Scanning probe microscopy

Scanning probe microscopy (english scanning probe microscopy, SPM) is an umbrella term for all types of microscopy, in which the image is not having an optical or electron- optical imaging ( lenses) is generated as in the light microscope (LM ) and scanning electron microscopy (SEM), but on the interaction of a so-called probe with the sample. The sample surface to be examined is scanned point by point by means of said probe in a scanning process. The resulting for each individual point readings are then composed into a digital image.

Operation

Grossly simplified, one can imagine the functioning of the SPM as the tactile sensation of a record by the needle. However, the record player, the needle is purely mechanically deflected by microscopic bumps in the groove. The SPM however, the interaction between the probe (needle) and the sample of a different nature. Depending on the nature of this interaction, the following thin SPM types:

  • Scanning tunneling microscope (STM ), Eng. scanning tunneling microscope (STM ): between the tip and the sample, which do not touch each other, a voltage is applied and measured the resulting tunneling current.
  • Atomic force microscope (AFM '), Eng. atomic force microscope (AFM, and SFM): The probe is deflected by atomic forces, typically van der Waals forces. The displacement is proportional to the force which can be calculated by the spring constant of the probe.
  • Magnetic force microscope ( MFM ), Eng. magnetic force microscope ( MFM): Here, the magnetic forces between the probe and the sample are measured.
  • Optical Rasternahfeldmikroskop, Eng. near-field scanning optical microscope (SNOM, NSOM also ): The interaction here consists of evanescent waves.
  • Acoustic Rasternahfeldmikroskop, Eng. scanning near -field acoustic microscope ( SNAM or NSAM )

Interesting is the following size comparison: Had the atoms of the investigated sample, the size of table tennis balls, so would the probe ( probe tip ) on the size of the Matterhorn. That you can sample such fine structures with such a coarse top, can be explained as follows. The tip of the probe can still be seen atomically so dull, although some of the atoms will be the top one. Since the interactions between the tip and the sample decreases exponentially with the distance between sample and tip, thus controls only the front (top ) atom of the tip with a significant contribution.

Resolution

By this method, resolution can be achieved up to 10 picometers (pm) (atoms have a size ranging from 100 pm ). Light microscopes are limited by the wavelength of light and usually reach only resolutions of about 200 to 300 nm, ie about half the wavelength of light. In the scanning electron microscope is therefore used instead of light electron beams. Here, the wavelength by increasing the energy, although theoretically be made arbitrarily small, but then the beam is so "hard" that he would destroy the sample.

But SPM can not only scan surfaces, but it is also possible to remove individual atoms from the sample and drop them off again at a defined place. Such nano manipulations were known was represented by the image of the IBM research laboratories, on which the logo of the company by individual xenon atoms (see chapter links).

Impact on the natural sciences

The development of scanning probe microscopes since the beginning of the 80s was due to the significantly improved resolution of well below 1 micron and the possibility of nanomanipulation an essential condition for the explosive development of nanoscience and nanotechnology since the mid- 90s. Starting from the basic methods described above, many other sub- methods are still distinguished, which respond to certain additional aspects of the interaction used and is reflected in a variety of advanced Abbreviations: STS, STL, XSTM, XSTS, SPSTM, VT -STM, UHV -AFM, ASNOM etc.

Similarly, emerged as Nano Research Biology, nanochemistry, nano Biochemistry, Nanotribology, nanomedicine and many more. An AFM ( atomic force microscope ) has now even been sent to the planet Mars to the surface to investigate (see links).

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