news & views Another limitation of this technology concerns the inability of alginate strands to penetrate deep into the gel, which consequently limits the amount of drug that can be refilled. This drawback could be resolved by increasing gel surface area and/or porosity. Inspired by the ability of nanotherapeutics to target specific tissues, Mooney and colleagues have developed a novel method for
blood-based refilling of drug-delivery devices. If translated into the clinic, it could offer several advantages, such as lower off-target toxicities and better comfort for the patient, in diverse applications ranging from cancer therapy to wound healing, drug-eluting vascular grafts and stents. ❐ Patrick Couvreur is at the Institut Galien, UMR CNRS 8612, Université Paris-Sud,
5, rue Jean-Baptiste Clément 92296 ChatenayMalabry, France. e-mail: [email protected] References
1. Brudno, Y. et al. Proc. Natl Acad. Sci. USA 111, 12722–12727 (2014). 2. Al-Shamkhani, A. & Duncan, R. J. Bioact. Compat. Polym. 10, 4–13 (1995). 3. Nakada, Y., Fattal, E., Foulquier, M. & Couvreur, P. Pharm. Res. 13, 38–43 (1995). 4. Kolb, H. C., Finn, M. G. & Sharpless, K. B. Angew. Chem. Int. Ed. 40, 2004–2021 (2001).
GRAPHENE NANORIBBONS
Chemical stitching
Junctions between graphene nanoribbons with different electronic properties can be created by using a bottom-up synthesis method.
Xinran Wang
T
he development of graphene-based electronics has been an important research goal ever since the material was isolated in 20041. For digital applications, the biggest hurdle for graphene is its lack of a bandgap. As a result, graphene field-effect transistors typically have large leakage current and cannot be completely switched off. Over the past decade, materials scientists have been trying different strategies to open a bandgap in graphene. So far, the most successful and technologically relevant approach is to slice graphene into one-dimensional ribbons that are only a few nanometres wide. A bandgap can open up in the electronic spectrum of these graphene nanoribbons as a result of the extreme spatial confinement, and transistors based on sub-5-nm graphene nanoribbons have been fabricated with an on/off ratio up to 106 (ref. 2), which is adequate for most logic applications. However, for many other applications such as diodes, bipolar junction transistors and photovoltaic devices, an added requirement is the fabrication of heterojunctions.
Heterojunctions form at the interface between two different solid-state materials. They serve as the fundamental building blocks for many modern electronic and optoelectronic devices. A simple example is a p–n junction, which is between materials with different electronic doping. The carrier diffusion results in a potential barrier at the junction interface, the socalled depletion region. The associated built-in electric field in this region can, for example, efficiently separate photoexcited carriers to give a photovoltaic effect. Writing in Nature Nanotechnology, Roman Fasel and colleagues now report the bottom-up synthesis of graphene nanoribbon heterojunctions, an important first step towards the realization of complex graphene devices3. The researchers — who are based at Swiss Federal Laboratories for Materials Science and Technology, Rensselaer Polytechnic Institute, Max Planck Institute for Polymer Research and the University of Bern — created p–n junctions from graphene
a
nanoribbons by merging two types of monomer precursor on a Au(111) substrate under ultrahigh-vacuum conditions. The monomers were the same except that two phenyl groups in one monomer were replaced by pyrimidine groups. Fasel and colleagues have previously synthesized pristine graphene nanoribbons using one type of monomer precursor and a two-step polymerization process4. Here, the team improved the technique by evaporating the two monomers consecutively on the same substrate. After annealing, they obtained heterojunctions along the chevron-type nanoribbons (Fig. 1a). Because nitrogen has one more valence electron than carbon, the pyrimidine-containing parts are electron-rich compared with the phenylcontaining parts. Therefore, as pyrimidine and phenyl portions are stitched together, the overall nanoribbon naturally contains many nanoscale p–n junctions connected in series. Scanning tunnelling microscopy studies revealed a band offset of ~0.5 eV at each heterojunction suggesting that b Ec
