Atomically thin transition metal dichalcogenides (TMDs) are a new family of semiconductors with exciting optical properties for practical applications in optoelectronic devices, as well as intriguing many-body behaviors for fundamental explorations in condensed matter physics. In bulk form they are van der Waals layered materials and have been traditionally used as industrial lubricants. When thinned down to monolayers they surprisingly become direct band gap semiconductors with exceptionally strong light-matter interactions dominated by exciton effects. In these monolayer semiconductors, excitons can be robustly observed even at room temperature, in contrast to conventional bulk semiconductors where the electron-hole binding energies are typically lower than the room temperature thermal energy. The strong excitonic effect is a direct result of both fundamental quantum confinement effects and reduced dielectric screening in a two-dimensional system. This dissertation presents experimental investigations on a few central questions in these materials: how strong is the electron-hole binding effect in the prototypical monolayer semiconductor MoS2 as well as in novel metal-chalcogenolate hybrid materials; how can exciton effects be tuned by carrier doping and controlled by electrostatic gating in real devices; and how can excitons coherently enhance nonlinear optical transitions for the development of novel nonlinear devices and imaging techniques.

The dissertation starts with an introduction to interband optical transitions including exciton effects in low-dimensional electronic systems. The focus is on providing a many-body physical picture for better understanding experimental phenomena, and not as much on a detailed quantitative treatment. Then three major research works surrounding this topic are included in the dissertation. First, we study carrier-induced quasiparticle band gap and exciton energy renormalization effects in monolayer MoS2. We show that both the quasiparticle band gap and the exciton binding energy can be largely tuned by applying electrostatic gating to a monolayer device. In the second work, we investigate exciton dominated optical properties of a family of “bulk 2D” materials: self-assembled hybrid metal organic chalcogenolate multi quantum wells. Owing to the weak interlayer electronic coupling, the exciton binding energy in these multilayered bulk materials are shown to be close to a true monolayer semiconductor. We also reveal the bright and dark exciton species by cryogenic photoluminescence excitation spectroscopy, providing further evidence of high exciton binding energy in the bulk layered crystal. In the third work, we come back to atomically thin TMDs and study how exciton resonances greatly enhance their nonlinear optical responses, leading to successful continuous-wave generation of nonlinear signals from monolayer TMDs. Finally, in addition to optical spectroscopy studies, we also present two examples of device applications of two-dimensional materials in micro-electromechanical systems (MEMS). In one example we demonstrate micro strain sensors based on the two-dimensional electron gas formed at the interface of AlGaN/GaN. In the second example we demonstrate efficient ultraviolet photodetectors using graphene/diamond heterojunctions. Together these studies provide crucial knowledge on the utilization and control of exciton enhanced linear and nonlinear optical properties of atomically thin semiconductors and related low-dimensional materials, and guidance towards practical optoelectronic device applications.

The efficient conversion of low-energy, near-infrared (NIR) photons to higher energies promises advancements across a range of disparate fields, from more efficient solar energy capture to advanced biologic studies and therapies. At the forefront of this effort, upconverting nanoparticles (UCNPs) contain an array of lanthanide ions alloyed into a ceramic matrix, most commonly NaYF4. These lanthanide ions possess a 4f orbital manifold with a series of ladder-like, long-lived electronic states, allowing the sequential absorption of NIR photons at relatively low photon flux. The electrons climb the 4f Stark levels, absorbing multiple photons from the excitation source before relaxing back to the ground state upon emission of a photon with an energy higher than those from the source. While UCNPs are among the most efficient systems for converting NIR light to visible, they still hold key limitations, namely, they only weakly absorb incoming photons, hindering their external quantum efficiencies and overall performance.

