Research ArticleCONDENSED MATTER PHYSICS

Giant enhancement of exciton diffusivity in two-dimensional semiconductors

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Science Advances  18 Dec 2020:
Vol. 6, no. 51, eabb4823
DOI: 10.1126/sciadv.abb4823
  • Fig. 1 Threshold-like spatial luminescence expansion of monolayer MoS2.

    (A) PL images collected from a typical suspended monolayer MoS2 under the incidence of different laser powers, as indicated by the white numbers (kilowatts per square centimeter in units). The time interval between each measurement is around 1 min. (B) Normalized spatial profiles of PL intensity across the monolayer MoS2. The spatial profiles of the focused laser beam (gray solid curve) and related fitting (dashed black curve) are also plotted. The right panel shows the as-measured spatial profiles of PL intensity without normalization. a.u., arbitrary units. (C) The size of the luminescence area (full width at half magnitude of the spatial profile) as a function of the incident power. The vertical dashed lines indicate the sudden spatial expansion. Error bars are not given in (C) for visual convenience because the error bars are comparable to the size of the dots.

  • Fig. 2 Effect of temperature increase and nonlinear exciton recombination on PL profile.

    (A) Typical spectral-spatial PL images collected from monolayer MoS2 at different environment temperatures (25° and 200°C) under the incidence of 1.0 kW/cm2, in which the vertical axis is the distance and the horizontal axis is the energy. The spectra at 200°C are broader than those 25°C, but the spatial distribution at 200°C is slightly smaller than that at 25°C. (B) Normalized spatial profiles of PL intensity at 25° and 200°C that are extracted from (A). (C) Schematically illustrates the PL spatial profile under linear and nonlinear exciton recombination. q1 and q2 indicate space-variant PL efficiencies. (D) Overlap of the measured PL profile (at 1.0 kW/cm2) with the PL profile expected for EEA-dominated exciton recombination and negligible diffusion, which is square root of the laser profile, as exp(−r2/w2). The Gaussian laser profile exp(−2r2/w2) is also plotted as reference.

  • Fig. 3 Threshold-like evolution of PL spectral features with the incident power.

    (A) As-measured (top) and normalized (bottom) PL spectra collected from monolayer MoS2 under different incident powers. (B) The PL’s spectral full width at half magnitude, (C) the PL’s peak position, and (D) the PL intensity (black circles) as a function of the incident power. The vertical dashed line in (B) to (D) indicates the threshold power for sudden spectra features change and PL intensity change. The spectral broadening and red shift caused by the laser-induced temperature increase are also plotted in (B) and (C), respectively. The inset in (D) schematically illustrates the ionization of excitons. Error bars are not given in (C) and (D) because the error bars are comparable to the size of the dots.

  • Fig. 4 Effect of trapped charges on the PL spatial expansion.

    (A) PL images collected from the same monolayer with different incident powers in sequence: 1.1 kW/cm2 (first measurement), 8.0 kW/cm2 (second measurement), 1.1 kW/cm2 (third measurement), and 1.1 kW/cm2 again (fourth measurement) after raising temperature to 100°C then cooling back to room temperature 25°C (RT). The time interval between each of the first three measurements is around 1 min. The heating process takes time of 10 min, and cooling takes 30 min. (B) Normalized spatial profiles of PL intensity collected at the different measurements using 1.1 kW/cm2 (first, third, and fourth measurement). (C) Spatial profiles of PL intensity collected at the first and third measurements. The orange arrows point to the peripheral region. (D) PL spectra collected at the first, third, and fourth measurements. (E) Schematic illustration for free charges and trapped charges resulting from exciton ionization.

  • Fig. 5 Effect of trapped charges on exciton recombination.

    (A) Pump-probe measurement results in suspended monolayer MoS2 under different pumping fluences. The inset is the result after being normalized to the value at 0 s. The measurements were performed with a pumping fluence of 1.9 μJ/cm2 per pulse (red), 12.8 μJ/cm2 per pulse (cyan), and 1.9 μJ/cm2 per pulse (black) in a sequence on the same spot. More experimental details can be found at our previous studies (21). The time interval between each of the measurement is around 10 min. (B) Normalized spatial-spectral PL images collected from monolayer MoS2 with low incident powers (1.1 kW/cm2) before (without trapped charges) and after (with trapped charges) the incidence of high photoexcitation (8.0 kW/cm2). They are collected at the first and third measurements in Fig. 4. The spectra at each location are normalized with respect to its own peak intensity, and the unnormalized results are plotted in fig. S7. The right panel shows typical spectra extracted from the spatial-spectra images, as indicated by dashed white lines.

  • Fig. 6 Enhancement of exciton diffusion coefficient.

    (A) Fitting and measured spatial profiles of PL intensity collected with low incident powers (1.0 kW/cm2) before (without trapped charges) and after (with trapped charges) the incidence of high photoexcitation (8.0 kW/cm2). The experimental results were collected at the first and third measurements in Fig. 4. The fitting results for the diffusion coefficient are given, as shown. (B) Comparison illustrating the consistence of the fitted PL intensity with the experimental result. (C) Schematic illustration for the screening of trapped charges for exciton scattering.

Supplementary Materials

  • Supplementary Materials

    Giant enhancement of exciton diffusivity in two-dimensional semiconductors

    Yiling Yu, Yifei Yu, Guoqing Li, Alexander A. Puretzky, David B. Geohegan, Linyou Cao

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    • Figs. S1 to S11
    • Sections S1 to S4

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