Research ArticleAPPLIED SCIENCES AND ENGINEERING

Highly efficient hot electron harvesting from graphene before electron-hole thermalization

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Science Advances  29 Nov 2019:
Vol. 5, no. 11, eaax9958
DOI: 10.1126/sciadv.aax9958
  • Fig. 1 Photoexcitation dynamics and corresponding carrier distributions in graphene.

    (i) Interband optical excitation leads to nonthermal electron-hole distribution. (ii) Electrons and holes each evolve into separate quasi-thermalized hot FD distributions with distinct temperatures (Te(h)*) and chemical potentials (Te(h)*) in ~10 fs, mostly through electron-electron intraband scattering. (iii and iv) Interband scattering across the Dirac point and optical phonon emission establish hot thermalized electron distribution with one temperature (Te) and chemical potential (μ) in ~100 fs, which cools down further through emission of optical and acoustic phonons in picoseconds.

  • Fig. 2 Scheme and characterization of Gr/WS2 heterostructure.

    (A) Scheme showing photoexcited hot electron transfer from graphene to WS2 underneath before electron-hole coalescence. (B) Optical image of a representative Gr/WS2 heterostructure. Scale bar, 5 μm. (C) Atomic force microscopy height profile of a Gr/WS2 heterostructure showing a ~0.9-nm height difference between WS2 and substrate and ~0.4 nm between graphene and WS2. (D) Scheme of quasi-particle band alignment between graphene and WS2. The barrier height between the graphene Dirac point and WS2 CBM is estimated to be ~0.9 eV without considering excitonic effect or interfacial effect. VBM, valance band maximum. (E) Reflectance contrast (δR) spectra of graphene, WS2 monolayers, and Gr/WS2 heterostructure. The summed spectrum of isolated graphene and WS2 monolayers is also shown for comparison, showing little contribution from interlayer CT transition.

  • Fig. 3 Hot electron transfer from photoexcited graphene to WS2 monolayer.

    (A) Color plot of TR spectra of Gr/WS2 heterostructure under 1.6-eV excitation, clearly showing the A exciton bleach of WS2 under sub-bandgap excitation. (B) TR kinetic at WS2 A exciton bleach in Gr/WS2 heterostructure showing ~25-fs rising and a ~1.2-ps decay process, corresponding to a hot electron transfer from graphene to WS2 and a subsequent back electron transfer process. The black line is the exponential fit. (C) Peak amplitude of TR signal for different photon energies as a function of absorbed photon density, showing a clear linear dependence. Solid lines are linear fits. Higher photon energy leads to larger peak amplitude due to hotter carrier distribution. (D) Experimentally determined QY (black circles) of hot electron transfer as a function of photon energies showing continuous increase with photon energy. Lines are predicted QY based on 2μPTE with different barrier heights (0.6, 0.7, and 0.8 eV). Relative hot electron injection yield (ℏν = 1.6 eV and Nphoton = 4 × 1012 cm−2) as a function of (E) sample temperatures (78 to 298 K) and (F) back-gating voltages (−40 to 40 V). The hot electron injection process shows negligible dependence on sample temperature. The electron inject yield is not affected by hole (p) doping but decreases with increasing electron (n) doping. a.u., arbitrary units.

  • Fig. 4 Comparison between conventional 1μPTE model and proposed 2μPTE model.

    (A) 1μ and (D) 2μ models where photoexcitation generates thermalized electrons characterized with one FD distribution in the former but quasi-thermalized electrons and holes with two FD distributions in the latter. Zero denotes the Dirac point. Nphoton increases as 2 × 1012, 4 × 1012, and 6 × 1012 cm−2, and ℏν is fixed at 1.6 eV. Electron distribution above the Dirac point in graphene with (B) 1μ and (E) 2μ models at different absorbed photon densities. Hot electrons above the barrier height (φB) can inject into the semiconductor conduction band. A much higher fraction of energetic electrons are observed in 2μ model. Color plot of predicted electron injection QY as a function of photon energy and density with (C) 1μPTE and (F) 2μPTE models. φB was chosen to be 0.7 eV, and all electrons above φB were assumed to inject into the semiconductor conduction band. QY shows a strong dependence on both photon energy and density in the 1μPTE pathway but remains nearly constant with varying photon density in the 2μPTE pathway.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/11/eaax9958/DC1

    Section S1. Determining doping level and chemical potential in graphene

    Section S2. Selective carrier extraction from Gr/WS2 heterostructures and charge recombination retardation

    Section S3. QY calibration using WSe2/WS2 heterostructure and direct high-energy excitation

    Section S4. Absorbed photon density calculation

    Section S5. Calculation with 1μPTE model and 2μPTE model

    Section S6. Hot electron transfer from photoexcited graphene to WSe2

    Section S7. Additional figures

    Fig. S1. Determining doping level and chemical potential in graphene.

    Fig. S2. Selective carrier extraction and charge recombination retardation.

    Fig. S3. CT in WSe2/WS2 heterostructure.

    Fig. S4. TR measurements of Gr/WS2 under above-gap excitation.

    Fig. S5. Beam profile and absorption properties of heterostructures.

    Fig. S6. Calculated carrier temperature and chemical potential with different models.

    Fig. S7. Calculated injection electron density.

    Fig. S8. Hot electron transfer from photoexcited graphene to WSe2.

    Fig. S9. PL spectra of WS2 monolayer and Gr/WS2 heterostructure.

    Fig. S10. All TR kinetics of Gr/WS2 heterostructure at WS2 A exciton bleach, at different excitation photon energies and densities.

    Fig. S11. TR results from another Gr/WS2 sample.

    Fig. S12. TR spectra of Gr/MoS2 and Gr/MoSe2 heterostructures.

    References (4548)

  • Supplementary Materials

    This PDF file includes:

    • Section S1. Determining doping level and chemical potential in graphene
    • Section S2. Selective carrier extraction from Gr/WS2 heterostructures and charge recombination retardation
    • Section S3. QY calibration using WSe2/WS2 heterostructure and direct high-energy excitation
    • Section S4. Absorbed photon density calculation
    • Section S5. Calculation with 1μPTE model and 2μPTE model
    • Section S6. Hot electron transfer from photoexcited graphene to WSe2
    • Section S7. Additional figures
    • Fig. S1. Determining doping level and chemical potential in graphene.
    • Fig. S2. Selective carrier extraction and charge recombination retardation.
    • Fig. S3. CT in WSe2/WS2 heterostructure.
    • Fig. S4. TR measurements of Gr/WS2 under above-gap excitation.
    • Fig. S5. Beam profile and absorption properties of heterostructures.
    • Fig. S6. Calculated carrier temperature and chemical potential with different models.
    • Fig. S7. Calculated injection electron density.
    • Fig. S8. Hot electron transfer from photoexcited graphene to WSe2.
    • Fig. S9. PL spectra of WS2 monolayer and Gr/WS2 heterostructure.
    • Fig. S10. All TR kinetics of Gr/WS2 heterostructure at WS2 A exciton bleach, at different excitation photon energies and densities.
    • Fig. S11. TR results from another Gr/WS2 sample.
    • Fig. S12. TR spectra of Gr/MoS2 and Gr/MoSe2 heterostructures.
    • References (4548)

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