Activity pubs.acs.org/jchemeduc
A Photocell Scanning Probe Microscope Model Maynard J. Morin* Science Department, Hingham High School, Hingham, Massachusetts 02043, United States S Supporting Information *
ABSTRACT: Concepts underlying scanning probe microscopy imaging are now part of high school curricula. This inexpensive hands-on activity allows students to experience probe microscopy data generation and analysis using plastic models of atomic surfaces. Furthermore, the apparatus can be easily modified by students to extend the original activity or create new ones.
KEYWORDS: High School/Introductory Chemistry, Laboratory Instruction, Analogies/Transfer, Computer-Based Learning, Atomic Spectroscopy, Laboratory Equipment/Apparatus
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entire measurement process provides a concrete basis for discussion of both systematic and random errors.
xperimental aspects of scanning probe microscopy (SPM) have been introduced to increasingly younger students, and even middle school students have learned to visualize atoms through SPM.1−6 What about students in schools with no access to such technology? Analogue model systems can provide a bridge. Activities based on simple plastic model atoms make it easier for students to interpret probe microscopy visualizations of real atoms. One hands-on activity previously described in this Journal uses a simple refrigerator magnet to mimic atomic force microscopy.7 The activity described here uses a simple scanning tunneling microscope model, made from readily available components, with a photocell circuit constructed after Gordon et al.8 A CdS photocell placed above plastic model atomic surfaces detects light from a light emitting diode (LED) placed below. (Light corresponds to tunneling current.) The CdS photocell, as part of a voltage divider, makes output voltage vary according to amount of light reaching it. More light reaches the CdS photocell when the model surface is close, which increases measured voltage. Voltage can thus be used directly as a measure of height. Voltage (z) measured over an x, y grid of points spaced 1 cm apart generates a table of (x, y, z) triplets. These triplets are analyzed by computer to produce threedimensional graphs using freeware such as QuikGrid9 or Gwyddion.10 QuikGrid can save data in VRML format for use with freeware such as MYRIAD Free Reader.11 Students without computer access can tabulate data in a 20 × 20 grid and color each square according to a voltage range to create a twodimensional data map. The apparatus can be easily and safely modified by students for additional open-ended experiments. Consideration of the © XXXX American Chemical Society and Division of Chemical Education, Inc.
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CONSTRUCTING THE MODEL The apparatus consists of a mechanical framework, two circuits, plastic “atoms”, a box, and a voltmeter. Construction details are provided in the Supporting Information. Essentially, a simple slider moves over a grid made from two meter sticks, one clamped to a bench (x-direction) and the other glued to an aluminum rail (y-direction). Students move the slider over this grid and collect voltage readings. This corresponds to measuring height as a function of position. Mechanical Framework
A wooden slider (Figure 1) holds an LED, a CdS detector, and associated electronics. The slider sits on top of an aluminum rail, an arrangement that allows the slider to move freely in the y-direction. The aluminum rail in turn is placed on top of two meter sticks clamped to a lab bench (Figure 2). This allows the aluminum rail to move freely in the x-direction. Coordinates are read directly from one clamped meter stick (x-direction) and another meter stick mounted on the aluminum rail (y-direction). The whole apparatus is shown in Figure 3. Electronics
Two circuits are necessary and are described in Gordon et al.8 First is a voltage divider circuit powered by a 9 V battery. It is made from a CdS photocell (Radio Shack 2761657) and a 1 kΩ resistor (RS 2711321). Voltage is measured across the 1 kΩ resistor. Second is a 10 kΩ micropotentiometer (RS 2710282)
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10 s average readings automatically. Good results can, however, be obtained in a shorter time by taking single readings.
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DATA ANALYSIS Import data into QuikGrid as metric data points to produce a two-dimensional data map (Figure 4) or a 3D surface (Figure 5).
Figure 1. Slider with LED, CdS detector, and aluminum rail.
Figure 4. QuikGrid 2D plot from paint palette target.
Figure 2. Aluminum rail on top of two meter sticks.
Figure 5. Three-dimensional view of painter’s palette from QuikGrid.
Although QuikGrid allows panning and zooming, exporting a VRML file for MYRIAD Free Reader allows greater flexibility in manipulation of the 3D graph. Figure 6 shows two measurements made by MYRIAD Free Reader on an inverted plastic painter’s palette, which compare favorably to actual measurements of 8 cm for the central feature and 3 cm for the surrounding wells. MYRIAD Free Reader also allows zooming and panning so that one gets a fly through over the “atoms”. Gwyddion, an open-source visualization tool for scanning probe microscopy, can read x, y, z data files and can correct data in several ways, such as leveling data and correcting horizontal rotation. It can also produce three-dimensional graphs that can be rotated, scaled, and sized. Sample output is shown in Figure 7. Height units shown here are arbitrary but can be scaled accurately. Alternatively, students can color squares according to a voltage range (e.g., less than 10 red, 11−15 orange, etc.) and
Figure 3. Model SPM apparatus showing slider on aluminum rail.
in series with a 9 V battery to power an LED light. Voltage is adjusted to match that recommended for the LED. Model Atom Targets and Support
Model atom targets that worked include egg-crate foam, an inverted ice-cube tray, an inverted plastic painter’s palette, and plastic eggs glued to jewel case covers. Targets attached to cardboard rest on wooden blocks inside a cardboard box.
