news & views the signal distortion for information transfer 3,11. With spin-lasers now operating at room temperature, these properties may enable a new class of high-performance spintronic devices. ❐ Igor Žutić and Paulo E. Faria Junior are in the Department of Physics, University at Buffalo, State University of New York, New York 14260, USA. Paulo E. Faria Junior is also at Instituto de
Física de São Carlos, Universidade de São Paulo, 13566-590 São Carlos, São Paulo, Brazil. e-mail: [email protected]; [email protected] References
1. Žutić, I., Fabian, J. & Das Sarma, S. Rev. Mod. Phys. 76, 323–410 (2004). 2. Cheng, J.‑Y., Wong, T.‑M., Chang, C.‑W., Dong, C.‑Y. & Chen, Y.‑F. Nature Nanotech. 9, 845–850 (2014). 3. Lee, J., Oszwałdowski, R., Gøthgen, C. & Žutić, I. Phys. Rev. B 85, 045314 (2012).
4. Rudolph, J., Hagele, D., Gibbs, H. M., Khitrova, G. & Oestreich, M. Appl. Phys. Lett. 82, 4516–4158 (2003). 5. Holub, M., Shin, J., Saha, D. & Bhattacharya, P. Phys. Rev. Lett. 98, 146603 (2007). 6. Iba, S., Koh, S., Ikeda, K. & Kawaguchi, H. Appl. Phys. Lett. 98, 081113 (2011). 7. Frougier, J. et al. Appl. Phys. Lett. 103, 252402 (2013). 8. Hallstein, S. et al. Phys. Rev. B 56, R7076–R7079 (1997). 9. Gerhardt, N. C. et al. Appl. Phys. Lett. 99, 151107 (2011). 10. Höpfner, H., Lindemann, M., Gerhardt, N. C. & Hofmann, M. R. Appl. Phys. Lett. 104, 022409 (2014). 11. Boéris, G., Lee, J., Výborný, K. & Žutić, I. Appl. Phys. Lett. 100, 121111 (2012).
2D MATERIALS
Valley currents controlled by light The asymmetry of light–matter coupling in momentum space of transition metal dichalcogenides drives valley photocurrents in WSe2-based devices.
Sergey Tarasenko
T
echnological and experimental advances in the study of twodimensional crystals such as graphene, monolayers of boron nitride and transition metal dichalcogenides (TMDCs)1, as well as of van der Waals heterostructures2, has triggered intense research on valley phenomena in solids. In these materials, conduction electrons occupy more than one equivalent band minima (valleys) in the Brillouin zone. In semiconductors, valleys hold great and yet unexplored potential for the realization of devices based on the valley degree of freedom, in addition to the charge and spin degrees of freedom3. In valley-based electronics, electron fluxes are generated and controlled in individual
valleys. The net electric current given by the vector sum of single-valley contributions is generally non-zero, but vanishes for the particular case of pure valley currents4. The excitation of valley currents is challenging to realize, and has only been demonstrated for two-dimensional silicon structures so far5. Other systems, in particular materials with strong spin–valley coupling such as TMDCs, can offer a more efficient control and read-out of valley states. Writing in Nature Nanotechnology, Yi Cui and colleagues at Stanford University, SLAC National Accelerator Lab, Peking University, and Innovation Center of Quantum Matter in Beijing report photogalvanic effects in accumulation layers on tungsten diselenide
a
b
Vg
σ−
ic Ion −K S
gel
j5 j4
σ−
+
σ
σ+ D
j6 jΣ j3
jΣ
j1 j2 Λ
Vg
K
r
afe Se 2 i w W O 2/S Si
ic Ion −K S
gel
j4
D
j6
j5
jΣ, jΣ = 0 j3
j1 j2 Λ
K
r
afe Se 2 i w W O 2/S Si
Figure 1 | Field-effect transistor on a WSe2 surface with a transparent ionic gel gate. a, Distribution of electron fluxes in valleys induced by circularly polarized light at oblique incidence. The total helicity-dependent electric current arises in the direction perpendicular to the light incidence plane. b, Distribution of electron fluxes in valleys for normally incident circularly polarized light. Ellipsoids show electron valleys in the Brillouin zone, represented by the hexagonal shape. Points K and –K are shown. S, source; D, drain; Vg, gate voltage. Left-handed (σ–; red) or right-handed (σ+; blue) circularly polarized light induces different currents (j vectors) in the electron valleys. The total current in a is shown by the red and blue arrows labelled jΣ. 752
