news & views CARBON NANOTUBES
Captured on camera
Images of individual carbon nanotubes with their respective optical spectra for chirality characterization are acquired directly on devices and growth substrates using a reflective polarized light microscopy set-up.
Matt W. Graham
Supercontinuum 0° polarization
Beamsplitter
Beamsplitter
Polarizer ~45° polarization
a
b
SWNT 4 45°
Monochromator
CCD
CCD
c 4 ΔI (a.u.)
S
ingle-walled carbon nanotubes can extend up to millimetres in length and rival the best bulk materials in terms of electric current transport properties; yet, they are only about a nanometrewide giving rise to quantum confinement effects. This peculiar dual nature makes nanotubes promising materials for a variety of nanoscale technologies such as molecularscale transistors, light-emitting diodes and charge-carrier multiplication devices in solar cells1. Indeed, the physics behind these emerging nanotube technologies has already been characterized at the singledevice level. However, attempts to scale up devices to the wafer scale have failed, largely because the devices almost always contain a mixture of carbon nanotubes with radically different electronic properties. To overcome this hurdle, either a method to reproducibly grow nearly identical carbon nanotubes, or a high-throughput technique that can rapidly characterize single nanotubes in situ is required. Writing in Nature Nanotechnology, Feng Wang and colleagues at the University of California, Berkeley have tackled the latter, and describe a spectroscopic and imaging technique that can determine the chirality of single tubes and image them at video rates on a wide variety of nanotube substrates2. Most applications of single-walled carbon nanotubes require a perfect separation of metallic and semiconducting nanotubes. However, the best synthetic methods still produce a myriad of tubes of varying lengths and diameters. The electronic properties of individual carbon nanotubes are best classified by the chiral indices (n,m), which correspond to the orientation of the vector along which a graphene sheet would be rolled to give the corresponding nanotube. What has long been desired is an efficient method for determining (n,m) to instantly identify the electronic properties of individual tubes in situ, on a chip. In 2012, researchers reported the unambiguous assignment of these chiral indices by measuring the optical scattering spectra of single-walled carbon nanotubes3. This result hinted at the tantalizing possibility that if the technique could be extended to acquire
(n,m)→(26,22)
3 2 1 0
1.6 2.0 2.4 Energy (eV)
Figure 1 | Characterizing individual carbon nanotubes on substrates using polarized light microscopy. a, The carbon nanotube is oriented at 45o with respect to the linearly polarized supercontinuum light source. The overwhelming scattering signal (red) from the substrate is largely subtracted by the polarizer, while the signal from the carbon nanotube (yellow) is largely preserved after the polarizer, owing to selective depolarization. The signal is split between a video CCD detector for imaging, and a monochromator and a second CCD detector for spectroscopy and chiral determination. b, A video snapshot of an individual single-walled carbon nanotube (SWNT) bridging a silicon/sapphire interface. Scale bar is ~4 μm. c, Acquisition of the optical spectra of the same tube readily identifies its chirality, in this case (n,m) = (26,22). Panels b,c reproduced from ref. 2, © 2013 NPG.
individual nanotube optical spectra in situ, the problem of wafer-scale characterization could be solved. The problem remained, a carbon nanotube only covers about a thousandth of the area of a diffraction-limited optical beam spot, meaning the scattered signal from the surrounding substrate, |Er|2, is strong enough to prevent detection of the desired scattered optical field from an individual nanotube (ENT) (ref. 4). In 2011, however, Jacques Lefebvre and Paul Finnie at the National Research Council of Canada had recognized that because carbon nanotubes selectively absorb light that is polarized parallel to the plane of their length, the signal from the substrate could be largely eliminated by using a linearly polarized excitation and a cross-polarized detection geometry5. Wang and colleagues improve this polarized light microscopy set-up by greatly enhancing the carbon nanotube signal compared with the surrounding
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2013 Macmillan Publishers Limited. All rights reserved
substrate signal. They can now acquire images of carbon nanotubes at video rates (~30 frames per second) along with their optical scattering spectrum, directly on substrates and devices. This result was achieved by orienting a field-effect transistor device so that the nanotube in the device is at an ~45° angle with respect to the incident linearly polarized light (Fig. 1a). The incident light scattered by the carbon nanotube gets selectively depolarized in a direction parallel to the tube. As the polarizer is cross-polarized to the signal scattered from the substrate, the collected signal ΔI is proportional to |Er + ENT|2 − |Er|2. While the scattered field from the substrate, |Er|2, is almost completely removed by the polarizer, the leftover heterodyne signal that reaches the CCD (charge-coupled device) detectors is proportional to 2|ENT| |Er|, that is, the remaining background signal enhances the desired nanotube signal. With this approach Wang and co-workers 1
news & views achieved a two orders of magnitude enhancement in the relative signal detected from an individual carbon nanotube. Although the general principle of the technique developed by the Berkeley group is conceptually similar to previous detection schemes already used to image nanotubes4,5, semiconductor nanocrystals6 and single molecules7, a few key modifications were needed to achieve video-rate imaging, such as the use of a high-intensity broadband supercontinuum laser source coupled with spectrally resolved CCD detection. Moreover, to further prevent the signal |Er|2 from reaching the detector, they had to mitigate objective-induced light depolarization effects without sacrificing numerical aperture, by optimizing the diameter of their incident laser beam. With these minor, but critical, modifications the researchers managed to collect images of hundreds of individual nanotubes and simultaneously record their scattering spectra to readily identify each tube’s chirality (Fig. 1b,c). But what is perhaps more impressive is that their method
