Inside the Black Box.

Byline: Yvonne S. Kao, Anthony Cina, and J. Aura Gimm Scientists often have to observe and study surfaces that are impossible or impractical to see directly, such as the ocean floor or the atomic surfaces of objects. Early in the history of oceanography scientists dropped weighted cables to the bottom of the ocean. By moving across the ocean at regular intervals and keeping track of how deep the cables went, the scientists produced a rough map of the ocean floor. Nowadays, scientists can calculate how deep the ocean is at any point by sending sound waves from a ship to the ocean floor and keeping track of how long it takes for the sound waves to return to the ship. The black box activity described in this article, created as part of the National Science Foundation-funded Internships in Public Science Education Program (IPSE) at the University of Wisconsin-Madison, introduces students to the idea of remote imaging and scanning probe microscopy (SPM). The activity can be incorporated into chemistry or other physical science classes to meet content standards on the structure and properties of matter, and technological design (NRC 1996, pp. 178 and 192). Students use a probe to collect data about a mystery landscape inside a "black box" and then use the data to build a marshmallow model of the contents of the box. This article is based on materials available on the "Exploring the Nanoworld" website (http://mrsec.wisc.edu/edetc/modules) under the heading "How can we see what we cannot see? " The activity was initially developed as part of the authors' Research Experience for Teachers (RET) program. SPM background SPM refers to a class of microscopes that use a probe to collect data about a sample's surface, much the same way that oceanographers have been able to collect data about the ocean floor. SPM was developed in the 1980s by Swiss scientists Gerd Binnig and Heinrich Rohrer-who were awarded the Nobel Prize in Physics in 1986 for their work-and is powerful enough to image individual atoms. [Editor's note: For an activity that allows students to construct a model SPM, refer to "Seeing the Unseen" on p. 58 in this issue. ] Scanning tunneling microscopy (STM) is a form of SPM that uses a probe that ends in a single-atom point. STM is used to image the surface of metals. The probe is typically made of tungsten or platinum and is positioned a few nanometers above the surface of the sample. Applying a small voltage to the gap between the probe and the sample causes a small current of electrons to flow through the gap. The strength of the current depends on the electron density at the probe tip-the electron cloud is denser near an atom's nucleus-and the amount of voltage applied to the tip. As the probe moves back and forth, a computer records the changes in voltage required to keep the electron flow constant. This data is used to assemble the final image (Bedrossian 2006). Atomic force microscopy (AFM), another form of SPM, can be used to image both metals and nonmetals. In AFM, the probe is again shaped like an extremely sharp needle that ends in few atoms. This probe is dragged across a material's surface and the tiny mechanical forces between the probe and the material are detected. These forces cause a cantilever to move up and down. These movements can be more closely monitored by reflecting a laser beam off the top of the cantilever into a photodetector that measures the displacements. The data is then transformed to create an image of what the material's surface looks like at the atomic level. Students' exploration of remote imaging with the black box (Figure 1) more closely resembles AFM than STM. Figure 1. Making the black box. Activity materials One black box per group of 2-3 students Thin sticks, such as barbecue skewers or straws Rulers (optional) Copies of grid or graph paper (2 sheets per 2-3 students) Markers or colored pencils (optional) Miniature marshmallows (be sure to take the marshmallows out of the bag and spread them out on waxed paper to allow them to dry for a day or two before the activity so they are harder and easier to use) or other small, uniform, cubic items to use as building blocks (e. …