The Scanning Theremin Microscope: A Model Scanning Probe

Dec 20, 2013 - A model scanning probe microscope, designed using similar principles of operation to research instruments, is described. Proximity sens...
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The Scanning Theremin Microscope: A Model Scanning Probe Instrument for Hands-On Activities Rebecca C. Quardokus, Natalie A. Wasio, and S. Alex Kandel* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: A model scanning probe microscope, designed using similar principles of operation to research instruments, is described. Proximity sensing is done using a capacitance probe, and a mechanical linkage is used to scan this probe across surfaces. The signal is transduced as an audio tone using a heterodyne detection circuit analogous to that used in the theremin (one of the first electronic musical instruments, invented in the early 20th century). The instrument is useful for demonstrations and hands-on activities that introduce fundamentals of scanning probe microscopy and, by extension, nanoscience and nanotechnology. The details of instrument construction are provided, along with instructions for assembly and troubleshooting. KEYWORDS: General Public, Elementary/Middle School Science, High School/Introductory Chemistry, Demonstrations, Physical Chemistry, Hands-On Learning/Manipulatives, Laboratory Equipment/Apparatus, Nanotechnology, Surface Science

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original and is limited both by the quality of the imaging optics as well as the wavelength of the light (or the electron beam) used. In contrast to this approach, scanning probe microscopy shrinks the detector to microscopic size. In STM, the oldest of these techniques, the detector is a metal needle that is sharpened down to a single-atom tip (though in practice, only a single atomically sharp asperity is required, allowing tips to be fabricated by any of a number of methods, including simple mechanical cutting). This tip is then positioned nanometers away from a sample while an electron tunneling current between the tip and sample is measured. This inherently produces a highly localized interaction, which is the basis for the extreme resolutions possible. The probe is then scanned back and forth across the surface, building up an image one pixel (or one line of pixels) at a time. Related techniques take advantage of a variety of different tip−sample interactions, including physical force (atomic force microscopy), optical adsorption or fluorescence (near-field scanning optical microscopy), and electrical capacitance (scanning capacitance microscopy).

canning probe microscopies, which include scanning tunneling microscopy (STM), atomic force microscopy (AFM), and a variety of other techniques, have become essential tools in the characterization of materials at the nanometer scale.1,2 STM, developed in 1982 at the IBM Zurich Research Laboratory, provided the first experimental, real-space images of individual atoms. The capabilities of this instrument were soon applied to outstanding problems in the structure of surfaces and materials, and Gerd Binnig and Heinrich Rohrer shared the 1986 Nobel Prize in Physics as a result. Scanning probe microscopes operate in an entirely different fashion from conventional (light or electron) microscopes. In conventional microscopy, a small sample is examined using a series of lenses or reflectors that create a one-to-one correspondence between the object and a much larger image. A student looking through the eyepiece of a benchtop microscope, for instance, can see what is at the focal point of the instrument because light maps the features of the object, scaled up 50−1000 times, onto the receptors of his or her retina. The same principle applies in electron microscopy, with electric and magnetic fields used to create lenses with scaling factors of 106 or greater, and film, phosphors, CCD cameras, or electron multipliers used for detection. In either case, resolution depends on the fidelity with which the image reproduces the © 2013 American Chemical Society and Division of Chemical Education, Inc.

Published: December 20, 2013 246

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Figure 1. Capacitance sensor for the SThM. IC1, IC2: 74HC14 Schmitt trigger inverter. IC3: 74HC74 D-type flip-flop. IC4: 74HC4040 binary ripple counter. IC5: LM78L05 +5 V regulator. T1, T2: 2N3904 NPN transistor. D1: variable capacitance diode (BB135 or NTE610, or similar). Not shown: power pins on all ICs should be bypassed to ground with 0.1 μF ceramic capacitors.

of scanning probe microscopy to a broad audience, either in public demonstrations or through classroom activities at the primary- and secondary-school level. The literature provides some successful examples of this type of outreach.9,10 However, we decided upon a different approach and worked with scanning probe principles to create a large-scale scanning device, the scanning theremin microscope (SThM). The SThM uses a capacitance-based proximity probe, can be built for approximately $20 US, and provides a hands-on experience that parallels in many ways the experience of operating a research scanning probe microscope.

