news & views GRAPHENE SYNTHESIS
Graphene closer to fruition
Cracks and defects induced during the transfer of large-area graphene on insulating substrates impair its excellent electronic properties. A defect-free transfer can now be obtained thanks to capillary bridges that anchor the graphene film to the substrate while the underlying growth layer is etched away.
Jaime A. Torres and Richard B. Kaner
I
magine a material that is one atom thick1 yet 200 times stronger than steel2. A material that conducts electrons better than copper but is as flexible as paper and is also the ultimate thermal conductor 3. Since its first isolation in a stable form in 2004, graphene has demonstrated this combination of spectacular properties and more. It has revolutionized multiple fields of research, excited many industries and captivated the minds of numerous futurists and writers. Despite its revolutionary potential, the commercial applications of graphene remain just out of reach because its successful integration into electronic devices such as transistors and mobile phones requires a high-fidelity automated growth and transfer process that has proved difficult to achieve. Reporting in Nature, researchers at the National University of Singapore now hope to bring graphene closer to real applications with the introduction of a single-step face-toface transfer method demonstrated on wafer scale4. Substantial progress has been made in the growth of graphene. Whereas the first few flakes of this wonder material were painstakingly isolated with cellophane tape, many avenues now exist for the production of ever-improving high-quality graphene5–7. In particular, chemical vapour deposition is proving to be one of the most promising approaches for the synthesis of graphene, enabling the fine control of the morphology 8 and doping level9 of this material, as well as the realization of heterostructures a
combining graphene with other two-dimensional materials10. But the growth by chemical vapour deposition requires the use of a metal catalyst such as copper or nickel on a rigid substrate. To be integrated into devices, the as-grown, high-quality graphene must be subsequently transferred to suitable substrates — typically silicon oxide or other insulating materials — without introducing cracks or defects that will deteriorate its extraordinary properties. However, the defect-free transfer of graphene from its growth media to usable platforms remains a formidable challenge. In one of the most used transfer protocols, a polymer support layer — typically polymethyl methacrylate (PMMA) — is placed on top of the graphene film, followed by removal of the underlying metal catalyst by wet etching. The resulting free-floating polymer– graphene film can be fished out and placed onto almost any substrate. Unfortunately, such wet transfers rarely give continuous large-area sheets and are not amenable to automated batch processing. Worse yet, interactions between the surface tension of the water and the underlying graphene film can cause wrinkles and tears that diminish its electronic properties. A potential breakthrough has emerged from the researchers at the National University of Singapore4. They found that the use of a nitrogen plasma treatment to clean the surface of the insulating SiO2–Si substrate before the deposition of the metal catalyst results in the generation of gas bubbles during the high-temperature
b
c Bubbles
Ions
Graphene Cu
N2 plasma
growth of graphene. This ‘bubble seeding’ creates capillary bridges between the graphene and the substrate. The effects of capillary forces are familiar — as when a paper towel is used to clean up a spill. The liquid fills the narrow pores in the paper because of cohesion and adhesive forces, seemingly defying gravity. Nature, too, draws on this phenomenon in the feet of terrestrial tree frogs11 and beetles who use capillary forces to stay adhered to leaves and surfaces even under flowing water. Likewise, the formation of narrow bubble channels between an insulating substrate and graphene produces long-range attractive forces, even in the presence of infiltrating