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| Here we see a student grade portable Scanning Tunneling Microscope and an image of the surface of graphite taken at the Engineering Expo in April, 1999.. |
In 1982 Binnig and Rohrer reported using a STM to see atoms in a silicon sample. For the development of the STM, they won the Nobel Prize in physics in 1986. Binnig, Christoph Gerber (IBM Zurich), and C. F. Quate (Stanford University) are credited with the development of the atomic force microscope (AFM). In 1987 Tom Alrecht was the first to see atoms using a AFM.
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| In this image we see a surface with a single atom on that surface and a collection of 6 more atoms on the surface. The "dull" probe in this view will not be able to characterize this surface very well as the features of the probe are larger that some of the features in the surface. |
For the probe to be effective in characterizing the surface, its features must be smaller than those of the surface being characterized.
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With the sharper tip, the surface can be better characterized as the tip can drop down in the area between the single atom and the layer of 6 atoms. To view a video of this, click here. |
To raise and lower the tip, piezoelectric crystals are used to allow for atomic sized steps up and down.
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The Scientific American, Oct. 2001, issue was a special dealing with Nanotech. The cover and pg. 33 show images of an atomically sharp probe. |
The principle of scanning probe microscopy can be illustrated with the use of an ordinary refrigerator magnet. A strip off of one end of the magnet will serve as the probe as shown in the figure below.
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Refrigerator magnets can be used to illustrate the principle of scanning microscopy by cutting a strip from one end to act as the probe strip. After the probe strip is detached from the magnet, it is drawn in the two directions shown in the middle set of images. The audience is then asked which pattern represents what they experienced with their probe. Refrigerator magnets are a stripped pattern so that they can act as a "horse-shoe" magnet and attach to the refrigerator door with the advertizement showing. |
In the figure below we see the basic components of scanning probe micrscopy, the probe tip attached to a cantilever, a laser, and a detector.
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An "atomically sharp" tip raster-scans over the surface. A laser is reflected off the back of the probe to the detector. Very small movements in the tip are amplified onto the detector. |
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| This is the surface of silicon as imaged by Prof. Max Lagally, University of Wisconsin - Madison. Note the various layers of atoms.The lighter colored rows represent layers on top of the bottom most layer. Also note the dark spots that represent holes in the layers where silcon atoms are missing. |
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Here we see a comparison of a STM image of graphite as compared to an AFM image of the same graphite. |
Atomic force microscopes (AFMs) can be used in air, in liquids, in gases, and under high vacuum and are being used throughout the scientific community. AFMs are member of the scanned-proximity probe microscopes, meaning the the probe does not come into contact with the sample, it just gets close enough to the surface to sense the forces of the sample. They can be used to determine the topography of the surface, as well as optical and magentic properties of the surface. They acquire their image by raster-scans of the surface, creating an image much like a television, one line after another.
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A desk top student grade atomic force microscope. |
In 2007 the first cases of identification of atoms in the surface of a material were report using a AFM. Each element in the alloy was first investigated to determine the amount of interaction there was with the probe. Then atoms of those elements were identified in the surface by "feeling" the atoms with the probe and identifiying the element based on the amount of interaction.
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| An atomic force microscope that operates at ultra-high vacuum. |
Don Igler and group and IBM used an AFM to create their now famous "Quantum Corral" by arranging iron atoms in a circle on a copper subtract. The atoms were arranged using the probe from the AFM to move the atoms across the surface. With the iron atoms in a perfect circle, the electrons in the copper surface set up a standing wave.
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Don Eigler arranged 48 iron atoms on the surface of a copper substrate. These images show the various stages of the process. Once complete the circular arrangement of the iron atoms forced the electrons in the surface of the copper to specific quantum states as shown by the rippled appearance of the surface. |
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"Quantum corral" of iron atoms on the surface of a copper substrate created the wave effect with the electrons in the copper atoms on the surface. |
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| Other arrangments of iron atoms on copper substrates. Note that only the perfect circle shows the standing wave of electrons. |
Some of the other forms of scanning probe microscopy include: Magnetic Force Microscopy, MFM; Chemical Force Microscopy, CFM; Scanning Force Micrsocopy, SFM; and Electrostatic or Electric Force Micrsocopy, EFM.