Direct determination of momentum-resolved electron transfer in the photoexcited van der Waals heterobilayer $\mathrm{W}{\mathrm{S}}_{2}/\mathrm{Mo}{\mathrm{S}}_{2}$

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Direct determination of momentum-resolved electron transfer in the photoexcited van der Waals heterobilayer $W S_{2} / Mo S_{2}$

Fang Liu, Qiuyang Li, and X.-Y. Zhu

Phys. Rev. B 101, 201405(R) – Published 18 May 2020

Abstract

Photoinduced charge separation in transition-metal dichalcogenide heterobilayers is being explored for moiré excitons, spin-valley polarization, and quantum phases of excitons/electrons. While different momentum points can be critically involved in charge separation dynamics, little is known directly from experiments. Here we determine momentum-resolved electron dynamics in the $W S_{2} / Mo S_{2}$ heterobilayer using time- and angle-resolved photoemission spectroscopy. Upon photoexcitation in the $K$ valleys, we detect electrons in $M$ /2, $M$ , and $Q$ valleys/points on timescales as short as ∼70 fs, followed by dynamic equilibration in $K$ and $Q$ valleys in ∼400 fs. These findings reveal the essential role of phonon scattering, the coexistence of direct and indirect interlayer excitons, and constraints on spin-valley polarization.

Received 18 September 2019
Revised 22 April 2020
Accepted 27 April 2020

DOI:https://doi.org/10.1103/PhysRevB.101.201405

Physics Subject Headings (PhySH)

Electronic structure Excitons

2-dimensional systems Dichalcogenides

Time & angle resolved photoemission spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Fang Liu , Qiuyang Li , and X.-Y. Zhu ^*

Department of Chemistry, Columbia University, New York, New York 10027, USA

^*Author to whom correspondence should be addressed: xyzhu@columbia.edu

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Vol. 101, Iss. 20 — 15 May 2020

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Figure 1
(a) Schematics of the TR-ARPES experiment. A femtosecond visible pulse serves as pump and the femtosecond EUV pulse is probe. The photoelectrons are detected by a hemispherical analyzer at various polar angles α from surface normal for momentum resolution. (b) Image of the $W S_{2} / Mo S_{2}$ heterostructure on 285-nm $Si O_{2} / Si$ , with blue color from optical contrast. Gold electrodes are deposited on the side for grounding. A zoomed-out image of the sample is shown in Fig. S1 [25]. (c) SHG mapped across 40 different points on the millimeter scale heterostructure. The uniform phase indicates uniform crystal orientation. The inset is the SHG polar plot of the 40 sampled points. (d) Calculated band structure of $W S_{2} / Mo S_{2}$ heterostructures with 0° alignment. (Reprinted, with permission, from Ref. [27]; copyright ©2018 American Chemical Society.) The color scheme indicates the contribution to wave functions from each layer. The inset is a schematic of the different points in the Brillouin zone. (e), (f) Experimental ARPES of the $W S_{2} / Mo S_{2}$ heterostructure measured along the (e) Γ- $K$ direction and the (f) Γ- $M$ direction, respectively. All TR-ARPES and ARPES measurements are performed at room temperature.
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Figure 2
TR-ARPES of the $W S_{2} / Mo S_{2}$ heterostructure (a) without ( $Δ t < 0$ ) and (b) with ( $Δ t = 0$ ) the visible pump excitation, respectively. The ARPES spectra are collected at the $M$ , $M$ /2, Γ, $Q$ , and $K$ positions in the momentum space. The color scheme is scaled up a lot to show the weak signal in the CB. (c) The corresponding electron energy distribution curve (EDC) near the CB edge, collected without ( $Δ t < 0$ , red) and with ( $Δ t = 0$ , green) the visible pump excitation, and the difference with and without pump (black). The EDCs are integrated within a $\pm 0.15 Å^{- 1}$ window of the center momentum at each point from raw photoionization signals. The visible pump is fixed at a photon energy of 2.2 eV and the probe at 22 eV. The sample is at room temperature.
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Figure 3
CB electron dynamics at the $K$ , $Q$ , $M$ /2, and $M$ points as a function of pump-probe delay. The integrated CB photoelectron intensities are shown as gray curves, with intensity at $Δ t < 0$ set to zero. The solid black curves are kinetic fits and the blue curves (scaled to experimental peak intensity) are convoluted Gaussians ( $σ = 140 fs$ ) representing laser pump-probe time resolution up to 70 fs ( $1 / 2 σ$ ). The dashed lines show zero intensity levels. The CB electron intensities are integrated from −0.5 to 2.5 eV. The corresponding TR-ARPES in 2D pseudocolor plots are shown in Fig. S5 [25]. The intensity is integrated within a $\pm 0.15 Å^{- 1}$ window of the center momentum.
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Figure 4
Schematics of intervalley electron scattering in the CB. The visible pump beam excites the electron from VB to CB in the $K$ valleys (green arrows), followed by ultrafast scattering (≤70 fs) of electron from $K$ to $M$ , $M$ /2, and $Q$ (black arrows). The dashed arrows represent subsequent electron scattering (∼0.4 ps) from $M$ /2 and $M$ back to $K$ and $Q$ . The double-ended and dashed arrows represent dynamic equilibria.
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Physical Review B

covering condensed matter and materials physics

Direct determination of momentum-resolved electron transfer in the photoexcited van der Waals heterobilayer $W S_{2} / Mo S_{2}$

Fang Liu, Qiuyang Li, and X.-Y. Zhu

Phys. Rev. B 101, 201405(R) – Published 18 May 2020

Abstract

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Physical Review B

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Direct determination of momentum-resolved electron transfer in the photoexcited van der Waals heterobilayer WS2/MoS2

Fang Liu, Qiuyang Li, and X.-Y. Zhu

Phys. Rev. B 101, 201405(R) – Published 18 May 2020

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Direct determination of momentum-resolved electron transfer in the photoexcited van der Waals heterobilayer $W S_{2} / Mo S_{2}$