news & views the ability to programme the electronic environment of active sites should have significant implications for the development of synthetic catalysts in which activity and selectivity can be carefully controlled. ❐ Avelino Corma is at the Instituto de Tecnología Química, Universidad Politécnica de
Valencia-Consejo Superior de Investigaciones Científicas, Avda. de los Naranjos s/n, 46022 Valencia, Spain. e-mail: [email protected]
3. Oliver-Meseguer, J., Cabrero-Antonino, J. R., Dominguez, I., Leyva-Perez, A. & Corma, A. Science 338, 1452–1455 (2012). 4. Okrut, A. et al. Nature Nanotech. 9, 459–465 (2014). 5. Tilekaratne, A., Simonovis, J. P., Lopez Fagundez, M. F., Ebrahimi, M. & Zaera, F. ACS Catal. 2, 2259–2268 (2012). 6. Öfner, H. & Zaera, F. J. Am. Chem. Soc. 124, 10982–10983 (2002). 7. Hwu, H. H., Eng, J. & Chen, J. G. J. Am. Chem. Soc. 124, 702–709 (2002). 8. Boyer, J. L., Rochford, J., Tsai, M.‑K., Muckerman, J. T. & Fujita, E. Coord. Chem. Rev. 254, 309–330 (2010).
References 1. Haack, P. & Limberg, C. Angew. Chem. Int. Ed. 53, 4282–4293 (2014). 2. Quintanar, L. et al. J. Am. Chem. Soc. 127, 13832–13845 (2005).
2D MATERIALS
Metallic when narrow
Subnanometre metallic wires can be engineered from semiconducting sheets of transition-metal dichalcogenides by means of a focused electron beam.
Wanlin Guo and Xiaofei Liu
R
educing the size of a crystal to the nanoscale can lead to a rearrangement of its atomic lattice and the creation of new material properties. Such rearrangements have been observed before in thin films and narrow wires fabricated from bulk materials of a single element 1,2. The use of materials made from more than one element has the potential to offer additional degrees of freedom because the composition of the crystals can also be modified3, and the atomic rearrangement of such structures has already been investigated using top-down methodologies such as electron-beam lithography 4. Molybdenum sulphide ribbons with a width of around 0.35 nm have, for example, been fabricated by creating holes in a MoS2 sheet using electron irradiation in a transmission electron microscope (TEM)4. Writing in Nature Nanotechnology, Junhao Lin, Wu Zhou and colleagues now report the fabrication of subnanometre wires in MoS2 and other semiconducting transition-metal dichalcogenide sheets, and show that these nanowires are metallic5. The researchers — who are based at Vanderbilt University, Oak Ridge National Laboratory, the University of Tsukuba, Fisk University and the University of Tennessee — used mechanically exfoliated MoS2, MoSe2 and WSe2 monolayer flakes and exposed them to an electron beam with relatively low energy in a scanning TEM. With the instrument, they were able to create holes side-by-side at designated sites in the monolayer flakes (Fig. 1a), and could observe in situ the detailed process of nanowire formation. The lighter atoms in the parent sheets can be removed by the energy beam through knock-off effects4 or ionization effects5, or a combination of both.
Thus, when two holes are created close to each other, the holes can rapidly increase in size and produce ribbons between them (Fig. 1a). When the ribbon width is narrowed past a critical size, the ribbon can turn into a wire with uniform width, and becomes robust enough to survive under the electron-beam irradiation. The team created, for example, three nearby holes in a MoSe2 flake, resulting in the formation of three wires in a Y-shaped connection (Fig. 1b). The subnanometre wires were found to be flexible and could rotate and bend under the electron irradiation while maintaining their atomic structure. The lattice structure of the nanowires was directly imaged from several viewing angles and found to be composed of stacked transition metal (M) and chalcogen (X) triangles in staggered positions. This lattice structure has been observed before in chemically synthesized pseudoone-dimensional ternary molybdenum chalcogenide crystals6, and nanowires of such molybdenum chalcogenides with a stoichiometry of MX have previously been shown to be metallic7. Lin and colleagues a
b
fabricated a two-terminal device with a MoSe wire and proved that the wires are metallic using in situ electrical conductance measurements in the TEM. This further confirms the MX structure of the formed nanowires. Fabrication of one-dimensional nanostructures — especially graphene nanoribbons — has been extensively studied by different bottom-up and top-down methods8–11. Narrowing twodimensional materials into ribbons by electron beam etching or chemical etching cannot produce ribbons with controlled widths and smooth edges8,9. Chemical bottom-up approaches can create highquality graphene nanoribbons and more complex structures (like T- and Y-shaped connections)10, but they are unlikely to be used on a large scale11. In contrast, the subnanometre wires produced in transitionmetal dichalcogenides by electron-beam irradiation have a definite structure, smooth edges and are reproducible4,5. Furthermore, Lin and colleagues show that it is possible to detect the variations of conductance during the phase transition from MX2 to MX, c
t=0 2
t = Δt
1
3
Figure 1 | Fabrication of subnanometre wires using electron-beam irradiation. a, Holes form and coalesce in a transition-metal dichalcogenide sheet exposed to an electron beam (pink) in a TEM4. b, Y-shaped connection of the subnanometre wires created by introducing and growing three holes adjacently using a scanning TEM5. c, The controlled creation of holes with nanometre spacing using focused electron beams.