water from the etching bath (Fig. 1). While the water-based etchant acts to remove the catalyst, voids are created allowing water to infiltrate the substrate/graphene interface, but the strong attractive capillary forces arising from bubble formation keeps the graphene anchored onto the surface of the substrate. After wet etching of the metal layer, the remaining water can simply be removed by baking the sample, and the graphene layer is deposited face-to-face on the insulating substrate without cracking or being damaged. This is demonstrated by the excellent electronic properties of the transferred graphene films measuring up to eight inches in diameter: the researchers measured a uniform conductivity along the wafers reaching up to 4,000 S cm−1, indicative of high crystallinity. Where will this discovery make an immediate impact? So far, the only limit
d PMMA Graphene
Bubbles
e PMMA Graphene Water
Cu
Graphene
SiO2
SiO2
SiO2
SiO2
SiO2
Si
Si
Si
Si
Si
Figure 1 | Schematic of face-to-face transfer of graphene following plasma treatment. a, Nitrogen ion species are adsorbed onto the surface of a rigid substrate. b, During graphene growth, ions form bubbles that are trapped in the Cu catalyst layer by the graphene film. c, Water infiltrates into the graphene/SiO2 interface during catalyst etching, forming capillary bridges. d, Bubble capillary bridges keep graphene anchored to the SiO2 surface. e, After baking, the water is removed and the graphene is transferred on the substrate. 328
NATURE MATERIALS | VOL 13 | APRIL 2014 | www.nature.com/naturematerials
© 2014 Macmillan Publishers Limited. All rights reserved
news & views to this approach is the need to use a rigid substrate that can withstand the high-temperature growth (>600 °C) of the graphene film. As a result, the microelectronics industry, which seeks the application of highly crystalline and continuous graphene for siliconbased integrated devices, will benefit the most. The elimination of the fishing step means that the whole fabrication process, from the growth of graphene to its deposition on an insulating substrate, can be automated and scaled. The nitrogen species implantation step uses readily available microelectronics equipment and has the added advantage of reducing the etching time of the catalytic growth
substrate. Notably, the resulting graphene films are uniform and unaffected by this nitrogen plasma pretreatment. By replacing the SiO2–Si substrate with a transparent material such as quartz or sapphire, the use of graphene as a transparent conductor can also be achieved. Substrates that lattice match with graphene such as hexagonal boron nitride or transitionmetal chalcogenides may potentially be used to give higher-performance integrated electronics. The potential for plant-scale production created by this face-to-face transfer may mean that commercial products containing highquality transferred graphene can soon be realized. ❐
Jaime A. Torres and Richard B. Kaner are in the Department of Chemistry & Biochemistry and Department of Materials Science & Engineering, University of California, Los Angeles, California 90095-1569, USA. e-mail: [email protected]; [email protected] References 1. Geim, A. K. & Novoselov, K. S. Nature Mater. 6, 183–191 (2007). 2. Lee, G-H. et al. Science 340, 1073–1076 (2013). 3. Seol, J. H. et al. Science 328, 213–216 (2010). 4. Gao, L. et al. Nature 505, 190–194 (2014). 5. Li, X. S. et al. J. Am. Chem. Soc. 133, 2816–2819 (2011). 6. Wassei, J. K. & Kaner, R. B. Acc. Chem. Res. 46, 2244–2253 (2013). 7. Li, X. et al. Science 324, 1312–1314 (2009). 8. Zhou, H. et al. Nature Commun. 4, 2096 (2013). 9. Reddy, A. L. M. et al. ACS Nano 4, 6337–6342 (2010). 10. Levendorf, M. P. et al. Nature 488, 627–632 (2012). 11. Federle, W., Barnes, W. J. P., Baumgartner, W., Drechsler, P. & Smith, J. M. J. R. Soc. Interface 3, 689–697 (2006).
BIOINSPIRED MATERIALS
Boosting plant biology
Chloroplasts with extended photosynthetic activity beyond the visible absorption spectrum, and living leaves that perform non-biological functions, are made possible by localizing nanoparticles within plant organelles.
