Tungsten Diselenide (WSe2) Quantum Dots for Optical and Catalytic Applications

Abstract

Two-dimensional tungsten diselenide (WSe2) quantum dots have drawn a lot of attention owing to their unique electronic-structure, exceptional room-temperature photonic emission, and outstanding catalytic properties. This dissertation presents novel top-down and bottom-up WSe2 QD manufacturing processes and establishes the underlying relationship between parameters governing the downsizing limit and WSe2 QD properties. The quality of as-produced WSe2 QDs was investigated in this dissertation using cutting-edge characterization techniques and validated by employing them in several real-world applications, including SERS applications for organic pollutant detection, optical applications for display, and catalytic applications for hydrogen production. In this dissertation, I propose three different synthesis methods for synthesizing WSe2 QDs. The first synthesis method is the top-down cryo-mediated procedure that produced 2H-phase WSe2 QDs with lateral diameters of under 8 nm. Using as-produced, high-quality WSe2 QDs, the surface-enhanced Raman spectroscopy (SERS) technique was used to detect organic molecules (RhB). WSe2 QDs demonstrate outstanding response for the RhB molecule, and the detection limit of the RhB molecule is as low as 10–10 M due to the high crystallinity and numerous edges. This thesis also proposes a theoretical model to explain the SERS mechanism of WSe2 QDs. Moreover, colloidal WSe2 QDs with monolayer thickness were synthesized using the bottom-up, hot-injection method. According to our study's findings, bottom-up, hot-injection WSe2 synthesis yielded lower lateral sizes (1.5 nm) than top-down, cryo-mediated WSe2 synthesis. The optical properties of WSe2 QDs produced by hot injection were investigated. The monodisperse and high-crystalline WSe2 QDs exhibit high-intensity PL emission at 422 nm. This thesis also highlighted how the excitation wavelength and WSe2 lateral size correlated with the PL emission wavelength. Furthermore, WSe2 QDs have excellent edge-catalytic activity for the hydrogen evolution reaction. However, the overall catalytic performance for hydrogen production was compromised by the limited charge conductivity of WSe2 QDs. To address the issue, a rational design of WSe2 QDs/Cu nanowire catalyst was produced by wrapping WSe2 quantum dots on copper nanowire to improve catalytic performance. By enhancing charge transfer, the onset potential of WSe2 QDs, which is 534 mV, is reduced to 259 mV for the WSe2 QDs/Cu (90:10) composite, showing great improvement in catalytic performance. In conclusion, this thesis proposes novel approaches for WSe2 QD synthesis and discusses potential optical and catalytic applications. This thesis also discusses key parameters that can be used to control the physical properties and downsizing limit of WSe2 QDs. As mentioned, novel synthesis methods and downsize limiting parameters will serve as a guide for new researchers interested in producing TMD quantum dots and other new nanomaterials for nanotechnology applications.