Demonstration pubs.acs.org/jchemeduc
Two Simple Classroom Demonstrations for Scanning Probe Microscopy Based on a Macroscopic Analogy Zdenka Hajkova,*,† Antonin Fejfar,‡ and Petr Smejkal† †
Department of Teaching and Didactics of Chemistry, Faculty of Science, Charles University in Prague, Hlavova 8/2030, 128 43 Prague 2, Czech Republic ‡ Department of Thin Films and Nanostructures, Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, 162 53 Prague 6, Czech Republic ABSTRACT: This article describes two simple classroom demonstrations that illustrate the principles of scanning probe microscopy (SPM) based on a macroscopic analogy. The analogy features the bumps in an egg carton to represent the atoms on a chemical surface and a probe that can be represented by a dwarf statue (illustrating an origin of the prefix “nano”), a plastic bottle, or a glass. The demonstrations allow students to visualize certain physical aspects of SPM, such as probing “hilly” atoms on a surface using changes in distance between probe and atoms. Both demonstrations are appropriate to introduce SPM into high school or undergraduate science education.
KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate/General, Demonstrations, Interdisciplinary/Multidisciplinary, Analogies/Transfer, Atomic Properties/Structure, Materials Science, Nanotechnology, Surface Science
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universities to accurately model the functions of SPMs.9−11 They were not intended to be used at the high school level or as a part of introductory university chemistry or physics courses. Apart from models mentioned above, very few visual aids for teaching about SPM have been published. Therefore, we developed two simple classroom demonstrations that illustrate the principles of SPM based on analogy, which are appropriate to introduce SPM into high school or undergraduate education.
anotechnology, nanoscience, and nanomaterials, everything with the prefix “nano”, are becoming popular because of the expected positive impacts on our lives. Nanotechnology and nanoscience are new interdisciplinary fields that encompass a wide range of tools, techniques, and applications that work in the so-called “nanoworld”. The size range that holds so much interest is ca. 1−100 nm, because it is in this range that materials can have different or enhanced properties compared with the same materials at a larger size.1 To explore the “nanoworld”, scientists need appropriate tools. Scanning probe microscopy (SPM) is an important technique used for studying the structure of matter at nanoscale level and for obtaining three-dimensional images of surfaces, even to atomic resolution. For this reason, the technique could be mentioned in high school, for example, in classroom discussions in chemistry or physics, science seminar, and so forth. Moreover, SPM can be integrated into undergraduate chemistry and physics courses. To date, there are only a few educational materials (other than texts, figures, and experiments) concerning SPM. Several interesting models of SPM have been published in this Journal2−5 and in others.6−13 Unfortunately, many of the resources cannot be used at a high school level because they are too complicated or expensive.7,13 There are some excellent teaching models of the AFM (atomic force microscope), for example, a coffee cup model,2 an inexpensive model that uses a modified phonograph stylus,6 LEGO models,3,4,8,12 and a wooden model.8 Some of the other macroscopic models of AFM were designed for advanced physics laboratories at © 2013 American Chemical Society and Division of Chemical Education, Inc.
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SCANNING PROBE MICROSCOPY Scanning probe microscopy (SPM) includes several related experimental techniques for forming three-dimensional images of surfaces using an extremely sharp tip of the probe that scans the sample. The best-known SPMs are scanning tunneling microscopy (STM)14 and atomic force microscopy (AFM),15 which were developed in the 1980s and have greatly affected the boom of nanotechnology. To generate an SPM image, the probe tip is mechanically moved step-by-step over the sample surface (explored in the first demonstration). When the tip moves close enough to the investigated sample, local interactions between the tip and the sample surface can be detected. The interaction can influence current flowing via a tunneling junction as in STM or it can influence the movement of the cantilever in AFM. The interaction between the tip and a sample surface is either directly measured or used for feedback control of the probe Published: January 22, 2013 361
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movement to keep the interaction constant. An image of the investigated surface is generated from the signal or the feedback recorded at each position and displayed on a computer screen. A schematic illustration of SPM is shown in Figure 1.
