2.3 Instruments of microscopy  (Page 9/16)

There are two types of scanning probe microscope: the scanning tunneling microscope (STM) and the atomic force microscope (AFM) . An STM uses a probe that is passed just above the specimen as a constant voltage bias creates the potential for an electric current between the probe and the specimen. This current occurs via quantum tunneling of electrons between the probe and the specimen, and the intensity of the current is dependent upon the distance between the probe and the specimen. The probe is moved horizontally above the surface and the intensity of the current is measured. Scanning tunneling microscopy can effectively map the structure of surfaces at a resolution at which individual atoms can be detected.

Similar to an STM, AFMs have a thin probe that is passed just above the specimen. However, rather than measuring variations in the current at a constant height above the specimen, an AFM establishes a constant current and measures variations in the height of the probe tip as it passes over the specimen. As the probe tip is passed over the specimen, forces between the atoms (van der Waals forces, capillary forces, chemical bonding, electrostatic forces, and others) cause it to move up and down. Deflection of the probe tip is determined and measured using Hooke’s law of elasticity , and this information is used to construct images of the surface of the specimen with resolution at the atomic level ( [link] ).

[link] , [link] , and [link] summarize the microscopy techniques for light microscopes, electron microscopes, and scanning probe microscopes, respectively.

Key concepts and summary

• Numerous types of microscopes use various technologies to generate micrographs. Most are useful for a particular type of specimen or application.
• Light microscopy uses lenses to focus light on a specimen to produce an image. Commonly used light microscopes include brightfield , darkfield , phase-contrast , differential interference contrast , fluorescence , confocal , and two-photon microscopes.
• Electron microscopy focuses electrons on the specimen using magnets, producing much greater magnification than light microscopy. The transmission electron microscope (TEM) and scanning electron microscope (SEM) are two common forms.
• Scanning probe microscopy produces images of even greater magnification by measuring feedback from sharp probes that interact with the specimen. Probe microscopes include the scanning tunneling microscope (STM) and the atomic force microscope (AFM) .

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Label each component of the brightfield microscope.

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