NEWS & VIEWS RESEARCH by the consumption of NAS are similar to those described in people with or developing diabetes remains to be seen. Many diseases associated with Western lifestyles have now been linked to environmentally induced alterations in the composition of the gut microbiota13. Questions remain regarding the precise mechanisms by which NAS disrupt the relationship between gut bacteria and their host. Studies to identify specific bacterial populations that promote resistance to weight gain or improve glucose tolerance may prove
useful for devising therapies that modulate bacteria or their metabolites. ■ Taylor Feehley and Cathryn R. Nagler are in the Department of Pathology, The University of Chicago, Chicago, Illinois 60637, USA. e-mail: [email protected] Gardner, C. et al. Diabetes Care 35, 1798–1808 (2012). Suez, J. et al. Nature 514, 181–186 (2014). Turnbaugh, P. J. et al. Nature 444, 1027–1031 (2006). Ley, R. E. et al. Proc. Natl Acad. Sci. USA 102, 11070–11075 (2005). 5. Turnbaugh, P. J., Bäckhed, F., Fulton, L. & Gordon, J. I. 1. 2. 3. 4.
MAT ERIALS ANALYSIS
Good vibrations A newly constructed electron-energy monochromator for an atomic-resolution transmission electron microscope has resolved spectroscopic signatures of chemical-bond vibrations that are spatially highly localized. See Letter p.209
I
t is extremely rare for developments in instrumentation to deliver an order of magnitude improvement in resolution or sensitivity for a particular analytical technique, while maintaining existing performance in other areas. A study by Krivanek et al.1 on page 209 of this issue, which focuses on highenergy-resolution electron-energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM), achieves just that. The results promise a new territory for experimental investigation of both fundamental physics and the structure of advanced materials at the atomic scale. The work is a true collaboration between a small manufacturer of electron microscopes and several academics, including teams from two US universities that have recently installed this STEM/EELS instrumentation. It represents a good example of a research community working together to drive forward a particular technique. The paper describes the practical implementation of a design2,3 for an electron monochromator — a device that produces an electron beam that has a narrow electronenergy distribution. The electron monochromator acts on the high-energy electron beam of a STEM, which is corrected for aberrations in the lens that focuses the electron beam into a small probe of atomic dimensions4. In a STEM, a high-energy electron probe is transmitted through a thin specimen, undergoing energy losses as a result of scattering from atoms in the sample. The probe is scanned over the specimen to form images of it as a function of probe position. If an electron spectrometer is added to a STEM, a spectrum of the transmitted electron intensity versus energy loss — the EELS data — is
also obtained. Krivanek and colleagues show that, with the inclusion of appropriate energystabilization schemes, the monochromator design that they have implemented can achieve an electron-energy resolution for EELS in a STEM of around 10 millielectronvolts (Fig. 1), while retaining a sufficiently small probe diameter with enough electron current to maintain atomic-resolution imaging and analysis.
This article was published online on 17 September 2014.
With an atomic-sized probe, STEM imaging using electrons scattered from atomic nuclei through high angles can already generate directly interpretable maps of the projected atomic structure of thin crystalline samples. In most materials, however, these images tend to be dominated by scattering from the heavier elements in the samples, making precise location of light elements difficult. EELS is an established technique in the STEM for studying electronic excitations in thin samples that are induced by the high-energy electron beam5. Such spectra can already provide maps of the distribution in elemental composition and chemical bonding at atomic resolution in two and, more recently, three dimensions. The electron-energy resolution for EELS demonstrated by Krivanek et al. now enables the identification of energy losses associated with lattice vibrations in solids, in particular those related to longitudinal optical phonons6.
