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Tuning ultrafast electron thermalization pathways in a van der Waals heterostructure

Nature Physics volume 12, pages 455–459 (2016)Cite this article

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Abstract

Ultrafast electron thermalization—the process leading to carrier multiplication via impact ionization1,2, and hot-carrier luminescence3,4—occurs when optically excited electrons in a material undergo rapid electron–electron scattering3,5,6,7 to redistribute excess energy and reach electronic thermal equilibrium. Owing to extremely short time and length scales, the measurement and manipulation of electron thermalization in nanoscale devices remains challenging even with the most advanced ultrafast laser techniques8,9,10,11,12,13,14. Here, we overcome this challenge by leveraging the atomic thinness of two-dimensional van der Waals (vdW) materials to introduce a highly tunable electron transfer pathway that directly competes with electron thermalization. We realize this scheme in a graphene–boron nitride–graphene (G–BN–G) vdW heterostructure15,16,17, through which optically excited carriers are transported from one graphene layer to the other. By applying an interlayer bias voltage or varying the excitation photon energy, interlayer carrier transport can be controlled to occur faster or slower than the intralayer scattering events, thus effectively tuning the electron thermalization pathways in graphene. Our findings, which demonstrate a means to probe and directly modulate electron energy transport in nanoscale materials, represent a step towards designing and implementing optoelectronic and energy-harvesting devices with tailored microscopic properties.

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Figure 1: Interlayer photocurrent of a G–BN–G device.
Figure 2: Two different regimes of interlayer photocurrent in a G–BN–G device.
Figure 3: Two-pulse correlation of interlayer photocurrent in a G–BN–G device.
Figure 4: Experimental signatures of direct Fowler–Nordheim carrier tunnelling.

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Acknowledgements

We thank V. Fatemi, L. Ju, L. Levitov, J. Rodriguez-Nieva, J. Sanchez-Yamagishi, E. J. Sie, J. C. W. Song and H. Steinberg for discussions. This work was supported by AFOSR Grant No. FA9550-11-1-0225 (measurement and data analysis, Q.M., T.I.A., N.L.N., N.G. and P.J.-H.) and the Packard Fellowship Program. This work made use of the Materials Research Science and Engineering Center Shared Experimental Facilities supported by the National Science Foundation (NSF) (Grant No. DMR-0819762) and of Harvard’s Center for Nanoscale Systems, supported by the NSF (Grant No. ECS-0335765). N.G. and C.H.L. have been supported by the Gordon and Betty Moore Foundation’s EPiQS Initiative through Grant GBMF4540 for the time-domain photocurrent measurements. F.H.L.K. acknowledges support by Fundacio Cellex Barcelona, the ERC Career integration grant (294056, GRANOP), the ERC starting grant (307806, CarbonLight), the Government of Catalonia through the SGR grant (2014-SGR-1535), the Mineco grants Ramón y Cajal (RYC-2012-12281) and Plan Nacional (FIS2013-47161-P), and support by the EC under the Graphene Flagship (contract no. CNECT-ICT-604391). W.F. and J.K. acknowledge the funding support by the STC Center for Integrated Quantum Materials, NSF Grant No. DMR-1231319.

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Author notes

  1. Qiong Ma, Trond I. Andersen and Nityan L. Nair: These authors contributed equally to this work.

Authors and Affiliations

  1. Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

    Qiong Ma, Trond I. Andersen, Nityan L. Nair, Nathaniel M. Gabor, Chun Hung Lui, Andrea F. Young, Nuh Gedik & Pablo Jarillo-Herrero

  2. ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels (Barcelona), Spain

    Mathieu Massicotte & Frank H. L. Koppens

  3. Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

    Wenjing Fang & Jing Kong

  4. National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan

    Kenji Watanabe & Takashi Taniguchi

  5. ICREA - Institució Catalana de Recerça i Estudis Avancats, 08010 Barcelona, Spain

    Frank H. L. Koppens

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Contributions

N.M.G. and P.J.-H. conceived the experiment; Q.M., N.L.N. and M.M. fabricated the devices; N.L.N., N.M.G., Q.M. and M.M. carried out the spatial and spectral photocurrent measurements; T.I.A. and C.H.L. performed the time-domain photocurrent measurements under the supervision of N.G.; Q.M., T.I.A., N.L.N. and M.M. analysed the data under the supervision of N.M.G., C.H.L., A.F.Y., F.H.L.K. and P.J.-H.; W.F. and J.K. grew the CVD graphene; K.W. and T.T. synthesized the BN crystals; Q.M., T.I.A., C.H.L., N.L.N., N.M.G., F.H.L.K. and P.J.-H. co-wrote the paper with input from all other authors.

Corresponding authors

Correspondence to Nathaniel M. Gabor or Pablo Jarillo-Herrero.

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The authors declare no competing financial interests.

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Ma, Q., Andersen, T., Nair, N. et al. Tuning ultrafast electron thermalization pathways in a van der Waals heterostructure. Nature Phys 12, 455–459 (2016). https://doi.org/10.1038/nphys3620

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