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Chapter 5 – Production of Few Layer Graphene in Organic Solvents Figure 5.11: a) 2D band position, b) FWHM(2D), c) G band position, d) FWHM(G), e) D band position, f) FWHM(D) as a function of sonication time with different centrifugation speeds. Highlighted regions show effect of centrifugation speeds at one sonication time (60 hr). The shape of the 2D band can be used to estimate the number of layers in the sample 48, 240. However, it remains challenging to establish the precise number of layers for LPE samples which have a broad and symmetric 2D band as observed in Figure 5.10. This broad 2D band is similar to that observed from turbostratically stacked graphene/graphite, probably due to the rotational disorder between the flakes breaking the electronic coupling between the individual layers.245 Figure 5.11a shows the change in 2D band position with sonication time (e.g., at 6000 rpm, 2D band position is 2662 ± 0.5 cm-1 at 24 hr to 2659 ± 0.6 cm-1 at 60 hr). It has been shown for MC flakes, that when a 2D band is fitted with single Lorentzian peak, as the number of layers increase, the position of the 2D band shifts to higher wavenumbers and reaches the maximum position which corresponds to the position of HOPG.353 Following this observation, empirically, it can be said that as the sonication time increases the number of layers decreases. Hence the 2D band position has a lower wavenumber for longer sonication time samples (Figure 5.11a). The difference in the 2D band position is clearer with different centrifugation speeds. The highlighted region in Figure 5.11a shows the variation of 2D band position at 60 hr with different centrifugation speed; 2665 ± 1.1 cm-1 at 2000 rpm to 2655 ± 0.8 cm-1 at 10000 rpm. This observation suggests that at higher centrifugation speeds, only the thinner graphene flakes remain dispersed, confirming AFM observations. The FWHM(2D) fall into 146PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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