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flakes by centrifuging at higher speeds. The sediment (which containing larger flakes) from the earlier step was redispersed in fresh solvent followed by subsequent centrifugation at lower speeds. These steps were repeated such that graphene flakes were separated based on their sizes as determined by TEM and Raman: ~3.5 ฮผm for 500 rpm to ~1 ฮผm for 4000 rpm. It is worth noting that the increase in thickness of graphene with decrease of centrifugation speed was observed.62 Such size-selection technique was even followed to establish the use of Dynamic Light Scattering (DLS) technique in measuring the lateral dimensions of 2d materials in liquid-phase.70 As mentioned earlier, due to the matching surface tensions, these organic solvents produce stable dispersions of graphene by LPE. However detailed solvent molecule- graphene interactions are not well explored.3 Liu et al. investigated the origin of surface charge of unfunctionalised graphene in organic solvents by electrophoresis and zeta potential (๐) measurements.71 They have proposed that the charge transfer between the graphene and solvent molecules are responsible for the grapheneโs surface charge. It was found that depending on the different donor and acceptor numbers of a solvent, graphene can be positive or negatively charged in the dispersion exhibiting different ๐ values. The electrostatic repulsion of charged graphene sheets facilitates stable dispersions. Such observations resulted in finding some unreported solvents such as tetramethylurea and benzoyl chloride for direct LPE of graphene.71 Figure 2.7: a) Zeta potential values of graphene dispersed in different organic solvents. b) Zeta potential of graphene dispersion vs. donor and acceptor numbers of the respective solvents.71 2.3.2.1.2. LPE in aromatic organic solvents Hamilton et al. used o-DCB โ a solvent with ๐พ values of 36.6 mJ/m2 and ability to interact with graphene via aromatic ๐ โ ๐ interactions, for LPE of graphite producing Chapter 2 โ Graphene: Properties and Production 39PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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