Ev
Figure 1 | Graphene nanoribbon heterostructures. a, Scanning tunnelling microscope image (main) and potential chemical structure of a series of heterojunctions3. (This schematic is just for illustrative purposes and may not correspond to the actual chemical structure of the nanoribbon shown in the scanning tunnelling microscope image.) Blue and grey areas are negatively doped (n-doped monomers) and pristine graphene, respectively. Scale bar, 2 nm. b, Band diagram of the heterojunction highlighted (dashed rectangle) in a. The band alignment is of type II, or staggered heterojunctions. Ec, conduction band; Ev, valence band. NATURE NANOTECHNOLOGY | VOL 9 | NOVEMBER 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
875
news & views the heterojunctions are type II with a staggered gap (Fig. 1b). The junction interface is extremely sharp (~2 nm), which is important to enable the efficient separation of charge carriers in diodes or photovoltaic devices. The significance of the work by Fasel and colleagues is further illustrated by the high precision with which they could control the nanoscale junctions. It is well known that the properties of graphene nanoribbons are highly dependent on edge structures. For example, in graphene nanoribbons with zigzag edges, magnetic edge states and half-metals are predicted, which can be exploited for spintronic devices5. Graphene nanoribbons with armchair edges, on the other hand, behave as semiconductors with tunable bandgaps6. The researchers convincingly demonstrated an ability to engineer the edge terminations by rational design of the precursors, making it possible to craft artificial junctions from the bottom up. Furthermore, the team showed that by controlling the number of pyrimidine groups in the monomer precursor, the band offset at the p–n junction could be
engineered. Such capability opens up an enormous space for materials scientists and electrical engineers to design the properties of heterojunctions on-demand. In this regard, collaboration between theoreticians and experimentalists could prove to be particularly beneficial. With graphene nanoribbon heterojunctions at hand, the next step is to fabricate devices and study their electronic transport properties. Some progress has already been made in this direction. To make contact to individual heterojunctions, first the heterojunctions have to be transferred from the metal surface on which they are grown to an insulating substrate such as SiO2/Si, avoiding structural damage. This was recently demonstrated by using a polymer support while etching the underlying metal substrate7, similar to the transfer of graphene grown by chemical vapour deposition8. Fasel and co-workers have also developed their own protocol for such transfer 3. Because the heterojunctions are randomly grown on a metal substrate and are typically shorter than tens of nanometres, state-of-the-art electron-beam lithography has to be used
to contact individual junctions8, which is by no means an easy task. When these technical difficulties are overcome, we can expect to see effects such as electronic rectification and resonant tunnelling in these graphene nanoribbon heterojunction devices. However, in order for these nanoscale heterojunctions to become technologically useful, scalable synthesis and processing also has to be developed. This remains a long-term goal in graphene nanoribbon research. ❐ Xinran Wang is at the National Laboratory of Solid State Microstructures, School of Electronic Science and Engineering, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China. e-mail: [email protected] References 1. 2. 3. 4. 5. 6. 7. 8.
Novoselov, K. S. et al. Nature 490, 192–200 (2012). Li, X. et al. Science 319, 1229–1232 (2008). Cai, J. et al. Nature Nanotech. 9, 896–900 (2014). Cai, J. et al. Nature 466, 470–473 (2010). Son, Y.‑W. et al. Nature 444, 347–349 (2006). Nakada, K. et al. Phys. Rev. B 54, 17954–17961 (1996). Bennett, P. B. et al. Appl. Phys. Lett. 103, 253114 (2013). Li, X. et al. Science 324, 1312–1314 (2009).
SINGLE-MOLECULE JUNCTIONS
Thermoelectricity at the gate
The electrical conductance and the Seebeck coefficient of molecular junction devices can be simultaneously enhanced using a gate electrode.
