Working Model of the Atomic Force Microscope
How is it possible to measure objects of increasingly small size when you can’t see them?
Objective: It could be difficult to have a conception of the magnitude of measurements made on an atomic scale, as accomplished by an instrument such as the atomic force microscope (AFM) or scanning tunneling microscope (STM). The two-fold purpose of this activity is to 1) help with the idea of atomic-scale measurement and 2) explain the operation of an AFM instrument.
Introduction: As a demonstration of surface measurements, construct a probe by attaching a baseball to a meter-stick. Clamp the meter stick loosely to a ringstand so it can move vertically. Attach a recording device (pencil, marker, laser pointer) to the meter stick and arrange paper so a pattern can be marked on it. The recording device could be attached at several places along the meter stick. Place the ball on a rough surface like egg-carton foam. Move the foam so the surface features can be recorded.
Assignment: After showing the demonstration to students, give them the assignment of improving the ability to scan a surface by either a) recording it with more accurate probe, or b) recording the features of a surface with features about an order of magnitude finer than the egg carton. To complete the assignment, the students would need to present their original data and use them to obtain the three-dimensional features of the surface they have measured. Calculations should be performed, clearly based on the original data.
Equipment: Have meter sticks available for student use along with a wide variety of miscellaneous household and laboratory materials. Make it possible for them to use materials from home. Provide surface samples for students for which the 3D features are known.
Questions:
- Describe how size of the probe affects your ability to measure detailed features of a surface.
- In what ways must technology respond to measuring surfaces with finer details?
- Describe the characteristics of an instrument you would use to observe the surface of a cell or the atomic surface on a slab of graphite.
Follow-up: Discuss the working of the AFM on micro- or nano-scale materials. Include technological modifications, such as PZT, laser use, and negative feedback. Show images that have been generated using this technique.
A similar project has been developed and can be found on the internet at the following site:
We discussed the idea of putting this entire apparatus in a box except for the recorder paper. We could ask the students to guess at the identity of the object being measured.
Results from the example built during the workshop: Two different probes were used. One probe was a votive candle with a diameter of 35 mm. The other was a pencil eraser with a diameter of 6 mm. Both probes were able to measure the distance between objects separated by 50-60 mm. Perhaps the 35 mm probe would not succeed if the spacing of objects was smaller. The smaller probe was better at determining the depth of valleys between objects, underestimating depth by about 35%, while the pencil underestimated the depth by about 5%.