news & views high shear rates that are typical in water purification devices will be an issue, and is likely to require large, continuous sheets of graphene that can be readily sealed into a membrane module. Supporting such large sheets and scalably creating nanopores in the graphene will also be challenges. Furthermore, ever-present issues in water purification processes are membrane fouling and concentration polarization, and this will be no different for graphene-
based membranes. Indeed, Mahurin and colleagues hypothesize that in their osmotic permeation experiments some of the graphene nanopores might be blocked by salt ions, which could explain the relatively low water permeabilities they observe under those conditions. Despite the many challenges facing such membranes, the work of Mahurin and colleagues suggests that practical, atomically thick membranes could, in the future, become a reality. ❐
Dong-Yeun Koh and Ryan P. Lively are at the School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332-0100, USA. e-mail: [email protected] References 1. 2. 3. 4. 5.
Li, X. et al. Science 324, 1312–1314 (2009). O’Hern, S. C. et al. Nano Lett. 14, 1234–1241 (2014). Kim, H. W. et al. Science 342, 91–95 (2013). Celebi, K. et al. Science 344, 289–292 (2014). Surwade, S. P. et al. Nature Nanotech. 10, 459–464 (2015).
NANOSCALE IMAGING
Tomography for plasmonics
Two-dimensional cathodoluminescence projections are used to reconstruct the plasmonic excitation of nanoscale crescents by tomography.
Marek Malac
T
omography is an imaging technique that allows the reconstruction of a three-dimensional object from a collection of two-dimensional projection images. Images of almost any type can be
z
used as long as the relationship between the two-dimensional projections and the object properties are known and satisfy the projection theorem: the image contrast should vary linearly with the property
γ
Polystyrene Gold
Detector
Far-field cathodoluminescence
e–
Figure 1 | Cathodoluminescence signal collection in a scanning electron microscope. A focused electron beam (red) is stepped over a nano-crescent made of a polystyrene core and gold shell. The light (gamma) generated by the incident electron beam at each position is linked to the plasmonic excitations at that location and can be collected by a spectrometer. The nano-crescents studied by Atre and colleagues are rotationally symmetric around the z axis. The symmetry reduces the requirements for the number of projections needed to reconstruct a three-dimensional representation of the plasmonic modes. For objects where the mutual orientation of the electron beam and the object has no effect, a single projection image is sufficient to generate a virtual tilt series that can be used to reconstruct the object in three dimensions. For plasmons, the mutual orientation of the beam and the excited object needs to be taken into account, in principle requiring a large set of projections. Nevertheless, Atre and colleagues achieve a good agreement between simulations and the plasmonic excitation map obtained from only seven projections. Image of prism reproduced with permission: © dip2000/Thinkstock. 386
of interest of the sample. Because of its generality, tomography, envisioned by Johan Radon in 1917, has been widely used, from probing the internal structure of the Earth1 to imaging the internal organs of living organisms. Now, writing in Nature Nanotechnology Ashwin Atre and co-workers from Stanford University and the FOM Institute AMOLF in the Netherlands show that tomography can be utilized to image plasmons in nanoscale objects using two-dimensional cathodoluminescence projections2. Imaging plasmon modes at the nanoscale is extremely challenging because the physical dimensions of the objects are much smaller than the wavelength of the light coupling to them. Researchers, therefore, have tried to use shorter-wavelength radiation, such as electron beams as used in cathodoluminescence (CL) and electron energy-loss spectroscopy (EELS)3. In CL imaging, a small electron beam probe is placed at a known location within the object; the electron beam excites the sample and the light emanating from the object is then detected in the far field, as shown in Fig. 1. EELS, on the other hand, analyses the energy of the electrons that pass through the sample offering good spectral and spatial resolution, as well as good collection efficiency 4. Detecting the emitted light by CL has the advantage that the spectral resolution usually exceeds that achievable by analysing transmitted electrons. The fact that EELS, unlike CL, probes all excitations means that radiative (light-emitting) and nonradiative modes cannot be discriminated. Comparing CL and EELS spectra offers this
