Atomically thin MoS2: A new direct-gap semiconductor

TL;DR

The electronic properties of ultrathin crystals of molybdenum disulfide consisting of N=1,2,…,6 S-Mo-S monolayers have been investigated by optical spectroscopy and the effect of quantum confinement on the material's electronic structure is traced.

Abstract

The electronic properties of ultrathin crystals of molybdenum disulfide consisting of N = 1, 2, ... 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 MoS2 monolayer emits light strongly. The freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of 1000 compared with the bulk material.

Figures (4)

• Figure 1: (a) Lattice structure of$\mathrm{MoS}_{2}$ in both the in-plane and out-of-plane directions. (b) Simplified band structure of bulk $\mathrm{MoS}_{2}$, showing the lowest conduction band $c 1$ and the highest split valence bands $v 1$ and $v 2$. A and B are the direct-gap transitions and I is the indirect-gap transition. Also indicated are indirect gap for the bulk material $E_{g}^{\prime}$ and the direct gap $E_{g}$ for the monolayer, the latter as predicted by the zone-folding analysis through isolating the AH plane.
• Figure 2: (a) Representative optical image of mono- and few-layer$\mathrm{MoS}_{2}$ crystals on silicon substrate with etched holes of 1.5 and $0.75 \mu \mathrm{~m}$ in diameter. (b) PL image of the same samples shown in (a). The PL QY is much enhanced for suspended regions of the monolayer samples, demonstrating its high quality. Note that the emission from the fewlayer sample is too weak to be seen in this contrast setting.
• 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 of $\mathrm{MoS}_{2}$ for number of layers $N=1-6$. (b) PL spectra of thin layers of $\mathrm{MoS}_{2}$ for $N=1-6$. The spectra are normalized by the intensity of peak A and are displaced for clarity. (c) Band-gap energy of thin layers of $\mathrm{MoS}_{2}$ for $N=1-6$. The band-gap values were inferred from the energy of the PL feature I for $N=2$ - 6 and from the energy of the PL peak A for $N=1$. As a reference, the (indirect) band-gap energy of bulk $\mathrm{MoS}_{2}$ is shown as dashed line.
• Figure 4: (a) Absorption spectra (left axis) normalized by the layer number$N$ in the photon energy range from 1.3 to 2.4 eV . The corresponding PL spectra (right axis, normalized by the intensity of the peak A) are included for comparison. The spectra are displaced along the vertical axis for clarity. (b) Photoconductivity spectra for mono- (red dots) and bilayer (green dots) samples. The data are compared with the spectral dependence predicted for 2D semiconductors using the model described in the text (blue lines). Contributions from both indirect-gap (pink dotted line) and direct-gap (green dotted line) transitions are required to describe the photoconductivity spectrum of the bilayer; a direct-gap transition alone is adequate to describe the photoconductivity spectrum of the monolayer. For comparison, the energies of the direct and indirect gap inferred from the PL measurements are indicated by arrows.