NATURE NANOTECHNOLOGY | VOL 9 | JUNE 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
413
news & views and apply rotation or bending load to the nanowire through electron irradiation for in situ mechanical testing. The researchers expect the fabrication method to be scalable, because all nanowires eventually reach a stable structure, regardless of the initial shape of the parent layer. This top-down approach is also versatile, and it could serve as a general energy-beambased top-down lithography technology for nanoscale devices that consist of subnanometre elements from binary or more complex two-dimensional materials (Fig. 1c). Using the technique, Lin and colleagues created holes at controlled spacings, with the smallest separation between two
nanowires being less than 100 nm. However, reducing this spacing further could be challenging. Furthermore, the efficiency of this fabrication method is limited by the low throughput of electron-beam lithography. Nevertheless, the work should inspire researchers to fabricate nanowires from other materials, and ultimately the creation of functional devices. ❐ Wanlin Guo and Xiaofei Liu are at the Key Laboratory of Intelligent Nano Materials and Devices of Ministry of Education, State Key Laboratory of Mechanics and Control of Mechanical Structures, Institute of Nano Science of Nanjing University of Aeronautics and Astronautics,
Nanjing 210016, China. e-mail: [email protected] References 1. Yanson, A. I., Rubio Bollinger, G., van den Brom, H. E., Agraït, N. & van Ruitenbeek, J. M. Nature 395, 783–785 (1998). 2. Kondo, Y. & Takayanagi, K. Science 289, 606–608 (2000). 3. Radisavljevic, B., Radenovic, A., Brivio, J., Giacometti, V. & Kis, A. Nature Nanotech. 6, 147–150 (2011). 4. Liu, X. et al. Nature Commun. 4, 1776 (2013). 5. Lin, J. et al. Nature Nanotech. 9, 436–442 (2014). 6. Tarascon, J., Hull, G. & DiSalvo, F. Mater. Res. Bull. 19, 915–924 (1984). 7. Venkataraman, L., Hong, Y. S. & Kim, P. Phys. Rev. Lett. 96, 076601 (2006). 8. Li, X., Wang, X., Zhang, L., Lee, S. & Dai, H. Science 319, 1231–1232 (2000). 9. Wang, X. & Dai, H. Nature Chem. 2, 661–665 (2010). 10. Cai, J. et al. Nature 466, 470–473 (2010). 11. Novoselov, K. S. et al. Nature 490, 192–200 (2012).
NANO-OPTICS
Steering Dyakonov-like waves
The propagation direction of low-loss electromagnetic surface waves can be tuned over tens of degrees by small changes in the dielectric environment.
Mikhail A. Noginov
E
lectromagnetic surface waves propagate along the boundary between two dissimilar media, their field intensities decaying exponentially on either side of the interface. The first such waves were discovered at the beginning of the twentieth century, when Jonathan Zenneck found that radio waves could be guided at the boundary between air and ground or water, and proposed to use the phenomenon for a wireless telegraph1. A century of subsequent research has revealed the fascinating underlying physics of surface waves, which today are starting to find their way in applications for chemical and biomedical sensing, electro-optic modulators and solar cells, to name but a few.