2
seems to work for almost any substrate. In particular, they reported sharp video-rate images on fused silica, in a sapphire dielectric layer and on carbon nanotube field-effect transistors over a wide range of doping conditions. There are other methods for identifying individual carbon nanotube chirality, including resonant Rayleigh and Raman microscopy, single-tube photoluminescence and photocurrent spectroscopy. However, at present, none of these approaches are compatible with wafer-scale nanotube characterization. Rayleigh scattering requires specialized substrates and collection geometries, whereas resonant Raman, photocurrent and photoluminescent spectroscopies are all prohibitively slow and substrate specific8. The set-up reported by Wang and colleagues has the potential to solve perhaps the greatest bottleneck hindering single-nanotube device development: that is, a technique for high-throughput, ‘on chip’ single-tube imaging and chirality identification. Further work is needed to
properly account for the tube’s absorption resonance spectral shifts and variable absorption peak widths in different dielectric media. Moreover, developing a self-calibrated scanning mechanism would widely increase the adoption of the technique at device fabrication facilities. Nonetheless, with this new tool the prospects for developments in carbon nanotube circuitry and optoelectronics look much brighter2. ❐ Matt W. Graham is at the Department of Physics of Oregon State University, 301 Weniger Hall, Corvallis, Oregon 97331, USA. e-mail: [email protected] References 1. Wang, C. et al. Chem. Soc. Rev. 42, 2592–2609 (2013). 2. Liu, K. et al. Nature Nanotech. http://dx.doi.org/10.1038/ nnano.2013.227 (2013). 3. Liu, K. et al. Nature Nanotech. 7, 325–329 (2012). 4. Berciaud, S. et al. Nano Lett. 7, 1203–1207 (2007). 5. Lefebvre, J. & Finnie, P. Nano Res. 4, 788–794 (2011). 6. Kukura, P. et al. Nano Lett. 9, 926–929 (2009). 7. Celebrano, M. et al. Nature Photon. 5, 95–98 (2011). 8. Joh, D. Y. et al. Nano Lett. 11, 1–7 (2011).
Published online: 10 November 2013
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2013 Macmillan Publishers Limited. All rights reserved
Captured on camera
Images of individual carbon nanotubes with their respective optical spectra for chirality characterization are acquired directly on devices and growth substrates using a reflective polarized light microscopy set-up.
Matt W. Graham
Supercontinuum 0° polarization
Beamsplitter
Beamsplitter
Polarizer ~45° polarization
a
b
SWNT 4 45°
Monochromator
CCD
CCD
c 4 ΔI (a.u.)
S
ingle-walled carbon nanotubes can extend up to millimetres in length and rival the best bulk materials in terms of electric current transport properties; yet, they are only about a nanometrewide giving rise to quantum confinement effects. This peculiar dual nature makes nanotubes promising materials for a variety of nanoscale technologies such as molecularscale transistors, light-emitting diodes and charge-carrier multiplication devices in solar cells1. Indeed, the physics behind these emerging nanotube technologies has already been characterized at the singledevice level. However, attempts to scale up devices to the wafer scale have failed, largely because the devices almost always contain a mixture of carbon nanotubes with radically different electronic properties. To overcome this hurdle, either a method to reproducibly grow nearly identical carbon nanotubes, or a high-throughput technique that can rapidly characterize single nanotubes in situ is required. Writing in Nature Nanotechnology, Feng Wang and colleagues at the University of California, Berkeley have tackled the latter, and describe a spectroscopic and imaging technique that can determine the chirality of single tubes and image them at video rates on a wide variety of nanotube substrates2. Most applications of single-walled carbon nanotubes require a perfect separation of metallic and semiconducting nanotubes. However, the best synthetic methods still produce a myriad of tubes of varying lengths and diameters. The electronic properties of individual carbon nanotubes are best classified by the chiral indices (n,m), which correspond to the orientation of the vector along which a graphene sheet would be rolled to give the corresponding nanotube. What has long been desired is an efficient method for determining (n,m) to instantly identify the electronic properties of individual tubes in situ, on a chip. In 2012, researchers reported the unambiguous assignment of these chiral indices by measuring the optical scattering spectra of single-walled carbon nanotubes3. This result hinted at the tantalizing possibility that if the technique could be extended to acquire
(n,m)→(26,22)
3 2 1 0
1.6 2.0 2.4 Energy (eV)
Figure 1 | Characterizing individual carbon nanotubes on substrates using polarized light microscopy. a, The carbon nanotube is oriented at 45o with respect to the linearly polarized supercontinuum light source. The overwhelming scattering signal (red) from the substrate is largely subtracted by the polarizer, while the signal from the carbon nanotube (yellow) is largely preserved after the polarizer, owing to selective depolarization. The signal is split between a video CCD detector for imaging, and a monochromator and a second CCD detector for spectroscopy and chiral determination. b, A video snapshot of an individual single-walled carbon nanotube (SWNT) bridging a silicon/sapphire interface. Scale bar is ~4 μm. c, Acquisition of the optical spectra of the same tube readily identifies its chirality, in this case (n,m) = (26,22). Panels b,c reproduced from ref. 2, © 2013 NPG.