Scanning probe microscopes are widely used in research and industry. The field of nanotechnology is currently one of intense interest, and scanning probe techniques are primary tools for characterization at the nanometer scale. Scanning probe instruments can operate at unmatched resolutions and, depending on the technique, can yield information complementary to that obtained from traditional microscopy. Scanning probe measurements can often be made with little or no sample preparation; this can be a distinct advantage compared to other types of measurements. Atomic force microscopy, in particular, can be used to study an extremely broad range of samples, from single molecules on crystalline surfaces to biological structures imaged in situ. At the same time, the current state-of-the-art capabilities of research scanning probe instruments help advance fundamental investigations in chemistry, physics, and materials science, visualizing individual molecules and individual bonds within molecules.3,4 The close proximity of the scanning probe to the surface allows it to be used not only for visualization but also for manipulation. The earliest research in this area involved writing “IBM” with xenon atoms, with each letter measuring 5 nm high.5 Quite recently, researchers at IBM made international news with the release of the “world’s smallest movie”, in which a stick figure traced out with carbon monoxide molecules dances, bounces a ball, and jumps on a trampoline.6 In addition to these proof-of-concept results, single-atom and singlemolecule manipulation also allows for construction of artificial structures that then, through their static and dynamic behavior, reveal properties of the surface or the adsorbates.7 In a very real sense, atomic manipulation can be used to create nanometersized laboratory equipment. Relatively inexpensive (under $10 000 US) scanning probe instruments can be introduced into the science curriculum in undergraduate laboratories;8 however, even this price range limits their use to institutions of sufficient size and means. Our interest over the past few years has been to introduce the idea

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SCANNING THEREMIN MICROSCOPE The SThM uses a macroscopic capacitance probe that is scanned manually; demonstrations and experiments with it are literally hands-on in nature. The choice of capacitance as the experimental observable is a serendipitous one when taken in a historical context. Research into the use of capacitance sensors to detect physical proximity led Russian researcher Léon Theremin to the development in 1920 of a circuit that produced audio-range oscillations in response to the presence of nearby objects.11 Used deliberately to create musical tones, this device became one of the earliest electronic musical instruments: the theremin. A performer plays the theremin by moving his or her hands in proximity to a pair of antennae (one for pitch and one for volume). The instrument lends itself to vibrato and portamento, resulting in an eerie or otherworldly sound that has found use, among other places, in the sound tracks of the science-fiction movies of the 1950s (The Day the Earth Stood Still is one example)though today’s students might be more impressed to hear the theremin in tracks by Nine Inch Nails or to see it played by Sheldon Cooper on The Big Bang Theory. By using a circuit much like Theremin’s original, the SThM is also designed to produce an audio signal proportional to a measured capacitance. This also makes the SThM well suited to 247

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combinations. This can be a hands-on way to learn about electronic music and additive synthesis, as well as human auditory processing; most notably, that harmonics played in unison are perceived as a different tone quality, more so than a different pitch. An alternate tuning method is shown in Figure 2 that eliminates the sometimes difficult to find variable-capacitance

a grade-school or high-school classroom, as headphones and speakers are readily available, whereas oscilloscopes and computer-based data acquisition are not. The fact that using the SThM can make a good deal of noise does not lessen its appeal in a classroom setting.12,13

