Seminarium Fizyki Ciała Stałego

Semiconducting transition metal dichalcogenides (S-TMDs) based on molybdenum andtungsten, i.e. MoS2, MoSe2, MoTe2, WS2, and WSe2, are the most well-known representativesof van der Waals materials. Their most distinguished hallmark is the transition from indirect- todirect-band gap, when thinned down from a bulk to a monolayer (ML). The optical response ofS-TMDs is caused mainly by excitonic effects, even at room temperature, due to the largeexcitonic binding energy at the level of hundreds of meV. Moreover, the exciton-phononcoupling (EPC) is significant in thin layers of S-TMDs, which leads to the rich Raman scattering(RS) spectra under their resonant excitation conditions.In my talk I will give an overview on excitons and phonons apparent in high qualityS-TMD MLs and bilayers (BLs) encapsulated in hexagonal BN. The first part of the lecturewill cover experimental and theoretical investigations of excitonic properties in ML and the BLof MoSe2 [1]. The measured magnetic field evolutions of the reflectance contrast spectra of theMoSe2 ML and BL permit one to determine g-factors of intralayer A and B excitons, as well asthe g-factor of the interlayer exciton. The dependence of g-factors on the number of layers andexcitation state will be explained using first-principles calculations. It will be demonstrated thatthe experimentally measured ladder of excitonic s states in the ML can be reproduced using thek·p approach with the Rytova–Keldysh potential that describes the electron–hole interaction.On the contrary, the analogous calculation for the BL case requires taking into account the outof-plane dielectric response of the MoSe2 BL.The second part of my talk will be devoted to the study of phonons in the four S-TMDMLs, i.e. MoS2, MoSe2, WS2, and WSe2, using Raman scattering excitation (RSE), which is anexperimental technique in which the spectrum is made up by sweeping the excitation energywhen the detection energy is fixed [2]. It will be shown that the outgoing resonant conditionsof Raman scattering reveal an extraordinary intensity enhancement of the phonon modes, whichresults in extremely rich RSE spectra. The obtained spectra are composed not only of Ramanactive peaks, i.e. in-plane E’ and out-of-plane A’1, but the appearance of 1st, 2nd, and higherorder phonon modes is recognized. Moreover, the intensity profiles of the A’1 modes in theinvestigated MLs resemble the emissions due to neutral excitons measured in the correspondingPL spectra for the outgoing type of resonant Raman scattering conditions. This results indicatesthat the strength of EPC in S-TMD MLs strongly depends on the type of their ground excitonicstate, i.e. bright or dark, resulting in different shapes of the RSE spectra.[1] Ł. Kipczak, et al., 2D Materials 10, 025014 (2023).[2] M. Zinkiewicz, et al., npj 2D Materials and Applications 8, 2 (2024)