The Chemistry of Solids, Part 6 - Atomic Force Microscopy

Greetings,

The experimental work of John Dalton provided convincing evidence for the existence of atoms, but scientists were not able to produce images of them until the early 1980's (scanning tunneling microscopy).  Improvements in resolution and versatility were provided by the invention of the Atomic Force Microscope  (AFM, 1986).

This post focuses on the "contact-mode" AFM, which is useful for more hard, rigid samples.  A "non-contact-mode" AFM is also in use for "soft" samples, such as biological specimens (http://en.wikipedia.org/wiki/Atomic_force_microscopy).

"Contact-Mode" AFM - Parts and Functions

A sample is prepared and mounted on a platform, called a substrate.  The sample is scanned by "dragging" a very sharp nanometer-scale probe (usually silicon) back and forth across a sample with a gradual motion in the perpendicular direction.  The probe is connected to a very tiny (micron-scale)"cantilever" with a light-relective surface.  The most common type of detection uses a laser beam focused on the cantilever.  As the probe encounters "bumps and valleys" (literally atoms and spaces between them!), the attached cantilever moves ever so slightly.  Motion of the cantilever causes the laser light to refelect at different angles which, in turn, are detected by a photo-diode-array positional sensor.  The sensor generates electrical signals which are sent to a computer for processing.  The following image provides a visual model of the AFM.

 

 

The image shows two closeup views - one of an actual image produced by scanning a sample of glass, and the other of a schematic showing the parts of a laser detection probe.

 

The Inability of Optical Microscopes to See Atoms

The wavelength of visable light ranges from 300 to 700 nanometers.  Atomic dimensions are on the order of ~1 nanometer.  In order to see something, the size of a substance cannot be smaller than the wavelength of light used to view it!  Consequently, we cannot see atoms with visable light.  However, scientists are able to determine the structures of ionic compounds and complex biological molecules (such as proteins) by the use of x-rays, which do have wavelengths in line with atomic dimensions (0.01-10 nm).

 

Thank you for reading.

 

 

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