Photoreflectance study of the near-band-edge transitions of chemical vapor deposition-grown mono- and few-layer MoS2 films

Abstract

Room-temperature photoreflectance (PR) and reflectance (R) spectroscopy are utilized to investigate the near-band-edge transitions of molybdenum disulfide (MoS2) thin films grown on sapphire substrates by a hot-wall chemical vapor deposition system. The layer thickness and optical properties of the MoS2 thin films are confirmed by Raman spectroscopy, atomic force microscope, and photoluminescence (PL) analysis. The B exciton shows relatively weak PL intensity in comparing with the A exciton even for monolayer MoS2 films. In the R spectrum of few-layer MoS2, it is not possible to clearly observe exciton related features. The PR spectra have two sharp, derivative-like features on a featureless background. Throughout the PR lineshape fitting, the transition energies are designated as the A and B excitons at the K-point of the Brillouin zone, but at room temperature there seems to be no distinguishable feature corresponding to an H-point transition for the mono- and few-layer MoS2 films unlike in bulk. These transition energies are slightly larger than those obtained by PL, which is attributed to the Stokes shifts related to doping level. The obtained values of valence-band spin-orbit splitting are in good agreement with those from other experimental methods. By comparing the PR lineshapes, the dominant modulation mechanism is attributed to variations of the exciton transition energies due to change in the built-in electric field. On the strength of this study, PR spectroscopy is demonstrated as a powerful technique for characterizing the near-band-edge transitions of MoS2 from monolayer to bulk.

Strong Substrate Strain Effects in Multilayered WS2 Revealed by High-Pressure Optical Measurements

“…The origin of the A* excitation has been a matter of debate since in general it could be attributed to either (i) an excited state of the A transition ( n = 2), (ii) a transition from the H -point of the Brillouin zone, (iii) an interlayer exciton, or (iv) charged excitons such as negative trions. ,− The latter interpretation can be ruled out since signals from trions are typically smeared out at temperatures above 200 K due to thermal exciton–electron scattering effects . Also, for the case of WS 2 , the interlayer exciton interpretation can be ruled out since the dark exciton energy is lower than that of the bright, while the A* feature seen in Figure exhibits higher energy than the A transition.…”

Hidden spin-polarized bands in semiconducting 2H-MoTe2

“…On the other hand, exploring the optical properties of MoTe 2 could help to clarify the ordering of the conduction bands at K or the nature of other interband transitions. For instance, the A* feature, which is ≈ 50 meV above the A transition, has been either assigned to excited excitonic states [10,11] or direct transitions at the H k-point [12][13][14]. More recently magneto-reflectance measurements showed that such feature could be attributed to an interlayer excitonic transition [15,16].…”

Phosphorous doped p-type MoS2 polycrystalline thin films via direct sulfurization of Mo film

“…The red-shifts are attributed to the Stokes shifts, and the magnitude of the Stokes shift is reported to increase with doping level. 36 I-V properties, resistivity, carrier concentration, and Hall-mobility of the undoped MoS 2 and MoS 2 :P samples were measured using a Hall-effect measurement system via the four measured points on the samples at the dark condition, as showed in Fig. 2.…”