Jeffrey B. Neaton
A
round half of the industrial energy consumption in the United States is lost through heat1. Collecting even a fraction of it to generate usable electricity could significantly reduce overall energy power consumption and, in turn, have a positive effect on the environment. One approach to transforming heat into electricity is through the thermoelectric effect — a phenomenon exhibited by certain materials in which a temperature difference induces a flow of carriers from a hot terminal to a cold terminal, creating an electrical voltage. The energy conversion efficiency of a thermoelectric material is proportional to the power factor, S2G, where S is the Seebeck coefficient and G the electrical conductance. However, developing materials that can offer both enhanced electrical conductances and Seebeck coefficients has proved to be a considerable challenge. Writing in Nature Nanotechnology, Pramod Reddy and co-workers at the University of Michigan2 876
now show that these two properties can be controlled and simultaneously enhanced in single-molecule junctions with the help of a gate electrode. The power factor of a bulk thermoelectric material can be modified by altering the carrier concentration via gating or doping. However, in many conventional bulk thermoelectrics, S and G trend in opposite directions with carrier density, limiting increases in thermoelectric efficiency. In particular, while G is commonly sensitive to the overall magnitude of the carrier density, S is related to the asymmetry of the empty and filled states of the junction within a range kBΔT around the Fermi level (kB is Boltzmann’s constant and ΔT is the temperature difference between electrodes; Fig. 1). In typical bulk semiconductor thermoelectrics, such as Bi2Te3 and Sr oxides, the greatest asymmetry in the density of states — leading to the largest voltage for a given ΔT — occurs near the band edge. But here the carrier concentration
(and conductance) is relatively low. More than a decade ago it was suggested that, by capitalizing on sharp asymmetric features in the densities of states of low-dimensional systems, nanostructured materials could perhaps be used to increase S and G simultaneously 3–5. Enter molecular junctions. These are systems that consist of individual or few molecules trapped between macroscopic electrodes. Owing to recent advances in experiment and theory, molecular junctions are proving to be increasingly useful in understanding non-equilibrium electronic structure and function at interfaces. By varying the organic molecule and the electrode material, junction properties can be tuned, and previous pioneering transport studies of molecular junctions6,7 have hinted at ways to improve thermoelectric performance8–10. A central quantity that describes the transport properties of molecular junctions in
NATURE NANOTECHNOLOGY | VOL 9 | NOVEMBER 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
blood-based refilling of drug-delivery devices. If translated into the clinic, it could offer several advantages, such as lower off-target toxicities and better comfort for the patient, in diverse applications ranging from cancer therapy to wound healing, drug-eluting vascular grafts and stents. ❐ Patrick Couvreur is at the Institut Galien, UMR CNRS 8612, Université Paris-Sud,
5, rue Jean-Baptiste Clément 92296 ChatenayMalabry, France. e-mail: [email protected] References
1. Brudno, Y. et al. Proc. Natl Acad. Sci. USA 111, 12722–12727 (2014). 2. Al-Shamkhani, A. & Duncan, R. J. Bioact. Compat. Polym. 10, 4–13 (1995). 3. Nakada, Y., Fattal, E., Foulquier, M. & Couvreur, P. Pharm. Res. 13, 38–43 (1995). 4. Kolb, H. C., Finn, M. G. & Sharpless, K. B. Angew. Chem. Int. Ed. 40, 2004–2021 (2001).
GRAPHENE NANORIBBONS
Chemical stitching
Junctions between graphene nanoribbons with different electronic properties can be created by using a bottom-up synthesis method.
Xinran Wang
T
he development of graphene-based electronics has been an important research goal ever since the material was isolated in 20041. For digital applications, the biggest hurdle for graphene is its lack of a bandgap. As a result, graphene field-effect transistors typically have large leakage current and cannot be completely switched off. Over the past decade, materials scientists have been trying different strategies to open a bandgap in graphene. So far, the most successful and technologically relevant approach is to slice graphene into one-dimensional ribbons that are only a few nanometres wide. A bandgap can open up in the electronic spectrum of these graphene nanoribbons as a result of the extreme spatial confinement, and transistors based on sub-5-nm graphene nanoribbons have been fabricated with an on/off ratio up to 106 (ref. 2), which is adequate for most logic applications. However, for many other applications such as diodes, bipolar junction transistors and photovoltaic devices, an added requirement is the fabrication of heterojunctions.