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USING THE MODEL TO OBTAIN MEASUREMENTS Voltage measurements are made by systematically moving the slider to different positions on, say, a 20 cm × 20 cm grid. A computer interface and software (e.g., Vernier’s Logger Pro) make data collection much easier. Vernier’s software can take B
dx.doi.org/10.1021/ed300546q | J. Chem. Educ. XXXX, XXX, XXX−XXX
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cube tray, and a plastic painter’s palette. In all cases satisfactory results were obtained. A typical run (Figure 6) shows “atomic” patterns and relative sizes clearly. Smaller atoms can be distinguished from larger ones. Measurements made by MYRIAD Free Reader are close to actual diameters measured by ruler (3.0 cm, 8.0 cm). The students’ results have been less than ideal because of room safety lights that cannot be shut off. Typically, this results in graphs with high edges (see the Supporting Information). Nonetheless, students were better able to appreciate scanning probe images from their textbook.
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EXTENSIONS OF THIS ACTIVITY Students can change several factors, such as color and size of LED light, size of CdS photocell, color of plastic eggs (or alternative target), distance between LED light and CdS photocell, size of x,y-grid steps, and can investigate repeatability of results, circuit drift over time, and so on. Students can devise their own x−y mechanical system, try new targets, or find a novel use for the circuits. For example, once the circuits have been built, it is easy to make colorimeters.
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CONCLUSION This low-cost photocell model of a scanning probe microscope is useful to generate data that can then be visualized, analyzed, and manipulated by computer. Concepts introduced serve as a very accessible bridge to introduce students to real scanning probe data and analysis. The model can be easily modified by students to do their own experiments.
Figure 6. Measurement using Myriad Free Reader.
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ASSOCIATED CONTENT
S Supporting Information *
Construction details; student handout; teacher notes; sample data; data templates. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS I would like to thank Janice Hall-Tomasik and Andrew Greenberg for initial discussions about this project. A prototype of this photocell project was supported by the Institute for Chemical Education and the Nanoscale Science and Engineering Center at the University of WisconsinMadison. This material is based upon work supported by the National Science Foundation under grant number DMR-0425880.
Figure 7. Gwyddion output of (A) painter’s palette, (B) egg crate foam, and (C) half ice-cube tray.
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create a two-dimensional data map. (See Table 1 in the Supporting Information.)
REFERENCES
(1) Glaunsinger, W. S.; Ramakrishna, B. L.; Garcia, A. A.; Pizziconi, V. J. Chem. Educ. 1997, 74, 310−311. (2) Rapp, C. S. J. Chem. Educ. 1997, 74, 1087−1089. (3) Zhong, C. J.; Han, L.; Maye, M. M.; Luo, J.; Kariuki, N. N.; Jones, W. E., Jr. J. Chem. Educ. 2003, 80, 194−197. (4) Aumann, K.; Muyskens, K. J. C.; Sinniah, K. J. Chem. Educ. 2003, 80, 187−193. (5) Lehmpuhl, D. W. J. Chem. Educ. 2003, 80, 478−479. (6) Margel, H.; Eylon, B.-S.; Schers, Z. J. Chem. Educ. 2004, 81, 558− 566.
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RESULTS AND DISCUSSION More light reaches the CdS photocell when the model surface is close, thus increasing measured voltage. Voltage can thus be used directly as a measure of height. Satisfactory results were obtained without corrections, such as for dark current. Four different targets were tested: plastic eggs glued to jewel case covers, thin (about 0.5−2 cm range) egg-crate foam, an iceC
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(7) Lorenz, J. K.; Olson, J. A.; Campbell, D. J.; Lisensky, G. C.; Ellis, A. B. J. Chem. Educ. 1997, 74, 1032. (8) Gordon, J.; James, A.; Harman, S.; Weiss, K. J. Chem. Educ. 2002, 79, 1005−1006. (9) John Coulthard. QuikGrid. http://www.galiander.ca/quikgrid (accessed Sep 2013). (10) Gwyddion from http://www.sourceforge.com/ (accessed Sep 2013). (Search for Gwyddion.) (11) Informative Graphics Corporation, Myriad Free Reader. http:// www.myriadviewer.com/myriadreader > Products (accessed Sep 2013).
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