Research ArticlePHYSICS

Giant gate-tunable bandgap renormalization and excitonic effects in a 2D semiconductor

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Science Advances  19 Jul 2019:
Vol. 5, no. 7, eaaw2347
DOI: 10.1126/sciadv.aaw2347
  • Fig. 1 A monolayer ReSe2 on a back-gated G/h-BN device.

    (A) Schematic illustration of a back-gated ReSe2/graphene/h-BN device. (B) Top view of the atomic structure of monolayer ReSe2. The two lattice vectors (a and b) are outlined by red lines. The lattice constants are a = 6.6 Å and b = 6.7 Å. The angle between a and b is 118.9°. (C) A representative STM image of a monolayer ReSe2 flake on graphene/h-BN. Inset shows the STM line profile along the step edge. (D) Atomically resolved STM image of monolayer ReSe2. The STM image reveals the unique 1D chains consisting of diamond-shaped Re4 units along a direction (highlighted by orange line). The cross (X) marks the position where the differential conductance (dI/dV) spectra were taken.

  • Fig. 2 STM images of moiré pattern in monolayer ReSe2/graphene.

    (A to C) Representative moiré patterns observed in the experiment. (D to F) Calculated moiré patterns obtained from the geometrical analysis. θ is the stacking angle between ReSe2 and graphene.

  • Fig. 3 Gate-dependent dI/dV and differential reflectance spectra of a monolayer ReSe2 on graphene.

    (A) dI/dV spectrum of monolayer ReSe2 (blue line) at Vg = 0 V together with the calculated LDOS (dashed red line). (B) Energy position of VB maximum (VBM; red points) and CB minimum (CBM; dark blue points) as a function of the gate voltage. (C) Gate-dependent dI/dV spectra of the monolayer ReSe2 on graphene/h-BN measured at 4.5 K. As-applied gate voltage is indicated above each STS curve. The VBM and CBM were indicated by light red and light blue points, respectively. (D) Gate-dependent differential reflectance spectra of the monolayer ReSe2 on graphene/h-BN measured at 5 K. The corresponding gate voltage is indicated on the side of each differential reflectance spectrum. Note: The original differential reflectance spectra after background subtraction (circles); fitted curves using the Lorentzian function (solid lines). a.u., arbitrary units.

  • Fig. 4 Gate-tunable bandgap renormalization and exciton binding energy of monolayer ReSe2 on graphene.

    (A) A plot of QP bandgap Eg (black points), optical bandgap Eopt (red points), and exciton binding energy Eb (blue points) as a function of gate voltage. Note: The Eopt = 1.47 ± 0.01 eV remains constant when the gate voltage increases from −40 to 40 V. Note: The same Eopt is used for the calculation of Eb at the gate voltage of −63, −60, −50, and +45 V. The solid blue line refers to the theoretically predicted Eb as a function of the gate voltage (refer to section S8 for more details). (B) Illustration of the screening of electron-hole interactions in monolayer ReSe2 by the gate-controlled free carriers in graphene. (C) Schematic illustration of gate-tunable Eg and Eb of monolayer ReSe2 at the gate voltage of −63 and +45 V, respectively.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/7/eaaw2347/DC1

    Section S1. Atomic force microscopy measurement of monolayer ReSe2

    Section S2. Moiré pattern of single-layer ReSe2 on graphene

    Section S2.1. Moiré patterns for various twist angles

    Section S2.2. A comparison between experimental and theoretical moiré patterns

    Section S3. Gate-dependent dI/dV spectra of a different ReSe2/graphene device

    Section S4. Probe the defects in monolayer ReSe2

    Section S5. Band structure of monolayer ReSe2

    Section S6. Differential reflectance spectrum and gate-dependent photoluminescence spectra of monolayer ReSe2

    Section S7. Gate-dependent dI/dV spectra of graphene/monolayer ReSe2

    Section S8. Calculation of Eb in monolayer ReSe2 as a function of the carrier density in graphene substrate

    Section S9. Charge transfer at the interface of ReSe2/graphene

    Fig. S1. Identify the thickness of monolayer ReSe2.

    Fig. S2. Moiré lengths of ReSe2/graphene as a function of twist angle.

    Fig. S3. Gate-dependent dI/dV spectra of a different device.

    Fig. S4. STM images and STS measurements of defects in ReSe2.

    Fig. S5. Band structure of monolayer ReSe2 calculated using the first-principle density functional theory calculations with the Perdew-Burke-Ernzerhof exchange-correlation functional using the QUANTUM ESPRESSO code.

    Fig. S6. Differential reflectance spectrum and gate-dependent photoluminescence spectra of monolayer ReSe2 on graphene/h-BN.

    Fig. S7. Gate-dependent dI/dV spectra of graphene/monolayer ReSe2.

    Fig. S8. Exciton binding energy (Eb) and Thomas-Fermi screening radius (rs) as a function of electron concentration (n) in graphene.

    Fig. S9. Charge transfer at ReSe2/graphene interface.

    Table S1. Geometrical properties of the moiré patterns of ReSe2/graphene.

    References (3746)

  • Supplementary Materials

    This PDF file includes:

    • Section S1. Atomic force microscopy measurement of monolayer ReSe2
    • Section S2. Moiré pattern of single-layer ReSe2 on graphene
    • Section S2.1. Moiré patterns for various twist angles
    • Section S2.2. A comparison between experimental and theoretical moiré patterns
    • Section S3. Gate-dependent dI/dV spectra of a different ReSe2/graphene device
    • Section S4. Probe the defects in monolayer ReSe2
    • Section S5. Band structure of monolayer ReSe2
    • Section S6. Differential reflectance spectrum and gate-dependent photoluminescence spectra of monolayer ReSe2
    • Section S7. Gate-dependent dI/dV spectra of graphene/monolayer ReSe2
    • Section S8. Calculation of Eb in monolayer ReSe2 as a function of the carrier density in graphene substrate
    • Section S9. Charge transfer at the interface of ReSe2/graphene
    • Fig. S1. Identify the thickness of monolayer ReSe2.
    • Fig. S2. Moiré lengths of ReSe2/graphene as a function of twist angle.
    • Fig. S3. Gate-dependent dI/dV spectra of a different device.
    • Fig. S4. STM images and STS measurements of defects in ReSe2.
    • Fig. S5. Band structure of monolayer ReSe2 calculated using the first-principle density functional theory calculations with the Perdew-Burke-Ernzerhof exchange-correlation functional using the QUANTUM ESPRESSO code.
    • Fig. S6. Differential reflectance spectrum and gate-dependent photoluminescence spectra of monolayer ReSe2 on graphene/h-BN.
    • Fig. S7. Gate-dependent dI/dV spectra of graphene/monolayer ReSe2.
    • Fig. S8. Exciton binding energy (Eb) and Thomas-Fermi screening radius (rs) as a function of electron concentration (n) in graphene.
    • Fig. S9. Charge transfer at ReSe2/graphene interface.
    • Table S1. Geometrical properties of the moiré patterns of ReSe2/graphene.
    • References (3746)

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