Investigation of Intrinsic and Tunable Properties of Two-Dimensional Transition-Metal Dichalcogenides for Optical Applications

Since the scotch-tape isolation of graphene, two-dimensional (2D) materials have been studied with increasing enthusiasm. Two-dimensional transition-metal dichalcogenides are of particular interest as atomically thin semiconductors. These materials are naturally transparent in their few-layer form, have direct band gaps in their monolayer form, exhibit extraordinary absorption, and demonstrate unique physics, making them promising for efficient and novel optical devices. Due to the two-dimensional nature of the materials, their properties are highly susceptible to the environment above and below the 2D films. It is critical to understand the influences of this environment on the properties of 2D materials and on the performance parameters of devices made with the materials. For transparent optical devices requiring electrical contacts and gates, the effect of transparent conducting oxides on the optical properties of 2D semiconductors is of particular importance. The ability to tune the optical properties of 2D transition-metal dichalcogenides could allow for improved control of the emission or absorption wavelength of optical devices made with the materials. Continuously tuning the optical properties of these materials would be advantageous for variable wavelength devices such as photodetectors or light emitters. This thesis systematically investigates the intrinsic structural and optical properties of two-dimensional transition-metal dichalcogenide films, the effect of substrate-based optical interference on the optical emission properties of the materials, and demonstrates methods to controllably tune the luminescence emission of the materials for future optical applications. This thesis advances the study of these materials toward integration in future efficient and novel optical devices. The specific transition metal dichalcogenides investigated here are molybdenum disulfide (MoS₂), molybdenum diselenide (MoSe₂), tungsten disulfide (WS₂), and tungsten diselenide (WSe₂). The thickness-dependence of the intrinsic in-plane crystal structure of these materials is elucidated with high-resolution transmission electron microscopy; thickness-dependent optical properties are studied using Raman and photoluminescence spectroscopies. This thesis investigates the optical interference effects from substrates with transparent conducting oxide layers on the optical properties of few-layer MoS₂ films. An understanding of these effects is critical for integrating MoS₂ into efficient optical devices. We predict contributions of optical interference effects to the luminescence emission of few-layer MoS₂ films. The predictions are experimentally verified. We also demonstrate the use of optical interference effects to tune the wavelength and intensity of the luminescence emission of few-layer MoS₂. This thesis explores the use of electric fields applied perpendicular to the films to continuously and reversibly tune the band gap of few-layer MoS₂ for future variable wavelength devices. To facilitate integration into devices, we demonstrate electric fieldinduced band gap tuning by applying electric fields with a pair of transparent or semitransparent conducting layers, and without the need for direct electrical contact to the MoS₂ films. The observed band gap tuning is attributed to the Stark Effect. We discuss challenges to maximizing the effect of electric field-induced band gap tuning. We demonstrate that optical interference effects do not prevent observation of band gap tuning via applied electric fields. We successfully combine two luminescence emission tuning methods: optical interference effects and electric field effects.