Investigating excitonic states in transition metal dichalcogenide monolayers and heterobilayers using resonance Raman spectroscopy

Abstract

This thesis investigates the excitonic states in transition metal dichalcogenide monolayers (TMDC) and heterobilayers using resonance Raman spectroscopy. The resonance Raman behaviour of monolayers of MoSe₂ and WSe₂ are probed at 4 K for both the A and B excitons and reveals the involvement of both neutral excitons and trions. For MoSe₂ when resonant with the B exciton several Raman peaks at 481.1, 531.2 and 581.1 cm⁻¹ exhibit an anomalous resonance behaviour. This is due to a double resonance with the neutral B exciton and a lower energy state at 1.802 eV, which is likely the 2s excited state of the A exciton. In monolayer WSe₂ resonance Raman behaviour is observed at the A and B excitons along with an additional resonance at 1.866 eV, which is attributed to the 2s excited state of the A exciton. A unique Raman peak at 494 cm⁻¹ is also discovered when resonant with the WSe₂ 2s state and has an asymmetric resonance profile. This asymmetry is the result of a double resonance process involving the 2s state at 1.866 eV and a higher energy state at 1.904 eV, which is attributed to either an excited state (3s/4s) or a dark excitonic state. Heterobilayer samples of MoSe₂/WSe₂ were investigated with twist angles of 57º (HS1) and 6◦ (HS2). For HS1 new low frequency Raman peaks are identified, which have a different characteristic spectra when exciting the MoSe₂ or WSe₂ excitons. These peaks are the result of zone folding with both the moir´e and crystallographic superlattices. The resonance profiles of the low frequency peaks demonstrate an unexpected coupling of these peaks only to trions which is not yet understood. For HS1 when resonant with the WSe₂ B exciton new Raman peaks at 290.7, 309.1 and 353.8 cm⁻¹ are observed. The peaks at 309.1 and 353.8 cm⁻¹ are assigned to the A00 2 (Γ) phonons associated with WSe₂ and MoSe₂ respectively. The presence of Raman peaks associated with the MoSe₂ layer at the WSe₂ B exciton suggests hybridisation of WSe₂ and MoSe₂ excitonic states. The symmetry associated with the A00 2 (Γ) phonons indicates that this hybridisation may involves the WSe₂ B exciton and an interlayer exciton.

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