news & views the competing states and allows the superconductivity to become established. Of course, such an interpretation does not straightforwardly explain the occurrence of superconductivity at room temperature. Still, one cannot exclude the possibility that the transition temperatures of the cuprates would in general be even higher 10 except for the inopportune occurrence of their many competing phases. An intriguing possibility reflecting scenario 3 is that the effect of driving the phonon actually dynamically stabilizes the superconductivity. Dynamic stabilization is a surprising but well known phenomenon whereby, under rapid periodic changes of its parameters, an oscillator can be stabilized in a configuration in which it might be otherwise unstable. A classic example of such a parametric oscillator is Kapitza’s pendulum11, in which a rigid pendulum can be stabilized with the mass above the pivot point (Fig. 2). Of course, under ordinary conditions such a configuration is not stable; the weight must hang below the pivot. But if the pivot of the pendulum is made to rapidly oscillate vertically, the behaviour of the pendulum changes dramatically. Within a certain range of driving motion, the inverted equilibrium can be stabilized. Although seemingly exotic, parametric oscillation is actually quite a common phenomenon: a child propelling themselves by shifting their centre of gravity up and down on a play swing is a more prosaic mechanical example. The optical parametric amplifier in which the intense mid-infrared pulses are generated in the present experiment are a related photonic realization of this phenomenon. How dynamic stabilization

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Figure 2 | A Kapitza pendulum. This can be constructed with a motor rotating a crank at a high speed. The crank drives a lever arm up and down on which the pendulum is attached to with a pivot. For a certain range of driving frequencies, the pendulum is stabilized in the ‘upside down’ configuration. θ is the angle of oscillation and a is the amplitude of vertical movement.

would work precisely in the present case is unclear, but there is the possibility that it could reduce the thermal fluctuations associated with the c-axis Josephson plasmon and the resulting degradation in the superfluid phase stiffness. The work of Hu et al. brings up some intriguing possibilities, and future work will hopefully reveal more details about this interesting photoexcited state. It is important to understand for instance the pathways that the energy (~4 mJ per cm2

per pulse) deposited in the system takes. Time-resolved photoelectron spectroscopy could be useful here. With regard to scenario 2, further structural and scattering studies on the photoexcited state would help to establish if fluctuations of a competing state are reduced by photoexcitation. It is important to keep in mind that even if superconductivity has been created at high temperatures, it is only of a transient nature. The most exciting possibility would, of course, be that superconducting correlations (or even fully fledged superconductivity) might be stabilized at room temperature in a continuous fashion with a non-pulsed optical system. For now this possibility still seems rather distant, but it’s a dream that will keep physicists going for a long while yet. ❐ N. Peter Armitage is at the Institute for Quantum Matter, Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, Maryland 21218, USA. e-mail: [email protected] References 1. Schilling, A., Cantoni, M., Guo, J. D. & Ott, H. R. Nature 363, 56–58 (1993). 2. Hu, W. et al. Nature Mater. 13, 705–711 (2014). 3. Lindner, N. H., Refael, G. & Galitski, V. Nature Phys. 7, 490–495 (2011). 4. Wang, Y. H., Steinberg, H., Jarillo-Herrero, P. & Gedik, N. Science 342, 453–457 (2013). 5. Averitt, R. D. & Taylor, A. J. J. Phys. Condens. Matter 14, R1357–R1390 (2002). 6. Tranquada, J. M. et al. Nature 375, 561–563 (1995). 7. Chang, J. et al. Nature Phys. 8, 871–876 (2012). 8. Fausti, D. et al. Science 331, 189–191 (2011). 9. Li, L. et al. Phys. Rev. B 81, 054510 (2010). 10. Lee, P. A. & Read, N. Phys. Rev. Lett. 58, 2691–2694 (1987). 11. Kapitza, P. L. Zh. Eksp. Teor. Fiz. 21, 588 (1951).

Published online: 11 May 2014

NANOSCALE FRICTION

Sliding charges

In contrast to the ultralow friction that exists between carbon layers in multiwalled carbon nanotubes, multiwalled boron nitride nanotubes are found to exhibit ultrahigh interlayer friction as a result of their ionic character.

