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]