Figure 2. Demonstration 1 is a simple model of SPM: a dwarf statue representing the probe that “scans” the molded-pulp egg carton representing the investigated surface.
mechanical contact (as even children have experience with attraction or repulsion of permanent magnets at distance). The tunneling of electrons between the tip and the sample in the STM may not be obvious. For this reason, the second demonstration has been designed using light transmission through a layer of water colored with ink. The solution is nearly opaque but sufficient transmittance can be obtained locally by immersing a transparent probe. For the second demonstration, the following materials are needed: an overhead projector (“old-fashioned” projector with a bulb at the bottom), a transparent (plastic) egg carton (e.g., from Kinder Eggs), a glass water bath (aquarium), drawing ink (or black tea), a glass (with a flat bottom), agar or gelatin, and glass balls or small stones (to hold the egg carton at the bottom of the water bath). The demonstrator fills the bumps in the transparent egg carton with agar or transparent gelatin before the demonstration. At the beginning of the second demonstration, the egg carton (filled with agar or gelatin) is put into the glass water bath (aquarium), which is placed on the overhead projector that is switched on (Figure 3A). The bumps in the egg carton
Figure 1. A schematic illustration of a scanning probe microscope (SPM) connected to a computer.
The probe−surface interactions (STM measures electric current and AFM measures interatomic forces between the tip and the surface) strongly depend on the distance between the tip and the sample surface. If the tip is far away from the sample (more than ∼1 nm), it is not possible to measure the interaction and visualize atoms of the sample. If the tip is closer to the sample, it is more likely that it will be possible to obtain the image of individual atoms and map the surface (explored in the second demonstration).
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HAZARDS Performing the demonstrations does not require special safety precautions because no dangerous or harmful chemicals or substances are used. In the case of the second demonstration, it is recommended to use latex or nitrile gloves if you immerse your hands into the ink in the water. Care is required as the ink can stain clothes, table, and so forth. When agar or gelatin gels are prepared to fill in the bumps in the egg carton, a hotplate or a microwave oven is needed. Be aware of the sources of heat and use the equipment in accordance with the instructions of the manufacturer.
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TWO DEMONSTRATIONS FOR SPM Two inexpensive classroom demonstrations of a visual analogy for SPM are presented. These demonstrations can serve as working aids to visualize certain physical principles of SPM. Before the demonstrations or while they are being performed, some basics of SPM should be mentioned, at least in context of the description above. The first demonstration is a simple model of SPM and can be used to introduce SPM. The supplies for this demonstration are a molded-pulp egg carton that represents the surface of the investigated object and an object with a “tip” (e.g., a plastic bottle, a pencil) that represents the probe (Figure 2). A dwarf statue is used as the probe to illustrate the origin of the prefix “nano”: the Greek word νάνος for dwarf, is pronounced as nanos.1 The demonstrator picks up the object with a “tip” and moves the “tip” (e.g., dwarf hat or bottle lid) step-by-step above the egg carton. The dwarf hat and the egg carton are held a certain distance apart and the demonstrator is “copying” the surface of the egg carton with the dwarf statue only a few centimeters above it. It is easy for the audience to imagine the force acting between the tip and the sample even though they are not in
Figure 3. Demonstration 2: (A) the demonstration setup; (B) after the ink is added to the water, a particular atom is visualized via using a glass as a probe; and (C−H) moving the glass (probe tip) closer to the egg carton (sample surface) results in more transmitted light (electric current). 362
