Research ArticleCONDENSED MATTER PHYSICS

Optical generation of high carrier densities in 2D semiconductor heterobilayers

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Science Advances  13 Sep 2019:
Vol. 5, no. 9, eaax0145
DOI: 10.1126/sciadv.aax0145
  • Fig. 1 Excitation density–dependent PL and Mott transition in the WSe2/MoSe2 heterobilayer.

    PL spectra (A) and intensity-normalized PL spectra (B) from a BN-encapsulated WSe2/MoSe2 heterobilayer with θ = 4° ± 2° angular alignment between the two monolayers. a.u., arbitrary units. The spectra were obtained with CW excitation at hν = 2.33 eV and calibrated excitation densities (neh) between 1.6 × 1010 and 3.2 × 1014 cm−2 at 4 K. The spectral region (hν ≥ 1.51 eV) corresponding to PL emission from monolayers WSe2 and MoSe2 is multiplied by a factor of 30. Also in (A) is PL from monolayer WSe2 (green) and monolayer MoSe2 (blue). Shown on the 2D pseudocolor (normalized intensity, I/IP, where IP is peak intensity) plot in (B) are contours of 50% (solid curve) and 25 and 75% (dashed curves) of IP. (C) Integrated intensities (left axis) of interlayer (1.2 to 1.5 eV, solid black circles) and intralayer (1.51 to 1.80 eV, open black squares) PL emission, full width at half maximum (FWHM) of the interlayer exciton peak (open red triangles, right axis) as a function of neh, and integrated intralayer PL intensities (solid gray squares) from a BN-encapsulated WSe2/MoSe2 heterobilayer with θ = 13° ± 2° angular alignment. (D) Computed joint electron/hole populations in the K valleys for interlayer exciton (black) and intralayer excitons in MoSe2 (blue) and WSe2 (green). The top of the figure is a cartoon illustrating the Mott transition in the WSe2/MoSe2 heterobilayer.

  • Fig. 2 TRPL emission from interlayer excitons in the WSe2/MoSe2 heterobilayer.

    The sample at 4 K is excited by pulsed laser (hν = 2.33 eV; pulse duration, 150 fs). The energy-integrated emission from the interlayer exciton [see spectra in (B)] is detected as a function of time (A) for initial excitation densities of (from bottom to top) n0 = 1.1 × 1010, 3.0 × 1010, 9.4 × 1010, 3.0 × 1011, 9.4 × 1011, 3.0 × 1012, 8.7 × 1012, 2.5 × 1013, and 6.0 × 1013 cm−2. (C) Initial decay time constants (solid circles) as a function of n0. The solid line is the biexponential fit to the data.

  • Fig. 3 Density-dependent transient reflectance spectra from the WSe2/MoSe2 heterobilayer.

    The WSe2/MoSe2 heterobilayer was excited at hν = 1.82 eV with initial excitation densities of n0 = (A) 1.0 × 1011, (B) 9.6 × 1011, (C) 5.6 × 1012, and (D) 3.4 × 1013 cm−2 at a sample temperature of 4 K. The excited sample is probed with a white light, and the pseudocolor scale is ΔR/R0 (blue , bleaching; red, induced absorption). Transient reflectance spectra at selected pump-probe delays (Δt) at n0 = (E) 1.0 × 1011 and (F) 3.4 × 1013 cm−2 are also shown. The probe regions around 1.55 eV are blocked out due to low intensity and noise from white light which was generated by 1.55-eV laser light. Kinetic profiles obtained from vertical cuts at (G) 1.351 and (H) 1.624 eV in the 2D pseudocolor plots at the four n0 values.

  • Fig. 4 Calculated optical spectra of the WSe2/MoSe2 heterobilayer.

    (A) Simulated reflectance spectra from theoretical optical spectra and experimental sample geometry at the indicated excitation densities (neh = 6 × 1011 to 3 × 1013 cm−2). (B) Experimental reflectance spectra at Δt = 1 ps at initial excitation densities of n0 = 9.6 × 1011 to 3.4 × 1013 cm−2. (C) Calculated optical absorptance spectra at neh = 1 × 1011 to 5 × 1014 cm−2. (D) Calculated relative optical absorptance as a function of neh at two photon energies used in the experiments.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/9/eaax0145/DC1

    Supplementary Text

    Fig. S1. 2D MoSe2/WSe2 heterostructure samples.

    Fig. S2. Determination of monolayer orientation via polarization-resolved SHG.

    Fig. S3. Determination of monolayer orientation via polarization-resolved SHG: The case of rotating laser polarization.

    Fig. S4. Low-temperature spectroscopy-microscopy setup.

    Fig. S5. Time resolution of TRPL and pump-probe experiments.

    Fig. S6. Calculated optical absorptances for monolayer WSe2, monolayer MoSe2, and WSe2/MoSe2 heterobilayer based on the reported dielectric constants in (17).

    Fig. S7. Calibration of steady-state excitation density.

    Fig. S8. Band structure model for the AA-stacked MoSe2-WSe2 heterolayer, including the high-symmetry points Γ and K, as well as the Q point in between.

    Fig. S9. Dielectric structure model.

    Fig. S10. PL of the misaligned heterostructure.

    Fig. S11. Comparison of steady-state and pulsed PL spectra at similar carrier densities.

    Fig. S12. Excitation density and temperature-dependent PL spectra from the WSe2/MoSe2 heterobilayer.

    Fig. S13. BN-encapsulated monolayer MoSe2 and WSe2 samples.

    Fig. S14. Electron-hole plasma dynamics in monolayers by transient reflectance.

    Table S1. Band edges, effective masses, and layer contributions for the AA-stacked MoSe2-WSe2 heterolayer according to the notation in fig. S8.

    References (4349)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Text
    • Fig. S1. 2D MoSe2/WSe2 heterostructure samples.
    • Fig. S2. Determination of monolayer orientation via polarization-resolved SHG.
    • Fig. S3. Determination of monolayer orientation via polarization-resolved SHG: The case of rotating laser polarization.
    • Fig. S4. Low-temperature spectroscopy-microscopy setup.
    • Fig. S5. Time resolution of TRPL and pump-probe experiments.
    • Fig. S6. Calculated optical absorptances for monolayer WSe2, monolayer MoSe2, and WSe2/MoSe2 heterobilayer based on the reported dielectric constants in (17).
    • Fig. S7. Calibration of steady-state excitation density.
    • Fig. S8. Band structure model for the AA-stacked MoSe2-WSe2 heterolayer, including the high-symmetry points Γ and K, as well as the Q point in between.
    • Fig. S9. Dielectric structure model.
    • Fig. S10. PL of the misaligned heterostructure.
    • Fig. S11. Comparison of steady-state and pulsed PL spectra at similar carrier densities.
    • Fig. S12. Excitation density and temperature-dependent PL spectra from the WSe2/MoSe2 heterobilayer.
    • Fig. S13. BN-encapsulated monolayer MoSe2 and WSe2 samples.
    • Fig. S14. Electron-hole plasma dynamics in monolayers by transient reflectance.
    • Table S1. Band edges, effective masses, and layer contributions for the AA-stacked MoSe2-WSe2 heterolayer according to the notation in fig. S8.
    • References (4349)

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