Like all other scanning probe microscopes, the atomic force microscope (AFM) utilizes a sharp probe moving over the surface of a sample in a raster scan. This microscope is used to show the roughnesses and measure their depth in the nanometer range.
Recording the position and showing the depth and height of the hills and valleys of a surface is called “topography”. The AFM probe is a tip on the end of a cantilever which bends in response to the force between tip and sample. We know that there are very small forces of attraction or repulsion between charged atoms, such forces are created between tip and surface atoms. By measuring the force between atoms in different points of the surface, the location of atoms can be determined on the surface. The hills and valleys are reported along the three axes (length, width and height) by this device. By deflection of the cantilever beam, the magnitude of the deflection is captured by a laser that reflects at an oblique angle from the very end of the cantilever. The distance between tip and sample surface, i.e. sample height, can be determined by measuring the force between atoms of sample surface and tip, which consequently is reported as roughness.
· Contact Mode
In contact mode, probe without creating vibration on the cantilever, operates in the region of repulsive Van der Waals forces between atoms of tip and sample. The deflection of the cantilever is measured by a feedback loop and kept at a fixed point. When the probe is scanning the surface, imaging is done by recording the applied voltage to the piezoelectric. The contact mode is suitable for investigating hard surfaces by thin, super sharp and hard tips.
· Non-Contact Mode
In non-contact mode, probe by creating vibration on the cantilever, operates in the region of attractive Van der Waals forces between atoms of tip and sample. In this case, the cantilever oscillates near a natural resonant frequency. The sample then approaches to decrease the cantilever amplitude to the specified value; in a way that, the interaction between tip and sample causes a sharp decrease in the amplitude; when the distance reaches the nanometer scale, the tip starts to scan the surface. In this mode, the oscillation amplitude is measured by a feedback loop and kept constant. This mode is more proper for the air and liquid media. In non-contact mode, soft samples and tip undergo less damage because a small force is applied to the sample.
· Tapping Mode
This mode is also same as the non-contact mode; but in tapping mode the oscillating tip of cantilever softly touches the sample and the oscillation amplitude is much larger than the non-contact mode. In this mode, imaging is done based on the oscillation amplitude of cantilever.