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Dynamic AFM Modes
Intermittent Contact Mode
- Effect on amplitude and frequency of an oscillating tip during approach
is a method for measuring the surface structure without the disadvantages. It is a low force, zero friction scanning method with a very high vertical resolution.
The cantilever of the AFM probe is driven at a fixed frequency near its fundamental resonance. If the oscillation amplitude is large enough the tip detects both, the long range interaction as well as the repulsive force. Detecting the oscillation amplitude of the cantilever during scanning allows for a highly sensitive vertical resolution that is on the same level as the contact mode.
The major advantage of the intermittent mode is that the vertical and lateral forces acting on the tip are extremely small, reducing tip wear and other artefacts of the contact mode, such as sticking, to a minimum.
Non-Contact Mode
- Regions of interactions (forces) between tip and sample
is a surface imaging method for characterizing both, topography and long range forces without surface contact.
The cantilever of the AFM probe is driven at a fixed frequency near its resonance frequency. Compared to intermittent mode the oscillation amplitude is small,
< 10nm. The cantilever oscillates solely in the long range attractive regime of the surface potential.
Non-contact mode is very useful for measuring long range forces, e.g. magnetic or electric forces. It allows characterization of magnetic or electronic structures. As the tip is never in contact with the surface, the force acting on the very end of the tip is negligible. This leads to a much longer lifetime of the probe. Due to the small interaction with the surface non-contact mode is ideally suitable for analyzing soft materials.
Non-contact mode also avoids the physical effect of adhesion hysteresis and chaotic phase changes due to the dynamics of energy dissipation that sometimes affect the imaging quality in intermittent contact mode
- Droplet with 4 coli bacteria inside, phase image as overlay
Phase Contrast Imaging
is a mode that uses oscillating cantilever methods. Phase refers to the lag between the driving oscillator and the oscillating cantilever.
In intermittent as well as in non-contact mode an oscillator drives the cantilever at its resonance. In these oscillation modes, also referred to as AC or driven cantilever modes, the phase difference between the driving oscillator, e.g. a small piezo actuator, and the cantilever is close to 90 degrees. The phase is very sensitive to the interaction of the tip with the surface.
The phase signal provides valuable information and additional contrast to the AFM data. Varying interactions due to local surface properties may not be visible in the feedback signal but show up in the phase signal. Therefore, as a standard feature of all Bruker AFMs the phase signal can be selected as an additional acquisition channel.
The phase is sensitive to short range forces (adhesion, elasticity) as well as long range magnetic or electrostatic forces).
Frequency Modulation Mode
- Topography of a medical tablet, 2 x 2 µm2, top, mapping of the resonance shift, bottom
is the most advanced oscillation mode. In FM mode, the resonant frequency of the oscillating cantilever is measured.In addition to the detection of the amplitude, the effect of surface interactions on the cantilever’s resonant frequency is measured. In a closed-loop operation the cantilever is always oscillating at its resonant frequency. Frequency and oscillation amplitude are measured simultaneously, and both signals are suitable for the distance control of the AFM.The resonant frequency is extremely sensitive to additional forces and therefore provides additional surface information and contrast. The FM mode also provides control over the quality factor (Q-factor) of the oscillating cantilever. It permits to either increase the cantilever’s sensitivity (set a low Q to high) or its response time (decreasing a high Q-factor).FM mode provides a very stable operation that is not affected by dynamic effects of the energy dissipation sometimes visible in the driven cantilever intermittent contact mode.
