2.3 Characterization of monolayer and bilayer graphene synthesized
by C-ion implantation
We have carefully examined the quality of the graphene layers formed on
these arbitrary substrates using this novel synthesis approach.Figure 4a schematically illustrates that removing the Cu-Ni
alloy by a simple thermal release tape leads to graphene directly
synthesized on arbitrary substrates. Figure 4b displays Raman spectra of
the as-synthesized monolayer graphene on Si, SiO2,
glass, and sapphire substrates, respectively, indicating the excellent
quality of monolayer graphene on these diverse substrates where three
distinctive bands of graphene: D-band, G-band, and 2D-band can be
recognized.[38-39] Figure 4c shows the
Raman spectra of monolayer and bilayer graphene structures on the
SiO2 substrate synthesized by this approach. For a
fluence of 4×1015 atoms/cm2, the
I2D/IG ratio is larger than 1.8,
indicating that monolayer graphene is
produced.[7,12,31] When the fluence of
8×1015 atoms/cm2 was used, the
corresponding I2D/IG ratio became
approximately 0.5, indicative of a bilayer graphene
structure.[7,23,35] The layer number of the
as-synthesized graphene films is separately confirmed viaUV-visible spectrophotometry considering the absorbance of the single
graphene layer is 2.3%. The comparison of absorbance shows that the
absorbance of the bilayer graphene is twice that of the monolayer
graphene (Supplementary Figure S4 ). The enlarged images in the
inset of Figures 4d and 4g reveal a honeycomb lattice
construction and a three-fold symmetry for the monolayer graphene and
the AB-stacked bilayer graphene.[40-41] These two
scanning tunneling microscopy (STM) images exhibit no visible lattice
defect, indicating that both the monolayer and bilayer graphene samples
have a high crystalline quality.[26] The selected
area electron diffraction (SAED) pattern (Figure 4e ) displays
one set of the hexagonal diffraction pattern and the band intensity
ratio (outermost to innermost) of ~1:2 (the inset ofFigure 4e ) implies the monolayer graphene is
formed.[42] The hexagonal diffraction pattern is
also recognized when the implantation fluence is
8×1015 atoms/cm2, although the band
intensity ratio increases to ~2:1 (the inset ofFigure 4h ), meaning the formation of an AB-stacked bilayer
graphene.[43-44] High-resolution transmission
electron microscopy (HRTEM) (the right side of Figure 4e andFigure 4h ) acquired from the crack-edges of the as-synthesized
films confirmed the characteristics of monolayer and bilayer graphene,
respectively. The crystallographic feature and domain size of the
as-synthesized graphene films were further examined by extensive SAED
patterns (Supplementary Figures S5-S6 ). The thickness
uniformity of the as-synthesized graphene is confirmed through Raman
intensity mapping, over a 30 μm × 30 μm. Figure 4f shows that
>90% of the film area has
I2D/IG ratios larger than 1.8,
indicating the film is a uniform monolayer graphene. Figure 4ishows that the film has no monolayer Raman signal
(I2D/IG>1.3) seen at any
pixel on the Raman mapping while ~90% of the film has
an I2D/IG ratio of 0.5, suggesting the
formation of uniform bilayer graphene.[45]