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Here, we report on the optical properties of manganese selenide (MnSe) layers grown by molecular beam epitaxy. We compare MnSe film growths on c-plane Al2O3 in various conditions. Samples were analyzed in situ using reflection high energy electron diffraction. This technique confirmed that in specific conditions growth of crystals well oriented to the substrate is possible and the surface of such crystals became flat after tens of nm, with typical grain size dependent on growth time. Further we studied samples using room temperature optical spectroscopy. Intensity of broad peak observed in photoluminescence spectra is strong for samples with large MnSe crystallites and very weak for samples with small crystallites. Origin of this peak is attributed to Mn2+ d--d transitions strongly hybridised with Se shells. Thanks to the transparency of Al2O3, transmittance measurements have been performed on obtained layers, revealing a broad absorption band in blue spectral range, which explains the yellow color of MnSe layers. Properties observed in transmittance and the Raman scattering -- large band gap and the Raman lines close to 250 cm-1 are in agreement with the data reported for the most stable polymorph of MnSe -- rocksalt structure (alpha-MnSe). [ABSTRACT FROM AUTHOR]

The Raman scattering has been studied in heterostructures composed of a thin MoS2 flake and a 1-1.5 nm layer of thermally evaporated gold (Au). There have been Au nanoislands detected in the heterostructure. It has been found that their surface density and the average size depend on the MoS2 thickness. The Raman scattering spectrum in the heterostructure with a few monolayer MoS2 only weakly depends on the excitation (resonant vs. non-resonant) mode. The overall Raman spectrum corresponds to the total density of phonon states, which is characteristic for disordered systems. The disorder in the MoS2 layer is related to the mechanical strain induced in the MoS2 layer by the Au nanoislands. The strain results in the localization of phonon modes, which leads to the relaxation of the momentum conservation rule in the scattering process. The relaxation allows phonons from the whole MoS2 Brillouin zone to interact with electronic excitations. Our results show that the Au nanoislands resulted from thermal evaporation of a thin metal layer introduce substantial disorder into the crystalline structure of the thin MoS2 layers. [ABSTRACT FROM AUTHOR]

The temperature evolution of the resonant Raman scattering from high-quality bilayer 2H-MoS 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 1g 2 (Γ ) and A 1g (Γ ) along with their Davydov-split counterparts, i.e. E 1u (Γ ) and B 1u (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 1g (Γ ) mode, which dominates the Raman scattering spectrum at T = 5 K quenches with increasing temperature. Surprisingly, at room temperature the B 1u (Γ ) mode, which is infrared-active in the bilayer, is substantially stronger than its nominally Raman-active A 1g (Γ ) counterpart. [ABSTRACT FROM AUTHOR]