One of the most researched surface waves are surface plasmon polaritons (SPPs), which propagate at the boundary between a medium with negative dielectric permittivity and one with positive dielectric permittivity, most commonly a metal and a dielectric, respectively2. The electric field of SPPs partly resides in the metal and partly in the dielectric. The fact that it resides in the dielectric means that SPPs are highly sensitive to changes in the dielectric environment and provides a basis for sensing. At the same time, in the metal, propagating surface plasmons experience optical loss, which hinders most of their existing and potential applications. Although propagation loss of SPPs can be
Incident beam
Incident beam
Prism
Reflected beam Liquid Al2O3 LBO
Grating z Surface wave
θ
Dyakonov-guided wave
Figure 1 | Excitation of Dyakonov-like surface waves guided by an ultrathin Al2O3 layer sandwiched between a biaxial LiB3O5 (LBO) crystal and a liquid with a high index of refraction. These waves propagate at the interface as a beam with narrow angular divergence, Δθ ≈ 1°, in the direction determined by the dielectric permittivities. Left: Prism coupling. Right: Grating coupling. 414
partly or fully compensated by optical gain in an adjacent dielectric3,4, spontaneous emission noise, heat dissipation and need for a strong pumping make this approach not very practical. Furthermore, the extensive ongoing search for metallic and non-metallic low-loss plasmonic materials, although promising, has not yet led to a major breakthrough5. Writing in Nature Nanotechnology, David Artigas and colleagues at the ICFO-Institut de Ciencies Fotoniques and Universitat Politècnica de Catalunya now report an experimental study of a new member of the family of surface waves6, which could potentially replace propagating surface plasmons in a variety of applications but without the major drawback of optical loss. The researchers report a surface wave that belongs to a family of waves first predicted by Mikhail Dyakonov in 19887. Dyakonov waves propagate at the interface between two lossless dielectric media: one having an isotropic dielectric permittivity, ε, and one having a uniaxial permittivity tensor (ε||, ε^, ε^) whose optic axis is parallel to the interface. Dyakonov surface waves exist only in a narrow range of dielectric permittivities that satisfy the condition ε|| > ε > ε^. If the birefringent medium is biaxial (εz > εy > εx), the condition above becomes εz > ε > εy (ref. 8). Dyakonov waves propagate at the interface as a beam with narrow angular divergence, Δθ
Valencia-Consejo Superior de Investigaciones Científicas, Avda. de los Naranjos s/n, 46022 Valencia, Spain. e-mail: [email protected]
3. Oliver-Meseguer, J., Cabrero-Antonino, J. R., Dominguez, I., Leyva-Perez, A. & Corma, A. Science 338, 1452–1455 (2012). 4. Okrut, A. et al. Nature Nanotech. 9, 459–465 (2014). 5. Tilekaratne, A., Simonovis, J. P., Lopez Fagundez, M. F., Ebrahimi, M. & Zaera, F. ACS Catal. 2, 2259–2268 (2012). 6. Öfner, H. & Zaera, F. J. Am. Chem. Soc. 124, 10982–10983 (2002). 7. Hwu, H. H., Eng, J. & Chen, J. G. J. Am. Chem. Soc. 124, 702–709 (2002). 8. Boyer, J. L., Rochford, J., Tsai, M.‑K., Muckerman, J. T. & Fujita, E. Coord. Chem. Rev. 254, 309–330 (2010).
References 1. Haack, P. & Limberg, C. Angew. Chem. Int. Ed. 53, 4282–4293 (2014). 2. Quintanar, L. et al. J. Am. Chem. Soc. 127, 13832–13845 (2005).
2D MATERIALS
Metallic when narrow
Subnanometre metallic wires can be engineered from semiconducting sheets of transition-metal dichalcogenides by means of a focused electron beam.
