서강대학교

초록/요약

We studied the optical properties of tungsten disulfide (WS2) and heterostructures based on WS2 through photoluminescence (PL) and Raman spectroscopy. First, the origin of the variation of the PL spectrum of monolayer WS2 is investigated systematically. The dependence of the PL spectrum on the excitation power shows that the relatively sharp component corresponds to excitons whereas the broader component at slightly lower energy corresponds to negatively charged trions. PL imaging and second harmonic generation (SHG) measurements show that the trion signals are suppressed more than the exciton signals near the edges, thereby relatively enhancing the excitonic feature in the PL spectrum and that such relative enhancement of the exciton signals is more pronounced near approximately armchair edges. This effect is interpreted in terms of depletion of free electrons near the edges caused by structural defects and adsorption of electron acceptors such as oxygen atoms. We also carried out the Raman spectroscopy on WS2/ReS2 van der Waals (vdW) heterostructures to investigate their optical properties. In the Raman spectrum of heterostructures, an interlayer peak at 26.5 cm-1 is observed, which confirmed that the constituent WS2 and ReS2 layers are well coupled. The strength of the interlayer interaction between WS2 and ReS2 is estimated by comparing interlayer mode with the linear chain model and is weaker than other transition dichalcogenides (TMDs) materials. It is also found that the interlayer mode has a polarization dependence using linearly polarized Raman scattering. Raman spectra of WS2/ReS2 heterostructures measured with up to seven excitation energies show that the main E2g1 and A1g peaks of the WS2 Raman spectrum are enhanced at some excitation energies, whereas the Raman modes of ReS2 are not. We suggested that this anomalous Raman enhancement effect is associated with the resonance effect from the exciton states of WS2 or the interlayer transition between WS2 and ReS2.