NATURE NANOTECHNOLOGY | VOL 10 | MAY 2015 | www.nature.com/naturenanotechnology
© 2015 Macmillan Publishers Limited. All rights reserved
news & views possibility. Alternatively, resolutions better than the wavelength of light can be achieved by collecting the emitted light in the near field, but this requires a complicated apparatus and may not be suitable for tomographic imaging 5. A drawback of CL is the poor lightcollection efficiency, as only a small fraction of the light generated by the electron beam reaches the detector. This fact, combined with the limited brightness of electron sources leads to data acquisition times that make collection of a standard tilt series of projection images for tomography impractical. Moreover, extensive electronbeam irradiation can damage the sample. Atre and colleagues circumvent these challenges by preparing crescents randomly oriented on a substrate and then only acquiring the projection that is at 90° with respect to the electron beam. From this single projection, the researchers generate, using a computer algorithm, a full standard tomographic tilt series by taking advantage of the symmetry of the object. The threedimensional tomographic reconstruction is then performed using traditional filtered back-projection of this virtual tilt series. The price to pay for this significant reduction in data collection is that not all of the plasmonic modes may be detected. For a particular mode to be excited, a favourable
orientation of the nano-crescent and the electric field associated with the incident electron beam is necessary. Collecting CL images from several nano-crescents with suitable orientation reduces or eliminates the possibility of missing an image of an excitation mode. When CL spectra are collected in a scanning (transmission) electron microscope the CL signal is integrated along the entire electron-beam path within the imaged object. As a result, the CL signal can be considered to satisfy the projection theorem, although this is far from obvious. In fact, because the CL signal depends on the mutual orientation of the electric field of the incident electron beam and the excited object, a rigorous treatment would require a full vector tomography reconstruction. However, the symmetry argument invoked by the researchers allows them to reduce the vectorial reconstruction problem to a scalar one. This simplification seems to be supported by a good agreement between the experiments and simulations. The work of Atre and colleagues has the potential to contribute to the many fields in which imaging plasmonic modes is desirable. It is worth noting, however, that in the case of imaging plasmonic modes, the highest possible spatial resolution does not depend solely on the experimental set-up (SEM plus CL). For example, plasmons
exhibit non-local effects that may outweigh the probe size; in addition, the dimensions of the examined object and the spatial delocalization of the low-energy excitations should also be considered5,6; the incident electron beam broadens as it goes through the sample, an effect that can be reduced by increasing the energy of the electron beam. One of the most attractive features of Atre and co-workers’ achievement is the fact that the experimental set-up is rather simple, consisting of an SEM with a far-field CL attachment. This should make the method accessible to many laboratories working in nanoplasmonics. To avoid artefacts, however, the symmetry argument should still be used with caution. ❐ Marek Malac is at the National Institute for Nanotechnology and in the Department of Physics, University of Alberta, Edmonton, Alberta T6G 2E1, Canada. e-mail: [email protected] References 1. 2. 3. 4. 5. 6.
Rawlinson, N. et al. Phys. Earth Planet. Inter. 178, 101–135 (2010). Atre, A. C. et al. Nature Nanotech. 10, 429–436 (2015). Garcia de Abajo, F. J. Rev. Mod. Phys. 82, 209–275 (2010). Krivanek, O. et al. Nature 514, 209–212 (2014). Haegel, N. M. Nanophotonics 3, 75–89 (2014). Egerton, R. F. Ultramicroscopy 107, 575–586 (2007).
Published online: 6 April 2015
NONLINEAR OPTICS
Tuning harmonics with excitons
Nonlinear generation of light from an atomically thin semiconductor can be controlled by electrical fields.
