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
news & views a
b Junction geometry
Vacuum LUMO
d Vg
HOMO
Au contact
Gate electrode
c
T
T + ∆T
µ
Vg
µ
Au contact
Vg µ
µ –dlnT(E)/dE
S T(E)
a coherent tunnelling limit is the transmission function, T(E), the energy-dependent tunnelling probability of charge carriers through a molecular junction. In a simplified representation, T(E) is a Lorentzian curve peaking ‘at resonance’: that is, at the energy of each molecular orbital in the junction (Fig. 1). The width of the Lorentzian is a measure of the strength of the coupling between the molecular orbital and the electrode. For small biases, transport properties are dominated by the value of T(E) at the Fermi level of the junction (μ in Fig. 1), and thus usually only by the resonance associated with the molecular orbital closest to μ — either the highest-occupied or lowest-unoccupied molecular orbital (HOMO or LUMO, respectively). The values of G = 2e2/hT(E) and S = –S0dlnT(E)/dE (with S0 a constant) are determined by the peak energy (relative to μ) and width of T(E), respectively11. (Here, e is the charge on the electron and h is Planck’s constant.) Both the peak energy and width of T(E) are highly specific to the interface chemistry binding the molecule to the leads on junction formation. In particular, junctions with sharp resonances near the Fermi level would maximize the power factor. This specific energy dependence of T(E) enables both the magnitude and asymmetry of the carrier density under a thermal bias to be simultaneously enhanced (Fig. 1c,d). In practice, the existence of interface states associated with the metal–molecule bond12,13 and other physical details of the metal–molecule junction, can introduce deviations from a simple Lorentzian picture. Specific electrodes, molecules and linkers that give rise to the best intrinsic power factors have not yet been determined. The three-terminal device used by Reddy and colleagues allows them to control the thermal gradient across the nanometre-size gap, while shifting the Fermi level closer to the resonance peak via the electrostatic gate. As a result, they are able to observe a simultaneous increase of the conductance and thermopower of two well-studied junctions: a C60 molecule or a biphenyl-dithiol (BPDT) molecule situated between a pair of gold electrodes. For BPDT, the HOMO level is closer to the Fermi level of the gold than the LUMO, and so holes dominate electric transport (p-type); for C60, the situation is reversed and electrons dominate electric transport (n-type). The demonstration that the Seebeck coefficient and the conductance can be increased simultaneously via a gate electrode indicates that enhancements in power factors are possible in molecular-scale set-ups. The next challenge is to learn what molecules and junction materials lead to the best — and experimentally achievable —
kBΔT G
HOMO
LUMO
E
E
Figure 1 | Working principle of a three-terminal molecular junction. a,b, Schematics depicting a metal–molecule–metal junction (a) and the gate electrode (b). The metallic leads are held at different temperatures (T, and T + ΔT), which generates an electric current through the junction. In the simple framework depicted here, the non-degenerate HOMO and LUMO resonance peaks are described by Lorentzians. c,d, The electrical conductance, G, is proportional (by a factor 2e2/h) to the transmission function value (T(E)) at the Fermi level μ. The HOMO resonance peak can be moved closer to the Fermi level by varying the gate voltage, Vg, increasing G (c) and S (d) at the same time. The inset in c shows the asymmetry of T(E) within an energy range kBΔT around the Fermi level. The red (blue) shaded regions indicate the electron (hole) carriers induced by ΔT. In the case depicted, more holes are induced than electrons (due to the asymmetry of T(E)), and S > 0.
