Comparison of Three Operation Modes of Microscope AFM Working Principle
contact mode
In contact mode, the tip is always in light contact with the sample, scanning in constant height or constant force mode. During scanning, the tip slides over the sample surface. Typically, contact mode produces stable, high-resolution images.
In the contact mode, if the soft sample is scanned, the sample surface may be damaged due to direct contact with the needle tip. If the force between the sample and the tip is weakened during scanning to protect the sample, the image may be distorted or artifacts may be obtained. At the same time, the capillary action of the surface will also reduce the resolution. Therefore, the contact mode is generally not suitable for studying biological macromolecules, samples with low elastic modulus, and samples that are easy to move and deform.
contactless mode
In non-contact mode, the tip vibrates above the sample surface, never in contact with the sample, and the probe monitor detects non-destructive long-range forces such as van der Waals and electrostatic forces on the imaged sample. Although this mode increases the sensitivity of the microscope, when the distance between the needle tip and the sample is long, the resolution is lower than that of the contact mode and the tap mode, and the imaging is unstable and the operation is relatively difficult. Imaging in liquid has relatively few applications in biology.
tap mode
In the tapping mode, the cantilever is forced to vibrate near its resonant frequency, and the oscillating tip gently taps the surface of the sample, making intermittent contact with the sample, so it is also called intermittent contact mode. Due to the tapping mode, it is possible to avoid the tip sticking to the sample, and there is almost no damage to the sample during scanning. When the tip of the tapping mode touches the surface, it can overcome the adhesive force between the tip and the sample by providing sufficient amplitude of the tip. At the same time, since the acting force is vertical, the surface material is less affected by lateral friction, compression and shear forces. Another advantage of the tapping mode compared to the non-contact mode is the large and linear working range, which makes the vertical feedback system highly stable and repeatable for sample measurements.
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Tapping mode AFM is achievable in both atmospheric and liquid environments. In the atmospheric environment, when the needle tip is not in contact with the sample, the microcantilever freely oscillates with the maximum amplitude; when the needle tip is in contact with the sample surface, although the piezoelectric ceramic sheet excites the microcantilever to oscillate with the same energy, the steric hindrance makes the microcantilever The amplitude of the cantilever decreases, the feedback system controls the amplitude of the cantilever to be constant, and the needle tip follows the ups and downs of the sample surface to move up and down to obtain the shape information. The tapping mode is also suitable for operation in liquid, and due to the damping effect of the liquid, the shear force between the needle tip and the sample is smaller, and the damage to the sample is smaller, so the tapping mode imaging in the liquid can be performed on active biological samples On-site testing, on-site tracking of solution reactions, etc.
lateral force mode
Lateral Force Microscopy (LFM) works similarly to AFM in contact mode. When the micro-cantilever scans above the sample, due to the interaction between the tip and the sample surface, the cantilever swings, and there are roughly two directions of deformation: vertical and horizontal. Generally speaking, the change in the vertical direction detected by the laser position detector reflects the shape of the sample surface, and the change in the signal detected in the horizontal direction, due to the different material properties of the material surface, the friction coefficient is also different. different, so in the process of scanning, the degrees of left and right distortion of the microcantilever are also different. The degree of torsional bending of the cantilever increases or decreases as the frictional properties of the surface change (increasing friction results in greater torsion). A laser detector measures and records topography and lateral force data separately in real time. Usually, not only the different components of the sample surface can lead to the distortion of the micro-cantilever, but also the change of the surface morphology of the sample can also cause the distortion of the micro-cantilever, as shown in the figure below. In order to distinguish between the two, usually LFM images and AFM images should be acquired simultaneously. Depending on the cause of the distortion of the cantilever, LFM can usually be used to obtain compositional images and "edge-enhanced images" of the material surface.
