Scanning tunnelling microscopy and spectroscopy of ultra-flat graphene on hexagonal boron nitride

Journal name:
Nature Materials
Volume:
10,
Pages:
282–285
Year published:
DOI:
doi:10.1038/nmat2968
Received
Accepted
Published online

Graphene has demonstrated great promise for future electronics technology as well as fundamental physics applications because of its linear energy–momentum dispersion relations which cross at the Dirac point1, 2. However, accessing the physics of the low-density region at the Dirac point has been difficult because of disorder that leaves the graphene with local microscopic electron and hole puddles3, 4, 5. Efforts have been made to reduce the disorder by suspending graphene, leading to fabrication challenges and delicate devices which make local spectroscopic measurements difficult6, 7. Recently, it has been shown that placing graphene on hexagonal boron nitride (hBN) yields improved device performance8. Here we use scanning tunnelling microscopy to show that graphene conforms to hBN, as evidenced by the presence of Moiré patterns. However, contrary to predictions9, 10, this conformation does not lead to a sizeable band gap because of the misalignment of the lattices. Moreover, local spectroscopy measurements demonstrate that the electron–hole charge fluctuations are reduced by two orders of magnitude as compared with those on silicon oxide. This leads to charge fluctuations that are as small as in suspended graphene6, opening up Dirac point physics to more diverse experiments.

At a glance

Figures

  1. Schematic device set-up and topography comparison of graphene on hBN and SiO2.
    Figure 1: Schematic device set-up and topography comparison of graphene on hBN and SiO2.

    a, Optical microscope image of the mechanically exfoliated monolayer graphene flake with hBN underneath and gold electrodes contacting it above. The wiring of the STM tip and back gate voltage is indicated. b, STM topographic image of monolayer graphene on hBN showing the underlying surface corrugations. The image is 100 nm×100 nm. The imaging parameters are tip voltage Vt=−0.3 V, tunnelling current It=100 pA. c, STM topographic image of monolayer graphene on SiO2 showing markedly increased corrugations. The imaging parameters are tip voltage Vt=−0.5 V, tunnelling current It=50 pA. d, Histogram of the height distributions for graphene on SiO2 (blue squares) and graphene on hBN (red triangles) along with Gaussian fits.

  2. Real space and Fourier transforms of Moiré patterns
    Figure 2: Real space and Fourier transforms of Moiré patterns

    a, STM topographic images of a Moiré pattern produced by graphene on hBN. The scale bar is 2 nm. The inset is a zoom in of a 2 nm region with a scale bar of 0.3 nm. The imaging parameters are Vt=−0.3 V and It=100 pA. b, Fourier transform of a showing the six graphene lattice points near the edge of the image and the long wavelength Moiré pattern near the centre of the image and around each lattice point. The scale bar is 10 nm−1. The inset is a zoom in around one of the lattice points with a scale bar of 2 nm−1. c, STM topographic image from another region of the same graphene flake showing a different Moiré pattern. The scale bar is 2 nm. The inset is a zoom in of a 2 nm region with a scale bar of 0.3 nm. The imaging parameters are Vt=−0.3 V and It=100 pA. d, Fourier transform of c showing the atomic lattice as well as the Moiré pattern. The scale bar is 10 nm−1. The inset is a zoom of the Moiré pattern with a scale bar of 4 nm−1.

  3. Spectroscopy of graphene on hBN as a function of gate voltage.
    Figure 3: Spectroscopy of graphene on hBN as a function of gate voltage.

    a, dI/dV spectroscopy showing a nearly linear density of states as a function of energy (tip voltage). b, dI/dV spectroscopy as a function of tip voltage and gate voltage. The white line corresponds to the minimum in the dI/dV curves and represents the Dirac point. c, Energy of the Dirac point as a function of gate voltage. The red curve is a fit assuming a linear band structure. d, Energy versus momentum dispersion relations for the case of graphene and hBN having the same lattice constant and zero angle mismatch (black curve) and two curves with 1.8% lattice mismatch. The blue curve has −5.45° angle mismatch and the red curve has −10.9°.

  4. Spatial maps of the density of states of graphene on hBN and SiO2.
    Figure 4: Spatial maps of the density of states of graphene on hBN and SiO2.

    a, Topography of graphene on hBN. b, Tip voltage at the Dirac point as a function of position for graphene on hBN. c, Tip voltage at the Dirac point as a function of position for graphene on SiO2. The colour scale is the same for b and c. The scale bar in all images is 10 nm. d Histogram of the energies of the Dirac point from b as well as a Gaussian fit. The inset shows the same data but also includes the histogram for SiO2 shown in red.

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Author information

Affiliations

  1. Department of Physics, University of Arizona, Tucson, Arizona 85721, USA

    • Jiamin Xue,
    • Philippe Jacquod,
    • Aparna Deshpande &
    • Brian J. LeRoy
  2. Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02138, USA

    • Javier Sanchez-Yamagishi,
    • Danny Bulmash &
    • Pablo Jarillo-Herrero
  3. College of Optical Sciences, University of Arizona, Tucson, Arizona 85721, USA

    • Philippe Jacquod
  4. Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan

    • K. Watanabe &
    • T. Taniguchi
  5. Present address: Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois, 60208 USA

    • Aparna Deshpande

Contributions

J.X. and B.J.L. performed the STM experiments for graphene on hBN. A.D. performed the STM experiments for graphene on SiO2. J.S-Y. and D.B. fabricated the devices. P.J. performed the theoretical calculations. K.W. and T.T. provided the single crystal hBN. P.J-H. and B.J.L. conceived and provided advice on the experiments. All authors participated in discussing the data and writing the manuscript.

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The authors declare no competing financial interests.

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