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Direct evidence for efficient ultrafast charge separation in epitaxial WS2/graphene heterostructures

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Science Advances  13 May 2020:
Vol. 6, no. 20, eaay0761
DOI: 10.1126/sciadv.aay0761
  • Fig. 1 Equilibrium band structure and photocarrier dynamics of WS2/graphene heterostructure.

    (A) Equilibrium photocurrent measured along the ΓK-direction with an unpolarized helium lamp. (B) Photocurrent for negative pump-probe delay measured with p-polarized extreme ultraviolet pulses at 26-eV photon energy. Dashed gray and red lines mark the position of the line profiles used to extract the transient peak positions in Fig. 2. (C) Pump-induced changes of the photocurrent 200 fs after photoexcitation at a pump photon energy of 2 eV with a pump fluence of 2 mJ/cm2. Gain and loss of photoelectrons are shown in red and blue, respectively. The boxes indicate the area of integration for the pump-probe traces displayed in Fig. 3.

  • Fig. 2 Transient band shifts after photoexcitation.

    Change in peak position of the WS2 valence band (A) and graphene π-band (B) as a function of pump-probe delay together with exponential fits (thick lines). The lifetime of the WS2 shift in (A) is 1.2 ± 0.1 ps. The lifetime of the graphene shift in (B) is 1.7 ± 0.3 ps.

  • Fig. 3 Energy- and momentum-resolved carrier dynamics.

    Pump-probe traces as a function of delay obtained by integrating the photocurrent over the area indicated by the boxes in Fig. 1C. The thick lines are exponential fits to the data. Curve (1) Transient carrier population in the conduction band of WS2. Curve (2) Pump-probe signal of the π-band of graphene above the equilibrium chemical potential. Curve (3) Pump-probe signal of the π-band of graphene below the equilibrium chemical potential. Curve (4) Net pump-probe signal in the valence band of WS2. The lifetimes are found to be 1.2 ± 0.1 ps in (1), 180 ± 20 fs (gain) and ∼2 ps (loss) in (2), and 1.8 ± 0.2 ps in (3).

  • Fig. 4 Transient hole density in graphene layer.

    Change of the number of holes in the π-band as a function of pump-probe delay together with exponential fit yielding a lifetime of 1.5 ± 0.2 ps.

  • Fig. 5 Sketch of ultrafast charge transfer deduced from tr-ARPES data.

    (A) Photoexcitation at resonance to the WS2 A-exciton at 2 eV injects electrons into the conduction band of WS2. The corresponding holes in the valence band of WS2 are instantly refilled by electrons from the graphene π-band. (B) The photoexcited carriers in the conduction band of WS2 have a lifetime of ∼1 ps. The holes in the graphene π-band live for ∼2 ps, indicating the importance of additional scattering channels indicated by dashed arrows. Black dashed lines in (A) and (B) indicate band shifts and changes in chemical potential. (C) In the transient state, the WS2 layer is negatively charged while the graphene layer is positively charged. For spin-selective excitation with circularly polarized light, the photoexcited electrons in WS2 and the corresponding holes in graphene are expected to show opposite spin polarization.

Supplementary Materials

  • Supplementary Materials

    Direct evidence for efficient ultrafast charge separation in epitaxial WS2/graphene heterostructures

    Sven Aeschlimann, Antonio Rossi, Mariana Chávez-Cervantes, Razvan Krause, Benito Arnoldi, Benjamin Stadtmüller, Martin Aeschlimann, Stiven Forti, Filippo Fabbri, Camilla Coletti, Isabella Gierz

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