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DESIGN AND CONSTRUCTION Capacitance, by definition, is the ability of an object to store electric charge. This can be measured by delivering charge to the object at a fixed rate (a constant current) and measuring the resulting potential versus timea large capacitance describes a system that charges up slowly, whereas a small-capacitance system charges up quickly. When playing the theremin, the musician’s hand is incorporated into the electrical circuit, causing small changes in capacitance as it approaches or recedes from the pitch antenna. Linking the pitch antenna to an oscillator circuit results in a frequency for this oscillator that is dependent on hand position: capacitance is larger when a hand is close to the antenna, resulting in a longer charging time and a lower frequency. This is the variable oscillator, and its frequency is measured with respect to an unchanging local oscillator by mixing the two signals together to obtain a beat (heterodyne) signal. Circuits originally used for the theremin employed oscillators with frequencies in the range of tens to hundreds of kilohertz. Hand position would change the frequency of the variable oscillator by parts per hundred or parts per thousand, resulting in a beat frequency in the acoustic range, ∼10−1000 Hz.14 The circuit for the scanning theremin microscope presented here works under the same principle, and a circuit diagram is shown in Figure 1. The variable oscillator frequency depends on a fixed capacitor C as well as any additional capacitance ΔC between the probe and sample. Along with a resistor R, this capacitance, C′ = C + ΔC, results in a charge or discharge time RC′ at the input of inverter IC1. Because the inverter is wired in an unstable configuration (the output, which wants to be the inverse of the input, is connected to the input itself), a squarewave oscillation15 results with frequency 1.1 f≈ RC′ The local oscillator operates in a similar fashion, with the addition of a variable-capacitance diode that allows fine adjustment of its running frequency. Interference between the two oscillators is minimized by using separate ICs for each oscillator. In addition, instead of comparing the oscillators directly, the variable oscillator frequency is first divided by a factor of 2 using a D-type flip-flop (IC3). Combining these signals using the same flip-flop directly produces the difference frequency between the two oscillators. Greater sensitivity to small changes in capacitance is achieved by running the oscillators at high frequency, with the result that the beat frequency can easily go outside the range of human hearing. This is remedied by the use of an additional frequency divider (IC4). Several of the divider’s outputs are added together using resistors, which reduces the need for tweaking of the circuit and produces a more interesting sound. The signal is then put out to headphones or speakers using an emitter follower (T2) and a coupling capacitor. Depending on the exact setup and adjustment of the SThM, a signal spanning multiple octaves is achievable. Students building a theremin as a longer project may want to initially have this part of the circuit on a prototyping board in order to experiment with different

Figure 2. Alternate design to simplify the tuning circuit; the remainder of the circuit is as shown in Figure 1

diode by directly adding a control voltage to the fixed oscillator. Although this does allow for a large frequency adjustment range, the dependence of frequency on control voltage is no longer monotonic, making the device slightly more difficult to use. One possible construction is shown in Figure 3, which shows the entire instrument (panel A) and the probe and sensing

Figure 3. Construction of the SThM. (A) Assembled instrument mounted on a wood pantograph for scanning. The control box circuit, which includes power supply, frequency divider, tuner, and output, is shown inset. The signal is sent from a standard 1/8” audio jack to headphones. (B) Sensor board with antenna−probe (top).

circuit (panel B). The oscillator circuit and mixer ICs are soldered directly onto a small piece of undrilled double-sided copper-clad circuit board (Figure 3B); this is more flexible and performs better than an alternate (though still viable) approach using perforated prototyping board.16 In either case, the antenna is mounted directly onto this board. The antenna ends 248

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in a small piece of copper-clad board (ours is approximately 0.5 cm2), which can be mounted on a small piece of plastic for mechanical support (a circuit-board standoff or a ballpoint-pen cap) and wired to the variable oscillator. It is the theremin antenna that functions as the scanning probe of the instrument. The switch, LED, and voltage regulator are placed on a separate board, along with the potentiometer and frequency divider (Figure 3A inset); the dashed line that goes down the middle of Figure 1 shows how the circuit can be divided. This control box is housed with a 9 V battery in an Altoids tin. A short cable connects the control box to the oscillators and mixer. Audio output is through a 1/8” headphone jack on the control box. Scanning is performed by mounting the oscillator board and antenna onto a pantograph. A simple pantograph can be built using 1-by-2 boards (or even meter sticks) and screws with a suction cup for a fixed point and hinge to allow the whole assembly to angle up and down for height adjustment. The magnification of stylus motion relative to antenna motion can be adjusted using the pantograph, depending on the resolving power desired. The assembled SThM is pictured in Figure 3A.

