Femtosecond photoluminescence from monolayer ${\mathrm{MoS}}_2$: Time-domain study on exciton diffusion

Transition metal dichalcogenides are expected to be used in transparent, flexible, and highly efficient light-emitting devices. Exciton diffusion is a key factor in device applications. In this study, we measured the photoluminescence (PL) decay of excitons in a ${\mathrm{MoS}}_2$ monolayer synthesized by chemical vapor deposition on a sapphire substrate. The PL decay of A and B excitons in the femtosecond regime was observed. The PL decay curves were analyzed based on a model of exciton trapping at the deactivation center via diffusion, and the diffusion time until trapping was obtained. The diffusion coefficient and the corresponding exciton mobility were also determined. This study demonstrates the two-dimensional exciton diffusion in the femtosecond regime in the ${\mathrm{MoS}}_2$ monolayer.

Figure 1

(a) Raman scattering spectra of ${\mathrm{MoS}}_2$ sample (circles) and sapphire substrate (dotted curve). Solid curve shows the result of curve fitting using four Voigt functions (dashed curves). Inset shows optical microscope image of the sample (scale bar: 100 μm). (b) Absorption (black curve) and steady-state PL spectra (circles) of ${\mathrm{MoS}}_2$ sample. Solid curve shows the result of curve fitting using two Voigt functions (dashed curves) for A exciton and trion.

Figure 2

PL decay curves of monolayer ${\mathrm{MoS}}_2$ sample at (a) 2.03 eV (B exciton band) and (b) 1.89 eV (A exciton band). The excitation photon energy was 3.10 eV, and excitation density was $8\mu \mathrm{J}{\mathrm{cm}}^{–2}$ per pulse (corresponding photon flux is $4\times {10}^{16}$ photons ${\mathrm{m}}^{–2}$ per pulse). Circles and solid curves indicate results of the experiment and fitting analysis. Insets in (a) and (b) show absorption and PL spectra and excitation density dependence of PL decay of the A exciton, respectively.

Figure 3

Schematic illustrations of exciton generation and decay. (a) Generation of excitons by an excitation pulse with a photon energy of 3.10 eV. The energy density of the excitation pulse was $8\mu \mathrm{J}{\mathrm{cm}}^{–2}$ per pulse (corresponding photon flux is $4\times {10}^{16}$ photons ${\mathrm{m}}^{–2}$ per pulse), and the absorbed photon flux was $8\times {10}^{15}$ photons ${\mathrm{m}}^{–2}$ per pulse. (b) Decay of B excitons. The B excitons decay due to (i) trapping at defects via diffusion, (ii) relaxation to the A exciton state, (iii) relaxation to the trion state, or (iv) other processes involving radiative recombination and multiphonon emission. (c) Decay of A excitons. The A excitons decay due to (i) trapping at defects via diffusion, (ii) relaxation to the trion state, or (iii) other processes. The B-to-A exciton relaxation generates A excitons.