Research ArticlePHYSICS

Highly mobile charge-transfer excitons in two-dimensional WS2/tetracene heterostructures

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Science Advances  12 Jan 2018:
Vol. 4, no. 1, eaao3104
DOI: 10.1126/sciadv.aao3104
  • Fig. 1 Construction of a 1L-WS2/Tc heterostructure.

    (A) Optical image of WS2 flakes exfoliated on a Si/SiO2 substrate. The 1L-WS2 is indicated by the dashed line. The square shows the area imaged by AFM in (B). Scale bar, 10 μm. (B) AFM image of the same 1L-WS2 flake in (A) with a Tc thin film deposited on top. Scale bar, 5 μm. (C) Schematic of the formation of CT excitons and the band alignment of the 1L-WS2/Tc heterostructure, showing the formation of a type II heterojunction. (D) Steady-state PL spectra of a 1L-WS2, Tc thin film, and a 1L-WS2/Tc heterostructure. The new emission band at 1.7 eV indicates the formation of interlayer CT excitons.

  • Fig. 2 Interlayer CT exciton emission and hole transfer from WS2 to Tc.

    (A) PL spectra of a 1L-WS2/Tc heterostructure, a 3L-WS2/Tc heterostructure, and a Tc thin film with excitation energy of 2.1 eV and 700-nm long-pass filter selectively exciting WS2 in the heterostructures and detecting only interlayer CT exciton emission. (B) Time-resolved PL measurements on the interlayer CT exciton in a 1L-WS2/Tc heterostructure, a Tc film, and a 1L-WS2 flake. The CT exciton PL decay is fitted with a stretched exponential function, as described in the main text. (C) Transient absorption dynamics probed at the A exciton bleach of the 1L-WS2 before and after Tc deposition with a pump energy of 2.1 eV (pump fluence, 50 μJ cm−2). Red solid lines are fittings with a biexponential function convoluted with an experimental response function. Inset: Band alignment shows the hole transfer process of the 1L-WS2/Tc heterostructure. (D) Transient absorption dynamics probed at the A exciton bleach of 2L-WS2 before and after Tc deposition with a pump energy of 2.1 eV, showing no hole transfer in the 2L-WS2/Tc heterostructure.

  • Fig. 3 Electron and energy transfer from Tc to WS2.

    (A) 1L-WS2 dynamics before and after Tc deposition. (B) 2L-WS2 dynamics before and after Tc deposition. Pump = 3.1 eV (pump fluence, 2.2 μJ cm−2) and probe = 2.0 eV. (C) Subtraction of 1L-WS2 dynamics from 1L-WS2/Tc dynamics fitted with a exponential growth function with a time constant of 2.1 ± 0.2 ps and subtraction of 2L-WS2 dynamics from 2L-WS2/Tc dynamics yielding a rise time constant of 44 ± 5 ps. (D) Schematic illustration of electron and energy transfer processes. In the heterostructures constructed from 2L-WS2 or thicker, type I heterojunctions are formed, and only exciton energy transfer is possible. (E) Energy transfer rate dependence on numbers of WS2 layers fitted to the electromagnetic model developed by Raja et al. (39), as described in the main text.

  • Fig. 4 Transport of interlayer CT excitons.

    (A) Exciton population profiles fitted with Gaussian functions at different delay times with the maximum ΔT signal normalized (Norm) to unity for the control 1L-WS2. The pump photon energy is 3.1 eV (pump fluence, 4.4 μJ cm−2), and the probe energy is 2.0 eV. (B) Embedded Image as a function of pump-probe delay time, with a linear fit to Eq. 3 (line) for the control 1L-WS2. Error bars of Embedded Image are the SEs estimated from Gaussian fitting to the spatial intensity distributions. (C) TAM image of the same 1L-WS2/Tc heterostructure shown in Fig. 1 taken with spatially overlapped pump and probe beams at 0 ps. Scale bar, 2 μm. The pump energy is 3.1 eV (pump fluence, 4.4 μJ cm−2), and the probe energy is 2.0 eV. (D) Exciton population profiles fitted with a sum of two Gaussian functions as described in the text at different delay times with the maximum ΔR signal normalized to unity for the 1L-WS2/Tc heterostructure along the line indicated in (C). (E) Embedded Image and Embedded Image as a function of pump-probe delay time, with a linear fit to Eq. 3 (line) for the 1L-WS2/Tc heterostructure.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/1/eaao3104/DC1

    Supplementary Notes

    section S1. Physical vapor deposition of tetracene

    section S2. AFM

    section S3. Microreflection microscopy

    section S4. Factors that limits the spatial precision of TAM imaging

    section S5. Thickness-dependent energy transfer from Tc to WS2

    fig. S1. Absorption and PL spectrum of Tc film and 1L-WS2.

    fig. S2. AFM line profile of the Tc film thickness.

    fig. S3. PLE measurements.

    fig. S4. Excitation intensity–dependent measurements of PL dynamics of CT exciton emission.

    fig. S5. Identification of numbers of WS2 layers using PL spectroscopy.

    fig. S6. TAM dynamics of 4L to 7L WS2 before and after Tc deposition.

    fig. S7. Energy transfer from Tc to WS2 measured with TAM (3.1-eV pump and 2.0-eV probe).

    fig. S8. Schematics of the TAM setup.

    fig. S9. CT exciton population profile fitted with a single Gaussian function and a sum of two Gaussian functions.

    fig. S10. Transient absorption dynamics of a 1L-WS2/Tc heterostructure measured at different pump fluences.

    fig. S11. TAM of CT exciton diffusion measured at 10.0 μJ cm−2.

    fig. S12. TAM of CT exciton diffusion measured at 20.6 μJ cm−2.

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Notes
    • section S1. Physical vapor deposition of tetracene
    • section S2. AFM
    • section S3. Microreflection microscopy
    • section S4. Factors that limits the spatial precision of TAM imaging
    • section S5. Thickness-dependent energy transfer from Tc to WS2
    • fig. S1. Absorption and PL spectrum of Tc film and 1L-WS2.
    • fig. S2. AFM line profile of the Tc film thickness.
    • fig. S3. PLE measurements.
    • fig. S4. Excitation intensity–dependent measurements of PL dynamics of CT exciton emission.
    • fig. S5. Identification of numbers of WS2 layers using PL spectroscopy.
    • fig. S6. TAM dynamics of 4L to 7L WS2 before and after Tc deposition.
    • fig. S7. Energy transfer from Tc to WS2 measured with TAM (3.1-eV pump and 2.0-eV probe).
    • fig. S8. Schematics of the TAM setup.
    • fig. S9. CT exciton population profile fitted with a single Gaussian function and a sum of two Gaussian functions.
    • fig. S10. Transient absorption dynamics of a 1L-WS2/Tc heterostructure measured at different pump fluences.
    • fig. S11. TAM of CT exciton diffusion measured at 10.0 μJ cm−2.
    • fig. S12. TAM of CT exciton diffusion measured at 20.6 μJ cm−2.

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