Ultrafast and spatially resolved studies of charge carriers in atomically thin molybdenum disulfide

Atomically thin molybdenum disulfide is emerging as a new nanomaterial with potential applications in the fields of electronic and photonics. Charge carrier dynamics plays an essential role in determining its electronic and optical properties. We report spatially and temporally resolved pump-probe studies of charge carriers in atomically thin molybdenum disulfide samples fabricated by mechanical exfoliation. Carriers are injected by interband absorption of a 390-nm pump pulse and detected by measuring differential reflection of a time-delayed and spatially scanned probe pulse that is tuned to an exciton transition. Several parameters on charge carrier dynamics are deduced, including carrier lifetime, diffusion coefficient, diffusion length, and mobility.

Figure 1

(a) Optical microscopy photo of the studied MoS${}_2$ flakes under white light illumination. (b) PL spectrum of the sample. (c) Schematics of the optical pump-probe setup. (d) Schematics of the band structure of bulk MoS${}_2$ and the pump/probe configuration. The blue and red vertical arrows indicate the pump and probe photon energies, respectively. (e) Geometry of the multiple reflection from the sample. The rays are vertically shifted for clarity.

Figure 2

(a) Differential reflection signal measured with a 390-nm pump and a 660-nm probe. The solid line is a single exponential fit with a time constant of 100 $\pm$ 10 ps. (b) Calculated $\Delta R/R_0$ as a function of $-\Delta \alpha /{\alpha }_0$ by using Eq. (1) over a rather large range of $\Delta R/R_0$. For small $\Delta R/R_0$ on the order of ${10}^{-3}$, the relation is approximately linear. (c) Calculated $\Delta R/R_0$ as a function of $-\Delta n/n_0$ by using Eq. (1).

Figure 3

(a) Spatial profiles of the $\Delta R/R_0$ signal measured by scanning the probe spot with respect to the pump spot, with several probe delays as labeled in each panel. The solid lines are Gaussian fits. (b) Square width of the spatial profiles deduced from Gaussian fits as a function of the probe delay. The solid line is a linear fit, corresponding to a diffusion coefficient of 20 $\pm$ 10 cm${}^2$/s.