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Atomic Force Microscopy (AFM components (Cantilever (Low spring constant,…
Atomic Force Microscopy
AFM modes
Contact mode
Constant force
Direct contact with sample
Tapping mode
Resonate just below resonance frequency
Keep constant vibration amplitude
Amplitude setpoint
Good images conditions
Enough energy to avoid tip sticking to sample
No other sample properties affecting the cantilever oscillation
Oscillation frequency affected by sample topography
Non contact mode
Small amplitude oscilallation
Above resonance frequency
Oscillation affected by
Van der Waals forces
Other long range forces
UHV required
Modes Summary
Contact mode
Advantages
High scan speed
Easiest mode for rough samples
Disadvantages
Lower resolution
Image distortion due to high normal and lateral forces
Sample damage due to tip scratching
Tapping mode
Advantages
High lateral resolution
Reduced normal and lateral forces
Almost no sample damage
Disadvantages
Low scanning speed
Non-contact mode
Advantages
Minimal normal and lateral forces
Atomic resolution in UHV
Disadvantages
Slowest scanning speed
Lower resolution in air as fluid contaminates the sample surface
Errors
Deflection error signal
Poor tracking
Cause
Not optimised feedback circuit
Solution
Increase gains
Decrease amplitude setpoint (tapping mode)
Decrease scan speed
Instability
Gains set too high
AFM components
Scanner
Non-linear response
Hysterisis piezo response
Must be calibrated
Cantilever
Low spring constant
High resonance frequency
Detector
Feedback circuit
Vibration isolation
Forces acting on tip
Attractive
Long range Van der Waals
Capillary forces
Thin fluid layer on surface
Repulsive
Columbic repulsion
