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Atomic Force Microscopy (AFM)

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AFM as a technique can be subdivided in either static or dynamic AFM. In static AFM, also known as contact AFM, a cantilever is brought into contact with the sample surface at its tip. Contact means that repulsive interactions take place between the cantilever tip and the sample. As a force is exerted to achieve contact, the cantilever will bend, resulting in a different optical deflection. A feedback loop will be used to keep the deflection at a constant value, ensuring a constant tip-surface distance. Using a conducting cantilever and applying a bias voltage will make it possible to perform conducting AFM, imaging local conductivity.

In dynamic AFM, the tip is generally kept at a greater distance from the sample surface, ensuring attractive interactions between the tip and the sample. The cantilever is then made to vibrate close to the resonance frequency at a certain amplitude, called the free amplitude. If the amplitude is kept small, the tip will stay in the attractive regime, leading to true non-contact AFM. Alternatively, a large (> 100 nm) amplitude can be employed, which will bring the tip into the repulsive regime at the bottom of the oscillation. This is considered intermittent contact or tapping mode, which is used most often in FYSC. Tapping mode offers advantages compared to both pure contact and non-contact AFM. Contact mode is influenced by friction or adhesion to the surface and can damage the sample, while non-contact generally offers a low resolution and can be hampered by a contamination layer (often water). Tapping mode eliminates these effects, as the intermittent contact avoids friction or adhesion and a large enough amplitude can measure through a contamination layer.

Under tapping mode conditions, the free amplitude will be dampened by the switch to the repulsive regime. The dampening, i.e. a percentage of the free amplitude, can be used as a setpoint value, and a feedback loop can be used to ensure a constant dampening. This set-up makes it possible to gain information about the topography of the scanned surface, as the tip-surface distance is kept constant this way. The amplitude on which the feedback loop is based can also be visualised, and will give a more sensitive image, although it can not be linked to the topography in a straightforward way. Furthermore, the phase difference between the applied and measured oscillation can give valuable information about the hardness or softness of the scanned material.