Chemical Education Today
Reports from Other Journals
Research Advances by Angela G. King
Toward a New Generation of Paper-Thin Loudspeakers In research that may redefine ear buds, earphones, stereo loudspeakers, and other devices for producing sound, researchers in China are reporting development of flexible loudspeakers, made from transparent carbon nanotube (CNT) films, thinner than paper that might be inserted into the ears with an index finger or attached to clothing, walls, or windows. Kaili Jiang, Shoushan Fan, and colleagues note that most of today’s loudspeakers are relatively bulky, complex, and inflexible, consisting of a permanent magnet fixed to a voice coil and a cone. To meet the growing demand for smaller speakers for portable digital consumer electronics devices, manufacturers need new technology, they say. The scientists describe the development of super-thin CNT films—1/1,000th the width of a single human hair—that are capable of transmitting music and other sounds. In laboratory tests, the researchers mounted a thin CNT film onto two electrodes to form a simple loudspeaker (Figure 1). When fed by sound frequency electric currents, the speaker produced loud sound with the same excellent quality as conventional loudspeakers, but without magnets and moving components. The researchers attribute this phenomenon to a thermoacoustic effect, due to the very small heat capacity/area of CNT film. This generates a wide frequency response range and a high sound pressure level with low total harmonic distortion. They also demonstrated that the flexible film could be used just as effectively to play music from an iPod and while pasted to a flexible, waving flag (to view the video, follow the directions in More Information or the links in
the Supporting JCE Online Material). “These CNT thin film loudspeakers are transparent, flexible, and stretchable, can be tailored into many shapes and mounted on a variety of insulating surfaces, such as room walls, ceilings, pillars, windows, flags, and clothes without limitations. Furthermore, CNT thin films can also be made into small area devices, such as earphones and buzzers. There is no doubt that more and more applications will be developed as time goes on. This technique might open new applications of and approaches to manufacturing loudspeakers and other acoustic devices.” More Information 1. Xiao, Lin; Chen, Zhuo; Feng, Chen; Liu, Liang; Bai, ZaiQiao; Wang, Yang; Qian, Li; Zhang, Yuying; Li, Qunqing; Jiang, Kaili; Fan, Shoushan. Flexible, Stretchable, Transparent Carbon Nanotube Thin Film Loudspeakers. Nano Lett. 2008, 8, 4539–4545. 2. The ACS Publications Web site at http://pubs.acs.org (accessed Apr 2009) has supporting information available (without charge) that includes QuickTime videos of CNT loudspeakers playing music while being stretched, CNT speakers playing a movie soundtrack via iPod, and CNT speakers pasted onto a flag. Simply look up the original paper (referenced above) and click on Supporting Info. 3. This Journal has previously published articles about the inte gration of nanotechnology into the chemistry curriculum including the development of a nanomaterials course, preparation of nanofiber composite films, and nanoscale patterning. See J. Chem. Educ. 2008, 85, 1406; 2008, 85, 1105; and 2007, 84, 1795. [See also pp 704A–725 of this Journal issue for additional nano-related articles.]
Figure 1. Carbon nanotube thin-film loudspeakers. (a) The CNT thin film was pulled out from a superaligned CNT array grown on a 4 in. silicon wafer and put on two electrodes of a frame to make a loudspeaker. (b) SEM image of CNT thin film showing that CNTs are aligned in the drawing direction. (c) A4 paper size CNT thinfilm loudspeaker. (d) The cylindrical cage shape CNT thin-film loudspeaker can emit sounds to all directions; diameter is 9 cm, height 8.5 cm. Reprinted with permission from Nano Lett. 2008, 8, 4539–4545. Copyright 2008 American Chemical Society.