thermoelectric properties. However, although previous work has hinted at how electrode composition, functionalization and molecule end-groups might yield a given energylevel alignment, there are few guiding principles linking molecular structure and junction composition to G and S. New theoretical tools for ab initio prediction of junction structure and level alignment are required to build this chemical intuition and optimize the parameters of the junction. Additionally, future work should concentrate on understanding thermal conductance, κ, at the molecular level14,15 with the aim of increasing the thermoelectric efficiency (measured by the figure of merit ZT = S2G/κ). In combination, such studies could then potentially allow the design of new cheap-toprocess, high-performance nanostructured thermoelectric materials that are synthesized from the bottom up. ❐ Jeffrey B. Neaton is at the Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA, the Department of Physics, University of California, Berkeley,
NATURE NANOTECHNOLOGY | VOL 9 | NOVEMBER 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
Berkeley, California 94720, USA, and Kavli Energy NanoSciences Institute at Berkeley, Berkeley, California 94720, USA. e-mail: [email protected] References
1. US Department of Energy, Industrial Technologies Program & Energy Efficiency and Renewable Energy Waste Heat Recovery: Technology and Opportunities in U. S. Industry (Prepared by BCS, 2008); available via http://go.nature.com/G73U2a 2. Kim, Y., Jeong, W., Kim, K., Lee, W. & Reddy, P. Nature Nanotech. 9, 881–885 (2014). 3. Hicks, L. D. & Dresselhaus, M. S. Phys. Rev. B 47, 12727–12731 (1993). 4. Venkatasubramanian, R., Siivola, E., Colpitts, T. & O’Quinn, B. Nature 413, 597–602 (2001). 5. Heremans, J. P. et al. Phys. Rev. Lett. 88, 216801 (2002). 6. Reddy, P., Jang, S. Y., Segalman, R. A. & Majumdar, A. Science 315, 1568–1571 (2007). 7. Widawsky, J. R., Darancet, P., Neaton, J. B. & Venkataraman, L. Nano Lett. 12, 354–358 (2012). 8. Finch, C. M., García-Suárez, V. M. & Lambert, C. J. Phys. Rev. B 79, 033405 (2009). 9. Bergfield, J. P., Solis, M. A. & Stafford, C. A. ACS Nano 4, 5314–5320 (2010). 10. Karlström, O., Linke, H., Karlström, G. & Wacker, A. Phys. Rev. B 84, 113415 (2011). 11. Paulsson, M. & Datta, S. Phys. Rev. B 67, 241403(R) (2003). 12. Widawsky, J. R. et al. Nano Lett. 13, 2889–2894 (2013). 13. Kim, T. et al. Nano Lett. 14, 794–798 (2014). 14. Lee, W. et al. Nature 498, 209–212 (2013). 15. Dubi, Y. & Di Ventra, M. Rev. Mod. Phys. 83, 131–155 (2011).
877
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
news & views a
b Junction geometry
Vacuum LUMO
d Vg
HOMO
Au contact
Gate electrode
c
T
T + ∆T
µ
Vg
µ
Au contact
Vg µ
µ –dlnT(E)/dE
S T(E)
a coherent tunnelling limit is the transmission function, T(E), the energy-dependent tunnelling probability of charge carriers through a molecular junction. In a simplified representation, T(E) is a Lorentzian curve peaking ‘at resonance’: that is, at the energy of each molecular orbital in the junction (Fig. 1). The width of the Lorentzian is a measure of the strength of the coupling between the molecular orbital and the electrode. For small biases, transport properties are dominated by the value of T(E) at the Fermi level of the junction (μ in Fig. 1), and thus usually only by the resonance associated with the molecular orbital closest to μ — either the highest-occupied or lowest-unoccupied molecular orbital (HOMO or LUMO, respectively). The values of G = 2e2/hT(E) and S = –S0dlnT(E)/dE (with S0 a constant) are determined by the peak energy (relative to μ) and width of T(E), respectively11. (Here, e is the charge on the electron and h is Planck’s constant.) Both the peak energy and width of T(E) are highly specific to the interface chemistry binding the molecule to the leads on junction formation. In particular, junctions with sharp resonances near the Fermi level would maximize the power factor. This specific energy dependence of T(E) enables both the magnitude and asymmetry of the carrier density under a thermal bias to be simultaneously enhanced (Fig. 1c,d). In practice, the existence of interface states associated with the metal–molecule bond12,13 and other physical details of the metal–molecule junction, can introduce deviations from a simple Lorentzian picture. Specific electrodes, molecules and linkers that give rise to the best intrinsic power factors have not yet been determined. The three-terminal device used