Growth and Functionalization of Monolayer WS2 Quantum Dots and Films for Photoluminescence Modulation and Photodetector application

Abstract

The present thesis focuses on the top-down controlled synthesis of WS2 quantum dots (QDs) by liquid exfoliation as well as the bottom-up controlled growth of large-area monolayer WS2 by chemical vapour deposition (CVD). The effect of the growth of WS2 via CVD technique has been extensively studied for a variety of substrates and different growth conditions. In particular, the salt-assisted growth gave rise to large area monolayer film. A detailed analysis of excitation wavelength-dependent photoluminescence (PL) spectra of the WS2 QDs is carried out and the interaction of these QDs with single walled carbon nanotubes (SWCNTs) is carefully examined. The PL of the highly fluorescent WS2 QDs is systematically quenched on complex formation with defective SWCNTs. We have fabricated self-biased p-n heterojunction photodetectors based on Si/WS2 QDs heterostructure and Schottky Si/Au/WS2 photodetectors by incorporation of Au nanoparticles in the system. The Au nanoparticle embedded Si/Au/WS2 QD Schottky photodetector exhibits highly suppressed dark current and fast photo response. We investigated the evolution of the PL spectra of CVD grown monolayer WS2 and the induced doping effect by decorating it with non-van der Waals Bi2O2Se QDs through a four-energy level model involving coupled charged transfer-based rate equations. The doping effect is quantified by estimating the change in the net electron density on the formation of the WS2/Bi2O2Se heterostructure. Lastly, we have carried out a further comprehensive analysis of the PL emission of the CVD grown monolayer WS2. We have encapsulated of the CVD grown WS2 film with large band gap ZnO and investigated the accompanied modulation of the PL of monolayer WS2 by the quantum well effect. The excitation power dependent studies enabled room temperature tuning of many-body effects, such as trion and bi-exciton populations in the quantum well. Thus, we have presented thorough investigative studies on the controlled synthesis of WS2 QDs and monolayer WS2 films and the fabrication of various heterostructures with SWCNTs, plasmonic Au NPs, non-van der Waals Bi2O2Se QDs and large band-gap ZnO for applications in photoluminescence modulation and photodetection. There is a vast scope to extend the present work to understand the fundamentals of light-matter interactions in two-dimensional transition metal dichalcogenide systems and in turn, utilize them for a broad range of applications.