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Temperature Affects Hydrophobicity of SWNTs A fresh discovery about the way water behaves inside carbon nanotubes could have implications in fields ranging from the function of ultra-tiny high-tech devices to scientists’ understanding of biological processes, according to researchers from the University of North Carolina at Chapel Hill. The findings relate to a property of so-called “nano-confined” water—specifically, whether hollow carbon nanotubes take in the liquid easily or reluctantly, depending on their temperature. As well as shedding light on the characteristics of humanmade nanomaterials, researchers note that such properties are relevant to the workings of biological structures and phenomena which also function at nano-scales. The team of scientists, led by Yue Wu, professor of physics in the UNC College of Arts and Sciences, examined single walled carbon nanotubes (SWNTs) measuring just 1.4 nm in diameter. The seamless cylinders were made from rolled up graphene sheets, the exfoliated layer of graphite, by laser ablation. “Normally, graphene is hydrophobic, or ‘water hating’—it repels water in the same way that drops of dew will roll off a lotus leaf,” said Wu. “But we found that in the extremely limited space inside these tubes, the structure of water changes, and that it’s possible to change the relationship between the graphene and the liquid to hydrophilic or ‘water-liking’.” The UNC team did this by making the tubes colder. The research team used 1H nuclear magnetic resonance (NMR) to observe water adsorption isotherms in SWNTs. Since no bulk water is condensed outside the SWNTs below the saturated vapor pressure (Po), the 1H signal is associated with water adsorbed inside the nanotubes. The NMR experiments allowed Wu’s lab to control the relative pressure (P/Po) and observe its effect on water adsorption. They also studied the impact of temperature on water adsorption. The scientists found that at 22 °C, the interiors of carbon nanotubes take in water only reluctantly. However, when the tubes were cooled to 8 °C, water easily went inside (Figure 2). Wu said this shows that it is possible for water in nano-confined regions—either human-made or natural—to take on different structures and properties depending on the size of the confined region and the temperature. In terms of potential practical applications, Wu suggested that further research along these lines could impact the design of high-tech devices (for example, nano-fluidic chips that act as microscopic laboratories) and microporous sorbent materials such as activated carbon used in water filters, gas masks, and permeable membranes. “It may be that by exploiting this hydrophobic–hydrophilic transition, it might be possible to use changes in temperature as a kind of ‘on–off ’ switch, changing the stickiness of water through nano-channels, and controlling fluid flow.” Wu also noted that this research relates to scientists’ understanding of the workings of many building blocks of life (such as proteins, whose structures also have nano-confined hydrophobic regions) and how their interaction with water plays a role in how they function. For example, such interactions play an important role in protein folding, which determines a protein’s eventual
Figure 2. Water adsorption isotherms. (A) Three isotherms at 8.0 °C (squares), 18.4 °C (triangles), and 22.1°C (circles) are shown. The lines are guides to the eye. The vertical error bars are shown when they are larger than the size of the symbols and the pressure uncertainty is less than 1% of P0. (B) An illustration of monolayer water in SWNTs with a diameter of 1.36 nm. (C) A logarithmic plot of water content versus [log10(P0/P)]2 for the isotherm at 8.0 °C. Reprinted from Science 2008, 322, 80–83. Copyright 2008 AAAS.
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Reports from Other Journals shape and characteristics. Misfolded proteins are believed to be a cause of several neurodegenerative and other diseases. “We don’t fully understand the mechanisms behind protein unfolding upon cooling,” Wu said. “Could this kind of coolinginduced hydrophobic–hydrophilic transition play a role? We don’t know but it’s worth investigating.” More Information 1. Wang, Hai-Jing; Xi, Xue-Kui; Kleinhammes, Alfred; Wu, Yue. Temperature-Induced Hydrophobic–Hydrophilic Transition Observed by Water Adsorption. Science 2008, 322, 80–83. 2. More details on research in Wu’s lab can be found online at http://www.physics.unc.edu/wugroup/index.php (accessed Mar 2009). 3. This Journal has published articles on making carbon nanotubes in an educational lab and using them in a demonstration. See J. Chem. Educ. 2002, 79, 203 and 2006, 83, 1511, respectively. 4. Research Advances has covered additional investigations of carbon nanotubes. See J. Chem. Educ. 2005, 82, 666 and 2005, 82, 1754. 5. http://mrsec.wisc.edu/Edetc/modules/index.html (accessed Mar 2009) has a wealth of information for teachers trying to incorporate nanotechnology into their curriculum. 6. Martin Chaplin maintains an excellent online site on water science and research at http://www.lsbu.ac.uk/water/ (accessed Mar 2009).
“Silver Nanoparticle” Microscope May Shed New Light on Cancer, Bone Diseases
photo by John Lupton, University of Utah
In a finding that could help speed the understanding of diseases ranging from cancer to osteoporosis, researchers in Utah report the development of a new microscope technique that uses “silver nanoparticle” mirrors to reveal hidden details inside bones, cancer cells, and other biological structures (Figure 3). The method also can help identify structural damage in a wide variety of materials, including carbon–fiber plastics used in airplanes, the researchers say.
Figure 3. A silver nanoparticle mirror shown next to a penny for scale. The new mirror could be used in microscopes to reveal hidden details inside bones, cancer cells, and other structures.
