Refractory Plasmonics Urcan Guler et al. Science 344, 263 (2014); DOI: 10.1126/science.1252722

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PERSPECTIVES tered substance for therapeutic or diagnostic purposes, will be thoroughly scrutinized by the existing regulatory and approval framework implemented by national and international agencies. In the meantime, we urge very careful characterization and rational selection of the graphene materials to be studied in specific biological models, based on a hypothesis-driven intended biomedical purpose. Only rational, well-designed studies of graphene interactions with cells, tissues, and organisms will help guide the best choices for the use of this exciting family of materials.

References and Notes 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

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O. N. Ruiz et al., ACS Nano 5, 8100 (2011). H. K. Na et al., Nanoscale 5, 1669 (2013). J. Russier et al., Nanoscale 5, 11234 (2013). W. Hu et al., ACS Nano 4, 4317 (2010). S. Liu et al., ACS Nano 5, 6971 (2011). S. Liu et al., Langmuir 28, 12364 (2012). C. Bussy, H. Ali-Boucetta, K. Kostarelos, Acc. Chem. Res. 46, 692 (2013). 23. H. Ali-Boucetta et al., Adv. Healthc. Mater. 2, 433 (2013). 24. G. P. Kotchey et al., ACS Nano 5, 2098 (2011). 25. C. M. Girish, A. Sasidharan, G. S. Gowd, S. Nair, M. Koyakutty, Adv. Healthc. Mater. 2, 1489 (2013).

Acknowledgments: The collaboration was supported by EC-FET European Graphene Flagship. 10.1126/science.1246736

APPLIED PHYSICS

Stable at high temperatures, refractory plasmonic materials could boost existing optoelectronic technologies.

Refractory Plasmonics Urcan Guler, Alexandra Boltasseva, Vladimir M. Shalaev

R

ity ittiv perm rt o f m y pa inar 1000 n @ Imag

Bulk melti ng point (° C)

efractory materials are defined as photonics, as well as to create new tech- ity and high losses (7, 8). Usually listed as those with a high melting point nological opportunities (6). Although sev- a problem for plasmonic applications, resisand chemical stability at tempera- eral proof-of-concept studies have been tive losses result in heating of the plasmonic tures above 2000°C. Applications based on reported, the realization of practical devices material, enabling a temperature rise in a refractory materials, usually nonmetallic, has been hindered by the challenges associ- confined volume around the nanostructure. span a wide range of areas including indus- ated with the properties of noble metals—in Several plasmonic applications with a great trial furnaces, space shuttle shields, and particular, poor chemical and thermal stabil- potential for practical use, such as photosemiconductor technology. Metals thermal treatment (9) and HAMR have also been studied as refracto(1), rely on the heating effects. ries; however, the optical properBecause of the local temperature W ties of those metals that have been rise, the mechanical and chemical tried for high-temperature applistability of plasmonic nanostruc3500 cations were not good enough tures are of paramount imporMo ZrN Ta to be used in plasmonic applicatance; refractory plasmonic mateTiN 3000 tions (these are almost entirely rials are therefore indispensable. based on noble metals, which are S/TPV technology is based on not good refractories). Refractory the idea of absorbing solar irradi2500 materials that exhibit reasonably ation with a broadband absorber, good plasmonic behavior would which results in heating of an 2000 undoubtedly enable new devices intermediate component and the 30 Al and boost such existing applicasubsequent emission of this ther1500 tions as heat-assisted magnetic mal energy in a narrow spectrum 20 recording (HAMR) ( 1), solar/ for efficient absorption by the Au 1000 thermophotovoltaics (S/TPV) (2), photovoltaic cell. Such devices Ag 10 plasmon-assisted chemical vapor can theoretically achieve energy 1200 1000 800 deposition (3), solar thermoelecconversion efficiencies up to 85% 600 ) th (nm tric generators (4), and nanoscale ( 2). However, the operational g 400 n le ve 0 200 v e r wa heat transfer systems (5). temperatures required for highCrosso The field of plasmonics offers efficiency devices are estimated the potential to greatly enhance at ~1500°C, and emitter materithe efficiencies of existing tech- A handful of alternatives. The low melting point and softness of metals pose als that can withstand prolonged nologies, such as electronics and problems when real-world applications are considered, especially in nano- exposure to such temperatures structures in which the melting point is reduced. Refractory plasmonic matehave not yet been developed. rials would provide a solution for high-temperature applications where corSchool of Electrical & Computer Engineering rosion and wear resistance are desired. Refractory metals exhibit plasmonic Engineered absorber and emitter and Birck Nanotechnology Center, Purdue resonances mostly in the near-infrared region with relatively higher losses. photonic crystals can be fabriUniversity, West Lafayette, IN 47907, USA, Transition metal nitrides mimic the optical properties of gold and provide the cated with refractory metals (10, and Nano-Meta Technologies Inc., 1281 superior material properties of the refractory materials. It is above the “cross- 11), but achievable operational Win Hentschel Boulevard, West Lafayette, IN 47906, USA. E-mail: [email protected] over wavelength” that a material becomes plasmonic. temperatures are still far below www.sciencemag.org SCIENCE VOL 344 18 APRIL 2014 Published by AAAS

