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Twistable electronics with dynamically rotatable heterostructures

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Science  17 Aug 2018:
Vol. 361, Issue 6403, pp. 690-693
DOI: 10.1126/science.aat6981
  • Fig. 1 Rotatable heterostructures.

    (A) Schematic cartoon of the device structure and the experimental technique. BN, hexagonal boron nitride. (B to D) AFM image of a fabricated device showing three different orientations of the top BN. The angle identified in each panel is the absolute angle referenced to the AFM coordinate system (θA). The images were acquired by the same AFM used to rotate the BN layer. Scale bars, 1 μm. (E) Schematic illustration of the moiré superlattice arising between graphene (red) and BN (blue) at zero angle. The moiré wavelength, λ, is shown. (F) Raman spectrum of the device shown in (B), (C), and (D) for θA between 34.2° ± 0.2°and 36.0° ± 0.2°. The black curve shows an additional measurement acquired at ~64°. a.u., arbitrary units. (G) FWHM of the 2D peak as a function of the absolute angle. All Raman measurements were taken with the gate bias held at Vg = 0 V. The peak FWHM position identifies zero-angle crystallographic alignment. Error bars represent the precision with which θ can be determined from the AFM topographic images. The dashed line represents the FWHM measured for all angles larger than ~2° away from perfect alignment, with the shaded region representing the associated uncertainty.

  • Fig. 2 Mechanical properties.

    (A) Schematic description of friction measurements. When the AFM tip encountered the BN structure, it canted, causing a repositioning of the reflected laser spot in the four-quadrant photodetector. The resulting voltage difference was proportional to the tip cant angle (referred to here as the tip deflection) and served as a measure of the torque force acting at the end of the tip. (B) Tip deflection versus time in a translational push of the upper BN structure. Different regimes of the measurement were identified: (i) As the tip dragged along the surface, tip-substrate friction resulted in a steady-state tip deflection. (ii) When the tip encountered the BN structure, it initially resisted translation, and the tip deflection increased. We refer to this as the static friction regime. (iii) Once the BN was in motion, the tip deflection relaxed slightly, providing a measure of the dynamic friction at the BN-graphene interface. (C) Tip deflection versus absolute angle, measured during a continuous rotation of the BN. Two peaks were observed, spaced 60° apart.

  • Fig. 3 Electronic transport properties.

    (A) Four-terminal resistance (R4P) as a function of Vg for different alignments of the graphene/BN structure, acquired at room temperature. (B) Position of the satellite peak in gate voltage as a function of the relative angle. The ±0.2° error in the angle reports the precision achieved with the AFM imaging in tapping mode. The vertical error bars represent the maximum uncertainty with which we can determine the position of the satellite peak owing to the broadening of the peak. (C) Linear dependence of the maximum value of the four-terminal resistance at the satellite peak at 300 K (red) and 1.7 K (blue) as a function of the moiré length. (D) Energy gap, measured by thermal activation, for the satellite peak (circles) and the charge neutrality point (diamonds) as a function of the relative angle. Open symbols represent a repeated measurement at a given angle after moving through other angles and thermally cycling. (E) Schematic band structure for native graphene and (F) for a graphene-BN heterostructure with a small twist angle. E, energy; k, momentum.

  • Fig. 4 Compilation of observations with the angle control technique.

    (A) Four-terminal resistance as a function of the relative angle measured at a carrier density of –1.9 × 1012 cm−1. (B) Tip deflection in friction measured simultaneously with the electronic transport. (C) FWHM of the 2D peak of the Raman spectrum as a function of the relative angle. All measurements were performed in the same device.

Supplementary Materials

  • Twistable electronics with dynamically rotatable heterostructures

    Rebeca Ribeiro-Palau, Changjian Zhang, Kenji Watanabe, Takashi Taniguchi, James Hone, Cory R. Dean

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods 
    • Figs. S1 to S9 
    • References 

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