Multilayer graphene (MLG) is an excellent anisotropic material whose electronic and mechanical properties are strongly dependent on the stacking order of individual graphene monolayers. However, the rhombohedral stacking order has been reported to occur in natural graphite (up to 30%). Throughout our research, we used X-ray diffraction (XRD) characterization to detect and quantify the contents of the rhombohedral phase in natural graphite.
Introduction
It was also reported that the content of the rhombohedral modification can be increased by using a mechanical milling (grinding) technique5. In that case, shear forces generated by high-energy milling can cause slippage of weakly interacting graphene sheets, resulting in the formation of less energetically stable depositional configurations (including ABC… stacking). Nevertheless, the ball milling process can significantly reduce the size of the crystallites and thus induce amorphization of graphite samples.
To obtain ABC-stacked MLG, we applied the “top-down” approach, which means exfoliating natural graphite with a relatively high content (~30%) of the rhombohedral phase. The most common and simple method for exfoliating graphite is the so-called “Scotch tape” method.6 The modified “Scotch tape” method to produce large surface area graphene flakes has recently been introduced. In our study, we used this modified mechanical exfoliation method to produce relatively large MLG flakes of ABA.
The rhombohedral phase content in natural graphite was evaluated using X-ray diffraction (XRD) analysis.
Literature Review
- Hexagonal and Rhombohedral Structure of Graphite
- Conversion to Rhombohedral Phase of Graphite
- Mechanical Exfoliation of Graphite
- Raman Spectroscopy of Graphene
- Conversion of multilayer graphene to Diamane …
But in nature, because the rhombohedral phase has the maximum content 30%, actual XRD patterns are shown as Fig 2.5 which are experimental XRD patterns of four different commercial graphite. Bracket value is the rhombohedral phase content The solid line is calculation data and dotted line is experimental data which are exactly the same as each other. Basically, planetary and vibrating mill systems have totally different energy delivery system to sample, so even though Fig 2.7-b) and c) have the same grinding atmosphere, two XRD patterns showed different peaks depending on their damaged structures.
Graphite exfoliation is commonly used in making graphene as a top-down approach because it is an easier method to obtain graphene than chemical synthesis of graphene. The thicker graphene showed brighter color than the thinner one. (Reproduced with permission from reference 6. Copyright 2004 Science). The main steps for the modified method were oxygen plasma and heat treatment to remove adsorbates on the SiO2 substrate and control the pressure inside the graphene by applying the heating process.
Graphene has characteristic peaks in the specific Raman shift and these peaks are deeply related to its properties. 2.10 showed the normal Raman spectrum of graphene.12. There are some reports that the position and shape of the G peak can be changed by applying an electronic or mechanical force such as uniaxial strain on graphene.13-14 The D peak is shown at 1350 cm-1, which is related to the defect in graphene and functionalizers that are connected to the carbon atom of graphene. Of course, due to the dangling bond at the edge of the graphene, the peak D also appears at the edge of the graphene.
In the case of multilayer graphene, 2D peak is good characteristic peak that can estimate the number of graphene layers.15 Fig. If the increase of the number of graphene layers, the 2D peak shows the lower intensity than G peak. When the number of graphene layers is closed for graphite, the 2D peak is split into 2 peaks.
Then, when it comes to the layer up to 5 layers, Hyeonsik Cheong et al.17 organized the comparison of 2D peak depending on the number of graphene layers and the stacking order. They also found 2 minor peaks which can distinguish the stacking order of graphene as G* peak (near 2450 cm-1), M peak (near 1700 cm-1), but 2D shape is seen more clearly than other minor peaks. 2.13 is 2D peak data for few-layer graphene with two different stacking orders under five different excitation energies.
Experimental Section
- Mechanically Exfoliation of Natural Graphite
- Raman Analysis
- XRD Analysis
- AFM Analysis
- Fluorination of Multilayer Graphene
- XPS Analysis
To distinguish the layer stacking order of the multilayer graphene, Raman spectroscopy was used with a Wi-Tec micro-Raman instrument using 488nm(2.54 eV), 532 nm(2.33 eV) laser excitation with a spot size of 250 nm. For the Raman mapping, I measured single spectra of the desired multilayer graphene for 60 x 60 spots with 1 s integration time, but the total measured time was dependent on the multilayer sample size. Rigaku SmartLab powder X-ray diffractometer was used in XRD analysis by using Cu-Kα source (λ = 0.154 nm) through Cu-Kβ filtering.
