AFM (Atomic Force Microscope)
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the force between a tip
and a sample surface to form an image
the sensor to monitor the
deflection of the cantilever (i.e. position of the tip) --> done by reflection of a laser beam --> it's targeted on the reflective backside of a cantilever --> intensity and position of the reflected laser beam is measured by a photon detector. The angle of reflection depends on the deflection of the cantilever (position of the tip), i.e. forces between the tip and surface that depend on the position of the tip above the sample surface and the surface topology.
Forces
The cantilever deflection s is related to the change of force acting on the surface FC via the spring constant kC of the cantilever.
For small deflections, the deflection is proportional to the force:
s =Fc/Kc
This system has a resonance frequency given by :
ωr = √kC/meff
--> the sensitivity of the system (Δz) increases with decreasing spring constant. But, the resonance frequency decreases and, therefore, its sensitivity to low frequency vibrations increases.
Modes of Operation
Non-contact
Intermittent mode
Contact
Attractive: When the tip approaches the sample surface, due to induced dipole and a consecutive dipole – dipole interaction an attractive force occurs (van der Waals). The tip experiences a force directed toward the surface.
Repulsive: On further approach, the wave function of tip and sample surface start to overlap. The electrons then have to occupy energetically higher levels (Coulomb interaction + Pauli
principle).
Constant Height
Constant Height
Constant Height
Constant Force
Constant Force
Constant Force
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Scan more easily rough samples with extreme changes in vertical topography
Lateral forces can distort features in the image or move objects on the surface
Forces normal to the tip-sample interaction can be high in air due to capillary forces from the adsorbed fluid layer on the surface
Combination of lateral forces and high normal forces can result in reduced spatial resolution
Only technique that can obtain “atomic resolution” image
High scan speeds
may damage soft samples