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4.3.3. Atomic force microscope AFM was used to measure the lateral dimensions and thickness of graphene sheets produced by various processing conditions. The measurements were taken in Bruker Dimension FastScan using FastScan-A (Bruker) probe in tapping mode. Samples for AFM measurements were prepared either by drop casting or spray coating on freshly cleaved mica sheets (Agar Scientific AGG250-2). For graphene produced in organic solvents, samples were prepared by spraying ~0.5 ml diluted dispersions (at 1.5 bar nitrogen pressure and 5-10 cm away) on mica sheet using an Evolution Solo Airbrush (Harder & Steenbeck). The mica sheet were heated up to 100 oC prior to spraying as it helped flash evaporate the solvent during the spraying procedure, reducing the probability of graphene aggregation. The mica sheets were again heated on hot plate at 150 oC to evaporate excess solvents. This procedure was generally repeated three to five times for good coverage. Residual solvents were removed by annealing the substrates under argon atmosphere at 400 oC for 4 hrs prior to AFM measurements. For graphene produced using biomaterials, AFM samples were prepared by dropping few drops (50 μl) of dilute dispersion on freshly cleaved mica and drying overnight at room temperature. DI water was used to wash the sample surface to remove excess lipid or proteins and gently dried using nitrogen gas. All the images were analysed using Nanoscope Analysis software. 4.3.4. Dynamic Light Scattering DLS also called as Photon Correlation Spectroscopy or Quasi-Electon Light Scattering is a non-invasive method for measuring size of the particles or molecules in dispersion typically in the submicron region. 4.3.4.1. DLS Principle Particles in dispersion undergo Brownian motion due to its collision with the surrounding solvent molecules. In DLS, the speed at which the particles diffuse due to Brownian motion is measured by determining the rate at which the intensity of scattered light fluctuates. The relationship between the velocity of Brownian motion and the particle size is defined by Stokes-Einstein equation;341, 342 Chapter 4 – Methods 119PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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