Sean P. Rodrigues and Wenshan Cai
D
ynamic control of light, which is essential for the creation and routing of optical signals, is intrinsically connected to nonlinear optics, a branch of optics that describes light–matter interactions where the induced dielectric polarization is related to the electric field component of light in a nonlinear manner. The birth of the field was marked in 1961 by Peter Franken’s demonstration of secondharmonic generation (SHG)1, which occurs when photons in a given medium can be combined and converted to a single photon with twice the energy of the input ones. This optical process is widely employed in physics, biology and materials science for diverse applications including the modification of the output wavelength in lasers, measurement of ultrafast pulses, and
high-resolution microscopy of biological tissues. Writing in Nature Nanotechnology, Xiaodong Xu and co-workers at the University of Washington, University of Hong Kong, Oak Ridge National Laboratory and University of Tennessee now show that the frequency-doubled output from a monolayer transition metal dichalcogenide semiconductor can be dynamically manipulated by gate biases, and its intensity tuned by up to an order of magnitude2. Electrically induced harmonic generation of light can be achieved through a variety of schemes. In 1962, Robert Terhune discovered that a large voltage applied to a calcite crystal can generate a second harmonic signal (Fig. 1a), which otherwise would not have occurred because the centrosymmetric crystal lattice of calcite would make its
NATURE NANOTECHNOLOGY | VOL 10 | MAY 2015 | www.nature.com/naturenanotechnology
© 2015 Macmillan Publishers Limited. All rights reserved
second-order nonlinear susceptibility χ(2) vanish3. The electrical-field-induced SHG stems from the interplay of the third-order nonlinear coefficient χ(3), ubiquitous in all materials, and the static or low-frequency electrical field induced by an applied voltage. Various configurations have been produced to realize electrically controlled frequency doubling. For example, a metallic surface placed in an electrolytic solution can be used as a nonlinear medium, where the SHG is sensitive to the surface charge accumulation induced by the electrical potential4 (Fig. 1b). In a metal–oxide– semiconductor structure or a p–n junction, the built-in electric field of the depletion region facilitates electrically enabled SHG in silicon5 (Fig. 1c). Recently, plasmonic resonators and photonic metamaterials have 387
based membranes. Indeed, Mahurin and colleagues hypothesize that in their osmotic permeation experiments some of the graphene nanopores might be blocked by salt ions, which could explain the relatively low water permeabilities they observe under those conditions. Despite the many challenges facing such membranes, the work of Mahurin and colleagues suggests that practical, atomically thick membranes could, in the future, become a reality. ❐
Dong-Yeun Koh and Ryan P. Lively are at the School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332-0100, USA. e-mail: [email protected] References 1. 2. 3. 4. 5.
Li, X. et al. Science 324, 1312–1314 (2009). O’Hern, S. C. et al. Nano Lett. 14, 1234–1241 (2014). Kim, H. W. et al. Science 342, 91–95 (2013). Celebi, K. et al. Science 344, 289–292 (2014). Surwade, S. P. et al. Nature Nanotech. 10, 459–464 (2015).
NANOSCALE IMAGING
Tomography for plasmonics
Two-dimensional cathodoluminescence projections are used to reconstruct the plasmonic excitation of nanoscale crescents by tomography.
