Research ArticleSPINTRONICS

Observation of ultralong valley lifetime in WSe2/MoS2 heterostructures

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Science Advances  26 Jul 2017:
Vol. 3, no. 7, e1700518
DOI: 10.1126/sciadv.1700518
  • Fig. 1 Ultrafast charge transfer process in the WSe2/MoS2 heterostructure.

    (A) Illustration of the ultrafast electron transfer process in the WSe2/MoS2 heterostructure. The WSe2/MoS2 heterostructure forms a type II heterojunction where the conduction band minimum and the valence band maximum reside in MoS2 and WSe2, respectively. Photoexcited electrons transfer rapidly to the MoS2 layer, whereas holes remain in the WSe2 layer. (B) Optical microscope image of a representative WSe2/MoS2 heterostructure. Blue, red, and black dashed lines encircle WSe2, MoS2, and heterostructure (HS) regions, respectively. Scale bar, 10 μm. (C) PL image of the WSe2/MoS2 heterostructure at WSe2 A-exciton resonance (1.65 eV) at room temperature. PL is quenched by four orders of magnitude in the heterostructure compared to the WSe2-only region due to the ultrafast electron transfer process in the heterostructure.

  • Fig. 2 Photoinduced CD signal of the WSe2/MoS2 heterostructure at 10 K.

    (A) Schematic of valley-polarized hole generation in the WSe2 layer within the heterostructure. Upon photoexcitation with LCP light, excitons are resonantly created in the K valley of the WSe2 layer (encircled with a dashed box). Ultrafast charge transfer process then efficiently transfers electrons to the MoS2 layer and leaves resident holes at the K valley in the WSe2 layer. (B) Top: Photoinduced CD spectrum at 3 ns. Bottom: RC spectrum of the WSe2/MoS2 heterostructure. RC spectrum is dominated by the optical absorption near the WSe2 A-exciton resonance at 1.72 eV. The CD spectrum, −Δ(RCσ+ − RCσ−), shows prominent resonant feature near the WSe2 A-exciton peak under LCP pump light at 1.78 eV (black arrow in the RC spectrum). (C) Decay dynamics of the resonant CD signal at 10 K. No decay is observed within 3.5 ns (inset). The decay curve over a longer time scale shows a significant slow decay component with a lifetime of more than 1 μs.

  • Fig. 3 An almost perfect valley polarization.

    (A and B) Resident holes in the K valley induce distinctively different absorption changes for the K (A) and K′ (B) valleys; RC of the heterostructure (blue solid line) is also shown for comparison. (A) The absorption change in the K valley features an overall reduction of the absorption oscillator strength and a slight blueshift of the exciton resonance. The inset illustrates the change of absorption spectrum, where the original exciton absorption (black dashed line) is modified in the presence of holes at the K valley (red solid line). This spectral change can be understood by the phase-space filling and Burstein-Moss effects. (B) The absorption change in the K′ valley shows a transfer of oscillator strength from exciton to trion absorption due to the formation of intervalley trions. However, the total oscillator strength is unaffected because there is no Pauli blocking effect. The inset illustrates the reduction of exciton absorption and emergence of intervalley trion absorption (red solid line) compared to the original spectrum (black dashed line). (C) The density of resident holes in the K and K′ valleys obtained by the integrated oscillator strength change of LCP and RCP light, which suggests near-perfect valley polarization (within 10% experimental uncertainty). The total hole density is normalized to 1.

  • Fig. 4 Ultralong valley depolarization lifetime.

    (A and B) Photoinduced difference (A) and sum (B) responses of the two valleys at a pump-probe delay of 3 ns (black), 80 ns (red), and 550 ns (blue). All spectra were measured at 10 K and normalized to 1. Both the difference and sum spectra show constant profile over time, except for a weak signal at around 1.66 eV in the sum response due to some defect states that decay slowly. (C) Decay dynamics of the total hole population p+ + p (black dots), the valley-polarized hole population p+p (red squares), and the degree of valley polarization P (blue triangles) obtained with a probe energy of 1.71 eV. The decay of the valley-polarized hole population of ~1 μs is mainly due to the total population decay. However, the valley polarization does not show any apparent decay at 2.5 μs, corresponding to an ultralong valley depolarization lifetime approaching 40 μs. (D) Temperature-dependent decay dynamics of valley polarization from 10 to 77 K (symbols). Solid lines are biexponential decay fitting of experimentally measured decay dynamics, with decay lifetime of dominant slow components summarized in (E). The valley depolarization lifetime changes strongly with the temperature, suggesting an energy-activated mechanism in the intervalley hole scattering.

Supplementary Materials

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    • fig. S1. CD signal of the heterostructure at 10 K with different pump fluence from 65 to 650 nJ cm−2.
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