Nano Activity Modeling A Scanning Probe Microscope

In this activity, Professor Campbell provides background information about Scanning Probe Microscopy (SPM) and ideas on how to build a LEGO® model of it.

Background Information. Scanning Probe Microscopy

You are likely familiar with microscopes that use light, but many objects like atoms are too small to be seen using light. Other microscopes use electrons to view even smaller objects, but these also have difficulty visualizing objects as small as atoms.

Scanning probe microscopy (SPM) is a method for mapping surfaces of materials on the atomic scale. It is useful to study the locations of atoms on surfaces because many chemical reactions, such as corrosion and catalysis, take place at solid surfaces. Most SPM techniques are variations of the same basic principles, illustrated in the picture. At the heart of the microscope is a probe called a cantilever. The cantilever is fixed at one end and can flex up and down at the other end like a diving board. A laser beam is shined onto the tip of the cantilever. The light bounces off the top of the cantilever onto a pattern of light sensors called a photodiode array.

To run the SPM, the cantilever is brought very close to a surface. The surface is moved back and forth under the cantilever, so the cantilever more or less scans the surface. Forces between the surface and a cantilever tip cause the tip to deflect up and down. Deflection of the cantilever shifts the position of the laser beam that reflects off the top of the cantilever onto a photodiode array. The movement of the beam between the photodiodes is used to measure the amount of cantilever deflection. A computer combines the information about the cantilever deflection with information about the back and forth movement about the surface to produce a three-dimensional map of the surface. This SPM technique is so sensitive that individual atoms can deflect the cantilever probe and therefore be detected!

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