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In the new study, John Lupton, Michael Bartl, and colleagues point out that one of the most powerful, widely used tools for imaging hidden biological structures is fluorescence microscopy, which requires the specimen be treated with fluorescent dyes or stains. But the dyes used to visualize the structures can kill living cells, limiting the effectiveness of the technique. A further complication is that many properties are controlled by the bulk arrangement of matter, which is best probed by transmission, not fluorescence. The University of Utah scientists addressed these issues by using an infrared laser to excite clusters of silver nanoparticles, each about 1/5000th the width of a human hair, placed below the material being studied. The particles focus intense beams of light up through the sample to reveal information about the composition and structure of the substance examined. In laboratory studies, they used the new technique to view the iridescent green exoskeleton scales of Lamprocyphus Augustus. The diminutive beetle was selected as a model to show how the need for facile high-resolution transmission microscopy crosses barriers between biology, physics, and materials science. L. Augustus, the so-called “photonic beetle”, was recently shown to possess near-perfect overlap of photonic stop bands formed in a diamond-based photonic crystal structure, resulting in iridescent coloring (Figure 4). Three dominant crystal domains of the diamond-based structure are oriented with their Γ-W, Γ-K, and Γ-X crystal axes normal or close to normal to the scale top surface. To resolve the different crystal domains, spatial resolution on the order of the diffraction limit of light is
Figure 4. Biological photonic crystals based on a cuticular exoskeleton diamond-structured lattice. (a) Photograph of the weevil Lamprocyphus Augustus. (b) Optical micrograph of iridescent beetle scales, recorded in reflection mode. (c) A typical bulk transmission spectrum of a single beetle scale exhibiting a broad composite photonic crystal stop band between 520 and 620 nm. The inset shows the white-light transmission image of a single scale. (d) SEM image of the top view of a single beetle scale, showing three distinct crystal facets. Reprinted with permission from Nano Lett. 2009, 9, 952–956. Copyright 2009 American Chemical Society.
Journal of Chemical Education • Vol. 86 No. 6 June 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
Chemical Education Today
needed. While it is possible to achieve this resolution with nearfield scanning microscopy, the soft, thick, and inhomogenous sample would make it a challenge to couple in light. The Utah scientists placed a beetle scale on a silver nanoparticle film, prepared by Tollens silver mirror reaction. The beetle scale adhered to the film due to strong van der Waals interactions. The team used a home-built wide-field fluorescence microscopy setup that allowed three modes of imaging: one-photon (440 nm) excitation, two-photon (920 nm) excitation, and broadband emission from the nanoparticle hotspots imaged through the beetle scale (Figure 5). The team of researchers’ new technique utilizes concepts from conventional multiphoton fluorescence with aperture microscopy. The technique can image absorption or scattering of light in volumes of subcellular cross sections. Their results may provide clues to designing new, more powerful solar cells and computer chips, the scientists say.
Figure 5. Silver nanoparticle hot spot white-light transmission microscopy of biological photonic cr ystals. (a) SEM micrograph of a beetle scale o n a To l l e n s s i l v e r S E R S substrate. (b) Fluorescence image of a beetle scale at 440 nm excitation. (c) Rayleigh scattering of 920 nm light from the same beetle scale. (d) Twophoton white-light beacons shining through the beetle scale, revealing the same overall spatial structure of the scale. Reprinted with permission from Nano Lett. 2009, 9, 952–956. Copyright 2009 American Chemical Society.
More Information
Supporting JCE Online Material
1. Chaudhuri, Debansu; Galusha, Jeremy W.; Walter, Manfred J.; Borys, Nicholas J.; Bartl, Michael H.; Lupton, John M. Toward Subdiffraction Transmission Microscopy of Diffuse Materials with Silver Nanoparticle White-Light Beacons. Nano Lett. 2009, 9, 952–956. 2. More information on research in the labs of Bartl and Lupton can be found online at http://www.chem.utah.edu/faculty/ bartl/webpage/home.html and http://www.physics.utah.edu/~lupton/, respectively (both sites accessed Mar 2009). 3. This Journal has previously reported the synthesis of silver nanoparticles in teaching labs. See J. Chem. Educ. 2007, 84, 322.
http://www.jce.divched.org/Journal/Issues/2009/Jun/abs670.html Abstract and keywords Full text (PDF) with links to cited URLs and JCE articles Supplement: QuickTime movie of CNT speakers pasted onto a flag
Angela G. King is Senior Lecturer in Chemistry at Wake Forest University, P.O. Box 7486, Winston-Salem, NC 27109;
[email protected].
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