Unmonochromated beam Monochromated beam Phonon signal Incident electron probe
1
Transmitted electron intensity
RIK BRYDSON
Cell Host Microbe 3, 213–223 (2008). 6. Ley, R. E., Turnbaugh, P. J., Klein, S. & Gordon, J. I. Nature 444, 1022–1023 (2006). 7. Turnbaugh, P. J. et al. Nature 457, 480–484 (2009). 8. Ridaura, V. K. et al. Science 341, 1241214 (2013). 9. Bäckhed, F. et al. Proc. Natl Acad. Sci. USA 101, 15718–15723 (2004). 10. Bäckhed, F., Manchester, J. K., Semenkovich, C. F. & Gordon, J. I. Proc. Natl Acad. Sci. USA 104, 979–984 (2007). 11. Qin, J. et al. Nature 490, 55–60 (2012). 12. Karlsson, F. H. et al. Nature 498, 99–103 (2013). 13. Cho, I. & Blaser, M. J. Nature Rev. Genet. 13, 260–270 (2012).
Thin sample
0 –150 –100 –50 Transmitted electron beam
250 9 ×300
0 50 100 150 200 250 Energy loss (meV)
Figure 1 | Spatially resolved vibrational spectroscopy. In a scanning transmission electron microscope corrected for aberration effects, a beam of high-energy electrons is focused onto a thin sample, forming a probe of atomic dimensions. The electron beam transmitted through the sample, which in this case comprises different types of atom and bond length, contains electrons that have experienced energy losses owing to scattering events with atoms in the sample. These electrons can be dispersed in terms of their energy using a magnetic prism spectrometer (not shown), enabling a spectrum of scattered electron intensity versus energy loss to be obtained. Krivanek et al.1 show that, if the incident electron beam goes through an energy monochromator (not shown), then the energy spread associated with a beam that undergoes zero energy loss can be improved from 250 millielectronvolts to 9 meV. This improvement allows the detection of spectroscopic signatures of spatially localized chemical-bond vibrations (phonons). The intensity scale of the phonon signal is magnified by a factor of 300 to show the phonon peak (blue arrow). (Plot adapted from ref. 1.) 9 O C T O B E R 2 0 1 4 | VO L 5 1 4 | NAT U R E | 1 7 7
© 2014 Macmillan Publishers Limited. All rights reserved
RESEARCH NEWS & VIEWS These are out-of-phase lattice vibrations in the plane of the thin sample, arising when neighbouring atoms in the lattice have different charge or mass. Until now, lattice vibrations were something electron microscopists have had to worry about only in terms of the sample damage that they induce, or when matching experimental images to simulations7. However, the main point of Krivanek and colleagues’ work is that optical phonons are key signatures of chemical bonds, particularly those involving light elements such as hydrogen, as is well established by the techniques of infrared and Raman (optical) spectroscopy. The implication is, therefore, that STEM–EELS may provide a route for the direct mapping of chemical bonding, including that associated with light elements, at near-atomic resolution. This achievement would present tremendous benefits in a number of highly topical areas of research into new advanced materials and devices. The improvements in overall energy resolution of EELS will undoubtedly aid the study of the local spatial variation of energy bandgaps in semiconducting structures, and the identification of localized collective oscillation of electrons in ‘plasmonic’ structures for light capture. The ability to detect and map light elements, including hydrogen, could extend the existing capability of analytical electron micro scopy to the study of organic materials such as polymers and pharmaceuticals, as well as energy-storage materials — if issues associated with electron-beam-induced damage can be addressed. Directly measuring phonons could potentially help to identify chemical reactions involving the surfaces of nanoscale hetero geneous catalyst particles, and could aid the investigation of the transmission of lattice vibrations across micro- and nanostructural features, such as interfaces and defects in thermal and optical materials. As with the emergence of any new technique, many additional research areas may ultimately prove to be most fruitful. Existing theory suggests that electrons that have undergone phonon scattering would be scattered through large angles and the resulting phonon signal would be spatially highly delocalized, preventing atomic-resolution analysis. However, Krivanek and colleagues present some initial findings which, together with recent theoretical predictions8, suggest that under appropriate conditions the phonon signal may be sufficiently localized for the study of vibrations at a spatial resolution better than that achieved by scanning probe tip-enhanced vibrational spectroscopies 9. The authors observed an exponential delocalization of the phonon signal as an electron probe is moved away from the surface of a sample and into the surrounding vacuum. However, there seems to be a more localized component of the signal that peaks in intensity close to the surface itself, and the researchers discuss a possible experimental geometry for signal collection that would
