Atomic Force Microscopy for Nanoparticles

Nanoparticles typically fall into one of two categories when it comes to sample preparation. The first category is nanoparticles rigidly attached to a solid structure. The second category is nanoparticles with weak adhesion to the substrate, such as dispersions of nanoparticles in liquid or dry mediums.

A good example of the first category is nanoparticles imbedded a solid matrix, as in the case of nanocomposites or nanoprecipitates . In such cases, typically a cross-section of the composite material is scanned to determine such properties as average particle size and spatial distribution.

Examples of nanoparticles in the second category are quantum dots, diesel soot particles, carbon black, and colloidal suspensions. Sample preparation for the second category of nanoparticles can be more complicated than for the first category. Because the AFM works by scanning a mechanical probe across a flat surface, any structure being imaged must have greater affinity to the flat surface than to probe tip. When nanoparticles do attach to the probe, the resulting images typically show reduced resolution. Streaking may occur if nanoparticles are not rigidly attached to the flat surface. To avoid such artifacts, close contact or vibrating mode is strongly recommended for such samples.

In certain cases it is necessary to affix nanoparticles in liquid or dry mediums to a sticky substrate. A cheap and easy way to do this is the use of double-sided sticky tape or other similar methods commonly used by microscopists. More refined techniques include the use of mica, "Tacky Dot" slides, and TempFix. Calcium phosphate nanocrystals mounted on TempFix can be seen in Figure 9.

Information on mica use is readily available in technical literature. Use of "Tacky Dot" slides and TempFix are well described in PNI's Application Note "Atomic Force Microscopy for Nanostructures."

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