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.

Light-Matter Interactions in Two-Dimensional Transition Metal Dichalcogenides: Dominant Excitonic Transitions in Mono- and Few-Layer MoX$_2$ and Band Nesting

“…We calculate a binding energy of the A exciton of 0.51 eV relative to the fundamental electronic band gap of 2.69 eV from our G 0 W 0 calculations, which fits well into the range of reported values for the A exciton of 0.2-0.57 eV from experiments [29,31,33] and to recent theoretical calculations [44,49,50]. Apart from the transition forming the absorption onset, the dielectric function features a prominent peak at an energy of 2.9 eV, which has been previously linked to the prominent 'C' feature in photoluminescence spectra at 2.8 eV [59] and consists of a A B I n t e n s i t y ( a r b . u n i t s )…”

Stacking-controllable interlayer coupling and symmetric configuration of multilayered MoS2

“…Therefore, the major contribution to VBS is SOC for an odd number of layers, while for an even number of layer, the dominant effect is interlayer interactions. [1][2][3]16,17 However, the structural symmetry in the AA(A…)-stacked multilayers is independent of the even-odd oscillation with the number of layers: all the layers should have broken inversion symmetry, [10][11][12][13] and thus, the observed VBS is mainly contributed by SOC. As shown in Figure 3c, the VBS phenomena exhibited a stacking-orientation dependence, with the AB(A…)-stacked crystal showing larger VBS than the AA(A…)-stacked crystal.…”

Dark excitons and the elusive valley polarization in transition metal dichalcogenides

“…The K K and ′ K K valleys can be selectively excited by σ ± circularly polarized photons, respectively [18]. In addition, the strong spin-orbit interaction in TMDs lifts the degeneracy of spin states particularly in the valence band giving rise to well separated A and B-excitonic transitions in absorption or reflectance spectra [6,[19][20][21]. The valence band splitting in TMDs is in the range of a few-hundred meV [3,5].…”