enhance this more localized contribution. Furthermore, if the probe is inside the sample, it seems that the delocalization could be screened at the interface between two materials with different electrical properties. The authors also demonstrate a method for remotely exciting such phonons at a surface using the inherent delocalization of the signal; here, the beam is located in the vacuum close to the edge of a sample, potentially helping to mitigate electron-beam-induced damage of radiation-sensitive samples. These investigations of the spatial resolution of the phonon signal represent a clear example of experiment leading theory in terms of the interpretation of the results, and is indicative of the new experimental landscape that this development in instrumentation unfolds. Undoubtedly, many more exciting experiments with this technology will follow, aided by the delivery, later in 2014, of a third-generation instrument to a shared user facility: the Engineering and Physical Sciences Research Council National
Facility for Aberration-Corrected STEM, or SuperSTEM10, in Daresbury, UK. I look forward to the community charting this new frontier of research. ■ Rik Brydson is at the Institute for Materials Research, School of Chemical and Process Engineering, University of Leeds, Leeds LS2 9JT, UK. e-mail: [email protected] 1. Krivanek, O. L. et al. Nature 514, 209–212 (2014). 2. Krivanek, O. L. et al. Phil. Trans. R. Soc. A 367, 3683–3697 (2009). 3. Krivanek, O. L. et al. Microscopy 62, 3–21 (2013). 4. Bleloch, A. & Ramasse, Q. in Aberration-Corrected Analytical Transmission Electron Microscopy (ed. Brydson, R.) Ch. 4, 55–87 (Wiley, 2011). 5. Egerton, R. F. Electron Energy Loss Spectroscopy in the Electron Microscope 3rd edn (Springer, 2011). 6. Mahan, G. D. Condensed Matter in a Nutshell (Princeton Univ. Press, 2010). 7. Williams, D. B. & Carter, C. B. Transmission Electron Microscopy 2nd edn (Springer, 2009). 8. Dwyer, C. Phys. Rev. B 89, 054103 (2014). 9. Hermann, P. et al. Analyst 136, 1148–1152 (2011). 10. www.superstem.org
A N I M A L BEHAV I OU R
Incipient tradition in wild chimpanzees The adoption of a new form of tool use has been observed to spread along social-network pathways in a chimpanzee community. The finding offers the first direct evidence of cultural diffusion in these animals in the wild. ANDREW WHITEN
S
ocial learning — learning from others — is one of the fastest-expanding research fields in animal behaviour 1,2. At the fundamental level of evolutionary biology, social learning provides a high-speed ‘second inheritance system’ that interacts with genetic inheritance to enrich behavioural evolution2. From a more anthropocentric perspective, animal social learning casts light on the evolutionary foundations of the cultural capacities that make our own species so successful. Studies of putative cultural variations in wild chimpanzees2–4, the primates with which (together with bonobos) humans last shared a common ancestor, have been particularly influential in our understanding of behavioural evolution. But these observed regional behavioural differences have displayed little change, making it difficult to investigate the workings of social learning. Now, writing in PLoS Biology, Hobaiter et al.5 describe a new form of tool use in the Sonso community of chimpanzees of the Budongo Forest in Uganda, and present a statistical technique for tracing the social transmission of this innovation.
1 7 8 | NAT U R E | VO L 5 1 4 | 9 O C T O B E R 2 0 1 4
© 2014 Macmillan Publishers Limited. All rights reserved
Chimpanzees at Sonso fold wads of leaves in their mouths to fashion a ‘leaf sponge’ that they dip into tree holes to extract water to drink. In 2011, the researchers observed the dominant male of the group, Nick, creating a sponge of moss gathered from a tree trunk and using it to drink from a small flooded waterhole — a behaviour not previously recorded in the 20-year research programme at the site. He was watched by the dominant female, Nambi. Over the next six days of intensive and often competitive use of the waterhole (which the researchers suspect contained unusual densities of minerals or other desirable content), Nambi and six other chimpanzees began to display the moss-sponging technique (Fig. 1a). More than 20 other individuals drank at the hole or in puddles around it, but either directly with their mouths or using leaf sponges rather than moss sponges. To establish whether this behavioural spread was due to social learning, the researchers developed a form of network-based diffusion analysis (NBDA). This statistical technique quantifies the extent to which the spread of a new behaviour is consistent with the prediction that it will follow the social network — a
useful for devising therapies that modulate bacteria or their metabolites. ■ Taylor Feehley and Cathryn R. Nagler are in the Department of Pathology, The University of Chicago, Chicago, Illinois 60637, USA. e-mail: [email protected] Gardner, C. et al. Diabetes Care 35, 1798–1808 (2012). Suez, J. et al. Nature 514, 181–186 (2014). Turnbaugh, P. J. et al. Nature 444, 1027–1031 (2006). Ley, R. E. et al. Proc. Natl Acad. Sci. USA 102, 11070–11075 (2005). 5. Turnbaugh, P. J., Bäckhed, F., Fulton, L. & Gordon, J. I. 1. 2. 3. 4.