The bragg angle range was fixed at 30 to 60 degrees because there are two characteristic rhombohedral peaks compared to two characteristic hexagonal peaks. Graphite rhombohedral and hexagonal phase reference data were obtained from the International Center for Diffraction Data (ICDD). The scale bar is 10 um, and the red, blue, black arrows indicate the multilayer graphene, which has 2~3 layers, over 10 layers, respectively graphite flakes.
Bruker Dimension Icon atomic force microscope was used for AFM analysis to check the number of multilayer graphene. The number of multilayer graphene was calculated by the information of one layer of graphene which is 0.34nm, which is the calculation data for the interlayer distance of graphite. The fluorine source was a XeF2 source which has a vapor pressure of ~3.8 Torr at ambient atmosphere.
In order to improve the fluorination for the sample, the chamber was kept at 60 degrees Celsius by heating elements.
Results and Discussion
Characterization
- XRD Analysis
- AFM Analysis
- Raman Spectroscopy
The average grain size of rhombohedral stacked graphite is about 30.6 nm, but as the meal duration increases, FWHM of rhombohedral (101) is also increased, so the grain size is reduced. In the case of mechanical exfoliation of natural graphite, the number of graphene layers is completely random, but we can estimate the number of layers by the contrast of graphene in optical microscopy. Based on the optical microscope image, we can measure the exact thickness by AFM analysis.
The marked value in image is the calculated value by the one-layer thickness is about 0.34 nm. Because the mechanical exfoliation using adhesive tape cannot clearly detach the graphite layer, so some spots showed that the graphene layer is folded or twisted. There is no relationship between the stacking sequence and AFM measurement image or optical microscopic image.
We can distinguish between ABA-stacked graphene and ABC-stacked graphene in the following reference paper. 17 ABC-stacked graphene showed the highest peak in the 2D peak region (~2700 cm-1) than graphene stacked with ABA. The color of the spectrum and the cross sign in the optical microscopic image means that each spectrum corresponds to each position. To visualize the stacking order, we calculated the ratio between the intensity of 2D1 and 2D2 across the entire graphene region.
We can see three situations for their stacking order through this calculation: Dominant ABC, ABC + ABA and dominant ABA- stacked within the same graphene region (Fig. 4.4). The brighter color in mapping data is locked to ABC stacked sequence and the darker color is locked to ABA stacked. Completely black area means the graphite because the graphite showed 2D splitting with dramatically lower 2D1 peak than 2D2.. calculate the "ratio" in the whole multilayer graphene region, the outside of the graphene region also showed the "color spot", but it should be noted that the outer side of graphene and edge side of graphene should be ignored while checking the stacking order of graphene.
The Raman mapping scale is in the same range from 0.9 to 1.25 a.u. a) predominantly ABC folded b) there are both ABA and ABC folded c) predominantly ABA folded.
Fluorination of Multilayer Graphene
When we fluorinated the multilayer graphene for the first time as exposure to the XeF2 source, the graphene showed the functionalized graphene peak that due to fluorine, the intensity of D-peak was increased and lower G-peak and 2D-peak. But inside the center of the multilayer graphene, they showed relatively larger G peak than the edge of the multilayer graphene, and at the same time, D' peak appears, which is related to D peak and defect in. 1620 cm-1. (Fig. 4.8 ) It means that the center of the multilayer graphene was not fully functionalized, and the color was also "graphene" color in optical microscopy, but it was a little blurred.
After the second exposure that applied the sample under harsher conditions than the first exposure, the multilayer graphene showed total optical transparency like monolayer fluorographene. But in the Raman spectrum (Fig. 4.9), there is no big difference between graphene assembled on ABA (blue signal) and graphene assembled with ABC (red and green signal). We speculate two possibilities of this functionalized multilayer graphene that one is the multilayer fluorographene which is fluorinated layer by layer and the other is locked in the diamond structure.
The red and green site is ABC-stacked graphene, the blue site is ABA-stacked graphene which was checked by Raman spectra before fluorination. (Figure 4.6).
Conclusions
Takahashi, T., Selective fabrication of ABA and ABC free-standing trilayer graphene with/without Dirac-zone energy bands. N.; Zheng, J.; Yang, T.; Englund, D.; Gao, H.-J.; Sutter, P., Reliable exfoliation of large-area high-quality graphene flakes and other two-dimensional materials. U.; Yoon, D.; Cheong, H., Energy-dependent Raman signatures of ABA- and ABC-stacked few-layer graphene.
S.; Park, N., Transformation of multilayer graphene into continuous ultrathin sp(3) carbon films on metal surfaces.
