AFM vs SEM
Need to explore the smaller world, has been rapidly growing with the recent development of new technologies such as nanotechnology, microbiology and electronics. Since microscope is the tool which provides the magnified images of the smaller objects, a lot of research is done on developing different techniques of microscopy to increase the resolution. Although first microscope is an optical solution where lenses were used to magnify the images, current high resolution microscopes follow different approaches. Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM) are based on two of such different approaches.
Atomic Force Microscope (AFM)
AFM uses a tip to scan the surface of the sample and tip goes up and down according to the nature of the surface. This concept is similar to the way in which a blind person understanding a surface by running his fingers all over the surface. AFM technology was introduced by Gerd Binnig and Christoph Gerber in 1986 and it was commercially available since 1989.
The tip is made of materials such as diamond, silicon and carbon nanotubes and attached to a cantilever. Smaller the tip higher the resolution of the imaging. Most of the present AFMs have a nanometer resolution. Different types of methods are used to measure the displacement of the cantilever. Most common method is using a laser beam which reflects on cantilever so that deflection of the reflected beam can be used as a measure of the cantilever position.
Since AFM uses the method of feeling the surface using mechanical probe, it is capable of producing a 3D image of the sample by probing all the surfaces. It is also enables users to manipulate the atoms or molecules on the sample surface using the tip.
Scanning Electron Microscope (SEM)
SEM uses an electron beam instead of light for imaging. It has a large depth in field which enables users to observe a more detailed image of the sample surface. AFM also has a more control in amount of magnification as an electromagnetic system is in use.
In SEM, the beam of electrons is produced using an electron gun and it goes through a vertical path along the microscope which is placed in a vacuum. Electrical and magnetic fields with lenses focus the electron beam to the specimen. Once the electron beam hits on the sample surface, electrons and X-rays are emitted. These emissions are detected and analyzed in order to put the material image on the screen. Resolution of SEM is in nanometer scale and it depends on the beam energy.
Since SEM is operated in a vacuum and also uses electrons in the imaging process, special procedures should be followed in sample preparation.
SEM has a very long history since its first observation done by Max Knoll in 1935. First commercial SEM was available in 1965.
Difference between AFM and SEM
1. SEM uses an electron beam for imaging where AFM uses the method of feeling the surface using mechanical probing.
2. AFM can provide 3-dimensional information of the surface though SEM only gives a 2-dimensional image.
3. There is no special treatments for the sample in AFM unlike in SEM where many pre-treatments to be followed due to vacuum environment and electron beam.
4. SEM can analyze a larger surface area compared to AFM.
5. SEM can perform faster scanning than AFM.
6. Although SEM can be used only for imaging, AFM can be used to manipulate the molecules in addition to imaging.
7. SEM which was introduced in 1935 has a much longer history compared to recently (in 1986) introduced AFM.