Marek Malac
T
omography is an imaging technique that allows the reconstruction of a three-dimensional object from a collection of two-dimensional projection images. Images of almost any type can be
z
used as long as the relationship between the two-dimensional projections and the object properties are known and satisfy the projection theorem: the image contrast should vary linearly with the property
γ
Polystyrene Gold
Detector
Far-field cathodoluminescence
e–
Figure 1 | Cathodoluminescence signal collection in a scanning electron microscope. A focused electron beam (red) is stepped over a nano-crescent made of a polystyrene core and gold shell. The light (gamma) generated by the incident electron beam at each position is linked to the plasmonic excitations at that location and can be collected by a spectrometer. The nano-crescents studied by Atre and colleagues are rotationally symmetric around the z axis. The symmetry reduces the requirements for the number of projections needed to reconstruct a three-dimensional representation of the plasmonic modes. For objects where the mutual orientation of the electron beam and the object has no effect, a single projection image is sufficient to generate a virtual tilt series that can be used to reconstruct the object in three dimensions. For plasmons, the mutual orientation of the beam and the excited object needs to be taken into account, in principle requiring a large set of projections. Nevertheless, Atre and colleagues achieve a good agreement between simulations and the plasmonic excitation map obtained from only seven projections. Image of prism reproduced with permission: © dip2000/Thinkstock. 386
of interest of the sample. Because of its generality, tomography, envisioned by Johan Radon in 1917, has been widely used, from probing the internal structure of the Earth1 to imaging the internal organs of living organisms. Now, writing in Nature Nanotechnology Ashwin Atre and co-workers from Stanford University and the FOM Institute AMOLF in the Netherlands show that tomography can be utilized to image plasmons in nanoscale objects using two-dimensional cathodoluminescence projections2. Imaging plasmon modes at the nanoscale is extremely challenging because the physical dimensions of the objects are much smaller than the wavelength of the light coupling to them. Researchers, therefore, have tried to use shorter-wavelength radiation, such as electron beams as used in cathodoluminescence (CL) and electron energy-loss spectroscopy (EELS)3. In CL imaging, a small electron beam probe is placed at a known location within the object; the electron beam excites the sample and the light emanating from the object is then detected in the far field, as shown in Fig. 1. EELS, on the other hand, analyses the energy of the electrons that pass through the sample offering good spectral and spatial resolution, as well as good collection efficiency 4. Detecting the emitted light by CL has the advantage that the spectral resolution usually exceeds that achievable by analysing transmitted electrons. The fact that EELS, unlike CL, probes all excitations means that radiative (light-emitting) and nonradiative modes cannot be discriminated. Comparing CL and EELS spectra offers this
NATURE NANOTECHNOLOGY | VOL 10 | MAY 2015 | www.nature.com/naturenanotechnology
© 2015 Macmillan Publishers Limited. All rights reserved
news & views possibility. Alternatively, resolutions better than the wavelength of light can be achieved by collecting the emitted light in the near field, but this requires a complicated apparatus and may not be suitable for tomographic imaging 5. A drawback of CL is the poor lightcollection efficiency, as only a small fraction of the light generated by the electron beam reaches the detector. This fact, combined with the limited brightness of electron sources leads to data acquisition times that make collection of a standard tilt series of projection images for tomography impractical. Moreover, extensive electronbeam irradiation can damage the sample. Atre and colleagues circumvent these challenges by preparing crescents randomly oriented on a substrate and then only acquiring the projection that is at 90° with respect to the electron beam. From this single projection, the researchers generate, using a computer algorithm, a full standard tomographic tilt series by taking advantage of the symmetry of the object. The threedimensional tomographic reconstruction is then performed using traditional filtered back-projection of this virtual tilt series. The price to pay for this significant reduction in data collection is that not all of the plasmonic modes may be detected. For a particular mode to be excited, a favourable
orientation of the nano-crescent and the electric field associated with the incident electron beam is necessary. Collecting CL images from several nano-crescents with suitable orientation reduces or eliminates the possibility of missing an image of an excitation mode. When CL spectra are collected in a scanning (transmission) electron microscope the CL signal is integrated along the entire electron-beam path within the imaged object. As a result, the CL signal can be considered to satisfy the projection theorem, although this is far from obvious. In fact, because the CL signal depends on the mutual orientation of the electric field of the incident electron beam and the excited object, a rigorous treatment would require a full vector tomography reconstruction. However, the symmetry argument invoked by the researchers allows them to reduce the vectorial reconstruction problem to a scalar one. This simplification seems to be supported by a good agreement between the experiments and simulations. The work of Atre and colleagues has the potential to contribute to the many fields in which imaging plasmonic modes is desirable. It is worth noting, however, that in the case of imaging plasmonic modes, the highest possible spatial resolution does not depend solely on the experimental set-up (SEM plus CL). For example, plasmons
exhibit non-local effects that may outweigh the probe size; in addition, the dimensions of the examined object and the spatial delocalization of the low-energy excitations should also be considered5,6; the incident electron beam broadens as it goes through the sample, an effect that can be reduced by increasing the energy of the electron beam. One of the most attractive features of Atre and co-workers’ achievement is the fact that the experimental set-up is rather simple, consisting of an SEM with a far-field CL attachment. This should make the method accessible to many laboratories working in nanoplasmonics. To avoid artefacts, however, the symmetry argument should still be used with caution. ❐ Marek Malac is at the National Institute for Nanotechnology and in the Department of Physics, University of Alberta, Edmonton, Alberta T6G 2E1, Canada. e-mail: [email protected] References 1. 2. 3. 4. 5. 6.
