Hi, would you please tell if we can calculate surface coverage or surface density by AFM or this technique is just qualitative?
Depends of what means "surface coverage or surface density". AFM is measurement tool and measure surface (mostly in the nano to micro scale) in x, y and z axis. It means the result is topography. So, for instance if you need to know surface area/real area AFM will give you answer.
For lower magnification (scans from 100x200um to 100x100mm)I prefer to use light interferometer as suitable technique for 3D measurements.
Using topography maps from AFM, you can quantify the surface coverage of features anywhere from a few angstroms to a few microns tall, as long as they sit on a substrate which is relatively flat compared to the height of those features. There should be a way to do this using your favorite AFM analysis software (I have done this using WSxM and Bruker NanoScope Analysis).
You can use "flooding analysis" or "bearing analysis" to find the area fraction or volume of features on top of a flat surface. You start by "flattening" or background subtracting the AFM image (using thresholding to exclude the particles from the background calculation) so that the background is all approximately the same height. Then, using the flooding/bearing analysis tool, find the background height. This tool will probably show your image "flooding" with a solid color as you increase the background height, and you want the background area completely covered, such that only the particles or film you're analyzing remains visible. Your software should then give you the area fraction covered by the features, and hopefully the volume of the features as well.
AFM is useful for looking at areas from 100um x 100um down to 100nm x 100nm or smaller. If your sample is very large relative to the size of the features you're interested in, then you may have to look at a few different areas to get a statistical idea of the coverage. As Luca suggested, you may want to confirm the coverage with a different technique such as calibrated X-ray fluorescence.
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