Robert Szoszkiewicz and Elisa Riedo

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rom the sliding of tectonic plates to the crawling of biological cells and the wear of orthopaedic metal implants, friction affects motion at any length scale1. Typically, friction is controlled by atomicscale surface protrusions that experience sub-terapascal pressure surges as they slide past each other within microseconds or less2. Ad hoc atomically flat surfaces — which can be produced by advanced engineering — placed in sliding contact

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could instead reduce friction and last longer, and ultimately lead to efficient miniaturized devices for microactuation, data storage and flexible electronics, for example. However, little is known about the physical laws that govern friction and energy dissipation between atomically flat surfaces. Reporting in Nature Materials, Niguès and co-workers demonstrate that, unexpectedly, nanotubes made of atomically flat boron nitride

surfaces sliding past each other undergo ultrahigh dissipation3. Niguès and co-authors attached the outer tubes of individual multiwalled nanotubes to a nanomanipulator on one side and to a sensitive tuning-fork force sensor on the other (Fig. 1) to measure the forces necessary both to tear apart the outer tube and to slide the resulting two pieces against the inner tubes. Driven by an external actuator glued to the tuning fork, both the tuning fork and

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Stiff tungsten tips

Nanotube glued via electron beam to tungsten tips

Precise x–y–z nanomanipulator

Tuning fork

Shifts and shape changes are measured Amplitude

the nanotube attached to it were made to vibrate with extremely small amplitudes (of the order of a few nanometres). Because the tuning fork behaves as a damped harmonic oscillator, changes in the frequency and amplitude of the vibrations give information about the forces acting on the attached nanotube (Fig. 1). By measuring those changes with precision, Niguès and co-authors were able to calculate variations in stiffness and dissipation forces during the stretching, fracture, and telescopic sliding of the fractured layers in multiwalled carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs). They observed that the stretching (elongation) of a multiwalled nanotube from its equilibrium state produced first a linear increase in the nanotube stiffness (elastic regime) followed by nonlinear behaviour (plastic regime). This preceded a sudden drop in stiffness, which corresponded to the fracture and separation of the outer layers of the nanotube, as verified by scanning electron microscopy. The almost linear decrease in stiffness measured on further stretching corresponded to the telescopic sliding of the fractured outer layers against the inner ones. Interestingly, for nanotubes of similar dimensions, the measured stiffnesses were much larger for BNNTs than for CNTs. Indeed, for the telescopic sliding of multiwalled CNTs, the authors measured a vanishingly small friction force that was independent of the contact area between the nanotubes, in agreement with previous reports showing ultralow intershell friction in centimetre-long double-walled CNTs4,5. However, for multiwalled BNNTs they found that the friction forces were orders of magnitude larger and increased linearly with sliding speed, and that the friction coefficient was proportional to the contact area between the sliding tubes. These results suggested that there are much stronger molecular interactions between layers in BNNTs than between layers in CNTs. The remarkably different friction behaviour of CNTs and BNNTs is surprising because the nanotubes have a similar crystallographic structure. Yet atomic interactions in the two types of nanotube are different: carbon atoms in CNTs are neutral, whereas in BNNTs boron and nitrogen atoms have ionic character (with partial charge separation of ~0.3–0.4e between B and N atoms). In fact, the authors suggest that, on sliding, negative and positive charges (which are distributed with hexagonal symmetry on the walls of the nanotubes) move past against each other, resulting in negative–negative (repulsive) and negative–positive (attractive) contacts (Fig. 2). Such electrostatic interactions, present in the BNNTs but not in CNTs,

Resonance frequency

Response of tuning fork

External piezoactuator

Frequency

Figure 1 | Measurement of interlayer friction in multiwalled nanotubes. Resonance curves (right) are measured by attaching the outer layer of a multiwalled nanotube to two stiff tungsten tips, one attached to a tuning fork and the other to an x–y–z step nanomanipulator. During displacement, both the position of the mobile tip and the mechanical response of the tuning fork are simultaneously recorded. Dissipative forces and stiffnesses are determined from measurements of the changes in the frequency and amplitude of the vibrations of the tuning fork during telescopic sliding of the nanotube.

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Figure 2 | Physical origin of the ultrahigh dissipative friction in boron nitride nanotubes. As two nanotube layers in contact slide past each other, the partial charges in the positive boron and negative nitride atoms in one layer alternately face charges of the same (inset, top) or opposite (inset, bottom) sign in the other layer. Viscous-like friction is ascribed to such breaking and forming of ionic boron–nitride bonds.

