In: Chemistry
Compare and contrast scanning tunneling microscopy (STM) and atomic force microscopy (AFM)? List at least two advantages and two disadvantages for both methods. Please reference at least one source in your discussion
Difference Between AFM and STM
AFM vs STM
AFM refers to Atomic Force Microscope and STM refers to Scanning Tunneling Microscope. The development of these two microscopes is considered a revolution in the atomic and molecular fields.
When talking of AFM, it captures precise images by moving a nanometer sized tip across the surface of the image. The STM captures images using quantum tunneling.
Of the two microscopes, the Scanning Tunneling Microscope was the first to be developed.
Unlike the STM, the probe makes a direct contact with the surface or calculates the incipient chemical bonding in AFM. The STM images indirectly by calculating the quantum degree tunneling between he probe and sample.
Another difference that can be seen is that the tip in AFM touches the surface gently touches the surface whereas in STM, the tip is kept at a short distance from the surface.
Unlike the STM, the AFM does not measure the tunneling current but only measures the small force between the surface and the tip.
It has also been seen that the AFM resolution is better than the STM. This is why AFM is widely used in nano-technology. When talking of the dependence between force and distance, the AFM is more complex than the STM.
When Scanning Tunneling Microscope is normally applicable to conductors, the Atomic Force Microscope is applicable to both conductors and insulators. The AFM suits well with liquid and gas environments whereas STM operates only in high vacuum.
When compared to STM, the AFM gives a more topographic contrast direct height measurement and better surface features.
Summary
1. AFM captures precise images by moving a nanometer sized tip across the surface of the image. The STM captures images using quantum tunneling.
2. The probe makes a direct contact with the surface or calculates the incipient chemical bonding in AFM. The STM images indirectly by calculating the quantum degree tunneling between he probe and sample.
3. The tip in AFM touches the surface gently touches the surface whereas in STM, the tip is kept at a short distance from the surface.
4. AFM resolution is better than the STM. This is why AFM is widely used in nano-technology.
5. When Scanning Tunneling Microscope is normally applicable to conductors, the Atomic Force Microscope is applicable to both conductors and insulators.
6. The AFM suits well with liquid and gas environments whereas STM operates only in high vacuum.
7. Of the two microscopes, the Scanning Tunneling Microscope was the first to be developed.
Advantages
AFM has several advantages over the scanning electron microscope
(SEM). Unlike the electron microscope which provides a
two-dimensional projection or a two-dimensional image of a sample,
the AFM provides a three-dimensional surface profile. Additionally,
samples viewed by AFM do not require any special treatments (such
as metal/carbon coatings) that would irreversibly change or damage
the sample. While an electron microscope needs an expensive vacuum
environment for proper operation, most atomic force microscopy
modes can work perfectly well in ambient air or even a liquid
environment. This makes it possible to study biological
macromolecules and even living organisms. In principle, AFM can
provide higher resolution than SEM. It has been shown to give true
atomic resolution in ultra-high vacuum (UHV) and, more recently, in
liquid environments. High resolution AFM is comparable in
resolution to scanning tunneling microscopy and transmission
electron microscopy.
Disadvantages
A disadvantage of AFM compared with the scanning electron
microscope (SEM) is the single scan image size. In one pass, the
SEM can image an area on the order of square millimeters with a
depth of field on the order of millimeters. Whereas the AFM can
only image a maximum height on the order of 10-20 micrometers and a
maximum scanning area of about 150×150 micrometers. One method of
improving the scanned area size for AFM is by using parallel probes
in a fashion similar to that of millipede data storage.
The scanning speed of an AFM is also a limitation. Traditionally, an AFM cannot scan images as fast as a SEM, requiring several minutes for a typical scan, while a SEM is capable of scanning at near real-time, although at relatively low quality. The relatively slow rate of scanning during AFM imaging often leads to thermal drift in the image making the AFM microscope less suited for measuring accurate distances between topographical features on the image. However, several fast-acting designs were suggested to increase microscope scanning productivity including what is being termed videoAFM (reasonable quality images are being obtained with videoAFM at video rate: faster than the average SEM). To eliminate image distortions induced by thermal drift, several methods have been introduced.
AFM images can also be affected by hysteresis of the piezoelectric material and cross-talk between the x, y, z axes that may require software enhancement and filtering. Such filtering could "flatten" out real topographical features. However, newer AFMs utilize closed-loop scanners which practically eliminate these problems. Some AFMs also use separated orthogonal scanners (as opposed to a single tube) which also serve to eliminate part of the cross-talk problems.