MAT ERIALS ANALYSIS
Good vibrations A newly constructed electron-energy monochromator for an atomic-resolution transmission electron microscope has resolved spectroscopic signatures of chemical-bond vibrations that are spatially highly localized. See Letter p.209
I
t is extremely rare for developments in instrumentation to deliver an order of magnitude improvement in resolution or sensitivity for a particular analytical technique, while maintaining existing performance in other areas. A study by Krivanek et al.1 on page 209 of this issue, which focuses on highenergy-resolution electron-energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM), achieves just that. The results promise a new territory for experimental investigation of both fundamental physics and the structure of advanced materials at the atomic scale. The work is a true collaboration between a small manufacturer of electron microscopes and several academics, including teams from two US universities that have recently installed this STEM/EELS instrumentation. It represents a good example of a research community working together to drive forward a particular technique. The paper describes the practical implementation of a design2,3 for an electron monochromator — a device that produces an electron beam that has a narrow electronenergy distribution. The electron monochromator acts on the high-energy electron beam of a STEM, which is corrected for aberrations in the lens that focuses the electron beam into a small probe of atomic dimensions4. In a STEM, a high-energy electron probe is transmitted through a thin specimen, undergoing energy losses as a result of scattering from atoms in the sample. The probe is scanned over the specimen to form images of it as a function of probe position. If an electron spectrometer is added to a STEM, a spectrum of the transmitted electron intensity versus energy loss — the EELS data — is
also obtained. Krivanek and colleagues show that, with the inclusion of appropriate energystabilization schemes, the monochromator design that they have implemented can achieve an electron-energy resolution for EELS in a STEM of around 10 millielectronvolts (Fig. 1), while retaining a sufficiently small probe diameter with enough electron current to maintain atomic-resolution imaging and analysis.
This article was published online on 17 September 2014.
With an atomic-sized probe, STEM imaging using electrons scattered from atomic nuclei through high angles can already generate directly interpretable maps of the projected atomic structure of thin crystalline samples. In most materials, however, these images tend to be dominated by scattering from the heavier elements in the samples, making precise location of light elements difficult. EELS is an established technique in the STEM for studying electronic excitations in thin samples that are induced by the high-energy electron beam5. Such spectra can already provide maps of the distribution in elemental composition and chemical bonding at atomic resolution in two and, more recently, three dimensions. The electron-energy resolution for EELS demonstrated by Krivanek et al. now enables the identification of energy losses associated with lattice vibrations in solids, in particular those related to longitudinal optical phonons6.
Unmonochromated beam Monochromated beam Phonon signal Incident electron probe
1
Transmitted electron intensity
RIK BRYDSON
Cell Host Microbe 3, 213–223 (2008). 6. Ley, R. E., Turnbaugh, P. J., Klein, S. & Gordon, J. I. Nature 444, 1022–1023 (2006). 7. Turnbaugh, P. J. et al. Nature 457, 480–484 (2009). 8. Ridaura, V. K. et al. Science 341, 1241214 (2013). 9. Bäckhed, F. et al. Proc. Natl Acad. Sci. USA 101, 15718–15723 (2004). 10. Bäckhed, F., Manchester, J. K., Semenkovich, C. F. & Gordon, J. I. Proc. Natl Acad. Sci. USA 104, 979–984 (2007). 11. Qin, J. et al. Nature 490, 55–60 (2012). 12. Karlsson, F. H. et al. Nature 498, 99–103 (2013). 13. Cho, I. & Blaser, M. J. Nature Rev. Genet. 13, 260–270 (2012).
