Atomic Force Microscopy

Short facts

• High vacuum is not necessary.
• Piezoelectric position control system.
• The cantilever: V-shaped or single beam. V-shaped: high deflection sensitivity, not sensitive to torque. Single beam: vice-versa.
• Material used: diamond, tungsten, silicon.
• Artifacts: drift, non-linear hysteresis of piezoelectric.

Basic setup and components

• AFM often mounted on rigid base with high damping capacity.
• Adjustable sample stage.

Movement:

• X-Y: Applying voltage of opposite signs across diagonal segments of piezoelectric tube. Will cause it to bend.
• Z: Apply voltage on tube that contracts or expands axial length.

Split photo detector:

• Solid state laser
• The deflected cantilever reflects a laser beam from two points on the cantilever (before and after deflection) and two beams are sent to the detector. The signal generated depends on the proporiton of light falling on each half of the photodiode. (Noen som kan forklare dette bedre?)
• Can also measure phase shifts.

Data collection:

• Artifacts! Non-linearity in piezoelectric response. Sample surface must be coplanar with x-y scan of probe tip.

Modes of operation

It is the displacement of the probe tip at the end of the cantilever that is measured. The modes have different resolution (can someone explain this?). Problem and material determines which mode is used.

Interaction model (surface forces) lies behind different modes:

• Strongly affected by surface adsorbates and gaseous/liquid environment.

Contact mode

In the contact region of the potential energy curve, repulsive forces dominate. This mode has the highest resolution.

Two sub-modes:

• Set height constant and measure repulsive force.
• Set repulsive force constant and measure height.

Tapping mode

In the semi-contact region of the potential energy curve. Attractive and repulsive forces have similar magnitude.

Non-contact mode

In the non-contact region of the potential energy curve, attractive forces dominate and are measured. What is observed is a an increase in the amplitude of the oscillations of the vibrating probe, translated into a change in attractive forces.