New plasmon-exciton coupling induced optical phenomena in monolayered WS₂ sandwiched in a metal-film-coupled nanocavity

Abstract

Recently, the study of two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs) has become one of the most interesting research topics due to their unique electrical and optical properties and their great potential for applications in optoelectronics devices. For example, the bandgap of Tungsten disulphide (WS2) is typically layer-sensitive. It exhibits an indirect band gap in multilayer form while it changes to a direct bang gap in monolayer form. Such flexible photoelectric characteristics allow a wide range of applications of 2D-WS2 in transistors, photodetectors and electroluminescent devices. However, the optical and vibrational properties of WS2 strongly depend on their fabrication methods, such as mechanical exfoliation (ME) and chemical vapour deposition (CVD). It is generally believed that ME prepared samples have higher quality than CVD prepared samples, but a comprehensive comparison between the two methods remains to be explored. The thesis provides a comparative study on the vibrational and optical properties of CVD-grown and ME-prepared monolayered and few-layered WS2 by using several optical spectroscopic techniques. Dramatic differences are observed in the Raman response, differential reflectance, photoluminescence (PL) efficiency and second-harmonic generation (SHG), which are all suggested to be related to the structural defects. Surprisingly, it is found that the PL of the CVD-grown WS2 bilayer shows almost no linear polarization dependence and exhibits no significant difference under the two different circularly polarized excitations, while these polarization dependences are much stronger for the ME-prepared WS2 bilayer. However, it can be observed that the CVD-grown WS2 bilayer has an unprecedentedly stronger SHG signal than the signal of both the CVD-grown and ME-prepared monolayers. The results not only point out the importance of improving current CVD-based growth methods to achieve large-area TMD materials with optical, electrical and structural properties comparable to small flakes prepared by ME methods, but also demonstrate the promise of using CVD-grown bilayer TMDs in general for nonlinear optoelectronics applications. In addition, the properties of the bright and dark excitons in monolayer WS2 have also been explored. Two respective states of the splitting conduction band, which are caused by the broken inversion symmetry and strong spin-orbital coupling, give rise to the bright and dark excitons by coupling with holes in the valence band. Normally, due to the selection rules, the radiative relaxation of dark excitons are generally forbidden, which means that the dark excitons are optically dark. Because the dark excitons have a longer lifetime, they can have potential applications in quantum communication and play an important role in fundamental research, such as Bose-Einstein condensation. To induce radiative emission of dark excitons in atomically thin TMDs, different approaches have been demonstrated in recent low temperature photoluminescence (PL) studies. However, the control of dark excitons and the observation of dark excitons at room temperature can be the remaining challenges. The second part of this thesis presents a way to observe the dark excitons under room temperature by metal-film-coupled nanocavities through polarization-resolved PL measurements. Firstly, the mechanically exfoliated WS2 monolayers are sandwiched into the metal-film-coupled nanocavities. Then, the polarization-resolved PL measurements are conducted to detect the dark excitons. A blue shift of the peak from the PL spectra can be observed when the detected polarization angle changes from 0 to 90 degree. To further examine the origin of this phenomenon, a fitting model is established to predict and fit all the experimental PL spectra. The fits are satisfactory in different dimensions and different characteristics of bright and dark excitons, such as peak width, peak intensity and polarization dependence can be directly observed. This part provides a simple method to observe the optical effects of dark exciton in room temperature. By controlling the detected polarization angle, the proportion of the bright and dark excitons can be changed. Compared with other works about brightening of dark excitons, this work has advantages in the simplification of the experimental condition (i.e. room temperature, simple set-up) and abundance of extracted information.