(WSe2) surfaces and conclude that optical pumping generates spin-coupled valley photocurrents6. Moreover, the researchers find that electron fluxes in individual valleys, as well as the net photocurrent measured at room temperature, can be controlled by the light polarization and by a gate electric field applied to the WSe2 layer. The results unambiguously demonstrate that the electron–photon interaction in TMDC accumulation layers is asymmetric in momentum space, and the asymmetry can be efficiently tuned by a gate voltage. These findings provide a basis for the study of symmetry breaking in TMDC layers, electron excitation and relaxation processes, as well as intervalley scattering and valley Coulomb drag, and may have implications for the design of optoelectronic devices. The researchers fabricated an electricdouble-layer field-effect transistor 7 in which the transparent gate was made of an ionic gel on a WSe2 single-crystal flake (Fig. 1). Charge redistribution in the gel produces a strong electric field, which is crucial for the study, at the solid/gel interface: the field forms an accumulation layer for electrons and breaks the inversion symmetry, which is a prerequisite for the photogalvanic effect to occur. Shining circularly polarized light of mid-infrared spectral range on the sample generates a d.c. electric current between the source and drain contacts. The photocurrent is measured in the absence of an external bias between the contacts, and flows in the direction perpendicular to the plane of incidence of the light. Importantly, the current reverses its sign when the radiation helicity is switched from left- (σ–) to right-handed (σ+) circular polarization,
NATURE NANOTECHNOLOGY | VOL 9 | OCTOBER 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
news & views demonstrating that it is driven by the photon angular momenta. The inherent relation between the photon and electron angular momenta, which leads to a significant spin–valley coupled polarization by optical pumping with circularly polarized light, has been demonstrated for TMDC monolayers8–11 and bulk crystals with broken inversion symmetry 12. Whereas in monolayers the electron valleys of interest are situated at two points (K and –K) of the Brillouin zone edge, the conduction band in bulk WSe2 has six valleys centred at the Λ (also denoted as T) points on the ΓK line13,14 (Fig. 1). The symmetry of each valley is very low. Therefore, optical excitation of electrons in valleys induces intravalley electron fluxes, the magnitude and direction of each flux depends on the valley number and light polarization. Figure 1 shows the possible distributions of electron fluxes in valleys generated by circularly polarized light for oblique and normal incidence of radiation. The distribution induced by normally incident
light (Fig. 1b) is invariant with respect to the rotation by 120° and the total electric current vanishes. In contrast, at oblique incidence the single-valley fluxes do not compensate each other, giving rise to a net photocurrent sensitive to the photon helicity (Fig. 1a). Such behaviour is observed experimentally. In the context of applications, the work by Yi Cui and co-workers raises questions on the efficiency and microscopic mechanisms of valley current generation. The theoretical estimations provided show that valley polarization in the generated photocurrents can be considerably large, yet the experiment does not yield information on the degree of valley- and spin-polarization of the current. Further studies, in particular experiments with high spatial and time resolution, may shed light on these questions. Spaceresolved pump–probe measurements can give direct access to the valley and spin separation in real space induced by valley currents. Experiments with time resolution can be highly efficient in discriminating the mechanisms of current generation due to
NATURE NANOTECHNOLOGY | VOL 9 | OCTOBER 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
the excitation and subsequent relaxation of polarized carriers. Finally, a challenging, yet natural, step in the field would be the study of valley photocurrents in TMDC single monolayers. ❐ Sergey Tarasenko is at the Ioffe Institute, Politechnicheskaya 26, 194021 St. Petersburg, Russia. e-mail: [email protected] References
1. Wang, Q. H., Kalantar-Zadeh, K., Kis, A., Coleman, J. N. & Strano, M. Nature Nanotech. 7, 699–712 (2012). 2. Geim, A. K. & Grigorieva, I. V. Nature 499, 419–425 (2013). 3. Xu, X., Yao, W., Xiao, D. & Heinz, T. F. Nature Phys. 10, 343–350 (2014). 4. Tarasenko, S. A. & Ivchenko, E. L. JETP Lett. 81, 231–235 (2005). 5. Karch, J. et al. Phys. Rev. B 83, 121312(R) (2011). 6. Yuan, H. et al. Nature Nanotech. 9, 851–857 (2014). 7. Fujimotoa, T. & Awaga, K. Phys. Chem. Chem. Phys. 15, 8983–9006 (2013). 8. Mak, K. F., He, K., Shan, J. & Heinz, T. F. Nature Nanotech. 7, 494–498 (2012). 9. Cao, T. et al. Nature Commun. 3, 887 (2012). 10. Zhenge, H., Dai, J., Yao, W., Xiao, D. & Cui, X. Nature Nanotech. 7, 490–493 (2012). 11. Sallen, G. et al. Phys. Rev. B 86, 081301 (2012). 12. Suzuki, R. et al. Nature Nanotech. 9, 611–617 (2014). 13. Zhao, W. et al. ACS Nano 7, 791–797 (2013). 14. Zhang, Y. et al. Nature Nanotech. 9, 111–115 (2014).