Rawlinson, N. et al. Phys. Earth Planet. Inter. 178, 101–135 (2010). Atre, A. C. et al. Nature Nanotech. 10, 429–436 (2015). Garcia de Abajo, F. J. Rev. Mod. Phys. 82, 209–275 (2010). Krivanek, O. et al. Nature 514, 209–212 (2014). Haegel, N. M. Nanophotonics 3, 75–89 (2014). Egerton, R. F. Ultramicroscopy 107, 575–586 (2007).
Published online: 6 April 2015
NONLINEAR OPTICS
Tuning harmonics with excitons
Nonlinear generation of light from an atomically thin semiconductor can be controlled by electrical fields.
Sean P. Rodrigues and Wenshan Cai
D
ynamic control of light, which is essential for the creation and routing of optical signals, is intrinsically connected to nonlinear optics, a branch of optics that describes light–matter interactions where the induced dielectric polarization is related to the electric field component of light in a nonlinear manner. The birth of the field was marked in 1961 by Peter Franken’s demonstration of secondharmonic generation (SHG)1, which occurs when photons in a given medium can be combined and converted to a single photon with twice the energy of the input ones. This optical process is widely employed in physics, biology and materials science for diverse applications including the modification of the output wavelength in lasers, measurement of ultrafast pulses, and
high-resolution microscopy of biological tissues. Writing in Nature Nanotechnology, Xiaodong Xu and co-workers at the University of Washington, University of Hong Kong, Oak Ridge National Laboratory and University of Tennessee now show that the frequency-doubled output from a monolayer transition metal dichalcogenide semiconductor can be dynamically manipulated by gate biases, and its intensity tuned by up to an order of magnitude2. Electrically induced harmonic generation of light can be achieved through a variety of schemes. In 1962, Robert Terhune discovered that a large voltage applied to a calcite crystal can generate a second harmonic signal (Fig. 1a), which otherwise would not have occurred because the centrosymmetric crystal lattice of calcite would make its
NATURE NANOTECHNOLOGY | VOL 10 | MAY 2015 | www.nature.com/naturenanotechnology
© 2015 Macmillan Publishers Limited. All rights reserved
second-order nonlinear susceptibility χ(2) vanish3. The electrical-field-induced SHG stems from the interplay of the third-order nonlinear coefficient χ(3), ubiquitous in all materials, and the static or low-frequency electrical field induced by an applied voltage. Various configurations have been produced to realize electrically controlled frequency doubling. For example, a metallic surface placed in an electrolytic solution can be used as a nonlinear medium, where the SHG is sensitive to the surface charge accumulation induced by the electrical potential4 (Fig. 1b). In a metal–oxide– semiconductor structure or a p–n junction, the built-in electric field of the depletion region facilitates electrically enabled SHG in silicon5 (Fig. 1c). Recently, plasmonic resonators and photonic metamaterials have 387