Gregory D. Scholes and Edward H. Sargent
T
he photosynthetic machinery is utterly ingenious. It uses the antenna effect, wherein many high-cross-section light absorbers — molecules, such as chlorophyll, that are embedded in the protein complexes that make up the photosynthetic unit of plants — funnel their energy (transiently captured as photoexcitations) into a much smaller number of reaction centres, the protein complexes that initiate a chain of reactions to convert the photoexcitations into chemical energy. Across the Earth, absorbed sunlight powers biochemical processes that produce a staggering 100 billion tons of biomass annually. Human-engineered analogues of photosynthetic systems include dye-sensitized solar cells1 and energygradient devices2. However, aspects of the photosynthetic system, such as the spectral cross-section for light harvesting, could be optimized further. Michael Strano and colleagues now report in Nature Materials that the localization of nanoparticles within plant chloroplasts aids photosynthesis through the broadening of the spectral capture of light and the scavenging of radical oxygen species3. Furthermore, the researchers report the first steps of what they term plant nanobionics: the enhancement of plant functions
through the combination of biology and nanotechnology. They also showed that living plant leaves can be embedded with nanoparticle-based sensors to monitor nitric oxide in real time. Strano and collaborators first showed that two nanoscale systems, singlewalled carbon nanotubes (SWNTs) and ceria nanoparticles, can traverse and localize within the lipid envelope of plant chloroplasts. SWNTs and ceria are attractive choices, as they have the potential to couple to the photosynthetic system. In particular, ceria is a well-known quencher of reactive oxygen species that may be produced by rogue photoexcitations, and SWNTs offer a broadband spectral capture, absorbing photons of energies lower than those typically absorbed by plants. Moreover, SWNTs can transport electronic excitations across extraordinary distances4. In fact, Strano and co-authors report that SWNTs show photoluminescence at longer wavelengths (785 nm) than natural photosynthetic reaction centres (chlorophyll excitation occurs at 700 nm). To demonstrate augmented photosynthesis in a much broader spectral bandwidth, however, further improvements to the SWNT–chlorophyll (or more generally nanoparticle–chlorophyll) hybrid system would be required. For instance, the
NATURE MATERIALS | VOL 13 | APRIL 2014 | www.nature.com/naturematerials
© 2014 Macmillan Publishers Limited. All rights reserved
reaction centre could be modified so that it can capture and process near-infrared excitations. Interestingly, the authors suggest a further possible element of sensitization within the photosynthetic apparatus by proposing that SWNTs may introduce electrons into the photosynthetic reactions. Structurally, the photosynthetic machinery is located in the thylakoid membranes within chloroplasts (Fig. 1). However, in some photosynthetic organisms, additional light-harvesting complexes dock on the stromal side of the membrane (in the case of cyanobacteria and red algae) or locate in the lumen (cryptophyte algae)4. Strano and colleagues show that SWNTs and nanoceria pass through the outer membranes of the chloroplast and locate in the stroma, and that, remarkably, both nanoparticles influence photosynthetic performance. It would be interesting to investigate the possibility to introduce nanoscale systems that sit in selected positions across the chloroplast system, thus potentially adding spatial localization to the new or augmented functionalities arising from the coupling of the nanoparticles to the biological components. This is particularly critical in the case of SWNTs, as their role in augmenting plant 329
Graphene closer to fruition
Cracks and defects induced during the transfer of large-area graphene on insulating substrates impair its excellent electronic properties. A defect-free transfer can now be obtained thanks to capillary bridges that anchor the graphene film to the substrate while the underlying growth layer is etched away.
Jaime A. Torres and Richard B. Kaner
I
magine a material that is one atom thick1 yet 200 times stronger than steel2. A material that conducts electrons better than copper but is as flexible as paper and is also the ultimate thermal conductor 3. Since its first isolation in a stable form in 2004, graphene has demonstrated this combination of spectacular properties and more. It has revolutionized multiple fields of research, excited many industries and captivated the minds of numerous futurists and writers. Despite its revolutionary potential, the commercial applications of graphene remain just out of reach because its successful integration into electronic devices such as transistors and mobile phones requires a high-fidelity automated growth and transfer process that has proved difficult to achieve. Reporting in Nature, researchers at the National University of Singapore now hope to bring graphene closer to real applications with the introduction of a single-step face-toface transfer method demonstrated on wafer scale4. Substantial progress has been made in the growth of graphene. Whereas the first few flakes of this wonder material were painstakingly isolated with cellophane tape, many avenues now exist for the production of ever-improving high-quality graphene5–7. In particular, chemical vapour deposition is proving to be one of the most promising approaches for the synthesis of graphene, enabling the fine control of the morphology 8 and doping level9 of this material, as well as the realization of heterostructures a