Wanlin Guo and Xiaofei Liu
R
educing the size of a crystal to the nanoscale can lead to a rearrangement of its atomic lattice and the creation of new material properties. Such rearrangements have been observed before in thin films and narrow wires fabricated from bulk materials of a single element 1,2. The use of materials made from more than one element has the potential to offer additional degrees of freedom because the composition of the crystals can also be modified3, and the atomic rearrangement of such structures has already been investigated using top-down methodologies such as electron-beam lithography 4. Molybdenum sulphide ribbons with a width of around 0.35 nm have, for example, been fabricated by creating holes in a MoS2 sheet using electron irradiation in a transmission electron microscope (TEM)4. Writing in Nature Nanotechnology, Junhao Lin, Wu Zhou and colleagues now report the fabrication of subnanometre wires in MoS2 and other semiconducting transition-metal dichalcogenide sheets, and show that these nanowires are metallic5. The researchers — who are based at Vanderbilt University, Oak Ridge National Laboratory, the University of Tsukuba, Fisk University and the University of Tennessee — used mechanically exfoliated MoS2, MoSe2 and WSe2 monolayer flakes and exposed them to an electron beam with relatively low energy in a scanning TEM. With the instrument, they were able to create holes side-by-side at designated sites in the monolayer flakes (Fig. 1a), and could observe in situ the detailed process of nanowire formation. The lighter atoms in the parent sheets can be removed by the energy beam through knock-off effects4 or ionization effects5, or a combination of both.
Thus, when two holes are created close to each other, the holes can rapidly increase in size and produce ribbons between them (Fig. 1a). When the ribbon width is narrowed past a critical size, the ribbon can turn into a wire with uniform width, and becomes robust enough to survive under the electron-beam irradiation. The team created, for example, three nearby holes in a MoSe2 flake, resulting in the formation of three wires in a Y-shaped connection (Fig. 1b). The subnanometre wires were found to be flexible and could rotate and bend under the electron irradiation while maintaining their atomic structure. The lattice structure of the nanowires was directly imaged from several viewing angles and found to be composed of stacked transition metal (M) and chalcogen (X) triangles in staggered positions. This lattice structure has been observed before in chemically synthesized pseudoone-dimensional ternary molybdenum chalcogenide crystals6, and nanowires of such molybdenum chalcogenides with a stoichiometry of MX have previously been shown to be metallic7. Lin and colleagues a
b
fabricated a two-terminal device with a MoSe wire and proved that the wires are metallic using in situ electrical conductance measurements in the TEM. This further confirms the MX structure of the formed nanowires. Fabrication of one-dimensional nanostructures — especially graphene nanoribbons — has been extensively studied by different bottom-up and top-down methods8–11. Narrowing twodimensional materials into ribbons by electron beam etching or chemical etching cannot produce ribbons with controlled widths and smooth edges8,9. Chemical bottom-up approaches can create highquality graphene nanoribbons and more complex structures (like T- and Y-shaped connections)10, but they are unlikely to be used on a large scale11. In contrast, the subnanometre wires produced in transitionmetal dichalcogenides by electron-beam irradiation have a definite structure, smooth edges and are reproducible4,5. Furthermore, Lin and colleagues show that it is possible to detect the variations of conductance during the phase transition from MX2 to MX, c
t=0 2
t = Δt
1
3
Figure 1 | Fabrication of subnanometre wires using electron-beam irradiation. a, Holes form and coalesce in a transition-metal dichalcogenide sheet exposed to an electron beam (pink) in a TEM4. b, Y-shaped connection of the subnanometre wires created by introducing and growing three holes adjacently using a scanning TEM5. c, The controlled creation of holes with nanometre spacing using focused electron beams.
