Coffee Cup Atomic Force Microscopy
David E. Ashkenaz, W. Paige Hall, Christy L. Haynes, Erin M. Hicks, Adam D. McFarland, Leif J. Sherry, Douglas A. Stuart,Korin E. Wheeler, Chanda R. Yonzon, Jing Zhao, Hilary A. Godwin, and Richard P. Van Duyne*
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois60208-3113
Supplemental Information
Ordering Information: Sources for laser pointers & cover slips
CompanyUNITCat. #DescriptionPrice
VWR1 oz12-540A18mm #2 square$17.39
cover glass
Fisher1 eaS686111.5V red laser$30.01
Model AFM Results from a High School Science Class
Class population / Correct h measurement (#-%) / Correct construction (#-%)n = 20 / 18 – 90% / 19 – 95%
n = 24 / 19 – 79% / 21 – 88%
Totals / 37 – 84% / 40 – 91%
Figure 1. This figure represents a summaryof measurements made by a class of 18 high school chemistry students in July, 2008.These three objects shows agreement between measured and actual object heights.
Answers to Questions
1.If multiple scans are taken across a sample, it is possible to create many line profiles of the surface. By knowing where each scan starts, the coordinates can be used to assemble a three-dimensional map of the surface.
2.The geometry of the cantilever tip influences the spatial resolution of the AFM. For example, if there is a small hole in a surface, a round, blunt tip would not resolve the hole well because there is very little change in the cantilever deflection as the hole is crossed. However, if a very sharp tip is used, a much greater change in the cantilever deflection will be measured because the tip will fall into the hole. The reduction of image resolution due to the tip shape is called tip convolution. Very sharp (10-30 nm) AFM cantilevers are manufactured with the same technology used to create the small features in microchip circuits.
3.Magnetic nanoscale features can be imaged using a magnetized tip. Instead of the cantilever being deflected by physical contact with the surface, tip attraction and repulsion with the magnetic particles causes the cantilever to move. This is known as magnetic force microscopy. Chemical forces such as electrostatics and friction can also be used in a technique known as chemical force microscopy. In these techniques, the cantilever is scanned in a side-to-side fashion. Attractive forces between the tip and the surface cause the cantilever to twist, thus resulting in a lateral deflection of the laser spot.
4.Magnification. The AFM uses magnification produced by laser deflection to visualize structures.The laser is deflected to a photomultiplier/position detector. This measurement calibrates the AFM, by allowing a magnification factor to be calculated by dividing the difference in distance between the laser spots by the actual object size.
Additional Activities
The instructor may wish to have the students creatively modify the coffee cup AFM so that they can make quantitative topographical measurements. The students can then use their modified AFM to create an image of a classroom object. Different AFM models may be referred to as specific ideas arise concerning various AFM techniques (reference 3, main text).
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