The optical response of bulk germanium sulfide (GeS) is investigated systematically using different polarization-resolved experimental techniques, such as photoluminescence (PL), reflectance contrast (RC), and Raman scattering (RS). It is shown that while the low-temperature (T = 5 K) optical band-gap absorption is governed by a single resonance related to the neutral exciton, the corresponding emission is dominated by the disorder/impurity- and/or phonon-assisted recombination processes. Both the RC and PL spectra are found to be linearly polarized along the armchair direction. The measured RS spectra over a broad range from 5 to 300 K consist of six Raman peaks identified with the help of Density Functional Theory (DFT) calculations: Ag1, Ag2, Ag3, Ag4, B1g1, and B1g2, which polarization properties are studied under four different excitation energies. We found that the polarization orientations of the Ag2 and Ag4 modes under specific excitation energy can be useful tools to determine the GeS crystallographic directions: armchair and zigzag.

Optical properties of thin layers of rhenium diselenide (ReSe 2 ) with thickness ranging from mono-(1 ML) to nona-layer (9 MLs) are demonstrated. The photoluminescence (PL) and Raman scattering were measured at low (T =5 K) and room (T =300 K) temperature, respectively. The PL spectra of ReSe 2 layers display two well-resolved emission lines, which blueshift by about 60 meV when the layer thickness decreases from 9 MLs to a bilayer. A rich structure of the observed low-energy Raman scattering modes can be explained within a linear chain model. The two phonon modes of intralayer vibrations, observed in Raman scattering spectra at about 120 cm −1 , exhibit very sensitive and opposite evolution as a function of layer thickness. It is shown that their energy difference can serve as a convenient and reliable tool to determine the thickness of ReSe 2 flakes in the few-layer limit.

The temperature evolution of the resonant Raman scattering from high-quality bilayer 2H-MoS$$_{2}$$ 2 encapsulated in hexagonal BN flakes is presented. The observed resonant Raman scattering spectrum as initiated by the laser energy of 1.96 eV, close to the A excitonic resonance, shows rich and distinct vibrational features that are otherwise not observed in non-resonant scattering. The appearance of 1st and 2nd order phonon modes is unambiguously observed in a broad range of temperatures from 5 to 320 K. The spectrum includes the Raman-active modes, i.e. E$$_{\text {1g}}^{2}$$ 1g 2 ($$\Gamma$$ Γ ) and A$$_{\text {1g}}$$ 1g ($$\Gamma$$ Γ ) along with their Davydov-split counterparts, i.e. E$$_{\text {1u}}$$ 1u ($$\Gamma$$ Γ ) and B$$_{\text {1u}}$$ 1u ($$\Gamma$$ Γ ). The temperature evolution of the Raman scattering spectrum brings forward key observations, as the integrated intensity profiles of different phonon modes show diverse trends. The Raman-active A$$_{\text {1g}}$$ 1g ($$\Gamma$$ Γ ) mode, which dominates the Raman scattering spectrum at T = 5 K quenches with increasing temperature. Surprisingly, at room temperature the B$$_{\text {1u}}$$ 1u ($$\Gamma$$ Γ ) mode, which is infrared-active in the bilayer, is substantially stronger than its nominally Raman-active A$$_{\text {1g}}$$ 1g ($$\Gamma$$ Γ ) counterpart.

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The temperature effect on the Raman scattering efficiency is investigated in $$\varepsilon$$ ε -GaSe and $$\gamma$$ γ -InSe crystals. We found that varying the temperature over a broad range from 5 to 350 K permits to achieve both the resonant conditions and the antiresonance behaviour in Raman scattering of the studied materials. The resonant conditions of Raman scattering are observed at about 270 K under the 1.96 eV excitation for GaSe due to the energy proximity of the optical band gap. In the case of InSe, the resonant Raman spectra are apparent at about 50 and 270 K under correspondingly the 2.41 eV and 2.54 eV excitations as a result of the energy proximity of the so-called B transition. Interestingly, the observed resonances for both materials are followed by an antiresonance behaviour noticeable at higher temperatures than the detected resonances. The significant variations of phonon-modes intensities can be explained in terms of electron-phonon coupling and quantum interference of contributions from different points of the Brillouin zone.

The dielectric environment of atomically thin monolayer (ML) of semiconducting transition metal dichalcogenides affects both the electronic bandgap and the excitonic binding energy in the ML. We investigate the effect of the environment on the in-plane magnetic field brightening of neutral and charged dark exciton emissions in the WSe2 ML. The monolayers placed in three dielectric environments are studied, in particular, the ML encapsulated in hexagonal BN (hBN) flakes, the ML deposited on a hBN layer, and the ML embedded between the hBN flake and SiO2/Si substrate. We observe that the brightening rates of the neutral and charged dark excitons depend on the dielectric environment, which may be related to the variation of the level of carrier concentration in the ML. Moreover, the surrounding media, characterized by different dielectric constants, weakly influence the relative energies of the neutral and charged dark excitons in reference to the bright ones.

Excitons in thin layers of semiconducting transition metal dichalcogenides are highly subject to the strongly modified Coulomb electron-hole interaction in these materials. Therefore, they do not follow the model system of a two-dimensional hydrogen atom. We investigate experimentally and theoretically excitonic properties in both the monolayer (ML) and the bilayer (BL) of MoSe2 encapsulated in hexagonal BN. The measured magnetic field evolutions of the reflectance contrast spectra of the MoSe2 ML and BL allow us to determine g-factors of intralayer A and B excitons, as well as the g-factors of the interlayer exciton. We explain the dependence of g-factors on the number of layers and excitation state can be explained using first principles calculations. Furthermore, we demonstrate that the experimentally measured ladder of excitonic s states in the ML can be reproduced using the k·p approach with the Rytova-Keldysh potential that describes the electron-hole interaction. In contrast, the analogous calculation for the BL case require taking into account the out-of-plane dielectric response of the MoSe2 BL.

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