Heterojunctions form at the interface between two different solid-state materials. They serve as the fundamental building blocks for many modern electronic and optoelectronic devices. A simple example is a p–n junction, which is between materials with different electronic doping. The carrier diffusion results in a potential barrier at the junction interface, the socalled depletion region. The associated built-in electric field in this region can, for example, efficiently separate photoexcited carriers to give a photovoltaic effect. Writing in Nature Nanotechnology, Roman Fasel and colleagues now report the bottom-up synthesis of graphene nanoribbon heterojunctions, an important first step towards the realization of complex graphene devices3. The researchers — who are based at Swiss Federal Laboratories for Materials Science and Technology, Rensselaer Polytechnic Institute, Max Planck Institute for Polymer Research and the University of Bern — created p–n junctions from graphene
a
nanoribbons by merging two types of monomer precursor on a Au(111) substrate under ultrahigh-vacuum conditions. The monomers were the same except that two phenyl groups in one monomer were replaced by pyrimidine groups. Fasel and colleagues have previously synthesized pristine graphene nanoribbons using one type of monomer precursor and a two-step polymerization process4. Here, the team improved the technique by evaporating the two monomers consecutively on the same substrate. After annealing, they obtained heterojunctions along the chevron-type nanoribbons (Fig. 1a). Because nitrogen has one more valence electron than carbon, the pyrimidine-containing parts are electron-rich compared with the phenylcontaining parts. Therefore, as pyrimidine and phenyl portions are stitched together, the overall nanoribbon naturally contains many nanoscale p–n junctions connected in series. Scanning tunnelling microscopy studies revealed a band offset of ~0.5 eV at each heterojunction suggesting that b Ec
Ev
Figure 1 | Graphene nanoribbon heterostructures. a, Scanning tunnelling microscope image (main) and potential chemical structure of a series of heterojunctions3. (This schematic is just for illustrative purposes and may not correspond to the actual chemical structure of the nanoribbon shown in the scanning tunnelling microscope image.) Blue and grey areas are negatively doped (n-doped monomers) and pristine graphene, respectively. Scale bar, 2 nm. b, Band diagram of the heterojunction highlighted (dashed rectangle) in a. The band alignment is of type II, or staggered heterojunctions. Ec, conduction band; Ev, valence band. NATURE NANOTECHNOLOGY | VOL 9 | NOVEMBER 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
875
news & views the heterojunctions are type II with a staggered gap (Fig. 1b). The junction interface is extremely sharp (~2 nm), which is important to enable the efficient separation of charge carriers in diodes or photovoltaic devices. The significance of the work by Fasel and colleagues is further illustrated by the high precision with which they could control the nanoscale junctions. It is well known that the properties of graphene nanoribbons are highly dependent on edge structures. For example, in graphene nanoribbons with zigzag edges, magnetic edge states and half-metals are predicted, which can be exploited for spintronic devices5. Graphene nanoribbons with armchair edges, on the other hand, behave as semiconductors with tunable bandgaps6. The researchers convincingly demonstrated an ability to engineer the edge terminations by rational design of the precursors, making it possible to craft artificial junctions from the bottom up. Furthermore, the team showed that by controlling the number of pyrimidine groups in the monomer precursor, the band offset at the p–n junction could be
engineered. Such capability opens up an enormous space for materials scientists and electrical engineers to design the properties of heterojunctions on-demand. In this regard, collaboration between theoreticians and experimentalists could prove to be particularly beneficial. With graphene nanoribbon heterojunctions at hand, the next step is to fabricate devices and study their electronic transport properties. Some progress has already been made in this direction. To make contact to individual heterojunctions, first the heterojunctions have to be transferred from the metal surface on which they are grown to an insulating substrate such as SiO2/Si, avoiding structural damage. This was recently demonstrated by using a polymer support while etching the underlying metal substrate7, similar to the transfer of graphene grown by chemical vapour deposition8. Fasel and co-workers have also developed their own protocol for such transfer 3. Because the heterojunctions are randomly grown on a metal substrate and are typically shorter than tens of nanometres, state-of-the-art electron-beam lithography has to be used
to contact individual junctions8, which is by no means an easy task. When these technical difficulties are overcome, we can expect to see effects such as electronic rectification and resonant tunnelling in these graphene nanoribbon heterojunction devices. However, in order for these nanoscale heterojunctions to become technologically useful, scalable synthesis and processing also has to be developed. This remains a long-term goal in graphene nanoribbon research. ❐ Xinran Wang is at the National Laboratory of Solid State Microstructures, School of Electronic Science and Engineering, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China. e-mail: [email protected] References 1. 2. 3. 4. 5. 6. 7. 8.
