Band Engineering of Large-Twist-Angle $\text{Graphene}/h\text{\ensuremath{-}}\mathrm{BN}$ Moir\'e Superlattices with Pressure

Band Engineering of Large-Twist-Angle $Graphene / h − BN$ Moiré Superlattices with Pressure

Yang Gao, Xianqing Lin, Thomas Smart, Penghong Ci, Kenji Watanabe, Takashi Taniguchi, Raymond Jeanloz, Jun Ni, and Junqiao Wu

Phys. Rev. Lett. 125, 226403 – Published 25 November 2020

Abstract

Graphene interfacing hexagonal boron nitride ( $h − BN$ ) forms lateral moiré superlattices that host a wide range of new physical effects such as the creation of secondary Dirac points and band gap opening. A delicate control of the twist angle between the two layers is required as the effects weaken or disappear at large twist angles. In this Letter, we show that these effects can be reinstated in large-angle ( $\sim 1.8 °$ ) $graphene / h − BN$ moiré superlattices under high pressures. A $graphene / h − BN$ moiré superlattice microdevice is fabricated directly on the diamond culet of a diamond anvil cell, where pressure up to 8.3 GPa is applied. The band gap at the primary Dirac point is opened by 40–60 meV, and fingerprints of the second Dirac band gap are also observed in the valence band. Theoretical calculations confirm the band engineering with pressure in large-angle $graphene / h − BN$ bilayers.

Received 17 May 2020
Accepted 22 September 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.226403

Physics Subject Headings (PhySH)

Band gap Pressure effects

Graphene

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Yang Gao ^1,2,3, Xianqing Lin ⁴, Thomas Smart⁵, Penghong Ci^1,2, Kenji Watanabe ⁶, Takashi Taniguchi⁷, Raymond Jeanloz⁵, Jun Ni⁸, and Junqiao Wu ^1,2,*

¹Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA
²Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
³Department of Engineering Mechanics, Zhejiang University, Hangzhou 310027, People’s Republic of China
⁴College of Science, Zhejiang University of Technology, Hangzhou 310023, People’s Republic of China
⁵Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA
⁶Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
⁷International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
⁸State Key Laboratory of Low-Dimensional Quantum Physics and Frontier Science Center for Quantum Information, Department of Physics, Tsinghua University, Beijing 100084, People’s Republic of China

^*Corresponding author. wuj@berkeley.edu

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Vol. 125, Iss. 22 — 27 November 2020

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Figure 1
Experiment schematic and fabrication process of the $graphene / h − BN$ field effect transistor in DAC. (a) A schematic of the experimental setup. The $graphene / h − BN$ field effect transistor is directly fabricated on the diamond culet and sealed inside the gasket chamber. Pt foils are manually mounted as feedthrough wires. The pressure is calibrated by ruby particles. (b) Schematic side view of the device. Graphite flakes are used as part of the electrodes. (c) A cartoon showing the $graphene / h − BN$ moiré superlattices with twist angle of $\sim 1.8 °$ . The red lines indicate the moiré wavelength. (d) Fabrication process of the device. All the 2D flakes are dry transferred. The diamond culet size is $300 μ m$ and the scale bars are $50 μ m$ .
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Figure 2
Transport measurements of the $\sim 1.8 °$ twisted $graphene / h − BN$ FET in DAC. (a) Two-probe $I − V$ curves of the FET in DAC. (b) Transfer curves of the same device in the DAC. Inset: transfer curve of a smaller-angle (0.5°) twisted $graphene / h − BN$ device recorded at ambient pressure. Clear electron-hole asymmetry is shown in both small and large twist-angle systems. (c) Drain-source resistance at the primary Dirac point, baseline resistance (at $V_{gate} = 10 V$ ) and the baseline-subtracted resistance at the primary Dirac point, under different pressures. (d) Pressure induced PDP band gap opening $Δ E_{g 1}$ with respect to the band gap at ambient pressure. The shadow area indicates the possible PDP band gap widening range considering the contact resistance.
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Figure 3
Calculations of the band structure of the $\sim 1.8 °$ twisted system. (a) the calculated and experimental band gap at the PDP vs pressure. The red circles depict the band gap evolution obtained from theoretical calculations and the gray area is the estimated band gap range from Fig. 2 using the gap value of 30 meV at 0 GPa from Ref. [31]. (b) Calculated band structure near the PDP at 0 GPa, 4.9 GPa, and 8.4 GPa, respectively. Inset: the $k$ path in the moiré Brillouin zone. (c) Calculated gap near the SDP in valence band and the drain-source resistance at $V_{gate} = - 10 V$ as a function of pressure. Blue dashed line indicates overlapped bands (i.e., zero gap). Inset: raw resistance data at $V_{gate} = - 8 V$ to $- 10 V$ . A none-zero SDP gap emerges at 7.3 GPa and an abrupt resistance increase on the hole side is also observed near 7 GPa. (d) Band structure near the SDPs in the valence band at 8.4 GPa. An indirect gap of 4.7 meV is opened.
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Physical Review Letters

Band Engineering of Large-Twist-Angle Graphene/h−BN Moiré Superlattices with Pressure

Yang Gao, Xianqing Lin, Thomas Smart, Penghong Ci, Kenji Watanabe, Takashi Taniguchi, Raymond Jeanloz, Jun Ni, and Junqiao Wu

Phys. Rev. Lett. 125, 226403 – Published 25 November 2020

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Band Engineering of Large-Twist-Angle $Graphene / h − BN$ Moiré Superlattices with Pressure