combining graphene with other two-dimensional materials10. But the growth by chemical vapour deposition requires the use of a metal catalyst such as copper or nickel on a rigid substrate. To be integrated into devices, the as-grown, high-quality graphene must be subsequently transferred to suitable substrates — typically silicon oxide or other insulating materials — without introducing cracks or defects that will deteriorate its extraordinary properties. However, the defect-free transfer of graphene from its growth media to usable platforms remains a formidable challenge. In one of the most used transfer protocols, a polymer support layer — typically polymethyl methacrylate (PMMA) — is placed on top of the graphene film, followed by removal of the underlying metal catalyst by wet etching. The resulting free-floating polymer– graphene film can be fished out and placed onto almost any substrate. Unfortunately, such wet transfers rarely give continuous large-area sheets and are not amenable to automated batch processing. Worse yet, interactions between the surface tension of the water and the underlying graphene film can cause wrinkles and tears that diminish its electronic properties. A potential breakthrough has emerged from the researchers at the National University of Singapore4. They found that the use of a nitrogen plasma treatment to clean the surface of the insulating SiO2–Si substrate before the deposition of the metal catalyst results in the generation of gas bubbles during the high-temperature
b
c Bubbles
Ions
Graphene Cu
N2 plasma
growth of graphene. This ‘bubble seeding’ creates capillary bridges between the graphene and the substrate. The effects of capillary forces are familiar — as when a paper towel is used to clean up a spill. The liquid fills the narrow pores in the paper because of cohesion and adhesive forces, seemingly defying gravity. Nature, too, draws on this phenomenon in the feet of terrestrial tree frogs11 and beetles who use capillary forces to stay adhered to leaves and surfaces even under flowing water. Likewise, the formation of narrow bubble channels between an insulating substrate and graphene produces long-range attractive forces, even in the presence of infiltrating water from the etching bath (Fig. 1). While the water-based etchant acts to remove the catalyst, voids are created allowing water to infiltrate the substrate/graphene interface, but the strong attractive capillary forces arising from bubble formation keeps the graphene anchored onto the surface of the substrate. After wet etching of the metal layer, the remaining water can simply be removed by baking the sample, and the graphene layer is deposited face-to-face on the insulating substrate without cracking or being damaged. This is demonstrated by the excellent electronic properties of the transferred graphene films measuring up to eight inches in diameter: the researchers measured a uniform conductivity along the wafers reaching up to 4,000 S cm−1, indicative of high crystallinity. Where will this discovery make an immediate impact? So far, the only limit
d PMMA Graphene
Bubbles
e PMMA Graphene Water
Cu
Graphene
SiO2
SiO2
SiO2
SiO2
SiO2
Si
Si
Si
Si
Si
Figure 1 | Schematic of face-to-face transfer of graphene following plasma treatment. a, Nitrogen ion species are adsorbed onto the surface of a rigid substrate. b, During graphene growth, ions form bubbles that are trapped in the Cu catalyst layer by the graphene film. c, Water infiltrates into the graphene/SiO2 interface during catalyst etching, forming capillary bridges. d, Bubble capillary bridges keep graphene anchored to the SiO2 surface. e, After baking, the water is removed and the graphene is transferred on the substrate. 328
NATURE MATERIALS | VOL 13 | APRIL 2014 | www.nature.com/naturematerials
© 2014 Macmillan Publishers Limited. All rights reserved
news & views to this approach is the need to use a rigid substrate that can withstand the high-temperature growth (>600 °C) of the graphene film. As a result, the microelectronics industry, which seeks the application of highly crystalline and continuous graphene for siliconbased integrated devices, will benefit the most. The elimination of the fishing step means that the whole fabrication process, from the growth of graphene to its deposition on an insulating substrate, can be automated and scaled. The nitrogen species implantation step uses readily available microelectronics equipment and has the added advantage of reducing the etching time of the catalytic growth
substrate. Notably, the resulting graphene films are uniform and unaffected by this nitrogen plasma pretreatment. By replacing the SiO2–Si substrate with a transparent material such as quartz or sapphire, the use of graphene as a transparent conductor can also be achieved. Substrates that lattice match with graphene such as hexagonal boron nitride or transitionmetal chalcogenides may potentially be used to give higher-performance integrated electronics. The potential for plant-scale production created by this face-to-face transfer may mean that commercial products containing highquality transferred graphene can soon be realized. ❐
Jaime A. Torres and Richard B. Kaner are in the Department of Chemistry & Biochemistry and Department of Materials Science & Engineering, University of California, Los Angeles, California 90095-1569, USA. e-mail: [email protected]; [email protected] References 1. Geim, A. K. & Novoselov, K. S. Nature Mater. 6, 183–191 (2007). 2. Lee, G-H. et al. Science 340, 1073–1076 (2013). 3. Seol, J. H. et al. Science 328, 213–216 (2010). 4. Gao, L. et al. Nature 505, 190–194 (2014). 5. Li, X. S. et al. J. Am. Chem. Soc. 133, 2816–2819 (2011). 6. Wassei, J. K. & Kaner, R. B. Acc. Chem. Res. 46, 2244–2253 (2013). 7. Li, X. et al. Science 324, 1312–1314 (2009). 8. Zhou, H. et al. Nature Commun. 4, 2096 (2013). 9. Reddy, A. L. M. et al. ACS Nano 4, 6337–6342 (2010). 10. Levendorf, M. P. et al. Nature 488, 627–632 (2012). 11. Federle, W., Barnes, W. J. P., Baumgartner, W., Drechsler, P. & Smith, J. M. J. R. Soc. Interface 3, 689–697 (2006).
