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Chapter 5 β Production of Few Layer Graphene in Organic Solvents of analysed flakes) as shown in Figure 5.4, which are assumed to be monolayer graphene. The increase in the thickness might be due to combination of scanning parameters and presence of residual NMP between the graphene and substrate and moisture content. With the knowledge of monolayer height and step height (~0.5 nm), the AFM height data can be converted into number of layers per flake (π). By analysing the AFM images, statistical data on the exfoliation state could be use derived. Statistical data was derived from at least 200 flakes for each condition. It should be noted that, despite pre-heating the mica substrates, the aggregation of flakes was unavoidable (Figure 5.3d and Figure 5.4a). However, through careful analysis of the AFM height image and corresponding amplitude image, the length can be measured (Figure 5.3 and Figure 5.4) reliably. Figure 5.5 shows the histogram of length of graphene flakes (πΏπ΄πΉπ) observed at different sonication times and centrifugation speeds used. In general, as the sonication time increased from 24 hr to 60 hr the πΏπ΄πΉπ decreased. The decrease is clearer at higher centrifugation speeds; At 10000 rpm, πΏπ΄πΉπ decreases from 150 Β± 80 nm for 24 hr to 80 Β± 40 nm for 60 hr. Lower centrifugation speeds retain larger flakes (> 500 nm) as seen in the histogram. Whereas, higher centrifugation speed removes the larger flakes and retains only the smaller flakes leading to relatively small length distribution. For e.g., at 24 hr, πΏπ΄πΉπ decreases from 290 Β± 170 at 2000 rpm to 150 Β± 80 nm at 10000 rpm, which is ~50 % reduction in mean size. This behaviour was generally observed for graphene and other 2d materials.60, 62, 352 As explained earlier, the AFM height data was converted to the number of layers, π, by determining the apparent monolayer height (~1 nm) and step height (~0.5 nm). Histogram data of π as a function of different sonication time and centrifugation speed is shown in Figure 5.6. 138PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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