NATURE NANOTECHNOLOGY | VOL 9 | JUNE 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
413
news & views and apply rotation or bending load to the nanowire through electron irradiation for in situ mechanical testing. The researchers expect the fabrication method to be scalable, because all nanowires eventually reach a stable structure, regardless of the initial shape of the parent layer. This top-down approach is also versatile, and it could serve as a general energy-beambased top-down lithography technology for nanoscale devices that consist of subnanometre elements from binary or more complex two-dimensional materials (Fig. 1c). Using the technique, Lin and colleagues created holes at controlled spacings, with the smallest separation between two
nanowires being less than 100 nm. However, reducing this spacing further could be challenging. Furthermore, the efficiency of this fabrication method is limited by the low throughput of electron-beam lithography. Nevertheless, the work should inspire researchers to fabricate nanowires from other materials, and ultimately the creation of functional devices. ❐ Wanlin Guo and Xiaofei Liu are at the Key Laboratory of Intelligent Nano Materials and Devices of Ministry of Education, State Key Laboratory of Mechanics and Control of Mechanical Structures, Institute of Nano Science of Nanjing University of Aeronautics and Astronautics,
Nanjing 210016, China. e-mail: [email protected] References 1. Yanson, A. I., Rubio Bollinger, G., van den Brom, H. E., Agraït, N. & van Ruitenbeek, J. M. Nature 395, 783–785 (1998). 2. Kondo, Y. & Takayanagi, K. Science 289, 606–608 (2000). 3. Radisavljevic, B., Radenovic, A., Brivio, J., Giacometti, V. & Kis, A. Nature Nanotech. 6, 147–150 (2011). 4. Liu, X. et al. Nature Commun. 4, 1776 (2013). 5. Lin, J. et al. Nature Nanotech. 9, 436–442 (2014). 6. Tarascon, J., Hull, G. & DiSalvo, F. Mater. Res. Bull. 19, 915–924 (1984). 7. Venkataraman, L., Hong, Y. S. & Kim, P. Phys. Rev. Lett. 96, 076601 (2006). 8. Li, X., Wang, X., Zhang, L., Lee, S. & Dai, H. Science 319, 1231–1232 (2000). 9. Wang, X. & Dai, H. Nature Chem. 2, 661–665 (2010). 10. Cai, J. et al. Nature 466, 470–473 (2010). 11. Novoselov, K. S. et al. Nature 490, 192–200 (2012).
NANO-OPTICS
Steering Dyakonov-like waves
The propagation direction of low-loss electromagnetic surface waves can be tuned over tens of degrees by small changes in the dielectric environment.
Mikhail A. Noginov
E
lectromagnetic surface waves propagate along the boundary between two dissimilar media, their field intensities decaying exponentially on either side of the interface. The first such waves were discovered at the beginning of the twentieth century, when Jonathan Zenneck found that radio waves could be guided at the boundary between air and ground or water, and proposed to use the phenomenon for a wireless telegraph1. A century of subsequent research has revealed the fascinating underlying physics of surface waves, which today are starting to find their way in applications for chemical and biomedical sensing, electro-optic modulators and solar cells, to name but a few.
One of the most researched surface waves are surface plasmon polaritons (SPPs), which propagate at the boundary between a medium with negative dielectric permittivity and one with positive dielectric permittivity, most commonly a metal and a dielectric, respectively2. The electric field of SPPs partly resides in the metal and partly in the dielectric. The fact that it resides in the dielectric means that SPPs are highly sensitive to changes in the dielectric environment and provides a basis for sensing. At the same time, in the metal, propagating surface plasmons experience optical loss, which hinders most of their existing and potential applications. Although propagation loss of SPPs can be
Incident beam
Incident beam
Prism
Reflected beam Liquid Al2O3 LBO
Grating z Surface wave
θ
Dyakonov-guided wave
Figure 1 | Excitation of Dyakonov-like surface waves guided by an ultrathin Al2O3 layer sandwiched between a biaxial LiB3O5 (LBO) crystal and a liquid with a high index of refraction. These waves propagate at the interface as a beam with narrow angular divergence, Δθ ≈ 1°, in the direction determined by the dielectric permittivities. Left: Prism coupling. Right: Grating coupling. 414
partly or fully compensated by optical gain in an adjacent dielectric3,4, spontaneous emission noise, heat dissipation and need for a strong pumping make this approach not very practical. Furthermore, the extensive ongoing search for metallic and non-metallic low-loss plasmonic materials, although promising, has not yet led to a major breakthrough5. Writing in Nature Nanotechnology, David Artigas and colleagues at the ICFO-Institut de Ciencies Fotoniques and Universitat Politècnica de Catalunya now report an experimental study of a new member of the family of surface waves6, which could potentially replace propagating surface plasmons in a variety of applications but without the major drawback of optical loss. The researchers report a surface wave that belongs to a family of waves first predicted by Mikhail Dyakonov in 19887. Dyakonov waves propagate at the interface between two lossless dielectric media: one having an isotropic dielectric permittivity, ε, and one having a uniaxial permittivity tensor (ε||, ε^, ε^) whose optic axis is parallel to the interface. Dyakonov surface waves exist only in a narrow range of dielectric permittivities that satisfy the condition ε|| > ε > ε^. If the birefringent medium is biaxial (εz > εy > εx), the condition above becomes εz > ε > εy (ref. 8). Dyakonov waves propagate at the interface as a beam with narrow angular divergence, Δθ
