Confocal absorption spectral imaging of MoS2: optical transitions depending on the atomic thickness of intrinsic and chemically doped MoS2

Abstract

We performed a nanoscale confocal absorption spectral imaging to obtain the full absorption spectra (over the range 1.5–3.2 eV) within regions having different numbers of layers and studied the variation of optical transition depending on the atomic thickness of the MoS₂ film. Three distinct absorption bands corresponding to A and B excitons and a high-energy background (BG) peak at 2.84 eV displayed a gradual redshift as the MoS₂ film thickness increased from the monolayer, to the bilayer, to the bulk MoS₂ and this shift was attributed to the reduction of the gap energy in the Brillouin zone at the K-point as the atomic thickness increased. We also performed n-type chemical doping of MoS₂ films using reduced benzyl viologen (BV) and the confocal absorption spectra modified by the doping showed a strong dependence on the atomic thickness: A and B exciton peaks were greatly quenched in the monolayer MoS₂ while much less effect was shown in larger thickness and the BG peak either showed very small quenching for 1 L MoS₂ or remained constant for larger thicknesses. Our results indicate that confocal absorption spectral imaging can provide comprehensive information on optical transitions of microscopic size intrinsic and doped two-dimensional layered materials.