Broadband optical properties of large-area monolayer CVD molybdenum disulfide

Recently emerging large-area single-layer ${\mathrm{MoS}}_2$ grown by chemical vapor deposition has triggered great interest due to its exciting potential for applications in advanced electronic and optoelectronic devices. Unlike gapless graphene, ${\mathrm{MoS}}_2$ has an intrinsic band gap in the visible which crosses over from an indirect to a direct gap when reduced to a single atomic layer. In this paper, we report a comprehensive study of fundamental optical properties of ${\mathrm{MoS}}_2$ revealed by optical spectroscopy of Raman, photoluminescence, and vacuum ultraviolet spectroscopic ellipsometry. A band gap of 1.42 eV is determined by the absorption threshold of bulk ${\mathrm{MoS}}_2$ that shifts to 1.83 eV in monolayer ${\mathrm{MoS}}_2$. We extracted the high precision dielectric function up to 9.0 eV, which leads to the identification of many unique interband transitions at high symmetry points in the ${\mathrm{MoS}}_2$ momentum space. The positions of the so-called A and B excitons in single layers are found to shift upwards in energy compared with those of the bulk form and have smaller separation because of the decreased interactions between the layers. A very strong optical critical point predicted to correspond to a quasiparticle gap is observed at 2.86 eV, which is attributed to optical transitions along the parallel bands between the $M$ and $\Gamma$ points in the reduced Brillouin zone. The absence of the bulk ${\mathrm{MoS}}_2$ spin-orbit interaction peak at ∼3.0 eV in monolayer ${\mathrm{MoS}}_2$ is, as predicted, the consequence of the coalescence of nearby excitons. A higher energy optical transition at 3.98 eV, commonly occurring in bulk semiconductors, is associated with a combination of several critical points. Additionally, extending into the vacuum ultraviolet energy spectrum are a series of newly observed oscillations representing optical transitions from valence bands to higher conduction bands of the monolayer ${\mathrm{MoS}}_2$ complex band structure. These optical transitions herein reported enhance our understanding of monolayer ${\mathrm{MoS}}_2$ as well as of two-dimensional systems in general and thus provide informative guidelines for ${\mathrm{MoS}}_2$ optical device designs and theoretical considerations.

Figure 1

Raman characterization of the CVD-grown ${\mathrm{MoS}}_2$ on fused silica using a 532 nm laser line with a 342 nm spot size and 12.5 μW power: (a) Optical image of the monolayer ${\mathrm{MoS}}_2$ on fused silica substrate. (b) Averaged Raman spectroscopy of a 5 × 5 μm area in the center of the area shown in (a). (c) and (d) Raman mapping images of the $E^{\prime }$ and $A^{\prime }$ peak intensity, respectively, taken in the whole area shown in (a) with a 250 nm grid spacing. Scale bar: 5 μm.

Figure 2

PL characterization of the CVD-grown ${\mathrm{MoS}}_2$. (a) Averaged PL spectrum (red curve) measured over 5 × 5 μm area in the center of the area in Fig. 1 and the imaginary part (ε${}_2$; blue curve) of the dielectric function measured by SE. (b) and (c) PL mapping images of the A exciton peak intensity and peak energy, respectively, taken in the area shown in Fig. 1 with a 250 nm grid spacing. Scale bar: 5 μm.

Figure 3

The schematic of ellipsometry measurement and the dielectric function of the CVD-grown monolayer ${\mathrm{MoS}}_2$. (a) Schematic of SE measurement. (b) The real (ε${}_1)$ part of the dielectric function of CVD-grown monolayer and bulk ${\mathrm{MoS}}_2$, inset: zoom-in of the spectra range from 1.6 to 3.2 eV. (c) The imaginary (ε${}_2)$ part of the dielectric function of CVD-grown monolayer and bulk ${\mathrm{MoS}}_2$. The inset is the absorption spectrum of monolayer and bulk ${\mathrm{MoS}}_2$ calculated from the measured dielectric function.

Figure 4

CP determination. Red curves are the fits to the second derivatives (black curves) of the real (ε${}_1)$ and imaginary (ε${}_2)$ parts of the dielectric function of monolayer ${\mathrm{MoS}}_2$. The inset is the comparison of the fitting results when using exciton and 3D lineshapes.

Figure 5

Determination and comparison of the optical band gap of monolayer (blue curve) and bulk (red curve) ${\mathrm{MoS}}_2$.