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Chapter 5 – Production of Few Layer Graphene in Organic Solvents were observed under AFM. These peaks were also reported by others and suggested it might be from air bubbles or dust particles.70 Figure 5.17: Example IPD of a non-centrifuged sample (PDI-1) and 10000 rpm centrifuged sample (PDI-0.23) In some cases, when the sample is very poly disperse, the built-in quality report of the software notifies the user and suggests that the sample is unsuitable for measurement. This is because the software struggles to fit the correlogram function effectively due to presence of various sizes, hence affecting the derivation of the diffusion coefficient and the hydrodynamic size. Specifically some poorly centrifuged samples which had sedimenting particles (usually larger) were flagged with such notifications by the software. Figure 5.17 show an example of such case, where a non-centrifuged (only sonication) sample was measured. This sample had a population of particles which were sedimenting under gravity. The software stated that the samples poly dispersity index (PDI) measured by the software was PDI-1. The reason could be that the presence of large sedimenting particles constantly affects the observed size population within a certain time period, resulting in drastic change in observed intensity fluctuations, affecting the correlation function. Such samples are characterised by a high intense peak (Figure 5.17) and poor correlation function. However, sample centrifuged at 10000 rpm showed a PDI- 0.23. The measured PDI of all the FLG/NMP and FLG/DMF samples was between 0.2-0.42. The value of PDI for a monodisperse sample is < 0.05, and such values are not possible for graphene dispersions as there is always a range of sizes present. However, empirically, it can be suggested that smaller the PDI lesser the poly dispersity. The DLS instrument can assess the sample’s size distribution using a number of measurements; 𝑧-Average diameter, IPSD, volume particle size distribution (VPSD) and 155PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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