Thin sample
0 –150 –100 –50 Transmitted electron beam
250 9 ×300
0 50 100 150 200 250 Energy loss (meV)
Figure 1 | Spatially resolved vibrational spectroscopy. In a scanning transmission electron microscope corrected for aberration effects, a beam of high-energy electrons is focused onto a thin sample, forming a probe of atomic dimensions. The electron beam transmitted through the sample, which in this case comprises different types of atom and bond length, contains electrons that have experienced energy losses owing to scattering events with atoms in the sample. These electrons can be dispersed in terms of their energy using a magnetic prism spectrometer (not shown), enabling a spectrum of scattered electron intensity versus energy loss to be obtained. Krivanek et al.1 show that, if the incident electron beam goes through an energy monochromator (not shown), then the energy spread associated with a beam that undergoes zero energy loss can be improved from 250 millielectronvolts to 9 meV. This improvement allows the detection of spectroscopic signatures of spatially localized chemical-bond vibrations (phonons). The intensity scale of the phonon signal is magnified by a factor of 300 to show the phonon peak (blue arrow). (Plot adapted from ref. 1.) 9 O C T O B E R 2 0 1 4 | VO L 5 1 4 | NAT U R E | 1 7 7
© 2014 Macmillan Publishers Limited. All rights reserved
RESEARCH NEWS & VIEWS These are out-of-phase lattice vibrations in the plane of the thin sample, arising when neighbouring atoms in the lattice have different charge or mass. Until now, lattice vibrations were something electron microscopists have had to worry about only in terms of the sample damage that they induce, or when matching experimental images to simulations7. However, the main point of Krivanek and colleagues’ work is that optical phonons are key signatures of chemical bonds, particularly those involving light elements such as hydrogen, as is well established by the techniques of infrared and Raman (optical) spectroscopy. The implication is, therefore, that STEM–EELS may provide a route for the direct mapping of chemical bonding, including that associated with light elements, at near-atomic resolution. This achievement would present tremendous benefits in a number of highly topical areas of research into new advanced materials and devices. The improvements in overall energy resolution of EELS will undoubtedly aid the study of the local spatial variation of energy bandgaps in semiconducting structures, and the identification of localized collective oscillation of electrons in ‘plasmonic’ structures for light capture. The ability to detect and map light elements, including hydrogen, could extend the existing capability of analytical electron micro scopy to the study of organic materials such as polymers and pharmaceuticals, as well as energy-storage materials — if issues associated with electron-beam-induced damage can be addressed. Directly measuring phonons could potentially help to identify chemical reactions involving the surfaces of nanoscale hetero geneous catalyst particles, and could aid the investigation of the transmission of lattice vibrations across micro- and nanostructural features, such as interfaces and defects in thermal and optical materials. As with the emergence of any new technique, many additional research areas may ultimately prove to be most fruitful. Existing theory suggests that electrons that have undergone phonon scattering would be scattered through large angles and the resulting phonon signal would be spatially highly delocalized, preventing atomic-resolution analysis. However, Krivanek and colleagues present some initial findings which, together with recent theoretical predictions8, suggest that under appropriate conditions the phonon signal may be sufficiently localized for the study of vibrations at a spatial resolution better than that achieved by scanning probe tip-enhanced vibrational spectroscopies 9. The authors observed an exponential delocalization of the phonon signal as an electron probe is moved away from the surface of a sample and into the surrounding vacuum. However, there seems to be a more localized component of the signal that peaks in intensity close to the surface itself, and the researchers discuss a possible experimental geometry for signal collection that would
