**2. Synthesis of graphene**

The high spatial resolution imaging and chemical specificity of ToF-SIMS make it a suitable tool to investigate the uniformity of graphene and determine the number of graphene layers. The high spatial resolution map using the C2 − ion as a marker for

*Surface Analysis of Graphene and Graphite DOI: http://dx.doi.org/10.5772/intechopen.108203*

#### **Figure 1.**

*The C2 − ion map was reconstructed by accumulating the signals for (a) 20 s, (b) 100 s, and (c) 300 s of Cs+ sputtering, respectively. (d) A plot of the C2 − ion intensity integrated from the area line scan is shown in (c). (e) ToF-SIMS C2 − ion image of graphene on a Cu substrate. (f) Optical image and (g) corresponding ToF-SIMS C2 − ion image of graphene on a SiO2/Si substrate [15].*

the hexagonal graphene domains provides information about the uniformity of its domains [15]. In addition, ToF-SIMS analysis of multilayer graphene was achieved by cycles of high lateral resolution imaging followed by slow Cs+ ion sputtering to remove the graphene materials [15]. As shown in **Figure 1**, with 20 s of sputtering time, a uniform graphene layer can be observed, while it takes approximately 100 s of sputtering for the removal of the first layer of graphene. Prolonging the sputtering for a longer time exposes all the buried adlayers underneath the top layer, and thus a total of three layers of graphene can be observed after accumulating C2 − signals for 300 s of sputtering. The intensity of the C2 − ion shows a stepwise increase with the number of graphene layers. Particularly, the C2 − ion intensity at each step shows a linearly proportional increase to the number of graphene layers, and up to six layers of graphene can be distinguished. These results are validated with analogous optical images showing six layers of graphene. The ToF-SIMS chemical image shows its capability of identifying the layer number of graphene on both Si/SiO2 and Cu substrates.

CVD has been commonly used to grow large-area graphene sheets on various metal substrates [16]. Cu foil is often chosen as a substrate and also a catalyst for monolayer graphene growth [17]. The weak interaction between the graphene and Cu foil allows the graphene films to expand over the grain boundaries with minimal structural disruption, resulting in electron transfer from the Cu foil to the graphene [16, 18]. ToF-SIMS is a suitable technique to determine the chemical structures created from this interaction. It was found that peaks related to the interaction between the graphene and Cu foil substrate, such as C2Cu− and C4Cu− ions, are present in the ToF-SIMS negative ion spectrum obtained at 450°C [19]. The ToF-SIMS images further verify that the C2Cu− ion shows a distribution pattern similar to that of the C2 − ion, which is a characteristic ion for graphene (**Figure 2**). Two areas (60 μm × 60 μm) from the ToF-SIMS image of the C2 − ion corresponded to areas with high graphene and Cu concentrations were then picked for analysis. Both the graphene-related peaks, i.e., the Cx − and CxH− (where x=1, 2, 3…) ion series, and

#### **Figure 2.**

*ToF-SIMS images of different ions of graphene on a Cu foil substrate: (a) C2 − , (b) C4 − , (c) Cu− , (d) C2Cu− , (e) CuO<sup>−</sup> and (f) Cu3 − ions [19].*

peaks related to the interaction between the graphene and Cu foil substrate (CxCu− ion series) have higher intensities in the areas with a higher graphene concentration, confirming the existence of the graphene-Cu interaction at the interface of the graphene and Cu foil substrate.

Moreover, the oxidation protection of graphene for the Cu foil substrate was found when comparing the ToF-SIMS negative ion spectra obtained from the areas with and without graphene coverage [19]. The intensities of the peaks related to the oxidation of the Cu foil substrate, such as the CuO<sup>−</sup> and Cu2O− ions, are significantly higher in the spectrum obtained from the area without graphene coverage. On the contrary, the ion intensity ratios of Cu cluster ions to the Cu<sup>−</sup> ion, such as the Cu3 − /Cu− and Cu4 − / Cu− , are higher in the area with a higher graphene concentration. The Cu3 − ion shows a distribution pattern similar to that of the C2 − ion but complementary to that of the CuO<sup>−</sup> ion (**Figure 2**). As the metal cluster ions are usually generated during the sputtering process by direct ejection from a metal surface, it can be anticipated that more Cu cluster ions are formed from pure metal than from its oxides. Therefore, the higher ion intensity ratios of Cu cluster ions to the Cu− ion in the areas with a high graphene concentration also confirm that less oxidation occurred in the graphene-covered Cu foil substrate than in the uncovered areas. The above results confirm that the growth of graphene on a Cu foil substrate can prevent oxidation of the Cu foil substrate during storage and annealing processes. The graphene on the Cu foil substrate can inhibit the diffusion of small molecules, such as O2 and H2O, into the interface between the graphene and Cu foil substrate, thus preventing the oxidation of the copper.
