Ultrathin Light Sources: Lanthanide Ions in Polyelectrolyte Films and Two-Dimensional Molybdenum Disulfide Interfaced with Gallium Nitride Optical Cavities

Abstract

Ultrathin light-emitting materials are necessary for the design of compact optical devices such as sensors, lasers, optical communication systems, and biological probes. Such materials must be structurally and chemically stable over years to be practical for scientific applications and modern consumer technology. We examine two different types of thin light-emitting materials to characterize their optical properties on different substrates and under different environmental conditions to determine their suitability for further possible applications and devices. These are trivalent lanthanide molecules embedded in charged polymer matrices and the two-dimensional transition metal dichalcogenide molybdenum disulfide. Trivalent lanthanides provide a stable emission source at wavelengths spanning the ultraviolet through the near infrared with uses in telecommunications, lighting, and biological sensing and imaging. Methods for incorporating these lanthanides into materials need to preserve their narrow linewidths, long fluorescence lifetimes, and relatively high quantum yields. We form an organometallic complex with one trivalent lanthanide bound to three organic dipicolinate molecules. We then embed this lanthanide-containing complex into polyelectrolyte multilayers to form uniform, optically active thin films bound to substrates such as silicon, fused silica, and thin gold film. Specifying the number of polyelectrolyte layers provides direct control over the thickness of the films. Utilizing different lanthanides such as europium and terbium, we are able to easily tune the resulting wavelength of emission of the thin film. The emission intensity and quality in these samples is preserved over a period of months. In addition, emission is not quenched when the europium-containing complex is brought within 20 nm of thin metallic films and may in fact be enhanced. Finally, we have found a correlation between lanthanide emission intensity and the number of multilayers. These results demonstrate the suitability of this platform as a thin film emitter source for a variety of photonic applications such as waveguides, optical cavities, and sensors. Two-dimensional transition metal dichalcogenides (2D TMDs) are 2D semiconductors that emit visible through near-infrared light via exciton recombination and hold promise for a variety of optoelectronic devices, such as photo-detectors, sensors, and lasers. Interfacing 2D TMDs with optical cavities provides enormous opportunities in engineering ultrathin devices with tailored optical emission properties. We use the 2D TMD molybdenum disulfide (MoS2), which has broad emission from 600-750 nm. We use gallium nitride (GaN) microdisk optical cavities as our testbed in an attempt to enhance this MoS2 emission at selected wavelengths corresponding to the modes of the optical cavity. Our GaN material contains integrated near-ultraviolet quantum well (QW) emitters that exhibit cavity mode emission and lasing behavior, thereby providing a way to characterize the optical cavities before the introduction of MoS2 and providing a secondary probe to better understand the interaction at the MoS2-GaN interface. We are able to do large scale growth of MoS2 flakes via chemical vapor deposition (CVD) on silicon dioxide on silicon and then use a polymer transfer method to move the as-grown material to GaN bulk substrate for initial optical characterization and then to as-fabricated GaN optical cavities. Our experiments show that a single monolayer of MoS2 quenches the QW cavity mode emission and lasing behavior of the GaN microdisks, suggesting strong coupling of the MoS2 to the optical cavity. Yet, at the same time, we do not observe MoS2 emission coupling to the optical cavity. We perform finite-difference time-domain simulations, which suggest that providing an intermediary dielectric layer between the GaN disk and MoS2 could improve the quality of the optical cavity in this heterostructure. We introduce a spacer layer of SiO2 between the MoS2 and GaN microdisk surface to recover the QW cavity modes and provide a qualitative model to understand the interaction between the GaN QW and MoS2 photons emitted in this hybrid system. These results have implications for determining how to optimize the optical behavior of 2D TMDs in on-chip device applications where multiple optical components must be placed in close proximity.