Local Probe and AFM Flashcards
What is the concept for a local probe microscope?
A sharp point is brought close to surface and some interaction between the probe and the surface is measured. The ‘height’ of the probe relative to the surface is adjusted until the interaction is a some predetermined value. Image is built up point at a time.
Why does the diffraction limit not apply to local probe?
The interaction between the probe and the surface does not rely on propagating waves.
How is the cantilever detected in Atomic Force Microscopy?
A laser is shone on it which will reflect back onto a photodiode. The difference in intensity between the top and bottom diodes gives the bending of the lever and hence deflection of tip.
Why are the diodes normalised?
To reduce the effects due to fluctuations in intensity of the laser due to temperature.
What does contact mode with ‘constant force’ involve?
A feedback loop adjusts the height of the base of the cantilever to keep the bending constant.
What causes long distance attractive forces between atoms?
Van der Waals forces
What causes short distance repulsive forces?
The Pauli exclusion principle - two electrons cannot be in the same place.
What are capillary forces?
Forces due to condensed water from the air between the tip and the sample.
What disadvantage does contact mode have?
Can push molecules out of the way.
