Light-emitting diodes by band-structure engineering in van der Waals heterostructures

Journal name:
Nature Materials
Volume:
14,
Pages:
301–306
Year published:
DOI:
doi:10.1038/nmat4205
Received
Accepted
Published online

The advent of graphene and related 2D materials1, 2 has recently led to a new technology: heterostructures based on these atomically thin crystals3. The paradigm proved itself extremely versatile and led to rapid demonstration of tunnelling diodes with negative differential resistance4, tunnelling transistors5, photovoltaic devices6, 7 and so on. Here, we take the complexity and functionality of such van der Waals heterostructures to the next level by introducing quantum wells (QWs) engineered with one atomic plane precision. We describe light-emitting diodes (LEDs) made by stacking metallic graphene, insulating hexagonal ​boron nitride and various semiconducting monolayers into complex but carefully designed sequences. Our first devices already exhibit an extrinsic quantum efficiency of nearly 10% and the emission can be tuned over a wide range of frequencies by appropriately choosing and combining 2D semiconductors (monolayers of transition metal dichalcogenides). By preparing the heterostructures on elastic and transparent substrates, we show that they can also provide the basis for flexible and semi-transparent electronics. The range of functionalities for the demonstrated heterostructures is expected to grow further on increasing the number of available 2D crystals and improving their electronic quality.

At a glance

Figures

  1. Heterostructure devices with a SQW and MQWs.
    Figure 1: Heterostructure devices with a SQW and MQWs.

    a, Schematic of the SQW heterostructure hBN/GrB/2hBN/​WS2/2hBN/GrT/hBN. b, Cross-sectional bright-field STEM image of the type of heterostructure presented in a. Scale bar, 5 nm. c,d, Schematic and STEM image of the MQW heterostructure hBN/GrB/2hBN/​MoS2/2hBN/​MoS2/2hBN/​MoS2/2hBN/​MoS2/2hBN/GrT/hBN. The number of hBN layers between ​MoS2 QWs in d varies. Scale bar, 5 nm. e, Optical image of an operational device (hBN/GrB/3hBN/​MoS2/3hBN/GrT/hBN). The dashed curve outlines the heterostructure area. Scale bar, 10 μm. f, Optical image of EL from the same device. Vb = 2.5 V, T = 300 K. 2hBN and 3hBN stand for bi- and trilayer hBN, respectively. g, Schematic of our heterostructure consisting of ​Si/​SiO2/hBN/GrB/3hBN/​MoS2/3hBN/GrT/hBN. hj, Band diagrams for the case of zero applied bias (h), intermediate applied bias (i) and high bias (j) for the heterostructure presented in g.

  2. Optical and transport characterization of our SQW devices, T = 7 K.
    Figure 2: Optical and transport characterization of our SQW devices, T = 7 K.

    a, Colour map of the PL spectra as a function of Vb for a ​MoS2-based SQW. The white curve is the dI/dVb of the device. Excitation energy EL = 2.33 eV. b, EL spectra as a function of Vb for the same device as in a. White curve: its jVb characteristic (j is the current density). c, Comparison of the PL and EL spectra for the same device. As PL and EL occur in the same spectral range, we measured them separately. dg, The same as in b,c but for the bilayer (d,e) and monolayer (f,g) ​WS2 QWs. The PL curves were taken at Vb = 2.4 V (c), 2.5 V (e) and 2 V (g); the EL curves were taken at Vb = 2.5 V (c), 2.5 V (e) and 2.3 V (g).

  3. Optical and transport characteristics of MQW devices, T = 7 K.
    Figure 3: Optical and transport characteristics of MQW devices, T = 7 K.

    a, Modulus of the current density through a triple QW structure based on ​MoS2. b, Its schematic structure. c,d, Maps of PL and EL spectra for this device. EL = 2.33 eV. e, Individual EL spectra plotted on a logarithmic scale show the onset of EL at 1.8 nA μm−2 (blue curve). Olive and red: j = 18 and 130 nA μm−2, respectively. f, Comparison of the EL (taken at Vb = 8.3 V) and PL (taken at Vb = 4.5 V) spectra.

  4. Devices combining different QW materials and on flexible substrates.
    Figure 4: Devices combining different QW materials and on flexible substrates.

    ac, EL at negative (a) and positive (c) bias voltages for the device with two QWs made from ​MoS2 and ​WSe2 schematically shown in the inset in d. Its PL bias dependence is shown in b, for laser excitation EL = 2.33 eV, T = 7 K. White curve: |j|–Vb characteristics of the device. d, Temperature dependence of EQE for a device with two QWs made from ​MoS2 and ​WSe2. Inset: schematic representation of a device with two QWs produced from different materials. e, Optical micrograph taken in reflection mode of a SQW (​MoS2) device on PET. f, Optical micrograph of the same device as in e taken in transmission mode. For e,f the area of the stack is marked by red rectangles; scale bars are 10 μm. g, EL spectra for the device in e,f at zero (blue dots) and 1% (red dots) strain. Vb = −2.3 V, I = −40 μA at room T.

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Author information

Affiliations

  1. School of Physics and Astronomy, University of Manchester, Oxford Road Manchester M13 9PL, UK

    • F. Withers,
    • A. Mishchenko &
    • K. S. Novoselov
  2. Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK

    • O. Del Pozo-Zamudio &
    • A. I. Tartakovskii
  3. School of Materials, University of Manchester, Oxford Road Manchester M13 9PL, UK

    • A. P. Rooney,
    • A. Gholinia &
    • S. J. Haigh
  4. National Institute for Materials Science, 1-1 Namiki Tsukuba 305-0044, Japan

    • K. Watanabe &
    • T. Taniguchi
  5. Manchester Centre for Mesoscience and Nanotechnology, University of Manchester, Oxford Road Manchester M13 9PL, UK

    • A. K. Geim

Contributions

F.W. produced experimental devices, led the experimental part of the project, analysed experimental data, participated in discussions, contributed to writing the manuscript; O.D.P-Z. measured device characteristics, participated in discussions, analysed experimental data; A.M. measured transport properties of the devices, participated in discussions; A.P.R. and A.G. produced samples for TEM study, analysed TEM results, participated in discussions; K.W. and T.T. grew high-quality hBN, participated in discussions; S.J.H. analysed TEM results, participated in discussions; A.K.G. analysed experimental data, participated in discussions, contributed to writing the manuscript; A.I.T. analysed experimental data, participated in discussions, contributed to writing the manuscript; K.S.N. initiated the project, analysed experimental data, participated in discussions, contributed to writing the manuscript.

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The authors declare no competing financial interests.

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