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USE IN THE CLASSROOM In introducing the concept of scanning probe microscopy, one can first explain how bringing a probe very close allows for direct measurement of properties as well as the potential for very high resolution. The function of the probe can then be demonstrated, potentially by flipping the pantograph upsidedown so that the antenna can be seen easily. With the antenna away from any object, use the frequency-adjust potentiometer to lower the pitch of the output as low as possible; usually, this can be set below the threshold of hearing. Once the reference frequency has been set, any change in capacitance will increase the frequency difference between the variable and local oscillators, raising the pitch. Moving a finger around the antenna will demonstrate its sensitivitythe pitch can usually be sent screechingly high by touching the probe itself. The sudden noise (especially given that the circuit produces a jarring, electronic tone) can make the presentation suddenly much more interesting to an audience. This can also be a good point to digress briefly to discuss the theremin as one of the first electronic musical instruments, which can be supplemented with an audio or video recording.17 The simplest mode of scanning is to move the pantograph to scan the probe across the surface at a fixed height, listening for changes in pitch. This may be enough when the exercise is carried out by young students, but older students can use the pantograph to transcribe images by hand. As the pitch will vary by octaves depending on the surface−probe distance, it is not hard to decide on multiple distinct ranges and to assign a pencil, pen, or crayon color to each; for example, high tone is red, medium tone is blue, and low tone is black. The work can be split between two students, with one positioning the probe and the other coloring in a sheet of graph paper, square by square. An example of such a scan is shown in Figure 4. Samples for scanning can be constructed from a variety of materials. Play-Doh and modeling clay both have a high water content, and because water has a large dielectric constant, these materials produce a large response when probed. Metals can also be detected with high sensitivity, again because of their extremely large dielectric constants. Aluminum foil can be wrapped around plastic or wood blocks for structural support to make metal samples with easily controlled shape and size.

Figure 4. (A) A simple SThM scan detecting a hexagonal lattice made of 1” ball bearings. High, medium, and low pitches were assigned colors of red, blue, and black, respectively. Only a fraction of the ballbearing array is imaged, as it is scaled up by the 2:1 ratio of the pantograph. (B) Scan of a lattice defect created by replacing one steel bearing with a rubber bouncy ball; even though the ball is the same height, it appears as a depression (blue-black) in the lattice.

Paper, plastic, and wood alone produce much weaker signals. A sample made from mixed materials can illustrate the important idea that it is the composition of the surface, and not just its topography, that determines the final appearance of an image this is demonstrated in Figure 4B, which shows that an apparent depression is formed by replacing a steel ball by a rubber ball of exactly the same size. This is directly analogous to the techniques of STM (topography and electronic state density) or contact AFM (topography and stiffness). The SThM can also detect a high-signal object buried beneath lowsignal material. This is one of the real-world uses of scanning capacitance microscopy, as well as the principle of operation for some commercial “stud finders”. This capability could be demonstrated by scanning to find thumbtacks embedded in the backside of a Styrofoam sheet (where they can be sensed but not seen) or metal objects buried in sand. Imaging at a constant height creates a problem of dynamic range: because capacitance depends steeply on probe−sample distance, it can be difficult to find a height where the probe is sensitive to lower features while not crashing into higher ones. A more intricate approach for producing an image is to vary the tip height while imaging, in order to keep the pitch constant. Once again, one student can position the probe while the other records the height. Alternately, the height can be recorded by building up a model using Legos underneath the probe, which provides a more immediate (and permanent) result. Varying height to image at constant pitch also provides the best analogy to the common acquisition methods for real scanning probe measurements: STM images are produced at constant tunneling current, for example, and AFM images can be recorded at constant deflection. 249

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We have taken the SThM to Science Alive!, an annual event at the St. Joseph County Public Library in South Bend, IN, and to Science Spooktacular at the ETHOS Center in Elkhart, IN. The instrument stands up well to repeated handling by passers-by, including small children, who can scan over a few sample objects as an activity that complements a poster presentation of actual scanning probe images. We have also used the SThM in hands-on activities as part of the University of Notre Dame’s Upward Bound program. This was more of a classroom setting, and it worked well to have small groups exploring the instrument and its response to objects (plastic, metal, rubber, and foam) as a function of distance and material. After that, scanning was done by a single operator, who called out grid locations (A1, A2, B1, etc.) while students colored in paper grids at their desks. Using this approach, the object being scanned could be hidden from the students, and they then compared their images with it afterward. When students have a longer-term involvement with the SThM, whether as a class project, a science fair entry, or a chemistry club activity, additional subtleties of the instrument can be explored. The resolution of the SThM is limited both by the probe size and the size of the grid used for scanning; this can be determined relatively quickly by scanning repeatedly with different grid sizes and then investigated more thoroughly by changing the size of the capacitance probe. Because a smaller probe will decrease sensitivity, this may necessitate moving the probe much closer to the sample to compensate, which will work well for finely corrugated but flat surfaces but will create additional problems for samples with high-aspect-ratio features. In concluding this discussion, we discuss the broader insight that might result from the operation of the SThM. To wit, using the SThM helps illuminate the connection between a physical effect (the capacitance between the probe and a sample) and a more complex measurement (an image of the sample). This connection is often lost when working with physical instrumentation, as a piece of assembled scientific equipment appears to students (and professionals) as a black box. Although the circuit in the SThM may appear opaque, the other aspects of the measurementscanning the pantograph, determining the resulting pitch, and translating this to an appropriate colorare entirely open.

