Atomic Force Microscopes (AFM) do more than just provide a topographical image of the surface.  So for the next few blogs I’m going to describe some of the “modes” of the AFM.  Let’s start with contact mode.

In contact mode AFM, interatomic van der Waals forces become repulsive as the AFM tip comes in close contact with the sample surface. The force exerted between the tip and the sample in contact mode is on the order of about 0.1-1000nN. Under ambient conditions two other forces besides van der Waals interactions, are also generally present. These forces include the capillary force from a thin layer of water in the atmosphere, and the mechanical force from the cantilever itself. The capillary force is due to the fact that water can wick its way around the tip, causing the AFM tip to stick to the sample surface. The magnitude of the capillary force should vary with the tip-sample distance. The mechanical force resulting from the cantilever is similar to the force of a compressed spring and its magnitude and sign (repulsive or attractive) is dependent on the cantilever deflection and the cantilever’s spring constant. Consequently, in contact mode AFM, the repulsive van der Waals forces arising for the AFM tip to sample interaction must balance the sum of the forces arising from the capillary force plus the mechanical force from the cantilever.

The thin layer of water present on many surfaces in air exerts an attractive capillary force and holds the tip in contact with the surface. Thus, when scanner pulls the tip away from the surface, the cantilever bends strongly towards the surface and the scanner has to retract further so that the tip can snap off of the surface. The cantilever returns to its original unbent status as the scanner moves the tip away from the surface beyond the snap-out point. In liquid, since the large capillary force is isotropic and the total force that the tip exerts on the sample can be reduced to some extent.

In order to improve imaging in air and liquid environments, Agilent offers Magnetic AC mode (MAC Mode). In MAC Mode, a cantilever that is coated with a magnetic material is driven into oscillation by an AC magnetic field that is generated by a solenoid positioned close by the cantilever. The result is a clean cantilever response that has no artifacts or background signals when the cantilever is vibrated in air or in liquid.

For more information on Agilent AFM’s - visit the www.agilent.com/find/afm

This entry was posted on Tuesday, July 10, 2007 at July 10, 2007 and is filed under General, Microscopy. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.