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Atomic Force Microscopy (AFM)
Atomic Force Microscopy, or AFM, is a surface characterization technique invented in 1985. The great advantage of AFM is the ability to probe the surface topography of a sample in high detail due to its nanometer-scale spatial resolution. Contrary to other SPM (Scanning Probe Microscopy) techniques, it is appropriate for any kind of surface imaging; conductive or semiconductive surfaces, polymers and tissues, biological samples, etc. In addition, the versatility of AFM operation has proven to be a valuable tool in the research fields of biotechnology, surface characterization, etc.
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How the System of an AFM works
The microscope typically uses a laser beam deflection system to image the surface topography of a sample. More specifically, AFM operates by using a tip attached at the free end of a cantilever, to probe the sample's surface. The tip may come into direct or close contact with the surface, so that they may both interact with each other through attractive or repulsive forces (e.g. electrostatic, Wan der Waals etc.), depending on the proximity of the tip to the surface. These forces (F) are measured by Hook's Law:
F = - kz
where k is the cantilever stiffness and z the distance of the cantilever deflection measured through the laser beam deflection system.
The laser beam is reflected by the upper part of the cantilever. A photodetector measures the deflection of the laser beam and the data is converted into a topographic image of the probed surface. Other measurements include the surface roughness, distance, and peak-to-valley measurements. These different types of analysis can be acquired through the different scanning modes that the AFM can operate with.
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Contact Scanning Mode
This is the most common mode of operation. The tip is kept is constant contact with the sample's surface and a piezoelectric crystal is used to raise or lower the sample in order to maintain this contact constantly. The system measures the lateral position of the sample to recreate its topography.
Although this mode can provide high resolution images, it is prone to image artifacts and data distortion originating from the frictional forces applied to the surface by the tip. These forces may deform the surface's topography and provide a false image.
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Semi-Contact or Tapping Mode
In semi-contact or dynamic force or tapping mode, the tip periodically touches or taps the surface of the sample. This is achieved with the aid of a piezoelectric crystal that causes the cantilever to oscillate in a vertical direction near its resonance frequency. A feedback system is responsible for maintaining the oscillation amplitude constant, typically within the range of 100 to 200 nm. When the tip passes over a depression on the surface, the cantilever has more room to oscillate and the amplitude increases. In a similar way, the amplitude decreases when the tip passes over a bump. The changes in oscillation amplitude are used to identify and measure surface features.
The alternating contact with the sample provides a higher resolution compared to the other scanning modes. It is also appropriate for fragile and soft samples that could be easily damaged by the shear or adhesion forces applied in the other modes. Such samples may be soft films onto hard substrates, cells or adsorbed proteins on a surface, etc.
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The tip in this mode oscillates near the surface of the sample without coming into direct contact with it - at a frequency slightly above the resonance frequency of the cantilever. The electronic system detects the weak attractive Wan der Walls forces of the order of pN (10 -12 N), between the tip and the sample. The changes in amplitude, phase and frequency of the oscillation are measured in order to construct the topography of the sample's surface.
The greatest advantage of the non-contact mode is that it can be used for fluid surfaces, since the tip does not come into direct contact with it and cannot deform the surface characteristics in any way. However, the weak forces that are detected may not be enough to provide a high resolution image of the surface.
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Other Modes of Operation
Apart from the scanning modes described, new ones have been developed that actually take advantage of and measure for example, frictional forces and the twist of the cantilever instead of its deflection (Lateral Force Microscopy). Other examples include the oscillation of the cantilever in a very high frequency compared to its resonant frequency (Force Modulation). The versatility of AFM operation makes it a great research tool and worthy competitor among other microscopy techniques.