformity causes variability, this ratio remains
close to 0.8 even at the highest power. On the contrary, the integrated
D and 2D peak intensity ratio is usually dependent on the power when
using 532 nm photons and discernable 2D-band features are not evident.
The 355 nm light excites single-photon mediated valence-to-conduction
band transitions. This produces electron-hole plasmas in the material at
all pulse energies studied. Since at least two 532 nm photons are
required to exceed the band-gap, a coherent multiphoton or incoherent
multiple photon process may be required to create a similar
electron-hole plasma. This is consistent with the 532 nm 2D-band
formation threshold of 2.8 megawatts per square centimeter which is
below the pre-ablation threshold of graphite.