AFM Capabilities in Nanoparticle Characterization
Qualitative Analysis
Using the AFM, individual particles and groups of particles can be visualized. Visual images are essential in research and development projects and can be critical when troubleshooting quality control issues. Unlike other microscopy techniques, the AFM offers visualization in three dimensions. TEM provides images that are merely 2D projections, while generating 3D images from light microscopy and SEM are nonstandard and require rigorous calibration to a known standard.  Figure 3: NIST traceable polystyrene microspheres from Duke Scientific scanned with the NANO-R™. Mean Ш of microspheres is 73nm. Scan size is 6чm x 6чm. In Figure 3, 73mm NIST traceable microspheres are shown in both perspective view and top view. 3D information is incorporated in both views. In the perspective view, the 3D nature of the image is obvious. In the top view, the intensity of the color reflects the height of the particle. In material sensing mode, the AFM can distinguish between different materials, providing spatial distribution information on composite materials with otherwise uninformative topographies. In Figure 4, material inhomogeneity can be seen on a topographically flat organic film. Nanocomposites can be similar analyzed for dispersion of particulate matter.  Figure 4: Inhomogeneities in organic film on polymer substrate. Using material sensing mode on the NANO-R™ allows observation of material inhomogeneities in nanocomposites with relatively flat topographies. Quantitative Analysis
Software-based imaging processing of AFM data can generate quantitative information on both individual nanoparticles and groups of nanoparticles. For individual particles, size information (length, width, and height) and other physical properties (such as morphology and surface texture) can be measured. In Figure 5, surface roughness data generated from a scan of a wood fiber is shown.  Figure 5: A wood fiber scanned with an AFM to measure roughness. Paper products containing such wood fibers can vary in quality based on the physical properties of the fibers. Statistics on groups of particles can also be measured through image analysis and data processing. Commonly desired ensemble statistics include particle counts, particle size distribution, surface area distribution and volume distribution. With knowledge of the material density, mass distribution can also be easily calculated. Image processing of an AFM image are shown in Figure 6. A scan of colloidal gold and the particle size distribution calculated by image processing is shown in Figure 7.  Figure 6: Polystyrene microspheres with mean diameter of 102nm. (a) Topographical scan image. (b) Image processing used to determine boundaries of individual particles. Ensemble data can be further processed to provide statistical information such as particle size distribution.  Figure 7: Scan of colloidal gold and associated particle size distribution. Mean Ш = 28nm. Whenever data from single-particle techniques is processed to provide statistical information, the concern over statistical significance exists. It is easy to attain greater statistical in AFM by combining data from multiple scans to obtain information on the larger population. Mediums
AFM can be performed in liquid or gas mediums. In contrast, TEM and SEM generally require high-vacuum conditions, which can alter the particles being examined. For example, with combustion-generated nanoparticles, a major component of the particles are volatile components that evaporate at low pressure. Dry particles can be scanned in both ambient air and under controlled environments, such as nitrogen or argon gas. Liquid dispersions of particles can also be scanned, provided the dispersant is not corrosive to the probe tip. Particles dispersed in a solid matrix can also be analyzed by topographical or material sensing scans of cross-sections of the composite material. Such a technique is useful for investigating spatial dispersion in nanocomposites. Precipitates in a nickel aluminum alloy are shown in Figure 8.  Figure 8: Ni3Al precipitates in a nickel aluminum alloy. (a): 27µm × 27µm topography image. (b): 3µm × 3 µm shading image.
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