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.
• Tip materials used: diamond, tungsten, silicon. Silicon nitride - high elastic rigidity.
• 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 use different tips, and therefore have different resolutions. 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. The tip most commonly used is silicion nitrid, because of its physical restitance to damage. The radii is about 20-60nm, and atomic resolution is therefore beyond reach.

Two sub-modes:

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

Artifacts that can be removed when operating in contact mode:

• Capilliary attraction from films of moisture (under atmospheric conditions).
• Electrostatic charging of surface

Tapping mode

In the semi-contact region of the potential energy curve. Attractive and repulsive forces have similar magnitude. Force on probe tip changes sign during cycle. The tip used is usually a silicon tip with radii in the order of 10nm, which result in a very high resolution. Phase image extremly sensitive to elastic properties.

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.

Maximum deflection sensitivity: Thin v-shaped cantilever. Si nitride tip minimizes damage to the tip. Soft samples: Constant pre-set cantilever deflection. (Kan noen utdype/omformulere siste setning?)