Abstract
THz conductivity of large area MoS2 and MoSe2 monolayers as well as their vertical heterostructure, MoSe2MoS2 is measured in the 0.3-5 THz frequency range. Compared to the monolayers, the ultrafast THz reflectivity of the MoSe2MoS2 heterobilayer is enhanced many folds when optically excited above the direct band gap energies of the constituting monolayers. The free carriers generated in the heterobilayer evolve with the characteristic times found in each of the two monolayers. Surprisingly, the same enhancement is recorded in the ultrafst THz reflectivity of the heterobilayer when excited below the MoS2 bandgap energy. A mechanism accounting for these observations is proposed.
Highlights
The family of two-dimensional semiconducting transition metal dichalcogenides (2D-TMDs) such as MoS2, WS2, MoSe2 and so on, has grown significantly [1] and it is likely to remain one of the leading topics in science for many years to come due to many facets of scientific findings and knowledge they can contribute to
Heterostructures formed by two TMD monolayers can exhibit type-I or type-II electronic band alignments [9,10], leading to the formation of interlayer radiative excitons where the bottom of the conduction band and the top of the valence band reside in different layers
From the saturation of the dynamic THz response of MoSe2MoS2 at excitation photon energy of 1.55 eV, i.e., below the MoS2-bandgap energy, we find that about 90% of the photoelectrons transiently transferred from MoSe2 to MoS2 layer
Summary
The family of two-dimensional semiconducting transition metal dichalcogenides (2D-. TMDs) such as MoS2, WS2, MoSe2 and so on, has grown significantly [1] and it is likely to remain one of the leading topics in science for many years to come due to many facets of scientific findings and knowledge they can contribute to. Optical excitation at 3.1 eV generates photocarriers in both the monolayer samples and the MoSe2MoS2-heterobilayer This modulates the photoinduced conductivity and makes it possible to infer the characteristic hot carriers’ relaxation kinetics in them. 3(b,c) and 3(e,f) represent the mean behavior of the fluence dependence The highlights from these results are: (i) only the slow relaxation component, (A1, 1) is present up to reasonably high pump-fluences whose time constant strongly decreases with the increasing fluence, while, the saturation in its amplitude is seen at a much higher value of ~150 J/cm, for both the MoSe2 and MoSe2MoS2, (ii) at any given pump-fluence, the magnitude of photoinduced THz reflectivity change in MoSe2MoS2 is about four times stronger than in MoSe2
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