Novoselov, K. S. et al. Nature 490, 192–200 (2012). Li, X. et al. Science 319, 1229–1232 (2008). Cai, J. et al. Nature Nanotech. 9, 896–900 (2014). Cai, J. et al. Nature 466, 470–473 (2010). Son, Y.‑W. et al. Nature 444, 347–349 (2006). Nakada, K. et al. Phys. Rev. B 54, 17954–17961 (1996). Bennett, P. B. et al. Appl. Phys. Lett. 103, 253114 (2013). Li, X. et al. Science 324, 1312–1314 (2009).
SINGLE-MOLECULE JUNCTIONS
Thermoelectricity at the gate
The electrical conductance and the Seebeck coefficient of molecular junction devices can be simultaneously enhanced using a gate electrode.
Jeffrey B. Neaton
A
round half of the industrial energy consumption in the United States is lost through heat1. Collecting even a fraction of it to generate usable electricity could significantly reduce overall energy power consumption and, in turn, have a positive effect on the environment. One approach to transforming heat into electricity is through the thermoelectric effect — a phenomenon exhibited by certain materials in which a temperature difference induces a flow of carriers from a hot terminal to a cold terminal, creating an electrical voltage. The energy conversion efficiency of a thermoelectric material is proportional to the power factor, S2G, where S is the Seebeck coefficient and G the electrical conductance. However, developing materials that can offer both enhanced electrical conductances and Seebeck coefficients has proved to be a considerable challenge. Writing in Nature Nanotechnology, Pramod Reddy and co-workers at the University of Michigan2 876
now show that these two properties can be controlled and simultaneously enhanced in single-molecule junctions with the help of a gate electrode. The power factor of a bulk thermoelectric material can be modified by altering the carrier concentration via gating or doping. However, in many conventional bulk thermoelectrics, S and G trend in opposite directions with carrier density, limiting increases in thermoelectric efficiency. In particular, while G is commonly sensitive to the overall magnitude of the carrier density, S is related to the asymmetry of the empty and filled states of the junction within a range kBΔT around the Fermi level (kB is Boltzmann’s constant and ΔT is the temperature difference between electrodes; Fig. 1). In typical bulk semiconductor thermoelectrics, such as Bi2Te3 and Sr oxides, the greatest asymmetry in the density of states — leading to the largest voltage for a given ΔT — occurs near the band edge. But here the carrier concentration
(and conductance) is relatively low. More than a decade ago it was suggested that, by capitalizing on sharp asymmetric features in the densities of states of low-dimensional systems, nanostructured materials could perhaps be used to increase S and G simultaneously 3–5. Enter molecular junctions. These are systems that consist of individual or few molecules trapped between macroscopic electrodes. Owing to recent advances in experiment and theory, molecular junctions are proving to be increasingly useful in understanding non-equilibrium electronic structure and function at interfaces. By varying the organic molecule and the electrode material, junction properties can be tuned, and previous pioneering transport studies of molecular junctions6,7 have hinted at ways to improve thermoelectric performance8–10. A central quantity that describes the transport properties of molecular junctions in
NATURE NANOTECHNOLOGY | VOL 9 | NOVEMBER 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