This thesis presents studies designed to enhance the efficiency and brightness of UCNPs. One method to circumvent the inherently weak absorption in UCNPs involves attaching organic dye antennae to the surface of the nanoparticles to absorb incoming light and funnel the absorbed energy into the UCNP. However, a detailed understanding of the energy transfer mechanism between dye and UCNP is required to fully realize the potential of this system. We propose, using a series of spectroscopic and kinetic measurements, that this energy transfer is mediated via the absorbing dye triplet state, and further that the intersystem crossing rate within the dye is enhanced by interactions with the heavy nuclei of the lanthanides within the UCNP. We then design a highly efficient dye:UCNP hybrid system utilizing this new understanding. Lastly we present a study highlighting the power and potential of using UCNPs for biological applications. Together these studies will advance the utilization of photon upconversion towards realizing its promise in applications.

The monolayer semiconducting transition metal dichalcogenides are a family of 2D crystals that, beginning with MoS2 and soon expanding to include tungsten and other chalcogen variants, has become a focus of much research in 2D materials. Their semiconductor character, with wide direct bandgaps in the visible and near-infrared energy spectrum, make them particular amenable to optoelectronic applications. The low-dimensional character leads the direct gaps to host tightly bound exciton states that are stable at room temperature, allowing for the experiments on exciton physics that were only accessible at low temperatures in classical 2D excitonic system such as GaAs 2D quantum wells. The crystal structure of TMDs further gives the excitons and other quasi-particles valley contrasting physics, which can be discriminated optically with circularly polarized light, and leads to phenomena such as the valley Hall effect. The combination of strong light-matter interactions in TMDs with novel physics makes optical spectroscopy and microscopy powerful tools in the characterization of these materials. However, the native length scales of the excitons, while larger than the lattice constant, are still far below the resolution of optical probes. This resolution is set by the diffraction limit of light, which limits the focusing of propagating electromagnetic fields to roughly half the wavelength. Optical probes will effectively average over large areas (200 x 200 nm2) of the crystal, obscuring the effects of highly localized and heterogeneous perturbations.

The need for local experimental probes in the study of TMDs has been highlighted by the recent discovery of single-photon emission from monolayer WSe2. Many far-field experimental works have shown a strong association of the emergence of these emitters and regions inhomogeneous tensile strain, leading to the hypothesis of quantum confinement of the excitons by the strain-created potential wells. However, experimental investigation of this hypothesis is challenging due to the scale mismatch of the localized excitons, which must be on order the exciton Bohr radius of ~ 1 nm, and the ~ 250 nm size of the optical probes.

The focus of this work is to bridge the resolution gap of optical microscopy and spectroscopy in the study of excitons in the 2D TMDs using scanning near-field optical microscopy (SNOM), which allows optical resolutions down to a few nanometers. Using this technique to measure spectra of photoluminescence (PL) and scattering from lattice vibrations (Raman scattering), I investigate the nanoscale interplay of strain and the excitons in several of the commonly studied semiconducting TMDs, including WS2, WSe2, and MoSe2. In this thesis, I first discuss methods of performing near-field optical studies on 2D materials, including aperture and apertureless SNOM methods, and demonstrate them by collecting real nanoscale PL (nano-PL) and nano-Raman on WS2 and WSe2. Following that, I show how to estimate the strain in nanobubbles from classical models of plate bending, and show by comparison with shifts in the lattice vibrations that such models on average correctly estimate the strain. Further, by combining high-resolution nano-PL of WSe2 with advanced microscopic theories, I show evidence that strain alone can efficiently localized excitons at room temperature due to formation of atomic scale wrinkles of the crystal. Using gap-mode nano-PL with nanometer resolution, I demonstrate that the spatial location of localized exciton emission in WSe2 occurs along a common axis over several nanobubbles, and that it is highly localized in the wrinkle-like rings in asymmetric nanobubbles. Lastly, investigation of nanobubbles in MoSe2/WSe2 heterostructures shows that similar wrinkle-like ring structures occur commonly in large nanobubbles, and that such structures host strong exciton complexes in energetic regions associated with the interlayer exciton. This emission is unusually strong at room temperature, and has emission peaks that extend passed 1000 nm, which has not been previous described in such heterostructures. The results presented here show the strong and rich interplay of excitons with strain, demonstrating the utility of strain-engineering in future TMD devices, but also represent a frontier in studying the physics of confined excitons both spectrally and spatially.