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REFERENCES

(1) Binnig, G.; Rohrer, H. Scanning tunneling microscopyfrom birth to adolescence. Rev.Mod. Phys. 1987, 59, 615−625. (2) Meyer, E.; Hug, H. J.; Bennewitz, R. Scanning probe microscopy: the lab on a tip; Springer: Berlin, 2003. (3) Gross, L.; Mohn, F.; Moll, N.; Schuler, B.; Criado, A.; Guitian, E.; Pena, D.; Gourdon, A.; Meyer, G. Bond-order discrimination by atomic force microscopy. Science 2012, 337, 1326−1329. (4) de Oteyza, D.; Gorman, P.; Chen, Y.-C.; Wickenburg, S.; Riss, A.; Mowbray, D.; Etkin, G.; Pedramrazi, Z.; Tsai, H.-Z.; Rubio, A.; Crommie, M.; Fischer, F. Direct imaging of covalent bond structure in single-molecule chemical reactions. Science 2013, 340, 1434−1437. (5) Eigler, D.; Schweizer, E. Positioning single atoms with a scanning tunneling microscope. Nature 1990, 344, 524−526. (6) IBM research: A boy and his atom. http://www.research.ibm. com/articles/madewithatoms.shtml (accessed Dec 2013). (7) Heinrich, A.; Lutz, C.; Gupta, J.; Eigler, D. Molecule cascades. Science 2002, 298, 1381−1387. (8) Zhong, C.; Han, L.; Maye, M.; Luo, A.; Kariuki, N.; Jones, W. Atomic scale imaging: A hands-on scanning probe microscopy laboratory for undergraduates. J. Chem. Educ. 2003, 80, 194−197. (9) Goss, V.; Brandt, S.; Lieberman, M. The Analog Atomic Force Microscope: Measuring, Modeling, and Graphing for Middle School. J. Chem. Educ. 2013, 90, 358−360. (10) Margel, H.; Eylon, B.; Scherz, Z. We actually saw atoms with our own eyes” - Conceptions and convictions in using the scanning tunneling microscope in junior high school. J. Chem. Educ. 2004, 81, 558−566. (11) Nesturkh, N. The theremin and its inventor in twentiethcentury Russia. Leonardo Music J. 1996, 6, 57−60. (12) Colwell, B. Me and my theremin. Computer 2003, 36, 8−9. (13) Kraftmakher, Y. Metal detection and the theremin in the classroom. Phys. Educ. 2005, 40, 167−171. (14) Moog, R. Build the EM theremin. Electronic Musician 1996, 86− 100. (15) CMOS oscillators (Application Note 118). Fairchild Semiconductor, San Jose, CA, 1974. (16) Hayward, W.; Hayward, R. The ugly weekender. QST 1981, 18−21. (17) Examples of theremin performance on YouTube. http://www. youtube.com/watch?v=w5qf9O6c20o; http://www.youtube.com/ watch?v=RSsstXfcRWw; http://www.youtube.com/watch?v= mW0B1sipLBI (all accessed Dec 2013).

ASSOCIATED CONTENT

S Supporting Information *

Building and troubleshooting instructions; electronics parts list. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*S. A. Kandel. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation under CHE-0848415, CHE-1124762, and DGE-0638723. The authors particularly acknowledge the help of Thomas Loughran, Patrick Mooney, and the Notre Dame Extended Research Community (NDeRC). 250

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