Atomically Thin ${\mathrm{MoS}}_2$: A New Direct-Gap Semiconductor

The electronic properties of ultrathin crystals of molybdenum disulfide consisting of $N=1,2,\dots ,6$ S-Mo-S monolayers have been investigated by optical spectroscopy. Through characterization by absorption, photoluminescence, and photoconductivity spectroscopy, we trace the effect of quantum confinement on the material’s electronic structure. With decreasing thickness, the indirect band gap, which lies below the direct gap in the bulk material, shifts upwards in energy by more than 0.6 eV. This leads to a crossover to a direct-gap material in the limit of the single monolayer. Unlike the bulk material, the ${\mathrm{MoS}}_2$ monolayer emits light strongly. The freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of ${10}^4$ compared with the bulk material.

Figure 1

Lattice structure of ${\mathrm{MoS}}_2$ in both the in- and out-of-plane directions and simplified band structure of bulk ${\mathrm{MoS}}_2$, showing the lowest conduction band $c1$ and the highest split valence bands $v1$ and $v2$. $A$ and $B$ are the direct-gap transitions, and $I$ is the indirect-gap transition. $E_g^{\prime }$ is the indirect gap for the bulk, and $E_g$ is the direct gap for the monolayer.

Figure 2

(a) Representative optical image of mono- and few-layer ${\mathrm{MoS}}_2$ crystals on a silicon substrate with etched holes of 1.0 and $1.5\mu \mathrm{m}$ in diameter. (b) PL image of the same samples. The PL QY is much enhanced for suspended regions of the monolayer samples, and the emission from the few-layer sample is too weak to be seen in this image.

Figure 3

(a) PL spectra for mono- and bilayer ${\mathrm{MoS}}_2$ samples in the photon energy range from 1.3 to 2.2 eV. Inset: PL QY of thin layers for $N=1–6$. (b) Normalized PL spectra by the intensity of peak $A$ of thin layers of ${\mathrm{MoS}}_2$ for $N=1–6$. Feature $I$ for $N=4–6$ is magnified and the spectra are displaced for clarity. (c) Band-gap energy of thin layers of ${\mathrm{MoS}}_2$, inferred from the energy of the PL feature $I$ for $N=2–6$ and from the energy of the PL peak $A$ for $N=1$. The dashed line represents the (indirect) band-gap energy of bulk ${\mathrm{MoS}}_2$.

Figure 4

(a) Absorption spectra (left axis, normalized by $N$) and the corresponding PL spectra (right axis, normalized by the intensity of the peak $A$). The spectra are displaced along the vertical axis for clarity. (b) Photoconductivity spectra for mono- (red dots) and bilayer (green dots) samples [27]. The data are compared with the 2D model described in the text (blue lines). $E_g$ and $E_g^{\prime }$ inferred from the PL measurements, indicated by arrows, are included for comparison.