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PERSPECTIVES the desired values, and adequate durability cannot be obtained even with protective layers made of refractory dielectric materials (11). One-dimensional photonic crystal emitters based on Si/SiO2 layers, although they are more vulnerable to degradation at high temperatures, have demonstrated the highest S/TPV efficiency when integrated with a carbon nanotube broadband absorber that has limited spectral selectivity and back-emission at longer wavelengths (12). Refractory plasmonic materials could be the solution for most of the major limitations, thus advancing the existing S/TPV technology. Ultrathin metamaterial absorbers and emitters made of the same refractory plasmonic material could be integrated as a narrow intermediate spectral converter that can be easily heated, owing to the increased surface-to-volume ratio (13). The absorber could be spectrally engineered so that backemission is minimized at longer wavelengths and a reduced absorber area ratio is no longer a problem. Larger absorber areas would enable higher temperatures and much higher efficiencies, also eliminating the need for light collection optics. More important, thinner structures achievable with metamaterials would reduce the mechanical load on the nanostructures, thus enabling high-temperature durability. Another field with a potential near-term impact in industry is HAMR. The demand for larger data storage capacity has resulted in a need for larger areal densities, and consequently smaller grain sizes, which in turn may

lead to thermal instabilities. One promising approach is to use high-coercivity materials (which have greater stability at room temperature) and to locally heat the material with a plasmonic nanoantenna, lowering its coercivity for a short time to write data. This idea was demonstrated in an experiment in which local heating on a 70-nm track was achieved with a gold nanoantenna (1). However, efficient heating of the high-coercivity material results in self-heating of the plasmonic component. Local temperatures reaching 400°C, along with tough operation conditions, impose an extra load on the antenna, which is located very close to a disk spinning at a high speed. Under these conditions, deformation of the nanostructure is unavoidable, especially for noble metals (14). In contrast to S/TPV, studies on HAMR have yielded some findings on the advantages of using plasmonic nanostructures, but have lacked materials with the required refractory properties. The solution for potential high-impact refractory plasmonic applications lies in material building blocks. Finding the proper constituent materials could open up a new avenue for high-temperature applications of advanced plasmonic and metamaterial devices, analogous to recent advances in silicon and silica fiber technologies. One example is transition metal nitrides, such as titanium nitride and zirconium nitride, that exhibit plasmonic properties comparable to those of gold in the visible and near-infrared spectrum (15) (see the figure). Coupled with their refractory properties, these materials