enhance this more localized contribution. Furthermore, if the probe is inside the sample, it seems that the delocalization could be screened at the interface between two materials with different electrical properties. The authors also demonstrate a method for remotely exciting such phonons at a surface using the inherent delocalization of the signal; here, the beam is located in the vacuum close to the edge of a sample, potentially helping to mitigate electron-beam-induced damage of radiation-sensitive samples. These investigations of the spatial resolution of the phonon signal represent a clear example of experiment leading theory in terms of the interpretation of the results, and is indicative of the new experimental landscape that this development in instrumentation unfolds. Undoubtedly, many more exciting experiments with this technology will follow, aided by the delivery, later in 2014, of a third-generation instrument to a shared user facility: the Engineering and Physical Sciences Research Council National
Facility for Aberration-Corrected STEM, or SuperSTEM10, in Daresbury, UK. I look forward to the community charting this new frontier of research. ■ Rik Brydson is at the Institute for Materials Research, School of Chemical and Process Engineering, University of Leeds, Leeds LS2 9JT, UK. e-mail: [email protected] 1. Krivanek, O. L. et al. Nature 514, 209–212 (2014). 2. Krivanek, O. L. et al. Phil. Trans. R. Soc. A 367, 3683–3697 (2009). 3. Krivanek, O. L. et al. Microscopy 62, 3–21 (2013). 4. Bleloch, A. & Ramasse, Q. in Aberration-Corrected Analytical Transmission Electron Microscopy (ed. Brydson, R.) Ch. 4, 55–87 (Wiley, 2011). 5. Egerton, R. F. Electron Energy Loss Spectroscopy in the Electron Microscope 3rd edn (Springer, 2011). 6. Mahan, G. D. Condensed Matter in a Nutshell (Princeton Univ. Press, 2010). 7. Williams, D. B. & Carter, C. B. Transmission Electron Microscopy 2nd edn (Springer, 2009). 8. Dwyer, C. Phys. Rev. B 89, 054103 (2014). 9. Hermann, P. et al. Analyst 136, 1148–1152 (2011). 10. www.superstem.org
A N I M A L BEHAV I OU R
Incipient tradition in wild chimpanzees The adoption of a new form of tool use has been observed to spread along social-network pathways in a chimpanzee community. The finding offers the first direct evidence of cultural diffusion in these animals in the wild. ANDREW WHITEN
S
ocial learning — learning from others — is one of the fastest-expanding research fields in animal behaviour 1,2. At the fundamental level of evolutionary biology, social learning provides a high-speed ‘second inheritance system’ that interacts with genetic inheritance to enrich behavioural evolution2. From a more anthropocentric perspective, animal social learning casts light on the evolutionary foundations of the cultural capacities that make our own species so successful. Studies of putative cultural variations in wild chimpanzees2–4, the primates with which (together with bonobos) humans last shared a common ancestor, have been particularly influential in our understanding of behavioural evolution. But these observed regional behavioural differences have displayed little change, making it difficult to investigate the workings of social learning. Now, writing in PLoS Biology, Hobaiter et al.5 describe a new form of tool use in the Sonso community of chimpanzees of the Budongo Forest in Uganda, and present a statistical technique for tracing the social transmission of this innovation.
1 7 8 | NAT U R E | VO L 5 1 4 | 9 O C T O B E R 2 0 1 4
© 2014 Macmillan Publishers Limited. All rights reserved
Chimpanzees at Sonso fold wads of leaves in their mouths to fashion a ‘leaf sponge’ that they dip into tree holes to extract water to drink. In 2011, the researchers observed the dominant male of the group, Nick, creating a sponge of moss gathered from a tree trunk and using it to drink from a small flooded waterhole — a behaviour not previously recorded in the 20-year research programme at the site. He was watched by the dominant female, Nambi. Over the next six days of intensive and often competitive use of the waterhole (which the researchers suspect contained unusual densities of minerals or other desirable content), Nambi and six other chimpanzees began to display the moss-sponging technique (Fig. 1a). More than 20 other individuals drank at the hole or in puddles around it, but either directly with their mouths or using leaf sponges rather than moss sponges. To establish whether this behavioural spread was due to social learning, the researchers developed a form of network-based diffusion analysis (NBDA). This statistical technique quantifies the extent to which the spread of a new behaviour is consistent with the prediction that it will follow the social network — a