by Reddy and colleagues allows them to control the thermal gradient across the nanometre-size gap, while shifting the Fermi level closer to the resonance peak via the electrostatic gate. As a result, they are able to observe a simultaneous increase of the conductance and thermopower of two well-studied junctions: a C60 molecule or a biphenyl-dithiol (BPDT) molecule situated between a pair of gold electrodes. For BPDT, the HOMO level is closer to the Fermi level of the gold than the LUMO, and so holes dominate electric transport (p-type); for C60, the situation is reversed and electrons dominate electric transport (n-type). The demonstration that the Seebeck coefficient and the conductance can be increased simultaneously via a gate electrode indicates that enhancements in power factors are possible in molecular-scale set-ups. The next challenge is to learn what molecules and junction materials lead to the best — and experimentally achievable —
kBΔT G
HOMO
LUMO
E
E
Figure 1 | Working principle of a three-terminal molecular junction. a,b, Schematics depicting a metal–molecule–metal junction (a) and the gate electrode (b). The metallic leads are held at different temperatures (T, and T + ΔT), which generates an electric current through the junction. In the simple framework depicted here, the non-degenerate HOMO and LUMO resonance peaks are described by Lorentzians. c,d, The electrical conductance, G, is proportional (by a factor 2e2/h) to the transmission function value (T(E)) at the Fermi level μ. The HOMO resonance peak can be moved closer to the Fermi level by varying the gate voltage, Vg, increasing G (c) and S (d) at the same time. The inset in c shows the asymmetry of T(E) within an energy range kBΔT around the Fermi level. The red (blue) shaded regions indicate the electron (hole) carriers induced by ΔT. In the case depicted, more holes are induced than electrons (due to the asymmetry of T(E)), and S > 0.
thermoelectric properties. However, although previous work has hinted at how electrode composition, functionalization and molecule end-groups might yield a given energylevel alignment, there are few guiding principles linking molecular structure and junction composition to G and S. New theoretical tools for ab initio prediction of junction structure and level alignment are required to build this chemical intuition and optimize the parameters of the junction. Additionally, future work should concentrate on understanding thermal conductance, κ, at the molecular level14,15 with the aim of increasing the thermoelectric efficiency (measured by the figure of merit ZT = S2G/κ). In combination, such studies could then potentially allow the design of new cheap-toprocess, high-performance nanostructured thermoelectric materials that are synthesized from the bottom up. ❐ Jeffrey B. Neaton is at the Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA, the Department of Physics, University of California, Berkeley,
NATURE NANOTECHNOLOGY | VOL 9 | NOVEMBER 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
Berkeley, California 94720, USA, and Kavli Energy NanoSciences Institute at Berkeley, Berkeley, California 94720, USA. e-mail: [email protected] References
1. US Department of Energy, Industrial Technologies Program & Energy Efficiency and Renewable Energy Waste Heat Recovery: Technology and Opportunities in U. S. Industry (Prepared by BCS, 2008); available via http://go.nature.com/G73U2a 2. Kim, Y., Jeong, W., Kim, K., Lee, W. & Reddy, P. Nature Nanotech. 9, 881–885 (2014). 3. Hicks, L. D. & Dresselhaus, M. S. Phys. Rev. B 47, 12727–12731 (1993). 4. Venkatasubramanian, R., Siivola, E., Colpitts, T. & O’Quinn, B. Nature 413, 597–602 (2001). 5. Heremans, J. P. et al. Phys. Rev. Lett. 88, 216801 (2002). 6. Reddy, P., Jang, S. Y., Segalman, R. A. & Majumdar, A. Science 315, 1568–1571 (2007). 7. Widawsky, J. R., Darancet, P., Neaton, J. B. & Venkataraman, L. Nano Lett. 12, 354–358 (2012). 8. Finch, C. M., García-Suárez, V. M. & Lambert, C. J. Phys. Rev. B 79, 033405 (2009). 9. Bergfield, J. P., Solis, M. A. & Stafford, C. A. ACS Nano 4, 5314–5320 (2010). 10. Karlström, O., Linke, H., Karlström, G. & Wacker, A. Phys. Rev. B 84, 113415 (2011). 11. Paulsson, M. & Datta, S. Phys. Rev. B 67, 241403(R) (2003). 12. Widawsky, J. R. et al. Nano Lett. 13, 2889–2894 (2013). 13. Kim, T. et al. Nano Lett. 14, 794–798 (2014). 14. Lee, W. et al. Nature 498, 209–212 (2013). 15. Dubi, Y. & Di Ventra, M. Rev. Mod. Phys. 83, 131–155 (2011).
877