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Magnets: From Modeling of Nanoscale Characterization to Composite Fabrication. J. Chem. Educ. 1999, 76, 1205−1211. (4) Campbell, D. J.; Miller, J. D.; Bannon, S. J.; Obermaier, L. M. An Exploration of the Nanoworld with LEGO Bricks. J. Chem. Educ. 2011, 88, 602−606. (5) Lorenz, J. K.; Olson, J. A; Campbell, D. J.; Lisensky, G. C.; Ellis, A. B. A Refrigerator Magnet Analog of Scanning-Probe Microscopy. J. Chem. Educ. 1997, 74, 1032A−1032B. (6) Bonson, K.; Headrick, R. L.; Hammond, D.; Hamblin, M. Working Model of an Atomic Force Microscope. Am. J. Phys. 2011, 79, 189−192. (7) Bosma, E.; Offerhaus, H. L.; van der Veen, J. T.; Segerink, F. B.; van Wessel, I. M. Large Scale Scanning Probe Microscope: Making the Shear-Force Scanning Visible. Am. J. Phys. 2010, 78, 562−566. (8) Planinsic, G.; Lindell, A.; Remskar, M. Themes of Nanoscience for the Introductory Physics Course. Eur. J. Phys. 2009, 30, S17−S31. (9) Greczyło, T.; Debowska, E. The Macroscopic Model of an Atomic Force Microscope in the Students’ Laboratory. Eur. J. Phys. 2006, 27, 501−513. (10) Zypman, F. R.; Guerra-Vela, C. The Macroscopic Scanning Force ‘Microscope’. Eur. J. Phys. 2001, 22, 17−30. (11) Guerra-Vela, C.; Zypman, F. R. The Poor Man’s Scanning Force Microscope. Eur. J. Phys. 2002, 23, 145−53. (12) Planinsic, G.; Kovac, J. Nano Goes to School: A Teaching Model of the Atomic Force Microscope. Phys. Educ. 2008, 43, 37−45. (13) Gadia, V.; Patel, R.; Roy, S.; Singh, R.; Venkatesh, N.; Lunagaria, S.; Layton, B. E. An Educational Model of an Atomic Force Microscope. The Nanotechnology Group 2005, 4, 1−8. (14) Binnig, G.; Röhrer, H.; Gerber, C.; Weibel, E. Surface Studies by Scanning Tunneling Microscopy. Phys. Rev. Lett. 1982, 49, 57−61. (15) Binnig, G.; Quate, C. F.; Gerber, C. Atomic Force Microscope. Phys. Rev. Lett. 1986, 56, 930−933. (16) Atkins, P.; de Paula, J. The Intensities of Spectral Lines. Atkins’ Physical Chemistry, 7th ed.; Oxford University Press: New York, 2002; pp 491−494.
(that represent atoms) must be oriented upward. It is necessary to have no bubbles below the egg carton. Moreover, the demonstrator must ensure that the egg carton will not rise in the water by weighing it down with transparent glass balls or small stones. A few drops of drawing ink (or black tea) are then added to the water and the water is gently stirred. The water above the egg carton turns nearly opaque, whereas the agar or gelatin in the egg carton remains clear and transparent. Although little light passes through the opaque water, if the demonstrator immerses the bottom-down oriented glass (which represents the tip of the probe) in the opaque water (Figure 3B), when it is close enough to the egg carton, it makes the particular bump (“atom”) clearly visible in the overhead projected image (Figure 3H). The quantity of transmitted light depends on the thickness of the layer of colored water between the glass bottom and the egg carton bump according to the Beer−Lambert absorption law.16 The demonstrator can show how the quantity of transmitted light increases as the glass is lowered and the layer of colored water becomes thinner (Figure 3C−H). Light transmission increases exponentially as the thickness of the water layer decreases, as is true of tunneling current in real STM. As some skills are needed to carry out this demonstration, it is recommended to go through the whole procedure beforehand. Alternatively, inverted shot glasses can be used to demonstrate the atoms instead of the egg carton.
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CONCLUSION The “nanoworld” is an interesting domain of research. Therefore, high school students (at least students with an interest in chemistry and physics) and undergraduate chemistry and physics students need to be introduced to tools to explore the structure of matter at nanoscale level, especially with the scanning probe microscopy, an important up-and-coming technique in both academia and industry. To facilitate teaching and learning about SPM, two classroom demonstrations were described that are useful visual analogies.
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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 Financial support of the MSM0021620857 and LNSM projects awarded by the Ministry of Education, Youth, and Sport of the Czech Republic is gratefully acknowledged. We would also like to thank for the support of the OPPA project CZ.2.17/3.1.00/ 32121. Moreover, we express our thanks to Kevin Wehrly for suggestions on how to improve the language of this article.
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REFERENCES
(1) The Royal Society and The Royal Academy of Engineering. Nanoscience and Nanotechnologies: Opportunities and Uncertainties, 2004. Nanotechnology and Nanoscience. http://www.nanotec.org.uk/ finalReport.htm (accessed Jan 2013). (2) Ashkenaz, D. E.; Hall, W. P.; Haynes, C. L.; Hicks, E. M.; McFarland, A. D.; Sherry, L. J.; Stuart, D. A.; Wheeler, K. E.; Yonzon, C. R.; Zhao, J.; Godwi, H. A.; Van Duyne, R. P. Coffee Cup Atomic Force Microscopy. J. Chem. Educ. 2010, 87, 306−307. (3) Campbell, D. J.; Olson, J. A.; Calderon, C. E.; Doolan, P. W.; Mengelt, E. A.; Ellis, A. B.; Lisensky, G. C. Chemistry with Refrigerator 363
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