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news & views may indeed explain the large viscous-like dissipation generated during interlayer sliding of multiwalled BNNTs. Beyond mechanistic considerations, the work of Niguès and collaborators should help spur studies aimed at understanding how frictional forces at the atomic scale depend on the presence of electromagnetic fields, external mechanical stresses and environmental conditions (such as temperature, relative humidity and ambient-gas composition). For example, in single-walled nanotubes both the presence of defects and nanotube chirality can significantly affect frictional forces, as experiments with a sliding nanosized tip have determined6,7. Yet whether these factors influence interlayer dissipation in multiwalled nanotubes remains to be studied. Similarly, one wonders whether interlayer frictional behaviour in BNNTs and CNTs differs from the friction between adjacent yet non-concentric nanotubes sliding

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past each other. Moreover, by developing the force-sensing tuning fork of Niguès and colleagues to allow for bidirectional nanomanipulation, it may be possible to excite and exploit multiple mechanical resonances in the nanotubes, and thus obtain higher precision in measurements of both radial and axial components of the forces during telescopic sliding 8. Furthermore, Niguès and co-workers’ approach has the potential to be able to measure frictional forces between flakes or nanoribbons of graphene, boron nitride or transition-metal dichalchogenides — materials with two-dimensional crystalline configurations that have been shown to have exceptional electrical, electro-optical and mechanical properties. Ultimately, understanding the friction mechanisms between atomically flat materials should allow us to fabricate structures with ad hoc frictional properties for applications in ultraminiaturized vibration-control and

vibration–actuation devices. With this in mind, playing a violin with strings of nanotube yarn may one day become a reality.



Robert Szoszkiewicz is in the Department of Physics, Kansas State University, Manhattan, Kansas 66506, USA. Elisa Riedo is in the School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332-0230, USA. e-mail: [email protected] References 1. Vanossi, A., Manini, N., Urbakh, M., Zapperi, S. & Tosatti, E. Rev. Mod. Phys. 85, 529–552 (2013). 2. Urbakh, M., Klafter, J., Gourdon, D. & Israelashivili, J. Nature 430, 525–528 (2004). 3. Niguès, A., Siria, A., Vincent, P., Poncharal, P. & Bocquet, L. Nature Mater. 13, 688–693 (2014). 4. Cumings, J. & Zettl, A. Science 289, 602–604 (2000). 5. Zhang, R. et al. Nature Nanotech. 8, 912–916 (2013). 6. Chui, H.‑C., Dogan, S., Volkmann, M., Klinke, C. & Riedo, E. Nanotechnology 23, 455706 (2012). 7. Lucas, M. et al. Nature Mater. 8, 876–881 (2009). 8. Ploscariu, N. & Szoszkiewicz, R. Appl. Phys. Lett. 103, 263702 (2013).

THE CLEAN AIR ACT Most poets probably harbour a hope that their poems might change the world, but none has taken that wish quite as literally as Simon Armitage, whose ‘In Praise of Air’ is the first ‘catalytic poem’. Displayed on a 10 m by 20 m panel on the side of a university building overlooking a busy road in Sheffield’s city centre, it is not just an ode to the vital joys of clean air but is actively producing that very stuff. The panel is coated with a layer of photocatalytic titanium dioxide nanoparticles that, when irradiated with sunlight (or indeed street lights), convert nitrogen oxides (NOx) adsorbed on their surface to nitrate. The project is a collaboration with Sheffield materials scientist Tony Ryan, and has been funded as part of the city’s Lyric festival of literature. As well as breaking down nitrogen oxides, the catalytic nanoparticles transform toxic volatile organic compounds into fatty acids. They are, of course, barely able to make a dent on the fumes from passing vehicles: each square metre of the display removes about 2 g of NOx a day, about as much as is produced by a single bus. But of course the point is to make a difference in another way: to

create a visible and arresting symbol of the need to tackle air pollution. Armitage’s image of “days when thoughts are fuddled with smog/or civilization crosses the street/with a white handkerchief over its mouth” will be all too familiar to many urban dwellers, perhaps especially in China, where today mobile apps tell users whether or not the PM10 index (the level of airborne particulate matter smaller than ten micrometres across) is low enough for children to play safely outside. That same objective motivates the technology from which this project arose: ‘catalytic clothing’, developed by Ryan with designer and artist Helen Storey, who specializes in art–science collaborations for fashion, design and technology. They have devised a process in which the titania nanoparticles can become attached to ordinary clothing fabric (so far cotton, but they are working on other fibres) during the laundering process, so that subsequently the wearer may combat air pollution simply by walking around. Ryan and Storey say that the effects are not insignificant: 30 people in catalytic clothing walking past a metre-width stretch of

PHILIP BALL pavement every minute could effect a noticeable drop in levels of NOx. The duo are still trying to bring the idea to the market. In Praise of Air is also singing the praises of materials. The technology is nothing particularly new, but Ryan’s work is a reminder that bringing a useful laboratory product to the attention of both investors and consumers is often a matter of engaging the imagination — and that this is where scientists can benefit from interactions with designers and artists. It shows too that serious problems can be tackled playfully and in ways that encourage the public to see that they can participate and not be merely the passive recipients of some cryptic and forbidding technology. ❐

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Nanoscale friction: Sliding charges.

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