Working group of Prof. Gauglitz
Atomic Force Microscopy (AFM)
AFM probes the surface of a sample with a sharp tip, located at the free end of a cantilever. Forces between the tip and the sample surface cause the cantilever to bend. This cantilever deflection is detected optically by focusing a laser beam onto the back of reflective cantilever. As the tip scans the surface of the sample, moving up and down with the contour of the surface, the laser beam is deflected off the attached cantilever into a dual element photodiode. This photodetector measures the difference in light intensities between the upper and lower photodiodes and then converts this signal into a voltage. This method enables a computer to generate a three- dimensional map of the surface topography [1].
There are several modes of operation for the AFM. The primary modes of operation are contact mode, non-contact mode, and dynamic contact mode (tapping mode).
Especially tapping mode imaging allows high resolution topographic imaging of sampe surfaces including on surfaces that are easily damaged, loosely held to their substrate, or otherwise difficult to image by other AFM techniques. In contact mode AFM, the probe tip is simply dragged across the surface and the resulting image is a topographical map of the surface of the sample. While this technique has been very successful for many samples, it has some serious drawbacks. The dragging motion of the probe tip, combined with adhesive forces between the tip and the surface, can cause substantial damage to both sample and probe and create artifacts in the image. Tapping mode imaging overcomes the limitations of the conventional scanning modes by alternately placing the tip in contact with the surface to provide high resolution and then lifting the tip off the surface to avoid dragging the tip across the surface.
AFM is being applied to a wide array of application areas, e.g. determination of surface roughness of different polymers, imaging of nanoparticle adhesion or measurement in fluid environment of study biological samples [2].