BIOINSPIRED MATERIALS
Boosting plant biology
Chloroplasts with extended photosynthetic activity beyond the visible absorption spectrum, and living leaves that perform non-biological functions, are made possible by localizing nanoparticles within plant organelles.
Gregory D. Scholes and Edward H. Sargent
T
he photosynthetic machinery is utterly ingenious. It uses the antenna effect, wherein many high-cross-section light absorbers — molecules, such as chlorophyll, that are embedded in the protein complexes that make up the photosynthetic unit of plants — funnel their energy (transiently captured as photoexcitations) into a much smaller number of reaction centres, the protein complexes that initiate a chain of reactions to convert the photoexcitations into chemical energy. Across the Earth, absorbed sunlight powers biochemical processes that produce a staggering 100 billion tons of biomass annually. Human-engineered analogues of photosynthetic systems include dye-sensitized solar cells1 and energygradient devices2. However, aspects of the photosynthetic system, such as the spectral cross-section for light harvesting, could be optimized further. Michael Strano and colleagues now report in Nature Materials that the localization of nanoparticles within plant chloroplasts aids photosynthesis through the broadening of the spectral capture of light and the scavenging of radical oxygen species3. Furthermore, the researchers report the first steps of what they term plant nanobionics: the enhancement of plant functions
through the combination of biology and nanotechnology. They also showed that living plant leaves can be embedded with nanoparticle-based sensors to monitor nitric oxide in real time. Strano and collaborators first showed that two nanoscale systems, singlewalled carbon nanotubes (SWNTs) and ceria nanoparticles, can traverse and localize within the lipid envelope of plant chloroplasts. SWNTs and ceria are attractive choices, as they have the potential to couple to the photosynthetic system. In particular, ceria is a well-known quencher of reactive oxygen species that may be produced by rogue photoexcitations, and SWNTs offer a broadband spectral capture, absorbing photons of energies lower than those typically absorbed by plants. Moreover, SWNTs can transport electronic excitations across extraordinary distances4. In fact, Strano and co-authors report that SWNTs show photoluminescence at longer wavelengths (785 nm) than natural photosynthetic reaction centres (chlorophyll excitation occurs at 700 nm). To demonstrate augmented photosynthesis in a much broader spectral bandwidth, however, further improvements to the SWNT–chlorophyll (or more generally nanoparticle–chlorophyll) hybrid system would be required. For instance, the
NATURE MATERIALS | VOL 13 | APRIL 2014 | www.nature.com/naturematerials
© 2014 Macmillan Publishers Limited. All rights reserved
reaction centre could be modified so that it can capture and process near-infrared excitations. Interestingly, the authors suggest a further possible element of sensitization within the photosynthetic apparatus by proposing that SWNTs may introduce electrons into the photosynthetic reactions. Structurally, the photosynthetic machinery is located in the thylakoid membranes within chloroplasts (Fig. 1). However, in some photosynthetic organisms, additional light-harvesting complexes dock on the stromal side of the membrane (in the case of cyanobacteria and red algae) or locate in the lumen (cryptophyte algae)4. Strano and colleagues show that SWNTs and nanoceria pass through the outer membranes of the chloroplast and locate in the stroma, and that, remarkably, both nanoparticles influence photosynthetic performance. It would be interesting to investigate the possibility to introduce nanoscale systems that sit in selected positions across the chloroplast system, thus potentially adding spatial localization to the new or augmented functionalities arising from the coupling of the nanoparticles to the biological components. This is particularly critical in the case of SWNTs, as their role in augmenting plant 329
