Broadband optical properties of large-area monolayer CVD molybdenum disulfide

Abstract

Recently emerging large-area single-layer 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, 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 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 MoS 2 that shifts to 1.83 eV in monolayer 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 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 points in the reduced Brillouin zone. The absence of the bulk MoS 2 spin-orbit interaction peak at ß3.0 eV in monolayer 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 MoS 2 complex band structure. These optical transitions herein reported enhance our understanding of monolayer MoS 2 as well as of two-dimensional systems in general and thus provide informative guidelines for MoS 2 optical device designs and theoretical considerations.

Rapid Wafer-Scale Growth of Polycrystalline 2H-MoS2 by Pulsed Metal–Organic Chemical Vapor Deposition

“…The two broad features centered at 2.04 eV (656.0 nm) and 1.89 eV (607.6 nm) are assigned to the A and B excitons, respectively, originating from the direct gap transition at the K point. The energy of the A and B excitons are consistent with measurements from natural MoS 2 samples, and the simultaneous presence of the two transitions generally suggests a multi-layer film 2,56,57 . As with Raman, existing literature on PL of MoS 2 focuses on single-crystal measurements.…”

Heterostructures of MoS2 nanofilms on TiO2 nanorods used as field emitters

“…been low. In contrast, the band gap of MoS 2 is around or even obviously lower than 2 eV [7,22]. Therefore, in comparison with TiO 2 , more electrons should have been available in the MoS 2 conduction band during the field emission.…”

Spectral Characteristics of Noble Metal Nanoparticle–Molybdenum Disulfide Heterostructures

“…[ 40 ] Observed Raman shift peaks for deposited MoS 2 at 384 and 408 cm −1 (4 cm −1 width) for E 2g and A 1g modes, [ 41 ] respectively, confi rmed existence of primarily mono-to few-layer MoS 2 fl akes in the sample. [ 8,43,44 ] Overall, no aggregation of drop-casted nanoparticles and/or MoS 2 was apparent in optical images, spectral signatures, or electron images. 7-9 Å height differentials at the fl ake edges (see Figure S2, Supporting Information).…”

“…A 556 nm (2.23 eV) LSPR was previously reported for 60 nm AuNS on monolayer MoS 2 under electron excitation. [ 43,44,51,55 ] Use of dielectric data for liquid-exfoliated MoS 2 , which exhibited a 1.85 eV A transition, could move simulated bandgaps closer to measurements. Little absorption enhancement of exciton features in the far-fi eld by plasmonic nanoparticles was reported in FDTD simulations of 150 nm AuNS on MoS 2 .…”