753
Física de São Carlos, Universidade de São Paulo, 13566-590 São Carlos, São Paulo, Brazil. e-mail: [email protected]; [email protected] References
1. Žutić, I., Fabian, J. & Das Sarma, S. Rev. Mod. Phys. 76, 323–410 (2004). 2. Cheng, J.‑Y., Wong, T.‑M., Chang, C.‑W., Dong, C.‑Y. & Chen, Y.‑F. Nature Nanotech. 9, 845–850 (2014). 3. Lee, J., Oszwałdowski, R., Gøthgen, C. & Žutić, I. Phys. Rev. B 85, 045314 (2012).
4. Rudolph, J., Hagele, D., Gibbs, H. M., Khitrova, G. & Oestreich, M. Appl. Phys. Lett. 82, 4516–4158 (2003). 5. Holub, M., Shin, J., Saha, D. & Bhattacharya, P. Phys. Rev. Lett. 98, 146603 (2007). 6. Iba, S., Koh, S., Ikeda, K. & Kawaguchi, H. Appl. Phys. Lett. 98, 081113 (2011). 7. Frougier, J. et al. Appl. Phys. Lett. 103, 252402 (2013). 8. Hallstein, S. et al. Phys. Rev. B 56, R7076–R7079 (1997). 9. Gerhardt, N. C. et al. Appl. Phys. Lett. 99, 151107 (2011). 10. Höpfner, H., Lindemann, M., Gerhardt, N. C. & Hofmann, M. R. Appl. Phys. Lett. 104, 022409 (2014). 11. Boéris, G., Lee, J., Výborný, K. & Žutić, I. Appl. Phys. Lett. 100, 121111 (2012).
2D MATERIALS
Valley currents controlled by light The asymmetry of light–matter coupling in momentum space of transition metal dichalcogenides drives valley photocurrents in WSe2-based devices.
Sergey Tarasenko
T
echnological and experimental advances in the study of twodimensional crystals such as graphene, monolayers of boron nitride and transition metal dichalcogenides (TMDCs)1, as well as of van der Waals heterostructures2, has triggered intense research on valley phenomena in solids. In these materials, conduction electrons occupy more than one equivalent band minima (valleys) in the Brillouin zone. In semiconductors, valleys hold great and yet unexplored potential for the realization of devices based on the valley degree of freedom, in addition to the charge and spin degrees of freedom3. In valley-based electronics, electron fluxes are generated and controlled in individual
valleys. The net electric current given by the vector sum of single-valley contributions is generally non-zero, but vanishes for the particular case of pure valley currents4. The excitation of valley currents is challenging to realize, and has only been demonstrated for two-dimensional silicon structures so far5. Other systems, in particular materials with strong spin–valley coupling such as TMDCs, can offer a more efficient control and read-out of valley states. Writing in Nature Nanotechnology, Yi Cui and colleagues at Stanford University, SLAC National Accelerator Lab, Peking University, and Innovation Center of Quantum Matter in Beijing report photogalvanic effects in accumulation layers on tungsten diselenide
a
b
Vg
σ−
ic Ion −K S
gel
j5 j4
σ−
+
σ
σ+ D
j6 jΣ j3
jΣ
j1 j2 Λ
Vg
K
r
afe Se 2 i w W O 2/S Si
ic Ion −K S
gel
j4
D
j6
j5
jΣ, jΣ = 0 j3
j1 j2 Λ
K
r
afe Se 2 i w W O 2/S Si
Figure 1 | Field-effect transistor on a WSe2 surface with a transparent ionic gel gate. a, Distribution of electron fluxes in valleys induced by circularly polarized light at oblique incidence. The total helicity-dependent electric current arises in the direction perpendicular to the light incidence plane. b, Distribution of electron fluxes in valleys for normally incident circularly polarized light. Ellipsoids show electron valleys in the Brillouin zone, represented by the hexagonal shape. Points K and –K are shown. S, source; D, drain; Vg, gate voltage. Left-handed (σ–; red) or right-handed (σ+; blue) circularly polarized light induces different currents (j vectors) in the electron valleys. The total current in a is shown by the red and blue arrows labelled jΣ. 752
(WSe2) surfaces and conclude that optical pumping generates spin-coupled valley photocurrents6. Moreover, the researchers find that electron fluxes in individual valleys, as well as the net photocurrent measured at room temperature, can be controlled by the light polarization and by a gate electric field applied to the WSe2 layer. The results unambiguously demonstrate that the electron–photon interaction in TMDC accumulation layers is asymmetric in momentum space, and the asymmetry can be efficiently tuned by a gate voltage. These findings provide a basis for the study of symmetry breaking in TMDC layers, electron excitation and relaxation processes, as well as intervalley scattering and valley Coulomb drag, and may have implications for the design of optoelectronic devices. The researchers fabricated an