can boost the performance of many heatassisted plasmonic devices, thus replacing the traditional noble metals or refractory materials with poor plasmonic properties. Materials research has been at the center of the most recent studies in the field of plasmonics and metamaterials. Among the alternative materials with desirable optical performance, those with complementary metal-oxide semiconductor compatibility, biocompatibility, chemical stability, tunability, and low losses have attracted attention (8). High-temperature stability is the next desired feature to develop in the field of plasmonics. References and Notes 1. W. Challener et al., Nat. Photonics 3, 220 (2009). 2. S. Fan, Nat. Nanotechnol. 9, 92 (2014). 3. D. A. Boyd, L. Greengard, M. Brongersma, M. Y. ElNaggar, D. G. Goodwin, Nano Lett. 6, 2592 (2006). 4. D. Kraemer et al., Nat. Mater. 10, 532 (2011). 5. E. Rousseau et al., Nat. Photonics 3, 514 (2009). 6. M. L. Brongersma, V. M. Shalaev, Science 328, 440 (2010). 7. A. Boltasseva, H. A. Atwater, Science 331, 290 (2011). 8. G. V. Naik, V. M. Shalaev, A. Boltasseva, Adv. Mater. 25, 3264 (2013). 9. L. R. Hirsch et al., Proc. Natl. Acad. Sci. U.S.A. 100, 13549 (2003). 10. V. Rinnerbauer et al., Opt. Express 21, 11482 (2013). 11. K. A. Arpin et al., Nat. Commun. 4, 2630 (2013). 12. A. Lenert et al., Nat. Nanotechnol. 9, 126 (2014). 13. C. Wu et al., J. Opt. 14, 024005 (2012). 14. N. C. Lindquist et al., Laser Photon. Rev. 7, 453 (2013). 15. U. Guler et al., Nano Lett. 13, 6078 (2013).

Acknowledgments: Supported by Army Research Office grants 63133-PH (W911NF-13-1-0226) and 57981-PH (W911NF-11-1-0359) and NSF MRSEC grant DMR-1120923. 10.1126/science.1252722

NEUROSCIENCE

Myelin—More than Insulation R. Douglas Fields

M

yelin is often compared to electrical insulation on nerve fibers. However, nerve impulses are not transmitted through neuronal axons the way electrons are conducted through a copper wire, and the myelin sheath is far more than an insulator. Myelin fundamentally changes the way neural impulses (information) are generated and transmitted, and its damage causes dysfunction in many nervous system disorders including multiple sclerosis, cerebral palsy, stroke, spinal cord injury, and cognitive impairments. A detailed underNational Institutes of Health, Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), Building 35, Room 2A211, MSC 3713, Bethesda, MD 20892, USA. E-mail: fi[email protected]

264

standing of myelin structure is therefore imperative, but is lacking. On page 319 of this issue, Tomassy et al. (1) provide a highresolution global view of myelin structure spanning the six layers of mammalian cerebral cortex. The findings are likely to spark new concepts about how information is transmitted and integrated in the brain. New techniques of automating collection of electron microscopic images taken in series through layers of tissue are becoming available to analyze neuron ultrastructure in large volumes (2). Using such methods, Tomassy et al. reveal myelin structure in the mouse cerebral cortex along individual nerve fibers, providing a coherent picture.

Unusual features of myelin in the mammalian cerebral cortex permit more complex forms of network integration.

Myelin is a coating of compacted cell membrane that is wrapped around the axon by non-neuronal cells called oligodendrocytes. These multipolar cells extend slender cellular processes to grip axons and spin up to dozens of layers of membrane around it like electrical tape. Many oligodendrocytes grasp a single axon to span its full length. The tiny space exposed between each grasping “hand” corresponds to a node of Ranvier, where voltage-gated sodium channels are concentrated. When the electrical potential across the axon membrane depolarizes by about 20 mV, these channels allow rapid influx of sodium ions that discharges the transmembrane potential, creating a voltage transient of ~0.1 V—the action poten-

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