individual nanotube optical spectra in situ, the problem of wafer-scale characterization could be solved. The problem remained, a carbon nanotube only covers about a thousandth of the area of a diffraction-limited optical beam spot, meaning the scattered signal from the surrounding substrate, |Er|2, is strong enough to prevent detection of the desired scattered optical field from an individual nanotube (ENT) (ref. 4). In 2011, however, Jacques Lefebvre and Paul Finnie at the National Research Council of Canada had recognized that because carbon nanotubes selectively absorb light that is polarized parallel to the plane of their length, the signal from the substrate could be largely eliminated by using a linearly polarized excitation and a cross-polarized detection geometry5. Wang and colleagues improve this polarized light microscopy set-up by greatly enhancing the carbon nanotube signal compared with the surrounding
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2013 Macmillan Publishers Limited. All rights reserved
substrate signal. They can now acquire images of carbon nanotubes at video rates (~30 frames per second) along with their optical scattering spectrum, directly on substrates and devices. This result was achieved by orienting a field-effect transistor device so that the nanotube in the device is at an ~45° angle with respect to the incident linearly polarized light (Fig. 1a). The incident light scattered by the carbon nanotube gets selectively depolarized in a direction parallel to the tube. As the polarizer is cross-polarized to the signal scattered from the substrate, the collected signal ΔI is proportional to |Er + ENT|2 − |Er|2. While the scattered field from the substrate, |Er|2, is almost completely removed by the polarizer, the leftover heterodyne signal that reaches the CCD (charge-coupled device) detectors is proportional to 2|ENT| |Er|, that is, the remaining background signal enhances the desired nanotube signal. With this approach Wang and co-workers 1
news & views achieved a two orders of magnitude enhancement in the relative signal detected from an individual carbon nanotube. Although the general principle of the technique developed by the Berkeley group is conceptually similar to previous detection schemes already used to image nanotubes4,5, semiconductor nanocrystals6 and single molecules7, a few key modifications were needed to achieve video-rate imaging, such as the use of a high-intensity broadband supercontinuum laser source coupled with spectrally resolved CCD detection. Moreover, to further prevent the signal |Er|2 from reaching the detector, they had to mitigate objective-induced light depolarization effects without sacrificing numerical aperture, by optimizing the diameter of their incident laser beam. With these minor, but critical, modifications the researchers managed to collect images of hundreds of individual nanotubes and simultaneously record their scattering spectra to readily identify each tube’s chirality (Fig. 1b,c). But what is perhaps more impressive is that their method
2
seems to work for almost any substrate. In particular, they reported sharp video-rate images on fused silica, in a sapphire dielectric layer and on carbon nanotube field-effect transistors over a wide range of doping conditions. There are other methods for identifying individual carbon nanotube chirality, including resonant Rayleigh and Raman microscopy, single-tube photoluminescence and photocurrent spectroscopy. However, at present, none of these approaches are compatible with wafer-scale nanotube characterization. Rayleigh scattering requires specialized substrates and collection geometries, whereas resonant Raman, photocurrent and photoluminescent spectroscopies are all prohibitively slow and substrate specific8. The set-up reported by Wang and colleagues has the potential to solve perhaps the greatest bottleneck hindering single-nanotube device development: that is, a technique for high-throughput, ‘on chip’ single-tube imaging and chirality identification. Further work is needed to
properly account for the tube’s absorption resonance spectral shifts and variable absorption peak widths in different dielectric media. Moreover, developing a self-calibrated scanning mechanism would widely increase the adoption of the technique at device fabrication facilities. Nonetheless, with this new tool the prospects for developments in carbon nanotube circuitry and optoelectronics look much brighter2. ❐ Matt W. Graham is at the Department of Physics of Oregon State University, 301 Weniger Hall, Corvallis, Oregon 97331, USA. e-mail: [email protected] References 1. Wang, C. et al. Chem. Soc. Rev. 42, 2592–2609 (2013). 2. Liu, K. et al. Nature Nanotech. http://dx.doi.org/10.1038/ nnano.2013.227 (2013). 3. Liu, K. et al. Nature Nanotech. 7, 325–329 (2012). 4. Berciaud, S. et al. Nano Lett. 7, 1203–1207 (2007). 5. Lefebvre, J. & Finnie, P. Nano Res. 4, 788–794 (2011). 6. Kukura, P. et al. Nano Lett. 9, 926–929 (2009). 7. Celebrano, M. et al. Nature Photon. 5, 95–98 (2011). 8. Joh, D. Y. et al. Nano Lett. 11, 1–7 (2011).
Published online: 10 November 2013
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2013 Macmillan Publishers Limited. All rights reserved