electricdouble-layer field-effect transistor 7 in which the transparent gate was made of an ionic gel on a WSe2 single-crystal flake (Fig. 1). Charge redistribution in the gel produces a strong electric field, which is crucial for the study, at the solid/gel interface: the field forms an accumulation layer for electrons and breaks the inversion symmetry, which is a prerequisite for the photogalvanic effect to occur. Shining circularly polarized light of mid-infrared spectral range on the sample generates a d.c. electric current between the source and drain contacts. The photocurrent is measured in the absence of an external bias between the contacts, and flows in the direction perpendicular to the plane of incidence of the light. Importantly, the current reverses its sign when the radiation helicity is switched from left- (σ–) to right-handed (σ+) circular polarization,
NATURE NANOTECHNOLOGY | VOL 9 | OCTOBER 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
news & views demonstrating that it is driven by the photon angular momenta. The inherent relation between the photon and electron angular momenta, which leads to a significant spin–valley coupled polarization by optical pumping with circularly polarized light, has been demonstrated for TMDC monolayers8–11 and bulk crystals with broken inversion symmetry 12. Whereas in monolayers the electron valleys of interest are situated at two points (K and –K) of the Brillouin zone edge, the conduction band in bulk WSe2 has six valleys centred at the Λ (also denoted as T) points on the ΓK line13,14 (Fig. 1). The symmetry of each valley is very low. Therefore, optical excitation of electrons in valleys induces intravalley electron fluxes, the magnitude and direction of each flux depends on the valley number and light polarization. Figure 1 shows the possible distributions of electron fluxes in valleys generated by circularly polarized light for oblique and normal incidence of radiation. The distribution induced by normally incident
light (Fig. 1b) is invariant with respect to the rotation by 120° and the total electric current vanishes. In contrast, at oblique incidence the single-valley fluxes do not compensate each other, giving rise to a net photocurrent sensitive to the photon helicity (Fig. 1a). Such behaviour is observed experimentally. In the context of applications, the work by Yi Cui and co-workers raises questions on the efficiency and microscopic mechanisms of valley current generation. The theoretical estimations provided show that valley polarization in the generated photocurrents can be considerably large, yet the experiment does not yield information on the degree of valley- and spin-polarization of the current. Further studies, in particular experiments with high spatial and time resolution, may shed light on these questions. Spaceresolved pump–probe measurements can give direct access to the valley and spin separation in real space induced by valley currents. Experiments with time resolution can be highly efficient in discriminating the mechanisms of current generation due to
NATURE NANOTECHNOLOGY | VOL 9 | OCTOBER 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
the excitation and subsequent relaxation of polarized carriers. Finally, a challenging, yet natural, step in the field would be the study of valley photocurrents in TMDC single monolayers. ❐ Sergey Tarasenko is at the Ioffe Institute, Politechnicheskaya 26, 194021 St. Petersburg, Russia. e-mail: [email protected] References
1. Wang, Q. H., Kalantar-Zadeh, K., Kis, A., Coleman, J. N. & Strano, M. Nature Nanotech. 7, 699–712 (2012). 2. Geim, A. K. & Grigorieva, I. V. Nature 499, 419–425 (2013). 3. Xu, X., Yao, W., Xiao, D. & Heinz, T. F. Nature Phys. 10, 343–350 (2014). 4. Tarasenko, S. A. & Ivchenko, E. L. JETP Lett. 81, 231–235 (2005). 5. Karch, J. et al. Phys. Rev. B 83, 121312(R) (2011). 6. Yuan, H. et al. Nature Nanotech. 9, 851–857 (2014). 7. Fujimotoa, T. & Awaga, K. Phys. Chem. Chem. Phys. 15, 8983–9006 (2013). 8. Mak, K. F., He, K., Shan, J. & Heinz, T. F. Nature Nanotech. 7, 494–498 (2012). 9. Cao, T. et al. Nature Commun. 3, 887 (2012). 10. Zhenge, H., Dai, J., Yao, W., Xiao, D. & Cui, X. Nature Nanotech. 7, 490–493 (2012). 11. Sallen, G. et al. Phys. Rev. B 86, 081301 (2012). 12. Suzuki, R. et al. Nature Nanotech. 9, 611–617 (2014). 13. Zhao, W. et al. ACS Nano 7, 791–797 (2013). 14. Zhang, Y. et al. Nature Nanotech. 9, 111–115 (2014).
753
