234 Atomic Force Microscopy Atomic force microscopy is a member of the

234 atomic force microscopy atomic force microscopy

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2.3.4 Atomic Force Microscopy Atomic force microscopy is a member of the microscopic techniques together known as scanning probe microscopy (SPM). The working principle of scanning probe microscopes is very different
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from those underlying light and electron microscopy. An SPM is used to study the surface properties of materials by scanning a very fine pointed probe over the surface. SPM is a relatively new technique and emerged with the development of the first working SPM by Gerd Binnig and Heinrich Rohrer in 1981. The first SPM was a scanning tunneling microscope (Figure 12). Figure 11 The probe in a scanning tunneling microscope is a very fine metal tip at a high voltage. The tip is brought in a proximity of the surface and scanned across the surface in a raster pattern. The quantity that is measured is the tunneling current flowing between the sample and the surface. The instrument can operate either in constant current mode or in constant height mode. In constant height mode, the tip scans the surface and current is recorded at each point. In a constant current mode, the current flowing between the tip and the sample is kept constant through a feedback loop that causes the sample stage to move closer to or farther from the tip; the signal obtained in constant current mode therefore is the distance between the tip and the specimen. An intrinsic limitation of scanning tunneling microscopy is its inability to study the non-conducting surfaces. This led to the development of other types of microscopes including atomic force microscope. 2.3.5 Atomic force microscope (AFM) Atomic force microscope is a type of scanning probe microscope that records the force between the probe and the specimen. The working principle of an AFM can be understood like this: Consider yourself to be in a dark room in front of a table. The table has a book, a pen, a
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wristwatch, a spoon, a fork, and a screw driver. Will you be able to selectively lift the spoon if asked to do so? The answer for most people is yes. You can distinguish two distinct objects by touching them with your fingers. In this example, your fingers act as the probes, your arm acts as the positioner of your fingers, and your brain works as the processing unit. An AFM works the same way; it has three basic components: a probe, a positioner, and a processing unit. Figure 6 shows the diagram and the working principle of an AFM. Figure 12 An AFM has a pointed probe attached to a rectangular base called a cantilever. The positioning of the cantilever with respect to the specimen is achieved by the piezoelectric elements, called scanners. The piezoelectric element can be connected either to the cantilever or the specimen stage. In the initial AFMs, the piezoelectric element was a piezoelectric tube (Figure 14A) that can be allowed to position the cantilever in the three-dimensional space. As the X, Y, and Z scanners in a piezoelectric tube are coupled, there is always some crosstalk between the scanners.
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  • Electron, Scanning Tunneling Microscope, Scanning electron microscope, Scanning probe microscopy, Atomic force microscopy

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