An atomic force microscope (AFM) works by scanning a tiny and extremely sharp tip that is mounted on the end of a flexible cantilever over the surface of samples - it’s similar to the stylus on a record player. So unlike all other forms of microscopy it has no lenses and doesn’t image the sample by ‘looking’, it does it by ‘feeling’. It is analogous to reading Braille with your fingertips. The resolving power of an AFM is determined by the sharpness of the tip and the accuracy of the scanner mechanism. When both are optimised AFM can achieve atomic resolution on hard crystalline surfaces. Even with a single-atom sharp apex on the tip, how does a scanner move with this accuracy? The breakthrough in developing these forms of microscopes (by Gerd Binnig and Heinrich Rohrer) was to utilise the piezo electric effect of certain ceramics (lead zirconate titanates). All of us have seen this phenomena in gas lighters. When you press the button or push the lever on a gas lighter to produce the spark what it does is squeeze a piezoelectric crystal which generates a high potential difference (voltage) on the opposite sides of the crystal due to the stress. The scanners in AFM’s use this method in a reverse way – they apply a high voltage to the opposite faces of a piezo crystal and it expands. By slowly ramping up the voltage the crystals expand in highly accurate and reproducible manner. The scanner shown in this video is shaped into a tube (the outside of which is sectioned into four faces) made from the piezo ceramic. By applying the voltage to opposite sides of the tube it swings left and right and forwards and backwards. Another electrode attached to the inside of the tube (which is not sectioned) can make it expand and shrink in height. As seen in the video that makes the scanner move in all three dimensions (x,y,z). In most AFM’s the sample to be examined is mounted on top of the scanner. As the sample is scanned the AFM tip tracks across its surface causing the cantilever to move up and down in response to the topography of the surface. The tip movement is detected by bouncing a laser beam off the end of the cantilever onto a photodetector . It is split into four sections so it can detect both vertical and lateral movement of the AFM tip. It is basically a form of mechanical amplification, called an optical lever. Just like when the sun reflects off your watch face; a tiny movement of your wrist produces a large displacement of the light beam. In an AFM this is so sensitive that it can detect movement of the tip right down to the size of single atoms. In addition to imaging, the atomic force microscope (as its name implies) can also measure forces between atoms and molecules. For example it can measure the force required to pull an antibody free from its target molecule (antigen). AFM is a member of a family collectively known as scanning probe microscopes, the first of which the scanning tunnelling microscope (STM), made the incredible breakthrough of directly imaging atoms for the first time ever and won the geniuses Gerd and Heinrich the Nobel prize in physics! If you want to know more about AFM and its applications buy a copy of my book: Atomic Force Microscopy for Biologists, 2nd edition (World Scientific Publishing, Singapore).
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